€mtll iitti»wsitg ffttatg BOUGHT WITH THE INCOME EROM THE SAGE ENDOWMENT FUND THE GIFT OF liettf g W. Sage 189X R-rs ^«-^^ .\M.^AM •357 CORNELL UNIVERSITY LIBRARY 3 1924 064 673 092 Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924064673092 A DICTIONARY OF ELECTRICAL ENGINEERING y^ 1^ / -fit_ BRUSH OR ROCKER RING FOR SIX-POLE GENERATOR [Frontispiece II. LIST OF ABBREVIATIONS USED THROUGHOUT THE DICTIONARY ac alternating current. A.I.E.E. American Institute of Electrical Engineers. amp ampere. amp hr ampere-hour. Am. Phys. Soc. American Physical Society. Ann. der Physik. Annalen der Physik. atm atmosphere. ats ampere turns. AWG American Wire Gauge. bdf break-down factor. bhp brake horse power. B.O.T. Board of Trade. B.P. British Patent. BThU British Thermal Unit. BTU Board of Trade Unit. BWG Birmingham Wire Gauge. cc continuous current. c emf counter electromotive force. cgs centimeter-gram-second. cm centimeter. cp candle power. cu cubic. Deutsch. Phys. Gesell. Deutsche Physikalische Gesellschaft. dm decimeter. dp double pole. D.R.P. Deutsches Eeichs Patent. dt double throw. ehp electrical horse power; extra high pressure. eht extra high tension. Elec. ' Electrician '. Elec. Eng. ' Electrical Engineering '. Elec. Engr. ' Electrical Engineer '. Elec. Eev. ' Electrical Review '. Elec. Times ' Electrical Times '. Elec. World 'Electrical World'. emf electromotive force. Eng. ' Engineering '. VI List of Abbreviations Engr. ' Engineer '. E.T.Z. ' Elektrotechnische Zeitschrif t '. fig- figure. ft foot or feet. ft lb foot-pound. g gram. gal gallon. g deg cal gram-degree calorie. hf high frequency. hp horse-power. hp hr horse-power-hour. h pr high pressure. hr hour. ht high tension. I.E.C. The 1908 Sub-committee on Nomenclature of the British Committee of the International Electrotechnical Commission. I.E.E. Institute of Electrical Engineers. ihp indicated horse power. in inch. IWG Imperial Wire Gauge. Journ.I.E.E. ' Journal of the Institution of Electrical Engineers '. kg kilogram. kg eal kilogram calorie. kgm kilogram-meter. km kilometer. km phps kilometers per hour per second. kva kilovolt-ampere. kw kilowatt. kw hr kilowatt-hour. lb pound. If low frequency. I pr low pressure. It low tension. m meter. mfd microfarad. mg milligram. min minute. mkg meter-kilogram. ml phps miles per hour per second. mm millimeter. mmf magnetomotive force. mphps meters per hour per second. pd potential difference. Pf power factor. Phys. Kev. ' Physical Eeview '. Proc.A.I.E.E. 'Proceedings of the American Institute of Electrical Engineers '. ProcLCE. ' Proceedings of the Institute of Civil Engineers '. Proc.LM.E. ' Proceedings of the Institute of Mechanical Engineers '. Proc. Koy. Soc. ' Proceedings of the Royal Society '. List of Abbreviations Vll Ref. References. rms root mean square. rpm revolutions per minute. rps revolutions per second. sec second. Soc. Int. Elect. ' Bulletin de la Soci^t^ Internationale des Electriciens *. sp single pole; single phase. sq square. St single throw. SWG Standard Wire Gauge. tp triple pole. Trans. Transactions. V.D.E. Verband Deutscher Elecktrotechniker. V.D.I. Verband Deutscher Ingenieure. w watt. w hr watt-hour. wpcp watts per candle power. yd yard. LIST OF PLATES VOLUME II Page Brush or Rocker Ring for Six-pole Generator (see page 453) Frontispiece Pipe-ventilated Motor 362 Back-geared Motor - 362 DxjDDELL Oscillograph for Use on 25,000 Volts 382 RoTORS^I 458 Rotors — II - 460 Stator Case for Induction Motor - 506 Thermit Welding (see page 608) - - - 506 Remote-control Switchboard at Lots Road Power Station, London (see page 528) 524 Motor-operated Oil Switches at Carville Power Station, Newcastlb- on-Tyne 524 4600-KVA Three-phase Transformer 576 600-KW Oil-cooled Three-phase Transformer 576 Rod Type of Trolley - - 582 Bogie Truck (see page 585) .... 582 Truck of the Motor Cars for the London-Port Stanley Railway, Ontario, with Two Combined Three-phase Continuous-current Motors, 120 bhp each 586 Rugby- Curtis Turbo -generator installed at Liverpool Corporation Electricity Works 588 2000-KW 1500-RPM Westinghouse-Parsons Turbo-generator 588 PouLSEN Receiving Station 628 Apparatus for Poulsen System of Wireless Telegraphy and Telephony 628 A DICTIONARY OF ELECTRICAL ENGINEERING Line Erection, the fixing in position of an overhead electric conductor. In the case of overhead trolley wires the procedure is generally as follows. The poles are laid along the road in approximately their right posi- tions, then a gang of men under a foreman dig the holes for the poles, followed by an- other gang who plant the poles. After five to eight days, which gives the poles time to set, the wire gang follows with its tower wagon, and a wagon drawn by one or two horses carrying a reel of a mile or less of trolley wire. The wire gang generally con- sists of one foreman, two drivers, three or four labourers, and two or three wiremen. The trolley wire is first anchored at the end of the line, then, say, 300 m are run out, or as great a length as the traffic will permit, and loosely suspended from the span wires by means of iron wire hooks, or from the bracket arms by means of wire loops. A ' come-along-clamp ' is attached to the free end of the trolley wire, and the latter is pulled tight, the exact pull depending upon the temperature at the time of erection, and temporarily anchored. Another 300 m or so is then run out, and this is repeated until the whole of the wire on the reel is paid off. At curves a permanent anchorage is made at each side of the curve, and enough slack allowed to go round. Finally the ears are soldered or clipped to the wire and the in- sulators screwed on to the ears, and sprung on to the span wire or fitted to the bracket arm as the case may be. [f. W.] Line Impedance denotes the impedance of a transmission line or cable, and is mea- sured in ohms. It is equal to ^iir^ ~ H^ + E^ where 2Tr~l is the reactance and E is the resistance. Line Inductance denotes the inductance or self-induction of a transmission line or cable in henrys. When the diameter, dis- tance apart, and length of the wires of a trans- mission line are known, the inductance may 2 log, - + i) cm per cm length of single conductor, where d is the distance of the return conductor and f I { Vol. II. 309 r is the radius of the wire. To convert this result to henrys divide by 10*. The in- ductance per cm length of drmit will have double the above value. The inductance of one wire of a three- phase line having the two return conductors equidistant from it is the same as that of one wire of a sp line. Where the three wires are not equidistant the inductance of each may be found by considering the separate effects of the two others upon it. [r. c] Line Loss. See Loss, Line. Line Material, the material necessary for the installation of an overhead line. The term is used more particularly in connection with overhead trolley lines, and in .some cases is taken to include the poles, bracket arms, trolley wire, &c., while in other cases only the smaller fittings, such as ears, straight-line hangers, frogs, &c., are referred to. Line of Force. See Line of Induc- tion; Maxwell; Field, Magnetic. Line of Induction, less accurately called line offeree, a conception introduced by Fara- day into the study of electricity and mag- netism to indicate the phenomena in the medium surrounding an electrically-charged body or source of magnetic force. Lines of induction may be imagined as existing in the medium in quantity at every point pro- portional to the flux of induction at the point, and everywhere in the direction of the induction (see Induction). A slightly different and more profitable way of regard- ing the matter is to imagine any area through which the flux of induction passes, as divided into elements, through each of which unit flux passes, and to suppose lines drawn through every point of the bounding curves of these elements in the direction of the in- duction, these lines being continued so as to divide the soace into tubes, the bounding surfaces of which are everywhere in the direction of the induction. It is these ' tubes of induction ' thai are usually known as lines of induction or lines offeree. A diagram drawn to represent a field of induction (see fig.) may be regarded as showing a section of these tubes. In the case of electric induction each 21 310 Line Poles tube starts at a unit positive charge, and ter- minates at a unit negative charge. In the case of magnetic induction the tubes re-enter on themselves, forming closed rings. It will be seen that the number of lines of induction passing through any area is the Lines of Induction total flux through the area. The ' line of in- duction' is accordingly a unit for the mea- surement of flux or flux-density of induction, and this, or some decimal multiple of it, is the unit most commonly used. See KlLO- line; Megaline; Maxwell; Field, Mag- netic, [f. w. c] Line Poles. — Electric conductors are often carried overhead, being supported at the desired height by insulators mounted on poles called line poles. For extra h pr -work line poles are being super- seded by line towers. Conceete-steel Line Pole, a support for overhead electric con- ductors, constructed of concrete, re-enforced in- ternally by a steel frame- work (see fig. 1). Such a pole has a greater transverse strength than many wooden poles, and will last a long time. A 10-m pole of triangular section will be about 38 cm wide at the bottom, tapering to about 15 cm wide at the top, and can be arranged to take an insulator at the top and a cross arm lower down. (Ref. 'Hydro-electric Plants', p. 466, R. C. Beardsley.) Metal Line Poles, metal poles for carry- ing overhead conductors. In Britain poles of this type are generally used in connection Fig 1.— Concrete-steel Line Pole with electric tramways, and the following figures are deduced from the standard speci- fication drawn up by the Engineering Stan- dards Committee. The poles shall be of mild steel, and of three classes — light, medium, and heavy, the outside dimensions being as given below. Sectional Poles Top. Middle. Bottom. Light Medium Heavy 14-0 cm 16-5 „ 19-0 „ 16-5 cm 19-0 „ 21-5 „ 19-0 cm 21-5 „ 24-0 „ Taper Poles Top. Outside Diameter i m from Base. Light Medium Heavy 12-0 cm 14-5 „ 17-0 „ 19-0 cm 21-6 „ 24-0 „ The thickness of the metal shall not be less than 6 mm, and in the case of sectional poles the length of the top section shall be 2'5 m, of the middle section 2'5 m, and of the bottom section 5 m. Sectional poles shall be either solid drawn, or lap-welded wrought steel, the sections being swaged together when hot, with the lap-welded seams at 120° from one another, and the joints 45 cm long. Five per cent of each class of sectional poles shall be tested by being dropped ver- tically, butt downwards, three times in sue- cession, from a height of 2 m on to a block of hard wood 15 cm thick, laid on concrete; and 5 per cent of each class of poles, whether sectional or taper, shall be tested for deflec- tion by being supported horizontally for 2 m from the butt, and a load applied at right angles to the axis of the pole 45 cm from the top. Under these conditions the figures in the following tables must at least be reached : — Light... Medium Heavy Load in ig for Temporary Deflection not ex- ceeding 16 cm. 340 kg 570 „ 900 „ Load in kg for Permanent Set not exceeding 13 mm. 450 kg 800 „ 1150 „ Should any pole fail, a further 5 per cent shall be tested, and if any of these fail, all the poles from which the samples are taken may be rejected. Line Poles 311 Terminal poles and poles that take the pull-off at curves have to stand the greatest stresses, and should be stronger than the poles on straight portions of the line. In ordinary soil, poles should be buried to a depth of about 2 m, the butts resting upon a slab of concrete, &c., to prevent them from sinking. There should be about 15 cm of concrete round each pole, and it should have a week or so to set before the trolley wire is erected. On straight portions of the track bracket-arm poles should be planted with a list backwards of about 8 cm, span-wire poles with 10 cm to 15 cm; terminal poles may have a list up to 30 cm. The trolley wires are either supported by bracket arms attached to the poles, or by span wires stretched across the road from one pole to another. Metal poles are also sometimes used on transmission lines, a special variety taking the form of towers. See TRANSMISSION Line, Steel Tovster. See interim report issued by the Engineer- ing Standards Committee entitled 'British Standard Specification for Tubular Tramway Poles'. Wooden Line Poles, wooden poles for carrying the conductors of an overhead elec- tric transmission line. In this country, Nor- way fir is the best-known wood for poles; in America, cedar, chestnut, northern pine, and redwood are all largely used. The following table from Bell's ' Electric Power Transmis- sion ' gives dimensions and weights of repre- sentative cedar poles : — Total length. m. Diameter at Top. cm. Diameter 2m from Butt. cm. Depth of Setting. m. Approx. Weight. kg. 10-7 12-2 13-7 15-3 19-0 19-0 20-3 20-3 32-0 33-0 35-6 40-0 1-7 1-8 20 2'1 300 400 450 590 Wooden poles should be creosoted if pos- sible, as their life is increased threefold or fourfold thereby; the process consists in sub- jecting the poles to dry steam in an air-tight vessel for several hours, and then impregnat- ing them with creosote (tar oil) under heavy hydraulic pressure. Prior to the creosot- ing, the bark is removed and the poles trimmed. If the poles are not creosoted, the butts should be coated with pitch, tar, or asphalt, and the pole-tops, which should be cut to a point, should be tarred or painted. The stresses in a pole are due to the following : — (a) The direct weight of the conductor and the downward component of the ten- sion on it, both of which are generally negligible. (6) Bending due to pull of conductors at corners, which may be very severe; when excessive, stays may be employed, the guy rope being of stranded steel wire of from 6 to 12 mm diameter. (c) Wind pressure on poles and wires, which may be taken at 2 kg per sq dm as a maximum, though allowance must be made for ice, which jn America is generally taken as resulting in each conductor having a dia- meter of 50 mm. This stress may be very great, and stays may have to be used in certain cases, to allow of the pole working under a reasonable factor of safety, which should be taken as from 5 to 8 or 10, the latter figures applying where the wind-pres- sure is excessive. It may be noted that in Britain the Board of Trade (see 'Board of Trade Eegulations for Overhead Wires' under Conductors, Overhead) stipulates that a wind pressure of 2-5 kg per sq dm must be allowed for, but that nothing extra need be added for snow effects; they also require a factor of safety of 10 to be worked to when the poles are of wood. It is also laid down in the same Regulations that the minimum height of an overhead wire above- ground is 7 m, and 8 m where a ht conductor crosses a road. In America, cross arms are generally of hard yellow pine. They vary in length from 1-2 m to 2-4 m, and in section, from 11 cm X 9 cm to, say, 13 cm x 10 cm. Cross arms above 2 m in length are generally braced. Fig. 2 shows the / arrangement of cross s/j, Om^ arms and braces on the ~ *' "^ line of the Missouri A .. >;> -— ^ River Power Company, "^^>^ which works at 57,000 ^f volts. The poles are of cedar, 11 m to 23 m long, and 23 cm to 30 cm in diameter at the top. The cross arms are of Oregon fir, and the insulator pins are of oak, boiled in paraffin. 'f R Fig. 2.— Cross Arms and Braces used by Missouri Eiver Power Company 312 Line Poles — Linseed Oil The cross-arms arrangement on the Niar gararBuffalo line is shown in fig. 3, and on 4e + + 1 \ 1 V + 1 1 Fig. 3.— Cross Arras of the Ifiagara-Buflalo line the Milano-Varese-Porto-Ceresio Railway line in fig. 4. Fig. 4.— Wooden Line Pole of tlie Milano-Varese- Porto-Ceresio Kailway Insulator pins, when of wood, are preferably of locust, though euca- lyptus and oak are also largely used. An insulator and pin for extra h pr, is illus- trated in fig. 5. Wooden poles are sometimes used in America and on the Continent to support the overhead equip- ment of trolley lines for tramways and light railways; in this coun- try, however, steel or iron poles are invari- g}jjg Fig. 5. — Insulator and /•i4 p Til ■ T. Pin for Extra-high Pressure (Kef. ' JCilectnC Jb'OWer Transmission Line Transmission ', Louis Bell ; 'Electric Transmission of Water Power', Adams J 'Hydro-electric Plants', Beardsley.) See Conductors, Overhead; Transmis- sion Line, Steel Tower; Line Material; Line, Overhead; Insulator; Insulator Pin; Line Erection; Cable, Aerial; Alu- minium, [f. w.] Line Reactance denotes the reactance of a transmission line or cable, and is mea- sured in ohms. It is equal to 2 tt ~ I where ~ = frequency in cycles per second, and I = inductance in henrys. See also Line In- ductance; Line Impedance. Line Towers. See Line Poles; Trans- mission Line, Steel Tower. Linear Expansion, Coefficient of. See Coefficient of Linear Expansion. Linear Hysteresis, hysteresis (which see) due to alternating flux as distinguished from that due to rotating flux. In the former case, the reversal is effected by dimin- ishing the magnetisation to zero and then increasing it in the opposite direction, while in the latter, reversal occurs without chang- ing the magnitude of the flux, but merely by rotating it in space. See 'Core Loss Curves' under Curve, Characteristic. Linen, Oiled. See Oiled Linen; Im- pregnated Insulating Materials. Linesman's Detector. See Detector, Magnetic; Electrolytic Detector. Linkage, Magnetic. See Magnetic Linkage; Inductance; Henry. Link Belting. See Gearing for Elec- tric Motors. Linked Switches. See Switches, Linked. Linolic Acid. See Linseed Oil. Linolite Lamp (Tubolite Lamp). See Lamp, Incandescent Electric. Linotape, the trade name of a varnished tape which is cut from the varnished cambric either 'on the straight' or 'on the bias'. ' On the straight ' means in the direction of the threads in the cloth, and ' on the bias ', diagonally to the direction of the threads. A bias tape is more suitable for taping small coils. Linoxyn. See Linseed Oil. Linseed Oil, the oil obtained from the seed of the flax plant, Linum usitatissimvm, which is cultivated both for its seed and its fibre in Russia, India, Canada, and America. Its principal constituents are the glycerides of two fatty acids, viz. linolic and linolenic acids. Its distinctive feature is the power of ab- sorbing oxygen from the atmosphere, forming Linseed Oil 313 a hard resinous compound termed Unoxyn. This property of 'drying' distinguishes it from all other oils, none of which has the power of absorbing oxygen to the same ex- tent; and has led to its almost exclusive use in the manufacture of paints and var- nishes. It expands on drying, and this fact, coupled with its high flash-point (about 260° C), has led to its adoption for the impreg- nation of fibrous materials. Its use for this purpose increases dielectric strength, and tends to render the treated material mois- ture-proof, but reduces mechanical strength. There is no known process for retarding the oxidation of linseed oil, and under the pro- longed influence of high temperature, fibrous materials impregnated with it ultimately become brittle. The drying of linseed oil is hastened by boiling, and for impregnating purposes it is preferable to use a boiled oil, since the raw oil often contains impurities. It is extensively employed in the manufac- ture of insulating varnishes, but is not suit- able for oil-cooled transformers, oil-break switches, and similar apparatus. Oxidation of Linseed Oil. — Linseed oil has the property of absorbing oxygen on exposure to the air, whether in large volumes or in thin films. In so doing, the oil ' dries ', forming a resinous compound known as oxy- linoleic add. This is capable of still further oxidation to Unoxyn, which is the final oxida- tion product. Linoxyn is a somewhat elastic, transparent solid, and is generally considered to be insoluble in water, but recent investi- gations tend to show that it will absorb moisture to some slight extent. The drying of linseed oil may be hastened by boiling, and adding 'driers'. Driers are oxygen- carrying compounds, but the manner in which they act upon the oil is not fully understood. Raw Linseed Oil. — There are two sys- tems of obtaining linseed oil, known as the European and the Anglo-American. They closely resemble each other, and may be briefly summarised thus. The seed of the flax plant Linum usitatissimum is first freed from dirt and other seeds, by sifting through sieves. It is next crushed under edge-run- ners, heated, and formed into cakes; these are subjected to hydraulic pressure, which is maintained for some little time, and causes the oil to exude from the seed. The oil is collected and undergoes a refining process, which consists of heating to a tem- perature of some 75° C; this causes the coagulation of any albumen which may have come from the seed during pressing. The solid impurities are allowed to settle, and the oil is drawn off and subjected to a treat- ment with dilute sulphuric acid. During this process the two are thoroughly mixed, and the acid destroys any impurities, and removes any water that may remain after heating. After thorough agitation, the mix- ture is allowed to stand, the acid layer settles to the bottom, is drawn off, and the oil is finally washed with warm water to remove any remaining traces of acid. The refined oil thus produced is sold as raw linseed oil. Boiled Linseed Oil. — This is raw lin- seed oil which has been subjected to a tem- perature of some 200° C. There are several methods of boiling, and the operation is a somewhat delicate one. The temperature and the length of time it is maintained affect the oil, as do also the quantity and nature of the 'driers' that are added during the process. The action taking place during boiling is somewhat uncertain, but it may be considered as more or less of a refining process, since water is driven off, and the oil decomposed to some extent; oxidation takes place, the oil is rendered darker in colour and becomes more viscous, whilst the specific gravity is raised. The boiled oil absorbs oxygen from the atmosphere more rapidly than the raw oil, and, for impregnating pur- poses, is to be preferred. Double-boiled Linseed Oil. — Eaw lin- seed oil is sometimes subjected to two boiling processes, and is then termed dmble-boiled linseed oil. This treatment renders the oil very dark in colour, but ensures the entire absence of moisture. For impregnating pur- poses a double-boiled oil is superior to a boiled oil. Blown Linseed Oil, sometimes termed blown oil, is boiled linseed oil through which dry air has been forced during the boiling process. It is considered by some authori- ties that oil boiled without the addition of driers does not dry rapidly; blown linseed oil dries quicker than raw linseed oil, though not quite so rapidly as an oil to which driers have been added. The process of forcing dry air through linseed oil removes any traces of water more thoroughly than simply boiling, and for this reason blown oil is to be pre- ferred for impregnating purposes. (Eef. 'Linseed Oil and other Seed Oils', Ennis.) See also Insulating Vaknishes; 314 Lion's Photometer — Local Action Impregnated Insulating Materials; Oiled Linen j Empire Cloth, [h. d. s.] Lion's Photometer. See Photometer, Actinic. Lippmann's Eleetrometer. See Elec- trometer; Electrocapillarity. Liquid Insulatingr Materials, Di- electric Strength of. See Dielectric Strength. Liquid Resistance. See Eheostats or Eesistances. Liquid Rheostat. See Eheostats or Eesistances. Liquid Starter. See Switch, Motor- starting. Liquid Starting Resistance. See Starting of Motors; Eheostats or Ee- sistances. Liter, the volume occupied by 1 kg of water; a volume equal to 1 cu dm; one- thousandth of 1 cu m; 1000 cu cm. Lithanode Accumulator. See Ac- cumulator. Litharge, ' lead oxide (PbO), used for active material in pasted-type accumulator plates. See Accumulator. Litholite, the trade name of a hard- rubber substitute which is manufactured in three qualities. They are all tough, hard materials, and resemble hard rubber in many of their properties. The best quality is unaffected by a temperature of 120° C. They will all take a good polish, do not absorb moisture from the atmosphere, and they withstand warm mineral oil without disintegration. See also 'Moulded Insula- tors' under Insulator. Live. — [' Ime means electrically . charged.' — From defini- tions accompanying Home Office 1908 Regulations for Electricity in Factories and Workshops.] See Alive; also Dead; Dead Wire. Live Wire. See Alive; also Dead; Dead Wire. Load, a generic name for the sum of the consuming devices connected to an electrical system. The load may consist of lamps, motors, metallic or liquid resistances, elec- tric furnaces, electrolytic cells, &c., or a combination of these. The term is usually applied to the output of a central station for the generation of electricity, Vhich is expressed in kw. It is generally of a fluctu- ating nature. The output of a generating station during a specified time, say one year, is also known as its load for that period, and is expressed in kielvins (kw hr). See also Central Station for the Gene- ration of Electricity; Kelvin. Load, Condensive. See Transformer, Voltage Drop in. Load, External, a term used, when con- sidering the internal actions in a generator, to denote the load which is connected across the terminals outside the generator. Load Characteristic. See Curve, Characteristic. Load Component. See Power Factor ; Current, Component. Load Curve. See Curves, Load, of Central Stations; Central Station for the Generation of Electricity. Load Dispatcher, an employ^ 0,t an electricity station for supplying electricity to an electric railway, whose duty consists in regulating the distribution of power over the entire system. The load dispatcher's office is equipped with a record board which automatically indicates by visual signals the apparatus and lines in and out of service. Load Factor, ratio of actual output in kelvins (or kw hr) delivered to consumers to the possible output if the maximum load were constantly in use throughout the year or period of supply. This equals : dumber of kelvins (or ' units ') sold x 100 Max. load on feeders in kw x hr during the supply period' [' The load factor of a machine, plant, or system is the ratio of the average povirer to the maximum povfer during a certain period of time. The average power is taken over a certain interval of time, such as a day or a year, and the maximum is taken over a short interval of the maximum load within that interval. In each case the interval of maximum load should be definitely specified. The proper interval is usually dependent upon local conditions and upon the pur- pose for which the load factor is to be determined.' —Paragraphs 50 and 51 of 1907 Standardisation Rules of the A.I.E.E.] [' Consumers' load factor, the number obtained by dividing the actual consumption during a given period of time by what that consumption would have been had the maximum load reached during that period continued in use throughout the whole of that period,' — I.E.O.] See Central Station for the Gene- ration OF Electricity. Load Loss. See 'Transformer Efficiency' under Efficiency. Load Side of a Meter. See Meter, Load Side of. Local Action, a galvanic action occurring in lead accumulator plates and in primary batteries, due generally to the presence qf Local Battery — Loop of Armature Coil 315 impurities. See Accumulator; Battery, Primary, Local Battery. See Battery, Primary. Local Circuit, a circuit subsidiary to, but in close conjunction with, a larger cir- cuit, e.g. the subsidiary circuit brought into operation by the action of a relay. See Eelay. Local Current, a term usually applied to the current flowing in the relay circuit at the receiving end of a telegraph line, or signalling line of any kind. The current coming over the line, being too feeble to operate the recording instrument directly, excites a sensitive relay magnet, which in turn closes a local circuit and makes the desired record. See Relay. The term is also sometimes used to denote parasitic and wasteful currents, such as flow in the zinc plate of a primary battery, due to impurities. See Local Action. Local Jack, a jack (connector) on a local (exchange) circuit in telephony. Locke Suspension Insulator, a type of link insulator. See 'Link Insulator' under Insulator. Locking* Device, an arrangement where- by the operation of one piece of electrical apparatus, as for instance a switch, is made dependent upon the position of another piece of apparatus, e.g. the contact-arm of a motor- starter, or the handle of a controller. In the case of a main switch interlocked with a motor-starter or controller, it is generally arranged that it shall be impossible to open or close the switch unless the starter or con- troller is in the 'off' position. The door of a switch-box or switch-pillar is sometimes interlocked with the main-switch in such a way that the door cannot be opened unless the switch is in the 'off' position, and that the switch cannot be closed unless the door is closed. See also Switch, Interlocking. Locomotive Jib Crane, Electric. See Crane, Electric. Lodestone, a natural magnet; an oxide of iron found in various parts of the world. All magnets, previous to the- discovery, by Oersted, of the magnetic field of an electric current, owed their magnetism, directly or indirectly, to contact with lodestone. See Magnet; Terrestrial Magnetism. Lodge Coherer. See Coherer. Lodge-Muirliead Coherer. See Co- herer. Lodge-Muirhead System of Wireless Telegraphy. See Wireless Telegraphy. 'Lohys' Steel. See Steel. Long-distance Telephony. See Tele- phony, Long-distance. Long-range Electromagnet. See Magnet, Long-pull. Long-shunt Dynamo. See Compound- wound Dynamo. Long-wall System of Coal-cutting. See Electric Coal-cutter. Looping-in, a phrase used in describ- ing a widely employed method of interior Looping-in wiring which to a great extent obviates making joints in the conductors. The fig. represents diagrammatically a circuit of three lighting points where the switches and lights are looped in. It is seen from the fig. that the conductors from the fuse-connections in the distribution box are, at each point where a connection is required, doubled back on themselves, and inserted into the terminals, and then, without being cut, carried on to the next lamp or switch. The remaining pairs of terminals are connected by short lengths of wire as shown. Loop of Armature Coil, the end of 316 Lorain Surface-contact System — Loss the V in an end connection. There are various types of loop, amongst which may be mentioned the radial half-turn loop, obligue ^ \ \ \ w I W \ 1 Fig. 1.— Eadial Half-turn loop Fig. 2.— Oblique Loop Fig. 3.— Composite Loop Loop of Armature Coil loop, and composite loop. These are shown in the accompanying illustrations. See CoiL, Form-wound; Hobart Type of End-con- nection. Lorain Surface - contact System. See Surface-contact System. Loss, Brush Frictional. — This loss, which is entirely due to the mechanical friction of the brushes on the surface of the commutator, is determined in the following manner : If ^ is the pressure on the brushes in kg per sq cm, and a is the total area of all brushes in sq cm, then the total pressure P is equal to pa. If « is the velocity of the commutator in m per sec, and if /a is the coefficient of friction, then the loss in m kg is equal to /n x P X « or in y MXPxt>x746 The value of /^ is 0'3 for carbon brushes, and 0'2 for copper brushes, p seldom rises above 0'16 kg per sq cm, and is more usually 0'09 to O'lO kg, the highest values being found in traction motors which are subject to considerable vibration. In ordinary motors, V usually lies between 10 and 14 m per sec, and rarely rises above 15. It has been stated that the frictional loss is greatly reduced by the application to the commutator of a very small amount of pa- raffin wax. This is said by Prof. Baily to reduce the friction to one-fifth its original value, and at the same time to impair in no way the collection of current from the machine. (Ref. F. G. Baily and W. Cleg- horne, ' Some Phenomena of Commutation ', Journ.I.E.E., vol. xxxviii, p. 160.) Loss, Copper. — This is occasioned by the energy that must be expended in a con- ducting circuit, to overcome its resistance. Consider any two points in a conducting circuit. If E be the difference of potential between these points, and I the current pass- ing, we have, by Ohm's law, E = IE. Now the rate at which energy is expended in w is given by the product E I, and therefore the energy expended between the two points is EI = (IE) X I = PE. Hence the electrical energy which is frit- tered away in heat due to the resistance of the conductor, is proportional to the resis- tance of the conductor and to the square of the current. See Ohm's Law. Loss, Core, a term applied to the energy dissipated in heat in the iron or steel circuits of electrical machinery when submitted to repeated changes of magnetic flux. It is made up of two components: — {a) The hysteresis or pure magnetic loss. (b) The eddy-current loss due to the cir- culation of wasteful currents (called eddy, or Foucault currents, or parasitic currents) in the substance of the iron itself. See Curve, Characteristic; Core Loss Current; Loss, Iron; Hysteresis; Eddy Current; Loss, Eddy-current; Lamina- tion OF Magnet; Laminated; Core Disks; Core, Armature; Energy Losses. Loss, Eddy-current,' the loss of power due to eddy currents induced in iron cores when subjected to alternating or rotating magnetisation. This loss is proportional to the square of each of the following: The maximum flux-density, B, the frequency, ~, and the thickness of the stampings. It is also proportional to the conductivity of the stamp- ings. To avoid eddy-current loss in iron, the stampings should be very thin (0-5 mm to 0-75 mm), especially when the frequency is high. Eddy-current losses also occur in copper conductors. See Eddy Current; Loss, Core; Loss, Iron; Lamination of Magnet; Lamination of Armature Con- ductors; Laminated; Coke Disks; Core, Armature; Energy Losses. Loss, Energy. See Energy Losses. Loss, Friction and Windage, in Dy- namo-electric Machinery.— The friction and windage losses are the two components of the mechanical loss in all rotating machinery. Loss 317 The journal friction for any particular machine will remain sensibly constant for that machine after it is once in a steady running condition, and, for a given diameter and length of bearing, is independent of load. The windage loss due to fan action in the rotating member increases at high speeds as the cube of the speed. The cor- rect evaluation of these losses is attended with much difficulty, and several methods have been proposed. For instance, the ma- chine may be directly coupled to a calibrated motor and run at various speeds, the w in- put to the driving motor being observed. Housman and Kapp (Elec, vol. xxvi, pp. €99, 700) for cc machinery run the machine under test as a separately-excited motor, and observe the relation between the emf and current at the armature terminals, graphi- cally separating the friction loss. Probably the most direct method is the retardation method of E.outin (' L'Eclairage Electrique', vol. ix, p. 169), in which the machine is run up to full speed, or to a little over speed, and then allowed to slow down under its own journal friction and windage, observations being made, at regular intervals of time, of the speed or of the armature emf induced by residual magnetism (in the case of cc machines). From the retardation curve the mechanical losses may be evaluated if the moment of inertia of the rotor is known, or this may be eliminated by taking a second retardation curve with the rotor coupled to a known inertia, such as a flywheel, or by applying to its pulley a known retarding torque hy means of a brake. With an elec- tric regenerative brake of the type devised by Drysdale (Engr., Nov. 24, 1905), the friction torque may be directly measured as a torque, without calculation, and independently of the moment of inertia of the moving parts. The plotted relation between speed and torque gives the complete separation of windage and journal friction. See Bearing Friction in Dynamos and Motors; Loss, Gearing; Loss, Brush-Friction AL; Losses, Commu- tator; Brush Pressure; Friction Co- efficient, [c. V. D.] Loss, Gearing. — The power lost in tooth gearing depends upon the form of teeth, the smoothness of finish, and the lubrication. With a knowledge of the pressure on the teeth, and the distance along which rubbing occurs, and the value of the coefficient of friction [i, the efficiency may be obtained, but great variations may occur even in a single pair of wheels. Goodman has given the following empiri- cal formula for a single pair of machine-cut wheels, including axle friction, which fairly represents the experimental values: — V = 0-96 ^ , ' 0-025 N' where N is the number of teeth in the smaller wheel. With rough unfinished teeth the value of the first term of the right-hand side of the formula falls to 0'90. The effi- ciency increases slightly with the velocity of the pitch lines. With a train of n pairs of wheels, the efficiency ij^ = rf^, where tj is the efficiency of one pair. See Gearing for Electric Motors; Loss, Friction and Windage, in Dynamo - electric Ma- chinery, [c. V. D.] Loss, Hysteresis, that part of the loss of energy occurring in iron, when cyclically magnetised, which is directly proportional to the speed of magnetic reversal when the ranges of magnetisation are the same. It represents a definite loss of energy for each reversal. The loss in w is therefore propor- tional to the frequency, and to V, the volume of iron, magnetised. It is found also to de- pend on about the 1-5 to 1-6 power of the induction density. Hysteresis loss in w = 1? X ~ X V X B^' X 10"' where tj = hyster- esis coefficient (or Steinmetz coefficient), ~ = frequency, in cycles per sec, B = density in lines per sq cm, and V = volume of iron in cu cm. To obtain a low hysteresis loss it is neces- sary to use well-annealed stampings of good transformer iron, which should preferably not be bent, cut, or machined after anneal- ing. See Steinmetz Coefficient; Specific Hysteresis Loss; Energy Losses; Loss, Core; Loss, Iron. [c. v. d.] Loss, Iron. — The iron loss in any piece of electrical apparatus is occasioned by changes in the magnitude and direction of the flux traversing its magnetic circuits. The loss is made up of two components, viz. hysteresis or pure magnetic loss, and eddy-current loss, due to the secondary cur- rents induced in the bulk of the magnetic material itself by the variation of its flux. The hysteresis loss for purely alternating flux has been shown by Steinmetz to vary approximately as the 1-6 power of the in- duction density, but for cases where the 318 Loss — Low Frequency magnetism rotates relatively to the core {e.g. armature cores of cc machines), the law is not so simple (see 'Core Loss Curves' under Curve, Characteristic). The eddy-current component may be made small by lamination in the direction of the flux, but will depend upon the care taken with the insulation between the separate laminations. It varies as the square of the product of the induction density, the square of the thickness of the lamination, and the square of the frequency. See Energy Losses; Loss, Core; Loss, Hysteresis; Lamination of Magnet; Laminated; Core Disks; Core, Arma- ture; Loss, Eddy-current; Eddy Cur- rent, [c. V. D.] Loss, Line. — The energy wasted on a transmission line may be divided into two parts. Firstly, the true ohmic loss produc- ing a drop in pressure along the line. For the same efficiency the ratio of the lost volts to the generator volts must be constant, and for a given power the current producing this drop varies inversely as the generator pres- sure. Hence the economy of transmission with a given conductor is proportional to the square of the generator pressure. Limits are imposed on this, however, by the increase of cost, at high pressures, of generating, trans- forming, and distributing plant, and by the character of the load. See Law, Kelvin's. Secondly, with increase of pressure the second component of the line loss increases. This component is due to leakage and silent discharge between the conductors. The creep- ing loss over the insulators in dry climates- is relatively small, but Scott, Mershon, and Watson have shown that the loss due to silent discharge becomes very appreciable at pressures above 40,000 volts, and rises very rapidly as the pressure increases. Loss, Load. See 'Transformer Effi- ciency' under Efficiency. Loss, Rheostatie. ^Whenever the con- trol of an electric current is affected by the insertion of a resistance or rheostat into the circuit, the rheostat becomes the seat of a cer- tain energy loss. This is numerically equal to the resistance of the rheostat multiplied by the square of the current carried by it, and constitutes the rheostatic loss. Loss of Synchronism. See Syn- chronism, Loss of. Losses, CommutatOF, the losses occur- ring at the commutator, in continuous-elec- tricity machinery, are primarily due to three causes : — 1. Friction of the brushes upon the com- mutator surface. 2. Ohinic resistance presented by the con- tact between brush surface and commutator surface. 3. Stray losses due to parasitic currents and sparking. The reduction of the heating due to these combined causes to within reasonable limits, is of the utmost importance. With copper brushes, the combined effect of these losses, even where there is slight sparking, may not lead to a very great temperature rise, but with carbon brushes, much care must be exercised, otherwise considerable temperature rise will result, due to the increased values of the first two causes, and to the greater length of brush arc, which largely affects the third. Each of these causes of loss is treated in detail under its special heading (which see). Parshall and Hobart, taking a peripheral speed of commutator of 13 m per sec, and a brush pressure of O'l kg per sq cm, put- the temperature rise from these causes at 1'3° C. per w per sq dm, with a total rise of temperature of 50° C. with continuous running. [c. V. D.] Losses, No-load. See No-load Losses. Losses, Windage, in Dynamo-eleetrie Machinery. — These are the components of the mechanical losses in all rotating machinery, which vary with the speed. For high values- of the speed, the windage follows a cube law, but there is reason to suppose that at lower speeds, the power absorbed more nearly fol- lows a square law. The windage losses in any particular machine depend very much upon design. Thus they may be relatively insignificant in induction motors with cage rotors presenting a smooth unbroken surface, and the beneficial effect of the cooling, due to fan-action, has often to be obtained by providing such rotors with fan blades at the ends of the core. With large flywheel al- ternators, on the other hand, windage may become of importance when the peripheral velocity of the flywheel is high and especially when the wheel has built-up arms. For methods of separation see Loss, Fric- tion AND Windage. [c. v. d.] Losses in a Meter. See Meter, Losses; in. Low Frequency. See Frequency, Low Low-hysteresis Steel — Luxmeter 319 Low-hysteresis Steel. See Steel. Low-load Adjustment for ce Energy Meters. See Coil, Compounding. Low Pressure. See Pressure; Cur- rent, High-pressure. Low-pressure Current. See Current, High-pressure. Low-pressure Mains. See Mains. Low -resistance Magnet, an electro- magnet excited by a few turns of thick con- ductor adapted for use on a 1 pr supply or (more usually) in series with a main circuit. Low-tension Installations. See In- stallations, Low-tension. Low-tension Network. See Network of Conductors. Lp, the preferable abbreviation for low pressure. L.S.W.G., Legal Standard Wire Gauge. See Wire Gauge. Lt, the preferable abbreviation for low tension. Lubricants for Electrical Machinery. See Bearings. Lubrication, Forced. See Bearings. Lug-hand of Accumulator. See Ac- cumulator, Lug-hand of. Lug of Commutator. See Commuta- tor, Lug of. Lumen, the unit of luminous flvx proposed by the 1896 Geneva Congress. The lumen is stated by Solomon, on p. 33 of 'Electric Lamps', to be equal to the flux of light from 1 pyr in unit solid angle. Solomon points out that the lumen is not a spherical pyr. The lumen equals — ^— spherical cp. The lumen is widely accepted as the stan- dard of luminous flux, and is the light sent out from a unit source through a unit solid angle. Lumenmeter. See Photometer, Globe. Luminescence. See Radiation. Luminous Absorption Power. See Power, Luminous Absorption. Luminous Arc. See Arc. Luminous Efficiency. See Efficiency, Radiant. Luminous Intensity. — The luminous intensity produced by a source of light at a point is proportional to the power of the source, and inversely proportional to the sq of the distance from the source. It is measured in candle m or candle ft. A single cp source at 2 ft produces a lumin- ous intensity of one-quarter of a candle ft. A source of 10 cp produces at 1 m an in- tensity of 10 candle m, which is a con- venient intensity for general photometric work, and is nearly the same as 1 candle ft. See Law of Inverse Squares. Lummer-Brodhun Photometer Head. See Photometer Head, Contrast. Lute. — When a joint is made in rubber- covered copper wire, it is very important to solder the joint quickly and thus not give the heat time to travel along the wire to the parts covered with rubber; for if pure rubber is heated it will be turned into lute, a material which O'Gorman describes as of a sticky nature and which never hardens. (Ref. O'Gorman, Electrician Primers, vol. ii, No. 46.) Lux. — The 1896 Geneva Congress pro- posed the lux as the unit of illumination. A lux is equal to a flux of 1 lumen per sq m, and is the illumination produced by 1 pyr at a distance of 1 m. On p. 33 of his treatise on ' Electric Lamps ', Solomon states that a lux is approximately -^ oi a candle ft. Luxmeter, Photometric. — This is a photometer of the 'cat's -eye' type. By means of a combination of lenses and prisms all the light is focused on the eye in a simi- lar manner to that employed in the Cornu microphotometer. Prisms placed at the centre of a tube are arranged so as to form a contrast photometer, the ends of the tube being covered by two diffusing disks. The cat's eye is composed of two shutters which move in opposite directions before a rect- angular diaphragm, and since the area ex- posed is proportional to the distance between the shutters, the sensibility of the apparatus is practically constant. The photometer is arranged for either laboratory use or for photometering street lighting. See Photo- meter. 320 M — Machine M M, the preferable abbreviation for meter, the unit of length (which see). Machine, Electpostatic Induction. See Electrostatic Induction Machine; Machine, Wimshurstj Machine, Holtz Influence. Machine, Holtz Influence, a type of cc generator acting by variation of capacity and electric induction. See Electrostatic Induction Machine; Machine, Wims- hurst. Machine, Influence. See Electro- static Induction Machine; Machine, Wimshurst; Machine, Holtz Influence. Machine, Overtype. — An overtype dy- namo or motor is a two-pole machine with a horseshoe field-magnet, so disposed that the armature is mounted above the yoke. In the undertype, on the other hand, the armature is below the yoke. See Gene- rator. Machine, Picking'-up and Paying- out, a term applied exclusively to the special machinery used on marine cable- laying and repairing ships for hauling in or paying out telegraph cables. Machine, Wimshurst, a generator of continuous electricity depending for its wimshurst Machine operation on variation of electric capacity. The mechanical energy put into it is con- verted into electrical energy by making it act against electrical forces of attraction. The essential parts of an ordinary Wims- hurst machine are two insulating plates or drums placed coaxially. On each plate are fixed a large number of strips of conducting material, which are equal in size and are equally spaced — radially if on a plate, and circumferentially if on a drum. The plates, or drums, are made to rotate in opposite directions. The capacity of the conductors therefore varies from a maximum when each strip on one plate is facing a strip on the other, to a minimum when the conduct- ing strips on each plate are facing blank or insulating portions of the other plate. There are three pairs of contact brushes, the mem- bers of two of the pairs being at opposite ends of diametrical conducting rods placed at right angles to one another; the third pair are insulated from one another and form the principal collectors, the one giving positive and the other negative electricity. The plates are revolved in opposite directions; thus if there be initially a charge on one of the conducting segments of one plate and an opposite charge on one of the conducting segments on the other plate near it, their potentials will be raised as the rotation of the plates separates them. Suppose that the conditions are initially as in the fig., i.e. that the segment Aj is positive and the segment Bj negative. Now as A.■^ moves to the left and Bj to the right, their potentials will rise on account of the work done in separating them aigainst attrac- tion. "When Aj comes opposite the segment Bj of the B plate, which is now in contact with the brush Y, it will be at a high posi- tive potential, and will therefore cause a displacement of electricity along the con- ductor between y and y', bringing a large negative charge on to Bj and sending a posi- tive charge to the segment touching y'. As Aj moves on, it passes near the brush z and is partially discharged into the external cir- cuit. It then passes on until, on touching the brush x, it is put in connection with x' and has a new charge, this time negative, driven into it by induction from B^. We thus have positive electricity being carried by the conducting patches from right to left on the upper half of the A plate, and nega- tive from left to right on its lower half. A similar process is going on on the B plate, bub in this case the negative is going from M'Intyre Joint — Magnet 321 left to right above, and the positive from right to left below. On the whole, there- fore, positive is being supplied to the left- hand main conductor z by both upper and lower plates, and negative to z'. The largest machine of this type so far constructed is one comprising forty-eight plates of about 1 m diameter. It requires over 2 hp to drive it, and gives with ease a considerable current at a pressure of over 100,000 volts. It was made in the private laboratory of the late Lord Blythswood at Renfrew. See Electrostatic Induction Machine; Ma- chine, HoLTz Influence. M'Intyre Joint. See Aluminium; Jointing Aluminium Conductors. Macula Lutea. See Yellow Spot. Magazine Arc Lamp. See Lamp, Arc. Magnalium, an alloy of aluminium and magnesium. It is lighter than aluminium, its specific gravity varying (according to the proportion of aluminium and magnesium) from 2-4 to 2-5. It is stated that its tensile strength is 2400 kg per sq cm. Its melting- point is from 650° to 700° C. It is claimed in the exploiters' circulars that the electrical conductivity of magnalium is greater than that of aluminium. The percentage of mag- nesium varies, according to the purpose for which the magnalium is required, from 2 per cent to 30 per cent, the percentage of alu- minium consequently varying from 98 per cent to 70 per cent. Magnalium is easily tooled, and makes good castings. Magnesium, a metal whose specific gra- vity is 1-74. Its melting-point is about 630° C, and its specific heat is 025. From tests made by Dewar and Fleming the specific resistance at 0° C. was found to be 4'36 microhms per cm cube, increasing 0'38 of one per cent per degree Centigrade. Magnet, a mass of iron (or other mag- netic material) which possesses the property of attracting iron (or other magnetic material) to its ends or poles (see Magnetic Pole). The strength of a magnet depends upon the average flux-density existing in the magnet and also upon the cross section and the length; that is, it depends upon the total flux and upon the separation of the poles. [' A mM/net is a body which possesses the property of magnetism.' — From McGraw, Electrical Hand- book, 1908, p. 201.] Permanent Magnets are those which require no magnetising coil. They are made of extremely hard steel (preferably of tung- sten steel). As compared with electromag- nets, they are relatively feeble. [' A perma/nent magnet is one which retains a con- stant amount of magnetism for an indefinite period. Those used in practice are most often made of hard- ened steel, and are produced by rubbing on a magne- tised body or by placing in a strong magnetic field.' — From MoGraw, Electrical Handbook, 1908, p. 201.] Electromagnets consist of coils of wire surrounding a soft-iron core (see also Elec- tromagnet). Horseshoe Magnet, U-shaped Magnet, a magnetised steel bar which has been bent into the form of a horseshoe with the ends faced to receive the armature. Electromag- nets of this type generally consist of two bar-iron cores (carrying exciting coils) bolted to (or screwed into) a straight yoke-piece, as in electric bells, indicators, &c. (see special heads). See Field, Magnetic. Magnet, Artificial, a name given to an ordinary steel permanent magnet to distin- guish it from natural magnets or lodestones. See LODESTONE. Magnet, Bar, a straight permanent magnet. Magnet, Compound, a term used to describe a strong permanent magnet built up of a number of steel strips separately magnetised before putting together. See Lamination of Magnet. Magnet, Controlling.— 1. A magnet arranged for the purpose of controlling a piece of apparatus, generally as regards pres- sure, current, or speed. See 'Regulating Mechanism of Arc Lamps' under Lamp, Arc; Regulation; Automatic Regula- tion OF Voltage; Circuit Breaker; Con- trol, Electromagnetic; Switch, Elec- tromagnetically-operated. 2. A small magnet whose position may be adjusted, which serves to control the direc- tion or intensity of the magnetic field of a magnetometer or other moving-magnet in- strument. See Galvanometer. Magnet, Damping. — 1. In electrical instruments, a small, fixed, permanent magnet adapted to produce eddy currents in an alu- minium or copper disk or cylinder, so as to retard the motion of such disk. In electric meters, such a disk often controls the speed of revolution of the meter-spindle. In hot- wire instruments an aluminium disk prevents sudden movements of the indicating part. 2. In certain electrical apparatus in which it is not convenient to provide a proper 322 Magnet damping appliance, such as an air chamber or a frame for the moving parts in which eddy currents may be induced, an external magnet is sometimes used by which a retard- ing force can be exerted on the motion, and by applying this at suitable times the oscil- lating system can be brought to rest. Such a magnet is known as a damping magnet. See Damper; Damping; Meter, Eetard- iNG Torque of Magnetic Brake; Dead- beat OR Aperiodic Measuring Instru- ments ; ' Damped Galvanometer ' under Gal- vanometer. Magnet, Electro-. See Magnet; Elec- tromagnet. Magnet, Field-. See Field-magnet. Magnet, Horseshoe. See under Mag- net. Magnet, Laminated. See Laminated; Lamination of Magnet; Magnet, Com- pound. Magnet, Lamination of. See Lamina- tion OF Magnet. Magnet, Long-pull, a magnet or elec- tromagnet designed to give an approximately Fig. 1.— Solenoid Brake Magnet Fig. 2.— Coll Mag- net used in an Arc Lamp uniform pull over the whole of or a large part of its stroke (generally a few inches). This may be done in various ways, the most usual being the provision of a conical plunger so shaped that, when the solenoid force is large, the plunger presents a small polar surface, and vice versa. Fig. 1 shows the form of plunger used in a certain type of electric brake solenoid, and fig. 2 represents a long-range magnet used in the design of an arc lamp. Magnet, Natural, or Lodestone, an oxide of iron. See Lodestone. Magnet, Permanent. See Magnet. Magnet, Polyphase. See Polyphase Magnet. Magnet, Portative Power of. See Magnet, Tractive Force of; Lifting Power of Magnet. Magnet, Relay, an electromagnet which forms part of a relay device. See Relay. Magnet, Ring, a magnetic circuit which is complete and which has uniformly high permeability throughout its whole length. The magnetisation of a ring magnet is not evident from external magnetic effects; but the energy of magnetisation shows itself in the spark on breaking the magnetising circuit, and in the high inductance of such a circuit. Cores built up of ring stampings are fre- quently used in testing samples of iron for per- meability or hysteresis. See Ring Method of Magnetic Testing; Iron and Steel Testing. Magnet, Tractive Force of. — This depends upon the square of the flux-density at the contact of the keeper or armature with the magnet pole, and also upon the area of contact. Unless the poles of the magnet are carefully formed, the contact surface is very variable, and the pulling power is not definite. In the case of magnets of the horseshoe type in which there are two polar surfaces, the portative power or attractive force of a mag- net is greatly reduced if even only one pole is not properly fitted. With magnets work- ing on low flux-density, in which the reluc- tance is entirely that due to the air gap, the flux depends so very much on the closeness of contact that any rounded edges give rise to a high flux-density, and so to a strong pull. To prevent sticking, a small piece of non-magnetic material should be provided; otherwise the magnet will not be demag- netised by the demagnetising power of the poles, and the armature may stick and not be properly released when the current in the exciting coil is stopped. To calculate the portative force of a mag- net, the following formula is used : — /=^dynes = 0-0405(j|^)^Akg. Where B = induction density in lines per sq cm. A = area of poles in sq cm. If B is not uniform, as when the polar surfaces are not properly constructed, the mean square value of B for the area must be Magnet Coil — Magnetic Circuit 323 taken, since the pull depends on the average square value of the flux. This formula does not apply in cases where a current-carrying layer lies in the gap. See Lifting Power of Magnet; Magnet, Long-pull. (Eef. 'The Electromagnet and Electromagnetic Mechanisms', Thompson.) [d. k. m.] Magnet CoiL See Coil, Magnet or Field. Mag-net Core. See Electromagnet; Magnet. Magnet Frame op Carcass, a term which is generally taken as denoting the yoke and the magnet cores, which, together, make up the stater part of the magnetic circuit of a continuous dynamo or motor, or of an alter- nator of the type with an internal rotating armature. In the magnet frame of an inter- pole machine for generating continuous elec- tricity, shown in the fig., M represents the Magnet Frame main poles or magnets, I the interpoles, and Y the yoke. Magnet Keeper, the armature of a mag- net ; a bar of soft iron which bridges across between the two poles, and so completes the magnetic circuit of the magnet. See Mag- net; Electromagnet. Magnet Pole. See Magnetic Pole; Field-Magnet. Magnet Pole, Secondary. See Poles, Consequent. Magnet Spool. See Spool, Field or Magnet. Magnet Wheel, the revolving system or rotor of an alternator, consisting of a fly- wheel to which the field-magnet coils are bolted. In the illustration the poles are nm Magnet Wheel of an Alternating Generator shown at p, and the yoke, which serves also as a flywheel, is shown at Y. Magnet Wire. See Wire, Magnet; Wire, Enamel - insulated; Enamelled Copper Wire; Wire, Cotton-covered. Magnetic Adherence. See Magnet, Tractive Force of; Magnetic Friction; Lifting Power of Magnet; Magnetic Clutch; Clutch. Magnetic Ageing. See Ageing; Per- manent Magnet. Magnetic Alloys of Non - magnetic Materials. See Heusler Alloys. Magnetic Attraction and Repul- sion. See Magnetic Pole; Magnet, Tractive Force of; Magnet, Long- Pull. Magnetic Axis, a straight line joining the two efiective poles of a magnet. Magnetic Blow-out Circuit Breaker. See Circuit Breaker. Magnetic Blow-out Controller. See Controller. Magnetic Blow-out Fuse. See Fuse. Magnetic Brake. See Brakes. Magnetic Brake in Meters. See Meter, Eetarding Torque of Magnetic Brake; Magnetic Drag. Magnetic Bridge. See 'Bridges for Magnetic Measurements' under Bridges. Magnetic Bunching. See Gap Ee- luctance. Magnetic Circuit, the complete path of a given set of lines of magnetic flux. In most apparatus and machinery the magnetic circuit is partly iron or steel and partly air. 324 Magnetic Circuit — Magnetic Declination The law of the magnetic circuit is similar to that of the electric circuit, namely, .. „ mmf magnetic iiux = — = — . ° reluctance Where the mmf is due to coils of wire carrying current, it is, in cgs units, equal to — X ats. 10 Closed Magnetic Circuit, a magnetic circuit consisting entirely of iron and having no air gap, as the core of a ring magnet. See Magnet, Ring. Air Magnetic Circuit, a magnetic cir- cuit in which the lines of magnetic force do not pass through iron; as, for example, the magnetic circuit of a coil of wire wound on a former with no magnetic or iron core. Double Magnetic Circuit. — This term generally applies to a magnetic circuit which -__.._ 1 / \ z' j 1 t 1 1 1 1 1 1 j I \ .,' Fig. 1.— Double Magnetic Circuit of a Shell-type Transformer is completed through two separate return magnetic paths; the central flux dividing into two parts as shown by the dotted lines in fig. 1, which represents the magnetic cir- cuit of a shell-type transformer. Fig. 2.— Magnetic Circuits of a Four-pole Motor A true double magnetic circuit consists properly of two complete magnetic circuits, which may, as in a four-pole motor, each divide their ilux on the return path. The dotted lines in fig. 2 show the paths of the flux in such a case. See Reluctance; Mag- netomotive Force; Magnetising Coil. Magnetic Clutch, a clutch for coupling two portions of a running shaft, generally of the face-plate type, and so designed that by exciting a coil of wire the faces of the coup- ling plates are magnetised and drawn to- gether. Current is led into the coil by two slip rings. The intensity of the clutching action can be increased gradually by using a regulator in series with the exciting coil. Controlling gear for magnetic clutches should also include a discharge resistance to prevent rise of pressure in the exciting coil when switching off, unless the coil obtains its vary- ing pressure from a tapping ofi" a suitably- graded resistance in parallel. See Clutch; Magnet, Tractive Force of; Magnetic Friction; Lifting Power of Magnet. Magnetic Conductivity and Conduc- tance. — 1. Permeability. 2. The reciprocal of the reluctance of a magnetic circuit. See Reluctance; Magnetic Circuit; Perme- ability. Magnetic Constants, numbers express- ing the quality of a magnetic material, as permeability, hysteretic coefiicient, reluc- tivity, coercivity, &c. (which see). Magnetic Continuity. — A joint in a magnetic circuit is said to preserve magnetic continuity when the surface of abutment of the two portions is large and well fitted. With alternating flux, steps must be taken to avoid the production of eddy currents at the joint. Magnetic Control. See Instrument Control. Magnetic Couple, torque on a magnet in a magnetic field; the product, mxl, of pole strength (m) into distance between poles (T). Magnetic Creeping, the small change which sometimes occurs in the magnetisation of an iron or steel bar when subject for a long time to a constant mmf. This change is specially noticeable in open magnetic cir- cuits. Magnetic Curves. See Magnetisa- tion, Curve of; Hysteresis Loop. Magnetic Cut-out. See 'Automatic Circuit Breaker' under Circuit Breaker. Magnetic Damping in Meters. See Meter, Retarding Torque of Magnetic Brake; Magnetic Drag. Magnetic Declination. See Angle of Declination; Terrestrial Magnetism. Magnetic Detector — Magnetic Lines of Force 325 Magnetic Detector. See Detector, Magnetic; Indicator, Pole or Polarity. Magnetic Dip. See Dip, Magnetic; Terrestrial Magnetism. Magnetic Discontinuity, an air gap (see Insulation, Magnetic). To prevent sticking of an armature to a pole, it is desir- able to provide a small stop or layer of non- magnetic material, so that the armature may not come into actual contact with the pole. See Magnet, Tractive Force of. Magnetic Dispersion. See Disper- sion, Magnetic. Magnetic Drag, retarding force arising chiefly through eddy currents, but (if iron is present) partly through hysteresis, when a mass of conducting material is moved rapidly across a strong magnetic field. See also Meter, Retarding Torque of Magnetic Brake; Magnet, Damping. Magnetic Elements.—!. The elements which show magnetism most powerfully are iron, some kinds of steel, nickel, and cobalt. Such materials are described as paramag- netic. 2. The term is also applied to the com- ponents of terrestrial magnetism at a place. See Terrestrial Magnetism; Paramag- netism; Heusler Alloys. Magnetic Elongation, the microscopic increase in length which generally occurs in an iron bar when magnetised. Such increase changes to a small contraction when the mag- netisation becomes intense. Magnetic Fatigue. See Ageing. Magnetic Field. See Field, Magnetic. Magnetic Field, Bunching of. See Gap Reluctance. Magnetic Field, Distortion of. See Armature Reaction. Magnetic Field, Resultant, of Dy- namo. See Armature Reaction. Magnetic Field, Rotary. See Rotary Magnetic Field. Magnetic Field, Rotatory. See Ro- tatory Magnetic Field. Magnetic Field of Force. See Field, Magnetic. Magnetic Flux. See Flux, Magnetic; Field, Magnetic. Magnetic Force. See Force; Current, Electric. Magnetic Friction, the increased fric- tion between magnetised surfaces. This in- creased friction is due to the tendency of magnetic lines to shorten. The friction is very intense when the magnetic flux-density VOL, II at a magnetic surface is large. See Clutch ; Magnetic Clutch; Magnet, Tractive Force of; Lifting Power of Magnet. Magnetic Friction Clutch. See Clutch; Magnetic Clutch. Magnetic Fringe at Edge of Pole Piece. See Gap Reluctance. Magnetic Gear. See Magnetic Gear- ing. Magnetic Gearing, toothless gear whose driving power is derived from magnetic fric- tion due to flux being made to cross at the point of contact. See Magnetic Friction; Gearing for Electric Motors. Magnetic Hysteresis. See Hysteresis. Magnetic Induction. See Induction; Induction, Magnetic; Flux, Magnf.tic; Unit of Magnetic Flux; Maxwell; Line of Induction; Field, Magnetic. Magnetic Insulation. See Insulation, Magnetic. Magnetic Intensity. See Force; In- tensity OF Magnetisation. Magnetic Lag. See Hysteresis. Magnetic Leakage, magnetic flux which strays or is pushed aside from the main mag- netic circuit, and so does not take part in producing the desired effect {e.g. induced emf or tractive force). In electrical machinery and apparatus, if it is desired to force mag- netic flux through any portion of a magnetic circuit by means of ats on another portion of the same circuit, then it is necessary that no alternative path or leakage path be avail- able, or that. the alternative path be of high magnetic reluctance. Where windings, carrying currents exert- ing opposing mmf, exist on the same magnetic circuit, as in transformers, and, to a lesser extent, in dynamo machinery, the leakage is generally proportional at each instant to the difference between such mmf. The effect of magnetic leakage is twofold. 1. It tends to increase the flux-density, and so diminish the magnetising power of the main or magnetising portion of the magnetic circuit. 2. It prevents the field from being stiff, viz. the field is easily distorted through leakage paths by the presence of still further mmf, as those due to the currents in an armature. See Flux, Leakage; Dispersion, Magnetic. Magnetic Limits. See Magnetisation, Limits of. Magnetic Lines of Force, lines indicat- ing the direction in which the north pole of 326 Magnetic Linkage — Magnetic Potential a magnet tends to move. This expression is also used instead of tubes of force, so that an induced emf is expressed by the rate of cutting lines of force. See Unit of Mag- netic Induction. The term lines of force is also used to express the magnetism inside a mass of iron. Induction density or magnetisation is measured by the number of such lines per sq cm. A unit line or tube of force is defined as a tube of 1 sq cm cross sectional area lying along a uniform magnetic field of unit strength. Such a field exists in the centre of a long coil of wire carrying 7— or 0'8 ats per cm length. See Unit of Magnetic Flux; Line of Induction; Maxwell; Field, Magnetic. Magnetic Linkage, the product of mag- netic flux into the number of turns of wire surrounding it. The rate of change of this product is the induced pressure or back pressure in the circuit containing the coil. See Inductance; Henry. Magnetic MemOPy. — This term refers to the fact that when iron has once been intensely magnetised in one direction it is more readily magnetised for some time after- wards in that direction; and this eifect con- tinues until the iron has been subject to magnetic changes of moderate intensity for a long period, or until the iron is subjected to an equally intense magnetising force in the opposite direction. The effect is removed by annealing. See Coercivity; Retentivity. Magnetic Moment, the product of the length of a magnet as expressed by the dis- tance between its effective poles into the strength of one pole. This product is the same as the turning force on a magnet when placed at right angles to a unit magnetic field. See Magnetic Couple. Magnetic Momentum of the Cur- rent, the product I x ? of the current I into the inductance or coefficient of self- induction (I) of the circuit through which the current flows. Magnetic Needle, a small steel bar magnetised longitudinally and pivoted at its centre. Magnetic - needle Pole or Polarity Indicator. See Indicator, Pole or Po- larity; Detector, Magnetic. See also Electrolytic Detector. Magnetic North, the direction of the horizontal component of the earth's mag- netism at the point considered. This varies from time to time, and differs from the true north by' different angles in different locali- ties. See Terrestrial Magnetism; Angle OF Declination; Declinometer; Devia- tion; Dip, Magnetic. Magnetic Parallels, lines on the earth's surface along each of which the vertical com- ponent of the earth's magnetism is constant. They do not coincide with the geographical parallels of latitude, though they have a general east-and-west direction. See Ter- restrial Magnetism. Magnetic Permeability. See Perc meability. Magnetic Pole. — When a bar of iron is placed within a coil of wire, carrying current, it is said to be magnetised, and it possesses properties, such as the power of attracting other pieces of iron to its ends or poles, which distinguish it from unmagnetised iron. If another bar be similarly magnetised, then it will be found that one end of this second bar will be attracted and the other end repelled from an end of the first bar. Two poles which behave in a similar way to a magnetised end of a bar are said to be similar poles. Thus the two ends of a mag- net are opposite poles. If a magnetised bar be suspended so that it can move freely in a horizontal plane, it will be found that it sets itself approximately N and S, so that the ends are named respectively N or N- seeking pole, and S or S-seeking pole. A magnetic pole (or a portion of one) oc- curs whenever magnetic flux emerges from or enters a mass of iron, or, generally, crosses any boundary where a change of permeability occurs. See Magnet; Mag- netic Lines of Force. Magnetic Pole, Unit, a magnetic pole whose strength is such that it attracts an equal and opposite pole at a distance of 1 cm with a force of 1 dyne, or one which repels a similar and equal pole with this same force. The flux issuing from such a unit pole is ix lines. See Magnetic, Potential. (Ref. chap, vii of Hobart's 'Electricity'.) Magnetic Potential, the work in ergs (whether positive or negative) required to carry a unit magnetic pole (see Magnetic Pole, Unit) from the point under considera- tion in a magnetic field to a point completely outside the range of such field. Difference of magnetic potential is mmf. The difference Magnetic Pull — Magnetic Testing 327 of magnetic potential between two points is measured by the work done in carrying a unit pole from one to the other. The con- ception of potential as applied to magnetic phenomena is similar to that of electric po- tential as applied to charged bodies. See Potential, Electric. Magnetic Pull. See Magnet, Trac- tive Force of; Lifting Power of Mag- net; Clutch; Magnetic Clutch; Mag- net, Long-pull. Mag-netic Pull, Unbalanced. See Un- balanced Magnetic Pull. Magnetic Quantity, a term expressing the strength of a magnetic pole. It is nu- merically equal to the magnetic flux issuing from a pole divided by 47r. See Magnetic Pole, Unit. Magnetic Reluctance. See Reluc- tance. Magnetic Reluctivity. See Reluc- tivity. Magnetic Rotary Polarisation. See Eotary, Magnetic Field; Field Rotating. Magnetic Saturation, a relative term expressing high magnetisation or flux-den- Curve of Magnetic Saturation E, Flux-density. H, Magnetising force. sity in iron. In the accompanying satura- tion curve the point K indicates the com- mencement of saturation. When magnetised beyond this point, known as the ' knee of the saturation curve ' (which see), a much greater increment of mmf is required to pro- duce a given increase of magnetisation, and the iron is said to be getting saturated. This is the state of affairs at S of the accom- panying curve. Magnetic Screen, a cylinder or ring of soft iron surrounding any instrument liable to magnetic disturbance. In the absence of a perfect magnetic insulator, this device serves to prevent external magnetic influence from reaching a given point. The external stray field traverses round both sides of the cy- linder, and so passes away and does not reach the inside owing to the relatively im- perfect magnetic conductivity of the air. See Insulator, Magnetic ; Shielding of Elec- trical Measuring Instruments. Magnetic Separator, an apparatus for separating filings or turnings of iron from those of brass or other metal. The turnings are allowed to fall slowly at a distance in front of a strong magnet. The iron filings are deflected sideways as they fall, and so pass into a separate box. The same prin- ciple has been applied to the separation of ores. See Separation of Ores. Magnetic Shell, a thin layer of magnetic material traversed by magnetic flux, magnet- ised, that is, with a N face and a S face. The distribution of magnetic field around it is the same as around a current-carrying wire situ- ated all around the edge of the shell. Hence its theoretical importance in connection with some ways of regarding magnetic problems. Magnetic Shield. See Shielding of Electrical Measuring Instruments; Magnetic Screen; Insulator, Magnetic. Magnetic Shunt, a device for diverting a portion of a magnetic flux of a magnet through an adjustable circuit, and so serv- ing to adjust the amount of magnetic flux through the main portion of the magnetic circuit. It is generally applied to moving- coil instruments in order to adjust their sen- sitiveness, and to electric meters to secure constancy. Magnetic Slot Wedge. See Slot Wedge. Magnetic Strain, the change produced by application of mmf. See Field, Mag- netic. Magnetic Stress, a magnetising force, i.e. a magnetomotive force (mmf). Magnetic Susceptibility.— For iron, this is approximately -— X permeability. Not ex- actly, however, since the terms are so de- fined that while the permeability {fi) of air is unity, its susceptibility {k) is zero. Thus yu , = 1 -f- 4 TT ^. See Permeability. Magnetic Tester. See Ewing Hyster- esis Tester; Step-by-step Method of Mag- netic Testing; 'Bridges for Magnetic Mea- surements' under Bridges; Tester, Epstein Hysteresis; Drysdale Method of Test- ing Iron and Steel; Permeameter; Iron and Steel Testing. Magnetic Testing. See Testing, Mag- netic; Iron and Steel Testing. 328 Magnetisation — Magnetism Mag^netisation, Curve of (Saturation Curve, B-H Curve), a curve expressing the relation between magnetising force and re- sulting magnetisation. The magnetising force or magnetic field is frequently denoted by H, and the magnetisation in lines per sq cm by B; such curves of magnetisation are therefore known as B - H curves. To eliminate the effects of hysteresis, this curve should be obtained by the method oj reversals (which see). The magnetisation curve for iron is Curve of Magnetisation. characterised by a slow rise at first, then a rapid one up to a point known as the knee of the saturation cwrve (see K in the fig.), and then a slower rise again. A knowledge of such a curve is important in all questions of design involving the use of iron or steel in a magnetic circuit. See also Hysteresis Loop. Magnetisation, Cycle of, a double re- versal of magnetisation between equal and opposite limits of induction density. See illustration accompanying definition of Mag- netisation, Limits of. Magnetisation, Intensity of. See Intensity of Magnetisation. Magnetisation, Limits of, the upper f + B Magnetic Cycle Area to (agitable scale) 47r = hysteresis loss in ergs per cc per complete cycle. B = Density. C = -{- Limit of magnetisation. D = - Limit of magnetisation. H = Magnetomotive force. and lower limits of cyclic induction density, as in transformer cores, denoted thus : — B = ±6500. In the accompanying fig., C represents the positive, and D the negative, limit of mag- netisation. Magnetisation, Uniform.— This exists in any space in which the flux-density is the same at all points. Magnetising Coil, a coil of insulated wire surrounding an iron core for the pur- pose of magnetising it when current is passed through the coil. The mmf of such a coil, in cgs units, is — -, or 1 -aS? times its ats, but in practice the mmf is usually given directly in ats. Magnetising Component (also known as Wattless Current). See Power Fac- tor; Current, Component; Magnetising Current; Current, Wattless. Magnetising Current, the current re- quired to magnetise to a given intensity; the field current (of cc machines, or exciters of ac machines). The magnetising current of a transformer is sometimes spoken of as that current which \ Magnetising Current A, True magnetising current. B, Total current. C, Phase of applied pressure. D, 'In-phase' component to over- come iron losses. the primary winding takes from the mains when working at normal pressure. The true magnetising current is only that component of this total no-load current which is in quad- rature with the supply pressure. The re- maining component has to overcome the various iron losses, and is therefore an 'in- phase' component. The relation between these two components determines the pf of the so-called 'magnetising current'. See fig. The true magnetising component is small if the transformer is well designed, and is worked at low flux-density. The magnetis- ing current of an induction motor is large, since it has to provide a sufficient mmf to overcome the reluctance of a magnetic circuit containing an air gap. See Power Factor; Current, Wattless; Current, Compo- nent. Magnetism, a general term covering all phenomena relating to magnets and magnetic flux. Magnetism — Magnetomotive Force 329 ['MagneUmi is a property possessed by certain bodies, by virtue of which they attract and repel each other according to determinate laws.' — From McGraw, Electrical Handbook, 1908, p. 201.] Magnetism, Ampere's Theory of.— In this theory the magnetised mass is sup- posed to consist of an aggregation of mole- cular magnets, each of which owes its mag- netisation to an electric current circulating undiminished in a path offering no resistance, so that the magnetism may be permanent, if, as in hardened steel, the average direc- tion of the particles can be maintained after the removal of the external magnet- ising force. Mag-netism, Ewing-'s Theory of.— In this theory it is supposed that each small particle of the magnetic material is in itself a small magnet which is capable of oscillating freely. When a bar is magnetised, these oscillating magnets attract neighbouring mag- nets in certain ways, and when the mag- netism is reversed, the magnets have to turn round into positions in which they are no longer attracted in the same grouping, but other pairs are attracted together, so that vibrations are set up which are equivalent to heat being generated. In this way the hysteresis loss arising through reversal of magnetism in iron may be explained, and also the main effects of temperature at dif- ferent flux-densities. Further, the absence of hysteresis when iron is subject to intense rotating magnetisation is readily explained by this theory. Magnetism, Free, a term relating to the phenomena occurring at a magnetic pole, that is, at a place where magnetic flux passes through a surface, on one side of which the permeability is different from that obtaining on the other side. [d. K. m.] Magnetism, Permanent. See 'Per- manent Magnets' under MAGNET; Flux, Kemanent. Magnetism, Terrestrial. See Ter- restrial Magnetism. Magnetism of Gases, effects arising from the small diflierences between the per- meability of air and that of other gases, the permeability of air being reckoned as unity. Oxygen is the most notable example of a paramagnetic gas. Magnetite, an iron ore consisting of oxides of iron. It is the lodestone (which see) of the ancients, and has the magnetic properties of the ordinary compass needle in setting itself, when freely suspended, in a north-and-south direction. See also 'Mag- netite Arc Lamp' under Lamp, Arc. Magnetite Are Lamp. See Lamp, Arc. Magneto. See Magneto Generator. Magneto BeU. See Bell, Magneto. Magneto-electric Machine. See Bell, Magneto. Magneto -generator, a small electric generator whose field magnets are formed of permanent magnets. It is used for gene- rating momentary currents in connection with ignition apparatus for petrol motors; also for supplying small currents at rela- tively high pressure for testing the insulation of supply circuits (see Ohmmeter), and for telephone call-signals. Magneto Inductor. See Magneto- generator. Magneto Speed Indicator. See Speed Indicator, Electric. Magnnietograph, apparatus for recording changes of (1) magnetisation in iron, or (2) the Earth's magnetic force. See Magneto- meter. Magnetometer, an apparatus for detect- ing small changes in the horizontal com- ponent of the Earth's magnetic field. An apparatus consisting of a small sus- pended magnet whose motion is controlled by the torsion of its suspension, or by an adjacent magnet, and used for the purpose of comparing weak magnetic fields, such as those in the neighbourhood of a bar magnet. The instrument follows a tangent law; but since, in mirror instruments, the deflections are of such small angular magnitude, the current flowing may, without appreciable error, be taken as directly proportional to the scale readings. Magnetometry includes all magnetic measurements made by a magnetometer. See Magnetometer; 'Bridges for Magnetic Measurements' under Bridges; Iron and Steel Testing. Magnetomotive Force (mmf), that which causes or tends to cause magnetic induction. The expression has been intro- duced, from analogy with electromotive force (which see), to represent a quantity standing in the same relation to the magnetic circuit as does emf to the electric circuit. Mmf is the line integral of the magnetic force. Taken round a closed circuit, it is proportional to the total electric current 330 Magnetomotive Force of Armature — Manganese flowing through the circuit (see Current, Electric); in fact the electric current is the source of mmf. Taken between two points, it is necessary to specify the path chosen. This, however, leads to no am- biguity in practice, as the points are always chosen on the same line of force, and the path in the direction of this line of force. Mmf are usually measured in ats, that is, the product of the current and the number of times it threads the magnetic circuit. To reduce this to cgs units it must be multiplied by 0-4 X. The name 'gilbert' (which see) has been proposed for the cgs unit of mmf. [f. w. c] Otherwise it denotes the measure of mag- netising force operating upon a part or the whole of a magnetic circuit. Where the mmf is due to a coil of wire, itself carry- ing current, it is properly measured by 47r X the ats, or one-tenth of this if expressed in cgs units. For many purposes, however, the mmf is simply given as the number of ats. For calculating the mmf required for the whole of a composite magnetic circuit, as, for instance, that of a dynamo, the expres- /■O "D 7 — . dl or 1, — is the most convenient to use. If iron is not present, reluctance = ^Hdl, where H, B, fj, have their usual signification, and I, dl is the length in cm of a portion or element of the magnetic circuit under con- sideration, measured in a direction along the lines of flux. See Ampere - turn ; Mag- netic Circuit. [d. k. m.] Magnetomotive Force of Armature, the cross-magnetising or distorting ats, due to the induced currents in the armature; the total mmf of the armature which is the resultant of the cross-magnetising mmf and the few back ats, due to the shift of the brushes. See Armature Reaction. Magneto Potential Regulator. See Regulator, Potential. Main Bus-bars. See Bus-bars. Main Dispersion. See Dispersion, Magnetic. Main Field, the magnetic field produced by the main poles; the useful magnetic field as distinguished from any leakage fields that may be present. See Magnetic Leakage; Dispersion, Magnetic; Flux, Leakage; Auxiliary Poles; Interpoles. Main Insulation. See Double Insula- tion. Main Poles, those poles of the magnet which supply the chief part of the magnetic lines of force. Used in contradistinction to aimliary poles {i.e. interpoles), which are used on certain kinds of electrical apparatus to provide a comparatively small and local field for special purposes. See Auxiliary Poles ; Interpoles. Main Switchboard. See Central Station for the Generation of Elec- tricity. Mains, conductors carrying electrical en- ergy to a distribution point, e.g. h pr mains carrying current to a substation, and 1 pr mains carrying current to individual con- sumers' premises. The term may also be applied to the main conductors in a house, &c., but would not apply to the small wiring from distribution boards to individual lamps. See also under the various classes of cables; also^ Distributing Mains. (Ref. 'Central Station Electricity Supply', Gay and Yea- man.) [f. w.] Maintenance of Electrical Plant. See Central Station for the Generation of Electricity. Maley Electro-mechanical Rail Brake, a type of track brake for electric tramcars, with a combination of electrical and mechanical control. There are three track shoes, but no wheel-brake shoes. The outer shoes are arranged for mechanical operation, while the centre shoe is a mag- netic track brake operated by the braking points on the controller. The middle shoe is capable of slight movement relative to the car, along the rail, and when in operation the magnetic attraction between shoe and rail causes a backward drag on the shoe. This is arranged also to bring the outer shoes into operation through the system of linkwork. The load on the motors during braking is then much less than in simple rheostatic braking. See also Brakes. Manchester System. See Tariff Sys- tems. Manganese, a non-magnetic metal whose chief use in the electrical industry is as a constituent of iron alloys. A small per- centage of manganese enormously weakens the magnetic properties and the conduc- tivity of iron, and increases the hardness. Thus manganese-steel rails are very suitable for curves and points, on account of the Manganese Steel — Mariner's Compass 331 greatly decreased rate at which they wear away. See Steel; Ferro-alloys; High- resistance Alloys; Wire, Resistance. Manganese Steel. See Steel; Ferro- alloys; Manganese. Mangcanin, a resistance alloy of exceptional permanence, extensively used for standard re- sistances. Its composition is approximately as follows: Copper 84 per cent, manganese 12 per cent, and nickel 4 per cent. Its spe- cific resistivity is about 47 microhms per cm cube, and its coefficient of increase of resist- ance for 1° C. is 0-00033 of its resistance at 0° C. For an explanation of some variations in the resistance of standard manganin coils, see Eosa and Babcock, Elec, vol. lix, p. 339; Jaeger and Lindeck, Elec, vol. lix, p. 626; Eosa, Elec, vol. Ix, p. 162. See High-re- sistance Alloys; Wire, Resistance; Rheostat. [j. s. s. c] Manhole, a chamber in an underground cable system large enough for men to enter Manhole for the purpose of making joints, pulling cables through ducts or pipes, or for simi- lar reasons. Manholes are placed at inter- vals of, say, 80 or 100 m from one another, and are generally lined with brick or tile. They vary in size up to chambers 3 m or so in depth. They should, in all cases, be pro- vided with watertight covers, and should be ventilated to prevent the accumulation of gas and the consequent risk of an explosion. The fig. represents a manhole containing a 15-duct conduit. Draw-in Pit. — Manholes provided for the purpose of pulling the cables through ducts or pipes are sometimes termed draw-m pits. See Cable, Underground; Conduit, Un- derground. Manila Paper.— Pure Manila paper is made from Manila hemp. It is tough, con- tains no mineral matter, and only a small per- centage of moisture. Its dielectric strength is high and uniform, and it is one of the very best papers for insulating purposes. Many so- called Manila papers only contain a small per- centage of Manila hemp. See Leatheroid; Horn Fibre; Press -spahn; Japanese Paper; Fibre. [h. d. s.] Marble as an Insulating Material.— Marble finds an extensive use for switch- boards and rheostat faces on account of its handsome appearance when polished. White Sicilian marble is the kind most com- monly employed, but it is apt to contain metallic veins and absorb moisture, so that in places where h pr are employed it is desirable to use suitable insulating bushings. See also Enamelled Slate; Slate as an Insulating Material. Marine Galvanometer. See Galvano- meter. Mariner's Compass, a magnetic needle or system of needles attached to a circular card marked at its circumference into thirty- two equal divisions or 'points' marked N, N by E, N N E, &c. In Lord Kelvin's com- pass there are eight parallel needles, each about the size of a sewing-needle, but with an eye at each end, through which a silk thread is knotted. The outer ends of the threads are fixed to the circumference of a card, which is merely an annular ring strengthened with aluminium wire, and the inner ends to a small central boss of alu- minium. A sapphire cup is let into the centre of the boss, and the whole is sup- ported on a platinum-iridium point. This construction has several advantages over the older forms of compass, in which a large and heavy needle was attached to a large card. Among these advantages may be mentioned that, firstly, the frictional error is very small and the period of vibration long; and that, secondly, the shortness of the needles pre- vents errors due to magnetic induction on adjacent parts of the ship. The compass bowl is mounted on gimbals, i.e. it is pivoted at the ends of a diameter to a ring of brass, which in turn is pivoted at the ends of a diameter at right angles to the first. This 332 Mass Resistivity — Mean Hemispherical Candle Power arrangement enables the compass to remain level in spite of motion of the ship. See Ter- restrial Magnetism; Magnetic North; Angle of Declination; Declinometer; Deviation; Dip, Magnetic. Mass Resistivity. See Eesistivity. Master Clock. See Electric Clock. Master ContPoUer. See Controller, Master; Relay. Master Switch, a switch, generally of small capacity, which controls another circuit, generally of much larger capacity, through the intervention of an electromagnet, or of a small motor that operates a larger switch, or of some other suitable device; e.g. the switch in the cage of a lift (see Lift, Elec- tric), or the small switch that is sometimes used to bring into operation a large oil switch or battery switch. The term is sometimes used as equivalent to 'master controller'. See Controller, Master. Materials for Resistances. See Rheo- stats OR Resistances; Wire, Resistance; High-resistance Alloys. Matthews Integrating Photometer. See Photometer, Integrating. Matthews Integrator. See Photo- meter, Integrating. Matthiessen's Meter-gramme Stan- dard of Resistance. See Resistance, Matthiessen's Standard of. Matthiessen's Standard of Conduc- tivity. See Resistance, Matthiessen's Standard of. Mavor's Variable - speed Induction Motor. See Spinner Motor, Three- speed. Maximum-adhesion Torque, the maxi- mum torque that can be developed by a trac- tion motor without causing the driving wheels to slip. See Adhesion, Coefficient of; Adhesion between Wheel and Rail; Adhesion Weight. Maximum and Reverse Circuit Breaker. See Circuit Breaker. Maximum Circuit Breaker. See Cir- cuit Breaker. Maximum Demand. See Central Station for the Generation of Elec- tricity; Tariff Systems. Maximum - demand Indicator. See Indicator, Maximum-demand. Maximum - demand System. See Tariff Systems. Maximum Induction or Magnetisa- tion, in ac apparatus, the peak of the mag- netisation occurring when the induced back emf is zero. See points (and 1) of fig. t + B f a/ -H / h"' -B \j Magnetic Cycle ^ ^ Area to (snitable scale) ^ hysteresis loss in ergs per cc per complete cycle. B = Density. C = + Limit of magnetisation. D = - Limit ol magnetisation. H = Magnetomotive force. Maximum Traction Truck. See Truck. Maxwell. — To explain the fundamental phenomena connected with magnetism and electromagnetism, 'lines of force' are sup- posed to exist as an invisible stream in the magnetic field, tending always to shorten in length, and to produce a side repulsion among themselves. A ' line of force ' is termed a maxwell, while many lines of force make up a magnetic flux. This flux can therefore be evaluated in maxwells. See Line of Induction; Field, Magnetic; Magnetic Lines of Force; Flux, Mag- netic; Induction, Magnetic; Mega- line. Mazda Lamp, the name of a metallic filament lamp introduced early in 1910 by various American manufacturers acting in conjunction with a view to obtaining the advantage of their combined experience, and by the British Thomson-Houston Company in Britain. The filament is dravm tung- sten, and is claimed to be an improvement on the tungsten filaments previously used by these companies. Lamps for all pressures are made, in units ranging from 22 cp up- wards. A special feature is the much larger sizes of units which have been produced to rival arc lamps for street lighting. See Tungsten Lamps with Drawn Filaments; Lamp, Incandescent Electric; Filament. Mean Conical Candle Power. See Candle Power. Mean Hemispherical Candle Power. See Candle Power. Mean Horizontal Candle Power — Merz-Price System 333 Mean Horizontal Candle Power. See Candle Power. Mean Spherical Candle Power. See Candle Power. Mean Turn of the Winding, the turn whose length is the average of the lengths of all the turns of the winding. It is a turn which, if it had the full pressure of the wind- ing applied at its ends, would take a number of amp equal to the number of ats of the winding. Thus we may state that the mean turn of the winding is the turn whose resis- tance in ohms equals the ratio — volts across the whole winding ats of the whole winding Mean Value of Sine Wave. See Sine Wave. Mean Zonal Candle Power. See Candle Power. Mechanical Clearance. See Clear- ance. Mechanical Ear. See Ear, Mechan- ical. Mechanical Energy. See Energy. Mechanical Equivalent of Heat. See Joule's Equivalent. Mechanical Equivalent of Light. See Light, Mechanical Equivalent of. Mechanical Quadrature (or Space Quadrature), ninety degrees displaced from one another. The primary windings of a two -phase motor are in mechanical (or space) quadrature. See Polyphase Systems; Alternating Current. Mechanically - Welded Joint. See Jointing Aluminium Conductors. Medium Pressure. See Pressure; also Current, High-pressure. Medium-pressure Current. See Cur- bent, High-pressure. Megakelvin. See Kelvin. Megaline, a unit of magnetic flux, being one million cgs magnetic units; principally used in specifying total flux in dynamo- electric machines. See Line of Induction; Maxwell; Unit of Magnetic Flux. Megger. See Ohmmeter. Megohm, one million ohms (which see); a unit used in evaluating large resistances, especially insulation resistances. For stan- dard megohm see Eesistanoe, Standard. Megohmit, a registered trade name ap- plied to mica plate built up by a process which, it is claimed, reduces the amount of adhesive material employed to less than 0*25 per cent; the manufacturers also supply a flexible kind of megohmit. See also Flex- ible Mica; Mica; Micanite; Mica Cloth; Mica Paper; Megotalc. Megotalc, a registered trade name ap- plied to built-up mica plate; the manufactur- ers also supply varieties of flexihle megotalc, megotalc cloths, and papers. See also Flex- ible Mica; Mica Cloth; Mica Paper. Meidinger Cell. — The positive electrode is amalgamated zinc, and the negative is copper. The electrolyte is ZnS04 (zinc sul- phate), or MgSO^ (magnesium sulphate). Concentrated CuSO^ (copper sulphate) is used as a depolariser. The emf of the cell is approximately 1 volt. See Battery, Pri- mary, and Cell, Standard, for descriptions of other types of primary cells. Mercury (chemical symbol = Hg), a metal which is liquid at ordinary tempera- tures, its melting-point being —39° C, and its boiling-point about 360° C. It is con- siderably used in the electrical industry in connection with contacts for apparatus, and as a constituent of certain primary batteries (see Battery, Primary). Mercury has a specific gravity of 13 '6, and a specific heat of 0'032. Its specific resistance at 0° C. is 94 microhms per cm cube, and its resistance increases by 0'072 per cent per degree Centi- grade increase in temperature. Mercury-arc Rectifier. See Rectifier. Mercury Break. See Coil, Induction. Mercury Motor -meter. See Meter, Mercury Motor-. Mercury Rectifier. See Rectifier; Automatic Interrupter; Interrupter, Simon; Interrupter, Wehnelt; Gratz Method of Rectifying Current. Mercury -vapour Lamp. See Lamp, Tubular. Mercury-vapour Rectifier. See Rec- tifier. Mercury Voltameter. See Volta- meter. Merz Indicator. See Merz Scale of Charges for Electricity. Merz-Price Automatic Balanced Pro- tective System, for ht feeders and distri- butors, transformers, generators, and other apparatus. The principle is that, under normal conditions, a balance exists between the power which is flowing at the points of entry into and exit from the cable or appa- ratus to be protected, and that the balance is disturbed when an excessive leak occurs. 334 Merz Scale of Charges for Electricity due to breakdown on the protected circuit, thereby operating relays and opening the switches serving that section. The application to a three-phase feeder is shown in the fig. Corresponding phases at the ends of the protected sections excite the primaries of current transformers. The sec- ondaries of these are connected in series- opposition; and since the instantaneous cur- rents at the two ends are equal, except for a small capacity effect, the small normal cur- rent between these secondaries has no action on the relays placed in the circuit. The relays are simply contact-makers for the oil- switch trip-coil circuit, being arranged to give instantaneous action; they may be either three single-core relays, one for each phase, or one triple-core type as shown. The chief methods of balancing are by opposing equal emf (as in illustration), and by employing the balance current to pro- duce equal and opposite mmf in the same magnetic circuit. The normal flux in the common magnetic core is very small; but when balance is upset, the flux produced excites a third coil on the core to which the Merz-Price Automatic Balanced Protective System A, Trip-coil switch. E, Three-pole relay. 0, Three-core, pilot cable. D, Three-phaae ht feeder cable. F, Current transformer with non-inductive shunt. O, Oil switch. E, Earth. relay is connected. The emf balance is found best for feeder-cable protection, while mag- netic balance is preferable for transformers and other station plant. Measuring instru- ments may also be operated from the current transformers. A three-core pilot cable is necessary when the curreht-transformer secondaries are star- connected as shown, but a two-core cable suffices with open reverse-delta connection. The particular method employed may differ considerably for different cases, but the same balance principle is maintained. The system prevents shock at the power station when a breakdown occurs, and saves synchronous plant from dropping out of step — isolating only the faulty section. Merz Scale of Charges for Electri- city, a scale arranged with the endeavour to distribute equitably the capital costs and the running expenses amongst the different consumers. A charge of a given rate per annum is made on the basis of the maximum demand (averaged over any one hour), and in addition to this charge there is a fixed rate per kw hr {i.e. per kelvin). Thus the total charge may, for instance, be made up of a charge of £8 per kw of maximum demand plus a charge of 0'3d. per kw hr consumed. Merz Indicator.— For determining the maximum demand (as averaged over one hour) the Merz Indicator is employed. This is an attachment comprising a clock arranged to automatically re-set an indicator at zero at the end of every hour, the integrated maximum value of the rate of consumption during that hour having been registered. Mesh-connected Motor Starter — Meter 335 The highest of these hourly readings for the period to be covered by the charges alone remains recorded. Such meters are only in- stalled for customers consuming a consider- able amount of electricity. See Tariff Systems. Mesh-coimeeted Motor Starter. See Starting of Motors. Mesh Connection. See Connections, Three-phase. Mesophotometer. See Photometer, Integrating. Metal-and-carbon Brush. See Brushes. Metallic Are. See Arc; Lamp, Arc. Metallic -filament Lamp. See Lamp, Incandescent Electric; Tungsten Lamp with Drawn Filament; Filament. Metals, Non- arcing Property of. See Lightning Arrester. Meter (abbreviation = m), approximately the one ten-millionth part of an earth's quadrant. According to the latest deter- minations the Earth's quadrant measured along a meridian is 10,000,856 m, but this does not affect the meter, which is the length of a Government standard. The m is the unit of length of the metric system. Meter (verb), to register the quantity of electricity supplied to a circuit. Meter, Adjustments of, the alterations of certain of the parts of a meter to adapt it to the circuit to which it is to be connected, and to bring it within the necessary degree of accuracy. In energy motor-meters for continuous electricity, there are two adjust- ments — the high-load adjustment which af- fects the speed of the meter, and the light- load adjustment, or friction-compensation, which affects the accuracy on light loads. The former is usually carried out by moving the adjustably-mounted brake-magnel (or magnets) relatively to the brake disk. The latter consists in an alteration of the com- pounding coil on light loads, and is very important, as all meters tend to go slow on light loads, due to increase of friction. The above two methods also apply to oscillating meters. In amp hr motor-meters with mag- netic brake (shunted type) only one adjust- ment is practically possible, namely, that at the high loads. ' It is mostly carried out by increasing or decreasing the drop across the armature brushes by altering the position of a movable contact on the low-resistance shunt of these meters. In certain types it can also be effected by altering the posi- tion of the brake-magnets when adjustably mounted. In sp induction meters for alternating electricity circuits, there are three adjust- ments — the high-load adjustment, the fric- tion-compensation for light loads, and the phase adjustment for inductive loads. The high-load adjustment is exactly the same as for energy motor-meters for continuous elec- tricity. The friction compensation affects the accuracy on the light loads only, and consists, in general, in producing an unsym- metrical distribution of the shunt flux by means of a pivoted strip of iron on the driving magnets. A slight supplemental driving-torque is obtained which tends to drive the disk of the meter either one way or thp other, and so increases or decreases its speed on the light loads. The phase- adjustment differs in induction meters ac- cording to the method of phase-compensation used (see 'Methods of Phase-Compensation' under Meter, Phase Compensation of an Induction-), and relates to the adjustment of the meter for correct registration on in- ductive loads. Meter, Ampere-hour, for Battery, apparatus for recording the input and out- put of a battery in amp hr. As the amount sent into a battery exceeds that which can be taken out, battery meters are sometimes ar- ranged so that the amount sent in is recorded too low by a predetermined amount, usually 10 to 15 per cent, in order that both the charge and discharge dials may register the same amount; but this practice is objection- able. It is better that both records should be accurate, and that the quantity of amp hr sent in should exceed the quantity taken out by a varying ratio according to the con- dition of the battery. See also Battery Meters and Battery-metering Systems. Meter, Ampere-hour Motor-, with Permanent Magnets, an amp hr meter consisting of a small electric motor, a per- manent magnetic brake, and an integrating mechanism. The motor consists of an arma- ture (either, without any iron core or with a stationary iron core), which rotates in the magnetic field produced by one or more per- manent magnets, and is actuated by the whole supply current, or by a fractional part of it. The motor drives the integrating mechanism and the magnetic brake. The former registers the amp hr (or kw hr when 336 Meter the supply-pressure is constant). The lattei' consists of a disk, or bell, of copper, alu- minium, or other suitable conducting, non- magnetic material, which is mounted on the armature -spindle and rotates between the poles of the driving-magnet, or magnets, or between those of other permanent magnets. The retarding torque of the magnetic brake is proportional to the speed. The driving torque exerted on the armature is propor- tional to the current. When balance occurs, the two torques are equal, and it follows that the speed is proportional to the current. A straight-line law between the current and the speed is never attained, owing to friction such as bearing-friction, brush-friction (in commutator meters), and friction of the counting train and gear. There are two classes of amp hr motor-meters with per- manent magnets— commutator and mercury motor-meters. In the mercury type (see Meter, Hookham and Meter, Ferranti) no commutator and brushes are employed, the mercury serving to conduct the current to and from the armature, which in this -case consists of a simple disk immersed in the mercury bath. Commutator motor-meters of the amp hr type are usually shunted meters (see Meter, Shunted), whereas mercury motor-meters up to 50 or 100 amp are un- shunted meters. Ampere-hour motor-meters are almost always used for continuous elec- tricity. They have no pressure circuit. They are placed in series with one of the mains leading to the consumer's circuit. [h. g. s.] Meter, Aron Clock-. — In this meter, invented by Dr. Aron, use is made of the principle of the electromagnetic interaction between currents in stationary and movable coils to influence the rates of oscillation of two pendulums, retarding the one and ac- celerating the other. The interior of a two-wire, house-service, w hr meter is shown in the accompanying illustration. The meter consists of two ac- curate pendulum-clocks, the pendulums of which carry the volt-coils, seen suspended just above the main-current coils in the illus- tration. These volt-coils are connected to- gether in series through a high resistance. They are energised by a current proportional to the supply-pressure, and oscillate over the two stationary main-current coils. The main- coils are in series with one another, and so wound that the current flows in a clockwise direction in the one, and in a counter-clock- wise direction in the other. The pendulum- coils are traversed by the pressure-current in the same sense, so that the one pendulum is retarded and the other accelerated. The difierences in speed set up in this manner by the passage of a current in the main circuit are proportional to the power in w expended in the circuit. These differences are continu- ously integrated through a differential gear and a counting-train, the dials of which in- dicate the units consumed (BTU, i.e. kelvins). Aron Clock-meter When no current is flowing in the circuit, the two pendulums oscillate at approximately equal rates. Any want of synchronism be- tween the pendulums is neutralised by the aid of a reversing mechanism, which prevents any difference in synchronism being recorded with no current. Acting alone, however, this reversing mechanism would destroy the differences in the rates of oscillation of the pendulums on the passage of a current, so that a commutator is employed to simultane- ously reverse the pressure-current in the pen- dulum-coils. The meter reading only depends on the differences in the oscillation rates of the pendulums when current is flowing, and not on the actual rate of either. It is un- affected by stray magnetic fields. Each pen- dulum is acted upon by a magnetic field ex- Meter 337 ternal to the meter in the same way and to the same extent; consequently, the differences upon which the readings depend remain un- altered. The meter can be used on both continuous and alternating circuits. The meter is provided with an electrical self- winding gear, so that the clocks do not have to be wound up by hand. For an alternat- ing circuit, the winding gear has to be wound to suit the periodicity of the circuit on ac- count of its high inductance. [h. g. s.] Meter, Astatic Switchboard Inte- grating. See Meter, Switchboard In- tegrating. Meter, Bastian Electrolytic. See Meter, Electrolytic. Meter, Battery. See Battery Meter; Meter, Ampere-hour, for Battery. Meter, Capacity of, the maximum num- ber of amp the meter is designed to carry at full load. Meter, Chemical Electric. See Meter, Electrolytic. Meter, Clock, a meter in which the periodic time of a clock is retarded or in- creased by the electromagnetic action of a current. The most successful clock meter is that invented by Aron (see Meter, Aron Clock-). Meter, Commutator Motor-, a quan- tity or energy motor-meter with a commu- tator. See Meter, Ampere-hour Mo- tor-, with Permanent Magnets; Meter, Energy; Meter, Motor-; Meter, Con- tinuous-current Energy Motor-. Meter, Continuous -current Energy Motor-, an energy meter of the type in- vented by Prof. Elihu Thomson, which con- sists of a small' cc commutator motor without iron in its armature or field system, a dial counter, and a magnetic brake. The motor drives the counter and the brake, which absorbs the work done by the motor, and consists of a copper or aluminium disk ar- ranged to rotate between the poles of one or more adjustably -mounted permanent mag- nets. The brake disk is mounted directly on the meter-spindle, and the counter is driven through a pinion or worm. The armature is air -cored, and is of the open -coil or closed-coil type, with its windings connected to a diminutive commutator on the shaft. The one brush of the commutator is con- nected to one supply main, and the other brush is connected in series with a high resistance and a compounding coil to the other supply main, so that the armature is energised by a current proportional to the supply pressure. The armature rotates in the magnetic field produced by one or two stationary coils, which constitute the field system of the motor. These stationary coils are traversed by the main-current flowing in the installation. The current in the main or field coils produces a magnetic flux, which is the driving field, and varies with the current. The current in the armature produces an- other field at right angles to the former, and which is proportional to the pressure. The two together exert a driving torque on the armature, which is proportional to the pro- duct of the two fields, and consequently also proportional to the w supplied. The brake exerts a resisting torque proportional to the speed. When the condition of steady motion is reached, neglecting friction, the two torques equalise one another, whence it follows that the speed is proportional to the w. The re- volutions of the meter executed in a given interval are thus proportional to the energy consumed in this period. This proportionality is not absolutely true on account of friction, such as bearing friction, brush friction on the commutator, air friction, friction of the counting train and gear. To compensate for friction, the compounding coil (see CoiL, Compounding) is used in the armature cir- cuit, and the object of the high resistance in the latter is to reduce the pressure across the armature. A continuous motor-meter may be regarded as a small motor-generator, the motor utilis- ing a fractional amount of the power sup- plied; the generator is a short-circuited mag- neto-dynamo, and absorbs the major portion of the work done by the motor. See Meter, Thomson Watt-hour. [h. g. s.] Meter, Continuous -current Watt- hour, an energy meter for use on a cc cir- cuit. See under Meter, Continuous-cur- rent Energy Motor-. Meter, Continuous - current Watt- hour Motor-j an energy motor-meter, usu- ally of the Thomson type, for cc circuits. See Meter, Continuous-current Energy Motor-. Meter, Coulomb. — The term cmlomb is practically obsolete in reference to meters, as the practical unit of electrical quantity is the amp hr, whereas the andomb is the scien- tific unit of quantity and denotes one amp see. See Meter, Quantity. 338 Meter Meter, Cubic. See Cubic Meter. Meter, Double-lag-ged Induction, an induction meter provided with an adjustment by means of which it can be adapted for use on either of two widely different periodicities, e.g. 133 cycles, or 60 cycles per sec. Most frequently employed in America. Meter, Double - tariff. See Metek, TwO-KATE. Meter, Driving Torque of a Motor-, the turning moment exerted on the armature, or rotor, of a motor-meter by the electrical system producing rotation. In energy motor- meters, it is proportional to the power in w. In quantity motor-meters with permanent magnets it is proportional to the current in amp. The driving torque represents the work done per radian (scientific unit of angular measure), and is usually expressed in centimeter-grams, or millimeter-grams. If P denote in g the force producing rotation (981 P = force in dynes), and ?• be the radius in cm of the body rotated, then Tr equals the driving torque T in cm g, and 2 tt T w is the work done per min in cm g at a speed of n rpm. Meter, Electric, a term usually applied to integrating meters, but also used for am- meters, voltmeters, and the like, as current meter for ammeter, and so forth. See Watt- hour Meter; Ampere-hour Meter; Elec- tricity Meter; Ammeter, Voltmeter; Phase Meter; Wattmeter. (Ref. 'Electri- city Meters ', Solomon ; ' Electricity Meters ', Gherardi; 'Industrial Electrical Measuring Instruments ', Edgcumbe.) Meter, Electrolytic, an instrument based on the chemical action of a current for the measurement of amp hr (quantity) and regis- tration of BTU (energy) on constant-pressure circuits. It is essentially a cc amp hr meter. Bastian Electrolytic Meter, an amp hr meter of the unshunted, electrolytic type, invented by Bastian, in which the decom- position of water is used for the measure- ment of the amp hr conveyed in a circuit. The whole of the current traverses the elec- trolytic cell, which consists of a wide tube filled with a solution of caustic soda, in which are immersed two nickel electrodes. The cell is closed at the top by a porcelain cap through which the rods of the electrodes pass. They are connected to the meter- terminals by means of flexible leads. On the passage of a current the water is decom- posed, the evolved gases escape into the atmosphere, and the level of the electrolyte falls. The difference in height between two readings of the graduated scale attached to the tube gives the units consumed in the interval between the two readings. The surface of the liquid-column is covered with a thin film of oil to prevent evaporation and loss of water by the evolved gases, and to facilitate reading the meter. It is only suitable for continuous electricity, and has to be periodically filled with water. HoLDEN Electrolytic Meter, an amp hr meter of the shunted, electrolytic type, invented by S. H. Holden, in which the de- composition of dilute sulphuric acid is used to measure the amp hr passing in a circuit by the volume of the hydrogen evolved. The dilute acid, and the platinum elec- trodes, are contained in a sealed glass U- tube, the smaller limb of which terminates in a bulb in which the anode is situated. The anode consists of a piece of platinum gauze, and the cathode is formed by a piece of platinum wire. Both are coated with platinum-black. The anode is only partially immersed in the electrolyte, being surrounded at the top by an atmosphere of hydrogen; the cathode is totally immersed, and is situ- ated in the larger, vertical limb, just above the bend. A resistance is used in series with the cell, and the cell circuit is connected across a low-resistance shunt, which is tra- versed by practically the whole current of the circuit to which the meter is connected. On the passage of a current the electrolyte is decomposed into hydrogen and oxygen. The oxygen is evolved at the anode, and at once combines with the hydrogen absorbed by the anode. , The hydrogen is liberated at the cathode, and collects in the vertical limb provided with a scale graduated in units cor- responding to the particular supply pressure. The meter is suitable for continuous elec- tricity circuits only. It has to be periodically re-set to zero, but requires no refilling. Wright Electrolytic Meter, an amp hr meter of the shunted, electrolytic type, invented by Wright, in which the electro- lytic decomposition of mercury from a mer- cury salt is used to measure the amp hr passing in a circuit. The fluidity of the mercury is made use of to measure the de- composition by volume instead of by weight. In the latest type, known as the new solution meter, the electrolyte consists of a double iodide of mercury and potassium, and prac- Meter 339 tically fills the whole of the electrolytic cell, which is hermetically sealed. The anode is composed of mercury, and an iridium cathode is used. The electrical circuit is shown in the fig. A is the anode mer- cury contained in the anode chamber, B is a platinum fence which prevents the mer- cury of the anode from being jerked into the cell-tube (not shown in the diagram), and c is the iridium cathode. A fine-wire Wright Electrolytic Meter resistance H is used in series with the cell- circuit to compensate for temperature varia- tions. The cell-circuit leads G, F are brazed to the manganin, low -resistance shunt K which bridges the terminals of the meter, of which E is the positive and D the negative. On the passage of a current in the circuit to which the meter is connected, a fractional part traverses the electrolytic cell, and mer- cury is deposited in minute globules on the iridium cathode, from which it descends into a tube provided with a scale graduated in units at the supply pressure. The scale is read off at the height of the mercury column. In the type provided with a siphon U-tube, when an amount of mercury corresponding to 100 kw hr (or kelvins) fills the U-tube, it siphons over the bend and collects in the larger tube, to which is attached a second scale, reading in 100 kw hr. The level of the mercury in the anode chamber at A is kept constant by means of an anode feeder. The meter can only be used for continuous electricity. It has to be reset to zero after a certain number of kw hr have been regis- tered; it does not, however, require any re- filling, [h. g. s.] Meter, Energy, an instrument which measures and registers the electricity sup- plied to, or consumed in, an electrical circuit in a given interval of time. Its action de- pends both on the main current flowing in the circuit to which it is connected, and on the supply pressure across that circuit. The main-current circuit of the meter is traversed by the current utilised in the circuit, and the pressure circuit of the meter is energised by the supply pressure. Its dials register direct in kelvins (i.e. in BTU), the electricity consumed, in units (BTU). It is also called w hr meter, kw hr meter, integrating wattmeter. Meter, EffoP - limits of a, the maxi- mum percentage errors allowable in a meter at difiierent loads. For commercial purposes the usual error-limit in this country is + 2J per cent at any load from one-tenth of full load up to full load. PM Ferranti Continuous-current Meter MeteF, FeFFanti Continuous-euFFent. -The Ferranti cc meter (Hamilton patent) 340 Meter is of the amp hr mercury motor type. A half-sectional side elevation is given in the accompanying illustration. It consists of a very simple motor driving a magnetic brake and a counting train. Eeferring to the fig., CD is the disk comprising the armature of the motor. It acts also as the magnetic brake-disk, and revolves in the mercury bath M B between the poles of two permanent mag- nets P M, only one of which is visible in the illustration. The spindle carrying the disk is counterweighted with the weight w to balance the upward thrust of the mercury, and gears with the counter through the worm wheel ww. The one magnet produces the driving field, which, acting upon the current flowing radially through the disk on that part between its poles, rotates the disk with a driving torque proportional to the current. The other magnet, together with the former one, produces the retarding torque proportional to the speed due to the eddies induced in the disk, as it cuts the fluxes of both magnets. Disregarding friction, when steady motion en- sues the two torques are equal, from which it follows that the speed is proportional to the current. The revolu- tions executed in a given time are thus propor- tional to the amp hr conveyed to the circuit in that time. For con- stant - pressure circuits the dial registers the units (kelvins or kw hr) direct. The mercury retards the disk as it rotates, especially at the high loads, when the armature speed in- creases. To compensate for this fluid-friction a compounding coil CC is used in series with the armature, so connected and arranged that the magnetic flux it pro- duces augments the flux of the driving mag- net, and decreases that of the retarding mag- net. As both magnets retard the disk, the total brake-torque remains unaltered by the compensation. The driving torque is, how- ever, increased at the high loads. The effect of fluid-friction is thus corrected. The whole current to be measured flows through the armature, a low-resistance shunt being used when the current in the circuit exceeds 100 amp. The meter is only suitable for con- tinuous-electricity circuits. [h. g. S.] Meter, Frequency. See Frequency Indicator or Meter. Meter, Friction Compensation in an Induction, a device in induction meters to compensate for the retarding efiiect of friction at light loads. In general, a strip of iron so arranged that it produces an unsymmetrical distribution of the shunt-flux in the air gap in which the meter-disk rotates. It is usu- ally adjustable so that the slight auxiliary torque produced in this way either assists or retards the meter. Meter, Hookham Continuous - cur- rent, an amp hr mercury motor-meter in- vented by Hookham, of which the accom- panying illustration gives an interior view of Hookham Continnous-cnrrent Meter the 1907 pattern, with the cover removed. It comprises a very simple motor, which drives a counting train and a magnetic brake. The armature consists of a disk of copper immersed in a mercury bath situated Meter 341 between the poles of a permanent magnet, and not only carries the current flowing in the installation to which it is connected, but acts also as the magnetic brake-disk. A is the permanent magnet with the wrought-iron poles BE, the pole-pieces of which are em- bedded in the ebonite blocks ee forming the top and bottom of the circular mercury chamber. The side of the chamber is formed by the flexible metal band C lined with leather. G is a counter-weight on the arma- ture-spindle to counteract the upward thrust of the mercury in the bath. The current is conveyed to and from the mercury by means of leading-in wires embedded in the ebonite, and flows radially through the disk beneath the poles. The interaction between the cur- rent in the armature disk and the magnetic field between the poles produces a driving torque on the disk proportional to the cur- rent, causing it to rotate. The rotation of the disk in the magnetic field induces Foucault currents in it which produce a retarding torque proportional to the speed. Neglect- ing friction, when balance occurs these two torques are equal, from which it follows that the speed of the armature is proportional to the current in amp. Thus the revolutions, executed in a given time, will be proportional to the amp hr conveyed in the circuit. The dials are calibrated to register direct in kw hr when the supply is a cc one at constant pres- sure. The meter can only be used for con- tinuous electricity. The coil on the right of the permanent magnet is wound upon an iron core, and is in series with the armature disk. Its object is to compensate for fluid friction, especially at the high loads, when the speed of the meter is relatively high. It will be seen that it is adjustable, as it can be moved nearer to or farther away from the pole- pieces. It produces a slight supplemental driving torque proportional to the square of the current. The meter is of the unshunted type, a low-resistance shunt being, however, used for heavy current circuits, [h. G. S.] Meter, Hour, a clock, the function of which is to register on a dial or counter the time in hours during which current flows in a circuit. On the passage of a current the clock starts and registers the time (indepen- dently of how the current and pressure may vary); when the current is interrupted the clock stops. Hour meters are largely used for special -tariff purposes, and are variously modified. They are not electricity meters, Vol. II i.e. they do not measure either quantity or energy. Meter, House-service, any type of elec- tricity meter used on consumers' premises for the determination of the amount of electricity supplied for lighting, heating, motive power, or any other industrial or domestic purpose. It is also called electricity supply meter, supply meter, integrating meter. Meter, Integrating. See Meter, House-service. Meter, Intermittent Registering, a meter consisting of an ammeter or wattmeter intermittently brought into gear with a clock- work mechanism, so that a counting-train is periodically actuated, the amount of motion imparted to it depending on the ctirrent or w passing through the meter. The deflec- tions of the ammeter, or wattmeter, are thus integrated on the counter, which is calibrated to read the units direct. Meter, Kilowatt-hour. See Meter, Energy. Meter, Load Side of a.— The load side of a meter refers to those terminals of the meter which are connected to the consumer's circuit. See Meter, Supply Side of a. Meter, Locliing Device of a, a me- chanical device by means of which the meter-shaft with the portions attached to it may be lifted off the jewel in the lower footstep bearing of the meter, and firmly clamped against the top bearing- stud, to avoid damage to the lower pivot or jewel during transit and carriage. The locking device must always be released before pass- ing current through the meter. Meter, Losses in a.— Power is wasted both in the main-current and pressure cir- cuits of a meter. The loss in power in the main-current (series or field) circuit of the meter is known as the series loss, and that occurring in the pressure circuit is known as the shunt loss. The current flowing in the main circuit produces a drop in voltage which reduces the pressure across the circuit on the load side of the meter. Both the series loss at full load and the drop in pres- sure are small, and only occur when current flows in the installation. They both reach a maximum when the current taken is a maximum, and are zero when the current is zero. The series loss is borne by the consumer. The shunt loss depends on the pressure of the supply circuit, and is the most important 23 342 Meter of the two. It is practically constant, and is continuous whether current be taken or not. It is entirely borne by the supply station, and represents a very considerable annual loss of revenue. If the power wasted in the pressure circuit of a meter be 1 w, the annual loss to the station is 8-76 kelvins per meter due to this cause. In practice the average loss is much higher. In an amp hr meter the waste of power is restricted to the series loss; in the w hr type both losses occur. See also Meter, Shunt Loss in a; Meter, Series Loss in a. Meter, Magnetic Drag" in a, denotes the retarding torque produced by the mag- netic brake used in motor-meters to maintain proportionality between the speed and the power in w, or current in amp. Meter, Mercury Motor-, an electricity meter of the motor type in which the arma- ture is immersed in a mercury bath. The mercury conveys the current to and from the armature, which usually consists of a disk, or bell, of copper protected from the action of the mercury, and so arranged that the current enters on one side and leaves either on the other or at the centre. See Meter, Ferranti; Meter, Hookham. Meter, Mordey-Fricker Electrolytic Prepayment. See Meter, Prepayment. Meter, Motor-, an electricity meter in which the electromagnetic or inductive action of a current is utilised to produce rotation of a small electric motor the revolutions of which, when current is passing in the circuit to which the meter is connected, are trans- ferred to a counting-train the dials of which register the units of electricity. Meter, O'K, an amp hr meter of the motor type without any brake system, invented by O'Keenan. It consists of a small magneto motor which drives an integrating mechanism. The armature of the motor revolves between the poles of a permanent magnet with a central, stationary iron core. It is connected in parallel, through its brushes on the com- mutator, with a low-resistance shunt which is placed in series with one of the mains of the circuit to be metered. When current is taken, a small fraction of it flows in the armature, which is accelerated until it attains a speed at which its back emf is equal to the drop across its brushes produced by the main current in the low-resistance shunt. When this condition is reached the speed is pro- portional to the drop, i.e. to the current in the installation. The number of revolutions executed in a given interval of time will be proportional to the amp hr conveyed in the circuit in that time. The dials, in conformity with the usual practice, read direct in kw hr {i.e. kelvins), on constant-potential, continu- ous-electricity circuits. It is only suitable for a system supplied with continuous electricity. Meter, Oscillating, a meter in which the electromagnetic action of a current is utilised to produce an oscillatory motion of a movable coil (the armature), in contrast to that of ro- tation in motor-meters. Such a meter is, in general, furnished with a magnetic brake to produce direct proportionality between the rate of oscillation and the load. Meter, Over - compensated Induc- tion, an induction meter in which the phase- compensation is too large, so that the angle between the two fluxes operating the meter, when the current and pressure are in phase, is greater than a right angle. An over-com- pensated meter reads high when the current is a lagging current, and low when the cur- rent leads in advance of the pressure. See Meter, Under-compensated Induction. Meter, Permanency of Calibration of a, the retention of the original accuracy of a meter after its installation in a con- sumer's circuit. It is dependent on the various causes which adversely affect the accuracy of a meter and tend in the course of time to destroy its calibration in everyday use. These causes depend on the electrical and mechanical features of the meter, its handling, installation, and location. The mechanical design is most important, espe- cially as regards the provision of a dust-, insect-, and moisture-proof cover or covers. All adjustments and regulating devices should be incapable of alteration when set. Permanent magnets should be properly aged, as imperfectly aged magnets lose their magnetism very rapidly, with a con- sequent very rapid decrease in the accuracy of the meter, as the retarding torque of the magnetic brake is proportional to the square of the magnetic induction in the air gap between the poles of the magnet. In a motor-meter, the higher the driving torque and the more effectively the disturbing in- fluences are eliminated, the greater will be the permanency of calibration of the meter. The more constant the working state of a meter, the longer it will retain its accuracy and function properly. [h. G. S.] Meter 343 Meter, Phase Compensation of an Induction. — In an induction meter (see Meters for Alternating Electricity) the rotation of the revolving element (disk) is produced by two alternating magnetic - fields, the one due to the pressure-winding of the meter, and the other to the main- current winding. These two fields differ in phase from one another, the pressure circuit of the meter being, as a rule, highly induc- tive. The angle of phase displacement of the fields when the current and pressure are in phase {i.e. when the load is a purely non-inductive one, such as incandescent lamps) is some 70° to 85°, varying in different meters. In order to make the meter suit- able for measuring the energy when the load contains inductance or capacity (arc lamps, induction motors, condensers, over- or under-excited synchronous motors), the angu- lar displacement of the two fields, on unity pf, requires to be artificially compensated until the angle is 90°, i.e. until the two fluxes are at right angles. When this condition is attained, the speed of the meter is propor- tional 'to the true power, and it registers cor- rectly on inductive loads. The arrangement by means of which this desired phase rela- tionship is achieved is known as the phase compensation. Methods of Phase Compensation of Induction Meters. — The phase compen- sation of an induction meter can be carried out in three different ways, according to which most induction meters may be broadly classified. The first method consists in using an impedance (choking) coil and a non-in- ductive resistance in connection with the pressure-circuit (shunt coil) of the meter. The second method consists in using a short-circuited secondary winding on the shunt magnet. The third method diflfers from the first two, both of which operate on the shunt flux, by operating on the main-current field. The main-current winding is composed of two halves, of which the one is made highly inductive and the other relatively non-induc- tive. By suitably adjusting the diflferent circuits of the meter, the shunt flux can be made to stand at right angles to the main- current flux, when the current and pressure are in phase. When this condition is ful- filled, the meter measures the energy cor- rectly when there is a pf less than unity. The methods given above in outline can be modified in a numl^er of ways. (Ref. 'Elec- tricity Meters ', H. G. Solomon.) Meter, Polyphase Induction, an in- duction meter (see under Meters for Al- ternating Electricity) specially arranged for measuring and registering the total en- ergy expended in a polyphase, or multiphase system. Usually restricted to the three- phase, three-wire type. Meter, Prepayment, in general, an electricity meter fitted with a prepayment or automatic slot -attachment so arranged that until a coin has been inserted in the slot, and a handle turned, current cannot flow in the circuit to which the prepayment meter is attached. This operation closes a circuit- switch in the meter, and at the same time produces a displacement of the prepayment mechanism to a predetermined extent. The meter works when current is taken, and gradually restores the prepayment mechanism to its original condition. This condition is reached when an amount of energy has been consumed corresponding to the value of the coin inserted, and the circuit switch is auto- matically opened. MORDEY - FrICKER ELECTROLYTIC PRE- PAYMENT Meter consists of an electrolytic cell fitted with an automatic slot attachment. The cathode is permanently fixed in the electrolyte. The anode consists of a thin copper strip, or ribbon, wound on a drum mounted above the cell. The strip is fed into the cell to a definite extent by means of the prepayment attachment, on the inser- tion of a coin and when the handle has been turned. The cell is in series with the circuit, which is completed through the solution when the anode is immersed in it. On the passage of a current, which flows entirely through the meter, the anode is gradually eaten away from the bottom upwards, the copper being deposited on the cathode. When the immersed portion of the strip has been consumed, the circuit is opened at the surface of the solution. Several coins can be successively inserted, the length of the anode immersed in the solution being cor- respondingly increased. Watson Prepayment Electricity Me- ter. — This is a purely mechanical type of slot meter. The design comprises, in addi- tion to the necessary clockwork mechanism, various details for the purpose of rendering it impossible for customers to interfere with its normal operation. 344 Meter Meter, Quantity, an instrument which measures the amount of electricity in amp hr conveyed by a current flowing in a cir- cuit during a given interval of time. Mostly used on cc systems. Its action is indepen- dent of the supply pressure. When the latter is maintained constant, the register of the meter is calibrated to record direct the kw hr of electricity supplied, or consumed, at this pressure. It is rarely used on ac networks on account of possible phase displacements between the current and the pressure. It is also called amp hr meter, coulomb meter, but the last term is practically obsolete. Meter, Retarding^ Torque of a Motor. — In a motor-meter the electrical system producing rotation of the armature, or rotor, exerts a driving torque (turning moment) on the moving system. The speed of rota- tion should always be proportional to the power in w (energy motor-meter) or current in amp (quantity motor-meter). This con- dition of steady motion is fulfilled by the employment of a brake system, which should exert an opposing torque which is the same function of the speed as the driving torque is of the power or current. When steady motion results, these two torques equilibrate one another, from which it follows that the speed is proportional to the power, or cur- rent, neglecting friction. The same applies to an oscillating meter. Meter, Retarding- Torque of the Magnetic Brake of a.— In motor-meters in which the driving torque produced by the electrical system is directly proportional to the power in w (energy motor-meter), or the current in amp (quantity motor-meter), the brake-system employed is the magnetic brake. It consists of a disk of copper, or aluminium, which is rotated between the poles of one or more permanent magnets. The disk, in rotating through the magnetic field, has eddy currents induced in it, which, interacting with the field, produce a resisting torque which is proportional to the speed. When steady motion ensues, the speed is propor- tional to the power, or current, neglecting friction. This also applies when the motion is one of oscillation instead of rotation as in motor-meters. The retarding torque of the magnetic brake may be expressed by the formula — T = KB2r2o). (T = retarding torque, B = induction in the air gap, r = radius of disk from axis to centre of poles; <« = angular speed, K = constant depending on the air gap, conduc- tivity of the disk, and shape of the poles.) Meter, Series Loss in a, the waste of power which occurs in the main-current cir- cuit of a meter. See Meter, Losses in. Meter, Shunted, an electricity meter provided with a low-resistance shunt which carries the major portion of the supply cur- rent, only a fractional part of the total cur- rent traversing the main-current circuit of the meter proper. The series circuit of the meter forms a parallel branch to the shunt. In small -capacity meters, mostly of the electrolytic (exception: Bastian Electrolytic Meter, for which see Meter, Electrolytic) and amp hr motor-meter types, the shunt is incorporated in the instrument itself and bridges the two terminals. When the cur- rent exceeds about 50 or 100 amp, the shunt forms a separate piece of apparatus. In this case the main-current terminals of the meter are connected to the volt terminals of the shunt, which is placed in series with the main over which the electricity to be metered is supplied. Meter, Shunt Loss in a, the waste of power which occurs in the pressure-circuit of a meter. See Meter, Losses in a. Meter, Single -phase Induction, a meter based on the principle of induction, consisting of a small split-phase induction motor, a magnetic brake, and a counter. Such a meter measures and registers the energy in a sp ac circuit. See under Meters for Alternating Electricity. Meter, Slot. See Meter, Prepayment. Meter, Supply Side of a.— The ter- minals of a meter to which the incoming supply mains are connected are generally referred to collectively as the supply side of the meter. It is of importance that an energy meter should be connected to the incoming supply mains and the consumer's circuit (load side of a meter) in such a manner that the current taken by its pressure circuit does not flow through its main-current circuit. This would happen if the incoming main to which the main-current circuit of the meter is connected were joined to the main terminal of the meter other than the common main and shunt terminal. See Meter, Load Side OF A. Meter, Switchboard Integrating.— The output of lighting, power, and traction Meter 345 systems is determined by the aid of meters usually mounted direct on the switchboard. They do not differ in principle from the ordinary house-service types to which they belong, but are variously modified to adapt them to the conditions of heavy and fluctuat- ing loads which occur. They are generally of the w hr type. For cc systems two methods are in vogue. The meter either has a high-capacity shunt, or it carries the whole of the current. The former method is usually restricted to amp hr meters, the shunt being mounted on the back of the switchboard and the meter on the front, connected by potential leads to the shunt. The advantage of this method is that the meter can be disconnected without inter- rupting the circuit. Errors may, however, arise due to the shunt. When the meters (Thomson, oscillating, and Aron clock types) carry the whole current to be metered, they are usually made astatic to render them, as far as possible, independent of the magnetic fields of neighbouring bus-bars, heavy-current cables, and connections. The Aron switch- board meter, although not astatic, is inde- pendent of stray fields (see under Meter, Aron Clock). The meter, when it carries the whole current, can only be disconnected when the circuit is opened, but the errors that may occur in consequence of the high- capacity shunt are eliminated. In an ac system the induction switchboard meters do not differ from the usual types, being merely arranged for mounting on the board. They are generally used in conjunc- tion with both pressure and current trans- formers, and are of relatively small capacity. In this manner they are not traversed by any h pr current, and can be handled with impunity. Each meter has its own current transformer, but not a separate pressure transformer. The general practice is to con- nect the pressure circuits of the meters in parallel to two It bus-bars energised by one or more h pr transformers. Astatic Switchboard Integrating Meters, switchboard meters for cc, in which the armature and field systems are so ar- ranged that the construction renders the meter astatic and practically independent of stray magnetic fields. For instance, in the Thomson astatic switchboard meters, instead of one armature as in the ordinary meter, two armatures are used in series and are oppositely wound. They are arranged, the one above the bus-bar of the meter, and the other below. The armatures are cut in op- posed directions by the flux produced by a current in the bus-bar, but rotate in the same direction. The brake system is also totally enclosed in an iron box. [h. G. S.] Meter, Tariff, a meter used in connec- tion with special systems of charging for electricity. Among those that may be thus described being two-rate meters, maximum- demand indicators, prepayment meters, hr meters. Meter, Testing Constant of a Motor-, the relationship which exists between the speed of the meter armature, or rotor, and the load. It does not refer to the meter- dials, and is only used in checking the speed of the meter at different loads. In amp hr motor-meters it gives the connection between the correct speed of the armature, or disk, and the current in amp. In energy motor- meters the testing constant connects the speed with the power in w. It is usually stamped on the meter-cover or meter-dials. This value is referred to as the ' declared ' value of the constant, and forms the basis of reference as regards the meter. It is very often expressed as the number of revolu- tions per BTU (i.e. kelvin), from which the current, or power, per rpm (or per sec) can be easily deduced. Meter, Test-terminals for. See Ter- minals, Test-, for Meters. Meter, Thomson Watt -hour. —This meter, invented by Elihu Thomson, is typi- cal of those w hr motor-meters in which use is made of the electromagnetic action of a current, to produce continuous rotation of a small continuous - electricity motor without iron in its armature or field system. The motor absorbs an exceedingly minute amount of power, and drives a counting train and a magnetic brake, the object of which is to keep the rate of revolution of the motor proportional to the power in w, neglecting the disturbing influences of friction. The counting train (dials, or cyclometer counter) shows the units (BTU) of electricity con- sumed in the circuit to which the meter is connected. The field system of the motor consists of two stationary, series-connected, main-current coils, very clearly shown in the accompanying diagrammatic sketch of the standard house-service meter. They are traversed by the current taken in the in- stallation by the lamps, or other current- 346 Meter consuming devices. Symmetrically situated between them is the armature, which is drum -wound, and connected to a commu- tator composed of silver segments, situated beneath the main-current coils. Two brushes of phosphor-bronze wires with silver bearing surfaces rest on the commutator, and convey the armature current to and from the arma- ture. The latter is in series with a com- pounding coil, C C, and a high resistance, R. R AAAAAAAAA- ri o Thomson Watt-hour Meter K, Kesistance. C, Compounding coil, i., Line. The compounding coil is in two sections, one in each main -current coil. The armature, or pressure, circuit (armature, compounding coil, and high resistance) is connected directly across the supply circuit. It is thus ener- gised by the supply pressure, and is con- tinuously traversed by a small current pro- portional to the latter. The armature is carried on a spindle which rests on a lower; jewelled, footstep bearing, and is guided at the top. The spindle also carries the brake- disk. It is geared to the counter through a gun -metal worm. When current flows in the main coils the armature rotates, and revolves the disk between the permanent magnets at the top of the instrument, at the same time driving the counter. Both the compounding coils and the magnets are adjustably mounted, the former to vary the compensation for friction at the low loads, and the latter to vary the speed at the high loads. The instrument can be used both for continuous and alternating electricity. It is more customary, however, to use in- duction meters in the latter case (see under Meters for Alter- nating Electricity). There are several meters of this class. They differ as regards the arrange- ment and number of the elements comprising the meter and also as re- gards the methods of adjustment. Meter, Three-phase Four - wire Induc- tion, a special poly- phase induction meter for metering the energy supplied to a four-con- ductor three-phase sys- tem. See Meters for Alternating Elec- tricity. Meter, Three-phase Three -wire Induc- tion, a polyphase induc- ■ tion meter for measur- p ing the energy supplied to a three-phase three- ■ wire network. It con- sists of a combination of two sp induction meters to form a single instru- ment (see Meters for Alternating Elec- tricity). It is connected to the network in accordance with the two-wattmeter method of measuring power. Meter, Three-wire.— The measurement of the electrical energy supplied to a multiple- wire system is readily obtained by dividing it into a series of independent two-wire circuits, in each of which a two-wire meter is placed, as, for instance, when two houses are wired as two-wire circuits from a three-wire net- work. In many instances the three-wire mains are taken into the consumer's premises. A single three-wire meter is then usually P, Load. Meter 347 employed, and consists, in general, of a spe- cially-arranged energy meter. When the system is a cc three-wire one, the three-wire meter is either of the w hr motor type, or is an Aron three-wire clock meter. In the former (Thomson type) two main- current coils are used. The one field coil is placed in series with the positive main, and the other in series with the negative main, and the armature circuit is connected either direct across the two outer conductors, or between one outer conductor and the third, or neutral, wire. The latter method of connection of the armature circuit should be avoided, since with it the errors due to want of balance between the two sides may become very considerable, and are much greater than with the former method. Neither method is absolutely correct except under conditions of perfect balance of the three-wire network. The Aron three-wire clock meter, however, measures the total three -wire energy correctly, however un- equally loaded the two sides may be, as it practically consists of two complete energy meters. Each main coil (see Meter, Aron Clock) is inserted in an outer conductor (the one in the positive and the other in the negative), and the pressure, or volt, coils of the pendulums are connected in series across the whole three-wire system, the junction of the two pendulum coils being joined to the neutral wire. In a sp ac three-wire supply the methods are similar, but the problem of correctly metering the energy is further complicated by possible phase displacements between the currents and pressures. A sp three-wire in- duction meter is used with its two main-coils connected as explained above, and its pres- sure-coils energised either by the total or half of the three -wire pressure. A Thomson three-wire meter with no iron in the arma- ture or field system can also be used (see Meter, Thomson Watt-hour) for a cc network. Both these types of so-called three- wire meters only correctly measure the energy under certain conditions, which are, however, never completely fulfilled in actual practice. For absolute accuracy an Aron three-wire clock meter, or a three-phase three-wire in- duction meter, should be used, the latter connected to the network on the two-watt- meter method of measuring power. Both these meters correctly measure the three-wire ac energy whatever the conditions that pre- vail. (Eef. Solomon, ' Limitations of Three- wire Energy Motor-meters', Elec. Eev., p. 327, vol. lix, 1906.) Meter, Torque, a dynamometer used in the measurement of the driving torque of a motor-meter. By means of it the force (usually in g) exerted in the armature, or rotor, is determined, the load on the meter being kept constant during the measurement. The product of this force and the radius of the armature, or disk, in cm gives the driving torque in cm g. If the driving force be in dynes, then the torque will be in cm dynes (1 g weight = 981 dynes). Meter, Tramcar, an electricity meter specially arranged for measuring the elec- trical energy taken by tramcars. The special features relate to providing a meter whose readings shall not be affected by the vibra- tion of the tramcar, by the inclination at which the meter must stand when the car is on grades, or by the centrifugal force on curves. Meter, Two-rate, an ordinary electricity meter fitted with two sets of dials, or coun- ters, and a change-over device by means of which the two integrating mechanisms are alternately connected to or disconnected from the armature, or rotor, according to the tariff, the change-over device being elec- trically or mechanically actuated by a time switch. The one counter, or dial, registers the units consumed during the high rate, and the other those consumed during the low-rate periods, so that the two consump- tions are separately registered. A simple device, such as a pointer, indicates (i.e. dis- tinguishes) the counter which is operating, and thus the tariflf. From the above defini- tion it follows that there are electrically- and mechanically-operated two-rate meters. The former consists of two separate pieces of apparatus — the two-rate meter and the time- switch electrically connected; in the latter the two are mechanically coupled, and the time -switch is mounted on the meter base. The only modification of the meter in either type is in connection with its counter and gear. Employed in connection with the two- rate system of charging for electricity supply (see Tariff Systems; Systems of Charg- ing). A two-rate meter is also called a Meter, Two-wire, an ac or cc energy meter for connection to a two-wire circuit (in contrast to a three-wire meter). 348 Meter — Meters for Alternating Electricity Meter, Under - compensated Induc- tion, an induction meter in which the phase- compensation is insuificient, so that the angle between the two fluxes operating the meter, when the current and pressure are in phase, is less than a right angle. An under-com- pensated meter reads high for a leading cur- rent and low when the current lags behind the pressure. See Meter, Over-compen- sated Induction. Meter, Watt -hour. See Meter, En- ergy. Meter Bridge. See Bridges. Meters for Alternating- Electricity. — ^The electricity expended in an alternating system is to-day almost exclusively measured by induction motor -meters based on the rotatory magnetic field principle first clearly enunciated by Galileo Ferraris (Atti della E. Academia delle Scienze di Torino, xxiii, p. 360, 1888). Continuous-electricity energy motor-meters of the Thomson type, without iron in the armature or field system, and the Aron clock meter, were originally used for this purpose, but have now been largely superseded by induction motor-meters on account of their simplicity, ease of adjust- ment, low cost, and low shunt losses relatively to the types just enumerated, the total elimi- nation of brush friction or rubbing contacts of any description, and the very small amount of friction. The sp induction motor-meter now uni- versally used is of the w hr type, i.e. it measures the electricity in the-sp alternat- ing circuit. The amp hr type, of which the Shallenberger was formerly largely used, is now discarded, and should not be employed, owing to possible phase displacements be- tween the current and the pressure. In the present w hr types the meter consists of a small induction motor, a magnetic brake, and a counting train (dial or cyclometer counter). The motor is really a split-phase motor, the rotor of which revolves in a two- phase rotary field, of which the one field is proportional to the supply pressure, and the other to the current flowing in the circuit to which the meter is connected. The revolv- ing element of the meter, the rotor, usually consists of a simple light disk of copper or aluminium, which revolves in the air gap of an electromagnet wound with two sets of coils. The one winding is energised by a current proportional to the supply pressure, and produces the shunt magnetic flux, the other winding is traversed by the circuit current, and creates the series magnetic field. These two alternating magnetic fields, which differ considerably in phase from one another, cut the rotor disk and induce eddies in it, which, reacting upon the magnetic fields, exert a driving torque on the disk propor- tional to the product of the two magnetic fields multiplied by the sine of the angle of phase -difference between these fields. The same disk revolves between the poles of a permanent magnet, which induces in it the eddy current producing the retarding torque proportional to the speed. A retarding torque is also produced by the driving fields them- selves, but is negligibly small compared with the retarding torque of the brake -magnet. When steady motion has been attained, then (disregarding friction) these driving and re- tarding torques are equal to one another. The speed is thus proportional to the product of the supply pressure and the main current multiplied by the sine of the angle of phase- difierence between the two alternating fluxes which they produce in the meter. The power expended in a sp alternating circuit is equal to the product of the supply pressure and main current multiplied by the pf. In sym- bols this is VC cos ^ (V = voltage, C = cur- rent, cos cj) = pf)- Hence in order that the meter-speed shall be proportional to the true power, the sine of the angle of phase-displace- ment between the fields on the disk must be equal to cos (j> (the power factor). In other words, when the pf is unity, i.e. when the current and pressure coincide in phase (on non-inductive loads), the two alternating mag- netic fields producing rotation of the disk should be displaced in phase by exactly 90°. The speed is then proportional to the true power. The revolutions of the meter-spindle are transferred through a worm, or pinion, to a counting-train which actuates the index- hands on a dial, or the number-wheels of a cyclometer-counter, which registers direct the kw hr consumed. The quadrature between the shunt and series fluxes acting on the disk is obtained by means of a phase compensation (see Meter, Phase Compensation of an Induction). Induction meters, of which there is a very large number on the market, differ mainly as regards the methods employed in com- pensating the meter so as to produce the desired phase relationship between the fluxes. For metering a sp three-wire network, a Meter-testing 349 sp three-wire induction meter is generally used. This method is, however, a faulty one. See under Three-wire Meters. In the case of a polyphase system the electricity supplied is either measured by several sp induction meters (two or three, depending upon the system), or by a single specially -arranged induction meter which is referred to as a polyphase meter. When the polyphase network is a two-phase or three- phase three- wire system, the polyphase meter is called a three-phase three-wire induction meter, and, in general, is simply a combination of two sp induction-motor elements operating upon a single disk, or upon two such disks mounted on the same spindle. The usual magnetic brake is used, and the revolutions of the spindle are transferred to a counter Tvhich registers in units the total polyphase energy. It practically amounts to two sp in- duction meters combined to a single instru- ment, and is connected to the polyphase system on the two-wattmeter method of mea- suring power (see under). When a single in- strument is not employed, two independent sp induction meters are connected to the cir- cuit in the same manner, the algebraic sum of their readings giving the total consump- tion. In a three-phase supply with four con- ductors, either three independent sp induction meters are connected to the system in such a manner that the pressure -circuit of each meter is between that supply-main in which its series-circuit is placed and the neutral (or fourth wire), the sum of the three readings giving the total consumption, or a special three-phase four-wire induction meter may be employed. In general, a three-phase three- wire meter will not measure the energy in a three-phase four-wire system. A two-phase four-wire circuit practically forms two independent sp circuits, in each of which a sp induction-meter is used. (For A discussion of the principles of sp and poly- phase meters, &c., see 'Electricity Meters', ■Solomon; 'Electricity Meters', G-herardi.) [h. g. s.] Meter-testing. — The examination of a meter for accuracy is conducted by means of time and speed tests. Either test may be applied to meters of the motor and oscillating types; whereas electrolytic meters and clock meters (Aron type) can only be checked by time tests. In a time test (usually made at full load only), both the current and the pressure are maintained constant during the test, the duration of which should be suffi- ciently long to enable reliable dial-readings to be taken. The true values of the readings are obtained by means of an ammeter and voltmeter (if cc), or by means of a wattmeter. A time test can also be carried out by using a standard testing meter, in which case it is unnecessary to keep the load constant. A speed test of a motor-meter consists in checking the revolutions of the meter arma- ture or rotor at different loads between one- tenth of full load and full load. For deter- mining the percentage errors at the different loads the testing constant of the meter must be known. When the speed of the meter is too high at full load to admit of accurate measurement by counting the revolutions, the testing dial of the meter is used, when the pressure must be taken into account. The instruments used in testing meters are stop-watches, ammeters, voltmeters, and wattmeters. Ammeters and wattmeters of different ranges should be employed, as it is very important to have the same degree of accuracy at each load, especially when the latter is a light one. Instead of an ammeter, a potentiometer (which see) or kelvin amp balance (see Kelvin Balance) may be used. When amp hr meters are being tested, ammeters only are required. With energy meters the circuit must include ammeters, voltmeters, and wattmeters. The latter are not essential for cc measurements, but are indispensable in testing ac meters. In test- ing energy meters care must be taken to connect the pressure circuits of the meters under test, the voltmeters, and the pressure circuits of the wattmeters to the testing cir- cuit in such a manner that the pressure currents do not traverse the main-current circuits of the instruments; otherwise faulty results will ensue. The best method is to employ two separate testing circuits — the one for the load, in which are the main-cur- rent circuits of the instruments in series, and the other for the pressure circuits of the in- struments connected to it in parallel. A three-wire meter is usually tested as a two-wire meter with its two field coils in series. It should be examined as a three- wire meter in a three- wire network with two wattmeters, one in each half of the system. An induction meter depends on the fre- quency, and this must therefore be known. It should be examined both on non-inductive and inductive loads. 350 Method of Reversals — Mg Three-phase, three-wire meters are usually checked by examining each half of the meter as a two-wire sp meter, when the two pres- sure circuits should be energised, whichever half is under current. This is of importance owing to the brake-action of the shunt fluxes. The best method is to test the meter in a three-phase, three-wire network by means of two wattmeters connected on the two-watt- meter method of measuring power. In addition to the time and speed tests, a meter should be examined for creeping (energy meters), effect of varying the pres- sure (energy meters), effect of varying the frequency (induction meters), and the effect of stray magnetic fields (usually restricted to cc switchboard' meters of the energy type). The power wasted in the main and pressure circuits of meters has also to be determined. The main current loss {i.e. the series loss) in cc meters is obtained either by measuring the drop at full load in the main circuit with a low -reading voltmeter, and the current by an ammeter; or directly with a wattmeter, and in ac meters by means of a wattmeter. The shunt loss of a cc meter is determined by measuring the ohmic resistance of its pressure circuit after the supply pressure has been applied for an hour, with the cover on. The square of this pressure, divided by the re- sistance, is the loss in w. It can also be de- termined by a wattmeter. In ac meters the shunt loss is found by the aid of a wattmeter, or by using the three-voltmeter or three-am- meter methods of measuring ac power. [h. g. s.] Method of Reversals.— This term ap- plies to the method of obtaining the satura- tion curve of a sample of iron, which is carried out as follows. The iron is first carefully demagnetised by successive re- versals of large, at first, and then gradually diminishing magnetising current. Observa- tions are then made of the change of mag- netism which occurs when each of these currents is reversed. These changes are noted by means of a ballistic galvanometer connected with a test coil or secondary cir- cuit. The method is only suitable for small samples, and in order to obtain accurate re- sults, the sample must either be in the form of a ring or of a long bar of such a length that its magnetised ends do not exert any appreciable demagnetising action upon that portion of the bar on which the secondary coil is wound. In the case of ring magnets, the internal and external diameters should differ as little as possible, since the ats operating on both the inside and the outside of a ring are the same, and if the internal and external dia- meters are very different, an appreciably greater field will occur on the inside of the ring than on the outside. See Stbp-by-STEP Methods of Magnetic Testing; Iron AND Steel Testing; Eing Method of Magnetic Testing. Methven Screen. — A secondary stan- dard of light which was formerly widely employed in photometry. On p. 781 of McG-raw's ' Standard Handbook for Electrical Engineers ' it is described by Dr. Louis Bell as comprising a powerful Argand gas burner fitted with a chimney, and having adjusted in front of it, 38 mm from the axis of the • flame, which is 76 mm high, a flat blackened plate provided with a slit termed the Methven- slit. The slit is located in front of the centre of the flame, and, when ordinary gas is used, the slit should measure 25 mm by & mm. The Methven screen was originally intended for a primary standard, but Bell ' states that it proved too sensitive to varia- tions in quality of the gas. It is, however, stated to afford a very convenient and steady secondary standard of about 2 cp. See Standard of Light. Methven Slit. See Methven Screen. Methyl Alcohol. See Alcohols. Methylated Finish, sometimes termed finish. This is methylated spirit eontaining- about 3 oz of resin per gal. Its use for thinning varnishes for insulating purposes is not good practice, but it may be employed for spirit varnishes. See ALCOHOLS; Insu- lating Varnishes. Methylated Spirit. See Alcohols. Metre Bridge. See Bridges. Metric Atmosphere, the preferable unit of pressure when dealing with steam cal- culations; a pressure of 1 kg per sq cm. The name follows from the circumstance that atmospheric pressure is usually of the order of 1-03 kg per sq cm, i.e. within some 3 per cent of the metric atmosphere. Metric System. See Units, Systems OF; GrRAM. Metric Ton. See Ton. Mfd, the preferable abbreviation for micro- fwrad. See Microfarad; Farad. Mg, the preferable abbreviation for milli- gram. Mg — Mica-sticking Varnishes 351 Mg, the chemical symbol for magnesium (which see). Mho, the unit of conductance. See Con- ductance. Mica, a transparent anhydrous silicate of aluminium and potassium or sodium. The chief deposits are found in India, Canada, and the United States, by far the largest supplies coming from India. Mica is mined in block form, and reaches the market in flat irregular pieces varying in size from about 5 cm square up to about 30 cm square and some 6 mm thick. Mica is classed accord- ing to its colour, size, degree of hardness, and freedom from impurities, all of which qualities aflFect the price. There are white, green, ruby, and amber micas. The latter variety is the softest, and it is considered good practice to employ it for commutator insulation; the others vary in degree of hardness, and find extensive use for various purposes. They sometimes have curious markings, due to small deposits of magnesia and earthy matter, but these do not necessarily have any injurious effect on the insulating qualities of micas. The insulation resistances and dielectric strengths of micas are high, but they are subject to considerable surface leakage. The insulating properties remain permanent, and are unaffected by any reasonable temperature. See also Flexible Mica; Mica Splittings; MiCANITE; MEGOHMIT. Tempered Mica, mica split into thin laminae and artificially softened by high tem- perature. The extent to which the softening is carried depends on the temperature and on the time it is maintained. (Eef. for pro- perties of mica, 'The Insulation of Electric Machines', Turner and Hobart, chap, v.) [h. d. s.] Mica, Moulded.— Mica plate built up of laminse, and with suitable adhesive material, may, on heating, be moulded under pressure to almost any shape. The cement when heated allows the laminae to slip over each other and conform to the shape of the mould; the plate is allowed to cool in the mould, the cement sets, and the plate retains its moulded shape. Experience will determine the mica and cement most suitable for the particular purpose in view. Mica in the raw state can- not be moulded. See Eing, Commutator. Mica Cloth. — This is a flexible mica with a thin cloth either on one or both sides, which acts as a mechanical protection to the mica. A layer of thin Japanese paper with suitable cement is sometimes employed to hold the mica in position on the cloth. This allows a greater thickness of mica to be obtained. A varnished cambric may also be used for this purpose. Either the Japanese paper or the varnished cambric may, if the materials are of the very best quality, serve to slightly increase the dielectric strength for a given thickness. The manufacturers of mica plate under the various trade names also supply lines of mica cloths. Mica-insulated, a term applied to insu- lation in which mica in one form or another is the dielectric relied upon. Mica Paper. — This is thin tough paper with a covering of one or more layers of thin mica laminae, held in position by a layer of thin Japanese paper and suitable adhesive material. The paper acts as a mechanical protection to the mica, and its use produces a good stout mica wrapping. The manufac- turers of mica plate, under the various trade names, supply lines of mica papers. Mica Plate. — This is mica sheet built up of layers of overlapping laminae and suitable cement. The sheet is then heated under pressure, which reduces the cement to a minimum, and causes the laminae to adhere together, forming a hard compact mica plate. In this form it can be cut and milled to suit requirements. See also Megohmit; Mego- talc; Micanite. Mica Splittings. — The block form in which mica usually reaches the market is of an inconvenient shape, size, and thickness for many purposes, and it is split into thin laminae to render it more flexible. These laminae vary in thickness from O'Ol mm to about O'l mm, and in this form can be con- veniently built up into sheets and moulded to any desired shape. Manufacturers of elec- trical apparatus still sometimes split their own block mica, but it is becoming more frequent practice for this to be done at the mines, on account of the cheapness of labour and the reduction in waste, and also on account of the further consideration that the transportation charges will be on the basis of the finally useful material, instead of on the crude blocks, much of which must be thrown away. Mica- sticking Varnishes.— These, as the name implies, are used for the manufac- ture of mica plate. They must have good adhesive powers, and be of a thin consist- 352 Mica Tube — Milking Cells of Accumulator ency, in order to reduce to a minimum the amount of adhesive material between laminae. Their insulating properties need not be high, but it is essential that they be moisture- proof, and that they shall not soften at a low temperature. See also Flexible Mica- STiCKiNG Varnishes. Mica Tube. — This is made from mica sheet built up in the same way as for mica- plate manufacture. The sheet is tightly wrapped round a wedge-shaped, split-man- drel, and placed in a mould. The mould and its contents are heated, the split-mandrel is expanded, and pressure is exerted on the sheet. It is allowed to cool under pressure, the sheet retaining the form of a tube. The manufacturers of mica plate under the vari- ous registered trade names also supply lines of mica tubes. Micanite. — This term has come to be ap- plied quite generally to mica plate built up from thin laminse and suitable adhesive ma- terial. It is, however, the registered trade name of a mica plate in which a special cement is used which the manufacturers claim renders the plate impervious to moisture. The manufacturers also supply lines of flex- ible micanite, and micanite cloths and papers. See also Flexible Mica, Mica Cloth, and Mica Paper. Micanite Rings. See Ring, Commutator. Micarta Tubes, the registered trade name of insulating tubes manufactured by a patent process from paper and mica, and sometimes exclusively from paper. MiCFO-ampere, a unit used in evaluating exceedingly small currents, as galvanometer currents; one-millionth of an amp. See Am- pere. Microfarad, the practical unit of capa- city, being the millionth part of a farad. See Farad. Microhm, one -millionth of an ohm; a unit used in evaluating small resistances. See Ohm. Micrometer Caliper. See Wire Gauge. Microphone. — Hughes discovered that if two pieces of conducting material be placed lightly in contact, the resistance of the con- tact varies with the slightest change of pres- sure between them. Thus the slight varia- tions of air pressure which constitute sound waves are sufficient to produce marked changes in the resistance of contact of two pieces of carbon, and consequently of the current flowing from one to the other, if they be connected to a battery. A telephone receiver placed in the circuit reproduces with wonderful exactness the sound which is affect- ing the microphone, thus showing that the variations in resistance correspond almost perfectly with the variations of pressure on the contact. Hughes showed that many sub- stances may be used, though carbon is prob- ably the most sensitive, and that the form of the material is of no great consequence. The transmitters used in telephony, and known by many names, are all modifications of the original microphone. The most usual form consists of a quantity of grains of car- bon, such as partially-burnt seeds, between conducting disks of metal or carbon. A microphone has yet to be constructed which will carry any considerable current. Pencil Microphone, a type of micro- phone in which the carbon is in the form of a pencil, sharp at both ends, resting lightly between cups of carbon. Microphone Relay. See Relay. Microtelephonic Transformer. See Transformer, Microtelephonic. Middle Wire of a System. — 1. The middle conductor of a three-wire system, that is, the conductor which is at mid poten- tial between the two outers of the system. 2. Also, though less often, the conductor con- nected to the neutral point of a polyphase system. See Feeders; Neutral Conductor; Compensator; Three-wire Distributing System; Neutral. Midg-et Are Lamp. See ' Miniature Arc Lamp ' under LAMP, Arc. Migration of Charged Particles. See Ionisation. Milking*, the process of specially charging weak cells of accumulators. During this pro- cess the acid becomes so highly charged with small bubbles of gas as to present a milky appearance. See Accumulator; Milking Cells of Accumulator. Milking Booster, a small dynamo ar- ranged to give a low pressure for specially charging weak cells, one or two at a time. It is usually driven by a motor. See also Milking. Milking Cells of Accumulator, a few cells employed for charging individual weak cells by themselves when they require it, thus avoiding having to charge the whole battery. Their use has been practically dis- Milking Clips for Accumulators — Mining Equipment 353 continued in favour of the milking booster (which see). Milking Clips for Accumulators, metal clips attached to cables used for spe- cially charging weak cells. See Milking. Milliammeter. See Ammeter. Milliampere, a unit used in the measure- ment of small currents, being one-thousandth of an amp. See Ampere. Millikelvill, one-thousandth of a kelvin, i.e. one-thousandth of a kw hr, i.e. one w hr. A millikelvin is the amount of energy re- quired to lift 1 kg through 367 m. Also it is the amount of energy which, if imparted to 1 kg of water, occasions a rise of tem- perature of 0-86° C. 1 millikelvin = 3600 joules. See Kelvin. Milling' Slots.— One method of forming the slots in an armature core is to assemble the blanked and insulated disks to the finished length, and then to mill out the slots in a milling machine in much the same manner as the teeth are often cut in gear wheels. The method is, however, not to be recommended, as it is liable to burr the disks into one another and so provide paths for eddy currents to circulate throughout the core. For this reason it is not often used nowadays. Millivolt, one-thousandth of a volt; a unit used in the measurement of small emf. See Volt. Millivoltmetep. See Voltmeter. Min, the preferable abbreviation for minute. Mineralised Carbons. See Carbons, Arc Lamp. Miniature Are Lamp. See Lamp, Arc. Minimum Circuit Breaker. See Cir- cuit Breaker. Minimum Relay. See Relay. Mining Equipment, Electrical.— At the present time electricity is very largely used for driving the machinery both of metalliferous mines and of collieries. Usu- ally each mine has its own generating station, but in some instances a single gene- rating station is used to supply current to a group of mines, considerable economy being effected thereby. Hydro-electric Station. — In the case of metalliferous mines in mountainous dis- tricts, water power is frequently available, and the electric generating station is then of the hydro-electric type; that is to say, the water power is utilised to drive turbines or waterwheels which are coupled to the electric generators. See Hydro-electric Generating Set. Turbine. — For moderate heads of water the turbine is most commonly employed. This machine consists essentially of two parts, one fixed and the other free to rotate. Upon each of the parts are mounted vanes having opposite curvature. The water is led by pipes to the turbine, and by flowing along the curved vanes it sets up a reaction, which causes the free portion to rotate. Pelton Waterwheel. — Where the head of water is great, the Pelton type of water- wheel is used. This is an extremely simple appliance consisting of a wheel having vanes secured to the rim. The water is brought by pipes to a nozzle, the jet from which impinges on the vanes and so drives the wheel round. Steam Plant. — In collieries the gener- ating plant is usually steam-driven, the boilers being fired with the inferior grades of coal. From the generating station the electric main's are led to the motors driving the various machines. As far as possible the mains on the surface are carried overhead on poles, owing to the lower cost of this method as compared with that of placing the mains underground. But in districts which are subject to frequent and violent thunderstorms, such as in some parts of South Africa, it has been found more eco- nomical to place the mains underground in order to avoid damage from lightning, since the cost of the damage is liable to exceed the saving effected by the overhead system. The most interesting and important ap- plications of electricity in mining are for pumping, hauling, winding, and, in col- lieries, for coal-cutting. Electrical Pumping. — In pumping, the pumps are placed down the mine as near as possible to the water, and, if of the ordinary reciprocating type, are usually driven from the motor by toothed gearing. In many cases high-lift centrifugal pumps are used in preference to reciprocating pumps. These consist of a series of centrifugal pumps coupled together, the delivery of the first being joined to the suction of the second, and so on. In one type of this class of pump the arrangement of vanes is similar to that of a turbine, the action being that of a turbine reversed: these are known as 354 Mining Equipment turbine pumps. Both centrifugal and tur- bine pumps require to run at high speeds, and their spindles can therefore be coupled direct to the spindles of the electric motors, gearing being dispensed with. L -B- + -e- H f— Fig. 1.— Endless-Tope Haulage M, Motor. F, Pulley. E, Tightening pulley. S, Bope. Electric Hauling. — Two principal sys- tems of hauling are in use. In the system illustrated in fig. 1 and known as the end- less-rope haulage system, an endless wire rope is driven continuously by a capstan -+- -S Po L, Tub. D nnnt DDDDD -+- D, Winding drum. Fig. 2.— Main-antl-Tail-rope Haulage s, Main rope. M, Motor. T, Tail rope. L, Train of tubs. drum; it passes round a pulley at the far end of the road, and is supported on idle pulleys or rollers at intervals along the road. The wagons or tubs are provided with clips by which they may be attached to, or dis- engaged from, the rope at any required point. The electric motor in this case runs continuously in one direction, so that the electric controlling arrangements consist only in fig. 2 and known as the main-and-tail- rope system, a train of tubs or wagons is used. The main rope, S, is attached to the head of the train, and is led to one drum of an electrical winding gear. The tail rope, T, which may be rather smaller than the main rope, is attached to the back of the train. It is then taken round a pulley at the far end of the road and led to a second drum on the wind- ing gear. The loaded trains are hauled by the main rope, and when empty are drawn back to the workings by the tail" rope. For this class of work it is best to use a shunt-wound reversing motor, and to gear the drums together permanently, the one paying out while the other takes in. With this arrangement, if the train meets with a down grade it drives the winding gear, and the regenerative feature of the motor comes into play and automatically prevents the train running away and becoming de- railed. See 'Regenerative Con- trol of Electric Cranes ' under Crane, Elec- tric; Regenerative Control Systems. For incline haulage, an electrical winding gear is placed at the head of the incline, Po p, Pulley. Incline Haulage of the usual starting-switch and fuses^ The system is suited both for the level and for inclines. In the second system, illustrated having two drums, one taking the load rope and the other the balance rope, and arranged so that as one drum takes in the other pays out (see fig. 3). At the end of the load ropes a platform n on wheels is attached, upon which the wagons or tubs are placed, and this plat- form runs on rails laid along the incline. A balance weight w is fastened to the end of the balance rope, being made flat so that it will pass under the platform when it meets it on the incline, and running on a narrow-gauge track laid inside the track upon which the platform runs. Generally the motor used, m, Mining Equipment 355 is of the reversing type, with a controller similar to those used on electric tramcars; but sometimes the motor is arranged to run continuously, and the starting, stopping, and reversing of the gear are obtained with fric- tion clutches. Where the loads to be dealt with are small, the balance weight is generally dispensed with. Electric Winding. — In winding, which is the term applied to the hoisting of ma- terials vertically up the main shafts, elec- trical gears have hitherto been used for what may be considered moderate powers only. The largest ones, in use in some of the German collieries, deal with an output of 1200 to 1500 tons per day, and this con- trasts with outputs of 4000 to 5000 tons per day dealt with by steam winders in the most up-to-date English collieries. Fig. 4.— ngner System Ilgner System. — Since, for winding, very high speeds are essential, high rates of ac- celeration are necessary, and in consequence the maximum force applied to the drum-shaft greatly exceeds the mean. If the electric winder consisted simply of an electric motor driving a winding drum and taking its cur- rent direct from the supply mains, as in the case of the small winders for electric haul- age, the fluctuations of current would be very considerable, and would give rise to serious difficulties. To avoid these, the Ilgner sys- tem has been devised. The arrangement is shown diagrammatically in fig. 4. A motor A is driven from the supply mains G, and is coupled to a shunt-wound generator B, a large flywheel C being mounted on the shaft which couples the two machines. A shunt- regulating resistance D and reversing switch E are placed in the magnet circuit of b, whereby its voltage may be regulated from zero to maximum and the direction of its current reversed. The winding-drum is driven by a shunt-wound motor F, the mag- nets of which are excited from the supply mains, while its armature receives current from B. With this arrangement, the con- troller, being in the shunt circuit of B, is small and inexpensive, as it does not have to carry the main cur- rent, and the loss of energy in resistances is practically neglig- ible. The fluctuating currents are provided by b; the energy for the large acceleration currents being provided by the flywheel, which stores this energy during the period when the winder is stopped; and the current taken from the mains by the motor A is relatively steady, and free from severe fluctuations. Regenerative Braking. — A further advantage is that 're- generative braking is secured; that is to say, when it is required to stop the winder the controller handle is moved back, inserting resistance in the magnet circuit of B, reducing its emf below the back emf of F, which now be- comes a generator driven by the momentum of the winding gear. B now becomes a motor driving C, which by this means stores the energy freed by the retard- ation of the winding gear instead of its being wasted in grinding the brake blocks. See Regenerative Control Systems. Ward-Leonard System. — The same ar- rangement, without the flywheel, was origi- nally invented by Ward -Leonard, and is known as the Ward-Leonard system. This arrangement has not the property of smooth- ing out large fluctuations of current, but it 356 Mirror — Modulus of Elasticity has the same advantages as the Ilgner as regards control and regenerative braking. When braking, as there is no flywheel to absorb the energy, the motor A becomes a generator and supplies current to the cir- cuit, G. The main supply for either system may be cc or ac, but the machines B and F must be cc machines. When the supply is alter- nating, a small exciter is coupled to the shaft of A and B to supply cc for the magnets of B and F. Electric Rock Drills. — In metalliferous mines, electric rock drills have been used to some extent. These consist of two types, the rotary and the percussive. In the rotary type the drill rotates continuously, being driven by speed-reducing gear from an electric motor. In the percussive type the drill reciprocates longitudinally at a high speed, striking the face of the rock and rotating slightly at each blow. In some cases the motion is obtained by an arrangement of cams and springs driven by an electric motor, while in others it is obtained by a plunger and solenoid supplied with ac. For mining purposes, either cc or ac may be used, the three-phase alternating being generally used for large mines or for groups of mines supplied from one generating sta- tion. See also Electric Coal-cutter. (Eef . ' Electrical Mining Equipment ', Journ. I.E.E., No. 178, p. 477; 'Electric Coal Cut- ting', Proc.I.C.E., vol. cxliv, p. 247; 'Elec- tric Drills', Proc.I.C.E., vol. el, p. 472; ' Home Offiee Rules for Electricity in Mines ', Journ.I.E.E., No. 184, p. 255; 'Electricity in Mining', Hutchinson and Ihlseng; 'Elec- tricity Applied to Mining', Lupton, Parr, and Perkin.) MiPFOP, Galvanometep. See Galvan- ometer. MiPFOP Galvanometer, synonymous with reflecting galvanometer. See 'Reflect- ing Galvanometer' under Galvanometer. Mippop Receiving- Instpument, the first practical receiving instrument for long- distance submarine telegraphy. It was in- vented by Lord Kelvin (then Professor Thomson), and solved the problem of the transmission of signals through long cables without the use of high potentials, which destroy the insulation. A coil of many miles of very fine copper wire (silk-insulated) is wound on a brass bobbin (inner diameter about 12 mm, outer 75 mm, length 50 mm). In the centre of the coil a small mirror, on the back of which are stuck three or four small magnets, is suspended by a fine silk fibre. A large magnet outside the instrument is used to control the suspended magnet system in order to avoid the necessity of placing the plane of the coil in a north-and-south direc- tion. A beam of light is thrown on to the mirror, and is reflected from it on to a white- paper scale. If a current is passing through the coil, its magnetic field turns the magnetic needles and with them the mirror, through an angle, thus causing the reflected beam of light to turn through double this angle. The beam is practically a very long pointer without inertia or weight, which can indicate a very small angular deflection of the mirror. The instrument is thus both extremely sen- sitive to small currents and rapid in action. Mixed -gas Voltametep. See Volta- meter. M kg, the preferable abbreviation for meter Mhgram. Ml phps, the preferable abbreviation for mile (or miles) per hour per second. See Ac- celeration. Mm, the preferable abbreviation for milli- meter. Mmf, the preferable abbreviation for mag- netomotive force (which see). Mn, the chemical symbol for manganese (which see). Modulus of Elasticity.— The modulus of elasticity of a metal is the ratio of the stress in kg per sq mm to the strain in cm per cm of length. Thus if the total stress applied is P kg, and the area over which it acts is A sq mm, then the stress in kg per p sq mm is -^. If the length under test is L cm, and if the resulting elongation is I cm, then the strain per cm of length is =-. Then E, the modulus of elasticity, is equal to P/A l/L' For very small elongations the strain will be proportional to the stress. If this should continue to be the case until the elongation equals 100 per cent, i.e. until / = L, then we should have =- ■ 1 and E = tL. A Thus we may define the modulus of elas- ticity as the stress in kg per sq mm which would produce an extension of 100 per cent Moisture 357 if the strain continued to be proportional to the stress. For copper and aluminium the moduli of elasticity are respectively — Modulus of elasticity of hard-drawn copper wire = 11,300 kg per sq mm. Modulus of elasticity of hard-dTa;vn aluminium wire = 6,300 kg per sq mm. So long as the strain is proportional to the stress, and thus the stress -strain curve .a straight line, the sample will return to its original length when the load is removed, the sample being, under these conditions, perfectly elastic. When the load is increased beyond the range where the strain is propor- tional to the stress, the strain increases more rapidly than the stress. The sample is no longer perfectly elastic, and when the stress is removed, the sample no longer returns to its original length, but is found to have ac- quired a permanent set. The maximum load for which the strain is proportional to the stress is called the elastic limit, and is pre- ferably expressed in kg per sq mm. The stress in kg per sq mm at which the bar breaks is called the ultimate strength or tensile strength. Moisture, Effect of, on Insulating Materials. — Moisture has an exceedingly C016OOO O140O0 > 212000 gioooo Ul eooo m 6000 (0 4000 s CC 2000 e ^ ^ y X ■ -^ A y y ^ ^•^ y ^ ^ y ^^ ^ /. ^ /, f / 0.5 1.0 1.5 2,0 2.5 MILLIMETERS THICKNESS Curves showing the Influence of Moisture on the Disruptive Strength ol Fullerboard A, As received. B, Dried out. deleterious effect on the insulating qualities of insulating materials, lowering both their insulation resistance and their dielectric strength. The latter effect is by far the more important. The curves show very Vol. II clearly the extent of the harm done by moisture in solid materials. With fibrous materials, such as paper, press-spahn, or cam- bric, the moisture is usually expelled by drying out in a vacuum oven. Effects of Moisture in Oil. — In trans- former oil the bad effect of small quantities of moisture is still more marked, as with otherwise equal conditions the disruptive voltage may be diminished to one-quarter of its original value by the presence of as little as 0-03 per cent of water. The presence of moisture in an insulating oil not only reduces the dielectric strength, but it may cause corrosion of immersed ap- paratus. A simple but efficient method of removing moisture is to heat the oil to a temperature of about 50° C, and force through it warm dry air. Heating the oil renders it less viscous, the warm dry air readily absorbs moisture, and rapidly rises to the surface. Percentage Moisture in Insulating Oil. — The exact amount of moisture present in an oil is difficult to determine, but under no circumstances should it be permitted to exceed one-hundredth of one per cent. Test for Moisture in Oil. — The follow- ing methods may be employed for detecting the presence of mois- ture in an oil: — 1. A slight rise in specific gravity may denote the presence of moisture. 2. A rise in the di- electric strength after 12 hours heating at 105° C. 3. Plunge a red-hot wire into the oil; the presence of moisture is detected by a faint crackling sound. If the oil is dry a pufi' of smoke only will be emitted. 4. Roast a few crys- tals of copper sulphate to a white powder, and immerse in a small quantity of oil. The presence of moisture is detected by the return of the copper sul- phate crystals to their original blue colour. (Ref. C. E. Skinner, 'Transformer Oil', The Electric Club Journal, vol. i, p. 227; 'Insu- 24 358 Molecular Energy — Monocyclic System lation of Electric Machines', Turner and Hobart.) Molecular Energy. See Energy. Moment, TuFnlng. See Turning Mo- ment. Moment of a Magnet, the product of the magnetic length of a magnet (measured between effective poles) and the strength of one pole. See Magnetic Moment. Moment of Inertia. — The moment of inertia of a mass m, about any axis of rota- tion, is the value of the expression mr^ where r is the distance of the mass in question from the axis of rotation, provided the dimensions of the mass are very small in comparison with r. The moment of inertia of a rotating body is the sum of the quantities obtained by mul- tiplying the mass of each portion by the square of its distance from the axis of rota- tion. If fig. 1 represents a cross section of a body rotating about the axis o at right angles to Fig. 1. — Moment of Inertia of a Mass the plane of the diagram, and if Sm repre- sents an element of the mass of the body, which may be taken as being situated at a distance r from the axis, the moment of in- ertia of this element about the axis o is r^Sm, and of the whole body "Zr^Sm, where the summation is extended to all portions of the body. The moment of inertia in dealing with an- gular motion is the counterpart of mass in translational motion; thus the kinetic energy of a rotating body is expressed by |Ia^, where I = moment of inertia, and a = the angular velocity, while the kinetic energy of a body moving with a motion of translation is ^mv^, where m = mass and v = linear velocity. The moment of inertia of a cross section about any axis in the plane of the cross section is the sum of the quantities obtained by multiplying each portion of the area by the square of its distance from the axis. Thus in fig. 2 the moment of inertia of the cross section about the axis xy is 2r^8o, where Sa represents an element of area, and Fig. 2.— Moment of Inertia of a Cross Section where the summation is extended over the whole cross section. In the theory of beams, the neutral axis is implied, unless some other axis is specified. [m. b. f.] Monochromatic Light. — The radiation emitted by gases differs from that emitted by solids, in that it is restricted to radiations of definite wave length depending on the complex vibrating period of the component parts of the particles of the gas. In an or- dinary spectroscope the light from an incan- descent gas appears as fine lines divided by dark spaces indicating complete absence of radiation. These lines are identical in posi- tion and intensity for the same gas under the same conditions. In the case of radiation from solids, the radiation is not confined to certain definite wave lengths, but the spectrum is in general ' continuous ', indicating that waves of every possible length between the measurable limits are being emitted. If the light emitted by an incandescent gas be separated into its various components by a spectroscope, the radiation of a single spectrum ' line ' may be isolated. Such light is termed monochromatic. Occasionally a coloured glass may be em- ployed to intercept other radiations than the one desired, either instead of or in addition to the use of a spectroscope. Monocyclic Alternator. See Alter- nator, Monocyclic; Monocyclic System. Monocyclic System, a system of alter- nating generation and distribution invented by Steinmetz, which has been used to a limited extent in America. The generator consists of an ordinary sp machine, but with a supplementary or teaser winding connected to the middle point of the main winding (see fig.). A is the main winding, to the centre of which the 'teaser' winding a is Monophase Current — Monorail Electric Railway 359 connected, the emf of the latter being in quadrature with that of the main winding. The distributing system is so arranged that where lights only are required, the pressure A is used, and where motors are required, the third wire connected to a is run. The con- nections are shown in the fig. The system < owo"0'0"0"o"0"owo"crooooooo > Monocyclic System A a, Windings of monocyclic generator. B, Transformer, c, Three-phase motor. is now obsolete, having been superseded by the three- and two-phase systems. Monophase Current. See Alternat- ing Current. Monorail Electric Railway, a railway in which the cars run upon a single rail. In the Barmen-Elberf eld Suspended Railway, an illustration of which is given, the track is carried overhead on steel columns, and the cars are suspended from trolleys running on the single rails. This system may be termed an undershmg monorail electric railway. Brennan Gyrostatic Monorail Sys- tem. — In this system the cars run above the rail, and are kept upright by means of gyrostats driven by electric motors. The underslung and the Brennan are the only true monorail systems since they are the only ones not requiring guide rails. n Behr Monorail System. — In this system there are five rails for each track, arranged 360 Monotooth Alternator — Motor like the letter A, the weight of the car being carried by the single rail mounted at the apex. Monotooth Alternator. See Alter- nator. MoFdey - Frieker Electrolytic Pre- payment Meter. See Meter, Prepay- ment. Mordey's Split-armature Method of Testing' Alternators. See Tests, Split- armature. Morganite Brush. See Brushes. Morganite Crucible Company's Pneu- matic Brush Holder. See Brush Holder. Morse Alphabet, an alphabet in which the letters and other signs are represented by combinations of dots and dashes, or of any two distinct signals, e.g. a high and a low note, a long flash and a short one, a motion to the right and to the left, and so on. This type of alphabet, or code, was invented simultaneously by Morse, in America, and by Lieutenant, afterwards Vice - Admiral, Colomb of the British Navy. Colomb's code was devised for signalling at night, and was transmitted by means of a lantern with a movable shutter, in the form of long and short flashes. Morse devised his code for electric telegraphs in order to simplify the complication of having five needles and their corresponding line wires required in the early Wheatstone instruments. A number of vari- ations have been made in the signals of the original code, and in America a code is in general use in which there are two elements added to the five of the ordinary Morse. In Morse, as used in Europe and elsewhere, the elements are (1) the dot, (2) the dash = 3 dots in length, (3) the space between dots or dashes in a letter = 1 dot, (4) the space between consecutive letters = 3 dots, (5) the space between consecutive words = 5 dots. In the American (Phillips) code there are in addition (6) a long dash, (7) a 2-dot space between elements of the same letter; this last being introduced to avoid the necessity for representing some letters by a number of dashes. Thus in Euro- pean Morse is represented by three dashes, in Aitierican Morse by three unequally spaced dots, so that the saving in time of trans- mission is appreciable. Morse Code. See Morse Alphabet. Morse Inker. See Morse Eecorder. Morse Receiver. See Morse Eecorder. Morse Recorder, a telegraphic receiver which records the dots and dashes of the Morse telegraphic code (see MoRSE Alpha- bet) on a paper tape. The mechanism con- sists of an electromagnet, actuated either directly or through a relay by the current from the distant sending station, above the poles of which is an iron block attached to an arm pivoted at one end. Near the outer end of the lever, and supported by the frame of the instrument, there is a small steel wheel the lower edge of which dips into a trough of ink. The paper tape is drawn through continuously between the upper edge of the ink-wheel and the lower side of the lever above mentioned, by clockwork, which also revolves the wheel. The lever is normally so supported by a spring that it just does not press the paper against the wheel. On a current passing through the electromagnet, the lever is drawn down and presses the paper against the wheel as long as the cur- rent lasts, leaving a short or long line of ink on the paper according to the duration of the current. Stops are provided to prevent the iron armature coming into actual contact with the electromagnet, and the tension of the spring is adjustable. Morse Telegraph. See Telegraph, Morse. Moscicki Condenser. See Condenser, Electric. Motor, any machine capable of producing mechanical power in a convenient form. The term was formerly applied mainly to elec- trical machines, but is now used to denote other sources of mechanical power also. See Electromotor. Motor, Adjustable-speed.— \^ Motor, Adjustable - speed. — Motors in which the speed can be varied gradually over a considerable range; but when once adjusted remains practically unaffected by the load, such as shunt motors designed for a considerable range of field variation." — Paragraph 48 of 1907 Standardisation Rules of the A.I.E.E.] See also Motor, Variable-speed; Motor, MuLTisPEED; Motor, Varying-speed; Mo- ^TOR, Spinner. Motor, Alternating-current, an elec- tric motor which is designed for operation on a circuit supplied with ac. Such a motor may be of the sp (see Single-phase Motor) or of the polyphase type (see Motor, Induc- tion ; Polyphase Motors with Commuta- tors), and may also be either synchronous (which see) or asynchronous (which see). Syn- chronous ac motors are identical in construe- Motor 361 tion with ac generators, and, like these, they require separate excitation for the field mag- nets (see Alternator; Dynamo -electric Machine). Asynchronous motors are of two classes: Induction motors, in which there is no electrical connection between the stator and rotor windings; and commutator motors, in which the rotors are provided with com- mutators, and closely resemble the armatures of cc motors (the repulsion motor is an inter- mediate type, having a cc armature and com- mutator, but no electrical connection between the stator and the rotor). See Single-phasb Motor; Motor, Induction; Motor, Com- mutator. Synchronous motors have to be brought into synchronism with the source of ac be- fore they can be run on load; asynchronous motors are usually self-starting (except sp induction motors) and run at a speed below synchronism if of the induction type, but are independent of synchronism if of the commutator type. [a. h. a.] Motor, Back-geared, is used to give considerable speed reduction beween the motor and the driven machine, the arrange- ment being more compact than belting and more eflScient than worm gearing, with the advantages of direct-drive when required. A pinion, fixed on the motor shaft, en- gages "with a spur wheel mounted on a countershaft, from which the drive is taken either by belt or gearing. The fig. on the Plate facing p. 362 shows a form of back- geared motor manufactured by the British Thomson-Houston Co. The same company have a form which can be arranged to work in different positions, on the ground, wall, or ceiling. This is effected by fixing the motor in a cast-iron cradle, with its pinion gearing into, the spur wheel on a countershaft sup- ported from the cradle. The bearings of the countershaft are capable of being rotated, as are also the motor endshields and bearings. The gearing is enclosed in a sheet-iron case, of which the side is easily removed for in- spection. See Gearing for Electric Motors. Motor, Blower, See Blower Motor. Motor, Cascade. See Cascade Motor. Motor, Commutator. — This term, while applicable to cc motors, is usually restricted to sp ac motors of which the rotors are pro- vided with commutators and closely resemble cc armatures. Such motors are of three types:— Series -WOUND Ac Motor. — In general design this is identical with a series-wound cc motor, but the field-magnet cores are laminated throughout, and a compensating winding is often employed to neutralise the armature ats, for the purpose of improving the pf. Repulsion Motor. — Similar to the fore- going, but in this case the armature is en- tirely disconnected from the field-magnet winding, and the brushes are short-circuited, the armature current being generated by induction from the field -magnet (this is therefore also an induction motor); the brushes are displaced 60° or 70° from the neutral axis. The repulsion motor was in- vented by Elihu Thomson. Compensated Eepulsion Motor. — A combination of the two types above-men- tioned; in the simplest form, there are two independent sets of brushes, one set being short-circuited, while the other set is in series with the field-magnet winding, as in the series ac motor. Invented simultane- ously by Latour, and by Winter and Eich- berg. These motors, unlike the sp induction motor, have a strong-starting torque, and the first and third have a high pf, while they can be run economically at speeds above syn- chronism; they are much used for electric traction with sp ac. For a much more exhaus- tive classification of sp motors, see Single- phase Motor. [a. h. a.] Motor, Compound-wound. See Com- pound-wound Motor. Motor, Concatenated. See Cascade Motor. Motor, Constant-current, an electric motor, usually of the cc type, which is de- signed to operate on a circuit in which the current is maintained constant, the voltage only being varied in accordance with the speed, while the torque is controlled by shunting part of the current past the field- magnet winding. Motor, Constant-pressure, an electric motor which is intended to be used only on a circuit supplied at constant pressure. Most types of motor, whether for cc or ac, come under this head. Motor, Constant-speed.— ['Motor, Constant -speed. — Motors in which the speed is either constant or does not materially vary ; such as synchronous motors, induction motors with small slip, and ordinary cc shunt motors.' — Paragraph 46 of 1907 Standardisation Rules of the A.I.E.E.] 362 Motor Motor, Continuous-current, an electric motor which is intended for operation on a circuit supplied with cc. Motor, Counter Emf of. See Elec- tromotive Force, Counter. Motor, Differentially -wound. See Compound-wound Motor. Motor, Electric. See Electromotor. Motor, Induction, an ac electric motor in which (in the simplest and most usual types) there is no electrical connection be- tween the stator and rotor windings, these being in the same relation as the primary and secondary windings of an ac transformer, with the secondary free to rotate. There are two types — the sp motor and the poly- phase motor. In the sp motor the magnetic field set up by the primary ats is of an oscil- lating character, and the currents induced by it in the secondary winding, (which is short-circuited), are at a maximum in a plane at right angles to the magnetic field when the rotor is at rest, so that there is no initial torque, and the motor is not self-starting. When the rotor has been set in motion by external means, the reaction between the magnetic field and the induced currents in the rotor is no longer zero, and a torque is produced which attains a maximum at a speed of the order of 5 per cent below syn- chronism, and falls to zero at or near syn- chronism. In the polyphase motor, on the other hand, which is supplied with two or more ac difTering in phase, a magnetic field is set up by the currents in the primary or stator windings, which rotates in space about the axis of the motor, inducing cur- rents in the winding of the secondary or rotor. The reaction between these currents and the rotating field creates a torque both at standstill and when the motor is running, so that motors of this type are self-starting. Owing to the rotation of the magnetic field (or fields, if there are more than one pair of poles), polyphase induction motors are also called rotary-field motors; they may be either two-phase or three-phase. By applying a start- ing winding to a sp induction motor, with an arrangement for splitting the phase {i.e. converting the sp current into two ac differ- ing in phase), sp motors may also be made self -starting, the supplementary winding being cut out when full speed is attained. Both single and polyphase induction motors are of the constant-speed type, their normal speed being a little below synchronism. A form of induction motor known as the re- pulsion motor, in which a cc type of armature is used, is described under Motor, Com- mutator, and under Single-phase Motor. Owing to the ability of induction motors to operate without attaining exact synchronism with the source of supply, they are also known as asynchronous motors. See Contact Me- thods OF Measuring Slip in Induction Motors; Drysdale Stroboscopic Method OF Slip Measurement; Single-phase Motor; Polyphase Motor; Motor, Al- ternating-current, [a. h. a.] Motor, Multispeed (two -speed, three- speed, &c.). — These are defined in Paragraph 47 of the 1907 Standardisation Eules of the A.I.E.E. as motors which can be operated at any one of two or more distinct speeds, these speeds being practically independent of the load, such as motors with two armature windings. See also Motor, Variable- speed; Motor, Adjustable -speed; Mo- tor, Varying-speed; Motor, Spinner. Motor, Narrow-g-aug-e, a traction motor specially designed for use in vehicles running on a track of narrow gauge, as for instance a motor with vertical shaft, driving the wheel axle through bevel gearing. Motor, Open, an electric motor of which the armature and field-magnet windings are fully exposed. Motor, Open - protected, an electric motor of which the armature and field-mag- net windings are covered by shields to protect them from accidental injury, without seri- ously hindering the free circulation of air about them. Sometimes called semi-enclosed motors. Motor, Pilot, a small electric motor used as an auxiliary to a larger one, or to carry out minor operations in connection with in- stallations of machinery; for example, to actuate the controller of the main driving motors of a railway coach or of an electric elevator, or to operate switchgear at a dis- tance from the controlling platform in a power station. Motor, Pipe-ventilated, is a totally-en- closed motor, each end-shield being provided with a flanged pipe-end for connection to pipes which serve as inlet and outlet for air supplied on the forced-draught principle (see Ventilation of Electrical Machinery). See illustration on Plate facing this page. A number of such motors can be served with air from a common source, and the a; ■2 o S Q w a: < w o u <: C3 o H o Q w H z > a. Motor 363 advantages of totally - enclosed motors are secured at reduced size of motor for the same output, as compared with those not so fitted. See Motor, Totally -enclosed; Motor, Ventilated. Motor, Polyphase. See Polyphase Motors with Commutators; Motor, Al- ternating-current; Motor, Induction. Motor, Railway. See Eailway Mo- tor. Motor, Reciprocating, an electric motor which operates with a reciprocating motion, such as the motor of an electric rock drill. In a wider sense, any device for producing reciprocating motion, such as a reciprocating steam or gas engine. Motor, Reversible, an electric motor which can be run in either direction without difficulty or injury by effecting a simple change of connections. In a cc motor the brush-leads are interchanged, or the brushes are interchanged without altering the con- nections; or the connections of the field-mag- nets may be reversed. In the elementary sp ac induction motor the direction of rotation depends only upon the direction in which the motor is started. In a polyphase induction motor the direction of rotation is reversed by interchanging any two of the leads of a three-phase motor, or by interchanging the leads of one of the phases of a two-phase motor. Ac synchronous motors are not strictly speaking reversible, as they have to be brought into synchronism with the source of supply by external means. Motor, Running" Torque of. See Torque, Running, of Motor. Motor, Semi - enclosed. See Motor, Open-protected. Motor, Separately-excited, an electric motor of which the field-magnet winding is supplied with current derived from a sepa- rate source, distinct from that whence the armature current is derived. Motor, Series-wound, an electric motor of which the field-magnet winding is designed to carry the whole or the greater part of the current supplied to the machine, in series with the armature, the excitation being therefore proportional to the current in the armature. Eailway motors are usually series-wound. In a series motor the speed decreases rapidly with increasing load. Motor, Shunt, a cc motor which is pro- vided with a shunt winding only on the field- magnets, that is to say, only a portion of the current supplied to the motor is used for the purpose of excitation, and this does not pass through the armature, but is ' shunted ' past the latter by way of the field-magnet wind- ing. The excitation is thus proportional to the pressure applied to the motor, and is independent of the armature current. In a shunt motor the speed is practically con- stant at all loads, but can be altered by altering the strength of the shunt field. See Motor, Variable-speed. Motor, Signal, a small motor which actuates the signal disk or arm in electric systems of railway signalling. Motor, Single - phase. See Single- phase Motor. Motor, Spinner, an induction motor in which the member carrying the primary winding, as well as that carrying the secon- dary winding, is so mounted as to be capable of rotation. The spinner motor is the in- vention of Mavor. At starting, that member which is not connected to the load is allowed to rotate. As there is no torque opposing this rotation, the power required is only that necessary to accelerate the moving parts, and to overcome friction. When this member has reached normal speed a brake is applied, and it is gradually brought to rest. The rotation is taken up by the other member and by the load, which gradually accelerates as the speed of the other member decreases. In this way a squirrel-cage induction motor can be started against a large torque without taking an excessive current. Three-speed Spinner Motor, a spinner motor in which the member which is not connected to the load, but is free to rotate, is controlled by a second induction motor arranged to run at a speed diflfering from that of the first. In the fig. A represents a squirrel-cage rotor coupled to the load; the primary is shown at B, and is mechanically attached to another squirrel-cage winding c. The outer member is stationary, and carries a second primary winding D. If desired, the primary and secondary windings may be interchanged ; thus C might carry a primary and D a secondary winding. If E be the synchronous speed between A and B, and E' that between C and D, it is obvious that if the two primaries are connected to the mains, so as to cause rotation in the same direction, the speed of the rotor will be E -f E' (neglecting slip). If the member carrying B and c be held stationary, the speed will be 364 Motor E, and if the connections to D be reversed, so as to occasion rotation in opposite direc- tions, the speed will be R — E'. Three speeds are thus available, and by reversing the connections to BA the same three speeds may be obtained in the reverse direction. (Eef. H. A. Mavor, 'Paper read before the Inst, of Engineers and Shipbuilders 7ZZZZZ.7ZZZZZZZ. y^y>7777. W J- Three-speed Spinner Motor in Scotland', Feb. 18, 1908; and before the Inst, of Civil Engineers, Dec. 7, 1909.) [w. B. H.J Motor, Starting". — Ac motors of the synchronous type, including rotary conver- ters, are not readily made self-starting; a small starting motor therefore is often pro- vided, the sole function of which is to run the main motor up to speed, after which the former runs idle, or is uncoupled. The start- ing motor is generally of the induction type. Similarly large cc motors are sometimes ar- ranged to be started by a smaller motor. Motor, Synchronous, an ac motor (see Motor, Alernating - current), of which the iield-magnets are excited with cc, and which can only run in synchronism with the source of supply, at a speed determined by the frequency and the number of its poles. Motor, Totally-enclosed, an electric motor of the ironclad type, the armature, commutator, and field-magnet windings being completely surrounded by an iron or steel shell often forming part of the magnetic cir- cuit of the machine, in order to exclude dust and damp from the windings, and, in the case of motors in mills, coal mines, &c., to prevent danger of fire or explosion due to sparking at the brushes. In the 1907 Wiring Eules of the I.E.E., which apply to the supply of electricity at 1 pr not exceeding 250 volts, a totally en- closed motor is defined as follows: — 'A totally enclosed motor is one in which all the live parts, whether insulated or not, are totally en- closed as in ventilated motors, bnt without provision for internal ventilation.' See Motor, Pipe-ventilated. Motor, Variable - speed, an electric motor which is provided with means for varying the speed of rotation as desired. In the case of cc motors the field-magnets are preferably shunt-wound, and the speed Motor — Motor Performance Curves 365 is varied by inserting resistance in series with the field-magnet windings. Ac motors are not readily adapted for the purpose unless they are of the commutator type, when the speed can be economically varied either by moving the brushes (repulsion type) or by means of transformers with multiple secondaries, from which several different pressures can be obtained and applied to the brushes. Induction motors can be connected in cascade (see Cascade Motor), giving two economical speeds, or they can be constructed with means for varying the number of poles, intermediate speeds being obtained with resistance in- serted in the rotor circuit. Induction motors have been built with several rotors on one spindle, working in stators having diflferent numbers of poles. See Control, Series- parallel; Double - COMMUTATOR Motor; Motor, Multispeed; Motor, Adjustable- speed; Motor, Varying - speed. (Ref. Journ.I.E.E., vol. xxxix, p. 648.) Motor, Vapying-speed. — The American Institute of Electrical Engineers has very un- fortunately introduced the above designation for 'motors in which the speed varies with the load, decreasing when the load increases, such as series motors' (paragraph 49 of 1907 Standardisation Eules of the A.I.E.E.). It is unfortunate, since by the term variable- speed motors (see preceding article) a much more general meaning is usually intended. Motor, Ventilated.— In the 1907 Wir- ing Eules of the I.E.E., which apply to the supply of electricity at 1 pr not exceeding 250 volts, a ventilated motor is defined as follows : — • ' A ventilated motor is one in which, while ventila- tion is provided for, access to the armature, field coils, and other live parts is only to be obtained by opening a door in, or removing a part of, the enclosing case.' See Motor, Pipe-ventilated. Motor-booster. See Booster. Motor Car, an electric railway or tram- way car equipped with motors for the pur- pose of self-propulsion. See also Electro- mobile. Motor-ear System, that mode of electric traction on railways in which all or several of the cars of a train are equipped with motors for its propulsion, no locomotive being required. Motor Characteristic Curves. See Motor Performance Curves. Motor Converter, synonymous with cascade converter. See Converter, Cas- cade. Motor Dynamo. See Motor Gene- rator. Motor for Electric Cranes. See Crane, Electric. Motor Generator, an electric motor driving, usually by direct coupling, a gene- rator. Either machine may be for either ac or cc, of any pressure, frequency, or phase. When both are cc, wound for different pres- sures, the combination is sometimes called a rotary transformer (see Transformer, Ro- tary), as it fulfils the same function for cc that an ordinary stationary transformer fulfils for ac. Motor generators are also used for changing the frequency of the supply current (see Frequency-changer), and for changing the phase, as from sp to three-phase (rare). Paragraph 14 of the 1907 Standardisation Rules of the A.I.E.E. states that 'A motor generator is a transforming device consisting of a motor mechanically connected to one or more generators'. Asynchronous Motor Generator, an electric generator driven by an ac motor of the induction type, which is therefore self- starting. Synchronous Motor Generator, an electric generator which is driven by a synchronous ac motor, at a fixed speed depending upon the frequency of the sup- ply current and the number of poles of the motor. See also Central Station for the Generation of Electricity. Motor-g-enerator Locomotive, a loco- motive intended for use on a sp electric railway, the wheels being driven by cc motors fed from a generator carried on the loco- motive, which in turn is driven by an ae motor supplied with power from an over- head conductor. In such a case the Ward- Leonard system of regulation is applicable. See also Ward-Leonard System. Motor-meter. See Meter, Motor-. Motor Performance Curves, curves employed to exhibit the running and start- ing properties of motors. When a motor is running, the following quantities can be measured: Input, output, torque, speed, cur- rent, copper losses, iron losses, mechanical losses. One of these being plotted horizon- tally, usually either the output or the torque, the others can be plotted as ordinates, as in 366 Motor Performance Curves fig. 1. At starting we can measure the pd, current, input, and torque, the current or o ' 2 J t s 6 Fig. 1.— Curves for Heyland 4-bhp Single-phase Motor the torque being plotted horizontally. Fig. 2 shows a typical set of starting curves. Other curves are sometimes given, show- ing the performance of a motor as it runs up to speed in conjunction with a controller, and other auxiliary gear. motor characteristics, but there is no more reason to apply this term to speed-torque Q U 140 a. (0 \ \ Q lU 130 \ \ A U. 120 \ V v ERCENTAGE 8 5 \ ^ C \ V D^ ^ \ -E ^ ?rw Q. ^ 16 ptb CLf\ 14 / 12 30 / / f> INPUT- KW 0> Q) O AMPERES _ 10 10 01 O 01 / / 4 V / / j / d k > 4 V / y /. r / / \j 4 10 / / y / /• 2 6 [^' y ^ r /. 2 4 6 8 10 12 TORQUE-METER KG Fig. 2.— Typical Starting Curves of Motor Motors of various types are specially characterised by their speed-torque curves. Such curves are sometimes broadly termed 40 SO 120 160 PERCENTAGE OF FULL LOAD TORQUE Fig. 3.— Torque-speed Diagram A, Full load. B, Co series. 0, Single-phase commu- tator. D, Cc shunt. E, Single-phase induction. F, Kated speed, o, Three-phase squirrel-cage. curves than to the other motor performance curves already mentioned; consequently it is preferable to term them speed-torque characteristics. In series - wound and shunt- wound motors, particularly the former, the speed of the ^00 motor decreases with the in- crease of torque as the motor is loaded; while with com- pound-wound motors it is possible to obtain practic- ally constant speed over the whole range of load. Where compound-wound motors are employed, however, it is frequently the custom ta connect the compounding in the opposite direction to that which gives constant speed, which assists the motor to start, but makes: its speed characteristic droop even more than that of a shunt machine. Fig. 3 shows examples of such speed curves. See also Curve, Char- acteristic, [c. V. D.] 14 16 Motor Starter — Multiple-voltage System 367 Motor Staptep. See Starting of Motors. Motop - stapling Resistance. See Starting of Motors; Rheostats or Ee- sistances. Motop-stapting- Switch. See Switch, Motor-starting. Motop Transfopmep. See Motor Generator. Motor Tpuck. See Truck. Moulded Insulatop. See Insulator. Moulded Mica. See Mica, Moulded. Moulding (op Casing) fop Intepiop Wiping". See Wiring Systems. Moulding System of IntepioP Wiping. See Wiring Systems. Movement. — Those parts of an electrical measuring instrument constituting the mov- ing system are termed the movement. Moving-coil Ammetep. See Ammeter. Moving-coil Galvanometep. See Gal- vanometer. Moving -coil Voltmeter. See Volt- meter. Moving-ipon Ammetep. See Ammeter. Moving-iron Voltmetep. See Volt- meter. M phps, the preferable abbreviation for mefer {or meters) per hour per second. Multi- filament Lamp. See Lamp, Incandescent Electric. Multi-gap Lightning Appestep. See Lightning Arrester. Multiphase. See Polyphase. Multiple-circuit Winding. See Wind- ing, Multiple-circuit. Multiple Coil. See Coil, Multiple. Multiple-contact Switch. See Switch, Multiple-contact. Multiple -font Electpically Opepated Type - composing Machine, a machine with a species of typewriter keyboard, each key operating an electric circuit which re- leases one letter belonging to one or other font, which letter is then automatically set as in other composing machines. Multiple-series Cipcuit. See Circuit, Electric. Multiple -unit ContPoUep. See Con- troller, Master. Multiple-unit System of Train Con- trol, a mode of controlling electric trains composed wholly or partially of motor cars, each of which is provided with apparatus for regulating the speed and direction of running, and one or more master controllers. All the master controllers are connected by a cable running from end to end of the train, and each is connected with the auxiliary controlling apparatus on its own car. By operating any one of the master controllers, the whole of the motors on the train are controlled exactly as if all the master con- trollers were simultaneously operated. The master -controller circuits carry only small currents, the switchgear which actually handles the heavy-power currents being mounted separately under the car, or in a special compartment. In the Sprague multi- ple-unit system the auxiliary switches (which are called contactors) are closed and opened, and the reverser, which determines the direc- tion of rotation of the motors, is operated, entirely by electrical means. Current for this purpose is derived from the main circuit on the car where the master controller is being handled, and conveyed to all the other motor cars by the cable above mentioned. Each motor car is provided with collecting shoes at both ends, which bear on the con- ductor rail, the car being entirely self-con- tained so far as the motive power is concerned, and independent of the rest except in respect of the control. In the Westinghouse unit- switch multiple^wnit system the general princi- ples are the same, but the contactors or unit- switches are operated by compressed air derived from the air-brake system, the master controllers being supplied with current from a small battery of accumulators, and actuat- ing the valves of the pneumatic contactors by this means. The reverser is also operated by compressed air. In earlier forms both these systems used, instead of contactors and unit -switches, controllers driven by motors or by compressed air. See also Con- tactors; Controller; Controller, Mas- ter; Coupler, Bus-line; Jumper Recep- tacle, Bus-line. [a. h. a.] Multiple-voltage System, an electrical system with three or more conductors (for instance, figs. 1, 2, and 3) so connected to the generating plant that the consuming devices may be supplied at two or more different voltages, as desired. For instance, incan- descent lamps and small motors may be supplied at 220 volts, and larger motors at 440 volts. In fig. 1 the various voltages are obtained by the use of a balancer machine, but a similar result may be attained with generators in series, as shown in fig. 2, or by a motor generator, as shown in fig. 3. 368 Multiplex Connection — Multi-voltage Speed Control Multi-voltage systems are frequently em- ployed where variable-speed cc motors are required (see also Multi-voltage Speed Control). Multi-voltage ac systems are generally obtained by taking a tapping from the centre of the secondaries of the supply transformers (see fig. 4). The well-known F -f- F ■ B Fig. 1 F Fig. 2 i tV-6 1 — r E F 4- -B H 1- F FG _1_J_ Fig. 4 Multiple-voltage System A, c = Outer conductors. B = Middle conductor. D = 60 volts. E = 160 volts. P = 220 volts. Q = 440 volts. H = 2000 volts. three-wire system (which see) is the type of multiple-vpltage system most frequently em- ployed. (Eef. 'Electrical Transmission of Energy', A. V. Abbott; 'Electric Motors', Hobart.) Multiplex Connection (of a Light- ning Arrester). — On p. 1061 of the Trans. A.I.E.E., vol. xxvi. Part II, Creighton gives the following definition: — 'On a lightning arrester, this commercial term means that there are certain cross-oonneotions betvreen phases of the arrester, above the earth oonneotion. These cross-connectiona may or may not have an appreciable resistance.' Multiplex Lightning Arrester. See Lightning Arrester. Multiplex Telegraphy. See Tele- graphy, Multiplex. Multiplicity of Continuous -current Winding designates the number of com- ponent windings. Thus a winding may be simplex, duplex, triplex, &c., and in these cases the complete winding comprises one, two, three, &c., component windings. (Eef. 'Armature Construction', Hobart and Ellis, p. 157.) See also Ee-entrancy. Multiplying Power of Galvanometer Shunt. — In order to use a galvanometer whose coils are wound for very small cur- rents, for measuring not only small currents but much larger currents, resistances called shunt resistances are provided. These are conveniently arranged in a shunt box (which see). Thus if it is desired to measure cur- rents of the order of 100 times the magnitude of the current for which the galvanometer coils are suitable, a resistance equal to one ninety-ninth of the resistance of the galvano- meter coils is shunted about the galvano- meter. A given deflection will then corre- spond to a 100 times greater curreilt than corresponds to this deflection when the gal- vanometer is unshunted. The multiplying power of this galvanometer shunt is said to be 100, and the shunt ratio is one ninety- ninth. See Galvanometer; Shunt Box. Multipolar Dynamo. See ' Continuous Generator ' under Generator. Multipolar Field. See Field, Multi- polar. Multipolar Generator. See ' Continu- ous Generator ' under Generator. Multi-speed Motor. See Motor, Mul- ti-speed. Multi-voltage Speed Control, control of the speed of a motor by connecting its ter- minals to supply leads at different voltages; for instance, in some works and factories three leads are provided, as shown in fig. 3 on this page, and the motors can be connected by means of controllers or special switchgear between a and B, A and C, and B and C as desired, with a corresponding variation in the speed of the motors. The fields of the motors are generally excited from a fixed pair of the leads, say from A and c, and the alteration of the connections is limited to the armatures. If the field connections were altered with the armature connections, the field of the motor would vary with applied voltage, and the speed, (which varies as the applied voltage and inversely as the field strength), would therefore remain approxi- mately constant. Intermediate speeds can, however, be obtained by field variation, though in some cases they are obtained by a resistance in series with armature. (Eef. 'Electric Motors', Hobart.) Mummified Winding — Natural Period of Oscillation 369 Mummified Winding-. See Winding, Mummified. Murray Type -printing- Teleg-raph. See Telegraph, Type-pkinting. Mushroom. See Arc. Mutual Induction. See Induction, Mutual. Mutual-induction Standard, a standard of induction depending on the interlinkage of the lines of force produced by an air-cored coil carrying a current, with the turns of a search coil. The magnetising coil is either in the form of a long solenoid, or is arranged so that the core forms a ring. The induction density may then be calculated. It is equal 4ir X ats to 10 X length of coil in cm" N Naphtha, a term applied to certain sol- vents. There are four distinct products called naphtha, viz.: (1) wood naphtha, (2) coal-tar naphtha; (3) petroleum naphtha; (4) shale naphtha. 1. Wood Naphi'ha. — This is sometimes termed wood spirit. It is crude methyl al- cohol, and is obtained from the distillation of wood. It contains more or less ethereal impurities which give it an unpleasant odour and nauseous taste, but render many of the resins and gums more soluble in wood naph- tha than in pure methyl alcohol. Recent improvements in its manufacture have over- come its objectionable taste and smell. The best qualities contain about 95 per cent of methyl alcohol. It is added to spirit var- nishes to render them quick-drying, and is ex- tensively used in the manufacture of methy- lated spirit. See ' Methylated Spirit ' under Alcohols. 2. Coal-tar Naphtha, sometimes termed solvent naphtha. This is a refined distillation product of tar, of complex composition, but consisting chiefly of compounds of the ben- zene series. It is a water-white liquid of rather unpleasant odour, and evaporates at ordinary air temperatures, leaving no residue. It is a good solvent for resins, pitches, and indiarubber, and is used for thinning water- proofing compounds. Coal-tar naphtha is sometimes sold as benzene (which see). 3. Petroleum Naphtha, sometimes termed bemolin£ or benzolene. This is a re- fined distillation product of petroleum. It is a water-white limpid liquid evaporating rapidly at atmospheric temperatures, leaving no residue. It has a specific gravity of some 0'69 to 0-72, and is used to some ex- tent for thinning core-plate varnishes. See also Petroleum; Shale Solvents. 4. Shale Naphtha, a refined distillation product of shale, sometimes termed shale spirit or paraffin naphtha. It is composed of a mixture of hydrocarbons of the paraffin and olefine series, the latter forming some 60 per cent of the naphtha. It is a water- white limpid liquid, evaporating rapidly at ordinary air temperatures, leaving no resi- due. Shale naphtha is insoluble in water, and readily dissolves many resins. It is ex- tensively used as a substitute for turpentine in oil varnishes. See Petroleum and Shale Solvents. [h. d. s.] Narrow-g-aug-e Motor. See Motor, Narrow-gauge. Natural Magnet. See Lod'estone. Natural Period of Oscillation of a System, the time occupied by one complete cycle of a disturbance of a system which has been acted upon by an external disturb- ing agency, and then left to itself. A pen- dulum, for example, if disturbed and then left to itself, will swing with a definite period of vibration; or if an electric disturbance be set up in a circuit containing inductance and capacity, an electric oscillation will result (see Electric Oscillation), of which the periodic time depends solely upon the con- stants of the circuit. The natural period of oscillation of a sys- tem is to be distinguished from the forced period of oscillation, which may result from some external agency. For example, al- though a pendulum, if left to itself, will swing with a definite period of vibration, it is obvious that it could be dragged back- wards and forwards at a diiferent rate by some external agency. Similarly, in the electrical example above-mentioned, an alter- nating emf of any given frequency might be introduced into the circuit. Thus the forced frequency must be distinguished from the natural frequency of oscillation of a system, and, similarly, the forced period from the natural period. See Discharge, Oscilla- 370 Needle — Neutral TORY; Discharge, Dead-beat; Oscillating Circuit; Electric Oscillation; Oscilla- tion, [m. b. f.] Needle, Astatic. See Astatic Needle. Needle Gap. — This consists of two sharp needles held, facing each other, in suitable holders mounted on insulating pillars. They are so arranged that the distance between their points can be accurately measured, and are connected to the electric circuit in such a way that a discharge can pass between them. Since a certain voltage is required for the discharge to pass with the needles a given distance apart, the gap may be used as an approximate measure of the voltage of a circuit. A table for use with the needle gap will be found under the heading of Sparking Distance. Needle-point Suspension. See Sus- pension IN Measuring Instruments. Negative Booster. See Booster. Negrative Brush of Dynamo or Motor. See Brushes. Neg-ative Carbon of Arc. See Arc. Negative Chargre.— A charge is mea- sured in electrostatic or electromagnetic units of quantity. See Negative Elec- tricity. Negative Current. See Current, Ee- TURN. Negative Electricity is distinguishable from positive electricity in a large number of phenomena, among which the very definite diiferences between the negative and positive ends of a discharge through a gaseous or liquid medium are perhaps the most charac- teristic. Recent theory and experiment point to the existence of negatively -charged par- ticles having about one-thousandth part of the mass of an atom of hydrogen. It ap- pears that these are more or less free, and that their diifusion through a conductor con- stitutes a current. They are also detectable in radium radiations, and in similar radia- tions. There do not appear to be any posi- tively-charged particles (or atoms of elec- tricity) so small as these; in fact, a positive charge is looked upon as an atom not fully stocked with negative particles. See also Electricity. [j. e-m.] Negative Electrode. See Electro- lysis. Negative Feeder. See Feeder. Negative Group, any number of nega- tive plates of an accumulator connected to- gether by a bar. Negative Plate. See Plate, Negative; also Accumulator Plates. Neptune Rail Bond. See Bond. Nernst Lamp. See Lamp, Incandes- cent Electric. Nett Core Length. See Core Length. Network of Conductors, a system of electrical conductors, all in continuous con- nection, joined to one or more sources of supply. The expression is often used with reference to the complete system of feeders and distributors used in the distribution of electricity in a town or district. Where h pr is supplied to transformers or to substations, the terms high-pressure (or tension) network and low-pressure (or tension) network are used in contradistinction. (Eef. ' Central Electrical rl|, Network of Conductors Stations', Wordingham; 'Modern Electric Practice ', vol. ii.) In the fig., the full lines represent a 1 pr network supplied from the substations indicated by the black squares. These substations are in turn supplied by the h pr feeders indicated by the broken lines. See Distributing Mains; Feeders. Neutral, the central point of an electrical system. For instance, in a three-wire system the neutral is the middle conductor, in a three-phase system the neutral conductor consists of the centre of a star arrangement of windings or of consuming devices. The neutral of a system is frequently earthed in order to limit the possible maximum differ- ence of potential between any part of the system and earth. See also Three-wire Distributing System. (Eef. ' Central Sta- tion Electricity Supply ', Gay and Yeaman.) Grounded Neutral, in an electrical sys- tem, a neutral point which is connected to Neutral Axis of Commutation — Neutral Point 371 earth. The Board of Trade Regulations under the Electric Lighting Acts require that in a three-wire system where the pres- sure between the outers exceeds 250 volts, the neutral of the system must be earthed at one point, and the current through the earth connection must be continuously re- corded. Should this current exceed one- thousandth of the maximum supply current, immediate steps must be taken to improve the insulation of the system. A recording ammeter is used to measure the current, and very often an electromagnetic switch is added to short-circuit the recording ammeter in the event of a very heavy current passing; sometimes it is also arranged that an electric bell shall ring, or other signal be given, when this happens. It is not desirable that there shall be a fuse in the earth connection unless it is arranged as a short circuit to a resist- ance, which is thus introduced into the cir- cuit when the fuse blows, and keeps the value of the earth current within such pro- portions as will not jeopardise the whole supply. There is considerable divergence of opinion as to the desirability of earthing the neutral of a three-phase system, and for pressures up to say 10,000 volts many engineers prefer not to connect to earth. Where, however, very high pressures are employed on long-distance transmissions, earthing is often resorted to in order to keep down the maximum possible pressure to ground, seeing that it is this pressure which has to be taken into account in providing insulation. (Ref. Taylor, 'Net- work Tests and Station Earthing', Journ. I.E.E., vol. xxxii, Feb. 1903). See Three- wire Distributing System; Compensator; Multiple-voltage System; Dobrowolski Three-wire Dynamo; Neutral Conduc- tor, [f. w.] Neutral Axis of Commutation. See Neutral Point on a Commutator; Ar- mature Reaction; Commutation; Neu- tral Zone. Neutral Conductor, in a three -wire system, the conductor midway between the two supply mains (a in fig. 1) (see Three- wire Distributing System); in a three- phase system a conductor connected to the central point of the star windings of the generators, transformers, &c. (a in fig. 2). (Ref. 'Three-wire System' in 'Central Sta- tion Supply', Gay and Yeaman; 'Modern Electric Practice'; 'Standard Polyphase Ap- paratus and Systems', Oudin; 'Alternating- current Phenomena', Steinmetz.) In the 1907 Wiring Rules of the I.E.E. for the supply of electricity at 1 pr not ex- ceeding 250 volts, the neutral conductor of a three-wire system is defined as 'the con- ductor which is at a potential intermediate Fig. 1.— Neutral Conductor in I'liree-wire System Fig. 2. — Neutral Conductor in Tliree-phase System between the potentials of the outer con- ductors'. See also Three- wire System; Neutral; and Conductor, Outer; Mul- tiple-voltage System. Neutral Feeder. See Feeder. Neutral Point, Thermo-electric. See Thermo-electricity. Neutral Point of a Y- connected Winding", the point at which the windings of the three different phases are brought together. See Neutral Conductor; Con- nections, Three-phase. Fig. 1 Fig. 2 Neutral Points o( Commutator Neutral Point on a Commutator, a position corresponding to a point on the 372 Neutral Wire — Non-combustible Insulating Covering armature at which the magnetism changes in polarity. At no load, and with a uniform distribution of flux on each side of the pole, neutral points will be midway between the poles, but will change as the load comes on, owing to the distorting efiect produced by the armature load-current. In fig. 1, which corresponds to no load, the neutral points are seen to be on an axis normal to the field axis. In fig. 2, which corresponds to full load, the neutral points are farther forward in the direction of rotation. See Neutral Zone; Commutation. Neutral Wire. See Neutral Con- ductor. Neutral Zone, that portion of the air gap between two adjacent poles of opposite polarity; in cc machines that portion of the interpolar space where commutation takes place. See Neutral Point on a Com- mutator; Commutation. Newell Eleetromag-netic Brake. See ' Electromagnetic Brake ' under Brakes. Ni, the chemical symbol for nickel (which see). Nickel, a slightly magnetic metal with a specific gravity of 8 -9 and a melting-point of about 1500° C. Its specific heat is O'll. The specific resistance at 0° C. is 12-4 mi- crohms per cm cube, and the resistance in- creases by from 0'5 to 0'6 of one per cent per degree Centigrade increase in temperature. Nickel Steel. See Steel. Nickelin, a resistance alloy of approxi- mately the following composition: Nickel 18-5 per cent, copper 61 -5 per cent, zinc 20 per cent, with traces of iron, cobalt, and manganese. Its resistivity is about 33 to 44 microhms per cm cube, and its coefiicient of increase of resistance per 1° C. is 0'00030 to ■00033 of its resistance at 0° C. See High-resistance Alloys; Wire, Eesist- ANCE. No-are Fuse. See Fuse. No-load Current, the current flowing through the primary winding of an induction motor or of a static transformer when there is no load on the motor or on the secondary of the transformer. See Magnetising Cur- rent. No-load Losses, a term applied to the energy consumed by any piece of electrical apparatus which is doing no external work. Thus in generators and motors the no-load losses are made up of — (a) Excitation Loss. — FK of exciting coils. (6) Gore Loss. — Hysteresis and eddy-current loss in iron cores due to alternating or rotat- ing magnetism. (c) Friction Loss. — Windage. Brush and journal friction. In addition to the above, there is a small armature copper loss, but on account of the smallness of the light-load current and the very low resistance of the armature, this is negligibly small. In the case of transformers with open secondary, the power consumed is practi- cally all core loss, since the magnetising current is always extremely small and the resistance of the primary comparatively low. No-load Release, an automatic device which opens a switch, in the event of the supply of electricity to the circuit controlled by the switch being interrupted. Usually employed in connection with a rheostat for starting an electric motor, in such a way that the whole of the starting resistance is inserted in the main circuit and the latter is opened, on failure of the supply. See also Switch, Motor-starting; No-voltage Eelease; Starting of Motors. No-load Saturation Curve. See Curve, Characteristic. Nodon Electric Valve. See Eectieier. Nodular Deposit, a solid deposit from a solution, in the form of nodules, i.e. in roughly hemispherical lumps, the formation of which is usually non-crystalline. Noegrerrath Homopolar Dynamo, a type of homopolar dynamo in which the emf is increased by connecting several armature conductors in series, with the aid of slip rings and brushes, and stationary conductors external to the armature. See Generator; Unipolar; Non-polar Generator. Nominal Rating- of Railway Motors. See Eailway Motors, One-hour Eating of. Non-arcing- Multigap Lig-htning Ar- rester. See Lightning Arrester. Non-arcing Property of Metals. See Lightning Arrester. Non- combustible Insulating- Cover- ing-, insulation designed to resist high tem- peratures without deterioration and to be non-inflammable, not merely 'fire-resisting'. See also Fireproof Covering; Fire-re- sisting Covering; Mica; Asbestos; Del- TABESTON MaGNET WiRE; WiRE, EnAMEL- INSULATED; INSULATION, LaVA; LAVITE Non-conductor — Normal Short-circuit Current 373 Insulating Material; Marble as an Insulating Material; Slate as an In- sulating Material; Enamelled Slate. Non- conductor, any substance which does not conduct electricity to an appreciable extent. It is simply an insulator defined from a different point of view. See Insu- lator; Insulativity; Dielectric. Non-ereeping Device, a device used in energy meters to prevent 'shunt running' or 'creeping' (see under Shunt Running); e.g. in meters with a magnetic brake the device usually consists of a tiny piece of iron on the brake disk. In the Aron clock meter a small coil is used in the pressure circuit of the meter so that it acts on one pendulum coil only, tending to drive the meter back- wards. The backward movement is pre- vented by a pawl attachment to the register. Non-inductive Load, a term usually applied to a resistance used for loading ac apparatus, which when traversed by the load current gives no magnetic effect, and hence there is no difiference of phase between the load current and the pressure at the terminals of the load. The pf of a non- inductive load is unity. The load may con- sist of ordinary banks of glow lamps, straight parallel wires or strips laid close to one another, and connected in such a manner that each alternate strip carries current in the opposite direction to the preceding one, or coils of wire which has been doubled back upon itself before the coils were wound. The ordinary open-wound spiral coils used in resistance frames are but very slightly inductive, and the ordinary water resistance is wholly non-inductive, in fact it may in some cases produce a slight lead in the load current, due to a capacity effect. A Twn-indudive load is defined in para- graph 52 of the 1907 Standardisation Eules of the A.I.E.E. as 'a load in which the cur- rent is in phase with the voltage across the load'. Non-inductive Resistance. See Rheo- stats OR Resistances; Resistance, Non- inductive; Non-inductive Load. Non-magnetic Steel. See Steel. Non- periodic Function, a function which does not follow the periodic law. See Periodic Function; Fourier's Method; Fundamental Waves and their Har- monics; Harmonics. Non -polar Generator, a type of cc generator invented by Prof. George Forbes, I Vol. II and described in ' Dynamo - electric Ma- chinery', by Prof. S. P. Thompson, p. 227 (2nd ed.). The principle is identical with that of the homopolar generator (see ' Homo- polar Generator' under Generator). The term non-polar is not now used. See also Unipolar; Noegerrath Homopolar Dy- namo. Non-sine Waves, waves which are not sine waves. In paragraph 83 of the 1907 Jig. 1.— Non-sine Waves Standardisation Rules of the A.I.E.E. xne phase displacement between two waves which are not sine waves is defined as that phase dis- placement between their equivalent sine waves (see Sine Wave) which would give the same average product of instantaneous values as the actual waves; i.e. the same electrody- namometer reading. In fig. 1 are shown two ^-N z s / \ / \ ^ 7 A r S Z Fig. 2.— Bquivalent Sine Wave non-sine waves of equal area, and in fig. 2 is shown an equivalent sine wave with a crest value equal to the mean of the crest values of the two non-sine waves in fig. 1, and witji the same area as these non-sine waves. Normal Current denotes the current which the machine, circuit, or other ap- paratus in question is designed to carry. Normal Rating. See Rating, Normal. Normal Short-circuit Current. See Current, Short-circuit. 25 374 Normal Voltage — Null Method of Measurement Normal Voltage. See Voltage, Nor- mal. Norwich TarifF. See Assessment Ta- riff. Nose Suspension. See Suspension of Traction Motor. Notched-end Fuse. See Fuse. Notches. See Controller. Notching Die. See Die. No -voltage Circuit Breaker. See Circuit Breaker. No-voltage Relay. See Eelay. No-voltage Release, a device arranged to trip a circuit breaker, oil switch, &c., automatically when the voltage of the system falls to zero, or to return the contact arm of a motor-starter to the 'off' position auto- matically under the same conditions, the object being to prevent the sudden rush of current that would occur if the circuit re- mained closed when the voltage of the system was restored to its normal value. The rush of current might prove injurious to apparatus on the circuit, as, for instance, to the armature of a motor that was at a standstill. In the case of a circuit breaker or oil switch the no-voltage release generally takes the form of a solenoid wound with a shunt coil, and so arranged that the core drops and releases the contacts, on the voltage failing; or of a small iron armature without any winding which is placed between the jaws of an electromagnet energised by a shunt coil, with the result that it is pulled into line with the electromagnet when there is a voltage on the circuit, while it is pulled by a spring into a position at right angles to this as soon as the voltage fails, and so trips the breaker. With ac the iron portions of the magnetic circuit are of course lami- nated. In the case of a motor starter the no- voltage release generally takes the form of a shunt magnet, which in normal working holds the contact arm in the full ' on ' posi- tion, but allows it to be pulled by a spring to the 'off' position when there is no volt- age on the circuit. See No-load Eeleasej Starting of Motors; Switch, Motor- starting. Nozzle. — In turbo -generating sets the steam passes through nozzles on its way to the turbine wheel. These nozzles are pas- sages the form and dimensions of which must be determined with great care by turbine designers if low steam consumption is to be attained. They are so proportioned as to impart to the steam a speed greatly in excess of that which it would attain if discharged through a plane aperture. De La\al Diverg- ing Nozzle. — The great importance of this point was first clearly appreciated in 1889 by de Laval, who invented the diverging nozzle illustrated in iig. 1. The invention is the subject of British Patent No. 7143 of 1889. It is interesting to contrast this 1889 invention with Hero's steam turbine, invented over 2000 years previously, in which rotation was also effected by means of jets of steam, as shown in fig. 2, reproduced from Stevens and Hobart's ' Steam Turbine Fig. 1.— De Laval Diverging 14'ozzle Fig. 2.— Hero's Steara-Englne Engineering ' by the kind permission of the publishers. Null Method of Measurement.— This term is applied to those methods in which the effect to be measured is balanced against some known value, with the result that the Oblique Loop — Ohmmeter 375 balancing instrument indicates zero. Ex- amples will be found in the Wheatstone Bridge, the Potentiometer, the Magnetic Bridge, the Sine Galvanometer, and the Siemens Dynamometer. See also Zero Methods of Measurement. o Oblique Loop. See Loop of Armature Coil. Odd Harmonics. — Odd harmonics are frequently confused with uneven frequency terms, whereas in reality they correspond to the even frequency terms. See Third Harmonic; Harmonics; Fundamental Waves and their Harmonics. Oerlikon Trolley. See Rod-type Trol- ley. Oersted. — l. A distinguished Danish physicist who first discovered the magnetic action of an electric current. 2. A name proposed by the A.I.E.E. (see Transactions, vol. xxii, p. 533) for the cgs unit of magnetic reluctance; not generally accepted or used. According to the A.I.E.E. definition, a magnetic circuit has a reluctance of 1 oersted when unit mmf produces unit flux. SeeEELUCTANCE; Magnetic Circuit. Ohm. — 1. A German physicist who first enunciated the principle now known as Ohm's Law (which see). 2. The practical unit of resistance in the electromagnetic system of units. It is in- tended for IC absolute cgs units, and is called for distinction the True Ohm; but inasmuch as the determination of a resistance in absolute units is a matter of great difficulty, and is liable to much greater errors than the comparison of resistances with one another, the ohm is defined also in terms of the resistance of a replaceable standard. Thus in Great Britain the ohm is legally defined as being 'represented by the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice, the mercury having a mass of 14-4521 g, and having a length of 106-3 cm, and a constant cross-sectional area '. The ohm defined substantially as above was recommended for adoption by the Inter- national Electrical Congress, Chicago, 1893, and has been given legal sanction by the chief governments of the world. It is known as the International Ohm, or more generally in this country as the Standard Ohm, or occasionally the B.O.T. {Board of Trade) Ohm, to distinguish it from certain provisionally- accepted units based on earlier determinar tions of the value of the true ohm. One of the first of these was the so-called B.A. Unit or B.A. Ohm, issued in 1865 under the direction of a Committee of the British Association for the Advancement of Science. Its value is 0-9866 of the International Ohm. A later unit, which has had some acceptance, was recommended in 1884 by a Commission of the International Congress of Electricians, Paris. This was called the Legal Ohm, al- though it was never given legal sanction. It is defined as the resistance at 0° C. of a column of mercury 106 cm in length, and 1 sq mm in cross section. Its value is 0-99718 of the International Ohm. Naturally the ohm is also the unit em- ployed in measuring quantities of the same dimensions as resistance, such as impedance, reactance, &c. (which see). [f. w. c] Ohmage, a word sometimes used to signify resistance or impedance; formed by analogy with voltage. Ohmaline, the registered trade name for a quick air-drying varnish which the manu- facturers claim remains plastic. See Insulat- ing Varnishes. Ohmic, pertaining to electric resistance; pertaining to the ohm. Ohmic Drop. See Fall of Potential. Ohmic Resistance. See Resistance, Ohmic. Ohmmeter, a direct-reading instrument for the measurement of resistance. It may a £7a Ohmmeter be either of the (1) deflectional or (2) zero type. Instruments of type 1 are either of the moving-needle, moving-coil (e.g. megger), or electrostatic pattern, the principle being much the same in each case, viz. to replace the control-spring or weight in a sensitive am- meter by a control proportional to the pres- 376 Ohm's Law — Oil sure. The fig. shows the arrangement of a moving- needle ohmmeter in diagrammatic form. The magnetic needle n is acted upon by two windings b and a, the former being connected in series with X the resistance to be measured, and the latter across the ter- minals of the testing battery B. If x is in- finite, b will carry no current, and the needle will be in the axis of coil a. For every value of X there will be a particular position of n, which is quite independent of the pres- sure of the battery B. The scale can, con- sequbntly, be graduated direct in ohms or megohms, as the case may be. Type 2 is a modification of the slide-wire Wheatstone bridge, and differs from it in be- ing self-contained with the galvanometer, and also sometimes with the battery or generator. It is thus sometimes termed a bridge megger. For insulation testing, a portable magneto generator is often used in place of a battery, with either type of ohmmeter. One of the most widely-employed types of ohmmeter is that sold under the trade name of Megger. Ohm's Law. — Ohm's law states that the current in a conductor is proportional to the emf applied to that conductor. The constant quotient of emf and current is called the resistance, and its value depends upon the material of which the conductor is composed, and its length and sectional area. If I be the current, E the emf, and R the resistance, then we have I = I E = IE, E = J If I is measured in amp, and E in volts, then E will be in ohms. The law is univer- sally true for steady currents. Where the cur- rent is changing in value, the 'inductance' of the circuit also has to be taken into ac- count, but Ohm's law still remains true when considering the instantaneous values of the current, if all the instantaneous electromotive forces in the circuit are taken into account. See Eesistance; Conductance; Imped- ance; Eeactance; Inductance. Oil, Effect of, on Mica.— It has for a long time been supposed that oil produced an immediate deleterious effect on the insu- lating qualities of mica. It is certainly the case that mica shows a much lower disrup- tive strength when its surface is covered with oil, but this seems to be entirely due to the difference between the specific inductive ca- pacities of oil and air. This forms an exact parallel to the picein-drop method used for testing insulating materials (see Picein-drop Method). This difference naturally affects the dielectric stress in the mica when under test, and so causes the observed difference of disruptive strength. In reality, oil appears to be entirely without chemical action on pure mica. In the case of built-up mica, or micanite, however, the oil is able to dissolve the binding medium, and so to effect the dis- integration of the material. The real harm done by oil on mica is thus due to three separate actions: 1. Dissolving of the binding material used (most commonly shellac). 2. Carbonisation of the oil under the action of sparks (see Commutator Insu- lation, Effect of Oil on). 3. Accumu- lation of carbon and other dust on the oily surface of the insulating material, so that leakage eventually follows. This last efl'ect explains why the design of oil throwers and catchers is of such vital importance in elec- tric machines (see Mica; Micanite). Oil, Fire Test of. — The temperature at which vapour is given off so rapidly that it burns continuously above the surface of the liquid is termed the fire test or bwrning-pdnt. Oil, Flash-point of an.^The flash-point of an oil is that temperature at which vapour is given off more rapidly than it can diffuse in the atmosphere. The test should be made in an enclosed vessel. Oil, Fusel. See 'Amyl Alcohol' under Alcohols. Oil, Insulating^, a term generally applied to a high flash-point mineral oil, sold for the insulation of transformers, oil-break switches, and similar apparatus. See also Oil, Trans- former; Oil Insulation; Oils for Insu- lating Purposes. Oil, Insulating", Evaporation Test for. — Mineral oils evaporate slightly at a temperature somewhat below their flash- point, and it is desirable to make an evapo- ration test when selecting an oil for insulat- ing purposes. The percentage loss in weight after six hours continued heating at 105° C. should not exceed 0-25 per cent. The purity of the oil can also be judged by the amount of discoloration that takes place. Oil, Linseed. See Linseed Oil. Oil, Transformer, oil used to insulate and to cool oil -cooled types of transfor- mers. Such oil should have the following characteristics: High insulating power, low Oil-break Fuse — Oil Varnishes 377 viscosity, high flash-point, freedom from acids and moisture, absence of any tendency to thicken under continued heating. The in- sulating power of oil is considerably reduced by a very small percentage of moisture, and for that reason it should be carefully dried before use, and kept in tin or iron tanks and not in wooden barrels. Specially-dried trans- former oil can be obtained which will stand 35 to 40 kilovolts (rms) between two flat disks placed about 5 mm apart. The oil usually employed for the purpose is a high flash-point mineral oil. It is quite general practice to employ one of the high flash-point paraffins obtained from the distillation of petroleum. The purity of the oil may be determined by — 1. Dielectric strength. 2. Loss by evaporation on heating at 105° C. for six hours. 3. Change in colour after heating for six hours at 105° C. See also Oil Insulation; Oils for In- sulating Purposes; Oil, Insulating, Evaporation Test for. (Ref. 'The In- sulation of Electric Machines', Turner and Hobart, chaps, x and xi; C. E. Skinner, 'Transformer Oil', 'The Electric Club Jour- nal ', vol. i, p. 227.) Oil-break Fuse. See Fuse. Oil-break Switch. See Switch, Oil- break. Oil Chamber of Bearing, a chamber formed in the cast- iron casing of a bear- ing, below the bearing bush, to contain the oil for lubricating pur- poses. The lower por- tions of the oil rings are thus immersed in oil. The fig. shows the bearing of a small motor, the oil chamber being denoted by A. See Oil Thrower; Oil Ring; Bearings. Oil Circuit Breaker. Breaker, Oil. Oil Insulation. — The use of oil for insu- lating transformers and ht switches is now common practice. For the former purpose the oil forms a homogeneous insulation for those parts not already insulated, and at the same time aids cooling. The oil nearest the transformer expands by reason of its rise in Oil Chamber of a Bearing See Circuit temperature, rises to the top, and the colder oil at the sides flows in to take its place; this causes a circulation of the oil, which continues as long as the transformer continues to radi- ate heat. For ht switches the oil acts as a very efficient insulation by reason of its high dielectric strength. It extinguishes the arc when the circuit is broken, keeps the con- tacts free from oxidation, and is considered by some authorities to allow more effective contact to be made. See also Oils for Insulating Purposes; Circuit Breaker, Oil; Switch, Oil -break; Oil, Trans- former. Oil Insulator. See Insulator. Oil Ring. — The most usual method of automatically lubricating the bearings of electric machines is to provide a recess or recesses in the top part of the bearing bush in one or more places, and allow a brass ring, or brass rings, to rest on the shaft. These rings are of such a large diameter that they reach into the oil-well underneath the bear- ing, and when the shaft revolves it also rotates the rings, which thus carry up a continuous supply of oil to the bearing. Compare sketch given under heading Oil Thrower (which see). See Bearings; Oil Chamber of Bearing. Oil Switch. See Switch, Oil-break. Oil Thrower. — To prevent oil travelling along the shaft, and so damaging the insu- lation on the windings, corrugations are pro- vided on the shaft just outside the bearing- line, but still within the bearing-caps, so that OH Thrower the oil mounts by centrifugal force to the head of the corrugations and is thrown off. See Oil Chamber of Bearing; Bearings. Oil Varnishes. — These may be either 378 Oiled Linen — ^ Open-circuit Battery baking or air -drying varnishes, according to their ingredients and the method of manufacture. They usually consist of a drying oil, resin, or gum, and 'drier', thinned to a suitable consistency with a volatile spirit. In the process of drying, the solvent is evaporated and the oil ab- sorbs oxygen, forming with the resin or gum a hard lustrous coat. The exact func- tion of the ' drier ' is not thoroughly under- stood, but it hastens the drying of a varnish. Oil varnishes dry with a tougher coat than spirit varnishes, and are more moisture- proof. See Insulating Varnishes. '' Oiled Linen, cambric impregnated with linseed oil, to ensure uniform dielectric strength. The cambric must not contain traces of any bleaching, and all moisture must be thoroughly dried out before im- pregnating. See also Linseed Oil; Im- pregnated Insulating Materials; Em- pire Cloth. Oiled Paper. — Paper, being more homo- geneous than cambric, can be impregnated with any oil, but linseed oil is most generally employed. The paper used should be free from mineral matter and thoroughly dried before impregnating. For the same thick- ness, it will give a higher dielectric strength than cambric, but will not be quite so flex- ible, and will become more brittle with heat. See also Impregnated Insulating Ma- terials; Paper eor Insulating Pur- poses; Japanese Paper; Manila Paper. Oils for Insulating Purposes.— These may be divided into two classes, namely, oils for impregnating fibrous materials, and used also in the manufacture of impregnat- ing compounds; and oils for the insulation of transformers, oil -break switches, and similar apparatus. For impregnating fibrous materials linseed oil is most suitable, on account of its drying properties, whilst for insulating apparatus it is usual to employ a high flash-point mineral oil. Eesin oil is sometimes used for this purpose, but is apt to vary in quality. For the insulation of apparatus an oil should possess the following characteristics : — 1. High flash-point. 2. High dielectric strength. 3. Freedom from acid, alkali, and moisture. 4. Low solidifying temperature. The last feature is only of importance in the selection of an oil for use with oil-break switches. The dielectric strength is a very c ^ good indication of the purity of an oil. The oils employed for the insulation of apparatus are generally sold under the names of in- sulating oil and transformer oil. See Oil In- sulation; Oil, Transformer. (Eef. 'The Insulation of Electric Machines', Turner and Hobart, chaps, x and xi.) O'K Meter. See Meter, O'K. Okonite. See Rubber. Oldham's Coupling^.— In this coupling the driving and driven shafts each end in a flange, diametrically across the face of which is fastened a bar of rectangular cross section. The power is transmitted between the flanges by a middle piece in the form of a thick disk. In each face of the disk is a groove which is a sliding fit on the bar carried by the corre- sponding flange. The two grooves run at right angles, so that the middle piece is supported entirely by the bars fltting in the two grooves. The whole thus forms a flexible coupling which is still effec- tive even if the shafts are appreci- ably out of line. The middle disk IS usually of steel, but sometimes hard rubber or fibre is employed so as to form an in- sulating coupling. See Coupling, Shaft; Coupling, Flexible; Plate Coupling; In- sulating Coupling; Coupling; Clutch. Omnibus Bar. See Bus-bars. OLdogpaph, Hospitaller. See Hospi- talier Ondograph. One-hour Rating- of Railway Motors. See Railway Motors, One-hour Rating op. One-layer Armature Winding. See Winding, Armature. One-man Car. See Single-ended Elec- tric Tramcar. Also called Demi-car. One - phase Alternator, synonymous with sp alternator. See ALTERNATOR. Opalescent Globe. See Globe. Opal Shade or Globe. See Globe. Open Are Lamps. See Lamp, Arc. Open Circuit. See Circuit; Circuit, Electric. Open -circuit Armature. See Arma- ture. Open-circuit Battery. See Battery, Primary; Cell, Standard. Oldham's Coupling Open-circuit Characteristic — Oscillation 379 Open - circuit Characteristic. See Curve, Characteristic. Open-circuit Control. See Control, Open-circuit. Open-circuit Emf. See Open-circuit Potential. Open-circuit Induction. See Induc- tion, Open-circuit. Open-circuit Potential, the pressure at the terminals of an electric generator when excited and running at normal speed but supplying no current to the main circuit. Open-circuit Windings, windings in an ac machine which, when not coupled to any external 'load', form open circuits. Open- circuit windings were formerly also used in certain types of cc dynamo-electric machines (notably arc-lighting dynamos), but these types are all now obsolete. See Generator. Open-coil Armature. See Armature. Open Motor. See Motor, Open. Open -protected Motor. See Motor, Open-protected. Open Slots. See Wide Open Slots. Optical Pyrometer. See Pyrometer, Electric. Ordinates. See Curve. Ore Separator. See Magnetic Sepa- rator; Separation of Ores. Os, the chemical symbol for osmium (which see). Oscillating Circuit, an arrangement of conductors and dielectrics in which a natural (or free) oscillation is possible. An ap- proximate condition is that 4L shall be greater than E^C, where L is the inductance, R the resistance, and C the capacity of the circuit. In practice a Leyden jar or con- denser may form the capacity, and a coil of wire the inductance. See Discharge, Os- cillatory; Discharge, Dead-beat; Natu- ral Period of Oscillation of a System; Electric Oscillation; Oscillation. Oscillating Current.— In paragraph 6 of the 1907 Standardisation Rules of the A.I.E.E. an oscillating current is defined as a current alternating indirection and of decreas- ing amplitude. See Oscillating Circuit. Oscillating Meter. See Meter, Oscil- lating. Oscillation, a to-and-fro motion or re- peated change. The idea of an oscillation involves the idea of time and, at least, one other quantity. The simplest case is a mathe- matical point oscillating in space. This in- volves time and space only, and is the standard case by means of which many others may be graphically represented, if one of the axes of co-ordinates in a diagram be taken to repre- sent time and the other the variable quantity. Other cases are a mass oscillating with time in space; oscillations, with time, of the pres- sure, potential, temperature, or density at a point; the to-and-fro movements of electricity in time and space in a conductor, or of elec- tric stresses in a non-conductor. The type of oscillation called simple harmonic is the simplest obtainable if matter or electricity moves, though there are simpler geometrical cases in which infinite velocities and dis- continuities occur such as are possible only to a purely mathematical point. A simple harmonic motion is represented graphically as a sine curve ; it is the motion of the foot of a perpendicular to a fixed diameter of a circle whose other end is through a point which revolves uniformly round the circum- ference. In its simplest symbolical form the distance of the foot of the perpendicular from the centre of the circle, at time t units from the time when the moving point was first on the fixed diameter, is given by the equation x = cos 6, where is the angle between the fixed diameter and the radius through the moving point, the radius of the circle being taken as unit length, and 6 being the angle between the fixed diameter and the radius through the moving point. But, since the revolution is uniform, 6 = Id; where A; is a constant. Therefore z = cos kt. To determine k suppose there is one revolu- tion in unit time, therefore for t = 1, 27r = k, since the angle 2ir isa complete revolution. If there be ~ complete revolutions, or period of vibration, per unit of time, we have x = cos 2Tr~t. ~ is here the number of complete vibrations, or cycles, per sec, and is called, somewhat indiscriminately, the frequency, the periodicity, or the rate of vibration. It is more usual to take the line of vibra- tion of the moving point as at right angles to that of zero angle, a method which gives the equation in the form x = sin 2Tr ~ t. If the line of vibration be in any other direc- tion the equation must be modified by the addition of a constant angle; thus x = cos {2'ir~t + a), or a; = sin {2Tr~t + P). Fourier, early last century, made the great discovery that any continuous periodic motion may be represented mathematically by series of sines or cosines in which ~ is given a dif- ferent but properly chosen value in each term 380 Oscillation — Oscillogram of the series; or, in other words, that any complex vibration may be analysed and will be found to be the sum of a number of simple harmonic (otherwise 'sine' or 'cosine') vibra- tions of diiferent frequencies. Thus x = cos If + cos 2t is a motion in which both the fundamental, or slowest, vibration and the second harmonic, or first overtone, are com- bined. X = cos f + cos 2t + cos 3t includes also the third harmonic, i.e. another simple harmonic or sine motion, having, however, three times the frequency of the funda- mental. This discovery of Fourier's renders the understanding of complex periodic mo- tions comparatively easy, and elucidates the theory of alternating electricity and all simi- lar motions, whether of electricity, matter, or such quantities as temperature. See also Continuous Oscillation; Electric Os- cillation; Fourier's Method; Natural Period of Oscillation of a System. Oscillation, Electric, a to -and -fro movement in which electrical quantities are concerned. Lord Kelvin showed that if a charged condenser having capacity C, be discharged through a circuit having induct- ance L and resistance E, the current may oscillate before dying away. The discharge is unidirectional and dies away without oscil- lations if 4L is less than E^C, and is oscil- latoiy if 4:L is greater than R^C. In the second case the frequency is ~ = l/27rvCL, and the period or time of one complete oscil- lation is T = l/~ . In the case of a charged condenser the oscillation dies out owing to dissipation of the initial energy through heat- ing and radiation. It is said to be 'damped'; the ratio of the amplitude of the second half- wave to that of the first is called the damp- ing, and the Napierian logarithm of the re- ciprocal of this ratio is called the logarithmic decrement. If in the expression for ~ the units taken are mfd and cm (1 henry = 10^ cm), it 5 X 10^ See Oscillation; becomes ~ Vol Oscillatory; Discharge, Decrement, Logarithmic; Discharge, Dead-beat; Cymometer. Oscillation Constant. See Cymometer. Oscillation Valve, a device which auto- matically rectifies an ac. There are many ways in which this may be done, most of them depending on electrochemical phe- nomena, since (at least for high frequencies of alternation) the construction of a suitable electromagnetic machine presents great difii- culties. The action of an electrochemical valve usually depends on the formation of a thin insulating film of oxide, or other compound, on the surface of a metal plate, while the current is flowing in one direction, and its destruction when the current is re- versed. Thus during one-half of every alter- nation, the current has an easy passage, while during the other half it is checked. As a result, the current becomes more or less unidirectional. A very large number of electrolytic cells show the phenomenon more or less markedly, particularly those containing aluminium, as do also various types of vacuum tubes. In the latter, the effect of heating one of the electrodes may greatly increase the efficiency of the rectifi- cation; thus a valve peculiarly suitable for hf currents may consist of an ordinary glow lamp in which the carbon filament, kept hot by an auxiliary current, is used as one elec- trode, the other being a cool metal plate inside the bulb but not touching the filament. See also Rectifier. [j. e-m.] Oscillator, Hertz. — Hertz showed ex- perimentally that it was possible to produce . . , — — »» electrical cur- '^ ^ ' ■ '^ rents of very high frequency by the disruptive discharge of elec- tricity beween two short con- ductors, and that free electrical O radiation took place from them during the pro- cess. Some of the forms of oscillator which he used are shown in the fig. The oscillator is in reality a condenser of very small capacity, the two conductors being separated by the air sur- rounding them. Oscillaitory Discharg-e. See Dis- charge, Oscillatory. Oscillatory Impedance. See Imped- ance, Oscillatory. Oscillogram, the term applied to the diagram obtained from an oscillograph. The horizontal scale usually denotes time, while the vertical scale denotes current or pd. The records are usually obtained photographi- cally, especially for non-repeated phenomena. But in the case of a repeated variation, the records may be obtained by tracing. a DC Hertz Oscillators Oscillograph 381 Oscillograph, an instrument for continu- ously recording the variation of current or potential in a circuit. The most notable types are described in the following paragraphs. The Duddell Oscillograph.— The com- plete oscillograph outfit (see the Plate) con- sists essentially of a modified moving-coil galvanometer combined with a rotating or vibrating mirror, a moving photographic film, or a falling photographic plate. The galvanometer portion of the outfit is usually referred to as the oscillograph, and the fig. represents diagrammatically the principle on which it works. In the narrow gap between the poles N, S of a powerful magnet are stretched two Diagrammatic Slietch of Worliing Parts of Dnddell Oscillograph parallel conductors, s, s formed by bending a thin strip of phosphor bronze back on itself over an ivory pulley P. A spiral spring attached to this pulley serves to keep a uniform tension on the strips, and a guide- piece L limits the length of the vibrating portion to the part actually in the magnetic field. A small mirror M bridges across the two strips as shown in the fig. The efiect of passing a current through such a 'vibrator' is to cause one of the strips to advance whilst the other recedes, and the mirror is thus turned about a vertical axis. This principle was first suggested by Blon- del, to whom belongs the credit of working out and describing in considerable detail the principles underlying the construction of os- cillographs. In the Duddell oscillograph each strip of the loop passes through a sepa- rate gap (not shown in the fig.) and the clearance between the edge of the strip and the sides of the gap is very small, varying from 0'04 to 0'15 mm according to the type. The whole of the 'vibrator', as this part of the instrument is called, is immersed in an oil-bath, the object of the oil being to damp the movement of the strips, and make the instrument dead-beat. It also has the additional advantage of increasing by refrac- tion the movement of the spot of light re- flected from the vibrating mirrors. In the hf instrument the natural period of vibration (undamped) of the loop is xwdtt sec, and the clearances being, as stated, extremely small, the damping effect of the oil is so great, that the instrument can be relied upon to give accurate results even when the periodicity of the current to be tested is 300 periods per sec. Small fuses mounted in glass tubes are provided to protect the strips from injury in case of accidental excessive current. The beam of light reflected from the mirror M is received on a screen or photo- graphic plate, the instantaneous value of the current being proportional to the linear dis- placement of the spot of light so formed. With ac the spot of light oscillates to and fro as the current varies, and would thus trace a straight line. To obtain an image of the wave form, it is necessary to traverse the photographic plate or film in a direction at right angles to the direction of the move- ment of the spot of light. Or a second mirror can be interposed in the path of the beam of light, and this mirror caused to vibrate or rotate so as to impart to the beam of light a uniform motion proportional to time about an axis which is at right angles to the zero position of the beam and also is in the initial plane of vibration. The spot of light will then trace out on a stationary screen or plate the time curve of the variation of the emf or current as the case may be. If the variations are periodic, as in ac, then the second mirror can be synchronised and the spot of light caused to trace out the wave form over and over again. (Ref. Blondel in ' Comptes Rendus', 1893, vol. cxvi, pp. 502 and 748; Duddell, 'Oscillographs', B. A. Reports, 1897; Elec, vol. xxxix, p. 636.) The Irwin Oscillograph. — This instru- ment depends upon an ingenious arrangement of a polarised hot wire. In order to over- come the inherent sluggishness of hot-wire instruments, the movement is put in series with a high resistance shunted by a con- denser for potential measurements, or the 382 Osmium — Oven movement is shunted by a highly inductive resistance for current measurements. (Ref. Journ.I.E.E., vol. xxxix, p. 617.) Glow -LIGHT Oscillograph, two alu- minium rods in a partially evacuated tube, their ends being about 2 mm apart. When an ac of any frequency passes between them, a sheath of violet light forms on one of the electrodes, passing over to the other when the current reverses during each cycle. The phenomenon may be observed or photo- graphed by means of a revolving mirror. Braun's Cathode-ray Oscillograph, a cathode-ray tube having a fluorescent screen at one end, a small diaphragm with a hole in it at its middle, and two coils of a few turns each, placed outside it at right angles to one another. These coils carry currents proportional to the pressure and current re- spectively of the circuit under observation. The ray then moves so as to produce an energy diagram on the fluorescent screen. The instrument is much used in wireless telegraphy, as it is capable of showing the characteristics of currents of very hf. See also Hospitalier Ondograph. Osmium, the heaviest known metal. Its specific gravity is 22-5. Its melting-point is very high, and it was for a time exten- sively employed for the filaments of incan- descent lamps. It has, however, been largely superseded for this purpose by tungsten. The so-called osmium, osram, and osmi lamps (see Lamp, Incandescent Electric) gene- rally have tungsten filaments. At 0° C. the specific resistance of osmium is some 10 microhms per cm cube. (Swinburne, 'Metallic- filament Lamps ', Journ.I.E.E., vol. xxxviii.) Osmium Lamp. See Lamp, Incandes- cent Electric. Outboard Bearing'. See Bearings. Outer. See Conductor, Outer. Outer Conductor. See Conductor, Outer. Outgoing Current. See Current, Outgoing. Output, the power given out by a genera- tor, motor, or transformer. Output = input — losses, or = input X efficiency. Output, Rated, the load in kw or hp for which an electrical machine is designed. See Rating, Continuous; One-hour Rat- ing OF Railway Motors. Output, Specific. See Specific Output. Output, Unit of. See Unit of Output, Output Coefficient, a coefficient indi- cating the relation between the linear dimen- sions of a dynamo-electric machine and its. output, and defined by the equation D2 X A„ X R where ^ is the output coefficient, D is the diameter of armature (at the air gap) in dm, A,g is the length of core in dm, w is the output expressed in watts, R is the revolutions per minute. ^ varies for the various types of machines^ rough limiting values being as follows : — 1. Induction motors, from 0'5 to 2-0. 2. Co machines, .. 0*5 „ 3'5. 3. Alternators, ... TO „ 2 5. Output in hp. — This term is applied to. the output of motors, which is usually mea- sured in hp. See Output. Output of cc Machines, Limit of. See' Limit to Output in Dynamo-electric Machinery. Output per Pole, the output of any dynamo-electric machine divided by the number of its poles. Oven, Baking. — In electrical manufac- ture, this term relates to an oven for drying out insulated coils and other apparatus before Vacuum Baking Oven A, Steam pipes. B, Vacuum pumps connection, c, Compound inlet pipe. D, Window. B, Tliennometer. DUDUELL OSCILLOGRAPH FOR USE ON 25,000 VOLTS [To/ace p. sSb. Overall Diameter — Ozone 383 and after treatment with varnish. The baking before treatment serves to remove all moisture (water) from the materials used. The baking after treatment serves to remove the solvent of the varnish, and to initiate the oxidation of the linseed oil if that is present. The oven may be heated either by electricity, steam, or hot air. It may also be arranged to have the air exhausted, when it is called a vacuum oven. In any case it should be arranged so that all vapour is removed as rapidly as possible. The fig. represents a vacuum baking oven. See also Electric Baking Oven; Vacuum Oven. Ovepall Diameter (of a Cable).— The term can be applied to any particular core layer, the whole core, or the finished cable. In either case it is the maximum diameter measured over the outside. For a core layer it is also pitch diameter + diameter of wire. See Pitch Diameter (of a Cable). Overall Efficiency. See Efficiency ; Central Station for the Generation OF Electricity. Over-compensated Induction Meter. See Meter, Over - compensated Induc- tion. Over-compounded. See Excitation. Overhead Conductors. See Conduc- tors, Overhead; Transmission Line; Line, Overhead; Line Poles. Overhead Electric Crane. See Crane, Electric. Overhead Line. See Line, Overhead; Transmission Line; Line Poles. Overhead System, a system of supply- ing electrical energy to moving tramcars or trains by means of an insulated wire sus- pended over the track, and connected with the source of supply by insulated feeder cables. The cars collect the current from the overhead wire by means of trolley wheels or bows. The return circuit is usually formed by the track rails. See Trolley; Trolley Wire; Eeturn Circuit. Overhead Trolley Wire. See Trolley Wire. Overhung- Type of Alternator. See Alternator. Overload, a load on a generator, motor, or transformer which is greater than its rated load, and which is intended to be carried only for a short time. Overload Capacity denotes the amount of overload which a given machine is capable of carrying. Overload Capacity, Ideal. See Ideal Overload Capacity of Induction Motors. Overload Relay. See Eelay. Overload Release, an automatic device by means of which a switch is opened, if the strength of current in the circuit controlled by the switch exceeds a predetermined limit. It is generally used in connection with a rheo- stat controlling an electric motor, to prevent damage to the latter through overloading. See also Switch, Motor-starting; Start- ing OF Motors. Overload Switch. See Circuit Breaker. Overrunning" of Glow Lamps. See Lamp, Incandescent Electric. Overshooting. — When a metallic-fila- ment lamp is switched on, the cp is, for the instant, much in excess of the value to which it settles down as soon as the resistance of the filament has increased to the value cor- responding to the high temperature at which it normally runs. The phenomenon does not occur in carbon incandescent lamps, since the resistance of carbon decreases with increasing temperature. Overtype Machine. See Machine, Overtype; Generator. Oxidation of Linseed Oil. See Lin- seed Oil. Ozite, the trade name of a high-melting- point impregnating compound. See Insu- lating Compound. Ozokerite, a dark-brown mineral resin with a melting-point of some 60° C. It resembles paraffin wax in its nature, but is not such a good insulator, and not quite so hard. It is moisture-proof, and does not de- teriorate by exposure to the atmosphere. It is used for impregnating the outside cotton- braided covering of cables. When bleached it is sold under the name of Ceresine or Cera- sin. See Insulating Compound. Ozone, a modification of oxygen in which the molecule consists of three atoms instead of the usual two. It is unstable, and there- fore much more active than ordinary oxygen gas. It is a strong disinfectant and bleacher on account of its action on organic substances. It is producible by the slow discharge of elec- tricity into air, and is hence found in nature on mountain tops and by the sea, particularly in fine weather, since it is in these places and circumstances that atmospheric electricity is greatest. Ht electrical apparatus is now on the market for the production of ozone, and 384 P — Panels is sometimes used in large buildings, such as hospitals. Too great a percentage of ozone in the air causes headache, catarrh, and other distressing symptoms. P, the chemical symbol for phosphorus (which see). Pacinotti Dynamo.— A type of cc gene- rator introduced in 1878 by Pacinotti had an armature made in the form of a disk, with a ring winding. The construction is now obsolete. See Genekator; 'Disk Ar- mature' under Armature. Palladium (chemical symbol = Pd), an element which, when pure, has at 0° C. a specific resistance of about 10 microhms per cm cube. Its resistance increases by 0-35 of 1 per cent per degree Centigrade increase in temperature. Paneake Coil. See Coil, Form-wound. Panchronous Alternatop. See Alter- nator, Panchronous. Panel, Board of Trade. See Board of Trade Panel. Panel, Feeder, a panel, generally of marble or of enamelled slate, equipped with apparatus for controlling a feeder circuit (that is, a circuit carrying current to a con- suming area), and for measuring the cur- rent and pressure in that circuit. The most common equipment in the case of a cc feeder panel comprises an ammeter, a dp switch, and dp fuses, or instead of the fuses a circuit breaker on one or both poles; in the case of a tramway feeder a choking coil and a light- ning arrester are often added. The equipment of a typical ht .three-phase feeder panel is an ammeter (sometimes three ammeters, one in each phase) operated by a current transformer, and a tp oil-break switch with two overload release coils, or three if the neutral of the circuit is earthed, the re- leases being operated by current transformers. The switch when on a large system is often in a cell some distance behind the panel, and is then controlled by a system of levers, or by a small motor which is started and stopped by a throw-over switch on the panel, in which case there is generally a lamp or lamps on the panel to show whether the switch is open or closed. Air-break switches or links are placed between the bus -bars and the oil switch to allow of the latter being isolated for inspection purposes, and as a general rule no apparatus carrying ht current is allowed on the front of the panel. With both cc and ac feeders a w hr meter is often added to show the total consumption of the circuit. The illustration shows a simple feeder panel equipped with circuit breaker A, Feeder Panel A, Circuit brealcer. B, Ammeter. 0, Voltmeter. D, Two single-pole switches ammeter B, voltmeter c, and two sp switches, D. (Eef. 'Electricity Control', Andrews; 'Standard Polyphase Apparatus and Sys- tems ', Oudin.) Panel, Fuse. See Fuse Panel. Panels, Switchboard, the panels, gene- rally of marble or enamelled slate, upon which are mounted the switches, instru- ments, and other apparatus of a switch- board. The marble or slate should be free from metallic veins, and for pressures above, say, 600 volts, live connections, terminals, Panel Switchboard — Paralleling Switch 385 &c., should preferably be insulated from the panels by ebonite, mica, &c., or removed from them altogether, as is generally the case with ht alternating gear where the switches are of the oil type. The bus-bars and connec- tions should be supported by the framework at the back of the board, or in separate cells, and the instruments should be operated at low potential through instrument transformers. Panels vary from some 35 mm to 50 mm in thickness, and the edges of the front of the panels are generally bevelled to give a fin- ished appearance. Where slate is used, the front and edges alone are enamelled, not the back, while with marble panels the front and edges alone are polished. The panels are generally held in position by bolting them to an angle-iron, or a strip- iron framework behind them. Sometimes, though now rarely, a wooden framework is employed for the sake of appearance, but this is not to be recommended owing to the extra fire risk incurred. (Ref. 'Electricity Control', Andrews.) Panel Switchboard. See Switch- board, Panel. Pantograph Trolley, an approximately diamond-shaped frame mounted on the roof of an electric car or locomotive, in order to support the bow or collector, to which it is designed to give free motion in a vertical plane. Springs or compressed air applied to the lower members of the frame are employed to raise the bow into contact with the over- head wire. The device is used mainly in connection with ac traction. See Collect- ing Shoe; Collector, Trolley. Paper, Fish. See Leatheroid. Paper, Japanese. See Japanese Paper. Paper, Leather. See Leatheroid. Paper, Manila. See Manila Paper. Paper, Oiled. See Oiled Paper. Paper for Insulating- Purposes.— The selection of a paper for insulating pur- poses will to some extent be determined by the treatment it will receive. There are two treatments generally employed, namely invpregnatmg and varnishing. For both pur- poses, the paper should be tough, flexible, and uniform in thickness. There must be no pinholes, and the paper should preferably have a smooth surface. For impregnating, the paper should contain no mineral matter; but for varnishing, this is not of great im- portance. Before either treatment the mois- ture should be dried out. For the insulation of laminations a tough thin paper, of uniform thickness, that will not char at 70° C. is required. It may be adhered to the iron sheet before punching, or it may be placed between laminae during the process of assembling the core. See Oiled Paper; Japanese Paper; Insulation of Laminations; Pertinax Tubes; Micarta; Impregnated Insulat- ing Materials; Horn Fibre; Press- spahn; Leatheroid; Manila Paper. Paper-insulated Cable. See Cable, Paper-insulated; Cable, Underground. Parabolic Reflector. See Reflector, Parabolic. Paraffined Wire, cotton- or silk-covered wire that has been impregnated with paraffin wax. This prevents the absorption of mois- ture after the cotton or silk has once been thoroughly dried out and impregnated. See ' Insulation of Magnet Wire ' under Insula- tion; Wire, Cotton-covered; Wire, Silk- covered. Paraffin Naphtha. See Naphtha. Paraffin Oil, sometimes termed kerosene oil, a distillation product of petroleum. It has too low a flash-point to be used for insulating purposes, although it has high dielectric strength. It is used to some slight extent in the manufacture of compounds, and of japan baking varnishes. See Oils FOR Insulating Purposes; Insulating Varnishes. Paraffin Wax, an opalescent white solid, the residue from the distillation of petroleum. It has a high insulation resistance and di- electric strength, both of which properties are permanent. It is acid and waterproof, but its low melting-point — some 50° C. — largely restricts its use. See Insulating Qualities. Parallel Circuit. See Circuit, Elec- tric. Parallel Connection of Cells. See Accumulators in Parallel; Cells, Grouping of. Parallel Control, Cascade. See Cas- cade Motor. Parallel Grouping of Battery. See Cells, Grouping of; Accumulators in Parallel. Parallel Running of Alternators. See Alternators, Parallel Running of; Irregularity Factor; Cyclic Irregu- larity; Amortisseur; Phase Swinging. Paralleling Switch. See Switch, Paralleling. 386 Paramagnetic — Partridge Safety Device Paramagnetic. — A material was called paramagnetic by Faraday if it acted like iron in regard to a magnetic field, i.e. if it was more susceptible to magnetism than the air surrounding it. When a material of this kind is placed in a magnetic field the lines of force converge into it, their density be- coming greater than that in the surrounding space. The body is then usually said to be magnetised by induction. The most power- fully paramagnetic bodies are iron, nickel, and cobalt. Since convergence or divergence of magnetic lines is the physical cause of what are usually called poles, the polarity of paramagnetic bodies is in the direction of the external inducing field. (Ref . ' Electrical Researches', Faraday; 'Papers on Electro- statics and Magnetism', Sir W. Thomson (Kelvin).) See Diamagnetic. Paramagnetic Bodies. See Para- magnetic. Paramagnetism. See Paramagnetic. Para Rubber. See Rubber. Parasitic Currents. See Eddy Cur- rent. Parliamentary Candle. See Candle Power. Parsons and Laws Leakage Wind- ing, a winding carrying the main current (or a current proportional thereto) from an alternator, applied to a leakage magnetic circuit attached to the alternator exciter, by Main Alternator Current Transformer Leakage Winding Diagram of Connections of Parsons and Laws Leakage Winding for Compounding Alternators means of which the terminal pressure of the alternator may be automatically compounded. The exciter is provided with an alternative path for the magnetic flux, the leakage path, in parallel with the armature core. This leakage path is furnished with a winding placed in series with the armature winding of the alternator, either directly connected or connected through a current transformer. When the current in the armature winding of the alternator rises, the current in the windings on the leakage path also rises. This chokes back the magnetic flux in the leakage path, and compels more flux to pass through the armature of the exciter. The terminal voltage of the exciter, and hence the field current of the alternator, is thus increased. Thus, as the load on the alternator increases, the excitation of the alternator field increases, and compensates for the drop in pressure at the alternator terminals due to the loading ■of the alternator. The leakage paths may be readily adjusted for any desired amount of compounding, or for any pf. The diagram illustrates the arrangement as applied to a three-phase h pr alternator. Partial Earth. See Earth. Partly Closed Slots.— In order to com- bine a good flux distribution with a low winding reactance, the slots of alternators and induction motors are now usually partly closed at the top. The opening is often no wider than a saw cut. Windings for this type of slot must either be threaded through an insulating tube, or the single wires of a par- tially formed coil may be fed in, one at a time, through the opening, into a horn-fibre or mica- nite envelope, which will afterwards be closed over and secured by a wedge. See also Slot. Partridge Safety Device, a device for use in transformer circuits to automatically short-circuit the primary winding, should a breakdown of insulation occur between the primary and secondary windings of the transformer. As shown in the upper figure, the device is held between knife contacts, and contains a number of brass disks sepa- rated by thin mica disks. Partly Closed Slots Partridge Sparklet Fuse — Percentage Error 387 When a fault occurs between the primary and secondary transformer windings, the mica insulation in x breaks down, permitting the S^^^=T3S i E TJ n Partridge Safety Device X, Safety device. A, Alternator. 0, Concentric cable. B, Earth. F, Fuse. T, Transformer. excess current to open the circuit. This will be clear from the diagram of connections. Partridge Sparklet Fuse. See Spark- let Fuse, Partridge. Party Line, Selective, a system in which the removal of a telephone from its hook automatically cuts out all other sub- scribers' instruments on the same party line. This is effected by means of electromagnetic relays at each substation. (Eef. ' Telephony ', Abbott.) See Party Lines for Telephone Service. Party Lines for Telephone Service. — A number of instruments belonging to Y Y Party Lines for Telephone Service different subscribers are connected in series in one circuit, which is usually earthed at its middle point. If the diagram represent a party line, A and are the exchange ter- minals, B is the earthed call-circuit terminal. Subscribers' instruments are Xj, x^, &c.; Yj, Yj, &c. E is the earth connection. To call Xj, the operator connects AB to battery cir- cuit and rings twice. The call is heard by all x's, but not by y's. Passburg Vacuum Oven. See Vacuum OvBN; Oven, Baking; Electric Baking Oven. Passenger Density, the number of pas- sengers carried in a given period, per mile of route on a railway or tramway. Pasted Filament. See Filament. Pasted Plate. See Accumulator Plates; Accumulator. Paste Process of Manufacturing Tungsten Lamp Filaments. See Lamp, Incandescent Electric; Filament. Paul Surface -contact System. See Surface-contact System. Pawlowski Valve. See Eectifier. Paying- out Cable. See Cable, Pay- ing-out. Pb, the chemical symbol for lead (which see). Pd, the preferable abbreviation for po- tential difference (which see). Pd, the chemical symbol for palladium (which see). Peak. See Central Station for the Generation of Electricity. Peak Load. See Central Station for THE Generation of Electricity. Peaked emf Curve, an emf wave-form more pointed than a sine curve; a curve of emf which, when compared with a sine curve of the same rms value, has greater mean height. See Non-sine Waves ; Harmonic; Sine Wave; Wave Form. Peaked Wave Form. See Wave Form; Peaked emf Curve. Pedestal Bearing. See Bearings. Peltier Effect. See Thermo-elec- tricity. Pelton Water Wheel for Hydro- electric Generating Stations. See Mining Equipment, Electrical. Pencil Microphone. See Microphone. Pendant Cord, a pair of insulated con- ductors, generally twisted together and flexible, which hang from a roof, bracket, or other support, and serve to support one or more electric lamps and their fittings, and to convey current to them. See also Cable, Flexible. (Eef. 'Electric Wiring, Fittings, Switches, and Lamps', Maycoek; 'Internal Wiring of Buildings', Leaf; 'Prac- tical Electric Wiring', Metcalf.) Pendant Fittings, shades and acces- sories hung from above and serving to carry incandescent lamps. See references under Pendant Cord. Pentane Standard Lamp. See Stan- dard OF Light. Percentage Error. — When an instru- ment is tested, the numerical values of the results of the test are usually concisely stated as percentage errors. The percentage 388 Percentage Moisture — Peripheral Speed error is a percentage fraction, the numerator of which is the actual error, i.e. the differ- ence between the instrument reading and the absolute value of the reading (obtained by means of standard testing instruments), and the denominator is either the instru- ment reading or its absolute value. When the denominator is the instrument reading, the percentage error refers to the actual reading of the instrument, and can be used in a direct and simple manner. When the denominator is the absolute value, the per- centage error refers to absolute accuracy and cannot be used in the same simple and direct manner as in the former method, which is the rational one. The whole ob- ject of the percentage error is to enable the true value of the instrument reading to be readily obtained from the known value of its percentage error. A positive percentage error means that the instrument is indicat- ing too much, so that the true value is less than that given by the instrument by an amount equal to its actual error. A nega- tive percentage error has the opposite mean- ing, the true value being greater than the instrument reading by an amount equal to the actual error. If, for instance, an elec- tricity meter should read 15 units during a given period on full load, and at this load its percentage error be -f- 10 per cent, then the actual error is 1'5 unit. The true value is 15 — 1'5 = 13 -5 units. The per- centage error refers to the actual readings of the meter. If, on the other hand, it referred to absolute accuracy, the above method of procedure would no longer hold, and the values would be incorrect. In this case (absolute accuracy) the actual error becomes 1'364 unit, and the true value is 15 - 1-364 = 13-636 units. (Eef. Solomon, ' The Calculation of Percentage Error ', Elec. Rev., vol. lix, p. 524, 1906.) Percentage Moisture in Cotton Covering's on Wires. — This varies with the humidity of the atmosphere, but is usually from 6 to 8 per cent of the weight of the cotton covering. Before treatment with varnish or moisture-proofing compound, it is essential to thoroughly dry out all moisture. (Ref. 'The Insulation of Electrical Machinery', Turner and Hobart, p. 71.) Performance Curves of Motor. See Motor Performance Curves; Curve, Characteristic. Period. — In dealing with ac and emf, the period is the time of one complete cycle, or the time after which the successive values of emf or current repeat themselves. Thus the period is the reciprocal of the frequency. See Periodic Function; Frequency; Alter- nating Current; Alternating -current System; Cycle; Phase. Period of Oscillation, the interval of time between the moment at which a body passes a given point and the next moment at which it passes, going in the same direction. For electrical oscillations the definition is similar, viz. the interval of time between successive moments at which all electrical conditions are the same. It is, in other words, the duration of a complete 'cycle', or vibration, or oscillation. The reciprocal of the period is the number of cycles per sec, and is called the frequency or periodicity. Some important periods are: Nutation of earth's axis (about 19 years); the year, the day, tidal periods; the 'seconds' pendulum (2 sec); lowest audible note (about -^-^ sec); highest audible note (about :j5^TrTs; ^^^)> ^c supplies {-^-g to yJ^ sec); wireless telegraphy ac (sTT^nj *o ^ sec); heat, light, and similar rays {^^^ sec and less). [j. E-M.J Periodic Currents and Emf. See Alternating Current. Periodic Function.— /(a) is said to be a periodic function, if f{x) = fix + a)= f{x + 2a) = &c. = f(x + na), where a is any constant, and n is any integer. f{x) may be a function which is periodic with regard to time, with regard to space, or with regard to any other variable. For instance, an ac or emf, which follows a sine law is a periodic function with regard to time, since all successive values of current or voltage are repeated after a definite interval of time. The interval represented by a in the above expression is known as the period. See Phase; Fundamental Waves and their Harmonics. [m. b. f.] Periodicity. See Alternating Cur- rent; Alternating - CURRENT System; Frequency; Cycle; Periodic Function; Phase; Period; Period of Oscillation. Peripheral Dispersion. See Disper- sion, Magnetic. Peripheral Speed, the velocity of any point on the circumference of a rotating body. Peripheral speed is preferably expressed in meters per sec. Permanent Magnet — Permeameter 389 The peripheral speeds of commutators are preferably not over 15 m per sec. The rotors of large turbo -driven generators are often well up toward 80 m per sec. For moderate- speed generators the peripheral speeds of the rotors are usually from 20 to 50 m per sec. PePmanent Magnet, used chiefly in in- struments. Generally the steel employed con- tains a high percentage of carbon and some tungsten. In order to retain the magnetic condition, the magnetic circuit should pre- ferably be nearly closed. The best method of magnetisation is to subject the steel to a magnetic blow, such as may be readily ob- tained by short-circuiting a large cc gene- rator by a few turns of wire wound round the specimen to be magnetised. (An auto- matic circuit-breaker must, of course, be put in series with the coil.) At the same time the magnetic circuit should be entirely closed by soft-iron keepers. Subdivided Permanent Magnet. — Per- manent magnets may be subdivided longi- tudinally with advantage in certain cases. In small magnetic needles, such as are re- quired in the Thomson Graded Galvano- meter, the magnet is thus rendered more permanent, as each part has a greater length compared with its cross-sectional area than one solid needle equal in bulk to the aggre- gate of the small needles. With nearly-closed horseshoe magnets, such as are used in mov- ing-coil instruments, no greater advantage results unless the magnet is so large that it cannot well be magnetised fully in one piece. Ageing. — The object of the process of ageing permanent magnets is to secure great constancy. The several means recommended are: (a) Boiling in oil for six to forty-eight hours; (6) storing the magnets with keepers on for several months before using. It is, however, more than doubtful whether either or both of these methods are thoroughly effec- tive. A better way is to first obtain the maxi- mum possible degree of permanent magneti- sation, and afterwards reduce the flux to about two-thirds of this maximum by ac. If the correct value is chosen for the flux, it will then be found that the magnet will re- main constant indefinitely. If the flux is not reduced sufficiently, the magnet will in time become weaker; if reduced too much, the magnet will become stronger. Tempering. — The degree of hardness re- quired in permanent magnets depends some- what on the shape and composition of the VOL. II steel used. Small needles should be tem- pered 'glass hard' on account of the great degree of retentiveness required. Large horseshoe magnets with nearly closed cir- cuits may, however, be left somewhat softer, as a higher degree of permanent magnetisa- tion may thereby be obtained. [l. m.] Permanent - magnet Moving - coil Ammeter. See Ammeter. Permanent-magnet Voltmeter. See Voltmeter. Permeability, the ratio of flux-density to magnetising force; a term expressing the b H-* Permeability ease with which a given piece of magnetic material can be magnetised. The number expressing permeability is given as a mul- tiple of the permeability of air, so that the permeability of air is unity. In the accom- panying fig. is a B-H curve, for any point of which the ratio of the ordinate to the ab- scissa equals the permeability. The perme- ability is usually denoted by /x. In the fig. we have Oa B li = 06 H" (Ref. 'The Electromagnet and Electromag- netic Mechanisms', Thompson.) Permeability Bridge. See 'Bridges for Magnetic Measurements ' under Bridges ; Drysdale Method of Testing Iron and Steel; Permeameter; Iron and Steel Testing. Permeameter, an instrument or an out- fit for testing the magnetic properties, notably the permeability, of magnetic materials, usu- ally iron or steel. Drysdale's Permeameter, a method of testing large bulks of magnetic material for permeability. A hole is first drilled in the mass of iron under test by means of a special tool which leaves a small rod in the centre. On to this rod is pushed a plug which con- tains a magnetising coil and search coil, and the test is made either by a ballistic galvano- 26 390 Permeance of a Magnetic Circuit — Petrol-electric System meter or by a secohmmeter. See Drysdale Method of Testing Iron and Steel. Thompson's Permeameter, a traction method of measuring permeability. The specimen consists of a round iron rod with one end nicely faced. This forms a bar of a Hopkinson ' bar and yoke ' arrangement, and the pull required to open the faced joint is measured by a spring balance. The mmf is varied by current variation. See 'Bridges for Magnetic Measurements', under Bridges. (Ref. 'The Electromagnet and Electromagnetic Mechanisms', Thomp- son.) Permeance of a Magnetic Circuit, analogous to conductance in an electrical cir- cuit; the reciprocal of magnetic reluctance. In a uniform portion of a magnetic circuit, it is given by permeability x cross-sectional area length Permittance. See Permittivity. Permittivity, a word used by Heaviside to denote specific inductive capacity (which see). For this quantity various expressions have been employed. Thus on p. 39 of the Journ. I.E.E. for Nov. 14, 1907 (vol. xl), Campbell made the following statement in his contri- bution to the discussion of Russell's paper on 'The Dielectric Strength of Insulating Materials ' : — 'Of the two points to which I wish to refer very shortly, the first is a question of nomenclature. I intended to explain to the meeting, because I was afraid some might not understand it, that Mr. Kus- sell's didectrie coefficient, which is quite a new term, means the same as specific inductive capacity, or, as it is sometimes called, dielectric constant. I would draw attention to the fact that both of these terms, as well as Mr. Russell's, seem objectionable, and ought to be changed. Specific inductive capcuiity is very clumsy ; we do not want three words. Didectrie constant is quite colourless, and may mean several things, and so may dielectric coefficient. One might as well talk of the insulation coefficient as meaning the resistance of an insulator. I would suggest that Mr. Heaviside's clear nomenclature should be adopted, which uses the word 'permittivity ' for this quantity. It is expressive and quite distinctive. I am glad to say that Mr. Heaviside's nomenclature is gradually being adopted; we talk of resistivity, resistance, in- •d/actance, conductance, iTtvpeda/nce, and so on, and now 'permittance' is coming in. I think it is time we adopted his terminology more completely, now that the great value of his work in connection with tele- phone cables is so universally recognised.' Permutator, a machine used for con- verting alternating into continuous current. The Bmgd-Faget type consists of a stationary, laminated-iron, cylindrical core, on the inner surface of which coils are wound in slots, as in the stator of an induction motor; the stator is provided with a hollow commutator, inside which are collecting brushes carried by a revolving rotor, and connected with slip rings, from which the cc is collected. The rotor is provided with a short-circuited winding, by means of which it starts up as a polyphase motor, and on attaining syn- chronism it continues to rotate as a syn- chronous motor, the winding being then fed with cc from the slip rings. Permutators of this type may be used with either single or polyphase currents, which are supplied to the stator windings. More complicated self- regulating types are made. The Amert- Ferrand type is based on the principle of the rectifier for sp current, having a two-part commutator which is driven in synchronism with the supply current. Between the two segments are introduced a number of smaller segments connected with intermediate points of a choking coil, the terminals of which are connected to the main segments. The latter are fed with ac by means of slip rings, and a rectified current is delivered at the brushes on the commutator. By shifting the brushes the ec pressure can be regulated. (Ref. 'Elec- tric Traction', Wilson and Lydall, p. 245.) See also Rectifier; Gratz Method of Rectifying Current. [a. h. a.] Pertinax Tubes, the trade name of in- sulating tubes manufactured of treated paper by a secret process. See Paper for In- sulating Purposes. Peschel Steel -tube System. Wiring Systems. Peschel System of Wiring. Wiring Systems. Petrol-electric Automobile System, a system in which the prime mover is a See See Elevation of Chassis of Fetrol-electric Car, showing relative sizes of the different parte A, Petrol engine. B, Dynamo, c, Controller. D, Motor. E, Pedal controlling forward and reverse motionc and brakes, F, Speed-varying lever. petrol engine, while the power is transmitted to the wheels of the car by means of electric Petrol-electric Car — Phase-advancer 391 motors. The engine drives a dynamo, which either supplies power direct to the motors, or charges a battery of accumulators, from which the motors are supplied, or both. There are many different methods of effect- ing the combination. The motors may be suppressed, the dynamo being used as a motor to start the engine and to assist it in climbing hills; a battery is indis- pensable in this case. A constant- power dynamo may be used, and a battery dis- pensed with, the dynamo and mo- tors in this case taking the place of a mechanical vari- able-speed gear. Ac generators and motors may also be em- ployed. The system is used both for road vehicles and for railway traffic on branch lines; it is also applicable to launches, and other vessels. A petrol-electric car is shown in the fig. [a. h. a.] Petrol - electric Car. See Petrol- electric Automobile System. Petpol-electric Omnibus. See Petrol- electric Automobile System. Petrol-electpic Propulsion for Ships. See Petrol-electric Automobile System. Petroleum and Shale Solvents.— These have been classified together, since in chemical composition and general charac- teristics they closely resemble each other. They are, however, two distinct products, the former being obtained from the distil- lation of petroleum, and the latter from Scotch shale. The petroleum solvents con- tain a larger proportion of the paraffin hy- drocarbons than shale naphtha, the only shale product that is employed for thinning varnishes. The petroleum solvents comprise the water- white limpid liquids, sold somewhat indis- criminately under the names of benzine, benzoline, and petroleum naphtha. These are refined distillation products, and are dealt with more fully under their respective names. See also Naphtha. [h. d. s.] Petroleum Naphtha. See Naphtha. Petticoat Insulator. See Insulator. Pf, the preferable abbreviation for power factor. Phase, applied to an ac quantity, denotes the angle turned through by the rotating vector reckoning from a given instant. Phase is usually measured in degrees reckoning from the zero value of the quantity con- sidered. In the fig., if P be the representa- tive rotating vector of the sine wave B, the phase of any point p will be the angle 6 or 360 Diagram illustrating Phase the angle moved through from P, the start- ing-point. In ae work we are generally concerned with phase differences between quantities, rather than with their actual phase at any instant. See Phase Difference; Lag; Angle of Lead. [r. c] Phase-advancer, an apparatus intended for use in conjunction with asynchronous Fhase-advancer alternating machinery to annul the bad effects of low pf and wattless currents on a distributing system. Such effects are mainly due to the wattless magnetising currents and also to the reaction between stator and rotor. In 1895 Leblanc proposed an arrangement of two exciters in series with the windings of a two-phase rotor, to improve the pf of the machine, and the phase advancer acts on a very similar principle with three-phase 392 Phase Angle — Phases r-nppT^iSW^ L^WNAAAA machines. It is, in fact, a special form of exciter connected in circuit with the rotor winding of an induction motor to which it supplies magnetising current generated at low pressure and low frequency — the 'fre- quency of slip' of the induction motor. Adjustment of the pf is obtained either by changing the speed of the phase-advancer or by rheostatic control of its iield-diverter. Induction motors may be worked at unity pf or even leading pf, and the slip adjustment I to give greater torque may be made an economical one by means of the phase advancer. The utility of induction generators will also be increased when working in conjunction with such a phase-correcting device. A diagram of connections is shown in the fig. The armature consists essentially of open-coil windings connected to a central star point, each winding having two coils relatively displaced by 120 electrical deg. The current is collected by the equivalent of very wide brushes, and passes round the main poles and compensating windings in the pole faces to the circuit to be controlled. The different circuits may be traced out by reference to the fig. The phase-advancer is the invention of Mr. Miles Walker. (Eef. Journ.I.KE, vol. xlii, 1910, p. 599.) Phase Ang^le. See Phase. Phase Compensation of an Induc- tion Meter. See Meter, Phase Compen- sation OF AN Induction. Phase Difference denotes the angle be- tween the phases of two or more ac quantities. Phase difference is usually measured in deg. See Phase. Phase Displacement denotes a change of phase of an alternating emf or current. Thus the introduction of inductance into a previously non-inductive circuit has the effect of causing the current to lag in phase behind that of the emf, and a phase displacement is produced. See Phase; Phase - splitting Device. Phase Errors in Instrument Trans- formers. See Transformer, Instrument. Phase Indicator. See Indicator, Phase; Electrogoniometer. Phase Meter. See Indicator, Phase; Electeogoniometer. Phase - splitting Device, a device for dividing up an ac into two components dif- fering from one another in phase. Such devices are frequently used for starting sp motors, and in other cases where a rotating magnetic field is required to be produced from a sp supply. An arrangement often used consists of making the main circuit Fig. 1.— Connection of Phase-splitting Device divide into two, one containing high induc- tance, and the other high resistance. In fig. 1 let L and r represent the induc- tance and resistance respectively connected to the windings of a rotating field device M, such as a motor. Then if the line o v in fig. 2 re- present the applied voltage, the current in the E circuit will be represented by some line oij, Fig. 2.— Vector Diagram of Pliase-splitting Device lagging a small amount behind ov, whilst the current in the L circuit will be represented by some line 08; lagging nearly 90° behind V. I will be the line current which has thus been split into two components i^ and i^. [r. c] Phases, Rotation of. — In a three-phase system of currents the phases follow one an- other in order at regular intervals through- out the cycle. The present definition relates Phases — Phono-electric Wire 393 to the order in which the currents grow in the various coils of a polyphase machine. In a three-phase winding it is the usual practice to place one side of the three successive groups of windings in the space between one pole and the next, i.e. within 180 electrical degrees. If the first of these three groups is called phase 1, the third such group is phase 2, and the intermediate group is phase 3 re- versed. The first wires of each phase, num- bered 1, 2, and 3, therefore enter slots placed over two pole-pitches at angles of 0, 120, and 240 electrical degrees apart. [h. w. t.] Phases, Succession of, an expression. applied to the order in which the various phases of a three-phase system grow and at- tain their maximum value. For instance, 2 1 2 3 2 3 12^ SnccesBlon of Phases may succeed 1, and 3 succeed 2, as indicated in the diagram; but if, on the other hand, the station ends of 2 and 3 are crossed over, the order then becomes 1, 3, 2, 1, 3, 2, &c. The matter is important in that the direction of rotation of polyphase machinery depends upon the order of succession of the phases, and in attempting to synchronise a new alter- nator or motor it is possible to have the phase on which synchronising apparatus is placed, in perfect synchronism, and yet have the other two phases in opposition if the two machines do not have the same succession of phases. If the phases are applied to an induction motor in the wrong order, the direction of rotation is reversed. [h. w. t.] Phase Swinging- (or Surging), an an- gular oscillation of limited magnitude super- posed on the rotatory motion of an ac dy- namo or motor. A periodic variation, such that the generated emf of the ac machine successively lags and leads with respect to the emf at its terminals. The phenomenon is only encountered when two generators or more are running in parallel, or when a gene- rator is driving motors, converters, or similar machines. The condition may be stable or unstable. If unstable, the superposed oscil- lation may tend either to die out, or may tend to increase in magnitude until the ma- chine falls out of synchronism. See Cyclic Irregularity; Crank-effort Diagram; Torque Diagram of an Engine; Surging; Damping Grid; Amortisseur; Damping; Irregularity Factor. [m. b. f.] Phase TransfoFmer. See Transfor- mer, Phase. Phoenix Fire Office Rules, a set of rules for observance in electric light, power, heating, and telephone installations, in order to minimise risk of fire. They apply mainly to installations with a pressure of not over 250 volts, and give particulars regarding conductors, insulation resistance, systems of wiring, accessories, lamps, systems of distri- bution, and testing of installations. Special rules apply to theatres, shops, and show win- dows, mills, central stations, and other places where additional precautions are necessary, First compiled in 1882, by the late Mr. Musgrave Heaphy, for the Phoenix Fire Ofiice, the rules have now (1910) reached the thirty-ninth edition. Phono - electric Wire, a kind of wire employed for overhead trolley lines. It is made from a special alloy intended to give the best combination of strength and conduc- tivity. Compared with hard-drawn copper wire, phono-electric wire has certain advan- tages, the chief being its greater homogene- ity, greater tensile strength, and longer life. Some of the properties of both materials, as published by the makers of phono-electric wire, are set forth in the table. When sub- jected to arcing, or current over-loads, the phono-electric wire is stated to be affected to a far less extent than is hard-drawn copper. Wire. Gauge Num- ber. B.4S. Dia- meter, mm. Breaking Strength. Ultimate Tensile Strength. Electrical Beaist- ance in ohms per 1000 m. Current to Fuse Trolley- wire, amp. Batio of Life. lb. kg. lb per sqin. kg per sqcm. Hard-drawn Copper < Phono-electric ... -j 00 00 9-27 8-25 9-27 8-25 5290 4710 7400 6300 2400 2135 3356 2850 55,600 57,000 70,620 75,820 3920 4010 4965 5382 0-254 0-322 0-633 0-795 1100 870 800 680 394 Phonoplex Telegraphy — Photometer The conductivity of phono-electric wire is seen, from the above table, to be only some 40 per cent of the conductivity of copper, and this is, of course, its chief disadvantage. Phonoplex Telegraphy. See Tele- graphy, Phonoplex. Phosphor Bronze, a class of bronze (copper-tin alloy) with 0-2 per cent and up- wards of phosphorus added. Its specific gravity is 8-9. It is used in electrical en- gineering for banding wires, shrink rings, end-bells, &c., where a non-magnetic material of great tensile strength is needed. This is often the case in those parts which are in the immediate neighbourhood of the current- carrying parts, and where a magnetic mate- rial would cause excessive magnetic leakage, or be the seat of hysteresis and eddy-current losses. The ultimate tensile strength of phosphor bronze may amount to 8200 kg per sq cm. Its specific resistance varies greatly with its composition. The advent of steam turbine-driven generators has caused a great increase in the use of this class of material. The results of a number of tests are given in the following table: — Per cent Specific Increase in Tensile Resistance Resistance at 0" C. in Microhms per deg. C. rise in Strength in kg per per cu cm. Tempera- ture. sq cm. Phosphor bronze (cop- per, tin, and phospho- ■ 1-6 0-39 4500 rus) — Hospitaller ... Phosphor bronze (cop- ■ per, tin, and phospho- ■ 5-6 0-39 8200 rus) — Hospitalier ... Phosphor bronze with 1 10 per cent of tin — 24-6 — — Abbott Phosphor bronze with - 1 9 per cent phospho- \ 32-5 — — rus — Abbott J See HiGH-KESisTANCE Alloys; Wire, Resistance; Phono-electric Wire. Phosphorescence. See Eadiation. Phosphorus, Electrical Reduction of. — Phosphate of lime, together with coal, coke, or anthracite, and a suitable flux of siliceous material, are fed into an electric furnace. The flux forms a conducting bath, and the current is so regulated as to main- tain sufficient heat to drive off the phos- phorus, which is led away by a pipe to a chamber in which it is condensed. The function of the electricity is simply to gene- rate sufficient heat for the process; the action is not electrochemical. See FuRNACE, Electric. [c. w. h.] Phosphorus Limit in Steel. See Steel. Photometer, a piece of apparatus de- signed for the comparison of two sources of light. The ordinary type of fixed photometer generally consists of a bench along which carriages can slide. The lamps under com- parison are supported at the ends, whilst the 'photometer head' (or comparison device) is movable between them. The cp of one source in terms of the other is calculated by the law of inverse squares. Since the pho- tometer head is thus usually the feature which distinguishes one type of photometer from another, there has arisen in some quar- ters the practice of designating some of the various types of photometer as types of photometer head. Thus formerly the contrast type was termed a contrast photometer. Re- cently it has come to be termed, in some quarters, a contrast photometer head. (Ref. 'Radiation, Light and Illumination', Stein- metz; 'Electric Lamps', Solomon; 'The Art of Illumination', Bell; 'Electric Arc Lamps', Zeidler and Lustgarten.) See also Photo- metry; Standard of Light, [c. c. p.] Photometer, Actinic, or Actinometer. — Various methods for measuring light in- tensity, other than by the eye, have been suggested, but such apparatus can in no way measure the physiological effect. These de- vices may be termed actinometers. Of these the effect of light on silver salts is used in the many actinometers for photographic pur- poses. Others have been developed, de- pending on the rate of combination between hydrogen and chlorine gases (Dessendier's), the rate of decomposition of nitrogen iodide (lAon's), and the change of electrical resistance of selenium (Selenium). The action of the last depends on the fact that the metal selenium lessens its resistance according as the intensity of the light incident upon it increases. A selenium cell used in connec- tion with a recording Wheatstone's bridge will give a record of the variation of sunshine during any period. (Ref. Elec, April 13, 1906; E.T.Z., 28, p. 510, 1907.) Photometer, Bougier, a crude form of photometer formerly employed, in which the upper and lower parts of an opaque screen were illuminated respectively by the two Photometer 395 sources of light which it was required to compare. A horizontal partition prevented the two sources of light from aflfecting more than their own half of the screen. The re- lative distances of the sources of light, when the two halves of the screen were equally illuminated, aflForded data for estimating their relative illuminating power. See Law OF Inverse Squares. PhotometeF, Dispersion.— In the mea- surement of the intensity of strong sources of light it is often important to reduce artificially the intensity on the photometric screen to such a degree that a small change is most readily appreciated by the eye. A negative lens has been employed for this purpose, producing a divergence of the rays after passing through it, and so weakening the intensity to the required degree. The absorption of light in passing through the lens must be in some way allowed for, and the somewhat complex calculation involved in addition, renders other methods preferable in general practice. [c. C. P.] Photometer, Flickep. See 'Flicker Photometer Head' under Photometer Head. Photometer, Globe, Lumenmeter, op Ulbricht Photometer, a photometer used for obtaining a value for the mean spherical cp of a lamp in one reading. It consists of a hollow sphere, whitened on the inside surface, with a small window of translucent glass at one point. When a lamp to be measured is supported at the centre, this window is illuminated by light diffused from the whole of the internal surface, and its illumination is proportional to the mean sphe- rical cp of the lamp. By comparing the light transmitted through this window with that given by a standard source, a measure of the mean spherical cp of the lamp is obtained. The lumenmeter must be itself calibrated with a lamp of known mean sphe- rical cp. [c. c. p.] Photometer, Illumination, or II- luminometer, is the term which is now applied to a portable form of photometer which is intended to be used for the measure- ment of the actual illumination existing in a building, street, &c., in practice. The instrument is thus primarily intended, not for the measurement of the cp of a source of light, but for the measurement of the degree of illumination falling upon any par- ticular surface. (For a description of such instruments and their method of use, see paper by P. S. Miller, Convention issue of the Illuminating Engineering Society Tran- sactions, 1907.) Preece and Trotter Illumination Photometer, a portable photometer in which an inclined screen or diffusing card is viewed through holes in a card of exactly similar quality. The inclined card is illu- minated by a standard electric lamp. As its inclination is varied, its illumination changes approximately according to the cosine law. A scale indicates the amount of the inclination of the screen, and is cali- brated directly in candle feet. An improved photometer on this principle has been devised and constructed by Everett, Edgcumbe, & Co. (Eef. Elec. Engr., May 16, 1907.) [c. C. P.] Photometer, Integrating, or Meso- photometer, a photometer which gives directly, by one reading, the average light emitted round a meridian line. If the source of light be turned about a vertical axis through definite angles, and cp read- ings be taken in each meridian, the mean spherical cp can be obtained by taking the average of these. Examples of mesophoto- meters are the Matthews integrator and the Russell- Leonard photometer. In the former, mirrors are placed at equal distances round a meridian, and at such angles that they reflect light from the source into the photo- meter. In the latter, it is possible to obtain an equal accuracy with fewer mirrors by increasing the number as the equatorial zone is approached and diminishing it nearer the poles. See also Photometer, Globe, Lu- menmeter, OR Ulbricht Photometer. [c. c. p.] Photometer, Spectro-, a photometer which enables two sources of light to be compared at different parts of their spectra. Thus if a carbon -filament lamp is to be compared against one with a metallic fila- ment, the photometer will separate out the spectra of the two beams to be analysed and place, say, the red band from one source in juxtaposition to the red band from the other, and so enable the relative intensity to be gauged. Similarly, comparisons may be made at other points in the spectrum. [c. c. p.] Photometer, Table, an arrangement of photometer specified by the Metropolitan Gas Referees for the measurement of the illuminating power of gas. The two lights 396 Photometer — Photometer Head stand side by side on a table and illuminate two portions of a small piece of paper placed behind a slot. The distance of this slot from the paper is adjusted so that the two illu- minated patches are exactly side by side without any dividing line. The slot-and- paper device is known as the photoped. One of the lights is moved or varied in intensity until equality of illumination is produced, (Ref. 'Notification of the Metropolitan Gas Referees '.) [c. c. P.] Photometer, Weber, a photometer com- prising a standard lamp scale and contrast device. It is chiefly used for the measure- ment of large sources of light, on account of its flexibility. It is arranged so that lights at any angle can be compared against the standard benzine lamp fixed at the end of the tube in which the contrast prisms are placed. The comparison is made, not, as is usually the case, by varying the distance of the source from the photometer, but by placing diifusing screens of different densities in front of the lights to be compared. The fine adjustment is made by varying the position of one of the diflfusing screens from the standard source. The photometer is also arranged for the comparison of lights of widely differing colours by viewing through red and {jreen glasses, thus obtaining rough comparisons in different wave lengths. (Ref. 'Photometrical Measurements ', Stine, p. 78.) [c. C. P.] Photometer Head.— Contrast Photometer Head. — A cer- tain degree of difficulty is experienced in deciding exactly when two surfaces appear to be evenly illuminated. A contrast pho- tometer produces in the field of observation a definite difference in the illumination of the surfaces compared. The surfaces are so arranged that the contrast between one light and one dark surface is compared with the contrast between another light and another dark surface. BuNSEN OR Grease -SPOT Photometer Head, probably the most widely used pho- tometer for general commercial use. In it crudest form it dates from 1841. The form generally employed is shown in fig. 1. The paper screen x, which receives light from both sides, has a spot in the centre which has been impregnated with grease. Light incident on one side will be transmitted through this spot to a greater extent than is the case with the light which strikes the remainder of the surface. Hence the spot represents to a large extent the light from the other side of the screen than that being viewed. By the aid of mirrors A and B the observer is enabled to see both sides of the grease spot simultaneously, and so to contrast the two illuminations. The theory of the grease-spot photometer is somewhat complex, 1 ? Fig. 1.— Bnneen Photometer although its construction is simple. (Ref. 'Photometrical Measurements', Stine, p. 62.) Lummer-Brodhun Photometer Head, a photometer head devised by Professors Lummer and Brodhun in connection with their work at the Reichsanstalt in Berlin — now widely used for accurate laboratory work. It is manufactured by Schmidt & Haensch, Berlin. The 'contrast' principle is employed in Fig. 2.— Lnmmer-Brodhun Photometer Head. Diagram ot Light Path the photometer, by which an area illuminated by source A on a background illuminated by B, is compared against an area of source B on a background of A. Two contrasts are thus compared together. The form of the photometer is shown diagrammatically in fig. 2. s is a white diffusing screen illumin- ated on either side respectively by the lights to be compared. Light from either side is reflected by the mirrors A and B into the right-angled prisms c and D. By an in- Photometer Head — Photometry 397 genious arrangement of the centre faces of the prisms, which lie together, portions of each beam of light are reflected or trans- mitted to a telescope E in such a way that they form a field of the description shown in the diagram at the side. L^ L^ are the por- tions illuminated by one source and L^L^ those illuminated by the other. (Ref. ' Pho- tometrical Measurements ', Stine, p. 70.) Flicker Photometer Head, a photo- meter head in which the illumination on the observed surface or surfaces is made to vary in such a way that a flicker is produced. Usually the field of view is alternately illuminated by the two sources to be com- pared, so that a flicker is seen so long as the illumination at the photometer head from the two sources is unequal. As the illumina- tion grows more equal, the flicker disappears, EooD Photometer Head, a form of flicker photometer head in which a lens is made to oscillate in front of the observing telescope, and ar- ranged alternately to trans- mit light from either of two illuminated sur- faces. A flicker remains so long as there is inequality of illumination. (Ref. Amer. Journal of Science, p. 194, 1899.) Simmance-Abady Photometer Head, a flicker photometer head in which (see fig. 3) a thick disk about ^ in wide is rotated in front of an observing tele- scope whose axis is at right angles to the axis of revolution. The ob- server thus sees a small portion of the periphery of the disk. An eccen- tric bevel is turned on either side of the disk (see diagram), so that when rotated the illu- minated surfaces of the two bevels are alter- nately seen by the ob- server. The flicker disappears as soon as these surfaces are equally illuminated. (Ref. Proc. Phys. Soc. of London, vol. xix.) Whitman Photometer Head, a flicker pTiotometer head in which a rotating sector revolves in front of an observing telescope. By this means, surfaces alternately appear in the field of vision which are illuminated by the two sources respectively. A flicker is thus produced when the surfaces are un- equally illuminated. (Ref. Physical Review, p. 241, 1896.) "Wild Photometer Head. — This flicker photometer head employs the grease- spot principle. It has a disk which revolves about a central axis normal to its sides. One-half of the disk is opaque and reflects light incident upon it ; the other half is ren- dered translucent in the same way as the Bun- sen screen, and transmits light incident from the other side. The disk is viewed through a telescope while rotating, and a balance is obtained when there is no flicker visible. Ritchie - wedge Photometer Head, a compact photometer represented diagram- Fig. 3.— Simmance-Abady Photometer Head Fig. 4.— Ritchie-wedg* Photometer Head matically in fig. 4. The two beams of light to be compared enter at x and Y and il- luminate the two sides of a wedge. The illuminations of these two faces are com- pared together. The chief disadvantage of this photometer is that an error will be introduced unless the light from either side strikes the two faces of the wedge at exactly equal angles. A slight rotation of the photo- meter in a horizontal plane will produce an appreciable error. See also Ritchie Wedge. Trotter Photometer Head, a device in which a card illuminated by one source of light is viewed through a hole (generally star-shaped) in a card illuminated by a second source. The one being seen on a background of the other enables the illumination to be adjusted until it is equal on the two cards. (Ref. Treatises indicated at end of definition of ' Photometer '.) [c. c. p.] Photometpy is the measurement of the intensity of luminous radiation at a point. In general, the intensity of radiation is measured by comparing the illumination of a surface produced by the radiation, with the illumination of a similar surface by a known intensity of radiation. Such methods require the inclination of the surface to the incident radiation to be known. Colour Photometry or 'Heterochro- 398 Photometry — Pilot Cell of Accumulator MATic Photometry ' denotes the process of comparing two sources of light which differ in colour. A distinction is usually drawn between the comparison of artificial lights which only differ comparatively slightly in spectral com- position — such as glow lamps and incan- descent mantles, for instance — and the com- parison of spectral colours, the latter being termed ' spectral photometry ' or spectra photo- metry. See Photometer, Spectro-. See also the references to treatises on Illumination, given with the definition of Photometer. [c. c. p.] Photometry, Law of Inverse Squares in. See Law of Inverse Squares. Photometry, Rotation of Glow Lamps in. See Lamp, Incandescent Electric. Photoped. See Photometer, Table. Photophone, Bell's. See Wireless Telephony. Phototelegraphy, a method of repro- ducing photographs at a distance by the action of electric currents. Korn's Phototelegraph. — In Kom's phototelegraph, a photograph on a celluloid film is mounted on a glass cylinder, which is slowly revolved on a vertical axis, and at the same time moved vertically. An intense beam of light reaches one point of the film through a small aperture. Owing to the rotary and vertical movement of the cylin- der, every point of the photograph is succes- sively subjected to the beam of light, the intensity of which after passing through the film is a function of the lights and shades of the photograph. The beam is afterwards re- flected to a selenium cell (which see) whose resistance is a function of the intensity of the light falling upon it. The selenium cell is in series with a battery and with the trans- mission line. The receiver comprises another drum on which a sensitive film is mounted. This drum is revolved at the same rate as the drum at the transmitting end. The strength of a beam of light thrown on the receiving film is modified from instant to instant by suitable means controlled by an electromagnet whose strength is a function of the current received from the transmitting end. Photographs have been telegraphed from Paris to London by the Korn photo- telegraph. See also Telautograph. Picein Drop Method of Testing In- sulating Materials. See Testing Insu- lation. Picking-up and Paying-out Machine. See Machine, Picking-up and Paying-out. Pickquiclc Coal-cutter. See Electric Coal-cutter. Pierced Core Disks. See Core Disks. Pigtail (for carbon brush), the length of flexible copper-braid conductor used to carry ^~^pi' \W Various Methods of Constrncting Pigtails for Carbon Bruslies the current collected by the brush to the ter- minal on the brush holder. This prevents abrasion of the surface in which the carbon is free to slide, otherwise abrasion is liable to occur by the passage of current and con- sequent sparking between the brush and the surface of the brush holder. Some methods of attaching and soldering the pigtail to the brush are depicted in the fig., which also shows the connector to which the free end of pigtail is soldered, and which is used under the terminal of the brush holder. One brush may have more than one pigtail. See Brushes; Brush Holder. Pillar, Feeder. See Feeder Box. Pillar-and-stall System of Coal-cut- ting. See Electric Coal-cutter. Pilot Brush. See Brush. Pilot Cell of Accumulator, a cell so called because it is used as a guide in the operation of the battery as a whole, readings taken from it being considered indicative of the condition of the remainder of the bat- tery. See Accumulator. Pilot Controller — Pitch 399 Pilot .ContPoUer. See Controller; Controller, Master. Pilot Motor. See Motor, Pilot. Pilot Voltmeter. See Voltmeter. Pilot Wire. See Wire, Pilot. Pin. See Insulator Pin. Pillion, a small spur wheel; in the case of a tramway motor, usually of solid steel, with ma- chine-cut teeth, fixed on the motor shaft. Pinions are sometimes made of raw hide. See Gear- ing FOR Electric Motors; Spur Geak Pipes for Under- ground Cables. See Conduit, Under- ground. Pipe-ventilated Motor. See Motor, Pipe-ventilated. Pit, Draw-in. See Manhole. Pitch. — The word pich is used in dynamo design in various senses. For definitions covering its use in armature wind- ings see Pitch, Winding. The conception of pitch as employed in dynamo design is that of an arc of the circumference, usually of the armature, as measured at the air gap. to the centre of the next. It is usually taken at the air gap, but for certain purposes must be also taken at the bottom of the slots, and also midway. Htch The line along which the pitch is measured, whether this be the air-gap circumference or some other circumference, is the pitch lime. The slot pitch is the distance between the centre line of one rotor or stater slot or tooth Winding Pitch The polar or pole pitch is usually measured at the air-gap circumference, and is equal to this circumference divided by the number of poles. Thus the pitch is the distance from the centre line of one pole face to that of the next, measured circumferentially. It is usu- ally denoted by t. In the case of the dynamo illustrated in the fig., A is the slot pitch and B is the polar pitch. Pitch, Winding. — This is usually de- noted by y, and is the number to be added to the number assigned to a conductor or coil of an armature winding in order to as- certain the number of the conductor or coil to which the first-mentioned conductor is to be connected. The conductors must be dia- grammatically represented and numbered in accordance with certain precise conventions which are set forth in textbooks on the subject (see chap, viii of 'Armature Con- struction', Hobart and Ellis). Back Pitch, the winding pitch at the back end of an armature {i.e. at the end remote from the commutator). 400 Pitch Diameter — Platinum Front Pitch, the winding pitch at the front end of an armature {i.e. at the com- mutator end). In the accompanying armature winding diagram, conductor No. 1 is connected over the back end to conductor No. 10, and con- ductor No. 10 is connected over the front end to conductor No. 21. Consequently the back pitch (yb) is equal to 10—1 = 9, and the front pitch (yf) is equal to 21 — 10 = 11. The average pitch (y) is in this case equal to yb + yf = 9 + 11 = 10 2 2 Pitch Diameter (of a Cable).— This Piteh Diameter of a Cable Ay Fitch diameter for the 6-stram1 layer. B, Pitch diameter for the 12-8trand layer. C, Overall diameter. is the mean diameter of any particular layer of strands; that is, twice the dis- tance from the centre of one strand to the centre of the core. See Overall Diameter (of a Cable). Pitch Factor.— It often hap- pens, for various reasons, that an armature coil does not embrace a complete pole pitch, and therefore the two sides of the coil do not lie in similar positions under adjacent poles with respect to the flux. The emf from instant to instant are not the same in the two sides of the coil, and the resultant emf will be less than if the coils spanned the complete pole. To allow for this, the ^ifcA factor is introduced into emf formulae, and is usually taken as the sine of half the angle of span of the coils, the full span being taken as 180°. See Kapp Coefficient; Winding, Fractional Pitch; Winding, Chord. Pith-ball Electroscope. See Electro- scope. Pivots, Measuring-instrument. See Suspension in Measuring Instruments. Pivot Suspension. See Suspension in Measuring Instruments. Plante Accumulator. See Accumu- lator. Plant Eificiency. See Efficiency. Plastic Insulating* Varnish. See Elastic Insulating Varnish; Insulat- ing Varnishes; Impregnating Varnishes. Plastic Rail Bond. See Bond. Plate, Earth. See Earth Plate. Plate, Ground. See Earth Plate. Plate, Neg'ative, that plate in a cell, from which negative electricity flows to the outer circuit, or into which positive electricity flows from the outer circuit. In primary cells, the negative plate is usually of zinc; in secondary lead cells, it is usually the grey, pure -lead plate. See also Accumulator Plates. Plate, Positive, the plate in a cell from which positive electricity flows to the outer circuit. In primary cells it is usually of copper, carbon, or platinum. In the lead accumulator or secondary cell it is the highly- oxidised, plum-coloured plate. See also Ac- cumulator Plates. Plate Coupling", a rigid coupling usually consisting of two dished pulleys bolted to- gether. A coupling of this type is illustrated in the fig. See Coupling, Shaft; Old- ham's Coupling; Coupling, Flexible; In- sulating Coupling; Clxjtch. Plate Coupling Plates, Accumulator. See Accumu- lator Plates. Plates, Core. See Core Disks. Plates, End. See End Plates. Platinoid, an alloy widely used as a re- sistance material. Its resistivity varies with its exact composition from 32 to 50 microhms per em cube, and its coefficient of increase of resistance for 1 deg. C. is 0-00021 of the resistance at 0° C. See High-RESISTANCE Alloys; Wire, Resistance. Platinoid Contacts. See Contact, Electric. Platinum, a metal with a specific gravity of 21-2, a specific heat of 0-032, a melting- point of 1780° C, and a specific resistance at 0° C. of 11 microhms per cm cube. Its re- sistance increases by 0-35 of 1 per cent per Platinum Contacts — Plug Dial for Resistance Bridge 401 degree Centigrade increase in temperature. The world's annual output of platinum is only a matter of 10 to 20 tons, and the price is £& per Troy oz., i.e. £193,000 per metric ton. Platinum Contacts. See Contact, Electric. Platinum Standard of Light. See Standard of Light. Plough CaFPier, the frame from which the plough is suspended beneath an electric tramcar operating on the underground con- duit system. See Conduit System of Electric Traction. Plough Collector. See Conduit Sys- tem of Electric Traction. Plough Pit, a chamber provided in the roadway, on an electric tramway operated on the underground conduit system, in which the collecting plough can be removed or replaced, automatically or otherwise. See Conduit System of Electric Traction. Plug, a device consisting essentially of a piece of metal, generally circular in section Fig. 1 Plugs K-2 and tapered, which makes an electrical con- nection when introduced into a suitable socket. The plug is frequently used to make con- tact between two jaw terminals (see fig. 1) ; this type is largely used in connection with resistance boxes for measuring purposes. In another type the plug makes contact between two bars, one below the other and insulated from it (see fig. 2); this design has been largely used on switchboards, when for in- stance it has been desired to connect any one of several feeders to any one of a number of generators. It is, however, now more common to run generators in parallel, and the use of plugs is becoming more restricted. Plugs are also used to connect portable apparatus (^e.g. a reading lamp) with a fixed point of supply, in which case the plug is generally provided with two contacts which make connection with two corresponding contacts in the socket into which the plug fits. Similar plugs are used with moving jib cranes, electric drills, portable electric machine tools, &c. See Jumper or Bus- line Jumper Receptacle; Plug Dial for Resistance Bridge; Bridges. Plug, Attachment. See Attachment Plug. Plug, Flush. See Flush. Plug, Infinity, a plug fitted to the ad- justable arm of a Wheatstone's bridge (see 'Post-office Form of Wheatstone's Bridge', under Bridges). On removing the plug the circuit is broken, as there is no connection behind the brass blocks. When measuring exceedingly high resistances, the plug is useful for detecting a broken circuit. Plug, Inspection, a removable plug in the lids of enclosed accumulator cells, fitted for observing the level of the acid, and for facilities in taking readings of specific gravity. See Accumulator. Plug, Switch. See Switch Plug. Plug, Wash-out, a plug fitted in a hole in a leaden box or a lead-lined wood box containing an accumulator. When the acid in the box is siphoned ofi", and the plug removed, the deposit from the plates, which has accumulated on the bottom of the box, can be washed out of the hole, into which the plug is afterwards returned. See Ac- cumulator. Plug Dial for Resistance Bridge.— A plug dial has a number of flat metal con- tacts (usually ten) arranged round a central one, to which any one of the outer contacts may be electrically connected by inserting a Hog Dial conical plug in a hole drilled partly in the central contact and partly in the separate outer contact. Nine resistance coils have each one end connected to one of the contacts, and the other end is taken to a common connection which is also served from the tenth 'no re- sistance' contact, by a simple connection of negligible resistance. One plug only is used, being placed in the hole corresponding to the resistance used on that dial. Several 402 Plug Fuse — Polarisation dials are usually provided (two to eight), which have their first resistances in the ratio of 1, 10, 100, and so on, the central contact of one being in series with the common con- nection of the next. See Bridges; Plug. Plug- Fuse. See Fuse. Plunger Magnet. See Electromagnet, Plunger. Plunger Switch. See Switch, Plunger. Pneumatically-controlled Switches. See Switch, Electro-pneumatically-con- trolled Unit; Control, Pneumatic, of Electric Apparatus. Pneumatic Brush Holder. See Brush Holder. Pneumatic Control. See Control, Pneumatic, of Electric Apparatus. Pneumatic Track Brake. See Brakes. Poggendorf's Solution, a combined exciting and depolarising fluid for batteries with zinc and carbon electrodes. Composi- tion: Concentrated sulphuric acid, 250 g; potassium bichromate crystals, 120 g; water, 1 liter. See Battery, Primary. Pohl's Commutator. See Key, Pohl's Commutator. Point. See Wiring Point. Point, Fusing. See Fusing Point; Fuse. Point, Neutral. See Neutral Point. Point, Wiring. See Wiring Point. Point Effect. — This is the effect pro- duced by a point in a charged conductor, of concentrating the electric potential in its immediate neighbourhood. It facilitates electric discharge from the point, the elec- tricity escaping either to other conductors near, or being used merely in charging the air. The same effect is to be observed in the neighbourhood of any sharp places on a con- ductor or on an insulator of high specific in- ductive capacity. See Dielectric Strength. Points, Electrical Operation of.— In some electrical systems of operating railways the points (or switches) are moved over by a small electric motor at the side of the track. The current which actuates the motor is regu- lated from the signal box, the electric switch being interlocked with that for the proper semaphore signal. See PoiNT-SHlFTEK. Point -shifter, apparatus by means of which the driver of an electric tramcar can set the points at a junction, so as to guide the car in the desired direction, without leaving his place or stopping the car. See Points, Electrical Operation of. Polar Angle. See Polar Face. Polar Are. See Pole Arc. Polar Curve. See Curve. Polar Extension. See Pole Tips. Polar Face, that face of a magnet from which the magnetic flux emerges into the air gap. Under certain conditions of load the cur- rents in neighbouring conductors may distort the field so that some parts of the polar faces do not carry any magnetic lines; the re- mainder of the face actually carrying lines is then known as the active polar surface of Polar Face the magnet. The angle subtended at the centre of the armature or rotor by the polar face is known as the polar angle (see Angle OF Polar Span), and the length of arc oc- cupied by the pole face is called the pole arc (a in fig.). The space between one polar face and the adjacent one is known as the inter- polar space (l in fig.). Polar Pitch. See Pitch. Polar Span, the span of the pole (or polar) face; the arc of the angle subtended by the pole (or polar) face (which see). This is also designated the polar (or pole) arc. See also Angle of Polar Span; Polar Face. Polarisation. See Polarisation, Elec- trolytic, IN Battery; Depolarise; Pog- gendorf's Solution; Battery, Primary. Polarisation, Counter emf of. See Electromotive Force, Counter. Polarisation, Electrolytic, in Bat- tery. — In a simple zinc-copper couple the hydrogen which is liberated at the copper electrode tends to set up a current opposed to the main current (see Counter emf of Polarisation). This effect may also take place in batteries provided with a depolariser, as under certain conditions such an excess of current may be taken from a cell that the depolariser is unable to oxidise the hydrogen Polarisation — Pole 403 sufficiently rapidly, and consequently polar- isation ensues. See Battery, Primary. Polarisation, Emf of. See Electro- lysis; also Electromotive Force, Coun- ter; Depolarise. Polarisation Capacity.— A battery be- comes polarised by the accumulation of hydro- gen on the positive electrode (see Polarisa- tion, Electrolytic, in Battery). The accumulation of gas is gradual, and assumes a maximum after a certain quantity of elec- tricity has passed through the battery. This quantity of electricity is a measure of the polarisation capacity. See Battery, Pri- mary. PolaFisation of the Medium, a hypo- thetical arrangement of the molecules in the dielectric of an electrostatic condenser. This arrangement is analogous to that assumed by iron filings when sprinkled on a piece of paper in the neighbourhood of a magnet, forming the well-known lines of force. The molecules in the dielectric are supposed to arrange themselves in chains in a similar manner. See Dielectric Polarisation; Dielectric. Polarised Ammeter. See Ammeter. Polarised Armature. See Armature, Polarised. Polarised Electromagnet, a magnet in which only a part of the magnetism varies with the current in the windings. Either the whole or a portion of the mag- netic circuit is of steel, which retains its magnetism to a large extent. If the mag- net is of horseshoe form, the cross-piece may be of steel and the limbs of soft iron. The advantage of polarisation is that the iron is maintained in its most sensitive con- dition, i.e. in a condition in which a small increase of the magnetising field causes a relatively large increase in the magnetic induction, or, in other words, the iron is worked at a steep part of the B-H curve. Polarised Rays. — The motions of elec- tricity, or ether, concerned in the propagation of electric waves (as also light and heat waves) are in general at right angles to the direction of the ray. (The same is approxi- mately true of surface waves in deep water — the wave travels along, the water goes up cmd dovm, i.e. moves at right angles to the direc- tion of propagation of the wave.) If the motion of the medium is confined to straight lines which are all parallel, the wave is said to be 'plane polarised'; if to parallel ellipses or circles, ' elliptically ' or ' circularly ' polar- ised. If not confined to any. particular type, such as these, but more or less a 'random' motion, it is non-polarised. In speaking of light, this last is called ' ordinary ' light. It is not ' ordinary ' in electrical work such as wireless telegraphy or telephony. A straight (Hertzian) oscillator gives out rays polarised in a plane through the axis, or length, of the oscillator. In this case, the electric current in the conductor travels to and fro in a straight line which is at right angles to the direction of maximum radia- tion, i.e. to the direction of the ray. A single ring-shaped oscillator would give out 'circularly polarised' waves; and 'non- polarised' rays would be given out by an oscillator formed of two concentric rings, each forming one-half of the oscillator. A polarised ray is more absorbed by a conductor exactly like the oscillator, and placed parallel to it, than by a conductor of any other form or in any other position. [j. e-m.] Polarised Relay. See Relay. Polarity, applied to magnets to express which pole is north and which south; applied to electrical circuits to express which terminal is positive and which negative. Polarity Indicator. See Indicator, Pole or Polarity; Detector, Magnetic; Electrolytic Detector. Pole (or Pole Piece), in dynamo-elec- tric machines, the iron limb from which the yjjjjjT /S/e Flece \Ufl7T\ VtfttQ / ?b\eShoe \ f^le Morn orTi/3 Pole or Pole Piece magnetism passes across the air gap into the armature, and which is embraced by the ex- citing windings. A typical shape is indicated in the accompanying sketch. Many methods of construction are in use. Among others, the pole is built up of stampings which are attached to the yoke ring by bolts, or again the pole shoes are separate from the main pole core, which in the case of small machines is cast in one with the yoke ring. In the smooth-core rotors now used upon 404 Pole — Pole Horns turbo-alternators, the pole may be defined as the region froin which the magnetic flux emanates. See Pole Shoe. Pole, Bushed, an arrangement introduced by Dobrowolsky for securing a concentration or a stiifening of the field towards the middle of the pole face. It consisted of connecting all the poles together by an iron ring, which produced a large leakage flux across the pole tips, so rendering them highly saturated and unable to carry the main flux, which is liable to be distorted under the action of the arma- ture when loaded. The arrangement is now practically obsolete. Pole, Comb. — With laminated poles, in order to produce a ' stiff field ', every other Fig. 1.— Comb Poles lamination is sometimes cut short of the pole face (see fig. 1), so that the fiux-density in the air gap at that region is doubled. Some- times only the pole tips are cut away in this manner (see fig. 2); this, however, being done Fig. 2.— Comb Poles more with a view to approximating to a sine distribution of the flux. See Pole Shoe. Pole, Consequent, a magnetic pole oc- curring in or near the middle of a magnet. Also, when two or more magnetic poles of similar polarity are placed together so as to form one pole, they are said to produce consequent poles. Such an arrangement of consequent poles is shown in the fig., which represents a system of field-magnets used in one type of electric generator. Consequent Poles Pole, Magnetic. See Magnetic Pole. Pole Are, the span between the extrem- ities of the tips on the same pole. It is sometimes expressed in actual length, but more often as a percentage of the pole pitch, or else in electrical degrees. The pole arc is thus the arc of the angle subtended at the centre of the armature by the pole shoe. It is the length of the pole shoe, measured circumferentially. A common value for the pole arc is two-thirds of the polar pitch. See also Polar Face; Angle of Polar Span. Pole Apmatupe. See Armature. Pole-ehanging" Switch. See Switch, Pole-changing. Pole Face, that surface of the pole shoe facing the armature. See Polar Face; Ec- centric Pole Face; Pole Shoe; Angle OF Polar Span. Pole Horns, those parts of the pole shoe which project beyond the pole core. Their purpose is to provide a suitable density and distribution, in the gap region, of the flux which has been confined in a much smaller area in order that sufficient room may be obtained for the exciting windings. In the fig., A denotes the pole horns. See also Pole Tips; Pole, Comb. Pole Indicator — Pole Tips 405 Pole Indicator. See Indicator, Pole OR Polarity; Detector, Magnetic; Elec- trolytic Detector. Poles, Commutating'. See Inter- Compensating. See Inter- poles. Poles, poles. Poles, Line. See Line Poles. Poles, Reversing. See Interpoles. Poles, Salient, a system of poles pro- jecting inwards or outwards from the main Salient Poles yoke ring, as on the field system of a fly- wheel alternator. The term is used to dis- tinguish such poles from poles which do not project, as those produced by the stator winding of an induction motor or by the revolving field of a turbo-alternator with a distributed field winding. The rotor in the fig. has twelve salient poles. Poles, Staggered. — In inductor alter- nators, in order that the coils in the two armatures may give current having the same phase relation, the polar projections on the crown of south poles must be displaced with regard to the poles on the north crown, so that a polar projection on one crown comes opposite a depression between two polar pro- jections on the other crown. If the poles are side by side, and the coils are wound directly under them, there will be a differ- ence of phase of 90° between the emf gene- rated in the two armatures. Pole Shoe, the extended extremity of the usual pole. It is generally a separate piece from the main pole, and is attached by means of screws or bolts when the coils have VOL. II been assembled upon the pole cores. It may be either solid or laminated. Bevelled Pole Shoes, a construction in which the pole face is bevelled ofij as indi- Kg. 1.— Bevelled Pole Shoe I', Fringe ol flux. cated in fig. 1, in order to obtain a good 'fringing' of the magnet flux. Skewed Pole Shoes. — This is one of the methods adopted with a view to obtaining a sinusoidal distribution of flux in the air gap. Two forms of the arrangement are indicated Fig. 3.— Skewed Pole Shoes P, Pole pitch. 8, Solid pole shoe. L, Laminated pole ehoa I, Outline of pole core. in fig. 2. See also Eccentric Pole Face; Pole, Comb. Pole Shoes, Chamfering of. See 'Bevelled Pole Shoes' under Pole Shoe. Pole Shoes, Laminated. See Lami- nated. Pole Span. See Pole Arc; Angle of Polar Span. Pole Tips, those edges of the pole shoe the general direction of which is parallel to the armature shaft. That edge of a pole shoe which is first met by a point rotating with the armature is called the leading pole Up or leading pole horn; the opposite edge is called the tailing pole tip or trailing pole horn. One or both of the pole tips are sometimes shaped with a view to obtaining a distribu- tion of the magnetic flux favourable to good 27 406 PoUak-Virag Writing Telegraph — Polyphase Motors commutation. These shaped tips are occa- sionally referred to as polar extensions, Pole tips are also known as pole horns (which see). See Pole Shoe. PoUak-Vipag" Writing Telegraph. See Telegraph, Writing. Polymorphic, adapted for the supply of more than one kind of electricity. Thus on p. 346 of Koester's 'Steam Electric Power Plants ' is described a polymorphic group at the St. Denis station. The group is made up of two motors and two generators. At each end of the shaft is one 550-volt cc gene- rator, and between them are one 3-phase, 25- cycle, 10,250-volt alternator, and one 2-phase, 42-cycle, 12,300-volt alternator, the latter being also arranged to give 6150 volts. In the middle of the group is an electri- cally operated mechanical clutch-coupling, making it possible to use the group in two sets, or, if required, the two alter- nators, each having a capacity of 750 kw, may operate together under a load of 1500 kw on the 550-volt service. See Double-current Generator. Polyphase. See Polyphase System. Polyphase Alternator. See Al- c" ternator. Polyphase Armature. See Arma- ture. Polyphase Asynchronous Motor. See Motor, Induction. Polyphase Coupling of Magnetic Cir- cuits denotes the arrangement of a polyphase transformer where the magnetic circuits of the phases are interconnected. With an n-phase system n cores are required connected at both ends by common yokes. This arrangement gives in effect a closed magnetic circuit for each core, as the return path of any indi- vidual one is through all the others in paral- lel. The algebraic sum of the fluxes being zero, it follows that the average magnetic potentials of the two yokes are the same, and hence the magnetic system has no external polarity. {N.B. — This statement must be modified in the case of a star-connected wind- ing where hysteresis causes the appearance of a small flux harmonic which passes exter- nally from yoke to yoke.) [r. c] Polyphase Currents. — This term de- notes collectively the currents flowing in the conductors of a polyphase system. See Polyphase System. Polyphase Energy. See Polyphase Power and Energy. Polyphase Generator. See Alter- nator. Polyphase Induction Meter. See Meter, Induction. Polyphase Induction Motor. See Motor, Induction. Polyphase Magnet, an electromagnet excited with polyphase currents. Polyphase Motor. See Motor, Alter- nating-current; Motor, Induction. Polyphase Motors with Commuta- tor. — While the development of commuta- tor motors for alternating circuits has been chiefly devoted to sp circuits, a number of applications to polyphase circuits have also been worked out. The development of these ■WWWVM WWVWW^C ■wwww^ www A wwwvi B Fig. 1. — Three-phase Compensated Induction Motor A, Phase I. ii, Phase II. c, Phase III. ideas has been so interwoven with the important developments associated with sp commutator motors that, in investigating the subject, careful study should also be made of sp motors. See Single-phase Motors. Polyphase Induction Motors. — The compensated polyphase induction motor consists of a polyphase stator of the usual type, with a rotor supplied with continuous-type wind- ing and fitted with three or more brushes on its commutator. These brushes are con- nected in shunt with the stator, usually through a transformer, as only a very few volts are usually required. As the machine speeds up, and as the frequency of the rotor currents relative to that of the stator de- creases, the impedance in the circuit of the commutator brushes decreases proportionally, i.e. for a given current transmitted by the brushes, the pressure across them decreases as the speed increases. In the neighbour- hood of synchronism the impedance is very low indeed, and at synchronism it merely corresponds to the I E drop. Consequently, at that speed we may feed sufficient cur- rent in through the brushes to magnetise the Polyphase Motors with Commutator 407 machine by the expenditure of a very small number of volt -amperes, thus saving the magnetising current in the stator, or even reversing it if the pressure fed into the rotor is sufficiently high (see fig. 1). Shunt Variable-speed Polyphase In- duction Motor (see also Single- phase Induction Motor (Atkinson's Form) ).— This motor consists, if we take its two-phase form, of a two-phase wound stator and a rotor with four brushes upon it, as shown in fig. 2, the axes of the brush circuits being the same as those of the stator circuits. Into the rotor circuits are fed emf in phase with those fed into the stator circuits having parallel axes, and by varying these emf the speed may be varied. We may compare each of the rotor circuits through opposite brushes to the armature circuit of a cc machine, and the flux at right angles to it to the flux of this machine. The latter we cannot conveniently vary, but we can vary the voltage applied to the armature circuits in the manner above described, and so vary the speed. Gorges in 1891 (D.RP. 61951) described a motor of the above type in which the rotor was fed by voltages in parallel with the stator, but Fig. 2. — Two-phase Variable- speed Induction Motor A, Phase I. B, Phase II. Gorges described his invention in the KT.Z. for December 25, 1891, p. 699. Fig. 3 represents diagrammatically a three-phase m n Fig. 3.— Three-phase Variable-speed Induction Motor N, Neutral point. it is difficult to ascertain precisely how far he carried his invention. It was revived in 1902 by Winter and Eichberg, who were the first to state explicitly and fully the possi- bilities of the machine. A-OU Fig. 4.— Phase Diagram of a Three-phase Variable-speed Induction Motor A, Phase I. B, Phase II. 0, Phase III. variable-speed induction motor, and in fig. 4 are shown the phase relations of the stator and rotor circuits. Polyphase Conduction Motor, Shunt Variable -SPEED. — This motor, proposed by Lamme, consists of a rotor with a con- tinuous type of winding upon which four or Fig. 6.— Two-phase Lamme Motor (shunt characteristic with variable speed) 0, Phase I. s, Phase II. three brushes are placed. Each brush circuit, as A B of fig. 5, for instance, is neutralised by a circuit placed in series with it producing an equal and opposite number of ats. Thus polyphase current flowing through the rotor and neutralising winding in series produces no flux across the air gap. To produce the 408 Polyphase Motors with Commutator — Porcelain required flux there is a separate winding on the stator in addition to the neutralising winding, and the speed may be regulated either by regulating the flux due to this winding, or by varying the voltage applied to the rotor. A main condition for the good working of the motor is the position of the stator winding in relation to the rotor wind- ing belonging to the same phase. These two windings should be 90° distant, if no J 1 J^ i-rrmnr r^ p-^stsaniu—r- 1 [ :^ i Fig. 6.— Series Modification of Lamme's Shunt Variable- speed Conduction Polyphase Motor A, Phase I. B, Phase II. E, Exciting Windings. N, Neutralising Windings. attempt is made to compensate for the dis- persion of the motor. Despite the rotor winding being neutralised by the series- wound stator winding, the dispersion of such motors is still considerable, and it is impor- tant to adjust the position between the shunt- wound stator winding and the rotor winding so as to produce a leading current at no load. The chief drawback to this machine is that it will usually require an auxiliary transformer, /T Fig. 7.— Gbrges Two-phase Series Motor A, Phase I. B, Phase II. as the rotor can only be wound for low volt- PoLYPHASE Series Motors. — The poly- phase motor with shunt characteristic and suitable for variable speed (Lamme's poly- phase motor) can be transformed into a series motor by leading the main current through the armature and neutralising winding and through the exciting winding, the latter being (just like the corresponding shunt winding) 90° displaced in relation to the neutralising winding, as shown in fig. 6. As two windings having difi'erent axes, but the same current, can be represented by a single winding, we arrive at once at the Gorges polyphase series motor, one of the earliest types of polyphase commutator motors. (Eef. D.E.P. 61,951; E.T.Z., p. 699, Dec. 25, 1891.) (See fig. 7.) Polyphase Power and Energy. —The measurement of the power expended and, consequently, the energy consumed in a polyphase system depends on the number of conductors of the system. In a three-wire polyphase system (three-wire three-phase, star or delta connected, or three-wire two- phase) the two-wattmeter method of measur- ing power is employed. When the system has four conductors (three-phase four-wire) three wattmeters are used. A two -phase four-wire system consists of two separate sp two-wire circuits differing in phase by a quar- ter period, which are treated as two distinct sp ac circuits. See Power, Methods of Measuring, in Polyphase Circuits. [h. g. s.] Polyphase System denotes an ac system of generating and transmitting electrical energy where the conductors between the generators and the absorbing devices are more than two in number, and between each conductor and the neutral of which there is an emf difiering in phase from the others. The impressed emf in such a system are symmetrical, i.e. the phase displacement be- tween one emf and the next in order of rotation, is equal for all the conductors. For a system of n phases, therefore, with n conductors, the progressive phase displace- ment is 360/w degrees. See also Alternat- ing-current System. [r. c] Polyphase System, Balanced. See Balanced Load. Polyphase System, Unbalanced. See Unbalanced Load. Polyphase Transmission System, an electrical system in which power is trans- mitted by means of polyphase currents. See Alternating-current System; Alter- nating-current Transmission; Three- phase Transmission; Overhead Line. Polyphase Winding. See Winding, Polyphase. Porcelain. — There are two distinct grades of porcelain, the 'hard' and the 'soft' porce- lains. The former only is used for electrical purposes, and is a hard, opaque solid, manu- factured from kaolin mixed with quartz and Porcelain Interiors — Potential 409 with a fusible silicate. It has high disrup- tive strength and insulation resistance, and is unaffected by exposure to climatic con- ditions. Its high insulating properties, which remain permanent, have led to its adoption for the manufacture of overhead line insu- lators. The fact that it can be moulded to almost any shape renders it suitable for the insulation of ht switch contacts, bushings, and fuse blocks, for all of which purposes it is extensively employed. (Eef. ' The Manu- facture of Electrical Porcelain', Dean Harvey; The Electrical Journal, vol. iv, p. 352, June, 1907.) [h. d. s.] Porcelain Interiors, blocks of porcelain on which the terminals and other line parts Porcelain Interior of small switches and junction boxes are mounted. In the illustration a T-jointing box with its porcelain interior is shown with the cover removed. Porous Cups (Porous Pots), vessels, generally of cylindrical or rectangular sec- tion, used for separating two liquids in an electrolytic cell and yet allowing free pass- age for the current. They consist of porous earthenware, and are unattacked by the electrolyte. Portable Cell. See Cell, Portable. Porteleetric Railway System, a mode of transportation of letters and parcels at very high speeds; a steel car is drawn along a special kind of track by solenoids distri- buted at intervals, each solenoid being auto- matically energised for a short time only as the car approaches it. The system has never yet come into commercial use. Position of Brushes. See Brushes, Adjustment of; Position of Least Spark- ing. Position of Least Sparking. — For every different load on a cc dynamo or motor (without interpoles) there is an angle of lead or lag of the brushes at which there is a minimum tendency to sparking. In other words, as load comes on, the brushes should be adjusted more forward from time to time to suit the load. But in modern machines the sparking tendency is usually so slight that no movement of the brushes is required. The machine in which brush movement is required would nowadays usu- ally not be regarded as a commercial article except in special cases. See Brushes, Ad- justment of. Positive Brush of Dynamo or Motor. See Brush. Positive Carbon of Arc. See Arc. Positive Current. See Current, Out- going. Positive Electricity. See Electricity. Positive Electrode. See Electro- lysis. Positive Feeder. See Feeder. Positive Group, any number of positive plates (of an accumulator) connected together by a bar. See Accumulator. Positive Plate. See Accumulator Plates. Post, Binding. See Binding Post or Screw. Post-office Bridge. See Bridges. Potential, Absolute. See Potential, Electric. Potential, Concentration of. See Concentration of Potential; Potential Front. Potential, Constant. See Generat- ing Systems; also Central Station for THE Generation of Electricity. Potential, Disruptive. See Disrup- tive Voltage; Dielectric Strength. Potential, Electric. — This term, as used in electrical engineering, may be defined as the tendency of electricity to flow. The potential of any conductor at a given instant denotes the tendency of the conductor to charge other conductors, just as the tempera- ture of a body denotes its tendency to give up heat to its surroundings. The absdhite potential of a body is measured by the work necessary to bring a unit charge up to the body from infinity, assuming that the charge on the body is unaffected by this operation. Hence the potential different between two bodies is measured by the work done in transferring a charge from one to the other. This last term — often contracted to pd for short — is frequently used in electrical work instead of 'voltage' and 'emf. Zero potential usually denotes the poten- 410 Potential — Potential Regulation tial of the earth which is taken as a zero of reference. Eqidpotemlial is a term applied to points in a field which are at the same potential. See also Equipotential Sukface. [r. c] Potential, Magnetic. See Magnetic Potential. Potential, Zero. See Potential, Elec- TEIC. Potential Diflfepenee, the emf between points, taken along a path which does, not pass through any region of electromotive activity (see Electromotive Force). The line integral of the electric force taken be- tween the points along such a path. See also Potential, Electric; Contact Elec- tricity. ■ [f. w. c] Potential Dividing- Method.— If a pd be applied to the terminals of any resistance, a pd proportional to any fraction of the total may be obtained by tappings. If the frac- tion of the whole resistance is known, then the fractional pd is accurately known. This is the principle of the potentiometer (which see). Potential Energy. See Energy; also Energy, Potential. Potential Front, the steep portion of a curve representing the space-variation of potential along a conductor. This term is usually confined to the space variation of potential along a conductor occasioned by some redistribution of electric charges in a system. If, for example, one end of a motor winding be connected to one pole of a circuit, the whole winding will rapidly be brought to the potential of the circuit; but, during the process, redistribution of electric charges throughout the winding occur, and are ac- companied by space variations of potential along the winding. See Field's extended ex- planation and discussion of potential front at p. 692 of vol. xxxii of the Journ.I.E.E. Potential Galvanometer. See Gal- vanometer. Potential Gradient, the difference of potential per unit distance at a point of space; the space rate of change of potential at a point; the slope of the curve connect- ing potential and distance in any direction. Thus, if a conductor be considered, along which a current is flowing, the potential gradient along the conductor is the rate of fall of potential along the conductor, but it is equally permissible to talk of the po- tential gradient in the dielectric surround- ing the conductor. In this case the potential gradient is usually measured in a direction at right angles to the axis of the conductor. [m. b. f.] Potential Regulation, the alteration and control of the emf of an electrical cir- cuit. Where a cc dynamo is the source of emf, the voltage is generally regulated by altering the resistance in the field circuit of the machine, and almost every dynamo is provided with a field regulator for this purpose. Cc dynamos are often compound-wound, that is, provided with a series field winding in addition to a shunt winding in order to automatically keep the pressure at the right value as the load changes; but even when so compounded, adjustable resistances are generally provided in the shunt winding to permit of variations due to alteration in temperature or in speed being rectified by hand regulation. Where it is desired to regulate the pres- sure of a single circuit, a booster is often used, the booster consisting of a generator in series with the circuit in question, driven by a motor connected across the main cir- cuit. When the single circuit is employed to charge accumulators, the booster gene- rator is, as a rule, excited across the main circuit and regulated by a field resistance, whereas when the single circuit feeds, say a distant load of lamps, the booster is gene- rally series-wound, with the result that the pressure it adds to the circuit is approxi- mately proportional to the current flowing in the circuit, which is just what is desired. The pressure at the end of a feeder can also be regulated by the insertion of resist- ance in series with the feeder, and this is sometimes done in the case of public supply systems where the drops down the feeders vary considerably. The pressure of a rotary converter depends upon the pressure of the ac supply, and field regulation has no direct effect. Pressure variation, when required, is obtained by hav- ing tappings on the transformers which feed the rotary, or by a compound winding on the field of the rotary. In another method, a resistance in the shunt field winding is made to affect the pressure of the cc side indirectly if reactance is present to a considerable ex- tent in the ac circuit. This is due to the fact that by altering the strength of field of the rotary the lead or lag of the current in Potential Regulator — Potentiometer 411 the circuit is altered, and a smaller or larger current is taken for the same power trans- mitted, and consequently a smaller or larger drop of volts occurs over the reactance, which is equivalent to an alteration in the pressure of the ac supply. The regulation of an ac generator is usu- ally effected by variation of the main field current, or of the field current of the exciter. Special methods of rendering ac generators self-regulating have been devised, but are not in general use. See Alteknatok, Com- pound-wound; ' Hey land Polyphase Gene- rator ' under Generator. Individual ac circuits can be adjusted by means of induction regulators, which are essentially transformers, the primaries of which are connected across the main cir- cuit, while the secondaries are in series with the feeders to be regulated. See ' Induction Potential Eegulators' under Regulator, Potential. Transformer pressures can be regulated by means of tappings taken from the windings and connected to multicontact switches. The pressure of a battery of accumulators is regulated by altering the number of cells in circuit through the agency of a multicon- tact switch, or by having a booster in series with the battery which either adds to or sub- tracts from the pressure as required. The pressure of a dynamo may be auto- matically regulated by the introduction of a suitable resistance into the field circuit as re- quired. See ' Tirrill Eegulator ' and ' Thury Regulator ' under Regulator, Potential. The special means of regulation adopted in the case of the old Brush and Thom- son-Houston constant-current h pr arc-light generators are referred to under Automatic Regulation of Voltage. See Regulator, Potential; Regulation. [f. w.] Potential Regulator. See Regulator, Potential. Potential Transformer. See Trans- former, Instrument. Potentiometer, an instrument for com- paring potentials by balancing them against the potential fall in a known fraction of a series of resistances carrying a uniform cur- rent. The essential parts are — (a) A resistance divided into known equal sections with a known pd at its terminals. (b) Sliding keys on switches capable of tapping off any fraction of the known pd. (c) A sensitive galvanometer to indicate when the potential to be measured is equal to the fractional pd tapped off the wire. The most notable types are described in the following paragraphs. 1. The Crompton Potentiometer. — ^In this instrument the resistance consists of four- teen coils, each of 10 ohms, in series with a straight wire, also 10 ohms resistance, thus forming a system of fifteen equal steps. Across the whole a pd of 1'5 volt is applied from a secondary cell, thus providing a pd of j\y volt per step. Any fraction is then tapped off by means of a radial switch on the resistance coils and a sliding contact on the wire. The standardisation is performed by adjusting a resistance in series with the whole until the standard cell employed indicates, by means of the galvanometer G, a balance at the Fig. 1.— Crompton Potentiometer point which represents its emf on the basis given above (see fig. 1). 2. The Nalder Potentiometer. — In this instrument the slide wire is dispensed with. There are a great number of stops with re- sistance steps between, and a small dial con- taining a few stops arranged to give decimals of the larger fractions. 3. The Leeds and Northrup Potentio- meter. — The special feature of this instru- ment consists of an arrangement of shunt and series resistances for reducing the poten- tial drop across the whole to a small known fraction of the original. This has the advan- tage of greatly increasing the accuracy when measuring small potentials. vwvw- Fig. 2.— Potentiometer as Amperemeter FF, Leads to potentiometer. Potentiometer as Amperemeter. — Pass the current to be measured through a resist- ance of known value, and take from its ter- minals a pair of leads to the potentiometer 412 Potentiometer Ratio Resistance — Power to measure the pd across it. For the maxi- mum accuracy, the pd produced by the cur- rent to be measured should be equal to the maximum reading of the potentiometer in volts. The resistance will then be, by Ohm's E law, E = _, where E = maximum reading of potentiometer, and I = current to be mea- sured (see fig. 2). Potentiometer as Voltmeter. — The potentiometer may be used directly as a voltmeter up to the total pd across it (usu- ally 1"5 to 3 volts). Above this a ratio re- sistance must be used (see Potentiometer Ratio Resistance). It is essential for this purpose that the potentiometer be first accu- rately adjusted to read directly in volts by means of a standard cell. Potentiometer Used to Compare Re- sistances. — Pass a suitable current through both resistances in series, and simultaneously measure the volt drop on each. The resist- ances will then be proportional to their re- spective readings. It is unnecessary for this purpose to accurately adjust the potentio- meter against a standard cell (see also Bridges). Potentiometer Used to Measure Power. — Simultaneous readings are taken of current and pd in a circuit by the me- thods given under 'Potentiometer as Am- pere Meter' and 'Potentiometer as Volt- meter'. The change is quickly effected by a multiple-contact switch, and care should be taken when making measurements on circuits at over 50 volts pressure, that the current and potential connections are both brought from the same side of the circuit, [l. m.] Potentiometer Ratio Resistance. — For the measurement of potentials greater than the maximum range of the potentio- meter, the potential to be measured is con- nected across the ends of a high resistance (about 100 ohms per volt) and a tapping taken at some known fraction (usually ^ or ihs)- "^^^ P'^ across the fraction is then measured directly oh the potentiometer, and, since no current flows through the galvano- meter circuit, this will bear the same relation to the potential required as the fractional re- sistance bears to the whole. When enclosed in a suitable box with terminals, the arrange- ment is often spoken of as a volt box. See Potentiometer. Potentiometep Volt Box. See Po- tentiometer Ratio Resistance. Poulsen System of Wireless Tele- graphy and Telephony. See Wireless Telegraphy and Telephony. Poulsen Telegraphone. See Tele- graphone. Power, Altemating-eurrent, denotes the power carried by an ac. See also Power, Electric. Power, Apparent. See Apparent Watts. Power, Electric.— The power of an electric current is the rate at which work is being done by the current. The power expended by a current of I amp flowing at V volts is equal to IV watts. In cc circuits, where the current has a steady value, the power flows outwards from the generator to the circuit, the emf and the current being always in the same direction. But with ac the effect of the reactance of the circuit is to cause the current at one part of the cycle to flow in opposition to the generator emf, and hence to return power to the generator. In other words, there is an oscillation of energy between the receiving circuit and the generator. To obtain the power in a cc cir- cuit it is sufficient to take the product of volts and ampj but with an inductive or conden- sive circuit carrying an ac, the product of current and emf rflust be multiplied by the pf which, with sine waves, is equal to the cosine of the angle of lag of the current. To measure the power directly a wattmeter should be used. See Wattmeter. [E. c] Power, Luminous Absorption.— With modern metallic-filament lamps, the light intensity is so great that it is desirable to modify it by shades of considerable luminous absorption power. The following are amongst the materials employed for the purpose: Alabaster, which absorbs some 20 per cent; various grades of ground glass, opal glass, and milky glass, which absorb from 20 to 60 per cent; and porcelain, which absorbs from 40 to 80 per cent. See Globe. Power, Methods of Measuring, in Polyphase Circuits.— Whereas the elec- trical measurements associated with the de- termination of power in cc and in sp ac circuits are comparatively simple, certain less evident principles are involved in the equivalent determinations in polyphase cir- cuits. Two-wattmeter Method of Measuring Power. — The power absorbed in a three- Power — Power Absorption 413 phase three-wire star- or delta -connected system, whatever the distribution of the load, may be measured by means of two watt- meters Pj and Pg connected as shown in the accompanying diagram. When the three- phase system is balanced, denoting the pd between any two of the supply mains by V, Two-wattmeter Method of MeaBuring Power the current flowing in a supply main by c (both rms values) and the angle of lag by <}>, then the mean power is P = Vc cos { - 30°) -f Vc cos (^ + 30°) = ^3 V c cos . As the system is balanced, ^, the angle of lag, is the same for each branch. The one wattmeter, Pj, measures Vc cos (<^ — 30°) and the other, Pj, measures Vc cos (), the result obtained will be the true w. Cos 4> is therefore equal to the pf. Assuming the diagram to show the full load conditions of the transformer, the angle of lag being 14° 2', the pf at full load is 0-97. With no external load on the transformer, the load component of the current is that necessary to make up the core losses. Taking this merely for purposes of illustration at 5 amp, while the magnetising current remains as before at 25 amp, the angle of lag becomes 78° 41' and the pf 0-196. Thus it will be seen that in ac transfor- mers, induction motors, &c., the pf is a function of the load. [Paragra,ph 54 of the 1907 Standardisation Rules of the A.I.E.E. states that 'the power factor in ao cir- cuits or apparatus is the ratio of the electric power in w to the apparent power in volt-amperes'. It may be expressed as follows : — True power _ watts Apparent power volt amperes _ energy current _ energy voltage "I total current total voltage "J See also Eeactive Factor; Angle of Lag; Power-factor Indicator — Pressure 415 Angle oi Lead; Lag; Phase; Apparent Watts. [c. w. h.] Power-faetop Indicator. See Indica- tor, Phase or Power-factor; Electro- goniometer. Power-faetoP Metep. See Indicator, Phase; Electrogoniometer. PoweP House, a building in which power is generated for any purpose, but the term is more particularly applied to a central station in which electricity is generated. See also Central Station for the Gener- ation OF Electricity. PoweP Houses, defined by the V.D.E. (which see) as premises which contain other than electrical machinery, as distinguished from electric power-rooms (which see), and which are not regularly accessible to un- authorised persons. (Eef. Journ.I.KE., vol. xli, p. 167.) Powep Load. See Central Station FOR THE Generation of Electricity. Power Station. See Power House. Powep Supply, Industpial, the supply of electric energy from a central station over a district for the purpose of driving machinery in many factories in that district, thus re- placing wasteful small steam and gas engines by clean and economical electric motors. The system also makes it practicable for small industries to be carried on in the workers' own homes. A notable example of this is at Lyons, France, where current from a central station is supplied to the ribbon- weavers, who, working in their own homes, use a J hp electric motor for each of their little ribbon looms. See also Central Sta- tion FOR the Generation of Electricity. Ppaetieal Eleetpieal Units. See Unit, Practical. Ppeece and Tpotter Photometer. See Photometer, Illumination. Ppepapation Costs. See Contract Demand System; Indicator, Maximum- demand; Tariff Systems; Merz Scale OF Charges for Electricity; Assess- ment Tariff. Prepayment Metep. See Meter, Pre- payment. Ppepayment System. See Tariff Systems. PpeSSboaPd, sometimes termed pressed howrd, a fibrous material closely resembling press-spahn. It is, however, not so tough as press-spSihn and has not such a highly glazed surface, but it is more flexible. See Paper for Insulating Purposes; Im- pregnated Insulating Materials; Horn Fibre; Press-spahn; Leatheroid; Fuller- board. Pressed Board. See Pressboard. Pressed - ribbon Resistance. See Eheostats or Resistances. Pressed Stranded Cable. See Cable, Pressed Stranded. Pressing Armature Coils.— After the insulating materials have been applied to an armature coil, it is usually found that it has not been possible to make them lie ab- solutely close down on the conductor, and the coil has not its proper shape. This must be restored, if valuable room is not to be wasted in the slot. The best way to do this is to press the coils in a hot vice for a few minutes, and then to allow them to cool off in a cold press. In this way they can be made to retain their shape after removal from the press. See Hot Vise; Winding, Forming, and Spreading Ma- chinery; Coil, Form-wound. Press-spahn, sometimes termed fvller- hoard, a homogeneous fibrous material, with a smooth glossy surface, supplied in thick- nesses ranging from 0-004 to 0-5 in (O'l to 12 mm). In the thinner grades it is tough, though pliable. It does not become brittle with heat, and is used for slot linings. In the greater thicknesses it is very hard and tough, but contains a large percentage of moisture. If this be thoroughly dried out, and the material impregnated with linseed oil, it forms an excellent material for washers, and for purposes where a hard, light, durable insulation is required. See Fullerboard; Paper for Insulating Purposes; Im- pregnated Insulating Materials. Press-spahn Mica, press-spahn with one or two layers of overlapping laminae held in position by a layer of thin Japanese paper, and suitable adhesive material. See Mi- CANITE; Micabta; Pertinax; Japanese Paper; Mica-sticking Varnishes. Pressure. — 1. The pressure in volts (or the . voltage) at the terminals of an elec- trical supply system. According to the Board of Trade Eegulations under the Electric Lighting Acts of 1882 and 1888, it is laid down that (a) where the condi- tions of the supply are such that the pres- sure at the terminals where the electricity is used cannot exceed 250 volts, the supply shall be deemed a low-pressure supply. (J) 416 Pressure — Primary Ampere Turns Where the conditions of supply are such that the pressure at the terminals where the electricity is used, between any two conductors, or between one conductor and earth, may at any time exceed 250 volts, but cannot exceed 650 volts, the supply shall be deemed a medium-pressmre supply, (c) Where the conditions of supply are such that the pressure at the terminals where the electricity is used, between any two conductors, or between one conductor and earth, may at any time exceed 650 volts but cannot exceed 3000 volts, the supply shall be deemed a high-pressure supply, (d) Where the conditions of supply are such that the pressure at the terminals where the electri- city is used, between any two conductors, or between one conductor and earth, may at any time exceed 3000 volts, the supply shall be deemed an extra high-pressure supply. The definitions accompanying the 1908 Home Office Eegulations for Electricity in Factories and Workshops read as fol- lows : — 'Pressure means the difference of electrical po- tential between any two conductors, or between a conductor and earth, as read by a hot-wire or electro- static voltmeter. ' Low pressure means a pressure in a system nor- mally not exceeding 250 volts where the electrical energy is used. 'Medium pressure means a pressure in a system normally above 250 volts, but not exceeding 650 volts, where the electrical energy is used. ' High pressure means a pressure in a system nor- mally above 650 volts, but not exceeding 3000 volts, where the electrical energy is used or supplied. ' Extra high pressure means a pressure in a system normally exceeding 3000 volts where the electrical energy is used or supplied.' 2. The force of a body acting on another, tending to produce compression. Pressure, Admission, the pressure at which steam is admitted to a steam-electric generating set. When the prime mover is a piston engine the economic pressures are usually well above 10 metric atm (see Met- ric Atmosphere); but for steam-turbine sets, although this range of admission pres- sures is still customary, the best overall economy, considering not only operating but capital charges, would in most instances correspond to distinctly lower admission pressures. See Steam Turbine; Pres- sure, Exhaust. (Eef. 'Steam Turbine Engineering', Stevens and Hobart.) Pressure, Applied. See Electromotive Force, Impressed. Pressure, Disruptive. See Disruptive Voltage; Electric Breaking Strength; Dielectric Strength. Pressure, Electric, a term frequently used to denote electromotive force (emf) and difference of potential (pd). Pressure, Exhaust, the pressure of the steam when it leaves a steam-electric gene- rating set and enters the condenser. For piston-engine plant, the best overall economy, when taking into account not only operating but capital charges, is probably in most cases from 0'15 to 0'20 metric atm (which see). For steam-turbine plant the most economical range will usually be from 0*10 to 0'15 metric atm. The tendency towards designing plant for a still lower exhaust pressure is usually a mistaken one, since the cost for piping, con- densing plant, pumps, and cooling towers, and the outlay for circulating water, all rapidly rise with decreasing exhaust pres- sure. See Steam Turbine; Condenser, Steam; Pressure, Admission. Pressure, Intensity of, in a Dielec- tric, the stress of an emf on a dielectric. The stress is affected by the area and shape of the electrodes, being increased by a needle point or sharp edge; it is probable also that in the case of alternating emf the frequency and wave form also affect the stress on a di- electric. To ensure uniform stress, the elec- trodes should have curved edges, and be larger in area than the thickness of dielec- tric. See also Tension, Electric. Pressure, Puncture, the pressure in volts at which a dielectric is punctured. See Dielectric Strength; Electric Break- ing Strength; Disruptive Voltage. Pressure, Rated. See Eated Voltage or Pressure. Pressure Drop. See Fall of Potential. Pressure Indicator. See Voltmeter. Pressure Recorder. See Eecording Instrument. Pressure Regulation of an Alter- nator. See Alternator, Pressure Ee- GULATION OE AN; EeGULATION. Pressure Rise. See Alternator, Pres- sure Eegulation of an. Pressure Transformer. See Trans- former, Instrument. Preventive Coil. See Coil, Kicking. Preventive Resistance Leads. See Leads, Preventive Eesistance. Primary Ampere Turns. See Ampere Turns, Primary. Primary Battery — Protective Device 417 Primapy Battery. See Battery, Pri- mary j Cell, Standard. PrimaFy Current, the current flowing in the primary winding of a transformer, an induction coil, or ac motor. See also Cur- rent, Secondary; Coil, Induction; Coil, Spark. Primapy emf, the emf applied to the terminals of the primary winding of a trans- former or ac motor. Primary Impedance, the impedance of the primary winding of a transformer or ac motor. See Impedance. Primary Voltage. See Primary emf. Prime Conductor. See Conductor, Prime. Prime Movers, Variation and Pulsa- tion in. See Variation in Prime Movers; Pulsation in Prime Movers; Cyclic Irre- gularity; Irregularity Factor; Crank- EiTORT Diagram; Torque Diagram of AN Engine; Pulsation in Alternators; Variation in Alternators. Principle of tlie Conservation of Energy. See Energy. Prismatic Reflector. See under Ee- flector. Prony Brake for Testing Electric Motors, probably one of the earliest forms 1 .— Frony Brake of friction dynamometers. It consists of a long lever a (see fig.), provided with a fixed wooden jaw J and flexible band of metal B at one end, between which the pulley of the machine under test may be clamped, and the friction varied by altering the tension on the flexible band by means of a screw and hand wheel w attached to it. At a definite radius from the axis of rotation a metal stud or point s is fixed in the long lever, which is arranged to press on the platform of a weigh- ing machine, and the pressure on this point can thus be balanced. If L be the distance in meters between the axis of rotation and the centre of the stud s, and if P be the pres- sure on the platform of the weighing ma- chine in kg, then the torque is PL = /r, where / is the frictional force between the surfaces of brake and pulley, and r is the radius of the pulley. If the motor makes n rps, then the power in kg m per sec is PL n = — 2irrn, T where P is in kg and L and r are in m; and the hp is therefore , 2n-7!,PL Since the entire energy delivered from the motor is converted into heat at the friction surfaces of the brake, special precautions have to be taken to carry this heat away as fast as it is generated; for this purpose special en- closed pulleys are often used, provided with water supply to their inner surfaces for cool- ing. Many variations on the above-described design have been employed. See Dynamo- meter, Absorption; Band Brake for Testing Electric Motors; Power Ab- sorption, [c. V. D.] Proof-plane or Sphere, a small insu- lated conducting disk or sphere used to test the charge of larger conductors by carrying samples to an electrometer. See Electro- meter. Propulsion, Electric. See Electric Propulsion; Electrification of Kail- ways ; Petrol-electric Automobile Sys- tem; Electromobile; Accumulator Car,; Cab, Electric. Protected Rail Bond. See Bond; Bonding Rail. Protection Cap. See Cap, Protection. Protective Device, Cardew, consists of two metal plates held in a horizontal po- sition, with an air space of about 6 mm be- tween them. A thin strip of aluminium foil is flexibly attached at one end to the upper plate, which is connected to the secondary circuit. The other end of the aluminium strip is supported near to, but normally not in contact with, the lower plate, which is earthed. If the pressure between the two plates rises above normal, the aluminium strip is attracted to and makes contact with 418 Protective System — Punga Equalising Connections the lower plate, thus earthing the secondary circuit. The aluminium strip remains in con- tact with the lower plate until the fuses are blown, the device thus being quite automatic. See Partridge Safety Device. PFoteetive System, Mepz-Price Auto- matic Balanced. See Merz-Price Auto- matic Balanced Protective System. Provisional Order. — The expression means a Provisional Order under the Elec- tric Lighting Acts (which see). (Electric Lighting Act, 1909, clause 25.) Pt, the chemical symbol for platinmn (which see). Public Supply.— [' PvMie supply means the supply of electrical energy (a) by any local authority, company, or person author- ised by Act of Pariiament or Provisional Order con- firmed by Parliament or by licence or Order of the Board of Trade to give a supply of electrical energy ; or (5) otherwise under Board of Trade regulations.' — From definitions accompanying Home OfSce, 1908, Regulations for Electricity in Factories and Work- shops.'] PuU-ofF, a fitting used on overhead trolley lines, consisting essentially of a bolt to take a trolley -wire ear, the insulation surrounding the bolt, and a cover with an arm to which a span wire can be attached. PuU-oflFs are used to support the trolley wires at curves in the line, and they are also often used to carry the ears when a span-wire construction is used, both when the span wire extends across the road and when a short length is used to give a flexible suspension from a bracket arm. See Bracket Arm; Bracket-arm Hanger; Insulated Hanger. (Eef . ' Modern Electric Practice ', vol. iv; ' Electrical Traction ', Wilson and Lydall.) Pull -rope System of Electric Lifts. See Lift, Electric. Pulsating" Continuous Current. See Current, Rectified; Current, Pulsat- ing; Rectifier. Pulsating" Current. See Current, Pulsating; Rectifier. Pulsating" Field. See Field, Alter- nating; Shifting Magnetic Field. Pulsating Stator Flux. See Flux, Pulsating Stator. Pulsation. See Reactance. Pulsation in Alternators.- In para- graph 62 of the 1907 Standardisation Rules of the A.I.E.E. the pulsation in alternators or ac circuits in general, is defined as the ratio of the difierence between maximum and minimum frequency during an engine cycle, to the average frequency. If n = number of pairs of poles, the variation of an alter- nator is n times the variation of its prime mover, if direct connected, and n/p times the variation of the prime mover if rigidly connected thereto in the velocity ratio p. See also Pulsation in Prime Movers; Variation in Alternators; Variation IN Prime Movers. Pulsation in Prime Movers.— Para- graph 60 of the 1907 Standardisation Rules of the A.I.E.E. defines the pulsation in prime movers as the ratio of the difference between the maximum and minimum velocities in an engine cycle, to the average velocity. See also Variation in Prime Movers; Pul- sation IN Alternators; Variation in Alternators; Cyclic Irregularity; Ir- regularity Factor; Crank-effort Dia- gram; Torque Diagram of an Engine; Flywheel Storage. Pump, Rees-Roturbo. See Rees-Ro- turbo Pump. Pumping, Electric See Mining Equip- ment, Electrical. PunchingS, a. name frequently applied to laminations. See Laminations, Arma- ture; Core Disks; Staggered Punchings; Armature Stampings. Puncture Pressure: Puncturing of In- sulation. See Pressure, Puncture; Dis- ruptive Voltage; Dielectric Strength; Insulation Breakdown; Electric Break- ing Strength. Funga Equalising Connections Punga Equalising Connections. — These connections are employed for the pur- pose of maintaining a uniform gradation of the potential over the segments between Pupillary Diameter — Pyrometer 419 brushes, where multiplex windings are em- ployed. The plan as applied to a duplex winding is illustrated in the fig. Were the equalising connections not employed, while there would be a definite progression of potential from segments 1 and 2 to segments 3 and 4, and from segments 3 and 4 to seg- ments 5 and 6, the use of the Punga connec- tions ensures a progressive growth of poten- tial between every two adjacent segments, and thus ensures that the maximum difference of potential between any two segments shall be as small as practicable. The invention is covered by Punga's British Patent No. 1 0861 of 1907. It was laid before various British manufacturers in 1907, but has not yet even been tried by any of them, although it has been quite widely used, and with excellent results, by Continental manufacturers. Pupillary Diameter, the diameter of the aperture controlled by the iris muscles in the eye. When a very brilliant light falls on the eye the aperture is partially closed, and thus serves to reduce, to its normal value, the illumination actually reach- ing the eyes. On the other hand, the aper- ture increases in diameter when the illumi- nation is very weak. The diameter of the pupil aperture is thus connected with the intensity of illumination of the rays striking the eye, and photometers have been con- structed utilising this principle. See Photo- meter; Photometry. Pupkiiye Effect, a physiological eflFect which occurs when lights differing in colour are compared photometrically. Purkinje found that the law of inverse squares does not hold over a wide range of illumination. For a given increase in stimulus, our sen- sation of light seems to increase more rapidly for the red end of the spectrum than for the blue end. Hence, while at strong illumina- tions a red surface may appear brighter than a green or blue one, the opposite may be true when the surfaces are very feebly illu- minated. These discrepancies do not occur to any serious extent at the illuminations and with the colours ordinarily met with in practice. According to the more recent theories of physiological optics, the eflfect is due to the peculiar action of two varieties of light-per- cipient organs, distributed over the retina. See Photometer; Photometry. (Ref.Lum- mer, Deutsch. Phys. Gesell., Verb. 6, 2, pp. 62-76.) Push Button, a small knob of insulating material which, when pressed, completes the circuit of an electric bell or other indicator, by pushing a spring-contact against a fixed one. Push-button System of Electric Lifts. See Lift, Electric Pyr. — A pyr is defined at p. 33 of Solomon's 'Electric Lamps' as a luminous intensity equal to ^ of the Violle plati- num standard. It is equal, very nearly, to 1 Hefner candle or 0-915 English candle. The unit was proposed in 1896 by the Geneva Congress. See Standard of Light. Pyro-electricity. — When certain crys- tals are heated, the ends of their axes become oppositely electrified. The pheno- menon has probably an intimate relation to the equally obscure subjects of contact and thermo-electricity, and, like these, is clearly of great importance in molecular theory. See Thermo-electricity; Element, Ther- mo-electric; Current, Thermo-electric; Generator, Thermo-magnetic; Genera- tor, Thermo-chemical Electric. Pyromagnetism. See Generator, Thermo-magnetic ; Thermo-electricity. Pyrometer, Electric, an instrument for indicating temperature, also spoken of as an electric thermometer or a tele-thermometer. Pyro- meters are of three classes — 1. Resistance Pyrometer, in which a wire, usually of platinum, is exposed to the temperature to be determined. Its resistance before and after heating is measured with a Wheatstone-bridge, a differential galvano- meter or a potentiometer, and from the in- crease in resistance the temperature is cal- culated. 2. Thermo-electric Pyrometer (often called after its originator the Le Chatelier pyrometer), in which the temperature is de- duced from the thermo-emf produced at the heated junction of a thermo-couple. For low-temperature measurements (say up to 500° C.) a copper and constantan couple is used; above this, platinum and an alloy of platinum and iridium is more durable, and can be used up to 1500° C. The thermo- emf being small is best measured on a po- tentiometer, but a sensitive high-resistance galvanometer is often used. Both patterns of pyrometer must be carefully protected from mechanical damage and from the action of corrosive gases, a porcelain tube being generally used for this purpose. 420 Q.E.S. System of Units — Quadruplex Telegraphy 3. Optical Pyrometer, based on Wiens law that for all 'black bodies' {i.e. those which absorb all wave lengths, e.g. soot and also any nearly closed cavity in a heated body) a definite relation exists between colour and temperature. A simple pyro- meter based on this law consists of a sighting tube through which the heated body is ob- served, and across it is stretched a glow- lamp filament capable of being raised to various degrees of incandescence, by means of a current passed through it. In use, the current is varied until the colour of the fila- ment appears identical with that of the glow- ing body behind it, the temperature being then deduced from the ammeter reading. 4. Radiation Pyrometer, based on the Stefan-Boltzmann law that the heat radiated from a glowing 'black body' (see Black Body) is proportional to the fourth power of its absolute temperature. The fig. shows the arrangement in the case of the Firy pyro- i >c In the sighting tube A is a parabolic mirror B which reflects the heat - received from the glowing body on to a constantan- _ copper thermo- couple ate. This couple, which is protected from the direct rays by a small shield, is connected to the terminals of a sensitive galvano- meter E which can be graduated direct in degrees. It can be shown that, so long as the image of the glowing body, thrown by the mirror B, is large enough to completely cover the thermo-junction, the reading will be independent of the distance of the junc- tion from the heated body. [k. e.] yj Kry Pyrometer Q Q.E.S. System of Units. See Units, Q.E.S. System. Quadrant. — 1. An arc of a circle sub- tending a right angle at the centre. 2. A name formerly used for the practical unit of inductance, which is 10' cm or an earth quadrant; synonymous with henry (which see). The unit of length in the metric system is the meter, which is ap- proximately the one ten-millionth part of an earth's quadrant. See Units, Practical; Henry; Units, Q.E.S. System op; Induc- tance; Induction, Self-; Meter. Quadrant Electrometer. See Elec- trometer. Quadrantal Deviation. See Devia- tion. Quadrature, a convenient term denoting a phase difiference of 90° between two alter- nating quantities. The two quantities are said to be ' in quadrature '. The illustration represents a pressure curve A and a current curve B for zero pf. In this case the current is said to be 'in quadrature' with the pressure (and vice versa), since the current is lagging 90° behind the pressure; that is to say, the current reaches its maximum value 90° or a quarter of a period later than the pressure. See Mechanical (ob Space) Quadrature; Time Quadrature; Polyphase System; Two-phase System; Alternating Cur- Qnadratnre rent System; Quarter - phase; Phase; Lag; Angle of Lag; Angle of Lead. Quadrature, Mechanical. See Me- chanical Quadrature. Quadrature, Time. See Time Quad- rature. Quadripolar Field. See Field, Quad- ripolar. Quadruplex Circuit. See Telegraphy, Quadruplex. Quadruplex Telegraphy. See Tele- graphy, Quadruplex. Quantity — Radiating Surface of a Conductor 421 Quantity, Electrostatic, quantity of electricity as measured on the electrostatic system. The unit quantity is that which, when placed at unit distance (1 cm) from an equal quantity, repels it with unit force (1 dyne). Quantity Meter. See Meter, Quan- tity. Quantity of Electricity. See Elec- tricity; Coulomb. Quantometer, a moving-coil instrument without spring control. The movement of the needle depends upon the flux change in a coil of wire connected to the moving coil. The instrument is generally employed for the purpose of determining the value of a magnetic field. See Grassot Fluxmeter; 'Ballistic Galvanometer' under Galvano- meter; Tester, Bismuth Spiral Density. Quarter of a Period denotes one quar- ter of the time taken by an ac or emf to ~\ ^ ■\ / \ y \ / \ \o ( I ; / \ x TO f \ / \ / S. ^ 1 A Complete Period pass through one complete cycle of values. The time of one cycle is called a period. The illustration shows a complete period, and represents a sine rate of variation with time or with the angle of displace- ment. It is the time taken to complete 360 electrical or magnetic degrees. A quarter of a period therefore consists of 90 electrical or magnetic degrees as represented by means of the dotted lines in the fig., to 90 being one quarter, 90 to 180 another quarter, and so on. See 'Cycle of Alternation' under Alternating Current; Period. Quarter-phase. — This term is often em- ployed instead of 'two-phase'. See Two- phase System; Alternating Current. Quarter-phase Armature. See Arma- ture. Quartz -fibre Suspension. — For sus- pending the moving parts of galvanometers, magnetometers, &e., where a torsionless sup- port is required, a single fibre of quartz crystal is employed. These fibres may be had with a thickness of under O'OOS mm. Quartz Lamp. See Lamp, Tubular. Quick Air-drying Varnish. See Core- plate Varnishes; Quick -drying Var- nish. Quick-baking- Varnish. See Core- plate Varnishes; Quick-drying Var- nish; Insulating Varnishes; Impregnat- ing Varnishes. Quick-break Fuse. See Fuse. Quick -break Switch. See Switch, Quick-break. Quick -drying" Varnish. — The term quick -drying is usually applied to baking varnishes, which, by reason of the addition of suitable driers, require a shorter time to dry, and in consequence are not so tough or flexible as the ordinary insulating varnishes. The term is, however, often applied to air- drying varnishes employed for repair work. See Core-plate Vaknishes; Insulating Varnishes; Impregnating Varnishes. E Racing of Motor. See Starting of Motors. Radial Half-turn Loop. See Loop of Armature Coil. Radial Truck. See Truck. Radiant Centre. — The inverse -square law in photometry is only strictly applicable to the theoretical 'point-source' of light, having no dimensions. In some cases it is permissible, in calculation, to regard the total light from an extensive source as con- centrated at a certain point, and this point Vol. II is termed the radiant centre. Thus Lieben- thal (E.T.Z., 1895, p. 657) found the radiant centre of the 1 cp pentane lamp to be midway between the axis of symmetry of the flame and the outer edge. See Photometry; Standard of Light; Law of Inverse Squares. Radiant Efficiency. See Efficiency, Eadiant. Radiating Surface of a Conductor, that portion of the surface of a conductor from which the heat generated in the con- 28 422 Radiating Surface of Armature — Radiation ductor can be radiated into the surrounding atmosphere. Cooling swrface would be the more accurate term, for reasons identical with those set forth in the following defini- tion (which see). See Heating Effect of Currents; Heat - emissivity of Various Materials or Surfaces. Radiating Surface of Armature or of Magnet. — Radiating surface is often used as synonymous with cooling surface. In dy- namo-electric machines, however, the greater proportion of the heat is got rid of by con- vection and conduction; very little is radi- ated; cooling swrface, is therefore the more accurate term. See CoOLiNG SURFACE; Temperature Eise of Electric Machines. Radiation, the transfer of energy which may take place through the luminiferous ether. Electric Eadiation. — Clerk - Maxwell showed (' Electricity and Magnetism ', vol. ii) that alternating electrical stresses might be propagated freely through a dielectric, and that the wave motion thus produced should be of the same nature as light. The exis- tence of such radiation and the means of producing it were discovered by Hertz (1887). These radiations were used as a means of conveying signals, by Lodge, Marconi, and Rutherford about 1896, and developed into a practical telegraphic system by Marconi in 1898. Hughes, in 1879, had observed similar phenomena, but did not publish his results. The radiations used in wireless tele- graphy and telephony are not usually ' free ', but are attached to a conductor (the earth), and correspond therefore rather to the phe- nomena observed by Yon Bezold in 1870 than to Hertzian waves. Temperature Radiation. — When the temperature of a body is above that of the surrounding medium, energy is radiated into space; and such radiation, being derived solely from the heated conditio!} of the body, is termed temperature-radiation. "When the temperature of the heated body is so high that the energy radiated is in a luminous form, the body is said to be incandescent, and the phenomenon is termed incandescence. The light yielded by the great majority of artificial illuminants, e.g. the candle, the gas flame, and the glow lamp is obtained by this process. Range of Visible Radiation, the por- tion of the spectrum which is visible to the eye as light extends from about A, = 04 /* (violet) to A. = 0'8^ (deep-red). This is therefore known as the range of visible radia- tion. Under certain conditions the eye is said to be able to perceive vibrations outside this range, but the above represents the ex- treme limits under normal conditions. Selective Radiation. — Lummer and Prinssheim (Deutsch. Phys. Gesell. Verb. 1899), Wien, and others have studied the law connecting the nature of the radiation of a black body with its temperature. If a body is not truly black it does not follow this law exactly, but shows a prefer- ence in favour of certain frequencies of radia- tion; under these circumstances the body is said to exert selective radiation. (The term, however, has also been used to describe the radiation from luminescent gases, &c., which commonly yield a discontinuous spectrum instead of the continuous one char- acteristic of an incandescent solid, and also to denote any radiation which is not of ther- mal origin. As this is apt to occasion some confusion, it is preferable to confine the meaning of selective radiation to that pre- viously mentioned.) Luminescence is the term applied to de- note any process other than temperature- radiation (see above) which results in the production of light. For instance, the mercury-vapour lamp is believed to be an example of luminescence, because the light yielded by it is not a direct consequence of its temperature, but seems to be obtained by the direct transformation of electrical energy into light. To this class of radiation belong 'fluores- cence ' and ' phosphorescence ' (see below). Phosphorescence. — This term is fre- quently used to denote effects which are preferably included under the term fhwres- cence (which see). Strictly, however, the term phosphorescence applies to substances which continue to ap- pear luminous after the stimulus is with- drawn. Fluorescence. — This term is applied to a special case of absorption and re-emission of radiant energy. It may thus be defined as the action of those substances which have the power of degrading the radiant energy im- pinging upon them into energy of a smaller wave length. When radiant energy falls on a black sur- face it is largely absorbed, the process being indicated by a rise in temperature and by Radiation — Railway Motor 423 emission of radiant energy in the form of heat of longer wave length than the initial incident radiation. If the radiation, however, be not 'degraded' to so great an extent as to be- come long heat waves, but remains still vis- ible, the phenomenon is termed fluorescence. Some substances, such as quinine and fluores- cine, will absorb invisible radiation in the ultra-violet, and will, in consequence, emit radiation, some of which is within the limits of the visible spectrum. The phenomenon is closely related to anomalous dispersion. It has been proposed to utilise the fluores- cent qualities of certain substances in order to improve the spectrum of the light yielded by mercury lamps, which is rich in ultra- violet. Radiation, Kirehhoff's Laws of. See Kirchhoff's Laws. Radiation Coefficient. See Heating Coefficient. Radiation Pyrometep. See Pyro- meter, Electric. Radiometer, Electric, a glass bulb con- taining small mica vanes mounted on a ver- tical axis. One side of each vane is blackened. When light falls upon the vanes, the in- equality in the absorptive properties of the two sides of each vane occasions a difference of pressure on it, tending to turn it round the vertical axis, which it does, if the friction of the bearings is not too great. The instru- ment is the invention of Sir William Crookes. See also Ceookes' Tube. Radiotelegraphy. See Wireless Tele- graphy. Radius of Gyration.— If I be the mo- ment of inertia of a rotating body about any axis, and m be the total mass, . /— is the 'Mm radius of gyration. Thus, if the whole mass were concentrated at the extremity of the radius of gyration, the moment of inertia would be the same as that of the rotating body. The radius of gyration of a cross-sectional area is similarly defined j thus, if the whole area be multiplied by the radius of gyration, the result is equal to the moment of inertia of the cross section about the axis in ques- tion, [m. b. f.] Rail. For particular types of rail see De- meebe Kail; Tramway Eail; Vignoles Eail. Rail Bond. See Bond; Bonding Rail. Rail Corrugation, the production of shallow waves upon the heads of the rails of a railway or tramway by the wheels of the cars or trains which run upon them. The cause of the phenomenon is unknown; it is very prevalent on electric railways and tram- ways, under most varied conditions, and steam railways are by no means immune from the disease. (Eef. Journ.I.E.K, vol. xxxix, p. 3. Elec, vol. lix, pp. 798, 979; vol. Ixi, pp. 563, 599, 995. Elec. Ely. Journ., vol. xxviii, p. 1180; vol. xxxiv, p. 317; vol. xxxv, p. 438. Tram, and Ely. World, vol. xix, p. 558; vol. XX, p. 44; vol. xxi, pp. 104, 394. Engineer- ing, vol. Ixxxiii, p. 763. Elec. Engr., vol. xxxvi, p. 595. Elec. Times, vol. xxviii, p. 9.) Rail Joints. See Bond; Bonding Eail; Continuous Eail Joint; Welded Eail Joints; Eomapac Eail; Eenewable Plate for Rail Joints. Rail-resistance to Alternating- Cur- rents. See Impedance of Steel Eails; Skin Effect. Railway Controller, a device by means of which an electric locomotive or train is started or stopped, and its speed regulated. In some cases this closely resembles the tram- car controller (see Control, Series-paral- lel; Control, Eheostatic). More often a master controller (see Controller, Master) is used, which governs auxiliary apparatus for effecting the necessary changes 'of connec- tions, the master controller carrying small currents only (see Multiple-unit System). The former type is used for continuous elec- tricity only, the latter for continuous, sp and alternating electricity. A special type of controller is employed on three-phase rail- ways (see Cascade Motor). Railway Generator, a type of electric generator developed with special reference to the requirements of railway or tramway working; usually of the continuous-electricity compound -wound type, generating at from 600 to 1200 volts, and coupled directly to the engine which drives it. See Generator. Railway Motor, a type of electric motor designed specially for the purpose of electric traction on railways or tramways, usually of the continuous-electricity series-wound type, working at about 500 volts. Such motors are now built for pressures up to 1000 volts when for continuous electricity. Alternating- electricity railway motors have sometimes been used in equipments which have been worked at much higher pressures (see Motor, Commutator). In fig. 1 is shown a series- 424 Railway Motor — Ranges of Armature Windings wound railway motor for continuous elec- tricity which, on the standard 1-hr test, rates at 240 hp; and in fig. 2 is shown a sp rail- way motor which, although it is of practic- ally the same weight and of the same dimen- sions, and although it runs at a higher speed, only gives half the output for the same tem- perature rise. Sp railway motors are inhe- Fig. 1.— Series-wound ce Railway Motor rently much heavier and larger for a given rated output than are railway motors for continuous electricity. See Single-PHASE Motor. (Eef. 'Electrical Traction', Wilson and Lydall; 'Electric Trains', Hobart.) Railway Motor, One -hour Rating of. — This is sometimes spoken of as the nominal rating of a railway motor. It is de- fined in paragraph 327 of the 1907 Standard- ising Rules of the A.I.E.E. as : — 'The hp output at the car axle, that is, including gear and other transmission losses, which gives a rise of temperature above the surrounding air (referred to a room temperature of 25° C), not exceeding 90° C. at the commutator and 75° C. at any other part, after one hour's continuous run at its rated voltage (and frequency, in the case of an ac motor) on a stand, with the motor covers removed, and with natural ventilation. The rise in temperature is to be deter- mined by thermometer, but the resistance of no elec- trical circuit in the motor shall increase more than 40 per cent during the test.' This definition is accompanied by a large number of notes, which will be found at pp. Kg. 2.— Single-pliase Eailway Motor 1100 to 1102 of the Proc.A.I.KE., for July, 1907. Railway Motor, Rating- of. See Eail- way Motor, One-hour Eating of. Raising" Transformer. See Trans- former, Eaising. Rake of Poles. — Side poles which carry span wires for electric traction have to sus- tain a horizontal pull of 200 to 500 kg near their upper ends, and are therefore appreciably bent when in position with the span wires taut. To counteract this efi'ect, and for appearance' sake, the poles are planted in the ground with a rake or inclina- tion of 8 to 15 or 20 cm from the vertical at the top, in the direction away from the track, so that when pulled up by the span wires they become approximately vertical. See Line Erection. Range of Visible Radiation. See Eadiation. Ranges of Armature Windings.— End connections of suitable shapes are de- Rated Capacity — Reactance 425 signed to connect active conductors in an armature to produce desired combinations of their induced emf and currents. One-range: When only a single shape of end connection is necessary in a machine, it is termed one-range. Two-range: When two shapes are necessary so that each crosses the other, it is termed two-range. Three, &c., ranges: Similarly for 'three' or more ranges. Figs. 1, 2, and 3, reproduced by permission of the publishers from Hobart and Ellis's Kg. 3 Sections through ends of ATmatnre Coils, shoiring one- range, two-range, and three-range windings. ' Armature Construction ', relate respectively to one -range, two-range, and three -range windings. Rated Capacity. See Eating, Nor- mal; Output, Eated. Rated Output. See Output, Eated j Eating, Normal. Rated Voltage op Pressure denotes the normal voltage {i.e. pressure), at which an electrical machine or apparatus is designed to work. Rate of Change of Current denotes the rate at which the current in a circuit is increasing or diminishing. The term is used chiefly in ae work in connection with the emf of self-induction, which is equal to the circuit inductance (t) multiplied by the rate of change of current ( — j, or E = Z -3-. In a circuit whose inductance is I henrys, to which an emf of E volts is applied, the initial "F rate of change of current is equal to -j amp per sec. [r. c] Rating, Continuous, denotes the load which an electrical machine is designed to carry continuously without overheating or deterioration. Heating and sparking in the case of commutating machines set the limit of the continuous rating, while for alternators and transformers, heating and regulation are usually the governing features. Rating, Intermittent, denotes the load which an electrical machine is designed to carry for a short period, or for short periods between which there will be a period of no load or light load for cooling down. For instance, a generator that is designed to carry its full rated load continuously, may be also able to stand 50 per cent overload for half an hour without serious overheating. Rating, Normal, denotes the load which a motor, generator, or transformer is designed to carry under service conditions. Rating of Fuses. See Fuse. Rating of Railway Motor. See Eail- WAY Motor, One-hour Eating of. Ratio Arm. See ' Wheatstone's Bridge' under Bridges. Ratio Errors in Instrument Trans- former. See Transformer, Instrument. Ratio of Field Ampere Turns to Armature Ampere Turns. See Arma- ture Eeaction. Ratio of Transformation. See Trans- formation Eatio. Ratio Tests of Transformers. See Testing Transformers. Raw Linseed Oil. See Linseed Oil. Raworth Traction System. See Ee- generative Control Systems. Reactance, the component of the impe- dance which when multiplied into the cur- rent, gives the wattless component of the emf. Eeactance is usually composed of one or both of two elements, viz. inductive or mag- netic reactance, sometimes called mchietance, and capacity reactance or condensance. Both represent components in quadrature with the energy component, but whereas inductive reactance represents a lagging component, condensance represents a leading one. If I be the inductance of an apparatus, and ~ the frequency of alternation; its re- actance, X, is a; = 2 IT ~ I. If the apparatus have capacity C but no inductance, its reactance is X = 2^~C" 426 Reactance — Recorder If the apparatus have both inductance and capacity in series, the reactance is 1 27r ~ Z- 2 7r~C" Fleming denotes the quantity 2 jt ~ by ^, and calls it the pulsation of the circuit. The reactance is then The practical unit of reactance is the ohm (which see). See Farad; Henry; Capacity Eeactance. [f. w. c] Reactance, Capacity. See Eeactance; Capacity Eeactance. Reactance, Equivalent.— In electrical conducting systems related inductively to one another, the question of magnetic leakage is very complicated. It is often convenient in electrical problems to imagine that there is substituted for such a system, a single coil whose reactance is such that the circuit with such a single coil would, so far as relates to this quantity, be the equivalent of the system of circuits inductively related to one another. See also Transformer, Equiva- lent Eeactance of. Reactance Coil. See Coil, Eeactance; Coil, Choking; Coil, Kicking. Reactance per Pole. — i. In alternating machinery, the reactance of an armature winding divided by the number of poles on the machine. 2. In commutating machinery the reactance of a turn (or of turns) momentarily short-cir- cuited under a commutator brush during the process of reversal of the current flowing in it. In a multiple-circuit single winding with one turn per commutator segment, the re- actance (i2) is given by the formula: — fi = KxA.„xExFx 10-8 where K is a function of the ratio of the gross core length (A.^) to the pitch (t), and is usually about 0*4. \ = Gross core length. E = Speed in rpm. F = Total number of face conductors on the armature. (Eef. 'High-Speed Dynamo-Electric Machin- ery ', Hobart and Ellis, p. 334.) Reactance Voltagfe, the reactance of any circuit multiplied by the current flowing in the circuit. In the design of commutat- ing machinery, reactance voltage means the reactance voltage of one armature coil whilst it is undergoing commutation; and since other neighbouring coils are undergoing com- mutation at the same time, a modified value of the inductance, I, is taken, to include not only the inductance of the one coil con- sidered, but also the mutual inductance of the other coils in which the current is being simultaneously reversed. Various compli- cated methods of estimating the reactance voltage of cc machines have been proposed. Less elaborate but more useful methods are given by Mavor, Journ.LE.E., vol. xxxi, p. 221, and at p. 37 of Hobart's 'Electric Motors'. See Eeactance per Pole; Commutation. Reaction of Armature. See Arma- ture Eeaction. Reaction Type of Brush Holder. See Brush Holder. Reactive Factor. — This is defined in paragraph 55 of the 1907 Standardisation Eules of the A.I.E.E. as— ' The ratio of the wattless volt amp {i.e. the product of the wattless component of current by voltage, or wattless component of voltage by current) to the total amp. It may be expressed as follows: — wattless volt amp _ wattless current total volt amp total current _ wattless voltage total voltage Pf and reactive factor are related as follows : — If p = pf, 5 = reactive factor, then vrith sine waves of voltage and current p' + ^ = 1. With distorted waves of voltage and current p' + q^ = or < 1. See also Power Factor; Impedance Factor; Inductance Factor. Reactor, a name put forward by the A.I.E.E. for any apparatus heretofore known as a reactance coil or choking coil. See also Eegulator, Potential; Coil, Eeactance; Coil, Choking; Coil, Kicking. Reading Telescope. See Telescope, Eeading, for Electrical Measurements. Receiver, in electrical methods of com- munication of all kinds, is the apparatus which converts the electric current received from the distant station into a signal or sound perceptible to the senses. See also Telephone Eeceiver. Receiving Antenna. See Antenna. Reciprocating Motor. See Motor, Eeciprocating. Recorder. See Instrument, Eecording. Recorder, Morse. See Morse Ee- CORDER. Recording Ammeter— Rectifier 427" Recording Ammeter. See Instru- ment, Eecording. Recording Electrometer. See Elec- trometer. Recording Instrument. See Instru- ment, Eecording. Recording Voltmeter. See Instru- ment, Recording. Recording Wattmeter. See Instru- ment, Eecording. Rectified Current. See Current, Rectified; Rectifier; Gratz Method of Rectifying Current. Rectifier, a device by means of which CO may be obtained from an ac supply. In 'Kecti&cation of an Alternating Current accordance with this definition, motor gene- rators (which see), motor converters (which see), and rotary converters (which see) are rectifiers. The term has, however, not been generally applied to these types of machine, although their purpose is the same as that for which other types of rectifier are em- ployed. The classes of apparatus to which the term rectifier is often applied may be broadly subdivided into mechcmical, electrolytic, and mercwy-arc rectifiers. Not only, how- ever, have various amongst these come to be more generally known under names not readily brought within these subdivisions, but also use is sometimes made of other phenomena which are in some instances too obscure to permit of classification. The general tendency is now to designate many of these rectifiers as valves. In the follow- ing treatment the various types have been grouped partly with reference to their general characteristics, and the individual members of each group are arranged alpha- betically. Reference should also be made to a paper entitled 'Electric Valves', by Moss (Journ.I.E.E., vol. xxxix, p. 652). A slightly narrower definition may be ap- plied to the term ac rectifier, which may be stated to be applicable to any piece of ap- paratus which when supplied with ac either suppresses the negative half of the supply wave or reverses it, thus setting up a pul- sating unidirectional current. In fig. 1, a complete sine curve is shown at A. In the next diagram (b), the negative half is sup- pressed, and in the third diagram (c), the negative half is reversed. Such rectifiers may be roughly classified as follows : — (a) Mechanical, in which a synchronously rotating commutator' is employed to com- mutate the negative half of the wave (see 'Ferranti Rectifier' below). (6) Electrolytic, in which one-half of the wave is suppressed by the formation of a film which is highly resistant to current passing in one direction between the elec- trolyte and electrode, but has little resistance to current in the opposite direction (see 'Nodon Valve' and 'Cooper-Hewitt Mercury- vapour Rectifier ' below). Electrolytic rectifiers must always be grouped in such a manner as to provide an alternative path, in order that the negative half of the supply wave may be utilised. See Gratz Method of Rectify- ing Current. Ferranti Rectifier. — This piece of ap- paratus is primarily intended to provide a unidirectional current from an alternating supply for purposes of arc lighting. The complete apparatus for this purpose consists of a constant - current transformer and a synchronously - driven commutator. The transformer is provided with two movable secondary coils and a fixed primary, while the current for driving the synchronous motor is provided by an auxiliary fixed secondary coil. The movable secondaries are counterpoised so that they are nearest to their primary when fully loaded, and are repelled as the load goes ofi'. The ends of each of these coils are taken to four brushes on the commutator, which alternately joins together two of them and connects the other two to the load. Electrolytic Rectifier or Valve. — If two metals are placed in an electrolyte and then subjected to a definite diflference of 428 Rectifier potential, it will be found that they will, under certain conditions of pd, exhibit an apparently greater resistance to the passage of a current in one direction than to its passage in the reverse direction through the cell. Metals of low atomic weight exhibit this so-called ' valve effect ' at high pd, while heavier metals produce it at low differences of potential. The property has been used by Nodon in his electrolytic valve (which see), in which the cathode is of aluminium or of an aluminium alloy, the other electrode, which has a much greater surface, being the lead-containing vessel. The electrolyte is a neutral solution of ammonium phosphate. The valve action in such an arrangement as this appears to be due to the formation of a film of normal hydroxide of aluminium (Al2(0H)g) over the surface of the aluminium electrode. This film, when current tries to pass from it to the other electrode, and the potential is below a certain value, presents an enormously high resistance, while if the current be reversed and flows from lead to aluminium, the film offers but little resist- ance to its passage. If, therefore, such a cell be supplied with an alternating pd, the effect will be that half the wave will be suppressed and a pulsating or unidirectional current will result, as already indicated in fig. 1. By coupling a series of cells in op- posed pairs, both halves of the alternating wave may be utilised. The eificiency of the film is largely dependent upon the tempera- ture, which should not, for the highest effi- ciency, exceed 30° C, and there is also a certain critical voltage above which the film breaks down locally, giving rise to a lumin- ous and somewhat disruptive discharge ac- companied by a rapid rise of temperature and fall in efficiency. See also 'Nodon Valve ' below and Gratz Method of Eec- TiFYiNG Current. (Eef. Norden, Elec. World, vol. xxxviii, 1901, p. 681; Cook, Phys. Eev., vol. xx, 1905, p. 312.) Nodon Valve. — This is an electrolytic rectifier in which the cathode is a rod of aluminium alloy held centrally in a leaden vessel which forms the anode and contains the electrolyte, a concentrated solution of ammonium phosphate. Only a short por- tion at the lower end of the cathode is utilised, the rest, which is rather smaller in diameter, being protected from action by an enclosing glass sleeve. The current density at the cathode ranges from 5 to 10 amp per sq dm. In the larger sizes the cells are made double, and a current of air is kept circulating between the walls by means of a motor-driven fan. In order to utilise both halves of the sup- ply wave, the Gratz method of connection is Hg. a— Plan view of Nodon Valve adopted (which see). The maximum efii- ciency is obtained at about 140 volts, and the efficiency lies between 65 and 75 per cent, and is practically independent of the fre* quency between the limits of 25 ~ and 200 ~ . Above a pressure of 140 volts, the efficiency Hg. S.— Elevation of Nodon Valve falls off very rapidly, owing to breakdown of the film. The pf is high, being over 90 per cent at full load. Temperature largely influences the action of the valve, and should never exceed 50° C. Figs. 2 and 3 show in plan and elevation a 5-amp Nodon valve. Fig. 4 is an oscillograph record of the Rectifier 429 original supply voltage and the correspond- ing pulsating current at the terminals of such a valve. Fig. 1.— Oscillograph Becord tmm Nodon Valve Fig. 5 is a set of performance curves for a 5 -amp valve. (Ref. Jolley, 'Some Observations on Alternating-current Eecti- fiers', Elec, vol. Ivii, 1906, p. 998; Moss, Journ.I.E.E., May, 1907; Boot, Elec. Rev., vol. Ivi, 1905, p. 211.) 1100 1200 monium borate. Biittner smm fsmmmmmm I DO -^ B Fig. 6.— Churcher Valve A and B, Transformer terminals, F, Anode. 0, Catliode I. D, Cathode II. 3000 J" 800 S 400 200 600 400 300 200 100 400 600 Watts, Output Fig. 5.— Performance Curves of S-ampere Nodon Valve. Constant Secondary Voltage Test. Loaded on Kon-inductive Sesistances Frequency SO'". Maximum power factor on valve 0*98. on valve 070. The Audion Valve. — This valve was in- vented by De Forest in 1900, and is practi- cally identical with the ' Fleming Oscillation Valve ' (which see below). BiJTTNER Valve, an electrolytic valve of the Nodon type employing a cathode of mag- nesium-aluminium alloy, and probably iron or lead as anode, with an electrolyte of am- claims that the borate is su- perior to the phosphate in that it does not attack iron, and will keep in good working condition for longer periods. (Ref. Central- blatt fiir Accu- mulatoren, vol. vi, 1905, pp. 66-97.) Churcher Valve. — This is an electroly- tic valve of a modified Nodon type, from which it differs in that it has two cath- odes of aluminium, and an anode of lead or platinum suspended in the one cell. This allows of the complete utilisation of both halves of the supply wave with one cell in- stead of the four re- quired in the Oratz method. The connec- tions of such a cell are shown in fig. 6. The secondary of the trans- former carries a central tapping, and is con- nected through the cc load to the central anode, while each of the cathodes is connected to the ordinary ter- minals of the transfor- mer itself. The prac- tical limits of the cell are 50 volts cc, or 130 volts at the transformer terminals ab. (Ref. Elec. World, vol. xliv, 1904, p. 308.) De Faria Valve. — This is an aluminium- lead rectifier. The cathode is a hollow cylin- der of aluminium placed concentrically in a larger cylinder of lead, and the whole im- mersed in an electrolyte of sodium phosphate in an ebonite containing vessel. Cooling is effected by promoting automatic circulation 140 135 130 Minimum power factor 430 Rectifier of the electrolyte by providing the lead cyl- inder with holes near its extremities; the heated electrolyte then rises in the lead cyl- inder, passes out at the upper holes, is cooled by contact with the walls of the containing vessel, and descends outside the lead cylinder. Hg. 7.— De Faria Valve It is claimed that this cooling action is suf- ficient to allow of a current density of 8 amp per sq dm of aluminium. Fig. 7 shows a section of such a valve. (Eef. La Eevue Electrique, vol. vi, 1906, p. 296.) Fleming Oscillation Valve, a valve depending for its action on the well-known ^^SM Fig. a— Fleming OsoiUation Valve Edison eflfect in glow lamps. The valve con- sists of a carbon-filament glow lamp with a simple central horseshoe filament. Around this filament inside the exhausted bulb is fixed a small cylinder of nickel, which is con- nected by means of a platinum wire sealed through the bulb to a third terminal (see fig. 8). The valve is used as follows. The carbon loop is made incandescent by a suit- able battery. The circuit in which the oscil- lations are to be detected is joined in series with a sensitive mirror galvanometer, the nickel cylinder terminal and the negative terminal of the filament of the valve being used. The galvanometer will then be traversed by a series of rapid discharges all in the same direction, those in the opposite direction being entirely suppressed. (Eef. Fleming, Proc. Phys. Soc. Lend., vol. xx, p. 179, 1906.) Grisson Valve. — This is an elec- trolytic valve in which the cathode is a sheet of aluminium, and the anode a sheet of lead ; supported, in the original form, horizontally in a vessel contain- ing the electrolyte, consisting of a solution of sodium carbonate. Cool- ing is effected by circulating water through metal tubes in the electrolyte itself. (Eef. Central Zeitung f iir Optik und Mechanik, vol. xxviii, 1907, p. 95.) Pawlowski Valve. — This is an electro- lytic valve employing a solid electrolyte. It consists of a copper plate which has been coated with a crystalline layer of carefully prepared copper hemisulphide, prepared by melting sulphur and copper together out of contact with air. The prepared plate is placed in contact with an aluminium sheet, and the combination is then 'formed' by submitting it to an alternating potential until sparking, which at first occurs, ceases. (Eef. Elec. Eev., N.Y., vol. xlix, 1906, p. 554.) Giles Electric Valve, a combination of spark gaps and capacity used to protect elec- trical apparatus against damage due to atmos- pheric discharges and resonance surges. The spark gaps are formed between the edges of sharp-rimmed disks of non-arcing metal. These disks are insulated from each other and from the central tube, which provides a support for the apparatus and also an 'earth'. The condenser effect is obtained by means of the annular disks and the central tube; an adjustable spark gap, a high resistance, and a fuse all connected in series, complete the valve. Mercury-arc Eectifier. — This type of valve depends for its action on the property of ionised mercury vapour of conducting electricity in one direction only. The recti- fier is usually in the form of a glass container. Rectifier 431 into which are sealed (for a sp valve) two iron or graphite anodes and one mercury cathode, and a small starting electrode; the container is filled with mercury-vapour under 1 pr. Since, as in the mercury vapour lamp (see under Lamp, Tubular), no current will flow until the starting or negative electrode resistance has been overcome by the ionisa- tion of the vapour in its neighbourhood, a device must be provided by which this may be accomplished. This may be done by raising the voltage to such an extent that a spark jumps the gap between the mercury cathode and the starting electrode, or by bringing these terminals (cathode and start- ing electrode) together in the vapour by tilt- ing and then separating them, thus drawing out the arc. When this has been done, cur- rent will only flow from the anode to the mercury cathode, and not in the reverse direc- tion. In order to maintain the action, a lag is produced in each half-wave by the use of a reactive or sustaining coil, and hence the current never reaches its zero value; other- wise the arc would have to be restarted. The losses in the tube are due to two causes: firstly, leakage from one anode to the other, or what is technically known as arcing of the rectifier; this is only serious at very high voltages. Secondly, there is a constant drop of 15 to 18 volts (the mercury- arc voltage), which is irrespective of the load. The energy represented by this voltage is converted into heat, which is dissipated at the surface of the containing vessel, and a little light in the bulb itself. According to Steinmetz the limit of voltage must be very high, as 36,000 volts has been rectified. The current output is limited principally by the leading-in wires to the electrodes, it being a difficult problem to seal into the glass container the large masses of metal re- quired for the conduction of large currents. Frequency has but little influence. The cc voltage ranges from 20 to 50 per cent that of the ac supply. The life of the valve depends somewhat upon its size, being longer in the small sizes, and never with fair usage less than 1000 hr. Cooper-Hewitt Mercury-vapour Eeo- TiFiER. — This consists of a large glass bulb filled with mercury vapour and mounted in trunnions so as to be capable qf tilting through a small angle. Into this bulb are sealed four electrodes in the case of a sp converter, and five in the case of a three- phase one. The electrodes are connected in circuit as shown in fig. 9 ; the terminals M N are joined by mercury when the bulb is tilted, and this serves to break down the high resistance which exists between the Fig. 9.— Cooper-Hewitt Mercury-vapour Rectifier M and N, Terminals, i', Mercury. L, Continuous- current lead. K, Reactance coil. mercury vapour and the surface of the nega- tive electrode. In order that the valve may be continuously operating, this current must not be allowed to fall to zero, even for the smallest fraction of time, and therefore the [sasaswrnusju /mi'ttmumiu] ■— L -'jraronnnr Fig. 10.— Constant-current Mercury-arc Rectifier S, Starting resistance. A., Anodes. L, Continuous-cur- rent load, u, Starting mercury electrode, c, Cathode. R, Reactance coils. I, Transformer or Auto-transformer. two halves of the supply wave must be made to overlap a very little. This is effected by inserting the reactance coil R in the lead between z and the load L. This causes a slight lagging in the currents and produces 432 Rectifier System of Arc Lighting — Re-entrancy the required overlap. The three-phase con- verter does not require this device, since the half-waves will ordinarily have the required overlap. CONSTANT-CURKENT MERCURY-ARC REC- TIFIER. — The regulation of a mercury-vapour valve is largely dependent upon the auxiliary apparatus with which it is used. The actual drop of volts in the bulb with increase of load is very slightly negative; that is, the lost volts in the bulb are slightly less on load than with no load. On the other hand, for constant-current service each anode is connected to the terminals of the auto supply transformer through a reactive coil which produces the required constant-current char- acteristic, bad voltage regulation; or a droop- ing characteristic, which is in no way a feature of the valve, but is produced simply by the inductive apparatus in its circuits (see fig. 10). Tube Eectifier, a term applied to any rectifying device whose electrodes are con- tained in an exhausted bulb. See also Oscillation Valve; Contact Rectifiers; Permutator; Automatic In- terrupter; Interrupter, Simon; Inter- rupter, Wehnelt; Gratz Method of Rectifying Current. [c. v. d.] Reetifier System of Arc Lighting-. See Arc Lighting, Rectifier System of; 'Ferranti Rectifier' under Rectifier. Red Fibre. See Fibre. Red Rope Paper, a fairly tough paper A singly re-entrant simplex winding is denoted thus A „ „ duplex made from the fibres of hemp rope. It varies in quality, but usually contains about 7 per cent mineral matter, and has a good smooth surface. It is uniform in dielectric strength and withstands heat well. As usu- ally employed, it is treated with an insulat- ing varnish. See also Manila Paper; Leatheroid; Horn Fibre; Press-spahn; Japanese Paper; Fibre. Reducing- Negative Plates of Ac- cumulators. See Accumulator Plates. Re-entrancy, the number of separate windings in a closed-circuit, cc armature. The component windings are insulated from one another at all points. Degree of Re-entrancy. — Single. — An armature winding which con- sists of a single conducting system forming a closed circuit is called singly re-entrant (if in a rotary converter, single re-entrancy involves one tap per slip ring per pair of poles). Double. — An armature winding which con- sists of two entirely independent closed cir- cuits completely insulated from each other (except, if ac windings, they are connected in parallel at the slip rings) is called doubly re-entrant. Triple, &c. — Similarly three, four, five, six, &e., entirely independent circuits give re- spectively trebly, gvadruply, quintwply, sextwply, &c., re-entrant windings. Simplex, duplex, triplex, &c., windings are defined under. triplex A doubly J1 duplex A „ )) quadruplex A triply n triplex A „ 1) sextuplex A doubly „ „ A sextuply „ „ The examples suffice to explain the basis of this nomenclature. As a matter of fact, the use of other than simplex and duplex windings is rare. So very large a proportion o (£) (ssu OO o o o " (SO® " " OOOOJDO of all the windings in practical use are sim- plex windings, that, where not specifically stated a winding may be taken as being simplex. See Windings. Rees-roturbo Pump — Reflection 433 Rees-roturbo Pump, a centrifugal water pump designed especially for high-speed running, with electric motor drive (but low- speed pumps of the same name are also manufactured). The impeller — or central rotating part by which the water is impelled — is of the drum type in high-speed pumps, and is so constructed that it consists of a pump and turbine part in series with a con- stant-pressure division between these two parts. Ideal conditions are thereby realised, in that the pump discharges into, and the turbine discharges from, a constant pressure. Pumping against high heads and high pres- sures (as in boiler-feed pumping) is effected by the water passing through several pressure chambers in series, each increasing its iinal pressure. The pumps can be supplied for working with the axis horizontal or vertical. See 'Electrical Pumping' under Mining Equipment, Electrical. Reflecting' Efiieieney. — The ratio of the radiation reflected from a plane surface of any material to that impinging on the surface is termed the reflecting efficiency of that material. In the case of polished metals and alloys, such as silver and speculum metal, the efficiency for visible radiation is high, some 90 per cent being reflected. For radiar tion of shorter wave length, however, the silver is much more transparent, and its re- flecting efficiency is considerably lower. The efficiency also increases with increasing angle of incidence, polished black glass reflecting an appreciable quantity of light impinging at nearly grazing incidence. In the case of solid substances without an optically-polished surface, a proportion of the incident radiation is also reflected, the amount depending on the material and state of the surface. Lampblack reflects very little of heat or light radiation incident upon it, while such substances as white paper, powdered glass, &c., reflect a large proportion of incident light. (Ref. ' Photometrical Measurements', Stine, p. 11.) See Eeflbc- "lON, Regular and Diffused, [e. h. r.] Reflecting- Galvanometer. See Gal- vanometer. Reflection, Angle of. See Angle of Reflection. Reflection, Coefficient of. See Co- efficient OF Reflection. Reflection, Regular and DiflEused.— - When rays of light fall upon a perfectly smooth and polished surface, the reflected light has an angle of reflection which is the same as the angle of incidence at which the rays strike the surface. The reflected rays corresponding to a parallel beam of light will, therefore, under these conditions, all issue from the surface at the same angle, and the source of light producing them can only be seen by reflected light when viewed at the particular angle of reflection of the Kg. 1.— Regular Reflection reflected beam. Light so reflected is said to be regularly reflected. An instance of such reflection is afforded by an ordinary silvered mirror (see fig. 1). When, however, rays of light fall upon a rough, unpolished surface, such rays as are reflected leave the surface at all possible angles corresponding to the effect of the Fig. 2.— Diffused ReflectloD minute particles of which such a surface is composed, as shown in fig. 2. Hence, in this case, the reflected light can be seen when viewed at all angles; but, owing to the light having been irregularly distributed, one does not see the image of the source of illu- mination, but instead, the reflecting surface appears to be illuminated. Light so distri- buted is said to undergo irregular or diffused reflection. See Coefficient of Reflection; also Reflecting Efficiency. [l. g.] Reflection, Total Internal.— If light be incident on the smooth boundary between two media at and PjO be the direction of the incident wave normal in the medium of higher refractive index, then if the angle of incidence P,ON be not too great, the light will pass out in a direction OPj^, where Sin P,^ONi _ iJ. of medium N Sin PjON [1. of medium Nj' As the angle PjON is increased, Pi^ONj increases to a limit of 90°; and if P^ON is further increased, it will ultimately reach a 434 Reflector value PgON, when all the light will be re- flected in the direction OP^, and the angle PjiQN will be equal to the angle PjON. The Total Internal It«flection minimum angle P2ON at which this takes place is termed the critical angle, [e. h. k.] Reflector. — When radiation reaches the boundary between two media, part is ab- sorbed by the second medium, part passes into the second medium, and part is restored as radiation penetrating the first medium. The radiation returning into the first medium may have in general two charac- teristics. It may be difiuse, such as that reflected from minutely divided solids such as red lead, or it may be ' specular ', such as is obtained from the surfaces of liquids or polished metals. In the latter case the direction of radiation reflected from the surface makes an angle with the normal to the surface equal to the angle of inci- dence of the radiation. In the case of dif- fuse radiation the reflected proportion is propagated equally in all directions into the first medium. In general, no substance acts either as a perfect diffuser or as a perfect reflector, having neither quality to the com- plete exclusion of the other. HoLOPHANE Re- flector, a glass re- flector the surface of which towards the source of light is smooth and may be of any required shape (see fig. 1). The outer surface is corrugated with ridges of an angle of 90°. The light falling on the two sides of the ridges in succession is totally reflected and returned in the direction of the incident Fig. 1.— Holophane Reflector light or other direction as required. See also 'Holophane Globe' under Globe. Parabolic Reflector. — A reflector of parabolic form reflects radiation in a parallel Fig. 2.— Parabolic Keflector beam equal in diameter to that of the re- flector, if the source of radiation be a point placed at the focus of the parabola (see fig. 2). This form of re- flector is used for lighthouse pro- jection, search- lights, and trac- tion headlights. Prismatic Re- flector. — The phenomenon of total internal re- flection (see Re- flection, Total Internal) is „. „ often made use Fig. 3 Figs. 3 and i. Fig. 4 -Prismatic It«flectors of instead of a mirror, where an alteration in the direction of light is required. The general case is shown diagrammatically in fig. 3. The most usual form is a block of glass Reflectoscope — Regenerative Control Systems 435 with two sides equal and at right angles, on which the light is incident normally or nearly so, and is totally internally reflected on the hypotenuse, as shown in fig. 4. The advantage of this form over a mirror is that there is no disturbing reflection as from the front surface of an ordinary mirror, which if silvered on the front rapidly tar- nishes and is easily spoilt. This principle is used in the Lummer- Brodhun photometer (see under Photometer Head), in some forms of the Ritchie photo- meter (see under Photometer Head, Ritchie Wedge), and in Holophane Reflectors and Holophane Globes. [e. h. r.] Reflectoscope, an apparatus with which, by means of electric arc lamps and other suitable arrangements, the image of any illustration, or even of the printed page of a book, may be thrown, enlarged, on a screen for the edification of an audience, thus avoiding the necessity for preparing lantern slides. Refraction. — The alteration of the direc- tion of radiation when passing from one medium to another, when incident at an angle inclined to the normal to the surface' between the two media, is termed refraction. It is especially used in reference to light radiation. Refraction of lines of magnetic force also occurs when there is a change in perme- ability; and of electrostatic force when there is a change- in the dielectric constant. The laws of magnetic and electric refrac- tion difi'er from those of light and heat radi- ation. See Angle of Incidence; Angle of Refraction. [e. h. r.] Refraction, Angle of. See Angle of Refraction. Refraction, Electrostatic. See Elec- trostatic Refraction. Refuse as a Fuel in Electricity Works. See Central Station for the Generation of Electricity. Regenerative Braking. See Brakes; Mining Equipment, Electrical; Regene- rative Control Systems; Crane, Elec- tric. Regenerative Cell. See Cell, Regene- rative. Regenerative Control Systems, sys- tems of control employed in various methods of electric driving, and characterised by the feature that when there is occasion to de- crease the speed of the driven apparatus. part of the energy stored up as momentum in the driven apparatus is reconverted into electricity and returned to the line. An instance of such a system is that of "Ward Leonard (see Ward-Leonard System). As developed with practically exclusive refer- ence to the requirements of electric traction, the two most notable instances are the John- son -Lundell Regenerative Control System and the Raworth Traction System. 1. The Johnson - Lundell Regenera- tive Control System is a system of electric traction in which the reversible property of the electric motor is made use of to econo- mise energy. In the latest type of this system the motor fields are connected as series windings when the machines are run- ning as motors, but as compound windings when they are acting as dynamos and return- ing energy to the line; in the latter case the machines act as electric brakes, but instead of wasting the energy thus absorbed, they recuperate (or, less accurately, ' regenerate ') a considerable portion of it. In order to avoid loss in regulating resistances, either four motors are used, or two motors with double-wound armatures, each having two commutators, and the series-parallel principle is applied twice, the four armature windings being put in series at starting, then two in series in two parallels, and finally all four in parallel; the field strength is also gradu- ated over a long range to give intermediate steps of speed. The same principles are adopted to enable recuperation to be car- ried on down to a very low speed, after which a mechanical brake is automatically applied. In addition to the usual controllers, modified to suit the system, a field-changer is provided to effect the change from series to compound winding; the field coils are subdivided in such a way that, by suitable grouping, they are wholly utilised whether as series or as compound windings. The most complete published description of this systein is contained in a paper pre- sented by E. H. Johnson before the Man- chester Association of Engineers in Nov., 1906, and entitled 'The Third Function of Electric Traction Motors'. Some of the principal British patents taken out for this system are: 7979 of 1899; 11,933 of 1900 26,668 of 1902; 329 of 1903; 11,304 of 1903 1374 of 1904; 13,564 of 1904; 176 of 1906 11,794 of 1907. 2. Raworth Traction System. — This 436 Registering Instrument — Regulation system, althougli of more recent date than the Johnson-Lundell, has been much more extensively employed, and is characterised by various unique features. The most com- plete published description is contained in a paper read at Leeds by Alfred JEiaworth be- fore the Institution of Electrical Engineers in Nov., 1906, and entitled ' Eegenerative Control of Electric Tramcars and Locomo- tives '. See also Crane, Electric ; Brakes ; Mining Equipment, Electrical. Registering Instrument. See Instru- ment, Eecording. Regulating Cells. See Cells, Regu- lating. Regulating Mechanism of Arc Lamps. See Lamp, Arc. Regulating Switch. See Switch, Re- gulating. Regulation. — The term regulation applies to the means adopted either to maintain con- stancy of a pressure, speed current, &c., or to obtain a predetermined variation in their values, as may be required by varying con- ditions. The most important cases of regulation requiring to be dealt with in electrical work are regulation of the voltage of generators on constant-potential systems, regulation of the current of generators on constant-current systems, and regulation of the speed of motors. Inherent Regulation. — In some classes of machine inherent regulation is obtained, that is to say the machine automatically effects its own regulation. This is notably the case with compound-wound dynamos, in which a portion of the magnetic excita- tion is obtained by coils traversed by the main current. As this current varies the magnetic flux varies, and this variation is utilised to obtain constant or varying poten- tial at the terminals of the machine accord- ing to the purpose for which it is required. Compound-wound alternators having inherent regulation have been constructed, but have not so far been used to any extent. In shunt-wound motors inherent regula- tion for constant speed at all loads may be obtained, where the design permits, by giving the brushes a backward lead such that the tendency to decrease of speed with increase of load due to increased voltage loss in the armature is balanced by the tendency to increase of speed due to the demagnetising effect of the armature current. This method of regulation is quite practicable, gives very good results, and costs nothing. Excitation Regulation. — It has been shown that inherent regulation may be ob- tained by automatic variation of the field excitation of a machine. The term excitation regulation is, however, sometimes applied to cases in which regula- tion, either constant or varying, is obtained by varying the excitation of a machine by means of a regulating resistance in its field circuit, the resistance being usually regulated by hand, but occasionally by an automatic device. Regulation of Shunt Dynamos. — Shunt dynamos are regulated by resistances con- nected in series with their field-magnet coils. Regulation of Series Dynamos.— Series dynamos, which are usually required to give constant current with varying potential, may be regulated by means of resistances con- nected in parallel with the field coils, or (as in the Thury system) by moving the brushes around the commutator. Speed Regulation of Shunt Motors. — The speed regulation of shunt motors may •be effected by inserting regulating resistance in the field circuit, so varying the excitation, or by connecting resistance in the armature circuit, so varying the voltage at the brushes of the machine. In cases where a wide range of speeds is required both methods are used, the two resistances being contained in one box, and varied by one switch lever. Control of Series Motors. — Series motors generally require to have their speed controlled rather than regulated, and this control is effected by resistances in the main circuit of the machine, resistances in parallel with the field being seldom employed. See also Potential Regulation; Exci- tation; Alternator, Pressure Regula- tion OF AN; Automatic Regulation of Voltage. [c. w. h.] The term regulation is still used in so many vague and indefinite ways, that, al- though they do not necessarily conform by any means with the most approved practice, nevertheless the following definitions from the 1907 Standardisation Rulesof theA.I.E.E. are useful for reference, pending the promul- gation of a comprehensive code of inter- national definitions covering the subject of regulation: — ' The regulation of a machine or apparatus in regard to Bome characteristic quantity Csuch as terminal vol- Regulation — Regulator 437 tage, current, or speed) is the ratio of the deviation of that quantity from its normal value at rated load to the normal rated-load value. The term regulation, therefore, has the same meaning as the term inherent regulation occasionally used. ' Constant Standard. — If the characteristic quantity is intended to remain constant {e.g. constant voltage, constant speed, &c.) between rated load and no load, the regulation is the ratio of the maximum variation from the rated-load value to the no-load value. ' Varying Standard. — If the characteristic quantity is intended to vary in a definite manner between rated load and no load, the regulation is the ratio of the maximum variation from the specified condition to the normal rated-load value. ' (a) Note. — If the law of the variation (in voltage, current, speed, &c.) between rated load and no load is not specified, it should be assumed to be a simple linear relation, i.e. one undergoing uniform variation between rated load and no load. '(b) Note. — The regulation of an apparatus may, therefore, differ according to its qualification for use. Thus, the regulation of a compound-wound generator specified as a constant-potential generator will be dif- ferent from that which it possesses when specified as an over-compounded generator. ' In constant-potential machines the regulation is the ratio of the maximum difference of terminal voltage from the rated-load value (occurring within the range from rated load to open circuit) to the rated-load ter- minal voltage. ■ In constant-current machines the regulation is the ratio of the maximum difference of current from the rated-load value (occurring within the range from rated load to short circuit, or minimum limit of opera- tion) to the rated-load current. ' In constant-power apparatus the regulation is the ratio of maximum difference of power from the rated- load value (occurring within the range of operation specified) to the rated power. ' In constant-speed direct-current motors and induc- tion motors the regulation is the ratio of the maxi- mum variation of speed from its rated-load value (oocurring within the range from rated load to no load) to the rated-load speed. ' The regulation of an induction motor is, therefore, not identical with the slip of the motor, which is the ratio of the drop in speed from synchronism, to the synchronous speed. 'In constant-potential transformers the regulation is the ratio of the rise of secondary terminal voltage from rated non-inductive load to no load (at constant primary impressed terminal voltage) to the secondary terminal voltage at rated load. ' In over-compounded machines the regulation is the ratio of the maximum difference in voltage from a straight line, connecting the no-load and rated-load values of terminal voltage as function of the load cur- rent to the rated-load terminal voltage. ' In converters, dynamoiors, motor generators, and frequency converters the regulation is the ratio of the maximum difference of terminal voltage at the output side from the rated-load voltage, to the rated-load voltage on the output side. ' In transmission lines, feeders, ikc, the regulation is the ratio of the maximum voltage difference at the receiving end, between rated non-inductive load and no load, to the rated-load voltage at the receiving end Vol. II (with constant voltage impressed upon the sending end). 'In steam engines the regulation is the ratio of the maximum variation of speed in passing slowly from rated load to no load (with constant steam pressure at the throttle) to the rated-load speed. 'In a hydra/ulic turbine or other water motor the regulation is the ratio of the maximum variation of speed in passing slowly from rated load to no load (at constant head of water, i.e. at constant difference of level between tail race and head race) to the rated- load speed. ' In a generator unit, consisting of a generator united with a prime mover, the regulation should be deter- mined at constant conditions of the prime mover, i.e. constant steam pressure, head, &c. It includes the inherent speed variations of the prime mover. For this reason the regulation of a generator unit is to be distinguished from the regulation of either the prime mover or of the generator contained in it, when taken separately.' Regulation, Potential. See Potential Eegulation; Regulation. Regulation of emf. See Potential Regulation; Regulation. Regulation of Pressure. See Alter- nators, Pressure Regulation of; Poten- tial Regulation; Regulation. Regulation of Series Dynamo. See Regulation. Regulation of Shunt Dynamo. See Regulation. Regulation Tests of Transformers. See Testing Transformers. Regulations, B.O.T. See Board of Trade Regulations. Regulator, Controller, a device by means of which the rate at which a con- troller is operated is regulated, indepen- dently of the will of the driver. The regu- lator is usually controlled by the main motor current, so that during acceleration the cur- rent shall not exceed or fall below predeter- mined limits, thus keeping the acceleration approximately constant. This class of de- vice is especially appropriate in connection with multiple-unit train control systems, in which each controller or contactor system acts independently of the rest, in obedience to the master controller^ and various modes of applying it have been developed. See also Controller-regulator for Electric Cars. Regulator, Potential (or Voltage), a device for regulating the pd applied to an electric circuit. This may take the form of a resistance in series with the circuit, a re- sistance in the field circuit of the dynamo supplying the circuit, a transformer or in- 29 438 Regulator ductive apparatus for adding to or taking from the voltage of an ac circuit, or a switch for altering the number of cells in a battery. Potential regulators may be hand-operated, as is the case with the majority of dynamo field regulators; or automatically operated, generally by magnets or small motors. Induction Potential Regulators, devices for regulating the pd of an ac circuit, the action of which _IU Fig. 1.— Stillwell Regulator A, Reversing switch. B, Oenerator. Fig. 2. ■General Electric Company's Induction Potential Regulator depends upon the inductive effect of one cir- cuit upon another. The most natural form for such a regulator to take is that of a transformer the primary of which is con- nected across the mains of the circuit to be regulated, while the secondary is in series with that circuit. Fig. 1 shows the connections of the Stillwell regulator, from which it will be seen that the amount of regulation can be varied by means of the multi-contact switch, and that the induced voltage can be added to or subtracted from the cir- cuit by alteration of the reversing switch. In order to prevent a short circuit of a coil while passing from one contact to the next, the switch arm is divided into two parts by means of suitable insulating partitions and a reactance is by this means in- troduced into the circuit. In a second form of induction regu- lator an iron core is movable, and the effect of the primary coils upon the secondary is thereby altered. Fig. 2 shows a regulator of this type manu- factured by the General Electric Com- pany of America. The primary and second- ary coils are fixtures in the four slots in the laminated stator, and the core (also lami- nated), which is moved by the hand wheel as desired, has no windings, but alters the magnitude and direction of the magnetic flux through the secondary due to the pri- mary, according to the position in which it is placed. The term induction regulator is, however, generally reserved for a regulator with cores and windings arranged as in an induction motor. Here also the primary is a shunt to the circuit while the secondary is in series with it, the primary generally be- ing on the moving- or rotor-portion and the secondary on the stator. The cores are of course of laminated iron. The primary wind- ings induce an emf in the secondary, and the amount and direction of this boosting emf is varied by altering the position of the rotor, that is, by rotating it through a certain angle, the maximum angle between full boost in one direction and full boost in the opposite direction being, for instance, 60° in a six-pole regulator. The General Electric Company of America have manufactured regulators of this type for many years, more particularly for use with rotary converters, and in their design the connection with the movable rotor is by means of flexible cable. The regulators can be made for any number of phases, and the rotor is turned by means of a hand wheel or by a small electric motor controlled by a simple throw-over switch. The efficiency of such a regulator may be somewhat greater Fig. 3.— Thnry Regulator than that of the corresponding induction motor, but as there is no constantly rotating part its power of dissipating energy is less, and hence in the larger sizes water cooling, or cooling by means of air under forced draught, has to be resorted to. Regulator 439 An induction regulator is rated according to the product of the secondary current and the maximum alteration of secondary voltage which is effected by the regulator. Thury Regulator, a device for auto- matically regulating the voltage of a circuit (see fig. 3). The shaft A, which is kept revolving either by a belt or by a small auxiliary motor, actuates the rocker B by means of the crank 0. Normally neither of the projections D and E engage with the teeth of the wheel F, the axis of which is connected to the switch-arm G; but should the voltage of the circuit rise, the coils H and I repel one another, the end K of the pivoted lever is depressed, engages with the stop L as the rocker comes in that direction, and so the projection D catches in a tooth of the wheel F, and the switch-arm is moved over one contact and cuts out a cell of a battery or adds resistance to a field, or other circuit, as the case may be. If the voltage is still too high, the process is repeated. Should, however, the voltage of the circuit fall, K is raised by the action of the spring M, the projection E catches in the toothed wheel, the switch arm is moved in the opposite direction, and a cell or step of resistance is cut out of circuit. The appa- ratus, with a few minor alterations, can be used on ac circuits. The parts R, S, and p are added to over- come any tendency of the apparatus to hunt, owing to the field of the generator not re- sponding immediately to a variation in the current in the field winding. TiRRiLL Eegulator, a device for auto- t n k %J!d X3^ .^wv Fig. 4.— Tirrill Eegulator A, Continnoue-current control magnet. B, Counter- weight and Dashpot. 0, Alternating- current control magnet. D, Bela;. matically regulating the voltage of a gene- rator. Fig. 4 shows diagrammatically the arrangement of the apparatus, c is an elec- tromagnet, the core of which is held up partly by the counterweight B and partly by the pull of the coil in shunt from the mains. As load comes upon the alternator the voltage falls, the coil referred to exerts less pull, and the core of the magnet falls, with the result that the main contacts close. This causes a current to ilow through the winding of the differential relay D; the relay contacts are closed, and the resistance in the shunt field of the alternator-exciter is short-circuited. The alternator voltage thereby rises again, the core of C rises, the relay contacts open, and the resist- ance is once again inserted in the field of the exciter. This process repeats itself rapidly, and the ex- citer field resistance is cut out of circuit to just the right ex- tent to give constant voltage at the alter- nator terminals; or, if an increased voltage with increased load is desired, this can be attained by the use of an adjustable com- pensating winding on c, fed from a cur- rent transformer in thealternatingmains. The regulator can also, with slight modification, be used to control the volt- age of a cc machine. EouTiN Regulator, an automatic device for maintaining constant for all loads the pressure of the supply from an ac generator. The apparatus is described in the following papers: Soc.Int. Elect., Bulletin 2, pp. 678- 704, Aug.-Sept.-Oct., 1902; Elec, Hi, pp. 772-774, March, 4, 1904; 'L'Eclairage £lec- trique', xxxiii, pp. 181-187, Nov. 8, 1902. Dick Automatic Potential Regulator, a voltage regulator devised by Dick and manufactured by the Austrian Siemens- Schuckert Company, Fig. 5 is reproduced from p. 315 of Elec. Eng. for Feb. 27, 1908. h3-' MAIN OR EXCITATION BUS BARSO.C.) Fig. 5. — Dick Automatic Potential Regulator 440 Regulator — Relay The apparatus, as may be seen from the fig., comprises an iron core C which is drawn up into a solenoid S. A plunger p carried by C controls the level of mercury in a vessel whose sides are composed of alternate layers of insulating and metal rings. The mercury short-circuits sections of a rheostat R tapped off to the metal rings, w and w are adjust- ing resistances. It will be readily seen that the apparatus can be so adjusted that any change in the pressure on the mains shall occasion such a change in the excitation of the main dynamo D as to tend to restore the pressure. Hand Eegulator, any device operated by hand for regulating an electrical machine or an electrical circuit. The purpose of such a device is generally to regulate the voltage of a circuit or the speed of a motor. See Switch, Voltage-regulating; Speed Re- gulation. Carbon Regulator, a resistance of carbon blocks, subject to a varying mechanical pres- sure, and varying in ohmic value accordingly. The resistance of a number of pieces of carbon decreases as they are pressed together more firmly, and increases as the pressure is re- laxed, and this fact has been taken advan- tage of in the design of such automatic voltage regulators as those used with the old Brush arc-lighting generators. See Automatic Regulation of Voltage; also Wall Regulator for Series Arc- light Dynamo; Potential Regulation; Alternator, Pressure Regulation of; Regulation. (Ref. 'Standard Polyphase Apparatus and Systems', Oudin.) [f. W.] In the 1907 Standardisation Rules of the A.I.E.E., potential regulators are defined as types of stationary induction apparatus comprising a coil in shunt and another coil in series with the circuit, the coils being so arranged that the ratio of transformation betweeil them is variable at will. They are of the following three classes: — {a) Compensator potential regulators, in which a number of turns of one of the coils are adjustable. (6) Induction potential regulators, in which the relative positions of the primary and secondary coils are adjustable. (c) Magneto potential regulators, in which the direction of the magnetic flux with re- spect to the coils is adjustable. {d) Reactors or Beacta/nce Coils, formerly called choking coils, are a form of stationary Fig. 1.— Elementary Belay induction apparatus used to produce react- ance or phase displacement. Reinforced Concrete. — As used for the construction of line poles, bridges, and in buildings, and other structures required by electrical engineers, reinforced concrete con- sists of concrete in which is embedded a steel framework to add strength. The brittleness of the concrete is counteracted, and at the same time the steel is fully protected from atmospheric influences. See Line Poles. Relative Calibration. See Calibra- tion. Relay, an instrument actuated by small currents from a distant point or station, and operating on a switch which opens and closes a local circuit in response to the impulses received from the distant point. Usually the flrst circuit contains an electromagnet which, when ener- gised, draws a keeper to it, and by means of this movement com- pletes the second circuit (see fig. 1). The local circuit may include a powerful battery, and may be employed to operate instruments requiring heavy currents. It is therefore unnecessary for the long main circuit to carry these heavy currents, with the consequent loss of power. The object of a relay is generally to act as a sort of electrical multi- plier, that is to say, it enables a compara- tively weak current to bring into operation a much stronger current. Relays are largely used in connection with h pr switch gear, where the small amount of energy derived from an ordinary current- or potential in- strument-transformer is insufiicient to trip a large oil switch or other gear. Relays are also in extensive operation in multiple- unit systems of train control (which see), and in connection with many other descrip- tions of apparatus. A relay may also be defined as a device in which an auxiliary circuit is automatically closed or opened by electrical means. Relays are much used in telegraph and telephone work and for the opening and (although more rarely) the closing of circuit breakers. For this purpose there are — 1. Overload or maximum relays, coming into Relay 441 action on the passage of a current exceeding some predetermined value. 2. No-voltage or minimum relays, coming into action so soon as the current or pressure falls below a given value. 3. Sever se- current or discriminating relays, acting immediately a given current flows in the reverse direction. Type 1, which is very largely used for the protection of feeders, is usually either of the solenoid pattern (in which an iron core or armature is attracted), an example of which has already been given in fig. 1, or of the induction pattern (similar to an induction ammeter). Type 2 is chiefly used for the protection of motors in the event of a tem- porary failure in the supply, and is very similar in construction to Type 1. Type 3, which is largely used to cut off a generator from the bus-bai]fe in the event of its failure from loss of field, or any other cause, con- sists, as a rule, of some form of wattmeter arrangement in which a forward current, no matter how large, has no effect, whereas even a small reversal is sufiicient to actuate the relay. It is often inconvenient that a circuit breaker should be opened immediately on the occurrence of what may prove to be merely a momentary overload, so that tims- lag attachments are frequently provided, par- ticularly with relays of Type 1. These devices, which may form part of the relay, or may be quite distinct from it, retard its action until the overload has lasted for a predetermined time, say 5 sec. Preferably the time-lag should depend on the extent to which the overload is reduced as the time elapses. In induction relays the time-lag device usually consists of a damping magnet, sometimes adjustable, which acts on the disk or drum. In solenoid relays some form of air or oil dash-pot arrangement is often employed. In the Statter time-lag device a disk D (fig. 2) attached to the plunger A rests on the bottom of a fixed cup E partly filled with oil. The weight of the plunger forces the oil out from between the two flat surfaces so that a considerable pull is required to separate them suddenly, whereas a much smaller force applied for a longer time will do so readily, consequently the time-lag becomes shorter the greater the overload (see also Hobart's Time-limit Device for CO Circuits). Fig. 2 shows a typical ar- rangement of relay circuit, to being the B TO RC circuit -breaker tripping coil, RC the relay coil carrying either the main current or one proportional to it, say through a trans- former. At c are the relay contacts, and B is an auxiliary source of supply, preferably cc. The current setting can be varied by altering the position of the core A in the solenoid. In ac stations where no cc is available the secondary current of a current transfor- mer working the coil R C is often employed to actuate tc. In this case the latter is nor- mally short-circuited at C, and is only opened when the relay acts, thus sending the second- ary current through TC. The supply volt- age should not be used in place of B, since in the event of a heavy overload or short-cir- cuit, it may fall too low to allow T c to act satis- factorily. Differential Relay. — This is a type of relay on the electromagnet of which two windings are provided. In normal working these oppose and neutralise one another. Should, however, either winding become stronger or weaker than the other, the bal- ance is upset, the magnet is energised, and the relay comes into operation. A modifi- cation of such a relay for ac is shown in Fig. 2. — a Typical Arrangement of a Relay Circuit and a Statter Time-lag Device Fig. 3.— Differential Selay Transformer, and Keverae- current Circuit Brealcer Discriminating Device fig. 3, from which it will be seen that when the currents are as indicated, the circuit A has the larger emf induced in it, whereas, should the main current reverse with refer- ence to the shunt current, the circuit B would have the larger induced emf. (See, in Andrews' 'Electricity Control', chapter 442 Relay — Reluctivity dealing with 'Discriminating Devices' for various types.) Automatic Cut-out Relay, a relay ar- ranged to be operated by the main current or by a shunt current of the circuit, and closing an auxiliary circuit which trips the cut-out. Relays are resorted to when the operating current is too small to trip the cut-out, or the voltage too high to be introduced directly, and they are generally met with in connec- tion with h pr oil switches operated through current and voltage transformers. The relays above-mentioned are such as are chiefly employed in heavy electrical en- gineering. It is impossible to distinguish broadly between the principles employed for such work and those employed in tele- graphy, telephony, and in signalling. The following types of relays are, however, chiefly such as are required in applications of electricity when very small currents are dealt with. Polarised Relay, a relay in which there is a permanent magnet in addition to the electromagnet. In a common form, one end of the permanent magnet is attached to the yoke of the electromagnet, while the other end is bent up so as nearly to touch the pivoted end of the moving tongue or ditton. The outer end of the ditton is free to move between the poles of the electromagnet. Owing to the permanent magnet the polarity of both poles of the electromagnet is the same, while that of the ditton is opposite. The ditton (which see) is thus in a position of instability, and therefore of great sensi- bility; a very small change in the relative strengths of the poles of the electromagnet, caused by a current in the windings, being sufiicient to deflect it to one side or the other and close the local circuit. See also Polar- ised Electromagnet. Microphone Relay, a relay whose action depends on a microphonic contact, i.e. on the variations of resistance in the contact between two pieces of carbon caused by small changes of pressure on the contact. The actuating current passes through the wind- ings of an electromagnet, the armature of which is attached to one of the carbon con- tact-blocks. The current from the local battery passes through the contact, and is therefore varied by increase or decrease of the actuating current. Box Sounding Relay, a sensitive sounder mounted on a thin wooden box to render audible the click due to the tongue striking the fixed contact. Compound-wound Relay, a relay with more than one set of windings on the electro- magnet. Signal-lamp Relay, a relay which is actuated from the signal box by a small current thus completing the lamp circuit. Thermoelectric Call Relay, a thermo- electric device used in wireless telegraphy to call the operator's attention to the fact that a distant station is calling him. See Circuit Breaker; Circuit-opening Devices; Switch; Telephone Relay; Time-limit Device; Bell, Relay. (Ref. 'Electricity Control', Andrews.) Relay Contact, the contact which is made when the relay comes into operation. See Relay. Relay Magnet. See Magnet, Relay. Release, No-voltag-e. See No-voltage Release. Reluctance. — The magnetic reluctance of a path is proportional to its length, and is inversely proportional to its cross-sectional area and to its permeability. Generally the reluctance of a magnetic circuit is the sum of such expressions as — where I, s, and « are Sfl respectively the length, cross-sectional area, and permeability of each element of the mag- netic circuit. See Magnetomotive Force; Oersted; Magnetic Circuit; Core Reluc- tance; Gap Reluctance; Permeability. Reluctance, Core. See Core Reluc- tance. Reluctance, Gap. See Gap Reluc- tance. Reluctance, Unit of, the magnetic re- luctance of a column of air having 1 sq cm cross-sectional area and 1 cm length. See Reluctivity; Oersted. Reluctance of Air Gap. See Gap Re- luctance. Reluctance of Teeth.— The reluctance of the armature teeth in electrical machinery should be low as far as relates to their func- tion of transmitting the main flux, but high from the standpoint of preventing excessive local flux around each armature coil, and to restrict armature interference in general. See Inductance, Slot; Armature Interfer- ence. Reluctivity, the reciprocal of magnetic permeability. See Permeability; Conduc- tivity; Insulativity. Remanent Flux — Renewable Plate for Rail Joints 443 Remanent Flux. See Flux, Remanent. Remanent Mag-netism. See Flux, Remanent. Remote-controlled Oil Switch, See Switch, Oil-break. Remote - control Switch Gear. See Switch Gear, Remote-control; Remote- control System; Relay; Controller, Master; Switch, Oil-break. Remote-control System, the control- ling of electrical apparatus from a distance, generally by electrical means. The appara- tus so controlled is for the most part either switch-gear or motors. It has become customary to place h pr oil switches in separate stonework cells, and, when of large capacity, to operate them by means of small electric motors fed from a separate source, as a rule from a cc circuit, and controlled by a reversing switch on an operating panel at some distance from the cell. In many large stations this system is adopted throughout, and the switch cells and operating panels are placed where most convenient, irrespective of their relative posi- tions. In other cases, where the capacity of the plant is not so great, and the expense of electrically-operated switches would not be warranted, the operating panels are placed near, but still some feet from, the oil-switch chambers, and the connection is eflfected by means of rods and bell-crank levers. This is also referred to as remote-control switch gear, but the term is more generally taken to refer to electrical operation of the switches. It is often necessary to start and stop electric motors from a distance, as, for in- stance, from the cage of a lift, or automati- cally from a hydraulic accumulator, or from a tank in which the level of the water must not exceed a certain height nor fall below a certain fixed limit. An electric motor driv- ing, say, a line of shafting may be controlled by an ordinary motor-starter at some dis- tance from the motor, but the distinguishing feature of the instances referred to above — that is, the feature that brings them under the classification of 'remote control', is the separation of the starting handle, tappet, &c., from the starter proper, that is, the piece of apparatus by which the circuit to the motor is closed or opened. Incidentally it may be noticed that, as a rule, in such cases of remote control, the starting force acts sud- denly, so that the gradual cutting out of the starting resistance has to be effected auto- matically. Many forms of remote-control gear have been designed, but most of them can be classified under three main headings : — 1. The type in which the starting force closes a switch and so starts a small auxiliary motor, which in turn moves the contacts of the motor-starting gear. 2. The type in which the starting force acts through a rod, rope, &c., upon a switch which closes the motor circuit with all re- sistance in; while a number of solenoids which are connected across the motor ter- minals draw up their plungers one by one as the emf across the motor terminals in- creases, and so operate switch contacts which each short-circuit a portion of the starting resistance. 3. The type in which the starting force, acting through a rope, rod, &c., operates a starting switch which is prevented by a dash-pot from cutting out resistance too rapidly. A further example of remote control is seen in the multiple-unit system of electric traction, in which the several motors in a train are regulated from a single master-con- troller which operates the various distant main-current switches either electrically or pneumatically. See Multiple - unit Sys- tem; Controller, Master. Remote control is indeed one of the most suitable fields for the use of electricity, and all manner of devices have been designed to meet special requirements. Among these it will here be sufficient to mention a special arrangement for switching transformers in a substation into or out of circuit from the main station as the load increases or de- creases; and the method of stopping a motor, driving, Say, a printing press or a rolling mill, by the pressing of a button near the work itself, and so tripping a circuit breaker on the switchboard or panel, and in this way stopping the motor instantaneously, when if it continued in operation it might cause damage to delicate work or lead to the in- jury of a workman. See Circuit Breaker; Relay; Switch Gear, Remote - control; Switch, Oil-break. [f. w.] Renewable -Centre, Trolley Wheel with. See Trolley Wheel. Renewable Plate for Rail Joints.— Under Continuous Rail Joints have been described a number of methods of improving the rail continuity in electric tramways and railways. The renewable plate illustrated 444 Repeater — Resistance in the fig. affords an alternative means for accomplishing the purpose. It is stated that the construction removes the difficulties caused by corrugations at joints. The rails are prepared on the track, to receive the renewable plates (see A in figure), by special Kenewable Flate for Bail Joints electrically driven machinery. See Con- tinuous Rail Joint; Eomapac Railj Bond; Bonding Rail; Welded Rail- Joints. Repeater, Repeating Coil, or Trans- lator. — This is an induction coil or trans- former with approximately equal primary and secondary windings. It is used for con- necting one telephone line to another, so that speaking and ringing can take place without direct metallic connection. Repeater, Telegraphic. See Tele- graphic Repeater. Repeating Coil. See Repeater. ' Repeating Station. See Telegraphic Repeater. Replenisher, Sir William Thomson's, a small influence machine mounted on an electrometer for the — ^ purpose of maintain- ^. ing the charge on ( the needle when the electrometer is used heterostatically. It consists of two fixed cylindrical segments FF, and two seg- ments R R capable of rotation about a ver- tical spindle coaxial with FF. Light spring contacts are ar- ranged to alternately short-circuit rr and to connect RR to ff whilst rotating, with the result that rotation of the carriers in one direction results in increasing, and in the other in decreasing, the pd between F Sir William Thomson's Replenisher and F by small increments. See Electro- meter, [l. m.] Repulsion denotes the force mutually exerted by similarly charged bodies or similar magnetic poles. The repulsion between two bodies charged with mj and m^ units of electricity or mag- netism and placed d cm apart is equal to tMh dynes. See Static Charge: Line d^ •' ' of Induction; Law, Coulomb's; Law of Inverse Squares; Torsion Balance. The term repulsion is also applied to a par- ticular type of ac motor invented by Elihu Thomson. See Repulsion Motor and Single-phase Motor. Residual Charge. See Charge, Resi- dual. Residual Magnetism. See Flux, Re- manent; Coercivity; Retentivity. Resin, a collective term denoting the nu- merous hard exudations from trees. These exudations are insoluble in water, and form the most important group of materials used for varnish manufacture. Resinous Electricity. See Electricity. Resistance. See Resistance, Electric. Resistance, Apparent. — 1. The ratio of efiective volts to effective amp for a con- ductor carrying an ac. See Impedance. 2. The ratio of volts to amp for any ap- paratus, such as an electric arc, in which energy is dissipated other than in pure re- sistance. Resistance, Brush. See Brush Re- sistance. Resistance, Car. See Traction, Co- efficient OF. Resistance, Compensating. See Com- pensating Resistance. Resistance, Electric, a molecular pro- perty of matter in virtue of which the energy of (electric currents fiowing in the matter is converted into heat. The name is derived from a mechanical analogy. Electric resist- ance is of similar physical nature to frictional resistance, being invariably dissipative; that is to say, the energy-transfer is non-revers- ible. The resistance of an infinitesimal portion of matter is a function only of its physical state, and is independent of the current flowing through it. It is proportional to the length of the element, measured in the direction of flow of the current and inversely proportional to its cross section. The con- Resistance 445 stant of the proportion is called the specific resistance of the material (which see). The same can be stated of a finite conductor provided (1) that the cross section of the conductor is constant, (2) that the current flows everywhere at right angles to the cross section, (3) that the conductor is composed of homogeneous material, (4) that the cur- rent is everywhere constant in direction and magnitude. A uniform wire or strip, of con- siderable length compared with its other dimensions, practically satisfies the first three conditions. If the fourth condition is not satisfied, however, the resistance of such a conductor depends on the rate of change of the current — the magnetic field due to the current itself causing it to distribute itself over the cross section in a non- uniform manner, and increasing the resistance. See Skin Effect. The energy converted into heat due to the flow of a steady current in a linear conductor is proportional to the resistance of the con- ductor, and to the sq of the current (see Joule's Law). In the case of a non-linear conductor, the distribution of the current depends on the position of the electrodes. Assuming the current to be steady, and not due to emf in the conductor itself, the energy converted into heat is the volume-integral throughout the conductor of the product of the specific resistance and the square of the current density. This quantity, divided by the square of the total current entering, may be defined as the resistance of the conductor with the given electrodes. The same defini- tion may be adopted for the instantaneous value of the resistance when the current is varying, provided that no part of the current is due to emf generated in the conductor by external fields (see Eddy Current). Since by the principle of the conservation •of energy (see Energy) the energy loss is equal to the energy input, or, with the above restrictions, to the product of the total cur- rent entering and the diff'erence of potential between the electrodes, it follows that the resistance of a conductor may also be defined as the ratio of the difiierence of potential be- tween the electrodes to the total current entering. See Ohm's Law. The chief physical condition affecting the resistance of a conductor is its temperature, an increase of temperature usually causing the resistance to increase. See Resistance, Temperature Coefficient of. The principal unit of resistance is the stan- dard or international ohm. See Ohm. [f. w. c] Resistance, Electpolytie. See Elec- trolysis; Polarisation. Resistance, Equivalent, the ratio of applied voltage to current in a machine or apparatus absorbing electrical energy. See also Transformer, Equivalent Eesist- ance of. Resistance, E.S.C. Standards of. See Resistance, Matthiessen's Standard OF. Resistance, External, a resistance used in connection with any machine or apparatus, which forms no part of the working windings of the machine or apparatus. Resistance, Fixed. See Rheostats or Resistances; Box, Resistance. Resistance, German-silver. SccRheo- stats or Resistances; German Silver; High-resistance Alloys; Wire, Resist- ance. Resistance, Inductive, denotes a resist- ance wound in such a way, or possessing such a form, as to ofier also an appreciable react- ance to an ac traversing it. Strictly speak- ing, of course, all resistances have inductance and are therefore 'inductive', but the term is usually applied to a resistance whose time- constant is appreciable, and whose impedance is sensibly larger than its resistance. See Time-constant; Impedance; Reactance; Inductance; Coil, Reactance; Coil, Choking. An inductive resistance is usually repre- sented diagrammatically, thus — Inductive KeBietance Resistance, JaCObi's Unit of, an arbi- trary unit of resistance proposed in 1848 by Jacobi, being that of a certain copper wire of 22-4932 g weight, 7-61975 m length, and 0-667 mm diameter (Maxwell's 'Electricity and Magnetism', vol. i, §336). Jacobi had copies made of his standard and sent to the leading physicists of his day. It is known as Jacobi's italon. Resistance, Legal Standard. See Re- sistance Standard. Resistance, Matthiessen's Standard of, a standard of comparison used largely in 446 Resistance the specification of copper conductors. Al- though Matthiessen's experiments were made long ago, the results being published in 1865, there was a certain indefiniteness possible which it was attempted to clear up in 1899, when a committee representing the General Post Office, the Institution of Electrical Engineers, and various cable - makers, re- solved (see Journ.I.E.E., vol. xxix, p. 169 seq.):— 1. That Matthiessen's Standard of 0-153858 standard ohm resistance for a wire 1 m long, weighing 1 g, at 60° F. be taken as the stan- dard for hard-drawn, high-conductivity com- mercial copper, hard-drawn copper being de- fined as that which will not elongate more than 1 per cent without fracture. See Hard- drawn Copper Wire. 2. That Matthiessen's Standard of 0-150822 standard ohm resistance for a wire 1 m long, weighing 1 g, at 60° F. be taken as the stan- dard for annealed high-conductivity commer- cial copper, copper to be taken as weighing 555 lb per cu ft at 60° F., which will give a specific gravity of 8-912. This committee, however, left an indefiniteness in the method of correcting for a variation of temperature from the standard, which might easily be taken advantage of to make a specimen ap- pear of higher conductivity than it should. See Eesistance, Temperature Coefficient OF. The matter was reconsidered by the Engi- neering Standards Committee, whose report was issued in 1906. Their resolutions agreed substantially with the above, except that their constants were not carried to so many decimal places, the resistance unit for hard- drawn high-conductivity commercial copper being taken as 0-1539, and for annealed high- conductivity commercial copper as 0-1508, whilst the specific gravity of copper was taken as 8-9. The E.S.C., however, also omitted to lay down proper rules for mak- ing the temperature correction. See Eesist- ance, Temperature Coefficient of. If R is the resistance, reduced to 60° F., of a wire I m long and weighing M g, the resistance of a meter-gramme is EM/Z^. This quantity divided into 100 times Matthiessen's standard value gives the percentage in which the conductivity of commercial copper is usu- ally expressed. [f. w. c] Resistance, Meter - millimetep Unit of. See Resistance, Siemens Unit of. Resistance, Non-inductivej denotes a resistance, the inductance of which is so small as to produce negligible reactance at com- mercial frequencies. The current through such a resistance may be assumed to follow Ohm's law, and to be in phase with the ter- minal emf. A non-inductive resistance is usually re- presented diagrammatically thus — Non-indnctive Eesistance See also Eheostats or Eesistances; Eesistance, Ohmic. Resistance, Ohmic, the actual resistance of a conductor under any particular condi- tions, being the ratio of the mean energy loss due to the current in the conductor, t» the mean square value of the current (see Eesistance, Electric). The expression is hardly necessary, but is occasionally employed to emphasise the fact that the actual resist- ance is intended, and not a manifestation , in which work is done against an emf. See Eesistance, Spurious; Insulation Eesist- ance; Eesistance, Non-inductive. Resistance, Siemens Unit of, a unit of -resistance proposed in 1860 by Werner von Siemens. It is defined as the resistance, at 0° C.,. of a column of mercury of a uniform cross-sectional area of 1 sq mm, and a length of 1 m. Its value is 0-9407 international ohm. This unit had considerable accept- ance on the Continent. Resistance, Specific, a physical con- stant devised to express in numerical form the electrical resistance of a conductor, in so far as it depends on the material of which the conductor is composed. It is the ratio of the fall of potential per unit length, mea- sured in the direction of the current, to the current-flow per unit area, or the resistance between opposite faces of a cube of the substance having its edge 1 cm in length. Specific resistance of metals is sometimes expressed in absolute units, and sometimes in microhms per cm cube. The term is synonymous with volume resistivity. See Ee- SISTIVITY. Resistance, Spurious, an expression variously and loosely used to denote the ratio of terminal volts to amp in a portion Resistance — Resistance Standard 447 of a circuit in which emf exists in some form or other. See Impedance; Reactance; Resistance, Apparent. Resistance, Temperature Coefficient of, the proportional increase of resistance per unit increase of temperature (= -j-r ). \R at / The increase of resistance with rise of tem- perature is not generally a small quantity whose square can be neglected — the total in- crease for copper between the freezing- and boiling-points of water being over 40 per cent. The resistance, however, usually changes with temperature almost according to a straight- line law. It follows that the temperature coefficient of resistance is not even approxi- mately constant, but varies almost inversely as the resistance, and therefore may change as much as 40 per cent between the freezing- and boiling-points. It is accordingly neces- sary, in giving the temperature coefficient of resistance, to state also the temperature on which it is based, and in any use made of it the resistance should be referred back to that temperature. The recommendations of the Institution of Electrical Engineers' Com- mittee on copper conductors (1899), and of the Engineering Standards Committee on the same (1906), are defective in giving the temperature coefficient of resistance for cop- per as 0'002S8 per degree F, without stating at what temperature this is correct (see Re- sistance, Matthiessen's Standard of). The American Institute of Electrical En- gineers' Committee on Standardisation, on the other hand, in dealing with the deter- mination of temperature rise by resistance measurements, give the temperature coef- fieient of resistance for copper as 042 per cent per degree C, from and at 0° C. This is definite. It is worthy of note that at ordinary climatic teHiperatures, the temperature co- efficient of resistance for many pure metals is approximately 0-4 per cent per degree C. It is not, however, exactly true that resist- ance is a linear function of the temperature. For coppfer conductors Matthiessen proposed the empirical formula: — E 40 20 ^^ - J* ^ J / / .^•"" ^_ /' '•** 1 !/ r j .^'' ^^ 1 v'' ,'' 1 ,-'' y*' 20 40 60 AMPERES Series Characteristic of the pd characteristic gives a point on the emf or internal characteristic. See Droop- ing Characteristic. [c. v. d.] Series Circuit. See Circuit, Electric. Series Distribution, a distribution of electrical energy in which the consuming devices (lamps, motors, &c.) are in series. Some years ago this method of distribution was very general in connection with arc lamps, and special dynamos of the Brush and Thomson-Houston types were in use capable of supplying up to as many as 125 arc lamps in series per machine. Such machines were, however, inefficient, and arc lamps are now generally supplied four or eight in series across the ordinary supply mains at, say, 220 volts or 440 volts pres- sure. Cc High-tension System. — Series dis- tribution is sometimes used in long-distance high-voltage transmission schemes, cc being employed. A number of generators are ar- ranged in series at the generating station or stations, and a number of motors are fed in series in one or more substations at the re- ceiving end of the line. This system has Series Excitation — Sheet Iron 467 "been developed in particular by Thury on the Continent, the chief plants being the St. Maurice-Lausanne, 35 miles, 27,000 volts, 4000 kw, with 6 double generators in series, each half-generator 373 kw, 2250 volts, 300 rpm; and the Moutier-Lyon, 112 miles, 57,600 volts, 4300 kw, with 4 double gene- rators, each half -generator 582 kw, 7200 volts, 300 rpm. (Ref. J. S. Highfield, 'The Transmission of Electrical Energy by Direct Current on the Series System', Proc.I.E.E., March 7, 1907.) See also 'Continuous-current Constant-current System' under Generating System; Circuit, Electric. Series Excitation. See Excitation; Generator. Series Loss in a Meter. See Meter, Losses in a. Series-parallel Circuit. See Circuit, Electric. Series-parallel Control. See Control, Series-parallel. Series Regulating Mechanism of Arc Lamps. See Lamp, Arc. Series System. See Series Distri- bution; Circuit, Electric. Series Transformer. See Transformer, Instrument. Series Winding. See Winding, Series; Generator. Series-wound Arc Lamp. See Lamp, Arc. Series -wound Dynamo (or Gene- rator). See Generator. Series -wound Motor. See Motor, Series-wound. Service Box. See Cable, Service. Service Cable. See Cable, Service. Service Lead. See Cable, Service. Servo-motor, synonymous with pilot motor. See Motor, Pilot. Sextuply Re-entrant Sextuplex Wind- ing. See Multiplicity of Continuous- current Windings; Re-entrancy. Shade. See Globe. Shaft, that central element on which the rotating member of a machine is mounted. It has to fulfil two functions, namely: (1) it must be of such a diameter that when sup- ported in its bearings, it shall not bend ap- preciably owing to the weight of the spider or armature which it carries, and the un- balanced magnetic pull of the field system; and (2) it must be able to transmit the necessary hp without twisting appreciably. To fulfil the first condition, machine shafts are usually made of varying thickness, hav- ing the largest diameter in the centre and tapering oflf in stages to that necessary at the pulley to efficiently transmit the turning effort. Since shafts vary in the span between bearings, and since the weights of the ro- tating parts vary considerably for given outputs, it is difficult to give a rule covering the first point, but it may be borne in mind that the deflection is proportional to — ~W where W = weight on shaft, I = span between bearings, and d = diameter of shaft. As regards strength for transmitting power, the following rule may be given for the case of such machine tools as constitute a steady load : — d = 10 3/^ N' where d = diameter of shaft in cm, P = horse power, and N = revolutions per minute. The coefficient 10 is for mild steel, and should be increased somewhat in cases where the machine has an irregular load and is subject to shocks such as a reciprocating pump or a rolling mill. In the case of dynamo machines the twisting moment on the shaft is usually some one-fifth to one- third of the bending moment, and it is gene- rally sufficient to design the shaft only for bending. (Ref. Livingstone, ' Mechanical Design of Electrical Generators', Elec, vol. Ivii, p. 569, July 27, 1906.) [h. w. T.] Shaft Coupling. See Coupling, Shaft. Shale Naphtha. See Naphtha. Shale Spirit. See Naphtha. Sharpness of the Tuning of a Wireless Telegraph Receiver. See Selectance. Shears, Guillotine. See Guillotine Shears. Sheathing, Lead. See Cable; Lead Sheathing; Cable, Underground; Lead. Sheathing of Cable. See Cable; Cable, Underground. Sheer Legs, Electric. See Crane, Elec- tric. Sheet Iron. See Core Disk; Lamina- tions, Armature; Iron and Steel Test- ing; Punchings; Armature Stampings. 468 Sheet Steel — Short-circuit Test Sheet Steel. See Steel. Shellac, the principal refined lac product, extensively used in the manufacture of spirit varnishes. Lac is an incrustation found on trees in India, produced by the female of the lac insect. Coccus Lacca. According to the ' Technological and Scientific Dictionary ', shellac 'consists of about 85 per cent resin, 5 per cent wax, colouring matter, &c. There are six varieties in commerce, viz. stick lac, lac dye, shell lac, seed lac, button lac, and shellac rods. It is largely used in the manu- facture of wood polishes and of spirit and other varnishes.' Shellac Varnish.— This is a spirit var- nish composed of shellac and methylated spirits. It is used to some extent for in- sulation purposes, but absorbs moisture rather readily, and on this account its use is decreasing. It is practically never used in the manufacture of dynamo-electric ma- chinery by progressive firms. Shell-type Transformer. See Trans- former, Shell-type. Shielded Galvanometer. See Gal- vanometer. Shielded -pole Induction Ammeter. See Ammeter. Shielding of Electrical Measuring Instruments, their protection from exter- nal disturbance (1) due to stray magnetic fields, (2) due to electrostatic charges on the instru- ment case. The instruments most afiected by external influences are those with weak magnetic fields (e.g. the dynamometer type); they can, however, be protected by enclosure in iron. A cast-iron case serves this purpose for switchboard instruments, and a soft-iron shield for portable instruments. Electrostatic voltmeters are particularly liable to disturb- ance from the second cause, and are pro- tected by covering the case inside with tin- foil, and the glass front with a conducting varnish in electrical contact with the tinfoil. Shifting Magnetic Field, a term ap- plied to that portion of the magnetic flux of a dynamo machine which fluctuates in space, as from the successive passage of armature teeth, &c. See Flux, Pulsating Stator. Shims, sheet-metal distance pieces used under the feet of machines to adjust the centres above the bedplate, or between the pole cores and the yoke to adjust the depth of the air gap.- With regard to the latter use it is often found, when a new type of machine has been built, that from the point of view of regulation or field heating it is desirable to alter the depth of the gap. In case a deeper gap is wanted, shim plates are often inserted in the first erecting of the machine so that they may be withdrawn if necessary, and a re-machining of the pole- seating at this late stage is thus eliminated. See Air Gap; Air-gap Depth. Ship, Cable. See Cable Ship. Shock, Electric, the shock experienced on touching any conductor carrying an emf of, say, more than 50 volts, in such a manner as to complete a circuit. The intensity of the shock from a given emf varies greatly with different individuals. Thus to the ma- jority of people a 50-volt shock would usu- ally be inappreciable, while to a few it would be severe, and to these a shock from even less than 50 volts might cause inconvenience. The physiological consequences of exposure to a given voltage are very dependent upon the resistance at the points of contact. Thus, if the shock is received between any two points of an individual's body, the severity of the shock is greatly dependent upon the condition and extent of the surfaces in con- tact. Static Shock, the shock obtained on dis- charging a piece of static apparatus, such as a charged Leyden jar. Shoe (in an accumulator), the support placed under the hangers of a cell of an accumulator to hold them in position, and take up any irregularity. Shoe, Brake. See Brake Shoe. Shoe, Collecting. See Collecting Shoe. Shoe, Pole. See Pole Shoe. Shoe-carrier, the device by means of which the collector or contact shoe of an electric car or locomotive is carried in the proper position to make contact with the third rail or other conductor rail. Short-chord Winding. See Winding, Fractional Pitch. Short Circuit. See Circuit, Electric. Short-circuit Characteristic. See Curve, Characteristic. Short-circuit Current. See Current, Short-circuit. Short-circuit Curve. See Curve, Characteristic. Short Circuit in a Cell. See Cell, Short Circuit in a. Short-circuit Test. — This is a test which is applied by some companies to their Short-circuiting Device — Shunt Box 469 ac machinery in order to obtain data for esti- mating the regulation. In the case of alter- nators the armature coils are closed on an ammeter of negligible inductance and resis- tance, and the machine is run at its normal speed with reduced excitation. The result- ing short-circuit currents are plotted against the corresponding values of the excitation, and this curve, together with the no-load saturation curve for the machine, is em- ployed in estimating the regulation. As thus applied to the determination of the regulation of alternators, the method is of but little use. In transformers a similar process is em- ployed : the secondary circuit is closed through an ammeter, and the primary sup- plied with a reduced voltage. If the volts, amp, and w supplied to the primary be mea- sured, and if the ratio of the windings be known, the equivalent resistance and react- ance of the transformer may be calculated, and the regulation deduced. As applied to the determination of the regulation of trans- formers the method is very useful. For induction motors the rotor is locked in position with its circuits completely closed, and the stator then supplied with reduced voltage, the value of the voltage and current being ascertained as before. These values, together with the corresponding value for the machine running with no load, may be used for determining the dispersion coefficient {i.e. the circle coeflBcient) of the machine, and give, therefore, a means of determining the diameter of the Heyland semicircle. See 'Short-circuit Characteristic' under Curve, Characteristic; Circle Diagram; Dispersion, Magnetic; Testing Trans- formers. Short-circuiting Device. — In most of the wound-rotor types of induction motors, slip rings are provided in order that resis- tances may be inserted with the object of improving the starting torque. At full speed all the resistance is cut out, and the current is merely passing through the slip-ring con- tacts to a neutral point immediately outside. As this passing of current heats the rings, and also inserts an unnecessary resistance in the rotor circuits, an arrangement, usually consisting of a disk with three contact blades, is provided, which can be moved longitudi- nally along the shaft, and thus, by making contact with spring contact-strips attached to the slip rings, short-circuit them from the in- VOL. II side, thus rendering unnecessary the passage of current to the outside of the motor. Short-circuiting devices have been de- veloped which, by the action of centrifugal force, not only short-circuit the windings, but gradually cut out a resistance which is at- tached to the rotor itself. Such devices, however, although extremely ingenious, have not withstood the test of practical working, although they have the advantage of auto- matically opening the short-circuit when the motor is shut down. With the usual attach- ment it is essential to see that the short-cir- cuiting device is out of contact when about to start the motor up, as otherwise a large rush of current will result on applying the full voltage to the stator terminals by closing the main switch. Short-shunt Dynamo. See Compound- wound Dynamo. Short Ton. See Ton. Shot-gun Diagram, synonymous with Target Diagram. See under Lamp, Incan- descent Electric. Shrink-ring Commutator. See Com- mutator. Shrouded Magnet, synonymous with Ironclad Electromagnet. See Electromag- net, Ironclad. Shunt, an electric circuit connected in parallel with an instrument, dynamo, or other apparatus so as to divert or shunt some of the current from such apparatus; more particularly a winding on the magnets of a dynamo or motor connected in parallel with the armature circuit. See Excitation. A shunt is sometimes provided for regu- lating purposes, in parallel with the main series winding of a dynamo, or in parallel with the series winding on the interpoles of a dynamo. Such a shunt is called a diverter (which see). See also Magnetic Shunt; High-resistance Alloys; Wire, Eesist- ANCE. Shunt for Ammeter, a resistance con- nected in parallel with an ammeter so as to increase its range; used particularly in con- nection with moving-coil or hot-wire am- meters (which see) owing to the fact that they can carry only small currents. Such shunts are usually made of a material having a negligible temperature coefficient (e.g. man- ganin or constantan). Shunt Box. — For convenience, a number of galvanometer shunts are fitted in one box with suitable plugs on a dial switch for 31 470 Shunt-breaking Resistance — Shunt Running varying the sensitiveness of a galvanometer over a wide range. The values of the shunt resistances are usually ^, -j^-, and -^^ of the resistance of .the galvanometer. Eeadings equal to J^, ^hv ^^^ T^on respectively of those without any shunt are thus obtained. Univeksal Shunt Box, a form of shunt box devised by Ayrton and Mather for use with galvanometers of widely different re- sistances. The galvanometer, as indicated diagrammatically at G in the fig., is connected across the ends of a series of resistances A B. The main wires are connected, one to end ' A ' of the series and the other to a travelling point whose position is varied by means of plugs or by a dial switch. The resistance o o t INF LINE UniTersal Shunt Box of the galvanometer should be low compared with that of the shunt. Compensated Shunt Box. — When it is essential that the total resistance of the cir- cuit may not be altered by an alternation of the galvanometer shunt, a compensated box should be used which automatically inserts a resistance for each shunt, in series with the shunted galvanometer, to bring the total resistance up equal to the unshunted value. Thus the current in the main circuit is not altered. See Multiplying Power of Galvan- ometer Shunt; Galvanometer, [l. m.] Shunt - breaking' Resistance. See under Eheostats or Eesistances. Shunt Characteristic, a curve giving the relation of pd to current for a shunt generator run at constant speed. In this case, as the shunt winding is connected across the brushes, the emf, which is practi- cally equal to the pd at no load, rises at once to its full value. As a greater current is taken from the machine, resulting in a drop of potential in the armature and brushes, and weakening of the field due to armature reaction, the pd falls, thus causing a reduction in the current round the shunt coils, and a further reduction of the magnetic field. The pd and emf characteristics therefore both fall away or droop with increasing load, and after a time bend round as shown in the fig. The internal emf can be approximately ob- tained from the pd by drawing a line of lost volts due to the armature and brush resist- ance as in the last case. For small machines 120 100 « 80 -1 O > 60 40 20 ^t-; „,,^^** — •*^.j. ■^^^Cl ::;:; r>- ^. "V ^1 ^ y > y^ i 25^^ X * -^ ^ -H. — 10 15 20 2& 30 36 AMPERES Shnnt Characteristic A, 1436 Bevolutions per minute. B, 1242 Sevolutions per minute, c. Armature drop. where the shunt current is of greater rela- tive importance, the construction is more complicated, but that given above is usually sufficient in practice.' [c. V. D.] Shunt Excitation. See Excitation; Generator. Shunt Field Winding. See Winding, Field. Shunt Loss in a Meter. See Meter, Shunt Loss in a; Meter, Losses in a. Shunt Ratio. See Multiplying Power OF Galvanometer Shunt; Shunt Box. Shunt Reg-ulating Mechanism of Arc Lamp. See Lamp, Arc. Shunt Regulating Rheostat. See Eegulation. Shunt Running. — In energy meters (more especially of the motor type) furnished with a friction compensation for the low loads there is a tendency of the meter to work with no current in the circuit to which it is connected, owing to the constant ex- citation of the pressure circuit of the meter, and the subsidiary driving torque which is always being exerted in the armature system. This is referred to as 'shunt running' or ' creeping '. A small increase in the voltage, a slight vibration, or a jar may be sufficient to start the meter. To prevent ' shunt run^ Shunt Winding — Silicon Bronze 471 ning', a non-creeping device is used which prevents the meter from starting until the current in the main circuit is about 1 per cent of the maximum. An energy meter should not 'creep' or 'run on the shunt' when the normal voltage of the circuit is increased by 20 per cent. See Meter, Friction Compensation in an Induction. Shunt Winding". See Winding, Shunt. Shunt-wound Arc Lamp. See Lamp, Arc. Shunt-wound Dynamo. See Excita- tion; Generator. Shunt -wound Motor. See Motor, Shunt. Shunt - wound Rotary Convertep. See EoTARY Converter. Shunted Galvanometer. See Gal- vanometer; Shunt Box. Shunted Meter. See Meter, Shunted. Shuttle, Winding-. — In winding small gramme ring armatures, the wire is usually first wound on a shuttle which is carried in the hand, the wire being gradually trans- ferred from the shuttle to the armature. See Hand Winding. Shuttle Armature. See Armature. Side -bar Suspension. See Suspen- sion OF Traction Motor. Side -pole Bracket-arm Suspension. See Suspension, Trolley-wire. Side -pole Suspension. See Suspen- sion, Trolley-wire. Side Slot. See Conduit System of Electric Traction. Siemens Armature. See Armature. Siemens Dynamometer. See Dyna- mometer, Siemens. Siemens Unit of Resistance. See Eesistance, Siemens Unit of. Signal, Interlocking", signals at points or cross-overs which are so connected me- chanically or electrically that they can only be operated in groups and in their proper order. Thus a signal cannot give 'line clear' until the points concerned have been set; or both up and down lines blocked at a cross- over. Signal-Lamp Relay. See Relay. Signal Motor. See Motor, Signal. Signalling Systems. — Electrically-oper- ated signalling systems may be divided into several classes. Among these the chief are : (1) semaphores, or similar visible signals at the side of the track ; (2) signals in the cab of the locomotive, operated through contact- rails at the centre or sides of the track, which are in electrical connection with the signal box. The latter system is a great advance, since it acts equally well in all weathers, and if adopted on all engines would obviate difficulties of traffic in fog. Either system may be made to a great ex- tent automatic, i.e. a train indicates its pres- ence on a section of line by automatically maintaining the signal immediately behind it at danger. Interlocking of points and signals is also easily accomplished electrically. See Block System for Railways. (Ref. Gardiner, ' System of Cab Signalling', Journ. I.E.E., vol. xlii, p. 125; Pigg, 'Automatic Cab Signalling on Locomotives', Journ.I.E.E., vol. xl, p. 62; Peter, 'Power Signalling as Installed by the Underground Electric Rail- ways Co. of London', Elec. Rev., vol. Ix., p. 437.) Signals, Electro - pneumatic, signals in which compressed air and electricity both play parts. Silent Arc. See Arc. Silent Discharge. See Discharge, Silent; Static Disturbances; Atmos- pheric Electricity; Brush Discharge; Saint Elmo's Fire; Storms, Electric. Silicate-of-SOda Paste, a paste made of finely-powdered mica and a solution of silicate of soda (water glass). It is used to plug up the cavities formed when the mica insulation between the segments of commu- tators becomes eaten away. The hole must first be scraped clean, and the paste must be packed in as hard as possible, and given time to set before using. The cavities in question are generally formed by excessive sparking at the brushes, especially when this is caused by an open circuit in the armature winding. Silicon Bronze, an alloy of copper, tin, and silicon. Its specific gravity is 8-9. Tests by Hopkinson showed the specific resistance to be a function of the tensile strength. The results are given in the following table : — Specific Resistance at 0° C. in microhms per cm cube. Tensile Strength in Itg per sq cm. 1-67 2'69 5-76 7-80 4500 6500 7500 10000 For the first of the above four grades the 472 Silicon Detector — Single-ended Electric Tramcar increase in resistance was ascertained to be 0'15 of 1 per cent per degree Centigrade in- crease in temperature. Silicon Detector. See Detector. Silicon Limit in Steel. See Steel. Silk as an Insulator.— Silk is here con- sidered for the insulation of magnet wire. It is used for wires of small diameter and is superior to cotton for this purpose, since it will give a thinner covering, and has a more finished appearance. It will generally con- tain less moisture than cotton, and for the same thickness it usually gives a higher in- sulation resistance. See also Cotton as an Insulator. Silk-covered Wire. See Wire, Silk- covered. Silk-fibre Suspension.— A single fibre of raw silk forms a good suspension in gal- vanometers, magnetometers, and other deli- cate instruments where a torsionless support is required. See Quartz-fibre Suspension. Silver (chemical symbol Ag), the best- known conductor of electricity and also of heat. Its specific gravity is 10-5, its specific heat 0'056, its melting-point 950° C, and its specific resistance at 0° C. is 1-48 microhm per cm cube. Its resistance increases at the rate of four-tenths of 1 per cent for every degree Centigrade increase in temperature. The world's annual output of silver is of the order of some 10,000 tons, and the price varies but little from 2s. per Troy oz., which corresponds to £3200 per metric ton. Silver Contact. See Contact, Elec- tric. Silver Voltameter. See Voltameter. Simmance-Abady Photometer Head. See Photometer Head. Simon Interrupter. See Interrupter, Simon. Simplex Tubing. See Conduit, In- terior; Wiring Systems. Sine Currents, alternating or undulatory currents the rise and fall of which can be represented by a sine-curve diagram. See Alternating Current; Sine Wave. Sine Curves. See 'Curve of Sines' under Curve; Quarter of a Period; Quadrature; Power Factor; Phase; Harmonic; Non-sine Waves; Field, Eo- TATiNG; Wave Form. Sine Galvanometer. See Galvano- meter. Sine Law. — If y and x be variables re- lated by the equation y = A sin (a; + e) where A and e are constants, y is said to vary according to a sine law. In electrical problems the important cases are when x is either a time or a space variant. Thus ii y = A sin pt where t represents time measured from any datum, and p is sl con- stant, the time variation of y follows a sine law. The variation of y is periodic, i.e. as time progresses the same values of y con- stantly recur in the same order. See Sine Curves. Sine Wave emf or Current, an emf or current varying according to a sine law; an emf or current of which the value at any instant, say at time t, measuring from a given time datum, may be expressed as A sin (pt + e) where A, p, and e are constants; an emf or current of which the wave form is a sine curve. Equivalent Sine Wave denotes a pure sine wave which may be used in calculation to replace a non-sinuous wave. For instance, a sine wave of emf applied to a circuit con- taining iron produces a distorted current wave. The latter may be regarded as equi- valent to a sine wave of the same rms value, and placed at such an angle to the emf wave that the cosine of that angle is equal to the pf found with the actual distorted wave. Defined in paragraph 82 of the 1907 Stan- dardisation Eules of the A.I.E.E. as a sine wave having the same frequency and the same efifective or rms value as the actual wave. See Non-sine Wave. Mean Value of Sine Wave denotes the true mean or average value of the equispaced ordinates of a sine wave as distinguished from their rms value. This mean value is equal to 2/jr or 0'637 of that of the maxi- mum ordinate. Ems Value of Sine Wave denotes the 'root of the mean of the squares' of the equispaced ordinates of a sine wave, as dis- tinguished from their true mean, or average value. The rms value is equal to l/s/^ or 0'707 of that of the maximum ordinate. See Sine Curves. [r. c] Sine Wave Form. See under Wave Form; Sine Curves. Single Bonding. See Bonding Eail. Single Coil. See Coil, Single. Single-eoil-field Dynamo. See under Generator. Single Control, Cascade. See Cas- cade Motor. Single -ended Electric Tramcar, a Single-fluid Voltaic Cell — Single-phase Motor 473 small car provided with a door and platform at the front end only; the passengers on entering or leaving the car have to pass the driver, who controls the car and collects the fares. An automatic device ensures that the car shall stop when a passenger desires to alight, independently of the driver's action. It is also called the demi-car or one-man car. (Ref. Kaworth's British Patent, No. 2192 of 1903.) Single -fluid Voltaic Cell. See Cell, Voltaic. Single-lamp Transformep. See Teans- fOEMEE, AdAPTEE. Single-layer Winding. See Winding, Barrel. Single-phase Alternator. See Alter- nator. Single-phase Armature. See ' Single- phase Armature' under Armature. Single -phase Armature Reaction. See 'Single-phase Reactions' under Arma- ture Reaction. Single-phase (Monophase) Current. See Alternating Current. Single - phase Electro - pneumatic Traction System. See Arnold Single- phase Electro-pneumatic Traction Sys- tem. Single -phase Energy Meter of the Induction Type. See Meter, Single- phase Induction. Single-phase Equipment. See Equip- ment. Single-phase Generator. See Alter- nator. Single-phase Induction Meter. See Meter, Single-phase Induction. Single-phase Locomotive, a locomo- tive equipped with motors, controllers, transformers, and regulators suitable for operation on a sp railway. See also Single- phase Railway. Single-phase Motor.— [Note. — There is probably at present no department of electrical engineering where there would be greater likelihood of unintentionally overlooking the work of various inventors, than in the case of the development of sp motors. The number of workers in the field is very large. The contributor of the following interest- ing article is one of these workers whose own contri- butions to the developments in this field are consider- able, and his view of the situation is certainly of great interest. It would be too much to expect, however, that his analysis of the situation would at all points conform with the views of rival workers in this field. (See also 2nd ed. of Hobart's ' Electric Motors ', part iii of which is devoted to the subject of sp motors. See also Fynn's ' The Classification of Alternate Cur- rent Motors' and Punga's 'Single-phase Motors'.) — The Editor.'] The sp system of distribution of alternat- ing electricity was extensively used in the early days of electric lighting. Consequently, even in these early days, a need was felt for small motors capable of running on the sp circuits then in vogue. This was at first met by the use of small sp series motors, which were the first of the sp commutator motors. In 1888, however, Ferraris and Tesla inde- pendently invented the polyphase induction motor, depending upon the principle of the rotating field. By means of phase-splitting devices Tesla also applied this principle to the sp motor. It was not, however, then generally realised that the sp induction motor would run without such devices when once brought up to speed. This fact only gradually came to be realised by engineers. C. E. L. Brown would appear to have been the earliest, or amongst the earliest, to real- ise it. In the meantime, Elihu Thomson (U.S.A. Patents Nos. 363,185 and 363,186 of Jan. 26, 1887) had, in 1887, invented the repulsion motor; and Wightman (U.S.A. Patent No. 476,346 of Nov. 14, 1888) the motor now known as the compensated repul- sion motor, and one form of the compensated shunt induction motor. In 1891, Gorges (D.R.P. 61,951 of Jan. 21, 1891) applied the commutator to polyphase motors, invent- ing the polyphase series motor (which see) and the polyphase shunt motor (which see), while C. E. L. Brown, Arnold, D^ri, and others in 1893, and thereabouts, developed several forms of motor with both commu- tator and slip rings, which started as repul- sion or series motors, and, at speed, ran as induction motors, the slip rings being short- circuited after the motor had been brought up to speed. It was also in 1893 that Eicke- meyer and Steinmetz first made the sp series motor a practical machine by the use of the neutralising coil. But the hf then in use prevented the introduction of their motor in practice. The next advance in the subject of sp com- mutator motors is due to the researches of Atkinson, which covered the period between 1895 and 1898. Atkinson foreshadowed much of the work which has since been carried out by others. The results of Atkin- son's researches are described in the Proc. I.C.E., 1898, vol. cii, and in his patents. 474 Single-phase Motor the principal of which is B.P., No. 838 of 1898. In these documents Atkinson ex- plains (1) various types of series induction motor with two brushes per pole pair; (2) shows how the 'squirrel-cage' types of single and polyphase motors are related to the commutator types of machine; (3) shows (in his patents) how 'phase compensation' may be effected; (4) shows a method of varying the speed of machines with shunt characteristics, from synchronism, by field regulation; (5) initiated the methods in use at the present day of regarding the theory of the subject. The diagrammatic method of drawing sp motors, of which frequent use is made in this Dictionary and elsewhere, is also largely due to Atkinson. Even at the present day, Atkinson's paper must be re- garded as one of the most important publi- cations upon the subject. Prior to 1902, however, these develop- ments took place almost entirely on paper, or, at most, in the laboratory. The only type of sp motor in extensive commercial use at that time was the sp induction motor of the type requiring to be started by some phase-splitting arrangement. The only types of sp commutator motor on the market were a repulsion induction motor made by the Wagner Electrical Company of St. Louis, employed chiefly for elevator work; a neu- tralised series motor made by Messrs. Ganz & Co. of Budapest, and by the Creusot Works in France; and a form of repulsion motor due to D^ri. In the year 1902, a paper read by B. G. Lamme of the Westing- house Company, before the A.I.E.E., describ- ing the application to heavy electric traction of the neutralised sp series motor operating at a frequency of 16J cycles per sec, and the thorough advertisement given to the possibilities of phase compensation and com- pounding by the work of Heyland and Latour, aroused great interest in ac com- mutating machinery of all kinds. At about this time the fuller possibilities of the com- pensated repulsion motor, and of the poly- phase shunt motor, were realised by Messrs. Winter and Eichberg and by Latour, and the former's machine (namely the compensated repulsion motor) was developed for traction purposes, first by the Union Elektricitats- Gesellschaft, and later by the AUgemeine Elektricitats-Gesellschaft. A large number of firms soon took up the manufacture of the sp series motor, and many thousands of hp of this motor are now in more or less successful use. The repulsion motor has also received a certain amount of development, and is in use to some extent, though not so widely as the series motor, or as the compensated re- pulsion motor. Motors with shunt charac- teristics have also received a gre9.t deal of development. In 1902 Fynn described (B.P., 22,712, of 1902) a type of compensated shunt induc- tion motor fitted with commutator and slip rings, and with a special rotor winding. In 1903 Latour proposed a type of motor with- out slip rings, and substantially similar to Wightman's motor of 1888, save that Latour stated that the circuit perpendicular to the stator axis may be excited by a transformer instead of being placed directly across the mains. He also used a different arrangement of brushes to that employed by Wightman. In 1904 Arnold and La Cour (D.R.P., 165,053, 165,054, and 165,055, of May 25, 1904) applied phase compensation to Atkin- son's method for varying the speed of sp shunt induction motors, while in 1 905 Punga (B.P., No. 10,585 of 1906; date of German application, May 5, 1905) described a large number of new methods of speed variation and combinations of these with phase com- pensation. He was soon followed by Perret and by Fynn (B.P., No. 2367 of 1906) who independently proposed much the same methods; while Greedy (B.P., 5136 of 1906) pointed out that inductance or capacity placed in series with the brushes perpendicular to the stator axis, allows of an ef&cient regula- tion of the speed above and below syn- chronism. Single-phase Induction Motors The sp induction motor in its most com- mon form consists of a stator provided with a sp winding for use during running, and also usually with an extra winding used with some phase-splitting arrangement for start- ing. The rotor is usually of the same type as in the three-phase induction motor, i.e. it is either of the squirrel-cage type, or else it is fitted with slip rings. Such a motor is not self-starting, as it has no torque when sta- tionary; but if it is started up by external means, as by some phase-splitting arrange- ment, a field is set up normal to the axis of the stator winding, and nearly 90° displaced in phase from the field in that axis. This Single-phase Motor 475 cross field produces the useful torque. The connections for starting and for running are indicated in fig. 1. This is the type of sp motor in most general use at the present Fig. 1.— Single-pbaee Motor A, starting position. B, Running position. I, External inductance. E, Rheostat. time, as it is the only one, except the syn- chronous motor, which has no commutator. One of its chief defects is that, except for that obtained by the use of phase-splitting devices, it has no starting torque, and even with the best phase-splitting arrangements, the starting torque is poor. The efficiency and pf also fall considerably behind those of the three-phase motor. These latter defects, however, decrease with increasing size, and the large sp induction motor, though devoid of starting torque, and much heavier than a three-phase motor of corresponding hp, will not fall far behind it in pf, although it is of low efficiency. Atkinson's Form of Single-phase In- duction Motor. — In Atkinson's form of sp induction motor described in his paper (ProcI.C.E., vol. cxxxiii, 1898), the squirrel- cage rotor which characterises the ordinary form of sp motor is replaced by a co armature and is provided with a commutator upon which rest two pairs of short-circuited brushes, spaced at 90° to one another. The motor is indicated diagrammatically in fig. 2. In view of the fact that this motor possesses pre- cisely the same character- istics as the ordinary sp induction motor, with the additional drawback of a commutator, it has not come into practical use without further modifica- tion. The presence of the commutator, however, al- lows the possibility of starting the machine as a repulsion motor, and of feeding emf into the rotor for the purposes of improv- ing the pf and altering the speed. Hence this basic type of motor is the foun- dation of all the types of sp commutator motor with shunt characteristics which have been proposed during the last few years, and some of which have already come into use. On this account it is desirable to consider rather more fully the principles which govern the action both of it and of the ordinary sp induc- tion motor. The flux in the direction of the stator axis or ' stator field ' is determined in magnitude by the applied emf, and is therefore constant. Consequently at a given speed the emf in the circuit of the brushes at right angles to the stator field, which emf is due to the conductors cutting that field, is also determined. The flux in the direction of this pair of brushes, that is, the flux at right angles to the stator field, is deter- mined in magnitude by this emf, in the same manner as the stator field is determined by the applied emf. This cross fiux therefore is also determined, and is proportional to the speed. Fig, 2.— Atlfinson's Single- phase Induction Motor 476 Single-phase Motor Owing to the rotation of the conductors in this cross flux, a counter emf is set up in the circuit of that pair of brushes which is parallel to the stator axis. This emf is opposite to that induced in the same circuit by transformer action from the primary, and consequently when these two emf are ap- proximately equal and opposite, the motor reaches its maximum speed. The machine may, in fact, be regarded as precisely analo- gous to a cc shunt motor in which the brush circuit parallel to the stator axis plays the part of the armature circuit, the working emf being induced in it by the transformer action from the primary, instead of being conducted-in directly. The cross flux per- pendicular to the stator axis plays the part of the motor field, and produces a counter emf exactly as in a cc motor. Compensated Single-phase Induction Motor. — The compensated single-phase in- duction motor consists of a stator with a Fig. S.— Compensated Single-phase Induction Motor main single-phase winding (with or without one or several auxiliary windings on the stator), whilst the rotor (in the principal form) has two windings; a normal three- phase short-circuited winding (preferably with slip rings) and a cc type of winding connected through brushes to auxiliary windings on the stator, or to the secondary of a small transformer, the primary of which is connected to the mains. The type is in- dicated diagrammatically in fig. 3, in which A represents the mains. Like the polyphase compensated induction motor (which see), the volt-amperes fed into the rotor by means of the brushes are very much less than the magnetising volt-amperes of the stator, which they replace; in fact, at synchronism they merely represent the IE drop. At synchronism it is clear that no current can be induced in the short-circuited winding, and it will consequently produce no effect. It has been found by Heyland that, at speeds in the neighbourhood of syn- chronism, the rotor current is divided be- tween the two windings so that the presence of the short-circuited winding does not pre- vent compensation from being effected. In 1891 (D.E.P., 61,951) Gorges proposed a motor similar to that described above, and appears to have been aware of the possibility of obtaining high pf, but no commercial development appears to have been attempted, and the important characteristics of the machine were first clearly pointed out by Heyland and by Latour. In the above motor, it should be noted that the axis of the brush connected to a given terminal is at right angles to the axis of the stator circuit to which the same terminal is con- nected. Compensated Shunt-induction Single- phase Motor. — The compensated shunt in- duction motor is similar to the Atkinson commutator type of sp induction motor (which see), save that into the circuit of that pair of brushes which is perpendicular to the stator axis, is inserted an emf in phase with that which is applied to the stator. Thus it will be seen that this type of motor is some- what similar to the compensated repulsion motor, or Winter -Eichberg motor (which see), save that instead of the circuit perpen- dicular to the stator axis being joined up in series with the stator circuit, it is now joined in parallel therewith, a transformer being usually employed in both cases. This machine has only a very slight starting torque, and is thus, as a rule, started as some form of repulsion motor. When run- ning at synchronous speed, however, by suit- able adjustment of the emf fed into the exciting circuit, the pf may be kept within a few per cent of unity over a very wide range of load. In fact, this type of machine excels all others except the synchronous in this respect. However, the efficiency, espe- cially in small sizes, is very low, and the machine is more costly than the ordinary induction motor, so that though many of the drawbacks of the latter machine are completely overcome, it has at present only come into very limited use. This machine, like the compensated repulsion motor, was first suggested by Wightman (U.S.A.Patent, 476,346 of 1888), who, however, proposed to connect directly across the mains, the circuit perpendicular to the stator axis. It was independently proposed, and its true Single-phase Motor 477 properties first pointed out, by Latour and by Fynn, the latter apparently being the first to reduce it to commercial form (B.P., 26,897, March 25, 1905). The transformer shown in the arrange- ment employed in fig. 4 is capable of being Fig. 4.— Compensated Shunt-induction Single-pliase Motor replaced by a coil placed upon the stator having the same axis as that of the primary winding, so that the flux produced by this latter induces in the coil an emf in phase with the primary emf. Such a coil will now 1)6 at right angles to the circuit to which it ds connected. In a similar manner a coil at light angles to the 'armature' circuit, i.e. the circuit parallel to the stator axis, if con- nected in series with that circuit, will also serve to compensate the motor. This con- nection was first given in D.R.P., 135,896 (Nov. 6, 1900) by Vogel (see fig. 5 of that patent). Fynn's Compensated Shunt-induction Motor. — It is possible, in certain cases, to Fig. 6— Fynn's Compensated Slinnt-induction Motor combine the compensated shunt-induction motor with the ordinary or squirrel-cage form. An example of this is Fynn's form of compensated shunt-induction motor (B.P., 22,712, 1902), in one form of which, in addi- tion to the ordinary drum winding on the rotor, there is another three-phase winding, into the 'star' of which the drum winding is connected. This second winding is con- nected to three slip rings which are short- circuited when the machine is up to speed. Upon the commutator are placed a pair of brushes connected to an auxiliary winding placed on the stator in such a position that the flux from the primary coil induces in it an emf of suitable phase to produce compen- sation. The same pair of brushes is also used for starting (see fig. 5). Milch's Compensated Shunt-induction Motor (B.P., 3589, 1904). — This is an- other example of a combination of the com- pensated shunt -induction motor with the ordinary induction motor. It is similar to Fynn's (which see), save that instead of the special three-phase winding on Fynn's rotor, three choking coils are tapped on to the equi- distant points on the ordinary drum wind- ing, and joined in star as shown in fig. 6, in which A A repre- sent inductances. As the speed rises, the frequency of the cur- rents in these coils becomes less and less, and with it of course the inductance of the coils, until as it ap- proaches synchron- ism, the coils short- circuit the winding in a manner which has often been proposed in the case of three- phase motors. Consequently no slip rings are required. Variable - speed Shunt - induction Single-phase Motor. — For many years it was thought that all sp shunt-induction motors, even when provided with a com- mutator, must run in the neighbourhood of synchronism. The first to break away from this idea was Atkinson, who in his well-known paper (Proc.I.C.E., vol. cxxxiii, 1898) proposed a modification of his com- mutator induction motor (see 'Atkinson's Form of Single -phase Induction Motor' above), in which an auxiliary coil is con- nected in series with the brushes which lie perpendicular to the stator axis. This coil has an axis parallel to that of the brushes Fig. 6.— Milch's Compen- sated Shunt-induction Motor A, Inductances. 478 Single-phase Motor to which it is connected, and consequently serves to modify the cross flux perpendicu- lar to the stator axis. It has been stated in connection with the commutator induction motor that the cross flux perpendicular to the stator axis corre- sponds to the field of a cc shunt motor, while the brush circuit parallel to the stator axis corresponds to the armature circuit. Con- sequently if we weaken or strengthen this cross flux by means of Atkinson's auxiliary coil, without changing its phase, we shall raise or lower the speed. The same idea is rather obscurely present in Deri's B.P., No. 26,870 of 1902, and Messrs. Arnold and La Cour have proposed to compensate the machine (i).E.P., 165,053, 165,054, 165,055 of May 25, 1904). Punga (B.P., 10,585 of 1906; German application, dated May 5, 1905) has shown that the speed may also be varied in a manner precisely analogous to variable - pressure control of ce shunt motors, by varying the emf applied to that rotor circuit, which corresponds to the armature circuit in a cc motor, i.e. the circuit parallel to the stator axis. He has also suggested a large number of combina- tions of this method with the previous one, and with various methods of compensating. Creedy (British Patent, 5136 of March 2, 1906) has pointed out that the speed may also be varied by putting self-induction or capacity in series with the rotor circuit per- pendicular to the stator axis, so as to weaken or strengthen the cross flux. This method is analogous to the shunt regulation of cc shunt motors. The use of resistance for this pur- pose disturbs the phase of this flux, and is consequently very unsatisfactory. These types of motor have also been described at a later date by Fynn (B.P., No. 2367, of 1906), Perret (French Patent, No. 359,946 of Nov. 29, 1905), and also by Gratzm tiller. Shunt- CONDUCTION Single-phase Mo- tor. — As an alternative to the shunt in- duction motor already described, Fynn has proposed a machine in which the emf in the circuit parallel to the stator axis, which, as mentioned'in connection with shunt induction motors, corresponds to the armature circuit of a cc machine, is conducted in from the mains instead of being produced by induc- tion from the stator circuit. In order to supply along the stator axis a constant field, suitable for producing the cross flux to which the torque is due by its action on the pircuit perpendicular to the stator axis, the 'arma- ture circuit', as it may be called, has a neutralising coil connected in series with it, so that 'armature circuit' and neutralising coil together, produce no flux. In addition to this, there is a magnetising coil along the same axis, which is connected across the mains and so produces the same flux as the Fig. 7.— B^n's Shunt-conduction Single-phase Motor primary coil in a shunt-induction machine. The method is shown in fig. 7. Fynn has proposed a number of methods of varying the speed and of compensating this machine. The machine .is, however, complicated in itself, and, moreover, is only suited for very low voltages, so that on ordinary circuits it would also need a separate trans- former. Single-phase Eepulsion Motor Thomson's Form (see also under Motor, Commutator). — In this machine, the inven- tion of Elihu Thomson, the sta- tor is only pro- vided with a single coil S (see fig. 8), and the axis of the rotor circuit is dis- placed by a certain angle from that of the stator circuit. The brushes are short circuited. The flux perpen- dicular to the Fig. 8.— Thomson Repulsion Motor brUsh line, which is that to which the torque is due, can only be produced by the stator current, and is therefore, until the motor gets saturated, proportional to it. Consequently the machine has a series Single-phase Motor 479 characteristic. The chief advantage of this machine is the complete independence of the rotor and stator windings. This feature renders it possible to keep the rotor voltage low in the interests of good commutation, while the stator may in many cases be wound for a fairly h pr. At synchronous speed the flux in this motor is a purely rotating one, and hence the commutation is equal to that of a ce motor. The motor is, however, not suitable for a wide range of speed, since the commutation beyond synchronism rapidly impairs. In this respect the equivalent series conduction motor (which see) fitted with a transformer is by far preferable. The repulsion motor principle is very fre- quently used to start up all kinds of sp motors with shunt characteris- tics. (Ref.- U.S.A. Patents, 363,185 and 363,186 of 1887.) Inverted Ee- PULsioN Motor. — This difiiers from the ordinary repulsion motor only in that the supply voltage is applied to the rotor, while the stator is short-circuited (see fig. 9). Atkinson's Form of Repulsion Motor. — Thomson's form of repulsion motor has Fig. 9.— Inverted Repulsion Motor Fig. 10.— Atkinson Repulsion Motor the disadvantage (which for many purposes is very serious) of being incapable of reversal except by moving the brushes. This is re- medied in Atkinson's form by dividing up the single primary coil employed in the Elihu Thomson motor into a coil t (see fig. 10), parallel to the brush axis, and a coil f per- pendicular thereto, connected in series with the coil T. The coil t serves exclusively to induce current in the secondary circuit, while F produces the field to which the torque is due. Thus by reversing this second coil, the motor can be reversed, and in general the regulation of this type of motor is easier than that of Thomson's original type. Apart from this, however, the theory and proper- ties of the two machines are identical. Compensated Repulsion Single-phase Motor (see also under Motor, Commuta- tor).— The stator of this machine, which Fig. 11.— Compensated Repnlsion Single-phase Motor may also appropriately be designated the Latour-Winter-Eichberg motor, is wound with a single distributed winding, shown at T in fig. 11. Upon the commutator rest four brushes, Aj A2 and Fj Fj. The axis of the pair A, Ag lies in a line with that of the stator coil T, while that of the pair FjFj is perpendicular to the axis of T. The sta- tor winding T is connected in series with the second pair of brushes, FjFj, while the first pair is short-circuited as shown. This machine difi'ers from the Atkinson repul- sion motor (which see), which also has a pair of short-circuited brushes, in that the circuit perpendicular to these is here upon the rotor, whereas in the Atkinson repulsion motor it is on 'the stator. This circuit is placed upon the rotor in order that a com- pensating emf may be induced in it by its rotation in the fiux along the axis of the coil T. This emf leading largely on the stator current, tends to raise the pf of the machine. In other respects, however, the characteristics of this motor are similar to those of the Atkinson repulsion motor. In its practical application a series transformer is inserted between the stator winding t and 480 Single-phase Motor the brushes FiFj, as shown in fig. 12. This is for the purpose first of making the stator current independent of the rotor current, secondly of regulating the field and the speed, and thirdly to weaken the exciting flux at starting, and thus to improve the commutation at starting (see publications Fig. 12.— Compensated Eepnlaion Motor with Transformer M, Mains. of Winter and Eichberg in the E.T.Z.). (Ref. Wightman, U.S.A. Patent, 476,346 of 1888; Winter and Eichberg, B.P., 23,288 of 1902; Latour, B.P., 1494 of 1904.) Latour's Compensated Eepulsion Single-phase Motor. — It has already been Fig. 13.— Latour's form of Compensated Eepulsion Motor pointed out that the sp compensated repul- sion motor (which see) was independently invented by Wightman, by Winter and Eichberg, and by Latour. In addition to the types described elsewhere, Latour has suggested a form connected as shown in fig. 13, in which the four brushes on the com- mutator are displaced 45° from their position in the other types. The effect of this method of connection is to improve the ' winding co- efficient' (which see) of the rotor winding, but the number of turns between the short- circuited brushes is now only one-half as great as in the normal type of machine, so that the rotor voltage is reduced about 30 per cent, and the current is raised in pro- portion. Winter-Eichberg Compensated Eepul- sion Single-phase Motor. — Messrs. Winter and Eichberg have proposed the two follow- ing methods of regulation for the compen- sated repulsion motor. Firstly, and this is the method usually employed, and that which was shown in fig. 12, instead of connecting the brushes FjFj directly in series with T, they are connected by means of a transfor- mer whose transformation ratio may be varied, thus varying the strength of the field perpendicular to the short-circuited brushes, and therefore varying the torque. This series transformer is required in nearly all cases, as it seldom happens that the volt- age most suitable for the rotor is that for which it is desired to wind the stator. Secondly, instead of short-circuiting Aj A^, an external voltage may be fed into these brushes from a transformer of variable ratio joined across the mains. Single-phase Eepulsion Motor with Excitation Winding in Series with EoTOR Winding (Self- excited Single- phase Series Induction Motor).— In this Fig. U.— Single-Phase Bepnlsion Motor with the Excitation Winding in Series with the Rotor Winding machine the stator carries two windings, s and F, at right angles to one another (see fig, 14). The axis Aj Ag of the rotor circuit is the same as that of s, but it is connected in series with F. The machine corresponds to a neutralised sp series motor (which see) in which the working emf is induced in the armature circuit by the coil s instead of being conducted to it directly. The charac- teristics of the machine are the same as those of a normal repulsion motor. Single-phase Eepulsion Motor with Voltage Applied to the Ector Brushes. Single-phase Motor 481 — This is defined later on as a series induction- conduction single-phase motor. D]£ri's Form of Single-phase Eepul- siON Motor. — D^ri has proposed (B.P., 374 of 1905) a form of repulsion motor hav- ing four brushes per pole - pair, arranged as shown in fig. 15. The motor is regulated ei- ther by varying the re- sistance R, or by moving the brushes CD. This motor is manufactured by Messrs. Brown- Boveri & Co., and is applied mainly to tex- tile work where smooth starting is an essential condition (see lecture delivered by Schnetzler before the Jahresver- sammlung des Verbands Deutscher Elektrotech- niker, 1907). Arnold and La Cour's Compensated Repulsion Motor, a modification of the compensated repulsion motor in which three brushes only are used. The brushes are con- nected as shown in fig. 16. A second wind- ing E is disposed on the stator in addition to the main or ' transformer ' winding T, and at L-A^A^ Kg. 15.— Deri's Repul- sion Motor with Movable Brushes A and B, Fixed brushes. and s, Movable brushes. M, Mains. Fig. 16.— Arnold and la Cour's Compensated Bepulsion Motor right angles thereto. This is connected in series with t and the brush Fj as shown. Speed regulation is effected by varying the number of turns on this winding. (Eef. B.P., 184 and 185 of 1905.) Single-phase Series Motor In the elementary form of this machine (see fig. 17) the stator carries a single winding Fig. 17.— Elementary Single- Phase Series Motor F which is connected in series with the rotor circuit, whose axis is at right angles to that of the coil F. In fact, the machine is pre- cisely similar to a ce series motor, save that the stator is laminated. The chief defect of this machine is the excessive self-induction of the armature, which may, however, be somewhat reduced by dividing the stator longitudinally along the axis of the coil F, so that while fluxes along this axis are not interfered with, fluxes perpendicular thereto are reduced as much as possible. If this precaution, familiar in cc prac- tice, is taken, the machine at very If and for very small outputs, does not fall hopelessly short of the cc motor, but at higher frequencies the neutralised series motor (which see) must be employed. One of the principal, differences between the cc and the ac series motor is due to the active emf of self-induction between field turns in the latter. To diminish this rela- tively to the armature counter emf, a weaker flux, lower flux densities, and more armature copper are used in the latter machine. Series Compensated Single-phase Mo- tor. — This term has unfortunately come into common use to signify two totally distinct types of machine. When used in conjunc- tion with the names of Winter and Eichberg, Wightman, Latour, Arnold, La Cour, the A.E.G., &c., it usually signifies what has been called in this Dictionary the compen- sated repulsion motor. When used in conjunc- tion with the names of Lamme, Finzi, Stein- metz, Eickemeyer, the Westinghouse Com- pany, the General Electric Company, the Siemens - Schuckert Werke, the Oerlikon Company, &c., it signifies the type which should preferably be termed the neutralised series motor, which is quite a different ma- chine. Series Neutralised Single - phase Motor. — Conductive Method. — Inalmost every modern sp series motor (which see) a neutralising coil is employed to diminish the armature self- induction. This neutralising coil, shown at 482 Single-phase Motor N in fig. 18, consists of a coil wound upon the stator at right angles to the exciting coil F, and in such a manner as to produce a mmf equal and opposite to that of the arma- ture. If such a winding is properly distri- buted so as to produce as nearly as possible Fig. 18.— Single-Phase Series Motor with Conductive Neutralisation the same form of mmf curve round the peri- phery as does the armature, and if it is con- nected in series with it as shown, there need only be a very small flux indeed crossing the air gap. Such a winding cannot of course affect that leakage flux which interlinks the armature circuit alone, and indeed the leak- age inductances of armature and neutralising coil are added together. Fig. 19.— Single-phase Series Motor with Inductive Neutralisation Inductive Method. — Although the conduc- tive method of neutralisation is now em- ployed in nearly all machines, it is possible merely to short-circuit the neutralising coil upon itself, instead of connecting it in series with the armature circuit. In this case the flux due to the armature circuit cannot be eliminated altogether, as sufficient must al- ways remain to produce enough emf to bal- ance that due to the residual impedance of the neutralising coil. It would be a mistake to infer, however, that on this account this method of neutralisation is less effective than the conductive one, since the residual flux simply serves to transfer to the armature circuit a drop in potential precisely equiva- lent to that due to the resistance and local self-induction of the neutralising coil in the conductive method. Fig. 19 represents a sp series motor with inductive neutralisation. F is the main field winding and N is the neutralising winding. Cramp's Single -phase Motor. — This machine consists, as shown in fig. 20, of a stator with salient poles like those of a Cramp's Single-phase Motor sp series motor, and a rotor with com- mutator and brushes of the usual type. The flux from the poles is, however, excited in the following way: instead of the ordi- nary field winding a coil is wound ring-wise round the body of the stator iron as shown in the space on either side of the pole. This coil is divided into two groups of turns, all those on the left-hand side of each north pole being joined in series, as also all those on the right-hand side. The left-hand coil is joined across the mains, and the right-hand coil across the brushes. The result of this is that the left-hand coil tends to produce a flux around the iron of the stator. This causes the right-hand coil to act as a secondary, and to produce a number of ats neai'ly equal and opposite. The two mmf thus produced act together, so that their vector sum produces the flux which proceeds from the field through the armature and back to the field, while their difference produces the transformer flux which passes round the stator. The emf across the brushes is that due to the secondary coil, and may thus bear any desired relation to the primary emf. In this way a transformer flux and field flux are produced in a manner essentially different Single-phase Motor 483 from that used in other motors. The ma- chine may be fitted with a neutralising coil also if desired. Series Induction -conduction Single- phase Motor (Repulsion Motor with Voltage applied to the Rotor Brushes). This machine is similar to the Atkinson repulsion motor, save that an auxiliary emf is fed into the rotor, which will in general be in phase with the emf across the inducing coil, though it may be convenient to vary its phase somewhat in order to raise the pf. The object of feeding in such an emf is to render the regulation of the fluxes parallel and perpendicular to the brush line, inde- pendent of the regulation of the rotor circuit. Fig. 21.— Funga'8 Series Induction-conduction Single- phase Motor It has been pointed out that the commuta- tion of the Thomson repulsion motor at syn- chronism is equivalent to that of a cc motor. The net effect of the auxiliary rotor emf used in this motor is to change the speed of best commutation from synchronism to some other speed which may be varied by varying the emf. Thus by making use of this form of regulation, the range of speed during which good commutation is obtained in the repulsion motor maybe considerably widened. Fig. 21 represents a series induction-conduc- tion sp motor of the Punga type. For the general theory of this motor see p. 267 of the E.T.Z., vol. xxvii, for 1906; also Elec, vol. Ivii, p. 27. Patents: B.P. No. 11,930 of 1904 (May 25, 1904), General EI. Co., Schenectady. B.P. No. 19,209 of 1904 (Sept. 5, 1904), Punga. Alexanderson Series Induction-con- duction Single-phase Motor. — The sp series repulsion (or series induction-conduc- tion) motor invented by Alexanderson and developed by the General Electric Com- pany of Schenectady, embodies several im- portant and interesting features. It starts as a repulsion motor (as shown in fig. 22, in which MM is the source of supply) with the rotor short-circuited at S and in series with the stator winding. The transformer winding T has twice as many turns as the rotor, and as a consequence the rotor current M ( )[h/Wj Fig. 22.— starting Connections tor the Alexanderson Single-phase Motor is twice as great as the stator current. By this arrangement it can be attained that the current passing switch S is only half the rotor current, i.e. the switching gear need only be provided for half the starting current on the rotor. At a certain speed the motor is reconnected as shown in fig. 23. In this Fig. 23.— Sing Connections {or the Alexanderson Single-phase Motor connection the behaviour of the motor is nearly identical with the motor described under 'Series Induction -conduction Single- phase Motor'. A feature embodied in this motor is the reduced winding pitch for the rotor, which gives a better distribution of the flux at the point of commutation. In practice the difi'erent manufacturing companies have developed sp railway motors 484 Single-phase Motor — Single-phase System incorporating various of the principles set forth in the preceding definitions. Oerlikon Single-phase Eailway Mo- tor. — The connections employed by the Oer- likon Company are shown in fig. 24. The windings f' of the auxiliary poles employed are excited from a series transformer T whose Fig. 24.— Oerlikon Single-phase Bailway Motor A, Armature. 0, Compensating winding. r. Main fleld winding, s', Auxiliary pole winding, n, Kheo- stat. T, Series transformer. primary is connected in the main circuit. R is a rheostat for adjusting the strength and phase of the current flowing in the windings of the auxiliary poles. A is the armature of the motor, F its main field wind- ing, and C its compensating winding. SlEMENS-SCHUCKEET SiNGLE-PHASE RAIL- WAY Motor. — In this motor the interpoles mimh T5wnnnn(w> R Fig. 26.— Siemens-Schuckert Single-phase Railway Motor A, Armature. 0, Compensating winding. P, Field winding. V, Shunt winding on commutation pole. K, Besistance. S, Series winding on commutation pole. carry two windings, S and F' (see fig. 25). S is connected in series in the main circuit, and F* (together with a controlling rheostat a) in shunt across the main circuit. A, F, and C are the armature, the main field wind- ing and the compensating winding on the main poles. Miscellaneous Single-phase Motors CoRSEPius Single-phase Motor, a com- mutatorless motor employing main and auxil- iary components. The stator A has both main and auxiliary parts, each wound with two sets of windings. Eotor B is of the squirrel-cage type and is free on the shaft D. Eotor C — ^keyed to the shaft — has windings leading to three slip rings. At starting, one of the auxiliary stator windings is shunted by a resistance, when the currents produce a revolving field, which starts the rotor B. The auxiliary set then acts as a phase splitter, and by suitable connections from this circuit to the main stator winding, the main rotor starts with a fair torque. The connections are by a conductor carried from the junction of the two windings of the auxiliary stator to the corresponding junction of the main Fig. 26.— Corsepius Single-phase Motor stator winding, and by the two main ter- minals; a sufficiently symmetrical polyphase supply being delivered to the main stator. Akno Single-Phase Motor. — This is described under Arno Single-phase Motor. See also Arnold Single -phase Electro- pneumatic Traction System. [f. c] Single-phase Railway, an electric rail- way operated by means of sp ac, which is supplied to the trains by means of a bare overhead conductor, and bow or trolley col- lectors carried on the cars. The locomotives or motor cars are equipped with transformers to reduce the pressure at which the current is collected (which may be 10,000 or even 20,000 volts) to a moderate value for supply to the motors; the latter are, so far as the system is as yet commercially developed, always of the commutator type. The trains may be hauled by locomotives, or may be composed of motor cars and controlled on the multiple-unit system. The return circuit is made by the rails. Two single-phase systems of railway electrification are already in operation in Britain. The most important is the South London elevated section of the London, Brighton, and South Coast Eailway. The other is a short section of the Midland Eailway at Heysham. See also Arnold Single-phase Electro-pneumatic Trac- tion System. Single-phase Rotary Converter. See EoTARY Converter. Single -phase System. See 'Single- phase Current' under Alternating Cub- Single-phase Induction Meter— Skeletonised Constructions 485 rent; Alternating -current System; Single-Phase Railway; Single-Phase Motor. Single -phase Three -wire Induction Meter, See under Meter, Three-wire. Single-phase Winding. See Winding, Single-phase. Single-pole Cut-out, a cut-out arranged for insertion in one main only of a circuit. Single -pole Switch. See Switch, Single-pole. Single Pull-off. See Insulated Hanger. Single Throw. See Switch Types, Designation of. Single-wound Gramme Ring, an an- nular core on which are wound by hand many turns, which are all connected in series. From this winding tappings are taken to the commutator segments from symmetrically located points. See ' Ring Armature ' under Armature; Winding, Armature. Single -wound Multiple-circuit Mul- tipolar Drum Armature. See Arma- ture; Winding, Armature. Single - wound Two - circuit Drum Armature. See Armature; Winding, Armature. Sinking Fund in Electrical Enter- prises. See Central Station for the Generation of Electricity. Sinusoidal Alternating Current, cur- rent varying according to a sine law. See Sine Wave emf or Current; Sine Curves. Sinusoidal Alternating emf. See Electromotive Force, Sinusoidal Alter- nating; Sine Wave emf; Sine Curves. Sinusoidal Curve, a curve of sines, or sine curve; a curve of the same shape as a sine curve. See Curve; Sine Curves. Siphon Condenser, a variety of ejector condenser. See Condenser, Steam. Situations in which there is Danger of Explosion, defined by V.D.E. (which see) as situations in which explosive ma- terial is stored, used for process of manu- facture, or manufactured. (Ref. Journ.I.E.E., vol. xli, p. 167.) See also Situations in which there is Danger of Fire; Satu- rated Power- or Work-rooms; Electric Power Rooms; Power Houses. Situations in which there is Danger of Fire, defined by the V.D.K (which see) as situations where easily inflammable ma- terial is stored or manufactured or used for processes of manufacture, or in which in- flammable mixtures of gases, vapours, dust, Vol. II or fluff may occur. (Ref. Journ.I.E.E., vol. xli, p. 167.) See also Saturated Power- OR Work-rooms; Electric Power Rooms; Power Houses; Situations in which there is Danger of Explosion. Six -phase System.— This system is similar to a three-phase system, but has a separate return conductor for each of the three phases. It is practically only used in connection with six-phase rotary converters, where the connections between transformers and the converter armature are six in num- ber. Six-phase generators are not used, but the six wires are derived from the windings of a three-phase transformer, or group of ^AAAA/^ ^AAAA/V /VWWN mmmm mmmm mmmm Six-phase System transformers, by bringing out both ends of the secondaries (see fig.). Besides the six- phase connection shown in the fig., there is another six-phase connection designated the Double-Delta Cmnection. This is described on p. 331 of Parshall and Hobart's 'Electric Machine Design '. Six-wire Three-phase System. See Six-phase System. Skate. See Contact Skate. Skeletonised Constructions, construc- tions of field spool windings and armature windings in which provision is made for the thorough circulation of air amongst the wind- ings. Such constructions are coming rapidly into extensive use, although prejudices are still in some quarters entertained against them on the ground that oil and moisture can more readily gain access to the windings. While this is so, nevertheless the continuous circulation of warm dry air is desirable, and by applying a forced blast of air through the machine when at rest, it may be thoroughly cleaned from any accumulations of foreign matter; whereas when the vnndings are covered in, foreign matter is nevertheless 32 486 Skew-coil Winding — Sleeve for Armature Core liable to work its way in, and it remains undetected and may ultimately occasion harm. See Ventilation of Electrical Machinery; Winding, Mummified. Skew-coil Winding". See Winding, Skew-coil. Skewed Pole Shoes. See Pole Shoe. Skiagraph. See Rontgen Rays. Skin Effect, the phenomenon of apparent increase of electric resistance of a more or less solid conductor of large cross -sectional dimensions when conveying an ac; the non- uniformity of current density in such a solid conductor when traversed by an ac, the cur- rent density being generally greatest at the exterior surface or skin. The disturbance of current- density may be looked upon as a self-induced eddy cur- rent in the conductor. It necessarily results in an increase of ohmic loss (as compared with a steady current) proportional to the square of the total current flowing, and con- sequently gives rise to an apparent increase of ohmic resistance. The coefficient of in- crease of resistance depends upon the dimen- sions and the shape of the cross section, the frequency, and the specific resistance. A similar but distinct efiect is experienced in conductors due to the neighbourhood of similar parallel currents. For example, in a heavy multicore cable the non-uniformity of current density in any core may be con- sidered as partly due to eddy currents in- duced by the currents in the neighbouring cores and partly to the self-induced eddy current. It is only the latter eifect which should rightly be considered as comprised under the term sldn effect. Hughes, about 1883, called attention to the fact that the resistance of an iron tele- graph wire was greater for rapid periodic currents than for steady currents. In 1888 Kelvin showed that when ac at moderately hf flow through massive con- ductors, the current is practically confined to the skin, the interior portions being largely useless for the purpose of conduc- tion. The mathematical theory of the subject has been developed by Kelvin, Heaviside, Rayleigh, and others. See also Fall of Potential. [m. b. f.] Slaby-Apco (Telefunken) System of Wireless Telegrraphy. See Wireless Telegraphy. Slate as an Insulating Matepial.— Slate finds some use for switchboards and rheostat faces. It is tougher than marble, but is not so ornamental. The varieties usually employed are known as ' red ' and ' blue ' slate. Slate is apt to contain metallic veins, and absorbs moisture somewhat readily. See also Enamelled Slate; Marble as an Insulating Material. Sleet-CUtteP, a device employed for re- moving from trolley wires or conductor rails the hard coating of frozen sleet which forms upon them under certain conditions of weather. Sleeve, Splicing, a sleeve of tinned cop- per made to fit neatly over the ends of two cables, wires, or other .conductors, and to be sweated to each to make a good electrical joint. The conductors are held in place by small wedges or in some other suitable man- ner, and are then soldered solid in the tube. Splicing sleeves are sometimes used on over- head trolley lines, but are not desirable in such positions, as the trolley wheel does not, as a rule, run smoothly over them. Splicing ears should be employed instead, wherever possible, or, better still, the breaks in the trolley wire should occur only at the section insulators. See Joint, Spliced; Joint, Sleeve; Jointing Aluminium Conduc- tors; Ear, Trolley. Sleeve for Armature Core.— In mount- ing small-sized punchings, the difficulty arises Sleeve lor Armature Core of providing means for the entrance of the ventilating air. Two methods are usually adopted: the first being to provide holes in the core plates near the centre and then assemble them straight upon the shaft, and the second is to assemble them on a spider which provides an entrance for the air be- tween its arms. The spider in this case is, as seen in the fig., more like a sleeve fitted upon the shaft, and hence its name. See Spider; Armature; Armature Hub; Core Disks. Sleeve Joint — Slot Linings 487 Sleeve Joint. See Joint, Sleeve; Sleeve, Splicing. Sleeying", a term applied to tubular braided cotton. See also TUBULAR Braid- ing. Slide Rails. — Small dynamo-electric ma- chines are provided with slide rails to serve as foundation plates or bedplates. Besides being provided with arrangements for bolt- ing down the machine, they usually have horizontal set-screws by which the machine may be adjusted for belt tightening, &c. See Bedplate; Foundation Plate; Slid- ing Base for Alternator Prame. Slide Wire Bridge. See Bridges. Sliding Base for Alternator Frame, an arrangement provided by some firms for facilitating inspection of the rotor and stator without necessitating disassembly. The base- plate is provided with a machined extension, so that the stator may be slid along in an axial direction from the region of the rotor. See Foundation Plate; Bedplate; Slide Eails; Bed Blocks. Sliding Bow. See Trolley, Bow-type. Sliding-contact Dial for Resistance Bridge. — Ten flat metal contacts are ar- ranged concentrically about a central mov- able contact, which has a finger extending over the outer contacts. The width of the finger is not more than that of one fixed contact, so that by revolving it, connection is made from the centre to any fixed contact, and thence through the particular resistance with which the contact is connected, to a common connection joining one end of all the nine resistances employed. The common connection is also joined, by a conductor of negligible resistance, to the tenth 'no-resist- ance' contact, and is in series with the centre- contact of the next dial. See Bridges. Sliding-contact Key. See Key, Slid- ing-contact. Slip, the amount ''by which the actual speed of an asynchronous motor differs, from synchronous speed (which see). The slip varies with the load, and is usually stated as a percentage of the synchronous speed. See Contact Methods of Measuring Slip in Induction Motors; Drysdale Strobo- scopic Method of Slip Measurement. Slip -ring Losses. See Losses, Com- mutator; Friction Coefficient; Brush Eesistance. Slip Rings. See Rings, Slip. Slip Tubing. See Tubing, Slip. Slipper, sometimes used as meaning the collecting shoe (which see); the part of a slipper brake which rubs on the track. See ' Slipper Brakes ' under Brakes. Slipper Brakes. See under Brakes. Slot. — The armatures of modern dynamo- electric machines, as also the stators and rotors of ac motors are almost invariably provided with slots in which the conductors forming the winding are placed. These slots are usually punched in the periphery of the laminations of which the armature is to be built, before assembling the laminations. The metal left between the slots builds up into projections which are known as the teeth. According as the slot width is the same throughout, or is more or less reduced at the periphery, we distinguish between a wide-open slot, a partly-closed slot, and a totally-closed slot. A partly-closed slot is sometimes termed semi- closed, and a totally-closed slot is often called a tunnel, and a winding in such slots is some- times called a tunnel winding. See Partly- closed Slots; Totally - closed Slots; Wide-open Slots; Inductance, Slot. Slot -conduit System of Electric Traction, a system employing a slot, gene- rally situated midway between the track rails, through which the collector runs, this collector, called the plough, serving to convey the electrical energy from the conductors en- closed in the conduit to the motors on the car. See Conduit System of Electric Traction. Slot Inductance. See Inductance, Slot. Slot Insulating Tubes. — More particu- larly in high-voltage machinery, specially- prepared slot insulating tubes are employed to insulate the conductors from the iron of the slot in which they lie. Such tubes are, of course, shaped to suit the slot, and gene- rally contain mica in their composition, with or without paper. Micanite, micarta, and per- tinax are among the most frequently used materials for this purpose. See Micanite; Micarta; Pertinax; Slot Linings. (Eef. 'Insulation of Electric Machines', Turner and Hobart, p. 89 and pp. 256 to 258.) Slot Leakage. See Leakage, Slot. Slot Linings. — As distinct from ordinary insulation on armature coils, slot linings are of two kinds. When the coil that is to oc- cupy the slot is itself fully insulated, a slot lining of treated paper is often employed to protect the coil from being injured by the 488 Slot Meter— Sodium core disks while it is being placed in the slot. For low voltages, the slot lining is sometimes depended on for the main insula- tion, the more or less uninsulated coil being placed in the slot, which is already provided with a lining. For this purpose the lining may consist of paper, treated cloth, or mica, or a combination of any of these. See Slot Insulating Tubes. Slot Meter. See Meter, Prepayment. Slot Wedge, the wooden strip inserted in the mouth of a slot when it has been filled with conductors, in order to retain the wind- ings in place against magnetic and centri- fugal action. Magnetic Slot Wedge, a slot wedge made of a magnetic material, used to produce a uniform flux distribution in the gap, and so eliminate the tufting of the flux occurring with open slots. See Tufting of Flux. Slot Winding. See Winding, Slot. Slots pep Pole per Phase, the total number of slots on one member of a poly- phase machine divided by the number of phases and by the number of poles. Slotted Armature. See Armature. Slotted Conduit. See Conduit System OF Electric Traction; Slot-conduit Sys- tem of Electric Traction. Slow Cyclic Method of Hysteresis Measurement. — In this method, due to ^It tvv^vvvwwwwwUf Slow Cyclic Method of Hysteresis Measurement Messrs. J. T. and D. K. Morris, the rate of change of flux through the specimen is kept constant, and the required rate of change of mmf is observed. In order to control the magnetising current a 'potential dividing resistance' rr' (see Potential Dividing Method) is connected across the battery terminals. Two sliding contacts ss' are then arranged by gearing to travel at equal speeds in oppo- site directions over the resistance. As these approach from the two ends, cross over at the centre, and finally recede to the opposite ends, a variable potential difference will be available which supplies the magnetising cur- rent read by A. This pd, and therefore the magnetising current, is altered at such a rate that the induced emf in a search coil wound on the specimen is kept constant. The not- able feature of the arrangement is the direct deduction of the flux value from the mea- sured emf induced in the search coil, and shown at 6. See also Scott Test or Me- thod OF Constant Induced Voltage; Iron and Steel Testing. [l. m.] Smashing Point. See Lamp, Incan- descent Electric. Smee Cell, a type of primary battery with a zinc positive electrode, a platinised- silver negative electrode, and an electrolyte consisting of 1 part of sulphuric acid and 7 parts of water. See Battery, Primary; Cell, Standard. Smelting of Ores, Electric. See Furnace, Electric. Smooth-core Armature. See Arma- ture. Smooth-core Winding. See Wind- ing, Smooth-core. Sn, the chemical symbol for tin (which see). Snap Switch. See Switch, Snap. Snatch Blocks. See Crane, Electric. Soaking In and Out, a term applied to an eflect observed when a condenser is charged or discharged. On the application of potential, a certain charge rushes into the condenser, after which there is a further flow, which goes on slowly for some time and is known as ' soaking in ' of the charge. Similarly, on discharge, there is, after the flrst rush, a gradual 'soaking out' of the charge. This may be observed with a Ley- den jar, from which several spark discharges may be obtained at intervals after the first one. See Charge, Eesidual; Condenser, Electric; Dielectric. Soames Absorption Dynamometer. See Band Brake for Testing Electric Motors. Sodium (chemical symbol, Na), a greyish- coloured metal with specific gravity of about 0-97 (depending greatly on the purity). Its speciiic heat between — 80° C. and 17° C. has been given by Schiiz as 0-283, and be- tween — 28° C. and 6° C. by Eegnault as 0'293. The electrical conductivity per unit of weight is higher than any of the common metals, the relation of this figure for sodium, aluminium, and copper is 1-00 : 0"70 : 0-33, that is 3-07 : : 2-14 : 1-00. The specific re- Sodium — Solder 489 sistance at 60° C. would be about 6 2 mi- crohms per cm cube. Electrolytic methods of preparing sodium have recently developed to a great extent; and the metal can be produced at Qd. to 7^d. per lb by the Castler process, and is expected soon to be obtained at id. per lb (£37 per ton) by the Ashcroft process. Sodium, Use of, as Electric Conduc- tor. — It has been proposed to use sodium as a conductor both for lines and for heavy wiring about a station. For this purpose the sodium would be contained in an iron tube with air-tight joints. The advantage claimed is entirely one of cost, for of all common metals sodium has the greatest conductivity by weight, being in this respect just over three times as good a conductor as copper. Including the cost of the pipe, it is estimated that a sodium conductor of large size would have 22 per cent of the cost of an equivalent copper conductor. Of course, this percentage varies with fluctuations in the prices of materials. The disadvantages would be: (1) increased cost of construction; (2) prohibitive cost of insulating on account of the large outside diameter, so that the proposal applies chiefly to bare conductors; (3) cost of maintenance, as the pipes would have to be painted at regular intervals ; (4) danger from fire, espe- cially if water obtained access to the sodium. No commercial use has yet been made of this suggestion. (Eef. Betts, Elec, Iviii, p. 218.) Soft-drawn Copper Wire. See Cop- per; Wire, Copper. Soft-iron Ammeter. See Ammeter. Solari Coherer. See Coherer. Solder, a generic name for fusible alloys used to unite different metal parts. In elec- trical engineering the solder used is prac- tically always an alloy of tin and lead. As the electrical conductivity of such an alloy is usually about one-seventh of that of cop- per, the best joint between copper conductors is made by bringing the copper surfaces as close together as possible and using a mini- mum of solder. For jointing, especially where work has to be done in awkward positions, it is essential that the solder should have a plastic stage between its liquid and solid states. The accompanying curve gives the melting- points of tin-lead solders as a function of the percentage of tin, according to tables pub- lished by the Smithsonian Institution. Au- thorities differ as to the exact values. Those given in Kemp's handbook result in a curve lying considerably below the curve in the fig., while Hiitte's pocketbook gives values re- sulting in a curve slightly higher. All, how- ever, agree in showing a marked minimum of the melting-point with about 60 to 65 per cent of tin. These differences are doubtless due to the degree of purity of the ingredients used. A good electrical solder complying with the conditions mentioned above contains 40 3ie I wo ^ 2ao \ \ "^ V \ \ • ISO IJO /so \ ,' -' \ X »■ i s i i to TO x X ia> FbrCanC oj Tin. Melting Points of Tin-lead Solders (Smithsonian Institution Tables) to 45 per cent of tin, and melts at about 230° to 220° C. with pure ingredients, or lower with commercial samples. 62 per cent of tin makes a very soft solder suitable for tinsmiths' work, but all alloys with more than 50 per cent of tin melt rather suddenly with no marked plastic stage. For ' tinning ' conductors before soldering, pure tin is often used, as it seems to dissolve copper less readily than ordinary solders, and hence does not render the wires so brittle if they are accidentally left some time in the molten metal. Care must be taken not to leave the conductor too long in the tinning bath as the section of the conductor may be appreciably reduced. In very hot solder, copper and brass dissolve freely, and their presence may entirely spoil the quality of the mixture. Large pots of solder, such as are used for automatic soldering of tramway armature coils to their commutators, if left exposed hot to the air, rapidly become richer in lead. This is because the tin oxidises in the air, and it must be corrected from time to time by the addition of pure tin. 490 Soldered Rail Bond — Solvents for Insulating Varnishes (q59S^5?5S^ Solenoid For information on the various solders used in other industries the special literature of these industries should be consulted. [j. s. s. c] Soldered Rail Bond. See Bond. Soldering, Elec- tric. See under Weld- ing, Electric. Solenoid, a magnet- ising coil (which see). The term is generally applied only to small coils operating on plun- gers or solenoid cores. [' A solenoid is a coil wound in the form of a helix. —From McGraw, Electrical Handbook, 1908, p. 201.] See Helix. Solenoid Blow-out, an arrangement for the extinction of the arcs produced in the operation of a tramcar controller, by means of the magnetic field set up by a solenoid without an iron core, which is traversed by an electric current. See Contkoller. Solenoid Controller. See Controller; Lift, Electric. Solenoid Core, the central part of an electromagnet, made of magnetic material, and influenced by the magnetising effect of the solenoid (which see) surrounding it. Solenoid Relay. See Eelay. Sole Plate. See Foundation Plate; Bed Blocks; Bedplate; Slide Eails. Solid Carbons. See Carbons, Arc Lamp. Solid Electrolyte. See Electroly- sis. Solid System of Cable Laying-, a system in which the cables are laid in troughing and the troughing filled in with a suitable compound. The troughing may be of wood, stoneware, asphalt, cast iron, or wrought iron, and the cables are supported in it on bridges of wood, stoneware, or com- position. Wooden bridges should be of hard wood boiled in paraffin wax or some similar compound ; stoneware bridges should be well glazed, with no rough edges. Composition bridges are manufactured, but have been but little used as yet. If the cables are bitumen-insulated, the fiUing-in material should be bitumen; but if they are lead-covered, a cheaper filling-in material may be used. The filling-in ma- terial is run into the troughs warm, and soli- difies as it cools. The troughs should be covered over, and for this purpose bricks or hard tiles from about 1 J in to 3 in thick are often used. The covering should project about 1 in beyond the trough on either side. Wooden, stoneware, or iron coverings are also fre- quently used. (Eef. 'Distribution of Electrical Energy', Snell; O'Gorman in 'The Electrician Primers', 2nd ed., vol. xi, 1906.) See Cable, Under- ground; Cable Troughing; Conduit, Un- derground. Solids, Dielectric Strength of. See Dielectric Strength. Solvent Naphtha. See Naphtha. Solvents for Impregnating Mate- rials. — Solvents are employed for thinning impregnating materials to reduce their vis- cosity, so that the impregnation of fibrous materials may be more thorough and uniform. The impregnating material should be readily and wholly soluble in the solvent, which, to ensure economical working, should not be of too volatile a nature. While the choice of a solvent will be influenced to some extent by the method of impregnation, it will be mainly determined by the nature of the impregnating material; where this is a mixture of several gums or resins, it may be necessary to employ more than one solvent, or a mixture of solvents may prove more economical than the use of one. Solvents that are products of distillation and refining processes, are generally more uniform in quality than natural products. The various solvents are dealt with under their respective names. See also Solvents for Insulating Varnishes; Impregnating Varnishes; In- sulating Varnishes; Naphtha; Benzine; Alcohol; Turpentine; Carbon Bisul- phide; Petroleum and Shale Solvents. [h. d. s.] Solvents for Insulating Varnishes. — Solvents or thinners are volatile liquids, easily convertible into vapour at low tem- peratures. They are added to varnishes to reduce them to a consistency suitable for the use to which they will be put. For insulating varnishes, to obtain high dielectric strength, it is essential that the solvent should completely evaporate, leaving no residue. They may be divided into five classes, viz.: — 1. Petroleum and shale solvents. 2. Coal-tar solvents. 3. Alcohols. Sonometer — Spans in Transmission Lines 491 4. Turpentine. 5. Artificial solvents. For their correct use, experience is neces- sary, since they are not all miscible with each other, and the various gums and resins are not soluble in them all. In thinning varnishes the solvent should be added slowly, and in small quantities, each addition being thoroughly mixed with the varnish. The various solvents are dealt with under their respective names. See also Solvents FOR Impregnating Materials; Insulat- ing Varnishes ; Impregnating Varnishes ; Naphtha; Benzine; Alcohol; Turpen- tine; Carbon Bisulphide; Petroleum and Shale Solvents. [h. d. s.] Sonometer, Hughes, a stretched wire mounted on a graduated sounding board, by means of which the number of vibrations of any given note may be obtained. It may be used in the determination of frequencies not exceeding 2000 or 3000 per sec, by deter- mination of the pitch of note given out by a transformer, or a revolving shaft with teeth on it against which a spring is pressed. See Frequency Indicator or Meter. Sonorous Circuit. See Circuit, Son- orous. Sounder, a simple telegraphic receiver, in very general use on land lines, by means of which the incoming Morse signals are converted into sound signals.- A short arm carrying a piece of soft iron is pivoted hori- zontally above an electromagnet, and is nor- mally held up against a stop by a spring. The current from the sending station ener- gises the electromagnet and draws down the arm against a second stop, thus producing a click. On the current ceasing, the arm flies up again, producing another click. The interval between these sounds indicates, by its length, the transmission of a dot or dash. See Telegraph Systems; Morse Alphabet. Sp, the preferable abbreviation for single pole and for single phase. Space, Clearance. See Clearance Space. Space, Faraday's Dark. See Fara- day's Dark Space. Space Factor, in apparatus where the winding is carried in slots, the ratio of the copper cross-sectional area to the slot area; more generally in any winding the ratio of the copper cross-sectional area to the total space taken up by the winding and its in- sulation. In all electrical windings, a certain amount of insulation must be provided to protect the conductors from one another and also from surrounding metallic objects. This insulation, as far as active material is con- cerned, is worthless, but must be provided to ensure the satisfactory working of the apparatus. To obtain some idea of the ex- tent to which a given winding space may be utilised, the percentage of copper section to the total section is calculated, and this is called the space factor. It varies with the voltage of the winding, the nature of the conductor, i.e. whether round or square wire or pressed cable, the quality of the insula- tion, and with the amount of space occupied by the winding; for whatever the size, the same thickness of insulating envelope is required for protection from surrounding objects. In the slot of a 500-volt generator, the space factor may be as much as 0*65, and in a 10,000-volt alternator as low as 0-15. Field coils wound with copper strip insulated with paper may have a space factor as high as 0'80. The subject is thoroughly discussed in Hobart's 'Electric Motors'. Space Quadrature. See Mechanical Quadrature. Spacers, the pieces inserted in the air ducts of an armature core to keep the passage open against the pressure of the end flanges. These pieces often consist of punched strips of sheet iron, bent in zigzag fashion, with punched-up nicks which en- gage in small holes specially punched in the core plates at each side of the duct. See Distance Piece; Duct, Ventilating; Ventilation of Electrical Machinery; Spacing Fingers. Spacing- Fingers, the spacing pieces used in the ducts of armature cores (see Spacers). The present term is used, how- ever, more in connection with the cast spacing pieces which are often used be- tween the end of the core and the flange, and which have projections (fingers, as it were) corresponding to each tooth, which keep the teeth portions of the last few plates from being bent back. See also Distance Piece. Spans in Transmission Lines, the dis- tances between supports in overhead trans- mission lines. As a general rule, with wooden poles, spans are from 40 to 50 yd in length; but with steel towers as supports, spans of 125 and 150 yd are common, and even much 492 Span Wire — Sparking of Electric Machinery greater spans are sometimes met with. See Transmission Line, Steel Tower; Line Poles; Conductors, Overhead; Line, Overhead; Line Erection. (Ref. 'Dis- tribution of Electrical Energy', Snell; 'Elec- tric Power Transmission', Bell; 'Electric Transmission of Water Power', Adams). Span Wire. See Wire, Span. Spare Plant. See Central Station FOR THE Generation of Electricity. Spark Balls. — In measuring the break- ing-down voltage of liquids or gases, spheres of polished bi-ass (preferably gilded) are em- ployed as electrodes. The size of the spheres, as well as their distance apart, should be noted in all tests. See Sparking Distanck Spark Coil. See Coil, Spark. Spark Discharge, a sudden current of electricity striking from one conductor to another through a medium which, up to the time at which the discharge commences, has been practically a perfect insulator, and which regains this property , when the discharge ceases. Spark discharges may occur through air and other gases, through liquid insula- tors, and through solids; in which last case the hole pierced is not self-sealing, and the resistance does not rise to its original high value after the spark has passed. Though the resistance of a gap of some cm in air is millions of ohms before the discharge com- mences, it may fall during the passage of the current to a very few ohms, the actual value depending on the strength of the cur- rent and on the nature of the electrodes. A spark discharge occurring very frequently has the appearance of an arc for steady cur- rent, but is very different in its properties. All kinds of discharge through a gaseous medium, from a slow intermittent spark to a steady arc, have now been obtained ex- perimentally. The intermediate types are used in wireless telegraphy and telephony. See Discharge, Electric; Discharge, Disruptive; Dielectric Strength. Spark Gap. See Sparking Distance; Spark Balls; Discharger. Spark Recorder. See Instrument, Eecording. Spark Timingr Apparatus.— In order to record the number of sparks per sec given by an induction coil, various types of ap- paratus have been devised by Fleming and others. The spark may be made to record itself on a moving paper tape on which equal intervals of time are also recorded by clock- work. The spark pierces a small hole in the paper as the latter is moved forward between the electrodes. Sparking*, Resistance to. See Dielec- tric Strength. Sparking Distance. — The least distance over which sparks will pass between con- ductors in any medium when they are at a given pd, is called the ' sparking distance '. This distance depends, among other things, on the medium and its condition, on ionisa- tion, on the shape of the electrodes, and on the wave-form of the applied voltage. A table is given below of the sparking dis- tances between needle points in air, and is taken from the Standardisation Eules of the A.I.E.E. In this case the chief disturbing elements would be the variation of the baro- metric pressure and the presence of light. Table op Sparking -Distances in Air between Opposed Sharp Nkkdle Points fob various Effective Sinusoidal Voltages Eilovolts. Distances. (Sq. Root of Mean Square.) Inches. Centimeters. 10 0-47 1-19 20 1-0 2-54 30 1-63 4-1 40 2-45 6-2 SO 3-55 9-0 60 4-65 11-8 70 5-85 14-9 80 7-1 18-0 90 8-4 21-2 100 9-6 24-4 120 11-9 30-1 140 14-0 36-4 160 16-1 40-7 180 18-2 46-1 200 20'3 51-4 220 22-4 56-8 240 24-5 62-1 260 26-5 67-3 280 28-5 72-4 300 30-5 77-4 See Dielectric Strength. Sparking Limit. See Limit to Output OF Dynamo-electric Machinery; Commu- tation. Sparking of Dynamo - electric Ma- chinery, the presence of an electric spark at the moving contacts of dynamo-electric machines. Sparking may be due to me- chanical causes, e.g. to insufficient tension in the brush-holder springs, to the brushes being badly bedded, to vibration of the whole or part of the machine causing the brushes to make varying contact with the Sparking Plug — Specific Heat 493 collector. Sparking may arise from these causes both in the case of slip rings in ac machines and in the case of commutators in cc machines. In the latter case, however, if the electric and magnetic constants of the machine are not suitably proportioned, sparking will occur even with perfect mechanical con- ditions. See Reactance Voltage; Arma- ture Reaction; Commutation. Sparking Plug", an attachment which is screwed into the cylinder cover of a gas or petrol engine, and has an insulated central spindle by way of which currents at a high voltage are conveyed to a spark gap at the end of the plug, the flow of the currents PORCELAIN CYLINDER WALLS Sparking Plug being so timed by a contact-maker running in connection with the engine shaft that the gases in the cylinder are exploded at the right instant. There are many forms of construction, and the above sketch shows a typical arrange- ment. Sparking Voltage, the pressure (i.e. the voltage) at which sparks will just pass be- tween two conductors under given conditions as regards distance, surrounding medium, &c. For the conditions governing this voltage see Discharge, Disruptive. For a table of sparking voltages between needle points in air see Sparking Distance. Sparkless Commutation. See Com- mutation. Sparklet Fuse, Partridge, a fuse spe- cially designed for h pr circuits to extinguish the arc formed when the fuse blows. The sparklet is a small metal globe containing carbon dioxide under great pressure, and sealed by fusible metal. Should an arc form, the seal is melted, and the discharge of gas from the sparklet cools and extinguishes the arc, since the sudden expansion of the gas greatly lowers its temperature. Two spark- lets are more frequently employed, one at either end of the fuse. See Fuse; Circuit Breaker. Spear, Battery. See Voltmeter. Specific Conductance. See Conduc- tivity. Specific Conductivity. See Conduc- tivity. Specific Electric Resistance. See Resistance, Specific; also Resistivity. Specific Gravity.— The specific gravity of a substance is the ratio of the weight of a certain volume of that substance to the weight of an equal volume of water. The volume varies with the temperature, but 15° C. is generally taken as the standard temperature of comparison. The average values of the specific gravity of a few amongst the many materials employed in electrical engineering are given in the fol- lowing table: — Platinum 21-2 Gold 19-3 Mercury 13-6 Lead 11-4 Silver 10-5 Nickel 8-9 Copper 8'9 Phosphor bronze ... 8 '6 Wrought iron 7"8 Cast iron 7'2 Zinc 7-1 Aluminium 2"65 Porcelain 243 Magnesium' 1"74 Bitumen 1-35 Vulcanised fibre ... 1'30 "Water 1-00 Sodium 0-97 Ice 0-93 Transformer oil 0'90 Petrol 0-70 Specific Heat. — The specific heat of a substance at any given temperature is the ratio of the heat required to raise its temper- ature by 1° to the heat required to raise, from freezing-point to 1° above freezing-point, the temperature of an equal weight of pure water. (Note. — This is independent of the particular units or temperature scale employed.) Since in raising 1 g of pure water from 0°C. to 1° C, the amount of heat energy required is 1 (small) calorie, the specific heat of a sub- stance is the number of calories per g, or great calories per kg, required to raise its temperature by 1° C. The specific heat, S, varies slightly with the temperature, but may be assumed constant for a small range of f C. about the given temperature T° 0. 494 Specific Hysteresis Loss — Speed Indicator when the heat in great calories required to raise the temperature of W kg of the sub- stance by f is H = W X Si X <. The table gives average values of specific heats (at 0° C.) of various materials, and de- ductions therefrom giving the electrical en- ergy required to raise the temperature by r C. Watt-Iiours Specific required to raise Material. Heat by 1° C. the atO"C. Temperature of | 1kg leu dm Water 1-00 1-16 1-16 Transformer oil ... 075 0-87 0-78 Ice 0-49 0-57 0-53 Sodium 0-29 0-338 0-33 Magnesium 0-25 0-290 0-51 Aluminium 0-21 0-244 0-65 Cast iron 0-13 0-151 1-09 Wrought iron 0-11 0-128 1-00 Nickel Oil 0-128 1-14 Zinc 0-095 0-110 0-78 Copper . . 0090 0-104 0-93 Silver 0-056 0-065 0-68 Gold 0-032 0-037 0-72 Lead 0-032 0-037 0-42 Mercury .. 0-032 0-037 0-50 Platinum . . 0-032 0-037 0-79 Specific Hysteresis Loss, the loss in w per lb, or per kg, through hysteresis alone, in well-laminated cores for B = +10,000 at 50 periods. See Loss, Hysteresis; Tester, Epstein Hysteresis; Figure of Loss; Iron and Steel Testing. Specific Inductive Capacity. See Di- electric Constant. Specific Output, the ratio of the output of a motor or generator to its linear dimen- sions or to its weight. Specific Resistance. See Eesistance, Specific. Specific Temperature Rise, the num- ber of degrees rise in temperature of any part of a dynamo-electric machine per w per unit of radiating surface. Specific Utilisation Coelficient. See Utilisation Coefficient, Specific. Spectral Photometry. See Photo- metry. Spectro - photometer. See Photo- meter, Spectro-. Spectro - photometry. See Photo- metry. Speed, Schedule, the average speed of a car or train over a given route, obtained by dividing the whole time occupied on the journey, including all stops, by the whole distance traversed. Speed, Synchronous, a term applied to ac motors to denote the speed at which the frequency of the induced emf in the motor is equal to that of the supply frequency. Applied to a synchronous motor, it is that speed at which the motor runs normally. The synchronous speed of an induction motor is that at which the slip would be zero. With a machine of p poles fed by a current whose frequency is ~ cycles per sec, the synchronous speed is (— X 120) rpm. See Slip; Synchronism. Speed Control. See Multivoltage Speed Control; Motor, Varying-speed;, Motor, Variable -Speed; Motor, Spin- ner; Motor, Multi-speed; Motor, Ad- justable-speed. Speed Counter. See Revolution Counter; Dial Register; Cyclometer Counter; Speed Indicator or Tachome- ter; Tachometer; Tachometer, Liquid; Vibration Tachometer. Speed Indicator or Tachometer, an instrument for showing by direct deflection the angular or linear velocity of a piece of apparatus. An Electric Speed Indicator or Tach- ometer is a speed indicator or tachometer based on some electrical principle and usually graduated direct in rpm, or (in case of an automobile speed indicator driven from one of the wheels) in miles per hour. They are of three classes: — 1. Magneto Speed Indicator, consisting of a small permanent-magnet dynamo driven at a speed proportional to that to be measured, and connected to the terminals of a low- reading voltmeter, usually of the moving- coil pattern (see Voltmeter) if the dynamo gives cc, or of the hot-wire pattern (see Voltmeter) if ac, graduated in suitable units (rpm, miles per hr, &c.). 2. Eddy-current Speed Indicator, in which a disk or drum is rotated between the poles of one or more permanent magnets at a speed proportional to that to be measured. The eddy-current torque so produced is measured by means of a spring attached to a pointer moving over a scale graduated in suitable units. 3. Besoncmee Speed Indicator, in which an Speed Regulating Resistance — Spiral Connections 495 alternating or intermittent current of a fre- quency proportional to the speed to be measured and produced by a dynamo or contact-maker is transmitted to a resonance frequency indicator which can be graduated direct in rpm or in units. See Dial Eegister; Cyclometer Cou\- ter; Eevolution Counter j Tachometer; Tachometer, Liquid; Vibration Tacho- meter. Speed Regulating Resistance. See Kegulation. Speed Regulation of Shunt Motors. See Eegulation; Motor, Variable-speed; Motor, Adjustable-speed. Speed-time Curve, a mode of represent- ing the motion of a car or train during a run 20 £ toS f20 (0 u ^ V ^ V \ Id A \ S 40 -J; ^ \ < ^" ^ \ \ 80 20 40 60 SECONDS Speed-time and Current-time Curves between two stations or stops, by plotting on squared paper the speed of the car at each moment as ordinates, and the time reckoned from the moment of starting as abscissse. Frequently also the distance traversed and the current or the square of the current are plotted on the same diagram, enabling a number of calculations to be readily made. A typical speed-time diagram is shown in the fig. The fig. also contains another curve showing the relation between current and time, from which it is seen that the maximum current is in this instance 100 amp, and that the supply of electricity is cut off at the end of the 49th sec, the current then falling to zero. Speed -torque Characteristics. See Motor Performance Curves. Spelter, the zinc used for generating hy- drogen in lead-burning apparatus employed for accumulators. See Lead Burning Ap- paratus FOR Accumulators. Sperm Candle. See Candle Power. Spherical Armature. See Armature. Spherical Reduction Factor of Lamps, the fraction by which the mean horizontal op of a lamp must be multiplied in order to obtain the value of the mean spherical cp for that lamp. The spherical reduction factor for ordinary carbon-filament lamps varies between 0'8 and 0'9 according to the type of filament. See Candle Power. Spider, the armed casting which is pressed upon the shaft and forms a mechanical sup- Spider A, Faced plate to which commutator bracket is bolted. B, Dovetail slots for receiving stampings, c, Flange for coupling to engine flywheel. port for the armature core of a cc machine, or for the comniutator of a cc machine, or for the laminations of which an alternator field system is sometimes built or for the rim of a flywheel. The accompanying illustration shows a typical spider for carrying the laminations of a cc machine. The word spider is sometimes applied to the stator frame of an alternator, this being a survival from the days when alternator practice changed from the use of the internal revolving armature to the now almost uni- versal internal revolving field. See Com- mutator Spider; Sleeve for Armature Core. [h. w. t.] Spike, Battery. See Voltmeter. Spinner, that member of a spinner motor which is constructed to rotate but is not connected to the load. In a three-speed spinner motor the member carrying the windings (b) and (c) is called the spinner. See fig. under Motor, Spinner. Spinner Motor. See Motor, Spinner. Spiral Coil is one in which, as seen in the fig., each successive turn lies entirely with- in the previous turn, starting with the outer- most turn of the coil. The successive turns of a spiral coil are thus not of the same size, and are not over- lapping as in a 'lap' coil (which see). Spiral Connections. See Connections, Butterfly. Spiral Coil 496 Spiral Density Tester — Spur Gearing Spiral Density Tester. See Tester, Bismuth Spiral Density. Spiral Winding", an almost obsolete term for the winding preferably designated two-circuit, and often also designated wave winding. See Winding, Two-circuit. Spirit Varnishes.— These are air-drying varnishes, and consist of resinous matter dissolved in a suitable solvent. On exposure to the air in thin layers, the solvent rapidly evaporates, leaving a hard lustrous coat. The insulation resistance of spirit varnishes is high, but their dielectric strength is un- certain. They absorb moisture to some ex- tent, and will not withstand the action of hot oil or of heat, so well as oil varnishes. They dry very rapidly, usually in from fifteen to thirty minutes. See Impregnating Varnishes; Insulating Varnishes. [h. d. s.] Spliced Joint. See Joint, Spliced. Splicing Compound, a term applied to a rubber compound tape, coated on a white drill, and used for covering joints, the ends of leads, and for similar purposes. Splicing Ear. See Ear, Trolley. Splicing Sleeve. See Sleeve, Splicing. Split - armature Tests. See Tests, Split-armature. Split -field Tests, Behrend's. See Behrend's Split-field Method; Tests, Split-armature. Split Fittings, couplings, tee pieces, junction boxes, and other fittings, which are in two parts so that they may be removed without disturbing the wiring located ivithin them. The fig. illus- trates a split tee piece for a system of interior conduit. Interior; Wiring Systems. Split Frame. See Frame, Split. Split Phase. See Phase-splitting Device. Split -pole Rotary Converter. See Rotary Converter. Split -ring Commutator. See Com- mutator. Split Spur Wheel, a spur wheel, usually of cast steel with machine-cut teeth, and divided on a diameter, to enable it to be put in place on a shaft or axle and bolted together without dismantling the shaft. See Gearing for Electric Motors; Spur Wheel. Split Fittings See Conduit, Split Tee Piece. See' Split Fittings. Spool, Field or Magnet, the coil of wire or strip which is fitted on to the pole of a dynamo machine and which carries the cc for exciting the magnetic circuit. See Coil, Field; Coil Magnet. Spool Winding. See Winding, Spool. Sprague Multiple-unit System. See Multiple-unit System of Train Control. Spray Arrester, a sheet of glass or ebonite placed on the top of an accumulator to prevent the diffusion of the acid spray given off during charging. See Accumu- lator. Spreader. See Winding, Forming, and Spreading Machinery. Spread Factor. — In order to minimise the stator impedance and so improve the regulation, the coils in the various phases of an alternator armature are divided up over a number of slots, instead of being con- centrated all in one slot. As a result of this, the emf in the various coils are not all in the same phase, and the resultant emf is not the arithmetical sum of the emf in the several coils. To correct for this, a factor, known as the spread factor, is introduced into emf calculations. For practical estima- tion this reduction factor may be taken as 0'96 in the case of a distributed three-phase winding, 0-90 for a distributed two-phase winding, and 0-84 for a sp winding distri- buted over two-thirds of the pole-pitch (see also Pitch Factor; ' Single-phase Armature ' under Armature). The spread factor is also used in calculating the effective value of the reactive ats in an armature. See Armature Reaction; Breadth Coeffi- cient; Distribution Factor, [h. w. t.] Spread of Winding, the angular dis- tance occupied under one pole by the wind- ing of one phase on an armature, stator, or rotor. For the effect of the spread on the emf generated, see Spread Factor; Breadth Coefficient; Kapp Coefficient. Spreading Armature Coils. See Wind- ing, Forming, and Spreading Machinery. Spreading Coefficient. See Gap Re- luctance. Spring-arm-type Brush Holder. See Brush Holder. Spring Control. See Instrument Con- trol. Spur Gearing, a system of transmitting motion from one revolving shaft to another spur Wheel — Standard of Light 497 by means of toothed wheels, the teeth being cut or cast on the external cylindrical peri- phery of the wheels. See Gearing for Electric Motors; Split Spur Wheel; Spur Wheel. Spur Wheel, a gear wheel provided with teeth on its periphery (see Spur Gearing). The term is commonly applied to the larger of the two gear wheels by which the motion of an electric motor (with single-reduction gearing) is communicated to the driving axle of a car or locomotive, or to the main shaft of a machine. The smaller wheel is usually termed the pinion. See also Gearing for Electric Motors; Pinion. Spurious Resistance. See Eesistance, Spurious. Sq, the preferable abbreviation for square. Square Root of Mean Square. See Eoot-mean-square. Square Wire. See Wire, Square. Squirrel-cage Rotor. See Rotor. Squirted Filament. See Filament. St, the preferable abbreviation for dngle throw. See Switch Types, Designation of. Stability of Electric Generator, a property of a shunt or compound -i^ound generator in virtue of which it maintains its terminal pressure in spite of considerable variations of load or of magnetising current. Stability is obtained by working at fairly high inductions in the iron parts of the mag- netic circuit. If the conditions of working are such as to bring the machine on the straight part of the magnetisation curve (see Saturation Curve), a slight increase of load or a slight decrease in the exciting current will cause the terminal pressure to fall to zero. Under these conditions the machine is said to be unstable. Staggered Coils. — In inductor alter- nators with unstaggered crowns of poles, the coils of the two armatures may be made to give emf corresponding in phase by dis- placing the coils of one armature relative to those in the other by a distance equal to half that between neighbouring polar projections. Such armatures or armature coils are said to be staggered. See Poles, Staggered; Alternator. Staggered Joints, joints in parallel rails so arranged that the joints in one rail do not come opposite the joints in the other rail. Where the joints are bonded, this plan re- duces the chance of the bonds in the two rails failing to make good contact at the same time and so breaking the continuity of the rail-return circuit. See Bond; Bond- ing Rail; Welded Rail Joints; Renew- able Plate for Rail Joints. Staggered Poles. See Poles, Stag- gered. Staggered Punchings.— In segmental armature laminations it is essential to the continuity of the magnetic circuit that the joints shall not always come in the same place. The segments may therefore be ar- ranged each with two dovetail slots, so that staggered Punchings each layer may overlap the next. In the fig. the segments in successive layers are indicated by full and broken lines. Staggering of Brushes. See Brushes, Staggering of. Stalloy. See Steel. Stampings, Armature. See Arma- ture Stampings; Laminations, Arma- ture; Core Disks; Staggered Punch- ings; Punchings. Stand, Insulator, for Accumulators. See Insulator Stand for Accumulators. Standard, B.O.T. See Board of Trade Standards; Board of Trade Regula- tions. Standard Cell. See Cell, Standard. Standard Condenser. See Condenser, Electric. Standard Frame. See Frame, Stan- dard. Standard of Light.— In civilised com- munities, artificial light is a commercial commodity, and consequently it must be measured in order that it may be sold by quantity. There is, as yet, no general agree- 498 Standard of Light ment in regard to standards by comparison with whicli commercial lamps shall be miea- sured. The chief standards employed in various countries may be defined as fol- lows : — Pentane Standard Lamp, a lamp in which pentane is used as fuel, and designed in such a manner as to form a standard of cp. Various pentane lamps have been de- vised, mostly by Harcourt. The chief of these are — Pentane air gas 1 candle standard, the pentane gas being made in a gasholder or portable reservoir. Pentane wick lamp (sometimes known as the Woodhouse and Eawson lamp). Pentane 10 candle standard. The latter has been officially adopted by the Gas Referees for testing Metropolitan Gas, and as the ultimate standard by the Engineering Standards Committee for the testing of glow lamps. It has no glass chimney, but the mixture of pentane and air is burnt at an open Argand burner. It gives a light of 10 British candles when burn- ing in an atmosphere of 760 mm pressure and containing 10 liters of water- vapour per cu m of dry air. (Ref. Notification of the Metropolitan Gas Referees ; Journ. I.E.E., vol. xxxii, p. 126, vol. xxxviii, p. 281 ; 'Gas Analyst's Manual', Abady.) Hefner Standard or Amyl- acetate Standard, the legal standard of light in Fig. 1.— Helner Lamp Germany; a small and compact lamp burn- ing amyl-acetate which passes specified tests for purity. As shown in fig. 1, the liquid is contained in a cylindrical reservoir which forms the base of the lamp. A wick dips into this and passes up a thin-walled Ger- man-silver tube projecting from the centre of the base. The flame is a lambent one, and resembles in general appearance that of an ordinary candle, except that it is cir- cular in cross section. The exact height at which it gives 1 Hefner candle (about 0'915 British candle) is 40 mm. In order to ad- just the flame to the correct height, the lamp is fitted with a sighting arrangement in which an image of the top of the flame is cast on a ground -glass disk and adjusted to a cross line. (Ref. Journ.I.E.E., vol. clxxxii, p. 283; Zeitschrift fiir Instrumentenkunde, 1893.) Carcel Standard of Light, the working standard of the French gas industry, and the ultimate working standard of the French electric-light industry. The lamp is shown in fig. 2, from which it is seen that it has Fig. 2.— Carcel Lamp glass chimney, and a wick of annular cross section to which a continual supply of pure colza oil is maintained by means of a clock- work pump. The lamp gives its standard cp when consuming 42 g of oil per hr. To measure the rate of consumption of oil, the lamp is counterpoised on a balance while measurements are in progress. The light given by the lamp is equal to about 9 '6 decimal candles, 9-8 British candles, or 10'75 Hefner candles. The decimal candle being legal in France, the values found for lamps measured against the Carcel must be multiplied by 9-6 to obtain their value in decimal candles. (Ref. Journ. I.E.E., vol. xxxviii, p. 284.) Violle or Platinum Standard of Light, a light standard in which one sq cm of a surface of molten platinum at the tem- perature of solidification is taken as giving a constant cp. One-twentieth part of this is taken as the standard of cp in France, and is known as the bougie didmale or decimal candle. standard Ohm — Starting of Motors 499 Primary Standard of Light, a standard of light the cp of which can be reproduced from a knowledge of its construction and method of use, without reference to any other standard of cp. Secondary Standard of Light, a reason- ably constant source of light which has been assigned a certain value in terms of a primary standard. An electric glow lamp which has been compared against a primary standard is an example of a secondary standard of light. [c. C. P.] Standard Ohm. See Ohm. Standard Wire Gauge. See Wire Gauge. Stand-by Charges. See Central Station for the Generation of Elec- tricity. Stand-by Losses. See Central Sta- tion FOR THE Generation of Electri- city. Star Connection. See Connections, Three-phase. Star - connection, Four -wire. See Four-wire Star-connection. Star Current in Polyphase System denotes the current in the legs or circuits of a star-connected system, this being equal to the current in the corresponding line. The apparatus into (or out of) which the current flows may be delta (A) connected, but it may nevertheless be useful to speak of the star current flowing into (or out of) the apparatus, meaning thereby, the current which would flow in each of the legs of an equivalent apparatus if wound Y instead of A. Star-delta Switch. See Switch, Mo- tor-starting. Star Grouping. See Connections, Three-phase. Star Point. See Connections, Three- phase. Star Points connected by a Fourth Wire. See Connections, Three-phase. Star Potential in Polyphase System denotes the pressure between each conductor and the neutral point of the system. In a three-phase system this is equal to l/Vs or 58 per cent of the pressure between lines. In a four-wire two-phase system it is equal to half the pressure per phase. Star Triphase Winding. See Wind- ing, Star Triphase; Connections, Three- phase. Starter. See Starting of Motors; Auto-starter. Starting Box. See Starting of Mo- tors. Starting Coil (of a Meter). See Coil, Compounding. Starting Compensator. See Auto- starter; Starting of Motors. Starting Current. See Starting of Motors. Starting Induction Motors. See Starting of Motors; Auto-starter. Starting Motor. See Motor, Start- ing. Starting of Motors and Rotary Con- verters. — In starting continuous-electricity motors, it is necessary, owing to the low resistance of their armatures, to insert re- sistance in the circuit in order to limit the initial flow of current to an amount which will not overheat the armature windings, or cause undue stresses on the mechanical parts. As the speed of the motor increases, and its counter emf rises, the resistance is gradu- ally cut out step by step until the full pres- sure of the circuit is applied to the terminals of the motor. Motor Starter, variously known as motor starting resistance, starting resistance, starting Fig. 1.— Motor starter rheostat, starting box, and starter.^The device employed for efTecting this purpose is gene- rally called a motor starter, and an illustration of a type suitable for a shunt-wound or com- pound-wound motor is given in fig. L In this illustration A, A are the coils of resistance wire, B is the contact-arm or switch-lever, c, C the contacts, and D the pivot on which the arm swings. In the circuit there are also the double-pole fuse F and the double-pole switch s. 500 Starting of Motors and Rotary Converters There are in addition two devices which should preferably be fitted to all motor starters. One of these is the no-volt release, and consists of a small magnet Mj so placed as to attract and hold the armature E, which is mounted on the switch lever, when the iever is in the ' full-on ' position. The lever is provided with a spring which normally keeps it in the 'ofi"' position. The magnet Mj is in the circuit of the shunt field of the mo- tor. The other device is the overload release. This consists of the magnet Mj, the winding of which is connected in the armature circuit of the motor, and which, when excited by a predetermined amount of current, attracts and pulls up the armature G, upon the end of which is the light copper contact H. When pulled up, G makes contact between two pieces of metal connected to the two ends of the winding of Mj, thus diverting the current from Mj, causing it to lose its magnetism, and to let go of the switch lever. The process of starting the motor is as follows. The double-pole switch S is closed, then the switch lever is moved slowly over to the right, pausing a little while on each contact to give the motor time to speed up. When the extreme right-hand position is reached, the magnet Mj holds the lever on. To stop the motor the double-pole switch is opened. The speed of the motor then begins to die down, and the current in the shunt circuit dies down with it. As the current dies down the magnet Mj loses its strength, until finally it lets go the switch lever, which flies to the off position. Should the motor be overloaded, the excessive current taken ener- gises the magnet Mj sufiiciently to pull up its armature, and stop the motor in the manner already described. Starting Torque of Motor. — A motor when starting has to overcome the friction of repose of the machinery to which it is connected, and in some cases it has to impart acceleration to heavy masses, such as fly- wheels. Its torque when starting requires therefore to be greater than its normal run- ning torque, and the starting resistances must therefore be proportioned to give a starling current which exceeds the normal by an amount sufficient to give the required torque. Liquid Starting Eesistance. — In place of the metallic resistance a liquid resistance may be used for motor starters. A liquid starter is illustrated in fig. 2, with reference letters relating to the same parts as on the metallic starter already described. T is the tank containing the liquid, usually a solution of caustic soda, and the switch lever B carries an iron plate which dips into the liquid. It will be noted that there is a no-volt release Fig. 2.— Liquid Starter Mj, but not an overload release. E, F, and S of fig. 2 correspond to E, F, and s of fig. 1. Graphite Starting Resistance. — Motor starters have been employed which are gene- rally similar to liquid starters, but in which finely-divided graphite is used in place of a liquid. These starters are in use to a limited extent. See ' Carbon Rheostat' under Rheo- stats OR Resistances. Racing of Motor. — The speed of a series-wound motor varies in some inverse proportion to the load; and if the whole load is thrown off, the motor-speed may run up to a very high amount. This is techni- cally known as racing. As the speed in- creases the current diminishes, consequently, in order to prevent racing, the no-volt-release magnet is sometimes connected in the main circuit. Then should the motor tend to run away, the diminution of current would cause the magnet Mj (see figs. 1 and 2) to let go the switch lever and stop the motor before any damage could be done. See 'Inverted Converter' under Rotary Converter. Three-phase Starters or Rheostats. — In starting three-phase motors having slip- ring rotors, the primary current is switched on to the stator by means of a three-pole switch. In the rotor circuit a starter is used of similar construction to those for cc machines, but having three sets of resistances and three switch levers joined together and turning on one pivot. Such a starter is shown diagrammatically in fig. 3. S is the stator winding of the motor and R the rotor winding. C are the collector rings or slip rings, s is the three-pole main switch, / is the three-pole fuse, b is the arm of the starter, moving over the contacts c. The starting of Motors and Rotary Converters 501 resistances between the contacts are indi- cated at a. Star Connection. — In fig. 3 the starting resistances in the rotor are joined in star or Y connection, that is to say, their extremities are connected together and the coils lead rig. 3.— star-connected Starter lor Three-phase Motors out in their branches like a star or letter Y; this is also sometimes known as a three-legged connection. See Connections, Three-phase. Mesh Connection. — The starting re- sistances may be joined in a mesh connection similar to that of the stator and rotor con- nections in fig. 3, but this is seldom adopted, as the star connection is usually much more convenient for starting resistances, and as a matter of fact, also for the stator and rotor windings. See Connections, Three-phase. Auto-transformers for Motor Start- ing. — For induction motors having squirrel- cage rotors, auto-transformers are used for starting. These are a simple type of trans- former having only one winding, the second- ary current being taken oflf at some inter- mediate point on the winding. They are placed in the stator circuit and reduce the pressure at starting, thus avoiding excessive currents in the stator and rotor windings and in the supply line. With this arrange- ment the lower the pressure provided at the motor, the less will be the torque against which the motor will start. See Auto- starter. Cascade Control. — In three-phase rail- way work, a method of starting and control- ling motors known as the cascade method has been adopted. It can only be used where motors are operated in pairs, and consists of an arrangement of controller by which, when starting or running slowly, the rotor winding of one motor supplies current to the, stator winding of the second one, so saving loss of energy in resistances. This is also referred to occasionally as tandem connection. See also Cascade Motor. Starting Frequency in the Rotor. — VOL. II The frequency in the rotor of a three-phase motor is proportional to the slip or difference between the speed of the rotor and the field. Thus the frequency in the rotor is a maxi- mum when the stator is first switched on to the circuit, and before the rotor has com- menced to revolve. The fre- quency decreases as the speed of the motor rises. This phe- nomenon is utilised in various of the special methods of start- ing induction motors described below. Starting of Rotary Con- verters. — Where cc is not avail- able, a rotary converter may be started by switching the ac on to its armature slip rings, the amount of current taken being limited by an auto- transformer as in the case of squirrel-cage induction motors. When up to speed the cc side is switched in. Where cc is available it is better to start on the cc side, employing the same method as for a cc motor. The ac side is switched on when the correct speed has been attained. There are many alternative ways of starting rotary converters. A considerable number of these are described in Parshall and Hobart's 'Electric Machine Design'. Starting Small Motors. — The starting of very small motors, whether for continuous or for alternating current, can be done with an ordinary switch without resistances. [c. W. H.J Special Methods of Starting Induc- tion Motors Boucherot's Methods. — In the first of Boucherot's methods he employed more than one concentric set of squirrel-cage conductors as shown in fig. 4. The inner sets are of lower resistance and higher inductance than the outer set, for the purpose of obtaining high starting torque without incurring large slip and low efficiency during normal running. In the second method two stators are employed. One of these stators is capable of being given an angular displacement rela- tively to the other stator. The shaft carries two rotor cores, but the bars of the squirrel- cage rotor run straight through both cores. At the middle point of their lengths, i.e. between the two rotor cores, these bars are all connected to a high-resistance ring. A good torque is obtained at starting by so 33 502 Starting of Motors and Rotary Converters displacing one stator relatively to the other as to oblige the rotor currents to traverse the intermediate high-resistance ring. For normal running, however, there is no angular displacement of the stators, and the currents in the rotor conductors do not make use of the high - resistance intermediate ring, but only of the low-resistance end rings. Con- sequently, for normal running, a low slip and high efficiency may be obtained. Fig. 4.— Boucherot'8 Type ot Squirrel-cage Sotor Boucherot's third type of motor is de- scribed in British Patent No. 9534 of 1900. It resembles the second type in having two stator elements and a high-resistance ring connecting the middle points of the rotor bars. But the two stators have a fixed position with relation to one another, and the phase conditions are suitably modified electrically by a phase transformer at the switchboard instead of by mechanically mov- ing one of the stators. The same result is thus obtained so far as regards conditions at starting and during normal running. In Gorges' method of starting induction motors, which is the subject of British Patent No. 21,141 of 1894 (German Patent No. 82,016 of 1894), the principle employed consists in so arranging the secondary windings at start- ing, as to have the emf in parts of the wind- ings oppose the emf in other parts, thus obtaining but a small resultant emf and current. For normal running, however, the windings are so reconnected that the emf in them are all co-operating. The Fischer - Hirnnen Method (Hungarian Patent No. 6308) is described in 'L'Eclai- rage Electrique' for July 28, 1900. It is illustrated in fig. 5. At starting, the period- icity of the induced emf in the rotor windings is so high that the greater part of the current must flow through the relatively high re- sistances in parallel with the windings on the iron rings. But during normal running, the periodicity of the emf induced in the rotor windings is very low, and hence also the reactance of the windings on the iron rings. Consequently the current may then take advantage of the low-resistance path Fig. 6.— The Fischer-Hinnen Method of Starting Induction Motors A, £eaotance. B, Kesistance. afforded by the reactance coils, and the motor runs with low slip and high efiiciency. Zani Method of Starting Induction Motors. — Zani's method is described in D.R.P. No. 105,986 of 1899, and difTers from the Fischer- starting of Motors and Rotary Converters 503 Hinnen method chiefly in automatically de- creasing the reactance of the windings on the iron rings (or other form of magnetic circuit), by a magnetic circuit so designed that as the motor increases in speed the reluctance of the magnetic circuit is increased in virtue of the increased air gaps occasioned by centrifugal movement of portions of the magnetic circuit. Hohart's Methods of Starting Induction Motors. — In one method devised by Hobart, use is Fig. 6.— Hobikrt's Double-wound Kotor made of the skin effect. The method is the subject of British Patent No. 8476 of 1900, and was devised in 1898 with the same general thought employed later by Zani and Mscher-Hinnen, except that use was made of the variation with the periodicity, of the skin eflfect of conductors, instead of variation in the reactance of the windings. In British Patent No. 25,744 of 1897 an- other method devised by Hobart is described. Two component windings A and B (fig. 6) are wound on the secondary element (in this case the rotor R) in a double spiral, so that they may be considered to be practically super- posed so far as relates to their position in regard to the stator s. These two com- ponent windings are represented diagram- matically side by side. The winding A is permanently tapped off to a common con- nection from three points aaa (the diagram is for two poles; for larger numbers of poles there would be three ta/ps 'per pair of poles). These connections, aaa for A, are the same at starting as for running. The independent winding B is tapped off at points hhh, corre- sponding with the points aaa of A, but has also a second set of taps from points c c c at a suitable angular distance away from Ihh, this angular distance being chosen to suit the conditions of starting torque required for each motor. The windings are shown as of the gramme-ring type merely for the sake of clearness in explaining the principle of the method. They would, of course, in practice generally be constructed as drum windings. The principle involved is that when the rotor R is at rest, and the current is switched on to the primary P of the stator S, currents will be induced in the two component secondary windings A and B of the rotor R. If these two component windings A and B were independently short-circuited from cor- responding points as aaa and hhh, the two sets of current-carrying conductors compos- ing the two windings A and B would co- operate electromagnetically in reacting on ,the system, and in the majority of cases the magnitude of this demagnetising reaction would be greater than that best adapted to secure the desired starting torque; but by making the common connection from the points ccc of the winding B, the points ecc are at the proper angular distance from the connections aaa of winding A, and therefore the resultant secondary reaction (of the two windings A and b) is decreased to the value most suitable for obtaining the desired elec- tromagnetic condition at starting. When sufficient speed has been acquired, the con- nections may, by any suitable device, be re- arranged by opening the connection of the three leads ccc, and connecting together the leads hhh, and the motor will then be pro- perly arranged for running. In 1895 Hobart devised a third method, which may be explained by reference to Fig. 7.— Hobart's Multiple-wound Botor fig. 7. The rotor is provided with more than one winding. In the case illustrated in fig. 7, four windings, denoted by I, II, III, and IV, are employed. If at starting wind- ing I is alone short-circuited, the resistance of the rotor is high, and good starting torque is obtained. During normal running all the windings are short-circuited, the resistance is low, and the motor runs with but small slip and with good efficiency. 504 Starting of Rotary Converters — Station A method proposed by Bradley for start- ing small induction motors consisted in ar- ranging the stator so that it could be moved longitudinally. At starting, the stator sur- rounds a high-resistance rotor, but during normal running it is moved longitudinally to surround a low-resistance rotor. (Eef. 'Starters and Regulators', Rudolf Krause.) See Rheostats or Resistances j Auto-starter ; Switch, Motor-starting; Motor, Starting; Winding, Auxiliary Starting. Starting' of Rotary Converters. See Starting of Motors. Starting Resistance or Rheostat. See Starting of Motors; Rheostats or Resistances. Starting" Switch. See Switch, Motor- starting; Starting of Motors. Starting Torque of Motor. See Start- ing OF Motors. Starting Winding. See Winding, Auxiliary Starting. Stassano Electric Furnace. See Fur- nace, Electric. , Static. See Static Disturbances. Static, End-gap. See End-gap Static. Static Capacity. See Capacity. Static Charge. — In electrostatics the earth is assumed to be at zero potential, and a conductor which has a potential above or below that of the earth is said to be elec- trified, or to possess a static charge. If the potential of the conductor is V, and its capa- city C, the charge Q that it possesses is equal to the product of potential and capacity, elec- trostatic units being employed: Q = VC. See Capacity, Electrostatic; Condenser, Electric; Farad. Static Disturbances, disturbances often occurring in an electrical system at the in- stant when the electrical arrangements are altered by operating switches. They are also occasioned by lightning, and by less manifest special atmospheric conditions. Peck states in his paper entitled ' Protective Devices for High-tension Transmission Circuits ', that the distinguishing features of static disturbances are their oscillatory nature and their extreme suddenness of action. He states that in the case of lightning discharge the frequency may be thousands or even millions of cycles per see. In considering static disturbances, it is quite customary to picture the system as being temporarily traversed by -a medium to which the name static is applied. Thus, in the paper to which allusion has just been made, we read : 'One of the best-known effects due to static is the concentration of potential (which see) upon the outer turns of the windings of electrical apparatus '. Peck has suggested that since the term static, as ap- plied to phenomena which are distinguished chiefly by the extretne suddenness with which they act, is an obvious misnomer, it would be preferable to discard the use of the ex- pression static phenomena and substitute there- for lightning phenomena, which could be sub- divided into two classes : — 1. External lightning phenomena, for desig- nating the phenomena occasioned by cloud lightning. 2. Internal lightning phenomena, for desig- nating the disturbing phenomena attending switching and other such operations. Accumulated Static Charge. — On p. 1060 of the Trans.A.I.E.E., vol. xxvi, part ii, this term is explained as applying to 'the charges of electricity which accumulate on a transmission line due to wind, rain, &c., and represent an electric displacement in the dielectric between line and earth '. See also Static Charge; Lightning Arrester; End-gap Static; Atmospheric Electricity. Static Electricity, electricity at rest; electricity in the form of a charge as distin- guished from dynamic electricity (which see). The expression is particularly used for the electricity induced in the neighbourhood of conductors carrying currents at high poten- tial. See Electricity. Static Friction, the resistance to rela- tive motion by sliding between two surfaces in contact, due to the roughness of the surfaces, and proportional to the pressure between them. The maximum value is usu- ally taken. This occurs when the surfaces are on the point of sliding but have not begun to slide. See Adhesion between Wheel and Rail; Friction Coefficient; Bearing Friction; Loss, Gearing; Loss, Brush Friction al; Loss, Friction and Windage, in Dynamo - electric Machi- nery. Static Phenomena. See Static Dis- turbances; End-gap Static. Static Shoclc. See Shock, Electric. Station, Central. See Central Sta- tion FOR THE Generation of Electricity. Station, Transforming' or Convert- ing. See Central Station for the Generation of Electricity; Substation. station Load Curve — Steam 505 Station Load Curve. See Central Station for the Generation of Elec- tricity. Station Load Factor. See Central Station for the Generation of Elec- tricity. Stationary Armature. See Armature. Stationary Cell. See Cell, Station- ary. Stationary Induction Apparatus.— In paragraph 22 of the 1907 Standardisation Eules of the A.I.E.E. it is stated that sta- tionary induction apparatus change electric energy to electric energy through the me- dium of magnetic energy. They comprise several forms, distinguished as follows: — (a) Transformers. (i) Auto-transformers and compensators. (c) Potential regulators. Stationary Transformer. See Trans- former, Stationary. Stationary Windings. See Windings. Stator, the stationary portion of an elec- tric generator or motor. The word is used chieily in connection with induction motors, and with the external armature of the in- ternal revolving - field type of alternators. The stator of an induction motor is usually that portion of the machine which is con- nected to the line wires, and consequently corresponds to the word 'armature', more often used in connection with cc work. The Plate facing p. 506 includes an illustration of the stator case of an induction motor. Stator Dispersion. See Dispersion, Magnetic. Stator Frame. See Frame, Stator. Stator Pulsating" Flux. See Flux, Pulsating Stator. Statter Time-lag- Device. See Eelay. Stays. See Line Poles. Steady Pin, a small steel pin engaging in a hole drilled in the bedplate and a cor- responding hole drilled in the foot of the frame of a machine. The placing of this pin in position, as soon as the machine is put on its bedplate, causes the foundation bolt holes to register. See Bedplate; Bed Blocks; Foundation Plate. Steam. — By imparting to water a sufS- cient quantity of energy it is converted into steam. The greater the pressure obtaining in the receptacle {e.g. boiler) in which the con- version is eflfected, the higher is the tempera- ture to which the water must be brought before the conversion into steam takes place. Thus, while at an absolute pressure of 1 metric atm (see Metric Atmosphere) evaporation takes place at 99° C, at a pressure of 10 metric atm a temperature of 179° C. must be at- tained by the water before it commences to evaporate into steam. The temperature at which evaporation commences to take place at any pressure is termed the temperature of evaporation or temperature of vaporisation at that pressure. The amount of heat required to bring the water from 0° C. to the tem- perature of vaporisation at any pressure is termed the water heat at that pressure. It is preferably expressed in kelvins per ton. The further amount of heat required to con- vert all the water into steam of this same temperature is termed the latent heat. The latent heat, in kelvins per ton, ranges from 680 at an absolute pressure of only 0'02 metric atm, down to 531 at an absolute pressure of 20 metric atm. The latent heat has two components. By far the largest of these is the internal latent heat, which is the amount of energy required to alter the mo- lecular condition of the water into that of the steam; and the external latent heat, which is the amount required to increase its volume against the external pressure from the volume of the water to the volume of the same weight of steam at the pressure in question. When steam at a given pressure has the same temperature as the water from which it has been converted, it is termed saturated steam. When only part of the water has been converted into steam, the mixture is termed wet steam. Thus wet steam is simply a mixture of saturated steam and water. The percentage of water in a given quantity of wet steam is termed the wetness factor of the steam. When the wetness factor is zero, the steam is sometimes termed dry steam. Thus dry steam is saturated steam with a wetness factor of zero. If to a given quan- tity of dry steam any further quantities of energy are imparted, the temperature of the steam increases above the temperature of vaporisation (which, by the way, is also sometimes termed the saturation temperature), and the steam is said to be superheated steam. The superheat is the number of degrees by which the temperature of superheated steam exceeds that of saturated steam, i.e. by which, at any pressure, the temperature of the steam exceeds the saturation temperature. The specific heat of steam at any temperature and pressure is the ratio of the energy required 506 Steam — Steel to raise a given weight of steam at that pressure and temperature by 1° C, to the energy required to raise an equal weight of water from 0° C. to 1° C. Investigators have not yet arrived at the precise values of the specific heat of steam; but although it is a function both of the temperature and of the pressure, it may be said in general to have values ranging between 0'48 at any temperature for low pressures, up to from 0'5 to 0"6, according to the temperature, for higher pressures. For rough calculations, it is sufficient to take 0'5 as the specific heat of steam. Although redundant, this is often termed the specific heat of superheated steam. In Hobart's 'Heavy Electrical Engineer- ing' are given tables of the properties of steam, in which the kelvin is employed as the unit of energy. This is the most con- venient form in which to have steam tables when dealing with electrical engineering cal- culations. See Kelvin; Energy. Steam, Convertible Energ-y of. See Energy, Convertible, of Steam. Steam Condenser. See Condenser, Steam. Steam Consumption, the amount of steam consumed (preferably expressed in kg) per kelvin {i.e. per kw hr) of output from an electric generating set. See Cen- tral Station for the Generation of Electricity. Steam Dynamo, the combination con- sisting of a steam engine coupled directly to a cc generator. The term is usually restricted to reciprocating engine sets, dy- namos driven by steam turbines being usu- ally known as turbo-dynamos or turbo-generators. Steam - electric Generating Set, an electric generator driven by a steam engine or steam turbine. Steam-turbine Dynamo, the combina- tion consisting of a cc generator coupled directly to the steam turbine by which it is driven. Steel. — The word steel usually refers to iron which has been decarbonised, and has subsequently had a definite quantity of car- bon added to it. More recently, however, processes have been devised for making steel direct from the iron ore. In any case the amount of carbon in the resulting metal is the chief determining factor as to its quali- ties. Low-carbon steels are, in general, soft and ductile, of high melting-point, and of good magnetic quality. High-carbon steels are, in general, hard and unyielding, of lower melting-point, and of inferior magnetic quality. Other ingredients also exert a very considerable influence on the properties of steel, as is seen in the so-called ferro-alloys and special steels. Sulphur should be reduced to the lowest limit commercially possible, as its presence renders the metal 'red-short'. Formechanical reasons, sulphur should never exceed 0'08 per cent, and where particularly good me- chanical strength is desired, the amount of sulphur should be limited to 04 per cent. In these amounts it does not seem to exert a very appreciable efiect on the magnetic qualities of the material. Phosphorus is also to be regarded as an impurity. Its presence renders the steel 'cold-short', and it should be restricted within about the same limits as sulphur. If kept to these, it will not produce a serious efiect on the magnetic qualities. Silicon Steel, an alloy of silicon and iron. For a sample tested by Hopkinson, the specific resistance at 0° C. was 62 microhms per cm cube. Its composition was as fol- lows : — Carbon 0-685 Manganese ... 0'694 Sulphur 0-024 Silicon 3-44 Phosphorus ... 0-133 Silicon hardens the steel into which it enters. For mechanical purposes the amount ^_^_^_^_^_^ of silicon is usually kept below 0"08 per cent, but more may be allowed if man- ganese is present. Silicon in certain proportions and under certain condi- tions possesses the peculiar property of decreasing the hy- steresis loss of steel and iron. This is probably due, at least in part, to its property of hinder- ing carbon from combining with iron, for it is the combined carbon which raises the hysteresis and retentivity of steel. Up to 4 or 5 per cent, silicon increases the suscepti- bility of steel, but above this quantity it has 3 1,2 cc ui 0. » Not given 75-0 Nickel Steel usually contains from 3 to 3-5 per cent of nickel (or more for special purposes). A sample containing 2*7 per cent of nickel gave an ultimate tensile strength of 115,000 lb per sq in (8100 kg per sq cm), while one containing 3-25 per cent of nickel gave 276,000 lb per sq in (19,500 kg per sq cm). Nickel steel is largely used for the shafts of high-speed machines, where its great toughness and freedom from molecular fatigue are of the greatest value. It is im- possible to obtain a sudden fracture with this material, so that sharp fillets and careless machining have less serious effect than in 508 Steel — Steel Core Bonds other steels. Its magnetic qualities are good, the permeability of some 5-per-cent samples being phenomenally high. A sample of Hadfield's nickel steel was tested by Dewar and Fleming. It contained 4-35 per cent of nickel. Its specific resistance at 0° C. was 29 '5 microhms per cm cube, and the resistance increased two-tenths of 1 per cent per degree Centigrade increase in temperature. Aluminium has the effect of softening steel and increasing its fluidity. For castings it has the valuable property of reducing the liability to blowholes. Mitis metal contains 0"05 to 0'2 per cent of aluminium; it has a high magnetic susceptibility at low inductions. Chrome Steel, or ferro-chrome, has great hardness. Steels with 1 to 2 per cent of chromium are exceedingly hard, and cannot be welded. They are used for tool steels and for chilled shot and shell, but they are not employed much in the electrical industry. Two samples tested by Hopkinson were of the chemical compositions and had the specific resistances set forth in the following table : — Sample. Carbon. Manganese. Percent Sulphur. jge of— Silicon. Phosphorus. Chromium. Specific Resistance in microhms per cm cube at 0° 0. 1. 2. 0-687 0-S32 0-280 0-393 0-02 0-02 0-134 0-220 0-043 0-040 1-195 0-621 17-9 19-4 Fbreo-tungsten is also a hard steel, but lacks the excessive brittleness of a very high carbon steel. It is used for cutting tools, and should be forged with care, as it is rather short when hot. A sample with 0-3 per cent of tungsten and 0-52 per cent of carbon gave an ultimate tensile strength of 173,000 lb per sq in (12,200 kg per sq cm). Hopkinson tested an annealed sample of the following composition: — Carbon ... ... 1-36 Manganese ... ... 0-36 Silicon 0-043 Phosphorus 0047 Tungsten 4-65 and determined its specific resistance at 0° C. to be 22-5 microhms per cm cube. Cast Steel is largely used for end plates and for the yokes and poles of cc machines, where its higher magnetic susceptibility more than compensates for its greater cost as com- pared with cast iron. A cast steel of good magnetic quality should not contain more than 0-25 per cent of carbon, 0-08 per cent of phosphorus, 0-05 per cent of sulphur, 0-2 per cent of silicon, and 0-5 per cent of man- ganese. The great failing of cast steel is that it is liable to have blowholes. As men- tioned above, this can be minimised by the addition of a suitable percentage of alu- minium. Forged Steel is used to an enormous ex- tent for magnetic parts in electrical apparatus, mostly in the form of sheets or laminations. Solid forged steel is sometimes used for pole shoes and magnet cores, but is only available where eddy currents are not to be feared. It is used also for the revolving field-magnets of turbo-alternators, but less so than formerly, as the laminated type is now being more de- veloped. Since it is possible to obtain forged steel much lower in carbon than cast steel, there is a corresponding advantage in de- creased hysteresis loss and increased perme- ability. Sheet Steel for punchings is generally very low in carbon, and there is usually no necessity for having especially good mechani- cal qualities. Ordinary laminations contain about 0-07 per cent of carbon, or, in other words, as little as is commercially possible, so that their ultimate tensile strength is as low as 40,000 lb per sq in (2800 kg per sq cm). Phosphorus and sulphur limits are kept as low as possible, but it is only in transformer work that special alloyed steels are com- monly employed. For high-speed rotors a rather higher carbon steel is used, as much as 0-15 per cent carbon being allowable. Thicker laminations may also be permitted here, as rotating parts are not in general subject to hf alternations of magnetic flux, and eddy currents will therefore be less serious. (Ref. 'Magnetic Induction in Iron and other Metals', J. A. Ewing.) [j. s. s. c] Steel and Iron Testing. See Iron AND Steel Testing. Steel Armouring of Cables. See Cable; Cable, Underground; Armour- ing OF A Cable. Steel Conduits. See Conduit; Con- duit, Interior; Wiring Systems. Steel Core Bonds. See Bond. steel Sheathing of Cable ~ Stethoscope 509 Steel Sheathing of Cable. See Cable; Cable, Underground; Armouring of a Cable. Steel Tape Armouring. See Cable; Armouring of a Cable. Steel Tower Transmission Line. See Transmission Line, Steel Tower; Line Poles. Steel Tube System of Wiring. See Wiring Systems; Conduit, Interior; Conduit. Steel Wire Sheathing. See Cable; Armouring of a Cable. Steinmetz Coefficient. See Hysteresis Coefficient; Loss, Hysteresis ; Loss, Iron. Steljes Type - printing Telegraph. See 'Type-printing Systems of Telegraphy' under Telegraph Systems. Step -by -step Method of Magnetic Testing. — There have been various step-by- step methods devised for making magnetic step-by-step Method of Magnetic Testing A, Ammeter. B, Battery. BO, Ballistic galvanometer. Bi B2 Bs, Besistances. Ej Ej K3, Switches or keys. M 0, Magnetising coil. S C, Search coil. tests. Some one of these methods of testing is usually adopted when a complete hysteresis loop is to be traced on a ring sample. In- stead of reversing the magnetising current as in the ordinary ring test (see Ring Method OF Magnetic Testing), it is suddenly al- tered from one value to another, and the resulting change of flux at each step is mea- sured by the throw of the ballistic galvano- meter. The total flux is then the algebraic sum of all throws up to each point, and they must therefore be determined with great care, as errors of observation are ad- ditive. Plotting the total magnetic change against the current, a hysteresis loop (which see) is obtained, from which the hysteresis loss can be derived. A modification of this method, and one ^ving more accurate results, consists in first connecting the magnetising circuit through ■a two-way switch, which in its first position connects the battery to the magnetising coil through a resistance of such a value as to only allow the full magnetising current to pass, and in its second position injects a second resistance which reduces the current by definite steps. Thus one always steps from the maximum to some smaller value of the magnetising force, returning always to the maximum before the next reading. The connections for this method of testing are shown in the accompanying fig. See Iron and Steel Testing, [c. v. d.] Step -down Converter. See Trans- former, Step-down. Step-down Transformer. See Trans- former, Step-down. Step-up Converter. See Transfor- mer, Step-up. Step -up Transformer. See Trans- former, Step-up. Stethoscope, Electrical, an instrument for use in medical work to magnify sounds such as are caused by the beating of the heart or the passage of air through the bronchial tubes, and to make such sounds easily audible. This has been done previ- Electrical Stethoscope ously without resort to electrical methods, but not to the degree that is now possible. The electrical stethoscope is based on the microphone principle, contacts of osmium- iridium being employed and adjusted to give a microscopic air gap, across which a stream of ionised air forms a conducting path. In the fig., A is a shallow metal case with a thin ebonite diaphragm in front. This is placed over the part under examination, caus- ing the diaphragm to vibrate, and the vibra- tions are transmitted by the air in a flexible tube B to a metal diaphragm D on the in- strument. D carries one microphone contact, while the other is attached to a reed in front of the poles of a permanent magnet, which latter carries a winding W. The function of w is to strike and maintain the microscopic air gap between contacts M, and then adjust- ment is made by the fine screw S tilting the magnet, change of tone and volubility being thereby obtained. 510 Sticking of Magnet Armature — Storage of Energy Current through the contacts M, the wind- ing w, and a telephone transformer T is sup- plied by the dry cell C. The magnified sotinds are heard in a telephone receiver connected at x, but a much more pronounced effect is given when the wires x are con- nected to a telephone relay, and thence to the receiver. An important feature is the possibility of transmitting the sounds over a long distance, by extending the length between the relay and the receiver. (Eef. 'A Telephone Relay' by Brown, Journ.I.E.E., May, 1910.) See Relay. Sticking of Magnet AFmature.— When an electromagnet with a good mag- netic circuit is excited and the armature drawn up to it, then when the current is interrupted, the remanent magnetism is often so large as to cause the armature to still be quite strongly attracted. A slight demagnetising action, such as that occurring when a small air gap is made by inserting a small distance-piece of non-magnetic ma- terial between the pole face and the arma- ture, is quite sufficient to entirely eliminate this effect. See Flux, Remanent; Coer- CIVITY; ReTENTIVITY. Stickoline, the trade name of a line of mica-sticking varnishes, which, the manu- facturers claim, have good adhesive proper- ties, and are of such a consistency that it is only necessary to use a very small quantity in the manufacture of mica products. This reduces the amount of adhesive material in the finished product to a minimum, and allows the use of more mica for a given thickness. See also MiCA - sticking Var- nishes; Flexible Mica - sticking Var- nishes. Stiff Field. See Field, Stiff. Stoker, Automatic. — In large electricity supply stations it is often economical to sub- stitute automatic for hand firing. The ap- paratus for this purpose is termed an automatic stoker. Automatic stokers are es- sential adjuncts to very large boilers, since it exceeds the limits of human strength and dexterity to convey the coal to the more remote parts of the grates. See Central Station for the Generation of Elec- tricity. Stoneware Cable Conduit, a conduit of stoneware or earthenware designed to carry and protect an electric cable or cables. See Conduit, Underground. Storage, Flywheel. See Flywheel Storage; 'llgner System' under Mining Equipment, Electrical; Energy, Kinetic OF Flywheel; Storage of Energy; Pul- sation IN Prime Movers. Storage Battery. See Accumulator. Storage Battery Plates. See Accumu- lator Plates. Storage Cell. See Accumulator. Storage of Electricity, an accumula- tion of energy brought about by electric charges or electric currents, and charac- terised by the fact that it manifests itself in electrical form during dissipation. The accumulated energy may, however, be other than electrical in form. A charged con- denser, or Leyden jar, may be said to con- tain a store of electricity in that — 1. Its charged state has been brought about by imparting definite charges of elec- tricity to it. 2. It can remain in this state for a long time, if adequate measures are taken to ensure absence of leakage. 3. During the process of discharge, the accumulated energy is manifested in the form of electric currents. .The term storage of electricity is more commonly associated with storage batteries, where energy is accumulated by virtue of the chemical transformation within the battery, which takes place when a current of elec- tricity is passed through it — the new chemi- cal state being essentially reversible, i.e. returning to the previous state if allowed to give out energy in the form of electric currents. See Accumulator; Leyden Jar; Condenser, Electric. [m. b. f.] Storage of Energy, accumulated ca- pacity for, or capability of doing work. Energy may be stored in a great many ways. For example, a body weighing 1 kg, after it has been raised vertically through a height of 10 m, is capable of doing more work by the amount of 10 kg m in virtue of its elevated position than it was in its original position. This capability of performing an additional 10 kg m of work remains latent in the body so long as it is supported in the elevated position — thus energy is said to be stored in the body to the extent of 10 kg m in virtue of its position. The energy thus stored is termed potential energy. Energy may be stored kinetically in a moving body, as in a flywheel (see Fly- wheel Storage), or chemically, as in coal, and in other ways. A store of energy in storms — Stranded Conductors 511 one form may be liberated and utilised in another form. For example, the energy stored chemically in coal may ^e liberated as heat energy, and the energy stored chemi- cally in a primary or secondary battery may be liberated as electrical energy. In what- ever form the energy is manifested, or pre- sent, it is preferable to express its quantity in kelvins. See Energy; Electricity; Kelvin; Storage, Flywheel. [m.b. f.] Storms, Electric. — Franklin proved that electricity as known in the laboratory is the cause of lightning, by showing that the at- mosphere is practically always in an elec- trified state which is subject to great varia- tions. He experimented with a kite attached to a wet string or conducting wire and ob- tained long sparks between the string and the earth, even in fine weather. Kelvin (then Sir Wm. Thomson) was the first to investigate atmospheric electricity with any degree of accuracy, and for this purpose he invented the type of instrument known as the electrometer. (See Papers on Electro- statics and Magnetism, Sir Wm. Thomson.) He found that the gradient of voltage with- in a few feet of the earth frequently amounts to several hundred volts per foot, particularly in dry fine weather, and is positive upwards. In thundery weather it is subject to great variations, being often reduced to zero and reversed in the course of a few minutes, attaining occasionally a gradient of 1000 volts per ft, and negative above. His ob- servations also showed that comparatively small masses of highly - charged air drift about in fine weather, particularly with an east wind (the observations were made in the Island of Arran, Firth of Clyde, where the east wind is usually dry and fine). Thunderstorms are usually associated with small cyclonic disturbances, and the very high difiierences of potential which occur during such storms are no doubt partly due to the motion in opposite directions of superposed layers of air carrying different charges. It appears probable that the enormous length of the sparks constituting lightning does not in reality indicate so very large a pd as might be deduced from laboratory experiments; nevertheless it cer- tainly amounts to some millions of volts. Thunderstorms reach a maximum in wet tropical regions and a minimum in dry regions whether cold or hot. Thus Central America, the West Coast of Africa, and Burma have over fifty thunderstorms per annum, while the Sahara has less than five, and Great Britain has about eight per an- num (see Bartholomew's Atlas of Meteor- ology). Thunderstorms in mountainous country frequently occur in the valleys with- out reaching up to the summits. Shocks are often felt during thunderstorms even by persons who have not been struck by a spark. These are due to sudden currents in the earth occasioned by the redistri- bution of potential consequent upon a discharge somewhere else. This type of current causes much trouble in wireless telegraphy, as it resembles the signals, and of course travels very long distances. Dur- ing electric disturbances brush discharges are sometimes seen, and heard, on outstand- ing conducting points such as the masts of a ship. The frequency of thunderstorms is related to the eleven-year sunspot and weather cycle, which indicates that the elec- trical conditions of the atmosphere are af- fected by the cause or eifects of sunspots. See Electricity; Atmospheric Electri- city; Static Disturbances; End -gap Static; Lightning Arrester; Saint El- mo's Fire; Brush Discharge; Discharge, Silent. [j. e-m.] Straight-through Joint. See Joint, Straight-through. Strainer, a clamping device to draw together the ends of two wires which are to be joined, or to be fixed to a single fit- ting such as a section insulator in an over- head trolley line. The term is sometimes applied to any device for tightening an overhead wire. See Insulator; Insulated Hanger. Strain Insulator. See Insulator. Stranded Conductors.— 1. For Armature Windings. — Besides the use of stranded cores for cables, stranded conductors have been employed for arma- tures,- especially in large machines. Their chief advantage is flexibility, which makes for decreased labour costs. An incidental advantage is avoidance or minimisation of eddy currents. The flexibility is not always an unmixed blessing, as mechanical rigidity is an essential quality in certain cases. The forms in which stranded conductors have been used are (1) round cable, (2) pressed cable, of rectangular section, largely used for hand-winding of ac armatures, (3) copper braid (little used), and (4) laminated copper, 512 Stranded Core — Stroboscope consisting of a number of copper ribbons laid together and insulated over all. See also Cable, Pressed Stranded. 2. For Cables. See Cable, Underground ; Cable, Rubber; Cable, Flexible; Con- ductors, Overhead. Stranded Core.— In order to increase their flexibility, and to obtain greater strength, cables for electric distribution are now almost always made with a stranded core, that is, a number of wires are used instead of a solid rod of copper. The num- bers of wires that will fit together to make a circular core are 1, 7, 19, 37, 61, 91, 127, &c. The stranding of the core increases the resistance per unit of length by a small per- centage, as the current mainly follows the spiral path of the wire, and only a small portion passes from wire to wire. See Pitch Diameter; Overall Diameter of a Cable. Strap - brake Dynamometer. See Dynamometer, Absorption; Prony Brake; Water-brake Dynamometer; Band Brake FOR Testing Electric Motors. Stray Current. — In electric tramway and railway work, in installations of electric power in engineering yards in the open air, and in similar cases, where it is impossible to maintain perfect insulation, a certain amount of current escapes from the positive main and makes its way across to the nega- tive main, so returning to the generator without having accomplished any useful work. Such a current is objectionable for two reasons. Firstly, it represents power uselessly developed in the prime mover, which may be termed stray power, and secondly, in its passage from the one main to the other, it may cause fire or the destruc- tion of metal work such as pipes, by electro- lytic action. In electric traction systems in which the rails are used as negative con- ductors, there is a tendency for a portion of the current to leave the conductors and -stray back to the generating station through the earth. Such currents do not cause any waste of power, but are apt to have destruc- tive electrolytic eifects on gas and water pipes. In this country the Board of Trade has issued strict regulations with regard to stray currents, and owing to this supervision, no serious cases of destruction have been recorded, although such have occurred in other countries where a more lax system prevails. See Earth Return; Earth Con- nection; Electrolysis; Wallis - Jones Automatic Earth - Leakage Cut - out ; Leakage Indicator. Stray Field. See Flux, Leakage. Stray Flux. See Flux, Leakage; Magnetic Leakage; Dispersion, Mag- netic. Stray Power. See Stray Current. Strength, Mag-netie Field. See Field, Magnetic. Stress, Dielectric. See Stress, Elec- tric; Dielectric Stress; Dielectric Strain; Dielectric Strength. Stress, Electric, an electrical action throughout a medium by which forces are applied to charged bodies. It is indicated by Faraday's lines of force, which show its direction and intensity at any point. See Dielectric Stress; Dielectric Strain; Dielectric Strength. Stress, Electromagnetic, the system of electromagnetic forces in a varying mag- netic field. Stress, Electrostatic. See Stress, Electric. Striking Distance. See Sparking Distance. Striking the Arc. See 'Regulating Mechanism' under Lamp, Arc. Stringing Line Conductor, the pro- cess of erecting the conductor of an overhead line or trolley wire in position, preparatory to stretching it and binding it to the in- sulators, or soldering it to ears. See Line Erection; Line Poles; Overhead System; Conductors, Overhead. Strip-bending Machine, a machine for bending copper strip on its narrow edge. As it is nowadays very customary to employ in electrical apparatus very heavy copper strips thus bent on edge, a strip-bending machine to meet a wide range of requirements may be a very substantial affair, but for lighter work of a simple character inexpen- sive tools suffice. Machines for these pur- poses are described and illustrated on pp. 274 and 275 of Hobart and Ellis's ' Armature Construction'. See Edge- WINDING for Field Spools. Strip-wound Armature. See Arma- ture. Stripping. See Electrometallurgy. Strobograph, a recording stroboscope. See Stroboscope. Stroboscope, a revolving disk or cylinder of opaque material with a series of equally- spaced slots in it, against a similarly slotted Stroboscopic Methods — Substation 513 fixed one, through which a moving object is viewed. The result is a succession of pictures on the retina of the eye, following one another at equal intervals of time. If the object observed be in periodic motion {e.g. a vibrating fork or revolving wheel) it will appear to be at rest if the interval between the passing of two successive slots of the stroboscope is equal to the period of the object's motion. It is possible to determine the rate of revolution of a shaft or wheel with great accuracy by this method. A tuning fork of known frequency with a second slotted plate attached to it may be used in conjunction with the disk or with a fixed slot. Stroboscopic efi'ects are often visible at central stations illuminated by alternate current, the spokes of a rapidly revolving flywheel appearing to be at rest or revolving slowly backwards or forwards. See Stroboscopic Methods. StFOboscopie Methods. — These are methods which may be applied to the mea- surement of any periodically recurring phe- nomena, and depend upon the production of rapid but uniformly periodic illumination; such illumination may be obtained by the spark produced in a suitable gap in the secondary of an induction coil if the primary is periodically connected to a source of supply by a uniformly -rotating contact maker or other similar means, or by a beam of light passing through slits in metal shutters at- tached to the prongs of a tuning fork main- tained in vibration by suitable means. Or again the light may be allowed to pass through radial slits in a uniformly rotating disk. An arc lamp supplied from a source of ac gives intermittent light having twice the frequency of the supply; and a glow lamp with a thin filament may also be used for the same purpose when supplied with an ac. If the light from any of these sources be allowed to fall upon a disk provided with a suitable geometrical device, and attached to a rotating shaft, the device will be visible and may either be stationary or it may appear to be rotating backwards or forwards according as the speed of the shaft is exactly a multiple of the flashes, or a little less or more than an even multiple of the flashes. If the flashes are extremely sudden, short and sharp, as in the case of the induction-coil spark, the definition will be very clear; sufiiciently clear, indeed, to allow the relative positions of a scale and pointer to be accurately de- termined on a rapidly-rotating coupled shaft. These methods may be applied with advan- tage to the accurate measurement of speed, frequency, and slip, and with suitably con- structed apparatus many other periodic phe- nomena may be studied, such, for instance, as torsion in shafts, hunting in rotary con- verters, phase swinging, sparking at commu- tators, balancing of high-speed rotating ma- chinery, iron losses in rotating magnetic fields, &c. See Stroboscope; Drysdale Stroboscopic Method of Slip Measure- ment. (Kef. Beneschke, Elec, vol. xlii, p. 676; M. von Hoor, Zeitschrift fiir Electro- technik, Vienna, 17, p. 211, 1899; Schwertzer, E.T.Z., p. 947, 1901 ; A. Meynier, 'L'Industrie Electrique', p. 560, 1902; C. V. Drysdale, Elec, Aug. 25, 1905; Elec. Eev., Sept. 7 and 14, 1906; Trans, of Opt. Soc, Nov. 16, 1905.) [c. V. D.] Strongly Saturated Field. See Field, Strongly Saturated. Stubs Wire Gauge. See Wire Gauge. Stud System, synonymous with surface- contact system of electric traction (which see). Sub-centre. See Sub-circuit. Sub -circuit usually denotes a branch from a main circuit, and separately protected by its own fuses. In interior wiring, a sub- circuit is a circuit emanating from a distri- buting fuse board. Such a term as 'sub- circuit' is not so rigorously employed as to be capable of being precisely defined. As illustrative of the sense in which it is often employed, reference may be made to the following quotations from p. 8 of the April, 1907, issue of the Wiring Rules of the I.E.E. Clause 21 on p. 9, reads: — ' Conductors must radiate from distributing centres, and in large systems from those centres to suh-eemtres, so that no final suh-cirouit carries more than 5 amp up to 125 volts, or more than 3 amp from 125 to 250 volts for incandescent lighting. The sub-circuits for small heaters must not carry more than 15 amp up to 125 volts, or more than 10 amp from 125 to 250 volts.' Paragraph 22 on the same page is to the efiect that 'Every sub-circuit must be pro- tected on each pole by a fuse'. Submarine Telegraphy. See Tele- graphy, Submarine. Substation.— [' Suhstation means any premises, or that part of any premises, in vrhich electrical energy is transformed or converted to or from pressure above medium pres- sure, except for the purpose of working instruments, 514 Substation for Currents — Superposition of Currents relays, or similar auxiliary apparatus ; if such premises or part of premises are large enough for a person to enter after the apparatus is in position.' — From defini- tions accompanying Home Office 1908 Regulations for Electricity in Factories and Workshops.] Substation for Conversion or Trans- formation of Current. See Central Station for the Generation of Elec- tricity. Substitution Process of Manufactur- ing" Tungsten Lamp Filaments. See • Lamp, Incandescent Electric. Subway, an underground passage for vehicles or pedestrians; especially an elec- tric railway or tramway laid underground near the surface of the roadway. Also a tunnel beneath the street in which water and gas pipes, electric cables, and sewers are laid. Cable Subway, an underground passage in which cables are carried. Such subways are frequently constructed in the neighbour- hood of large generating stations to take the large number of important cables out to the point at which they divide to go in diiferent directions. The passages should be of suf- ficient size to allow of men easily inspecting the cables and the racks which carry them. See Manhole; Conduit, Underground; Cable, Underground. (Eef. ' Central Sta- tion Electricity Supply ', Gay and Yeaman.) Succession of Pliases. See Phases, Succession of. Sucking Booster. See Booster. Sulphating*, excessive chemical combina- tion of the sulphur element in the electro- lyte (of an accumulator) with the active material of the plates. See Accumulator. Sulphur Limit in Steel. See Steel Sumpner's Differential Test, a test applicable to two similar transformers, and Sumpner's Differential Test substantially a modification ofthe well-known Hopkinson test used on cc machines. See Kapp's Method of Testing Dynamos and Motors. As shown in the fig. the h pr windings of the two transformers are directly coupled together, and the two 1 pr windings are also coupled through an ammeter A, wattmeter Wj, and the secondary of a small auxiliary transformer T, arranged to inject into the circuit of the two coupled transformers a small emf. The low-voltage windings so connected are then supplied with power from the mains through a second wattmeter Wg, and the primary of the auxiliary transformer is also connected across the mains with a suitable adjusting resistance K in series. It is evident that if the primary of the auxiliary transformer t be open-circuited, Wg measures the sum of the core losses in the transformers. If the primary of T be closed, a current will circulate in the 1 pr windings, due to the disturbance of balance produced by the small secondary pd of T, and by adjusting R to a suitable value the ammeter A may be made to read the full- load current. Wj reads the power supplied by T to this circuit, corresponding to the copper losses in the transformers. There- fore Wj -f Wj gives the total loss of power in the two transformers, and half this sum represents the power loss of one transformer. The circuit pressure is indicated by the volt- meter V. The main advantages of the test lie in the fact that a full-load test may be made on large units with comparatively small expen- diture of energy, as only the losses in the two transformers have to be supplied by the mains, and further, the h pr circuits once joined round are not again handled during the test. See Testing Transformers. [c. V. D.] Superheat. See Steam. Superheated Steam, See Steam. Superposed Flux. See Flux, Super- posed; Flux, Eesultant. Superposition of Currents denotes the combination of two or more currents in a wire to form a resultant current. The cur- rents may be of different frequencies (as in the case of a non-sinusoidal current wave), or they may consist of continuous and alternat- ing currents in the same wire. In the case of the armature of a rotary converter we have the cc of output superposed upon the ac of input, the resultant current being of curiously irregular shape. Another instance of superposition occurs in the case of an Supply — Surface-contact System 515 auto -transformer where the current in the portion of the winding enclosed by the lower- voltage terminals is equal to the difference between the primary and secondary currents. Note. — ^Where continuous and alternating currents flow simultaneously in the same wire, the heating effect is equal to the sum of the heating effects due to each current alone. Also the effective value of the resul- tant current is equal to the sq root of the sum of the squares of the components. See Compensator. [k. c] Supply, Area of. — The expression means any area within which any local authority, company, or person is authorised to supply electricity. (Electric Lighting Act, 1909, Clause 25.) Supply Electricity in Bulk, To.— The expression means to supply electricity — (a) to any local authority, company, or person authorised to distribute electricity to be used for the purposes of distribution; or (6) to any local authority authorised by any general or special Act to undertake or contract for the lighting of streets, bridges, or public places, or to supply electricity, to be used for the purposes of lighting streets, bridges, and public places. (Electric Light- ing Act, 1909, Clause 25.) Supply Fuses. See Supply Mains. Supply Mains, mains usually entering buildings from under the pavement level, and terminating in double-pole fuses in the basement. These fuses are under the con- trol of the servants of the supply company, and are termed the supply fuses. See Mains. Supply Meter. See Meter, House- service. Supply Side of a Meter. See Meter, Supply Side of a. Surface Condenser. See Condenser, Steam. Surface - contact System, a mode of electric traction on roads, in which electric power is supplied to the cars by means of metallic contact-studs or rails laid in the roadway between the track rails, and pro- jecting slightly above the surface. Normally all the studs are dead except the one under a car, which is connected with a feeder cable by an automatic switch, and usually the track rails form the return circuit. There are many methods of carrying out the system, of which only a very few are actually in operation, although several of them have been given a trial. DOLTER System. — In this system the auto- matic switches are placed below the studs, and are actuated by magnetic attraction ex- ercised by the iron collecting skate, which extends nearly the full length of the car, and is magnetised by electromagnets carried under the car. The skate is long enough to make contact with a stud before leaving the previous one, so that the current is not in- terrupted by the switches, which fall open by gravitation directly the skate leaves the studs. G.B. (Griffiths -Bedell) System. — In this system the skate is replaced by a flexible iron chain, and the studs are flush with the surface of the roadway. The chain is mag- netised by an electromagnet on the car, and is thus attracted into contact with the studs, which are of cast iron; at the same time a plunger inside the stud is drawn downwards by magnetic attraction, and makes contact with an iron distributing cable, which is carried beneath the roadway on insulators in a stoneware pipe. When the chain leaves the stud, the plunger is raised by a spring, disconnecting the stud from the cable. Lorain System. — This is in general simi- lar to the Dolter, the principal differences being in the details of the automatic switch. Diatto and Thompson -Walker Sys- tems. — In these systems the iron plungers of the switches are partly immersed in mer- cury, which forms part of the electric circuit, and supports the plungers by flotation. A skate and electromagnet actuate the switches. The Diatto system is described on p. 170 of vol. i of Wilson and Lydall's 'Electrical Traction '. Paul (Schuckert) System. — The Paul system, originally exploited by the Schuckert Company, employs contact studs without moving parts, the automatic switches being grouped in underground cast-iron boxes, and being actuated in succession by electromag- nets. The skate consists of a chain, held against the studs by springs. Johnson-Lundell System. — In this sys- tem the switches are similarly grouped and operated, but the studs are replaced by sections of rail laid in the roadway, and unconnected with one another, the succes- sive sections being automatically made alive as the car passes over them. KiNGSLAND System. — The Kingsland sys- tem is operated by mechanical switches, which are closed and opened by striker-bars carried 516 Surface Fittings — Suspension by the car, and projecting through a slot in one of the rails; each stud is thus energised in turn as the car passes over it, the current being collected by a skate. Claret- VuiLLEMiER System. — This is one of the earliest surface-contact electric railway systems, a section of which is still in opera- tion in Paris. It is described on p. 187 of vol. i of Wilson and Lydall's 'Electrical Traction'. Still another type is the Whe- LESS System. Notable success has not been achieved with any surface-contact system of electric traction. [a. h. a.] Surface Fittings. See Flush; Buried Work. Surface Leakage. See Leakage, Sur- face. Surface Work. See Buried Work. Surface -wound Armature. See Ar- mature. Surging, phase - swinging, accompanied by periodic increase and decrease of cur- rent in the circuit. See Phase-swinging; Cyclic Irregularity; Irregularity Fac- tor; Crank -effort Diagram; Torque Diagram of an Engine; Damping Grid; Damping; Amortisseur; Variation in Prime Movers; Pulsation in Prime Mov- ers; Flywheel Storage. Susceptance, the component of the ad- mittance which when multiplied into the applied emf gives the wattless component of the current. If r is the resistance, and x the reactance, the susceptance s is given by — a; S = r^ + x^ (See ' Alternating - current Phenomena ', Steinmetz, § 40.) The unit of susceptance is the mho. See Admittance; Conductance. [f. w. c] Susceptibility, the coefficient of induced magnetisation, being the ratio of the mag- netic force to the intensity of magnetisation produced by it. It is usually represented by the symbol k, and is connected with the magnetic permeability [j. by the equation — /u. = 1 -+• 47ri. See Intensity of Magnetisation; Per- meability, [f. w. c] Suspending Wire of Cable. See Cable, Suspending Wire of. Suspension, Bifllar. See Bifilar Sus- pension. Suspension, Catenary. See Catenary Suspension. Suspension, Flexible. See Flexible Suspension. Suspension, Trolley - wire. — Trolley wires may be suspended either from span wires or from bracket arms. The span wires are usually stretched be- tween wood or steel poles planted on either %ide of the road (side-pole span-wire suspension), or between rosettes fixed in the walls of TroUey-wire Suspension buildings, the trolley wires being attached to ears carried by but insulated from the span wires. See Ear, Trolley; Trolley Hanger; Insulated Hanger. Bracket arms are carried either by poles at the roadside (side-pole bracket-arm suspension) or by poles planted in the middle of the roadway* (centre-pole suspension); the insula- tors may be rigidly clamped on the arms, but are preferably, for the purpose of in- creasing the flexibility, suspended by ordi- nary hangers from short span-wires carried by the arms, as shown in the illustration (bowstring suspension). In any case provision should be made in the suspension for double insulation, i.e. two insulators in series be- tween the wires and the poles. Suspension (in Measuring Instru- ments), the method employed for support- Suspension — Switch 517 ing the moving parts of an instrument. The commonest are: (1) Pivots working in jewels (preferably sapphires); (2) needle-points in steel cups; (3) steel or agate knife edges; (4) ligaments (usually of wire or thread in ten- sion). Of these, 1 is the most used, or 2 when the working forces are small (e.g. elec- trostatic voltmeters), and 4 for laboratory or special instruments. See Unipivot Mea- suring Instruments. [k. e.] Suspension (of Traction Motor).— A traction motor must be suspended from the truck in such a way that while the distance between the centres of the armature shaft and the driving axle is maintained constant, a large proportion of the weight of the motor is borne on springs. There are two usual ways of effecting this. In nose suspension, sometimes termed yoke and illustrated in fig. 1, about Fig. 1.— Nose Suspension of Traction Motor half the weight of the motor is carried directly by the axle, by means of suitable bearings; the remainder rests upon a cross- bar parallel with the axle, which supports the motor-case by means of a lug or by set- bolts, and is itself carried at each end by springs connected to the side-frames of the truck. In side-bar suspension almost the whole of the weight of the motor is carried by bars running at right angles to the axle, and spring-supported at one or both ends from the truck. In cradle suspension, side-bars are used, but the weight is divided practically as in the Fig. 2.— Cradle Suspension ol Traction Motor case of nose suspension. Fig. 2 shows the cradle suspension applied to a two-motor equipment. The total weight of the motor is in this case taken by the axles. Yoke suspension is equivalent to nose sus- pension, and central suspension to the side-bar type. In gearless motors the armature is con- centric with the driving axle, and the whole of the weight of the motor is sometimes spring- supported. See Gearless Motor; Truck. Suspension Insulator. See Insulator. Vol. II Swamp, Voltmeter. See Voltmeter. Swamping Resistance. See Volt- meter. Sweating Thimble, a metal lug into which the end of a cable is fixed. SWG, abbreviation for Standard Wire Gauge. See Wire Gauge. Switch, a piece of apparatus for making, breaking, or changing the connections in an electric circuit. The switch has an almost infinite variety of forms. See also Circuit- opening Devices ; Circuit Breaker. ^Eef. 'Electricity Control', Andrews; 'ElekMas. Apparateund Anlagen', Niethammer, vol. iii.) Switch, Accumulator, an arrangement for varying the number of cells connected to either the charge or discharge circuit. Usually there is a double row of contacts, so that the number of cells in either circuit can be varied at will. If, however, no cur- rent is required (at a constant potential) while charging is being done, one regulator can be employed for both the charge and discharge circuit. When heavy currents have to be dealt with, the moving contact is actuated by a screw. Accumulator switches are nowadays much less often employed for varying the number of cells in circuit, boosters having largely taken their place. See Boos- ter; Booster, Reversible; Accumulator, End Cells of an ; Switchboard, Accumu- lator; Switch, Multiple-contact. Switch, Air-break, a switch in which the break occurs in air. Most switches are of this type, and the term is generally used 34 518 Switch in contradistinction to 'oil -break siintch' (which see). See Switch, Air-type. Switch, Air-type, a switch in which the points at which rupture of the circuit is eflfected are surrounded by air, in contradis- tinction to an oil switch (which see), in which the rupture is effected under oil. See Switch, Air-break. Switch, Automatic, a switch which is operated automatically. See Circuit Breaker; Eemote-control System; Con- tactors; Switch, Oil- break; Switch, Electromagnetically-operated. Switch, Automatic Transfopmer, a switch used on the primary of a transformer. or bank of transformers, the function of which is to open or close the primary circuit according to the demand on the secondary side. The desirability of using such switches on a bank of transformers which is only at full load during a small part of the twenty- four hours, is evident, as a considerable saving in w hr is effected, due to the decrease in the aggregate daily core loss. Thus the all-day efficiency of the bank is raised considerably. Many attempts have been made to produce satisfactory switches for this purpose, but up to the present they have not been very widely used. See Switch, Berry Transformer. Switch, Berry Transformer, a device Berry Transformer Switch for automatically switching in and out of circuit a small auxiliary transformer, whose windings are arranged in series with those of a main transformer, the large no-load losses of which it is desired to avoid during periods of low load. The diagram of con- nections for a two-wire sp circuit are shown in the accompanying fig. At periods of low load the load is taken up by the smaller transformer, and the losses occurring are those due to the smaller transformer. The function of the switch is to automatically cut in or out this smaller transformer as the load on the secondary circuit decreases or increases. If the load is small, i.e. if the smaller transformer is in circuit, then when the load increases the action of the switch is as follows: The relay solenoid A draws down the lever arm, and causes the lower carbon-contact Cg to close. This energises coil Kg of the magnet M, so that the movable armature is raised, thus closing switches S and P. These short-circuit the primary and secondary windings of the smaller trans- former at one and the same time. The load is then taken by the large transformer. As the armature of the magnet M is raised, a rocking lever R disconnects the coil Kg and places in circuit coil Kj. When the load falls again, the arm of the relay solenoid A makes contact with the upper contact Cj, thus energising Kg and pulling off switches p and s. When once in position the switches P and S no longer require any force to keep them in place. The apparatus can be ar- ranged for three-phase circuits. Switch, Bus Sectionalising, a switch for dividing bus-bars into sections. Such switches are of service when occasion arises to disconnect part of the bus-bars with their attachments and thus put them out of use, for inspection, alteration, or repair; or to run one part of the system independently of the re- mainder. They are some- times used on ac switch- boards, but their use is by no means general. (Ref. Elek. Mas. Apparate und Anlagen ', Niethammer, vol. iii.) Switch, Canopy, a switch placed in the roof of a tramcar or railway carriage or locomotive, by which the main supply from the trolley line can be cut off. A canopy switch generally takes the form of an automatic circuit breaker in a suitable box or casing. Switch, Condenser, a switch devised to switch in gradually the condensers in a condenser installation. It is operated by remote control, and in the form shown in the accompanying fig. the switch is operated by ropes passing over pulleys to the attendant's place at a safe distance from the condensers. In the fig., which relates to an installation Switch 519 also including a Moscicki condenser (shown at c), a condenser switch is shown at A. See 'Moscicki Condenser ' under Condenser, Condenser Switch A, Condenser switch. B, Line, c, Condenser. D, Choking coil. E, Earth Connection. F, Fuse, o, Transformer. Electric; 'Giles Electric Valve' under Kectifier. Switch, DistFibuting:, a switch on feeders or branch circuits where they separate from the bus-bars or mains, as opposed to generator switches or incoming main switches. Switch, Double -break, a switch ar- ranged, as shown in the fig., so as to have two breaks, in series. This type of switch has the advantage that no current need be carried through the pivot, and that the arc on breaking circuit is subdivided and there- fore less likely to be injurious than when all Donble-break Switch sfto c ® Donble-poie Switch E Double-throw Switch the energy has to be dissipated at a single contact. Switch, Double -pole, a switch with two blades arranged to break connection in two places at once (see fig.). One blade is in the positive side of a cir- cuit and the other blade in the negative side, and on opening the switch the cir- cuit is entirely isolated. Switch, Double-throw, a switch with contacts on either side, so that connection may be made through the centre contact with either of two cir- cuits. It is often specified that the hinge of the switch must carry no cur- rent, in which case two centre contacts have to be provided as shown in the fig. Switch, Electromagrnetically - oper- ated, a switch of which the movable portions are operated by means of electromagnets. The majority of automatic circuit breakers come under this heading, as well as many switches arranged for operation from a dis- tance, as for instance certain designs of oil- break h pr switches, of lift controller switches, and of contactors or switches used in mul- tiple-unit train systems operated from a small master switch. See also Automatic Circuit Breaker; Oil-break Switches; Eemote-control System; Contactors. Switch, Electropneumatically - con- trolled Unit, a switch on a coach of a train worked on the multiple-unit system, which is operated by compressed air, the valves being controlled electrically from a master controller. The compressed air is obtained from a motor-driven compressor on the train. See also Contactors'; Master Controller; Multiple-unit System. (Eef. 'Electrical Traction', Wilson and Lydall.) Switch, Emergency, a switch to be operated, generally automatically, when the ordinary control fails to act, or would not act rapidly enough. As an example may be mentioned the switches which are often fitted to hoisting or travelling gears, to be auto- matically operated by a catch or by a nut 520 Switch moving on a screwed shaft, and so thrown open when the gear has reached the end of its proper travel. Switch, Equaliser, a switch for con- necting the armature side of the series wind- ing of a compound-wound cc generator or rotary converter to a common lead or bus- bar, to which the corresponding terminals of other machines can be connected when it is desired to run them in parallel. The simplest arrangement of this kind is shown in fig. 1. This connection has the effect of fixing the direction of the current through the series winding of the machine, and therefore, even if the voltage of the machine falls and it is run as a motor, the current in the series winding is not reversed; if it were reversed, the field of the machine would be still further weakened and its speed would in- crease, sometimes to a dangerous extent. In general, without the equalising connection, the operation of machines in parallel would be much more sensitive, and practice has shown that a connection such as that de- scribed, is necessary for satisfactory and reliable operation. It is required that the equalising connection should be made and current allowed to circulate in the series Fig. 1 Fig. 2 Kg. s Various Equaliser Switch Connections Pig- 4 winding before the machine is connected to the live bus-bars, as otherwise the rush of current through the series winding would alter the voltage of the machines at the in- stant they were paralleled. The equalising switch is therefore generally sp, or double- pole in connection with the switch at the other end of the series winding, as shown in fig. 2; if triple pole in connection with the two main switches, there should be another switch or circuit breaker on the opposite terminal of the machine, which can be closed last. This arrangement is shown in fig. 3. The equalising connection need not be as large as either of the main connections, and in practice is generally of half the capacity. It is, however, essential that it should not be too small, since if it is, the resistance be- tween the machines will be too great to allow of sufficient interchange of current. In order to cut down the length of equalising cable, and therefore not only its cost, but also its resistance, it is general with large units to place the equalising switch on the frairie of the machine, and to run an equalising cable from machine to machine, in place of having an equalising bus-bar on the switchboard, as is done with small units. The equalising switch is sometimes com- bined as a double-pole switch with a switch for cutting out the series winding when it is desired to run the generator as a shunt ma- chine (see fig. 4); this is common in stations where compound machines are required for a traction load, and shunt machines for the lighting load. Switch, Feeder, a switch for opening or closing a feeder circuit, the type depending upon the nature of the supply system. See Switch, Distkibuting. Switch, Flush. See Flush. Switch, Horn -break, a switch with horn-shaped extensions to the contacts, ar- ranged as shown in the fig., so that the arc on breaking circuit is formed at the base of the horns where they are nearest together. It is a principle in electromagnetism that a circuit will tend to move so as to embrace the largest possible number of the lines of force set up by it; hence the arc that starts between the horns where they are near to- gether rises between them until it becomes so attenuated that it is extinguished. Prior to the introduction of the oil-break switch, horn-break switches were used extensively on h pr ac circuits, but, owing to the arc. Switch 521 they were apt to cause surges on the line, with consequent destructively high poten- tials, and had. in addition the disadvantage that they required considerable space for the n Horn-break Switch free play of the arc. See 'Horn-type Ar- rester' under Lightning Arrester. (Ref. 'Electricity Control', Andrews.) Switch, Hospital, a switch provided in the controller of an electric tramcar to enable a faulty motor to be cut out of action. Switch, Interlocking, a switch so ar- ranged that it cannot be opened or closed unless some other piece of apparatus is in some prearranged position; for instance, a motor-starting switch may be interlocked by means of a rod with a circuit breaker, or with a main switch, in such a manner that the latter cannot be closed unless the starter is in the 'ofiF' position. Switch, Knife, a switch in which the moving contact con- sists of a blade which completes the circuit by coming down, knife fashion, between the almost touching jaws of the fixed contact. See fig. Switch, Limit, a switch arranged to be operated (generally to be opened) when a moving cage, carrier, &c., reaches the limit of its travel. Such a switch is, as a rule, operated direct by a catch or tappet on the moving object, or by a nut moving on a screw which rotates with the axle of the winding gear or other part. Switch, Master. See Master Switch; f- ^ Enife Switch Controller, Master; Relay; Multiple- unit System of Train Control. Switch, Motor-starting-, a switch used for starting a motor. The term is generally applied to a multicontact switch, each con- tact of which cuts out a portion of a resis- tance which is inserted in the circuit at the commencement of the starting process in order to keep down the magnitude of the starting current, the resistance often being combined with the switch to form a single piece of apparatus which is called a motor starter, or even a motor-starting switch, though strictly the latter term should be confined to the switch portion only. See Starting of Motors; Rheostats or Resistances. The counter emf of a motor increases with its speed, being a minimum when the motor is at rest; and if the motor were switched directly on to the supply mains, a very large current would pass through it, which would not only damage the armature windings and cause the motor to start with a jerk, but would also cause a variation in the supply emf, and so interfere with other consumers connected to the mains. Hence the initial value of the starting resistance is such that the current at the moment of first switching the motor through to the supply circuit is, say, only three-quarters of the normal full- load current of the motor. As the motor gathers speed, the resistance is cut out step \)Y step, until after, say, a quarter to half a minute (the exact time depends upon the size of the motor), all the resistance has been cut out, and the motor is running direct on the circuit. The current taken by a motor of a given power is the greater the lower the supply pressure, and for this reason many supply-station engineers who have a three- wire system under their charge insist that all motors above 5 hp shall be connected across the outers of the system. Very small motors, for instance \ hp motors, may usu- ally be switched direct on to the mains with- out the aid of a starting resistance, for their armature resistance is high in itself, and no very serious rush of current can result. A typical motor-starting switch is shown in fig. 1, from which it will be seen that the switch arm is held in the full-on position by the electromagnet A, the winding of which is in series with the field, and that should the supply pressure fail, or the field circuit be broken for any reason, the contact arm will be at once drawn back to the ' off' posi- 522 Switch tion by the action of the spring B. Such an electromagnet is called a 710-voltage release, and the majority of cc motor starters are provided with this adjunct. In addition, an overload release c is often provided; this comes Oil -Typical Motor-starting Switch tor Cantinuous-current Shunt Motor into action if the current through the motor exceeds a prearranged value, and acts by short-circuiting the no -voltage release, so allowing the spring to bring the contact arm to the ' off ' position. Owing to their wide field of application motor-starting switches are made in almost innumerable forms and designs, and are ar- ranged for use with series, shunt, or com- pound motors, and may be adapted to vary- ing the speed of the motor by increasing the permanent carrying capacity of the resist- ances, and for reversing, by the addition of suitable extra contacts. They can also be arranged for use with induction motors on polyphase circuits, in which case the switch is generally provided with three arms, and is inserted in the rotor circuit of the motor; if reversing is required, a second switch is necessary to reverse two of the stator leads. It may here be noted that induction motors with short-circuited rotors, if of small size compared to the generating system, are some- times started by switching the stators direct on to the mains, though in this case the rush of current is great, often four or five times the normal full-load current of the motor. To avoid such an excessive current, the stator windings may be connected to a so-called star-delta starting switch, which connects the stator windings in star for starting, and after- wards in delta for normal running; in this case the starting torque will be reduced to, say, one -third of normal full -load torque, while the current-rush will be, say, one and a half times the normal full -load current. Yet another method of starting a short- circuited-rotor motor is to employ an auto- transformer or auto-starter, which is a choking coil connected to the supply with a tapping, or tappings, and a two-way or multi-contact switch, so arranged that the stator of the motor is first connected across part only of the supply voltage; when the motor is running at full speed the autotransformer is dis- connected from the mains by the switch. (See Auto-transformer; Auto-starter.) In addition there are many forms of automatic start- ing switch in which the starting is effected from a distance, as in the case of a passenger lift, hydraulic accumulator, &c. See Eemote- CONTROL System. Liquid Starter. — This is a distinct type of starting apparatus, in which a liquid re- sistance is employed which is varied as required by the greater or less immersion of a plate or plates in the liquid. This type was at first used in connection with cc crane motors, in which service it did not meet with much approval, but more recently it has been adapted for the starting of polyphase in- duction motors with consider- able success. If well designed, the apparatus is simple and in- expensive, and permits of a uniform altera- tion of the re- sistance in circuit. A typical liquid starter is shown in fig. 2, from which it will be seen that in the full-on position the moving por- tion makes metallic contact with the fixed portion and so short-circuits the liquid. The liquid itself is generally a solution of soda. Liquid starters can be made for the starting Fig. 2.— Liquid Starter tor Polyphase Induction Motor Switch 523 and speed-control of large motors, e.g. of 500 hp to 1000 hp, and in these cases a cooling tank is generally added, with a small pump driven by an auxiliary motor to circu- late the liquid; further, in such large sizes the plates are often stationary, and the re- sistance is varied by alteration of the level of the liquid in the resistance tank. See Starting of Motors. [f. w.] Switch, Multiple - contact, a switch having a number of contacts with any one of which the moving contact can make circuit as desired. Multiple-contact switches are largely used in connection with field regulators and other forms of voltage regu- lators, with motor starters (see Switch, Motor-starting), and with secondary bat- teries. See Switch, Accumulator. Switch, Oil. See Switch, Oil-break. Switch, Oil-break, a switch in which contact is broken under oiL Such switches are almost invariably used on high-potential ac circuits, owing to the fact that the oil has the property of breaking the circuit when the current wave is at its zero value, and thus prevents the heavy arcing which would occur with an air-break switch, and the con- sequent surges in the line which are so often the cause of breakdown of the insulation of the system. Further advantages of the oil- break switch are its comparatively small size, its compactness, and the fact that it is to- tally enclosed; these features are of such importance that oil-break switches are also used to a very large extent upon low- potential alternating circuits, for instance to control induction motors, more particularly on underground circuits in mines. The oil break cannot be satisfactorily employed with cc, as these present no zero current position at which the oil can flow in and break the arc, and the result is that the persistent arc is not only apt to cause explosions under the oil, but also in a short time acts upon the oil chemically and makes it unfit for further use. The contacts of a simple oil-break switch are generally of copper, carried by a support of hard wood; the terminals are brought through porcelain insulators mounted in the iron frame plate, and the oil container, which is often of sheet iron, is fixed to the frame by means of bolts and fly-nuts. Such a switch can be arranged for mounting at the back of a switchboard panel, with the handle projecting through to the front of the board, and automatic trip coils can also be affixed for the purpose of tripping' the switch under the action of an overload, or on the occasion of a failure of the supply voltage, or of a reverse -power current, the break being ef- fected by gravity or by the action of a spring. See 'Automatic Circuit Breaker' under Circuit Breaker. Oil -BREAK High-tension Switch. — In the case of larger types of switches which are used on high -voltage three-phase sys- tems, each phase is placed in its own fire- proof cell of brickwork, or other construc- tion. The switch is opened and closed by the auxiliary motor which can be run in either direction from a small throw - over switch on the switchboard, the necessary current being obtained from a subsidiary source of supply, often a small set of ac- cumulators. A set of motor- operated oil switches is shown on the Plate facing p. 524. Two lamps are generally mounted on the board, a red and a green, which are switched on or off by the action of the small throw-over switch, or by the tripping of the oil switch, and serve to indicate when the latter is closed and when open. Remote-controlled Oil Switch. — In other designs the oil switch is operated by Hg. 3.— Bemote-contTolled Oil Switch solenoids, or by compressed air controlled by electrically -operated valves, and the ad- vantage of any of these methods of control is that the oil-switch cells may be at a con- siderable distance from the operator's panel. Eemote control, as it is called, is also in some instances secured by means of levers and bell cranks, which allow of the switch cells being placed some distance from the operating handles on the panels. An in- stance of a remote-controlled oil switch in which mechanical means of this sort are employed, is shown in the fig. 524 Switch Oil switches are often used on systems with generator powers of many thousands of kw. It is consequently essential that the switches shall be able to break not only their nor- mal current, but also the greatly increased current that would flow if a short circuit or partial short circuit occurred — ^indeed some manufacturers go so far as to state that every oil switch connected to a system should be able to break the full capacity of the generating station; but this is prob- ably a counsel of perfection involving an unnecessary expense, seeing that, in the event of such a dead short-circuit occurring, the load on the generators would be greatly increased and their voltage greatly reduced. The oil used in oil-break switches may be the same in quality as that used in oil-cooled transformers, and it should be inspected at intervals of — say — one month to three months, and renewed if found to have de- teriorated (see Oil Insulation). A vent hole should be left in the container to al- low of the egress of any gases that may be formed, and a small gauge to indicate the level of the oil is also sometimes added. (Ref. 'Standard Polyphase Systems', Oudin; 'Electricity Control', Andrews; 'Elek. Mas. Apparate und Anlagen', Niethammer.) [f. W.] Switch, Overload. See Circuit Breaker, Automatic. Switch, Paralleling, a switch for con- necting generators in parallel. As a rule no special switch is used other than the ordinary switches in each lead from the generators. With cc shunt-wound genera- tors the incoming machine is run up to the pressure of the bus-bars, and the main switches or circuit breakers closed; with cc compound-wound generators the series wind- ing of the incoming machine is iirst switched into circuit, then the pressure of the ma- chine is brought up to that of the bus-bars, and finally the machine is connected to the bus-bars by a switch or circuit breaker on the until -then disconnected side (see also Generator Panels; Switch, Equaliser). An ac generator has not only to be run up to the pressure of the bus-bars, but it also has to be synchronised with the bus-bars, that is to say, it has to be run at such a speed that its emf wave exactly coincides with the emf wave of the bus-bars, before the main switch {e.g. a three-pole oil switch for a h pr three-phase machine) is closed and the machine connected to the bars in parallel with the other generators. See also 'Synchronising Bus -bars' under Bus- bars; Synchronising Dynamo - electric Machines; Synchroniser. Switch, Plug, a switch which consists essentially of a plug and a corresponding socket, the circuit being made by inserting the former into the latter. A type of plug . ^^' > < t 1 B m. f V'^r i '(^ Plug Switch A, Brass. B, Ebonite. switch employed in laboratories for circuits carrying small currents is illustrated in the , fig. The plug is termed a smtch plug. Switch, Plunger, a switch which takes the form of a plug making contact on pass- ing into a suitable socket. One of the best-known forms of this switch is that as- sociated with the name of the Westinghouse Company, which consists of a number of in- sulated metal plugs which, on being pressed home, each makes circuit with a metal con- tact in a porcelain cylinder. On opening the switch the arc is broken, partly by the rapid absorption of heat by the walls of the porcelain tube, and partly by the expulsion of the heated air through an opening in the tube. This type of switch has been used on alternating circuits operating at from 1000 to 2000 volts, but is now superseded to a large extent by the oil-break switch. See Switch, Oil-break. [f. w.] Switch, Pneumatically Controlled. See Switch, Electro - pneumatically Controlled Unit. Switch, Pole -changing, a switch ar- ranged to alter the connections between the coils in the stator of an induction motor, and so vary the number of poles produced by the stator and consequently the speed of the motor (see fig.). Such a switch be- comes difficult to make and handle if large currents or h pr have to be dealt with, and REMOTE-CONTROL SWITCHBOARD AT LOTS ROAD POWER STATION, LONDON (see p. 52S) MOTOR-OPERATED OIL SWITCHES AT CARVILLE POWER STATION, NEWCASTLE-ON-TYi\E [Tojacep. J34. Switch 525 consequently pole changing is only employed as a rule with small motors, say up to 20 or 30 hp, and even then is but rarely re- sorted to. The changes practic- ally possible are Tery few (gene- rally nothing more than in the ratio of 1:2 is attempted), and in- termediate speeds are not obtained, which considerably limits the useful- ness of the arrange- ment. Switch, Quick-break, a switch so de- signed as to abruptly effect the opening of the circuit irrespective of the rate at which the operator moves the handle of the switch. ^ Pi y Pole-changing Switch Fig. 1.— Quick-break Switch Fig. 2.— Switch without Quick-break Feature Almost all modern switches are of some quick- break type, but formerly switches without this feature were also widely employed. Two ordinary hand-operated lever switches, the one without and the other with the quick- break feature, are shown in the accompanying figs., the quick-break switch being shown with part of its blade withdrawn, and when the handle is pulled out farther, the other part of the blade will be withdrawn from the contact and the springs will occasion a quick break. Switch, Regulating", a switch, generally of the multicontact type, for controlling an electric circuit. Regulating switches are of many types, but in the majority of instances the name is applied to a multicontact switch which alters the resistance in the circuit, which may be the main circuit or a dynamo or motor field circuit. See Potential Regu- lation; Regulator, Potential. Switch, Reversing-, a switch by means of which the direction of the current in a Beverslng Switch circuit may be reversed. In the throw-over switch shown in the fig., the upper position serves to send the dynamo current in the one direction through the external circuit, and the lower position serves to send the current in the other direction through the external cir- cuit. Such switches are largely used to reverse the direction of rotation of an electric motor, in which cases they are often combined with the motor controller or starter. The ma- jority of tramway controllers have a separate handle and drum for operating the reversing contacts (see Controller). A cc motor is generally reversed by reversing the direction of the current in the armature, the direction of the current in the field windings being kept constant; a three- phase motor is reversed by reversing any two of the leads going to the stator terminals; a two-phase motor by re- versing the two stator leads of either phase. Switch, Segment, a switch in which the contacts form the segments of a circle. The fig. illustrates a usual type of segment switch. Segment Switch 526 Switch Switch, Selector, a switch to connect a generator, feeder, &c., on to any one of two or more sets of bus-bars. In some large ht stations two sets of bus-bars are provided, and it is possible to connect each generator or feeder to either set at will. The switches for effecting this are often independent of the main switches, and are not intended to break circuit while the load is on. Switch, Single-pole, a switch arranged to make and break contact on a single main. In the 1907 Wiring Rules of the I.E.E. the term is defined as follows: 'Sp switches are switches interrupting one conductor only of a circuit'. See Switch Types, Designa- tion OF. Switch, Snap, a switch in which the action of breaking, aided, as a rule, by a spring, is very rapid. The term is generally •confined to small switches for the control of one or a few electric lamps. These switches are generally mounted on a porcelain base, and have a metal or a porcelain cover. Switch, Star-delta. See Switch, Motor-starting. Switch, Three-phase, a three-pole switch for use on a three-phase circuit. Swit?ch, Three-point, a switch with three contact arms, so arranged that on moving the handle all three arms move over contacts at the same time (see fig.). Such switches are used in connection with starters of in- duction motors, where it is desired during starting to gradually cut out resistance from the three legs of the rotor circuit. The term is also taken, in some cases, as being synonymous with three-way switch (which see). Switch, Three-way, a switch which can be so moved as to make circuit through any one or more of three separate contacts. See fig. Switch, Throw - over. See Switch, Double-throw; ^^^.^^^ ^^^^^ also fig. under Switch, Re- versing; Switch Types, Designation of. Switch, Time, an ordinary clock fitted with a mechanical or electrical change-over switch which is actuated by the clock at definite hours. The clock is either hand- Three-polnt Switch wound or is furnished with an electrical self-winding gear. When used in conjunc- tion with a two-rate meter (see under), the clock, through its switch, brings the one or other dial into gear with the meter at the time when the tariff changes. The tariff times are set on the clock, and usually a wide range of setting is attainable. The clock indicates on its dial the hours of the day, and the times when the high- and low- rate charges become operative. It is also called change-over clock, dmhlertariff clock, two- rate clock. See Meter, Hour; Meter, Two- rate ; Tariff Systems. Switch, Traclc, Electrically - oper- ated. See Track Switch, Operated. Switch, Trip, an automatic circuit breaker. The term is also sometimes applied to a switch which is opened or closed by a rod or catch fixed to a moving object, as, for instance, the cage of a hoist. Switch, Tumbler, a well-known form of snap switch (which see) for small currents up to about 5 amp. The handle takes the form of a small metal knob. (Ref . ' Electric Wir- ing, Fittings, Switches, and Lamps', May- cock; 'Electric Wiring', Clinton.) Switch, Two-pole. See Switch, Double-pole. Switch, Two-way, a switch which can be moved so as to make circuit through either one of two contacts. See fig. Switch, Unit, a switch or circuit breaker actuated by compressed air, and used in the Westinghouse unit- switch system of train control. See Multiple-unit System. Switch, Voltage-regulating', a switch used for potential-regulation purposes (see Potential Regulation). Each form of regulation has a corresponding type of switch, but the most common is the multi- contact hand-operated type used in con- junction with a multi-step resistance in the field circuit of a generator. Switch, Water-break, a switch with auxiliary contacts dipping into water pots, and so arranged that the circuit is first broken at the main contacts, and finally, as the handle is moved farther, at the water- immersed contacts, so that the arc on rup- ture of the circuit is quenched by the water. Such switches are rarely met with, but have occasionally been used on high-potential ce Two-way Switch Switch — Switchboard 527 circuits. (Ref. 'Modern Electric Practice'; 'Electricity Control', Andrews.) Switch, Watkin. See Watkin Switch. Switch and Fuse Combined. See Combined Switch and Fuse. Switch Box. See Box, Contact. Switch Types, Designation of.— /Sp, an abbreviation for single pole. Thus a sp switch is a switch which, when opened or closed, interrupts or makes a circuit at but one point. In the same sense dp is used for double pole, and ip for three pole, the latter term usually being employed in connection with three-phase work where there are three conductors. Si and dt are corresponding widely employed abbreviations for single throw and double throw. A single-throw switch is a switch only operating to make a single definite circuit combination, whereas a double- throw switch can be employed to make, at Fig. 1.— Single-pole Single-throw Switch Fig. 2.— Single-pole Double-throw Switch Fig. 3.— Donble-pole Single- throw Switch Fig. i Double-pole Double- throw Switch a i i ' I @ M Fig. 6.— Triple-pole Single-throw Switch Fig. «.— Triple-pole Doable-throw Switch will, either of two circuit combinations. With respect to the number of poles and the sub- division into st and dt types, we may have the following kinds of switches: — Single pole, single throw, double Double , single , j» , double , Triple , single „ »1 , double , sp St sp dt dp st dpdt tp st tpdt . 1. 2. 3. 4. 5. 6. The letters qb are widely employed as an abbreviation for quick break Switchboard, a panel or panels with switchgear mounted upon them. See Panels, Switchboard; Generator Panels OF Switchboard; Panel, Feeder; Switch- board, Cubicle or Cellular Type of; Board of Trade Panel. ['Sieitchhoard means the collection of switches or fuses, conductors, and other apparatus in connection therewith, used for the purpose of controlling the current or pressure in any system or part of a system.' — From definitions accompanying Home OfBce 1908 Regulations for Electricity iii Factories and Work- " ■] Switchboard, Accumulator, the neces- sary apparatus for controlling the charge and discharge of a battery, usually mounted on an enamelled-slate or polished-marble base. The instruments and switchgear vary according to tie work the battery is called upon to do. 528 Switchboard — Switchgear Where a battery forms but a part of the plant for supplying electrical energy, the apparatus for controlling its operation, in- cluding the material on which it is mounted, is spoken of as a lattery panel and forms part of the main switchboard. See Switch, Accumulator; Booster; Booster, Rever- sible; Accumulator, End Cells of an; Switch, Multiple-contact. Switchboard, Board of Trade Panel of. See Board of Trade Panel. Switchboard, Central, the principal centre at which the circuits are controlled. Switchboard, Common Battery, a tele- phone switchboard suitable for central-battery working. See Central-battery System. Switchboard, Cubicle or Cellular Type of High -pressure, a design of switchboard in which the apparatus is placed in a number of separate cubicles or cells, the walls of which are of slate or artificial stone, or other suitable material, and sometimes wholly or in part of iron. In the older Ferranti type of such boards, the cells were built against a wall with the fronts open, each generator or feeder having its own cell partitioned off vertically from its neighbours, but in more recent designs it is usual to have only 1 pr gear accessible from the front of the board, and to arrange the cells with the h pr switches, connections, &c., behind the face of the board. Horizontal partitions are often employed in each cell to separate from one another the various pieces of appa- ratus on the same circuit. A further modification is seen in the re- mote-control type of h pr switchgear in which a flat board is erected to carry the 1 pr gear such as the instruments, which are supplied through instrument transformers, and the handles for operating the h pr switches, while the h pr oil switches themselves are each in a separate stonework or brickwork cell behind the board, and the bus-bars in yet another cell. In this design the oil switches are operated by bell-crank levers from the handle on the front of the board, or electrically by means of small motors con- trolled from a 1 pr circuit from a switch on the board. On the Plate facing page 524, a remote-control switchboard at Lots Road Power Station is illustrated. See 'Remote- controlled Oil Switch' under Switch, Oil- break; 'Remote-controlled Switchgear' under Switchgear; Remote-control. The advantages of a cellular board are that a fire, if started, is likely to be confined in one cell, and that the apparatus in any one cell can be safely inspected and handled even though the rest of the board- may be alive. (Ref. 'Standard Polyphase Apparatus and Systems', Oudin; 'Elek. Mas., Apparate und Anlagen', Niethammer; 'Electricity Control', Andrews.) [f. w.] Switchboard, Generator Panels of. See Generator Panels of Switchboard. Switchboard, Main. See Central Station for the Generation of Elec- tricity. Switchboard, Panel, a switchboard in which the switchgear is mounted on panels, as opposed, for instance, to one in which the gear is mounted on a number of pillars, or in a number of cubicles or cells. See also Panels, Switchboard. Switchboard, Trunking, in telephony the switchboard at which the trunk wires end (the trunk wires being the connections between separate exchanges). Switchboard Integratingr Meter. See Meter, Switchboard Integrating. Switchboard Mat. — An indiarubber mat some 3 ft or more in width is generally to be seen on the floor in front of a switch- board. Its object is, of course, to minimise the possibility of the attendant receiving shocks to ground while operating the switch- gear. It is, however, often not in such a condition that any reliance can be placed on it. See Shock, Electric. Switchboard Measuring Instrument. See Instrument, Switchboard Measuring. Switchboard Panels. See Panels, Switchboard. Switchboard Passage-way.— [' Switchboard passage-way means any passage-way or compartment large enough for a person to enter, and used in connection with a switchboard when live.' — From definitions accompanying Home Office 1908 Regulations for Electricity in Factories and Work- Switchboard Wattmeter. See Watt- meter. Switches, Linked,— In the 1907 Wiring Rules of the I.E.E. this term is defined as follows: — ' Linked switches are sp switches (which see) fixed on conductors of different polarity, linked together mechanically so as to operate simultaneously.' Switchgear, a collective and rather in- definite term for all the switches, circuit breakers, instruments, &c., used to control, Swivel Truck — Synchronising Dynamo-electric Machines 529 protect, and regulate a circuit or circuits, and to measure the currents, voltages, (fee, in them. Where the gear is mounted upon panels the latter are generally included in the term, as are also motor starters, though controllers are generally excluded. Eemote-controlled Switchgeak, switches, &c., operated from a distance. The term is generally applied to the oil switches of h pr ac systems, when they are operated either through levers and bell-cranks from handles on the front of the board, or by means of small electric motors or electro- magnets worked by 1 pr current through a small switch on the switchboard. The ad- vantages of a distant-controlled switch are that both the switch and the switchboard can be placed where most 'ionvenient, irre- spective of one another, and that the switch can be placed in an isolated cell. See Ee- MOTE-CONTROL SYSTEM; ' Eemote-controUed Oil Switch' under Switch, Oil-break; Switchboard, Cubicle or Cellular Type OF. (See 'Standard Polyphase Apparatus and Systems', Oudin; also 'Elek. Mas., Apparate und Anlagen', Niethammer.) [f. w.] Swivel Truck. See Truck. Synchronise, to correspond in point of time; to cause to correspond in point of time; to switch together two circuits carrying ac, at the critical moment when the phases and the emf correspond. Synchroniser, an apparatus for indicat- ing the phase relation of two alternators with a view to throwing them into parallel. The simplest arrangement consists of syn- chronising a lamp, or, preferably, a voltmeter connected across one pole of a two-pole switch, connecting the incoming machine on to the bus-bars, the other pole of the switch being already closed. If the machines are out of step the lamp will fluctuate in bright- ness, or the voltmeter-pointer will oscillate, as the case may be, the pulsations becoming less and less as the incoming machine ap- proximates more and more nearly to the correct speed. Exact synchronism is shown by the lamp remaining out, or the voltmeter at zero. On ht systems two transformers, Tj and Tg, are often employed, connected as shown in the fig., BB representing the bus- bars and M the incoming machine. The secondaries of the two transformers are con- nected to VM, the voltmeter (or synchronising lamp) in series, and synchronism is shown as UlUtfiJ nnrmr^ _^t^ -nnnnr T2 before, except that the transformer can be so connected that equality of phase is shown by maximum pressure on the voltmeter or lamp, instead of by zero, whereby the sensi- tiveness is greatly increased. This arrange- ment has practically superseded the double- wound synchronising transformers, which had two primaries and a single secondary, owing to the inefficiency of such apparatus. In place of the lamp or voltmeter, a vibrating reed is sometimes employed (see Frequency Indicator, 'Eesonance Pattern'), the indi- cations being either visual or acoustic. Rotary synchronisers have now largely superseded the devices just described. In these instruments, which are really special forms of phase in- dicator (see Indicator, Phase; Electro- goniometer), synchronism is shown by a pointer coming to rest in a particular posi- tion. Moreover, B.B. L lfC))M ^° ^°°S *^ *^^ ^^^' quencies are dif- ferent, the pointer rotates in one '^J"^ direction or the other, according to whether the SynohroniBer incoming machine is running too fast or too slow. Automatic synchronisers are used to a limited extent, and include means for ensuring simultaneous equality of (1) voltage, (2) phase, (3) speed. They comprise, as a rule, differential solenoids for 1 and 2, and a time-lag arrangement (see Eelay) for 3. See Synchronoscope. [k. e.] Synchronising', used or provided for the purpose of synchronising, as synchronising lamps, synchronising bus-bars, &c. Synchronising Bus -bars. See Bus- bars. Synchronising Devices. See Syn- chroniser. Synchronising Dynamo-electric Ma- chines. — The process of connecting in paral- lel ac machines designed for operating syn- chronously, at the correct instant when the amount and phase relations of the emf agree. For this purpose auxiliary apparatus is neces- sary, such as a synchronoscope, asynchronising lamp, or a voltmeter. Where synchronising lamps are used they are connected to the incoming alternator and to the source with which it is to be synchronised, so as to indi- cate by their brightness the difference of emf between the two. A voltmeter is further 530 Synchronising Gear — Synchronoscope connected to the incoming alternator, and to the source to which it is to be connected. The process of synchronising is as follows. The incoming machine is run up to about the correct speed, and excited until the volt- meter indicates the same voltage as that of the source to which it is to be coupled. The speed is then further adjusted until the lamps indicate by their brightness that the phase of the incoming alternator agrees with that of the source to which it is to be coupled. When agreement has been reached, the main switches are closed. It is usually found that the incoming alternator, before the main switches have been closed, will not maintain phase agreement for any length of time. This is indicated by the lamps becoming alternately bright and dark, with a more or less regular pulsation. The lamps can be connected so that the correct moment for closing the main switches is when they are at their brightest, or when they are dark. In place of lamps, synchronising voltmeters are frequently employed — the correct mo- ment for closing the main switch being indi- cated by the voltmeters, either when they are at their maximum deflection, or at zero, according to the system of connections adopted. See Synchroniser; Synchrono- scope; Alternators, Parallel Running of; Switch, Paralleling; Synchronising EoTARY Converters. [m. b. r.] Synchronising Gear, Automatic. See Synchroniser. Synchronising" Lamp. See Synchron- iser. Synchronising' Rotary Converters.— The principle of synchronising rotary con- verters is the same as that of synchronising any other type of generator or motor, but the procedure is somewhat varied. Eotary converters are sometimes run up to speed by a small independent motor, then excited, and synchronised in the usual way, after which the cc voltage is further adjusted, if necessary, and the machine is paralleled on the cc side. Eotary converters may, how- ever, be run up to speed from the cc side as cc motors. The speed and the voltage on the ac side is then adjusted for synchron- ising. When following this procedure it is advisable to disconnect the rotary converter from the cc supply immediately before syn- chronising, in order to avoid heavy current rushes. It is frequently arranged that the cc switches are automatically tripped as the ac switches are closed, and afterwards the cc switches are again closed. See Synchron- ising Dynamo-electric Machines. Synchronising Torque. See Torque, Synchronising. Synchronising Transformer. See Synchroniser. Synchronising Voltmeter. See Syn- chroniser. Synchronism, agreement as regards time, of two or more events, motions, or phase re- lations, e.g. the pendulums of electric clocks may swing in synchronism; two alternators operate in synchronism as regards their phase relations with time. Loss of Synchronism. — The departure from the state of synchronism is termed loss of synchronism; the state where two or more alternators, though connected in parallel, have ceased to run at the same frequency. The emf of alternators running normally in paral- lel synchronise with one another, i.e. the emf waves agree in point of time. Should one alternator be accelerated, so that its emf wave is slightly advanced with respect to that of the others, an interchange of current will occur, which in effect will tend to retard the leading alternators, and to accelerate the lagging alternator. Thus a tendency exists to preserve synchronous runnine (see Torque, Synchronising). If, however, the emf of one alternator be allowed to advance or lag beyond a certain limit, the tendency of the alternators to keep step will be overcome, and thereafter the relative phase relations and speeds of the various alternators will bear no definite relation to one another. Synchronism, Cascade, the speed at which two cascade-connected motors run at no load. When the motors have the same number of poles, this is half the synchronous speed of one of the motors. If, however, one of the motors has, for instance, four poles, and the other six poles, the cascade -syn- chronous speed is 4 4-1-6 0-4 of the synchronous speed of the four-pole motor, or T^ = 0-6 4-1-6 of the synchronous speed of the six-pole motor. These two speeds are obviously identical. See also Cascade Motor. Synchronoscope, an apparatus for pro- Synchronous — Tachometer 531 viding a visual indication of the approximate phase relations at any instant of two ac, or emf, and is usually employed for deter- mining the correct moment for switching two alternating circuits into parallel. Synchron- ising gear may consist of lamps, voltmeters, &c., suitably connected; but the term syn- chronoscope is more particularly confined to an instrument provided with windings to be attached to the two circuits to be compared, and a pointer capable of moving over a dial which by its position or motion indicates when the phases of the two circuits agree or disagree, or when the periodicity, of the one is slightly greater or slightly less than that of the other. See Synchroniser. Synchronous, designed to operate in synchronism, e.g. a synchronous motor is a motor designed to run on an ac circuit at a constant speed depending solely upon the frequency, i.e. without slip. Such a motor makes a single revolution in either one, two, three, or more complete cycles. Synchronous Converter. See Kotary Converter; Converter; Converter, Cas- cade. Synchronous Motor. See Motor, Syn- chronous. Synchronous Motor Generator. See under Motor Generator. Synchronous Speed. See Speed, Syn- chronous; Slip; Synchronism. System.— ['System means an electrical system in which all the conductors and apparatus are electrically connected to a common source of emf.' — Home Office 1908 Eegulations for Electricity in Factories and Work- shops.] Systems of Charging for Electrical Energy. See Tariff Systems. T Tachometer, an instrument for directly indicating the angular velocity of a shaft or the linear velocity of a belt. The principle most usually employed is very similar to that of an engine governor, the movement of the mass on which the centrifugal force is allowed to act being measured by a dial and pointer. See also Speed Indicator OR Tachometer; Vibration Tachometer; Tachometer, Liquid; Revolution Coun- ter; Cyclometer Counter; Dial Ee- GISTER. Tachometer, Liquid, an instrument for speed measurement based on the centrifugal force exerted by a paddle wheel revolving in a liquid. Denoting the force by F we have F = 0-0000284 rR2, where F = force in lb per lb of liquid, r = radius of paddle wheel in in, E = speed of paddle wheel in rpm, and is independent of the liquid employed. The force is caused to support a column of liquid so that balance is obtained between the force and the pressure of the column; "and since the pressure of the column in lb per sq in per lb of liquid is also independent of the liquid employed, all liquids will give practically the same results except for minor differences due to viscosity and surface ten- sion. As a representative type of such instru- ments the Veeder tachometer may be de- scribed. The paddle wheel A is mounted on a shaft, with a ball thrust bearing at the end, and has radial channels to permit of operating in either direction. B is a reservoir which com- municates by pas- sages c with the paddle case and gives the liquid access to both sides of the paddle, at its- centre. Eadial apertures are arranged in the paddle case in com- munication with an annular space around the inside of the case from which leads the glass tube D contain- ing the registering column of liquid. A scale E is placed behind D and is graduated In rpm. F is a rubber displacement-plunger fitted in the passage between b and the paddle case, to permit of the adjustment of zero when the levels in b and d should be almost the same (allowing for capillary attraction). Veeder Liquid Tachometer 532 Take-off — Tangent Scale A check valve G is provided to choke the passage to D and so minimise oscillation of the liquid in D and make the instrument ' dead-beat '. A tube H is provided for fill- ing purposes and to serve as overflow should the speed become excessive, and to convey liquid back to B. The liquid employed must be mobile at all normal temperatures, non-inflammable or else of high flash-point. Coloured alcohol is generally used. The scale is very open at the higher speeds, and is seldom graduated between zero and one-third of the maximum speed. Take-off, the terminal of an end or regulating cell. The design varies somewhat according to the amount of current to be dealt with. See Accumulator, End Cells of an. Talbot Disk of Sector Disk, a disk out of which two or more sectors have been cut. It is capable of being rotated at a high rate of speed, and the amount of light pass- ing through is proportional to the size of the openings. When rotated in front of a photometer head the amount of light reaching the photometer from that side is diminished. The relation between the open and closed areas of the disk forms a measure of the intensity of one light in terms of the other. (Eef. Bulletin of Bureau of quently employed is termed fre/mh, chalk. See French Chalk for Assembling Core Plates. Tandem Connection. See 'Cascade Control' under Starting of Motors; Cas- cade Motor. Tangent Galvanometer. See under Galvanometer. See also Tangent Law. Tang"ent Law, the law of the combined action of two magnetic fields upon a magnetic needle. If two magnetic fields are at right angles in direction as indicated in the figure, H Talbot Disk Standards, vol. ii, No. 1, Govt. Printing Office, Washington.) See Photometer; Photometer Head; Law, Talbot's. Talbot's Law. See Law, Talbot's. Talc, native hydrated silicate of magnesia. It is an extremely soft mineral, and can be ground to a fine powder and used as a dry lubricant. The variety of talc most fre- ->M Tangent Law the resultant field is obtained by the paral- lelogram of forces and makes an angle <^ with one of the component fields such that tan <^ = jj. where M and H are the strengths of the com- ponent fields. In the tangent galvanometer this principle is employed in the measure- ment of currents. A magnetic needle is pivoted in a field of known strength. The current to be measured is passed round a coil or coils which generates a field at right angles to the original field. The needle then lies along the direction of the resultant field, and by finding the tangent of its angle of deflection, and knowing the field strength produced by unit current in the coil, the current strength can be found. Tangent Scale Tangent Scale, for tangent galvano- meters, magnetometers or other instruments following a tangent law. As shown in the Tangent Voltage — Tariff Systems 533 figure, a straight line is drawn at a tangent to the circle, passing through zero, and equal divisions are marked on it. If the points thus found are joined to the centre of the circle, the points where these lines cut the circle will represent a direct-reading tangent scale. Tangent Voltage.— As a gauge of the degree of saturation at which a machine is ^^^ ^/""^^^^ =£: ^^ m \- -1 o > • r ■ 7 / / AMPERE TURNS Tangent Voltage T, Tangent voltage. N, Koimal voltage. working, a tangent to the curve at the point of normal voltage may be drawn and extended to cut the Y axis of the saturation curve, as indicated in the figure. The height at which this axis is cut may be expressed as a per- centage of the normal voltage. The voltage where the tangent cuts is known as the tangent voltage, and the percentage value varies between 30 and 60. As a rule this percentage is lower for alternators than for cc machines. [h. w. t.] Tantalum, a metal nowadays widely em- ployed for filaments of incandescent lamps. The melting-point of tantalum is some 2300° C; its specific gravity is 16-5, and its tensile strength is said to be higher than that of mild steel. At 0° C. its specific resistance is 16-5 microhms per cm cube. At 100° C. the specific resistance has increased to 21 '5, and at 350° C. to 31 '5. At the temperature of the filament when operated at an efiiciency of 1'5 wpcp, the specific resistance has in- creased to 85 microhms per crii cube. Tantalum Lamp. See Lamp, Incan- descent Electric. Tape, Jaconet. See Jaconet Tape. Tape for Insulating Purposes.— Tape is extensively used in the manufacture of VOL. II armature coils, largely as a mechanical pro- tection to the insulation. For this purpose it is essential that the tape be thin, strong, of uniform thickness, with only slight elon- gation under tension ; it should be uniformly woven, preferably free from nap, and if bleached it should contain no traces of the bleaching agent. See also Friction Tape; Jaconet Tape. Taping Machine. See Armature Coil Taping Machine. Tapper, an electromagnetic hammer like that of an electric bell; used in wireless telegraphy to decohere the coherer employed in Marconi's original system. See Wireless Telegraphy. Tapping Closed-circuit Windings for Alternating Current, connecting symmet- rical points in closed-circuit windings to slip rings (the angle between adjacent tappings in any one phase depending on the number of phases; being respectively 180, 90, and 120 electrical degrees for one-, two-, and three-phase windings). Taps or Tappings, connections into in- termediate points of active conductors, e.g. armature tappings to slip rings as in rotary converters (see Winding, Eotary Con- verter); transformer taps to obtain pres- sures higher or lower than normal. See Leads, Preventive Resistance; Connec- tions, Equipotential. Target Diagram of Lamps. See Lamp, Incandescent Electric. TariflF Meter. See Meter, Tariff. Tariff Systems. — The cost of the genera- tion and supply of electricity for commercial purposes depends on the amount of the sup- ply, the rate at which energy is taken, and the maximum possible demand which can at any time be made on the supply undertaking by the various classes of consumers connected to its network. This cost is made up of the 'preparation and production charges. The former are the heavier of the two, and in- volve the expenditure incurred, inclusive of annual charges on capital outlay, in prepar- ing to supply, i.e. they depend on the maxi- mum demand. The latter vary directly with the amount of energy generated, i.e. they comprise the running costs incurred in main- taining the supply. Consumers differ very largely from one another, not only as regards the total amounts of their consumption, but also as regards rate of consumption, maxi- mum demand, duration of demand, and the 35 534 Tariff Systems time when electricity is taken. The object of the special tariff systems in vogue is the equitable treatment of all classes of con- sumers, consistent with a fair profit to the supply company, and with the general adop- tion of electricity for lighting, heating, do- mestic, and power purposes. All the special systems of charging for the supply of elec- tricity are based on the principle of pro- portionality of charge to cost of supply, and only differ as regards its method of applica- tion in practice. Flat-kate Systems. — In flat-rate systems, with and without discounts, as the name im- plies, a uniform price is charged per unit, in many instances without discounts (rarely en- tirely used), and in others with discounts based on the quantity consumed. The latter system is, however, not equitable, or ad- vantageous to the company, as the long-hour consumer is placed on the same level as the short -hour consumer, who is not by any means so profitable as the former, although his actual total consumption may be larger. The reason is that the short-hour consumer not only makes heavier demands, but de- mands of shorter duration, on the electricity undertaking, and therefore costs more to supply. Manchester System. — In the so-called Manchester system, first introduced into Manchester by Hopkinson, a fixed charge is made per quarter (or per annum) per kw of maximum demand (or maximum lights installed), and the actual units con- sumed, measured in the ordinary manner by a meter, are charged for at a low price per unit. Maximum-demand System. — In the max- imum-demand system, first introduced at Brighton by Wright, also known as the Wright or Brighton system, two charges are made. The one is a high price per unit on all units based on the maximum demand of the consumer, i.e. on his maximum rate of consumption. The other is a low price per unit on all units in excess of the demand units. The demand units are not directly measured in this system, but are based on the maximum current taken in a given period (e.g. quarter), the duration of the demand, which is fixed by the supply undertaking {e.g. 1 hr per diem, or more), and the pres- sure of supply. If V denote the supply pressure in volts, C the maximum current in amp taken at any time during the quarter of, say, 90 days, h the duration of the de- mand in hr per diem, then the demand units which must be consumed in the quarter before the low price per unit is charged are — — — . The demand units, as just ex- 1000 ' ■• plained, are given by an instrument, called a maximum-demand indicator (see Indica- tor, Maximum-demand), which measures the maximum current. The actual units consumed are directly registered by an elec- tricity meter in the usual manner. The reading of the demand indicator gives the demand units, and the meter-reading, the actual units. If the latter be in excess, then the difference gives the units charge- able at the low price. If, on the other hand, the demand units exceed the actual units consumed, this implies that the consumer has been using his current uneconomically, making heavy demands for short periods, and all the units (those registered by the meter) are in this case chargeable at the high price. The demand indicator is so arranged that it does not record small in- crements of current due, for instance, to momentarily switching on extra lights, un- less based for a time exceeding the time-lag of the instrument. By using the lights eco- nomically, and not allowing any one maxi- mum demand to exceed the average daily maximum demand, the benefit of the low price is mostly obtainable. On special occa- sions, festivities, &c., it is arranged that the demand indicator shall be short-circuited, and the maximum demand on such occasions is not registered. Two-EATE OR Double-tariff System. — In a two-rate or double-tariff system, two distinct charges are made according to the time of day when current is taken. During the periods when the station is heavily loaded in the evening, the price per unit is high, and during the remainder of the day the price per unit is much lower irrespective of the purpose for which current is required. The units consumed during the high-rate and low-rate periods are separately registered. This is effected by using a two-rate or double- tariff meter in combination with a time-switch. The meter has, in general, two dials, of which the one gives the units chargeable at the high price, and the other those chargeable at the low price. An index pointer shows which dial is registering, so that the two consump- tions can always be determined, as well as T-die — Telautograph 535 the tarifif in vogue. The tariff- time can, further, he varied to suit the time of year. In many cases when the load is practically constant, instead of installing an expensive meter an horwr meter is used. It simply re- gisters the hours during which current is taken. Knowing the value of the load, and the hours of supply from the hour meter, the units consumed are readily deduced. Hour meters are also used in various ways in conjunction with ordinary meters. In one system the meter proper registers the actual units consumed, and the hour meter is arranged to measure the hours during which the current taken exceeds a predeter- mined amount. The tariff is based on the length of time of using current. Prepayment System. — The prepayment system of charging does not constitute any special tariff, but is merely a convenient pay- ment system, the object being to reach a class of consumer otherwise difficult to obtain, or unprofitable. It consists in making the con- sumer pay for his energy before he uses it, on the basis of a fixed price per unit« For this purpose a prepayment, or automatic slot, meter is used. The consumer has to insert a coin of the right denomination in the in- strument and turn a handle. This operation closes a circuit-switch in the meter. He can then take electricity until he has consumed an amount corresponding to the value of the coin inserted, when the circuit-switch is auto- matically opened. For a further supply he must insert another coin. Successive coins can be inserted up to a certain number, when the instrument is locked and further prepay- ment cannot be made until at least one coin's worth of electricity has been consumed. See Assessment Tariff; Indicator, Maximum-demand; Meter, Hour. [h. g. s.] T-die. See Die. Teaser, an obsolete term. In the early days of the electrical industry, shunt wind- ings were termed teasers. At a later date the subsidiary windings in the generators of the monocyclic system (which see) were called teaser lomdings. Teeth, Armature, the projections from the main part of the core stamping, between which the armature windings are placed, and which serve as a mechanical support to retain them in their place against the lateral forces experienced when the machine is loaded. See fig. 1. Detachable Teeth. — In order to use a former -wound coil with a closed slot, the A. E. G. of Berlin, among other firms, have adopted a construction in which each tooth Fig. 2.— A. E. G. System ol Detach- able Teeth Fig. 1.— Armature Punching with Teeth at the Periphery is dovetailed into the punchings. Fig. 2 shows the idea applied to the rotor of a turbo-alternator. The formed coil is placed in position upon a laminated or cast rotor, and the teeth ^^^^ii^^^5^^ are then inserted and held in place by a key. In the centre of the pole the teeth are of steel, while the few teeth situated between poles may be of a non-magnetic material such as phosphor-bronze. Sayers' Detachable Teeth, a construc- tion forming the subject matter of British Patent No. 12,801 of 1904, in which alternate teeth are arranged to be de- tachable, thus per- mitting of the use of form-wound coils in rotors or stators with closed - over slots. The idea is illustrated in fig. 3. See Slot; To- tally - CLOSED Slots ; Partly - closed Slots; Wide-open Slots. Telautog^raph, or Writing- Tele- graph, a telegraphic instrument which transmits written characters and line draw- ings. The principle on which the instru- ment is based is that of two variable co- ordinates. The pencil with which the writ- Fig. 8.— Sayers' System of Detachable Teeth 536 Telebarometer — Telegraph Instruments ing is done at the transmitting station is connected to two sliding rods, each of which controls the current in a circuit. The cur- rent is therefore proportional in each circuit to the length of the co-ordinate of the point at which the pencil may be placed at the moment. On the receiving board the posi- tion of the pencil is controlled by rods corre- sponding to the co-ordinates of the sending board, the actual length of each co-ordinate being controlled by the current from the same co-ordinate of the sending board. The pencil on the receiving board therefore takes up the same position as that of the trans- mitter, and in passing from point to point traces whatever curve the transmitter be made to trace. The telautograph was origi- nally invented by Elisha Gray. Crzanna's Telautograph, a telautograph in which, at the transmitting end, the emf in two circuits are controlled respectively by the horizontal and vertical components of the movement of a pencil with which the operator writes his message or makes the sketches to be transmitted. At the receiving end of the line, light from a miniature glow lamp is thrown upon two pivoted mirrors whose positions are controlled respectively by currents in the two transmitting circuits. The resultant ray of light makes the record upon a photographic film. POLLAK-VlEAG WRITING TELEGRAPH. — A small mirror throws a beam of light on a sensitised photographic strip. The mirror is controlled by two magnets at right angles to one another, the current in each being proportional to the co-ordinate of the point on the transmitting slip in the process of transmission. The result is that the spot of light traces the characters on the sensitive strip, reproducing the actual characters on the transmitting slip. It is claimed that a speed of 2000 words per min has been at- tained by this instrument. The writing transmitted is readable but none too clear. (Eef. 'Modern Electric Practice', vol. vi.) In the most recent form the transmitter is operated with perforated strip, the perforator being a species of typewriter, and the char- acters transmitted are much clearer than in the early instruments. (Eef. 'The Telegraphic Transmission of Photographs ', Baker.) See also Phototele- graphy. Telebarometer, an electrical arrange- ment by means of which the reading of a mercurial or other barometer may be taken at a distance. The motion of the barometer causes a change in resistance in a circuit which is recorded as a change of current at the observing station. Telefunken System of Wireless Tele- graphy. See under Wireless Telegraphy. Telegraph, Automatic, a telegraph in- strument in which the transmitter is con- trolled by a punched tape and not directly by the hand of the operator. The speed of transmission is therefore not limited to the capacity of one operator, since several may be simultaneously employed in punching tape for one machine. At high speeds it is necessary that the receiver should be a re- corder, as it is not possible to read and transcribe a message at speeds much above 35 or 40 words per min. Telegraph, Dial, a telegraphic instru- ment having a dial on which are printed the letters of the alphabet and the numerals. The transmitter and receiver are similar in appearance, except tha*- the transmitter has a handle on a movable arm which is turned until it stands opposite the letter to be sent, and the receiver has only an index hand or pointer. The transmitting arm is moved round, always in the same direction, from one letter to another. As this is done it makes a temporary current as it passes over each letter. This current, acting on the receiver, moves the pointer over the same letter on its dial. The receiving pointer therefore stops opposite the same letter as the transmitter does, i.e. it points to the letter sent. The instrument is sometimes used in country offices where the operator is not a trained telegraphist. Telegraph, Electric. See Electric Telegraph. Telegraph, Wheatstone A B C, a step-by-step instrument. Current waves are sent out by a magneto generator, the position of the transmitter determining their number. Each impulse moves the receiving pointer forward one letter round a circular scale on which the characters are marked. The pointer thus stops at the proper letter signalled; it returns to zero between the letters. See Telegraph, Dial. Telegraph Conductor, Capacity of. See Capacity of Telegraph Conductor. Telegraph Instruments, in general, instruments for varying the rate of trans- mission of electrical energy in accordance Telegraph Lines — Telegraph Systems 537 with a prearranged alphabetical code {e.g. Morse code) and for detecting, at the receiv- ing end, the variations caused by the trans- mitter. Transmitting instruments all involve the use of a key, whether hand-operated or automatic, which interrupts or varies the current; receiving instruments consist es- sentially in some device for rendering the variations of current obvious to the ear or eye. See Transmitter; Receiver; Eelay; Sounder; 'Siphon Recorder' under Tele- graphy, Submarine; Morse Recorder; Telegraph, Dial. Telegraph Lines, Capacity and Self- induction of. — ^These quantities determine to a large extent the possible speed of sig- nalling. The capacity (to earth or return conductor) is practically a shunt circuit for varying currents, and thus distorts and weakens the signals at the far end. Self- induction has a counteractive effect, and the proper combination of the two is essential to good transmission, especially in tele- phony. (Eef. ' Papers on Electrostatics and Magnetism', Kelvin (Thomson); 'Electrical Papers ', and ' Electromagnetic Theory ', Heaviside. See Capacity of Telegraph Conductor. TelegPaph Repeater. — The transmission of telegraphic signals through long cables is much complicated by the fact that the current waves become distorted, losing their definite character, and thus giving illegible signals. To avoid this, intermediate stations are erected where possible, at which the signals are transmitted either by hand or by means of a type of relay termed a repeater. The repeater receives signals from one sec- tion of the cable and transmits them afresh on to the next, the power being provided by a local battery. See Relay; Repeater. Telegraph Systems.— Morse Telegraphy. — In Morse's system of telegraphy the letters are formed by com- binations of dots and dashes, i.e. of currents of short and long duration. These are re- corded by a small ink-wheel pressed against a moving paper tape by the action of the current on an electromagnet. As long as the current is flowing, the wheel is making its mark on the tape; hence, since the tape moves uniformly, the marks made correspond in length to the periods for which the current flows. Dots and dashes are therefore re- corded. The instrument is called a Morse inkwriter or inker. The inker is not now much used in ordinary land-line telegraphy, the signals usually being read by sound, the dots and dashes being distinguished by the clicking of an armature of soft iron suspended over an electromagnet by a spring as it strikes two fixed stops. Multiplex Telegraphy, the transmission of two or more sets of signals simultaneously over one line wire. Duplex Telegraphy, the transmission simultaneously of two messages in opposite directions. There are two principal methods by which signals may be simultaneously received and transmitted. 1. By the use of a differential galvanometer, through the coils of which the outgoing current passes in opposition, so that the magnetic field is zero and the needle is unaffected, while the incoming current passes through them in series and moves the needle. The balance is preserved by means of a rheostat between the receiving instruments and the earth, and condensers are also used in the circuit if the capacity of the line is subject to variation. 2. The second method is called the Bridge method, from the resemblance of the circuits to those of a Wheatstone bridge. In this method the outgoing current passes along two arms of one half of a Wheatstone bridge, in parallel, while the receiver is placed in the diagonal and is therefore unaffected. The resistances are shunted by condensers, in order to balance the inductance of the line wire and thus prevent momentary distur- bances of the receiver; a rheostat is also interposed between receiver and ■ earth to make up for the difference in resistance between the earth connection and the line wire. DiPLEX Telegraphy, the transmission of two simultaneous messages in one direction. See 'Quadruplex Telegraphy'. QuADRUPLEX Telegraphy, two sets of signals simultaneously in each direction over one line wire. The circuits most in use resemble those of the differential duplex system. There are, however, at each end two relays, one polarised, and one non-polar- ised and of less sensibility. There are also two transmitting keys at each end, one being a reversing key and the other an increment key which increases the voltage in the circuit. When the reversing key is pressed it actuates only the polarised relay of the receiver, the current it makes being insufiicient to pull over the unpolarised relay. When the in- 538 Telegraph Systems crement key only is depressed the nonpolar- ised relay is alone actuated, since the current is in the direction which does not affect the polarised one. If both keys are simultane- ously pressed the current is large enough to actuate the nonpolarised relay and is in the right direction for the polarised one, hence both act. It is thus possible, by means of these additions to the differential duplex circuit, to send two sets of signals in each direction without interference. The bridge method of quadruplex telegraphy is also in use, but the difficulties of maintaining a balance are considerable, particularly over a land line. Both bridge duplex and quad- ruplex are used to some extent in cable telegraphy, where the electrical dimensions Quadruplex Telegraph System A, Polarised relay. B, Kon-polarised relay. C, Be- versing key. E, Earth. G, Galvanometer. B, Variable resistance. E, Increment key. are more constant than on land lines. It is possible to increase the number of simulta- neous messages by extension of the bridge method; the circuits, however, become com- plicated, and the difficulty of balancing in- creases with the complication. The most promising method for extension of the system is different in principle. It depends on the superposition at the combined transmitter, and analysis at the receiver, of a number of alternating or interrupted currents of different frequencies. The analysis at the receiving end may be by means of mechani- cally-tuned resonators such as electrically- driven tuning forks, whose frequencies of vibration correspond to those of the incoming currents. In wireless telegraphy (which see) duplex and diplex working have been achieved by somewhat similar resonance methods. Two transmitters having different frequencies act on one aerial, and two receivers, equif requent to the two transmitters respectively, are connected to the receiving aerial. Simul- taneous messages may thus be transmitted in one direction. The resonance in this case is purely electrical. Since a difference of 5 per cent in wave length, and frequency, is found to be sufficient to prevent interference of neighbouring stations, it is possible to have a considerable number of parallel wire- less connections in the same direction. Phonoplex Telegraphy, a system of multiplex telegraphy in which discrimination between the messages sent over the wire is accomplished by means of resonance. In the circuit of each transmitter there is a tuning-fork interruptor which makes and breaks the circuit a certain definite number of times per sec. The frequency of inter- ruption of each transmitter is different. At the receiving end there are similar forks actuated electromagnetically by the received currents. Each fork, however, will fall into vibration only when affected by intermittent current impulses properly timed to its own frequency. Thus a message sent by the transmitter having a frequency of say 100 per sec affects only the receiver of the same frequency. It is possible, therefore, to send several messages simultaneously in the same or in opposite directions. High-speed Telegraphy. — In high-speed telegraphy the transmission and reception are both necessarily accomplished by auto- matic instruments, since the hand is not able to send, nor the eye or ear to read, Morse signals at speeds greater than some 40 words per min. The transmitter is usually fed with a paper tape which has had holes punched in it corresponding to the Morse signals, several operators being detailed to each transmitter. The receiving instruments are of various types; the most rapid in action depend chiefly on photographic regis- tration of movements of a beam of light reflected from the moving part of a species of galvanometer. The highest speeds have been attained by the PoUak-Virag system (which is described under the heading Telautograph). In Green's system the message is recorded by the action of the current on a chemically-prepared tape (see also 'Hughes', 'Baudot', and 'Murray Type- printing Systems ', defined on p. 539). The Wheatstone Automatic High-speed Telegraph is most used in Britain. Speeds up to many hundreds, indeed some thousands, of words per min have been obtained. This is not a type-printing telegraph, but records in the Telegraph Systems 539 Morse code on paper type running at from 20 to 60 ft per sec. The apparatus consists of three principal parts: (1) the perforator, (2) the transmitter, and (3) the receiver. The perforator is an arrangement of five steel punches actuated in combinations by three keys which print respectively the signs for a space, a dot, and a dash. The spacing key punches a row of holes down the centre line of the tape. There are side punches which operate at the side of centre line, and punch the combination •• for a dash and ^ for a dot. The transmitter is a species of double- current key which is controlled by the holes in the slip. There are three rods corre- sponding to the three rows of holes, the central row controlling the timing of the whole motion. Each rod rises and falls continuously, but only actuates the current apparatus when it goes through a hole in the slip. The receiver is a sensitive polar- ised relay with an ink-wheel attached. A speed of 600 words per min may be attained by careful adjustment. The Wheatstone automatic is in use in this country for high- speed work. The most recent type of trans- mitter is termed the magnetic bias transmitter. It is practically a relay adjusted to an un- stable or neutral position. (Eef. 'Tele- graphy', Herbert.) FiRE-ALAKM TELEGRAPH SYSTEM, a radial system of circuits, centred at the fire station, with circuit-making keys in wall-boxes or pillars at convenient points throughout the district. The keys are usually protected by glass, which must be broken before they can be actuated. There is an indicator board at the station showing the locality from which the call is made. In the better types a telephonic connection is included, or at least a bell in the pillar indicates that the call is being attended to. In most cities this system has been in use for twenty or thirty years, fire-alarm pillars being placed at about every second street corner. Type-printing Systems of Telegraphy In type - printing systems of telegraphy the receiver prints the message in ordinary type on a tape or page. Its advantages relate to the small liability to error in transmission, and, in most cases, to the higher speed of transmission. The instru- ments, however, are in general somewhat complicated, and may require the attention of a skilled operator capable of adjusting them, though this is not always the case. Short descriptions of some of the principal systems are given below. Baudot Type-printing Telegraph. — A special code is employed, the instrument being manipulated by means of five keys, each character being represented by a cer- tain combination of these. Perforated slip is used, and the receiver prints in ordinary type. Steues Type-printing Telegraph. — This instrument is the natural development of the Wheatstone ABC (see Telegraph, Wheatstone A B C), which merely indi- cates the letters by pointing to them. The method of transmission is similar. Murray Type - printing Telegraph, a rapid type - printing telegraph which uses a special code, each character being trans- mitted by pressing a combination of keys. Perforated slip is used in the transmitter. The speed is higher than that of most type printers, reaching as much as 160 words per min. The code used is somewhat similar to that of the Baudot, which is much used in India. Hughes Type - printing Telegraph. — This is a kind of synchronous type-printing telegraph. It differs from all others in two important points — (1) that a single current only is required for each letter, and (2) that the type wheel revolves continuously and is not checked when a letter is printed. The working currents are of equal duration, but are separated by unequal intervals of time. There are two absolutely synchronous type- wheels at the two stations. At the trans- mitting station there is a keyboard which actuates radial pins in a plate round which a carriage sweeps. The carriage carries a lever which makes a contact when it strikes one of the pins. Connected to the carriage is a type-wheel which prints the letters at the transmitting station. The current im- pulse sent, not only prints the corresponding letter at the receiver, but also synchronises the motion of the receiving and transmitting type-wheels automatically. The system is capable of being duplexed by the bridge method. It is largely in use in France on lines where ordinary manual transmission is not sufficiently speedy. (Eef. ' Telegraphy', Herbert.) Buckingham-page Type-printing Tele- 540 Telegraph Transmitter — Telegraphy GRAPH. — Wheatstone punched slip is used in the transmitter, while the receiver prints the characters in ordinary type in page form. Rowland's Page Type-printing Tele- graph. — In this instrument the signals are sent directly by manual operation of a key- board, and are printed in page form by the receiver. (Ref. 'Modern Electric Practice'.) See also Telegraphy, Submarine j Wireless Telegraphy. [j. e-m.] Telegraph Transmitter. See Trans- mitter. Telegraphic Code, a set of symbols re- presenting either letters, words, or combina- tions of words (sentences or phrases), and suited for telegraphic transmission. The Morse (see Morse Alphabet) and similar letter codes are used for transmission, letter by letter. In order to reduce the cost of transmission of long sentences commonly occurring in business communications, codes have been constructed such as the ABC, the "Western Union, and the Unicode, in which one word is made to stand for a phrase, the phrases being classified alpha- betically under subject headings. In send- ing a telegram to a long distance it is therefore usual to translate it into a few code words, which are then transmitted, letter by letter, in the Morse code. Private, i.e. secret, codes are frequently constructed by merely taking from the code book, words which are a iixed number of lines farther down the page than the words in the ordinary code. [j. e-m.] Telegraphone, an instrument which takes a record of a telephonic conversation. The recorder consists of a small electro- magnet in the telephone circuit over which a thin band of steel is passed by clockwork. Each current wave in the circuit records itself as a magnetised patch on the band, so that if the band be afterwards passed through a small coil connected to a tele- phone the varying induction from the mag- netised patches induces varying currents which reproduce the original sound. The apparatus is the invention of Mr. Valdemar Poulsen, and, though very ingenious, has not yet been made of practical use, prob- ably because of the fact that the record is invisible, and that the long steel strips are comparatively expensive and inconvenient for storage. Mr. A. P. Hanson, in Berlin, has also invented a system of this class. Telegraphy. See Telegraph Systems; Electric Telegraph. Telegraphy, Submarine. — Submarine telegraphy differs from land-wire telegraphy in several particulars. 1. Since the surround- ing medium, the sea, is a conductor, the wire must be insulated from it along its entire length, and not only at its points of support as is the case with a land wire. 2. The central conductor and the sea or outer sheath- ing, form the coatings of a condenser; thus the current at the far end does not at once rise to its full value but increases gradually. This eflFect, which is the chief hindrance to rapid signalling through a long cable, was first explained by Kelvin. The possible speed of signalling is increased by sending a reverse current (see Reverse- current Working) immediately after the direct cur- rent. This, as it were, kills out the tail of the first, or positive, current wave, leaving the line clear for the next positive impulse. At first great trouble was experienced, and frequent and long interruptions in the service were caused, on account of the currents pro- duced in the line by differences of potential between terminal stations of long cables. Since both ends of the wire were connected to earth, it is clear that a difference of poten- tial at these points of the Earth's surface must cause a current along the wire. The obvious method of using a return wire, i.e. a completely-insulated metallic circuit instead of earth, is precluded on the score of expense. The difficulty has, however, been overcome in recent years by avoiding a metallic connec- tion between the end of the conductor and the earth, by working through a condenser. In this method the conductor of the cable is completely insulated, the current -impulses which constitute the elements of the signals being transmitted to the cable through a condenser which forms a perfect barrier to cc. In recent years the speed of signalling, which at first did not exceed 6 words per min, has been greatly increased, the maxi- mum on transatlantic cables in one direction being about 50 words per min, i.e. 100 per min over the cable where duplex working is used. The receiving instruments used are still of the siphon - recorder pattern in- vented by Kelvin, which was the first ' moving-coil ' galvanometer and at the same time produced a legible record on paper strip. The improvements have been mainly Telegraphy — Telephone Cable 541 in relays and other details of the methods of working, and in proper proportioning of the electric ■ constants of the line as indi- cated by mathematical theory. The chief treatises on the subject are by Wunschen- dorff and by Charles Bright. [j. E-M.] Telegraphy, Wireless. See Wireless Telegraphy. Telephone, Electrostatic, a small con- denser of which one of the plates is of thin flexible metal, the dielectric between the plates being air. The variations of air pres- sure constituting sound cause the thin plate to move to and fro, increasing or decreasing the capacity of the con- j^ ■denser. If the plates be I charged, this variation in ^ C|[|J the capacity will cause a ^ current in the wires lead- Fig. ing to them which will be similar in form to the sound waves. This type of telephonic transmitter is peculiarly suitable for wireless telephony, since a small capacity can convey a large current if the frequency of alternation of the latter be high •enough. The late Prof. A. E. Dolbear of Tufts College did a great deal of early work in the investigation of the subject of electro- static telephony. Telephone Cable. — An anti-induction tele- phone cable is constructed so that the electric and magnetic inductions between neighbour wires, whether of the same or different cir- cuits, are as small as possible, to prevent cross -talk between adjacent circuits. The first requirement is fulfilled |_ by the use of a dielectric nnUT^ of low specific inductive /^ _ capacity, e.g. air and dry paper; the second by twist- Kg. 2.—' ing the two wires forming Inductance. — The distortion of the wave forms which occurs in long telephone lines and cables, and which is the cause of in- distinct articulation in the receiver, may be to a large extent corrected by the addition of inductance to the line. This should pre- ferably be uniformly distributed along it, but as such an arrangement is not easily practicable it is usual to insert inductance coils at intervals. In a long line the electric capacity between the two wires forming the circuit (or between one wire and earth if an earth return be used, which is not now usual) is by no means 1 1.— Telephone Cable with No Inductance negligible. If we suppose that the capacity is not uniformly distributed but is concen- trated at intervals along the line (see fig. 1), and that there is no inductance, it is clear that a rise of voltage at A, the transmitting end of the line, will cause a flow of current into the condenser c, and that this will become charged, and will therefore have transmitted all the current which it can transmit with the given voltage, by the time the voltage between its terminals t-^ and t^ has risen to a maximum. On the whole the energy thus short-circuited or diverted by the capacities, and therefore not transmitted to the distant station B, will be greater the more rapid the L/ -nm^ ^e a circuit, round one another like the strands of a cord. A bare wire in a paper tube of much larger diameter than the wire forms an clement, and from a number of such elements the cable is built up. The whole bunch is surrounded by a leaden tube or other insu- lating and protective covering. The effect of a varying magnetic field in inducing current in the twisted circuit is practically zero, since at each half-turn the wires change places as regards , the lines of force. The induction therefore changes, and the induced emf is reversed. The resultant effect on a con- siderable length is thus nil. Telephone Cable with Distributed Telephone Cable with Distributed Inductance rate of change of the voltage, i.e. the higher the frequency of the waves. Thus the higher overtones will suffer most in transmission, and the distinctive characters of the vowel and consonant sounds will be lost in trans- mission. In addition to this there will be a large phase difference between the current and voltage (current leading), and the pf will consequently be small. Now suppose that, as in fig. 2, inductances L, Lj, Lj are interposed at points along the line. The result is that as the voltage rises the current can no longer rush into the condensers so rapidly; it is checked by the inductances; and if the inductances are pro- 542 Telephone Drop — Telephone Exchange perly proportioned the current will no longer lead, but will reach its maximum when the voltage does, and therefore the voltage will already have fallen by the time the condensers become fully charged. Hence their charges will be less, i.e. the energy short-circuited through them will be less than with no inductance in circuit. More energy will therefore reach the receiving instrument at the end of the line. Also, since the currents of higher frequencies are most easily trans- mitted across a condenser, the proportionate reduction of these, at the receiver, will be less, and articulation will be better. The principle may be shortly stated thus : If there be a condenser in a circuit to the terminals of which an alternating „voltage is applied, the condenser can never become fully charged, i.e. to the extent correspond- ing to the maximum circuit voltage, unless the current is leading by 90° in phase, since otherwise the current is still flowing into the condenser when the voltage has commenced to fall, and therefore the maximum charge, which does not occur till the current stops flowing into the condenser, corresponds to some voltage lower than the maximum volt- age applied to the circuit. [j. E-M.] Telephone Drop, a small metal plate hinged at its lower edge and held by a catch at its top. The catch is released by an electromagnet when a current is sent over the line. The drop is used as an indicator in telephone exchanges where the central- battery system is not used. It has been superseded in modern exchanges by the lamp as an indicator. See Annunciator. Telephone Exchang-e, the cen- tral station from which the sub- scribers' wires radiate, and through which one subscriber obtains a con- nection with any other subscriber, either in the district or through the trunk lines to another district. In early types of exchange a local battery at the subscriber's instrument was the source of energy. This system has, however, been superseded by the central-battery system, in which the power for all the lines is supplied by a single battery in the exchange. The chief advantages of this system are the omission of the magneto and subscriber's battery at every substation, and the great simplification of the calling apparatus. The result is both cheaper and more rapid work- ing. A typical subscriber's apparatus is. shown diagrammatically in fig. 1, and the circuits of a telephone exchange are shown in fig. 2. By taking the telephone ofif the hook the subscriber makes the circuit through his transmitter, actuating relays in the ex- change and thus lighting up the signal lamp under his number. On noticing this the exchange operator inserts the 'answering :E § C Mb Fig. 1.— Subscriber's Apparatus A, Receiver. B, Transmitter, c, Switcli liook. D, Induction coil. E, Polarised bell. F, Condenser. plug', which cuts out the call-signal apparatus and enables her to connect her telephone to the subscriber's circuit by pressing the listen- ing key. She asks the subscriber the num- ber required, and on receiving his answer, tests to see if the called line is engaged. If the line is not engaged, she inserts the call- ing plug, and presses the ringing key. The calling plug cuts out all exchange apparatus Mg. 2.— Speaking Connections of a Telephone Exchange 0, Condenser, s, Subscriber. E, Exchange. R, Bepeater. except the relays of two supervisory lamps-, one under each number, which light up when the telephones are placed on the hooks at the conclusion of the conversation. When both lamps light up, the operator discon- nects the subscribers by pulling out the plugs. Automatic Telephone Exchange. — For many years attempts have been made to obviate the necessity of having the connec- tions between one subscriber and another Telephone Induction Coil — Telephony 543 made by hand in the exchange. These have at last been successful, and automatic ex- changes are now working in various parts of the world. The principle of action usually depends on a series of current impulses transmitted by the subscriber in sliding a contact arm over a number of contacts cor- responding to the number of the subscriber wanted. In its simplest form this would only be applicable to a very small exchange, but the system is easily extended to deal on the same principle with large ones. In an automatic exchange the mechanism informs the subscriber if the line wanted is engaged, connects him to it, and rings up the moment it is clear. The Strowger system is the best known, but A. P. Hanson of Berlin has also invented a system which has been covered by a large number of patents. Telephone Induction Coil.— In tele- phony the microphone current in the trans- mitter is usually transformed to higher voltage by the use of a small transformer, sometimes called the induction coil. Telephone Jack, the spring - contact switches into which the plugs for connecting subscribers' lines are pushed. If the body of the plug is made of insulating material, the conductors being connected to metal rings on its surface, it is possible to make several circuits in one movement of the hand, thus economising time. See Jack, Five-point; Answering Jack; Jack-box; Jack-sleeve. Telephone Receiver.— The electromag- netic telephone invented by Bell in 1876 is still universally used as the receiver in tele- phony. It consists of a thin iron disk sup- ported at its circumference, so that its centre is very near the end of a polarised electro- magnet or of a ' permanent ' magnet having a coil of fine wire wound on it. Variations in the incoming current cause corresponding variations in the strength of the magnet, and hence make the disk play to and fro in time with them. Since the to-and-fro motion is similar to the variations of the current, and since the latter are controlled by the original sound at the transmitting station, the telephone reproduces the original sound, i.e. gives out waves of air-pressure exactly similar to those spoken into the transmitter. Telephone Relay, a term usually applied to a type of relay (which see) suitable for use in the transmission of speech. Its action must therefore vary continuously with the current, and not be intermittent as in a tele- graph relay. In connection with telephone exchanges, relays of the usual electromagnetic type are used to actuate the signal lamps or other call arrangements in the exchange, thus avoiding the necessity of sending con- siderable currents through the subscribers' circuits when the exchange is to be called. See Relay. [j. e-m.] Telephone Traffle.— In suiting a tele- phone service to the conditions under which it is worked, the following considerations are the most important: originating traflftc, trunk traffic, statistics of circuits and stations, labour, flat-rate service, measured-rate service, party-line service. Since, from the point of view of the subscriber, the value of a tele- phone connection increases with the number of subscribers in the district, it is usual to accept subscribers to party lines at rates which are not directly remunerative, but which add to the number of subscribers and therefore to the popularity and value of the service, thus increasing the number of sub- scribers at higher and more remunerative rates. (For an exhaustive discussion of the subject see Abbott's 'Telephony', pt. vi, chap, viii.) Telephony, Diplex, a method of trans- mitting simultaneously two conversations in the same direction over one wire or wireless connection. Not yet achieved by wire, but probably quite feasible by wireless methods, employing hf currents of difi"erent wave lengths for each conversation. Methods have been patented by Fessenden in America. Telephony, Long-distance.— The chief difficulty in long-distance telephony is the distortion of the current wave owing to the fact that the higher harmonic components tend to die out more rapidly than the lower as the distance is increased. The character of the tone is thus lost and articulation becomes imperfect, although the volume of sound may be maintained to a large extent. It has been found possible to overcome this difficulty by adding self-induction to the line, usually by interposing special coils at regular distances along it (see Telephone Cable). These, however, have their disadvantages in that they increase the ohmic resistance and may cause reflections of the electric waves as they travel along the wire (see Oliver Heaviside's Electrical Papers, and works on modern telephony). Distances of from one to two thousand miles are the greatest that 544 Telephony — Temperature Rise of Electric Machines have as yet been achieved on land lines, while on cables the distance is very much less. It is probable that at great distances, telephony will be more easily carried on by wireless methods than by wires, particularly where the latter have necessarily to be en- closed in insulating cables, as under the sea. [j. E-M.] Telephony, Wireless. See Wireless Telephony. Telescope, Reading*, for Electrical Measurements. — A telescope is arranged so that an observer may see through it the Beading Telescope for Electrical Measurements image of a fixed scale reflected from the mirror attached to the moving deflected part of a galvanometer, electrometer, &c. A hair line drawn across the telescope indicates the exact reading. See Galvanometer. Tele -thermometer. See Pyrometer, Electric; Thermo-electricity. Telpherage, a mode of transporting goods and materials by means of cables sup- ported on posts, and trolleys provided with grooved wheels to run on the cables. The skips or buckets in which the loads are placed are suspended from the trolleys and coupled together to form a train, the leading trolley carrying an electric motor for haulage. Power is derived from a bare conductor sus- pended above the main cable, connection being made with it by means of a collector carried on the motor trolley. The motor trolley may be used alone, the load being carried in a skip slung beneath it. See PORTELECTRIO EAILWAY SYSTEM; MONO- RAiL Electric Railway; Schilowsky Monorail System. Temperature Coefficient of Resist- ance. See Resistance, Temperature Co- efficient OF. Temperature of Evaporation. See Steam. Temperature of Vaporisation. See Steam. Temperature Rise by Resistance.— Since the resistance of a conductor usually varies with its temperature (see Resistance, Temperature Coefficient of), it can be employed to determine the temperature. This is very frequently done in the case of the windings of electrical machinery. See Temperature Rise of Electric Machines. The average temperature coefficient for copper between 0° C. and 100° C. is 00042 per degree C, the resistance at the freezing- point being taken as standard. Thus if R is the resistance at f C. and Rj that at t^ C. : — J, ^ 1-1- 0042 < J, _ 238 -f- t T, l-f 0-0042 . Also we have Vj = I R where R is the value of the non-inductive resistance, /, V2 = Vi^ -f V/ + 2 Vil R cos , \ Power, Vjl cos = _ ya _ V^2 _ v^; 2R The connections for the test are shown in the fig. E.av O^- Ihree-ToltmeteT Method of Measuring Alternating- current Power E.S.V., Electrostatic voltmeter. A, Ammeter. B, In- ductive load. 0, Non-inductive resistance. See Power, Methods of Measuring, in Polyphase Circuits. [c. v. d.] Three -wattmeter Method of Mea- suring Power. See Power, Methods OF Measuring, in Polyphase Circuits. Three-wire ac and cc Compensator. See Compensator; Three-wire System. Three -wire Balancer.— The machine Three-wire Balancer I, Primary leads. a, Secondary leads. shown in the fig. is known as the 'C.M.B.' auto-converter, and may be used as a three- wire balancer, although originally developed as a cc compensator for reducing supply Three-wire cc Traction — Three-wire Distributing System 559 pressures for working metal-filament lamps, arc lamps, &c. In the fig. the armature con- stitutes a gramme ring, the brushes a a being connected to the primary source of supply, and the brushes hi short-circuited to form one of the terminals of the secondary supply S. In the two-pole machine shown, each pole is divided into two parts, and mag- netically separated, the object being partly to reduce the effect of the armature mmf caused by the short-circuited brushes and partly to obtain regulation of the secondary pressure. The armature revolving in the field set up by the armature reaction mmf induces an emf which causes large circulat- ing currents to flow between the short-cir- cuited brushes, and these currents, if not properly controlled, would make the opera- tion of such a machine practically impos- sible — either causing the machine to race or to lose speed. To overcome this difficulty, series coils AjAg are placed on the main poles in addition to the shunt coils. These series coils are connected to the short-cir- cuited brushes 6 h, and thus each carries nor- mally half full-load current. If a short- circuit current is produced, it will flow in opposition to the load current in one coil and in conjunction with the load current in the other coil. Thus, supposing that the current in coil Aj were less than the current in coil Aj, then a larger pressure would be generated under the pole s„ than under n^, thus opposing the pressure which is setting up the short-circuit current. See Compen- sator; DoBROwoLSKi Three-wire Dynamo. Three-wire ee Traction, a system of supplying trains or tramcars with electric power in such a way as to secure the ad- vantages of transmission at h pr, while feed- ing the motors at a lower pressure. Two insulated conductors are used, with a pd of, say, 1000 volts between them, and 500 volts between each conductor and the track rails, which act as a third conductor. According to one method of operation, about half the cars or trains are supplied with current from the positive conductor, and half from the negative, the two sets being in point of fact in series with one another; any excess of current from one side or the other is carried back to the power station or substation by the track rails, which otherwise act merely as the connecting link between the two sets of cars, and as a rule carry very little cur- rent. A preferable method of operation is one in which two motors on a single car, or two sets of motors on one train, are put in series between the two insulated conductors, the middle point remaining in connection with the rails for balancing purposes, and to limit the maximum voltage on either motor. The control of the motors is effected by placing them all in series between one insu- lated conductor and the track rails during starting, and for the running condition by changing the track-rail connection to the other insulated conductor, thus putting the motors all in series across the insulated con- ductors. This method is employed on the Krizik system at Tabor-BeehynS with. 1400 volts between outers. (Kef. McMahon, 'The City and South London Eailway: Working Eesults of the Three-wire System applied to Traction, &c.', Journ.I.E.E., vol. xxxiii, pp. 100-200; Krizik, Street Railway Journal, vol. xxiv, p. 1042, Description of the Tabor- BechynS Line.) Three-wire Distributing System (see also Three-wire System), a system of elec- trical supply in which fK TT 1 1 a neutral wire is used ^ 11 . I [ midway between the O AAA AA two supply mains, ^ TTT TT and the lamps or rig. 1.— ContinuouB-current Other consuming de- Three-Wire Distributing Sys- yices are connected tem with Two Generators in , , . Series between the mams and the neutral, the object being to reduce the total amount of copper required assuming a given voltage across the terminals of the lamps. When -| — |- the supply is cc the 6 o neutral may be -" — I— brought out from Fig. 2.-Continuous-current the Connected ter- Three-wire Distributing Sys- . ■• j. . tem with Balancer minals 01 two gener- ators in series (see fig. 1), or from the common terminal of two machines connected across the outers as shown in fig. 2, such a pair of machines constituting Fig. 8.— DobrowolsiJi Three-wire Distributing System a so-called balancer; or from the central or approximately central point of three choking 560 Three-wire Dynamo — Tikker coils connected to three slip rings which tap the armature winding of the generator at three equidistant points (see fig. 3). The neutral conductor is frequently made half the area of either outer, and in this case the weight of copper as compared with a two-wire system working at half the voltage between the outers is in the proportion of 31'25 to 100. When the supply is alternat- ing, the neutral is obtained by tapping the Fig. 4.— Three-wire Tianstonner System secondaries of the transformers at the mid- point as shown in fig. 4. See DoBEOWOLSKi Three-wire Dynamo; Compensator; Neutral; Neutral Con- ductor; Multiple-voltage System; Mul- ti-voltage Speed Control; Three-wire System. Three-wire Dynamo. See Dobrowol- SKi Three-wire Dynamo; Compensator; Three-wire Distributing System. Three-wire Lighting System, a three- wire distributing system for lighting pur- poses. See Three -WIRE Distributing System; Neutral Conductor; Compen- sator; Multiple-voltage System; Three- wire System. Three-wire Meter. See Meter, Three- wire. ' Three -wire Network, a network of conductors for the supply of electrical energy on the three -wire system. See Network of Conductors; Three -wire Distributing System; Neutral; Neutral Conductor; Multiple-voltage System. Three-Wire Neutral. See Neutral; Neutral Conductor; Three -wire Dis- tributing System; Multiple - voltage System. Three - wire Speed Control. See Multi-voltage Speed Control; Multi- voltage System. Three -wire System.— As defined by the 1907 Wiring Eules of the I.E.E. for the supply of electricity at 1 pr not exceeding 250 volts— 'A three-wire system, is one in which three con- ductors are maintained at different potentials, the conductor at a potential intermediate between the highest and lowest being common to all lamps or other consuming devices supplied on either side of the system'. See also Neutral; Neutral Conductor; Outer Conductor; Multiple - voltage System; Three -wire Distributing Sys- tem. Three-wire Transmission, the trans- mission of electrical energy by means of the three-wire system (see Three-wire Distri- buting System). The term, however, is but rarely used, as the name transmission is generally reserved for long-distance trans- mission, and the three-wire system being operated at a comparatively low voltage is not suited to such transmission. A three-phase transmission, although em- ploying three conductors, is scarcely ever referred to as a three-wire transmission. Three -wire Two -phase System, a two-phase system in which two of the con- ductors are joined together. Seeing that the two circuits of a two-phase system are quite independent of one another, they may be joined at any one point without upsetting the operation of the system. This permits of two of the leads being joined together to form a single third conductor, which must, however, be V^ times the area of each of the single conductors, since the current it has to carry is increased in this proportion. See also Alternating - current System. (Eef . ' Alternating - current Phenomena ', Steinmetz; 'Standard Polyphase Apparatus and Systems', Gudin.) Throttle Switch or Magnet, an elec- tromagnetic device used in pneumatically- operated controllers, to actuate the valves which admit or release the compressed air in accordance with the movements of the master controller. See Switch, Electro - pneu- matically - controlled Unit; Control, Pneumatic, of Electric Apparatus. Throw -over Switch. See Switch, Double-throw. Thrust Bearings. See Bearings. Thury Constant -current System. See Series Distribution; Generating System. Thury Regulator. See Regulator, Potential. Thury System. See Series Distribu- tion; Generating System. Tie-rod Type of Stator Frame. See Frame, Stator. Tiklcer, a vibrating interrupter used in the Poulsen system of wireless telegraphy Time — Time-limit Device 561 to make intermittent contacts in the receiving circuit with the object of integrating the energy of the received current, and rendering its presence audible in the receiving tele- phone. See Wireless Telegraphy. [j. e-m.J Time, one of the three fundamental phy- sical conceptions. The other two are dis- tance and mass. Thus in the cgs system the units of distance, mass, and time are respectively the centimeter, the gram, and the second. Time-constant, the ratio of the coefiBcient of self-induction (i.e. the inductance) to the resistance of an electric circuit. This ratio = is a physical constant of a circuit of dimensions MIT, and determines the rate of growth of the current in a circuit when a given difference of potential is suddenly applied and maintained at its terminals. The time-growth of current in these circumstances is given by the expression where V is the applied steady emf in volts, E the resistance in ohms, I the co-eflBcient of self-induction (i.e. the inductance) in henrys, and t the time in seconds. The rate of growth of current is given by the expression where T is the time constant, e being the base of the Napierian system of logarithms. [m. b. p.] Time -current Curve, a diagram ob- tained by plotting on squared paper the current passing in any circuit at each moment as ordinates, against the time as abscissae. Thus the variation of the output of an elec- tricity generating station throughput the day may be shown in the form of a load diagram, or the current consumed by an electric car during a journey may be similarly exhibited. Some load diagrams of central stations are given in the definition Central Station for the Generation of Elec- tricity and in the definition CxjRVE, ChaEt acteristic. See Speed-time Curve. Time Cut-outs. See Time-limit De- vice; Time Element. Time Element, a feature incorporated in circuit breakers whereby the circuit is only interrupted when the overload is of some prearranged duration. See Circuit Breaker; Time-limit Device. Time-lag- Devices. See Eelay; Time- limit Device; Circuit Breaker. Time Lag- of Magnetisation, the time interval occurring before the magnetism reaches the value corresponding to a given magnetising force. This delay of magnetis- ation is almost exclusively due to existence of eddy currents; and the promptitude with which the magnetism follows the magnetising force depends on how completely the mag- netic circuit is laminated to avoid eddy currents and upon the hysteretic quality of the material. See Lamination of Magnet; Laminated; Hysteresis; Eddy Current. Time -limit Circuit Breaker. See Circuit Breaker; Hobart's Time-limit Device for cc Circuits; Time - limit Device. Time-limit Device, a device to extend and control the time taken by a switch, circuit breaker, relay, &c., in coming into operation. A fuse is in a sense a time-limit dcATice in so far as it takes time to heat up the mass of the fuse to the melting-point. A large momentary rush of current, if its duration is suificiently short, may not melt the fuse; and this is a valuable feature, seeing that such a momentary rush does no harm, or, at any rate, very little harm in the majority of cases, and is, as a rule, preferable to a discontinuance of the supply until the circuit can be again completed, and perhaps first inspected and tested. On the other hand, should an unduly increased cur- rent pass for a comparatively lengthened period, the mass of the fuse will have time to heat up to the melting-point, and the circuit will be broken when the safety of the apparatus on it demands that it should be broken. The term time -limit device is, however, generally applied to a mechanical contrivance designed to delay the opening of a circuit by a circuit breaker. The device may take the form of a clockwork train acting upon a ratchet attached to the plunger of the circuit breaker and driving the vanes of a small fan which forms a damper; in this case the greater the pull on the plunger — that is, the greater the overload on the circuit — the faster the fan will revolve and the quicker the breaker will trip, which is a most de- sirable result. Indeed in the ideal time- limit device the time lag is inversely pro- 562 Time-limit Relay — Torpedo portional to the severity of the overload until, on short circuit, the operation is in- stantaneous. Dashpots in connection with the plunger of the circuit breaker are also used as time limits; and thermal devices, in which the action depends upon the expansion of a portion of the circuit when heated by the current, have been adopted in some cases. Time-limit devices can be attached to relays which operate circuit breakers or oil- switches, in a similar manner to that in which they are attached direct to the circuit breakers themselves. (Eef . ' Elek. Mas. Apparate und Anlagen ', Niethammer.) See Circuit Breaker; Ho- bart's Time-limit Device for cc Circuits; Eelay. Time-limit Relay. See Relay; Time- limit Device. Time Phase. See Time Quadrature. Time Quadrature. — One periodically varying quantity is in time quadrature with another when it passes through its maximum (or zero) value one-quarter of a cycle later or earlier than the other quantity does. When the two quantities pass through their maxi- mum values at the same instant, they are sometimes said to be in time phase, or, more usually, in phase. The terms time quadrature and time phase have been considerably used recently by McAllister (see Section 8 of McGraw's Standard Handbook for Electrical Engineers). See also Quadrature; Me- chanical Quadrature. Time Rate of Change of Flux, the change per sec of magnetic flux linked with a coiL When multiplied by the number of turns of wire on the coil, this gives the in- duced voltage in cgs units. Time Switch. See Switch, Time. Tin (chemical symbol Sn), a metal with a melting-point of only 230° C. Its specific gravity is 7 '3 and its specific heat is 0'056. At 0° C. the specific resistance of tin is 13"1 microhms per cm cube, and its resistance increases by four-tenths of one per cent per degree Centigrade increase in temperature. See Solder. Tinned Tubing. See Tubing, Tinned; Conduit, Interior; Wiring Systems. TiFFill Regulator. See Regulator, Potential. Ton. — Unfortunately, several values of the ton are in extensive use. Every effort should be made to employ the metric ton as frequently as practicable, and to discontinue employing other values. The metric ton is equal to the weight of a cu m of water. The British ton is equal to the weight of 1'015 cu m of water. The American ton (the short ton) is equal to the weight of 0'906 cu m of water. Thus the American ton is so widely different from the others as to often lead to serious error when it is not known which value of the ton has been employed. In 99 per cent of all engineering work, the 1"5 per cent difference between the metric ton and the British ton will not be of any consequence, but never- theless it is highly desirable to employ the metric ton. The point is, that since the difference between the two values is so slight, it becomes at once practicable to adopt the metric ton in new work and take old values of quantities expressed in British tons as sufficiently correct for practically all engineer- ing purposes. Ton-calorie, the amount of energy re- quired to raise the temperature of 1 ton of water from 0° C. to 1° C. 1 ton-calorie = 1"16 kelvin = 427 ton meters = 4,190,000 joules. See Energy; Kelvin; Kilogram-meter; Joule; Erg. Ton-kilometer, a unit of measurement used in electric or other systems of traction as a basis of comparison, the quantities to be compared being divided by the product of the total weight transported and the distance carried in each case, in terms of tons and kilometers. When the distance is measured in miles, the unit becomes the ton-mile. Ton-meter, the quantity of energy re- quired to raise one ton through a height of one meter. I ton meter = 0-00272 kelvin = 2-34 kg cal = 9810 joules. See Energy; Kelvin; Kilogram-meter; Joule; Erg. Ton-mile. See Ton-kilometer. Top-groove Insulator. See Insulator. Toplis Ventilated Commutator. See Commutator. Topping-up an Accumulator. See Accumulator, Topping-up an. Torpedo, Sims -Edison, a locomotive torpedo, propelled and steered electrically, Torque — Torque Diagram of an Engine 563 which can be discharged either from shore or from a ship in motion, and its direction controlled by an operator either on the shore or ship as the case may be. The torpedo is divided into three compartments, the for- ward one containing the explosive with the usual concussion exploder, the centre one the electric motor and cable, and the after one the steering magnets and a polarised relay to direct the main current into the coil of the port or starboard magnet as required. Current for the motor and steering magnets is provided by a dynamo, and for the relay by a small battery of accumulators, and is conducted to the torpedo by a duplex flexible cable, a common earth -return being used. On the shore or ship, a switch is provided to start and stop, and regulate the speed of the motor on the torpedo, and a second switch to operate the polarised relay of the steering apparatus, so as to direct the course of the torpedo. To secure the correct degree of submersion the torpedo is hung from a float which runs on the surface of the water. [c. w. H.] Torque, Hysteretie, the retarding force due to hysteresis on the armature of a dynamo or other electrical machine. Owing to hysteresis, the magnetism is more intense on the armature surface just behind each pole than just in front. The poles there- fore exert a drag on the revolving armature. See Meter, Magnetic Drag in a; Meter, Retarding Torque of the Magnetic Brake of a; Ewing Hysteresis Tester; Blondel Hysterbsimeter. Torque, Maximum - adhesion. See Maximum-adhesion Torque. Torque, Synehronising:, the torque mutually exerted by ac machines when running in parallel as soon as any ten- dency to depart from synchronism arises. The torque is due to the interaction of the magnetic fields and armature currents, and is in such direction as to help to main- tain synchronous running. Thus, if two alternators, A and B, are running in parallel, and together supply a definite load, and if for any reason A lags behind B, a greater load is taken up by B, and a lesser load by A. This results in a tendency to maintain synchronism. The synchronising torque may be said to be due to a motoring current, i.e. the lagging machine acts less as a generator, and becomes more of a motor, in very much the same way as when two cc machines are running in parallel. In such a case, it is well known that if the supply of power to one generator by the prime mover is interrupted, there will be a tendency for the generator to con- tinue to run, being driven from the other after the manner of a motor. See Alternators, Parallel Running of; Phase Swinging (or Surging); Surg- ing; Damping; Irregularity Factor; Cyclic Irregularity. [m. b. f.] Torque Diagram of an Engine.— ' Torque ' is the technical expression for the tendency of a force to produce turning or twisting. It is measured in pound-feet or kilogram -meters. A torque Fig. 1.— Torque Diagram of a Single-cylinder Gas Engine diagram is a curve indicating the variation in the turning efibrt on a crank shaft for various stages of an engine cycle. For ex- ample, fig. 1 shows the torque variation of a single-cylinder gas engine whose complete cycle occupies two revolutions. It will be seen that at some stages the turning effort is nega- tive, the stored energy in the flywheel being the source which keeps the engine turning. X /or S/ng/e ocCtfiQ *«, Ci^/Jnc/er ^rtQJm of ^ai/o/ut/an ■ One /fero/uCion ' Fig. 2.— Torque Diagrams of Steam Engines wiUi Single- acting and Double-acting Cylinders Fig. 2 shows the diagrams obtained from two steam engines with double-acting cylinders respectively. It will be seen that the curves in fig. 2 are more regular than those in fig. 1, and that, in the case of the double-acting cylinder, at no part of the cycle does the torque attain a negative value. With a com- pound or triple-expansion high-speed engine, a much more uniform diagram would be obtained. See also Cyclic Irregularity; 564 Torque EflEiciency — Total Works Cost Irregularity Factor ; Crank-effort Diagram; Variation in Prime Movers; Pulsation in Prime Movers; Variation IN Alternators; Pulsation in Alter- nators; Fly-wheel Storage. Torque Efficiency. See Efficency. Torque in Meters. See Meter, Ee- TARDiNG Torque of the Magnetic Brake of a; Meter, Driving Torque of a Motor-; Meter, Eetarding Torque of A Motor-. Torque Meter. See Meter, Torque. Torque of Motor, Running, the torque produced by a motor when running at its appropriate speed, as distinguished from the torque it exerts when standing still, or under the special conditions introduced in the cir- cuit at starting. See Starting of Motors. Torque - speed Characteristic. See Motor Performance Curves. Torque Transmission Dynamometer. See Dynamometer, Transmission; Torsi- ometer. Electrical. Torsiometer, Electrical, an instrument for measuring the angular deflection of a shaft due to torsion, while the shaft is re- volving; especially useful in marine work to determine the power transmitted by the propellor shaft. A recent form designed by Messrs. Denny and Johnson has two sleeves turned to a sliding fit over the shaft under test and rigidly fixed to the shaft only at their outer ends. The inner ends are ar- ranged to carry electromagnets with pole- faces closely adjacent, so that any deflection of the shaft between the fixed ends of the sleeves causes a change in the depth of the air gap between the pole - faces of these electromagnets. One magnet winding is traversed by an alternating current of constant eflfective value which induces an emf in the other winding. The induced emf will be practi- cally inversely proportional to the length of the air gap, and a suitable instrument of the voltmeter type can then be calibrated to read the shaft-deflection direct. (Many torsion - measurement instruments not em- ploying electrical principles are in use with either mechanical or optical registering de- vices.) (Eef. Mech. Eng., vol. xxv, p. 221.) See Dynamometer, Transmission. Torsion Balance, Coulomb, an instru- ment by which Coulomb measured the forces between charged bodies and between magnet poles. One of the bodies was suspended from a torsion head by a fine silver wire and the other was fixed. The force was measured by the amount of twist given to the head, and the distance between the bodies was read oflF by a scale on the side of the glass con- taining-box. See Law, Coulomb's. [l. m.] Torsion Galvanometer.— In the article entitled Galvanometer, a number of instru- ments are described in which the force re- straining the moving parts is due to torsion. Such galvanometers may be classed as torsion galvanometers. See Torsion Head. Torsion Head, an arrangement whereby the torque produced in an instrument may be measured by bringing the movement back to its zero position, thus balancing the torque against that of a spiral spring or torsion wire. The angular strain imposed on the spring or wire is measured in degrees on a dial. See Torsion Galvanometer; Galvanometer; Dynamometer, Siemens; Torsion Sus- pension, [l. m.] Torsion Meter, for measuring shaft power. See Torsiometer, Electrical. Torsion Suspension, a vertical suspen- sion, by means of which a controlling force is exerted on the moving part of an instru- ment. It usually consists of a fine silver or phosphor-bronze wire or strip. See Gal- vanometer; Torsion Galvanometer; Dynamometer, Siemens; Silk-fibre Sus- pension; Quartz-fibre Suspension; Tor- sion Head. [l. m.] Totally Closed Slots.— Some years ago it was often the practice to make the slots of alternators and induction motors completely closed, as shown in the fig., the bridge across the face being, of course, very thin. Although this gave an even flux distribution, the reactance of the winding was high, and poor regulation and lower pf resulted. See also Slot; Induc- tance, Slot; Wide-open Slots; Partly Closed Slots. Totally Enclosed Motor. See Motor, Totally Enclosed; Motor, Pipe-venti- lated. Total Works Cost of an Electric Machine. — This term applies to the cost of production of a machine apart from the cost of selling it, and consists of the cost of all labour and material, which is usually termed the prime cost, plus the cost of Totally Closed Slot or Tunnel Tower — Tractive Effort 565 management, designing, upkeep of tools, &c. (Ref. 'Factory Accounts', Garcke and Fells; 'Continuous-current Dynamo Design', Hobart.) Tower, Cooling, a tower from the top of which the warm circulating water from a condenser trickles down over surfaces (which are sometimes kept cool by currents of air). The circulating water is thus considerably reduced in temperature by the time it reaches the bottom of the tower, and is again avail- able for admission to the condenser. See Condenser, Steam. Tower, Steel. — In the majority of the more modern transmission lines, the con- ductors are supported on steel towers, which are light structures of steel provided with insulators to which the conductors are se- cured. For the spacing between towers, and other important data relating to the use of steel towers, see Transmission Line, where will also be found illustrations of typical towers. See also Line Poles j Line Erec- tion; Line, Overhead; Cable, Aerial. Tower Wagon, an elevated platform, either of fixed height or adjustable to various heights, mounted upon a four-wheeled wagon, which may be hauled by horses or propelled by mechanical power. The apparatus is used to facilitate the erection or repair of over- head trolley wires. See Line Erection; Line Material: Conductors, Overhead. Tp, the preferable abbreviation for triple pole. See Switch Types, Designation of. Track Bond. See Return Circuit in Electric Traction; Bond; Bonding Rail. Track Brake. See Brakes; Maley Electro-mechanical Rail Brake. Track Gauge, the distance of one rail from the other measured perpendicularly between the inside faces of the rail heads. If the sides are tapered the measurement is taken halfway down the rail head. Track Switch, Electrically-operated, a rail switch in the permanent way for diverting cars on to one of two alternative branches or from a main line on to a branch. The tongue of the switch is operated by electromagnets situated underground but controlled from the car approaching the junction. Many electric tramway systems are equipped with the device — overhead contacts, insulated from the trolley wire, are arranged so that the trolley wheel passes over them. The contacts are in connection with the Vol. II electromagnets and, in some systems, the motor-man on the car must have the con- troller in the ' off' position when passing such contact, if he does not wish the track switch to operate; but the switch is actuated if the controller is left 'on'; current then passes from the line through the electro- magnets and overhead contact to the car and rails. The tongue is usually arranged to automatically return to a definite position after the car passes. Trackless-trolley System. See Trol- ley Omnibus System. Traction, Coefficient of, the ratio of the tractive effort required to propel or haul a car or train at a given constant speed on a level track to the total weight of the car or train, preferably expressed in kg per ton. Its value depends upon the state of the rails, the speed, curvature of the line, &c. Also called tractive resistance, car resistance, and train resistance. See Adhesion, Coefficient OF; Adhesion between Wheel and Rail; Adhesion Weight; Maximum -adhesion Torque. Traction, Electric, the employment of electric power for haulage of vehicles and boats; the propulsion of vehicles by elec- tric power. See Electric Propulsion; Electrification of Railways; Torpedo, Sims-Edison; Petrol- electric Automo- bile System; Elbctromobile; Accumu- lator Car; Cab, Electric; Arnold Single-phase Electropneumatic Trac- tion System; Monorail Electric Rail- way; Schilowsky Monorail System; Canal Traction, Electric; Tractor; Crane, Electric; Portelectric Rail- way System; Telpherage. Traction, Generating Systems for. See Generating System. Traction Accumulator. See Accumu- lator. Traction CelL See Accumulator. Traction Dynamo. See Generator; Railway Generator. Traction Load. See Central Station for the Generation of Electricity. Traction System, Arnold Single- phase Electropneumatic. See Arnold Single-phase Electropneumatic Trac- tion System. Tractive Effort, the force required to propel or haul a car or train in opposition to the resistance due to friction, inertia, gravi- tation, &c. Also called tractive force, draw- si 566 Tractive Force — Transformer bar pill, and (improperly) tractive power, and frequently confused with tractive coefficient (see Traction, Coefficient of). The term draw-bar pull is preferably confined to the tractive effort exerted by a locomotive in hauling, measured at the draw-bar. See also Magnet, Tractive Force of; Lifting Power of Magnet. Tpaetive Force. See Tractive Effort; Magnet, Tractive Force of. Tractive Resistance. See Traction, Coefficient of; Tractive Effort. Tractor, a locomotive running either on rails or on a road, propelled by mechanical or electrical means, and used for hauling. Usually restricted to an electrical locomotive running on the towpath of a canal, and used for hauling barges, power being con- veyed to it by means of overhead trolley wires. See Traction, Electric; Canal Traction, Electric. T-rail. See Vignoles Eail. Trailer, a tramcar or railway coach which is not equipped with electric motors, and is adapted only to be hauled or pushed by a car or locomotive so equipped. See Motor Car. Trailer Truck. See Truck. Trailing" Brush Edge, that side of a brush at which contact between the brush and the commutator bar is finally broken. Trailing' Pole Tip or Pole Horn. See Pole Tips. Train - control. See Multiple - unit System of Train Control; Contactors; Controller, . Master. Train - destination Indicator. See Indicator, Train-destination. Train of Waves, a succession of waves which may be either damped or undamped. In the former case, each wave is less than the one before it, while if the train be un- damped, all the waves are exactly alike and constitute an ac. See Oscillation, Elec- tric; Tuning. Train Resistance. See Traction, Co- efficient OF; Tractive Effort. Tramcar Meter. See Meter, Tramcar. Tramway Motor. See Eailway Mo- tor. Tramway Poles. See Line Poles. Tramway Rail. — The running rail em- ployed on the permanent way of a tramway system. There are very many different sec- tions used for tramway rails in various locali- ties, but an attempt to standardise tramway rails has been made by the British Standards Committee, and the results are embodied in the report of that committee which gives particulars of various sections. Standard Eails of Sections No. 1, 2, 3, and 4, weigh 90, 95, 100, and 105 lb per yard respectively. standard Tramway Kail (Section No. 3) The fig. shows the form and dimensions of Section No. 3. See Demerbe Eail; Vig- noles Eail. (Eef. Eeport of British Stan- dards Committee.) Tramways, Guard Wire for. See Guard Wire. Transformation Ratio, the ratio be- tween the primary and secondary voltages of a transformer. Except for a small differ- ence at full load, due to ' drop ', the transfor- mation ratio is the same as the ratio of the number of turns. See Transformer. Transformer denotes, in general, a piece of apparatus which receives electrical power from one system and delivers it into another system with or without a change of voltage, frequency, or number of phases. Trans- formers may be divided into two broad classes — 1. Botary Transformers. 2. Stationary Transformers. Class 1 covers that type of machine more usually known as a rotary converter, which is used when cc is to be transformed into ac, or vice versa. (See EoTARY Con- verter.) Class 2 covers that type of ap- paratus which transforms ac of one voltage or phase system into ac of another voltage or phase system, the frequency remaining Transformer 567 unaltered. Every such transformer consists of three essential parts — (1) a primary elec- trical circuit, (2) a secondary electrical cir- cuit, and (3) a magnetic circuit with which (1) and (2) are both interlinked. The pri- mary circuit is connected to the supply source, and the secondary to the system or apparatus to be supplied at the changed voltage or changed number of phases. The power delivered by the secondary circuit is equal to that absorbed by the primary circuit minus the power lost in the transformer and dissipated as heat. The percentage thus lost varies in commercial transformers from 1| per cent of the output in large sizes, up to 10 per cent and more in small sizes. The stationary transformer is thus a highly effi- cient piece of apparatus, there being no moving parts to cause friction losses. The following transformations are the principal ones for which stationary trans- formers are used: — 1. Single-phase to single-phase. 2. Three-phase to three-phase. 3. Three-phaSe to two-phase. 4. Two-phase to two-phase. 5. Three-phase to six-phase. Transformation from a polyphase to a sp system, or vice versa, is not possible in per- fection, because in a sp system the flow of power is a pulsating one, whereas a balanced polyphase system deals with a steady flow of power. In the 1907 Standardisation Eules of the A.I.E.E., transformer is defined as a type of stationary induction apparatus (which see) in which the primary and secondary wind- ings are insulated from one another. (Eef. 'The Transformer', Kapp; 'The Alternate Current Transformer', Fleming; ' Transformers : Some Theoretical and Practi- cal Considerations', FlemingandFaye-Hanseii in Journ.I.E.E., vol. xlii, p. 373.) [r. c] Transformer, Adapter.— An adapter transformer or an adapter compensator is a transformer or a compensator usually em- ployed in connection with house wiring, in order to obtain some other (usually lower) pressure than that for which the house is wired. The first extensive demand for adapter transformers arose in 1907 and 1908, when the merits of metallic-filament lamps first became extensively realised. At the date of writing, the progress made in the development of 200-volt metallic-filament lamps, even of rather large cp, is far from satisfactory, as the filaments are very fragile. Most types of 8 to 16 cp metallic-filament lamps can only be obtained for pressures of 100 volts or less. The smallest sizes of com- mercial 200-volt lamps are 25 cp, and these are very fragile. 50 cp 200-volt lamps are reasonably immune from premature break- age. Almost all interior wiring has been carried out for pressures of 100 or 200 volts, and consequently the introduction of me- tallic-filament lamps has been accompanied by a marked increase in the cp per lamp. vwwwwwvww AAA Fig. 1.— Adapter Transfor- mer for supplying only one secondary circuit iAAAAAMM/WWW Fig. 2. — Adapter Compen- sator for supplying only one secondary circuit In order, however, to retain the better dis- tribution of light provided by subdividing a given aggregate cp amongst as many different lamps as possible, adapter trans- formers and adapter compensators have come into extensive use. Where the secondary voltage is much less than half the primary voltage, as in stepping down from 200 to 25 volts, there is insufficient advantage in employing a compensator (or auto-trans- former, which see), and a transformer should be employed. But when the secondary pressure is of the order of 30 per lAM^A/\A^AAA|A/V\^ cent and more of the primary pressure, a com- pensator may be built with distinctly less ma- terial and at less cost than a transformer. Considerations of this sort should generally be the determining factor in deciding between an adapter transformer and an adapter com- pensator when only one secondary circuit is to be supplied, as indicated in figs. 1 and 2. But if several circuits are supplied from a distribution board, then an adapter compen- sator is more appropriate, even when the pressure of each of the circuits is very low. The arrangement may then be as indicated in fig. 3. An adapter compensator, when used as shown in fig. 3, is sometimes called Fig. 3.— Adapter Com- pensator supplying four low-voltage circuits 568 Transformer a balancer (which see), or a balancer compen- sator, since the coil possesses the property of tending to preserve some approximation to equality of voltage at the various circuits independently of any inequality in the loads on the different circuits. A compensator transformer is a device (see B.P. No. 28652 of 1907, D.R.P. No. 214579 of ^VWWWWWVH 1908, and U.S.A.P. No. 925105 of 1909) com- bining certain properties of a transformer and of a compensator, with the result that the voltage on any circuit, while still varying with variations in the total load, is inde- pendent of variations in the load on any particular circuit. The connections of one form of compensator transformer are shown in fig. 4. Other forms are described in the patents above mentioned. Single-lamp Transformer. — An adapter transformer for a single lamp is often called J CJ /VW\ AAM Fig. 4.— Compensator Transformer V // .■■/ ^^ > Fig. 6.— Single-lamp Adapter Transformer a single-lamp transformer. The first com- mercial single-lamp transformer for metallic- filament lamps of from 8 to 16 cp appears to have been developed by H. W. Turner (see B.P. No. 23418 of 1907). Fig. 5 is a section through a single-lamp transformer. An addi- tional feature employed in some of Turner's designs consists in an arrangement such that by turning the holder through a certain angle, the lamp can be switched to a lower potential tap of the secondary of the trans- former and thus be burned at a lower cp. A further angular movement extinguishes the lamp. Developments in adapter transformers and in adapter compensators are at present proceeding rapidly, with the natural conse- quence that such apparatus is being placed on the market under a considerable variety of suggestive names, such as balancer trans- formers, halamcer compensators, and balancers, these in turn being sub-classified as 2-circuit, 3-, 4-, &c., circuit balancers. See also Trans- former; Auto-transformer ; Balancer; Compensator. Transformer, Air-blast, a transformer which is cooled by a blast of air supplied by a blower. Such transformers are provided with spaces between the coils and with ducts in the iron which allow the air free access to all parts. The power required to drive the blower need not exceed a small fraction of 1 per cent of the transformer capacity. The transformers are banked together over an air passage and are all fed from the one blower. The copper and iron in air-blast transformers may be run at higher densities than is permissible in a natural draught transformer, owing to the greater rate at which the heat is abstracted by the air-blast. See Transformer, Cooling of. Transformer, Air-cooled, a name given to all transformers whicb are cooled by cur- rents of air, whether the air is circulated by natural convection or is forced through core and coils by means of a fan. See Trans- former, Natural-draught ; Transformer, Air-blast; Transformer, Cooling of. Transformer, Air-core, a transformer having its magnetic circuit of air or other non-magnetic material. For ordinary com- mercial frequencies such transformers are never used on account of the large amount of copper they would require, and the large magnetising current they would take at no load. For very high frequencies, however, such as are used in wireless telegraphy, such transformers are used, and consist of coils with comparatively few turns of wire. The use of iron in the core of a very hf trans- former is not feasible on account of the screening effect of eddy currents. Transformer, Ammeter, a transformer Transformer 569 employed for the express purpose of enabling an ammeter to be located in a secondary cir- cuit instead of in the circuit the current in which is to be measured by the instrument. The transformer often consists simply of a ring of sheet iron which is threaded on the ht bus-bar the current in which is to be measured. The iron ring has a secondary consisting of a number of turns. The secondary is short - circuited through the ammeter. An ammeter transformer is a variety of instrument transformer. See Transfoemer, Instrument. TransfoFmep, Armature - testing", a transformer which has a saddle-shaped mag- netic circuit made to fit an armature of a particular size, and which is used to test the inter -turn insula- tion of the armature, when wound. In the fig., A repre- sents an armature and B is the transformer core, which is wound with a few turns of thick conductor, In use, the transformer is lowered upon the arma- ture, and an alternating voltage is applied to the coil for a certain time. The armature is then turned, and the test applied again, and so on until every coil has been threaded by the flux. The voltage per turn induced in the armature coils between the poles will, of course, be the same as that applied to the magnetising coil. Any short circuit is at once detected by the heating of the faulty coil. See Testing Armatures. Tpansformer, Auto-. See Auto- transformer; Auto - STARTER; Starting OF Motors; Transformer, Adapter. TransformcF, Balancer. See Trans- former, Adapter. Transformer, Berry, a particular type of transformer having the primary and second- ary coils on a central leg of the core, the magnetic circuit being completed by radiat- ing bunches of laminations. In the fig., A represents the concentric primary and secondary coils, while the iron core B is split into a number of bunches which radiate from the centre, pass down the outside, and return back through the central hole. Annature-testing Trans- lormer Berry Transformer Transformer, Booster, a transformer the primary of which is placed in shunt across the mains, the secondary being placed in series in the circuit in which it is desired that the pressure shall be raised. See Booster. Transformer, Bus -bar. See Trans- former, Instrument. Transformer, Closed Iron Circuit, a transformer whose magnetic circuit con- sists wholly of iron. Such transformers are now universally used, except for special re- quirements, such as occur in telephonic or wireless-telegraphic work. Transformer, Commutating, a name sometimes, though rarely, applied to a rotary converter (which see). Transformer, Constant - current, a transformer which gives constant current on the secondary, irrespective of the value (within limits) of the resistance of the load. Such a transformer is sometimes used for series arc lighting, and maintains an ap- proximately constant current through the lamps, no matter how few or how many may be in circuit. The arrangement by which this is usually accomplished consists in making the primary and secondary coils relatively movable. The mechanical tendency 570 Transformer of the coils is to close together, but electrical repulsion forces them apart. The two forces are balanced against each other, so that over a large range of secondary voltage (i.e. of ex- ternal impedance) the current remains very nearly the same. See Three-wire Balancer. TransformeF, Continuous - cuppent. See Transformer, Eotary; Three -wire Balancer. Tpansfopmep, Cooling of, refers to the means adopted to dissipate the heat due to the energy wasted in a transformer. See Transformer, Air-blast; Transformer, Air-cooled; Transformer, Water-cooled Oil; Transformer, Oil-cooled; Trans- former, Natural-draught. Tpansfopmep, Cope Loss of, denotes the loss occurring in the iron core of a transformer due to the rapid reversal of the magnetic flux. Also called the iron loss. The core loss in a transformer is due to hysteresis in the iron, and to eddy currents which flow in the thickness of the lamina- tions. The magnitude of the hysteresis loss depends on the quality of the iron, the in- duction density, and the frequency. The eddy loss depends on the resistance and thickness of the core plates, the induction density, and the frequency. Core loss may be measured by means of a wattmeter (see ' Transformer Efficiency ' under Efficiency). The losses due to hysteresis and eddy currents may be separated by tak- ing measurements at constant induc- tion and varying frequency. The analysis may be made on the as- sumption that the hysteresis loss is proportional to the first power of the frequency, and the eddy loss to the square of the fre- quency. Tpansfopmep, Core - type, a name given to that class of transformer consisting of a cen- tral core of lami- nated iron, round which the coils are wound. Pk Core-type Transformer A usual form of core-type transformer con- sists of a rectangular core A, round the two long limbs of which the coils P and S are wound, the It coil being placed next the coi'e. A core-type transformer is usually charac- terised by a large number of turns in the winding (relatively to the ' shell ' type), and hence the voltage per turn is low. Tpansfopmep, Cuppent. See Trans- former, Instrument; Transformer, Am- meter. Tpansfopmep, Dpy, a transformer in which oil is not used as a cooling and in- sulating medium. See also Transformer, Air-cooled; Transformer, Air-blast. Tpansfopmep, Eapth-shield, denotes a transformer wound with a metal sheath be- tween its windings. This device is used as a protection against the possibility of the It circuit becoming charged to a high potential by contact between the It and ht windings inside the transformer. The sheath is usually of copper, and is connected to earth. Should the insulation of the transformer break down between the ht winding and the shield, the ht side is automatically earthed. If another earth exists on the primary circuit, the cut- outs act and discoiinect the transformer from the supply. The protection afforded by an earth-shield is limited to the case where contact occurs within the transformer. See Partridge Safety Device; Protective Device, Cardew; Ground Wire. Tpansfopmep, J Equivalent Re- actance of, de- notes the react- ance in ohms which may be considered to re- place the actual reactance in a transformer, and which will pro- duce all the effects of the latter. We may con- sider a transfor- mer as consisting of two hypothet- ical windings P and s between which no magnetic leakage exists, having a reactance L in series with the primary. The value of L, which gives the same inductive effects as are actually found lAAAAAAAAAAA NAAAA/VVWWl Equivalent Reactance of Transformer Transformer 571 IWWWWWW k/WVWWWWM s due to the magnetic leakage between P and s in the real transformer, is the equivalent reactance of the transformer referred to the primary circuit. [r. c] Transformer, Equivalent Resistance of, denotes the resistance in ohms which may be considered as replacing the resistances in a transformer, and which will produce all the effects of the actual resistance of the windings. We may consider a transformer as consisting of two resistanceless windings P and S, with a resistance E in series with one of them. This combination will give the same results as the actual transformer, if the value of E be cor- rectly chosen.. If a trans- former have a ratio of P: 1, then any resistance E placed in its secondary will have the same effect as a resistance of E X P^ placed in its primary circuit. Hence the equivalent pri- mary resistance of a trans- former having a P : 1 ratio, and whose primary and secondary resistances are E and S ohms respectively, will be equal to (E + P^S) ohms. Its equivalent se- condary resistance will be equal to (E/P^ + S) ohms, or l/F^ of the first value. The above reasoning only holds if the currents in the two windings are inversely proportional to the turns and are opposite in phase. This condition is, in commercial transformers, fulfilled sufficiently nearly for all practical purposes. [k. c] Transformer, Ferraris-Arno Phase-. See Transformer, Phase. Transformer, Hedgehog", an open magnetic circuit type of transformer made by Swinburne in the early days of trans- formers. It was so called because the core was constructed of a bundle of iron wires whose ends were spread out hedgehog fashion in order to reduce the reluctance of the air-path. This type of transformer is obso- lete, except for special cases. Transformer, Humming- of. See Humming of Transformer. Transformer, Instrument, a trans- former used with an instrument for the pur- pose (1) of increasing the range, or (2) of insulating it from a ht system. There are — Equivalent Besist- ance of Transformer 1. Current or series trcmsformers, in which the current is transformed up or down. 2. Potential, presswe, or voltage transformers, in which a high voltage is transformed down to, say, 100 volts at the terminals of the instrument. In the case of ammeters and voltmeters it is essential (unless the instrument can be calibrated in conjunction with its own trans- former) that the ratio of transformation should be known and constant. For use with wattmeters it is further necessary that the secondary current and pressure should be accurately in phase (or rather, just 180° out of phase) with the primary. Both these conditions can be approximately complied with in potential transformers, but in current transformers the design is less easy, and a phase displacement of less than 1° is difficult to attain. The resulting errors in the in- struments connected to the transformers are spoken of as ratio errors and phase errors respectively. Current transformers arranged to slip over a conductor which acts as the primary are known as bus-bar transformers. See Transformer, Ammeter. [k. e.] Transformer, Insulation -testing-, a small step-up transformer used for testing the insulation of machines, or of small samples of insulating materials, for dielectric strength. Such a transformer is usually provided with several terminals on the ht side, so that a range of voltage may be obtained. Whenever possible, the primary voltage of such a transformer should be adjusted by varying the exciting current of the alternator supplying it, as the introduc- tion of resistance or reactance tends to change the wave shape and increase the maximum value of the emf wave applied to the insu- lation under test. Owing to the effect of capacity current in increasing the ratio of transformation, the ht voltage should be directly measured and not inferred from the It voltage, unless the reactance of the trans- former is small. See Transformer, Vari- able Voltage. [r. c.] Transformer, Magnetic Circuit of, that part of a transformer which carries the magnetic flux common to both primary and secondary windings. The magnetic circuit may be of air, as in the air-core transformer, or of iron, as in the ordinary transformer. See Transformer, Core-type; Transfor- mer, Shell-type j Transformer, Hedge- hog. 572 Transformer Transformer, Magnetic Leakage in, denotes the action of a part of the primary flux of a transformer which does not thread the secondary coil. The primary flux may be divided into two components. The first of these exists in the iron core, and is pro- duced by the resultant action of the primary and secondary ats. This flux threads both primary and secondary coils. The other primary component threads the primary coil only, and completes its path in the space between primary and secondary coils. It is proportional to the primary ats, and in- versely proportional to the reluctance of its path. By winding the coils closely together or intermixing the coils, this reluctance may be increased and the magnetic leakage mini- mised. Magnetic leakage is objectionable because it absorbs primary voltage, thereby decreasing the secondary induced emf. See Transformer, Reactance of. [r. c] Transformer, Magnetising- Current of, denotes the principal component of the current flowing in the primary of a transfor- mer when there is no load on the secondary. It is frequently called the ' no-load ' current. The current which flows under the no- load condition fulfils two functions. It mag- netises the core to the density required to induce a counter emf equal to the applied emf, and it supplies the power required to make up the losses due to hysteresis and eddy currents in the core. It may therefore be divided into two components — one a power component in phase with the emf, and one a wattless component lagging 90° behind the emf. Except in very small transformers or those for specially .If, the magnetising current of the modern closed -circuit trans- former is generally not more than 5 per cent, and is frequently less than 2 J per cent of the full-load current. See Magnetising Cur- rent; Current, Wattless; No-load Cur- rent; Transformer, Power Factor of. [r. c] Transformer, Microtelephonic, a small transformer used in converting the micro- phone current into one of higher voltage, a plan frequently adopted where a local battery is used at each subscriber's instru- ment. Transformer, Movable Secondary of, the secondary coil of a variable -ratio transformer. See 'Induction Potential Regu- lator' under Regulator, Potential; also Transformer, Constant-current. Transformer, Natural-draught, a transformer which has no artificial means of cooling, but whose heat is dissipated by natural convection currents of air. Some transformers of this type have sheet-iron cases with open top and bottom to form a chimney and assist in the air circulation. See Transformer, Cooling of. Transformer, Oil-cooled, a transformer immersed in oil, and provided with ducts in coils and core to allow the oil to circulate by convection and thus carry off the heat to the case. See Transformer, Cooling of. Transformer, Oil-insulated, a trans- former immersed in oil for the purpose of insulation as well as cooling. Extra high voltage transformers are almost invariably oil- insulated, and ducts are left between primary and secondary coils, to be filled with oil. As good transformer oil has a breakdown volt- age of over 6000 volts per mm between flat disks, the advantage of oil over air insulation is obvious. See Oil, Transformer; Oil Insulation; Oils for Insulating Pur- poses; Sparking Distance; Dielectric Resistance; Dielectric Strength. Transformer, Phase, a name given to a polyphase transformer whose secondary voltage difl'ers in phase from its primary voltage. An induction regulator may be used as a ' phase transformer ', and the phase of the secondary voltage may be set at any desired angle to that of the primary by turning the rotor. Ferraris-Arno Phase Transformer, a method of obtaining polyphase currents from sp circuits, and consisting in principle of starting up a polyphase induction motor un- loaded, from a sp circuit, and when it has attained its normal running speed, using it as a transformer fed by sp electricity and supplying polyphase electricity from suitable points of its windings. It may thus be re- garded as a phase-splitting device (which see). The Ferraris-Arno phase transformer is the subject of B.P. No. 7504 of 1895. Bradley experimented with phase -splitters on this principle, at a still earlier date. Transformer, Polarity of, a term referring to the relative polarities of the primary and secondary terminals of a trans- former. In paralleling a bank of transformers it is essential that leads of the same polarity be connected together, otherwise the secon- dary emf will act in series through the secon- daries so connected, and produce a short Transformer 573 circuit. In the case of three-phase trans- formers it is also necessary to have regard to the phase of the secondary voltage, where primary and secondary are not both 'star' or both 'mesh' connected. For instance, with a 'star' connected secondary, the changing of the neutral connection to the opposite ends of the windings is equivalent to turn- ing the phases through an angle of 60°, and such a change would render the trans- former incapable of being paralleled with a similar one having the original connec- tions. Transformer, Polyphase, a single-unit transformer for transforming polyphase cur- rents. Such transformers are coming into more general use, as they are lighter, cheaper, and require less floor space than single- phase transformers of the same aggregate capacity. The three-phase transformer is the only type of polyphase transformer used to any extent. Transformer, Potential. See Trans- former, Instrument. Transformer, Power Factor of, the ratio of the true w to the volt amp, or apparent w, absorbed by the primary of a transformer. On no load the pf of a trans- former is comparatively low, owing to the magnetising component being fairly great as compared with the power component supply- ing the core loss. The value of the no-load pf may be from 0-7 or 0-8 for a 100-cycle transformer down to 0*2 for a If transformer. As soon, however, as the transformer has even a very small load, the primary pf be- comes practically equal to that of the second- ary, and remains so up to full load and far beyond the safe overload. At very excessive currents the pf again decreases, but not within practical working limits. See Trans- former, Magnetising Current of. [r. c] Transformer, Pressure. See Trans- former, Instrument. Transformer, Primary of, the pri- mary winding of a transformer, or the winding which receives the power from the primary system and delivers it by electro- magnetic induction to the secondary wind- ing. See Current Induction; Trans- former, Secondary of. Transformer, Primary Turns of, the number of linkages which the primary cir- cuit of a transformer makes with the mag- jietic circuit. Transformer, Raising-, a transformer so proportioned as to deliver electricity from its secondary at a higher pressure than the pressure at which it receives electricity into its primary. Synonymous with step-up trans- former. See Transformer, Step-up. Transformer, Reactance of, denotes the reactive effect due to the magnetic leak- age in a transformer (see Transformer, Magnetic Leakage in). This reactance may be considered and treated as that due to a coil placed in series with the primary or secondary winding. It is measured by short-circuiting one winding and measuring the impedance of the other. The resist- ances being known, the reactance may be deduced. See also Transformer, Equiva- lent Reactance of. Transformer, Revolving-field, an in- frequently used term, but one which denotes a polyphase transformer of the induction motor type, in which, instead of there being separate fluxes, one corresponding to each phase, there is one common flux which rotates in direction and links successively with the primary and secondary coils of eack phase. The induction regulator or polyphase booster is a transformer of this type. Its structure is that of an induction motor, the rotor being movable by hand. One member is in series with, and the other in shunt to, the line. Rotation of the rotor changes the phase re- lation of the series winding emf (or secondary emf) to that of the line, and increases or diminishes the resultant voltage. See Po- tential Regulation; Regulator, Poten- tial, [r. c] Transformer, Rotary, a cc machine which is used to transform the pressure of the current supplied to it from one value to another, the current strength undergoing a corresponding (inverse) variation. The ma- chine often consists of a single field-magnet frame fitted with a double-wound armature having two commutators, the ratio of the number of turns in the two windings being practically equal to the ratio of transforma- tion of the voltage. Instead of the double- wound armature, separate armatures may be used, with separate field-magnet frames, the machine then becoming a motor generator (which see). Rotary converters were for- merly often called rotary transformers. See Rotary Converter; Starting of Motors AND Rotary Converters. Transformer, Secondary of, the secon- 574 Transformer dary winding of a transformer, or the wind- ing which receives the power by electro- magnetic induction from the primary, and delivers it to the external circuit. See Cur- rent Induction; Transformer, Primary OF. Transformer, Series. See Trans- former, Instrument; Transformer, Am- meter. Transformer, Shell-type, a name given to that class of transformer which consists Shell-type Transformer of an assemblage of primary and secondary coils, round and through which the iron core is built. P and S are separately wound and insulated coils consisting usually of only one or two turns per layer, of copper strip. They are separately wound and insulated, and after being placed iij position ,the core is built up round them. A shell-type trans- former has, in general, fewer turns, and a larger voltage per turn, than the core type. See Transformer, Core-type; Trans- former, Hedgehog; Transformer, Berry. Transformer, Single-phase, a trans- former used for the transformation of sp currents. See Transformer, Polyphase. Transformer, Starting, a transformer used for starting an induction or synchronous motor. See Auto-starter ; Starting of Motors. Transformer, Static— This term has come into frequent use to denote a sta- tionary transformer, or one which has no rotating parts. Static is a rather undesirable term when used in this connection, as it had long previously been used to denote pheno- mena and apparatus connected with ' static ' electricity. Stationary transformer is a better term. See Transformer; Transformer, Stationary. Transformer, Stationary, a trans- former without rotating parts, as distin- guished from a rotary converter. See Trans- former; Transformer, Static; Eotary Converter; Transformer, Eotary. Transformer, Step -down, a trans- former used to convert power at a high voltage into power at a lower voltage. Such transformers are used at the receiving or distributing end of a ht transmission line. Transformer, Step-up, a transformer used to convert power at a low voltage into power at a higher voltage. Such trans- formers are used at the generating end of a transmission line to raise the voltage of the generators to such a value as will enable the power to be economically transmitted to the distant point. See Transformer, Eaising. Transformer, Subdivided, a trans- former either of whose windings is divided into several parts by tappings so that various voltages may be obtained. See Transfor- mer, Variable-voltage. Transformer, Synchronising, a small transformer used on switchboards for re- ducing the voltage of a ht machine, or of ht bus-bars, to a suitable value for operating synchronising apparatus. One such trans- former is usually connected permanently to the bus-bars, and one to each machine. By means of plugs or small switches the secon- dary of any machine transformer may be connected to the bus-bar transformer and the synchroniser, and the corresponding machine synchronised. See also Synchroniser; Syn- chronising Dynamo-electric Machines. Transformer, Tesla.— A Leyden jar, L, or other suitable condenser, is supplied from the secondary of the induction coil, i, of the fig. The Tesla coil, t, is connected as shown. Its primary consists of but a few turns of wire. The oscillatory discharge sent through these few turns from the Ley- den jar induces very high voltages in the secondary of the Tesla coil, which is com- Transformer 575 posed of a layer of fine wire, the maximum eflfect being obtained when the natural fre- quencies of oscillation of the two circuits are the same. The coil is sometimes immersed Ifl 1 w^ Y Tesla TranBtonner in oil. See Coil, Tesla ; Coil, Induction; Leyden Jar; Condenser, Electric. Transfopmer, Three-phase, a single- unit transformer for transforming three- phase currents. A three-phase transformer is made either core-type or shell-type. In the former, illustrated in fig. 1, there are three cores A, B, and C, joined by yokes D and D^. This forms a three-phase magnetic circuit, ps D B Ik /^ 1 \ / j ^ 1 1 ^o^ V- ',/ k 1 n1 bX y^ 1 fN y ^ 1 ' 1 1 1 V- 1 'r/ L \ ' r^y /^ ' — :-\ y ' ^ 1 1 1 1 1 V [—4^ ™ Nj^ 1 rT D' Fig. 1. — Three-phase Transformer (core type). since the instantaneous sum of the fluxes is zero. Each core is wound with a primary coil P, and a secondary coil S. (In the fig. the primary winding of each phase is divided into three coils to ensure better insulation.) The primaries and secondaries may be con- nected ',star' or 'mesh'. The core B has a shorter return path than A and C, which causes the magnetising current in that phase to be less than in the A and C phases. This has sometimes been obviated by placing the three cores at the corners of an equilateral triangle. The extra trouble involved in this arrangement, however, is not justified, as the unbalancing is a no-load condition, and prac- tically disappears when the transformer is loaded up. The figs, on the Plate facing p. 576 are photographs of large three-phase transformers of the core type. Fig. 2 shows the shell arrangement, which consists practically of three separate trans- formers in one unit. The flux paths are here separate, each pair of coils being threaded by its own flux, which does not, as in the core type, return through the other coils. This gives the shell type an advantage over Fig. 2.— Three-phase Transformer (shell type). the core type ; for should one phase burn out, the other two may still be used, especially if the faulty coils be short-circuited. The effect of such short-circuiting is to prevent all but a very small flux from threading the faulty coil. [r. c] Tpansforinep, Three - wire, a trans- former with a three-wire secondary consisting of two windings so disposed as to have but small magnetic leakage, the junction of the two windings being connected to the neutral wire of the system. See Compensator; Three-wire Distributing System; Trans- former, Adapter. Transformer, Two-phase to Three- phase, a transformer, or set of transformers, which receives two -phase power on the primary, and gives out three-phase power on the secondary. The best-known system for efifeeting this purpose is that due to Scott. Two trans- formers, A and B, are connected as shown diagrammatically in figs. 1 and 2. The ratio of B is made 1-155 times that of A. The secondary voltage of B is therefore equal to 0"866, or ^- times that of A, and as it is in quadrature with the latter, the leads X, Y, and z give a three-phase voltage. The cur- rent in Sj is in phase with its voltage (assum- ing a non-inductive load), but that in s, is 30° displaced, lagging in one half, and lead- ing in the other. Due to this idle current 576 Transformer the secondary Sj requires 15 per cent more copper than Sj. Pj and Pg are the primaries of the two transformers. Kaaaaa/n lAAAAAA/N WWVVM Y Z Fig. 1 Fig. 2 Scott's System of Two-phase to Three-pliase Transformation A system patented by Woodbridge is shown in figs. 3 and 4. One three-phase or three sp transformers are used. Two have double secondaries, and the third a single secondary, with tappings. As shown in fig. 4, two 'meshes' are formed, and con- nected on opposite sides of the ' base ' formed by the , third secondary. The three-phase side is connected in mesh, and the two phases in quadrature are derived, one from the extended base B, C, and one from the points A and D. An advantage of this system is that a single three-phase unit may be used. vwwwwwJ IvwwwwwTlwwwww mM WM wm mm mmmi Either of these systems may of course be used for two- to three-jphase transformation, or vice versa. [r. c] Transformep, Variable- VOltag"e, a transformer whose ratio of transformation can be varied, so that, with fixed primary voltage, it will give a variable secondary voltage. This variation is usually accom- plished by means of a multi- contact switch on primary or secondary, whose contacts are connected to tappings brought from the winding. The short- circuiting of adjacent contacts is provided against, either by a double -fingered switch arm to which is connected a small reactive- coil, or by a snap switch which passes rapidly from one contact to the next. In the figure, the double- AAAAAAAA p, w QQQQQML I ■ ■ ■ [iiMJa B Fig. 3 Woodbridge's System of Two-phase to Three-phase A disadvantage is that the phases on the two -phase side are interconnected at the middle point. Variable-voltage Transformer fingered switch arm is shown diagrammati- cally. a and b are the two contacts con- nected by a small reactance R. W is the winding tapped out as shown. The coil R ofi'ers a high reactance to a cur- rent circulating from a to b, but a small reactance to the line current which flows equally in the two halves towards the centre. See Eegulator, Poten- tial; Coil, Kicking; Leads, Preventive Eesistance. Transformep, Voltage. See Trans- former, Instrument. ./ \ ^( /s ^^ ^ B < / o ^\ [/^ D Fig. 4 Transformation 4600-KVA THREE-PHASE TRANSFORMER (Messrs. Brown, Boveri, &Co., Ltd.) 630-KW OIL-COOLED THREE-PHASE TRANSFORMER INSTALLED AT ASTON MANOR GENERATING STATION (Messrs. Johnson & Phillips, Ltd.) Tofacep.S76. Transformer — Transformer Switch 577 Tpansfopmep, Voltag-e Drop in, de- notes the drop of voltage occurring across the secondary terminals of a transformer with load. This drop is due to two causes — (1) the resistance of the windings, (2) the reactance or magnetic leakage of the wind- ings. On non-inductive load, the reactive drop, being in quadrature, produces but a slight effect, but on inductive loads it causes ■the voltage to drop, and on condensive or leading-current loads it causes the voltage to rise. As the voltage drop of a good trans- former does not exceed 3 or 4 per cent, even on inductive load, direct accurate measure- ment is difficult. It is best to measure the copper loss with short-circuited secondary by means of a wattmeter, and at the same time the voltage required to drive full-load current through. From the watts the resistance drop can be found, and from this and the imped- ance voltage, the reactive drop may be calcu- lated. From these data a simple vector dia- gram will give, nearly enough for all practical purposes, the drop for any pf. Or the following formula may be used, which has been deduced from the vector diagram. R = per cent resistance drop. X = „ reactive drop. P = „ pf of load. W = „ wattless factor of load ( Vl D = „ resultant secondary drop. Then P^). D = V(W -1- Xf + {n + P)^ - 100. For non-inductive loads where P = 100 and W = 0, we have D = VX2"h- (100 + B)2 - 100. In the case of leading currents W should be considered negative. [r. c] Tpansfopmep, Watep-eooled Oil, de- notes a transformer immersed in oil and provided with cooling pipes through which water is circulated. The pipes are generally in the form of a coil in the oil above the transformer. The oil acts as a conveyor of heat from the transformer to the coil of piping above, a circulation being kept up by convection. Where water is plentiful, the use of such transformers is economical, as they can be made smaller than a plain oil- cooled transformer, owing to the rate at which the heat can be abstracted by the water. In the upper fig. on the accompany- ing Plate, the cooling pipes above the trans- former may be seen. See Transformer, Cooling of; Transformer, Oil-cooled. Tpansfopmep, Weldings, a transformer whose secondary is wound to give a very low voltage and large current. The secondary winding of such a transformer often consists of a single turn of heavy copper bar, to whose ends are attached the clamps for holding the metal pieces to be welded. See Welding, Electric. Tpansfopmep, X-pay. See X-ray Transformer. Tpansfopmep Core, that portion of the iron magnetic circuit of a transformer which is surrounded by the windings. The term is also often used to denote the entire iron circuit of a transformer, especially when dealing with the core type. Tpansfopmep Cuppent. See Current, Transformer. Tpansfopmep Efficiency. See under Efficiency. Tpansfopmep Electpomotive Fopce. See Electromotive Force, Transformer. Tpansfopmep Oil. See Oil, Trans- former. Tpansfopmep Regulation, a term syn- onymous with drop. See also Transformer, Voltage Drop in. Tpansfopmep Stampings, pieces of thin iron of which the magnetic circuit of an iron-cored transformer is built up. These are usually made with a punch and die, and fit together to form the core or shell. The thickness of stampings for transformer cores usually varies from 0-35 mm to 0-50 mm, rarely exceeding the latter vahae, although with the modern high-resistance materials, the economical limit of thickness is higher than with materials of ordinary resistance. See Steel. Tpansfopmep Switch, Beppy. See Switch, Berry Transformer. Tpansfopmep Switch fop Lighting Installations. — Many installations employ metallic-filament lamps of low voltage sup- plied from auto-transformers. To save the no-load losses in the transformer when no lamps are in use, an automatic switch may be provided to cut out the transformer from the mains. In one form of such a switch, mercury cups are provided through which the circuit is completed by a contact on a moving armature of an electromagnet. The first lamp will be switched in circuit across the mains, but in series with a high resist- 578 Transformer Tank — Transmission Line ance coil on the electromagnet. The latter then becomes energised, pulls over the arma- ture, and switches in the auto-transformer. Precautions are taken against sticking be- tween magnet and armature, and the sensi- tiveness of the switch can be adjusted to start when only one lamp is switched into circuit. Sefe Switch, Berry Transformer; Switch, Automatic Transformer. Tpansformer Tank, the containing ves- sel of an oil-immersed transformer. Such tanks are usually of cast iron or sheet iron, and are frequently corrugated or else pro- vided with ribs to furnish a large cooling surface. The tank for a 4600 kva trans- former may be seen at the left in the upper iig. on the Plate facing p. 576. TpansformeP Tap, an intermediate con- nection made to a transformer winding to provide a voltage lower than the full ter- minal voltage. Such a connection only allows of a reduced output being obtained from the transformer, as the current-carrying capacity of the winding is the same. Taps on the ht winding of a step-down trans- former are frequently provided, so that the ratio may be varied by a few per cent to suit local conditions, such as line drop. See Transformer, Variable-voltage. Transfopmer Winding, an electric cir- cuit which is linked with the magnetic circuit of a transformer. See Transformer, Pri- mary OF; Transformer, Secondary of. Tpansfopmep Winding, Space Factor of, denotes the ratio of the total sectional area of copper in a transformer winding to the area of the winding space, i.e. of the space surrounded by the iron, and through which the windings thread. The value of this factor depends upon the space devoted to insulation, which depends upon the volt- age for which the transformer is wound. A low-voltage transformer may have a factor of 0*4: or 0"5, but in a small-sized high-voltage transformer it may be as low as O'l, and even much less in extreme cases. See also Winding Space; Space Factor. Tpansfopmeps, Ageing of. See Ageing. Tpansfopmeps, Bank of, a group of transformers connected in parallel between the same primary and secondary mains. Tpansfopmeps, Bell, small transformers the primaries of which are wound for the pressure employed for the lighting circuit. The secondaries have several terminals, so that, say, either 3, 5, or 8 volts may be obtained, according to the pressure required for the electric bell. By the use of a bell transformer, the need for a primary cell is avoided. The objection is, of course, the continuous waste of energy occurring in the transformer, chiefly in core loss, since the primary is in circuit for the entire time. This waste is not of any great amount, since such a transformer is very small indeed, weighing only some 2 '5 kg. An ordinary bell transformer costs only a few shillings. See Bell, Electric; Annunciator. Tpansfopmeps, Testing of. See Test- ing Transformers. Tpansfopming Machinery. See Cen- tral Station for the Generation of Electricity. TpanslatOP. See Repeater. Transmission, ac. See Alternating- current Transmission. Tpansmission, Coefficient of. See Coefficient of Transmission. Transmission Dynamometep. See Dynamometer, Transmission; Torsio- meter. Electric. Tpansmission Line, the conductors, together with their supports, &c., by means of which electrical energy is transmitted from one place to another. The term is generally confined to cases of transmission over a considerable distance by means of overhead conductors. See Overhead Con- ductors; Line Poles; Insu- lators; Line Erection; Line Material; Conductors, Over- head; Cable, Aerial; Alu- minium. Steel-tower Transmission Line, a transmission line in which the electrical conductors are sup- ported by steel towers. These are stronger and more durable than wooden poles, and enable longer spans to be adopted. On the other hand, they are more expensive, and the total cost of the line may exceed that of a wooden-pole line. Each case has to be considered on its merits. The steel-tower line has also the disadvantage that should an in- sulator break, the conductor will at once make a good earth, and the line will almost to a certainty be shut down completely. With wooden poles, a conductor may remain supported on Fig. 1,— Steel Tower Transmission Line — Transmitter 579 a wooden cross arm for a considerable period without the supply being affected, at any rate long enough for the fault to be dis- covered and the supply transferred to an- other circuit, or until the most important working hours are over and a shut-down for repairs will not be attended with disastrous consequences. A good representative steel-tower line is that at Guanajuato, in Mexico, which is of the type shown in fig. 1, being 13"7 m high; the spans vary from 150 m to 180 m, and the conductors are of hard -drawn copper cable equivalent in area to No. 1 B and S, that is, about 0425 sq cm. Windmill Type of Steel Tower. — This is stated by Abbott, on p. 680 of the Fig. 2.— Windmill Type ol Steel Tower 'Standard Handbook for Electrical Engin- eers' (McGraw, 1908), to be the most eco- nomical design. The spread of the legs is from one-fourth to one-third the height. A tower of this type, designed to carry two three-phase lines, is illustrated in fig. 2. As an example of a very long span, that across the Straits of Carquinez, in the 225 km, 60,000 volt, Oakland, California line, may be mentioned. The distance from an- chorage to anchorage of the span is 1900 m, the length of the span from steel tower to steel tower being 1350 m. The conductor is a 19-strand galvanised-steel cable 2-22 cm diameter, which is equivalent to a No. 2 B and S copper wire; its breaking strength is 44,500 kg, its weight 3210 kg, the factor of safety 4, and the dip 30 m. At the anchor- age points special mica-strain insulators are used, and at the main towers the conductors are carried over saddles, each mounted on six porcelain cups, each 43 cm diameter. (Ref. 'Electric Power Transmission', Bell; 'Electric Transmission of Water Power', Adams; Mershon, 'The Transmission Plant of the Niagara, Lockpprt, and Ontario Power Company', Proc.A.I.E.E., Sept., 1907.) Tpansmission Line Spans. See Spans IN Transmission Lines. Transmission of Electric Energy. See Energy, Electric Transmission of. Transmission of Power. — From an electrical engineering point of view this ex- pression means the electric transmission of energy. See Energy, Electric Trans- mission OF. Transmission System, the complete plant for the generating of electrical energy and transmitting it to a distance. The term is, however, sometimes taken to cover only the transmitting conductors with their sup- ports, &c., to the exclusion of the generating plant. See Transposition of Conductors in Transmission System. Transmitter, in telegraphy, the apparatus used in a telegraph station to send out the electrical energy in accordance with the system of telegraphy and the code used. In telephony, the apparatus which converts the sound waves into electrical waves or varying currents of similar form. A transmitter consists essentially of a source of cur- rent, a key or microphone to vary the current, and a line wire leading to the receiving station, or, in wireless tele- graphy, a short aerial con- ductor, to spread the alter- nating current waves. The Blake Telephone Transmitter was one of the first forms of microphone to give good articulation. The microphonic contact is between a small bead of platinum (see fig.) and a boss of carbon mounted on parallel straight springs. The vibrating disk attached to the mouthpiece presses the bead against the carbon. The disk is insulated, the current flowing only from the bead to the carbon, the leads being attached to the springs. An indiarubber band is wound round the spring of the bead Blake Transmitter 580 Transparent Varnish — Transposition of Conductors to damp its natural vibrations. The Blake transmitter gives excellent articulation, but is somewhat deficient in power. Ofcher types have therefore taken its place for long-dis- tance work. Carbon Teansmittek, carbon in some form, whether granular or in pencils or plates, used in the microphone of a tele- phonic transmitter. See Mickophone. Another type of microphone in use in tele- phony is termed the loose-carbon transmitter. Condenser Transmitter, a telegraphic instrument in which a condenser plays an important part. Thus, in cable-working, a condenser is used to block the steady cur- rents which would be caused by terrestrial ' magnetic ' storms, while the impulses which constitute the signals are transmitted through it (see Dielectric). In wireless telegraphy the term may be applied to a transmitter in which there is an oscillating- current circuit containing a condenser. In telephony, a condenser transmitter is a device for con- trolling the electric current and so trans- mitting speech. The dielectric is air in this case, and one of the plates is thin and movable so that the variations of air pressure, due to the voice, cause it to move to and fro, thus varying the capacity of the condenser and therefore the current in the transmitting aerial wire. The condenser transmitter was invented by the late Prof. Dolbear. See Telephone, Electrostatic. Aerial Transmitter or Antenna, the elevated conductor used in wireless trans- mission. It may be made in many forms, varying from a single nearly-vertical wire, to a single horizontal wire at a small distance above the ground. Common forms are the favrshaped aerial, the umbrella aerial, the in- verted pyramid, the horizontal plane. See Wireless Telegraphy. [j. e-m.] TranspaFent Conducting Varnish, See Varnish, Transparent Conducting. Tpansporter, Electric, See Crane, Electric. Transposition Insulator, See Insu- lator. Transposition of Conductors in Transmission System, changing the re- lative positions of the conductors in order to neutralise inductive efiects from neigh- bouring circuits. Such transpositions are chiefly required in connection with long- distance three-phase lines, and the guiding principle in all cases should be to bring each conductor into the same position relatively to the other conductors, for equal distances. A simple case to consider is a single three- phase circuit; if the conductors are arranged so as to form an equilateral triangle, as shown in fig. 1, the inductive efi'ects are equal for each wire and no transposition is ne- cessary. If, however, two such circuits are J "^ ^ Fig. 1.— Three Equidis- tant Conductors of a Three, phase XransmiBslon Line Fig. 2.— Two Parallel Three- phase Circuits with the conduc- tors of one circuit transposed run on the same poles, and the length of the line and the current carried are such that undue interference would result, then the wires of one circuit should be transposed, as shown diagrammatically in fig. 2. The transpositions should occur at one-third and two-thirds of the dis- tance between the ends of the line, or the points where !s^ Fig. 3. — A Three-phase Transmission System with the three conductors in line Fig. i. —Transposition re- quired for the Conductors of fig. 3 tappings are taken from the line. It may be mentioned that if such transpositions are not made, and if the two circuits are operated at the same frequency, the drop in one cir- cuit will be increased by the mutual induc- tion and will be decreased in the other; while if the frequencies are dissimilar, the potential in either line will rise and fall in accord- ance with the phase-differ- ence between the circuits. In cases where the three conductors are in line, as shown in fig. 3, they must be transposed twice be- tween the ends or tapping- points, so as to divide the distance into three equal portions, as shown in fig. 4. A two-phase circuit, arranged as in fig. 5, is symmetrical, and the mutual inductive efi'ects Fig. 6.— Symmetrical Arrangement of Con- ductors in Two-phase Transmission Line Transverse Resistance of Brush — Trinidad Bitumen 581 are neutralised, conductors be ar- ranged as shown in fig. 6, it is necessary to re- verse the wires of one phase be- tween the ends, or tapping-points (fig. 7). (Ref. 'Electrical En- gineer's Pocket Book ', Foster, under ' Imped- ance ' and ' Re- actance'; 'Long- distance Electric Power Transmis- sion', Hutchinson.) OR Lines. Transverse Should, however, the four [L DC Figs. 6 and 7.— A Two-phase Transmission System with the four conductors arranged in one line, and the necessary transposi- tion which must be made in one of the two phases A, Phase A. n. Phase B. See Trunk Circuits, [p. w.] Resistance of Brush. See Brush Resistance; Brushes, Grad- ing OF; Brush. Tray, a receptacle for sawdust or sand, placed under the glass box of an accumu- lator in order that the pressure on the bottom of the box may be evenly distributed. When lead boxes are used, the 'tray' is simply a flat board. See Insulator Stand FOR Accumulators. Treated Filament. See Filament. Treeingf, a spongy lead growth which sometimes appears on the negative plates of accumulators. It is capable of 'growing' until it reaches across to the positive plates. See Accumulator. Tree System of Wiring:. See Wiring Systems. Trembler Coil, an ordinary induction coil fitted with an automatic switch or Trembler Coil A, Make-and-break contact, B, Battery, c. Condenser. trembler, which makes and breaks the pri- mary circuit, as in a simple electric bell, producing a pulsating current in the secon- dary of the coil. A condenser is usually VOL. 11 connected across the contacts of the make and break, to suppress the sparking. The illustration gives the connections of a trembler coil provided with a condenser. Trifureating Box. See Box, Trifur- cating. Trimming of Arc Lamps. See Lamp, Arc. Trinidad Bitumen.— The bitumen used for insulation purposes in electrical work has a fundamental constituent of bitumen (which see), and its chief source is Trinidad, where, with other admixtures, it forms the natural pitch or asphalt for which Trinidad is famous. This crude asphalt covers the land in varying thickness, over a considerable area, but is emitted in greater quantities from openings in the so-called ' Lake ' (which alone covers 110 acres) than from fissures which are believed to exist around the ' Lake '. An average analysis of the asphalt is: Water, 27 per cent; mineral matter, 28 per cent; bitumen and other organic matter, 45 per cent. The asphalt, after being semi- refined by boiling down and removing the coarser impurities, becomes ^purd lake bitumen. There is found also, in certain Trinidad mines, a very brittle but almost pure bitumen, called manjak. The commercial product for industrial use may be formed either by treating the ^pur^ bitumen with a flux, or by treating the manjak with a flux (usually 10 to 15 per cent of crude bituminous oil) and mixing with a certain quantity of the ^pur6 material, but gypsum or similar mineral matter has in some cases also been added to the extent of 30 per cent. The first of these products — obtained only from the ^pur^ and manjak materials— should strictly be called refined Trinidad bitumen, as the term Trinidad bitumen alone might be taken as referring either to the crude or to the dpur^ material. There has been recent litigation on this point, and from the verdict it seems justifiable to call any product 'Trinidad bitumen' which has a basis constituent of crude bitumen from Trinidad. In the preparation of the refined product the quality depends greatly on the kind of flux employed — a variety of petroleum being more general, which reduces the boiling- point (to about 105° C.) and decreases the brittleness of the crude material. The natural ash in ' lake ' bitumen is retained in the re- fining processes since it prevents the material 88 582 Triphase — Trolley Bushing from cracking should the ground sink — the proportion of ash may be about 28 per cent. The specific gravity of refined Trinidad bitu- men is about 1-35. Bitumen; 'Vulcanised Bitumen' under Cable, Underground. Triphase. See Three-phase. Triple Concentric Cable. See Cable, Triple Concentric. Triply Re-entrant Armature Winding. See Ee-entrancy. Trolley, originally a small wheeled carriage running upon an over- head conductor, and towed along by the metallic cable which conveyed the current to the car; now applied to any overhead collecting device. Wheel-type Trolley (see fig. 1). — This L Fig. 1.— Wheel-type Trolley Fig. 2.— Bow-type Trolley consists of a small wheel carried at the end of a long steel pole, and pressed upwards against the trolley- wire by means of springs. The wheel is grooved to prevent it from leaving the wire under the condi- tions of normal running. The Bow-type Trolley (see fig. 2) con- sists of a contact-bar of U section, sometimes made of aluminium, the groove being filled with lubricant. This type of collector is some- times called a how collector. The bar is car- ried by a light frame, pivoted on the roof of the car, and is pressed upwards against the trolley- wire by springs. Sometimes a copper roller takes the place of the bar. EoD-TYPE Trolley, a type of trolley which has been developed by the Oerlikon Com- pany for h pr sp railways; shown on the accompanying Plate. It is so devised as to automatically maintain contact with the over- head wire throughout its course, even though the relative positions of the roof of the car and of the overhead wire vary greatly at different points of the route. [a. h. a.] Trolley, Double, a system in which two insulated overhead trolley wires are used, in- stead of one wire with earth return. Rarely adopted, on account of the difficulty in main- taining the insulation between the positive and negative conductors, especially at junc- tions and crossings. See Thrjee-wire cc Traction; Trolley-omnibus System. Trolley Base, a steel casting bolted to the roof of a tramcar, and carrying a pivoted Trolley Base frame, free to rotate in a horizontal plane; the frame in turn carries the trolley A (see fig.), which is pivoted to move in a vertical plane, and is pressed upwards against the overhead conductor by springs B attached to the frame. Trolley Bushing, a tube of metal or graphite forming the bearing surface of a trolley wheel, and running upon a steel pin, ROD TYPE OF TROLLEY BOGIE TRUCK (see p; ,585) \Tofacep. f&. Trolley Car — Trolley Retriever 583 througli which the current passes from the trolley wheel to the harp. See Trolley Wheel. Trolley Car, an electric tramcar equipped with the trolley type of collector. Trolley Cord, a rope depending from the trolley head, by means of which the trolley can be removed from the wire or replaced upon it. See Trolley. Trolley Ear. See Ear, Trolley. Trolley Hanger, a bronze or malleable- iron fitting provided with an insulated stud or bolt, which is screwed into a socket in the trolley ear (see Ear, Trolley) after the latter has been attached to the wire. The hanger is either clipped' on to a span-wire supported by side-poles or rosettes, or is provided with eyes for the attachment of pull-off wires. The insulator consists usually of some special com- position, moulded to shape, such as 'Aetna' or ' Ambroin ', or it may be of porcelain. See Insulated Hanger; Suspension, Trolley- wire, [a. h. a.] Trolley Harp, a gun-metal fitting in the shape of a fork, with a steel pin fixed between the arms to carry the trolley wheel. In the case of a fixed trolley head, the base of the fork is provided with a socket to fit on the end of the trolley pole; if the head is of the swivelling type, the base forms a pivot, car- ried vertically in an extension of the head, and preferably running on ball bearings, to enable the trolley wheel to follow the direc- tion of the wire freely. In the latter case the arms of the harp are carried up to the rim of the wheel, to prevent entanglement with the overhead construction in the event of the wheel being de- wired. See Trolley; Trolley Head. [a. h. a.] Trolley Head, a fitting consisting of pole- socket, harp, and trolley wheel, fixed on the . II I k 3: Fig. 1.— Fixed Trolley Head «nd of the trolley pole to collect the current from the overhead wire. In a fixed trolley head (fig. 1), the harp is combined with the socket; this type of head is suitable only for straight-under-ruimmg, the trolley wire being suspended over the track at a distance of not more than 2 ft from the centre on either side. In a swivelling trolley head (fig. 2), the part forming the socket is prolonged to provide a bearing for the base of the harp, which is free to rotate on an approximately vertical axis, so as to enable the wheel to follow the wire, which in this case may be as much as 10 ft from the centre line of the track. The socket cast- ing is provided with guards al- Fig. 2— Swivelling Trolley Head most touching the wheel and harp, to prevent the overhead wires from becoming entangled with the head if the wheel jumps off the wire. [a. H. a.] Trolley Line, Anchoring'. See An- choring Trolley Line. Trolley-omnibus System.— This sys- tem, sometimes called the trackless trolley system, combines some of the advantages of the electric tramway and of the independent automobile. Trolley wires, positive and negative, are erected at the side of an ordi- nary road, and an omnibus is fitted with electric motors and controllers for propulsion, and two trolleys of the swivelling type to collect the current from the wires. The omnibus is free to vary its position with regard to the trolley wires whilst travelling along the road, in order to pass other ve- hicles, without losing connection with the source of power, and the roadway needs no special preparation, thus greatly reducing the capital cost. The system has recently been adopted for a few roads in Britain. [a. h. a.] Trolley Pick-up, a device attached to the trolley harp or fork, to facilitate the replacing of the trolley wheel on the wire. Trolley Pole, a taper steel tube 4 to 5 m in length, which is carried by a universal- joint device bolted to the roof of a tramcar, and which supports at its upper end the trolley head, which should be insulated from the pole by an insulating sleeve. The in- sulated cable connecting the trolley head with the car -wiring is carried down inside the pole. Trolley Retriever, an automatic device which, when the trolley wheel jumps off the 584 Trolley Reverser — Trolley-wire Suspension Trolley Eeverser It is usual to divide wire, immediately lowers the trolley below the level of the latter, in order to prevent damage to the overhead construction. Trolley Reverser, a triangular arrange- ment of trolley wires at the terminus of an electric tramway, so devised that on start- ing the car away from the terminus, the trolley wheel t runs along two sides of the triangle, and in doing so, automa- tically reverses the trolley pole, so that it trails behind the car in the proper way. Trolley Section. the overhead trolley wire into lengths of half a mile or more, called sections; each section is separated from the adjoining sec- tions by section insulators, which take the place of ordinary hangers, and the ends of the adjacent sections are, at the roadside, connected with switches, by means of which they may be coupled together or to the trolley feeders. Several sections are usually supplied by one feeder. Trolley Wheel, a wheel from 3 to 4 in in diameter, usually of gun metal, and pro- vided with a deep U-shaped groove in its periphery, loosely fitting the trolley wire, from which it collects the current. Trolley Wheel WITH Renewable Centre. — The sides of a trolley wheel have a very long life, say 80,000 km or more. But the wear on the centres is far greater, and the centres have a life of only some 8000 km. These circumstances have led to the development of trolley wheels with renewable centres. In some designs the renewable centre is simply a gun-metal disk. A graphite bush, the life of which is some 13,000 km, is employed in this design of trolley wheel. The design illustrated in fig. 1 also has a renewable Self-oiling Trolley Wheel witli Renewable Centre centre, but in this instance, the bvsh is of gun metal and is self-oiling. Trolley Wire, a conducting wire of hard- drawn copper or bronze, from 0*3 to 0'5 in in diameter, which is suspended from in- sulators over or alongside an electric tramway or railway, for the purpose of conveying to the cars a continuous supply of electrical energy derived from a stationary source. The current is collected by a trolley or bow- • » • Sections of Trolley Wire 1, Circular. 2, Figure-8. 3, Grooved. collector carried 'on the car and making contact with the wire. The trolley wire is usually circular in section. Grooved and figure-8 wire are also sometimes used. Some typical sections are shown in the fig. See Hard-drawn Copper Wire; Phono-elec- tric Wire. Trolley-wire Conductor. Se^ Trol- liTy Wire; Conductors, Overhead. Trolley-wire Crossing, a device placed at the intersection of two overhead trolley Fig. 1. — Fixed Trolley-wire Crossing wires above a track-crossing, to enable the trolley wheels to pass the crossing without ^ leaving their respective wires. Crossings are ' right-angle ' or ' diagonal ' according to the angles at which the tracks intersect, and both kinds can be made 'adjustable' to Fig. 2.— Adjustable Trolley-wire Crossing accommodate slight variations in the angles. Fig. 1 shows a fixed crossing, and fig. 2 an adjustable one. Insulated crossings effect the same purpose without permitting the inter- secting wires to make electrical contact with one another. Trolley-wire Suspension. See Sus- pension, Trolley-wire. Trotter and Preece Photometer — Trunk Circuits 585 Trotter and Preece Illumination Photometer. See Photometer, Illu- mination. Trotter Photometer Head, See Pho- tometer Head. Truck, a steel framework mounted on four wheels by means of journal boxes, and carrying a spring-supported seating for the car body. There are three principal types — rigid, swivel, and radial trucks. The rigid truck is bolted directly to the car body, and is equipped with either one or (more usually) two electric motors. Swivel trucks are always in pairs, the car running on eight wheels; such a car is called a double-truck car or bogie car, and the trucks are bogie trucks. Examples of two typical designs of bogie trucks are given on the Plates facing pp. 582 and 586. In the latter case, two motors are shown, one carried by each axle. The car body is pivoted on the trucks in such a way that the latter are free to rotate sufficiently to adjust themselves to curves in the track. Bogie trucks may have four equal wheels, in which case each axle may be driven by a separate motor; or they may have two large and two small wheels, the axle of the former only being driven by a motor, when the Warner Non-parallel Axle (Eadial) Truck greater part of the weight of the ear is arranged to fall upon the larger wheels to secure the maximum adhesion possible, and consequently such trucks are known as maxi- mum-traction trucks. A radial truck (see fig.) is one in which the axles are not held rigidly parallel, but are given a certain amount of freedom to enable them to set themselves approximately along radii from the centre of curvature of the track, while the truck is bolted to the car body. In a double-truck car, if only one truck is equipped with motors, the truck thus equipped is known as the motor truck, the other being styled the trailer truck. In addition to carrying the motors, the trucks are equipped with the brake gear, sand boxes, safety appliances, &c. True Efficiency denotes the real efficiency of a motor or transformer, or the ratio be- tween true w output and true w input. The term is used to denote the above as dis- tinguished from the 'apparent efficiency'. See Efficiency; Apparent Efficiency. True Ohm. See Ohm. True Watts denotes the true power in w delivered by or to a circuit. The true w are equal to the product of volts, amp, and pf. Or, true w = apparent w x pf. See Apparent Watts; Power Factor. Trunk Circuits, or Lines, the circuits connecting telephone exchanges. Where the distance between the exchanges is great, i.e. in long-distance trunk lines, the two wires composing the circuit do not run parallel, but are so connected to the insulators on the poles that each makes a complete turn about the other for every four (or more) poles. The circuit therefore presents to the induc- tion from any other circuit on the poles, the appearance of a series of elongated loops; and since first one and then the other wire is uppermost, the inductive action on suc- cessive loops is equal and opposite. On the 586 Trunking Switchboard — Tungsten Lamps whole, therefore, induction from neighbour- ing circuits produces no resultant current in the trunk circuit, and consequently eliminates cross talk. The usual arrangement is to make the two wires forming a circuit, screw round one another with such a pitch that each makes a complete revolution about the other every eight posts. The induction from neighbour- ing wires thus causes equal and opposite effects on every section of four posts, and is therefore practically negligible. See Transposition of Conductors in Transmission System. [j. e-m.] Tpunking- Switchboard. See Switch- board, Trunking. Trunnion Type of Alternator. See Alternator. Tube, Electric, a tube railway worked by electric power. See Tube Eailway. Tube of Induction. See Line of In- duction; Maxwell; Field, Magnetic. Tube of Magnetic Flux. See Unit of Magnetic Flux; Line of Induction; Field, Magnetic. Tube Railway, an underground railway carried in a tunnel of circular cross section, lined with cast-iron segments bolted together in the form of a tube. Such railways are either electric or cable, almost always the former. Tube Rectifier. See Rectifier. Tubing, Bitumened Paper. See Con- duit, Interior. Tubing", Screwed, tubing used for in- terior conduits, in which the junctions be- tween successive sections of tubing are eflFected by screwed joints. See also Con- duit, Interior; Wiring Systems. Tubing", Simplex. See Conduit, In- terior; Wiring Systems. Tubing, Slip, tubing used for interior conduits, in which the junctions between successive sections of tubing are effected merely by means of a sliding fit. Such tubing is very cheap; but since it must be enamelled to preserve the metal, the con- tinuity of the conductivity of the entire system of tubing is usually very inferior to that obtained with screwed tubing (see Tubing, Screwed). See also Conduit, Interior; Wiring Systems. Tubing", Tinned.— Tinned brass tubing is sometimes employed for conduits for in- terior wiring. One advantage over ena- melled tubing relates to the greater readi- ness with which the tubing can be made electrically continuous throughout. One system employing tinned brass tubing is known as the Kalkos conduit system. See also Conduit, Interior; Wiring Systems. Tubular Braid (sometimes termed sleev- ing), cotton, silk, or other fibrous material, woven in such a way as to form a hollow tube. The diameter may be varied by the use of different sizes of thread and by vary- ing the number of threads. Tubular Electromagnet. See Elec- tromagnet, Ironclad. Tubular Lamp. See Lamp, Tubular. Tudor Accumulator. See Accumu- lator. Tufting" of Flux.— In machines with open slots, the distribution of the flux in Tufting of Flux the gap is not uniform, as the magnetism on leaving the pole face narrows down towards the tops of the teeth, as indicated in the accompany fig. This collection of the flux towards the tooth tops is sometimes spoken of as tufting. See Gap Eeluctance; Flux Distribu- tion; Fringe of Flux. Tumbler Switch. See Switch, Tum- bler. Tuned - reed Frequency Indicator. See Frequency Indicator or Meter. Tung OiL See Chinese Wood Oil. Tung'Sten, a metal now widely employed for the filaments of incandescent lamps. Specific gravity = 19'0. Specific heat = 0-033. Melting-point = 1700° C. See also Steel. Tungsten Lamp. See Lamp, Incan- descent Electric. Tungsten Lamps with Drawn Fila- ments. — Tungsten has properties admirably adapted to use for metallic-filament lamps; but great difiiculty has been experienced in producing the filaments, especially for the ■5, o K ^ H ffi H i o" K 2 u ■< < H w z o p- X >-" CQ <; o ^ M d CO ^ pi c< O H > O w g J z h <11 Z H W C/2 Pi o Pi u Pu c/; 1 ID O o S Q z Z >— H O H kJ Z O W U K H w en Pi <: O ffi i^ ^ VI w « w < pi CJ K hi Pi o O H w O z s t— 1 pa W S o fe O w u D Pi H I^M^.. Tuning — Turnbuckle 587 higher voltages. The methods previously employed were by squirting a colloid or finely -divided metallic paste, and by re- placement of carbon on a carbon filament; but in 1910 a dravm-vf ire filament was pro- duced, which has great advantages both in manufacture and use. Examples of lamps provided with such filaments are the 'Mazda' (which see), and the 'One- watt' lamp recently placed on the market. (Eef. under 'Tung- sten ' in paper by Swinburne, entitled ' New Incandescent Lamps', Journ.I.E.E., vol. 38, p. 225.) Tuningf, a term borrowed from musical technical language. It implies the equalisa- tion of frequencies of vibration or wave lengths. Its advantage in wireless trans- mission is that the current in a circuit, having a natural frequency of oscillation, grows in amplitude if the successive impulses received from outside are timed exactly to its natural frequency. Thus a current whose natural frequency is 10® per sec will induce a much larger current in a circuit whose natural frequency is 10® than in one whose natural frequency is 10^ or 10'^. Indeed tun- ing has now been made so sharp that a much smaller difference of frequency than this will make a great difference in the receiver cur- rent. Poulsen claims that a difference of 1 per cent frequency is sufficient to practi- cally cut out the receiver. Similar results have been obtained by other experimenters. In general, a difference of 5 per cent in fre- quency is sufficient to prevent interference between neighbouring stations. For marine signalling, such sharp tuning as this is not advisable, as it is difficult to maintain the exact frequency on board ship. With sharp tuning the receiver only responds to a very limited range of frequencies. See Aerial, Tuning of; Kesonance, Electric; Re- sonance, Selective. [j. e-m.] Tuningf Fork, a U-shaped piece of steel of such dimensions as to give a strong vibra- tion of definite frequency. Used as time keeper for records of rapid motions on a revolving drum (chronograph), also in fre- quency meters and in stroboscopic methods of determining rates of rotation. See FRE- QUENCY Indicator or Meter; Drysdale Stroboscopic Method of Slip Measure- ment; Vibration Tachometer. Tunnel. See Slot. Tunnel Armature. See Armature. Tunnel Winding. See Slot. Turbine, Water-power, for Hydro- electric Generating Stations. See Mining Equipment, Electrical. Turbine Pump. See Mining Equip- ment, Electrical; R?;es-boturbo Pump. Turbo - alternator. See Turbo-gene- rator; Turbo-generating Set. Turbo -dynamo. See Turbo -gene- rator; Turbo-generating Set. Turbo - generating Set, one or more dynamo-electric generators, either for supply- Fig. 1.— Horizontal Type of Turbo-generating Set ing continuous or alternating current, direct connected to a steam turbine. Sometimes the condenser is also built into the set and constitutes a component of it. In most types, the shaft is horizontal (see fig. 1), but in the Curtis and a few other types (see fig. 2) the shaft is sometimes vertical and carries the electric generator at its upper end. In these ver- tical sets the steam turbine is frequently arranged between the condenser, which constitutes the base of the set, and the electric generator, — which is at the top of the set. In the more recent Curtis sets, horizontal shafts have usually been employed. Two ty- pical designs of turbo-generating sets are shown on the Plate facing p. 588. Turbo-generator, an electric generator designed for use in combination with a steam turbine, the high speed of this type of prime mover necessitating the exercise of special precautions in the construction of the gene- rator, which differs widely from the ordinary type. See Turbo-generating Set. Turnbuckle, Insulated, a device used in the overhead construction of electric tramways, for communicating tensile stress Fig. 2.— Vertical Type of Turbo-generating Set 588 TurnbuU-McLeod Booster — Two-phase System without allowing the electric current to flow along the wire or cable in the course of which it is interposed. It is provided with a screw for adjusting the tension in the wire attached to it. One variety of this device is the Brooklyn strain insulator. See Insulator; Insulated Hanger. TurnbuU-McLeod Automatic Rever- sible Booster. See Booster, Eeversible. Turning' EflFOFt, the tangential action of a force upon a shaft, tending to produce twisting or turning. The idea is especially important in electrical engineering in connec- tion with the irregular rotatory motion pro- duced in steam and gas engines, due to the intermittent admission of the propelling agent. See Torque Diagram of an En- gine; Pulsation in Prime Movers; Ir- regularity Factor; Variation in Prime Movers; Torsiometer, Electrical. Tupning Moment, synonymous with Turning Effort (which see). Turns, Dead, of a Dynamo.— For the sake of mechanical symmetry and balance, a 'dummy' coil is sometimes employed on a dynamo-electric machine, but is not connected into the circuit. The turns of this coil are spoken of as dead turns. Turns per Commutator Segment, the number of turns in series between ad- jacent commutator segments. Turns per Phase, the ratio of the total number of turns to the number of phases in a machine. Turns per Pole, the ratio of the total number of turns to the number of poles in a machine. Turpentine. — There are three varieties of turpentine met with commercially. These are known as American, French, and Eussian respectively. They are all distillation pro- ducts from the exudations of various species of pine trees. Turpentine has the property of absorbing oxygen from the atmosphere, forming a resinous film. This property dis- tinguishes it from all other solvents. Ameri- can turpentine absorbs more oxygen than either French or Eussian turpentines, and is the most suitable for thinning insulating var- nishes. It has a specific gravity of 0'865 to 0-875, and a flash-point of 35° to 38° G. In drying, it acts as a binding agent. Varnishes thinned with it dry with a good smooth tough surface, that will age well, and retain their brilliancy of finish; they do not, however, uaU3,lly give very high dielectric strength, and the use of turpentine on this account, and by reason of its high price, is decreasing. In cases where punchings are not annealed, it is convenient to use turpentine when punching out core disks; it is a very good lubricant for this purpose, and obviates the necessity of cleaning, if the punchings are to be varnished. See Die; Dies. There are many turpentine substitutes at present on the market, chief among which is rosin spirit. This is a distillation product of rosin, but is not suitable for thinning insu- lating varnishes, as it is liable to contain traces of rosin oil, which prevents thorough drying. See Thinner; Solvents for Im- pregnating Materials; Solvents for In- sulating Varnishes. [h. d. s.] Twaddell Hydrometer Scale. See Hydrometer Scales. Twin Cable. See Conductor, Twin. Twin-carbon Are Lamp. See Lamp, Arc. Twin Conductor. See Conductor, Twin. Twin Flexible. See Cable, Flexible. Twin Flexible Cable. See Cable, Flexible. Twin Lead -covered Cable, a lead- covered cable enclosing two independently- insulated conductors. See Cable; Con- ductor, Twin. Twisted -strip Galvanometer. See Galvanometer. Two -meter Battery System. See Battery Meters and Battery Metering Systems. Two-part Commutator. See Com- mutator. Two -phase, synonymous with quarter- phase (which see). See under Alternating Current. See also Two-phase System. Two-phase Alternator. See Alter- nator; Two-phase System. Two -phase Armature. See Arma- ture. Two-phase Circuit. See Two-phase System. Two-phase Current. See Alternat- ing Current; Two-phase System. Two -phase Generator. See Alter- nator. Two-phase Motor. See Motor, Alter- nating Current; Motor, Induction. Two-phase Rotor. See Eotor. Two -phase System, a system of ac generation and distribution in which two RUGBY-CURTIS TURBO-GENERATOR INSTALLED AT LIVERPOOL CORPORATION ELECTRICITY WORKS 200O-KW, isoo-RPM WESTINGHOUSE-PARSONS TURBO-GENERATOR [Tafaccp.sSS. Two-phase System — Unbalanced Load 589 emf are employed, having their phases in quadrature. Two-phase currents may be conveyed by four wires, or by three wires. In the four-wire two-phase system there is a pair of conductors to each phase. Kepre- senting the generator winding by two wind- ings AB and CD (fig. 1) placed in quadrature, A Kg. 1.— Fonr-wire Tiro-phase System the four-wire system requires two conductors for each phase, each phase having a separate return. In the three-wire two-phase system the phases are connected together at one end, and to a common return as at A and C (fig. 2). The current in this wire is v'2 A Fig. 2.— Three-wire Two-phase System times that in the other two, and the system really becomes an unsymmetrical three-phase system, having a right-angled triangle of voltages between the three conductors. The system is also unsymmetrical with regard to the resistance drop on the two phases, since the drop in the common return conductor is 46° out of phase with either current. Power measurement in a two-phase four- wire system is efi"ected with two wattmeters, placed one in each circuit. In a two-phase three-wire system the meters are placed in two of the three wires, with the potential coils connected between either wire and the third. The quantity of copper required in a four- wire two-phase system to transmit a given power with a given loss at a given maximum voltage between wires is the same as with a sp system, whereas a three-wire two-phase system requires 146 per cent of this amount. See also Alternating-current System. Two-phase Transmission, Transposi- tion of Conductors in. See Trans- position OF Conductors in Transmission System. Two-pole Switch. See Switch, Double- pole; Switch Types, Designation of. Two-rate Clock. See Switch, Time. Two-rate Meter. See Meter, Two- rate. Two-rate System. See Tariff Sys- tems. Two-wattmeter Method of Measur- ing Power. See Power, Methods of Measuring, in Polyphase Circuits. Two-wire Meter. See Meter, Two- wire. Two-wire Plant. See Two-wire Sys- tem. Two-wire System, an electrical system in which all the consuming devices are con- nected between the two terminals of the system, as opposed to systems in which there are more than two main terminals at different potentials from one another, e.g. the three-wire and five-wire systems. Type-printing- Telegraph. See Tele- graph Systems. u Ulbrieht Photometer. See Photo- meter, Globe. Ultimate Strength. See Modulus of Elasticity. Ultra-violet Light. See Light, Ultra- violet. Umbrella Aerial Transmitter. See Transmitter; Wireless Telegraphy. Umbrella Type of Alternator. See Alternator. Unarmoured Cable. See Cable. Unbalanced Load denotes the condition obtaining on a three-wire system when the middle wire carries current, or on a polyphase system (which see) when, due to unsymmetri- cal loading of the phases, the total power 590 Unbalanced Magnetic Pull — Unidirectional Current supplied is not a constant quantity, but fluctuates to a greater or less extent. See Balanced Load; Three-wire Dis- tributing System; Compensator; Dobro- woLSKi Three-wire Dynamo. [r. c] Unbalanced Magnetic Pull, the dif- ference between the magnetic forces exerted on the rotor of a dynamo-electric machine by the stator by reason of the unevenness of the air-gap depth, due to eccentricity of the rotor (which see) in the stator bore. This unbalanced pull exerts a bending mo- ment on the shaft, and tends to deflect it, drawing the rotor nearer to the stator sur- face. It should be taken account of in shaft calculations. See Shaft; Magnet, Trac- tive Force of. Unbalanced Polyphase System. See Unbalanced Load. Under-compensated Induction Meter. See Meter, Under-compensated Induction. Under-compounded. See Excitation. Under- contact Rail, a conductor rail for electric railways which is supported from above, in such a way that the collectors carried by the trains make rubbing con- tact with its under side, the upper side being provided with a guard to prevent accidental personal contact with the rail. The Wilgus-Sprague under-contact third- rail is shown in the fig. See Third-rail Electric Railway. Underground Cable, Underground. Underground Canalisation, a term denoting conduits or subways for the loca- tion of cables for transmitting and distri- buting electricity in cities. See Cable, Underground; Conduit, Underground; Conduit System; Ducts. Underground Conduit. See Conduit, Underground; Conduit System of Elec- tric Traction; Cable, Underground; Ducts. Underground Conveyor, Electric- Mechanical conveyors of difierent types are used for removing the coal from the coal face, and may be driven by electric motors. Wilgus-Sprague Under-contact Protected Third Rail See Cable, Several types of these conveyors have been introduced. In one method, steel troughs are laid along the face; in the trough runs a steel chain driven by an electric motor; the chain carries scraper-blades which carry the coal along the trough and discharge it at the end into the tubs. In another type, a textile band, instead of the chain scraper, is used to carry the coal. One of the latest coal-face conveyors is GihVs end -discharging carriage. This consists of a long, low car- riage, built up of jointed sections; the length of each section is about 6 ft, the total length may amount to 30 or 50 ft. The carriage is drawn along the face by an electric haul- age gear. When the carriage reaches the road at the end of the face, gearing auto- matically comes into operation which allows the motor to drive a scraper chain conveyor running inside the carriage, which discharges the coal into the tubs. The haulage is then reversed and the carriage returned to the coal face. The rapid adoption of coal-cutters (see Electric Coal-cutter) and the increased amount of coal obtained from a given face by their use, renders very imperative the employment of some mechanical means for rapidly removing the coal and freeing the face for another cut. See Electric Coal- cutter; Mining Equipment, Electrical. [w. B. H.] Under - running, a term applied to a trolley which consists of a wheel running under and in contact with a trolley wire suspended over the centre of the track. See Trolley; Under-contact Eail. Under - running Third Rail. See Under-contact Eail. Underslung Monorail Electric Rail- way. See Monorail Electric Railway. Undertakers. — The expression means- any local authority, company, or person, authorised to supply electricity, to whom the Electric Lighting Acts apply (Electric Lighting Act, 1909, Clause 25). Undertype Machine. See Machine, Overtype; Generator. Underwriters' Wire. See Wire, Underwriters'. Undulatory Current. See Current, Pulsating. Unidirectional Current, a current which flows in one direction only, with or without pulsations. See Current, Recti- fied; Current, Pulsating; Rectifier. Unidirectional emf — Unit of Magnetic Flux 591 Unidirectional emf, an emf which tends to set up currents, with or without pulsa- tions, in one direction only. See Unidikec- TiONAii Current. Uniformly Distributed Current. See Current Distribution. Uninsulated Conductor. See Con- ductor, Uninsulated. Uniphase, a synonym of single-phase and monophase. See ' Monophase or Single-phase Current' under Alternating Current; Single-phase Motor. Unipivot Measuring Instrument, a type of instrument in which only a single pivot is used, about which the moving coil rotates. The pivot is screwed into a vertical staff carried by the coil, and works in a jewelled cup at the exact centre of the coil, this jewelled cup being attached to a stud fixed to the frame. The whole moving sys- tem, including the pointer, is balanced so that the centre of gravity is on the pivot, and thus the necessity of levelling the in- strument is eliminated, as the controlling spring suflBces to hold the coil in its normal position. The chief advantage of the uni- pivot instrument is that, owing to the fact that the moving system is supported on a single jewel, it may be entirely lifted off when the instrument is out of use. This is effected automatically in some patterns by raising the instrument, and in others by closing the lid of the instrument. See Instrument, Switchboard Measuring; Ammeter; Voltmeter; Wattmeter; Test- ing Set; Meter, Electric. Unipolar, sometimes also called homopolar, denotes that type of generator or motor whose conductors always move in a field of constant direction, so that the induced emf does not reverse, and hence no commutator is required. Such machines have not come much into practical use, chiefly owing to the fact that several conductors in series have to be used, which entails many slip rings and correspondingly heavy losses. The mainten- ance of the slip rings and brushes is a very serious item. Such machines have also been called acyclic machines. See Noegerrath's Homopolar Dynamo; Generator; Non- polar Generator. Unipolar Current Induction denotes the generation of current on the unipolar principle. See Unipolar. Unipolar Dynamo. See Unipolar. Unit, Absolute, a unit chosen to be consistent with a recognised system of fun- damental units, as distinguished from a unit defined with reference to an arbitrary standard. Thus the true ohm is an absolute unit, whilst the Siemens unit of resistance is an arbitrary wnit. The standard ohm is really an arbitrary unit, but is chosen as represent- ing the best estimate of the value of the true ohm (see Ohm). The ohm per cm cube (see Specific Eesistance) is not an absolute unit, as the cm is not the unit of length in the Q.E.S. system (see Units, Q.E.S. Sys- tem of), to which the ohm belongs. The term is sometimes used as synonymous with cgs unit. See Units, Centimeter-gram- second System of. [f. w. c] Unit, Board of Trade. See Board OF Trade Unit; Kelvin; Energy. Unit, Derived. — Derived units are units used in the measurement of quantities which are expressible as functions of those to which the fundamental units apply. Thus the ' mile per hour ', as unit of velocity, is derived from the fundamental units of distance and time, the mile and the hour respectively. See Units, Fundamental. [f. w. c] Unit, Practical, a unit of measurement in general use in any art, particularly the units in which electrical quantities are usually measured, i.e. those of the Q.E.S. system (see Units, Q.E.S. System of), the term being used to distinguish them from those of the cgs system, of which they are simple multiples. Thus the amp is -J^ cgs unit of current, the ohm is 10^ cgs units of re- sistance, and the volt is 10* cgs units of emf. [f. w. c] Unit Current. See Current, Elec- tric; Ampere. Unit of Capacity. See Capacity, Electrostatic; Capacity, Electromag- netic; Farad; Microfarad; Condenser, Electric. Unit of Conductance. See Conduc- tance; Mho. Unit of Electrostatic Capacity, a condenser which has unit charges on its plates, when charged to unit potential See Farad; Microfarad; Condenser, Elec- tric. Unit of Magnetic Flux, a line of flux or magnetic induction; a unit tube of mag- netic flux. The name given to the unit of magnetic flux is the maxwell. A portion of a magnetic field or magnetised substance whose sides are everywhere along 592 Unit of Magnetomotive Force — Uviol Lamp the direction of the magnetic force (in air) or magnetic induction (in iron); and whose cross-sectional area taken at any point along the tube is such that when multiplied by the average field strength (in air) or induc- tion density (in iron), the product is unity. [d. k. m.] Unit of Magnetomotive Force. See Gilbert; Magnetomotive Force; Ampere Turn. Unit of Output, the output of a motor, since it is mechanical power, is usually mea- sured in hp. The output of a generator or transformer, since it is electrical power, is usually measured in kw or kva. See Output. Unit of Pliotometrical Intensity.— If P is the cp of a source of light, and D the distance of a point from it, the intensity of P illumination at the point is =-^. If P and D^ are numerically equal to one another, then unit intensity of illumination exists at the point. This is usually measured in candle-feet or candle-meters. See Candle- foot ; Candle-meter; Photometry; Stan- dard of Light. Unit of Supply.— The unit of supply has, until recently, generally been designated as the Board of Trade Unit (BTU). This is an amount of energy equal to one kilowatt- hour (kw hr), a term used and understood all over the world, and hence distinctly preferable to BTU. The word kelvin is, however, a still more suitable name for this unit of energy, and it will probably soon be widely if not universally employed. See Kelvin; Kilowatt-hour; Board of Trade Unit; Energy; Electricity. Unit of Work.— The unit of work in the cgs system is that done by a force of 1 dyne acting through a distance of 1 cm, or 1 dyne-cm. It is known as 1 erg. In the pound-foot system the unit of work is that done by a force equal to the weight of 1 lb acting through 1 ft, or 1 ft-lb. Unit of Electric Work. — The unit of electric work in the cgs system is that done by unit quantity flowing at unit emf, or by unit current flowing at unit emf for 1 sec. The practical unit of electric work or energy is the joule. The joule is equal to 1 watt second or to 10* ergs. See JoULE; Erg; Kelvin; Energy; Electricity, [e. c] Unit Strength of Current. See Absolute Unit of Current; Ampere. Unit Switch. See Switch, Unit. Units, Centimeter - gram - second System of, the system of units based on the cm as unit of length, the gram as unit of mass, and the sec as unit of time. This is the basic system from which most electrical and magnetic units are derived. See Unit, Practical; Unit, Absolute, [f. w. c] Units, Fundamental, the basic units in terms of which others can be defined. In the dynamical sciences, those of mass, length, and time are chosen as fundamental units. See Unit, Derived; Unit, Practical. [f. w. c] Units, Metric System of, the system of units based on the meter as the unit of length, and the gram as the unit of mass. [f. w. c] Units, Q.E.S. System of, the quadrant- eleventh-second system, that is, the practical system of electrical units (see Unit, Prac- tical), these being based on the earth quad- rant, or 10* cm as unit of length, lO'^^ g as unit of mass, and the sec as unit of time. [f. w. c] Universal Shunt Box. See Shunt Box. Universal Spreader, an adjustable tool which can be used for spreading any size of loop-wound coils. See Winding, Forming, and Spreading Machinery. Upward's Battery. See under Bat- tery, Primary. Useful Life of Lamps. See Lamp, Incandescent Electric. U-shaped Electromagnet. See under Magnet. Utilisation Coefficients, Speciflc— In connection with the design of dynamo- electric machinery, Dr. S. P. Thompson em- ploys two coefficients, a and /8, relating respectively to the ' specific electric loading ' and the 'specific magnetic loading' of the active zone of the machine. See p. 575 of vol. i of the seventh edition (1904) of Thompson's 'Dynamo-Electric Machinery'. Uviol Lamp. See Lamp, Tubular. Vacuum Augmentor — VaCriation in Alternators 593 V Vacuum Augmentor, a device intro- duced by Parsons in 1904 to supplement the main condenser in steam-turbine plant, by increasing the degree of vacuum. A small condenser C of about one-twentieth the cool- ing surface of the main condenser B is con- nected thereto by a pipe D, which lies 2 or 3 ft below the bottom of B. A portion of d Vacuum Augmentor is contracted, and the nozzle of a steam jet E enters the pipe at this point, giving axial- flow towards c. The action is similar to the draught production in a locomotive chimney, sucking from B the residual vapour with any air leakage which may be present, and de- livering to the air pump F. G serves as a water seal, preventing passage back to B. The jet consumes about IJ per cent of full- load steam supply, and improves the vacuum by 1 J to 2 in, which may give 5 per cent to 10 per cent decrease of steam consumption, according to size of plant. See Condenser, Steam; Pressure, Exhaust. Vacuum Drying Oven. — As the name implies, this is an oven used for drying coils and other parts of electrical machinery, both before and after impregnation with insulat- ing varnishes or compounds. The air is ex- hausted by pumps, and the oven is heated, generally by steam pipes. The chief advan- tages of the vacuum are: (1) the rapid and complete removal of vapour, whether of water or of varnish solvent, and (2) the forcing-in of the impregnating material into the crevices and pores of the insulation when the air pressure is again applied. Some ovens are arranged so that a pressure above that of the atmosphere may be put on after the vacuum has been relieved. Vacuum dry- ing ovens are widely used by large manufac- turers of electrical machinery. Passburg Vacuum Oven, a particular type of vacuum oven used for drying insulat- ing materials, coils, &c. Its name is taken from that of one of the pioneer inventors and manufacturers of this class of apparatus. See Oven, Baking; Electric Baking Oven. Vacuum Tube, a vessel containing a gas at very low pressure and small density. At extremely low pressure and at high {e.g. at- mospheric) pressures, gases appear to be non- conductors; at intermediate pressures they are somewhat conductive, particularly so if the discharge or current flows in a closed path in the gas, and has not to enter or leave by a metallic electrode. See Crookes' Tube; Hittorf's Tube; Geissler Tube; Eadiometer, Electric; Lamp, Tubular; Rectifier. Vacuum Tube Lamp. See Lamp, Tubular. Vagabond Currents, synonymous with eddy currents, foucauU currents, and jparasitic currents. See Eddy Current. Valentine Conduit. See Conduit, Un- derground. Valves. See Rectifier. Vaporisation, Temperature of. See Steam. Vapour Tube Lamp. See Lamp, Tubu- lar. Variable Resistance. See Rheostats OR Resistances. Variable-speed Motor. See Motor, Variable -SPEED; Motor, Adjustable- speed; Motor, Multi- speed; Motor, Varying-speed; Motor, Spinner. Variable-voltage Control. See Multi- voltage Speed Control. Variable-voltage Transformer. See Transformer, Variable-voltage. Variation in Alternators.— Paragraph 61 of the 1907 Standardization Rules of the A.LE.E. defines the variation in alternators, or ac circuits in general, as the maximum difference in phase of the generated voltage wave, from a wave of absolutely constant frequency, expressed in electrical degrees (one cycle equals 360°), and may be due to the variation in the prime mover. See Variation in Prime Movers; Pulsation IN Prime Movers; Pulsation in Alter- nators; Cyclic Irregularity; Irregu- larity Factor; Crank-effort Diagram; Torque Diagram of an Engine. 594 Variation in Prime Movers — Varnished-cambric Tube Variation in Prime Movers.— Para- graph 59 of the 1907 Standardization Rules of the A.I.E.E. defines the variation in prime movers which do not give an absolutely uni- form rate of rotation or speed, as, for in- stance, in reciprocating steam engines, as the maximum angular displacement in position of the revolving member, expressed in de- grees, from the position it would occupy with uniform rotation, and with one revolu- tion taken as 360°. See Pulsation in Prime Movers; Variation in Alternators. Varley Slide, more particularly the Thomson- Farley slide, consists of two boxes Big. 1.— View of Varley Slide containing resistances, with circular switch dials on top. Box A usually contains 101 coils of 1000 ohms each, and box B 100 coils of 20 ohms. The switch arm in box A has a split contact blade, the two halves of which press simultaneously on alternate contacts, mrnmm-m m-w Fig. 2.— Diagram of Connections of Varle; Slide V, Applied line voltage. p, Bequiied potential. while B has only one contact arm. All the resistance in B (2000 ohms) is connected be- tween the two switch arms on A, thus shunt- ing two coils (2000 ohms) of A as shown in fig. 2. Hence it will be seen that the resist- ance of the whole arrangement is 100,000 ohms, and that a fractional potential vary- ing between zero and the applied voltage by steps of , , , Too ^ Too "" 10,000 of the whole may be obtained between one end of A and the arm on B. See Potentio- meter Eatio Eesistance. [l. m.] Varnish, Copal. See Copal Varnish; Insulating Varnishes. Varnish, Transparent Conducting", a varnish used for coating the glass fronts of electrostatic voltmeters to shield them from external static influences. One such varnish (described by Ayrton and Mather in the Proc.Inst.E.E. for April 24, 1894), which is applied hot, con- sists of \ oz of transparent gelatine, melted at 100° C, in 1 oz glacial acetic acid, and added to half its value of dilute sulphuric acid (1:8 by volume). When hard, it should be covered with transparent anti-sul- phuric enamel. Varnished Cambric. See Cam- bric FOR Insulating Purposes; Empire Cloth. Varnished-cambric Cable.— The General Electric Company of America has developed a line of cables in which the cores are insulated with varnished cambric. Insufificient information is available to further describe the composition and pro- perties of these cables. They should be less hygroscopic than paper-insulated cables. See also Cambric for Insulating Purposes; Varnished-cambric Tubes; Cable; Cable, Underground. Varnished-cambric Tube. — Insulating tubes for the slot linings of high-voltage dynamos and for other purposes have been made of thin cambric, impregnated with suit- able insulating varnishes, and rolled up into tubes, whose walls are thus composed of many layers of varnished cambric. Linseed oil is one of the impregnating materials which is suitable for the purpose. Such tubes are, in the first instance, usually tightly rolled up circular, and are afterwards moulded into the desired form. The process is usually more or less similar to that employed in the manufacture of insulating tubes of paper or of mica, or of both these materials in suit- able proportions. See Mica Tube; Mican- ite; Micarta Tubes; Pertinax Tubes; Slot Insulating Tubes. Varnishes — V End-connections 595 Varnishes, Cloth. See Cloth Var- nishes; Impregnating Varnishes; In- sulating Compound; Insulating Var- nishes. VaFnishes, Core-plate. See Core- plate Varnishes. Varnishes, Elastic Insulating*. See Elastic Insulating Varnishes. Varnishes, Finishing-. See Finishing Varnishes. Varnishes, Flexible Mica-sticking-. See Flexible Mica-sticking Varnishes. Varnishes, Insulating. See Insulat- ing Varnishes; Elastic Insulating Var- nishes; Impregnating Varnishes; Insu- lating Compound. Varsulat Insulating Varnishes, the trade name of a line of insulating varnishes manufactured of Chinese wood oil (which see). The manufacturers claim that they are more permanent and moisture-proof than linseed-oil varnishes. See Insulating Var- nishes. Varying-speed Motor. See Motor, Varying-speed; Motor, Variable-speed; Motor, Adjustable-speed; Motor, Multi- speed; Motor, Spinner; Multi- voltage Speed Control. V - connection : (1) a name sometimes given to the opevrdelta method of connection for three-phase circuits. This method is shown diagrammatically in the fig. (2) Also applied to the end-connections of some types of armature windings. See End- conneotions; v end - connections for Bar-wound Armatures. V. D. E., the abbreviation for Ferband Deutscher ElektrofecJmiker (which see). V. D. I., the abbreviation for Verein Deut- scher Ingenieure (which see). Veeder Tachometer. See Tachometer, Liquid. Velocity of Signalling.— The speed at which signals are transmitted is usually reckoned by the number of words trans- mitted per min. The so-called ' word ' used consists of five V's (the letter V being three dots and a dash, thus •••—). As V is a long letter, in code, five V's correspond to an average word of seven or eight letters. Despatches can be transmitted by hand in land-line telegraphy at speeds up to about 35 ^"M^^ V-Connection words per min, and can be read by sound up to about 50 words per min. Higher speeds are only attainable by means of automatic transmitting and receiving instruments or by diplexing. With Wheatstone instruments, 400 or 500 words per min may be sent, while with the Pollak-Virag system it is claimed that 14,000 words may be trans- mitted in the same time. Very high speeds are not commonly used in telegraphy, as it is generally simpler to use several line Wires than to install and keep running the intri- cate and delicate mechanism necessary for using very high speed. The staff of clerks required is practically proportional to the number of words per min whatever form of transmitter be used. Hence practically the only possible economy in automatic transmission is in the capital cost and up- keep of the additional line wires, while against this are the similar charges for the automatic instruments. The cost and up- keep of ordinary instruments is very small in comparison. [j. e-m.] V End-connections for Bar-wound Armatures. — This is the name given to Fig. 1. — V End-connections for Bar Winding the connectors joining two consecutive con- ductors of a bar-wound armature in eases where the bars themselves are not made to Fig. 2.— Methods of Securing V End-connections to the Face Conductors act as connectors. The connector takes its name from its shape, which is more or less like the letter V. The connectors can best 596 Ventilated Commutator— Ventilation of Machinery be explained by reference to the figs. A flat strip of copper is split down the middle and bent open into the form of a V, as shown in fig. 1, this connector being secured to the conductor-bars by methods similar to Fig. 3.— Han View of Conductor Bars and the V End- connections joining them those shown in fig. 2. Fig. 3 shows diar grammatically four consecutive conductor- bars connected by V end-connectors. The reason for the special shape is to allow the adjacent connec- tors to nest into one another at the points of cros- sing, as shown in fig. 4. The pre- cise form given above is not very often used, as the adjacent connec- tors would not clear each other unless a little strip ,/'«• *-t"^*«'' "J^^-'b *^^ ^J -^ the V End-connections are packed IS punched out together instead of just malting a slit as above described. The more usual method is to bend a bar of copper to the shape shown in the last diagram of fig. 1. See End-connections; Dead Wire. Ventilated Commutator. See Com- mutator. Ventilated Motor. See Motor, Ven- tilated; Motor, Pipe-ventilated. Ventilating' Distance Piece. See Distance Piece; Duct, Ventilating; Spacers; Spacing Fingers. Ventilating- Duct. See Duct, Venti- lating; Distance Piece; Spacers; Spac- ing Fingers. Ventilation, Forced. See Ventilation OF Electrical Machinery. Ventilation of Armature. See Ven- tilation OF Electrical Machinery; Duct, Ventilating; Distance Piece; Spacers; Spacing Fingers. Ventilation of Commutator. See Ventilation of Electrical Machinery; Commutator. Ventilation of Electrical Maehinepy. — The conversion of energy in the dynamo- electric machine involves, as do all systems for converting energy, a certain waste, which appears in the form of heat in the armature core, the armature conductors, the field spool, and, sometimes, in the pole cores. In order that the temperature of the various parts shall not rise to an excessive amount and so deteriorate, if not destroy, the insulation, ample provision should be made for the cir- culation of air through the machine. Ventilation of Armature. — In design- ing armatures, air ducts are provided. These are parallel with the shaft and lead to spaces left between the armature core plates. In these spaces, fan vanes are sometimes placed, so that when the armature revolves, air is drawn in along the ducts and blown out at the periphery. The air, in its passage through the armature, draws away heat both from the core and from the conductors, so keeping down the temperature. The armature conductors on the end-bends should be arranged apart from one another, so that air may circulate freely. If neces- sary, the field coils should also be ventilated. In the case of totally-enclosed machines where all the heat must be dissipated from the out- side of the casing, it is also important to allow for ventilating passages throughout the interior of the machine, in order that no part shall become unduly hot, and that an efficient circulation of air may carry the heat rapidly to the sides of the machine. See Distance Piece; Duct, Ventilating; Spacers; Spacing Fingers. Ventilation of Commutator. — In some cases the armature air-ducts are continued through the commutator, which is made hollow so that the air on its way to the armature cools the commutator bush and the inner surface of the bars. See Commu- tator. Ventilation of Magnets. — It is becom- ing customary to provide field magnets with air spaces through which the current of air from the armature can pass. For the same purpose a space is left between the magnet- core and coil, and when the winding is deep, the coil is wound in two or more parts with air spaces between. See Skeletonised Con- structions. Forced Ventilation. — In some classes Verband Deutscher Electrotechniker — Viscosity 597 of work, such as railway work, motors of considerable power have to pack into very small spaces and have to be totally enclosed to exclude dirt and wet. Such motors can- not be self-ventilating, so they have to be cooled by air currents generated by fans or blowers placed at a distance and driven by independent motors, the air being led through suitable pipes. See Motor, Pipe -venti- lated. Altogether, it cannot be too strongly pointed out that the ventilation of electrical machinery is an important matter, and that efficient ventilation materially increases the output obtainable from a given weight of machine. [c. w. H.] Verband Deutscher Elektrotech- niker (abbreviation V.D.E.), a body of Ger- man Electrical Engineers, a body equivalent to the Institution of Electrical Engineers (I.E.E.) in Britain. Eules and regulations for many branches of electrical engineering work in Germany are issued by and under the control of the V.D.E. (Kef. Eules of the Society of German Electrical Engineers, Journ.I.E.E., vol. xli (1908), p. 166.) See Engineering, Electrical. Verein Deutscher Ing-enieure (ab- breviation V.D.I.), a body of German en- gineers more or less comparable to the Institution of Civil Engineers (Inst.C.E.) in Britain. See Engineering, Civil. Vernier Calliper. See Wire Gauge. Vernon - Harcourt Pentane. See Standard of Light. Vibrating Contact. See Contact, Vi- brating; Trembler, Coil; Bell, Elec- tric. Vibration, Sympathetic. See Reson- ance; Oscillation; Natural Period op Oscillation of a System. Vibration Period, the interval of time between the moment when the moving quantity is at a certain point of its course and the moment at which it is next at the same point and moving in the same direction ; the time of a complete cycle. See Period; Period of Oscillation; Sine Curves. Vibration Tachometer, a speed indi- cator on a direct vibratory reed principle. The tachometer transmits vibrations to the reeds which will be synchronous with the speed of machine. A sufficient number of reeds are provided to indicate speeds ex- tending over a considerable range in the neighbourhood of the normal speed. See VOL. II Frequency Indicator; Tachometer; Tachometer, Liquid. Vibrator, Electromagnetic, usually applied to a mechanism like that of the ordi- nary trembling electric bell. The essential parts are: (1) an electromagnet; (2) an iron armature mounted on a spring; (3) contacts carried by the armature and forming part of the circuit of the electromagnet. A cur- rent passing through 1 energises it and at- tracts 2, causing the circuit to be broken at 3; the magnet therefore loses its power, and the spring carries the armature back, closing the contacts and thus recommencing the cycle. See Bell, Electric; Contact, Vi- brating; Trembler Coil. Vignoles Rail, headed rail similar rrZ^A^^i^^Ti^TTA Vignoles Hail a flat - bottomed bull- in section to the fig. This type is exclu- sively used in America for main -line track rails. The flanges rest directly on transverse sleepers, being secured thereto by fang-bolts or by spikes. The usual weight varies from 85 to somewhat over 100 lb per lineal yard. Many tube railways employ the Vignoles section for running rails, and it is frequently used for the conductor rail on electric railways. The modern tramway rail is a devolopment from a grooved Vignoles section. The rail is named from its inventor, but is also termed the T-rail. See also Eail. Violle Standard of Light. See Stan- dard OF Light. Virtual Amperes denotes the root mean square (rms) or 'effective' value of an ac, i.e. the number of amp of cc which, passed through the same resistance, would produce the same heating. See Current, Average; Eoot-Mean-Square. Virtual Volts, denotes the root mean square (rms) or 'effective' value of an ac voltage, i.e. the cc voltage, which, applied to the same resistance, would produce the same heating. See EooT- Mean -Square; Effective Voltage. Viscosity. — The viscosity of a liquid may be defined as an attribute resulting from the friction between the particles of the liquid when the different parts are moving with different velocities. See Oil, Transformer; Damping; Dead-beat. 39 598 Viscous Hysteresis — Voltage Viscous HysteFesis.— The tevmhysteresis (which see) is generally used to signify the lag of the magnetising eflfect behind the magnetising force, but there is a different lagging effect which takes time to develop. Thus when the magnetising force H is suddenly applied and maintained constant, the fiux density B will at once assume a definite value, which will, however, gradually increase if left undisturbed. An appreciable change is noticeable after one or two minutes, and in some materials the value of B may increase to 50 per cent in excess of the initial value. The effect is analogous to " creep " in con- denser charging, and to the mechanical effect on a viscous body; hence its name, viscous hysteresis. Owing to the time element it is not so marked in circuits undergoing rapid cycles of changes as in those where steady magnetising forces are applied. See Charge, Eesidual; Flux, Eemanent; Coeecivity; Retentivity. Visible Radiation. See Eadiation. Vitreous ElectFicity. See Electricity. Vogfel Arc Lamp. See Lamp, Arc. Volt, the unit of emf (which see) and of pdj 10^ cgs units. It is defined as the emf required to send a steady current of 1 amp against a resistance of 1 ohm. As the amp can be determined in absolute units with a much higher degree of accuracy than the ohm (which see), the accuracy of the deter- mination of the volt depends chiefly on that of the ohm. Accordingly different volts have been used corresponding to the several pro- posed values of the ohm. Thus the B. A. wit, the legal volt, and the international volt correspond respectively to the B.A. ohm, the legal ohm, and the international ohm. The international volt is now legally recognised in most civilized countries. [i". w. c] Volt, B.A., abbreviation for British As- sociation volt. See Volt. Volt, International. See Volt. Volt, Legal. See Volt. Volt -ampere denotes the product of volts and amp, as distinguished from the true watts carried by an ac. It has become usual to rate alternators and transformers in kilovolt amperes (kva), this giving the load in current which the apparatus is capable of carrying, irrespective of pf. The letters va are used as an abbreviation for volt-amperes. See Apparent WATTS ; Kilo- volt-ampere ; Power Factor. Volt Box. See Potentiometer Ratio Resistance; Varley Slide. Volt-coulomb, an infrequently-used term which denotes a unit of electrical work equal to the watt-second, and equal to 10' ergs or cgs units of work. Volt Indicator. See Voltmeter. Volta, Law of. See Law of Volta. Voltage, difference of potential in volts. See Potential Difference; Volt; Pres- sure; Electromotive Force. Voltage, Alternator, the voltage (».«. the pressure or the pd) at the terminals of an ac generator. Voltage, Applied. See Electromotive Force, Impressed. Voltage, Automatic Regulation of. See Automatic Regulation of Voltage. Voltage, Average. — In alternating elec- tricity the pressure (i.e. the ' voltage ') varies from zero to a maximum in one direction, then through zero again, and to a maximum in the other direction, and again to zero. This constitutes one cycle, and it is repeated many times per sec. If there are 50 such complete cycles per sec, the periodicity of the alternating electricity is said to be 50 cycles per sec. If the pressure -variations follow a sine law, then the maximum pres- sure is equal to 1-57 times the average pres- sure. See Current, Average; Root-Mean- Square ; Effective Voltage. ' Voltage, Disruptive. See Disruptive Voltage. Voltage, Effective. See Eoot-Mean- Square. Voltage, Impedance, a term applied chiefly in connection with alternators, mo- tors, and transformers. It denotes the volt- age which is absorbed by the passage of the load- current through the reactance and re- sistance of the winding or windings. For instance, the impedance voltage of a trans- former is that voltage required to drive full- load current through the primary when the secondary is short-circuited. See Imped- ance. Voltage, Internal. See Internal Voltage. Voltage, Normal, as applied to a ma- chine or a supply system, denotes the volt- age {i.e. the pressure) at which either is designed to work. Voltage, Primary. See Primary Emf. Voltage, Rated. See Rated Voltage OR Pressure. Voltage — Voltmeter 599 Voltage, Running, the voltage at the terminals of a motor when running at its appropriate speed, as distinguished from the modified voltage frequently applied for start- ing purposes. See Starting of Motors; Auto-starter. Voltage, Terminal. See Terminal Voltage. Voltage Preventer. See Excess-volt- age Preventer. Voltage Ratio, the ratio between the primary and secondary voltages of a trans- former. At no load, this ratio is exactly the same as that between the numbers of pri- mary and secondary turns. See Transfor- mation Ratio. Voltage Regulator, a device for regu- lating the voltage of an electric circuit. See Eegulator, Potential; Automatic Eegu- LATION OF the VOLTAGEj REGULATION. Voltage Transformer. See Trans- former, Instrument. Voltaic Cell. See Cell, Voltaic; Cell, Standard; Battery, Primary. Voltaic Electricity. See Electricity. Voltaic or Galvanic Battery. See Cell, Voltaic; Battery, Primary; Cell, Standard. Voltaic Pile, an arrangement of small couples in series, using a porous material to contain the electrolyte. In the original pile of Volta, disks of copper and zinc were used, between which were placed disks of flannel soaked in dilute sulphuric acid. The three kinds of disk are arranged in rotation thus: zinc, flannel, copper, zinc, flannel, and so on. The pile should terminate at one end with a zinc disk, and at the other with a copper disk. The emf of any pile will then equal the emf of one couple multiplied by the number of couples. See Contact Elec- tricity; Contact Series; Electrolysis; Zamboni's Dry Pile. Voltalac Insulating Varnish, the trade name for a line of black elastic-plastic insu- lating varnishes, which, the manufacturers claim, have good heat-conducting and radiat- ing properties combined with high dielectric strength. See Heat-dissipating Impreg- nating Materials; Elastic Insulating Varnishes; Insulating Varnishes; Im- pregnating Varnishes. Voltameter. — The amount of chemical action produced by passing a current through an electrolyte is proportional to the quantity of electricity passed. An instrument for measuring quantities of electricity in this way is termed a voltameter. Gas Voltameter. — In this instrument slightly acidulated water is employed for the electrolyte. The water is decomposed into hydrogen and oxygen by passing a current between two platinum electrodes immersed in the water. The gases may be collected in one tube, and the total bulk may be measured, in which case the apparatus is called a mixed gas voltameter, or they may be collected in separate tubes. Silver Voltameter. — This voltameter has been selected as the means of determina- tion of the legal standard amp. The elec- trolyte is a solution of silver nitrate, and the silver is deposited on a platinum plate form- ing part of an accurate balance for weighing the deposit. See Ampere. Copper Voltameter. — This usually con- sists of two copper plates dipping into a solution of copper sulphate. The deposit is measured by weighing the cathode before and after the current has been passed. Mercury Voltameter. — This form pos- sesses the advantage that the deposit may be measured by volume. A suitable com- bination is that of a mercury anode, platinum cathode, and a mercurous nitrate solution. See Meter, Electrolytic; Electrolysis. [l. m.] Voltametric Law. See Law, Volta- METRic; Electrolysis. Volta's Fundamental Experiment.— Volta demonstrated by means of a gold-leaf condensing electroscope that zinc when in contact with copper is positively electrified. A bar of copper, the end of which is soldered to a bar of zinc, is held in the hand by means of the zinc, and the copper is brought into contact with the lower plate in the electro- scope. At the same time the upper plate is momentarily touched with the other hand. On removing the rod, and then the upper plate of the electroscope, a divergence of the leaves is obtained indicating a charge of nega- tive electricity. See Voltaic Pile; Elec- trolysis; Zamboni's Dry Pile. Voltax, the trade name for an impreg- nating compound, which, the manufacturers claim, remains permanently plastic. See In- sulating Varnishes; Impregnating Var- nishes; Elastic Insulating Varnish. Voltmeter, an instrument for the direct measurement of pressure, and usually gradu- ated so as to read direct in volts. With the 600 Voltmeter — V-rings for Commutator exception of the electrostatic pattern (see below), voltmeters may be described as low- current ammeters of high and constant re- sistance. In order that the voltmeter resist- ance may remain constant, independently of temperature, the copper working-coil is con- nected in series with a high swamping resist- ance (or swamp) having a negligibly-small temperature coefficient. Voltmeters fall, naturally, into the same groups, and have much the same characteristics, as ammeters (which see), that is — 1. Moving-iron. 2. Permanent-magnet mov- ing-coil. 3. Hot-wire, e.g. the Cardew volt- meter (see Voltmeter, Cardew). 4. Induc- tion. There remains 5, the electrostatic volt- meter, in which the pressure is measured by the electrostatic attraction between fixed and moving plates, the latter being either pivoted or suspended. In electrostatic voltmeters the working forces are small, particularly in It instruments, and to increase them, the con- struction known as the multicellular is used, in which a number of fixed plates, say 10, act on the corresponding number of movable vanes attached to a common spindle. Volt- meters graduated in milli- volts are spoken of as millirvoltmeters. When used to measure the drop of volts over a fairly low resistance they are known as drop voltmeters, or drop irir dicators. Battery voltmeters, often called cell testers, are usually of the permanent-magnet moving-coil type, and are graduated to three volts on either side of zero. In order to make connection with the lead bars of the cells, hand spikes or battery spears are used. Pilot Voltmeter, a voltmeter connected by pilot wires with the distant end of a line, in order that the pressure at that point may be read at the central station. Compensated Voltmeter, a voltmeter arranged to indicate the pressure, not at Compensated Voltmeter its own terminals, but at the end of a line or feeder of known resistance (R). Such an instrument, which may be of any of the usual types, is arranged as shown in the fig. with two opposing windings, one carry- ing a current proportional to the voltage, and the other the main current, or a current proportional to it, if a transformer or shunt is used. The ratio of the turns in each winding can be so arranged that the volt- meter indicates V — I R, i.e. the pressure at the farther end of the line. The fig. shows diagrammatically a moving-coil compensated voltmeter, ij and ig being the current and pressure windings respectively, and s a shunt carrying the main current. See Electrometer; Electroscope; Battery Gauge. [k. e.] Voltmeter, Cardew, a hot-wire instru- ment devised by the late Major Cardew for the measurement of 1 pr. It consists essen- tially of a long platinum-silver wire, which is fixed at the ends and passes over pulleys four times up and down a long tube which is part of the instrument case. The top pul- ley, near the centre of the wire, is movable, and to its frame is attached a wire which passes round a grooved pulley and is kept taut by a spring. The grooved pulley oper- ates the pointer -spindle through a pair of light gear-wheels, magnifying the motion five times. The ends of the wire are brought out to two terminals on the case, and when connected across the pressure to be measured, the current passing through the wire heats it, and causes it to expand, the variation of length of the wire being proportional to the voltage across its ends. The instrument has a proportional scale. The instrument was the first type of hot- wire meter. It is cumbersome, and is now superseded by hot-wire instruments of the ' sag ' type. See 'Hot-Wire Ammeter' under Ammeter; 'Hot- Wire Wattmeter' under Wattmeter; 'Irwin Oscillograph' under Oscillograph. Voltmeter, Galvanometer as. See Galvanometer. Voltmeter. Potentiometer. See Po- tentiometer. Voltmeter, Recording-. See Instru- ment, Recording. Voltmeter, Synchronising. See Syn- chroniser. Volts Lost. See Fall of Potential. Volume Resistivity. See Resistivity; also Resistance, Specific. V-ring Commutator. See Commu- tator. V-rings for Commutator. See Rings, Commutator. Vulcabeston — Wall Socket and Wall Plug 601 Vulcabeston, the trade name of a moulded composition which is manufactured in several qualities. The best qualities are of a hard, tough nature, though inclined to be brittle. Insulating parts made of vulca- beston take a good polish and retain their appearance well. They are moisture-proof, and withstand warm mineral oil without dis- integration. See also 'Moulded Insulators' under Insulator. [h. d. s.] Vulcanisation. See Vulcanise. Vulcanise. — Rubber is said to be vul- canised when, as the result of subjecting it to a certain process, it is brought into an elastic but non-plastic condition, in which it deteriorates less rapidly and is fairly un- affected by moderate temperatures. The process by which this is accomplished con- sists in mechanically mixing dry rubber with a suitable proportion of sulphur, and main- taining the mixture at some 120° to 140° C. for a considerable time. The rubber is then said to be vulcanised, and the process is termed vulcanisation. See Cable, Rubber; Cable, Underground. [h. d. s.] Vulcanised Bitumen Cables. See Cable, Underground. Vulcanised Fibre. See Fibre. Vulcanised Rubber. See Vulcanise; Cable, Rubber. Vulcanite, a term applied to a black, hard substance produced by vulcanising a mixture of indiarubber with from 20 to 30 per cent of sulphur. It takes a high polish, and is useful as an insulator in electrical work. [h. d. s.] w W, the preferable abbreviation for watt (which see). W, the chemical symbol for wolfram, more usually known as tungsten (which see). Wagon, Emergency. See Emergency Wagon. Wagon, Tower. See Tower Wagon. Walker Commutator.— Miles Walker, in B.P. No. 5371 of 1906, describes a type Walker Commutator of shrink -ring commutator in which the brush-bearing surfaces are vertical. The con- struction will be understood from the fig. This commutator is a type of side-bearing commutator. Walker Fan-Brake Dynamometer. See Dynamometer, Absorption. Wall - entrance Insulator. See In- sulator. WaU Plug. See Wall Socket. Wall Regulator for Series Are-light Dynamo, a regulator suitable for mount- ing on a wall. It may comprise a number of carbon blocks pressed together more or less tightly by an electromagnet so that their aggregate resistance, which is connected as a shunt to the series field of the generator, varies automatically, and so adjusts the volt- age of the dynamo to suit the number of arc lamps in series. This type of regulator has been used with the Brush design of arc -light dynamo. See also Automatic Regulation of Voltage; ' Entz Booster' under Booster, Re- versible; Regulator, Poten- tial; Regulation; Potential Regulation. Wall Socket and Wall Plug, an arrangement which is attached to the wall of a build- ing with the object of supplying a ready means of connecting any pair of flexible leads to the source of supply. The general nature of the appa- ratus is shown in the accompanying fig. Two terminals, in connection with the mains in the building, are mounted on a suitable insulating base, usually composed of porce- lain. These are protected by a cover of the same material, which contains two apertures through which two pins can penetrate. These two pins are mounted in a suitable insulating holder, constituting the plug, and 602 Wallis-Jones Automatic Earth-leakage Cut-out are attached to the two flexible wires which it is desired to put in connection with the source of supply. The flexible mains re- ferred to, pass through an aperture in the Wall Socket and Plug insulating head to which the pins are at- tached. Thus when the plug is inserted in the socket, connection is made, but no live conducting surface is left exposed. Wallis - Jones Automatic Earth - leakag^e Cut-out, an instrument which so protects a cc circuit that the circuit is broken whenever a leak occurs from either main to •S ^O, JJ-B CgW Wallis-JoneB Earth-Leakage Cut-Ont earth, and so that the circuit cannot be permanently re-established until the leak has been removed. The instrument and its connections may be explained by the aid of the accompanying diagram, in which Tj and Tj represent the points of connection from the mains, and T3 and T4 the points of connection to the circuit to be protected. S„ Sj, and S3 will preferably be ordinary tumbler switches, but they are diagrammatically represented as plain bar switches, their fixed contacts being diagram- matically represented by dotted circles. When the three switches Sj, Sj, and S3 are closed, the current passes from T^to T3 through the small resistance Kj, through circuit L to T4, and back through the resistance R2 to Tg, In shunt with Rj and Kg are the two moving coils Cj and G^ working in the magnetic field of the magnets NS, NS, and rigidly fixed on one spindle, which is broken electrically by an ebonite block E. The points of connection to the shunts are adjusted so that when the same current passes out through one and back through the other, the effect on the two coils is equal and opposite, and there is thus no movement. Should, however, any minute portion of the current through R^ leak to earth instead of returning via Rj, the balance is disturbed, Cj becomes stronger than C^, the system is deflected, and a contact is made by the arm A at B, no matter in which direction the coils deflect. The system is similarly de- flected for a leak on the other lead. In the diagram these contacts are shown at right angles to their normal plane. As soon as the contact is made, the electro- magnet M is energised, the arm of Sj is re- leased and the spring at once pulls it off its contact, at the same time breaking Sj. The positions of the blades when the switches are open are shown dotted. | The only means the user has of closing the circuit is by putting on S3 by the handle H, which is outside the locked box. The first efiect of putting on S3 is to break its circuit; it then by means of the slotted bar P begins to pull on Sg and Sj, which can thus be closed again, and held closed by the trigger as before. The circuit, is, however, still broken till Sg is pushed back. Then if the leak is still on, the slot in p allows Sj and Sg to at once open as before. It is therefore impossible to keep the circuit closed while the leak exists. The working condition of the instrument can be tested at any time by switching a lamp on in the circuit and depressing one of the keys k„ Kg. This short-circuits Rj or Rg, throws the coils out of balance, and the switch opens. The contact -arm is enclosed in an inner dust-tight case, and it will be noted that it makes contact only; the break occurs at the switches, thus avoiding any sparking. Since the two coils work in the two gaps of one and the same field, changes in the strength of the magnets have no effect. Ward-Leonard System — Watkin Switch 603 The apparatus is enclosed in a locked metallic box, and the only part to which the user has access is the handle H, and, if desired, the testing keys % Kg. See Circuit Beeaker; Leakage Indicator. Ward -Leonard System, a method of controlling cc motors combining the qualities of high starting torque, economy of energy, and smoothness of acceleration. The genera- tor supplying the motor or motors is used for no other purpose. In starting, the volt- age of the generator is gradually raised from zero to the full value by varying its excitation, and the motors connected with it are thus brought up to full speed without the use of series resistances. The fields of the dynamo and motor are separately excited. The system has been applied to elevators, printing machinery, revolving turrets, sp locomotives, &c. This system has the property of permitting of the return of energy to the line during braking. The regenerative feature is widely employed in various special processes to which the system has been applied, and presents points closely analogous to those comprised in some of the systems of re- generative control which have been developed with special reference to systems of electric traction. See Eegenerative Control Systems; Mining Equipment, Electrical. Washburn and Moan Wire Gaugre. See Wire Gauge. Waste Magnetic Field. See Magnetic Leakage; Flux, Leakage; Dispersion, Magnetic; Leakage, Slot; Inductance, Slot. Water, Electrolysis of. See Electro- lysis. Water - brake Dynamometer. — This dynamometer is very suitable for testing electric motors. It was, however, originally invented by Froude for the testing of marine engines for the Admiralty, and has since been modified by Eeynolds. It consists essentially of a specially - shaped turbine- wheel which can be keyed to the shaft of the machine under test. The bowls or hemispheres of this wheel are fitted with inclined vanes raked forward in the direction of rotation. The wheel rotates in an outer case supported on the axis of rotation and which is itself free to rotate. This case is provided with vanes corresponding to those on the wheel, but raked back in the opposite direction. Water is admitted along the axis of rotation and passes out from the case, and the power is adjusted by controlling the passage of this water at its exit. The torque is measured by bringing the case to its original position by means of a weight sup- ported from a lever-arm attached to the case. The brake is specially adapted to the ab- sorption of large powers, as the difficulty of heating is almost entirely eliminated. Some trouble which was experienced in the original design in controlling the power absorbed, has been overcome in the more recent modi- fications. (Eef. Froude, Proc.I.M.E., p. 237, 1877; Hopkinson and W. Froude, Proc.I.C.E., vol. xcvi.) See Dynamometer, Absorption. Water-column Lightning' Arrester. See Lightning Arrester. Water Consumption of Electric Gene- rating Set. See Central Station for THE Generation of Electricity. Water -cooling for Stator Cores, a plan for carrying off the heat generated by eddy currents and hysteresis in the stator core of an induction motor. It was proposed some years ago and has recently been revived by a German firm. The plan consists of building up the armature laminations against a continuous iron surface in the housing, at the back of which cold water is circulated and so cools the core, in the same manner in which the water in the jacket of a gas engine cools the gas-engine cylinder. See Motor, Pipe-ventilated ; Ventilation of Elec- trical Machinery. [h. w. t.] Water - dropping Lightning Arres- ter. See Lightning Arrester. Water Heat. See Steam. Water- jet Aerial in Wireless Tele- graphy. See Aerial, Water-jet, in Wireless Telegraphy. Water-jet Lightning Arrester. See Lightning Arrester. Water Power for Driving Electric Generating Stations. See Central Sta- tion FOR THE Generation of Electricity; Mining Equipment, Electrical; Hydro- electric Generating Set; Accumulator, Hydraulic. Waterproofing Compound. See under Insulating Compound. Water Wheel, Pelton Type, for Hy- droelectric Generating Station. See Mining Equipment, Electrical. Watkin Switch, a combined switch and controller for continuous or alternating cir- cuits consuming comparatively small amounts 604 Watson Electricity Meter — Wattmeter of energy, such as some circuits for lighting, cooking, and accessories. It is particularly useful in lighting, to control the illumination, several stops being provided by which the light is diminished according to the value of the resistance inserted in series. The whole switch for lighting circuits is enclosed in a case consisting partly of the usual tumbler-switch cover. A knob-control oper- ates a lever moving over the contact studs inside. It is claimed from tests that the energy saved (depending on the stop em- ployed) is 12 to 80 per cent of the energy consumed when ' full on '. Series rheostatic control is necessarily a very uneconomical method, and the waste of energy on the last stop is a large percentage of the total energy then consumed by the circuit. For small- power circuits it is often legitimate to incur such a loss in order to obtain the convenience of control. The switch is especially useful for lighting and fans in hospitals and theatres. See Transformer, Adapter. Watson Prepayment Electpicity Meter. See Meter, Prepayment. Watt (preferable abbreviation w) denotes the unit of power in the practical, or volt- ampere, system of units. The w is equal to 10^ cgs units of work per sec, i.e to 10^ ergs per sec. Watt Component denotes the component of an ac which is in phase with the impressed voltage. The current in an inductive circuit may be considered as comprising two com- ponents — one in phase with the voltage and called the watt or power component, and one in quadrature with the voltage and called the idle or wattless component. The watt component is equal to the true w divided by the voltage. See also True Watts; Angle OF Lag; Current, Component; Power Factor; Current, Wattless; Magnetis- ing Current. Watt-hour (preferable abbreviation w hr), a unit of energy. 1 w hr represents the amount of work done by an electric current which for 1 hr gives out power at the rate of 1 w. As an instance of the use of this unit, it may be noted that in electric traction the energy taken to propel a train is often reckoned in 'watt-hours per ton kilometer' (or per ton mile). The w hr may also be defined as the quan- tity of electricity which is transformed into heat in a wire of 1 ohm resistance when a current of 1 amp flows through it for 1 hr. The w hr is equal to one -thousandth of a kw hr. Since 1 kelvin is equal to 1 kw hr, 1 millikelvin is equal to 1 w hr. The me- chanical equivalent of 1 w hr is 367 kg m, i.e. it is the amount of energy required to lift 1 kg through 367 m. Iwhris the amount of energy absorbed by a kg of water in raising its temperature from 0° C. to 1° C. 1 w hr is equal to 3600 joules, or 3600 X 10' ergs. See Energy; Kelvin; Joule; Erg; Kilogram-meter ; Electricity. Watt-hour Meter. See Meter, Energy. Watt -hours per Train Kilometer, the ratio of the total amount of energy ex- pressed in w hr that is required to move a train a certain distance, to the distance tra- versed in km; a quantity used in electric traction for purposes of estimation and com- parison. Similarly, watt -hours per car kilo- meter. If the result is further divided by the weight of the train or car in tons, the expression becomes watt -hours per ton kilo- meter; and if the distance is measured in miles, watt-hours per ton mile. Watt-minute, a unit of energy equal to 60 w sec. See Watt-Second. Watt- second, a term which denotes the energy given out in 1 sec by an electric cur- rent which does work at the rate of 1 w. This amount of energy is equal to 1 joule or 10' ergs. The w sec is, strictly speaking, the unit of electric work or energy in the practical system. The w sec is s.soo.ooo ^h of a kelvin or kw hr. See Unit of Work; Energy; Joule; Erg; Kelvin; Kilogram- meter; Electricity. Wattless Component denotes that com- ponent of an ac which is in quadrature (either leading or lagging) with the impressed volt- age. See Current, Wattless; Magnetis- ing Current; Power Factor; Current, Component. Wattless Current. See Current, Wattless; Wattless Component. Wattless Load. — This term (which, strictly speaking, is self - contradictory) is used to denote the wattless component of an apparent load whose pf is less than unity. Wattless Volt-amperes. See Watt- less Load. Wattmeter, an instrument for the mea- surement of electrical power. Four types are available, the first two being the most used :— 1. Dynamometer Pattern, consisting of a fixed coil, carrying the main current (or a Wattmeter 605 current proportional to it), which acts on a moving coil, connected in series with a non- inductive resistance across the mains. This pattern is further described under Watt- meter Dynamometer, Zero Type. See also Dynamometer, Siemens; Kelvin Balance. 2. Induction or Ferraris Type, con- sisting of two electromagnets acting on a common copper or aluminium disk or drum. The winding on one of the magnets carries the main current (or one proportional to it), and the other magnet is wound with fine wire and connected across the mains. The eddy currents induced in the disk or drum by the one magnet react on the flux due to the other, and, if proper precautions are taken as regards phase displacement, a torque is produced proportional to the true w. 3. Hot-wire Pattern, consisting of two hot wires of which the difference in expan- sion is measured by the pointer. One wire carries a current proportional to V -f- 1 (where V and I represent the instantaneous value of volts and amp respectively), while the other carries a current proportional to V— I. Since the expansion of each wire is propor- tional to the square of the current flowing through it, and since the pointer measures the difference, the deflection will be propor- tional to (V + 1)2 - (V - 1)2 = 4 V I, i.e. to the true w. Irwin's hot-wire wattmeter is de- scribed at p. 617 of vol. xxxix of Journ.I.E.E. See Voltmeter, Cardew. 4. Electrostatic Pattern (see also under Voltmeter). — The connections are shown diagrammatically in fig. 1, where A is the moving vane, while B and C are fixed plates acting on it, and S is a shunt carrying the main current and connected to B and C. The torque exerted on A will be proportional to (Va X V6) - (Vcs X Vc) = Va (V6 - Vc) = Va X I = watts, where Va, V6, and Vc represents the poten- tials of the respective vanes. In order that the above wattmeters may indicate the true w on an ac system, irre- spective of the pf, i.e. in order that they may indicate the value of VI cos <^ (where is the angle of lag or lead), it is essential that the currents in the various circuits, or rather the effects which they produce, should be in phase with the main current and pres- sure respectively. The method of ensuring this, varies with the type of instrument. The dynamometer pattern is by far the most universally applicable and accurate, being in- dependent of frequency and wave-form, and can be used for cc or ac indiscriminately, as Fig. 1.— Diagrammatic Bepresentation of an Electrostatic Wattmeter can also the hot-wire and electrostatic pat- terns. The electrostatic pattern forms a satis- factory laboratory instrument, but, owing to CO. Continuous Electricity or Single-phase Alternating Electricity CC. _rgpggggpi__ CC Unbalanced Two-phase Electricity (if balanced, one wattmeter can be omitted) v.c CC. Balanced Three-phase Electricity (Power = reading x 3) CO. CC. Unbalanced Three-phase Electricity (Power = sum of readings) Fig. 2.— Connections for Power Measurements en Various Systems the small pd available at the shunt terminals, it is lacking in power and has to be read by 606 Wattmeter — Wave means of a mirror and scale. The induction arrangement is chiefly used for switchboard wattmeters, where the frequency is constant, since it is considerably affected thereby, as also by change of temperature. The con- nections for power measurements on the vari- ous systems, assuming a dynamometer or induction instrument, will be gathered from fig. 2, where C.C. represents the current coil and V.C. the volt or pressure coil, in each case. It will be observed that two watt- meters are sufficient for a three-wire, three- phase load however much out of balance, since the current in the third wire is, at every instant, equal to the sum of the other two. The two movements are often attached to the same spindle, so that the pointer in- dicates the total power. The arrows in each case indicate the direction in which the power is flowing. In the cases illustrated, the power consumed by the pressure circuits of the watt- meters is included in the measurement, but it is, as a rule, negligibly small. Compensated Wattmeters are sometimes employed in which a number of turns equal to that of the coil CO., but wound in the opposite direction, are connected in series with V.C. and act upon it, so as to eliminate the efiect of the pressure - circuit current flowing in C.C. See also Power, Methods of Measur- ing, IN Polyphase Circuits; Meter; Dynamometer; Dynamometer, Siemens; Instrument, Eecording. [k. e.] Wattmeter, DeJQection, a wattmeter in which the deflection is a function of the power to be measured, as distinguished from a zero type of dynamometer wattmeter, where the power is measured by the amount by which the head of the instrument must be turned to bring the deflection to zero. See Watt- meter; Dynamometer, Siemens; Watt- meter Dynamometer, Zero-type. Wattmeter, Idle-current. See Indi- cator, Phase or Power-factor. Wattmeter, Integrating:. See Meter, Energy. Wattmeter, Recording. See Instru- ment, Eecording. Wattmeter Dynamometer, Zero- type, an instrument for measuring the power in a circuit by the torque produced between two coils set at right angles, one carrying the current and the other a small current proportional to the pd. The chief types are: — 1. The Siemens (see Dynamometer, Sie- mens). 2. The Ayrton-Mather-Duddell. — In this instrument the fixed current coils are subdivided so that currents varying in the ratio of 10 to 1 may be used to produce the full deflection. The moving potential- coil is rendered astatic by reversing half its turns. Damping is provided by means, of a mica vane moving in an air box, and the torque is balanced against a spring as in the Siemens type. See also Kelvin Balance; Wattmeter. [l. m.] Wattmeter Method of Testing- Iron and Steel. — This method constitutes one of the best ways of measuring iron losses under working conditions. A closed core is built up of stampings, and is provided with a single winding of such a number of turns that, when working at the required flux-density, it will induce a counter volt- age about equal to the available voltage of the ac supply. The winding is then coupled to the mains through the series coil of a. wattmeter whose shunt terminals are across those of the magnetising coil of the iron under test, or, still better, across a second coil wound on the iron. The wattmeter- reading shows the hysteresis and eddy- current loss in the iron. To separate these losses, the test should be carried out at different frequencies, but with the same flux-density, and the values of the quotient, J , should be plotted against the frequency frequency. The resulting line, projected back to the ordinate of zero frequency, gives the hysteresis loss in joules {i.e. watt- seconds) per cycle. See also Testing Sheet Iron and Sheet Steel; Iron and Steel Testing. [d. k. m.] Watts, Apparent. See Apparent Watts; Volt-ampere; Kilovolt- am- pere. Watts, True. See True Watts. Watts per Square Inch or per Square Decimeter. See Cooling Sur- face. Wave, Electric, a moving system of electric forces. An electric wave may either travel freely through the ether like a light wave, in which case it moves in a straight line if the medium be homogeneous and isotropic {i.e. the same everywhere and in every direction); or it may be guided by a, conductor. In the latter case it will follow Wave Detector — Way 607 the conductor, though the surface is not plane. On account of the inductance of a bent surface it will, however, tend to pass along the surface in such directions as are most nearly straight, rather than over curved portions. It has thus been found by wire- less telegraphists that where the waves have to cross a mountain range they are most strongly received over the lowest point of the range. Their progress is much affected by the resistance and dielectric constant of the conducting surface of the earth. If the surface be a perfect conductor, there will be no dissipation of energy; and the same is true if the entire medium be a perfect in- sulator. In actual cases of transmission over sea and land, great diflFerences are found owing to the dififerences in conduc- tivity and dielectric constant. The latter is of as much importance as the former. The greater ease with which waves may be sent over sea is mainly due to the very high di- electric constant (80) of water, as compared with an average of only 5 for land. Waves are guided along, or reflected from, a surface of high dielectric constant just as they are by a conductor. The velocity of propagation of a wave along a plane or straight wire is approximately the same as that of a free wave, i.e. as that of light, viz. 186,000 miles per sec, or 3 x IQi" cm per sec. Hertz showed experimentally that such waves can be reflected and refracted in exactly the same way as the shorter wave which we know as light. See Wireless Telegeaphy; Wireless Telephony. [j. e-m.] Wave Detector. See Detector. Wave Distortion, divergence of wave shape from the sine curve. Variation of wave shape from normal. The emf wave of an alternator may be varied or distorted according to the nature of the load. See Wave Form; Sine Wave; Harmonic; Sine Curves; Sine Law. Wave Form, the curve representing the time-change of a variant such as emf, current, magnetic flux, &c. Time is usually plotted horizontally, and the values of emf, current, &c., as the case may be, vertically. Distorted Wave Form denotes an ac wave or alternating emf wave whose shape is distorted, due to the properties of the cir- cuit. For instance, the effect of hysteresis in an iron core introduced into a coil is to dis- tort the current wave by adding harmonics, so that the ascending and descending parts of the half-wave become dissymmetrical in shape, as shown in fig. 1. See also Wave Distortion. Pig. 1.— Distorted Wave Form Peaked Wave Form denotes an ac or alternating emf wave form which has a large maximum compared with its efiiective value, and which rises quickly to a high peak (fig. 2). Fig. 2.— Pe^ed Wave Form Such waves have, in general, a high form factor. See Form Factor. Sine Wave Form a term applied to the wave form of an ac quantity which follows a sine law of variation with time, ie. whose Fig. 3.— Sine Wave Form value at any instant is proportional to the instantaneous projection of a vector which makes one revolution per cycle of the quantity considered (see fig. 3). See Sine Curves; Sine Law. Wave Shape. See Wave Form. Wave Winding. See Winding, Wave. Way, a switch is sometimes said to have a certain number of ways instead of number 608 Wearing Depth of Commutator — Welded Rail Joint of throws (see Switch Types). Thus dt switches (see Switch Types, Designation of) are often termed two-way switches. The iiiiiii^ . ""' J«" If lllllll-L .J .-J IIIUU ^f-^f w III ; , llfMlMMI '"If lit - i;f - 'ir ,„ - 1 ■ m ' ^^ ■ - 1 iiiiiiiii iiiiiiiiiij ■->■-' ^'"IL 1 Br """- ' Fig. 1.— Twelve-way Distribution Board expression is usually restricted to the case of small switches. The word way is also employed in stat- ing the number of branches from the main connections in distributing boards. Thus a twelve-way distribution board is illustrated in fig. 1 (this is a double-pole board, i.e. there is a fuse in each side of the circuit). Conduit Ways, tubing fixed in concrete or other flooring at the time the floor is being laid down. Fig. 2 is an illus- tration of a con- duit way. Wearingf Depth of Commutator. See Commutator, Wearing Depth of. Webber Winding" Form, an improved winding form devised by Webber to facilitate the preparation of armature coils of the Eicke- meyer type. This type of form is illustrated under Winding, Forming, and Spreading Machinery. See Coil, Form-wound. Web Bond. See Bond. Weber, a distinguished German physicist who did much to further the use of absolute units in electricity. A name proposed for the practical unit of magnetic flux, being 10^ cgs units; not generally accepted or used. See Maxwell; Megaline; Line of Induction, [f. w. a] Mg. 2.— Conduit Way Weber Electro - dynamometer. See Dynamometer, Weber Electro-. Weber Rail Joint. See Continuous Eail Joint. Weber's Absolute Unit of Resistance. See Eesistance, Weber's Absolute Unit OF. Wedge, Slot. See Slot Wedge. Wehnelt Break. See Coil, Induction. Wehnelt Interrupter. See Interrup- ter, Wehnelt; Coil, Induction; Recti- fier. Weight Coefficient of Electric Motor, the ratio of the total weight of a motor to its output reduced to a standard speed. See Output Coefficient; Utilisation Coef- ficients, Specific. Welded Rail Bond. See Bond. Welded Rail Joint, a joint in which the parts are united by actual fusion or by compression whilst in a plastic state, the rails thus becoming practically continuous. There are three methods of welding joints. In Electrically - WELDED Joints, the rails are laid in position, and steel bars of suitable shape are clamped one on either side of the joint. An ac of great intensity (up to 25,000 amp) at 5 to 7 volts is then passed through the joint from side to side, quickly bringing the bars and the web of the rail to welding temperature, when the parts are squeezed together with a hydraulic press. This process is performed at the middle and at each end of the bars, after which the surface of the rails is ground true. The Falk Cast Weld is made by apply- ing a mould to the rail ends and pouring in molten cast iron from a ladle, thus casting a massive sleeve round the joint, as shown in the fig., which represents a section through a joint. The pro- cess ensures both mechan- ical strength and high elec- trical conductance. The iron is melted in a port- able cupola, about 100 lb of metal being required for each joint. In Thermit Welding the intense affinity of aluminium for oxygen is utilised. Finely- powdered aluminium and iron oxide are placed in a crucible, with a pinch of aluminium and barium peroxide on the top, and a mould is fitted about the ends of the rails. The igniting compound is then kindled with a match, and the reaction proceeds throughout Talk Cast Weld Welding — Whole-coiled Winding 609 the mass, resulting in a charge of molten iron covered by a thick layer of liquid slag. The crucible is tapped, allowing the intensely- heated iron to flow into the mould, followed by the slag, which covers the top of the joint, thus ensuring uniform heating. The complete outfit is shown on the Plate facing p. 506. Although the metal round the joint weighs only 8 to 10 lb, a thoroughly sound me- chanical and electrical connection is effected. See BoNDj Bonding Eail; Continuous Eail JoiNTj Renewable Plate for Rail Joints; Welding, Electric, [a. h. a.] Welding", Autogenous, a method of welding in which the two parts to be welded are brought to the required temperature by heating in the flame of the oxy-hydrogen blowpipe. See also 'Benardos System' under Welding, Electric. Welding, Eleetrie. — Thomson - Hous- ton System. — The Thomson-Houston system of electric welding is based on the fact that when a current is passed through a resistance, heat is generated proportional to the resist- ance and to the square of the current. Thus if the ends of two metallic bars be placed together and a current passed through them, greater heat will be generated at the point of contact than in the body of the bars, because at the point of contact the resistance is greater. If the current be increased to a sufficient amount, the metal at the ends of the bars will be melted and will run together, forming a perfect joint. Electric Brazing. — If when the ends are brought together, we cover them with a suitable flux and powdered brass, the current required will be merely that which will generate sufficient heat to melt the brass and cause it to alloy itself with the surface of the bars, so forming a brazed joint. Electric Soldering. — In place of the powdered brass we may use solder, so obtain- ing a soldered joint. As solder melts at a low temperature, the current required to make a soldered joint is less than would be required for a brazed or welded one. Benardos System. — The Benardos system of welding is chiefly used for tubes and sheets. In this system the articles to be welded are connected to the positive supply main, an arc-lamp carbon on a flexible lead being joined to the negative main. After the edges of the articles have been brought to- gether and covered with a strip of the same metal, the operator holds the carbon in his hand and applies it for a moment to the metal, whereupon an arc is formed. He then draws the arc along the edges, causing them and the strip to melt together, the process being very similar to lead-burning with the oxyhydrogen flame. By this means steel sheets are joined together to form complete sheets of large size, from which baths, cis- terns, ship's boats, &c., can be stamped in one piece. See Welding, Autogenous; Welded Eail Joint; 'Welded Rail Bonds' under Bond. [c. w. h.] Westing-house Unit-switch System. See Multiple-unit System of Train Control. Weston Ammeter. See Ammeter. Weston Cadmium Standard Cell. See Cell, Standard. Weston Standard Cell. See Cell, Standard. Wetherill Process of Ore Separation. See Separation of Ores. Wetness Factor. See Steam. Wet Steam. See Steam. Wheatstone's Bridge. See Bridges. Wheatstone's Rheostat. See Rheo- stats OR Resistances. Wheel Base, the distance between the centres of the axles mounted in a four-wheel truck; practically identical with the distance between the points of contact with the rail of the two wheels on one side, but this varies slightly, especially in the case of a radial truck. See Truck. Wheel-type Trolley. See Trolley. Wheless Surface - contact System. See Surface-contact System. Whip, Electric, used for lift and hoist purposes. A whip serves to wind a rope on a cylindrical drum (or barrel) rotated about a horizontal axis. The drum is driven through suitable gearing by an electric motor, and control is effected either by electric con- trol of the motor, or by operating a friction clutch interposed between the motor-gear and the drum. See Crane, Electric; Lift, Electric; 'Friction Clutch' under Clutch. White Fibre, a term sometimes applied to white vulcanised fibre. See Fibre. White Light. See Light, White. Whitman Photometer Head. See Photometer Head. Whitworth Wire Gauge. See Wire Gauge. Whole -coiled Winding. See Wind- ing, Whole-coiled. 610 W hr — Winding Wide-open Slots W hP, the preferable abbreviation for watt-hour (which see). Wide-open Slots (see %).— The slots in cc machinery are invariably open at the face. In ac machines the open slot is less fre- quently used, it be- ing necessary only when the coils are completely insulated before being placed in the machine. See Slot; Partly-closed Slots; Totally- closed Slots; Inductance, Slot. Wild Photometer Head. See Photo- meter Head. Wimshurst Machine. See Machine, Wimshurst; Electrostatic Induction Machine. Windage Losses. See Losses, Wind- age, in Dynamo-electric Machinery. Winder. See Winding, Forming, and Spreading Machinery. Winder, Armature. See Wind iNG, Forming, and Spreading Ma- chinery. Winding", an organised system of insulated conductors suitably arranged with reference to the magnetic circuit of an electrical machine. The two chief types of winding are exciting windings (field coils) (see Excitation), and wind- ings in which emf is induced by the flux. These latter may be stationary {e.g. the secondaries of static transfor- mers) or movable {e.g. the windings on the armatures of dynamos and motors). The subject of armature windings is exten- sive, and cannot be more than indicated in the following definitions. (Eef. 'Armature Windings', Parshall and Hobart; 'Armature Construction', Hobart and Ellis; 'Armature Windings ', Cramp). [t. s.] Winding, Alternating -current Ar- mature, the windings of the armature of a generator in which alternating "^mf are gene- rated and ac flow. Winding, Armature, a system of con- ductors arranged on the armature core, and interconnected in such a manner as to give the required pressure. Cc armature windings may be either mul- tiple-circuit windings (see Winding, Mul- tiple-circuit), or two-circuit windings (see Winding, Wave; Winding, Two-circuit), the latter requiring only 2/No. of Poles times as many conductors as the former for the same voltage. Either of these types of winding can be simplex or multiplex windings (see Mul- tiplicity OF Continuous-current Wind- ing), and if the latter they can be singly or multiply re-entrant (see Ee-entrancy). Ac armature windings are either sp or polyphase. They may be spiral windings (see Winding, Spiral), wave windings (see Winding, Wave), or lap windings (see Wind- ing, Multiple -circuit). Spiral windings are either half -coiled or whole -coiled (see Winding, Half-coiled; Winding, Whole- coiled), and they are wound in one, two, or three ranges (see RANGES OF Armature Windings). [t. s.] Winding, Auxiliary Starting.— Sp in- duction motors (without commutators, and with a single stator winding) have no start- ing torque, but by providing an auxiliary £ D F 1 1 Auxiliary Starting Winding starting winding on the stator, that is, a winding which is in circuit during starting, to increase the torque, and which is cut out of circuit during normal running, and by causing the current in this winding to be out of phase with the current in the main winding, a starting torque can be produced. The usual practice, as shown in the dia- gram, is to wind the stator as for three-phase Y, connect the outer end of one phase C to the junction D of a resistance F and induct- ance e placed in series across the sp mains during starting, and connect the other phases A and B in series, and directly across the mains. This arrangement gives an approxi- mation to a revolving field as obtained in polyphase working, and is sufficient to start the motor unloaded. For normal running, phase c is left idle, the resistance and in- Winding 611 ductance being cut out of circuit by the switch G. Various other starting schemes follow the same principle of splitting the sp supply to an auxiliary winding, and producing during starting a type of polyphase system. See Single -PHASE Motor; Phase - splitting Device; Starting of Motors. Winding", Bar, a winding the conductors of which are of such large section as to neces- sitate making it up of separate bars, the ends of which are sometimes coupled together in pairs by separate V end-connections (which see). An alternative plan, which is often employed for windings the conductors of which are of relatively smaller cross section, is to take single bars of copper, bend them to the shape they will assume in the finished armature, and insulate them completely be- fore putting them in the armature slot. Such windings usually consist of but few conduc- tors per slot, and their name distinguishes them from the other common type of arma- ture windings consisting of numerous single wires per slot, which are often threaded into micanite tubes, and have their ends insulated after they have been put in place on the machine. See ' Bar-wound Armature ' under Armature; V End-connections. Winding", Bar and Cable. See Wind- ing, Cable and Bar. Winding, Barrel, a method of arranging the ends of armature coils as they pass from one pole to the next, in which, instead of using involute or butterfly connections, V-shaped end- connections are used which lie on a cylindrical surface, which is a continuation of the arma- ture surface. The coil ends must of necessity be arranged in two layers, but the method may be used for either one or two coils per slot, the difference in arrangement for these Pig. 1.— Single-layer Barrel Winding Hg. 2.— Double-layer Barrel Winding two cases being indicated in figs. 1 and 2. This type of winding was introduced by C. E. L. Brown about 1892. (Ref. 'Dynamo- electric Machinery', Thompson, vol. i., p. 448.) In a single-layer barrel wimding (see fig. 1), B ^ ^ each slot is occupied by but one side of one coil. A double-layer barrel winding is illustrated in fig. 2. In this winding the opposite sides of two separate coils occupy space in the same slot. The coils, on emerging from the slot, bend in opposite directions, and if one side of a coil occupies the bottom portion of a slot, its other side usually occupies the top portion of a slot distant from the first slot by the polar pitch. Winding", Basket, more often known as the chain type of winding (see Winding, Chain), in which the coils belonging to the three separate phases are laid out in two ranges, the centre of one coil or set of coils being occupied by the side or sides of the adjacent coil or coils on opposite sides. Winding, Bipolar, a winding of an arma- ture which is designed to give the desired volt- age when running in a magnetic field set up by two field-magnet poles. Winding, Cable and Bar, a two-layer winding in which the bottom conductor in the slot is made of solid copper bar, and the upper one of pressed strand. This construc- tion tends to minimise the eddy-current losses in the armature wind- ings, since, owing to the reactions of the flux con- stituting the slot leak- age, these losses tend to occur to a greater degree in those conductors at the upper part of the slot. See Inductance, Slot; Cable, Pressed Stranded. Winding, Chain. —The system of ar- ranging a three-phase winding on two ranges results in what is known as a chain winding. The adjacent coils link one another as in a chain, the centre of one set of coils being occupied by the sides of coils of the other phases. It is sometimes also termed basket winding (see Winding, Basket). This wind- ing is similar to a skew-coil winding (see Winding, Skew-coil) See also Ranges of Armature Windings. Winding, Chord, an armature winding in which the end-connections are so coupled that demagnetising turns do not exist, or are reduced to a less amount than result from the ordinary symmetrical winding. The name chord has been applied to such windings because the end-connections appear Cable and Bar Winding A, Cable. B, Bar. 612 Winding on the winding diagram as unequal chords of a circle. See Winding, Fractional-pitch. Winding, Compensating. See Com- pensating Winding; Coil, Compensating; Single-phase Motor. Winding, Compound, a winding (usu- ally for setting up a flux in a magnetic circuit) comprising two parts. One part is called the shv/at and takes the full voltage Compound Winding of the supply {i.e. it shunts part of the sup- ply). The other part is, in cc apparatus, in series with the 'load', and the magnetism resulting from it varies with the load. See Compound - wound ; Excitation ; ' Com- pound-wound Generator ' under Generator. Winding, Concentrated, a winding in which the conductors are concentrated in but a very few slots per pole. See fig. 1 accompanying the definition of Winding, Distributed. Winding, ConcentFated Field, a field- magnet winding (see Winding, Field-mag- net) in which all the turns are wound on one iron core, as distinguished from windings which are distributed amongst two or more slots, and the turns of which are therefore separated by iron projections or teeth, as in the usual types of compensating windings on sp commutator motors. See SiNGLE-PHASE Motor. Winding, Continuous - cuprent Ar- mature. — 1. Windings which are employed on the armatures of cc machines. 2. These windings are of a characteristic distributed type; and when, as frequently occurs, windings of this general type are employed in ac apparatus, such apparatus is said to have a cc type of winding. Winding, Cumulative Compound, a compound winding in which the two parts are so connected that they assist each other in producing a magnetic field, as distinguished from differential windings. See Excitation; Winding, Differential; Lamp, Arc; Com- pound-wound Motor; Compound-wound Dynamo; Compound-wound. Winding, Differential, a winding com- posed of two portions whose mmf are in opposite directions. The resultant mmf is thus equal to their difference, and the wind- ing is termed a differential vmding. Such windings are often required in automatic switchgear, in measuring instruments, in motors, boosters, balancers, and other special machinery. See Lamp, Arc; Compound- wound Motor; Winding, Cumulative Compound. W//M//^ff/X V//y/y//y//M Kg. 1.— Concentrated Winding Winding, Distributed, a winding spread uniformly over the whole of the periphery of i^^^^Si^ Z'y///yyyMy,//. J I '//yy/yyjyy,y,//^ Fig. 2.— Thoroughly Distribnted Winding Winding 613 a machine. In the earlier days the entire surface of the armature was often covered with wires, and these windings represented the extreme case of distributed windings. In most modern machines the windings are embedded in slots, and it is only in rare instances that much more than half i of the entire surface is occupied by windings, the remainder being occupied by the heads of the teeth. In such machines, the winding is said to be more completely distributed the greater the number of slots amongst which Y/y^//y//f//yy/, J I '/y///////y//y. J I '/y////////,^,/. Hg. 3.— Semi-diBtrlbuted Winding the conductors are divided. The accom- panying figs, represent cases which may be described as concentrated winding (fig. 1), thormghly distributed winding (fig. 2), and semi'distributed winding (fig. 3). These wind- ing diagrams have been reproduced from Hobart and Ellis's 'Armature Construction' by the kind permission of the publishers. Winding, Distribution of. See Wind- ing, Distributed; Distribution of Wind- ings. Winding, Double-layer. See Wind- ing, Barrel. Winding, Duplex. — The term is some- times applied to a winding for a magnet, in which bare wire is used, the adjacent turns being separated from one another by a cord of silk or other suitable material, wound between the wires. Sheets of paper are em- ployed between the layers. In the fig. the conductors are denoted by A, the separating cords by b and the insulation between the layers by c. For another sense, in which the term duplex winding is used, see MuLXl- Suplex Winding plicity of Continuous - current Wind- ing and Ee-ENTRANCY. Winding, Edge-, for Field Spools. See Edge-winding for Field Spools. Winding, Electric, Ilgner System of. See under Mining Equipment, Elec- trical. Winding, Embedded Length. See Embedded Length of Winding. Winding, Exciting. See Winding; Excitation; Winding, Field-magnet. Winding, Fed-in, a type of winding pos- sible with slots, open or only partially closed, in which coils previously formed into shape are introduced, only a few conductors at a time if necessary, into the slot from the top, the slot being provided with a lining of horn fibre or other suitable material, which is finally closed over and secured in place by means of a wedge, or by some other suitable means. The coil -ends are afterwards insu- lated with tape and varnish. See Partly- closed Slots. Winding, Field-magnet, that winding in a motor or generator which carries the so-called exciting current (which is cc), pro- ducing a magnetic flux in one direction only, which flux constitutes the magnetic 'field' of the motor or generator. Winding, Forming, and Spreading Machinery, machinery employed in the manufacture of coils for electrical apparatus. The terms are more usually applied to ma- chinery employed in the preparation of coils for armatures of dynamo-electric machinery. Sometimes the coil is wound strictly on the former, which may be defined as a full-size model or shape of a part of an electric ma- chine arranged for convenient preparation on it of a coil of insulated conductor which, when completed, is to be put in place on the machine. A former devised by Webber for a coil for the armature of a 25-hp tramway motor is shown in fig. 1, which also shows the coil after it has been taken from the former and insulated ready for insertion in 40 614 Winding the armature. Various designs of formers are described in chap, xi of Hobart and Ellis's ' Armature Construction '. In other eases the coil is wound on a suit- able piece of apparatus and is afterwards Fig. 1.— Webber Winding Form transferred to a former, by means of which it is given the required shape. The first piece of apparatus could then be termed a winder, but this is unusual, and the term mnder customarily denotes the operator. Thus an armature mnder is an artisan who winds armatures. Armature winding is an operation in which long experience is re- quired in order to attain any considerable degree of proficiency. Fig. 2.— Turner's Combined Winder and Spreader Combined Winder and Spreader, a tool designed to be attached to a coil-wind- ing lathe, and which can be used to spread the coil after it has been wound in the form of a flat loop upon it. Fig. 2 illustrates a combined winder and spreader, devised by H. W. Turner, which is attached to the face- plate of a winding lathe. Spreading Armature Coils. — One method of winding armature coils is to wind the wire in a flat loop, and then obtain the final shape by spreading. The part of each side of the loop which is to lie in the slot is gripped between flat surfaces, Fig. 8.— Spreading Armature Coils and the two sides are then pulled apart, producing a coil of the well-known 'dia- mond' shape. This method only produces entirely satisfactory results in the case of small wire-wound armatures. A method of spreading armature coils is shown in fig. 3. Winding, Fractional-pitch, an arma- ture winding in which the span of the coil, i.e. the winding pitch, is smaller than the pole-pitch of the machine. A full-pitch loindmg is the ordinary type of winding, in which any one coil spans a distance equal to the pole-pitch. Hence windings where the span is less than the pole-pitch are designated fractional-pitch wind- ings. These windings are also described as short-chord windings. One of the chief advan- tages relates to the shorter length of the end connections, which efiects a saving in weight of copper, armature resistance, and overall length of the armature. See also Winding, Chord. Winding, Free Length of. See Em- bedded Length of Winding. Winding, Half-coiled, an ac winding in which there is only one coil per phase per pair of poles; only half of the poles being subtended by coils. The illustration shows Winding 615 diagrammatically the difference between half- coiled and whole-coiled windings. Fig. 1 re- presents a six-pole winding with four con- ductors per pole connected as a half-coiled winding, and fig. 2 the same conductors connected as a whole-coiled winding. See rig. 1.— Half -coiled Winding Fig. 2.— Whole-coiled Winding Winding, Whole-coiled; Winding, Hemi- TROPic Armature. Winding*, Hand. See Hand Winding. Winding, Hemitropic Armature, an alternator winding having the coils in any one phase situated opposite alternate poles only. The most usual three-phase winding is of this type, as the three sets of coils are equispaced over a pair of poles. A hemi- tropic winding has only one coil in the slot, and has a slightly higher reactance than a winding in which two distinct coils are used in the same slot, one going forward and the other backward. The term hemitropic wind- ing has been widely employed, but the more recently suggested term half-coiled winding would appear preferable. See Winding, Half-coiled. Winding", Imbricated, a spiral-coil winding (see Spiral Coil) in which the end connections are built up one above the other, either in a radial or in a horizontal direction. The winding is used especially on the stators of turbo-dynamos and alterna- tors, and is sometimes used for the compen- sating coils of certain sp commutator motors. See figs. 24 and 25 of a paper by Miles Walker read before the LE.E. on March 10, 1910. Winding-, Lap. See Lap Winding. Winding, Leakag"e. See Parsons and Laws Leakage Winding. Winding-, Multiple Armature, a term which has been used to indicate windings of the lap type (see Lap Winding), as dis- tinguished from the wave or two -circuit windings. In this sense it is preferable to employ the expression multiple-circuit winding. See Winding, Multiple-cikcuit. The term has also been employed to desig- nate other than simplex windings, i.e. to designate duplex, triplex, &c., windings. It is, however, preferable that these should be termed multi- plex vmdings. See Winding, Multiplex. Winding-, Multiple-circuit, a cc armature winding in which there are as many conducting paths or circuits from negative to positive brushes as there are poles in the machine. It is sometimes called a lap winding (see Winding, Lap). It may be simplex, duplex, &c. (see Winding, Multiplex; Mul- tiplicity OF Continuous-current Wind- ing). But it is important to carefully discri- minate in employing the terms multiple-circuit wimding and multiplex winding, since a mul- tiple-circuit winding may be simplex or multiplex, and a multiplex winding may be of the multiple-circuit type or of the two- circuit type. Winding, Multiple - circuit Drum, a winding in slots distributed over the surface of a cylindrical or annular armature core, with as many circuits from negative to posi- tive brushes as there are poles in the field magnet. A typical multiple-circuit winding diagram is shown on p. 299. Winding, Multiplex.— The term multi- plex is applied to the group of windings individually designated as duplex, triplex, quadruplex, &c. Thus all windings that are not simplex windings are multiplex. Most windings in practical use are, however, sim- plex windings. These terms are very difficult to define ; their understanding will be facili- tated by consulting the definitions of Multi- plicity OF cc Winding and Ee-entrancy. Winding, Mummified.— When a wind- ing, after being covered with tape or other absorbent material, is saturated in an insu- lating compound and baked until the whole is solidified, it is said to be mummified. The treatment was formerly extensively employed in the manufacture of field-spool windings, and is still often appropriate in special cases. Nevertheless, the modern tendency is to go to the opposite extreme and employ skele- tonised constructions in the manufacture of 616 Winding field spools. See Skeletonised Construo TioNS; Ventilation of Electrical Ma- chinery. Winding, One -layer Armature, a winding in which all the active conduc- tors are laid side by side in a single layer, as distin- guished from the more usual arrangement in which the conductors occupy two cylindrical layers. See Winding, Barrel. Winding", Open - cir- cuit. See Open-circuit Windings. Winding-, Parsons and Laws Leakage. See Parsons and Laws Leakage Winding. Winding, Pin.— 1. A method of former winding in which the shape of the coils is set by wooden pins fixed in a shaped block. The wire is wound around the outside of the series of pins, which thus represent the various corners of the final winding in place on the armature. 2. The name sometimes given to a method of hand winding an armature, the slots first being filled with pins or dummies which are withdrawn, one at a time, to make place for the wires of the winding. See Winding, Threaded- iN; Hand Winding. Winding, Polyphase, a winding consisting of a number of sp windings arranged with 90 electrical degrees be- tween similar parts for a two-phase machine, 120 electrical degrees for three-phase, &c. Parts of the wind- ings of the different phases are said to be similar when they are situated in magnetic fields of like direction and intensity. Winding, Re-entrant, an arma- ture winding which, after once com- pleting the circuit for the desired volt- age, re-enters the magnetic field with a duplicate, triplicate, or more separate circuits in parallel. Each such circuit, of course, generates equal emf, and by paralleling at the brushes, the current output of one circuit is multi- plied by the number representing the degree of re-entrancy (which see) to get the output Doubly Be-entrant Winding of the armature. The fig. illustrates a doubly re-entrant lap winding (for six poles). For singly, doubly, quadruply, and sextuply re- entrant windings, see Ee-entrancy. [t. s.] Single-phaBe Rotary-converter Winding Winding, Rotary Converter, an ar- mature winding connected at one end of the Winding 617 active parts, to a commutator and at the other end (the magnetic circuit is between the ends) to slip rings having symmetrically- placed connections from each slip ring to suit- able points of the armature. The illustration shows a six -pole sp rotary-converter wind- ing. Winding, Sayers. See Sayers Wind- ing. Winding, Series.— 1. A winding which is coupled into a circuit so that the main current flows through it in series with the armature, e.g. the field winding of the stan- dard traction cc motor. See Motor, Series- wound; Bunching of Series Windings. 2. A term formerly widely employed to denote that type of armature winding which has also been termed a wave winding, but which is preferably termed a two -circuit winding. See Winding, Two -circuit; Winding, Wave. Winding, Sliunt, any winding which diverts a part of the current flowing through a circuit. It is commonly applied to the field winding of approximately constant-speed cc machines. A winding which shunts or by- passes a portion of the main supply. See Excitation. Winding-, Shunt Field, a field winding which is connected so that it shunts a part of the current away from the main circuit. See Excitation. Winding, Single - layer. See Wind- ing, Barrel. Winding, Single - phase, a winding carrying sp ac. Winding, Skew-eoil, an arrangement of the free parts of ac windings in which the ^ Ezzzz^Yp ^ Skew-coil Winding ends are shaped or skewed as shown in the fig. This is a design in which all the coils of one machine are of one pattern. See also Winding, Chain; Winding, Basket. Winding, Slot, a winding in which the conductors are distributed in slots in the armature surface. See Slot. Winding, Smooth - core, the winding on a surface -wound armature. See under Armature. Winding, Spiral.— This is a form of ac winding in which the coils constituting one group (i.e. one phase per pole in the case of whole-coiled windings, and one phase per pair of poles in the case of half-coiled wind- ings) assume the form of a spiral, as shown in fig. 2 illustrating Winding, Distributed; the inside coil occupying the two innermost slots of the group, the next coil the next two slots farther apart on the armature, and so on. Thus the fig. referred to above shows four component coils per phase per pole, which for the sake of clearness are repre- sented by lines only and not complete coils. This is the form of coil most widely used for the armature winding of ac machinery. Fig. 3 of the definition of Winding, Distributed, is also a spiral winding. See Spiral Coil. Winding, Spool, a coil of insulated con- ductor such as is used on field-magnets. It is thus named from its resemblance to the commercial spools of sewing cottons. Winding, Spread of. See Spread of Winding. Winding, Star Triphase, one whose dia- grammatic representation resembles a three- pointed star. It is also called Y connection. The three windings, one for each phase, start from the common connection or so-called neu- tral point, and advance symmetrically, being located so that each consecutive conductor of Phase 1 is one-third of a cycle behind the corresponding conductor of Phase 2, and two- thirds of a cycle behind that of Phase 3. A diagram of the winding is seen in fig. 3 under CONNECTIONS, Three-phase, and the representation is as in fig. 1 under that heading. Winding, Teaser. See Teaser; Mono- cyclic System. Winding, Threaded-in, a type of wind- ing adopted where closed or partially closed slots are used, with an opening not wide enough to lay in a single conductor. The conductor is pushed into the slot from one side of the machine to the other, being kept in place until its neighbouring conductors are wound, by the use of dummy sticks or rods, which are withdrawn one at a time un- til the coil is completed. For this reason it is sometimes called ^n winding. An efficient 618 Winding — Wire length of conductor is often taken to wind a set of two or three concentric coils belonging to one phase. The ends of the coils are after- wards insulated with tape and varnish. See Winding, Pin; Hand Winding. Winding", Three -coil Armatupe, of Alternator, an ac winding each element of which is divided into three parts, each part occupying a single pair of slots. Winding, Two - circuit, a winding of the type illustrated by the diagram on p. 399, and characterised by the property that the pitch (see Pitch, Winding) is always posi- tive. A two-circuit simplex cc winding has two paths through the armature, from the positive to the negative brushes, indepen- dently of the number of poles on the ma- chine. See Winding, Wave. Winding, Wave, an armature winding which roughly resembles in its diagram the section of waves. It gives two circuits be- tween + and — brushes whatever the num- ber of poles. (Compare Lap Winding.) The preferable term is two-circuit winding. Winding", Whole-coiled, an ac winding in which there is one coil per phase per pole; the whole (every one) of the poles being sub- tended by coils. See illustrations accom- panying Winding, Half-coiled. Winding" Bobbin. See Bobbin, Wind- ing. Winding" Coeflieient. See Winding Factor; Dispersion, Magnetic. Winding Diagram, a line drawing show- ing each separate insulated conductor and its connections, or else representing groups of conductors by single lines, so that the elec- tric circuit or circuits can be followed out and understood therefrom. For the purposes for which the diagram is required it is usu- ally desirable to represent the face conductors by radial lines. Sometimes, however, they are represented by points, and sometimes so-called developed armature winding diagrams are employed. These are diagrams in which, instead of representing the face conductors by radial lines, they are represented by paral- lel lines. Winding diagrams and sketches will be found on pp. 29, 108, 109, 126, 127, 298, 299, 302, 399, 418, 425, 596, 612, 613, 615, 616, and 617. Winding Factor or Coefficient, a re- ducing coefficient used in the alternator formula for the emf, to allow for the partial inactivity of the windings. It is a combina- tion of the pitch factor (which see) and the spread factor (which see). See also Kapp Coefficient; Dispersion, Magnetic. Winding Gear, Electric, for Main- and-tail-rope System of Haulag-e. See Mining Equipment, Electrical. Winding Gear for Electric Lifts. See Lift, Electric. Winding Pitch. See Pitch; Pitch, Winding. Winding Shuttle. See Shuttle, Wind- ing. Winding Space, the space available for conductors and insulation. Winding Space of Transformer, a term applied to the cross section of the space in- cluded within the magnetic circuit of a transformer, and which is available for the conductors which are linked with the magnetic circuit. In a core-type transformer the winding space is usually an elongated rectangle. In the shell type, a greater variety of cross sections are employed for the winding space. Instances of each of these types are shown in the two figs., in which the winding space is denoted by A. In the shell type the cross section of one side is taken as the cross section of the winding A Winding Space of a Core-type Trans- tonaei Winding Space ol a Sliell-type Transformer space. The apertures themselves are often designated the winding windows. See Space Factor. Winding Windows. See Winding Space. Windings, Current Distribution in. See Current Distribution. Windmill Type of Steel Tower. See under Transmission Line. Wiping Contact. See Contact, Wip- ing. Wire, Beacon, a trade name for a wire employed chiefly for resistances on account of its high specific resistance (85 microhms per cm cube at 0° C.) and its very low temper- Wire 619 ature coefficient. Its specific resistance at 100° C. is 91 microlims per cm cube. Its specific gravity is 8"1. See High-kesist- ANCE Alloys; Wire, Eesistance. Wire, Bimetallic, a wire having a dis- tinct coating of one metal on a base wire of another metal. An instance of such wire is a steel wire with an electrolytic deposit of copper, which is so closely adherent that the two metals behave as a homogeneous wire. It is chiefly used in telegraph and telephone work for stay -wires, braces, &c., but it is not so largely used as a conductor. The copper acts simply as a preservative for the steel. The breaking strength of a 400-lb wire is about 2160 lb. Another special use of a bimetallic wire is to combine the separate properties of two metals having respectively positive and nega- tive temperature coefficients. When the de- posit is of the correct thickness, the resulting conductor should have zero or negligible tem- perature coefficient over a wide range of tem- perature. (See WiKE, IaIa; Wire, Resist- ance; High-resistance Alloys.) Wipe, Binding. See Binding Bands. Wire, Braided, a magnet wire covered with a fine braiding of cotton or silk. It has been found possible to put on magnet wire a good close braiding of cotton with a total thickness no more than O'OIO in to 0'016 in, which is rather more than the thickness of double cotton covering. Braid- ing is not to be recommended, as it is in- ferior to ordinary covering alike in thinness, closeness, and mechanical endurance. Its chief advantage is a slightly decreased ten- dency to peel away if cut at one point, but this should not prove a serious difficulty with good spiral covering. See Wire, Mag- net; Wire, Cotton-covered; Wire, Silk- covered. Wire, Code, a term applied to a class of wire manufactured in the United States and presumed to conform with certain (rather lax) requirements of the Board of Fire Un- derwriters. Regarding such wire, Langan states, in a paper entitled 'Standardising Rubber-covered Wires and Cables', printed at p. 191 of vol. XXV (1906) of the Trans. A.I.E.E.:— 'Cables, some appear to think, are all alike. It they possess the common identity of copper, this establishes a relationship of equality, and labouring under this very general illusion, rubber-covered cables are called for and purchased with no other restriction as to quality, than that they shall conform to the National Code or the Board of, Pire Underwriters' rules. All this wire made subject to ' code ' rules, passes under the general but misleading name of ' rubber-covered ', when, as a matter of fact, there is not an ounce of rubber in its composition. It is com- posed of cheap ingredients, rubber substitutes, and the like, and bears about the same relation to fine rubber that a burnt cinder does to a lump of coal. Katurally, such insulation as this will not last long, since, having no rubber, it possesses no vitality, no dielectric strength, no capacity for work, and con- sequently becomes, in a short time, an easy prey to variations of temperature and climatic conditions. The fault here lies, not with the manufacturer, but with the rules, for the rules impose no provision as to quality. All they require is a certain diameter of insulation; but on the question of whether this di- ameter should be wholly or partly composed of mud or rubber, they are altogether silent. Of course, no one doubts the good intentions of the f ramers of these rules. But as indicating how wide of the mark they go in fulfilling any requirement of good insulation, let me quote what the rules say about testing the wires before installation. 'Each foot of the completed covering must show a dielectric strength sufficient to resist for five (5) minutes the application of 3000 volts per 1/64 of an inch thickness of insulation ' ' For example, they would require a conductor with 10/32 in wall of insulation to stand a test of 60,000 volts ; or a No. 14 wire which calls for 3/64 in wall, a test of 9000 volts. The absurdity of these tests must be evident to anyone having the slightest familiarity with the practical side of electrical en- gineering. From what source such data were de- rived is hard to tell ; for there is not the slightest warrant either in theory or in practice for it. Even the highest grade of fine Para would stand no such tests, much less the cheap, bituminous rubbish that enters virtually into every foot of what is termed ' code wire '.' See Cable, Rubber; Cable, Flexible; Cable, Association; Bitumen; Trinidad Bitumen; Rubber. Wire, Copper. — Hard-drawn copper wire is defined by the Engineering Standards Committee as that which will not elongate more than 1 per cent without fracture. Copper wire not complying with this de- finition is termed soft-dravm copper wire. Hard-drawn does not have quite so high a conductivity as soft -drawn copper wire. The difference in conductivity is, however, slight, whereas in various other physical properties the difference is very marked in- deed. See also Copper; Conductor; Con- ductors, Overhead; Hard-drawn Copper Wire; Enamelled Copper Wire. Price of Copper Wire. — The hasis price is the component of the total cost, which is dependent on the market price of copper. It is cost of material plus cost of rolling and 620 Wire preliminary drawing. The latter is gener- ally about £4 per ton. The extra price is that component of the total cost which covers drawing into wire (finer than the preliminary drawing), tinning, and winding on spools or drums. It varies considerably, depending on the size and finish of the wire. Thus it costs about £9 per ton to draw and to single-tin No. 14 S.W.Gr., while the same charge is made only to draw No. 25 S.W.Gr. See also Copper; Conductor; Conductor, Overhead. Wire, Cotton - covered.— This term is generally applied to wire spirally lapped with cotton spun yarn. Spun yarn may be distinguished from thread by its com- paratively loose twist, and by the fibres all being twisted in the same direction. The thickness of covering can be varied by the number of coverings or the diameter of the yarn. To obtain a good mechanical cover- ing which will withstand abrasion, two coverings are generally employed, lapped on in opposite directions. A process for spinning a cotton covering from cotton roving has recently been in- vented which produces a tougher and thin- ner covering than by lapping. (Eef. 'Conductors for Electrical Distribu- tion', Perrine; 'The Insulation of Electric Machines', Turner and Hobart; 'A New Process of Wire Covering ', Elec. Eev., Dec. 1, 1905.) See also Wire, Magnet; Cotton AS AN Insulator; Wire, Braided. Wire, Enamel -insulated, wire coated with one or more thin layers of enamel. In such wire the diameter over the insulation is but very slightly greater than the bare diameter. Consequently while with cotton- covered, and even with silk-covered wire, the space taken by the insulation is, for wires of very small diameter, commensur- able with, and (in extreme cases) even much greater than, the space occupied by the wire itself, this is not the case with enamel- covered wires. Until recently, enamel-in- sulated wires have not been reliable; but since 1907, developments in the manufac- ture of enamel-insulated wires have been rapid, and more than one manufacturer now has such material on the market. See Wire, Magnet; Enamelled Copper Wire. Wire, Eureka. See Eureka Wire; High-resistance Alloys; Wire, Eesist- ance. Wire, Figure -8. See Figuee-S Wire. Wire, Fish, a wire left in a conduit at the time of its erection, and subsequently serving to draw through the electric con- ductors. See Wiring Systems; Conduit, Interior. Wire, Grounded. See Grounded Wire. Wire, Guard. See Guard Wire. Wire, lala, a bimetallic wire having con- stituents of copper and nickel, a layer of one metal being electrolytically deposited on the other. The special object of this is to com- bine the positive and negative temperature coeflBcients of the constituents, and to aim at producing a material with a zero temperature coefficient for a wide range of temperature when the deposit is in the correct proportion to the basis metal. The wire is permanent, and not affected by moisture. See Wire, Bimetallic; Wire, Ebsistanoe; High- resistance Alloys. Wire, Idle. See Idle Wire. Wire, Magnet. — This name is applied to wire which is already insulated for use in winding coils for electrical machinery. The materials used for the covering are usually either cotton, silk, or enamel. Cotton is by far the most widely used, and is both cheap and satisfactory. It is applied in the form of unspun fibres wrapped closely round the wire. Where more than one covering is put on, the layers are alternately right- and left- handed. Double covering is the most com- monly used, and the radial thickness of tliis covering is generally somewhere between O'OOS and 0'014 in. Silk is used on small wires where space is valuable, as it is pos- sible to apply it in thinner layers than cot- ton. Double-silk covering is generally about one-half the thickness of double-cotton cover- ing, but is much more expensive. Coils wound with cotton or silk -covered wires should always be dried and treated with varnish before use, as neither of these mate- rials is in itself a first-class insulator. Enamelled Magnet Wire is more or less of a new departure. It is claimed that the difficulty of obtaining a thoroughly tough, flexible enamel has now been overcome. The thickness of the enamel is usually only one or two thousandths of an inch, but at least one firm now supplies a wide range of thick- nesses. Another new magnet wire is known as acetate wvre. In this the covering consists of cellulose tetracetate, for which peculiar ad- vantages are claimed in the way of flexibility, durability, and resistance to chemical action Wire 621 and heat. The space factors for enamelled wire and acetate wire are excellent. See Enamelled Copper Wire; Wire, Enamel- insulated. Aluminium Magnet Wire. — Bare alu- minium is always covered with a thin layer of oxide. This oxide is a sufficiently good insulator to permit of employing bare alu- minium wire for winding very low-voltage magnet coils. It is not suitable when the pressure between any two adjacent turns is ■at all high, consequently adjacent layers of the winding must be separated from one another by some suitable insulating ma- terial. See 'Insulation of Magnet Wire' under Insulation; Wire, Braided; Wire, Cot- ton-covered; Wire, Silk-covered; Wire, Enamel-insulated. Wire, Paraffined. See' Paraffined Wire. Wire, Phono -electric. See Phono- electric Wire. Wire, Pilot, a small auxiliary wire run- ning from a generating station or a substa- tion to the end of a main conductor, or to some particular point in a main conductor, to enable the voltage at that point to be measured at the station, or to allow of tests being made. Pilot wires are generally multi- core — each core being about 7/20 in size — and are as a rule insulated and protected in •a similar manner to the main cables in con- nection with which they are used. They are laid with or near the main cables. Pilot wires are sometimes used with light- ing or power mains to enable the voltage at "the distributing points to be read in the •station, and are necessary on tramway circuits in this country to allow of the voltage drop in the rail return being measured as required by the Board of Trade. Telephone wires are often used in conjunction with the pilot wires, the whole forming a multicore cable with any desired number of cores. Bare •overhead pilot wires are also sometimes used. ■{Ref. 'Central Station Electricity Supply', Gay and Yeaman; 'Modem Electric Prac- tice'.) [f. w.] Wire, Resistance, wire of materials of high specific resistance. Chiefly employed for rheostats (where iron is particularly use- ful) and in the construction of electrical instruments and apparatus, and especially for the resistance coils of bridges. In the following table are given the specific resist- ances at 15° C, and the specific gravities, of a number of these materials: — Specific Kesistance in Microhms per cm cube. Specific Gravity. Iron Platinoid Manganin lala ... Eureka Resista Krupp... Wolffin Beacon 11 41 42 50 50 77 85 86 86 7-8 8-9 8-9 8-6 8-9 8-2 8-1 8-2 8-1 Hecnum, a trade name for a resistance wire which is claimed to be non-corrosive. The specific resistance is 48 microhms per cm cube. The temperature coefficient is 0-00014 per degree Centigrade. The melting-point is stated to be over 1200° C. It is stated that the tensile strength before annealing is about 10 tons per sq cm. See High-re- sisTANCE Alloys; Constantan; German Silver; Nickelin; Phosphor Bronze; Platinoid; Manganin; Eureka Wire; Wire, Beacon; Steel. Wire, Sills -covered, a term applied to copper wire spirally lapped with ' floss ' silk, which may be distinguished from spun silk by its having little or no twist in the fibres. The use of 'floss' silk produces a covering, either for a single or double lapping, which is considerably thinner than it is possible to obtain with the finest cotton spun yarn, but is a more expensive covering and is only employed for fine wires. See 'Insulation of Magnet Wire' under Insulation; Silk AS an Insulator. (Ref. 'Conductors for Electrical Distribution', Perrine; 'The In- sulation of Electric Machines', Turner and Hobart.) Wire, Span, wire suspended between two overhead supports by which an electric trolley wire is in turn supported. Span wire is generally of about 7/16 SWG mild steel strand, and is fixed to poles or to rosettes attached to the walls of buildings. It is also generally insulated near the supports so that there may be double insulation between the trolley wire and earth. See also Suspen- sion, Trolley-wire. Wire, Square. — 'Square' magnet wire is not in reality quite square, as it has a small radius at the corners. It is, however, sufficiently rectangular to fill a given wind- 622 Wire — Wire Gauge ing space more efficiently than does round wire. In other words, the space factor is higher. It is therefore often used for the windings of both field and armature of elec- tric machines in preference to round wire. The chief eases where the use of square wire is inadvisable are: (1) small armatures where the square wire would be too fine to wind cheaply; (2) magnet coils of fine wire, for the same reason; (3) hand- winding of armatures. See Space Factor. Wire, Trolley. See Teglley Wire; FiGURE-S Wire; Phono-electric Wire. Wire, Underwriters', a wire covered with fireproof insulating material consisting of a braiding strongly impregnated with an inorganic paste. It is largely used for leads to and from apparatus, such as rheostats and resistance frames, which normally operate at a high temperature. Wire Gaugfe. — The simplest form of wire gauge is a thin steel plate provided with notches of difierent sizes. These notches are numbered, and the size of wire which fits any particular notch is known by the number of that notch. The sizes of the notches are quite arbitrary, and their num- bers are no indication of the actual sizes of the wires. These gauges were adopted in very early days, when no means existed for the measure- ment of the actual diameter of wires, and some means of grading was of course neces- sary. In these early days, owing to the difficulties of intercommunication, diiferent wire gauges were constructed in different countries, and even in different parts of the same country, so that a large number were constructed, of which some still survive. The principal survivors are the Birmingham Wire Gauge for iron and steel (BWG), the Birmingham Wire Gauge for gold and silver (BWGgj), the American or Brown and Sharpe (B & S) Gauge, the French Wire Gauge (FWG), the French Wire Gauge for gal- vanised wires (FWGg), the Washburn and Moen (W&M), Stubs Gauge (SG), and the British Standard Wire Gauge (SWG), some- times also called the Legal Standard Wire GoMge (LSWG). Some of these gauges were constructed subsequent to the invention of simple means for measuring the diameter of wires, and their continued existence is there- fore quite indefensible. The only really com- mon-sense gauge, and therefore naturally the only one which has received practically no recognition, was that proposed by Sir J. Whitworth, in which the numbers of the notches indicated the diameter of the wires in thousandths of an inch. Wire Tables. — In order to lessen the con- fusion caused by all these different gauges, the diameters of the wires have been mea- sured and tables have been prepared giving their actual sizes, weights, strengths, re- sistances, &c. Dealing with size only, the following are the diameters of a No. 13 wire in the diiferent gauges: BWG = 0'095 in, BWGg, = 0-036 in, B & S = 0-072 in, FWG = 0-0752 in = 1-91 mm, FWGg = 0-07874 in = 2-0 mm, SWG = 0-092 in, Whitworth = 0013 in, W & M = 00915 in, and SG = 0-182 in. Micrometer. — Over twenty-five years ago a simple instrument, called a micrometer, was Micrometer put on the market. With this instrument the diameters of wires can readily be mea' sured to less than the thousandth of an inch. The chief mechanical principle embodied in the construction of this instrument, of which an illustration is herewith given, is that of a screw free to move in a fixed nut. An open- ing, to receive the work to be measured, is afibrded by the backward movement of the screw, and the size of the opening is indi- cated by the graduations. The pitch of the screw, c, is forty to the inch. The graduation of the hub, a, in a line parallel to the axis of the screw, is forty to the inch and is figured 0, 1, 2, &c., every fourth division. As the graduation conforms to the pitch of the screw, each division equals the longitudinal distance traversed by the screw in one complete rotation, and shows that the calliper has been opened one-fortieth (or 0-025) in. The position of the head b is capable of slight adjustment. The bevelled edge of the thimble, D, is gra- duated into twenty-five parts and is figured every fifth division, 0, 5, 10, 15, 20. Each division, when passing the line of graduations on the hub, indicates that the screw has Wire Gauge — Wireless Telegraphy 623 made one twenty-fifth of a turn and that the opening of the calliper has been increased one twenty-fifth of one-fortieth, or one-thousandth of an inch. Hence, to read the calliper, the number of divisions visible on the scale of the hub must be multiplied by twenty-five, and the number of divisions on the scale of the thimble from zero to the line coincident with the line of graduations on the hub must be added. Vernier. — The Vernier is an older device than the micrometer, and its manipulation Vernier is more difficult than the latter. It is, how- ever, occasionally used for measuring the diameters of wires, and a description and illustration are therefore given herewith. On the bar of the instrument is a line of inches numbered 0, 1, 2, &c., each inch being divided into ten parts and each tenth into four parts, making forty divisions to the inch. On the sliding jaw is a line of division (called a Vernier, from the inventor's name) of twenty-five parts, numbered 0, 5, 10, 15, 20, 25. The twenty-five parts on the Vernier correspond, in extreme length, with twenty- four parts or twenty-four fortieths of the bar, consequently each division on the Vernier is smaller than each division on the bar by one-thousandth part of an inch. If the slid- ing jaw of the calliper is pushed up to the other jaw, so that the line marked on the Vernier corresponds with that marked on the bar, then the two next lines to the right will differ from each other by one-thousandth of an inch, and so the difference will continue to increase, one-thousandth of an inch for each division, till they again correspond at the line marked 25 on the Vernier. To read the amount by which the calliper is open, one must first notice how many inches, tenths, and parts of tenths, the zero point on the Vernier has been moved from the zero point on the bar. Then one must count upon the Vernier the number of divisions, until one is found which coincides with one on the bar, which will be the number of thousandths to be added to the distance read off on the bar. The best way of expressing the value of the divisions on the bar, is to call the tenths one hundred thousandths (O'lOO) and the fourths of tenths, or fortieths, twenty-five thousandths (0'025). Eef erring to the illus- tration it will be seen that the jaw is open two-tenths and three-quarters, which is equal to two hundred and seventy-five thousandths (0-275). Now suppose the Vernier to be moved to the right so that the tenth division coincides with the next one on the scale. This will make ten thousandths (O'OIO) more to be added to two hundred and seventy-five thousandths (0"275), making the jaws open two hundred and eighty -five thousandths (0-285). The majority of dynamo and motor makers have largely discarded the use of wire gauges, and specify the sizes of wires by their actual diameters. [c. W. H.] Wire-g-auze Brush. See under Brush. Wire Tables. See under Wire Gauge. Wireless Telegrraphy, op Radiotele- graphy, a method of communicating over long distances by electrical means without the use of a wire between the stations. In all the long-distance systems at present in use the method of transmission over the earth or sea is the same. As in wire tele- graphy the electric lines of force are guided hy the wire, so in wireless telegraphy they are guided by the earth, or rather by its sur- face, since ac such as are used do not pene- trate far into a conductor. The earliest types of wireless telegraph were of a different char- acter from modern methods. In the former there was a complete circuit, and intermittent currents of continuous electricity were used. The principle was that of a shunted galvano- meter, the earth being the shunt. The trans- mitter was a line-wire earthed at both ends, in which were placed a battery and key. The receiver was a parallel line-wire also earthed at both ends, in which was a gal- vanometer or other current indicator. For details of these systems see Fahie's ' History of Wireless Telegraphy'. Modern methods are entirely different in principle. There is no closed circuit, hence a current of con- tinuous electricity cannot be used, since it cannot be maintained for any appreciable time in a circuit, part of which is dielectric 624 Wireless Telegraphy and not conducting. The current used is therefore alternating, and radiates over the Earth's surface in waves travelling outwards in concentric hemispheres from the trans- mitting station. The lines of electric force are thus vertical, or approximately so, and travel at right angles to their length. Hertzian Telegraphy, in which the radiation is free, and therefore in straight lines, al- though it constitutes the origin of the modern system, has only been used successfully up to distances of a mile or two. (Ref. 'Elec- tric Waves', Hertz, translated by Jones.) There are three chief methods by which the hf ac used are generated: (1) the discharge of a condenser which gives a temporary oscil- latory current under certain conditions, and which dies out (i.e. is damped) after a few al- ternations; (2) a direct or If ac circuit having a short gaseous section (or arc), in parallel with which is a condenser circuit; (3) a hf ac dynamo. It is not commercially practi- cable to employ ac of If for radiotelegraphic or open-circuit transmission, because of the enormous aerial conductor or capacity which would be necessary in order to radiate suf- ficient energy. The higher the frequency, however, the greater is the energy, if the capacity and voltage are the same. Thus all modem systems use ac, whether damped or undamped, of very hf, i.e. from 50,000 cycles per sec up to about 10 million per sec. Alternating dynamos giving a useful output are as yet in the experimental stage for fre- quencies above 50,000, but they may very probably come into general use before long. The unstable arc (2) resembles an organ pipe in which the fluid moving is electricity in- stead of air, while the condenser circuit re- presents the resonating column of air in the pipe. By its aid it is therefore possible to keep up a uniform ac for any desired period of time. The spark discharge of a condenser (1) is, however, as yet by far the most gene- rally used method, a fact accounted for by the simplicity and cheapness of the apparatus required. Some of the types of conductor used are enumerated in the definition Trans- mitter. These may be classed according to the closeness with which the main oscillator is coupled inductively or conductively to the earth. Marconi and Lodge both made their first experiments with small symmetrical Hertzian oscillators placed horizontally. The earth played only an unimportant part in transmission. Marconi next increased the length and capacity of one-half of the oscil- lator, placed it vertically, and substituted a short, direct connection to the earth for the lower half. The earth being practically infi- nitely large as compared with the vertical con- ductor, the latter controlled the rate of elec- trical vibration of the system. The change thus introduced is an essential one. The radi- ation was no longer free, but is tied to the earth, since the latter actually forms part of the oscillator. It is somewhat remarkable that though this fact was pointed out by Erskine- Murray in 1900, there are still those who speak of free radiation in connection with earthed systems. The Lodge-Muirhead oscil- lator is symmetrical in itself, but is placed unsymmetrically with regard to the earth, since in its vertical position the capacity of the condenser formed by the lower conductor and the Earth's surface is much greater than that formed by the earth and the upper con- ductor. In all probability, therefore, the lines of force forming the electric waves radiated are tied to the Earth's surface, and travel outwards, with their ends on it as in the Marconi system. The chief difference is that as the oscillator is only weakly coupled to the earth, its vibration is much more per- sistent, i.e. less damped than with a con- ducting connection to earth. This condition renders syntonic working more easy, and improves the energy-efficiency of the whole system. In some other systems the lower half of the oscillator is a conductor placed close to the earth, but not conductively con- nected. The results obtained in this way are intermediate between those obtainable with the direct earth and the nearly inde- pendent oscillator. The receiving aerial conductor system in most cases is the same as that used in trans- mission, differences being confined to local circuits. In receiving, however, it is not uncommon to use several separate earth-con- neetions, each branched off from nodal points on a line containing inductance and capacity, such that the proper oscillatory current passes forward to the receiver, while disturbing cur- rents are filtered off by the earth connections. The Marconi X-stopper and Slaby's double- earth connection are examples of this arrange- ment. (For further details of the various systems see below : Marconi System, Lodge- Muirhead System, Fessenden System, De Forest System, Slaby-Arco (Telefunken) System, Poulsen System, Lepel System. Wireless Telegraphy 625 See also Wireless Telegraphy, Direc- ted.) (Eef. 'Electric Wave Telegraphy', Flemings 'Handbook of Wireless Tele- graphy', Erskine-Murray; 'Wireless Tele- graphy, Sewall; 'Leitfaden der Drahtlosen Telegraphic ', Zenneck.) Marconi System, the earliest practical method of radiotelegraphy, and now the largest organised radiotelegraphic system in the world. The chief technical features leading to its success were the invention by Marconi of a reliable coherer, and the discovery of the superiority of long waves through the use of an earth connection. Subsequent improvements, such as the mag- netic detector and the revolving-disk gene- rator, have maintained its position. The original Marconi transmitter used on short ranges (up to 100 miles) consists of an A > \ vllxliL — b' C E e' Fig. 1.— Marconi's Earthed System (Early Form) A, A', Aerials. B, B', Spark gaps. c,c', Colls. E, I/, Earth connections. induction coil or transformer, having a key in its primary circuit, and with the aerial and earth wires connected to the terminals of its secondary, or ht winding. The aerial wire is attached to one terminal of the ht winding and the earth to the other. Be- tween the two is a spark gap of about a cm. Fig. 1 shows an early form of Marconi's earthed system. For greater distances, or where accurate and sharp tuning is required, an oscillating circuit containing capacity and inductance is interposed between the coil and the aerial. The result is that a train of perhaps thirty or forty eflfective waves is given out instead of one of from five to ten effective waves in the simpler system mentioned above. It is possible therefore to utilise more efficiently the properties of a resonating circuit in the receiver, and to thus obtain not only sharper tuning but also greater range of transmis- sion. In the case of the transatlantic station, and of all large stations, a special form of current generator is now used which pro- duces uniform jigs, i.e. uniform ao of very hf; these are interrupted at regular inter- vals (some hundreds of times per sec), in order that their effects on the detector at the receiving statjion may be audible. The discharge by means of which these currents Fig. 2.— Marconi's High-frequency Generator (Disk Type) H, High-pressure continuous-current dynamo. I, I, Chok- ing coils. K, Condensers. L, I/, Inductances. 0, C, Elec- trodes. A, Revolving electrode. E, F, Main oscillating cir- cuit. I, Turbine. are produced is between rotating disks of metal in air, the interruptions being obtained by using a series of metal studs on the prin- cipal disk, as electrodes. Fig. 2 is a diagram of the apparatus. The original Marconi receiver consisted of a coherer, connected to the aerial and earth- wires and to a battery and relay; and a Fig. 3.— Marconi's Coherer Receiver (Early Type) A, Aerial wire. J, Coherer, c, c, Chokers. D, Dry cell. T, Tapper. R, Relay. B, Battery. M, Morse Instru- ment. E, Earth. tapper or electromagnetic trembler actuated by a second battery brought into action by the relay. The stroke of the tapper on the coherer shook up the filings and thus de- cohered it. In parallel circuit with the tapper was a Morse inkwriter or other tele- graphic indicator. Fig. 3 shows the early form of Marconi's receiver. This form of receiver has been displaced, for ordinary work, by the magnetic detector. 626 Wireless Telegraphy and for long distances by the Fleming recti- fying valve. The Marconi magnetic detector consists of an endless band of fine iron wires kept moving, by clockwork, through a small tube on which are wound two coils of fine wire, one being connected to the aerial and to earth wires, and the other to a telephone. A magnetic field is main- tained by per- manent mag- nets whose ends are near to the moving iron wire on either side of the fixed coils. As the wire moves, this field is deformed by the retentive power of the iron dragging the lines of force with it, but springs back to its original shape when the received hf currents pass through the coil. In doing so, lines of force cut the coil con- rig. 4.— Marconi Tuner A, Aerial wire. E, Eartli. B, c. Variable inductances. P, o, H, Vari- able capacities. R, Director. nected to the telephone and induce a current which produces a sound in the latter. An arrangement of receiving circuits called a tuner is used on the larger stations. It consists of an aerial, an intermediate, and a detector circuit, with variable condensers and coupling. A Marconi tuner is shown dia- grammatically in fig. 4. Marconi's Belay is a type of relay specially suited to working with a coherer in wireless telegraphy. A condenser is used across the local-current contact-gap to reduce sparking; the ditton or moving tongue is controlled both magnetically and by a short and some- what stiff helical spring which allows only a very small amount of play. The coherer circuit windings on the polarised electro- magnets is very long, and has a resistance of about 10,000 ohms (see also Eelay). For Marconi's magnetic detector see 'Wave De- tector ' under Detector. See also Coherer ; Coherer, Carborundum; Coherer, Fil- ings. LoDGE-MuiRHEAD SYSTEM. — This System, which is illustrated diagrammatically in fig. 5, differs from other systems in the use of a Fig. 6.— Diagram illustrating Complete lodge-Muirhead Transmitting and Keceiving Station ST, Sending tranformer. M, Motor, af, Alternator field. A, Alternator. Ai, Ammeter, v, Voltmeter. SE, Sending Key. DOA, Upper capacity area. LOA, Lower capacity area. MS, Multiple spark. I, Inductance. WR, Wheel coherer. E, k. Condensers. B, Kecorder. B, Battery. nearly symmetrical oscillator as radiator. No earth connection is used. The radiator con- sists of two horizontal networks of wire, one placed from 40 ft to 120 ft above the other. From the centre of each network a wire is led to the instrument room, one being con- nected to one terminal and the other to the other terminal of the sending and trans- mitting instruments. A special form of co- herer consisting of a slowly-revolving steel disk with a razor edge dipping in oiled mercury is generally used. The electrical Wireless Telegraphy 627 dimensions of the sending and receiving circuits are carefully adjusted to give the same natural frequency of vibration. The tuning is both accurate and sharp, it being possible to exclude signals whose wave length diflFers from that proper to the sta- tion by about 3 per cent or less. Fbssenden System. — The most distinc- tive feature of the Fessenden system is the method of sending employed. The key does not interrupt the current entirely, but merely cuts out some of the inductance in circuit, and therefore alters the frequency of the transmitted current. Another feature is the barretter, which in its original form consisted simply of a very fine wire with a large tem- perature coeificient, and of a rheostat of such proportions that the received current was suf- ficient to alter its resistance appreciably. The form used now is, however, of the electroly- tic type, being an exceedingly fine wire, say 0"001 mm diameter, dipping into a conduct- ing fluid, preferably nitric acid. The other electrode in the liquid may be larger. The instrument works much like a Wehnelt in- terrupter, i.e. under a perfectly steady volt- age only a very small constant current passes; but if the voltage be rapidly varying, or surging (attained in the Wehnelt by the introduction of a large inductance in cir- cuit), the current through the apparatus suddenly becomes a large but intermittent unidirectional current. In the barretter the exciting current is that from the receiving aerial, and the source of steady voltage is a local battery. In the latest installations hf alternators giving from 2 kw at 100,000 cycles per sec have been used directly in the main oscil- lating circuit, which is constructed to have the same natural frequency of oscillation. This type of transmitter may, of course, be used either for telegraphy or telephony by simply substituting a proper microphone for the telegraphic key. In earlier installations a spark-gap in compressed air was used which made it possible to use greater voltages with the same length of gap, and as the resistance of the gap during the actual dis- charge is roughly proportional to its length, caused less waste of energy in heating. In recent long-distance stations a comparatively If (about 70,000 cycles per sec) has been adopted, since such currents are found to be less interfered with by varying atmos- pheric conditions, and give practically as good signals by day as by night. Fig. 6 shows some details of the receiving apparatus. The greatest range as yet attained has been from Machrihanish in Argyllshire to Brant Eock near Boston, a distance of 3000 miles, Fig. 6.— Feesenden Receiver with Interference PreTenter A, Aerial wire. B, Inductance. c,c>,F, Variable capa- cities. D,E,Di, El, Transformers. Q, Detector. T, Tele- phone, p, Potential divider. H, Earth. A,B,C,D,H is timed to desired frequency. A,B, Ci,Di,H is then ciosed and timed to cut out interference. The interfering wave passes equally down both sides, and does not affect the detector circuit. the height of the masts carrying the aerial being 415 ft. De Forest System. — The distinctive features of this system are principally the detectors invented by Dr. Lee De Forest. The first of these was his resporider, a tube with electrodes and containing an electrolys- able mixture of glycerine with a salt of lead and fine lead filings. On application of a steady voltage, lead trees are formed which are broken up by the passage of the received hf current. The increase in resistance is indicated by a click in the telephone con- nected in circuit with the detector. The Avdion is an ionised gas relay, i.e. it allows the local battery current to pass more easily when the hf current is acting. A somewhat similar device is the hunsen flame detector, in which the ionisation is produced by chemical action, while in the Audion it is by an elec- tric current. All these detectors are prac- tically continuous in their action. Slaby-Aeco (Telefunken) System, a spark system of telegraphy without wires, developed in Germany by Slaby and Arco. In general features it is intermediate be- 628 Wireless Telegraphy tween the Marconi spark system and the Lodge-Muirhead, ie. the jigs (trains of waves) radiated are less damped than the one and more than the other. This comparison ap- plies mainly to the first standard form put on the market; modifications and improvements are continually being made in all systems, and the most recent types of all are approximat- ing to one another in regard to their possi- bilities of sharp and accurate tuning. One interesting feature of this system is the double earth connection. In one form this is from the top of the aerial through a suitable in- ductance, in another it is from a point near the lower end through a revibrating circuit having the proper inductance and capacity for the frequency of the transmitted current. In the latter case the stray currents of other frequencies go direct to earth, while the trans- mitted current excites the revibrating circuit in which the detector is placed. Details of this system will be found in ' Wireless Tele- graphy' by Gustav Eichorn, and in various German books. PouLSEN System. — Poulsen discovered that the Duddell arc (shown in fig. 7) could y^B Fig. 7.— Duddell Arc B, Gaseons section, l, Inductance. D, Chokers. 0, Capacity. be made to give currents of frequencies suffi- ciently high for use in wireless telegraphy by enclosing the arc in an atmosphere of hydrogen. The available frequency was thus raised from about 40,000 to over 1,000,000 per sec. It was thus possible to transmit a useful amount of energy with the comparatively small capacities practicable in wireless. In order to utilise to the greatest advantage the continuous, or uniform, ac produced, a receiver was invented in which the detector is a vibrator which only closes the circuit intermittently about 600 times per sec. During the intervals when the detector is not closed, the oscillating current in the main receiving circuits has time to grow under the influence of the current from the transmitter, the natural frequencies of both being made as nearly equal as possible. [80 (70- jeo- 4- and hence revibration or resonance takes place. The detector therefore only takes energy from the main receiving circuit at moments when the current and voltage are a maximum, and does not act as a constant drag on the circuit. The detector is called a tikker. It is an intermittent single-contact coherer.. The voltage obtained in the trans- mitter is only about 2000 volts, but as the current is continuously alternating, the rate of transmission of energy is greater than in spark systems using much higher voltages. A standard pattern is on the market which is rated at 1 to IJ kw. This is suitable for telegraphic trans- ~\/\/\/\/\/ mission up to -|V V V V V ^^^^^ jQQQ jj^jjgg^ and for telephony to about 250 miles. High-speed trans- mitting and re- ceiving instru- ments have been designed which are said to give clear signals up to 120 words per min. For a de- scription of the most recent appa- ratus see Elec, April 24, 1908. Fig. 8 shows the current in arc circuits, and the figs, on the accompanying Plate show the connections at a transmitting and a re- ceiving station. Lepel System. — In the Lepel system the electrodes which form the ends of the gaseous section of the circuit are metal plates, large in comparison with the gap between them. The discharge is therefore very quickly cooled, and becomes an exceedingly rapid series of dead-beat unidirectional rushes. The effect of a current of this type on a closely-coupled secondary circuit is the production of a prac- tically uniform hf current having a single frequency without any parasitic waves or overtones. The tuning, and therefore the freedom from interference, are thus unusu- ally good. A supply of continuous or al- ternating electricity is used, the discharge gap being so short that the supply voltage is sufficient to cause a discharge across it. A shunt circuit containing a comparatively large capacity and small inductance, forms the primary oscillating circuit. To this is Fig. 8.— Current in Arc Circuits in Poulsen System ID C H POULSEN RECEIVING STATION A, Aerial Wire; c, Condensers; D, Detector; h, Helices; t, U'elephone Cr M Ct APPARATUS FOR POULSEN SYSTEM OF WIRELESS TELEGRAPHY AND TELEPHONY n, Aerial Wire ; cb, Receiving Condensers ; Ci, Transmitting Condensers ; c, Arc Geneiator ; L, Inductances: m. Microphones VIofacep. 62S. Wireless Telegraphy — Wireless Telephony 629 coupled, either inductively or conductively, or in both ways, a secondary circuit, part of which forms the antenna. A If oscillator capable of giving any desired musical note is connected to the supply circuit. The Fig. 9.— Diagram of Transmitting Connections in tlie Lepel System A, Aerial wire. G, Generator. Ci, C2, Capacities. Iq, L2, Coupled inductances. L3, Aerial inductance. Si, Sj, Supply terminals. E, Earth. The current in Oi, L,, Q lias usually the same frequency as A, Lj, Lj, Q, E. receiver has three circuits — the aerial, inter- mediate, and detector. The latter contains a galena-graphite, thermo-electric detector. The aerial and intermediate circuits are pro- vided with variable condensers for tuning purposes; and the coupling between them is Fig. 10.— Diagram of the Eeceiving Connections in the Lepel System Cx, C2, Variable capacities. L], I/j, L3, Inductances in tuned circuits. L4, Coupling of detector circuit D to intermediate circuit. E, Earth. variable. Fig. 9 shows the transmitting connections, and fig. 10 the receiving con- nections of this system. [j. e-m.] Wireless TelegFaphy, Dipected.— A single vertical aerial wire constitutes an os- cillator which radiates equally in every hori- zontal direction. If the aerial be not vertical, or if it consists of a grid of wires in a vertical plane, the radiation is no longer symmetrical, being greatest in the direction of the lower end of a sloping or |~ shaped aerial, and perpendicular to the plane of a grid. The Vol. II radiation is also unsymmetrical if two or more vertical wires be employed which are placed so that the waves radiated are out of phase and therefore interfere along one line, thus giving a minimum or zero in this direc- tion and a maximum at right angles. It is also possible by these means to determine the direction from which signals are coming. Another method is to use a triangular aerial, forming a nearly-closed circuit in a vertical plane. With two of these placed at right angles, in connection with a special trans- former, the direction of the transmitter may be located within a few degrees, [j. E-M.] Wireless Telephony, the transmission of speech over long distances without the use of connecting wires between the speaker and hearer. The first instrument invented for the purpose was Bell's photophone. In this the transmission was by a beam of light whose intensity varied with the sound waves of the voice. The beam fell on the receiver, which was a selenium cell connected to a battery and telephone. Since the resistance of selenium varies with the intensity of the light falling on it, the current in the tele- phone varied exactly in accordance with the transmitted sounds, and therefore reproduced them. Communications over distances up to about ten miles have been achieved by Ruh- mer by this method, in the practical applica- tion of which a searchlight controlled by a microphone is employed as a transmitter, and a selenium cell with battery and telephone as receiver. Modern wireless telephony is purely electric, transmission being on the same prin- ciple as wireless telegraphy. It is necessary, however, that the hf currents transmitted should be continuous, i.e. an intermittent spark will not do unless it occurs at least 30,000 times per sec. Hf alternators, or the unstable arc, or some other form of hf dis- charge across a gaseous space, are used as generators of the current, while a microphone controls the strength of the radiation. The use of hf currents is necessary on account of the diflBculty of sending out enough energy by means of a If current while using moder- ate-sized apparatus. The electric waves are therefore not exact replicas of the sound waves, as in wire telephony, but are varia- tions in strength of the hf current, the fre- quency of which must therefore be very much greater than that of the highest-pitched note transmitted. This is clearly illustrated by the transmitted, transmitting, and receiv- 41 a 630 Wireless Telephony — Wiring Systems ing waves shown in the fig. Frequencies of from 50,000 per sec upwards (usually about 300,000 per sec) are used. The fact that the electric frequency is different from that of the sound waves provides a means for pre- venting cross-talk between stations, since the stations may be electrically tuned in pairs by using wave lengths for each pair differing by about 5 per cent. The receivers of one pair will then not respond to the currents from any other pair. At the same time, by alter- ing its wave length to the pre-arranged value, any station is able to call up any other. The microphone may be placed in any one of a number of positions in the transmitting circuits. (1) It may act through a transfor- ''• i#^f||l|^^ Wireless Telephone Transmission a, Sound waves of voice, b. Transmitted electiic waves, reproduced waves in telephone receiver. mer on the supply current to the arc if an arc is used. It may (2) be placed in the aerial wire itself, and influence the strength of the hf current directly. It may (3) act inductively on the aerial through a third winding on the coupling transformer be- tween the main oscillating circuit and the aerial. It may be used to modulate either the strength or the frequency of the trans- mitted hf current. In the latter case the receiver must be sharply tuned so that the variations of frequency may be suflBcient to make an appreciable difference in the strength of its response, and therefore cause a change in the detector current through the telephone. The great difficulty at present is in the micro- phone. It is found that even the best of micro- phones will only vary from 5 per cent to 10 per cent of the total energy. Consequently much more energy is required for telephony over a given range than is necessary for tele- graphy. Wireless telephones are now fitted on many ships of the U.S. Navy, and dis- tances up to about 250 miles overland have been spoken across with powerful installa- tions. Poulsen reports that he has trans- mitted speech without wires from Berlin to Copenhagen, and Majorana from Eome to Sicily, while Fessenden states that he has spoken between Boston and New York. These distances are all over 200 miles. Twenty-five sets of radiotelephonic appa- ratus were installed by De Forest on the ships of the American Navy toward the end of 1907. Sharman has invented a leakage current and inductive method in which the electric current has the same wave form as the sound, and is not of hf. While very convenient at short distances, this type has the disadvantage that the energy falls off very rapidly with the distance, rendering it unsuitable for employment at ranges of more than a very few miles. (Ref. 'Wireless Telephony', Ruhmer, translated by Erskine- Murray; also 'Wireless Telephones', by Erskine-Murray.) [j. e-m.] Wire-wound Armature. See Armature. Wiring Point.— This is a very indefinite term. In article No. 55 of vol. ii of the 'Electrician Primers' it is stated that ' Wherever a lamp, or group of lamps, is connected to the distributing circuit through a ceiling-rose, wall-socket, or plug, it is called a wiring point. This term includes the switch controlling the lamp if away from the distributing board.' In estimating for interior wiring the cost 'per point' is often given. See also Con- duit, Interior; Wiring Systems. Wiring Systems. — Interior wiring is usually located either in wooden mouldings or in metal conduits, or else it is exposed, and secured to insulators. These three sys- tems are termed respectively — Moulding System of Interior Wiring. Conduit System of Interior Wiring. Flexible System of Interior Wiring. Various types of each system have been developed, together with complete lines of the requisite fittings. While, as regards the disposition of the wires, the systems may be classified as above indicated, a parallel classi- fication, covering the general plan of distribu- tion, divides interior wiring into two groups relating respectively to the Tree System and the Distribution-board System. c, Sound' Wiring Systems 631 Moulding System of Interior "Wir- ing. — In this system the wires are laid in grooves in wooden moulding. A piece of Fig. 1. — Wooden Moulding such wooden moulding (or casing) is shown in fig. 1. Distribution - BOARD System of Wir- ing, a system in which the mains are run to a distribution board at the entrance to the Fig. 2.— Distribntion-board System of Wiring A, Tliird floor. B, Second floor. 0, First floor. D, Ground floor. I', Main fuse. M, Meter, s, Main switch, x, Lamps. consumer's premises after passing through the supply company's fuses, switches, and meters. The distribution hoard contains bus-bars, switches, and fuses, by means of which the main circuit is split up into cir- cuits running to Branch Distribution Boards at various parts of the consumer's premises. vol. II At these branch boards the circuits are still further subdivided. The system is repre- sented diagrammatically in fig. 2. Tree System of Wiring, a system of interior wiring which was once extensively used, but which is now almost completely superseded by the Distribution -board Sys- tem of Wiring (see above). The Tree System of Wiring is illustrated diagrammatically in fig. 3. From the point where the main cable -Tree System of Wiring L, Lamps. F, Main fuses, s, Main switch. enters the consumer's premises, it is carried on through the premises, and is branched as may be required to extend it to the various rooms where lighting is required. At each branch, fuses are inserted, and at the farther side of the fuse a conductor of reduced size, in accordance with the reduced current to be carried, is employed. No distribution boards are employed. In Britain the sys- tem would no longer be countenanced, even by contractors. Flexible Wiring Systems, systems of interior wiring in which flexibles are sup- ported on insulators. Such systems of wir- ing are very cheap, and have given satis- faction on the continent of Europe, where they have been widely used. Peschel System. — This is a system of wiring houses by flexibles supported on in- sulators. It has been considerably used in Germany for the last fifteen years, and it is claimed that while it is cheaper than wood casing, it has a less fire risk. The system is 41 i> 632 Wolfram — Yellow Spot described and illustrated at p. 80 of vol. xxxix of the Journ. I.E.E. in Schwartz's paper on 'Flexibles'. Recently a tube system of wiring has been placed- on the market under the name of Peschel Steel-tube System, but it is the Flexible Wiring System above mentioned which is generally known under the name of the Peschel System. See Cleat Wiring; Conduit, Interior; Conductors, Hidden; Tubing, Screwed; Tubing, Slip; Tubing, Tinned. (Ref. Clinton's 'Electric Wiring'; Metcalfe's 'Practical Electrical Wiring'; Leaf's 'Internal Wiring of Buildings'.) Wolfram (chemical symbol W), synony- mous with Tungsten. See Tungsten. Wood as an Insulating Material. — For mechanical reasons, it is sometimes de- sirable to use wood for insulation purposes. Well - seasoned wood should be employed, those finding most use being ash, teak, and lignum vitse. It is essential that the wood be thoroughly dried, and then impregnated with a*' waterproofing compound. When thoroughly dried and treated, it should with- stand some 10,000 volts per 1 in thickness, i.e. some 4000 volts per cm. No reliance should, however, be placed on the insulating properties of wood. Wood Naphtha. See Naphtha. Wood Spirit. See Naphtha. Wood System of Electric Canal Traction. See Canal Traction, Elec- tric. Woodbridg-e Split -pole Rotary Con- verter. See Rotary Converter. Wooden Line Poles. See Line Poles. Woodite, the trade name of a moulded composition which is manufactured in several qualities. It was one of the first moulded materials introduced for electrical work, and it is claimed that the degree of hardness and flexibility can be varied to suit difi'erent re- quirements; it is said to withstand the action of warm mineral oil without disintegration. See also 'Moulded Insulators' under Insu- lator. Work, Thermal Equivalent of. See Thermal Equivalent of Work; Joule's Equivalent. Work, Unit of. See Unit of Work. Worm Gearing. See Gearing for Electric Motors. Wound Rotor. See Rotor. Wpcp, the preferable abbreviation for watts per ccmdle power. Wriggle. See Balancing Machine. Wright Electrolytic Meter. See under Meter, Electrolytic. Wright Maximum - demand Indi- cator. See Indicator, Maximum-demand. Wright Maximum -demand System. See Tariff Systems. Wrought Iron. See under Iron. Wrought-iron Conduit. See Conduit, Underground. Wurtz Lightning Arrester. See Lightning Arrester. X X-ray Reflector, a name given to a type of metallic reflector in common use for the purpose of concentrating, in any desired direction, the light from artificial illumin- ants. See Reflector. X-ray Transformer, a ht transformer suitable for the production of the very high voltages necessary for the generation of X-rays. X-rays. See Rontgen Rays. Y Connection. See Connections, Three-phase. Yd, the preferable abbreviation for yard. Yellow Spot. —The 'yellow spot' or macula lutea is a small oval spot situated near the centre of the retina of the eye, which possesses characteristics different from the surrounding portion. The chief peculi- arity of this yellow spot is that it contains practically only the light-perceiving 'cone' organs. In colour photometry the result obtained has been found to depend very greatly on whether the image of the illuminated sur- face observed falls entirely within the yellow spot or no. Hence readings involving com- Yoke — Zero 633 parisons of light which differ in colour de- pend upon the angle subtended at the eye by the illuminated surfaces used in the photometer. See Photometer; Photo- metry, [l. g.] Yoke, that portion of the iron of a mag- netic circuit which plays no active part in the mechanical or electrical purposes for which the electromagnetic mechanism is de- signed, but which forms a low -reluctance return for the magnetism, from the South to the North polar region. In the case of a cc machine it is the out- side ring-shaped casting from which the poles project inwards, and in an alternator it is usually the flywheel to which the poles are bolted. See Corej Yoke, Laminated. Yoke, Brush. See Brush Yoke. Yoke, Eddy Cuprents in. See Eddy Current. Yoke, Laminated. — In certain types of cc machines in which it is necessary that the magnet circuit should quickly respond to variations in the exciting current, the yoke is built up of laminations. Amongst such machines may be mentioned dynamos, mo- tors, or boosters, in which the compounding action must take place instantly, since in ma- chines with solid yokes it is often some sec before these actions occur, owing to the eddy currents which are set up in solid masses when changes in the flux occur. The yokes of some types of turbo-alternators are made up of sheet-steel laminations, as it is claimed that it is easier to obtain each sheet of the equal strength than it is to obtain a large steel casting or forging which will be homo- geneous throughout. See Yoke. [h. w. t.] Yoke for Open -conduit System, a U-shaped iron casting which is used to hold the slot-rails apart against the pressure of the pavement under heavy traffic. The in- terior of the yoke is of the same dimensions Yoke for Open-conduit System as the conduit, and the slot-rails are bolted to its extremities. Extended yokes are also used, which are provided with arms to carry the track rails after the manner of sleepers. See Conduit System of Electric Trac- tion. Yoke Frame. See Frame. Yoke Ring". — The yoke of a cc machine, when built up of laminations, is more often spoken of as the yoke ring (or more shortly as the ring) than as the yoke frame. See Yoke; Yoke, Laminated. Yoke Suspension. See Suspension of Traction Motor. Y-potential of Three-phase System. See Star-potential in Polyphase System. z Zamboni's Dry Pile.— This consists of a large number of disks of paper coated with zinc foil on one side and dioxide of man- ganese on the other; the moisture in the paper acts as the electrolyte. The resistance is enormous, but a very large number of disks may be packed in a tube, and a pile giving a very high emf may thus be obtained. Such a pile is useful for charging the quad- rants of an electrometer, or for some forms of gold-leaf electroscope. See Voltaic Pile; Contact Electricity; Contact Series; Electrolysis; Volta's Fundamental Ex- periment; Battery, Primary; Cell, Vol- taic; Law of Volta. Zani Method of Starting Induction Motors. See special methods of 'Starting Induction Motors ' under Starting of Mo- tors. Zeeman Effect. — If a source of light giving a line-spectrum be placed in a dense magnetic field, the lines are seen to be broadened or divided into two or more separate lines. This effect was discovered by Zeeman, and is known by his name. Zero, Inferred or Set Up.— In order to obtain greater sensitiveness in an instru- ment, the length of scale may be made to only include a short part of the possible scale between zero and the required reading. This may be achieved in several ways: — L By giving the controlling system a 634 Zero Methods of Measurement — Zone Dynamos definite measured set whose value is nearly equal and is opposite to the reading desired. This is a very convenient method in instru- ments having a spring or torsional control. 2. By subjecting the instrument to a cur- rent or emf of definite known amount, in opposition to the current or emf to be mea- sured. Thus a galvanometer may have two emf in opposition acting upon it, and will measure their difference by its deflection. [L. M.] Zero Methods of Measurement.— This term applies to those measurements in which the pointer is brought back to zero by a measured force, opposed to the effect to be measured. The term may be fairly applied to instruments with torsion heads and to dif- ferential types. See also Null Method of Measurement; Dynamometer, Siemens; Kelvin Balance; 'Sine Galvanometer' under Galvanometer. [l. m.] Zero Potential. See Potential, Elec- tric. ZeFO-type Dynamometep. See Dyna- mometer, Zero-type; Dynamometer, Sie- mens; Zero Methods of Measurement. Zigzag Dispersion. See Dispersion, Magnetic. Zinc (chemical symbol, Zn), a metallic element of a bluish-grey colour and having a crystalline structure. Its sp gr is 7'0. The resistance of one cm length of zinc, with a cross-sectional area of one sq cm, is 5 '6 microhms at 0° C. Zinc is largely used for the positive or active electrode {i.e. negative pole) in primary batteries, in which case it is usually amalgamated to prevent loss by local action when the battery is out of use. Zinc is an important constituent of several alloys, e.g. brass, spelter, britannia metal, &c. See Battery, Primary; Zamboni's Dry Pile. Zine-lead Accumulator. See Ac- cumulator. Zirconium Lamp. See Lamp, Incan- descent Electric. Zircon Lamp. See Lamp, Incandes- cent Electric. Zircon - Wolfram Lamp. See Lamp, Incandescent Electric. Z Lamp. See Lamp, Incandescent Electric. Zn, the chemical symbol for zinc (which see). Zone, Commutation. See Commuta- tion. Zone, Neutral. See Neutral Zone. Zone Dynamos and Motors, a type of cc machine in which the field-magnet winding is given a sinuous shape, for which certain advantages are claimed (see Elec. Eev., vol. Iv, pp. 203-204, and 273-274). LIST OF BOOKS REFERRED TO BY CONTRIBUTORS Abady, J. — Gas Analysts' Manual. Spon, London; Van Nostrand, New York. 1902. Abbott, A. V. — The Electrical Transmission of Energy. Crosby Lockwood, London; M'Graw, New York. 5th ed., 1904. Telephony. 6 vols. Spon, London. 1903-5. Adams, A. D. — The Electrical Transmission of Water Power. M'Graw, New York. 1906. Allsop, F. C. — Practical Electric Light Fitting. Whittaker. 6th ed. 1905. Andrews, L. — Electricity Control. Griffin. 1904. Ayrton, Mrs. H. — The Electric Arc. 'Electrician.' 1902. Baker, T. Thorne. — The Telegraphic Transmission of Photographs. Constable. 1910. BoTTCHEB, A. — Cranes. Translated by A. Tolhausen. Constable. 1908. Beardsley, E. C. — Hydro-Electric Plants. M'Graw, New York. 1907. Bell, L. — The Art of Illumination. Constable. 1903. Electric Power Transmission. Constable. 5th ed., 1908. Bobchers, W. — Electric Furnaces. Longmans. 1908. Clerk-Maxwell. See Maxwell, J. Clerk-. Clinton, W. C. — Electric Wiring. Murray. 1906. Cooper, W. E. — Electrician Prim^^rs. 'Electrician.' 2nd ed., 3 vols., 1906. Cramp, W. — Armature Windings {of the Closed-circniit Type). Biggs. 1907. and Smith, C. F. — Vectors and Vector Diagrams. Longmans. 1909. Davies, F. H. — Electric Power and Traction. Constable. 1907. Dawson, P. — Engineering and Electric Traction Pocket Book. 'Engineering.' 3rd ed., 1903. Edgcumbe, K. — Industrial Electrical Measuring Instruments. Constable. 1908. EiCHHORN, G. — Wireless Telegraphy. Griffin. 1906. Ernst, A. — Die Hebeseuge. Kirner, Berlin. 4th ed., 1903. Erskine-Murray, J. — A Handbook of Wireless Telegraphy. Crosby Lockwood. 2nd ed., 1909. Wireless Telephones. Crosby Lockwood. 1910. 636 636 List of Books referred to by Contributors Everett, J. B.—Illustations of the C.G.S. System of Units. Macmillan. 1902. EwiNG, J. A. — MagneUc Induction in Iron and other Metals. 'Electrician.' 3rd ed. 1900. Fahie, J. 3.— A History of Wireless Telegraphy, 18S8-1899. Blackwood. 1900. Faraday, M. — Experimental Besearches in Electricity. 3 vols. Quaritch. 1839-55. Fleming, J. A. — The Alternate Current Trmsformer. 2 vols. 'Electrician.' Vol. I, 3rd ed., 1900; Vol. II, 1st ed., 1892. The Principles of Electric Wave Telegraphy. Longmans. 1906. Foster, H. A. — Electrical Engineer's Pocket Book. Constable. 5th ed., 1908. Fynn, V. A. — The Classification of Alternate-Cua-rent Motors. 'Electrician.' 1906. Garcke, E., and Fells, J. — Factory Accounts. Crosby Lockwood. 5th ed., 1902. Gay, a., and Yeaman, C. H. — Central Statim Electricity Supply. Whittaker. 1899. Gm.-BAB.m,'^. B..— Electricity Meters. 'Electrician.' 1906. GooDCHiLD, G. F., and Tweney, C. F. — Technological and Scientific Dictionary. The Gresham Publishing Company. 1906. Goodman, J. — Mechanics applied to Engineering. LongmanS. 1904. Hawkins, C. C, and Wallis, F.—The Dynamo. 2 vols. Whittaker. 5th ed., 1909. Hay, a. — Alternating Currents. Harper & Bros. 1905. ' Heaviside, 0. — Electromagnetic Them-y. 2 vols. 'Electrician.' 1899. Electrical Papers. 2 vols. Macmillan. 1892. Herbert, T. 'K.— Telegraphy. Whittaker. 2nd ed., 1907. Hertz, H. — Electric Wa/iies. Translated by D. E. Jones. Macmillan. 1893. Hobart, H. M. — Elementary Principles of Coniinuous-Cwrent Dynamo Design. Whittaker. 1906. Electricity. Constable. 1909. Electric Motors. Whittaker. 2nd ed., 1910. Electric Trains, Harper & Bros. 1910. HeoAiy Electrical Engimerin,g. Constable. 1909. and Ellis, A. G. — Armature Construction. Whittaker. 1907. ■ High-Speed Dyna/mo-Electric Machinery. Wiley, New York; Chapman & Hall, London. 1908. Hutchinson, E. W., Jun. — Long-Distance Electric Power Transmission. Van Nostrand, New York; Spon, London. 1907. Hutchinson, R. W., and Ihlsing, M. — Electricity m Mining. Van Nostrand, New York. 1909. Kapp, G. — Tramformers. Whittaker. 2nd ed., 1908. Kelvin, Lord. — Papers on Electrostatics and Magnetism. Macmillan. 1884. Kempe, H. R. — A Ha/adhooh of Electrical Testing. Spon. 7th ed., 1908. Kennedy, A. B. W, — The Mechomics of Machinery. Macmillan. 1886. Kershaw, J. B. C. — The Electric Fu/rnace in Iron amd Steel Production. 'Electrician.' 1907. Electro-Metallurgy. Constable. 1908. List of Books referred to by Contributors 637 Krause, E. — Starters and Begulators for Electric Motors. Translated by C. Kinzbrunner and N. West. Harper & Bros. 1904. Leaf, H. M. — The Internal Wiring of Buildings. Constable. 3rd ed., 1906. LiKBENTHAL, E. — PmUische Photometric. Vieweg, Brunswick. 1909. Livingstone, R. — The Mechcmical Design a/nd Construction of Commmtators. 'Electrician.' 1907. LuPTON, A., Pare, G. D. A., and Perkin, H. — Electricity as applied to Mining. Crosby Lockwood. 1903. M'GrRAW. — Standard Handbook for Electrical Engineers. M'Graw, New York; Spon, London. 1908. Maclean, Magnus. — Modern Electric Practice. 6 vols. Gresham Publishing Company. 2nd ed.. 1909. Martin, T. C. — Inventions, Researches, and Writings of Nicola Tesla. 'Elec. Engineer', New York. 2nd ed., 1894. Maxwell, J. Clerk-. — Electricity amd Magnetism. 2 vols. Clarendon Press. 3rd ed., 1892. Maycock, W. p. — Electric Wiring, Fittings, Switches, and Lamps. Whittaker. 4tli ed., 1909. Metcalfe, C. C. — Practical Electric Wiring. Harper & Bros. 1906. Moissan, H. — The Electric Furnace. Arnold. 1904. NiETHAMMER, F. — Bercchnung und Entwurf EleUrischer Maschinen, Appa/rate, und Anlagen. 3 vols. Stuttgart. 1903. OuDiN, M. A. — Standard Polyphase Apparatus and Systems. Van Nostrand, New York. 5th ed., 1907. Parr, G. D. A. — Electrical Engineering Measuring Instruments. Blackie. 1903. Parshall, H. F., and Hobart, H. M. — Armatme Windings. Van Nostrand, New York. 1895. Electric Machine Design. 'Engineering.' 1906. Electric Railway Engineering. Constable. 1907. -Perrine, F. a, C. — Conductors for Electrical Distribution. Van Nostrand, New York; Crosby Lockwood. 1903. PxraGA, F. — Single-Phase Commutator Motors. Translated by R. F. Looser. Whittaker. 1906. Richards, R. 'B..—Ore Dressing. 4 vols. M'Graw-Hill, New York. 1903-9. Rider, J. B..— Electric Traction. Whittaker. 1903. RuHMER, E. — Wireless Telephony. Translated by J. Erskine-Murray. Crosby Lockwood. 1908. Russell, Alex. — The Theory of Electric Cables and Networks. Constable. 1909. Russell, Stuart A. — Elecfrio-IAght Cables. Whittaker. 2nd ed., 1901. Rutherford, E. — Radio-Activity. Cambridge University Press. 2nd ed., 1905. Sewell, C. H. — Wireless Telegraphy. Crosby Lockwood. 1903. Snell, J. F. C. — !rhe Distribution of Electrical Enm-gy. Thomas Reed, Sunderland. 1907. Solomon, H. G. — Electricity Meters. GrifBin. 1906. Solomon, M. — Electric Lamps. Constable. 1909. 638 List of Books referred to by Contributors Steinmetz, C. p. — The Theory and Calculation of Alternating Cwrrent Phenmma. M'Graw, New York. 4th ed., 1908. Steinmetz, C. P. — Radiation, Light, and Illumination. M'Graw-Hill, New York. 1909. Stevens, T., and Hobart, H. M.— Steam Turbine Engineering. Whittaker. 1906. Stine, W. M. — Photometrical Measurements. Macmillan, New York. 1900. Thompson, S. F.— Dynamo-Electric Machinery. 2 vols. Spon. 7th ed., 1905. Electricity and Magnetism. Macmillan. 4th ed.j 1897. The Electro-Magnet and Electro-Magnetic Mechanisms. Spon. 2nd ed., 1892. Polyphase Electric Gwrents. Spon. 1895. Thomson, J. J. — Conduction of Electricity through Gases. Cambridge University Press. 2nd ed., 1906. Not^ on Becent Researches in Electricity and Magnetism. Clarendon Press. 1893. Thomson, Sir W. See Kelvin, Lord. Turner, H. W., and Hobart, H. M. — The Insulation of Electric Machines. Whittaker. 1905. Unwin, W. C. — Elements of Machine Design. 2 vols. Longmans. New ed., 1909. "Welz, F. — The BerlAn-Zossen Electric Raihvay Tests of 1903 (Translation). M'Graw, New York. 1905. Wilkinson, H. D. — Submarine Cable Laying amd Repairing. 'Electrician.' 2nd ed., 1908. Wilson, E., and Lydall, F. — Electric Traction. 2 vols. Arnold. 1907. WORDINGHAM, C. H. — Central Electrical Stations. Griffin. 2nd ed., 1901. Wright, J. — Electric Furnaces and their Industrial Application. Constable. 1906. Zeidlbr, J., and Lustgarten, Z.— Electric Arc Lamps. Harper & Bros. 1907. LIST OF ARTICLES IN PERIODICALS AND JOURNALS TO WHICH REFERENCE IS MADE ELECTBIGAL REVIEW (London) C. V. Drysdale. — Accurate Speed, Frequency and Acceleration Measurements. 1906. Vol. lix, pp. 363, 403. B. H. Peter. — Power Signalling as installed by the Underground Electric Railways of London. 1907. Vol. Ix, p. 437. H. Boot. — Some Tests conducted with an Electrolytic Eectifier and Motor Generator. 1905. Vol. Ivi, p. 211. H. G. Solomon. — Limitations of Three-wire Energy Motor Meters. 1906. Vol. lix, p. 327. H. G. Solomon. — The Calculation of Percentage Error. 1906. Vol. lix, p. 524. H. F. Joel.— ' Zone ' Dynamos and Motors. 1904. Vol. Iv, pp. 203, 273. T. Bolton.— Hard-Drawn Copper Wire. 1907. Vol. Ix, p. 131. H. Boot. — Correspondence on 'Rail Corrugation'. 1906. Vol. lix, p. 701. W. R. BOWKER. — Correspondence on ' Rail Corrugation '. 1906. Vol. lix, p. 863. 'Doubtful.' — Correspondence on 'Rail Corrugation'. 1906. Vol. lix, p. 744. L. C. Harvey. — Correspondence on ' Rail Corrugation '. 1906. Vol. lix, p. 783. H. Rayner. — Correspondence on ' Rail Corrugation '. 1906. Vol. lix, p. 823. W. S. BoULT. — Correspondence on ' Rail Corrugation '. 1906. Vol. lix, p. 863. R. B. Leach. — Correspondence on 'Rail Corrugation'. 1906. Vol. lix, p. 863. R. B. Leach. — The Eomapac System of Permanent Way Construction. 1906. Vol. Iviii, p. 245. R. B. Leach. — ^A New Process of Wire Covering. 1905. Vol. Ivii, p. 889. ELECTRICIAN (London) A. 0. Jolley. — Some Observations on Alternating Current Rectifiers. 1906. Vol. Ivii, p. 998. [M. E. J. Brunswick]. — Boucherot's Squirrel-Cage Motor with large Starting Torque. 1906. Vol. Ivi, p. 1050. [Abstract from article by M. E. J. Brunswick in 'L'Industrie iHecirique'.] A. Schwartz and W. H. N. James. — Low Tension Fuses. 1905. Vol. Iv. p. 8. [Also Journ.LKE., vol. xxxv.] A. Schwartz and W. H. N. James. — Zinc Fuses. 1906. Vol. Ivi. p. 184. A. Schwartz and W. H. N. James. — Aluminium Fuses. 1906. vol. Ivi, p. 468. 640 List of Articles in Periodicals [F. Punga].— A New Type of Eepulsion Motor. 1906. Vol. Ivii, p. 27. [Abstract from article by F. Punga in E.T.Z.] A. G. Betts. — The Use of Sodium as a Conductor in place of Copper. 1906. Vol. Iviii,, p. 218. [Abstract from Electrical World.] H. S. Meyer. — Voltage Regulation in Alternating-Current Systems. 1904. Vol. lii, p. 772. [From Paper before Liverpool Engineering Society.] W. W. Beaumont. — The Origin and Production of Corrugation of Tramway Rails. 1907, Vol. lix, p. 798. [From Paper before British Association, Leicester.] A. L. C. Fell.— Rail Corrugation. 1907. Vol. lix, p. 979. [From Paper before Muni- cipal Tramways (6th) Conference.] C. A. Carus Wilson. — Rail Corrugation. 1908. Vol. Ixi, pp. 563, 599. [Paper before Congress of Tramways and Light Railways Association.] C. A. Carus Wilson. — Some Opinions on Rail Corrugation. 1908. Vol. Ixi., p. 995. M. B. Field.— Idle Currents. 1906. Vol. Ivi, pp. 845, 884. [Also Journal LE.E., xxxix,. p. 636.] R. Livingstone. — Some Notes on the Mechanical Design of Electrical Generators. 1906. Vol. Ivii, pp. 569, 646, 687, 768, 804 (Axles, 569; Keys, 646; Bearings, 687; Pedestals,. 768; Spider and Frame, 804). C. Baur. — On the Electric Strength of Insulating Materials. 1901. VoL xlvii, p. 758. G. Benischke. — Stroboscopic Methods of Testing Small Motors. 1899. Vol. xlii, p. 676.. [From Paper before Berlin Electrotechnischer Verein.] C. V. Drysdale. — Measurement of the Slip of Induction Motors. 1905. Vol. Iv, p. 734.. G. Kapp. — The Separation of Foucault and Hysteresis Losses. 1891. VoL xxvi, p. 699. R. H. HouSMAN. — Graphic Method for Analysing Losses in Armature Cores, &c. 1891. Vol. xxvi, p. 700. E. B. Rosa and H. D. Babcock. — The Variation of Manganin Resistances with Atmos- pheric Humidity. 1907. Vol. lix, p. 339. [From Paper before American Physical Society, Washington, April, 1907.] W. Jaegab and St. Lindeck (Charlottenburg). — The Variation of Manganin Resistances, with Atmospheric Humidity. 1907. Vol. lix, p. 626. E. B. Rosa. — The Variation of Manganin Resistances with Atmospheric Humidity. 1907. Vol. Is, p. 162. E. GuMLiCH.— Magnetic Alloys of Non-Magnetic Metals. 1905. Vol. Iv, p. 94. [Abstract of article in E.T.Z., of March 2, 1905.] C. E. GuiLLAUMB. — Theory of the Magnetic Alloys of Manganese. 1906. Vol. Ivii, p. 707. [Abstract from Bulletin de la SocUU International des EleeMdens, June 1906.] J. C. M'Lennan. — On the Magnetic Properties of Heusler's Alloys. 1907. Vol. lix, p. 844. [Abstract from Physical Review, June, 1907.] P. HuMANN. — ^Dielectric Losses with High Pressure Alternate Currents. 1906. Vol. Iviii, p. 170. [From Thesis for Doctor of Philosophy, University of Bonn, and Elekirische Bahnen und Betriebe, August, 1906.] B. MoNASCH. — On the Loss of Energy in the Dielectric of Condensers and Cables. 1907. Vol. lix, p. 416. B. MoNASCH. — Poulsen's Apparatus for Wireless Telephony. 1908. Vol. Ixi, p. 49. B. F. Bailey.— The Slip of an Induction Motor. 1905. Vol. Iv, p. 340. [Abstract from Electrical World of New York and E.T.Z., February 28, 1901.] W. C. Clinton. — Measurement of Slip in Induction Motors (Correspondence). 1905. Vol. Iv, p. 388. List of Articles in Periodicals 641 E. H. Johnson.— The Third Function of Electric Traction Motors (Regenerative Control). 1906. Vol. Iviii, p. 336. [Abstract of Paper before Manchester Association of Engineers, November, 1906.] ELECTRICAL ENGINEER (London) G. Kapp. — The Determination of the Efficiency of Dynamos. 1892. Vol. ix (new), pp. 87, 102. W. E. SuMPNER and E. W. Weekes.— The Hopkinson Test as applied to Induction Motors. 1904. Vol. xxxiv, p. 310. [Paper before British Association, Cambridge.] E. B. Holt.— Eail Corrugation. 1905. Vol. xxxvi, p. 595. ELECTRICAL TIMES (London) Traction Topics.— The Corrugation of Eail Treads. Vol. xxviii, p. 9, 82. ELECTRICAL REVIEW (New York) [F. Pawlowski].— An Electrolytic Eectifier. 1906. Vol. xlix, p. 554. ELECTRICAL WORLD AND ENGINEER (New York) E. E. Hellmund. — Starting Torque of Induction Motors. 1906. Vol. xlvii, p. 666. E. E. Hellmund. — Influence of the Slot-ratio upon the Starting Torque of Induction Motors. 1908. Vol. lii, p. 672. E. E. Hellmund. — Cascade Eotary Converter. 1905. Vol. xlvi, p. 230. [From article in E.T.Z., July 13, 1905.] K. Norden.— The Theory of the Electrolytic Eectifier. 1901. Vol. xxxviii, p. 681. K. Norden. — Aluminium Alternating-current Eectifier. 1904. Vol. xliv, p. 308. H. M. HOBART and F. Punga. — ^A New Method of Testing Alternating-current Gene- rators. 1905. Vol. xlv, p. 759. W. H. Browne, Jun. — A Method of Measuring the Slip of Induction Motors. 1900. Vov. xxxvi, p. 574. i ELECTRIC RAILWAY JOURNAL— STREET RAILWAY JOURNAL (Chicago) F. Krizik. — The 1400-volt Direct Current Eailway between Tabor and Bechyne (Bohemia). 1904. Vol. xxiv, p. 1042. [Paper before Vienna Convention, International Street Eailways Association.] F. Krizik. — Eail Corrugations on the Boston Elevated Eailway. 1906. Vol. xxviii, p. 1180. F. Krizik. — Eec^nt Eeport on Eail Corrugation in Europe. 1909. Vol. xxxiv, p. 317. F. Krizik. — Eail Wear on London's Underground Lines. 1910. Vol. xxxv, p. 438. TRAMWAY AND RAILWAY WORLD (London) G. Moyle.— Eail Corrugation. 1906. Vol. xix, p. 558. F. F. Aman.— Eail Corrugation. 1907. Vol. xx, p. 44. E. W. Western. — Eail Corrugations. Vol. xxi, p. 104. E. W. Western.— The Causes of Eail Corrugation. Vol. xxi, p. 394. [From Official Circular, Tramway and Light Eailway Association.] 642 List of Articles in Periodicals ENGINEERING (London) R. W. Western. — The Corragation of Rails. Vol. Ixxxiii, pp. 422, 763. 0. Lasche. — The Construction and Systematic Manufacture of Alternators. 1901. Vol. Ixxii, p. 173. ENGINEERING AND MINING JOURNAL (New York) 0. Lasche. — The Murphy Air-hammer Rock Drill. 1905. Vol. Ixxx, p. 362. LA LUMIERE ULEGTRIQUE M. Depres. — Th6orie Graphique des Machines Dynamo-l3ectriques. 1881. Vol. v, p. 325. L'ECLAIRAGE EILEGTBIQUE J. L. RouTiN. — Nouvelle M^thode pour la Determination des Rendements. 1896. Vol. ix, p. 169. A. POTIER. — Sur la Reaction d'lnduit des Alternateurs. 1900. Vol. xxiv, p. 133. F. Copp:6. — Systfemes de Compoundage des Dynamos k Courant Continu pour Vitesse Variable. 1902. Vol. xxxiii, p. 181. L'INDUSTRIE ELEOTEIQUE A. Meynier. — Mesure du Glissement des Moteurs Asynchrones. 1902. Vol. xd, p. 560. M. E. Wedekind. — Sur les Composes Magn^tiques d'EWments Non-magn^tiques. 1907. Vol. xvi, p. 74. LA REVUE ELEGTRIQUE M. E. Wedekind. — Redresseurs de Courants Alternatifs. 1906. Vol. vi, p. 296. JOURNAL OF INSTITUTION OF ELECTRICAL ENGINEERS, (London) M. E. Wedekind. — Report of Committee on Copper Conductors. Vol. xxix, p. 169. W. B. SayerS. — On the Prevention and Control of Sparking in Continuous-current Dynamos without Winding on the Field Magnets, and Constant Pressure Dynamos without Series Winding. 1893. Vol. xxii, p. 377. W. B. Sayers. — Reversible Regenerative Armatures and Short Air-space Dynamos. 1895. Vol. xxiv, p. 122. J. A. Panton. — Rail Corrugations. 1907. Vol. xxxix, p. 3. H. A. Mavor. — ^Design of Continuous-current Dynamos. 1902. Vol. xxxi, p. 221. A. Gardiner.— System of Cab-signalling. Vol. xliii, p. 125. J. PiGG. — Automatic Cab-signalling on Locomotives. 1907. Vol. xl, p. 62. A. Siemens. — Notes on the Metrical System of Weights and Measures. 1902. Vol. xxxii, p. 283. J. A. Fleming. — The Photometry of Electric Lamps. 1902. Vol. xxxii, p. 119. R. Thrbleall. — The Testing of Electric Generators by Air Calorimetry. 1904. Vol. xxxiii, p. 38. C. P. Sparks. — Electrical Equipment of the Aberdare Collieries of the Powell Duffryn Company. 1906. Vol. xxxvi, p. 477. List of Articles in Periodicals 643 W. 0. Mountain. — Electric Winding in Main Shafts, considered Practically and Com- mercially. 1906. Vol. xxxvi, p. 499. G. HoosHWiNKEL. — Electric Winding considered Practically and Commercially. 1906. Vol. xxxvi, p. 506. G. HooGHWiNKEL. — Home Office Special Eules for the Installation and Use of Electricity in Mines. 1907. Vol. xxxix, p. 255. L. J. Hunt.— A New Type of Induction Motor. 1907. Vol. xxxix, p. 648. A. M. Taylor.— Network Tests and Station Earthing. 1903. Vol. xxxii, p. 872. E. E. Crompton.— The Cost of Electrical Energy. 1894. Vol. xxiii, p. 396. A. Siemens. — High-speed Electric Eailway Experiments on the Marienfelde-Zossen Line. 1904. Vol. xxxiii, p. 894. 0. Lasche. — High-speed Eailway Car of the Allgemeine Elektricitats-Gesellschaft, Berlin. 1901. Vol. xxxi, p. 24. W. H. Patchell.— The City of London Works of the Charing Cross, West End, and City Electric Supply Company, Ltd. 1905. Vol. xxxvi, p. 66. Alex. Eussell. — Mean Horizontal and Mean Spherical Candle Power. 1903. Vol. xxxii, p. 631. F. W. Carter. — Technical Considerations in Electric Eailway Engineering. 1906. Vol. xxxvi, p. 231. P. V. M'Mahon.— The City and South London Eailway: Working Eesults of the Three- Wire System applied to Traction, &c. 1904. Vol. xxxiii, p. 100. A. P. M. Fleming and K. M. Faye-Hansen. — Transformers: Some Theoretical and Practical Considerations. 1909. Vol. xlii, p. 373. A. Eaworth. — Eegenerative Control of Electric Tramcars and Locomotives. 1907. Vol. xxxviii, p. 374. J. S. Peck. — Protective, Devices for High Tension Transmission Circuits. 1908. Vol. xl, p. 498. S. Brown.— A Telephone Eelay. 1910. Vol. xliii. C. C. Paterson. — Investigations on Light Standards and the Present Position of the High Voltage Glow Lamp. 1907. Vol. xxxviii, p. 287. F, G. Bailey and W. S. H. Cleghorne. — Some Phenomena of Commutation. 1907. Vol. xxxviii, p. 160. G. Stoney. — Discussion on Direct Current Turbo Generators. 1908. Vol. xl, p. 638. H. Hirst. — Eecent Progress in Tungsten Metallic Filament Lamps. 1909. Vol. xli, p. 636. E. W. Moss.— Electric Valves. 1907. Vol. xxxix, p. 692. J. Swinburne. — New Incandescent Lamps. 1907. Vol. xxxviii, p. 211. Alex. Eussell. — ^The Dielectric Strength of Insulating Materials and the Grading of Cables. 1907. Vol. xl, p. 6. J. S. Highfield, — The Transmission of Electrical Energy by Direct Current on the Series System. 1907. Vol. xxxviii, p. 471. W. P. Steinthal. — Eules of the Society of German Electrical Engineers. 1908. Vol. xli, p. 166. J. T. Irwin. — Hot-wire Wattmeters and Oscillographs. 1907. Vol. xxxix, p. 617. H. W. Handcock and A. H. Dykes. — Electricity Supply Prospects and Charges as Affected by Metallic Filament Lamps and Electric Heating. 1908. Vol. xli, p. 332. 644 List of Articles in Periodicals J. A. EwiNG.— A Magnetic Tester for Measuring Hysteresis in Sheet Iron. 1895. Vol. xxiv, p. 398. Miles Walker. — The Improvement of Power Factor in Alternating Current Systems. 1909. Vol. xlii, p. 599. M. B. Field. — A Study of the Phenomenon of Resonance in Electric Circuits by the Aid of Oscillograms. 1903. Vol. xxxii, p. 647. A. SCHWAKTZ.— Flexibles, with Notes on the Testing of Eubber. 1907. Vol. xxxix, p. 31. G. Stoney and A. H. Law.— High Speed Electrical Machinery. 1908. Vol. xli, p. 286. C. W. Hill.— Crane Motors and Controllers. 1906. Vol. xxxvi, p. 290. Mervyn O'Gorman. — Discussion on Eussell's 'Dielectric Strength of Insulating Materials'. 1907. Vol. xl, p. 30. C. V. Drysdale. — A Permeameter for Testing the Magnetic Qualities of Materials in Bulk. 1901. Vol. xxxi, p. 283. W. M. MORDEY.— On Dynamoes. 1897. Vol. xxvi, p. 532. W. E. Ayrton and T. Mather. — An Astatic Station Voltmeter. 1894. Vol. xxiii, p. 380. C. H. Merz and W. M'Lellan. — Power Station Design. 1904. Vol. xxxiii, p. 696. G. Forbes. — The Electrical Transmission of Power from Niagara Falls. 1893. Vol. xxii, p. 484. H. S. Hele-Shaw, a. Hay, and P. H. Powell. — Hydrodynamical and Electromagnetic Investigations regarding the Magnetic Flux Distribution in Toothed-cored Armatures. 1904. Vol. xxxiv, p. 21. E. N. Tweedy and H. Dudgeon. — Notes on Overhead Equipment of Tramways. 1906. Vol. xxxvii, p. 161. Miles Walker. — The Short-Circuiting of Large Electric Generators and The Design of Turbo-Field Magnets. [Papers read before the I.E.E. on March 10, 1910.] PROCEEDINGS OF THE INSTITUTION OF CIVIL ENGINEEBS (London) C. W. Hill.— Electric Cranes. 1905. Vol. clx, p. 368. L. B. Atkinson. — The Theory, Design, and Working of Alternate Current Motors. 1898. Vol. cxxxiii, p. 113. H. A. Mayor. — Marine Propulsion by Electric Motors. 1909. Vol. clxxix, p. 234. E. KiLBURN Scott. — Coal Cutting by Machinery. 1906. Vol. cxliv, p. 247. T. GiLLER. — On Compressed-air and Electric Boring Machinery for Tunnels. 1902. VoL cl, p. 472. C. N. EussELL. — Combined Eefuse Destructors and Power Plants. 1899. Vol. cxxxix, p. 181. E. HoPKiNSON and E. E. Froudb. — In Discussion on Friction Brake Dynamometers, by W. W. Beaumont. 1889. VoL xcv, p. 37. E. E. B. Crompton. — The Cost of the Generation and Distribution of Electrical Energy. Vol. cvi, p. 2. PROCEEDINGS OF INSTITUTION OF MECHANICAL ENGINEERS W. Froude. — On a New Dynamometer for Measuring the Power delivered to the Screws of Large Ships. 1877. P. 237. List of Articles in Periodicals 645 PROOEEDINGS OF AMERICAN INSTITUTE OF ELEGTBICAL ENGINEERS (New York) H. 0. Lacount. — Standardization of Enclosed Fuses. 1905. Vol. xxiv, p. 893. J. E. WooDBKiDGE. — Some Features of Eailway Converter Design and Operation. 1908. Vol. xxvii, p. 191. E. D. Mershon. — The Transmission Plant of the Niagara, Lockport, and Ontario Power Company. 1907. Vol. xxvi, p. 1273. R. D. Mershon. — Practical Testing of Commercial Lightning-arresters. 1907. Vol. xxvi, p. 1097. E. E. F. Creighton. — Protective Apparatus Engineering. 1907. Vol. xxvi, p. 1049. B. A. Behrend. — A New Large Generator for Niagara Falls. 1908. Vol. xxvii, p. 1057. W. L. Waters. — Modern Development in Single-phase Generators. 1908. Vol. xxvii, p. 1069. A. E. Kennelly and E. E. Shepard. — The Heating of Copper Wires by Electric Cur- rents. 1907. Vol. xxvi, p. 969. P. M. Lincoln. — The Grounded Neutral with and without Series Eesistance in High Tension Systems. 1907. Vol. xxvi, p. 1585. F. G. Clark.— The Grounded Neutral. 1907. Vol. xxvi, p. 1597. G. L Rhodes. — Experience with a Grounded Neutral on the High Tension System of the Interborough Rapid Transit Company. 1907. Vol. xxvi, p. 1605. G. L Rhodes. — Grounded Neutral — Discussion. 1907. Vol. xxvi, p. 1611. H. W. Fisher. — Data Relating to Electric Conductors and Cables. 1905. Vol. xxiv, p. 397. J. Langan. — Standardizing Rubber-covered Wires and Cables. 1906. Vol. xxv, p. 191. E. M. Hewlett. — A New Type of Insulator for High Tension Transmission Lines. 1907. Vol. xxvi, p. 1259. A B. Field. — Eddy-currents in Large Slot-wound Conductors. 1905. Vol. xxiv, p. 761. J. W. Howell. — A New Carbon Filament. 1905. Vol. xxiv, p. 838. D. B. RusHMORE. — The Mechanical Construction of Revolving Field Alternators. 1904. Vol. xxiii, p. 253. A. Meyers.— High Tension Outlets. 1906. Vol. xxv, p. 865. ELECTRIC JOURNAL (Formerly ELECTRIC CLUB JOURNAL) C. E. Skinner.— Transformer Oil. 1904. Vol. i, p. 227. H. J. Ryan. — Compressed Gas as an Insulator. 1905. Vol ii, p. 429. Miles Walker. — Problems in Commutation. 1907. Vol. iv, p. 276. [Based on Lecture at Engineers' Club, Manchester.] Harvey Dean. — The Manufacture of Electrical Porcelain. 1907. Vol. iv, p. 352. Harvey Dean. — The Design and Testing of Electrical Porcelain. 1907. Vol. iv, p. 568. PHYSICAL REVIEW S. R. Cook.— On the Theory of the Electrolytic Rectifier. 1905. Vol. xx, p. 312. G. W. Pierce, — Crystal Rectifiers for Electric Currents and Electric Oscillations. 1907. Vol. xxv, p. 31. F. p. Whitman. — On the Photometry of Differently -Coloured Lights and the 'Flicker' Photometer. 1896. Vol. iii, p. 241. 646 List of Articles in Periodicals PROCEEDINGS OF THE ROYAL SOCIETY (London) J. A. Fleming and E. A. Hadfield. — On the Magnetic Qualities of Some Alloys not Containing Iron. 1905. Vol. Ixxvi, p. 271. J. Erskine-Murray. — On the Contact Electricity of Metals. 1898. Vol. Ixiii, p. 113. [Communicated by Lord Kelvin.] C. V. Drysdale. — On Luminous Efficiency and the Mechanical Equivalent of Light. 1907. Vol. Ixxx, p. 19. [Communicated by S. P. Thomson.] G. Marconi. — On Methods whereby the Eadiation of Electric Waves may be Mainly Confined to Certain Directions, &c. 1906. Vol. Ixxvii, p. 413. [Communicated by J. A. Fleming.] G. Maeconi. — A Note on the Effect of Daylight upon the Propagation of Electromagnetic Impulses over Long Distances. 1902. Vol. Ixx, p. 344. [Communicated by J. A. Fleming.] PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY H. L. Callendar. — On the Practical Measurement of Temperature (Cavendish Labo- ratory, Cambridge). 1887. A. vol. elxxvii, p. 161. [Communicated by J, J. Thomson.] PROCEEDINGS OF THE PHYSICAL SOCIETY OF LONDON J. A. Fleming. — The Construction and Use of Oscillation Valves for Rectifying High Frequency Electric Currents. 1906. Vol. xx, p. 177. Simmance-Abady. — Simmance-Abady 'Flicker' Photometer. 1903. Vol. xix, p. 39. F. W. Lanchester. — Pendulum Accelerometer. 1903. Vol. xix, p. 681. TRANSACTIONS OF THE ROYAL SOCIETY OF EDINBURGH Prof. Tait. — First Approximation to a Thermo-electric Diagram. 1873. Vol. xxvii, p. 125. PHILOSOPHICAL MAGAZINE W. E. Ayrton and E. A. Medley. — Tests of Glow-lamps and Description of the Mea- suring Instruments Employed. 1895. Vol. xxxix, 5th Series, p. 389. TRANSACTIONS OF THE OPTICAL SOCIETY, LONDON C. V. Drysdale. — Stroboscopy. 1905, Nov. 16. TRANSACTIONS OF THE INSTITUTE OF MINING ENGINEERS S. Mayor. — Practical Problems of Machine Mining. 1906. Vol. xxxi, p. 378. W. E. Garforth. — Applications of Coal-cutting Machines to Deep Mining. 1902. Vol. xxiii, p. 312. AMERICAN JOURNAL OF SCIENCE 0. N. EOOD.— On the Flicker Photometer. 1899. Vol. viii, 4th Series, p. 194. JOURNAL OF THE FRANKLIN INSTITUTE W. E. Harrington. — Cast-Weld and Surface-Contact Bonds. 1900. Vol. cxlix, p. 401, List of Articles in Periodicals 647 ELEKTROTECH'NISGHE ZEITSGHRIFT (Berlin) G. Seibt. — Messung der Schliipfung Asynchroner Motoren. 1901. Vol. xxii, p. 194. C. Baur. — Das Gesetz der Elektrischen Durchschlage. 1904. Vol. xxv, p. 7. B. Walter. — Ueber das Elektrische Durchschlagsgesetz fiir Atmospharische Luft. 1904. Vol. xxv, p. 874. F. PUNGA.— Ein Neuer Einphasen Kommutatormotor. 1906. Vol. xxvii, p. 267. O. BusSMANN. — Die Quarzlampe von Dr. Kiich, eine Quecksilberlampe fiir Hohe Spapnung Geringe Energieverbrauch und lange Brenndauer. 1907. Vol. xxviii. A. Schweitzer. — Messung der Schliipfung Asynchroner Motoren nach der Stroboskop- ischen Methods und mit Hiilfe der Braun'schen Rohre. 1901. Vol. xxii, p. 947. B. Walter. — Ein Verfahren zer Bestimmung der Elektrischen Durchschlagsfestigkeit Hochisolierender Substanzen. 1903. Vol. xxiv, p. 796. E. LlEBENTHAL. — Ueber die Abhangigkeit der Hefnerlampe und der Pentanlampe von der Beschaffenheit der Umgebenden Luft. 1895. Vol. xvi, p. 655. E. LlEBENTHAL. — Dynamomaschinen, Transformatoren und Zubehor; Umformer Stehender Anordnung. 1907. Vol. xxviii, p. 510. ELEKTROTEGHNIK UND MASGHINENBAU (Vienna) (Zeitschrift fur Elektrotechnik) E. LlEBENTHAL. — Elektrische Beleuchtung, Heiznutz; Elektrische Antriebe, Arbeitsma- schinen. 1906. Vol. xxiv, p. 423. M. VON HoOR. — Ueber eine Methode zer Bestimmung der Schliipfung von ein und Mehrphasigen Iifductionsmotoren. 1899. Vol. xvii, p. 211. GENTRALBLATT FtJB AGGUMULATOREN E. SiEG. — Die Letzten Neuerungen auf dem Gebiete Transportabler Accumulatoren, und Alkalischer Sammler (Jungner-Edison). 1905. Vol. vi, p. 66. PHYSISGHE GESELLSGEAFT H. M. VoGEL. — Ueber Photographische Momentaufnahmen des Kunstlichem Lichte. 1902. Vol. vi, p. 62. GENTRAL-ZEITUNG FUR OPTIK UND MEGHANIK H. M. VOGEL. — Gleichrichter und Drosselzellen. 1907. Vol. xxviii, p. 95. COMPTES BENDUS A. Blondel. — Oscillographes : Nouveaux Appareils pour I'Etude des Oscillations Elec- triques Lentes. 1893. Vol. cxvi, p. 502, 748. [Pr^sent^e par M. Potier.] BRITISH ASSOGIATION REPORTS W. Duddell. — Oscillographs: An Instrument for Eecording Eapidly Varying Potential Differences and Currents. 1897. Toronto, p. 575. [See Electrician. 1897. Vol. xxxix.] E. A. Hadfibld. — The Production of Magnetic Alloys from non-Magnetic Metals. 1904. Cambridge, p. 685. 648 List of Articles in Periodicals BULLETIN DE LA SOCIHT^ INTERNATIONALE DE8 ^LECTBICIENS March, 1904, p. 173; Aug., Sept., Oct., 1902, pp. 678-704. MISCELLANEOUS PAPERS C. A. Adams. — Trans. International Congress at St. Louis. 1904. Vol. i, p. 707. Moore. — Transactions of American Illuminating Engineering Society. 1907, May, p. 288. Parker & Clark. — American Physical Society, New York. 1906, December 29. P. S. MuLLER. — Convention Issue of the Illuminating Engineering Transactions. 1907. ScHNETZLER. — Jahresversammlung des Verbands Deutscher Elecktrotechniker. 1907. J. A. Fleming. — Eoyal Institution Lectures on Electric Heating and Pyrometry. 1910, June 4 and 11. J. A. Fleming.— Proceedings Eoyal Institution. 1910. Vol. xix. MISCELLANEOUS RULES AND REGULATIONS Electric Lighting Act, 1909, Clause 25. Wiring Rules of the Institution of Electrical Engineers. 1907. Vol. xxxix, p. 231. Standardization Rules of American Institute of Electrical Engineers. 1907. Vol. xxvi, p. 1796. Home Office Special Rules for the Installation and Use of Electricity in Mines. Jouriu I.E.E., 1907. Vol. xxxix, p. 255. Home Office Regulations for Electricity in Factories and Workshops. 1908. Board of Trade Regulations under Electric Lighting Acts, 1882 and 1888; revised 1906. Board of Trade Regulations for Overhead Wires and Guard Wires. Interim Report of the Engineering Standards Committee on British Standard Specification for Tubular Tramvray Poles. Interim Report of the Engineering Standards Committee on British Staiudards for Electrical Machinery. Bulletin of Bureau of Standards. Government Printing Office, Washington. Vol. ii. No. 1. Phoenix Fire Office Rules. Phoenix Assurance Company, Ltd., Phoenix Fire Office, 19 Lombard Street, London, E.G. Notification of the Metropolitan Gas Referees.