'0M0'0^MM00M'M iU '-{a GOEHELL UNIVERSITY LliEiRY. g :| This bdok^is ^f to be taken | ©^ from tho^^ffeading Room. ^ '(??) WHEN„06nE WITH, RETURN W^ONCE TO (^©. SHELF 1^^ ©SM^' ©■!^'t^® '!• 0^'0>f; A Cornell University f Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031240520 '^»S?i™?rii,Sli?™,.''y"^"'°" electric machines ,. 3 1924 031 240 520 olin,anx MAGNETO- AND DYNAMO- ELECTRIC MACHINES. A New Series of Handbooks for Students and Practical Engineers. Crown Svo. With many Illustrations , MAGNETO- AND DYNAMO-ELECTRIC MACHINES. By W. B. EssoN. 7s. 6i. GAS ENGINES. By W. Macgregoe. 8s. U. BALLOONING. By G. May. 2S. 6i. ELECTRIC TRANSMISSION OF ENERGY. By C Kapp, 7s. dA. ARC AND GLOW LAMPS. By J. Maier. ^s. 6d. ON THE CONVERSION OF HEAT INTO WORK. By Wm. Anderson. 6s. Ready shortly > — THE TELEPHONE. By W. H. Preece and J. Maier. GALVANIC BATTERIES. By Prof. George Forbes. MAGNETO- AND DYNAMO- ELECTRIC MACHINES. WITH A DESCRIPTION OF ELECTRIC ACCUMULA TORS. A PRACTICAL HANDBOOK. TRANSLATED FROM THE GERMAN OF GLACER DE CEW BY F. KROHN. SECOND EDITION, ENLARGED. With a Preface and an Additional Chapter on the LATEST TYPES OF MACHINE. BY W. B. ESSON. LONDON : WHITTAKER & CO., 2, White Hart St., Paternoster Square. GEORGE BELL & SONS, York .Street, Covent Garden. 1887. cornell\ university! LIBRARY LONDON : PRINTED BY J. OGDEN AND CO, 29, 30 AND 31, GT. SAFFRON HILL, EsC PBEFAGE. In the preface to the second edition of this work, a fitting opportunity is presented for brief remark on pro- gress made since the first edition appeared. The construction of the dynamo has in recent years passed into the hands of mechanicians who, thoroughly conversant with the principles underlying the designing of machines generally, have completely remodelled it from a mechanical point of view. To this is to he attributed in no small degree, the greater capacity of modern machines as compared with the capacity of those designed but a few years ago. Simultaneously with improvements effected in its me- chanical construction, its general proportions have been influenced to an unexpected extent by a more correct apprehension of the principles upon which its action depends. From a crude appliance, mechanically and electrically imperfect, has been evolved within a period ii PREFACE. comparatively short, a machine which as an agent for the transformation of energy must be classed amongst the most eflicient. The whole of the energy of which a machine is the recipient must reappear in some form or other, and the commercial efficiency of the dynamo is judged by the proportion directly serviceable of the reappearing energy. In modern machines the energy appearing between the terminals and available for lighting and other work reaches from 80 to 90 per cent of that given to the pulley. In machines manufactured a few years ago, the useful return rarely exceeded 60 per cent. The increase of commercial efficiency is directly attri- butable to progress in two directions. First, the internal losses arising from the generation of eddy ciurrents in the armature cores and supports have been almost eliminated ; secondly, the electrical energy expended in forcing the current through the armature and in exciting the magnets has been reduced to a very small amount. Progress in the first direction has been achieved by giving special attention to the method by which the armature is attached to the spindle and by properly laminating its iron core. As a result of successive im- provements, the combined losses arising from eddy currents, and from friction of bearings and collector brushes have been reduced in ordinary working to about 5 per cent of the power given to the driving pulley, the conversion efficiency thus reaching 95 per cent. In the second direction progress has been made by PREFACE. iii putting into the field magnets and armature cores more iron than it was formerly the practice to employ, the double result being a stronger magnetic field created by the expenditure of a smaller amount of energy and an armature of low resistance in which the electrical waste is less. In modern machines there appears between the terminals from '9 to '95 of the energy converted. In other words the electrical efficiency varies from 90 to 95 per cent. Although this step by step progression is due to the ac- cumulated labours of many workers in many fields, yet taking a prominent position amongst the factors which have influenced it, is the recognition that the path which the lines of force take in a machine form a closed circuit, the magnetic resistance of which ought to be as small as pos- sible. The magnetic field is proportional to the excitement supplied in amp6re-turns divided by the combined resist- ance of the field magnets, armature core and air-space which together make up the complete circuit. The resist- ance of each of these parts is separately found by dividing its length measured in the direction of the lines of force by its cross section and multiplying by a coefficient which in the case of the magnet and armature core has a value depending on the degree of saturation employed. These coefficients are easily obtained from experiment and as a consequence the behaviour of dynamos can be pre- dicted previous to their construction with close approxi- mation to accuracy. The cross section of the armature core determines the number of lines of force which can be enclosed by the iv PREFACE. rotating coils, and hence the e.m.f., which can be produced. The latter is quite independent of the shape of the core or of the extent of the polar surfaces, but on the other hand with an extended surface the field is produced by a smaller exciting current due to diminished magnetic resistance. If the armature core is too small, a great number of force lines leak from pole to pole without going through the coils, and energy is consequently wasted in creating a field which cannot be utilised. That uniformity of opinion concerning the best ratio of the cross section of the armature to that of the magnet has not yet been reached, is evident fi-om the fact that in modem machines this ratio varies from '5 to 1"0. Given a machine having an armature core of definite cross section and the efficiency, so far as the armature is concerned, can be increased by making the lines of force in the core more dense, since then a greater number is enclosed by a given length of wire. Although the density is always greater in machines having armature cores relatively small, a high degree of saturation in the arma- ture means a proportionally increased leakage across the poles. Bearing this in mind, it is not unlikely that the best result wiU be secured by making the cross section of the iron in the magnetic circuit uniform throughout, even if the armature core is thereby saturated to a less degree. The exact process by which modem dynamos have been made electrically more efficient, wiU be readily com- prehended. The e.m.f. produced at a given speed depends, other things being equal, on the product of the PREFACE. V armature core area into the number of convolutions. The energy wasted in the armature depends on the number of convolutions. By reducing the latter factor of the pro- duct and increasing the former, or, in other words by diminishing the copper and increasing the iron, the energy expended in the armature has been reduced to a very small quantity. But a diminished quantity of copper on the armature means a diminished air-space, and from this, combined with the increased masses of iron in the fields and armature, has resulted a magnetic circuit of low resist- ance in magnetising which there is expended an amount of energy comparatively small. In the present edition of the work the writer has undertaken the task of describing the latest types of machine ; but it will be found that, in several particulars, those of the best makers differ from each other. This is explained on the grounds that the object sought to be ob- tained is different in different cases, and designs will continue to be modified to suit circumstances and in accordance with the conditions under which the machines have to work. At the present time we find the same manufacturers constructing machines of widely differing types to suit requirements as widely differing. Descrip- tions of the latest of these will be found in Chapter XII., and it is hoped that the work has been rendered more valuable by leaving the original translation intact with the descriptions of the older forms untouched, inasmuch as there is thereby afforded a direct comparison between machines old and new. The student is asked to read this chapter in conjunction with the original text when he will vi PREFACE. be able to see more clearly the direction of recent progress. In conclusion the writer has to thank Messrs. Mather and Piatt, K. E. Crompton and Co., Paterson and Cooper, W. H. Allen and Co., Chamberlain and Hookham, The Anglo-American Brush Electric light Corporation, The Elwell Parker Co., and The Grulcher Electric Light Co. for the illustrations and descriptions of their machines, and the proprietors of " The Electrical Eeview," " The Electrician," and " Industries " for the use of several illus- trations. W. B. ESSON. London, January, 1887. G0NTENT8. Practical Units xi Introduction : tiie historical [development of magneto- and dynamo-electric generators— Induction phenomena— Tlie prin- ciple of Pixii's generator — The commutator — Stohrer's magneto- electric generator — Siemens' cylinder — Pacinotti's ring — Cuirents in a ring armature — Direct influence of poles — Construction of Pacinotti's machine — Wilde's machine — Principle of dynamo-electric machines — Ladd's machine with two cylinders 1 — 28 CHAPTER I. Machines generating alternating currents— The Alliance ma- chine — Dr. Meritens' machine — Holme's machine — Weston's plating- machine — Mbhring and Bauer's plating-machine — Gramme's alternat- ing current-machine — Siemens' alternating current-machine — Brush's dynamo 29 — 49 CHAPTER II. Machines generating direct currents — Gramme's ring and collec- tor — Generator with laminated magnet — Gramme's dynamo — Path of the current in the coils of tripolar magnets — Fein's generator — Schuckert's generator — Heinrich's generator with grooved ring- armature — Fitzgerald's generator — Jiirgensen's generator — Giilcher's dynamo — Hefuer-Alteneck's drum-armature — Course of the current in the drum-armature — Magneto-electric machine with drum-armature — Siemens' dynamo — Dynamo-electric generator for obtaining pure metals — The latest dynamo-electric generator of Siemens and Halskg — Connections in this generator — Weston's dynamo — Maxima dynamo — Wallace-Farmer's generator — Lontin's dynamo— Biirgin's dynamo — Niaudet's generator — The large Edison machine , . . 50—87 Till CONTENTS. CHAPTER in. Particular' applicability of thie various electric generators- Alternating current machines— Direct current machines— Magneto- electric machines— Dynamo-electric generators: their disadvantages— Advantages of magneto-electric generators with electro-magnets 88 — 94 CHAPTER IV. Automatic "-switches and current regulation— Siemens' switch —Maxim's regulator— Double-wound machines— Self-regulating ma- chines — Storage necessary 95—103 CHAPTER V. Electrical storage — Plante's elements — Their constructions — Forming of the Plants element — The Elwell-Parker accumulator — Faure'a batteries — Comparison of the Plants and Faure elements — Trustworthy data as to the efficiency of accumulators — Reynier's statements — Swan, Sellon-Volckmar accumulators — Brush's accumulator — Theory of action in accumulators — Applications of aocumulators^Their use as reservoirs and regulators — Their limited use as portable stores — Calculation of their value for electric railways and tramways — Their application in permanent connection with an electric-motor . . . 104 — 13* CHAPTER VI. The physical laws bearing on the construction of electric generators, and their practical application.- Relation of the electromotive force of a generator to the external work — Theoretical deduction of the law bearing on this point— Modification of the theo- retical data in practice — Internal resistance and external resistance in theory and practice — Relation of the strength of current to the number of turns of wire on the armature — The rate of rotation and its influence — The increase of temperature of the coils, and the calculation of the ' resistance thus caused — The intensity of the magnetic field — Special laws for the calculation in the case of dynamo-electric generators — Sir William Thomson's theory with regard to the advantageous dimensions of the armature coils and electro-magnet cores of dynamos. 140 — 163. CHAPTER VII. Construction of the several parts of electric generators. — The field-magnets — The manufacture of steel magnets — Haker and Elias' coefficient for the portative power of a magnet — Jamin's investi- gations on the distribution of free magnetism — Frankenheim's investi- gations —Elias' method for making steel magnets — Deprez' small CONTENTS. ix electro-magnetic motor — The coefficients found for various kinds of iron by Plueker and Barlow — The construction of the armature — Varia- tion — Period of change in the magnetisation of the iron core — Position of the brushes — Heating of the iron core, and reasons therefor — Most advantageous way of placing the armature with respect to the magnetic poles — Collectors and commutators — Diminution of the surfaces of fric- tion — Distribution of the formation of sparks — Edisons' collector — Mechanical construction of electric generators . . . 164 — 181 CHAPTER VIII. The employment of electric generators for producing the electric light. — Special rules for the construction of electric light gene- rators — The "Trinity House" report — ^Advantages of large generators — Tables of comparison — ^Report of the Military College at Chatham — Comparison between the Gramme and the Siemens generators — Com- mittee of the Franklin Institute in America — Tresca's experiments on expenditure of work and efficiency — Gramme's experimepts with electric light generators 182—202 CHAPTER IX. Various applications of electric generators. — Generators for galvano-plastic purposes — Current interrupters and circuit closers — Efficiency of some galvanoplastic generators — Gfenerators for obtaining pure metals and preparing ozone — ^Employment of electric generators for the melting of iridium, platinum, and steel — Sir WiUiam Siemens' smelting apparatus — Schwendler's experiments in telegraphy — ^Arrange- ments of the " Western Union Company " in New York — Employment of small generators worked by hand or foot — Electrical transmission of energy and its future 203 — 211 CHAPTER X. Formulae for the construction of electro-magnets —General formulse for electro-magnets— Dependence of the magnetic moment and the power of attraction on the resistance of the coil — The most advan- tageous diameter of wire — The best ratio of the magnetising coil to the diameter of the iron core — Conditions for maximum on shunt cir- cuits 212—225 X CONTENTS. CHAPTER XI. Instruments for measurements in connection with electric generators. — Ayrton.and Perry's dynamometer — The Commutator- Ammeter — Commutator- Voltmeter — Ammeter and Voltmeter without commutator — Ammeter and Voltmeter with springs — ^Ammeter and Voltmeter with cogwheel and gear— Energy measurer— Dr. Tobler's formulae for taking measurements in connection with dynamo-electric generators — Measurements for dynamos whose magnets are excited by the main current — Measurements for dynamos whose magnets are excited by a shunt-current 225 — 241 CHAPTER XII. Latest types of generator — Alternating current machines — Con- tinuous current machines — Field magnets — Pole-pieces — ^Armatures — Working temperature — ^Sparking at the brushes — Weight, size and output — De Meritens' magneto generator — The Gordon generator — Ganz's generator — The Ferranti generator — The Thomson-Houston dynamo — The Brush dynamo — 'The Raffard-Breguet dynamo — The Goolden-Trotter dynamo— The Manchester dynamo — The Phoenix dynamo — The Kapp dynamo — The two-pole Gulcher dynamo — The Crompton dynamo — The Elwell-Parker dynamo — Gulcher multipolar dynamo — The Victoria dynamo — The Edison-Hopkinson dynamo — The Edison dynamo — The Weston dynamo — The Chamberlain and Hookham dynamo — The Thomson-Houston incandescence dynamo . 242 — 301 PBAGTIGAL UNITS. I. Mechanical Units :- Unit of Time Unit of Space Unit of Force Unit of "Work Unit of Power II. Elbctkical Units:— Unit of Current Unit of Quantity Unit of E. M. F. Unit of "Work Unit of Power Unit of Resistance One Minute. One Foot. The Force which wiU support a weight of one pound. One foot-pound = a force of one pound exerted through one foot. The horse-power = 33,000 foot-pounds per minute. The Ampere. The Coulomb = the quantity of Electricity passing in one second if the current is one Ampfere. The Volt = a little less than the e.m.f. of a Daniell's cell. The Joule = one Coulomb at an e.m.f. of one volt (sometimes called the volt- coulomb). The "Watt or volt-ampire = 60 Joules per minute. The ohm = a resistance such that a current of one Ampfere flows due to an e.m.f. of one volt = the resistance ofEered by a column of mercury 106 centi- metres in length by one square milli- metre section. III. Electro-Mechanical Equivalents: — One horse-power = 746 "Watts. One watt = 44 •236 foot-pounds per minute. One foot-pound = 1'356 Joules. One Joule = "73726 foot-pounds. I"V". Unit of Output of CrBNEBATOKS = 1,000 "Watts. INTEODUCTION. HISTORICAL DEVELOPMENT OF MAGNETO-ELECTRIC AND DYNAMO-ELECTRIC MACHINES. So long as only galvanic batteries were employed in practice for generating electric currents, that is, so long as these currents were obtained solely by chemical action, it was but natural that, for doing large quantities of work, the application of electrical energy should be very limited For the cost of maintaining a battery is too high as compared with its efficiency, and it is almost impossible to obtain in this way constant currents of great quantity and intensity. A larger field for the application of electrical energy was opened up when use began to be made of electric currents, produced by the conversion of mechanical energy, through the invention of electric machines. Faraday's researches on the phenomena of induction, formed the theoretical basis for the construction of electric machines. He had shown that when a current circulates in a wire, A A', Fig. 1, a, forming part of a circuit, momentary currents will under certain circimoistances be induced in a neighbouring wire, B B', parallel to the first. These B 2 INTRODUCTION. currents will flow in the direction of the primary current, A A', or in an opposite direction, according to circum- stances ; and this direction can easily be observed by the deflections of the needle of a galvanometer connected with the wire B B', A current in a direction opposite to that of the primary current, that is in a direction from B' to B, will be generated : (1) at the moment when the primary current Fig. 1. V^ is started; (2) when the wires A A' and B B' are ap- proached to each other ; and (3) when the current in A A' is strengthened. A current is set up in the same direction as that of the primary current, that is from B to B' : (1) at the moment when the current in A A' is interrupted ; (2) when the wires A A' and B B' are moved away from each other ; (3) when the current in A A' is weakened. The discovery of the currents generated on the approach of the wires to each other (approximation currents), and on their being moved away from each other (retrocession INDUCED CURRENTS. 3 currents), was of special importance for the construction of electric machines. Far stronger induction currents are generated if the primary wire, as well as the wire in which currents are to be induced (the secondary wire) are coiled into spirals, or helices, and if both are so placed that the separate turns of the one can act on those of the other, as shown, for example, in Fig. 1,6. In this case, the strength of the induced current B B' increases, generally with the num- ber of turns or convolutions in the two wires ; for under the conditions assumed, small currents are induced by each turn of the primary wire A A', in every neighbour- ing turn of the secondary wire B B', and these unite to form a total strong current. The practical importance of this fact is only fuUy learned from the results of investigations by Ampere, who dis- covered that a magnet may be considered to be a piece of iron perpetually encircled by parallel electric ciirrents, and that by approaching a magnet to a conducting wire or by moving the magnet away, currents can be induced in the wire, in the same way as if there were used a wire spiral through which a current is flowing. It is to Pixii that the honour is due of having made the first practical application of this discovery. He con- structed the first magneto-electric machine in 1832. The mode of action of this machine, illustrated in Fig. 2, will be made clear from what follows. According to Ampere every magnet is encircled by parallel electric currents in such a way that if the north pole is pointed towards the observer, they circle in a direction opposite to that of the hands of "a clock, whilst, if the observer faces the south pole, the currents flow in the direction of the hands of a clock. INTRODUCTION. In Pixii's machine. Fig. 2, there is a compound horse- shoe magnet which can revolve on its axis, and above the poles of this magnet. is fixed the armature. This consists of two wire coils,, whose convolutions form a continuous helix. These coils contain two soft iron cores, which at every approach of the poles of the magnet are themselves converted into magnets. Fig. 2. Fig. 3 shows more clearly in what way the coils of wire are wound, and this figure serves better for explanation. Whenever the pole iVof the magnet approaches the soft iron core a of one of the coils, a will become a south pole, for a magnet always magnetises a piece of iron close to (but not touch- ing) it in such a way that a south pole is induced opposite the north pole, and a north pole opposite the south pole. At the same time, according to Ampere's law, electric currents wiU be generated, and these will then circulate round the iron core of the coil in the direction of the hands of a clock. But as soon as these currents are started, they will induce others in the turns of the wire coil, which are seen in the figure to flow from right to left. Simultaneously, however, 6 wiU become a north-pole on account of the pole 8 approaching it, and Amp^rian PIXII'S MACHINE. currents will commence encircling the iron core of the second coil from, right to left. The moment these are started, they induce currents in the coils surrounding the iron core, which flow in the direction of the hands of a clock. A careful examination of Fig. 3 will show that the two currents simultaneously generated in the two coils, though seeming to flow in opposite directions, ^ ' really form one current in the wire system, as indicated by the arrows. This current traverses the wire system in the same direction, from j3 to ^ jj'j and can be conducted away by the terminal wires p and- p'. An opposite current, one from p' to p, is induced in the wire system of the coils as soon as the poles of the magnet, N S, begin to move away from the iron cores a and b, in continuing their revolution. For the resulting gradual demagne- tisation of the iron cores causes a weakening of the Amperian currents encircling them, and, according to Ampere's law, this weakening must induce currents in the surrounding turns of wire of the opposite direction. Finally, the poles of the magnet will again approach the iron cores, in such a way that N approaches b, and S approaches a, and since a now becomes a north-pole and b a south-pole, a current will be induced in the wire coils 6 INTRODUCTION. opposite in direction to the original current of approach, but which is only a continuation of the current produced by the preceding demagnetisation of the iron cores. All this the reader will easily perceive if he makes an analysis of the separate processes. After the second current of approach follows a retrocession current, and so on. From what has been said, it will be seen that in every complete revolution of the magnet N S round its axis, the current in the turns of the wire coils changes its direction twice, the changes taking place at the moments when the poles N S pass the ends of the iron cores. In order to convert the two opposite currents, gene- rated during each complete revolution of the magnet N S, into a current of single direction, where this is desirable, a commutator is added to the machine. Fig. 4 shows the principle on which it is constructed. One end of the conducting wire of the armature coil is connected with the metallic segment A, Fig. 4, a ; the other end of the wire is connected with segment B. The segments are separated from each other by a strip of insulating material, i i, and the commutator is fixed in such a way that it revolves once round its axis simulta- neously with the magnet (or simultaneously with the armature in the machines to be subsequently described). Now, if the induction current be supposed to flow through the turns of the helix, in the direction from A to B, when the pole N approaches a, and the pole S ap- proaches b, it is clear that it wiU flow in the direction from i to i' in the conducting wire L L', whose terminals bear on the metaUic segments. When the poles N and S recede from a and b, and the retrocession current is started, the current in the spiral changes its direction, and now COMMUTATOR. y traverses the latter from B to A. In order to avoid a change of current in the conducting wires, the com- mutator is arranged so that at the moment when there occurs the change of current, and when the helix is momentarily currentless, the conducting wires L L' bear on the insulating portion of the commutator, Fig. 4 b. Simultaneously with the commencement of the new cur- rent, L bears on A, and L' on B; consequently, although the current in the spiral of the armature flows from B to A, it will continue to flow in the original direction in the conducting wires, that is, from L to L'. Thus the two opposite currents, generated diu-ing each complete revolu- tion of the magnet, are made to flow in the same direction in the external circuit by means of the commutator. Pixii's machine had this practical disadvantage, that the heavy compound magnet rotated in front of the armature. Subsequent constructors, as Saxton, Clarke, and others, modified the machine, making the lighter armature rotate in front of the magnet. Besides this, Saxton placed the magnet as well as the bobbins of the armature in a hori- 8 INTRODUCTION. zontal position, whilst Clarke retained the vertical position of the magnet, but placed it with the poles downwards, and made the bobbins of the armature rotate at its side. It seems also, that Saxton was the first to employ the commutator previously described. Stohrer considerably increased the eflSciency of magneto- Fig. 5. electric machines by increasing the number of the magnets as well as the number of armature-bobbins. In this machine six armature-bobbins rotate before the six poles of three compound magnets. The coils of the bobbins are wound in such a way that at each approach of the coils to the magnet poles currents flowing in the same direction are generated, and these unite to form one current ; again, every time the bobbins of the armature recede from the poles of the magnets, currents are induced STUHSER'S MACHINE. g which flow in the coils in a direction opposite to that of the currents of approach. Accordingly, in each complete revolution of the armature six compounded currents of approach, and six compounded currents of retrocession are generated in the wire coils of the armature-bobbins, and each of these, again, is composed of six elementary currents. By means of a commutator, connected with the machine, these alternating currents can be rectified before reaching the external circuit. The results obtained with Stohrer's machine were so satisfactory that subsequent inventors, adopting his idea, continued to increase the number of the armature-bobbins and magnet-poles, and in this way succeeded in obtaining currents of remarkable strength, following each other in rapid succession. Thus, by degrees, were developed the large magneto-electric machines for alternating currents, such as those of the " Alliance Company " constructed by NoUet, and the machines of Holmes, Lontin and others, which, side by side with the machines for continuous cur- rents, are still, under certain circumstances, used in practice, especially for the electric light in lighthouses. All these machines, which in principle are only enlarged Pixii machines, will be dealt with in the chapter on " Machines generating alternating currents." A notable m.odification in the construction of magneto- electric generators was made by Dr. Werner Siemens, of Berlin, in 1857, who greatly improved the shape of the armature-bobbins. Experiment soon showed that the strength of the ciu:- rents produced by a magneto-electric generator is in- creased when the coils of the armature are brought as near as possible to the magnetic poles ; and that the efficiency of a machine depends further on the interruption lO INTRODUCTION. being as short as possible when the current changes its direction. Siemens took both these experimental laws into account in the construction of his armature, the simplest form of which is represented in Fig. 6. An iron cyUnder, a b, Fig. 6 a, is provided with two grooves, and the spirals of the conducting wire are wound round this cyUnder, parallel with its axis, in such a way that the two grooves are filled up, the complete cyUndrical Fig. 6. form being thus restored, Fig. 6 b. One of the terminal wires is connected with the shaft of the armature, whilst the other is connected with a ring which is insulated from the shaft. Two springs, one of which bears on the in- sulated ring, and the other on the shaft, conduct the cur- rents to the external circuit. In Fig. 7, a small Siemens machine is illustrated, and from this figure can be seen the position that the rotating cylinder takes up between the poles of the magnets. The magnets are usually numerous. The poles of the magnets, SIEMENS' ARMATURE. ii Fig. 6 c, have semi-circular pieces cut out, so that the cylinder is well-surrounded ; in fact in the whole con- struction of Siemens' machine, the inductive action of the permanent magnets is far more completely utilised than in the magneto-electric generators previously described. One special advantage in the form of the cylindrical armature is that not only is the armature magnetised as completely as possible, but the poles of the steel magnets Fig. 7. can also directly exert a strong inducing action on the spirals of the wire coils, and this considerably strengthens the currents induced by the magnetism of the soft iron core. Besides, in the cylindrical armature the time during which the current is interrupted is reduced to a minimum, as the change of poles takes place in an extremely short interval of time, when the cylinder is rotated rapidly; and this circumstance greatly increases the eflSciency of the machine. The form of cylindrical armature just described (proba- INTRODUCTION. Fig. 8. bly now used in Siemens' alarm bell indicator only), was later on considerably modified, and so much improved that cylindrical armatures still very successfully hold their position, and have not been supplanted by the " ring " armature, which we now proceed to describe. The ring armature, which caused quite a revolution in the construction of electric generators, was invented by Dr. Antonio Pacinotti, of Florence, in 1860, and by its means he succeeded, for the first time, in obtaining, with an electric ma- chine, continuous elec- tric currents flowing in I a constant direction, without raaking use of a commutator. As Pacinotti's ring arma- ture is to be found in principle in all ma- chines constructed on the " Gramme " sys- tem, and as the prin- ciple of its construction is employed in almost all continuous-current machines, a carefid analysis of its action will be necessary. When a ring of soft -iron. Fig. 8, is placed between two magnetic poles, N S, a. south pole will be induced oppo- site the north pole N, and a north pole opposite the south pole. Now, if the iron ring is caused to revolve round its centre from right to left, new portions of the ring will in turn be magnetised, whilst the portions of RING ARMATURES. 13 the ring previously magnetised will lose their magnetism. Accordingly, the poles of the iron will travel along the ring from left to right. If this ring, as shown in Fig. 8, be encircled by a continuous coil of copper wire, insulated from the iron ring, currents will be induced in the wire, and the direction will be different in the several turns of the coil, accordingly as these are nearing the north pole or the south pole of the iron ring ; moreover, as the poles of the ring change, in consequence of its rotation, the currents also in the convolutions of wire surrounding it will change their direction. To understand better what takes place during the re- volution of the ring, it may be considered that the ring is fixed. For, in any case, its poles will always lie in the line N S, and it can be assumed that the windings of the coil move from left to right round the ring, and thus the maximum inducing action of S. Fig. 9. u are brought under the poles N and For the sake of simplicity, the inducing effect on a single turn of the coil need only be considered, and it may be supposed that the ring, is divided in two, so that two north poles meet opposite S, and two south poles opposite N. In this case, Amperian currents will circle round the ring, as indicated by the arrows in Fig. 9. Let us com- 14 INTRODUCTION. mence our observation at the moment when the turn x y is at the point where the dotted line h cuts the ring, and when it commences to move from left to right. Con- sidering only the nearest portions of the ring, we see that the turn or loop approaches the divisions A and B, re- ceding at the same time from divisions and H. Now let us call the current + when its direction is from the circum- ference toward s th e centre of the ring ; and — when its direc - tion is from the centre towards the circumference. Let us also denote the current induced in the loop by the two ad- joining segments of the ring by + 2 or — 2, and that in- duced by the portions of the ring which are more than 45° away by + 1 or — 1. We shall then be able to express the current induced in the loop at the moment when it com- mences to move from h towards the right by the sum of the factors + 1 +2—2,-1, i.e., by ; for a ciirrent of retreat will be induced in the loop x y hj its moving away ' from the segments of the ring G and H, and, according to the law of induction, this current will flow in the same direction as the Amperian currents in the iron ring. On the other hand, by its approach to the segments A and B, a current will be induced in the loop, traversing the latter in a direction opposite to that of the Amperian currents in these portions of the ring. Accordingly the two currents will neutralise each other at the moment when the loop quits h, for they will be of equal strength. If the loop has arrived at the point a, and commences its further course round the ring, it will be possible to express the current of approximation induced in it by the segments B and C, together with the current of retrocession induced by the segments H and A as the sum of the following factors, + 1+2— 2 + 1=+ 2. When the loop X y leaves b, the sum equals +1+2 + 2 + 1, RING ARMATURE THEORY. IS that is, = -f- 6. On quitting c, the induced current can be represented by +1— 2 + 2+1 = + 2, and aga,in, when the loop has passed d, we can express the action by the sum of the factors — 1 — 2 + 2 + 1, that is, by 0, and the loop has, therefore, no current induced in it. Now, when the loop reaches the point e and continues to move towards that portion of the ring which is most strongly influenced by the north pole N, the current in- duced in it after it leaves e will be expressed by the sum of the factors — 1 — 2+2 — 1, that is, by — 2, the cur- rent, therefore, has changed its previous direction. At / the current, which is now flowing in a new direction, will reach its maximum, and be equal to — 6; at g its strength will again have fallen to — 2 ; and after the strength at h has sunk to 0, a change of current will again take place, and the expressions for the strength of the induction currents will have the sign + prefixed. What we have said of the one turn x y oi the coil, of course equally applies to turns or groups of turns which come into the respective positions; and from the preceding considerations we see that the whole wire system which surrounds the ring can be regarded as being traversed by two opposite total currents meeting at p and p'. Fig. 8. Besides, according to the explanation, we shall be able to denote that current by +, which is compounded of all the separate currents in the turns which surround the portion A B CD/of the ring, whilst the other current, which is compounded of the currents traversing the turns surround- ing the section E F H oi the ring may be distinguised by the prefix — . The rotation of the ring only so far changes the condition of things that new turns and groups of wires cojQ^ajitly come into the regions where the two i6 INTRODUCTION. opposite currents dommate, and accordingly in every complete revolution of the ring, each turn passes both the neutral points once. What has been said, however, refers only to the inducing action that the magnetised iron ring exerts on the wire coils surroimding it ; all that we have now to do is to analyse the action of the fixed poles 8 and N, Fig. 8, on the wire system rotating between them. We shall retain the position of the poles as shown in Fig. 8. Fig. 10. In this position the turns of the coils are perpendicular to the plane of cross-section of the poles when they pass the latter. Fig. 10 will serve best for an explanation. In this A B represents a portion of the rotating ring, the turns of wire on which have been assumed to be moved a little asimder, though in reality they are very close together. S indi- cates the south pole, which in the figure is supposed to be imder the ring. The arrows indicate the direction in which the Amperian currents encircle the ring. We will suppose the ring to be moving in the direction from A to B, and willjonly consider those portions of the coils that RING ARMATURE THEORY. 17 are on the outside of the ring, and indicated by full lines in the figure ; the portions on the inside of the ring are indicated by dotted lines, and are omitted from con- sideration for the present. We then perceive that in turns I. and II., currents of approach will be generated, having a direction opposite to that of the Amp^rian currents at x (from top to bottom in the figure), whilst in the turns VIII. and IX., currents of retrocession will be generated (also flowing from top to bottom in the figure) that are due to the direction of the Amp6rian currents at y, from which they are moving away. In other words, currents of the same direction are in- duced in all the turns, left and right of the south pole, and this direction is exactly the same as that of the cur- rents induced by the magnetism of the iron ring. Now, if we take those portions of the turns into con- sideration that lie on the inner side of the ring, which are indicated by the dotted lines, we see that, if the iron core of the coils were not present, tKe south pole, S, would induce currents which would also flow from top to bottom in the figure. Accordingly, the currents induced in the internal and external portions of the turns would oppose each other ; and if the portions of the turns on the inside of the ring were as close to the pole of the magnet as the portions lyiiig on the .outside, the induced currents in the inner portions would be equally as strong, and would destroy the others. This, however, is not the case, for, firstly, the portions of the turns which are on the inside of the ring are farther away from the pol^ S than the por- tions on the outside, and the currents induced in the latter would, therefore, in any case, have the greater power, even if the iron core were not present ; secondly, the iron ring really does separate the inside portions of the turns; c i8 INTRODUCTION. from the magnet, and completely annuls the action of the magnet on those portions, so that the currents generated in the parts of the turns lying on the outside of the ring act unopposed. As these currents flow in exactly the same direction as those induced by the magnetised iron ring, they considerably strengthen its action. In our explanation we have, however, oidy spoken of the turns lying right and left of the pole, and we have not con- sidered the turns III., r\'., V., VI. and YII. The action of the pole S on these turns is equal to zero, as can be seen from what follows. The current induced in turn IV. will principally be a retrocession current from x, and it will accordingly flow in the direction of the Amperian current at x. In turn YI. a current of approach wiU predominate, and this will flow in the same direction as the previous one, because its direc- tion must be opposite to that of the Amperian current at y. The receding from x and the approach to y will exert an equally powerful influence on turn V — ^that is, a current of double strength will be generated in it, flow- ing in the same direction as the currents in IV. and VI. Now, if all the turns are the same distance apart from each other, which is the case in the figure and in reality, the opposite currents in 11. and IV. wiU have the same strength, and will neutralise each other, and a change of current will take place in turn III., for which reason this turn will be eurrentless. Sinularly, there will be no cur- rent in turn VII., on account of the currents in VI. and VIII., which neutralise each other ; and as currents I. and IX. are equidistant from V., the double current in V. will be annulled by currents I. and IX., which are opposite in direction to it ; all the tiums from III. to VII. win accordingly be eurrentless. As a result, therefore. RING ARMATURE, CONNECTIONS. 19 we only obtain the currents generated in the turns right and left of the south pole, and this, as we have observed, strengthens the current generated in the coils of wire by the magnetism of the iron core. It need scarcely be added that the action of the north pole on the neighbouring turns is quite analogous, and, from the preceding con- siderations, we can clearly see that the two total currents of opposite direction, generated by the magnetism of the iron ring unite with the currents produced by the direct Fig. 11. influence of the magnet poles on the coils, and which also flow in contrary directions. The two intensified currents which are thus generated in the wire system, surrounding the ring can be collected at the neutral points. Fig. 8, p p', where their directions meet ; and as shown in Fig. 11, a, they can be united into a single current and be conducted away, like as the cur- rent from the cells of a galvanic battery, connected for quantity. Fig. 11, 6. Pacinotti has effected this collection by dividing the wire coiling surrounding the ring- armatiu:e into groups, and by giving to the separate parts of his machine the form shown in Figs. 12 and 13. INTRODUCTION. The iron ring, which moves between two magnetic poles, has sixteen grooved spaces in it, and these serve to receive sixteen wire coils. Fig. 12, all wound in the same direction. The end of each coil is soldered on to the commencement of the next, so that the turns of all the coils unite to form one continuous helix. Wooden wedges are driven into the recesses of the iron ring, and separate the coils from each other. From the joints at which the terminal wire of one coil and the ^' ^^' wire commencing the next are soldered together, cop- per wires branch ofiF and run from the inner side of the ring to the shaft. There these lead to several brass terminal pieces, in- sulated from each other, and forming a ring, fixed to the shaft of the ma- chine, Fig 13. Two con- tact rollers, k k, bear on this ring, and are plaxjed in such a position that they are always in contact with those brass pieces that are attached to branch wires leading from the solderings, p p'. Fig. 8, momentarily at the neutral points. If the conducting wires are connected with these contact rollers, sixteen total currents will traverse the circuit at every revolution of the ring ; for every one of the sixteen solderings passes each of the neutral points once. AU these currents will flow ia the same direction without a com- mutator being necessary; here, then, is a basis for the construction of electric machines for continuous currents. PACINOTTI'S MACHINE. 21 The construction of the ring-armature was afterwards considerably improved by the Belgian inventor, Gramme, and the ring-armature employed by him will be more fully described in the chapter on machines generating con- tinuous currents. A further advance in improvement of electric machines was made by H. Wilde, of Manchester, in 1866. Wilde conducted the currents generated in a Siemens' eylinder- Fig. 13. armature, e c, Fig. 14, by the permanent magnets M M, through the coils of a large electro-magnet, E E, after they had been rectified by a commutator. This electro- magnet induced currents in a second Siemens cylinder- • armature, K K, and these, of course, were of considerable intensity, because the armature was moving in a very powerful magnetic field. With a machine constructed in this way results were obtained such as had not been ap- proached previously by any electric machine. Wilde later on constructed a machine with which he obtained stiU stronger currents. This he did by leading the currents 22 INTRODUCTION. induced in the armature K K through the coils of a second electro-magnet of very great dimensions, and by making this magnet act on a third cylinder-armature, in the wire of which was generated the current to be used for doing work. Wilde's machines soon found wide application in practice, and a number of them were employed in the celebrated galvano-plastic works of Elkington, in Bir- mingham, for obtaining galvano-plastic deposits on a large scale; some were used for the production of the electric light in photographic studios, and others again were employed in Whitechapel for preparing ozone as a bleaching agent. Nevertheless, even these machines left much to be de- sired. It appears that through the heating of the iron cores, the current was considerably weakened after a working time of several hours, and it could not be kept constant long enough to be employed with advantage in the pro- duction of the electric light for lighthouses. The causes of the heating of the iron cores are discussed in another part of this book. In all the machines yet described, the electric currents were induced by means of steel magnets, or, as in Wilde's machine, by magnets that were magnetised by the cur- rent produced in another machine. Such machines are usually called " magneto-electric" machines, to distinguish them from the " dynamo-electric " machines. In the latter the inducing magnets, " field " magnets, as these are termed, have cores of soft iron, which, at starting, only possess a very small trace of magnetism ; this trace is however, sufficient to induce a weak current in tie coils of the rotating armature, which current is then used to strengthen the magnetism of the field or inducing WILDE'S MACHINE. 23 magnets. This is done by the current being conducted through the wire coils surrounding the soft iron cores, which are accordingly magnetised more strongly, and now Fig. 14. induce a new but stronger current in the coils of the armature. The new current again increases the mag- netism of the iron c^res, by being conducted, like the previous one, through their wire coils. By this reciprocal 24 INTRODUCTION. action^ currents are generated in the armature-coils far surpassing in strength any that can be produced with machines of the same size, having only steel field-magnets. The machines without permanent magnets have been named dynamo-electric machines, or, shortly, dynamos. Siemens, in Berlin, and Wheatstone, in London, almost simultaneously discovered the principle of the dynamo- electric machines. Siem.ens perhaps, deserves the right of priority, as in December, 1866, he had made experiments, before several Berlin scientists, with a machine in which there were no permanent magnets.* In the middle of January, 1867, he communicated his discovery to the BerMn Academy of Science : whilst it was only in February that "Wheatstone communicated the result of his dis- covery to the Koyal Society in London, in a paper entitled: "On the Augmentation of the Power of a Magnet by the Eotation thereon of Ciurrents induced by the Magnet itself." These results agreed fully with those obtained by Siemens. Curiously enough, the discovery of the Berlin physicist was made known by his brother Dr. William Siemens, at the same meeting of the Eoyal Society as that at which Wheatstone delivered his lecture ; and Wheatstone's conmiunication immediately followed that of Siemens. The simplest kind of dynamo is shown in Fig 15. E E are two plates of soft iton, each about 60 cm. long 50 cm. wide, and 10 cm, thick. They are united at one end by a third plate, P. Each of the plates E E is sur- * The German scientific writeis, have in their good fellowship for their countryman, generally overlooked the fact that "Wneatstone most also have experimented ; and fhete are several scientific men who claim to have seen his machine at work at the earlier time indicated as that occupied by Siemens with exhibition in Berlin. The honour, however, can well be divided. DYNAMO-ELECTRIC PRINCIPLE. 25 rounded by coils of well-insulated and rather thick copper wire, about 27 m. long. The coils on each plate are wound in such a way that they may be considered as constituting a single coil, whose terminals, c, d, lead to the binding screws b and a, 7 is a Siemens' cylinder-armature, which rotates between the projecting ends of the iron plates. One ter- minal of its coil is connected with the shaft A, and the other terminal is connected with a copper ring, fixed to the shaft, but insulated from it. A spring, 2, in connection Fig. 15. with the screw b, bears on the shaft A, whilst a spring, 1, connected with the screw a, bears on the copper ring jR. Before using the machine for the first time, the soft iron plates E E are slightly magnetised, by connecting the wires c and d with a galvanic cell, and causing a cur- rent to traverse the coils surrounding the iron cores ; when this ceases a small quantity of residual magnetism is left. However, it is not necessary to do even this, for through the influence of terrestrial magnetism, a slight magnetic polarity is induced in all soft iron plates that lie in a certain position, and this magnetism is quite sufiBcient to induce the first current in the armature of a dynamo- 26 INTRODUCTION. electric maxjhine. After such a machine has once been made tx) give a current, there is always enough magnetism remaining, at any time, to generate electric currents. If the cylinder-armature is set in motion, alternating currents will be generated in its coils, during its revolu- tion between the poles. These currents are rectified by means of a commutator, omitted in the figure ; and as the terminals of the armature-bobbin are in connection with the terminal wires of the coils surrounding the iron cores, through the screws a and b, the currents encircle the iron cores, which will finally, after a certain speed of revolution of the armature has been attained, be mag- netised to the maximum extent possible ; in other words, the machine reaches the maximum of its efficiency, and the powerful currents generated in the armature manifest themselves by a great spark if the circuit is broken at any point. This spark can, for instance, be used to explode mines or torpedos, and so forth. Or the powerful currents of the machine can be used as in Siemens' " alarm-bell inductor," to ring large signal-bells in stations, and to work other signaUing apparatus. An important modification of the dynamo-electric ma- chine was carried out by an Englishman, Ladd, only four weeks after the date of the papers previously mentioned. He sent a description of his dynamo, to the Eoyal Society on the 14th of March, 1867, and exhibited it at the Paris Exhibition in the middle of May, 1867. The specialty of this machine was that the two iron plates were not connected with each other, but were con- verted into two electro-magnets, between whose poles rotated two armatures. Fig. 16, one of which served to Strengthen the magnetism of the iron plates, according to the dynamo-electric principle, whilst the currents from LAUD'S MACHINE. 27 the other could be used in doing work, that is, in pro- ducing the electric light, or obtaining galvano-plastic deposits. Later, the firm of Siemens and Halske, of Berlin, also constructed machines with several armatures, and obtained particularly good results with, a machine consisting of three pairs of plates lying horizontally and parallel to each' other. These were converted into six electro-magnets when the machinery was set in motion. Six armatures Fig. 16. revolved between the twelve poles, and the currents generated could be combined in the most varied ways by means of specially-constructed commutators. Other constructors, too, gradually changed and im- proved the dynamo-electric machines in various ways. All these improvements, however, contained nothing new in principle, and only dealt with the arrangement of the separate parts. A change in the principle of the electric machine has not taken place since the discovery of the 28 ISTRODUCTION. dynamo-electric principle. Accordingly, all the machines described in subsequent chapters, and at present used in practice, depend on modifications and combinations of the historical apparatus we have considered. CHAPTER I. MACHINES GENERATING ALTERNATING CURRENTS. Although electric machines may be classified in various ways, as they may be considered from different points of view, and although they are usually divided into magneto- electric and dynamo-electric machines, yet, for a rational classification, it is preferable to group them according to the nature of the currents generated. In the two follow- ing chapters, therefore, they have been divided under " Machines generating alternating currents," and ' Ma- chines generating continuous currents. The first large alternating current machines were con- structed by the " L' Alliance " Company, and were intended for the production of the electric light in lighthouses. They have become known as the Alliance machines. The inventor of the Alliance machines, which are still manu- factured by the same Company, was NoUet, Professor of Physics at the Military College of Brussels. NoUet's machine has been much modified and improved, on the one hand by Professor Masson, who, amongst other things, omitted the commutator, formerly employed with this machine ; on the other hand, by Van Malderen, engineer to the Alliance Company. The following is the construction of the present pattern of Alliance machine (Fig. 17). 30 MACHINES GENERATING ALTERNATING CURRENTS. Several brass disks are fixed to the shaft of the machine, one behind the other. Each of these carries 16 armature bobbins, which are attached at equal intervals, and each disk revolves, with its bobbins, between the poles of strong compound permanent magnets. These magnets are fixed radially to the horizontal bars of the cast-iron frame, so that two opposite poles face each other, and that the magnets arranged in a line are of alternate polarity. By means of this arrangement each bobbin is brought, when the brass disks revolve, between poles of opposite polarity, and the iron cores are converted into magnets, which induce powerful currents in the wire coils of the armature- bobbins. These coils are wound in such a manner that they may be considered to form a single large helix, one "ALLIANCE" MACHINES. 31 end of which is fixed to the shaft of the machine, the other leading to a ring fixed on the shaft, but insulated from it. The alternating currents are conducted into the external circuit through springs which bear on the ring and shaft. The Alliance machines usually carry, on the shaft, 4' or 6 of the brass disks, and accordingly the number of armature-bobbins, that move past the poles of the 40 or 56 magnets, is 64 or 96. As, in each revolution of the disks, each side of the 16 armature-bobbins passes 16 mag- netic poles, it follows that the current changes 16 times in each revolution ; and as in practice the shaft makes 400 revolutions per minute, when a steam-engine of about 5-horse power is used, there are about 100 changes of current per second. The intervals between the currents are, therefore, so short that they scarcely come into pon- sideration, and, for certain purposes, the currents, taken together, may be regarded as a single current. As regard* the dimensions and construction of the separate parts of the Alliance machine, the following data, given by Count du Moncel, may be of interest. The- horseshoe magnets are made at the works of Alvarre ; each weighs 20 kgrs., and each is composed of 5 or 6 steel laminae about 1 cm. thick, screwed together. In order to have the poles as uniformly magnetised as' possible, bars of soft iron are attached to the ends of the steel magnets. Each complete compound magnet can carry three times its own weight. The bobbins of the Alliance machine have gradually undergone much modification. At present they consist of iron tubes, which are slit through the whole length. These are enclosed in brass cylinders, also slit along the middle, and on which the wire-coils are wound. Each 32 MACHINES GENERATING ALTERNATING CURRENTS. bobbin is 10 cm. long, and has a diameter of 4 cm. The length and cross-section of the wire depends on the resistance of the external circuit, also on the number of bobbins and the work that the machine is intended to do. Fig. 18. Jil*^^^^^ In the machines for the electric light, strands of wire are used, which consist of 8 wires, each about 1 mm. thick and 30 m. long. The wires are wound with cotton ; and, to insure as perfect insulation as possible, they are dipped, before winding, into a solution of resin and tur- pentine, a very good insulating solution, which only very slightly increases the thickness of the wires. DE MERITENS' MACHINE. 33 With an Alliance machine of this construction, having a shaft carrying 4 disks, a light can be obtained, when a steam-engine of 5-horse power is employed, equal in intensity to that of 1,110 candles or 150 Carcel lamps (one Carcel lamp being equivalent to 7*4 standard candles). If a machine with 6 disks is used, a light of 1,480 candle-power can be obtained. The results obtained with the Alliance machines are good, and the machines are used for the electric light in many lighthouses (those of Cape La Heve, near Havre ; Cape Grriz-Nez, near Calais ; Kronstadt, Odessa, and others) ; but only a small number are made, for the con- struction is very complicated and costly, compared with that of more recent machines. De Meritens' Machine is considered by many en- gineers to be the best alternating current machine, and certainly very good results are obtained with it. This machine, especially the latest pattern, is not unlike the Alliance machine in appearance,: but differs considerably in the peculiar construction of the armature. The armature consists of an eight-spoked wheel, to the ,rim of which 16 bobbins are fixed in such a way that the iron core of one bobbin forms the prolongation of the iron Core of that preceding it. All the iron cores together form a ring, as shown in Fig. 18. The cores consist of 50 iron plates, 1 mm. thick, and at each end they have iron pole-pieces composed of similar plates. Fig. 19. The pole-pieces of each bobbin are con- nected vrith those adjoining by bars of copper. All the coils of these bobbins are wound round the iron cores in the same direction, forming a single long helix, whose terminals lead to two copper rings on the shaft of the machine, but insulated from it and from each other. 34 MACHINES GENERATING ALTERNATING CURRENTS. Two copper springs bear on these rings and conduct the Guirent into the outer circuit. The 8 inducing, or field, magnets, which are fixed to the outer framework of the machine, Fig. 18, are compound steel magnets, and are provided with pole-pieces, Fig. 19, so that the armature-bobbins revolve as near the magnetic poles as possible. The magnets are placed in such a way ?ig. 19- that poles of opposite polarity are always nearest to each other. When the armature is caiised to revolve, the bobbins pass close under the poles of the magnets ; the soft iron cores are magnetised, and alternating currents are induced in the wire coils, which are intensified by the direct action of the magnets on the coils. This arrangement, which brings the wire coils of the armature into the direct magnetic field of the permanent magnets, is one of the chief advantages of the De Meritens machine. The construction is ..ko particularly prac- WESTON'S PLATING MACHINE. 35 ticable, as the separate parts of the armature can, if necessary, be easily removed, without disturbance of the other parts, an absolute impossibility in many other machines. Another generator, which closely resembles the Alliance machine, is that of Holmes. Indeed, it is almost identical in construction with the Alliance machine, only , that Holmes, who has constructed a number of different pat- terns, has, in his last, replaced the steel magnets by V-shaped cores of soft iron, wound with wire coils, to form electro-magnets under the current from the machine. The cores of soft iron are attached to a disk which rotates in front of a second fixed disk, to which the armature-bobbins, are bolted. The coils of these bobbins are not connected with each other, but are united in groups, so that the machine can generate several currents at the same time, each of which can be used independently, so that one machine can work several /separate electric lights at the same time. In order to rectify the currents, the machine is provided with a commutator. Weston's machine for galvano - plastic purposes (Weston's light-machine is described in Chap. II,) con- sists of an iron drum, on the inner surface of which six cast-iron electro-magnets are fixed radially, each reaching to about the middle of the radius of the drum. Six smaller magnets, fixed radially to the hub and shaft of the machine, revolve close under the poles of the larger magnets, Fig. 20, These smaller electro-magnets form the armature, and the coils of two are united into one circuit, so that three horseshoe magnets are obtained, in the coils of which alternating currents are generated, and these are collected and, rect;ifi|ed by a, spepialjy-con- 36 MACHINES GENERATING ALTERNATING CURRENTS. structed commutator. The wire coils of the six field- tuagnets form one circuit ; but the wires are wound so that Adjacent poles are of opposite polarity. In order to mag- netise these electro-magnets, the currents generated in the armature are conducted through their coils on the dynamo-electric principle. To prevent the coils of the electro-magnets becoming overheated, their iron cores are made hollow, and are in ■ connection with a cir- Flg. 20, , ,. , . culatmg water system. In Weston's machine, the commutator, which Rectifies the currents generated in the coils of the armature, con- sists of a broad cylin- drical wheel, fixed on the shaft, having three teeth, and into the spaces between these teeth metal plates are inserted. These are con- nected with each other, but are insulated from the wheel. Three terminal wires, of the same sign, lead from the armature to the teeth, whilst the other three are con- nected with the little metallic plates. As each of the metallic plates is diametrically opposite to a tooth of the wheel, it is only necessary to let two springs bear on the commutator, directly opposite each other, to collect the currents. These will, at the same time, be rectified, for the metaUic plates are alternately in connection with one or the other of the springs. This construction of Weston's machine is principally MOHRING AND BAUR'S MACHINE, 37 used for galyano-plastic purposes, and it is provided with a " current interrupter," the object of which is instantly to interrupt the electric circuit, so that any current generated by the polarisation of the electrodes in the galvanic bath, cannot reach the coils of the electro- magnets, when the machine is going too slow or stops. Under certain circumstances, this would reverse the polarity of the machine, and currents would be induced in the armature, whose action would redissolve the galvano-^ plastic deposit. This current interrupter is described amongst the apparatus in Chap. IV. Weston's machine was improved by H. Gr. Mohring, of Frankfort-on-the-Maine, and Grustav Baur, of Stuttgart. The dynamo - electric machine of Mohring and Baur also has 6 field-magnets, and an armature consist-; ing of 6 electro-magnets. The magnets are, however, attached to the inner surface of the cover of a cylinder,' and, by means of a screw, the armature-magnets can. be approached to or moved away from the field-magnets,, thus making it possible to regulate the machine at any time. Besides this, the coils of the field-magnets do not form a single helix, as in Weston's machine; but each helix sur- rounding a field-magnet is separate from the others. The currents induced in the turns of the armature-bobbins,: are conducted to an insulated pin, after leaving the com- mutator, and thence they enter the six wires, leading to the coils of the field-magnets, which are magnetised as in, Weston's, machine, so that adjoining magnets are of dif- ferept' polarity. The currents pass from the six terminal, wires of the magnet coils to a second pin, and thence flow^ intq the external cir.ciiit. If required, the, currents can; 38 MACHINES GENERATING ALTERNATING CURRENTS. be conducted separately through different circuits and can then be united. This machine is also provided with a current-interrupter, as It is intended for galvano-plastic purposes. Lontin's alternating current machine resembles that of Weston very much in the arrangement of the separate patta, excepting that the twenty-four field-magnets are fixed to the hub of a wheel on the shaft, and revolve in front of twenty-four armature-magnets, which are attached radially to the inside of a drum-like frame, and converge towards the centre ; as do the field-magnets in Weston's machine. The wire coils of the inducing electro-magnets form a connected helix, and opposite poles are placed next to each other, as in the machines previously described. The current for magnetising the magnets is produced in a separate Lontin machine for continuous currents (see Chap. II.), the star-shaped wheel of which is fixed to the principal shaft of the alternate current generator. The coils of the armature-bobbins are connected with each other in pairs, and in such a way that, together, their cores form horseshoe magnets. One of the terminal wires leads from each to a binding screw on the right-hand side of the machine ; and the second leads to a binding screw on the left-hand side. Conducting wires branch off from the different binding screws on both sides of the machine, and twelve alternating currents are sent through them during each revolution. By a very simple commutating apparatus these currents can be combined for quantity or intensity, or they can be united into smaller groups. " The special adyantage of this machine is that neither a commutator nor contact brushes are necessary to con- duct the main currents away. Brushes, or contact pieces, usually wear rapidly, fw the sparks which occur burn or GBAMME'S .ALTERNATING CURRENT MACHINE.. 39 very much oxidize the metal patts^ if the currents are strong. This machine also possesses the advantage that if necessary the separate parts can be easily replaced with^ out the necessity of taking the machine to pieces. ..1 When the working rate is 320 revolutions per minute Lontin's machine generates 12 currents, each of which is able to produce an electric light of 740-candle power. A very efficient alternating current machine has also been constructed by M. Gramme, the inventor of the well- known machines for continuous currents, with the object of producing a suitable generator for the Jablochkoff candles. The following is the construction of Gramme's alternating current machine. Eight electro-magnets, K K (Figs. 21 and 22), provided with pole-pieces, are fixed to a steel shaft, F, by means of two cast-iron crowns, H, and an eight-sided cast-iron hub, /. In the older patterns of the generator, the current traversing the coils of these magnets was obtained from a small separate exciting machine, and conducted through two brushes bearing on two insulated rings. In the newest pattern this current is generated in a Gramme's ring- armature, which is attached to the principal shaft of the machine, and is brought under the influence of two electro- magnets. The armature of Grramme's alternating current machine consists of a broad ring of soft iron, on which are wound thirty-two bobbins completely separated from each other, their turns running parallel to the axis of rotation. These bobbins are divided into eight groups, and at certain periods, during the revolution of the field-magnets, each of these groups is opposite the pole of one of the magnets, and the polarity of the magnets alternates from one 40 MACHINES GENERATING ALTERNATING CURRENTS. to another. Accordingly, when one of the groups of thfe armature is opposite a north pole, each of the adjoining groups is opposite a south pole, and vice-versa. The posi- tion of the coils, with respect to the magnetic poles, is so Fiff. 21. symmetrical that when the field-magnets pass before them, currents of equal intensity are induced in all the coils, a ; similarly, the currents induced in the coils 6 are equal to each other in intensity ; and currents are induced in the coils c and d, their strength corresponding to their posi- tion. If the direction of the currents, therefore, is to be GRAMME'S MACHINE. 4* the same in all the similarly lettered coils, all that is necessary is that the bobbins, which are situated opposite different polarities, during the rotation of the field-mag- nets, should be coiled in opposite directions. As the terminals of each coil lead to separate binding screws, attached to the frame of the machine, it is possible to FJK. 22. conduct thirty-two separate alternating currents from the machine, or to combine these currents for quantity or intensity, as desired. The framework consists of a cast-iron standard, D, at front and back, and both nearly circular. These are fixed to a cast-iron base, R, their rigidity being further ensured by eight brass bars, E, and an iron support, U. To protect thfe electro-magnet bobbins from injury, by action of centri'- 42 MACHINES GENERATING ALTERNATING CURRENTS. fugal force, two thin disks, T, are attached to them, and these are firmly connected with the shaft. Gramme con- structs three sizes of this pattern of alternating current generator. The machiue described generates current for sixteen Jablochkoff candles, and to work it 16-horse power is neces'* sary. Its length, including the driving pulley, is 89 cm. ; width, 76 cm, and height, 78 cm. ; it occupies a space of ^ cub. m., and weighs 650 kg., 103 kg. of which are the copper wire. The maximum velocity of rotation is 600 revolutions per minute. The next size is intended for six Jablochkoff candles. To work it 6-horse power is necessary ; it is 70 cm. long, 40 cm. wide, and 52 cm. high, occupying a space of 0*15 cub. m. It weighs 280 kg., 40 kg. being due to the weight of the copper wire. The maximimi velocity of rotation is 700 revolutions per minute. The third size supplies four Jablochkoff candles, and re- quires 4-horse power. It is 55 cm, long, 40 cm. wide, and 48 cm. high, and occupies a space of 0'18 cub. m. The weight is 190 kg., of which 28 kg. are copper wire, and the maximum velocity of rotation is 800 revolutions in the minute. As already stated, the latest pattern of Gramme's machine for alternating currents contains the generator for magnetising the field-magnets in itself. Gramme made this change because in the generator previously de- scribed transmission of the power by belt was diflScult, and a disturbance was frequently caused in the uniformity of the light. In the latest pattern, Gramme's ring-armature (see Chap. II.) is fixed to the shaft of the machine, and two of the eight field-magnets are directed radially towards this . ' 1 ^ SIEMENS-HALSKE MACHINE. 45 ring, and are provided with pole-pieces j which generate two tr3,velling poles in it (see Pacinotti's ring). The current produced in the coils of the ring is then conducted througlj a popper wire, which can be replaced by another of different cross-section and length, if the strength of current in the 1 wo machines is to be modified. The current, as in the older pattern, is then taken to two annular disks, both of which are insulated from the shaft and from each other ; and from these, it passes into the coils of the field-mag- nets. Two sizes are built of this pattern. The largest machine of this kind weighing 470 kg., generates current for twenty-four candles each of 148 to 220 candle power, or for sixteen candles each of 296 to .370 candle power. The smECUer generators weigh 280 kg,, and supply twelve candles each of 148 to 220 candle power, or eight candles each of 296 to 370 candle power. Experience with the new machines shows that they are superior to the machines of the older type, especially in the production of a imiform, steady light. The firm Siemens and Halske also construct machines for alter- nating currents. The Siemens^Halske alternating current machines have also been gradually modified ; the fundamental principle has not, however, been changed. The following is the construction of the latest pattern : Cast-iron standards are fixed to a base, and are held to- gether at the top by a bar. Each of these standards carries twenty-four magnets on its inner face, and they obtain their magnetising current from a small Siemens continuous cur- rent exciting machine. This current magnetises the iron cores in such a way that adjacent magpets, as well as mag- nets facing each other, are of opposite polarity. The pro- jecting ends of the iron cores of the electro-magnets are 44 MACHINES GENERATING ALTERNATING CURRENTS. provided with pole-pieces, consisting of flat pieces of iron, and these serve to strengthen the inducing action on the armature-coils. In the older pattern the armature-coils are wound on an iron ring, somewhat as in the Gramme machine. In Fig. 23. the new machine, shown in Fig. 23, they axe fixed to the rim of a wheel, and do not contain any iron, but have wooden cores, which are perforated for the sake of ven- tilation. This so far gives the machine a considerahle advantage over those previously described, as there is no change of polarity in its movable iron parts. The heating of the armature-coils, which would otherwise occur, is SIEMENS-HALSKE MACHINE. 45 obviated, and there is not the waste of work due to this heating and change of magnetism. Another advantage arises from the arrangement of the diiferent parts, by which the wire coils are made to rotate through fields of high magnetic intensity, formed by the powerful electro-magnets placed opposite each other. The number of armature-bobbins, and of the magnetic fields, produced by the pairs of opposite poles of the field-- magnets which face each other, is the same ; the coiling of FifT. 24. the bobbins changes from one to another. If by a we denote all the bobbins in which the coils are wound in one direc- tion, and h, all those which are coiled in the opposite direction, we see that at a given moment, as the armature revolves, all the alternate bobbins a will be opposite the north poles on the anterior side of the machine, and all the bobbins 6 opposite the south pole. Immediately afterwards the bobbins a will be opposite the south poles on the same side, and the bobbins h will be opposite the north poles. Accordingly, the. current will change with each change of position of the bobbins ; the number. 46 MACHINES GENERATING ALTERNATING CURRENTS. therefore, of alternating currents generated will be equal to the niunber of armature-bobbins or of magnetic fields. The alternating currents induced in the armature-coils are conducted to contact rings, which are fixed to the shaft, but are insulated from the latter and from each other. They are then conducted to the external circuit by means of contact springs or brushes, which are connected • with the conducting wires leading to the external circuits For electric lighting, the Siemens alternating current machine renders good service, and it has proved of special use in connection with the Hefner-Alteneck dififerential lamps. Brush's Machine is, perhaps, the most original, and, at any rate, the most efficient of the machines that pri- marily generate alternating currents (Figs. 24, 25, 26). The armature of this machine is very much like Paci- notti's ring in outward appearance ; but the connection of the wire coils is different. The cast-iron core of the armature contains a vertical groove, shown in Fig. 25, which nearly completely divides it; it also has deep recesses of rectangular section on both sides, which are meant to receive the coils. The teeth thus formed are again traversed by three deep grooves. This grooving of the solid iron core serves, on the one hand, to prevent in- terfering induction currents being generated in the ring, which would weaken the current and heat the iron ; and, on the other hand, it aids ventilation diuring the revolu- tion of the armature, and serves thus to keep cool the wire coils. The wire coils, of which there are eight, completely fill the rectangular recesses in the iron core, and every pair of diametrically opposite coils are connected with each other, whilst the terminal wires are conducted to the four com- BRUSH'S MACHINE. 47 mutator rings, where they are attached to two segments, insulated from each other. The current generated in the two bobbins is conducted from these segments by means of brushes, consisting of slit pieces of sheet copper. Fig. 24 illustrates a machine for lighting sixteen arc-lamps. The commutator, which consists of four copper rings, differs considerably from all those that have been de- scribed, and is undoubtedly one of the most interesting features of the Brush machine. The construction of a commutator ring is shown in Fig. 26. Every ring consists of two segments, SS, which approach each other very closely at one point. They are ther« insulated from each other by a small air space, whilst between the other two ends there is a piece of metal, T, which is insulated from them, and correspond- ing to |~r evolution. This is called the " insulator," and serves to exclude that pair of bobbins from the circuit whose coils are at the moment in the neutral positipn, during the revolution of the armature. For when the bobbins are in this position, one of the brushes presses against the " insulator," the pair of bobbins is excluded from the principal circuit, and, what is more important, no current can traverse their coils, as there is no closed circuit. The four brushes, each of which bears on two of the commutator rings, can be shifted concentrically around the shaft of the machine, the proper regulation of the generator at any time being thus possible. They are presged against the commutator rings by strong clamps. The conductors leading from the brushes are of thick strips of copper. The armature, whose coils are separated by segmental cylindrical . sections -of the iron ring, similar- to the 48 MACHINES GENERATING ALTERNATING CURRENTS. wooden wedges in Pacinotti's ring, rotates very near to and between the poles of a pair of powerful horizontal horseshoe magnets, the iron cores of which are a little flattened, and have similar poles facing each other. To prevent the shaft of the machine from being displaced lengthways, it has grooved bearings in the journals as in the case of the shafts of screw-steamers. The segmental ring-shaped pole-pieces exert a very powerful inducing Fig. 25. Fig. 26. action on the whole armature, with the exception of the bobbins in the neutral position. In a lecture given by Brush in America, he explains that the arrangement of the field-magnets is such that the current generated by each pair of coils is alternately conducted through the coils of the electro-magnet and the external circuit. Thus, during each complete revolu- tion, every pair of bobbins once supplies the magnets with a current and once the lamps. In one position of the ring some of :the armature -coils send their current through the Coils of the magnets, and the others are in BRUSH'S MACHINE. 49 connection with the external circuit. During the next eighth part of the revolution of the ring, those bobbins that were connected with the coils of the magnets send their current into the circuit, and vice versa. According to Brush, the armature of the sixteen-light machine offers a resistance of about 4 ohms, and with a speed of 750 revolutions per minute, will supply 16 to 18 lamps, each of which has an arc of about 2mm, and offers a resistance of 4^ ohms, so that the machine generates a current which is able to overcome an external resist- ance, including that due to the electro-magnets, of about 80 ohms, or a resistance nearly twenty times greater than that of the arma,ture. A larger Brush machine has been constructed for forty lights ; and (according to Brush) actuated by 30-horse' power, it gives a current of 10 amperes, with an electro- motive force of 2,200 volts. In " Engineering," Vol 31, 1881, p. 55, the following data are given in connection with the sixteen-light machine represented in Fig. 24. The diameter of the ring is 20 ins.; the wire on the eight bobbins is No. 14, B. W. Gr. (= 2-15 mm. dia- meter); the weight of the wire in each coil is about 20 lb. (== 91 kgrs.) ; length of wire in one coil is about 900 ft. (= 275 m.) ; resistance of the four limbs of the , electro-magnets is 6 ohms ; the resistance of the machine from binding-screw to binding-screw is 10*55 ohms. When working sixteen lamps, the machine made 770 revolutions a minute, and the working power was 15*5 horse power ; the electro-motive force was 839 volts, the strength of current 10 amperes, and the resistance of one lamp 4*5 ohms. CHAPTKR II. MACHINES GENERATING DIRECT CUREENTS. Tbde fundamental basis on which the majority of direct or continuous current generators is constructed is the ring- armature of Pacinotti, -which, however, only attains its full importance in connection with Gramme's ingenioiisly con- structed collector. Zenobe Theophile Gramme, whose alternating current generator was described in the previous chapter, was formerly one of the employes of the " Alliance Company," and independently constructed a ring-armature in 1871, without knowledge of Pacinotti's work. In principle it agreed with the ring-axmature invented by the latter. The following is the construction of Gramme's armatiu-e and collector (perhaps incorrectly termed commutator by Pacinotti). In order to prevent interfering, heating, false, or so called Foueault currents, the core of the ring is constructed of annealed soft iron vrire, the copper conductor woimd round it being composed, as in the Pacinotti ring of groups of helices. These, however, are not separated from each other by projecting iron teeth, but follow one another closely. The wire commencing each coil is con- GRAMME ARMATURE. 51 nected with the wire ending the previous one, and con- sequently all the coils together form one continuous circuit. The number of coils varies in dififerent machines, and each consists of 300 or more turns. The points of junction all Ue on the same side of the ring, as shown in Fig. 27, and are connected with strips of copper, bent at right angles, one arm of which, M, lies radially and edge- wise along the side of the wooden hub, whilst the other arm is within this ring and runs parallel ^'■S' 2^- to the shaft. These copper strips, which are equal in number to the bobbins, are separated from each other by an insulating^, material, and their horizontal arms form a hoUow Cylinder (compare Fig. 29) through which the shaft passes. Two contact brushes, consisting of fine copper wires, bear on this cylinder, and are always in contact with those strips that are in connection with the poittts of junction in the neutral positions, p p'. Fig. 8, that is the points through the connection of which the two total currents generated in the wire system flow in the same direction. Direct currents, consequently, flow through the two brushes when they are in contact with the respective collector strips. Gramme constructs his riug-armature machine in several sizes, and with various modifications. Some of 52 MACHINES GENERATING DIRECT CURRENTS. them are arranged to be worked by hand or foot power, and are intended only for laboratory use, or for smaU quantities of work ; others are constructed to be driven by steam, and differ more or less from one another, acoordiug to the object for which they are intended. As, Fig, 28. however, the principle is the same in all these generators, a brief description of some of the most useful and most frequently employed types will suffice. Of the machines intended for use in physical labora- tories, the pattern shown in Fig. 28 is the best ; it is constructed by Breguet, of Paris. This is a magneto- electric machine, the field magnet of which is a so-called J AM IN' S MAGNETS. 53 laminated magnet) consisting of several steel plates, held together by clamps, a b, but dividing out a little at the poles. These plates are provided with massive pole-pieces, that nearly enclose the ring-armature which revolves be- tween them. The French physicist Jamin, to whom the Fig. 29. construction of this kind of magnet is to be attributed, calls it a normal magnet, as in it the maximum magneti- sation of the steel plates is attained. It therefore pos- sesses far greater portative power than magnets of the same size composed of simple steel bars. The Gramme generator with steel magnet; a§ con- 54 MACHINES GENERATING DIRECT CURRENTS. strueted by Bregaet, gives a current equal to that from three Bunsen's cells of ordinary size. Amongst the large Gramme generators, we shall specially mention those for the electric Ught. The framework of these generators consists of two iron standards, held together at top and bottom by two stout cylindrical cross-bars of soft iron. These cross-bars are converted into tripolar n^agnets, when the current induced in the ring-armature traverses the coils surroimdingthem. These coils are wound in such a way that all the poles of the tripolar magnets, opposite each other, are of different polarity. A glance at the direction of the current in Fig. 30 will make this clear. When the current generated in the coils of the ring- armature enters the helices of the lower tripolar electro- magnet at p', and traverses them in the direction of the arrows the right-hand half of the lower magnet has a south pole induced at s', and a north pole at n'. If we follow the coiurse of the current further, we see that there are formed in the right-hand half of the upper tripolar magnet, a south pole, s', and a north pole, n'. Again, the left-hand half of the upper magnet, n, becomes a north pole, and s a south pole ; and in the left-hand half of the lower magnet, a north pole is obtained at n, and a south pole at s. Therefore, in the middle of the upper tripolar magnet, a north pole, N, is produced, consisting of the portions n n', and in the middle of the lower tripolar magnet, a south pole, S, is formed, of the two portions s s'. This is the action in the electro-magnet cores of the Gramme machine, represented in Fig. 29, and in order to utilise the double poles more completely, they are pro- vided with heavy pole-pieces of soft iron. These nearly enclose the ring-armature, which is constructed as GRAMME'S MACHINES. 55 described, and revolves on a steel shaft. The brushes and collector are shown in Fig. 29, on the right-hand side of the machine, and the currents, which the brushes conduct away, are not only employed to do work in the external circuit, but also to excite the field-magnets on the dynamo- electric principle, already explained. Grenerators of this kind weigh 180 kgrs, ; are 0*60 m. high, 0*35 m. wide, and 0*65 m. long (including the belt- Fig. 30. pulley, shown on the left-hand side of the figure). The copper wire used for the field-magnets weighs 28 kgrs., whilst the copper wire coils of the ring weigh 4-5 kgrs. The generator usually works at a speed of 900 revolutions per minute, and produces a current which will maintain an electric light of 10,656 candle power. Gramme's plating-machine, constructed in 1873, also deserves mention. This generator weighs 177"5 kgrs., 47 kgrs. of which are due to the weight of the copper. It is 0"60 m. in height, and has a width and breadth of 0-55 m. With it a deposit of 600 grs. of silver per hour 56 MACHINES GENERATING DIRECT' CURRENTS. is obtained in an electrolytic bath, and to do this, f horse- power is necessary for driving the machine. The field- magnets are not wound with copper wires, as in the machine previously described, but both halves of each tripolar magnet are completely surrounded with a copper sheet, so that altogether the copper conductors of the field-magnets consist only of four broad copper strips or sheets. The coils of the ring-armature, which is almost com- pletely enclosed by the pole-pieces, as in the light-machine, consist of thick wire, flattened. The armature is thus made very strong, and is protected against the action of centrifugal force. This arrangement of the copper windings is suitable for a generator for galvano-plastic purposes, such generators having to produce currents of low intensity, but in large quantity, and this object is to be attained only by means of coiling with thick copper wire, offering a low resistance. The field-magnets obtain their exciting current from the armature, on the dynamo-electric principle, and an auto- matic current interrupter is connected with the generator, as in Weston's and Mohring's generators. This prevents polarisation currents reaching the machine from the baths, and traversing it in the opposite direction. Gramme's generator for the electrical transmission of power, differs somewhat from the other machines of the same constructor, in that four pairs of electro-magnets are used, which are fixed to the inside of an octagonal frame, Of these magnets, two adjacent are at right- angles to each other, and at the point where they con- verge, carry a common pole-piece. ■ In this way four poles are generated, which are alter- nately of ppposite polarity, and nearly completely sur- FEIN'S MACHINE. 57 round the ring-armature, generating four travelling poles. One of the deficiencies which some inventors claim to exist in Gramme's machines is that only the portion of the coils situated on the outside of the ring-armature is exposed to the inducing action of the fixed magnets. This Fein, of Stuttgart, endeavours to avoid. Fein's dynamo, Fig. 31, contains a cylindrical ring- armature, It B, the core consisting of a number of very thin strips of iron, insulated frorn each other ; it is fixed to a brass star-shaped hub, >S S, through the centre of which the shaft passes. The terminals of the coils pass through openings in the star, S S, provided with collars or thimbles pf insulating naaterial, and are led to the collector, (7, which is attached to the portion of the shaft shown on the right-rhand side in the figure, 58 MACHINES GENERATING DIRECT CURRENTS. The field-magnets, as in Grramme's dynamo, have iron cores, which are converted into tripolar magnets by the method of winding the coils. These are provided with pole-pieces, M M', which approach the external portions of the armature coils. In addition, the peculiarly-shaped prolongations, A A, are screwed on to these pole-pieces. They enclose the inside and back portions of the windings of the coils, so that nearly every part of the wire is under the inducing action of the magnetic poles. The construction of Schuckert's flat-ring generator, Fig. 32, is, too, such that the inducing action of the magnets on the coils is more completely utilised than in Gramme's machine. The way in which this is effected differs from that followed in Fein's machine. Whilst in Fein's machine the ring has a cylindrical form, Schuckert has made use of a flattened ring. This flat ring is nearly completely surrounded by the pole- pieces of the field electro-magnets. The core of the iron ring consists of thin insulated plates of sheet-iron ; and the collector and wire brushes are similar in construction to the corresponding parts of Grramme's machine. Schuckert constructs various sizes and patterns of his generators, as well as machines for galvano-plastic pur- poses, which, with the exception of the flat ring, are almost identical in construction with Gramme's machines. He also constructs generators with two collectors, one placed at each end of the shaft. This arrangement is also employed by Gramme, and makes it possible for only a portion of the current to be used for exciting the field-magnets, and the larger part of the current solely for doing work. The advantage is that when these machines are used for galvano-plastic pur- poses, a polarisation current, entering the machine from SCHUCKERT'S MACHINE. 59 the baths, cannot produce an inversion of the polarity of the field-magnets. For the same purpose Schuckert constructs generators with two flat rings ; and these are also provided with an arrangement for combining the currents for intensity or quantity. Heinrich's generator is another design for better Fig. 32, utilising the inducing action of the magnets on the armature. In this machine, the ring-armature, the core of which is composed of a bundle of thick iron wires, has a horse-shoe shaped cross-section, and the wire coils which surround it only lie close upon it on the outside, crossing over the hollow on the inside. In order to bring those parts of the wire that lie on the outside of the iron ring as completely as possible under the inducing action of the electroi-magnets, the 6o MACHINES GENERATING DIRECT CURRENTS. pole-pieces form nearly a complete ring, interrupted in two places. The cross-section of this, too, is horse-shoe shaped, and it thus very nearly surrounds the armature on the outside. The hollowing of the armature is in- tended to aid ventilation. Desmond G. Fitzgerald's generator has an arma- ture somewhat similar to Brush's ring. The coils are separated from each other by iron wedges, and the electro- magnets, which completely surround the ring, consist, for this purpose, of several pieces, which together make up a hollow ring. In this class of generators, however, when the armature is intended to rotate as close as possible to the magnetic poles, and if, at the same time, very rapid revolution is required, the builder has to over- come a great many technical difficulties. For, unless these machines are built very symmetrically, there is great danger of the armature and pole-pieces of the electro- magnets rubbing against each other, and, of course, this would soon remove the insulation, and make the machine useless. A better solution of the problem, how to utilise the armature coils as completely as possible, is obtained in Jiirgensen's generator. In this generator there are electro-magnetic field-magnets inside as well as outside the ring, but as the cross section of the ring and wire spirals is comparatively small, it is only important that the horizontal portions of the wire coils should be ex- posed to the inducing action of the magnetic poles ; and the short vertical portions can be neglected ; the poles of the magnets need not, therefore, take any particularly complicated shape. To prevent Foucault's currents, the core of the arma- ture in Jiirgensen's generator is composed of separate GULCHER'S MACHINE. 6i rings, which are insulated from each other, and consist of iron wire ; and it is worth mentioning that the wire coils of the electro-magnets increase in thickness towards the poles, to obtain a greater concentration of magnetism at those places. A generator noteworthy for its thorough practical construction is Giilcher's dynamo. The dimensions of the few thick copper coils surrounding the electro- magnet cores, and composed of wire strands, show that the machine is intended for currents of large quantity, but low intensity. The flat ring-armature, which is wound much in the style of Pacinotti's ring, as may be seen from Fig. 33, rotates between four pole-pieces, which enclose it like clamps. Proceeding round the armature, the polarity of these changes from one to the other, and each unites two of the electro-magnets, whose similar poles face each other. "The four currents induced in the coils of the armature are united for quantity, and are conducted into the external circuit by means of two contact brushes. The generator, shown in Fig. 33, will supply six Giilcher lamps of 1,300 candle power, when working at^the rate of 940 revolutions per minute, and using 10-horse power. Amongst the advantages of the machine are that its magnet-coil resistance is very small, on account of the four pairs of electro-magnets being coupled parallel to each other ; and that, through the arrangement of the coils, the internal resistance of the machine is only 0*265 ohm, for both the ring and the electro-magnets. The dynamo-electric generator of the firm of Siemens and Halske, in which the driun-armature invented by Hefner-Alteneck is used, differs very much from the machines previously described. fe MACHINES GENERATING DIRECT CURRENTS. DRUM ARMATURES. 63 The simplest form of drum-armature is shown in Fig. 34. In this figtoe N N and S S are the poles of the fieldrmagnets, whilst s a, n n include a hollow cylinder, which rotates with the shaft, and round which the wires are wound parallel with the axis of rotation. Now, as travelling poles are induced in the cylinder diuring its rotation, as in the case of Grramme's ring, and as the interval between the poles of the magnets and- the arma- ture is very small, the coils Inove in a field of high in- tensity. The turns of wire woUnd on to the armature-drum I'ig. Si. parallel to the axis of rotation are divided into from eight to tweilty^eight groups, and form a continuous wire circuit. The terminal wires of the separate groups are connected with the segments of the collector, which has as many segments as there are groups of wire on the drum. The method of connection of the terminals with the parts of the collector is such that the two total ciurrents of opposite direction, generated in the wire system, always meet in two opposite segments of the collector, and can thence be conducted into the external circuit by means of contact brushes. The way in which the connections are made, for this purpose, is shown in Fig. 35. This figure represents a 64 MACHINES GENERATING DIRECT CURRENTS, drum-armature, on which are wound eight wire groups. The separate segments of the collector are denoted by the do letters a, b, x', c, d, e, x, f, and the several portions of the same group are denoted by similar numbers. DRUM ARMATURES. 65 If we start from the collector, and follow the course of the wire of a coil in the figure, we see that when the wire leaves the segment under consideration, it runs to the circumference on the front of the cylinder, then runs parallel to the axis along the cylinder, again crosses the back of the cylinder, and runs along the opposite side, and is finally connected with another segment of the collector. Duringthe rotation, one half of such a rectangularly-bent group or coil is exposed to the influence of the north pole of the inducing field-magnets, and the other half is ex- posed to the influence of the south pole ; accordingly, cur- rents of opposite direc- tion are generated in both halves. For in- stance, in the lower half of the wire, the current will flow from left to right, whilst in the upper half, it will be directed from right to left ; yet both together, form a current of the same direction, as may be seen at a glance, . from Figure 36. However, as the drum rotates, that half of the coil, which at one moment was at the top, will, after a semi-revolution, form the lower half. In other words, during a complete revolution of a coil, each half turn of wire gets once into the position where the north pole dominates, and once into' the position where the south pole dominates. The current will be reversed immedi- ately after the two sides of the coils have passed the neutral points, which are equidistant from the north and south pole. The collector serves to conduct all currents of one direction simultaneously to one terminal, whilst it conducts the currents of the opposite direction to the F Kg. 36. L f ll ^ 1 ^ Jt 66 MACHINES GENERATING DIRECT CURRENTS. other terminal ; that is, the currents of opposite directions are, by it, united to form currents of the same direction. This is made possible by the ingenious manner in which the inventor of this collector has arranged the connection of the wires of dififerent coils with the separate segments of the collector. This arrangement is such that the opposite currents meet, as in Gramme's collector, in two diametri- cally opposite segments of the collector, and they are, as it were, coupled for quantity. If by + we denote, in Fig. 35, the ends of the wire in which the cixrrent flows from the circumference of the collector disk to the centre, and by — , the ends in which it has a direction from the centre to the circumference, we see that two + wires luiite at the collector segment .v, and two — wires at the collector segment x'. In Fig. 11, we have already shown how it is possible to obtain a direct current by connecting the conducting wires with the respective segments. The method of conducting the current away is, accordingly, exactly the same in the Siemens-Halske drum generator, as that employed in the Gramme generator ; that is, the conducting wires are always in contact with the two metallic segments of the commutator in which the currents meet ; and through the revolution of the drum, all the commutator segments come into this position in turn. The drum-armature now described is employed in a large number of generators constructed by the firm Siemens and Halske ; and as generally, these machines have a similar construction, it will be suflBcient to describe only a few of them. Fig. 37 represents a magneto-electric generator by this firm. This machine is constructed to be driven with a belt, and is very similar to the small magneto-electric SIEMENS-HALSKE MACHINE. 67 generators of the same firm, which are worked by the hand or foot. The drum-armature of this machine rotates between the pole-pieces, which closely clasp it, of fifty V-shaped steel magnets, twenty-five of which are at the top, and Fig. 37. ;^WM\\V-m' twenty-five at the bottom, placed with their similar poles •opposite each other. The poles of the top and bottom pairs are connected by soft iron pole-pieces, screwed on ; and magnetic fields -of fairly high intensity are thus produced, in which the ■drum-armature rotates. The collector and brushes are shown on the right-hand of the figure. When the drum is caused to rotate, ciurrents are in- 68 MACHINES GENERATING DIRECT CURRENTS. dueed in the coils surrounding it, by the direct actioii of the field magnets. These currents are made to flow in the same direction by the collector, and are con- ducted to the external circuit by the contact brushes, jK R'. From the construction of the drnm-axmature it wiU be perceived that in this type a greater proportion of the total length of armature conductor may be brought under the influence of the magnets than in the Gramme ring. In the former the whole of the wire is active with the exception of that crossing the ends of the drum whereas in the latter all the wire inside the ring is inactive. In the drum-armature we require less length of wire for the production of a given e.m.f., the advantage over the Gramme becoming more apparent as the length of arma- ture increases, but as a set off against this, the difficulty of removing a faulty coil in machines of the Siemens type ought not to be forgotten. In order to prevent the heating of the iron cylinder, (which occurs in machines for generating large quantities of electricity, in consequence of an undivided metalHc mass moving in the magnetic field), Siemens and Halske constructed generators in which the iron core of the drum is fixed. In these generators, the armature coils are wound on a drum consisting of a sheet of german-silver, which rotates round the iron drum, at a little distance from it and from the enclosing magnetic poles. The posi- tion of the magnetic poles is the same as shown in Fig. 38, and the magnets (the horizontal portions of which are round in the older patterns) are magnetised on the dyna- mo-electric principle. The rate of rotation is 450 revolutions per minute, at which is produced a light of 14,000 candles, with an ex- penditure of 6 -horse power. SIEMENS' DYNAMO-MACHINE. 69 In one respect the fixing of the iron core of the cylinder is a great improvement, but experience has shown that the construction of the machines is made far more difficult, especially in the winding of the armature-coils. Accord- ingly, this construction is now seldom employed, and in the small generators and in those of medium size, the Fig. 38. coil.s are wound directly on to a cylinder composed of iron wires. The complicated connection, too, of the separate coils is not retained in all Siemens' generators. Thus, in the well-known Siemens machines with flat magnets, Fig. 38, a drum-armature is used, the wire coils of which are divided into a great number of groups connected similarly to Gramme's method, and lead to a collector of the Grramme pattern. In these generators, as in that shown in Fig. 37, metallic contact brushes are used, and not contact rollers, 70 MACHINES GENERATING DIRECT CURRENTS. as in the older generators. Siemens and Halske construct generators of this kind in \-arious sizes. One size is 757 mm. long, 700 mm. wide, and 284 mm. high. The drum is wound with 28 wire coils, and the collector consists of 56 pieces. The generator weighs 200 kgrs. Its maximum velocity of rotation is 700 revolutions per minute, and 3^ horse power is necessary to work it. It will generate an arc light of 4,000 candles. The smaller generators of this pattern are 698 mm. long, 572 mm. wide, and 233 mm. high. The armatiu'e in these generators is also wound with 28 wire coils, and accordingly the collector consists of 56 pieces. The weight of the generator is 115 kgrs. Its maximum speed of rotation is 900 revolutions per minute, and 1^ horse power is necessary to drive it. With these generators a light of 1,400 candles is obtained. Small machines of this kind are also constructed with vertical electro-magnets, as shown in the exciting machine, Fig. 23. A very interesting type, the Siemens plating machine, is represented in Fig. 39. These generators are for the preparation of pure metals, and there are three employed in the Eoyal Foundries, at Oker. With each of these five to six himdredweight of copper is precipitated daily, eight to ten horse-power being reqiiired. The electro-magnets of these machines are encircled by thick, square- sectioned copper bars, which make seven tiurns round each Umb of the magnets. The armatiure also carries only one layer of windings of the bands of copper, which correspond to the coils in the other genei-a- tors, and are connected with the collector by suitably bent pieces. The contact-brushes which bear on the collector SIEMENS' PLATING MACHINE. IT- are of stout plates of copper. The several copper parts are insulated with asbestos ; therefore the insulation is not destroyed however hot the copper conductors of the machine may become. In outward appearance the latest dynamo designed by the firm of Siemens and Halske is very like the Siemens 72 MACHINES GENERATING DIRECT CURRENTS. alternating current generator. It is constructed as follows. Two iron standards are fixed to a sole-plate, and each of these carries an even number of electro-magnets, arranged in such a way that each is of opposite polarity both to the magnet facing it and to those right and left. The coils which surround the cores of the magnets form Fig. 40. a continuous circuit, and in the magnetic fields of the poles (provided with flat pole-pieces) there revolve the armature-coils, wound on wooden cores, as in the alternat- ing current generator. However, the number of these bobbins is two less than the number of the magnets attached to each of the standards. The distances, therefore, between the bobbins are greater than the intervals laterally between the magnets ; and during revolution the bobbins do not all simultaneously arrive directly opposite the magnetic poles. This is only the case with two of them, whilst the others are SIEMENS' CORE LESS ARMATURE. 73 at greater or less distance from the magnetic poles which they are about to approach. The maximum strength of current does not, therefore, occur at the same moment in all the bobbins ; bat occurs at successive intervals of time in successive bobbins., The coils of the armature form a single uninterrupted circuit, but the coiling changes in direction from bobbin Fig. 41. to bobbin. The direction of the currents induced in the bobbins during the revolution of the armature is easily determined if we suppose the armature coils and magnets to be tiu-ned lengthwise to the observer, as in Fig. 40, when we compare the direction in which the wire of the armature-bobbins is coiled with the direction of the Ampdrian currents circulating round the iron cores of the field-magnets. If on a certain side of a bobbin the direction in which 74 MACHINES GENERATING DIRECT CURRENTS. the wire is wound is the same as that of the AmpMan currents of the magnetic pole that this side of the bobbin is approaching, a current will be induced in the wire opposite in direction to its mode of coiling. On the other ha^d, when the direction of the coiling of the bobbin, and the direction of the Amp^rian currents of the magnet, are opposite on the sides facing each other, currents will be induced circulating in the direction of the coiling. Let us denote all the bobbins and all the magnetic fields by similar symbols, when the bobbins are so placed between magnetic poles that the direction of the coils of the bobbins corresponds with the direction of the Amp^r- ian currents of the poles opposite them. The position occupied by the armature-bobbins, with respect to the magnetic fields, will be seen from the diagram, Fig. 41. In this diagram, the small black and white circles repre- sent the bobbins, and the rectangular figures the magnetic fields. We see that all the bobbins that approach corre- sponding magnetic fields lie in one half of the rotating armature, whilst all bobbins which approach opposite fields are situated in the other half; and whatever the position we give to the armature, such a division will always be possible. In that half in which bobbins and magnetic fields correspond, a current will traverse the bobbins, flowing in the direction of the coiling. Since the coils of the separate bobbins form a single uninterrupted circuit, currents of opposite direction will meet at two points in it, as in the coils of Pacinotti's ring. All, therefore, required is to unite the two currents in one circuit. This is done in the following way. If only eight bobbins are present, as in the case illus- trated by the diagram, eight insulated metallic rings are SIEMENS' CIRCULAR DYNAMO. 75 fixed to the shaft of the generator, one behind the other. From the points at which two bobbins are soldered to- gether, wires branch off, and are in metallic contact with the rings. In the diagram, these points of junction are denoted by the numbers 1 to 8, placed on the circle representing the armature between the symbols for the bobbins. They are so connected with the rings that the point of juncture 1 is connected with ring 1 ; juncture 2 with ring 2, and so on. Besides these rings, there is also a collector cylinder, and it is composed of forty pieces. This cylinder, too, is represented in the diagram, and its position is correct as regards the angle that its separate parts make with the position of the armature- bobbins and magnetic poles at the given moment. The forty parts of the collector are divided into five groups, each of which contains the numbers 1 to 8. All parts numbered 1 are connected, by wires, with the ring 1 on the shaft, and that again is connected with the point of junction number 1, by the branch wires ; all divisions numbered 2 are connected with the ring which is in connection with juncture number 2, and so on. It has been stated that the position of the armature - bobbins relatively to the magnetic poles is such that, when the armature-system rotates, it is divided into two equal parts, in which currents of opposite direction are induced, and that where these currents meet they can be conducted away and be united. In Fig. 41 the armature is supposed to be revolving from left to right ; and at the given moment, all bobbins to the right of the dotted Hne (pass- ing through the points of juncture 3 and 7) are approaching like magnetic poles, and those to the left of the line are approaching unlike poles. Therefore, accord- ing to the explanation previously given, the currents must 76 MACHINES GENERATING DIRECT CURRENTS. meet at the junctures 3 and 7 ; and as these are in metallic connection with all parts of the collector num- bered 3 and 7, the currents can be conducted away at all those places. As shown in the diagram, the collect- ing brushes, which are indicated by the arrows marked -f- and — , are situated opposite two such collector portions numbered 3 and 7 ; and as the position of the collector- cylinder remains constant relatively to the armature- bobbins, with which it simultaneously revolves, the brushes will always bear on such portions of the collector as are connected with those points of junction in which op- posite currents meet. Accordingly, currents of the same direction will always reach the circuit. The number of armature -bobbins and magnetic fields can be varied according to certain laws, without change to the mode of action of the generator. For instance, instead of using n bobbins (as in the case given), and n+2 mag- netic fields, we can have n+2 bobbins, and n magnetic fields or keeping n + 2 magnetic fields, we can use 2n bobbins ; that is, we can double the number of bobbins. One great advantage of machines constructed on this principle is that, as in Siemens' alternating current generator, the mode of coiling is extremely simple, and the bobbins being wound on wooden cores, Foucault's currents are prevented. Weston's djmamo-electric light machine, Fig. 42, is also worth noting on account of its advantageously constructed armature. The iron core of the armature consists of 36 thin perforated iron disks, which have 16 indentations on their circumferences, so that they look like 16 toothed wheels. These disks are fixed one behind the other on the shaft in such a way that, when looked at along the shaft, all the teeth cover each other. The disks, WESTON'S MACHINE. 77 however, are not directly connected with each other, but are separated by the insertion of small washers, and, con- sequently, after the wire coils have been wound on, a current of air can constantly circulate in the interior of the armature, and most effectively counteracts the heating of the wires. The wire coils are wound like tl^ose in Siemens' cylinder armature; and they lie in the 16 grooves formed by the indentations in the rims of the 36 disks. In a new pattern of the generator, 12 cylindrical field- Fig. 42. magnets are united into 6 pairs, of which the three upper ones carry a common pole-piece, whilst the three lower pairs are also united by a common pole-piece of opposite polarity. The magnet coils form an uninterrupted circuit, which receives its current on the dynamo-electric principle. The way the electro-magnets of this machine act is especially characterised by their inducing action not being the same simultaneously on all parts of a wire, but proceeding from the centre to the ends, and inversely. This is brought about by the pole-pieces, between which the cylinder -armature moves, being composed of parallel 78 MACHINES GENERATING DIRECT CURRENTS. tongues of the same length, the ends of which form an elliptical figure on each side of the armature. The collector of Weston's generator has quite a special shape. It does not consist of separate segments running Fig. 43. parallel with the shaft, as in Gramme's collector, but the segment-strips, although parallel to each other, form spirals. By this means the contact brushes (which consist of from 10 to 12 elastic- copper plates, divided into 3 parts by slits), are made to bear simultaneously on several seg- ment-strips and the current is thus taken up very uniformly. MAXIM'S MACHINE. 79 In outward appearance Maxim's dynamo -electric generator is like a Siemens generator, of the pattern shown in Fig. 38, whose electro-njagnets are placed vertically. It is represented in Fig. 43, with its regulator, which will he described in Chapter IV. The armature of this generator consists of a cylinder-ring, round which the coils are wound as in the method employed by Gramme. Each coil, however, has four layers of wires, and the terminals of each layer are connected with two segments of the collector. For this purpose the collector consists of 64 parts. In some of Maxim's generators there are two collectors, one at each end of the machine, and the coils are connected so that coils 1, 3, 5, etc., are in connection with one, and bobbins, 2, 4, 6, etc., with the other collector. There is also an arrangement by which the currents obtained from the two collectors can be coupled up for quantity or intensity. A generator favourably known in the United States is that of Wallace Farmer (Fig. 44). The armature of this machine consists of two disks, which are fixed to the shaft, close together ; and on the outer sides these disks carry 25 flattened bobbins, with perforated cores. The coils of these form an uninterrupted circuit. Each bobbin contains four separate coils, the wires of which are connected in series. From the point where the wires of two bobbins are soldered together branch-wires lead to the segments of a collector, as in Gramme's ma,chine. There are four field magnets, and these are united in pairs by the iron standards of the framework, forming horseshoe magnets. Poles of opposite polarity face each other, and induce currents of the same direction in the armature bobbins. These currents, after traversing the 8o MACHINES GENERATING DIRECT CURRENTS. coils of the electro-magnets for the purpose of exciting them, pass into the circuit. Fig. 45 represents Lontin's dynamo - electric generator. The armature of this generate r^co^sists of a cylinder of soft iron, which carries one or mdre series of conical bars of iron fixed radially, and forming the cores of the armature-coils. The field-magnets are two vertical iron columns placed one on each side of the armature. Fig. a. All the armature-bobbins are coiled in' the same direction, and the terminal wire of each is connected with the wire commencing the next ; their wires accordingly form an uninterrupted circuit. From the points at which the wires of every two coils are joined, branch wires lead to the seg- ments of the collector ; and by means of springs or brushes, the direct currents generated in the armature are conducted into the external circuit. As in Pacinotti's ring, these currents are, of course, composed of two opposite and parallel ones, and the internal process in Lontin's generator BURGIN'S MACHINE. 8x is exactly the same, when the bobbins are coiled as de- scribed as that in the coils of the ring. In order to convert this generator into one for alternating currents, it is necessary only to change the direction of coiHng, from bobbin to bobbin. A generator with which very excellent results are ob- tained in practice is Burgin's dynamo. The armature of this generator consists of eight six-sided wheels, con- structed of iron wire, fixed on the same shaft, one behind the other. Looked at from the front, however, their sides do not cover each other, for every wheel is displaced 7^° relatively to the one preceding it. The 6 sides, or chords, of each wheel-rim form the cores of armature bobbins, and accordingly, there are 48 of these. The cores of the bobbins are wound with 6 coils of copper wire, each of which contains 15m. of wire of l'5mm. diameter. The G 82 MACHINES GENERATING DIRECT CURRENTS. several coils are connected with each other in such a way that if we imagine all 48 projected on a plane, the terminal of each bobbin is connected with the wire of the bobbin following next. The wire coiling of the armature, accord- ingly, forms an uninterrupted circuit. From the points of junction of two coils a wire branches off to a segment of the collector ; and, as in the generators previously described, the current is first conducted through the coils of the electro-magnets, on the dynamo-electric principle, and then into the external circuit. The strength of the induced current is greatly increased by the armature-coils of Burgin's machine moving close between the pole-pieces of the field-magnets. The resistance of the armature in the Burgin generator is 1-6 ohms, and that of the 4 electro- magnets, 1*2 ohms ; total resistance, therefore, of the generator is 2*8 ohms. When the machine is making 1,500 revolutions the electro-motive force of the current generated is 195 volts, and it attains 206*5 volts, when the rate is 1 ,600 revolutions ; the resistance of the external circuit being 13* 16 ohms. The manufacture of this machine in England has been discontinued by Messrs. E. E. Crompton & Co. the makers for some time. A description of the dynamo now made by this firm will be found in Chapter XII. An efficient magneto-electric machine for continu- ous currents is that of Niaudet, Fig. 46. In outward ap- pearance it somewhat resembles Clarke's generator. Two parallel horseshoe magnets are fixed to the base of the machine by one of their limbs, and are so placed that poles of opposite polarity face each other. A disk rotates between the 4 poles, and on one of its sides there are 12 perpen- dicular iron bars, arranged in a circle, and forming the cores of as many armature bobbins. The coils of these bobbins NIAUDET'S MACHINE. 83 are all wound in the same direction, and the terminal of each coil is connected with the wire commencing the next. On the outside of the disk, 12 metallic strips, on each of which the wires of two coils are joined together, converge radially towards the centre, being insulated one from another. Two springs bear on these strips and take up the currents. Suppose the disk of the armature to be rotating in the direction of the arrow, and that the anterior horseshoe Fig. 46. magnet has a south pole at the top and a north pole at the bottom. Then with reference to this magnet (the magnet behind only strengthens the action) aU bobbins left of a line that halves the disk and passes through the poles, are retreating from the north pole and approaching the south pole. In the lower ones a retrocession current wiU pre- dominate, and in the upper ones, an approximation current ; both currents wiU however have the same direction. On the other hand, in aU the bobbins to the right of the line, retrocession currents wiU be induced, relatively to the south 84 MACHINES GENERATING DIRECT CURRENTS. pole, and currents of approximation, relatively to the north pole. These currents too will flow in the same direction, but this wiU be opposite to that of the currents circulating in the bobbins on the left half of the disk. The two opposite currents will accordingly meet at those points of junction in the armature which are exactly opposite the north and south pole, that is, at both ends of the vertical diameter of the armature disk. The springs which serve to bring these currents to a single direct current and to the outer circuit, must therefore bear on the metallic strips which are situated on this diameter as shown in Fig. 46. Niaudet's generators have not found any very wide distribution in practice. Edison's dynamo-electric generator is remarkable for its dimensions, and well deserves the attention of the technologist on account of its practical construction, which follows certain theoretical laws. Fig. 47 represents one of the .12 large generators that are at present set up in New York, in a large central station whence a portion of the city receives the current for lighting with incandescent lamps. The armature of Edison's generator is a Siemens cylinder armature, the construction of which has, however, undergone slight modification. The core of the cylinder consists of iron plates, which are fixed to the shaft, one behind the other. They do not touch, but are insulated from each other with paper. Fou- cault currents are thus prevented. The copper coiling of the armature does not consist of insulated wires, as is generally the case, but of thick copper bars, as in Siemens' generator for depositing metals. These bars have a trapezoidal section, and are insulated from the iron cores EDISON'S MACHINE. 85 and from each other by air spaoesi At its anterioi end each bar is connected with a copper disk, which has the same diameter as the disks that compose the core ; this disk again is connected with a bar, which lies diametrically opposite the first ; and this bar also is connected with a copper disk situated at the back of the cylinder. This disk is connected with a third bar, and so on. Accord- ingly all the bars and copper disks, together represent an Fig. 47. iminterrupted circuit. Of course the copper disks, like the bars, are insulated from each other, and together with the disks of the core form a solid cylinder, thus considerably increasing the mechanical strength of the armature,, ■ The special advantage of the copper disks is that by their \ise, the internal resistance of the generator is reduced to a minimum, especially at the ends of the armature on which the magnets cannot exert any inducing action. In order to protect the bars (which run pajaJlel to the axis of the generator) against destruction from the centri- 8fi MACHINES GENERATING DIRECT CURRENTS. fiigal force during the rotation of the armature, they are held together, at different points, by ties. The cylinder armature of the generator has a diameter of 27*8 ins., and a length of 61 ins. without, and of 79 ins. with the commutator. The diameter of the commutator is 12f ins. The steel shaft round which the armature revolves is 10 ft. 3 ins. long, and 6^ ins. diameter. All these dimensions show that the armature is very strongly made, and that disturbances in its working will in conse- quence not be likely to occur so frequently as in generators of a more complicated pattern. Water circulates under the journals in which the shaft turns, so as to prevent their getting over-heated. Besides this, there are arrangements for automatically oiling the machine, by which the oil is carried along the shaft, whilst it is carried off before it reaches the commutator, where on account of its insulating properties, it would interfere in the electric conduction between the commutator and the brushes. The field-magnets are 12 cylindrical cores of soft iron, excited by coils arranged on the dynamo-electric principle, and connected with each other on one side by 4 connect- ing pieces. Eight of these iron cores terminate in an upper pole-piece and 4 in a lower pole-piece, between which the armature rotates. The width of the pole pieces is 49 ins., and they are 61^ ins. high. The length of the soft iron cores is 57 ins., the diameter of the 8 upper cores is 8 ins., and that of the 4 lower ones 9 ins. The connecting pieces are 1 1 ins. wide and 9 ins. thick, and the total length of the field- magnet is 94 ins. The system of field-magnets is insulated magnetically from the iron sole plate by a zinc plate 3 ins. thick. EDISON'S MACHINE. «7 The weight of the parts is distributed thus : — ■ Armature and shaft . . . 9,800 lbs. Shaft journals .... 1,340 lbs. System of field-magnets . . . 33,000 lbs. Zinc base ..... 680 lbs. Total weight 44,820 lbs. 3440 lbs. of this is the weight of the copper : — The copper bars of the armature weigh 590 lbs. The copper disks of the armature . 1,350 lbs. The wire coils of the magnets . . 1,500 lbs. CHAPTEE III. PARTICITLAR APPLICABILITY OF THE VARIOUS ELECTRIC GENEBATOES. The last two chiEipters will have given the reader a general idea of the construction of different electrical generators, and he may now naturally ask " What are the particular advantages of the various kinds of machines." If we follow the division in Chapters I. and II., the ques- tion can be answered somewhat as follows. The alternating current machines are necessary in electric lighting with Jablochkoff, Jamin and other " candles." They are always preferable to other machines when it is more desirable to burn away the carbon points in the lamps evenly, and to prevent residual magnetism in the electro- magnets of these lamps, than to advantageously utilise the energy employed. Accordingly the improved patterns of these machines, for instance, de Meritens' machines are extensively employed in lighthouses. When, however, it is principally a question of illuminat- ing a large area, as, for instance, in the lighting of streets or large halls, then the machines for generating continuous, or direct, currents are preferable, as in this case the uneven consumption of the carbons in the lamps, is an advantage ; for when the carbons have a vertical position, and the upper carbon point is the positive pole, the "crater" which is there formed plays the part of an efficient RELATIVE ADVANTAGES. 89 reflectoar and directs a considerable part of the rays of ligtt downwards. Numerous experiments have also proved that the machines for continuous currents are always to be preferred when it is desired to obtain the largest possible yield of work. For the conditions of resistance and rate of rotation being the same, these, machiaies. give about 35% more effect in the luminous arc than the alternating-current machines; Besides, machines for continuous currents will always have to be employed in cases where direct currents are absolutely necessary, for instance, in galvano-plastic operations, or in the preparation of pure metals. For alternating current machines, whose currents have to be rectified by a commutator, are not, as a rule, so efficient as other machines under similar circumstances. For not only is a considerable percentage of the current lost in the com- mutation, but, as we have before pointed out, the commutator very quickly wears, on account of the production there of sparks, whence constant disturbances occur in the working of the machine. If we divide electric machines into magneto-electric and dynamo-electric generators and if we compare these two classes we can but arrive at the conclusion that, in principle, magneto-electric machines, and especially those in which the field magnets are electro-magnets, are the most advantageous. We shall now try to establish the above statement. First of all, as far as alternating- current machines are concerned, it scarcely needs proving that for working electric candles they are absolutely necessary. This follows from the construction of the electric candles them- selves, and the very fact that the application of electric candles for. illuminating purposes, is gradually increasing, go PARTICULAR APPLICATIONS. makes it necessary to construct alternating-current machines. For through the use of these candles the problem of the distribution of light in moderately powerful foci has been successfully solved. In fact, constructors like Gramme and Siemens whose specialty is reaUy the construction of direct-Current generators, were obliged to make alternating-current generators as well, so as not to lose the manufacture. At the present time, it is true, great improvements in the lamps of other constructors, and especially the invention of the Hefher-Altenech differential lamp, has driven the Jablochkoff candle considerably into the background, as the electric exhibi- tion in Paris proved. Through this the alternating- current generators lose some of their importance. The table subsequently given (taken from the " Eeport of the Trinity House"), shows that, economically, alternating-current machines are far inferior to those giving direct currents. From this table we see that notwithstanding its costliness and comparatively large size, the Alliance machine generated only a current capable of produciiig a concentrated beam of 465 to 593 candles per horse power, whereas the current of- the small Siemens generator, No. 68, produced a concentrated beam of light of 2,080, and Gramme's generator. No. 2, a beam of 1,257 candles. It cannot be denied that the recent alternating-current generators give much better results; nevertheless economically, they are still very far behind the continuous- current machines. In a number of operations too, for which electric generators are used, alternating currents cannot be employed, so that this restricts the construction of alternating-current generators. We now return to our statement relative to the RELATIVE ADVANTAGES. 91 magneto-electric and dynamo-electric generators. Experi- ence has shown that the latter have great imperfections, for which as yet, no radical remedy has been found. As explained in the introduction, the magnetism of the field magnets in dynamos depends on the strength of the currents generated in the armature coils ; and, as this again depends on the greater or less rate of rotation of the armature, it follows that the intensity of the magnetic field fluctuates with every change in the rate of rotation ; of course, again causing a corresponding reaction on the currents produced in the armature. The result is, that the strength of the current cannot remain constant, as long as a constant rate of rotation of the armature is not maintained, and this is scarcely possible even with the best steam engine or other motor. For these motors never work with perfect uniformity, and there are difficulties in the uniform transmission of the motion by belting, &c. Each irregularity, however, in the working (caused by the slipping of the strap or some similar occurrence) is accompanied by a corresponding irregularity in the strength of the current of the dynamo. Variations of current strength from the cause of inequality in the rate of rotation can, however, be easily maintained below 5%. Another still more fatal source of disturbance in the strength of current of a dynamo, are the changes which occur in the external circuit. If, for instance, the current is used for generating the electric light between two carbon rods, each change in the arc, not only causes a corresponding but a proportionally increased variation in the strength of the current of the generator. Before the lamps are lighted the carbon points are in contact with each other, and a comparatively weak current only should be necessary for starting the light at their 92 PARTICULAR APPLICATIONS. point of contact. In a well-constnicted regula,tor-lamp, the small electro-magnets ought then immediately to separate- the carbon points, thus instantly employing the; strong currents produced in the mean time in the dynamo- eleetric generator. If, however, the carbons do not instantly separate, the intensity of the magnetic field in the generator rises with each revolution of the armature, and rapidly increases to such an extent that, the armature can be moved through the magnetic field only with great difficulty, and often the machine is brought to a, stop. But even if the carbon points are instantly separated, the strength of current is constantly subjected to disturbing influences. For the automatic regulation of the lamp by the regulator produces a continuous reaction on the machines, causing fluctuations in the strength of current, and thus again fluctuations in the length of the arc. The most disagreeable part, however, in this interdepend- ence of strength of current on the varying resistance in the circuit, is that the current is weakened just when strength is most wanted, whilst it is increased, when there is no necessity for a strong current. Thus for instance when combustion increases the distance between the carbon points the arc gets longer, and when, therefore, a stronger current is wanted to overcome the greater resistance, this increased resistance weakens the current; again when the carbon points are very near together, and a weaker current would suffice, the strength of current in the generator is increased. A good regulator lamp, it is true, modifies these occurrences ; but as yet no lamp has been constructed so perfect as quite to prevent a disturbance in the strength of current. Other changes in the resistance of the circuit, whatever SOME DIFFICULTIES. 93 their nature, react in a similar way on the strength of current generated. If, for instance, oil or dirt gets be- tween the brushes and the commutator, or if the binding screws get dirtied, the resistance of the circuit is iu- creased, and the current of the generator weakened. Although these occurrences caii be reduced to a minimum by careful supervision, and by great cleanliness in the handling, as well as by employing uniformly working motors, and a method of uniform transmission of work,. there still remains this defect in dynaino-e'lectric gene- rators, that the intensity of their magnetic fields, and consequently the current, varies with the resistance in the curcuit. In our next chapter we shall see by what preventive arrangements these fluctuations can be modi- fied ; they are not present in magneto-electric machines with steel magnets. The magnetic fields of these machines are of constant intensity, and do not depend on the rate of rotation of the armature. Also magneto-electric generators, with electro- magnets, which are excited by currents from a separate generator, are not influenced by the disturbances in the circuit — a circumstance which gives this arrangement great advantage over the ofdiuary dynamo-electric ma- chines. If the question be asked, what are the special ad- vantages of the various generators described in Chapters I. and II., the answer is not so easy. For although gene- ral conclusions can be arrived at as to the efficiency of the various machines from a consideration of their construc- tion, trustworthy data are wanting. There are, it is true, numerous reports, which seem to give the reader a good idea of the advantages and drawbacks of the several ma- chines, but, as a rule, the data given are vitiated by 94 PARTICULAR APPLICATIONS. private or national interests. For the data are either taken from reports of constructors, or from reports of national committees, and naturally in these an absence of party feeling is scarcely to be expected. Besides, the basis of comparison of the several machines varies in almost all published reports, and tables of comparison, which would be of real value to electro-technical science, could only be constructed by an international committee, provided with all the necessary facilities. It is much to be regretted that nothing was done in this respect during the Paris Exhibition, although a comparative investigation of the several machines would have been easy at that time ; but however great, in other respects, the utility of this exhibition was, absolutely nothing was done for the advancement of electro-technical science, to throw light on the mysterious darkness which prevents a clear com- prehension of the efficiency of the various magneto and dynamo-electric generators. As, however, some of the data given by national com- mittees, and by constructors and physicists known for their veracity, are a useful aid to the technologist, those most important have been reprinted and explained in a subsequent chapter. CHAPTER IV. AUTOMATIC SWITCHES AND CURRENT REGULATION. To prevent the extremely troublesome disturbances men- tioned in the last chapter, as occurring in the working of dynamo-electric generators, in consequence of changes in the external circuit, various devices have been de- signed. To these belong the so-called " switches," by which an artificial resistance is inserted in the circuit, actuated either by an overseer or automatically, and generally when for some cause the external circuit is interrupted. Siemens, Sawyer and other constructors employ these switches. That of Siemens depends on the action of a small extra magnet, through the coils of which the current is conducted when the machine is working regularly, and which, during this time, holds a small keeper connected with an extra circuit. As soon as the current in the external circuit is interrupted, a spring pulls away the keeper from, the magnet ; this action introduces into the circuit of the machine the extra circuit, which has the same resistance as the external circuit. Many of the switches in use are constructed on a similar principle. Another m.ethod of regulating strength of currents to follow the requirements of the circuit, is exemplified by 96 CURRENT REG ULA TION. Hiram Maxim's current regulator, which excited great interest at the electric exhibition in Paris. It is re- presented in connection with the generator in Fig. 43. As ahready mentioned, the electro-magnets of Maxim's generator are excited by a current from a small machine. In order to supply the electro-magnets with a weak or strong current as required, so as to regulate the current of the generator itself, the coUector-brushes of the exciting- machine are fixed to a rocking frame, by means of which they can be shifted round the collector-cylinder. As the strength of the current taken up by the brushes depends on their being more or less advantageously situated with respect to the sectors of the collector, this current in- fluences the strength of current of the generator. In order to alter the position of the brushes according to the requirements of the generator, the current of the latter is conducted to an electro-magnet connected with the regulator of the exciting-machine, and according to Schellen, the following action takes place (vide Schellen's " Die Magnet-elektrische und Dynamo-elektr. Maschinen,'' p. 509) ; " The electro-magnet lifts a pawl by means of a keeper attached to the end of a lever, which moves up and down between two set-screws. The pawl is caused to catch in the lower or upper of two ratchet-wheels, and is moved backwards and forwards by an oscillating bar, moved by a small crank which has a comparatively slow rotatory motion imparted to it from the shaft of the generator. If the pawl catches in one of the ratchet- wheels, the motion of the latter, as it turns, is transferred to a horizontal pin, and thence to the carrier of the brushes of the exciting-machine, by bevel pinions. The rocking-frame is turned in one or the other direction when the light-producing current is too weak or too MAXIM'S REGULATOR. 97 strong. The keeper of the electro-magnet of the regulator is pulled down more or less, and in consequence, the upper or lower ratchet-wheel is turned. This, first of all, strengthens or weakens the exciting current, and next, the current for generating the electric light." A very effective method for preventing a sudden rise in the strength of current from a dynamo consequent upon the resistance in the external circuit being lowered, is that of exciting the electro-magnets by a shunt current, which was first recommended by Wheatstone, in England, and afterwards applied by Siemens and others with the most satisfactory results. In this arrangement, with a lighting mafehirfe, for instance, only the lamp, the armature-Coils and conduct- ing wires are united to form the main circuit ; the electro -magnets are inserted in a branch-circuit, generally taken from one brush of the collector to the other. If the resistance in the external circuit is reduced to zero, a small portion only of the current passes into the coils of the electro-magnets, and as the iaducing action of the latter is thus weakened, the strength of the current is at the same time reduced. The reader should here note that with a: machine con- nected on this " shunt " method,- the removal of resistance from the working circiut causes a decrease of current in this circuit ; but that with a machine connected with the electro-magnets in the same circuit in which work is done, or in " series," as this arrangenieiit is termed, the removal of resistance from the working circuit is produc- tive of an increase of current in this circuit. In other words, with these two systems of connection, the same action of removing resistance from the working circuit produces opposite results. H 98 CURRENT REGULATION. To prevent reversal of citrrent in tlie external circuit influencing the magnets, C. F. Brusk surrounds the latter, in some of his generators for plating purposes, with a second coil of very fine wire whiph is connected with the collector-brushes, and thus forflis a shunt circuit to the main or working circuit. This system of double-winding of the electro-magnets of dynamo-machines as epiployed by Brush for preventing the demagnetisation of the '^ field "-magnets, was first used by Pftget Higgs, in 1880t-81, to maintain ^, constant electromotive forpe, under variations caused in the ex- ternal cirpuit by addition or removal of lamps from the circuit. When electrip lamps are ranged along the two conducting-^ires leading from the dynamo, side by side, one of the two terininals of the lanip being connected to each conducting wire, the l£|,mps are said to be put in " parallel arc " or in " n^ultiple arc." The greater the number of laiftps, the grefiter therefore the nupiber of ways for the electric current to flow fropi the positive to the negative conductor ; and the greater the number of ways, thp greater wiU be the total flow of purrent. Now, with a " series " machine (in which the electro-magnets are included directly in the working circuit), the greater number of lp,mps causing an increased flow of current, a higher electromotive force is obtained, consequent upon heightened magnetic intensity of the field-magnets, caused by the increased current circulating in the electro- magnet coils. If this increase of magnetism were pro- portioned to the number of lamps added or to the flow of current, aU. would be easy work for the electrical engineer ; unfortimately for him, the increase of magnetism does not follow any such convenient law. Besides, let us suppose that ten units of flow of current were necessary to pro- DOUBLE-WOUND MACHINES. 99 diice a certain intensity of magnetism, to which would correspond the electromotive force necessary to properly light the lamps, then it is quite clear that five lamps would not open ways sufficient to cause enough • flow of current to magnetise the magnets to that intensity neces- sary to produce the proper electromotive force (which for all practical purposes we may consider to be the same for one lamp, as for one hundred, when the lamps are arranged in parallel arc). And if more than ten lamps are in- cluded in the circuits between the two main conductors, then it is also pretty evident that a higher intensity of magnetism would occur from the greater flow of current around the magnets, and a higher electromotive force would result, and this would cause too much current to be forced through the lamps, probably more than they were intended to withstand, ending in their destruc- tion. On the other hand, if the electro-magnets were in- cluded in a " shunt " circuit taken from one of the conducting mains to the other, or from one brush of the ^machine to the other-=the electro-magnets being thus in parallel arc to the lamps--^increase of the number of lamps above the normal number would cause a too great number of ways to be opened for the current to pass by the electro-magnets, subtracting current from these magnets, and therefrom the magnetism would be de- creased, with the consequence that the electromotive force would also be decreased (in a much more than a pro- portional amount), resulting that the lamps. would de- crease in the amount of light given by each by this addition to their number. We see that adding lamps, in the one case of the " series " machine would cause the destruction of those already loo CURRENT REGULATION. arranged' in circuit from too high excitation, and in the other case of a " shunt " machine, reduction of current and light ensues : from these opposite effects it is not difficult to comprehend that a possible combination of these two methods of arranging the electro-magnets — "shunt" and "series"-— would result in maintaining, under the case of adding, or subtracting, lamps in the circuit, a constant magnetic intensity and consequently a constant electromotive force. Such is the principle of the ma- chines termed compound wound, or self -regulating ; and in these machines a constant speed being given, with the lamps arranged in parallel arc, very great variations in the external circuit may occur, without variation in the electromotive force. The steady maintenance of a constant electromotive force, with only one unit of current passing over the coils of the electro-magnets (ten units being supposed to main- tain saturation under a "series" arrangement), clearly necessitates that the magnetic field shall initially be raised to its proper intensity. This need of an " initial field " was first proved mathematically by M. Marcel Deprez, in 1881, independently of Paget Higgs (but several months after application for patent by the latter), in a valuable contribution to the French journal La Lumiere Electrique. M. Marcel Deprez proposed to maintain this initial field by employing two machines, one being the generator, the electro-magnets of which were wound with two equal and similar wires ; the other an exciting machine. One of the wire circidts on the electro- magnets of the generator was put into connection with the exciting machine, only to maintain the " initial field ;" the other wire circmt was arranged in " series " in the main circuit in which work was to be done. The system SELF-REGULATING MACHINES. loi adopted by Paget Higgs at once gave similar results, with the use of only one machine. It is frequently astonishing to find how nearly earlier inventors were in attaining results that have finally been produced with so much thought and labour. An old ma- chine by Hjorth, a Swede, was patented thirty years pre- viously, in which permanent magnets were used in com- bination with electro-magnets in the same machine. The permanent magnets were then doubtless intended to give the necessary magnetism to start the electric current, the dynamo-electric principle being then unknown ; but to- day, constructed in proper proportion, Hjorth's machine could be used to g^ive constant electromotive force, or as a self-regulating machine. Indeed, Professors Ayrtwn and Perry have recently perfected a dynamo, in which perma- nent magnets are employed to produce an initial field with electro-magnets to provide current for the remainder of the work. An objection to such a machine will be pro- bably found in the great size required, which is one of the most detrimental drawbacks to magneto-machines with permanent magnets. Self-regulating compound wound dynamo-machines have been usually constructed with the main circuit, or " series " electro-magnet coils wound on the same arm or limb of the electro-magnet, as contains the " shunt " coils ; some makers, like E. E. Crompton, putting the main coils, of thick wire, on first, the " shunt " coils, of finer wire, being wound above or over these other coils ; Siemens, on the contrary, puts the main coils outside and the shunt coils nearest the core of the electro-magnet. Paget Higgs, however, prefers to assign one limb of the electro-magnet to the shunt coils, and the other limb to the series or main coils ; very many more points that occur I02 CURRENT REGULATION. in practice are covered by this arrangement, and it has been shown to be possible by a suitable combination to obtain a machine so regulated as to produce a normal current only, any deviation of a marked character from this normal current causing a cessation, or great diminu- tion of current. The first published mathematical con- sideration of double-wound machines is due to Mr. E. H. Bosanquet, of St. John's College, Cambridge, and to this reference will be made in a subsequent chapter. M. Marcel Deprez, to whom electricians are indebted for the first logical enunciations of the laws relating to current electromotive force in dynamo machines, has also shown that with a properly-arranged initial field, and with the remaining coils of the electro-magnets in a circuit shunted from the main or working circuit, it is possible to maintain a constant current (the former arrangement referred to maintaining a constant electromotive force), flowing through a " series " of lamps— that is through a succession of lamps on a single circuit — when the number of lamps is varied, provided the speed of rotation of the armature of the machine is maintained constant. But this has only been successfully accomplished with two machines, one being used as an exciting machine, and the other, the chief generator, wound as to its electro-magnets with two equal and similar wire circuits. One of these circiiits is connected to the exciting machine, the other is arranged as a shunt from main to main of the working circuit. In all these double-wound machines, certain relations have to be maintained between the resistances of electrical circuits of the machine itself, and between the amount of current and number of turns of wire led around the electro-magnets, but to this it is not now necessary to refer. STORAGE NECESSARY. 103 So far, however, these methods of regulation have not furnished us with an absolutely reliable remedy, especially as to any variation in the speed of the motor or irregu- larity of its motion. However, a comparatively short time ago, great advance was made in electro-technology, which renders it possible, to completely sever the connection of the electric generator with the circuit in which elec- tricity is converted into work. This advance is the em- ployment of the improved secondary batteries as reservoirs of electrical energy. In this respect the recent improvements have made the subject of storage batteries sufficiently important to re- ceive separate consideration. J04 CHAPTEE V. JXECTRICAL STORAGE. When two platinum electrodes are immersed in hydric sulphate, and are connected with a galvanometer, after a current has been sent through the whole voltametric arrangement, a current is found to flow from one electrode to the other, in a direction opposite to that of the original current. This current is called a polarisation current, and was first observed by Gantkerot in 1802. It is pro- duced by the evolution of hydrogen at one pole, oxygen at the other ; these gases change the potential of the electrodes, by partly adhering to the metallic surfaces, and partly by penetrating into the metal ; and as soon as metallic contact in the external circuit between the electrodes is established, a current flows tending to reproduce eleptrical equilibrium. This polarisation current may be very powerful ; and as early as 1803, the Grerman physicist Eitter, of Jena, constructed a kind of voltaic battery in which only one metal was employed, the disk-electrodes of which were rendered active by polarisation. This secondary battery, by Eitter, can be regarded as the first electric storage battery or accumulator. Plante, the celebrated French physicist, however, de- serves the merit of having been the first who applied the polarising of electrodes to the construction of an eflScient PLANTE'S ACCUMULATOR. 105 battery that could be used in practice. In 1859, he con- structed a secondary or storage battery, the efficiency of which depended on the chemical behaviour of lead. The following is the construction of Plantfi's Blement, Fig. 48. A broad sheet of roUed lead is placed on a second sheet of the same size, so that one sheet covers the other. However, to prevent contact between the two sheets, thick strips of indiarubber are laid between them. Each strip of rubber is 1 cm. wide, 0*5 cm. thick, and of the same length as the lead sheets ; when similar india- Fig. 48. rubber strips have been laid on the upper lead sheet, the two sheets are rolled into a spiral on a wooden cylinder. To make the arrangement stronger, the lead spiral is held together at one end by gutta-percha clamps. The construction of the element is completed by placing this roll of lead, each sheet of which is provided with a connecting strip, in a cylindrical vessel, closed with an ebonite cover. The cover is provided with openings for the connecting strips, and an opening for pouring in the liquid, which consists of water containing ten per cent, of hydric sulphate. io6 ELECTRICAL STORAGE. Now, if the poles of such an element are connected with the poles of two Bunsen's elements, so as to cause a cur- rent to circulate through the Plante cell, lead peroxide (Pb 0") will be formed on the lead sheet by which the current enters the cell, or the anode ; and, on the other hand, on the lead sheet, or the cathode, hydrogen will be generated, tending to precipitate lead in the metallic state. The cathode obtains thus a rough granular sur- face, and the anode a brown coating. Now, when the Bunsen battery is removed, after the current has been allowed to work for some time (the current should only be allowed to traverse the Plante element till small bubbles of oxygen show themselves at the anode), if the poles of the Plante element are joined, a strong current results ; for now the oxygen of the peroxide powerfully attracts the hydrogen of the hydric sulphate ; and the peroxide is de-oxidised ; whilst the oxygen Kberated combines with the lead of the cathode, and oxide of lead is formed there. The current continues as long as the cathode takes up oxygen. When the dis- charge current Ceases, the conditions for a new polarisa- tion current can be again obtained by recharging with the Bunsen elements ; and as the discharge need not take place immediately, but can be proceeded with after several days, it appears that in a Plante cell we really have a reservoir for electrical energy. This element, however, cannot take up a large charge immediately, but its efficiency increases to a useful de- gree vrith repeated charging and discharging. The element has to be " formed," as Plante expresses it, and this " forming " is a long, troublesome process. When first charged, only a small quantity of peroxide is formed; and accordingly, in the discharge, a current of PLANTS 'S ACCUMULATOR. 107 but short duration is obtained. The poles have now to be reversed ; that is, the pole which was previously negative must now be oxidised, and the pole which was previously positive must be reduced. The reduced lead sheet now takes up oxygen, and lead peroxide is formed on it ; whilst the plate which was previously oxidised is reduced by the hydrogen. According to Plante, this process of reversion must be repeated frequently. On the first day the element must be charged and discharged from six to eight times, commencing with a quarter of an hour, and gradually in- creasing the length of time to an hour ; the element is allowed to stand charged over night. On the second day it is discharged, and then recharged in the opposite way during two hours ; again discharged, recharged afresh in opposite direction, and finally is allowed to stand charged for eight days. After eight days it is again charged during some hoiu-s without being reversed, and is then allowed to stand charged for fourteen days, and so on. In this way the capacity of the element is more and more increased. With a well-formed Plante element, a thick platinum wire of 1 mm. diameter Can be made to glow, and can be kept glowing during ten minutes ; a platinum wire of y^ mm. diameter can be kept glowing for an hour. On account of the spiral arrangement of the electrodes, the Plante element has very small internal resistance. It has a high electromotive force as well, and accordingly its construction is very advantageous. The results obtained with the elements would undoubtedly have ensured their extensive application in practice, had not their " forma- tion " offered such a great obstacle. This "forming" is the reason why this original Plante element is scarcely employed in practice, excepting for galvano-caustic pur- io8 ELECTRICAL STORAGE. poses, although it is an extremely convenient source of electricty when once formed. In 1883, M. Graston Plante patented a process for the rapid formation of the well-known secondary battery, and it consists in immersing the sheets of lead (similar to those devised by him in 1860) in nitric acid, diluted with from once to twice its volume of water, for about twenty- four hours before submitting them to the action of the primary current. The cells are then emptied, thoroughly washed, filled with water acidulated with about one-tenth of sulphuric acid, and submitted to the action of the current from the primary source of electricity of which they are intended to accumulate and store up the energy. By this preliminary immersion in nitric acid, a small quantity of lead is, of course, dissolved, but the thickness of the sheets suffers no sensible diminution as, by reason of their metallic porosity, the chemical action is not limited to the mere surfaces of the sheets of lead, but penetrates into the interior of the metal, creating new interstices and enlarging the natural pores already exist- ing, and consequently facilitating the ulterior electro- chemical action produced by the primary current. The sheets of lead intended to be employed in the construction of these secondary cells may be submitted to the action of the dilute nitric acid and washed before being rolled up or arranged in cells ; but the process is equally applicable to cells already constructed. Secondary cells thus formed, after having been submitted for a few hours to the action of the primary current, give off a discharge current lasting for a long period, whereas when they have not previously been attacked by the nitric acid, several weeks of electric action are required, as has been shown, before they will give the same results. Re- ELWELL-PARKER ACCUMULATOR. 109 versing the direction of the primary current, which is so useful for the operation that Gaston Plante described in 1872 under the name of " formation," is equally efficacious in the present case, without it being necessary to so frequently effect this change. About the same time that M. Plante made his remarkable discovery, Mr. Bedford Elwell and Mr. Parker, of Wolverhampton, hit on nearly the same thing. They found that by immersing lead plates in a dilute mixtmre of nitric and sulphuric acids very important advantages were secured. The method of making the ElweU-Parker secondary battery may be thus described : — Strips of sheet lead 9 ins. wide and any convenient length, weighing 2*16 lb. to the square foot, are passed through a machine which first punches holes entirely through them, and then impresses them with indentations^ which act as distance pieces to keep the layers of each plate apart. The holes secure a free circulation to the electrolyte. These strips are then roUed spirally into cylinders contq^ning, in the small cells, three thicknesses of plate each, the joints being made secure by fusing with a soldering iron, and a con- ducting piece of much thicker lead being fused on at the same time. Each cell contains eight of these cylinders \ in. apart. The lead cylinders are first placed in a bath containing a dilute solution of nitric and sulphuric acids, and left there for twenty-four hours. The effect of this bath is to minutely honeycomb the lead plates, putting them into the most favourable condition for " formation " by the electric current. There is also formed upon the surface of the plates a deposit of siilphate of lead, the greater part of which is subsequently reduced to peroxide, part of it being first washed off. The plates on being taken from the bath are washed, and then placed in the no ELECTRICAL STORAGE. ordinary dilute sidphurie acid solution in the cell. They are then charged in one direction for six hours with a current of 12 amperes, discharged in about three hours through ten Swan 45 volt, 20 candle lamps — ^twenty- two cells give 45 volts — ^and charged again in the reverse direction. They are now ready for use. There is then no sulphate visible, the peroxide plate being of a rich, dark brown colour, smooth, hard, and orystaUine in appearance, and the negative plate presenting a clean surface of ordinary lead colour. The plates or cylinders are retained in position by notched vnlcaoite frames underneath, and notched dis- tance pieces of the same material on the top, thus leaving the centre space between the plates, and a space under- neath them, open for the free circxilation of the electrolyte. Earthenware cells are generally used, but the company manufacturing under the patent also use wood cells, coated inside with a composition of gutta percha, which are preferable where strengUi and lightness are required. The quantity these cells will give out at an electromotive force of two volts, or rather more, is about 40 ampere- hours when sent firom the works, that is, supposing an accumulator is required to give a current at an electro- motive force of 45 volts, twenty-two of these cells will give a current of 10 amperes for four hours before any of the cells are exhausted. But the capacity of the cell may be greatly increased by occasionally reversing the charging current, as in the original Plante cell. The arrangement of the battery is very workman-like and convenient. A ceU can be taken to pieces and put together agmn in about two minutes ; and the way in which the cylinders are put together gives great stiffness and prevents deflection or bending, while the plates being free FAURE'S ACCUMULATOR. in to expand or contract, can do so without losing their shape. It is to a Frenchman named Faure that the honour is due of having in practice and by somewhat mechanical means rendered the long " forming " of the elements un- necessary. He uses lead in a powdered condition for the Fig. 49. construction of his elements. These consist of two lead plates, each 200 mm. wide, one 600, the other 400 nxm. long, and 1 and 0-5 mm. thick, which he coats as thickly as possible with a paste of red lead (Pbj O4) and water. On the larger plate he puts 800 grms., on the smaller are 700 grms. of red lead. To keep the paste on the metallic plates, he places over them strips of parchment paper, and 112 ELECTRICAL STORAGE. on that a strip of felt. He then rolls up the whole system as in the Plante element, and places it in a cylindrical vessel, Fig. 49. The liquid is the same as that used in the Plante element. When the element is thus prepared, a current is passed through it, and a thin layer of lead peroxide and lead sulphate is, according to Dr. Aron, formed on the outer surface of the red lead. This layer of sulphate then gets reduced to lead, whilst on the other plate pure lead peroxide is formed. In due course the stratum of red lead beneath gets attacked, and the action goes on till by degrees the cell is made capable of taking up a large charge. Charging the element two or three times is sufficient to make it ready for use ; the coating of red lead on one electrode has then changed completely into lead peroxide, whilst the red lead on the other electrode has been re- duced to lead. There were few trustworthy data with regard to the efficiency of the Faure batteries up to the beginning of last year ; and whilst, on the one hand, Faure's improve- ment of the Plante battery was lauded to the skies in ridiculous advertisements, on the other hand, in conse- quence of this quackery, so unworthy of a scientific in- vention, physicists were inclined to think lightly of the Faure battery. One of the first to truly investigate the merits of the Faure batteries was Mr. Frank Geraldy, who published the results of experiments carried out by him, in partnership with Mr. Hospitaller, in the electrical journal, La LuTniire Mectrique, in which he showed that a Plante element is almost as efficient as a Faure's element of the same weight. The advantage of the Faure element over the Plante element, consists in the former being FAURE'S ACCUMULATOR.' 113 ready for use almost immediately after its construction, whereas the latter requires the tedious " forming." We have now reliable data as to the value and efficiency of Faure's elements of the newest construction. These data are given in the journal mentioned, Vol. VI., No. 10, in which there is a full report of experiments made by a French committee, with Faure's batteries at the Conserva- toire des Arts et Metiers. The committee were Messrs. Allard, Blanc, Joubert, Potier, and Tresca, and a memdir on their experiments was also placed before the Freitdh Academy of Sciences on March 10, 1882. The battery placed at the disposal of the committee consisted of thirty-five Faure elements of the newest construction ; each element, with its liquid, weighed 43*7 kgs., the lead-electrodes '^ere covered with red lead in the proportion of about 1 kgr. to the square metre. The liquid consisted of distilled wdter Containing about 10 per cent, by volume of hydric sulphate. The gene- rator placed at their disposal by Faure was a Siemens dynamo. The resistance of the armature was 0"2t ohm, that of the field-magnets 19*45 ohms. The coils of the latter were supplied by a shunt current. By means of a regula- ting apparatus, employed by Faure, the exciting current was kept during the entire experiment at a strength varying between two and five ampetes. The object of the experiments was to measure : — 1. The mechanical work necessary to charge the battery. 2. The quantity of electrical energy stored during the charg- ing. 3. The electrical work actually performed during the discharge. For this object the electro-motive force and the resis- I 114 ELECTRICAL STORAGE. f« o o o o o o o o o ^ll Tj- M 00 lO 00 ON i-< d ^ O^ t^ t^ M NO ectriea donei \\ ... . H 1 h -a Q O O O O O O o o .5 ■5l- M H 00 NO t oe vo vo N fo -a'S-l O t-« <^ o 00 3 •*vo %n N 00 Fl 1 M II S 1 H •a o o o o a o o o ^ lit VO M op VO ■* f,o6 M N 00 1 "sfl 0© o\ O lO H H CI to NO Ih II < §-§" cq PI 1 M o o o o o h^ ■sqraoxnoo m o o o o o H jfiaW«g am jCq dn ua:^Bi VO •* «« ■* § rt jJ^TOu^oata; JO jtfpu^n^ M O H VO 61 N N ON NO ■sai^dray m ^nawnj) Sni ^0 M CSQOO ■*00 OO M 5 < -ijioxg; JO ^^isuaiml uua]^ N ^ ^ k n (n «^ -ci-viS u •saj^dniy m ^aauno Sui On a\ ON ro hH -SMqoi JO jfjisua^ui n^aj^ b f. i«->i> W •s^IOA ni 'ija^Ba aqi H* OO >-< \o 00 o o\ o H JO imijaatoj jo -jgip OTaJ^ N H IS! N 00 On 0\ On »^ « M 00 00 o 00 O ON 1-^ N On u") ■* Xq pajns^am ! ■uitIi On N 00 l^ -^i «« VO V5 M t^ Tj- CO' Snoo NO O moo o NO ■sjS3;ui3[Mj^pora8qoai5 « N P(S 1-1 On II II 00 ■^:^n^Ira jad snopniOAag OK N to m r*. t^oo 00 lo^Biauaf) jO paadg uBajj o o o M M M M a ^ d a' a ■^J 1 6 1^ lO fo 1 M ■«^ ■+ oil J ^' J^' ^' J 1 ^ to t^ t^ N g - " - '-' ' ■* io\o t^' 1 ,« ^1 ■S »i o o O o o O O 00 1-c On 's'.a O NO N 00 g ID N M to «t3 W . >. T. . o o o o 00 00 NO ON ^ N On NO d O' 1. In iH'actice, the importance of the distribution • of the magnetism is usually under-estimated, because, in i68 CONSTRUCTIONAL LAWS. nwst generators with steel magnets, the inductive power of the poles only is employed. Marcel Deprez, how- ever, has proved, in his small electro-motor, that this is erroneous; for with this motor very powerful effects are ohtained, the inventor having placed a Siemens cylinder armature between the branches of a horseshoe magnet-, with its axis parallel to these branches. The .rotating armature is thus influenced by the longest por- tions of the limbs. We give bere the dimensions of one of the small motors, as well as the values of its working power. The length of the horseshoe magnet, measured from the faces of the poles to the top of the bend ,,,... The distance between the limbs The thickness of the magnetic battery Diameter of the cylinder armature . Length of the iron core . Weight of the field-rinagnet Weight of the whole motor With a motor having ttese dimensions, Deprez obtains, using the apparatus as a generator, aU the effects that can be produced with three Bunsen's elements. On the other hand, if it is used as an eleptro-mag^netic motor, the work produced is as follows : — With 1 Bunsen's cell . . . 0-04 kg. metres. 2 „ ceUs . , . 0-20 . 145 mm . 33 » . 25 n . 32 n . 60 » . 1-70 kg. . 2-83 ^^ 3 4 5 8 0-45 0-75 1-10 1-80 In making electrormagnets, the construction of which PARTS OF GENERATORS. i69> is important both for the inducing portion and for the- armature of electric generators, it is necessary to take carefully into account the formulae given in the previous chapter, as well as to select good material. Unfortunately, the value of the co-efficient k, with which the intensity of the field and the volume of the magnetic core have to be multiplied, to obtaia the mag- netic momer,t, is not known with certainty for all kinds of iron ; from the investigations of Barlow and Pluckher, however, we know the following values, taken from Fleeming Jenkin's work.* Soft wrought iron Cast iron . Soft steel Hardened steel Soft cast steel . Hard „ „ . Nickel . Cobalt . k = 32-8 k = 23-0 k = 21-6 k = 17-4 k = 23-3 k = 16-1 k = 15-3 k = 32-8 If we wish to construct an electro-magnet, it is best to use another electro-magnet as a pattern, and to make the dimensions proportional. Then, according to Dub's laws, the quantity of current required to satiurate the core of the new magnet is proportional to that of the pattern electro-magnet, in the ratio of the cubes of the homo- logous dimensions of the two cores. The dimensions of the coil are calculated by the laws given iu Chapters VI. and X., taking into account the external resistance. Professor Silvanus P. Thompson, in his " Cantor Lec- * Electricity and Magnetism, p. 124. T70 CONSTRUCTIONAL LAWS. taxes " points out that to magnetise a piece of iron requires the expenditure of energy ; but when once it is magnet- ised, it requires no further expenditure to keep it magnetised, provided the magnet is doing no work. Even if it be doing no work, if the current flowing roimd it be not steady, there will be loss. If it do work, say, in attracting a piece of iron to it, then there is an immediate and corresponding call upon the ■ strength of the current in the coils, to provide the needful energy. This point Professor Thompson illustrates by the following experi- ment : — Let a current from a steady source pass through an incandescent lamp, and also through an electro- magnet, whose cores it magnetises. If now the magnet is allowed to do work in attracting an iron bar toward itself, the light of the lamp is seen momentarily to fade. When the iron bar is snatched away, the light exhibits a momentary increase ; in each case resuming its original intensity when the motion ceases. Now, in a dynamo where, in many cases, there are revolving parts con- taining iron, it is of importance that the approach or a retrocession of the iron parts should not produce such re- actions as these in the magnetism of the magnet. Large, slow-acting field-magnets are, therefore, advisible. And the body of the field-magnets should be soUd. Even in the iron itself currents are induced, and circulate round and round whenever the strength of the magnetism is altered. These self-induced currents tend to retard all changes in the degree of magnetisation. They are stronger in pro- portion to the square of the diameter of the magnet, if ■cylindrical, or to its area of cross-section. A thick magnet, wiU, therefore, be a slow acting one, and will steady the current induced in its field. It is important to have a sufficient mass, in the PARTS OF GENERATORS, 171 magnets that saturation may not be too soon attained. The softest possible iron should be used, and this can only be obtained by very careful annealing. The magnets should be made as short as possible for two reasons. First, a long magnet means a heavy machine with a small out- put for its weight. Secondly, if the magnets are long the lines of force have a longer distance to travel in complet- ing their circuits, and, experiencing a greater resistance, a greater field excitement is required for their creation. To make the path of the force lines as short as possible it is customary to utilise for the winding of the field coils as great a length of the magnets as the construction of the machine will permit. The shortness of the magnets is limited only by the requirements of the coils as regards smrface for the radiation of their heat. Pole-Pieces. — In modern machines the pole-pieces have been done away with or greatly modified in shape, a result following upon more correct notions regarding their functions. Xo good machines will be found now-a-days with pole-pieces as in Figs. 31 and 32. The former carried into the interior of the armature resulted from •erroneous ideas concerning the theory of generators. It is true that by bringing a pole-piece inside the armature the interior wire is made active, but while the leng^th of active wire is increased, it must be remembered that its activity per unit of length is correspondingly reduced if the total lines of force entering the armature remain as before. If pole-pieces are required they should be of shapes really adapted to their functions. The distribution of the €.m.f. in the armature coils depends very greatly on the shape of the pole-pieces. If the bed-plates of dynamos are of cast iron, care 172 CONSTR UCTIONA L LAWS. should be taken that these bed-plates do not short-circuit the magnetic lines of force from pole to pole of the field- magnets. Masses of brass, zinc, or other non-magnetie metal may be interposed ; but are at best a poor resource. In a well-designed dynamo, there should be no need of such devices. The Armature. — In connection with this portion of an electric generator, another important point regarding electro-magnets has to be considered, that is, the periods of change in the magnetisation of an iron core. A magnet- does not acquire its magnetism instantaneously, nor lose it instantaneously. When the circuit is closed or the current is started, the magnetic moment of the core in- creases rather rapidly, and reaches a maximum ; again on opening the circuit or stopping the current, the magnetic moment decreases and reaches a limit more or less nearly zero. The magnetism never entirely disappears, but a small amount of "residual" magnetism remains. The duration of the increase and decrease of the magnetism depends on various circumstances ; the principle is, that the coercive force of iron is never equal to zero. The separate molecules of the metal possess a certain inertia that prevents their instantly resuming the position they had before the iron was magnetised.' Another reason is the occurrence of extra currents produced by induction. These have the same direction as the principal current, when the circuit is opened or the current disappears, consequently, they tend to prolong the primary current, as well as its magnetising action. From this fact it follows that the magnetic maximum of the iron core in the armature of an electric generator does not begin to decrease when, according to theory, it should do so, but at another point. If, for instance, we PARTS OF GENERATORS. 173 take Fig. 8 for the basis of our consideration, and assume that the ring moves in a direction opposite to that of the hands of a clock, then the maxiintuii magnetisation of the iron core, and therefrom the maximuin current, does not decrease immediately after the turns of wire have passed the poles S and N ; but the magnetism of the iron core, and the current in these turns of the wire, remain at the same intensity for a few moments, so that the decrease commences a few degrees to the left of S and to the right of iV. The neutral points also ai'e displaced in con- sequence, and have to be sought below p and above p', and not at p and p'. This makes it necessafy to change the position of the brushes, and not place them at the ends of the horizontal Une joining the points p and p', but at those points which are really neutral. Affairs are made still more complicated by the fact that the amount of displacement of the neutral points depends on the greatet or less rate of rotation of the armature. For when the armattlre rotates rapidly from right to left, each of the respective points of the ring has advanced some distance before the decrease of the magnetism of the iron core, and of the current induced in the wire windings, has reached the minimum ; on the othet hand, when the armature mores slowly, the theoretical and actual neutral points of the armdtnre lie closer together. The difference in the position of the neutral points is further increased when the rate of rotation is rapid, because the magnetism of the field-magnets, and of the iron core of the armature, and consequently also the reaction on the armature coils, is increased, whereas when the rotation is slow these factors are of small value. It is therefore obvious that we should seek an automatic arrangement of the brushes to place them always on the points which are for the time 174 CONSTRUCTIONAL LAWS. being neutral. This is already partially effected by some regulators. Another consequence of the fact that the maximum magnetism does not immediately disappear, is the heating of the iron core of the armature, which greatly influences the efficiency of an electric generator. In any case it is necessary to prevent the peripheral current in the mass of iron of the core. This is effected to some extent by slitting the iron cores, that is, in other words, by structurally dividing the core in planes normal to the circuits round which currents are induced, which statement may be generally accepted as meaning, in planes at right angles to the direction of the wire windings, or in planes parallel to the lines of force to the direction of the motion. Cores are also built up of varnished or insulated iron wire, or of thia sheet iron separated by varnish, asbestos-paper, or mica, to realise the required condition. The heating of the armature, however, may not be a consequence of the residual magnetism in the iron core, but may arise from the resistance of the armature being too great. Also a part of the armature-ooils are not exposed to the action of the field-magneta, and oppose a great re- sistance to the current which has to traverse these coils. To overcome the resistance, work is necessary, which manifests itself as heat. One of the principal problems for the constructor is, where possible, to expose all parts of the coils of the armature simultaneously to the influence of the field- magnets, or where this cannot be done to temporarily exclude those parts of the armature from the circuit which are not accessible to the influence of the field-mag- nets, or are in the neutral positions, as is attained for instance in Brush's generator. PARTS OF GENERATORS. 17S The hollowing of the armature core and the condiicting of water through it, is a very incomplete remedy against heating. The injurious consequences can thus, it is true, be modified, but the work lost in the heating is not re- gained. Similarly the perforation of the armature for the sake of cooling by ventilation cannot be recommended, as the surface of the armature is thus increased, and the work which has to be spent in overcoming the atmos- pheric resistance, is lost as far as the efficiency of the generator is concerned. The rational remedy for this evil of the heating of electric generators is not to be found in modifying the consequences of the heating, but in avoid- ing the causes. The position of the armature with respect to the mag- netic poles must be such that the armature moves in as strong a magnetic field as possible, and this wiU be the case when it reYolves as close as possible to the magnetic poles ; for the intensify of a magnetic field is equal to the magnetising force of the pole divided by the square of the distance firom the pole. In order that the armature may rotate as close as possible to the pole, the parts of its surfaces which are turned towards the pole must be comparatively even, and we must therefore wind the coiling of the armature very symmetrically, or bring its core very close to the pole-pieces of the field-magnets by a suitable construction, as in Paccinotti's generator. The collectors and conmmtators of electric generators are the parts which perhaps require most care in their construction. If they are badly constructed they wear out quickly in consequence of friction and the for- mation of sparks, and badly constructed collectors or commutators are the cause that a large amount 176 CONSTRUCTIONAL LAWS. of the working power of a geneirator is spent use- lessly. The loss of energy through the friction of the rubbing parts of a generator is proportional to the number of revolutions of the shaft and to the diameter of the rub- bing surfaces. For these reasons the surfaces ought to be diminished as much as possible, not only in the journals of the generators but also, as Professor Perry suggests, in the conducting brushes and commiltators. But, besides, we must diminish the destnictive action in these parts caused by sparking. This can be done by distributing the sparking over various portions of the eoUector, so that only small sparks cdn be formed, which are unable to melt or oxidize. In order to reduce the sparking oti the collectors of large generators to a minimum, Edison increases the width of the insulation ttj, a^ a^ (fig< 50), between the segments of the collector. He makes the conducting sectx)rs b^, b^ 63, narrower at one end of the collector- cylinder A, and on each side of this portion of the cylin- der, he places a single brush e which he calls the insulated brush, the contact point of which is not in a line with the principal brushes. The insulated brush is not directly connected with the principal brushes, d d, but first with an interruption •cylinder B by means of the brushes Aj, h^. This cylinder has conducting and insulating sectors which correspond with those on which the insulated brush e bears, and it can be attached separately to one end of the shaft of the generator, or may form a continuation of the coUector-cyUnder A, as shewn in the figure, in which case its conducting strips c^ Cj must be insulated from tho«e of the coUector-cylinder A. In working the generator the local current and part of the principal cur- PARTS OF GENERATORS. 177 rent continue to flow through, each of the insulated brushes, and across each commutator segment, after having ceased traversing the principal brushes, so that no sparks are generated at the ends of the latter. When an insulated brush quits a strip of the collector, the current traversing it is interrupted on the interruption-cylinder B, and as the same thing occurs simultaneously on the collector-cylinder Fig. 50. A, through the insulated brush e, the spark is thus greatly subdivided and much weakened. All the hints given in this chapter, with reference to the construction of the separate parts of electric .genera- tors, are of great importance to the manufacturer. They do not, however, by any means exhaust the details which he has to take into consideration. For the difBculties which the constructor has to overcome are of too varied a nature to be all mentioned here. We have only been able to draw attention to the prin- N lyS CONSTRUCTIONAL LAWS. cipal points, which, if kept in view, fonn the basis of good construction. The constructor has, besides, to attend to peculiarities of construction connected, (1). With strength and simplicity of mechanical con- struction. (2). With easy access to ^nd repair of the several parts of the generator. (3). With the cost. A discussion of these points, however, does not come within the scope of a technological work. We shall only mention a few exanjples which may be regarded as typical of what has to be attended to, and of what has to be neglected under the three he^idinga we have mentioned. The generators of Brush and Edison, and the collector of Gramme's generator, and sonie of their imitations, are specially noteworthy for strength and simplicity of construption. Brush's generator too serves as a very good type, in which the parts are very accessible and easily repaired. With regard to the latter advantage, however, we must specially mention the altematingrcurrent generator and the latest dynamo-electric generator of Sien^ens and Halske, in which each bobbin can he replaced without dis- turbing the other parts. This is also true of the Bxirgin generator. The question of post is at present one of the principal obstacles in the way of an extensive applipation of electric generators, and again indirectly depends on complicated or simple construction. It is to be expected that there will be a considerable reduction in this direction as soon as the dimensions of generators are increased. For as DIMENSIONS OF GENERATORS. 179 regards their efficiency, large generators are much cheaper than small ones, as will be seen from the details in the next chapter ; and upon the whole, experience has taught that it is always advantageous to concentrate as large an amount of working-power at one point as possible, and then to distribute it in different directions. The secondary batteries described in Chapter V. promise to be of great service in this particular. As regards the relation of size to efficiency. Professor Thompson has pointed out, in the Cantor Lectures, de- livered by him before the Society of Arts, the much more than proportional efficiency of large machines. If we assume that the size of any machine can be increased n times in every dimension, and that though the dimensions are increased the velocity of rotation of the shaft remains the same, whilst the intensity of the magnetic field, per square 'centimetre, is also constant, the following laws of increase of size will hold good. The area the machine stands on wiU be increased n^ times, and its voliune and weight n' times. The cost wiU be less than n^, but greater than n times. If the same increase of dimensions in the armature coils be observed (the number of layers and of turns re- maining the same as before), there wiU be in the armature coils a length n times as great, and the area of cross section of the wire will be n^ times as great as before; the 1 resistance of these coils will be — ^th part of the original resistance. If the field magnet coUs are increased similarly, they wiU offer only — th the resistance of those of the original machine. i8o CONSTRUCTIONAL LAWS. The electromotive force will be increased v? times, the speed of the shaft being the same. To correspond, we may assume the whole circuit to be increased in section, so that its wire wiU carry the larger current, its resistance will then be — th of its previous value. If our theoretical machine is "series" wound, an 1 electromotive force, n^, working through — ^resistance will give a current 7i' times as great as before. In this respect, as the iron of our field magnets is v? times as great in mass, we need not so nearly saturate it as before to gain the same magnetic field, or to get 'n? times the area of surface magnetised to the former average intensity per square centimetre. Hence, the number of coils may be reduced in the proportion of 7i^ to v?, or to — ^th of its already dmivnished value, correspondingly reducing the resistance on which work has to be done. As in the larger machine, therefore, the electromotive force is increased n^ times, and the current n^ times, the work of the machine will be n^ x n^ = n^ times greater than with the smaller machine. Or, a machine doubled in all its linear dimensions will not cost eight times as much, and will be electrically thirty-two times more powerful than the smaller machine. If the machine be " shunt " wound, then to produce the field of force of n? times as many square centimetres area, will require (if the electromotive force be n^ times as great) that the absolute strength of the current remain the same as before in the field magnet coils. This can be eifected by using wire of the same size as before, and in- DIMENSIONS OF GENERATORS. i8i creasing its length v? times, to allow for n times as many turns, of n times as great a diameter each, in the same number of layers of coils as before. The current being the same, therefore, in the shunt circuit as before, but under n^ the e.m.f., the work here is only n^ times as great ; whilst in the whole machine it is n^ times greater than with the smaller machine.* Following wholly different lines of reasoning, M. Marcel Deprez has arrived at the conclusion that for similar machines the " statical effort " increases as the fourth power of the linear dimensions. But as this " statical effort " is a force of mutual reaction between two elements of the system of conductors ; and as work is represented by force multiplied by distance ; and as, again, in the similar machine whose dimensions are increased n times, the available distance through which the new force can act is also n times greater, the value n^ also here obtains. * Unfortunately the reasoning here given is valueless, for if con- structed on this' basis the larger machine would reach a temperature destructive to the insulation. A current of n^ through —resistance would produce re' times the heat which with a radiating surface of n? would raise the temperature to rfi times that of the smaller machine. The cur- rent is limited by the temperature to which the machine will rise and it will be found that with this limitation the output is about proportional to the weight if the number of revolutions is inversely as the linear dimen- CHAPTER Vin. THE EMPLOYMENT OF ELECTEUC 6ENEEIAT0RS FOR PBODUCINa THE ELECTRIC LIGHT. Although it is not our intention to discuss folly the applications of electric generators, we think it advisable to briefly review these applications, as far as this will con- tribute to the understanding of the properties of the electric generators. We have also included in this chap- ter the more important comparative experiments relating to the efficiency of the light-generators ; for, from the data as to the intensity of the light, the construction of the light-machines and the expenditure of motive power, tolerably correct conclusions can be drawn as to the in- dividual value of the various generators. The most important use to which electric generators have been put is undoubtedly their employment in the production of the electric lights and it was only by the invention of electric generators that electric illumination became possible on a large scale. As has already been said, generators intended for the purpose of electric illumination must not only produce a powerful current in the external circuit, but the current must also be at considerable tension, and this again must be maintained within certain limits, for if the tension is too great the luminous arc becomes unsteady SPECIAL DETAILS. 183 therefore, the internal resistance of the generators must be judiciously arranged. The internal resistance of the normal Siemens light- machine is about 0*7 or 0*75 ohms ; Gramme's generators have on the average a resistance of about 1 ohm, but Gramme also constructs generators which have a resist- ance of only 0*6 ohm. Of course, the greater the in- ternal resistance of a generator, the greater, generally, the e.m.f. of the current, and with generators that pro- duce a current of low tension, we can only supply one arc-light in series ; as for instance with the normal generators of Siemens and Burgin. The Brush generator, on the other hand, is capable of producing a ciurent of such e.m.f. that 20, and even 40 lamps can be inserted in a single circuit. As regards the cost of the conductors- this is a great advantage, but currents of such high in- tensity very considerably affect the colour as weU as the steadiness of the light. With currents of comparatively large quantity and low tension the light is steady, and in its colour resembles sunlight ; for a greater part of its lighting-power is due to the incandescence of the electrodes. It is not easy to determine accurately the comparative values of generators constructed for electric lighting ; for, it is difficult to establish a fair basis of comparison. Different machines suit different conditions, and one cannot be selected to give the best results under aU circumstances. One of the most interesting communications on com- parative experiments with light-machines of various con- struction is to be found in the report by Tyndall and Douglass, known as the " Eeport to the Trinity House," which embodies the result of experiments by an English i84 ELECTRIC LIGHT GENERATORS. committee, in 1877, with generators for producing the light in light-honses.* The generators employed in these experiments were — 1. A Holmes magneto-electric generator, for producing alternating currents. 2. An Alliance machine, also for generating alternating currents. 3. A Gramme generator. 4. Two Gramme generators, coupled up. 5. A large Siemens generator. 6. A small Siemens generator. The Gramme and Siemens generators were dynamos, producing continuous or direct currents. The Siemens generators were from the works of Siemens Brothers, in London, and the Gramme generators had been furnished by the " British Telegraph Manufac- tory " in London, conducted by Robert Sabine, to whom many suggestions as to the mode of measuring the light are due. The Alliance and the Holmes machines were already in use at the South Foreland lighthouses, where the experi- ments were carried out. After various preliminary experiments, the generators were tried on January 18th, with respect to the intensity of the light produced. These intensities were determined for diffused light, and for the light concentrated by re- flectors. However, in the following table, we have to deduct about sixty per cent, from the intensity of illu- mination obtained with the dynamos, if we wish to regard • These experiments, although at this date to he Tegarded as old and as made with machines the efficiency of which has been considerably increased, are yet valuable to the student as affording a comparison of the performance of earlier types now become historical with that of the later types described in the concluding Chapter. SPECIAL DETAILS. - 185 the light-intensity as a sign of the efficiency of the gene- rator; for the carbons in the lamps were placed more advantageously for these generators than for the alter- nating current generators. The table contains the mean value of the intensity of light obtained from several experiments. 1. Holmes' Generator 2 3. Alliance „ 4. Gramme „ 6. Siemens „ 7 Concentrated light. Diffused light. Standard Candles. 1494 1494 2721 2721 1953 1953 5333 3215 9126 5501 14573 8784 5920 3568 The intensity of illumination by a Gramme generator to that of the small Siemens generator (No. 58) was in the ratio of 100 to 100*6, and of the Holmes generator to the Siemens generator in the ratio of 100 to 384. The intensity of illumination with the No. 58 Siemens generator to that with No. 68 generator was in the ratio of 100 to 109-5. The two lighthouses on South Foreland have different heights ; the light of the one was 211*7 m., and that of the other 180*6 m. distant from the engine-room. This difference in distance was accordingly employed to measure the loss of illuminating power for each generator when the current was conducted from the engine-room tp the lamps, on the high and the low lighthouse. The cable employed was made up of two cables, connected with each other, each consisting of seven copper wires of No. 14 Birmingham wire-gauge, and the circuit was led i86 ELECTRIC LIGHT GENERATORS. from the engine-room through both lighthouses, and back to the lamp in the engine-room. The loss of iUuminating power, due to the resistance interposed by these cables, was, With the Holmes generator . . 29-8 per cent. Gramme „ • • 58"6 „ Siemens „ • • SO'4: „ On March 6th, the measurements were continued, after the collector and the brushes in the Siemens generators had been replaced by new ones ; the intensity of illumi- nation then was — With No. 58 Siemens generator, 4,446 standard candles. With No. 68 Siemens generator, 6,513 „ For both generators together, therefore . . . 11,009 „ When generators 58 and 68 were coupled together electrically, they gave a light of 13,179 standard candles, i.e., 19*7 per cent more than the sum of the intensities of illumination with the separate generators. The Holmes and the Alliance machine, which had both been put up in the lighthouses in 1872, had lost con- siderably since then in strength of current and lighting power by the weakening of the magnetism of the steel magnets — the Holmes machine about 22 per cent., and the Alliance machine about ten per cent. Various other experiments were tried, which we cannot here describe. The results are apparent from the adjoining Table I. We must not, however, forget that the report of these experiments refers to generators of older construction. Since then electric generators, and especially the Gramme generators, have been much improved, as can be seen from the results which were obtained with them, accord- SPECIAL DETAILS. 187 •Sni^nut JO lapiQ vo m Tj- -^ ro N M 1 1 1 m m t-^ r^ U-) t^ t-* t^ 10 10 0\ <> « N i>» *N N H M t-i •ra-ni ui sjniod noqjBo ai[4 jo uopoas ssoxQ X X X X X X X XXX m 10 !>. t-^ lio «^ t-* t^ 10 10 ON On « N !>. W N N r* *^ l-l M H M M M M M Qtensity illumina- ion per rse-power standard andles. *.^BI ^O ^O00 00 w -^ -^ . On C\ « H •Avi p9:j vO fO r^ t^ N « t> --^ IT) 10 !-■ 00 00 rOOO Tt- w^-^ a " ja.S -'BJ:^u^oaoo •rh 10 N w 10 in i_t w. w >-< N ■* M •sg-g _ ■Xui PO COO '^ « 000 " CTv N to w M CO ro ro HI VO N itensity iiminati standa sandles. pasnjgia: vo OS fO M M M T^ '^00 ro -^ PO CO ro toco On ""d- K ^ 00 lO VO 00 p. so ■* •Avi p8^ N in\o w covo M OS ro in on^o vb CO inoo 00 to M S^-S -■Bi^naonOQ M M vo ■ "^ >nvo N H Tj- M M M •3;nnini jgd 0000000 N N CO u^ m N 10 snoiinxoAag jo laqnili^ -^d- ■* ^ Tl- '^CO CO •* ■*oo HH •.laAvod-sBiGq N \p 50 f^OO }0 PO 10 u^\p ni '3[.ioAi JO ajn:npnadxa CO CO in in a\ ro ro MS b 10 !>. M m in C4 HH H Tj- - h^ ■sanini'Bj9o[i3 "! W^rajiV iCO 00 CO CO CO N 00 fO |a CO CO r* t-*. r>- r>- ir^ t-l M ro r^ r^ ON M r^ t*ft CO r^ VT) M fi •q^Sag^; Cn N 00 00 '«d-NO OS t^ N ■^ CO t^ r^ M \o NO ON 10 CO hH M M M i-i l-i ■^ m 000 •gp m aoijj m C\ w w NO « ^ m -^ CO to w •-< •H ►. SO N 00 00 06 so 4^ invo =8 :; 00 1 tz; . P^ 10 S c a ~ a - - g QQ [0 DO a a !zi o-| g S W Ogq M -4JO S N N CI i88 ELECTRIC LIGHT GENERATORS. < 1 The respective light-measure- j-ments were car- ried out with in- clined oarhons. Total light of two lamps with reflectors. "SF m 93ii*•■* >o ■* -panic^qo ^iom t'B0u:j08ia aip jo agK}n33i3j to O\00 to 00 I t^OO 00 00 oo 1 -j34od-3SJoq m 3[I0Al JO 31IL)ipa3lIz3 M lO O -■h 1-1 j^ lo y>y> to to N On OnvO M l-l IH •33J0J 3AponioJi33ia; o to « r>- to r^ r^ ON r^ 1 b>oo to ON OO 1 t^OO 00 VO 00 -:)ii3iin3 JO q^nsj^^g t 1 00 On CM On t~ « N 00 1 to to " -H 00 1 00 On OnOO no •snoi;n]OA3i jo jsqton^ O O to O to o 00 O t^ O t^ o NO to -^ M 00 to M -i i Two Siemens, medium size, coupled up, parallel Two Giamme, type A, coupted up, parallel Wilde, Navy pattern SPECIAL DETAILS, 189 ing to the report on the experiments carried out at the School of Military Engineering at Chatham, in the winter 1879-80. This report is important in so far as the data given were obtained from a large number of carefully-executed experiments ; it is, however, impossible here to reproduce the whole. The results of this report are given in Table II., from which we see that, calculating the mean intensity of illumination per horse-power obtained with the several generators, the Grramme generators this time gave the best results. The intensity of illumination per horse- power in standard candles is as follows : For two Siemens generators, coupled side by side ■ . 1428 Gramme-generator, type D . . . . 1821 „ C . . . . 2048 Two Gframme-generators, coupled side by side . 1916 Wilde's generator 877 The general advantages and disadvantages of the Grramme generator D and of the Siemens generators are summed up in the following criticisms : GSAMMfi, Type D. Advantages : 1. This generator gives a considerably stronger light than any of the other generators tried. 2. The generator can be entrusted to less experienced persons without fear of the wires suffering from heating or sparking. 3. During six hours continuous working, under the same conditions as with the Siemens generators coupled side by side, and with a current of 58"5 amperes, the tern- igo ELECTRIC LIGHT GENERATORS. perature of the wires only increased by 71°F. Under the same conditions the temperature of the dmm of the Siemens generators was raised IICF. and that of the electro-ma^ets 85°F., with a current of 55 amperes. The electro-magnets of the Gramme generator get more heated than the revolving ring, so that the maximum rise of temperature can be observed without stopping the machine. 4. Absence of sparks.— The sparking at the brushes is extremely feeble and often imperceptible; and con- sequently the wear of the collector and brushes is very slight. The brushes can easily be brought into the right position, and are so arranged that if necessary they can be shifted parallel to the axis of the collector. 5. Simplicity. The connections are very simple and can be easily followed. 6. With a circuit of 0*498 ohm external resistance, 47*8 per cent, of effective work was done in the arc. 7. The number of revolutions is less than in the two medium-sized Siemens generators, and less than half of the number in the Grramme C machine (1200) ; in con- sequencfe, there is less weaj and tear of the generator and of the rubbing parts. DisadvaTitages : The cost of a Grramme generator, type B, is £360, and, accordingly, about 1^ times as great as that of the Siemens generator. GrBAMHE, TyPE C. Advantages : 1. The generator can be attended to by persons who have not much experience, without fear of the wires being damaged by over-heating. In this respect, this generator surpasses aU the others experimented with. SPECIAL DETAILS. 191 2. During six hours continuous working, under the same conditions as with the two Siemens machines and the Gramme, type D, and with a strength of current of about 83* 15 amperes, the temperature of the wire in- creased only 30*' F. 3. Absence of sparks, see advantage 4 of Grramme D. 4. Compactness, see advantage 7 of Gramme B. The price of this generator is £240, which is about the same as that of two Siemens medium-sized gene- rators. Disadvantages : 1. The intensity of illumination is only 19,500 CEmdles, which is about as g^eat as that of the two Siemens gene- rators, and about 30 per cent, less than that of the Gramme D, when making 500 revolutions. 2. The great speed of 1,200 revolutions per minute would probably cause considerable wear of the machine and its rubbing parts.* Two A Grammes, coupled side by side. Advantages : 1. Cheapness. — The price of the two generators is only £170. 2. These generators have about the same slight amount of heating as the other Gramme machines. 3. Absence of sparks, see advantage 4 of Gramme D. 4. Using the generators separately, two lights can be produced. Disadvantages : 1. The amount of light obtained with these generators is only 18,500 candles, and this is not sufficient for military ptirposes, * This increased wear of tlie rubbing parts of properly-constructed high speed machines does not accrue in practice, — Editor. 192 ELECTRIC LIGHT GENERATORS. 2. When the generators are coupled side by side an inversion of the magnetism easily occurs, thus causing great disturbances and much loss of time. Two Siemens machines of medium size, coupled up side by side. Advantages : 1. By using the generators separately, two lights can be produced. 2. The intensity of illumination is considerably greater than with the other machines that were tried, excepting the Grramme generators Z) and C Disadvantages : 1 . The wires are easily heated if the persons in charge are not very well acquainted with the working of this generator ; it is also a disadvantage that the revolving drum is more strongly heated than the electro-magnets. 2. When the generators are coupled up side by side an inversion of the magnetism easily occurs, which causes great disturbances and much loss of time. 3. If the lamps work irregularly, there is great spark- ing at the brushes, causing rapid wear of the collector and the brushes. For these reasons more experience is necessary in order to work these generators satisfactorily than with the Granmie generators. A report of an American committee on comparative experiments with light-machines of various construction is published in the Journal of the FranMin Institute (1878, vol. 103, pp. 289-361), and the results of these experiments can be seen from Table III. The American rejwrters. Profs. Thomson and Houston, remark, that if the results obtained by the American com- SPECIAL DETAILS. 193 » used ofth rod. 1 ■* 00 to 1 J^ VO 1 po >o 1 O U1 Ill o o o o st> + IH ON HI 1 t< to 0*0 0*0 t-wH^ •s^ntod uoq.tBO aq^ jo azig X X 1 X X MIOO doo r+dr+d .3 •aipuBo jad .■* 1^ to M ID 1 t^OO ts» spnnod looj OO M 1 N -la^od ON to to 00 1 " to nten min can -asjoq jaj tr> 1 11 to O o to O \r> ■pioi o OS t) ■* o 00 -^ t^ VO VO o\ ■* •jaMod-asjojj « 1^ 1 00 00 rr> to 1 to - , VO 00 ■* t« O "it- ■* ON 1— 1 ■durei jad spmiod ^oo j ■* 1 >o Ov 1 oo" o w tfl fi VO w >-4 ^ V pq t= — X — =^-. (1) g 9 g^ Fmrther, the length of a turn, in the layer which lies directly on the core, is — and the length of a turn in the outermost layer is — c + 2a - gr ^T 2 ' consequently the total length of the wire in each of these two layers is — 6-c+gf ,6„c+2a — or — 2ir — s-^, and — 27r ^ 9^9 2 Now, as the layers between these two form an arith- metical progression, the first and last terms of which are given by the two preceding expressions, and as the number of terms is equal to — , we find the total length of the wire in the exciting helix from the equation — „_ h 27r (c + gr + c + 2ct — jj) g _ wba{a+ c) 1 4 J- P -^2) 214 CONSTRUCTION OF ELECTRO-MAGNETS. The values of t and H are therefore functions of those for a, 6, c and g ; and having measured the thickness of the insulated wire, and the thickness and length of the coil, and having counted the number of turns in the coU, we can easily calculate the length and number of the turns wound on a magnetic core, if we divide the length of the coil by the number of turns, and determine the difference between the external and internal diameter of the coil. Similarly, we can, if necessary, find the values for a, and g from the above equation, or establish other expressions for A and H. The electro-magnetic force of the magnet is found by taking into account the laws of Jacobi, Dub, and MuUer, who have determined that the actual strength of a mag- net — or, according to the physical term, its magnetic moment, M — is equal to the strength of the current flowing through the magnetising helix, multiplied by the number of turns of wire, and that its power of attrac- tion, A, is equal to the square of the magnetic moment. From this we get the formulae — ilf =,^and^=. ^^ B + H -{R + Hf Now, if in these formulae we substitute the values for t and H previously found, we obtain the equations — tf_ Sab ■'" — r» 9 . i — 7 — ; — ^ and M^ + iTOa(a + c) A = lRc^ + 7rba{a + c)J- These equations show that we can obtain maximum values of strength for M and A in various ways, accord- ingly as we change the value for a, b, c or g. The prin- MAGNETIC MOMENT AND ATTRACTION. 215 cipal conditions on which these maxima depend are : 1, The resistance of the coil ; 2, The ratio of its diameter to the diameter of the core ; 3, The dimensions of the magnet itself. In practice, it will always be desirable to find an ex- pression for iJ, which expression will be a function of the resistance of the magnetising coil. This is arrived at in the following way. First of aU we divide g, which denotes the diameter of the wire and its insulation by a coefficient, /, in order to obtain the diameter of the bare wire, as it is that alone which comes into consideration when we calctda.te the resistance. (In practice, this coefficient mav be taken as 1"6 for very fine wires, and as 1'4 for medium wires.) The diameter of the bare wire is therefore -^ ; and if by q we denote the ratio of the efficiency of conductor M to that of conductor H (including the constants, referring to conductivity per unit of cross-section, which is 0"000016), we obtain ^—f^ as the reduced value for R. Because, with the increase in thickness {-?-) of the wire, not only the resistance of a coil of constant diameter, but also the length of its wire, is diminished — two values which vary at the same rate — ^the resistance, H, of the coil will be inversely proportional to gr*, instead of to g% but the quantity, rj^ , will remain inversely proportional to g% so that for the nominators of M and A we get the following expressions : qRg* +4:ba{a + c)f^; \' qRg*+ 4:ba {a+ c)P ^^ /V L TV J 16 CONSTRUCTION OF ELECTRO-MAGNETS. The values for M and A themselves are : M = qRg*+f^wba{a + c) ^ (3) lqRg^+r7rba{a + c)S' Having thus determined the values for M and A, the question arises as to what are the conditions for obtaining maximum values for A and M. We shall first con- sider — I. The dependence of the maximum values of A and M on the resistance of the magnetising coil. This dependence would have to be ascertained if we were desirous of employing an electro-magnet of given dimiensions, and wished to select such a thickness of wire as would enable us to obtain a maximum for the values A and M, when the resistance of the external circuit is given ; or if, with a given external resistance, we wished to employ a particular kind of wire for the magnetising coil, and wished to calculate the dimen- sions of the latter, with which M and A would obtain their maximum values. In the first case, the variable is g, or the diameter of the wire ; in the second case, it is the thickness of the coil. If we now take into consideration that, in the formulae (3), the diameter of the wire is not g, but -K-, in which expression / denotes a constant, we see that the calcida- tion is not so simple as it would seem to be at first sight. The physicists who first occupied themselves with this question had assumed that they might neglect/ entirely in the calculation, and might put g for the diameter of the wire and its insulation. Even when the value for / was taken into account — as, for instance, in the formulae MAXIMA. 217 (3) — it was supposed that this value could not be ex- pressed by a constant when the value of g varied, and in consequence the final results obtained were different. Considering the question from the simplest point of view, we see that the maximum values for M and A are obtained when qRg^ _Trba{a + c) 7^ ~ 9' ' that is, when R = H;or,in other words, the most ad- vantageous diameter of the wire will be that which makes the resistance of the magnetising coil equal to that of the external resistance. If we take the thickness of the insulating covering into consideration, the coil that will render the best service wUl be that of which the resistance is to the resistance of the external circuit as the diameter of the bare wire is to the diameter of the insulated wire. Another law which can be deduced from these formulae is: Of different magnetising coils that have wires of the same diameter, but contain different numbers of turns, the most efficient will be that in which the resistance is to the resistance of the external circuit as the thickness of the coiling and of the iron core is to the thickness of the coil alone. II. Dependence of the maximum strength of A and M on the ratio of the thickness of the magnetising coil to the thickness of the iron core. As the strength of an electro-magnet increases with the dimensions of the core, and as the resistance of the exciting coil also increases proportionally to these dimen- sions, we must, of course, ultimately reach a maximum for the magnetic intensity ; and it is important, in the con- 2i8 CONSTRUCTION OF ELECTRO-MAGNETS. stniction of electro-magnets, to know the law with regard to this maximuni. This law is obtained from the following equations, where X is a coefficient, with which the length of the wire in the coil must be multiplied in order to obtain a maxi- mum in the circuit ; and if the values of A and M are ascertfiined with regard to c and have been equated, it will be seen that maximum values are obtained for them, when a = c, that is, when the thickness of the exciting coU is equal to the diameter of the iron core. The calculation for the maximum of A and of M is now very simple, since, tmder the conditions found for the maximum, the expression for the length of the wire in the magnetising coil becomes — — ^ — , and if we express the length 6 of the magnets as a function of the diameter c, by multiplying the latter by m., which experiments shew to be equal to 12, for the two branches of the magnet, we obtain the expression 2irc^m 75-4 x c* ,,, -^-or-^,- (5) in which the values of c and g occur. For the number of turns t, we then get 12ca Now, we see that A and M are maximum values when R = ff, a =^ c, b ^ cm, and it follows that as ^r is not known we must find a value for iJ, that is a function of g, and this we get from the value of rr 27r c* m LENGTH OF CORE. 219 which is equal to R, and we write qR^ _ 2Trc*m From this it follows that 9'=Pji-^l (6) and, finally, because is a constant, which is composed of known numbers and is 0'00020106, we get the equation 9 = y f l/-^ X 0-00020106. (7) in. The dependence of the maximum strength of M and A on the length of the iron core. From what has been said, it appears that it is important for the calculation, that the length of the magnetic core be expressed as a function of its diameter, but it is still a question whether the iron core may be lengthened indefinitely or whether we can also determine a maximimi value for this length. Siuce according to MiUler's law, the values of A and M are proportional to the square root of the diameter, we can find no maximum values in these cases, as long as 6 varies ; but if we express 6 as a function of the diameter c, the power of attraction becomes proportional to c th. '^/~c or to cf ; if we now take into consideration all the condi- tions found so fer for the maximum, strength, we get [ii(;* + 2,rc»m]a ^^' in which equation JS is represented by a certain length of wire of the coil. From this equation we then farther obtain 27r c'm t = 11 JL 220 CONSTRUCTION OF ELECTRO-MAGNETS. In other words : we can increase the dimensions of the iron core till the resistance of the magnetising coil is eleven times as great as the resistance of the external circuit. In that case we get ^ 2 m = 11 s-^3 and from this' we see that m is eleven times as great as the ratio of the resistance R of the external circuit, to that of the coil, for which latter in this case we have the expression — g-. Now, because in order to obtain the maximum intensities for A and M, the two resistances, as we have seen, ought to be equal, it follows that their ratio is equal to 1, and m in consequence =11. Practi- cally, however, we must assume w- to be = 12, on account of the magnetic poles usually having rather larger dia- meters than the core, the wires, too, as a rule, not being directly wound on to the iron, and on account of similar minor causes, on which the deviations from theory depend. From the preceding formulae, some important conse- quences may be deduced : — 1. For equal circuit resistance, the diameter of the electro-^magnets under maximum conditions should be proportional to the electromotive forces employed. 2. For equal electromotive forces, the diameter should be in inverse ratio of the square root of the resistance of the circuit, comprising that of the generator. 3. For equal diameters, the electromotive forces should be proportional to the square roots of the resistance of the circuits. 4. For a given electro-magnetic force, and with electro- magnets under their conditions of maximum, the electro- motive force of the generators should be proportional to the square roots of the resistance of the circuits. CO-EFFICIENT K. 221 If it be considered that the expression P t^ cf , which represents this force, may be converted by successive El substitutions in the values of /, of t, and of c, into -p|- x - a formulae in which Q is a constant equal to 2228 (if the electromotive force be taken in unity of a Daniell's element, and the resistance in metres of telegraph wire), there is attained for the same attractive force the ratios pl^ = -~ or -™-= — 7= . As in the values E and R there come in the numbers n and n' of elements employed, these numbers may be easily calculated, knowing the values of the constants e and p of the elements employed, for — r— , = — ^ "*" =, and if the accentuated quantities n' e' -J n' p' -<- r' refer to those of a known typical electro-magnet, the value of n may be easily deduced. If, in the problem to be solved, the attractive force of the electro-magnet is expressed as a weight P, and modifying this value by a co-effcient K, which represents F' the ratio -gj deduced from data from the typical electro- magnet, there may be taken for the value of this con- 0- 002297 26-85 0-00008555. It may be remarked that the quantity F in this ratio represents, for the typical electro-magnet, the formula P t^ cl or its equivalent ; and it will result from putting nei ^ Q=PK,the,t {np+r)i /2 ne = ^ltp-T~r x /^/ ^T^P^g^ stant for attraction at 1 millimetre, „^ ^^ , or 222 CONSTRUCTION OF ELECTRO-MAGNETS. and if the two constants Q and K be combined, -^^^ = (0-0225 l^W^\ Representing by M the parenthetic quantity, which may be easily calculated. n = ^ 2e^ If instead of a simple circuit, there were x derivations from the same pole of the battery, and on [these deriva- tions are interposed electro-magnets of equal resistance and dimensions, the quantity of current on each derivation would be iE hE -or ;^ , Xj^P + H where i is the number of series of k cells in parallel circuit ; and the values of i, h, or n will be the same as in the more simple case, but multiplied by x. As a numerical example of the preceding formulae, suppose, in an electro-magnet, it be desired to have an attractive force of 273 grammes, at 1 millimetre distance from the armature, on a circuit of 50 kilometres resist- ance, say 500 ohms, with a bichromate of potash sand battery. In this battery the value of the electromotive force, e, of each element is 2 (that of a Daniell being 1), and the resistance, p, about 1,000 metres of telegraph wire, say 10 ohms. According to the formulae — A = 0-0225 P^ ^ 1-37* x 273^ = 0-09 ; and ^^=0-0081 x^='gl+//(^)V o-oo8i^50>oo.Q ^n.i25; SHUNT CIRCUIT MAGNETS. 223 whence c = ^'^Z^'-' X 0-173 = 0-01553 metres. V 61,125 This gives for the length of each bobbin 0*0932 metre ; for the diameter of the wire, with its covering, 0-39 millimetre, and without, 0-28 millimetre ; for length 1,861 metres ; number of turns, 19,078 ; for quantity of current (not amperes, of course), 0-0001859 ; for value of c^, p-001935. Squaring the values of /and t, and mul- tiplying by c^, there is obtained 0-024378, which repre-. sents the electro-magnetic force, and this value, com- pared with that of the typical electro-magnet (experi- mented with by Du Moncel), which is 0-002297, gives the ratio 10-6, nearly that of the two weights 273 and 26-85, representing the attractive forces in grammes. Conditions for Maximum on Shunt Circuits. — The preceding deductions suppose that the permanent state of electric propagation is established, that the reactive effect of the extra current from the electro-mag- net does not occur, that the iron of the electro-magnet is magnetically saturated, and that the exterior circuit, R, is perfectly insulated. When these conditions do not exist, Hughes' experiments have shown that the resistance of the helix must be considerably reduced. To consider the most simple case, that of a single de- rivation u established on a circuit of resistance I, with a common resistance B in the battery circuit, the at- tractive force A of the electro-magnet interposed on I will be — , E^v^ [B{u + l+H) + u{l + H)f' and if, for t and H, be substituted their true values, 224 CONSTRUCTION OF ELECTRO-MAGNETS. drawn from the equations previously given, there is oh- tained in relation to g, considered as variable — qg^ fj -^ ^ \ _ irba(a+ c) J^V^ E + u) - ^^' an equation consequently corresponding to the conditions of maximum. In varying the thickness of the helix, the quantity g remaining constant, these conditions of maximum are represented by — qg^ (, , Bn \ _ nba^ 'pV^R+u) - g^ • Now, in the first of these equations, the second mem- ber represents the resistance of the wire of the helix, and the first member is the total resistance of the external circuit, expressed in units of the same order as those tised in the evaluation of the length of the heUx wire. But this total resistance is taken in inverse sense, because that which is under consideration is really represented by lu R + V . In this case, the total resistance should be supposed as if the part common to the two derived cur- rents were represented by the shunt circuit I, and as if the part really common, R, were only a simple shunt circuit. In the second equation, the first member represents, as before, the total resistance of the circuit taken in in- verse sense ; but this total resistance, as the resistance R of an insulated line, shoidd be considered as being smaller than that of the helix in the ratio of 1 to 1 + -, to satisfy the conditions of maximum relatively to the variable a. Definitively, it may be deduced that the laws of electro- magnetic maxima, on circuits to which shunt-circuits are VALUES OF K. 225 attached, are the same as for simple circuits, but only by supposing that the resistance R, on which they are based, is represented by the total resistance of the ex- ternal circuit with its shunts, and by admitting that this total resistance is considered as if the battery were sub- stituted for the electro-magnet in the circuit. And as the total resistance of a circuit from which shunt-circuits are taken is less than its own resistance, the helix should have less resistance than this latter. If E in the preceding formulae is expressed by the electromotive force of a Daniell element taken as unity, and if R is evaluated in metres of telegraph wire, K = 0*172175, and the figure obtained represents fractions of a unit. - When referring the values of K and R to the British Association unit — that is to say, to the volt and ohm — K = 0-015957. In conclusion we would call attention to the fact, already mentioned in a previous chapter, that other things being equal, the strength of M and A also depends on the quality of the iron employed. ?26 MEASURING INSTRUMENTS. CHAPTER XI. INSTRUMENTS FOR MEASUREMENTS IN CONNECTION WITH ELECTRIC GENERATORS. Believing that a short description of a few of the most useful instruments for carrying out measurements in connection with electric generators will form a Welcome supplement to our book, we have selected the dynamo- meter, the am-meter and the volt-meter, of Professors Ayrton and Perry. By this selection we do not mean to imply that the instruments named are those only useful; for, amongst others, the electro-dynamometer and the torsion galvano- meter of Siemens, as well as the ampere-meter and volt-meter of Marcel Deprez are also excellent instruments. Ayrton and Perry's dynamometer. The Ayrton and Perry dynamometer is constructed as follows : A cross-shaped piece KK, Fig. 51, is wedged on to the shaft W of the motor that drives the electric generator, and the four ends of this cross-piece R are connected, by strong spiral springs, with a pulley which transmits the power. When the transmission is made by means of belts, S represents a belt-pulley slipped loosely over the shaft ; whereas if the shaft, to which the work has to be transmitted, lies in the continuation of the shaft W, S is firmly fastened to it. Now, if W is turned in the direction of the arrow, the motion is transmitted to S by means of the springs ; and the larger the amount of work transmitted the more are DYNAMOMETER. 227 the springs extended, thus causing the relative positions of K and S to be changed. At C a small bar ABG is attached to S. From Fig. 51a it will be seen that this bar is supported against a point in the plane of rotation, and from Fig. 51c that it is supported against a point perpendicularly to this plane, and is connected with a continuation of one of the pieces iJ by a small bar BB'. Now if, during the rotation, S Fig. 51. lags a little behind W, A will approach the shaft, and the greater the amount of work transmitted the greater will be this approach ; so that the positions 0-5 shown in Fig. 516, indicate various degrees of this approach. In order to make the position of the bar ABG visible to the eye, during the rotation of the machine, a small bright button is attached at A. When the rotation is rapid this describes a luminous circle, and the radius of this circle, which is a factor in the measure of the work transmitted, 228 MEASURING INSTRUMENTS. can be read off from a scale placed on a level with the shaft. If the number of revolutions, which is important as the other factor in the measurement, is known, it is easy to calculate the amount of work transmitted. A great advantage of this dynamometer is that it occupies only a comparatively small radial space, and, according to the inventors, it costs little more than a rig. 52. flange-coupling usually employed in connecting two shafts. Ayrton and Perry's am-meter and Volt-meter, The commutator am-meter is an aperiodic galvanometer, obtaining its name from the circumstance that the strength of current can be read off from the scale directly in " amperes." Am-meter is an abbreviation of ampere- meter. It is connected with a commutator, which allows of the 60 convolutions of wire of the instrument being coupled up either one following the other, or as ten AM-METERS AND VOLT-METERS. 229 abreast. In the latter case the resistance of the circuit is only iryrpr of ths Tcsistance in the first case. •' 100 The following is the construction of the apparatus, which is shovm in Fig. 52. A small magnetic needle, connected with a light alumi- nium pointer, moves in the strong field of a powerful horse-shoe magnet ; the coils of wire are arranged in such a way that the deflections of the needle, and, therefore, of the pointer, are proportional to the strength of current. In order to gauge or to graduate this galvanometer, the commutator is turned so that the 60 turns of wire are connected in series, and a cell, of which the electro- motive force, E, is known, is then connected with the binding-screws B and PS; the position of the pointer is then read off; call it 8. By removing a plug, shown on the left in the figure, we insert a resistance of one ohm, which is connected with this circuit, and take a second reading ; caU this second reading G^. The resistance of the instrument, together with that of the cell and of the conducting wires is, then — Gi — (?2 and the strength of current necessary for the first deflec- tion is— — ^— '„ amperes. We accordingly find strength of current in ampdres, which corresponds to a deflection of any number of degrees, by multiplying the reading by E {G, - ga G,G^ ' 230 MEASURING INSTRUMENTS. when the convolutions of wire in the am-meter axe coupled up for intensity ; or by when the convolutions are coupled up for quantity. If for instance in graduating we use a Leclanche ceU (large pattern) of which the electromotive force is 1*26 volts, and the first reading gives us 6-2*, whilst after inserting the resistance of 1 ohm the pointer indicates 4*25 4-25, then the resistance is _ - = 1'53 ohms, and the strength of current for the deflection of 6'5 is 1'26 — — = 0*83 amp^s ; accordingly, to deflect the pointer X*Oo 0*82 one degree, a current equal to -^^ =0*125 amperes is necessary, when the convolutions of wire are coupled up by the commutator, one following the other, and equal to 1*25 amperes when they are coupled up, parsdlel or abreast. Some of the Ayrton and Perry instruments are so constructed that, when the wire turns are coupled up abreast, one degree of the scale measures 1 ampere; whilst in others 1 degree means 2 amperes, in some even 5 amperes. With the latter instruments currents of more than 200 amperes can be measured. Although the greater sensitiveness of the commutator am-meter, when arranged with its coils in parallel circuit by the action of the commutator, was originally only designed by the inventors for the purpose of graduating the instrument, it is now taken advantage of for deter- mining small strengths of current of 0*5 to 2 amperes that occur in measurements with single incandescent lamps. The commutator- volt-meter is a modiflcation of the commutator-volt-meter: 231 Instrument just described ; the resistance of the coils of wire is 400 ohms (each of the ten coils = 40 ohms), when coupled up in series, and 4 ohms when coupled up abreast ; whereas in the am-meter the resistance is only 0*3 when the coils of wire are coupled up in series, and 0"005 when coupled up abreast. Some of the volt-meters now constructed have a resistance of 200 ohms per coil, and accordingly when coupled up abreast, the parallel re- sistance is 20, and when in series, the resistance is 2,000 ohms. Whilst the am-meter is graduated when the coUs of wire are in series, and is usually employed in practice with the coils arranged parallel, the volt-meter is graduated with the coils parallel, and is used coupled up in series. In some instruments one degree corresponds to one volt, in others to five volts, so that by a complete deflection of 45° in the 5 volt instrument, the needle indicates an electromotive force of 225 volts. But, as the am-meter can be employed, if necessary, with the coils of wire in series (when, for instance, we wish to measure currents of comparatively small quantity that occur with single incandescent lamps), we can also in exceptional cases employ the volt-meter with the coils of wire coupled iip parallel ; as for example, when we wish to measure electromotive forces of only 2 to 3 volts, as with accumulators. In order to graduate the volt-meter, we couple up the coils of wire parallel, by means of the commutator, and then conduct a current through them from a cell whose electromotive force, E, is known. We take a reading of the deflection, and then remove the plug seen at the left hand of the figure. This inserts a resistance, equal to 4 ohms in this instrument. We then take a second reading. 232 MEASURING INSTRUMENTS. The electromotive force corresponding to one degree is 10 {G,-G^)^ Gi G^ when the turns of wire are coupled up parallel, and G1G2 when they are coupled up in series (the usual arrangement of the coils of wire in the instrument). The construction of the am-meter and volt-meter is such that in the am-meter the artificial resistance can be in- serted only when the coils of wire are coupled up in series by the commutator ; whilst in the volt-meter this inser- tion is possible only when the coils of wire are connected parallel. In this way damage to the resistance coil by fusion is avoided. In order to protect the galvanometer coils of the instrume^it against fusion — which might easily occur when the instruments are arranged for mea- suring currents of small quantity in the one case, or low intensity in the other — each instrument is provided with three binding-screws, marked B, (P S), and P,in the figure which will represent either a commutator am-meter or a commutator volt-meter, as both have exactly the same form. In the am-meters of the latest construction, P can be used only for thick wires, and B only for thin wires, whilst (P (S) is alone suitable for both kinds of wire ; conse- quently the wires of an electric generator can only be connected with (P S) and P, and not with B ; and as a current can only flow between (P S) and B, when the coils of wire in the instrument are coupled up parallel, it will be interrupted in the case where the coils of wire would be accidentally coupled up in series ; in this way th& SIMPLE AM-METER. 233 instrument is kept from injury by fusion of the wires, or by the needle being too strongly deflected. Similar precautions are taken in the construction of the volt-meter. The coiling of the wire of these instruments is such that the needle is deflected towajrds that binding screw which is in connection with the positive pole. Am-meter and volt-meter without commutator. In large establishments using electric lighting where many am-meters and volt-meters are constantly employed, it is of course unnecessary that each instrument should have its own arrangement for graduation. It is sufficient to have a few of the instruments with commutators, and the others can then be compared with these ; accordingly am-meters and volt-meters without commutators are con- structed. All these measuring instruments by Profs. Ayrton and Perry are constructed in such a way that the magnetic needle and the pointer swing on pivots, the axis passing through the centre of gravity, so that the deflection of the needle, and of the pointer, will be the same for any position of the instrument. The construction of the pivots is similar to that in watches ; it is so fine that the friction is not increased by giving the instrument a slanting posi- tion. The permanent magnets of the instruments are sufficiently strong that the deflections of the needle are not easily affected by the proximity of electric machines. One disadvantage which has been felt in the use of the instrument described is that the magnetism of the per- manent magnets becomes in time modified by the electric currents. This would not be of importance if the instru- ment were always graduated with a DanieU cell; but Daniell cells are not always to be obtained at plabes where 234- MEASURING INSTRUMENTS. the am-meters and volt-meters are employed, and the fact' that graduation is necessary is prejudicial to the value of the instruments. To prevent the weakening of the magnetism of the per- manent ma,gnets, the inventors have added a keeper to be put on when the instruments are not in use ; later, how- ever, the employment of permanent magnets has been given up and, am-meters and volt-meters with springs made. In these instruments the needle is not controlled by the pole of a magnet, but by a flat or cylindrical spiral spring, and the soft iron needle makes an angle with the axis of the galvanometer coil, which is less than 90° when the instrument is at rest (an angle of 90° would disturb the aperiodic character of the instniment). The inventors have determined by a large number of experiments and by calcidation, what angle the needle must make with the axis of the galvanometer coil (when the pointer is at 0°) in order to obtain the best results. When these new instruments are well made, deflections up to 45° can be obtained, which are directly proportional to the current ; and the instruments of this construction have the advantage that they can be used for alternating currents, and by a simple adjustment of the spring (by turning a smaU pointer moving over a scale), we can set the large pointer in such a way that in the am-meter it does not quit the zero position until the current has at- tained a certain strength, and in the volt-meter until it has attained a certain electromotive force. This method, which permits of the spring being adjusted, and the degree of adjustment read off the scale, con- siderably increases the sensitiveness of the instrument. Let us assume for instance that with a particiUar in- strument we have to measure currents of about 30 am- SPRING VOLT-METERS. 235 p^es or of a strength varying from 25 to 35 ampferes. In this case we do not adjust the needle so that weaker currents will deflect it, but so that the pointer quits the zero position only with a current of 25 amperes. If the instrument were so adjusted that the deflection com- menced with a current of 1 ampfere, the 45 degrees would be distributed to 35 amperes ; assuming 35 amperes to be Fig. 53. the maximum of the currents to be employed : in other words, each increase of 1 ampere in the strength of current would deflect the needle 1"3 degrees further whereas if the deflection of the pointer is made to com- mence with 25 amperes, the 45 degrees are distributed to 10 amperes ; or 4*5° to each ampere. The sensitiveness of the instrimient is, therefore, nearly four times greater than in the first case. 236 MEASURING INSTRUMENTS. Instruments allowing of still more accurate measure- ments are the Am-meter and volt-meter with cog-wheel and gear. — In this instrument there is a very fine cog-wheel W (Fig, 53) in the axis of the magnetic needle, and the teeth of this engage in the gearing P on the axis of the pointer. By this arrangement the deflection of the pointer is made ten times that of the magnetic needle, and if the con- struction of the instrument is such that up to 36 degrees the deflections of the needle are proportional to the strength of current, 360° of the pointer's deflection will have this proportionality. Besides this, the axis of the needle, as well as that of the pointer, is connected with a very sensitive spiral spring S and S' respectively. These springs can either be used simultaneously in carrying out the adjustment, or only one of them may be allowed to act ; <8" can be freed from the gearing by means by the lever A and screw H. When the springs are equally strong, we can increase the sensitiveness of the instrument exactly a hundredfold by turning this screw, H, and freeing S'. This sensitive- ness may be adjusted to a greater or less degree by bring- ing the strength of the spirals into certain relations. To each spring there is attached a small pointer, allow- ing of such an adjustment of the instrument that a com- paratively few imits of current in the one case or of electro motive force in the other, can be distributed over the 360 degrees deflection of the principal pointer. This is, for instance, desirable when small variations of a tolerably uniform current have to be measured ; and by means of this construction of the am-meter and volt-meter, we obtain instruments that, although intended for large 'Currents, or of high electromotive force, are as accurate and ELECTRICAL MEASUREMENTS. 237 sensitive as the more delicate instruments specially made for measuring weak currents and low electromotive forces. Another instrument for measurements in connection with electric machines is the horse-power measurer, or ergometer, the scale of which is graduated for readings g Volts ^ Amp. of horse-power, or ^ — ; and, therefore, simul- taneously measures the strength and electromotive force of the current. We shall not, however, describe this in- strument, although it is very useful in some cases, nor the ohia-nxeter, for measuring resistance in ohms, because the apparatus described are quite sufficient for all the measurements that usually occur in practice. And we shall conclude this section by stating a few physical equations for finding the principal values in the measure- ments usually to be carried out. Equations occurring with measurements in con- nection with electric generators.— In order to calculate the work done in the various parts of an electric machine, as well as in the external circuit, the equations given by Dr. A. Jobler, of Ziirich, may be used with advantage. Let r be the resistance of the armature, W that of the electro-magnets, and w the resistance of the external circuit, in which, for instance, a number of electric lamps of unknown resistance are inserted. The resistance of the latter wiU usually be sufficiently great that the re- sistance of the conducting wires may be neglected. In Fig. 54, the resistances are denoted by the abscissae, and the potential by the ordinates ; .the slope of the straight line (/) {d) or J, accordingly represents the quantity of current G ; a and b denote the terminals of the armature coil ; a and c those of the generator ; and a and (d) correspond to the same pole of the generator. MEASURING INSTRUMENTS. i- I- -W ""' a) "^o V. -V. V. ELECTRICAL MEASUREMENTS. 339 ' In the expressions for the various differences of potential, Fq is assumed to be equal to o, and V—^^ for instance, denotes the difference of potential between the armature terminals, whilst v — v^ denotes that between the poles of the generator, and Fq represents the total electromotive force. From Fig. 54 6, we can then deduce the following geometriced relations : Vq — Vq _ r + W + w ^ Fq — Vq _ r + W + w ^ F— Vq~ W + w ' V —Vg ~ w ' ^"^ "»= W+w ^^"""^ = " w ^^ ~ ■"o^- (^) ^0 - ^ ^Z^ ^ slope of J- = 0. (3) w = -IL^ ^^. (4) V-v W W(v —Vr.) The work E, in horse^power, done in the above circuit, is obtained from the equation _ G\r+ W+ w) _ _Fo C^ 9-81 X 75 ~ 735-75 ' and, as according to equation (1), v — Vq = G w, ■ve get ^_ [g(r+ W) + {v-Vo)]G ^ 9-81 X 75 The work e^ lost in the wires, and the work e^ obtained in the external circuit, can then easily be found by the aid of the equations e, = (73 (r + W) 63 = C^ *o. 240 MEASURING INSTRUMENTS. The above formulae, however, can only be used when the eleotaro-magnet of the generator, in connection with which the measurements are being taken, is inserted in the main circuit ; other formuks are necessary for generators in which the magnet is inserted in a shunt circuit. The diagrams, Fig. 54, d, e,f, will serve to determine the formulae for such generators. Here, too, r denotes the resistance of the armature, W that of the electro- magnets, w the resistance of the external circuit, and V—Vf, the difference of potential between the terminals of the armature. From the diagrams we then get — ^3 = (slope of/) = C; Z^»=(^slofeo£j") = G"; The reduced resistance of the electro-magnet coils and of the external circuit is w W w+ W K this value is taken on the axis of the abscissae, we easily get Og and Fq (Fig. 54/). The work, in horse-power, of the current, is then dis- tributed as follows : Total work : 9-81 X 75 735-75 ' Work in the armature coils : ** " d-Sl'x 75_^-^- ELECTRICAL WORK. 241 ATork in the coils of the electro-magnet : _ G'^W _ G'{V-v,) *i ~ 9-81 X 75 9-81 X 75 ^•^• Work in the oviter circuit : G"^w '^.= Zi^-:}H.p. 9-81 X 75 9-81 X 75 With the help of these formnlse and of FrohUch's equations given in Chapter V., almost all meas\irements can be carried out that occur in practice in connection with magneto- and dynamo-electric generators. CHAPTEE XII. LATEST TYPES OF GENERATORS. Enotjgh has been said in the preface to show the direc- tion in which during the past few years improvements have been made, but it remains before describing them to- classify in some way the types of machines now in use. Grenerators are primarily specified as of the alternating or continuous class, accordingly as the current generated alter- nates in direction or flows in one direction only. We have consequently established to start with a classification which depends on the character of the current produced. Alternating-Current Machines may be broadly divided into two groups, comprising (a), machines in which the armatures are fixed while the magnets revolve, and (6), machines in which the magnets are fixed while the arma- tures revolve. In the former, group, the armature consists of a number of independent coils which may be in the same electrical condition at the same instant, or of coils grouped in two or more sets which may be at the same instant in different electrical phase. In the latter case, there must be as many circuits as there are sets of coils. AU the coils of each set can be connected up together in parallel or in series, and the machine can thus be made to yield according to requirements, large currents of low e.m.f. or small currents of high e.m.f. In machines belonging to the second group, the individual coils of the revolving armatures are gener- ally connected up in series or in parallel to suit the requirements of the circuit, the two ends of the one coil CONTINUOUS-CURRENT MACHINES. 243 thus formed being brought to two insulated rings on the spindle off which the current is collected. Continuous-Current Machines : Adopting the sugges- tion of Professor S. P. Thompson, continuous-current generators may be described as belonging to the open-coil or closed-coil class accordingly as the e.m.f. is due to the sum or difference of a number of independent coils thrown into the circuit at intervals, or to the motion of a closed conductor from which the current is collected at two points diametrically opposite each other and fixed relatively to the magnetic-field. Of the former class, there are only two at present before the public, namely the Thomson-Houston and Brush machines. AU the others described in this chapter belong to the closed-coil class. Having classed machines, first according to the nature of the ciurent yielded and secondly according to the way in which the current is collected, there remains a third distinction depending on the method of winding the arma- ture. AU continuous-current machines have armatures with iron cores, which are said to be ring-wound or drum- wotmd accordingly as the conductors pass through an interior opening, Fig. 27, or He on the exterior surface of the core otily, Fig. 34. To the former class belong the Eaffard-Breguet, Goolden and Trotter, Manchester, Phoenix, Kapp, Crompton, Griilcher, Victoria and ElweU- Parker machines, while to the latter class belong the Edison-Hopkinson, Weston, Chamberlain and Hookham and Thomson-Houston incandescence machines. In con- sequence of the former class of armature having in some machines the greater dimension of the core at right angles to the spindle and in others the greater dimension paraUel to the spindle, they have been termed respectively disc and cylinder armatures. The table on other side ex- 244 LATEST TYPES. plains this classification of machines made with reference to the winding of the armature. i-Ring-wound. — Brush. rOpen Coil. -\ •-Drum- wound. — Thomson-Houston. Con- tinuous . Current Dynamos. ■-Oloaed Coil. -Ring - woundc I- -Disc. — ^Victoria, Giiloher. Cylinder. — Raffard-Bre- guet, Goolden& Trotter, Manchester, Phoenix, Kapp, Crompton, Two- pole Giiloher, ElweU- Parker. ■Drum-wound. — ^Edison-Hopkinson, Weston, Chamberlain and Hootham, Thomson- Houston incandescence. Field-Magnets : Whether the magnetic-field is obtained from a large number of sma;ll magnets, as in the various >■■ ^ Fig. 55. I Fig. 57. H---^ S -N 8 alternating-current machines, or from one large horse- shoe magnet, as in some machines giving continuous currents, the chief object of the designer is to make the magnetic resistance of the path through which the lines FIELD MAGNETS. 245 of force flow a minimum. For this reason the magnetic circuit is mostly completed in iron, so that for a given excite- ment there is produced maximum magnetic induction. The path which the lines of force take in the various alternating machines described is shown in Figs. 55, 56 and 57. In Fig. 55, A and B represent the armature - — N ^ < > 1 i i 4-POLE DOUBLE-MACNETS . magnets should be as short as possible consistent with ample cooling surface for the magnetising coils. Pole-pieces : At one time massive pole-pieces were 248 LATEST TYPES. considered requisite for high efficiency, but in several machines of modem build no pole-pieces are employed. The necessity for pole-pieces is purely accidental to the construction of the machine, their function being to convey from the magnet cores to the armature the lines of force without offering to their flow appreciable resistance and to present to the armature a large surface, so that the resistance of air-space is as small as possible. In Figs. 58, 59 and 60 no pole-pieces are shown, the magnets being bored out to obtain the requisite polar surface. In Fig. 58 the cross section of the magnet must not be diminished by boring out to less than half its original section, while in Fig. 59, the core might be cut through to a knife edge without influencing detrimentally the magnetic-field. In machines with cylindrical cores, pole-pieces are always employed in order that the requisite polar surface may be obtained; and from a study of the generators illustrated the reader will conclude that in the particular cases in which they are now employed it would be difficult to obtain without pole-pieces an air resistance sufficiently low for the economical working of the machine. Armatures, Great attention is now given to the proper subdivision of the armature cores. In modern dynamos the cores are constructed of soft iron wire coiled on a frame of iron discs or washers laid close together and insulated from each other ; or of iron ribbon coiled on a supporting ring with the adjacent convolutions insulated from each other. Not only must the core be subdivided to prevent generation of Foucault currents, but care must be taken that in the method of attachment to the spindle no circidt is formed which would allow such currents to flow. In some of the earlier machines, the power absorbed due to Foucault currents flowing in the driving attachments was WORKING TEMPERATURE. 249 considerable, but the loss from this cause has been in modern machines almost, if not completely, eliminated. Working Temperature. The temperature to which a dynamo, running continuously, will ultimately rise, de- pends primarily upon the ratio of the energy expended in heating its coils to the area of the radiating surface. This area would alone determine the temperature were the armature of the machine at rest, but a very considerable portion of the heat generated is due to magnetic friction in the armature core. From the facts that it is in rapid motion and that a current of air is frequently made to flow through its interior, much less radiating surface is required in the armature for each watt expended in heating its coils than is required on the field-magnet coils for each watt expended in excitation. The effective radiating surface for the magnets is considerably greater than the surface of the coils alone, as the whole machine gets gradually heated up in continuous running, the sur- face eflfective in dissipating the heat being thus expanded. The increase of surface due to this heating of the machine frame, it is, however, difiicult to calculate, and it has been the author's practice to allow, for dynamos running under ordinary circumstances, a radiating surface on the magnet coils of 1"5 to 1'75 square inches per watt expended in heating them. For the armature coils, half this surface will be found sufficient if the internal ventilation is good. Where the machines have to work in hot, badly ventilated rooms, it is often advisable to double the surface just given. With a radiating surface of 1'75 inches per watt, the magnet coils have been found by the author to attain in continuous running a temperature 33° C. above that of the surrounding air. In the test for continuous running •of a 16 unit Edison-Hopkinson machine the armature and 250 LATEST TYPES. magnets attained respectively temperatures 61° C. and 18° C. above that of the air in the test room. A radiating surface of 1"75 square inches per watt for the magnets and 1 square inch for the armature wiU be quite sufficient if the dynamos are in weU-ventilated rooms above ground, but for ship-lighting, or where the machines are underground and in close proximity to steam boilers and engines, the surface should not be less than 2*5 square inches per watt for the magnets and 1*5 for the armature. Sparking at the Brushes. It is almost unnecessary to state that such clumsy devices as those illustrated on page 177 for the reduction of sparking have become things of the past. The amount of sparking in the best dynamos is now-a-days quite inappreciable, and this ex- cellent result has been obtained by giving to the magnets a proper shape by reducing the turns on the armature so that the distribution of the field-lines is not seriously modified by the currents flowing in them, and by approxi- mating the number of collector bars to the number of convolutions on the armature. In series machines, where the current in the magnets increases at the same rate as the current in the armature, adjustment of the brushes for dif- ferent currents is seldom required, the lead being constant. In machines compounded for constant potential a slight adjustment forward, as the current increases, will gener- ally be found necessary to avoid sparking ; while in shunt machines, this adjustment, but to a greater extent, will always be required. Much depends on the way in which the brushes are set on the collector. These ought to bed fairly down all the way across the surface and have a con- tact width about equal to that of a bar and a half. The pressure of the brushes ought to be as slight as possible consistent with their being prevented from jump- WEIGHT. SIZE AND OUTPUT. 251 ing, and, vrith. care, the wear of the collector in well- designed machines may be made exceedingly small. In the Thomson-Houston dynamo, which is of the open- coil class, the brushes are, in order to effect the regulation, purposely placed off the neutral Une. This is an except- ional case, for in all other generators the brushes are adjusted so as to make the sparking a minimum. Weight, Size and Output. In the machines described, great differences will be foimd in the ratio of output to weight, and it may be weU to explain briefly the cir- cumstances upon which this ratio depends. The electro-motive force produced by any dynamo may be expressed by the formula : E = Nxaxnxp. where E is the total e. m. f. in volts ; N the number of revolutions per minute ; a, the cross sectional area of the iron in the armature core in square inches ; n, the num- ber of turns of wire on the exterior surface of the arma- ture, and p, a coefficient depending on the degree to which the core is saturated. The value of p varies from •000035 to -000040 according to the type and make of machine, but its exact value for the machines described can easily be found by the student where the requisite data are given. From the formula it will be seen that the e.m.f. de- pends directly on the speed of rotation, hence for a machine of given weight the total electrical energy de- veloped per pound of material is proportional to the number of revolutions. Although the total watts pro- duced are proportional to the speed, the waste in the machine remains constant if the weights of copper and iron are unaltered. We have, therefore, the electrical effi- 252 LATEST TYPES. ciency reduced at a lower speed and increased at a higher speed. To illustrate this, let us suppose we have a machine which at a speed of 1,000 revolutions produces 10,000 watts total electrical energy. Of this amount let 9,500 appear between the terminals and 500 be absorbed in the machine. The electrical efficiency wiU then be 95 per cent. If the current density remains the same, this machine will, at 500 revolutions, produce 5,000 watts, but since 500 of these are, as before, absorbed in the machine, the electrical efficiency will now be to§§ = •9 or 90 per cent. If the speed is further reduced, say to 250 revolutions, the total watts become 2,500 and the efficiency |^^ = -8 or 80 per cent. We see then that in machines of similar size, having similar weights of copper and iron, the ratio of energy converted to total weight and to weight of copper increases directly as the .speed, the output or energy appearing between the ter- minals increasing rather faster. We also see, that with this increase of output there is an increase in the electrical efficiency, although due to magnetic friction and Foucault currents the conversion efficiency may be somewhat lower. Imagine now that in making a slow speed machine we are at Hberty to increase the amount of copper, the iron part remaining as before. The electrical waste is made up of a portion in the armature coils and a portion in the magnet coils. If the iron part of the machine is to re- main unaltered, it is very unlikely that the former can be reduced, since the only method by which this could be done would be by increasing the gauge of wire. For this we have no room, and we cannot therefore increase the efficiency so far as the armature is concerned. But with the magnet coils it is different. Here by increasing the weight of copper we can make the waste smaller and WEIGHT, SIZE AND OUTPUT. 253. compensate, to some extent, the lower eflSciency conse- quent on the slower speed. In the example above, let us suppose that of the 500 watts absorbed internally 300 are spent in the armature and 200 in the magnets. If we put about twice the weight of wire on the magnets we can reduce the latter quantity to 100 watts, making the total loss therefore 400 instead of 500. At 500 revolutions the efficiency wiU then be °°°o7o*°° = "92 or 92 per cent instead of 90 per cent as formerly. Here by increasing the weight of copper we have raised both the output and efficiency. But for slow speed machines of higher efficiency, it becomes necessary to effect a reconstruction by putting more weight into the iron part. Since, other things remaining the same, the e. m. f depends on the product of the armature core area and number of convolutions, it is- evident that n may be diminished as fast as a is increased.. With the diminution of n the efficiency rises, since the resistance is smaller ; but it will be observed that a is nearly proportional to the weight of iron in the machine,, since if a be increased the field-magnet cross section must be increased in the same proportion. It follows then that high efficiency machines are necessarily heavier than machines of low efficiency, which give at the same speed similar output : it also follows, that in machines of similar efficiency and output the ratio of output to weight diminishes more rapidly than the speed. Among the machines which foUow wiU be found ratios varying from 5 to 12 watts per lb. of material employed; but the student will be able to place these different machines on a fair basis of comparison from the considerations above set forth. In machiaes of similar type and of similar efficiency, it is found that the output is nearly proportional to th& 254 LATEST TYPES. weight, provided the speed of rotation is inversely as the linear dimensions, and that the machines are, in all cases, raised to the same temperature. (1.) Alternating Current Machines. The De Meritens Magneto Generator still holds its own for lighthouse illimiination, and several machines of large size have been recently erected. The machine illus- trated in Fig. 18 has an armature, consisting of one ring of sixteen coils, but in some of the later machines as many as five rings of twenty-four coils each have been mounted on the spindle side by side to form the armature. The two machines for the lighthouse of Tino had each five rings of sixteen coils and forty permanent magnets, ranged in five circles of eight. Professor Adams gives the following particulars of three machines, intended for lighthouse illum.ination, which he tested at the South Forelaaad in 1884. Each machine had an armature, consisting of five rings of twenty-four coils and sixty permanent steel horse- shoe magnets, ranged in five circles of twelve. Each coil ■consists of four layers of wire, and is 27 millimetres deep by about 100 millimetres wide. The 120 coils, when coupled in parallel circuit for lighthouse work, have a re- sistance of about one-twentieth of an ohm. The diameter of the armature is 2 ft. 6 in., and the normal speed 600 revolutions per minute. The e. m. f. on open circuit, is about 75 volts, falling to 37 volts at the terminals, when a current of 135 amperes flows. The great fall in potential is to be attributed chiefly to the weakening effect on the magnets of the current flowing in the armature. The machines described are the largest De Meritens gene- rators yet constructed. THE GORDON GENERATOR. 255 The Gordon Generator, constructed by the Telegraph Construction and Maintenance Company, is an alternating current machine of colossal magnitude. The illustration, Fig. 62, represents a machine said to be capable of sup- plying 5,000 Swan lamps, of twenty candle-power, at a speed of 140 revolutions per minute. Its weight, com- 256 LATEST TYPES. Fig. 63. plete, is 22 tons. Apaxt from its size, it will be seen that the Gordon machine differs in several points from that of Siemens', shown in Fig. 23. In the latter, the coils in which the currents are induced revolve and contain no iron ; in the former, the coils in which the currents are induced are stationary and contain iron cores. In the Siemens* machine, the coils rotate between two circles of magnets ; in the Gordon machine the field- magnets rotate between two circles of coils. The advantage of the latter arrangement lies in the faci-Uty with which the coils can be grouped to give, in the lamp circuit, a smaU. current at a high difference of potential, or a large current at a low difference of po- tential. But, on the other hand, the revolving part is necessarily heavy, and there are good reasons for believing that machines of this type must be less efficient than those with revolving coreless ar- matures and stationary magnets. The magnets in the present instance weigh 7 tons, and consist of thirty-two soft iron cylindrical cores, which pass right through the carrying rings and face with each end, but with opposite polarity, a fixed coil. The magnet wheel has a diameter of 8 ft. 9 in., and is constructed of wrought-iron plates, strongly braced together, and riveted to a substantial cast-iron centre- piece keyed on the driving shaft. The fixed coils are idiiged in two circles of sixty-four on each side of the revolving magnets, their cores being formed of a piece of GORDON GENERATOR. 257 soft iron plate, bent back upon itself (Fig. 63). The coils are slipped on these cores, flanges of sheet German sUver divided and slit as shown serving to keep them in their places. A screwed tail-piece is welded into the back of each core, and by these the coils are firmly secured to the cast iron frame which supports them, a thickness of wood intervening between the cores and the frame. It will be observed that there are twice as many fixed coils in the circle as there are field-magnets. By this construction only half the coils are active at once, and the weakening effects of mutual induction on neighbouring coils are ' avoided. As an exciter for this generator a Biirgin ma- chine is employed. The machine above described was constructed in 1882 ; but for a plant recently erected at Paddington, to give a supply equal to 30,000 gas Hghts, three Gordon generators of much larger dimensions have been laid down. Each of these machines weighs 45 tons, the rotating field-magnets weighing 22 tons. There are motmted on the magnet wheel, which is 9 ft. 8 in. diameter, twenty-eight magnets having cylindrical cores 3 ft. long by 6 in. diameter, the fixed coils being placed on each side of the wheel in two circles of fifty-six in each. Here an improvement on the former machine is apparent, the inducing portion in these later machines being much larger as compared with the induced portion. The machines rim at a speed of 146 revolutions per minute, and are wound for a difference of potential of 150 volts. The exciting current is fur- nished by Crompton dynamos, driven direct by WiUans' engines at 500 revolutions per minute. Ganz's Generator, constructed by Messrs. Ganz and Co. of Buda-Pesth, is another alternating-current machine of gigantic dimensions. Constructed to supply current for 258 LATEST TYPES. 70 arc lamps and 600 incandescence lamps, it has a total weight of 15 tons, and runs at 180 revolutions per minute. The stationary coils, in which the current is generated, are of flat form. Fig. 64, thirty-sis being laid side by side round the interior circumference of a wire drum. The thirty-six field-magnets are mounted on a ring inside this drum and form a fly wheel 8 ft. 2^ in. diameter and 18 in. wide. These are similar in form to the revolving magnets of the Gramme alternating-current machine shown in Figs. 21 and 22. Mounted concentrically on the same shaft is the exciter, which consists of a six-poled ^'-- ^*- Gramme ring of 3 ft. 1 1 in. diameter revolv- ing between 12 pairs of field-magnets. The exciter , generates a current of 88-8 am- peres at a difference of potential of 36"4 volts. The alternating current is said to mea- sure 1,516 amperes at a difference of potential of 57-6 volts, and the electrical efficiency of the exciter and generator combined is given as 85 per cent. The Perranti Generator on its first appearance four years ago was considered an alternating-current machine of some promise. The main point of difference between it and the Siemens' machine, illustrated in Fig. 23, lies in the construction of the armature, which is shown diagramma- tically in Fig. 65. The latter is without iron, and consists of a continuoiis ribbon of copper, or of several ribbons in FERRANTI GENERATOR. 259 parallel, formed into a zig-zag and separated, the adjacent folds from each other, by a strip of vulcanised fibre, woiuid Fig. 65. Fig. 66. along with the ribbon. Two insulated rings on the spindle, to which the opposite ends of the ribbon are connected, serve to distribute the current through white metal rubbers 26o LATEST TYPES. kept in contact with the rings by suitable springs. In Fig. 66 is shown the complete armature. The ribbon conductor is secured to the spindle by a phosphor-bronze hub through which pass muntz metal rivets, insulated from and lying in the hollows of the zig-zag. The complete machine is illustrated in Fig. 67, the carcass consisting of Fig. 67. two side checks of cast-iron on each of which are cast in a circle 16 pear-shaped magnet cores. These are magnetised by coils, afterwards placed on them, so that adjacent cores present alternately N and S poles, those coming opposite THOMSON-HOUSTON DYNAMO. 261 ■when the checks are bolted together, presenting faces of opposite polarity. The construction of the armature ought to he compared with that of the Siemens' machine before referred to. In the latter the armature is made up of separate coils secured to a central piece by German silver side plates,the result being a structure obviously inferior to that of Fer- ranti as far as strength and rigidity are concerned. There being no supports interposed between the moving conductor and the magnets in Ferranti's machine, the opposite magnet faces can be brought closer together and the necessary intensity of field produced by a smaller excitement. But given the same intensity of field and the same speed, the weight of copper in the armature necessary to produce similar effects is the same in both machines, the smaller weight of copper in the Ferranti being due only to the greater speed at which the armature rotates. The Ferranti machine for 1,000 lights of 20 candle-power has an armature 30in. diameter rotating at a speed of 1,400 revolutions per minute. The weight of the machine complete is 32 cwts., the armature weighing 96 lbs. The armature resistance is given as -005 ohm. For exciting the magnets, a small continuous current Siemens' machine is employed as illustrateid on page 44. (2.) CONTINUOUS-CuitRENT MACHINES. (Open-Coil Class.) The Thomson-Houston Dynamo illustrated in Fig. 68, is an arc Hghting machine which exhibits in its con- struction every shade of electrical heresy. Although most of the principles of correct design, or of design 262 LATEST TYPES. hitherto considered correct, have been violated in its con- struction, it seems, disregarding the question of efficiency, that a high degree of success has attended its working. It is largely employed in the United States, but has only recently been introduced into England. The armature, THOMSON-HOUSTON DYNAMO. 263 externally resembling a sphere, is wound Siemens fashion with three coils. The coils are joined together at one end, the three free ends being connected up to three separate circular plates having each an angular width of 120°, and which when mounted on an insulating piece end to end and separated by an air space form the commiitator. The armatiue core is of soft iron wire, coiled on a spheroidal frame, formed by a number of arched, wxought-iron bars aj L£N SCOTT \ Ci let into holes in the outer edges of two convex cast-iron plates keyed on the shaft some distance apart. In Fig. 69 is shown a section through the magnets. These con- sist of cast-iron tiibes, BB, partially closed at the two inside ends, next the armature, which constitute the poles. Be- tween the flanges on the outside ends stretch a number of yoke bars, A A, the exciting. coils being wound on the out- side of the cast iron tubes as shown. The action of the machine is as foUows : — In Fig. 70 let the lines A, B and C represent the planes of the three armature coUs, the 264 LATEST TYPES. inside ends being joined together and the free ends attached to the three commutator segments as shown. The dotted Kne D E represents the neutral position, a coil when its plane coincides with this Kne being idle, or furnishing to the circuit no electro-motive force. The current is collected by what is equivalent to two brushes having an angular width of 60° under the normal con- ditions of working. This angular width is varied by a regulating apparatus in the main circtdt, each brush consisting of two copper plates connected electrically. From the bnishes F and G, the current is led through the regulator to the lamp circuit. Since each brush has an angular width of 60°, there is between the positive and negative brushes an angle of 120°, which is equal to the width of one commutator segment. It foUows that from the time one segment touches till the time it leaves a brush, the armature must rotate through an angle of 180°, or make exactly half a revolution^ In its normal condition then, each segment is always under one brush or the other, and changes over from the positive to the negative at the moment the corresponding coU occupies the neutral position, the ciurent always flowing through one coil in series with two in parallel. Consider what happens in half a revolution from the time a segment touches till it leaves the brush. In Fig. 70, coil A is shown in the neutral position and just changing over from the left hand to the right hand brush. For 60° onwards from the position shown, A is in parallel with B, these two being in series with G. For the second 60°, A alone is in contact with the brush, but in series with B and G, which are in parallel. For the remaining 60° ot the half revolution, A is in parallel with G, these two be- ing in series with B. In other words, for one third of the THOMSON-HOUSTON DYNAMO. 265 time the segment is under the brush its coil is in paral- lel with the one in front of it, for one third it is in series with the other two in parallel and for one third it is in parallel with the one behind it. It then passes to the next brush and the changes are repeated. It wiU be observed that a coil which is neutral is put Fig. 70. in parallel with an active coil when the latter has only moved 30" from its position of maximum activity, also that it is withdrawn from parallelism with an active coil when the latter is 30" from its position of maximum activity. Undoubtedly from this continuous short circuit- ing of an active with a neutral coil we have large local currents flowing, which must be a source of loss and to which is due a good deal of the sparking which occurs at the brushes. By an apparatus actuated by an electro- 266 LATEST TYPES. magnet in the main circuit, the current is kept constant, the regulation being affected by increasing or diminishing the angular width of the brush, and giving to it at the same time a backward or forward displacement. If the current by the short circuiting of a number of lamps gets too great, the rear plate of the brush is moved backwards and the leading plate ^ as much forwards, the effect of this being to bring the coil segments under the brushes Fig. 71. igs. 72, 73. before their coils become neutral and while yet they are active in an opposite sense to that in which they should be active in order to contribute to the e.m.f. of the circuit. By increasing the angular width of the brushes, the dis- tance between them is made less than 120°, the width of a commutator segment. The two brushes, in consequence, touch one segment, and the result is that two active coUs are short-circuited through a commutator segment six times in each revolution. If the current gets too Jlow, the rear brush plate is moved forwards and the leading BRUSH DYNAMO. 267 plate backwards, the effect of which will be understood from what has been already said. The regulation of the machine is thus effected by an automatic deter- mination of the fraction of a revolution for which the armature shall be short-circuited on itself and of the stage at which the respective coils shall be cut into paral- lel with coils more active. A blower is attached to the machine, which, by sending a puff of air at the right time against the point of the leading brush plate, blows out ', the spark. That the action of the machine will ever be completely understood seems highly improbable, from the complicated reactions taking place. The reader may be helped in his study by cutting out a three-segment collector in cardboard and rotating it on a sheet of paper, on which he can draw the brushes with various angular widths and displacements. The Brush Dynamo of to-day differs from that shown in Fig. 24 chiefly on account of the cast-iron armature core having been abandoned in favour of the wrought- iron laminated core, part of which is shown in Figs. 71 to 73. This core is built up of thin wrought-iron ribbon, B, coiled on an interior ring. A, there being secured between the successive convolutions I pieces of iron of the same thickness as the ribbon, Figs. 72 and 73. These I pieces have webs equal to the width of the rib- bon, the flanges consequently protruding beyond on each side to form the Pacinotti projections between which the coils axe wound. They are held firmly in their places by radial rivets, which, in addition, serve to bind the whole armature securely together. The results obtained from this armature are a great improvement on those derived from the old cast-iron one, for at the same speed and with the same current, there is a gain of 30°/o in the e.m.f. Due 268 LATEST TYPES. to elimination of eddy currents, which, in the older arma- ture were made so apparent by heating, there is also a great gain in economy. It is stated that there is required, to drive the machines at the higher output above given, a less- horse-power than was formerly required for the cast-iron machine. Another difference in the modern machine lies in the fact that when coil is cut out of circuit it is opened at both ends, the commutator now employed being shown in Fig. 74. Formerly one end of the coil was, while cut out, left in contact with the brush. Fig. 26. Now, when cut out, it is completely insulated from the circuit. Lately a Brush machine of enormous magnitude has been constructed in America for the electric smelting of Fig. 74. aluminium. This machine, which is f^ — ^ shunt-wound, is shown in Fig. 75,* and HI _ _ Bill weighs about 9| tons. There are eight field-magnets, each having a cyliadrical cast-iron core, 11 in. diameter and 16 in. long. The armature is 42 in. diameter, the iron in it weighing 1,600 lbs. The weight of wire on the magnets is 5,424 lbs., that in the 16 armature bobbins being 825 lbs. The space occupied by the machine is 15 ft. long by 4 ft. wide by 5 ft. high, and the machine is said to furnish, at a speed of 450 revolutions per minute, a current of 3,200 amperes at a difference of potential of 80 volts. ' The Author is indebted to the proprietors of The Electrician for this engraving. RAFFARD-BREGUET DYNAMO. 269 (3.) CONTINTJOUS-CUEaENT ]\IaCHINES. (Closed-Coil Class.) The RaflFard-Breguet Dynamo,* illustrated in Fig. 76, is manufactured by the Maison Breguet, and may be re- garded as the latest development of the Gramme machine in France. Its configuration resembles that of the original machine, shown in Fig. 29, and the various modi- . fications effected in its construction do not appear to have kept pace with those introduced by English makers. For the original armature core of soft iron wire, has been sub- stituted one made up of soft iron washers about -^ in. thick, separated by paper. The paper is thickly varnished and the varnish serves to hold the core together, until, by longitudinal taping, it is made rigid enough to receive the conductor coils. The core is 5 in. long by 4| in. internal and 7^ in. external diameter. It is wound in the usual Gramme fashion with 23 lbs. of 87 mUs. double cotton-covered copper wire, the coils being connected to a 60-part collector. The complete armature measures 8^ in. in diameter. On the spindle is a twelve-sided cast- iron hub, and 12 wedges are driven in between this and the inner side of the armature to keep the latter in posi- tion. Whether this is an improvement on the older method of securing the armature is an open question, but in this country methods of driving by friction have been condemned long ago. All leading makers now in- * For the illustration the author is indebted to the proprietors of Indus- tries. In this excellent joui-nal the first description 'in English of the dynamo appeared. 270 LATEST TYPES. sist, and properly, that the armature core shall be firmly- secured to the spindle by rigid mechanical connections only. In the machine described, the frame is in two pieces, the magnet cores, pole-piece and half the yoke on GOOLDEN -TROTTER DYNAMO. 271 each side being cast in one piece. The magnet cores are of cast-iron, their lesser naagnetic permeability, as com- pared with wrought-iron, being compensated for by ex- cessive section. They are of oval form and measure 6| in. by 2 in., being series wound with 78 lbs. of 160 mils, wire. The total resistance of the machine cold is '8 ohm. At a speed of 1,400 revolutions per minute, a current of Fig. 77; 35 amperes is generated at a difference of potential of 110 volts. The over-all dimensions are, height, 22 in. ; breadth, 14 in. ; length, inclusive of spindle, which pro- trudes on each side, 33 in. The cast-iron frame weighs 31 cwts., the armature 65 lbs., and the whole machine a Httle over 5 cwts. The output is, nearly 7 watts per lb. of material used in its construction. The Goolden-Trotter Dynamo, shown in Fig. 77, resembles more closely than the machine last described the original Gramme shown in Fig. 29. Into its design, 272 LATEST TYPES. however, have he&o. incorporated improvements which are comm.on to all good modern machines. In place of the armature being driven by the friction of the internal wires pressing on a central hub, the iron core is attached to the spindle by gun-metal arms and driven in a positive manner, charcoal iron washers separated by paper having been substituted for the wire in its construction. In the Crramme machine, the area of the armature core was much smaller as compared with that of the field-magnets Fig. 78. than in the machine of Groolden and Trotter, the section of the armature in the latter being about two-thirds that of the magnets. The armature core is also thicker as compared with its length than in the Gramme machine. The cast- iron side checks receiving the wrought-iron magnet cores have been thickened up, and on these cores are fastened massive pole-pieces of cast-iron, between which the arma- ture rotates. The armature is wound, Fig. 78, so that there is on the exterior surface only one layer of wire. The 12,000 watt machine runs at 950 revolutions per minute and weighs 2,4641bs., the output being at this speed rather less than 5 watts per pound of material used. Kg. 79.— (ro face page 273.) THE MANCHESTER DYNAMO. 273 The Manchester Dynamo, recently introduced by Messrs. Mather and Piatt of Salford, is shown in Fig. 79. The machine is of the two-pole double-magnet type and is interesting on account of the disposition of its magnet coils, these lying nearly at right angles to the line joining the points of contact of the brushes, instead of being paral- lel thereto, as is generally the case in double-magnet machines. The poles are formed of massive blocks of cast- iron, which are bored out _and fitted on the wrought-iron cylindrical cores, the object of thus letting the cores into the cast-iron being to obtain a contact surface equal to twice the sectional area of the cores. The armature, which is Gramme-wound, has a core formed of charcoal, iron washers separated by paper, the coils, being joined up to a 40-part collector, having bars of drawn copper insulated by mica. The illustration is ^ full size ; and the machine, which is compound-wound for incandescence lighting, gives at a speed of 1,050 revolutions per minute 130 amperes at a difference of potential of 100 volts. Its total -weight is l,3721bs., the armature weighing 252lbs. The output is therefore 9-48 watts nearly per lb. of material. The electrical data of a compound machine running at 1,050 revolutions andintended for a current of 200 amperes at 110 volts are given by the Engineer for Aug. 7th, 1885, as follows : — Armature : core, 12in. diameter ; 12in. long, wound with 120 convolutions of wire 203 mils, diameter connected to a 40-part collector. Field-magnets : core 7^in. diameter, compound-wound, and having on each limb 42 turns of triple 203 mils, wire and 1,680 turns of 65 mils. wire. Length of magnetising coils, 12^xa. The resistances are: armature, -023 ohm; series coils, '012 ohm; shunt, coils, 19'36 ohms. The radiating surface of the field T 276 LATEST TYPES. double-magnet type is one of the latest designs for ship- lighting and is mounted on a cast-iron bed-plate fitted Fig. 80- with screw rope-tightening gear. The engraving is one- tenth full size and the machine weighs with the bed-plate THE PHCENIX. DYNAMO. 277 complete, 1,519 lbs., the output being 14,580 watts at 1,200 revolutioiis,with an electrical efficiency of 92 per cent. In several of the Phoenix dynamos, the armatures are built up of charcoal plates, or washers with projections, as in Fig. 81, these when laid together forming a ring with protruding teeth, between which the conductor coils are wound. Fig. 82. In others, the armatiure is built up of plain washers, the core thus presenting a smooth ex- terior. The object of the protruding teeth is to lessen the resistance of the magnetic circuit and to produce the requisite field density with a less quantity of wire on the magnets. The dynamo here _'_ ' illustrated has a toothed armature, the plates being separated by- paper and clinched together by insu- lated bolts passing right through the core,these latter being firmly secured by their projecting ends to gun- metal supporting wheels, which are keyed on the spindle. The armature is milled in 48 slots, in each of which is wound a coil having 6 convolutions of wire 148 mils, diameter. The coils are Gramme connected to a 48-part collector. The clearance between the armature projections and the polar surfaces is ^ of an in., the magnets being bored out to 13 J in., and the armature measuring, over the teeth, 13f inches. The armature core is 7 in. long by 2 in. thick measured from the bottom of the slots, the area of the iron in it being 12 square in. The arched magnet bars are 7 in. wide, cor- responding to the length of the armatiire core, and 3 in. 278 LATEST TYPES. hi Q M 9 H 1^ Pi THE PHCENIX DYNAMO. 279 thick, their area being 21 square in. At a speed of 1,200 revolutions per minute, a current of 90 amperes is de- livered at a difiference of potential of 162 volts at the terminals. The field-magnet coils present a radiating surface of 2*5 square inches for each watt expended in heating them. The results tabulated below were obtained when a machine of this class was tested at the makers' works, the motive power being supplied by a Gwynne " Invincible " engine driving on to a counter-shaft, from whence was driven the dynamo. In the efficiency column is given the fraction of the I.H.P. actually available for lighting pur- poses. Phcenix Dynamo Tests. Eevolutions. I.H.P. Amps. Volts. Watts. E. H. P. A. X V. Efficiency E.H.P. Engine Dynamo A. V. A. X V. 746 I.H.P. IS8 1205 26'90 88 i66-o 14,608 19-58 -727 140 1244 26-14 85 168-0 14,280 19-14 -732 142 1256 27-65 «5 170-4 14,484 19-54 -706 143 1260 27-16 ^3 170-0 14,110 18-91 -696 144 1270 27-71 ^5 172-0 14,620 19-59 •707 153 1370 2b-33 79 191 -2 15,104 20-24 -714 From the above it will be seen that the mean efficiency is •713, or 71-3 per cent of the power expended in the engine cylinder can be obtained at the terminals of the dynamo. In some of the later machines of Messrs. Paterson and Cooper's make, the output has exceeded 12 watts per lb. of material employed in construction, the speed of ro- tation being 1,200, with an electrical efficiency of over 90 per cent. 280 LATEST TYPES. The Kapp Dynamo, manufactured Messrs, by W, H. Allen and Company, is shown in Fig. 83, -^ full size. It belongs to the two-pole, single-magnet type, and is shunt- Kg. 83. wound for a current of 125 amperes at a difference of potential of 82 volts. The machine weighs 24 cwts. and runs at a speed of 425 revolutions per minute. The field- magnets have wrought-iron cylindrical cores, their lower ends fitting into the bed-plate and their upper ends being turned taper to receive the cast-iron pole-pieces. THE TWO-POLE GULCHER DYNAMO. 281 The armature is 13 in. long by 13 in. diameter, is^ Grrainme-wound, and has a core made up of soft charcoal iron wire coiled on a gun-metal supporting cylinder, the latter resembling a number of double-flanged pulleys laid side by side and separated by a narrow space. The iron wire is coiled between the flanges, which have projecting from them, and rising above the iron wire, a number of teeth, transmitting, in a positive manner, the driving force to the conductors carrying the current. The air passing between the flanges, from the interior to the ex- terior of the armature, carries off the heat from the in- terior conductors and from the iron core. On the armature of the machine described, there are wound 216 convolu- tions of double 134 mils, wire, the area of the actual iron in the cross section of the core being 22'5 square in. The wire is two layers deep on the exterior, and the resis- tance, hot, is "049 ohm. The cores of the field-magnets have an area of 75 square in., and the magnet-coils , have a resistance of 16 ohms. The watts expended in the armature are 830, and the electrical efficiency is rather over 89 per cent. The radiating surface of the armature is nearly 1 square in. per watt. The Two- Pole Giilcher Dynamo, shown in Figs. 84 and 85, is another machine of recent design belonging to the single-magnet class. It has a Grramme-wound arma- ture, the core of which is formed by coiling on a gun- metal cylinder soft charcoal iron wire of rectangular section. The advantage of employing rectangular over round wire is two-fold ; first, the length of conductor re- quired per volt is considerably reduced, because the area of iron, enclosed by a given perimeter, is increased by 25 per cent. ; secondly, an armature so constructed is mechanically strong, no flanges being required at the ends 282 LATEST TYPES. of the gtm-metal cylinder to keep the wire on. The core when completed is of square section and measures 3| in. each way. It contains in its cross section 12"6 square in. Fig. 84. of iron and has an external diameter of 11^ in. It is wound with 240 turns of copper wire of rectangular section 120 mils, by 83 mils, laid flatways, one layer deep on the external siu-face and five layers deep internally. The coils are connected up to a 48-part collector, and the armature THE TWO-POLE GULCHER DYNAMO. 283 has,' when hot, a resistance of '077 ohm. The field- magnet rests on a gun-metal block interposed between the'poles and the bed-plate, and is provided with wing- Fig. 85. plates which enclose the armature on each side as shown. The cores of wroughfc-iron have a cross section of 25'5 square in., the area of the magnets and armature being consequently equaJ. The magnet-coils are 5^ in. long. 284 LATEST TYPES. the total weight of copper on the two being 43 lbs. The weight of copper on the armature is 14^ lbs., which brings the weight up to a total of 57^ lbs. The dynamo, complete, weighs 784 lbs. and gives at 1,000 revolutions 80 amperes at a difference of potential of 65 volts or 5,200 watts. This is at the rate of 6*64 watts nearly per lb. of material used. Fig. 86. SSS«&^«^SsS^S?S««>^'««SS;^>>>Xi!SS^^^ The Crompton Dynamo, manufactured by Messrs. K. E. Crompton and Co., is shown in Figs. 86 and 87. The armature of this machine is built up of thin charcoal iron washers about -^ of an inch thick, which are insu- lated from each other by every alternate one being coated on both sides with Stannic paint. On the inner circum- ference of the washers are cut, equidistant from each other, three dove-tailed notches. Fig 86. These form, when the washers are laid together, three longitudinal dove-tail grooves running from end to end along the interior of the core. In the spindle are cut equidistant THE CROMPTON DYNAMO. 28s from each other three deep grooves, which come opposite the grooves in the core ; and lying radially along the •whole length of the armature, dove-tailed one side into the grooves in the core and the other into those in the spindle, are strong phosphor-bronze plates by which the power is transmitted from the spindle to the €ore. By interposing several fibre distance pieces, which keep the washers apart, the armature core is 286 LATEST TYPES. virtually converted into a number of comparatively narrow rings between which air can pass from the interior to the exterior to cool the core and conductors. The armature is ring-wound, the field-n^gnets being of wrought-iron and of the double horse-shoe type. The machine illustrated is designed for coupling direct to a high-speed engine mounted on the same bed-plate, but which is omitted in the drawing. One end of the spindle has a solid flange-coupling for connection to the engine. The magnets are supported on the cast-iron bed by gun- metal chairs. The following particulars of a similar machine are given by the Engineer : core of armature, 1 2 in. diameter, 2 J in. deep and 28 in. long ; air space between core and pole-pieces, '47 in. ; core of field-inagnets, 4| in. thick by 24 in. wide ; conductor on armature, 300 mils, by 180 mils, wound over the core in 96 turns ; resistance of armature, '021 ohm. The machine is intended for a current of 200, amps, at a difference of potential of 110 volts ; speed 450 revolutions per minute. Kecently Messrs. Crompton have devised a new method pf winding armatures, having in view the prevention of FoucauUs currents where copper bars are necessary for the safe carrying of the current. If tae angular width of the conductor on the exterior of the armature is such that the density of force lines is unequal all over it, there are generated Foucault currents which pass in opposite directions along the two halves of the bar A, Fig. 88. Making the bars in two pieces, B, does not help us, as, being parted in the middle, the current is still free to flow in the direction of the arrows or along the two halves as before. But if the bar is made in two and the two halves crossed over as in G, the resulting current is nil, for the electromotive forces tending to produce Foucault cur- THE ELWELL-PARKER DYNAMO. 287 rents oppose each other. In Messrs. Crompton's arma- tures for large currents, one half of the bar dips under in the middle to cross over the other half, and by these means Foucaul^ currents in the conductors are greatly Fig. 88. A G 2 :^3 B [|i =^1 2 riz r5 l 3 c=- B^ c^ ^ i]2 ^^ 3 reduced. A further moral may be pointed. When armature conductors consist of a number of small wires laid together, if Foucault currents are to be avoided they should be stranded and not simply laid side by side. The Elwell-Parker Dynamo is shown in Fig. 89. It belongs to the single magnet type, having fom: poles, as shown in Fig. 60. Its armature, of the ring type, has 288 LATEST TYPES. a core consisting of iron wire coiled direct upon two sets of gun-metal supporting arms. The conductor is wound Fig. 89. one layer deep, on the exterior, and is connected up in the usual fashion to a Gramme collector, the bars of which THE GVLCHER MULTIPOLAR DYNAMO. 289 are insulated with mica. The magnets are of wrought- iron, and are mounted on a strong cast-iron bedplate, which also carries the bearings for the spindle. The machine illustrated is compound wound and has a capacity of 35 units, generating at a speed of 500 re- volutions per minute, 350 amperes at a difference of potential of 102 volts. The resistance of the armature is •01 ohm, that of the series coil '0017 ohm, and that of the shunt 17-3 ohms. The loss in the series-coil is 350 X -0017 = -5950 volts. The shunt, being coupled up between the brushes, has flowing in it a current of ^"rr-V^ = 5-93 amperes nearly. The total energy con- verted is 37,741 watts, of which 2,041 are absorbed inter- nally, leaving 35,700 available for lighting purposes. The electrical eflBciency is about 94*6 per cent. The overall dimensions of the machine are height, 2 ft. 7 in. ; length, 5 ft. 6 in. ; width, 4 ft. 4 in. The Giilcher Multipolar Dynamo, shown in Fig. 90 and manufactured by the Griilcher Company, is a machine with a history. Coming to England in crude form with an armature-core, constructed mostly of wood, Fig. 33, since its first appearance it has been completely remodelled by English electricians. After passing through many changes it seems that finality has been reached in the form of machine illustrated, which is of the latest design. It is of the double magnet type ^nd has 8 poles. The magnet cores are of wrought-iron on which are mounted ■cast-iron pole-pieces, as shown. The armature is shown in section in Fig. 91 and consists of a gun-metal wheel casting with a j[_ rim, on which are coiled, one on each side of the centre rib, two continuous ribbons of soft iron, the successive convolutions being separated from each other by asbestos paper wound along with the ribbons. U ago LATEST TYPES. ig. 90. THE VICTORIA DYNAMO. 291 The armature is turned up true and the outside rounded before the conductor is coiled on, the object of the round- ing being to make sure that all the lines of force enter the armature by the edge of the ribbon, the generation of Foucault currents being thus avoided. The conductor is generally wound on the armature in one or two layers ; Fig. 91. I and an additional advantage of the semi-circ,ular section lies in the fact that there is required less length of wire per volt than if the section is rectangidar. The Victoria Dynamo, made by the Anglo-American Brush Electric Light Corporation, is shown in Figs. 92 and 93. Like the machine last described it belongs to the double-magnet type, and may have four, six or eight poles according to the output required. Some years ago 2g2 LATEST TYPES. the makers, in order to meet the demand which had arisen for dynamos suitable for incandescence lighting, took up Kg. 92. the manufacture of the Schuckert machine, and from that, by a long series of progressive steps, has been evolved the "Victoria" dynamo. When made according to the Schuckert pattern, it was found that machines wound for THE VICTORIA DYNAMO. 293 294 LATEST TYPES. incandescence lighting evinced the same tendency to sparking at the brushes as did those wound for arc lighting. Imagining that the polar slabs, Fig. 32, might have some- thing to do with this, they were gradually cut down until the pole pieces had an angular width about equal to twice the diameter of the magnet cores. By narrowing down the pole-pieces and giving them a somewhat different shape, Fig. 95. Fig. 94. Fig. 94, the sparking was made to disappear, and as a farther result there was found room round the armature for four poles instead of two. By this modification the capacity of the armature and output of the machine at the same speed were doubled. At one time the sparking was thought to be incidental to a great angular width of pole-piece. This of course is not the case, for in disc machines, as in others, pole-pieces embracing a large pro- portion of the armature circumference are perfectly con- sistent with absence from sparking if the proper configur- ation is given to them. This has been proved by the THE VICTORIA DYNAMO. 295 Corporation in their recent practice. In their later machines, the pole-pieces instead of having the shape shown in Fig. 94 have been extended as in Fig. 95, a decided advantage having been gained by the reduction thus obtained in the resistance of the magnetic circuit. More iron has been introduced into the field-magnets and armatures of recently constructed machines, the area of ■the armature core having been increased relatively to the field-magnets, and made of greater width relatively to its depth than was at one time the practice. Independ- ently of the number of poles, the equi-potential strips of the collectors in all the machines are cross connected so that only two brushes are required. A four-pole com- pound machine weighing 13|^ cwts., of the type shown in Fig. 92, gives, when running at 800 revolutions per minute, a current of 150 amperes, at a difference of potential of 75 volts. The external diameter of the armature is 21 inches, and the core consists of a ring of wrought iron ^ inch thick, upon which is coiled a 2f inch soft iron ribbon of No. 30, B. W. G. to a depth of 3,^ inch. The convo- lutions are separated by paper, and the area of iron in the ■cross section is 7*8 square inches. In order to reduce the waste from eddy currents, the core has cut in it a few ■radial grooves from the exterior a short distance inwards, which divides the outside turns of ribbon into a number of comparatively narrow strips. The armature has 360 ■convolutions of 165 mils, wire connected up in 60 sections and wound two layers thick. The resistance of the armature is "0106 ohm, and the wire has a current- density of 1,800 amperes per square inch. The field- magnets have wrought iron cores 3| inches diameter, on which are shrunk the cast-iron pole-pieces. The out-put ■of the machine at the speed given is 31 '4 watts per lb. of 296 LATEST TYPES. copper, or 7*3 watts per lb. of material used in its con- struction. The cooling surface of the magnet coils is 1*61 inches per watt expended in them. The Edisou-Hopkiuson Dynamo, manufactured by Messrs. Mather and Piatt, is a modern developnient of the original Edison machine, due to Dr. J. Hopkinson. In the original machines, Fig. 47, the magnets consisted of a niimber of separate cores connected to one conunon pole-piece, this construction being common to the smaller machines for isolated lighting, as well as to those intended for central stations. One of the most important improve- ments consisted in substituting, for these multiple cores, one heavy core of large section and of much shorter length, whereby the cross section of iron which could be employed for an armature of given length was greatly increased, the length of wire required for excitement being at the same time greatly reduced. ^ In the original machines the iron discs, of which the armature-core is built up, were held together by uninsulated bolts passing right through all the plates, the result of this being a heavy loss due to the generating of eddy currents. In the Edison-Hopkinson machine, Fig. 96, these bolts are omitted and the plates are held together by nuts screwed on to the spindle itself, which thus serves as a bolt. Con- comitantly with the increase in the section of the mag- nets, the armature section has been increased, the distance between the steel spindle and the inner edges of the discs being merely sufficient to prevent any serious magnetisation of the former. The area of the armature core is rather more than "8 of the cjross section of the magnets. The final results due to these improvements are : (a), the reduction of the resistance of the magnetic circuit and an intensely strong field created by a minimum I" a. <; ^ 60 THE EDISON DYNAMO. 297 expenditure of energy; (b), due to the strong field, a less length of wire per volt in the armature and a cor- respondingly reduced resistance. In the Fig. is shown a 10 in. long armature, shunt-wound machine, which, at a speed of 750 revolutions per minute, generates a current of 320 amperes at a difiference of potential of 105 volts. The armature is built up of about 1,000 charcoal iron discs, 9ii in. diameter, separated by sheets of thin paper. It is wound with 40 complete turns of a conductor con- sisting of 16 wires in parallel, 69 mils, in diameter. The collector has 40 bars of drawn copper insulated with mica,, the connections to the coils being made by gold-plated spoons to ensure good contact, while, at the same time,, facilitating repairs. The armature resistance is "009947 ohm. at 13"5° C, and the weight of wire wound on it is 55 lbs. The magnets and pole-pieces are of wrought-iron, the cores measuring 18 in. in length by 9| in. in thickness, and having a cross sectional area of 171 square in. Each Umb is 24 ins. long, and has wound on it 1,544 turns in eight layers of 95 mils. wire. There are 205 lbs. on the magnets, and the resistance is 16'93 ohms. Electrical eflSciency, 95 per cent. Weight of machine, 51^ cwts. Weight of armature, 5i cwts. These dynamos have been tested by coupling together, as described on page 274,, part of the dynamometer gear employed being shown in the Fig. When thus coupled they gave for both genera- tor and motor a commercial eflSciency of about 93 per cent. The Edison Dynamo.* The manufacturers of the Edison machine in America have been led to modify their designs very considerably, their latest dynamos. Fig. 97, * The author' is indebted for the illustration to the proprietors of Industries. 298 LATEST TYPES. closely resembling those of the Edison-Hopkinson type. The wrought-iron magnets are of circular form and have been made shorter and thicker than was at one time the practice. The pole-pieces are of cast-iron and are swelled out externally to reduce the weight, while at the same time ensuring a sufficiently large area for the lines' of force to pass through without throttling. The illustration clearly indicates the changes which have been made, the machine shown giving at 750 revolutions 400 amperes at a difference of potential of 113 volts. The armature re- sistance is "0072 ohm. The Weston Dynamo is another machine of the drum-wound type which has undergone in recent times considerable modification. Shown as originally designed in Fig. 42, the improvements effected have been of a simi-. lar character to those described in connection with the Edison-Hopkinson machine. The magnet cores have been made more massive and the pole-pieces and yokes much heavier. More iron has been introduced into the armature core, there being left in the centre of the toothed discs an opening only large enough to admit the spindle. The shunt-wound machines, on account of their low armature resistance and small number of convolu- tions, are said to be almost self-regulating, a machine for 100 twenty candle-power lamps having only 24 complete turns on its armature connected up to a 24-part collector. The Chamberlain-Hookham Dynamo, a machine of recent date, is illustrated in Fig. 98. It belongs to the double-magnet drum-wound type, having an armature core built up of soft iron toothed plates separated by paper. The illustration shows a 10 unit machine, giving, at a speed of 1,250 revolutions per minute, a current of 175 amperes at a difference of potential of 59 volts. 97.— [To face page 298.) THOMSON-HOUSTON INCANDESCENCE DYNAMO. 299 'The armature is 7:^ in. diameter by 9 in. long, and the sum of the areas of all the projections at any time under a pole-piece is equal to the cross section of the armature core. There are on the armature 41 complete turns of two rectangular wires in parallel, which have, when con- nected up, a resistance, hot, of -007 ohm. The magnet cores are of wrought-iron, over which are cast the yokes and pole-pieces. The yokes are in two parts, bolted to- gether, the magnetising coils being wound directly on the wrought-iron cores. The brush adjustment, by a pinion and toothed segment cut in the brush-cradle, is clearly shown in the engraving. The following particulars of a 25 imit machine of similar type are given in Indus- tries of Oct. 1, 1886. Armature: length, 13 in.; dia- meter, 10 in. ; cross section of iron-core, reckoning from base of slots, 30 square in. : winding 35 complete turns of 12 wires in parallel in two layers, inside layer, of 104 mils., outside layer of 116 mils. Eesistance, hot, '003 ohm. The field-magnets have a cross section of 42 square in. and are shunt-wound with 2,856 turns of 109 mils, wire ; resistance, hot, 8 ohms. ; current through shunt, 7'13 amperes. At a speed of 900 revolutions per minute, a current of 450 amperes is generated at a difference of potential of 57 volts, the total weight of the machine being 25 cwts. This is at the rate of 9*16 watts for each lb. of material used. The Thomson-Houston Incandescence Dynamo is shown in Fig. 99*. The machine is of very recent design, its field-magnets resembling those of the arc machine described on page 262. The spheroidal armature is re- tained, being in this machine built up of annealed iron * The Author is indebted to the Proprietors of The EUctrieal Meinew for the illustration. 30O LATEST TYPES. Fig. 99. THOMSON-HOUSTON INCANDESCENCE DYNAMO. 301 Tings carried by hubs on the shaft. For the open-coil has been substituted the ordinary closed-coil winding, the sections being connected up to a several part collector. The field-magnets are more massive than in the arc dynamo and are compound-wound, the series-coil being situated in front of the magnet cores and at a certain angle to the magnetic axis of the latter in order to com- pensate distortion of field due to the current flowing in the armature. Electrical details are not yet to hand, but the machine appears to occupy the position of being the most costly to manufacture and the most awkward to repair. Employed for incandescence lighting, it may, not Tinjustly, be regarded as illustrating the persistence of a type when the circumstances which led to its development have ceased to exist. INDEX. Accumulators : The first, 104. Plante's, 104. The Elwell-Parker, 109. Favu'e's, IH. Sellon-Volckmar, 121. Defects in connection with, 131. Aimant Feuillete (Laminated Mag- net) of Jamin, 53. • Allard, 113. Alliance Machine : Construction of, 9, 29, 30. Application of, 29. Alternating Current : Machines, 29, 49, 242, 254. Production of, 5. Machine of the Alliance Com- pany, 29. Machine of Brush, 46, 267. Machine of De Meritens, 33, 88, 254. Application of, Machines, 88, 90. Machine of Gramme, 39. Machine of Holmes, 35. Machine of Gordon, 255. Feri-anti, 258. Ganz, 257. Machine of Lontin, 39. Machine of Mbhring and Baur, 37. Machine, Siemens-Halske, 43. Machine of Weston, for Gal- Tano-plastic purposes, 35. Ammeter : Commutator, of Ayrton and Perry, 228. Ayrton-Pen'y's, without Com- mutator, 233. Ayrton-Perry's, with Springs, 234. AMPtRE : Theory of Magnets, 3. Law of Magnetic Induction, 3. Amperiau Cm'rents, 13. Direction of, about the S. and N. Poles, 3. Direction of, in the Armature of Pixii's Machine, 4. Arc : Arrangement of Lamps in Multiple and Parallel, 98. Modification of the Law of greatest efficiency of a Machine in its employment for the Elec- tric, 144. Steadiness of, -Light, 183. Tresca'a Experiiueuts on the relation of the work expended to- the work done in the, 194. Armatures, Construction of, 172, •24S. King, Pacinottl, 12, 243. Drum, construction and theory, 62, 243. Causes of heating of, and pre- vention, 174, 249. liolutive position of, and the Field Magnets, 175. Coiling of, 125. Resistance and Electi-o-motive force of, when tlie Bobbins are variously coupled up, 145. Inter-dependence of the Elec- tro-motiTe force and quantity of current (1) on the number of convolutions in, 147. (2) on the rate of rotation of, 149. Ratio between current and rate of rotation of, 151. Ratio of etteetive magnetism in Dynamo-eleetnc Generators to rate of rotation of, 151. Ratio of wire turns in, to turns on Field Magnets in Dynamos, 157. INDEX. 303 Asbestos : Insulation, 71. Attractive force of Electro-magnets, 221. Automatic regulation of the position of the Brushes on the Collector, 173. Atkton and Perry's experiments with Fauve Accumulators, 124. — ■ — and Perry's Dynamometer, 22fi. and Periy's Ammeters and Voltmeters, 228, 239. Battery : Plante's Secondary, 104. Baur : Dynamo Electric Machine, 37. Bobbins : Method of Coupling up an Armature, to obtain the maxi- mum efficiency, 145. BosANQUBT, R. H. : Mathematical consideration of double-wound Machines, 102. Bkegttet : Gramme .Machine for physical laboratories, 52. Brush : Alternating-current Ma- chine, 46, 178, 267. Eegulation of. Machine, 47. Commutator, 47, 268. Double-wound Machine of, 98. Improved Method of "form- ing '' Secondary Batteries, 132. Proposal for the construction of the plates of Accumulators, 135. Brushes : Sparking at the, 250. Displacement of Neutral Points, and consequent disposi- tion of, 173. of Brush's Machine, 47. of Gramme's Machine, 51. of Siemens' Plating Machine, 70. of Weston's Light Machine, 78. BuRGiN : Machine, 178. Candles : Employment of Alter- nating-current Machines with electric, 89. Carbons : Advantage of the unequal consumption of, 88. Influence of the distance be- tween, 200. Carcel-burner, 119. Cell : Elwell-Parker, 110. Centrifugal force: Protection of Edison's Armature, 86. Chamberlain and Hookham : Dynamo, 298. Change in direction of current in the coils of an Armature during its revolution, 6. Charge : Retention of, by Accumu- lators, 126. Cause of the spontaneous loss of, by the hydrogen and oxygen plates in Secondary Batteries,127, 129. Charging : of Plante's elements, 106. of Accumulators, 112, 125. of the Elwell-Parker Accumu- lator, 110. of the Faure Accumulators, 114. Effects of, on the durability of the peroxide of lead coating, 130. Clarke : Modification of Pixii's. Machine, 7. Closed-coil dynamos, 269. Coils, Ratio of the diameter of the Electro-magnet to the diameter of the Coil, 217. Length of, 247. ■ • Radiating surface of, 249. Collector : Construction of, 20. Construction of Gramme's. 178, 50. Commutating : Lontin's Ma- chine, 38. Commutator, Principle of, 6. Segments of, 6. Method of taking the Current. from, 6. "Wear of, 89, 251. Continuous Current Machines, 243. Convolutions on Armature, 251. Cooling : Electro-magnet Cores itt Weston's Machine, 36. Armature in Heinrich's Ma- chine. 60. Journals in Edison's Machine,. 86. Armature Core, 175, 249. Surface of Coils, 249. 304 INDEX. 'Core : Construction of Armature, 248. Shape of Field-Magnet, 246. 'Cost : Question of, of a Machine, 178. Relation of, of a Machine to its dimensions, 179. Oeompton Dynamo, 284. 'Currents : Direction of, in the Ring Armature, 13. Ampferian, their direction about the N. and S. Poles of Magnets, 3. Change in direction of, in the CoUs of an Armature during its revolution, 6. Foucault, 50, 248. 287. Total, 51, 63. Direction of, in the Drum Armature, 65. Variations of, in Dynamos, 91. Actions of, in a regulator lamp, 92. Regulation, 95. Method of obtaining a con- stant, 102. Uniform, 136. Production of, in Plante Ele- ment, 106. Discharge, 106. Ratio between, and rate of Rotation in Dynamps, 151. Ratio of increase of, to the effective magnetism in Dynamos, 151. Frohlich's "current curve," 153. Method of determining the, necessary, for saturating the Core of an Electi'o-magnet, 169. Self-induced, m Electro-mag- nets, and their retarding action on the magnetisation of the iron Cores, 170. Steadying of, 170. Inversion of, from polarisa- tion, 204. Curve: Frohlich's Current, 153. Dead revolutions, 154. Defects of Pixii's Machine, 7. of Alliance Machine, 33. Defects of Faure's Accumulator, 121. in connection with Accumula- tors, 131. and Advantages of various Ma- chines, 89, 93. Db Meeitens' Generator, 33, 254. Drum Annature : Direction of cur- rent in, 65. of Siemens and Halske, 61, 66. Siemens' Magneto-Electric Ma- chine with, 67. Siemens' Dynamo with, 61, 67. vfiib. German-silver Cylinder, 68. Dtjb, Law of, in connection vrith Magnets, 214. Dynamo, Principle of, 22. Discovery and application of principle, 24. Thomson-Houston, 261, 299. Brush, 46, 267. Raffard-Breguet, 269. Goolden-Trotter, 271. Manchester, 273. Phoenix, 275. Eapp, 280. Crompton, 284. ElweU-Parker, 287. Victoria, 291. Edison-Hopkinson, 296. Chamberlain and Hookham, 298. Simplest kind of, 24. Edison's, 84, 297. Fein's, 57. Lontin's, 80. Burgin's, 81. ■ — - Giiloher's, 61, 281, 289. Maxim's, 79. Siemens', 66, 71. Weston's, 76, 298. Weston's, for galvano-plastic Ladd's, 26. Baur and Mohring's, 37. Historical development of, 1, 28. Formulse for measurements in connection with, 237. INDEX. 30s Dynamo, Dr. Frohlich's measure- ments in connection witli, 153. Employment of Accumulators in connection with, 125. Physical laws beaiing on the construction of, 140, 151. Employment of large Magnets in, 170. Employment of, for plating purposes, 204. Dynamometer Applicatien of the Easton-Anderson, 115. Ayrton- Perry's, 226. Edison: Dynamo Machine, 84, 297. Hopkmson Machine, 296. Collector for diminishing sparking, 176. Efficiency of a Machine, 9, 140, 163, 251. of Alliance Machine, 33. of Lontin's Machine, 39. of Gramme's Alternating-cur- rent Machi'ne, 43. of Brush's Machine, 49, 183. of Breguet-Gramme Generator, 54. of Gramme's Direct-current Light and Plating Machines, 55, 182. of the Siemens-Halste Drum- armature Machines, 68, 70. Relative working of different Machines, 89, 182, 202, 251. Relation of the dimensions of a Machine to its, 179. of the Faure Accumulators, 120, 137. Increase of, in the Plante ele- ment, 106, 107. of the Elwell-Parker Accumu- lator, 110. of the Faure Accumulator, 112, 114. Relative, of the Faure and Sellon-Yolckmar Accumulators, 124. Electrodes, 104. Electrolyte, 110. Electro-Magnets: Construction of, 169. Electro-Magnets : Employment of, as Field-Magnets, by Wilde, 21. Double winding of, 98. Relation of the strength of an, to the resistance of its Coils, 216. FormnlsB for the construction of, 212. Determination of the strength of current necessary for satu- rating the Core of, 169. Electro-motive force : Method of maintaining a constant, 96; Calculation for, 251. and resistance of the Faure Battery, 117. Relation of, to the work done, 141. ^^— Relation of, to an opposing Eleotro-motive force, 142, 144. of the AiTuature, when the Bobbins are coupled up in series, or for quantity, 145. Inter-dependence of, and quan- tity of current, (1) on the number of convolutions of the Armature, 147. (2) on the rate of rotation of the Armature, 149. (3) on the intensity of the magnetic field in which the Ar- mature moves, 150. Dependence of, on the shape of the Pole-pieces, 171, 288. of large Machines relatively to that of small Machines, 179. Relation of the dimensions of an Electro-magnet to, employed, 220. Electrotyping, 203. Elias : Determination of the co- efficient in Haker's formulse, 165. method of forming powerful Steel Magnets, 167. El well-Parker Aocumulator,109. Dynamo, 287. Ergometer, 237. Exciting, the Field-Magnets of a Dynamo, 25. Employment of an. Machine, with Lbntin's Alternating-cur- rent Machine, 38. j06 INDEX. Exciting Field -Magnets of Gramme's Alternating-current Machine, 39. Field-Magnets of Brush's Ma- chine, 49. Ferranti Generator, 258. Field : Magnetic, 21, 93. Intensity of the magnetic, 175. Initial, 100. Avoidance of edges and cor- ners in Pole-pieces, if a uniform field is desired, 171. Magnets of Dynamos, 151, 241. Intensity of the magnetic, and its influence on theElectro-motive force of the Machine, 151, 251. Magnets in a shunt-circuit, 97. Double-wound, Magnets, 98. Magnets, strength of, 91. Magnets, construction of, 164, 198. Magnets, resistance of, 160, 161. Magnets, advantageous dimen- sions of, 170, 244. Magnets, relative position of the Armature and, 175. FoNTAiNK : Eeport on the relation of the illuminating power to the work expended, 199. Force: Attractive, of Electro-mag- nets, 221. Forming : The Plantd element, 106. Rapid process of, 108. Faure's method of shortening the forming process, 111. Brush's process of, 132. process in the case of Plates coated with coherent lead, 134. FoucAtTLT currents, 50, 248, 287. Prevention of, in Gramme's Armature, 50. Prevention of, in Jiirgensen's Machine, 61. Prevention of, in Edison's Ar- mature, 84. Avoidance of, in Pole-pieces, 171. Prevention of, in iron Cores, 174. Fkankenheim : Increase of the "permanent moment " of Mag- nets, 166. Relation of the permanent magnetism of an annealed steel Magnet to the magnetism attain- able with a given magnetic field, 166. Free Magnetism : Jamin's experi- ments on, 166. Friction : Wearing of Collectors and Commutators, and loss of energy by, 175, 176, 261. Feohlich's, Dr. F., theory, 151. Current curve, 153. Fkommb and Frankenheim's ex- periments, 166. Ganz Generator, 257. Gordon Generator, 255. Gooldbn-Tkottee Dynamo, 271. Gramme, 21, 50. System. 12. Alternating-current Machine, 59. Generators : De M^ritens', 254. Gordon, 255. Ganz, 257. Ferranti, 258. Gramme Collector, 50, 178. Ring-Armature, 50. Dynamos employed by Treses in his experiments, dimensions, 174. Data in connection with Ma- chines of, 184, 202. Current interrupter, 205. GtTLCHER dynamo, 61, 2S1, 289. Hakbe's formulae for the portative power of magnets, 165. Heat : Production of, in the interior of a Machine, 114, 240. .Conversion of electrical energy into, 142. Heating: Influence of, on the re- sistance of the Armature, 149. Prevention of, in Weston's Machines, 36, 77. Prevention of, in the Siemens- Halske Drum-Armature, 68. INDEX. 307 Heating : Causes of, in Armature- cores, and prevention, 171, 175. Hefnbk-Alteneck, Differential lamps, 46, 90. Drum-Armature, 61. Heinkich's Generator, 59. Hjorth's Machine, 101. Holmes's Alternating-current Ma- chine, 35. Horse-power : Measurer, 237. One h.p. accumulator Cell, 122. Hospitalier's, Messrs^ Gekaldy and, investigation on the efficiency of the Faure Accumulator, 112. Hughe's experiments on Electro- magnets, 223. Illuminating : Experiments and data on the relation between Power expended and the Work, 199. • Relative advantages of diffe- rent Machines for, purposes, 88. Illumination, 183. Intensity of, with various Ma- chines, 183, 202. Induced current strengthened, 82. Inductive : Action of Pole-pieces in Brush's Machine, 48. Action of Field Magnets in "Weston's Light Machine, 77. Initial Field,- 100. Insulation: of Commutator Eings of Bru-sh's Machine, 47. Destruction of, 60. Asbestos, 71. Paper, 84. between the iron wires or sheets, of Cores, 174. of wire in the Alliance Ma- chine, 32. Internal resistance : Relation of the, of a Machine to the resistance in the external circuit, 145. Resistance of Light Machine, 183. Interrupter : Current, 37, 204. in connection with Gramme's Plating Machine, 56. Baur and Mohring's, 205, 206. Interruption Cylinder of Edison's . collector, 177. Inversion of the direction of the current in a Machine, 204. Jacobi : Law of Lenz and, 147. Laws of Dub and MiiUer, 214. Jamin : Normal Magnet, 52. Relative advantages of Ma- chines for Electric Ligliting with, candles, 88. on the co-efficient A in Biqt and Coulomb's formula for the distribution of Magnetism on the surface of a Core, 167. JouBEET : method of determining difference of potential, 115. Joubert, 113. JoBLEE : Equations for determining the work in the Circuit and Ma- chine, 237. Joule's law, 142. - — Equation, 161. JiJRGBNSBN's direct current Ma^ chine, 60. Kapp's Dynamo, 280. Kohlfubst: Employing Electric Machines in Telegraphy, 209. LaSd's dynamo, 26. Laminated Magnet; Gramme Ma- chine with, 52. Lamp : Action of current in a Regu- lator, 92. Application of Maxim Incan- descent, 115. Employed in Tresca's experi- ments, 196. Lead: Sheets in Plante's element, 104. Perforated, sheet Electrodes, 109. Red, employed in Accumula- tors, 111, 121. conducting power of Peroxide of, 127, 129. Effect of charging on the durability of the Peroxide of, 130. Effects of the Sulphate of, 131. Employment of Plates with a reduced, coating, 133. 308 INDEX. Lenz, law of, 147. LoNTiN : Alternating Current Ma- chine, 38. Dynamo, 80. Loss : Explanation of Charge of Accumulators, 127, 129. Magnets : Construction of Steel, 164. — — Formulae for the Construction of Electro, 212. Shape of in Dynamos, 245. Strength of a, on what it depends, 165, 166. Haker's formulse for the por- tative power of, 165. Permanent moment of, 166. Magnetic : Induction, 3. Ampere's Law of. Induction, 3,4. Field of Magneto-electric Ma- chines, 93. Resistance, 245. Eetardation of the attainment of the maximum Moment in Iron Cores of Electro-magnet, 172. Intensity of field, 175. Field in Electric Machines, 151, 152. Moment (law), 214. Magnetisation : Maximum, of Steel Plates, 53. of the Tripolar Field-Magnets, 64, 58. Expenditure of energy in the, of iron, 170. Periods of change in the, of an Iron Core, 172. Magnetising : Influence of the Ar- mature-coiling on the effective Magnetism, 156. of Steel Bars (by simple and double touch), 164, 165. Determination of the Moment of different kinds of iron, 169. Residual, 25, 88, 172. Effective, 151, 152, 156. Strength of, of Field Magnets, 91. Distribution of free, 166. Distribution on the surface of a Core (formulse), 167. iing: Eetardation in the maximum magnetisation of an Iron Core, and in the disappear- ance of the, 172. Maldeeen ; Improvement of the Alliance Machine, 29. Manchester Dynamo, 273. Masson : Improvement of the Alliance Machine, 29. Maxim Machine, 79. Incandescent Lamp, 15. Maximum : Conditions for, strength of Electro-Magnets, 214. Laws for Electro-Magnetic, on Shunt Circuits, 223. MfiRlTENS, De, Machine of, 33, 88, 254. MoHRiNQ and Batir, Dynamo of, 37. Current interrupter, 206. Moment : Increase of permanent, 166. Law of Jacobi, Dub & Miiller, 214. Influence of duration of the action of a current, and the per- manent of a Machine, 166. Value of the co-efficient, in determining the magnetic, for different kinds of iron, 169. MoNCEL, Du, Construction of Elec- tro-Magnets, 212. Morton, Prof. Hen5y ; Measure- ments in connection with the efficiency of the Sellon-Volckmar Accumulator, 122. MirLLER, Law of Jacobi, Dub and, in connection with Magnets, 214. Multiple Arc, Arrangement of lamps in, 98. / Neutral points, 15,, 65, 51. Displacement of, and conse- quent position of the brushes, 173 Niaudet's Machine, 82. NoLLBT, Inventor of the Alliance Machine, 29. Normal Magnet, 53. Ohm's Law, 141. Oiling, in Edison's Machine, 86. Open-Coil Dynamos, 261. INDEX. 309 Ozone, Employment of Electric Generators for the preparation of, 22, 207. Paoinotti, Dr. Antonio : Ring- armature, 12, 50. Method of collecting currents from Bing-armature, 19. ■ Relative advantages of, and Gramme's King-armatures, 50. Parallel Arc, Arrangement of lamps in, 98. Paterson and Coopek's dynamo, . 275. Permanent moment of Magnets, 166. Peeky : Perfected Dynamo, 101. Experiments on Faure Accu- mulators, 124. Dynamometer, 226. on the transmission of energy, 210. Phcenix Dynamo, 275. Photometric determinations in con- nection with Faure Accumula- tors, 119. Pixii Machine, 3, 7. . Planes, Division of Cores, into, to prevent Foucault currents, 114. Plante : Secondary Battery, 105. Process for rapid forming of elements, 108, 109. Plates : Perforated, for Batteries, 109, 121. Employment of corrugated, for receiving chemically-deposited lead, 133. Employment of other metals for supporting the Peroxide of Lead, 134. JPlating, 203. Gramme's, Machine, 55. Siemens', Machine, 70. Machine of Weston, 35. Machine of Baur andMohring, 37. Machines, Construction, 204. Current Interrupters and Closers in connection with, Ma- chines, 205. .Plating, Prevention of change in polarity of. Machines, 204. Platinum, Fusion of, 207. Pluoknbr and Bablow : Values of the coeff. IC., for various kinds of iron, 166. Polarisation : 104, 142. Inversion of current in Plating Machines from, 204. Currents, 60. Polarity : Prevention of change of Field Magnets in Plating Ma- chines, 204. Pole-Pieces, size of, 171, 247. Distribution of the Electro- motive force in the sections of the Armature Coils, depends on the shape of, 171, 294. Construction of, to avoid -Foucault currents, 171. Direction of the Ampirian- currents round the S. and N., 3. Influence of the fixed, on the Coils of the Eing-armature, 12, 19. Travelling, in the Core of the Eing-armature, 13. Distance of the Armature Core from, 175. Power : Carcel burner, 119. Candle, 119. Electric^ transmission of, 56, 210. Primary current, 2. Ebgnibr's statements in connec- tion with Faure Accumulators criticised, 137. Regulation : Current, 95, 97. Brush's Machine, 47. Eegulator : Maxim's, 96 . Eesldual Magnetism, 25, 88, 172. 204. Resistance and Electro-motive force of the Faure Battery, 117. Increase of, in Secondary Bat- teries, 129. Relative, 143. Relation of the internal, of a Generator to the resistande of the external circuit, 145, 147. of the Armature when the Bobbins are coupled up in series or for quantity, 146. of Armature Core, 174. 310 INDEX. Resistance, Siemens' Formula for calculating the increase of resist- ance of a wire with the increase of temperature, 150, Relation of, to tension of Current, 183. Internal, of the normal Sie- mens' Light Machine and Gramme Machine, 183. Relation of, of the Coils of an Electro-Magnet to its strength, 216. Diminution of, in the Arma- ture Coils of Edison's Machine, 85 Ring Armature, influence of the fixed Magnets on, 13, 19. Pacinotti'a, 12, 20, 50. Gramme's, 60. — ^- Theory of, 12. Commutator of Brush's Ma- chine, 47, 268. Armature of Gramme's plating Machine, 56. Flat, Armature, 58. Armature of Heinrich's Ma- chine, 69. Collector of Siemens' circular Dynamo, 75. RiTTEE : Voltaic Battery with one metal, 104. Rollers, contact, 20, 69. Saturation : Method of determining the strength of cuiTent necessary for, of the Core of an Electro- magnet, 169. Sawyer's Switch, 95. Saxton : modification of Fizii's Machine, 7. ScHucEEBT : Machines, 68, 59. ScHWENSLER : Experiments on the employment of Magneto-electric Machines in Telegraphy, 208. Secondary: Batteries {see "Accu- mulator"), 103. Currents (production and di- rection), 1, 3. Wire, 3. Sellon-Volckmar Accumulator, 121. Series Machines, 98. Shunt Circuit : suggested by Wheatstone, 97. ShuntCircuit: "Value of the, 162, 206 Condition for Electro-magnetic Maximum on, 223. Electrical work in, 240. Siemens : Cylinder Armature of Dr. Werner, 10. Small Machine, 10. Halske Dynamos, 27. -■ Halske Alternating Current Machines, 43. -Halske Direct Current Ma- chines, 63, 76. Application of Wheatstoue's Shunt-circuit, 97. Method of winding the Field Magnets of Self-Regmating Ma- chines, 101. Formula for calculating the increase of the resistance of a wire with rise of temperature, 150. 'ing: at the Collectors and Commutators, 175, 250. Edison's Collector for the re- duction of, 176. Steadying Current in Dynamos by employing large Field-Magnets, 170. Steel, Magnetising of, bars, 164. Elias' method of forming powerful. Magnets, 166. Stohrer's Machine, 8. Storage Batteries : The First (Hit- ter's), 104. Strength : Increased, of Induction Currents, 3, 9. Coulomb's Law for, of geome- trically similar Magnets, 165. Instruments for measuring, of Currents, 228, 233, 234, 236. Swan, Sellon-Volckmar Accumula- tor, 121. Switches, of Sawyer and Siemens, 95. System, Gramme, 12, 50. Telegraph : Employment of Electric Generators in the, 208. Telegraph: Employment of a Sie- mens' Generator by the Western Union Company, 209. Temperature, Law of the Increase of the resistance of the wire with, 149. INDEX. 3ir Temperature, Siemens' JFormula for calculating Increase of Resistance with rise of, 150. Employment of Electric Ma- chines for obtaining high, 207. Theoretical Principles of the elEcient construction of Electric Machines, 140. Theory : Ampere's, of Magnets, 3. of the Sing Armature, 12. of the Drum Armature, 63. of Siemens' Coreless Armature, 74. Thomson's, in connection with Dynamos, 157. Thompson, Prof. Silvanus : Conclu- sions respecting the Advantage of increased dimensions of Electric Machines, 179. on the Magnetism of a piece of iron, 170. Thomson, Sir W., Suggestion for the Copper Winding of an Arma- ture, 157. Thomson-Houston Dynamos, 261, 299. Time, Influence of, during which the Machine works, on the rela- tion between the work expended and the intensity of illumina- tion, 201. Transmission of Energy : Employ- ment of Electric Machines for, 210. Tkbsoa, Experiments, 194. Trinity House Report, 90, 183. Tripolar Magnets, 54. Turns of Wire on Armature and Field Magnets, 147, 251. Ttndall and Douglass on compa- rative Experiments with light Machines, 183. Uppenboen : Modifications of the law of the relation that the inter- nal resistance of a Msichine bears to the external resistance, 147. Values: Comparative, of Generators, 183. Variations in the strength of Cur- rent of a Machine, 92. ViOTOBi A -Dynamo, 291. Volckmak-Sellon, Accumulator, 121. Voltaic Battery, Ritter's, with one metal, 104. Voltmeter, with Spring, 234. Commutator, 230. without Commutator, 233. with Cog-wheel and Gear, 236. Wearing : of Commutators and Collectors, 89, 175, 251. Weight, size and output, 251. ^-^ Weston : Plating Machine, 35. Light Machine, 76, 298. Current closer, 205. Wheatstone : Method of exciting the Field-Magnets of a Dynamo by a Shunt Circuit, 97. Dynamo Electric principle, 24. Wilde : Magneto-electric Machines of, 21. Employment of Electro-mag- nets as Field-magnets, 21. Winding, Methods of. 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Technical School and College Buildings, being a Treatise on the Design and Construction of Applied Science and Art Buildings, and their suitable Fittings and Sanitation, with a Chapter on Technical Education. By Edward Cookworthy Robins, F.S.A., Fellow of the Royal Institute of British Architects, Member of the Institute of Surveyors, Member of Council of the Sanitary Insti- tute of Great Britain, &c. &c,. Member of the Executive Committee of the City and^ Guilds of London Institute for the Advancement of Technical Education. In one vol. Demy 4to, pp. 260, with 24 Illustrations in the text, and 25 double (16 x ii) and 40 single Plates (11 X 8), cloth, ;^2 los. The remarkable movement in favour of more efficient Technical training to combat the ever-increasing international competition heu called for aviore exact treatise on the peculiarities of plan and structural arrangements and fittings qf buildings required for its development^ and to give effect to the desire now so urn- versally expressed. It is generally admitted that foreign nations have been beforehand with us in this mattery and have long since provided most amply for the Technical education of all classes in their communities, in noble buildings specially erected^ and admir- ably fitted up for the purpose^ and well stored with singularly complete industrial andfine-art collections. The more immediate cause, however^ of the acceptance by the author of tht responsible task of expounding the principles and practice of Technical School Building has been the great interest shown^ both at home and abroad, in the various papers read by him on the subject at certain scientific societies, which papers. Privately circulated in a collected fonn, were so ably and flatteringly re- mewed in *' Nature " {March i^th, i386) by Professor T. E. Thorpe» qfthe Science Department at South Kensington. Professor Thorpe's capacity for appreciating the points of the papers is best evidenced by the singularly successful chemical laboratories designed by kirn, in association with Mr. Wateriiouse^ at the Yorkshire College, Leeds. Professor Thorpe obsetves: '' // would be quite impossible, within the s^ace at my disposal, to attempt to follow Mr. Robins in his analysis of the distinctive feaUtres of tJte various English mid Foreign Institutions he has visited and described. He has treated the mass of material which he has brought together in a re^narkahly clear and Ittcid manner. Nothing more certainly ii^icates the trained prof essional eye than the manner in which characteristic differences are detected and commented upon, audit is the evidence of this diagnostic faculty which constitutes one of the most valuable features of the book." " This collection of papers," continues Professor Thorpe, " is certain to be of the greatest service to any Architects engaged in the erection of Buildings of this class; it comtitutes, indeed, a sort of ^Vade Mecum^ * to the Building Committees whomay be responsible for the selection of th^ Architects plans; and the cause of Technical education in this country is under a debt of gratitude to Mr, Robins for the service which he has thus rendered to it. " NEW AND RECENT BOOKS. This generotis cofnmendation has incited t/ie author tq further effort^ and a still more extended observation of ike most recent erections. And thus, while the substance of the Jbrmer papers has been incorporated with the present work ^ the whole has been revised, rewritten, extended, and elaborately illustrated, so as to form a book of reference on Technical School Building unique of its kind, and brought down to the latest period. SALOMONS {SJR DA VID\ Complete Handbook on the Management of Accumulators. By Sir David Salomons, Bart., M.A., A.I.C.E., M.S.T.E. Second Edition, revised and enlarged. Demy 8vo, pp. 31. With Illustrations. Cloth, price 2j. The contents of this little pamphlet is the result of years of labour, and on all points ?iumerous experiments have been made regardless of expense, time, and trouble, " Will be found very valuable." — Industries. " A very important work, indispensable to scientists engaged in electric lighting.*' — The Colliery Guardian. SHAKESPEARE'S FLA YS, with Text and Introduction in English and German. Edited by C. Sachs, Prof. Ph.D. 8vo, cloth, each Play or Number, lod. Now Ready : 1. Julius Caesar. 2. Romeo and Juliet. 3. King Henry VIII. 4. King Lear. 5. Othello. 6. Hamlet. 7. A Midsummer Night's Dream. 8. Macbeth. Others to follow. "This edition will be quite a godsend to grown-up students of either language, for the ordinarj^ class reading books are too childish to arrest their attention. Tlie parallel paging saves the labour of using a dictionary, and the series is so low in price as to place it within the reach of all." — Saturday Review. SHAKESPEARE REPRINTS, i. King Lear. Parallel Texts of Quarto i and Folio i. Edited by Dr. W. Vietor, of Mar- burg. Square l6mo, cloth, 1$. 6d. The texts of the first quarto and folio, ■with collations from the later quartos attd folios, are here printed in a compact and convenient volume, intended as a class-book in the University. " A fairly accurate reprint of the texts, and for its handy form should be wel- comed by all interested in the textual study of Shakespeare s Plays."— y^e /4M«- nisum. Now Ready, SHUMWAY {E. S.). A Day in Ancient Rome. With numerous Illustrations. By Edgar S. Shumcoay, Professor, Rutger's College, New Brunswick. Small 4to, cloth, 5j-i WHITTAKER AND CO/S " THE SPECIALISTS' SERIES^ A New Series of Handbooks for Students and Practical Engineers. Crown 8vo, cloth. Illustrated throughout with original and practical Illustrations. Now Ready i MAGNETO- AND D YNAMO - ELECTRIC MA- CHINES, with a description of Electric Accumulators, A Practical Handbook, with use of the German of Glacer de Cey. Second Edi- tion, enlarged. With a Preface and an Additional Chapter on the latest Types of Machine. By W. B. Esson, M.S.T.E. With 99 Illustrations. Crown 8vo, pp. xi.-3i2, cloth, 7^. 6d, In successive chapters the author considers electrical units; the historical deve- loPment 0/ magneto- and dynanw-electric generators ; machines generating alter- nating and direct currents ; the particular applicability of the various electric generators; automatic switches and current regulation; electrical storage; the physical laws bearing on tfie construction of electa ic generators ; the constntction of the several parts of electric generators ; tf^ employment of t/tese machines in pro- ducing tfie electric light ; and for various other purposes. "Almost all the best-known machines are described and illustrated, with the dis- cussion of certain theoretical questions." — Electrician. " Will be welcomed by those who wish, without studying the matter for profes- sional purposes, to possess a scientific knowledge of modern electrical machines." — English Mechanic. Presents in condensed form an epitome of electrical progress up to recent dates." — Scientific American. Inthe second edition the latest types of machine have been described, and the book is brought up to date. GAS MOTORS. Gas Engines. Their Theory and Manage- ment. By William Macgregor. With 7 Plates. Crown 8vo, pp. 245, cloth, Sj. 6d, List of Contents. Introductory— Direct Working Engines without Compression— Gas Engmes work- ing with Compression— Compression En- gines with Compressing Pump — Theory of the Gas Engine — Relative Speed of Com- bustion in Gaseous Explosive Mixtures— Witz's Theoretical Cycles of Gas Engines —Some further Theoretical Data— Clerk's Theory of the Gas Engine— The Gas En- gine Indicator-Diagram— Index. " Mr. Macgregor has collected a considerable amount of information on his sub- ject of a kind which may prove valuable to many readers. All who desire to be well informed in gas engines will be able to get wliat they want from these pages."— Engineerijig. ^' From the Abb6 Hautefeuille's powder machine, invented in 1678, to the Maxim Patent of 1883, is a long record of progress fully detailed in Mr. Macgregor's useful and interesting book." — Saturday Review. ' '* This handbook may be safely recommended to all students who wish to acquire a thorough practical knowledge of the mechanism and theoretical principles of the gas engine. ^—The Buildiiig News, "This book should find a place in every engineer's \ihxdxy. '' -^yoiutial of Gas Ifighting. BALLOONING. Ballooning : A Concise Sketch of its His- tory and Principles, From the best sources, Continental and Eng- lish. By G. May. With Illustrations. Crown 8vo, pp. vi.-97, cloth, 2s. 6d, *^,* This deals, not with the Possibilities qf aeronautics on vague assumption, but gives informatwn from a practical view of what has been doHe^ showing the present position. NEW AND RECENT BOOKS. List of Contents. ing Power — Present State of Ballooning and Recent Proposals for Steering Bal- loons—The Cost. Introduction — First Practical Experi- ments—Resources and Incidents — The Practical Application of Aeronautics — Military Applications of Ballooning — Steer- , " Mr. May gives a clear idea of all the experiments and improvements in aero- navigation from its beginning, and the various useful purposes to which it has been applied." — Contemporary Review. " It confines itself to the statement of facts, and should fulfil completely the pur- pose for which it was written." — TAe Graphic. "Full of valuable matter." — Western. Morning News. "Brings his record down to the latest experiments." — United Service Gazette. ELECTRIC TRANSMISSION OF ENERGY, Electric Transmission of Energy, and its Transformation, Subdivision, and Distribution. A Practical Handbook by Gisbert Kapp, C.E,, Asso- ciate Member of the Institution of Civil Engineers, &c. With 119 Illustrations, Crown 8vo, pp. xi.-33i, cloth, 7^. dd. *#* It has beejt the aim of the author to present the sciettiific part of the subject in as sitnpie a form as possible^ giviftg" descriptions of work actually carried out. He has endeavoured in this way to place before tfie reader an unbiassed report on the present state of electric transmission of energy. "This is *a practical handbook' par excellence— a book which will be read, studied, and used not by electricians merely, but by most engineers. It contains a vast amount of original matter, and it bears the signs of much patient thought assisted by practical experience, '* We cannot speak too highly of this admirable book, and we trust future editions will follow in rapid succession." — Electrical Review. " A valuable work ; written with regard to facts only." — Electrician. " This excellent work ... it must be accorded a prominent place among the few standard works on the subject." — Electrician {New York). " The book is one which will be read with great interest by all who give thought to the subject." — Saturday Review. ELECTRIC LIGHTING. Arc and Glow Lamps. A Practical Handbook on Elfectric Lighting. By Julius Maier, Ph.D., Assoc. Soc. Tel. Eng., &c. With 78 Illustrations. Crown 8vo, pp. viii.-376, cloth, 7^. 6af. The whole system of modern electric illumination is dealt with in this volume. It gives a detailed description of all the principal modem generators and lamps, together with condtictors attd the oilier appliances required in electric light instal- lations. It contains also a full account of the various Applications of Electric Lighting up to recent date, " The author has collected all the most recent available information concerning the process of manufacture, life, &c.,of arc and glow lamps in a very convenient and readable form. Indeed, we do not know any work in which the subject is, on the " whole, so fully handled." — The Engineer. "An excellent and useful hoQ\fi.— Glasgow Herald. " We have no hesitation in recommending ii."—The Builder'. "This valuable handbook ought to be in the possession of every one interested in artificial lighting." — The Gas World. ON THE CONVERSION OF HEAT INTO WORK. A Practical Handbook on Heat-Engines. By William Anderson, M.Inst. C.E. With 62 Illustrations. Crown 8vo, pp. viii.-2S2, cloth, 6j. Ill Chapter I. the A uihor deals with a brief investigation of the lams of motion. In the next chapter he considers the principles involved in vacillations or vibror 10 WHITTAKER AND CO.'S iions. The7i follows the third chapter on the properties of gases. Thefojtrtk chapter deals with Carnot's laws, sources of heat, and the properties of fuels. The fifth chapter takes up the bletst furnace, and contains an investigation of the action of a gun as a heat engine. The sixth chapter deals with heat engines proper ; while in the seventh, and last, he considers various forms of the steatn engine and their characteristics. " From beginning to end the book is written for engineers, and it is therefore likely to prove more useful to engineers than any work with a similar object produced by a non-practical man for students of physical science. " We nave no hesitation in saying there are young engineers — and a good many old engineers too — who can read this book, not only with profit, but pleasure ; and this is more than dkn be said of most works on heat. ' — The^ Engineer. "The volume bristles from beginning to end with practical examples culled from every department of technolo^. In these days of rapid book-making it is quite re- freshing to read through a work like this, having originality of treatment stamped on every page." — Electrical Review. 'The book is the work of a thoroughly practical engineer of high standing in the profession. " — Engineering. STUTZER (A.). Nitrate of Soda : Its Importance and Use as Manure. A Prize Essay. By A. Stutzer, Ph.D., President of the Agricultural Experiment Station, Bonn. Re-written and Edited by Paul Wagner, Professor, Ph.D., President of the Agricultural Experi- ment Station, Darmstadt ; according to the Views of the Committee of Judges, partly Considering the Second Prize Essay by Prof. A. Damseaux, Gembloux. Crown 8vo, pp. vii.-gS, sewed, 2s. 6d. SuRTEEs' Society's Publications. iVWf Volumes. Just Published, YORKSHIRE DIARIES AND AUTOBIOGRAPHIES IN THE SEVENTEENTH AND EIGHTEENTH CEN- TURIES. With Portraits. 8vo, pp. ii.-i73, cloth, ^s. 6d. MEMORIALS OF THE CHURCH OF SS. PETER AND WILFRID, RIPON Vol. II., 8vo, pp. xii. -398,. cloth, 15J. Ready, 2 vols., %vo, pp. 711-970, cloth, £1 "js. TECHNOLOGICAL DICTIONARY OF THE ENG- LISH AND GERMAN LANGUAGES. Containing Words and Phrases employed in Civil and Military Engineering, Shipbuilding and Navigation, Railway Construction, Mechanics, Chemistry, Chemi- cal Technology, Industrial Arts, Agriculture, Commerce, 'Mining, Physics, Meteorology, Metallurgy, Mathematics, Astronomy, Mine- ralogy, Botany, &c. In Co-operation with P. R. Bedson, O. Brandes, M. Briitt, Ch. A. Burghardt, Th. Carnelly, J. J. Hummel, J. G. Lunge, J. Liiroth, G. Schaffler, W. H. M. Ward, W. Carleton Williams. Edited by Gustavus Eger, Professor of the Polytechnic School of Darmstadt, &c. Vol. I. — English-German. Vol. II. — German-English. " A really valuable work, which treats the two languages well and exhaustively, and, best o\ all, correctly. We can confidently recommend it to every one who haa to work in English and German technical terms." — EngineeriTig. NEW AND RECENT BOOKS. II Just Published, Vol. /., English-Spanish, in super-royal Svo, pp. 873, bound in half -morocco, £1 ids. TECHNOLOGICAL DICTIONARY. 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