%W(^p^^'^^:'^ BOUGHT WITH THE INCOMK FROM THE SAGE ENDOWMENT FUND THE GIFT OF licnrg W. Sage 1891 A. ya-7 3V-3 ^^7^ '^f arV1947 '^°™" """'•'•»'*y Library O oiin,an^ ^^24 031 201 787 4- lii The date shows when this volume was taken. HOME USE RULEST All Baoks subject to Racall. Books not needed for, instruction or re- search are returnable within 4 weeks. ' ' Volumes of, periodi- cals and of pamphlets lire held in the library as much as possible. Per special purposes they are given outi for a limited time. 1 ' Borrowers should not use their library privileges for the bene- fit of Other persons. Books not needed during recess periods should be> returned to the library, or arrange- ments made for their return during borrow- er's absence, if wanted. pooks needed by more than one person are held on the reserve list. Books of special value and gift books, when the giver wishes it, are not allowed to circulate. Marking books strictly for- bidden. Readers are asked to report all aases of books marked or muti- lated. 7 Cornell University WM 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/cu31924031201787 TELEPHONE LINES AND THEIR PROPERTIES TELEPHONE LINES THEIR PROPERTIES BY WILLIAM J. HOPKINS PROFESSOR OF PHYSICS IN THE DREXEL INSTITUTE OF ART, SCIENCE, AND INDUSTRY, PHILADELPHIA NEW EDITION REVISED AND ENLARGED NEW YORK LONGMANS, GREEN, AND CO. LONDON AND BOMBAY 1899 & A. 73^3 Copyright, 1893, bv LONGMANS, GREEN, AND CO. Copyright, 1894, by LONGMANS, GREEN, AND CO. All righls reset ved FIRST EDITION. MARCH, 1893 REPRINTED, FEBRUARY, 1894, DbCeMBER. 189S, FEBRUARY, 18; March, 1S99 TROW DIRECTORY PniKTING AND BOOKBINDING COMPANY NEW YORK PREFACE. Fully realizing the meagreness of the available literature on the subject of telephone lines, I have written the following chapters in the hope that my experience may prove of benefit to others. I have not attempted to go into the subject of line construc- tion in any detail ; but as its omission would have left the book incomplete, I have given an outline of the methods of design and construction. I hope that the applied mechanics of line construction may be fully treated in the near future by someone fully competent to do so in the light of personal experi- ence. The subject of exchanges has been taken up prin- cipally to round out the scope of the book ; and for much of the matter under this head 1 am indebted to the writings of Messrs. A. S. Hibbard, F . A. Pick- ernell, and J. J. Carty. In covering the ground which comes most properly under the title, the properties of telephone lines, I have endeavored, so far as possible, to avoid mathe- matics, and to treat the subject in a way which would vi Preface. prove most interesting and instructive to the general reader as well as to the student. Where it has seemed best to put in a mathematical demonstration, it has been put in a foot-note. The results of investigations in regard to the prop- erties of telephone lines, I have endeavored to state clearly in a general discussion. They are not sus- ceptible of tabulation, and the most important prop- erties it is impossible to formulate exactly. I have treated rather fully the questions of inter- ference with the telephone currents from outside sources, especially the troubles from electric rail- ways; for my practice in that direction, during the last four or five years, has been considerable, and en- ables me to deal with that subject in tlie light of personal experience. This question is still of con- siderable importance in this country, and its treat- ment here may be of assistance to managers of ex- changes in practical work. It was my intention that the book should be one which would prove useful to the practical man, as well as that it should serve as a basis for a lecture course to students. I have therefore thought it best to introduce some matter in which correct elementary ideas of matter and energy are developed, so as to lead up to the most modern conception of the method of propagation of electro-magnetic disturb- ances. In conclusion, I wish to express again my indebted- Preface to the Fourth Edition. vii ness to the writings of Messrs. Carty, Pickernell, and Hibbard, as well as to well-known works of Dr. Fleming, Dr. Lodge and others, and my further ob- ligation to Mr. Carty for the results of his experi- ments on static induction. William J. Hopkins. PREFACE TO THE FOURTH EDITION. Since the last revision of this book, the changes in practice in the matters treated are not sufficient to warrant changes in the text. There is one point, however, in regard to which it seems necessary to be somewhat explicit. It is stated in the preface to the first edition that the book is intended both to be useful to the practical man and to serve as the basis for a lecture course to students. In the most progressive technical schools of the present day, lecture courses are provided, at an advanced period, upon special branches of engineer- ing. These lectures are giveaby specialists, and each series covers, so far as circumstances allow, the prac- tice in that particular branch, as an immediate prep- aration of the student for contact with active pro- fessional work. A general comprehensive training in mathematics, science, and other engineering subjects is presupposed, and no attempt is made in these courses viii Preface to the Fourth Edition. to supply such matter, which the student has usu- ally acquired pretty thoroughly at this stage of his education. One special subject of this class is telephone engi- neering; and so far as that subject includes telephone lines and their properties, the matter must be sub- stantially that outlined here. Transmitters and re- ceivers, which would also belong in such a course if they had not previously received thorough considera- tion, will be treated in a similar manner in a little book now in preparation. Any elementary scientific matter herein contained is intended, not to take the place of a thorough mathematical and scientific training, but as a remind- er to the man who has had that training and to help the practical man who has not had it to profit by the book. I cannot refrain from expressing my gratification at the evidences which have come to me of the useful- ness of the work to those for whom it was primarily written. William J. Hopkins. Philadelphia, January, 1898. TABLE OF CONTENTS. CHAPTER I. Design and Construction of City Lines. PAGB THE SYSTEM 2 Sub Exchanges, . . ■ . . . . . .3 Party Lines, 4 POLES, 4 Cross-arms and Pins, 5 Setting Poles, 5 Corner Poles, 7 WIRE 8 Corrosion of Copper Wire, 9 STRINGING, 10 Joints and Ties, 11 Guy Wires, 12 CHAPTER II. Underground Work. CONDUITS OF THE FIRST CLASS— VEGETABLE, . 15 Valentine Conduit, 15 Wyckoff Conduit, 16 Paper Conduit, 16 CONDUITS OF THE SECOND CLASS— IRON, . . 16 Johnstone Cast-iron Conduit, . . . . . 16 Wrought-iron Pipe Conduits, . . . . .17 Cement-lined Iron Conduits, 18 Tabic of Contents. PAGB CONDUITS OF THE THIRD CLASS— CEMENT, . . i8 DoRSETT Conduit iS Lake Conduit — Terra-cotta, .... 19 Zinc Tube in Cement 20 Chenoweth Conduit, 20 MANUFACTURE OF WOODEN CONDUITS, 21 Destruction of Cable Sheath, 21 MAN-HOLES 23 Precautions Against Explosion 24 DRAWING IN CABLES 25 RODDING 26 Drawing In, 26 Testing and Making Joints, 27 DISTRIBUTION 28 Cable Boxes, 28 Connections to Cable Head 30 Lightning Arresters, 32, 33 CABLES 32 INSULATION OF CABLES 33 "Conference Standard" Specifications, . . 34 Armor and Sheath 35 CHAPTER III. Long Distance Lines. POLES . 38 Cross-arms, ^ Pins 40 Standard Pole, 4, Transposition Pole, 42 Setting Poles . . 42 Methods of Guying, 4c Table of Contents. xi PAGE STRINGING, 52 "Running Board," 52 "Banjo," . . . 53 Setting Up and Tying 53 Formula for Calculating Dip and Pull, , . 54 CHAPTER IV. Wire. MANUFACTURE, 59 Hot Rolling and Drawing, 59 Cold Rolling, . ' 60 Inspection, 61 Testing Wire 62 Formulae for Calculating Properties of Wire ' from Tests, . .' 64 Properties of Wire, . ' 65 Iron and Steel, ..." 66 Aluminum Bronze and Silicon Bronze, ... 66 Life of Iron, Steel, and Copper, ... .68 CHAPTER V. Insulators. MATERIALS FOR INSULATORS, 69 Tests as to Proper Form, 71 Hibbard's Transposition Insulator 75 CHAPTER VI. Kxchanges. OFFICE BUILDING 77 Gable Shaft, 78 Battery and Power Room 78 xii Table of Contents. FAGS Cable \'ault 79 Distributing Room, 79 Distributing Board, 80 Testing Room, 81 CHAPTER VII. Switchboards. "GROUPING" SYSTEM, 82 MULTIPLE SWITCHBOARD, 83 Connections, . . 85 METALLIC CIRCUIT BOARDS 86 Connections, Metallic Circuit Board, , . .87 Connections, Multiple Board, Adapted kor Metallic Circuits 88 Branch Office Wires, 88 Combination Boards, 89 Repeating Coils, 92 Party Lines 92 The "Law" System, 93 Automatic Exchanges, 94 CHAPTER VIII. The Propagation of Energy. MOLECULAR STRUCTURE OF MATTER, ... 95 The Ether, 96 WAVES 97 Sound-Producing Vibrations 98 Transverse Vibrations 99 Transference and Transformation of Energy in Case of Sound 100 Identity of Light and Heat, 102 Table of Contents. xiii PAGE Propagation of Electro-magnetic Disturbance, 102 Harmonic Vibration, 103 - Velocity of Propagation 103 Transformations of Energy in Circuit Carrying a Current, 106 Formula for Direction and Rate of Transfer- ence of Energy, 107 Transformations in Telephone Circuit, . . 107 Work, 109 Sir Wm. Thomson's Vortex Theory, . . .110 CHAPTER IX, The Telephone Current. TRANSMISSION OF SPEECH 11 1 The Human Voice, 112 PITCH, CHARACTER, AND CLEARNESS, . . .113 PROPERTIES OF ALL LINES 114 Alternating Currents, 114 INCREASED RESISTANCE FOR HIGH PERIODS AND EFFECT UPON TELEPHONE CURRENT, . .116 Transmission Through High Resistance, . . 117 SELF-INDUCTION, 119 Mutual Induction, 121 Self-induction in Magnetic Material, . . .121 Hysteresis 125 Retardation Due to Self-induction, . . . 126 Impedance 126 Unequal Retardation for Different Periods, . 128 Effects of Electro-static Capacity, . . . 129 CROSS-TALK, 132 Mr. Carty's Experiments, 134 xiv Table of Contents. PAGB Self-induction in Iron Wires 145 ELECTRO-MOTIVE FORCE AND VOLUME OF TELE- PHONE CURRENT, MS CHAPTER X. Measurement MEASUREMENT OF IMPEDANCE AND RETARDA- TION, 148 Measurement ok Volume 152 CHAPTER XI. Properties of City Lines. RESISTANCE AND IMPEDANCE, 156 CAPACITY 157 INSULATION, 158 RETARDATION 160 DISTURBANCES, 160 CROSS-TALK, 161 Formula for Cross-talk, 164 CHAPTER XII. Interferences from Outside Sources. AIR AND EAR H CURRENTS, 165 TELEGRAPH INDUCTION, 167 INDUCTION FROM ELECTRIC LIGHTING CIRCUITS, 167 Nature of the Disturbances, 168 Proposed Remedies, 169 Proper Arrangement of Circuits, .... 171 Belt Circuits, 174 Re-arrangement of Belt Circuits, .... 175 Table of Contents. xv PAGB Summary of Methods of Preventing Disturbance FROM Lighting Circuits, 176 INTERFERENCE FROM ELECTRIC RAILWAYS, . 176 The Single Trolley System, 177 Nature of the Disturbances, 178 Plans for its Prevention, 179 THE DOUBLE TROLLEY SYSTEM, . . . . i8o Metallic Circuits for Telephone Lines, . . .181 McCluer System, i8i Use of Repeating Coils, 186 Wiley-Smith System, 187 Condenser in Telephone Line, 187 Sabold System of Ground Rods 188 Instance of Interference from Electric Railway, . 188 MAPPING OUT GROUND POTENTIA.L, ... 193 CHAPTER XIII. Properties of Metallic Circuits. ADVANTAGES OVER GROUNDED CIRCUITS, . . 194 DESIGN 19s THE "CR FORMULA," 196 RESISTANCE AND IMPEDANCE, .... 196 CAPACITY, 197 Formula for Pole Lines, . . ,x ■ ■ 200 Methods of Diminishing Effects of Capacity, . 201 INSULATION, 202 SELF-INDUCTION, 203 Mutual Induction, 204 CROSS-TALK, 204 TRANSPOSITIONS, 206 Transpositions in the New York-Chicago Line, . 209 xvi Table of Contents. PAGE Transposition Insulators, 210 LOSS m VOLUME IN TRANSMISSION, . . .211 Very Long Lines, 2'2 PARTY LINES ON METALLIC CIRCUITS, . . .213 CARTV'S BRIDGING BELL SYSTEM, ... 214 CHAPTER XIV. Cables. FIBRE AND RUBBER COMPOUNDS 217 MANUFACTURE OF CABLES 218 "Dry" Cables, 219 Paper Cables, 219 DIMENSIONS OF UNDERGROUND LINES, . . .220 PROPERTIES OF CABLES— IMPEDANCE, . . 222 Capacity 222 Derivation of Formula, 224 Specific Inductive Capacity, 227 Methods of Reducing Effects of Capacity, . 228 INSULATION, 231 SELF-INDUCTION AND RETARDATION, ... 231 CROSS-TALK— TWISTED PAIRS 232 CONCENTRIC CABLES, 234 Capacity of Concentric Cables, 235 EFFECT OF CAPACITY IN RECEIVING AND IN TRANS- MITTING, 236 SPIRALLED PAIRS, 236 FUTURE IMPROVEMENTS, 239 APPENDIX A.— Oscillations 241 APPENDIX B.— Mr. Carty's Experiments on Induc- tion, 246 INDEX 269 TELEPHONE LINES AND THEIR PROPERTIES. CHAPTER I. OVERHEAD CITY LINES. When, on the introduction of the telephone, it became necessary to construct lines for the transmis- sion of its currents, the knowledge of what were the requirements for an efficient telephone line was con- spicuously wanting. In regard to the properties of such a rapidly alternating current but little — in fact, practically, nothing — was known. The lines must be built, however ; so by applying the small stock of knowledge which had been acquired in telegraphy, and by more or less skilful guessing, shift was made to get up something that would work, although not very well. Since that beginning rapid advances have been made in the improvement of apparatus, and no less rapid strides in the knowledge of what consti- tutes a good line. We are beginning to know, too, chiefly as the result of experiment, what are the char- 2 Telephone Lines and Their Properties. acteristics of the telephone current ; its magnitude, its effects on other currents and on itself, and the effects of external and internal conditions upon it ; what characteristics we wish to preserve, and what we wish to alter or destroy. From this knowledge, which is still very incomplete, we can to some extent determine the proper form, dimensions, and material to make use of in building a line; and it is most necessary that the maximum efficiency of transmis- sion should be obtained, for the most perfect appa- ratus can accomplish nothing if the requisite charac- teristics of its current are lost or destroyed. For a successful telephone system the first re- quisite is efficient service ; and this can be obtained only by good construction and careful inspection and maintenance. I shall therefore take up, first, the de- sign and construction of the system. THE SYSTEM. The exchange, or central office, should, of course, be so situated that the total length of line to be erected shall be a minimum. It is seldom possible, however, to place the exchange in the most desirable location. The routes for the lines must be secured, and as the so-called " house-top " construction is not now considered good practice, the lines must run in streets, either on poles or under the surface. The method which has been found the best, and the most Telephone Lines and Their Properties. 3 economical for maintenance, is to run trunk-lines on poles or in conduits, as far as possible. From these main trunk-lines, of which there may be three or four, branches are run off at intervals, and the indi- vidual wires are distributed from definite points. Lines designed in this manner are much easier to maintain in good order than those put up in a hap- hazard way. If on poles, they are not so unsightly, and they do not form the net-work overhead of which there has been so much and such well-founded complaint. If the lines are underground, this is the only prac- tical way to run them. The cables, containing many wires or circuits, are laid in conduits underground, the branches being led off from the main trunk-lines in the same manner as in the case of pole-lines. At the proper points the distribution of wires is made on poles, the cables being led up the poles and con- nected in cable-boxes to the individual wires. In many systems, covering large areas, the sub- scribers may be divided to advantage into several groups. This is the case in all large cities where the distances are great, and where the communication be- tween different groups or classes of subscribers is not very frequent. In such cases the main exchange is so placed as to serve to the best advantage the busi- est districts. A sub-exchange is established for each outlying district, and is connected by a trunk-line of comparatively few wires with the main exchange. 4 Telephone Lines and Their Properties. This question of sub-exchanges has been much dis< cussed. The use of sub-exchanges saves a great deal of wiring, which is a very important consideration in cities where the available space for wires is limited. On the other hand, the use of sub-exchanges causes delay in making connections, and increases the lia- bility to make mistakes. It will probably be neces- sary, however, to adhere to this plan, notwithstand- ing its disadvantages, as the question of space is the determining one. In the earlier days of the telephone it was a very common custom to place a number of subscribers on the same wire. In small exchanges, where the sub- scribers are somewhat scattered, and some at a con- siderable distance, this is still done to advantage; but in an exchange of any size the practice is usually not advisable, and, indeed, the old system of " looping- in '' cannot be defended on any ground. Such an ar- rangement is prolific of bad service and trouble, and should be avoided wherever possible. When "party lines," so called, are used, they should be connected according to the " bridging- bell " method.* POLES. In the best construction of overhead trunk-lines the poles are preferably of white Canada cedar — sound, live wood — not less than seven inches in diam- • Described on page 214. Telephone Lines and Their Properties. 5 eter at the top, and for city lines they should be a little larger than that. They should be stripped and shaved, and the gains for the cross-arms cut before setting ; and they should be set not less than six feet into the ground. In many cases the cross-arms may be bolted on before setting the pole. The pole is not subjected to any chemical treatment before setting; but, after setting, the poles in a city line should be given two coats of good paint, both for the looks of the line and for the preservation of the wood. The proper height of pole depends on the condi- tions met with. It is generally necessary to use a rather tall pole for city work, in order that the line may be thoroughly clear of the street, and not be too much of an obstruction to the adjoining buildings. The cross-arms should be of Norway pine, well coated with a mineral paint of good insulating prop- erties. They are usually 4 J^ by 31^ inches, and 10 feet long; and are sent bored and painted from the factory. Cross-arms should be set 2 feet apart, on centres, JjLhj ^ZI and braced to the pole with C iron straps, placed as shown in the figure. The best pins are of locust, i^ inch in di- ^""^ ameter, and are set 12 inches on centres in the cross- arm, the two centre pins being 1 5 inches apart, to clear the pole. The poles are set from 100 to 120 feet apart, the Tekplionc Lines and Tlwir Properties. closer the better ; and they are so set that on ad- jacent poles the cross-arms face each other, as in the figure. If they are set in this way the arms are not likely to be pulled off ; but when set all facing the same way, it sometimes happens that a whole line will go down at once, all the cross - arms being pulled off the poles. For if, through an excessive strain at one point, or some accidental cause, such as, perhaps, a fallen pole, a cross-arm happens to be wrenched off, the strain in the same direction is transferred to the next pole, that arm comes off, and so on, like a row of blocks, until the strain is relieved, and a considerable length of tangled wires lies upon the ground. On the other hand, if the poles are set alternately facing, and back to back, as in the figure above, it is almost impossible to pull off more than two sets of arms ; and in many cases a broken pole is kept from falling, and the grounding of all its wires avoided. It is important, both for the looks and mainte- nance of a line, that the tops of the poles should be as nearly as possible in line. This condition will generally be attained in cities by using poles of the Telephone Lines and Their Properties. same height, as the surface of the street is suffi- ciently level ; but in country lines, as will be seen later, inequalities of the ground must be compen- sated for by different heights of pole. Especial attention must be paid, also, to corner poles. Whenever possible, it is best to take off branch trunk-lines in opposite directions from the same pole, or in such directions that there shall be no resultant sidewise pull on the pole. When this is not possible, the angle at which the pole is to be set must be calculated, and the pole strongly guyed. Excessive inclination of poles is to be guarded against, however, as the strain thus caused on the pole is apt to be as great as when the angle is too small, and is more concentrated. In any case, cor- ner poles should be of greater di- ameter than the line poles, and the cross-arms so set that the strain will pull them against the pole, and not away from it. One of the best methods of turn- ing a street corner is that shown in the diagram. In this case two poles, A and B, are used at the corner, 8 Telephone Lines and Their Properties. both anchored at the corner diagonally opposite, as shown, to C. It is not necessary, in this construction, to cant the cross-arms, but it may be necessaiy to use what are known as " Y guys," to prevent the poles from bending near the top. The Y guy is shown below. When a single pole is used at a corner, it should be doubly guyed. The former construction is the better one.* There are many forms of insulator in use, none of which are altogether desirable ; but for mechanical reasons the insulator should be of such form that the wire is tied near the base of the pin, thus keeping the leverage small. WIRE. It has been in the past almost the universal cus- tom to use galvanized iron wire for city lines. Iron wire possesses just two advantages — strength and cheapness. Electrically it is altogether undesirable, especially for telephone lines, and it is easily and quickly corroded by the acids which are always to be found in abundance in the air of cities. Hard-drawn * For additional methods of guying, see chapter on Long Distance Lines. Telephone Lines and Their Properties. 9 copper wire is now produced possessing sufficient mechanical strength, and copper resists corrosion for a long time — we do not yet know how long, for there are now no copper lines which have been up for a sufficiently long time to suffer from this cause. For electrical reasons copper is infinitely superior to iron, and in the long run will be found to be cheaper. Assuming, therefore, that copper wire is to be used, the size is next to be determined. It is not possible, nor would it be profitable, to calculate the exact size which should be- used at any point, as in electric lighting. It is necessary to use a wire sufficiently large to give the requisite mechanical strength, and in general, for pole-lines, the larger the wire the better the line. This is not absolutely true for all cases, as there are other conditions to be con- sidered ; but in a general sense it is true. The wire which has been quite generally used, where copper has been employed at all, is 0.08 inch in diameter. It breaks at about three hundred and twenty-five pounds, and has proved to be very satisfactory wher- ever it has been used. Copper wire, as has been said, resists corrosion for a long time. On exposure to the air and weather a thin, dark-greenish skin, probably a chloride, forms on the outside of the wire, and protects the inside of the wire from further action. The length of time necessary to form this skin varies, of course, accord- lO Telephone Lines and Tlieir Properties. ing to the conditions of exposure, and the corrosion might, after long exposure under adverse conditions, penetrate to a considerable depth. I have not, how- ever, as yet, found any as thick as ^oV^ of an inch, and there is every reason to think that the length of life of a line of copper would be practically unaf- fected by corrosion due to exposure. STRINGING. In stringing the wire the best practice is to pull it up rather tight. By following this plan, with the comparatively short spans recommended, the " sag,'' or "dip," is very slight, and it is impossible for the wires to blow together, as they can swing but little. Moreover, the appearance of a line so constructed is much more regular and pleasing than when a long span and a large dip are employed. When using copper wire, it is customary to put up a line with a dip greater or less according to the weather.* In cold (freezing) weather it is pulled as tight as the linemen can pull it ; and in warm weather (say 65° to 80° F.) a span of one hundred and twenty-five feet is allowed a dip of about one foot at the centre. t The wire is tied fo the insulator with copper wire of the same size as the line wire, * For method of stringing wire, see page 52, long distance lines. f Tliis puts a ratlier severe strain on the wire in very cold weather, especially when loaded with snow or sleet ; and the practice in thig respect might be slightly modified to advantage. Telephone Lines and Their Properties. il but the tie wire must in some cases be annealed. The tie is shown in the diagram, each end being wrapped five times about the line wire. Line joints in copper wire may be made in vari- ous ways, as the well-known " American " joint ; but :^III]IMIP^^ffiIIffiff^ the best and quickest, as well as the most perma- nent, is made by means of the " Mclntire " sleeve. This sleeve consists simply of two small copper tubes, of sizes to fit the wires to be joined, the tubes being brazed together. The wires are passed through the tubes from op- posite ends, one in each, the ends turned up, and cut off close. The tubes are then twisted together by Mclntire Joint Complete. special pliers, and the joint is complete. No solder is used. The twisting not only makes the joint 12 Teleplionc Lines and Their Properties. mechanically stronger, but draws the copper tube close around the wire, insuring good contact, and the inner surfaces remain clear and bright. For guy wires, iron or steel is used on account of its strength, and the proper size for a guy wire must be determined by the strain on the pole, using a large factor of safety. When very large guy wires are used, it is better to have them stranded. When possible it is best to run the guy wires to the base of the next pole, but it is frequently neces- sary to set posts for attaching the guy wires, and in such cases care must be taken that the posts are strong enough and properly set. Much annoyance has been caused by the failure of posts which were faulty or improperly set. An instance of this, which came under my own observation, is shown below. The first post. A, was too weak, and the wire was fas- tened near the top. This post soon showed signs of giving way, and the post B c . was put m, but was also improperly set, and the guy wire AB fastened to its top. This in turn it was soon necessary to re- enforce by the post C, which bore practically all the strain. This was in a city street, and is a bungling and unsightly piece of work, besides costing more than if it had been properly done in the first place. By far the greater number of problems which arise Telephone Lines and Their Properties. 13 in putting up a city pole line, are individual, and must be solved according to circumstances. Strength should never be sacrificed, and the aim should be to make the line look as well as possible. In general, the line which is the best will look the best. CHAPTER II. UNDERGROUND WORK, The tendency at present, at least in the larger cities, is entirely toward underground systems ; and in these the difficulties to be met with are far greater than in the case of overhead lines. The general planning of routes has already been outlined, but in- stead of setting a line of poles, conduits must be pro- vided. Very many methods of putting wires underground have been tried. The plan of burying them perma- nently, without providing means for getting at them for purposes of alteration and repair, is very expen- sive and not admissible ; and the plan of drawing the wires into permanent ducts, with man-holes at convenient distances, has been almost universally adopted. Whatever the system, the conduit must be straight, or nearly so, between man-holes, and there must be no sharp corners to abrade the cable sheath, or cause difficulty in drawing in. The man- holes are usually placed about two hundred feet apart, but a cable may continue through a man-hole with- out being cut. So many systems of subway construction have Telephone Lines and Their Properties. 15 been proposed that only the most prominent among them will be described. The systems may be roughly divided into three classes : 1st. Conduits of vegetable material. 2d. Iron conduits. 3d. Cement or concrete. In the first class there are the — I (a). Valentine creosoted box. I (b). Wyckoff creosoted tube. 1 (c). Paper conduit. In the second class, there are the — 2 (a). Johnstone sectional cast-iron conduit. 2 (b). Wrought-iron pipe in hydraulic cement. 2 (c). Wrought-iron pipe in asphaltic concrete. 2 (d). Cement-lined wrought-iron pipe. In the third class — 3 («). Dorsett. 3 {b). Lake conduit (terra-cotta), 3 (c). Zinc tube in hydraulic cement. 3 (d). Chenoweth. CONDUITS OF THE FIRST CLASS. I (a). The Valentine conduit consists of a rectan- gular box, divided into ducts 3 inches square, into which the cables are drawn. The box is made of yellow pine, i inch thick, from which the sap has been withdrawn, and which has been treated i6 Telephone Lines and Their Properties. with a dead oil of tar or creosote. The wood par- titions between the ducts are treated in the same way. These boxes are made in lengths of about twelve feet, and are buried from two to three feet be- neath the surface. The abutting ends are carefully adjusted and joined, and the whole outside is thor- oughly coated with tar. This is the cheapest form of conduit. I {b). The Wyckoff conduit is very similar to the Valentine. It consists of tubular ducts bored in blocks of creosoted wood, the blocks being grooved and splined together. It is laid and the joints made in the same way as the Valentine conduit. 1 (c). The paper conduit consists of paper tubes im- pregnated with a compound of asphaltum, a number of these tubes being laid in a wooden box and filled about with the same compound. It is said that this compound is free from the faults of cracking and other deterioration which have proved fatal to so many similar devices ; but none of this conduit has yet been laid a sufficiently long^ time to furnish any reliable data concerning its life. Severe laboratory tests on this material have given excellent results. CONDUITS OF THE SECOND CLASS. 2 (rt). The Johnstone sectional cast-iron conduit is used chiefly for electric lighting. It is made in 5 feet lengths, which are laid one above another and held Telephone Lines and Their Properties. 17 by clamp pins. The cross-section is rectangular, and each section is divided by removable partitions into several ducts. The cover is flush with the surface of the street, and hand-holes are provided for distribu- tion to houses. This conduit is laid directly in the earth, without cement or concrete, and is efificient for electric lighting purposes. 2 (b'). Wrought-iron pipe in hydraulic cement. 2 (c). Wrought - iron pipe in asp halt ic concrete. This conduit consists of wrought-iron pipes of 3 inches diameter, in 20-feet lengths, the iron being nearly 3^ inch thick. The method of laying is as follows, the only difference between the two sys- tems being in the materials used for the concrete: The bottom of the trench is first levelled to grade and the sides braced with plank. A thick layer of concrete is then put in and rammed. On this a row of pipes is laid, close together. Any desired number of pipes is used, and a second layer of concrete, thinner than the first, is rammed between and over them. Upon this is placed a second row of pipes, then concrete, and so on ad libitum. The concrete is laid thicker on the bottom, sides, and top, to give the whole additional strength, and over all is laid 2-inch creosoted yeliow-pine plank as a protection against pickaxes in subsequent exca- vation. The joints are made with sleeves having a ta- pering or vanishing thread, which is tight, and ena- 1 8 Telephone Lines and Their Properties. bles them to be laid \'cr)- rapidly. The pipes are laid free from burrs and have a smooth inner sur- face. In some cases they are asphalted. Much of the New York subway system consists of this form of conduit. , 2 {d). Cement - lined wrought - iron pipe conduit. The ccmcnt-lincd pipe is made in single or multiple tubes, of from i to 7 or more ducts, by holding verti- cally a sheet-iron tube, with a smaller brass tube in- side. In the space between the two tubes is poured semi-liquid cement. When this has solidified the brass tube is withdrawn. The method of laying is much the same as that last described, the pipes being in lengths of 8 feet, connected by ball-and-socket joints. A great deal of this conduit is laid in the New York subways. CONDUITS OF THE THIRD CLASS. 3 («). The Dorsett cojiduit is one of the oldest forms. The material used is a composition of pitch, coal-tar, and gravel, which is moulded into blocks 4 feet long, having tubular openings 25^ inches or 3 inches in diameter. These blocks are butted to- gether and joined by pouring in a mixture of pitch and tar which hardens quickly. This liquid is pre- vented from entering the ducts by sleeves, which are placed inside the ducts at the joints. The great fault of this conduit is that the blocks get out of align- Telephone Lines and Their Properties. 19 ment at the joints, leaving sharp edges in the ducts and allowing gases to leak in. A considerable amount of this conduit has been in use in Chicago for some years, and some in New York. 3 (1^). Lake conduit. This consists of short lengths of terra-cotta pipe, of rectangular section, usually di- vided vertically into two ducts. Three lengths of 4 feet each are joined before the pipe is laid, the joints being made by butting the ends and wrapping them with burlap. The burlap is then thoroughly tarred, and the length of 12 feet laid directly in the ground, being joined to the abutting pipe in the same way. This conduit has given good satisfaction in several cities. The method of laying this conduit, as used in Washington, D. C, differs somewhat from that just described, and makes a more substantial structure. The conduit itself is of terra - cotta, rectangular in cross-section, as shown in the illustration. Each piece is formed with two ducts, and one is laid on another, the bottom of the upper section forming the top of the lower. Joints are made by bracket-shaped pieces which cover the abutting ends, m^ 20 Telephone Lines and Their Properties. and the whole is laid in concrete, which holds the brackets in place and affords mechanical protection to the conduit. 3 {f). Zi)ic tube in hydraulie cement. In this con- duit the zinc was intended simply as a mould. It was expected that the zinc would corrode and leave a smooth duct of cement. It was found, however, that the zinc tube was very frail and gave way before the cement was hard ; and although there is a short length of this conduit in successful operation, the construc- tion is so expensive that it has not been extended. 3 {d). Chenoweth conduit. The object of this method is to provide an easy and inexpensive means of constructing a continuous cement conduit. A wooden mandrel, divided longitudinally into three parts, and wound with a spiral of sheet metal covered with soapstone, is placed in position and cement tamped about it. The wooden pieces are then with- drawn, leaving the metal spirals in the duct ; and another length is constructed in the same way. The adjoining ends of the metal spirals are connected to- gether as they are laid, and when the cement has thoroughly hardened the whole length of metal lin- ing, from man-hole to man-hole, is easily drawn out, leaving a smooth cement duct. Of all the different forms of conduit described above, but a few have been used to any extent for telephone work. The creosoted wood conduits, as they are the cheapest to put down, were the first to Telephone Lines and Their Properties. 21 be used ; but their use resulted in rapid deterioration, and sometimes destruction of the lead sheath of the cables. In making these conduits the sap is first withdrawn from the wood and its place filled with some preserv- ative substance, usually creosote. It is necessary that this should be done thoroughly ; for if a portion of the sap remains, fermentation eventually takes place, resulting, in the formation, within the duct, of vegetable acids, which attack the lead of the cable- sheath and rapidly destroy it. The same result will follow if the preservative substance contains any or- ganic acid. Improperly prepared creosote often con- tains sufficient acetic acid to attack lead, and acetic acid tends to form, with lead, salts which are unstable in the presence of carbonic oxide, changing into the carbonate, and thereby again setting free the acid. This again attacks the lead, and the process is re- peated until the sheath of the cable is destroyed. An unlimited supply of carbonic oxide is usually at hand, from the illuminating and other gases which permeate the soil of our streets, and the conduit is seldom tight enough to exclude them. This destructive corrosion may be recognized by the appearance of the lead. It produces a white scale, irregular pits, or a white efflorescence covering a pit in the lead, and must not be confounded with another form of corrosion which is harmless. In the latter form the lead shows a uniform hard, dark- 22 Telephone Lines and Their Properties. brown coating, which protects the pipe from further action. Even if the material of the conduit does not con- tain the destructive agent, it may be admitted from soil containing decaying or fermenting organic mat- ter, and the chemical process go on, although with much less rapidity than in the first case. To provide against this, both ducts and man-holes must be gas- tight. It is probable, however, that if the wood of which the ducts are made is properly and carefully prepared, there would be no such destructive action, and the life of the conduit would be practically with- out limit. The iron-pipe conduit is more expensive to put down than the creosoted wood, but it is more per- manent in character, and does not cause any corro- sion of the cables. For this reason iron pipe, espe- cially the cement-lined pipe, is meeting with great favor, and being used to a greater extent than any other one kind. The cement-lined wrought-iron pipe makes probably the best conduit now known for lead-covered cables. It is said by some German authorities that cement has an injurious effect on the lead; but we have as yet failed to notice any such action in this country. Stoneware or cement conduits, although they do not affect chemically the sheath of the cable, are me- chanically much weaker than the iron-pipe conduits, and on that account not so good. Several forms of Telephone Lines and Their Properties. 23 stoneware conduits, however, have met with good success, especially when the conduit is strengthened by being embedded in concrete. A conduit of this kind has been recently put down in the city of Washington, the concrete being rammed around the terra-cotta ducts as in the case of iron pipes. It is said that a length of 50 feet of this conduit will sus- tain itself unsupported without injury. In New York City considerable trouble has been experienced from the proximity of steam-heating pipes. The only precaution which can be employed is to avoid the steam-pipes whenever possible, and to use a suitable cable. Evidently, even with the most suitable material for a conduit, if water and gases are allowed to leak in from the street, the sheath of the cable is still liable to corrosion. It is necessary, therefore, that the joints in the ducts and in the man-holes should be water- and gas-tight ; and in this respect the con- duits laid in concrete are best. MAN-HOLES. Man-holes, as the name implies, are holes or chambers in the ground, breaking the continuity of the conduit ; and their purpose is to give access to the ducts for drawing in or drawing out cables, mak- ing connections, or repairing. A man-hole should be of such size that a man can work easily in it. It 24 TclLphouc Lines and Their Properties. should be water- and gas-tight, but provision should be made for drawing off any water that may collect in it. Man-holes should be not less than six feet in diameter, and of such a depth that the ducts enter three or four feet from the bottom. The best practice at present is to build them of brick, laid up in concrete cement, with a hydraulic cement bottom. The outside of the walls is coated thoroughly with cement to prevent the entrance of gas or water. The roof is either arched or supported on iron girders. Double covers are provided, the lower one having a rubber gasket at the joint, and being funnel-shaped, so that any water coming from the street will drain away from the joint. The lower cover is fastened down by a cross-bar and gun-metal bolt, while the street cover, weighing about three hundred and fifty pounds, is loose on its seat. A man-hole must be placed at every point where the conduit makes a sharp turn. It is usual to locate them, in the straight portion, at street crossings or about two hundred feet apart. The importance of keeping the man-holes, as well as the ducts, free from gas and water can hardly be exaggerated. If gases and moisture once find their way in, it is practically impossible to get them out ; and the life of the cables may be very materially shortened by their presence. Moreover, the pres- ence of illuminating gas mixed with air may be the cause of dangerous explosions. A mixture of one Telephone Lines and Their Properties. 25 part gas to eight parts of air is very explosive, and there are many ways in which it may be ignited, even in conduits containing no electric-light cables. The greatest trouble in the New York subways is due to the leakage into them of gas, with which the soil of the streets is permeated. Many methods of ventilation have been tried, with but partial success, and it is now necessary to open the man-holes once a day to allow the gas . to escape and thus guard against explosions. Many explosions have been caused by the leakage of the gas through the pipes containing electric -light wires up a pole to an arc lamp. Here the gas is ignited, and the flame runs back into the duct, causing an explosion. DRAWING IN CABLES. In order to place the cable in the finished duct it is necessary to provide some means of pulling it in at the end. In some forms of cement conduit a small cord is put through each duct as it is laid, and the cord is left in place to be used whenever it is desired to draw a cable into the duct. A rope is then at- tached to the end of the cord and drawn through, the cable being in turn attached to the end of the rope and drawn in by means of it. This method, however, was found not to be a good one, as the cord rotted in the duct, and the use of so much cord involved a 26 Telephone Lines and Their Properties. needless expense. It is now the practice to put the rope through by the process of " rodding," as it is called. RODDING. In this process a number of short rods is used, the aggregate length being sufficient to reach from one man-hole to the next. There are various forms of rods — continuous steel, spiral gteel, or short lengths of wood. The best are of cane, and can be attached to- gether by screw-and-socket joint or by a pin and slot (bayonet joint). One rod is pushed into the duct at a time, being attached to the rod ahead of it at the joint, until the other end of the duct is reached, at the next man-hole. A mandrel which just fills the duct loosely is usually placed on the end of the first rod to clear away all obstructions. A rope is then fastened to the last rod, and the whole pulled through, the rods being detached one at a time as they come out of the duct. DRAWING IN. When the rope has been drawn in it is attached to the end of the cable. This must be done with some care; for it is necessary that the strain should come principally upon the conductors and not upon the lead sheath. Moreover, there must be no bunches to catch and stick in the duct. A swivel should be provided in the rope, near the cable. The farther end Telephone Lines and Their Properties. 27 of the rope is then passed around a windlass fastened in the pavement at the edge of the man-hole, and the cable is drawn in by turning the windlass. The reel of cable should be mounted at the edge of the first man-hole, and the cable guided into the duct by a man stationed in the man-hole, taking care that the cable sheath does not run against the edge of the opening so as to abrade or injure the covering. But one cable should be drawn into a duct, and a cable should never be drawn over others. The telephone cables now in most general use are of such a size as to most economically fill a 3-inch duct, leaving room enough for the drawing in. TESTING AND MAKING JOINTS. When the cable has been drawn in, it should be cut and carefully tested. If found perfect, the con- nections are made immediately, each joint wrapped, a lead sleeve slipped over the joint, and the whole sealed perfectly tight. This is usually done by a soldered " wiped joint." If the succeeding section is not ready to be connected, the end of the cable should be sealed up. It was formerly the practice, in many cases, to run the cables direct from duct to duct, across the man- hole. This resulted in filling the duct with a con- fused mass of cables, and rendered any subsequent work in the manhole very difificult. Cables running 28 TclepJwnc Lines and Tlicir Properties. into a manhole should be led from the ducts, in order, around the sides of the manhole, supported by shelves or hooks on the wall. This leaves the centre free for work, and makes inspection or testing easy. DISTRIBUTION. The distribution of the circuits in the conduit to individual subscribers is done from poles by short overhead lines, as it would obviously be impractica- ble to run single underground wires, on account of the great cost. For the location of a distributing pole a point is selected which is as nearly as possi- ble in the centre of a group of several subscribers. A cable is run from the nearest manhole to this pole, and led up the pole, through iron pipe, to the cable- box. It is necessary to use a cable-box for two reasons : The cable must be protected from injury by lightning, and a lightning arrester must therefore be inserted in every line, between the overhead por- tion and the cable ; and the cables now universally used for telephone work must be hermetically sealed to prevent the entrance of moisture, which would gradually destroy the insulation. Various forms of cable-box have been used; but the best is that put into use by the American Telephone & Telegraph Company. This is shown in the diagram. This arrangement is very complete, and has been designed not only for the protection of the cable, but Distributing Pole, Philadelphia. Telephone Lines and Their Properties. 29 also with a view to convenience and ease of access. The box is of iron, 30 inches in diameter, and en- circles the pole. It is weather-tight, with over- lapping doors, and is pro- vided with tubes for the cable and wires, which en- ter at the top, just under the eaves. Below the box is a circular platform, 6^^ feet in diameter, sur- rounded by an iron rail- ing, and firmly supported both from the cross-arm and from the pole. Ac- cess is had to this plat- form through a trap-door in the floor, and iron steps are provided on the sup- porting rods, to reach the cross-arms above. On this platform the lineman can work with ease and quick- ness, and the results are infinitely better than when he was obliged to cling to the pole and cross-arms for support. Inside the cable-box the connections are made from the cable to the overhead wires, the arrange- 30 Telephone Lines and Their Properties. merit being as indicated in the diagram. Tiie wires in the cable having been connected to their respec- tive binding posts in the cable -head, the head is closed and filled with paraffine, the sheath being joined to the sleeve through which it en- ters the cable -head by a wiped joint of solder. The end of the cable is thus hermetically sealed. The bridle wires are covered with water- proof insulation. They should be joined to the line wires either by Mclntire sleeves or by soldering; but they must not be soldered to the hard-drawn cop- per between supports, as the heating reduces the strength of the hard-drawn copper. The bridle wires are then led along the under side of the cross-arms ^■^^\x~~\ ^nd down the pole into the cable-box. Within the cable- box the bridle wire is led first to the "plate " arrester, which is connected to earth ; then to the fusible coil arrester, which is connected to the proper binding-post in the cable- head. This plan admits of ready change in the ar- Telephone Lines and Their Properties. 31 rangement of circuits when desired, by changing the cross-connections in the cable-box. " From the cable fixture bridle wires, not less than .0403 inch in diameter, insulated to ^^j- of an inch with the best quality of waterproof insulation, ex- tended through cleats on the under side of the cross- arm to wooden ducts fastened to the back of the cross-arms, or through properly constructed cables, should lead to the cable-box or tower. The bridle wires should be joined to the line wires either by Mclntire sleeves or by soldering. Care must be taken not to solder to the line wire between sup- ports, as heating the hard-drawn copper anneals it and reduces its tensile strength. " Inclosing the cable terminal, we have used a dust- and moisture-proof iron cable tower, cylindrical in shape, built completely round the pole, and supplied with a suitable platform and railing. If a wooden box is used, it should be made of i^ inch pine, tongued and grooved, having double doors hinged at the top and opening together. Its interior dimen- sions should be sufficient to give a space ample for placing the cable head, lightning arresters, and bridle wires. The box should be well oiled on its interior and painted thoroughly on the outside before plac- ing in position, the joints being white-leaded before the box is put together. The terminal tower or box is to be locked, and it is recommended that a stand- ard padlock be adopted and used for this purpose. In this tower or box the bridle wires are connected to the plate and fusible arresters. To protect the cable against heavy electrical discharges, we provide at the exterior terminal fusible coils and properly constructed plate or pointed lightning arresters. The fusible coil at this point is so designed that it burns out with a current of one ampere, thus having 32 Telephone Lines and Their Properties. twice the carrying capacity of the central office ar- rester ; this protects the cables and pre\'ents a large number of burn-outs at points remote from the office, locating most of them at the office terminal, where they may be quickly repaired. The outside wires ex- tend first to the plate arrester, which discharges the lightning to ground at the cable pole and prevents the burning out of many fusible coils. The ground plates upon the arresters should be connected to moist earth by means of conductors run independ- ently of the cable sheath, and having three times the conductivity of the line wire. For this purpose three No. 12 hard-drawn copper wires twisted together may be used. The arresters should be connected to the cable heads by waterproof insulated wire. " Tlie binding posts on the cable terminals should be arranged in pairs, so that the two wires of a pair will be exactly opposite each other horizontally. If a form of cable is used not requiring a permanent head, it may be connected directly to the lightning- arrester binding posts in the order recommended above. We recommend, especially at the cable head, that nothing but lock-nut binding posts should be used."* It is probable that lightning arresters consisting of /°\ (^^ plates with straight edges would be found \^ (^ more efficient than the comb arresters now in general use. CABLES, f The cables now used for underground telephone lines are, in this country at least, of uniform pattern, * From " A New Era in Telephony," Carty, Pickernell and Hib- bard. f See also chapter on Cables, page 217. North River Cable Terminal, New York End. Telephone Lines and Their Properties. 33 although the material of which they are made de- pends upon the manufacturers. The early cables used consisted simply of insulated wires grouped together and covered with a protecting sheath of metal. The wire was of any size that chanced to suit the ideas of the maker, usually very small, with a very thin insulation; the main object being apparently to get as many conductors as possible into the smallest pos- sible space. Through successive stages of develop- ment the wire and its insulation both grew. The difficulties due to induction gave rise to numerous devices, "induction killers" and "anti-induction cables," none of which were practically successful. The only sure method of preventing this trouble consisted in twisting the two wires of a circuit about each other ; and this method has now been univer- sally adopted. INSULATION. The insulation in telephone cables now consists universally of some fibrous material such as cotton, jute, or paper. The rubber compounds have been used to some extent, but have been found, on the whole, inferior to fibre, although possessing some ad- vantages. In a cable with rubber insulation it is not necessary to seal' up the ends, as the insulation is it- self waterproof, nor is a metallic sheath necessary, except for mechanical protection. But the disad- vantages of a material of this nature are insurmount- 3 34 Telephone Lines and Their Properties. able, practically. The rubber is likely to deteriorate with age, and admit moisture, or it softens with heat and allows the wire to become displaced from the centre.* Worst of all, its specific inductive capacity is so high that it is, on that account alone, unfit for use in telephone cables. Fibre is cheap, per- manent when protected from moisture, and it cannot spread, so that permanent centring of conductors is assured. The present uniformity and excellence in tele- phone cables is due in great measure to the fact that each year the representatives of the leading tele- phone companies of the country meet and discuss all matters connected with telephony. After much dis- cussion in regard to cables, specifications have been fixed upon known as the " Conference standard." These have been modified somewhat in minor details from year to year, but the main features are likely to remain unchanged for some time. The dimensions of " Conference standard " twisted-pair cables are as follows : Diameter of conductor, .035 inch. Outside diameter of insulation, . 125 inch. Length of twist, 2} inches to 3J inches. Maximum outside diameter of cable, 2J inches. Thickness of lead sheath, -J- inch. *Dr. W. \V. Jacques says : "Where cables are subject to large variations of temperature, say 50 to 150 degrees, iiidia rubber or gutta-percha, or any of their compounds, undergo very rapid de- terioration." Telephone Lines and Their Properties. 35 The lead sheath is coated with asphaltic paint, then with a braided jacket or with tape, and this jacket is also saturated with asphaltic paint. The outside is then thoroughly coated with powdered soapstone, so that it may be easily drawn into the duct. As it was found that a sheath composed of pure lead was rapidly destroyed when placed underground, the lead was alloyed with tin, which gives very good results. The alloy is harder than lead and resists destruction very much better.* The percentage of tin which can be successfully alloyed with lead is limited. The specifications now most generally fol- lowed, for armor, are as follows : Lead, 97 per cent. ; tin, 3 per cent. For if- inch pipe, 3S pounds per foot li 3 If 2.7s li 2.5 li 2.25 I 2.00 * 1-75 f 1-5 4 1. 00 The desired number of circuits, or " pairs," as they are called, is made up into a cable, not promis- cuously, but in concentric layers, each layer having a slight twist, spirally, in the direction opposite to the direction of twist of the adjacent layers. This * See page 21 on destruction of sheath. 36 Telephone Lines aud Their Properties. twist of the layers gives flexibility to the cable. The conductors are of the best Lake Superior copper, and the "Conference" specifications call for a con- ductivity of not less than 9S per cent, of pure copper. These dimensions have been determined from con- sideration of all the known conditions of trans- mitting efficiency, available space, and expense ; and they undoubtedly give the best cable that has yet been in use. CHAPTER III. "LONG DISTANCE" LINES. The first lines connecting cities were constructed on the same plan as the telegraph lines, that is, they were of iron wire and were grounded circuits. Less attention, too, was paid to the material and method of construction than there should have been. The spans were long, the poles usually not high enough, nor strong enough, many poorly set, and with de- fective guying. Clearly, with such lines as these, it would be impossible to talk to any great distance, except in the most favorable weather and under the best conditions of apparatus. When, therefore, the first long distance line, a portion of the present line between Boston and New York, was projected, it was recognized that only the best that could be built would be satisfactory. The results have more than justified the large expenditure, and comparing the Boston and New York line with a City Exchange line, each with its apparatus, the former is under air conditions far superior. The more recent trunk lines between cities, or what are commonly known as the " Long Distance " lines, 38 Telephone Lines and Their Properties. erected by the American Telephone & Telegraph Co., are of tlie most substantial construction. The number of wires carried on a long distance line varies from twenty, or occasionally less, up to one hundred ; but the most economical and best line to build and operate is a line of fifty wires on fifty- foot poles. The cost of such a line, complete, is in the vicinity of $3,500 per mile. POLES. The poles are of white cedar, or chestnut, obtained principally from Canada, and carefully selected. They must be alive and sound when cut ; and al- though the company's specifications call for " 'live, green cedar wood," it is most necessary that all wood used, whether for poles or cross-arms, should be thoroughly seasoned. If the wood is painted or set into the ground when green, with the sap in it, dry rot is sure to set in, and the pole, although pre- senting a fair exterior, may have entirely lost its structure and its strength. If the precaution of sea- soning has been neglected in the case of any consid- erable number of poles, the weakest will some day fall from some cause usually insignificant, and its fall may bring down several miles of line. The poles must be not less than seven inches in diameter at the top — for curves, eight inches — and of reasonable diameter at the butt. They are squared Heavy Pole Line Construction, West Street, New Vokk. Telephone Lines and Their Properties. 39 at both ends, and the top afterward cut to a wedge- shape. Every pole is stripped and the knots cut close, and the gains for the cross-arms cut before set- ting. They are subject to inspection before being accepted, and if these requirements are not complied with, and if they are not reasonably straight and of proper proportions, they are rejected. The standard 'cross-country line now consists of thirty-five foot poles, having an ultimate capacity for four cross-arms, each carrying ten wires. These are not all put in place when the line is first built, but all four gains are cut in each pole, so that the addi- tional cross-arms can be put on when they are need- ed. In actual lines, this standard height of thirty- five feet is a minimum. Obstacles are frequently met with in running lines along highways, and these obstacles must be cleared by the lines running above and not beneath, the lowest wires clearing by not less than four feet. As a matter of fact, therefore, the poles vary in height from thirty-five to one hun- dred feet. In consequence of the rule of the Ameri- can Telephone & Telegraph Co. that their lines must run above obstructions, seventy-foot poles are not un- common and there is a short length of line where the poles reach the great height of one hundred feet. The minimum height of pole for long distance lines was formerly fixed at forty feet ; but it was found that this height, when the pole was heavily loaded, sometimes caused an excessive strain at the base. 40 Telephone Lines and Tlicir Properties. CROSS-ARMS AND PINS. The standard cross-arm is of Norway pine — some- times of juniper — free from sapwood and bad knots. It is ten feet long and 2% x 4j{ inches in section. 1 1 i ■■\'t' II rf iji r^-i !; :; il' 1 )4 1 o Cr'i* O oi 1 1 ° f o o ° 1 It is bored as shown in the diagram and painted with two coats of metallic paint before delivery. The cross-arms are set twenty-four inches apart on centres, the upper arm being ten inches from the top of the pole. Each arm is fastened to the pole by a single galvanized iron bolt, and is braced to the pole with two iron straps, also galvanized. The braces are If 28" — ^ -^ I ;. 26 held to the pole by a fetter drive bolt, five inches long, passed through both straps and driven (-— , into the pole ; and to the cross-arms by car- T riage bolts. All this iron work is well gal- : vanized, and must stand the test of four = dippings into sulphate of copper solution v without removing the zinc. The method of bracing is shown in the diagram. Telephotie Lines and Their Properties. 41 CIS: m ivfwv ^=3 The pins are of the best lo- cust, ij^ inch in diameter. They are set twelve inches apart, except the two nearest the pole, which are set sixteen inches apart, for clearance. Each pin is held in place by a nail driven through from the side of the arm. The insulator used is that shown in a subsequent chapter. It is of white glass and rather small. It has been the practice to provide every fortieth , 8 § a g 9 r 1 8 • 8 a s a ^ f -S § a § a. & 8 § a !L ^ a 8 a 9 1^ 8 a § a_ _a a 8 M 9 8 8 8 8 3-- B a 8 s s .J a_ tL j^,, . " . ./» SCALE t" TO I FOOT 42 Telephone Lines and Their Properties. pole with a double set of cross-arms and insulators, on opposite sides of the pole ; and every twentieth pole has the alternate cross-arms duplicated in the same way. This is for the purpose of making the transpositions.* Latterly, however, a new form of insulator, devised by Mr. tlibbard, of the American Telephone & Telegraph Co., has been used, which makes the duplication of cross-arms unnecessary. The pins for the transposition insulator are substantially the same as the standard pin, the thread, however, being cut farther down toward the base. SETTING POLES. On straight lines, all poles are set into the ground six feet. On curves they are set deeper than that, usually six and one-half to seven feet. The holes are dug large enough to set the poles in easily, and the earth is tamped hard around the butt while it is slowly shoveled in. When the ground is too soft to hold the pole properly, artificial foundations are resorted to. One of the simplest methods consists in filling about the pole with a grouting of cement, sand, and broken stone, mixed in the proportion of one part cement to two * See page 206. Telephone Lines and Their Properties. 43 parts sand and five parts stone. A thin layer of grout should be placed in the bottom of the hole, and the butt of the pole, resting on two crossed planks, placed upon this foundation. The space is then filled in with grout. ,8 ft When the ground is very bad the pole maybe sup- ported by braces from a platform built of plank, pil- ing being used when necessary. The plank braces are all fastened by standard cross-arm bolts. When piling is used, the heads of the piles should be cut to receive the ends of the braces, as shown in the detail. 44 Telephone Lines and Their Properties. On straight lines, poles are spaced one hundred and thirty feet apart, as nearly as obstructions will allow, and the cross-arms are set alternately facing each other and back to back. When the span is very long, however — say two hundred feet — the cross-arms on the poles at each end are so set that the pull on the long section will set the arms against the poles. The same practice is followed at the terminals of lines, the cross-arms on the last two poles being placed on the side away from the line; and on curves the arms must always be placed on the sides of the poles facing the middle of the curve. The conditions met in building a 'cross-country lin£ are in many respects different from those obtain- ing on lines built along a railroad or in a city. In selecting poles for a 'cross-country line, and in deter- mining what height of pole to use at a given point, the conformation of the country must be considered, and the height of pole adapted to it, so that the tops of the poles shall be as nearly even as possible. This gives an approximately even strain on all the poles. If this is not done there may be at some points an upward pull on the poles, which will result Telephone Lines and Their Properties. 45 in pulling off the insulators, as at A in the diagram below, where the dotted line shows the position the wires would naturally assume. Care must be taken that the poles be not given too great an inclination on curves; for in such a case, when the wires are drawn up, the tops of the poles are bent over toward the outside of the curve. This, instead of relieving the pole, as it would if the inclination were too slight, buckles it, and tends to concentrate the strain at one point, so that it is apt to break. Cedar poles, in breaking, do not split and make a long break, but are apt to snap off short ; and it has been found that the points at which this break is most likely to occur are at the butt, just above the ground, and at a point just under the cross-arms. In fact, cases have been known where the pole has snapped in both places at once. It is now considered the best practice to set all poles perpendicularly and to provide against any side strain by guying. GUYS. Hard drawn steel or silicon or aluminum bronze should be used for guys. Soft iron has been used 46 Telephone Lines and Their Properties. for this purpose to a considerable extent ; but soft iron stretches so readily that a strain is soon put upon the pole and the guy does not fulfil its purpose. Bronze wire-rope would be a very desirable material, the only objection to its use being its high cost.* In the use of hard steel, considerable care must be exercised in making joints. It is in many cases very brittle, and a sharp bend, if made carelessly, will cause it to break off short. Care in inspection and manipulation are the only safeguards against this difficulty. Guys should always be in the form of cable, and the steel should be thoroughly galvanized. The cable or rope should consist of not less than seven strands, each 0. 109 inch in diameter. The wire should be free from flaws and scales, sand or splin- ters. It should withstand a pull of nearly five times its own weight per mile without breaking, and should take not less than fifteen twists in a length of six inches. The seven strands of the rope should be laid up with a twist not more than three and one-half inches in length. This insures the necessary flexibility. An equal weight of a smaller wire, with a shorter " lay," would give a better rope, although a more expensive one. The rope is required to be furnished in coils weigh- ing between 140 and 160 pounds. * For the properties of bronze wire, see the next chapter. | Telephone Lines and Their Properties. 47 The guy rope is attached to a rod, eight feet long, having a nut and washer at its lower end. The mmBiOiiigi lower end of this rod is passed through a log which serves as an anchor. In attaching the guy rope to the pole it is passed twice around the pole and fastened by means of the guy clamp, of malleable iron. This clamp has two grooves running through it, to take the rope, and is tightened by means of three bolts. The rope is held from slipping on the pole by staples driven down upon it. \ ■ ■ c : : (:) > — \ No general rule can be laid down in regard to the method of guying to be employed. The particular guy to be used in any case depends entirely upon cir- cumstances, and must be determined specifically for each case. The head guy runs from just above the upper cross- arm to the base of the next pole, or to the regular 48 Telephone Lines and Their Properties. anchor guy-rod, and takes a strain in line with the wires. The side-guy runs, in the same manner, at right angles to the line, and takes the side pull. The cut shows a pole both side- and head-guyed. The Y-guy is a very useful form, and it is almost always advisable to use this form when the pole carries more than two or three cross-arms. In such a case the use of a single guy is likely to result in the bending over of the top of the pole in one or the other direction, as shown. Guy stubs are used wherever it is necessary to raise Telephone Lines and Their Properties. 4g the guy to a sufficient height to clear an obstacle or to prevent the obstruction of a thoroughfare. The method of attaching guys to the stub is shown in the diagram. The anchor -guy is attached to the stub three inches below the pole -guy, and both guys are held in place by staples, in the regular manner. The pole brace is used only where it is impossible to use a guy. The brace is held to the pole by three wraps of No. 6 iron wire and by two nails at the top. The wire is prevented from slip- ping up on the brace by a five-inch fetter drive-bolt. The anchor is a log five feet long, buried six feet below the sur- face ; and the brace is fastened to it by a cross-arm bolt. 50 TelepJione Lines and Their Properties. The best practice in guying is outlined below: All poles having a side strain are guyed against that strain wherever possible. Terminal poles are head-guyed, and, if possible, side-guyed in both directions ; and an additional pole is set within seventy-five feet of each terminal pole and head-guyed to it. This applies also to the entrance to a curve. Road crossings are made at an angle of forty-five degrees, as nearly as may be, the pole at which the turn is made being head-guyed, and having also, if it can be located, a side guy to take the strain of the crossing. In every case of road-crossing the addi- tional pole, seventy-five feet away, is head-guyed to the base of the turning pole. When neither head nor side anchor guys can be located, an additional head -guy is run from the third pole to the base of the second. Two methods of guying at a street corner have been shown in the first chapter. Another method for a sharp turn is shown in the diagram. As with every pole terminating a straight line, the additional pole is head-guyed to Telephone Lines and Their Properties. Si the corner pole. The cross-guys are best run through dead-eyes to equalize the strain, and each corner pole is thus guyed to two anchors. It should be noted that, of each of these double guys, one part is in the direction of the pole-line and the other part is to- ward the centre of the curve. On every curve the side-guys should pull directly away from the centre, and sufficient head-guys should be used to take the whole strain of the line. Even in straight lines, guys are placed at regular intervals. A line carrying between ten and twenty wires is double head-guyed and double side-guyed at intervals of one mile; and lines carrying more wires than that are guyed in the same manner at propor- tionately frequent intervals. 52 Telephone Lines and Their Properties. In hilly country, head-guys should be so placed as to take the downward strain due to the weight of the line. STRINGING. The stringing of the wire for long-distance lines has now been reduced to such a simple method that the operation is a very rapid one. The poles for a distance of one-half to three-quarters of a mile have been set, the cross-arms and insulators are in place. A rope, called the " running rope," is then carried along over the cross-arms for the whole distance, and attached at one end to a board known as the " run- ning board." This running board is roughly shown in the diagram. At this end of the line the reels containing the wires are mount- ___^_^^ t Running Rope. ed, and the wires, I lA .a ,a ,A -J^ each on its sep- 1 I I ' I arate reel, are attached to the running board at A, A, A. When the wires are all made fast to the running board in the manner described, a team of horses is hitched to the other end of the running rope, and they "walk away" with it, pulling the running board and attached wires over the cross- arms. The wires are guided in passing by linemen stationed on each pole. In stringing wires over the lower cross-arms a divided running board is used, one half passing each side of the pole. Telephone Lines and Their Properties. 53 When the wires have been drawn over, they are pulled up tight. This was formerly done by means of the " banjo." The " ban- jo " consists of a wooden drum fastened upon a kite- shaped board. The wire is wound around the . drum, and a small double \ jm '((f block and fall is attached at T. Each wire in turn is drawn tight, and at a given signal is tied on all the poles at once by the linemen stationed there. An improvement on this method consists in the use of a clamp to grip the wire while it is pulled up. This clamp operates on the prin- ciple shown in the illustration. The wire is gripped between the jaws, which are so shaped as not to injure it, and the strain on the chain forces the arm more into line with the wire. This turns the cam and closes the jaws the more tightly, so that the grip on the wire increases with the pull on the chain. While the wire is held by the tackle, a clamp is put on it at the last pole and is fastened to the cross- arm, holding the wire until the next length beyond is pulled up and connected to it. The neglect of this precaution used to result in making kinks in the wire at each insulator. For when the tackle was taken off 54 Telephone Lines and Their Properties. after the wire was tied, the strain would cause the tie to slip around somewhat on the glass, and a little sharp bend would be made in the line wire just back of each insulator. This weakened the wire. The tie is made with wire of the same size and material as that used for the line, and is shown in the sketch. By the it is easy to put on all the wires, dip.* As many method described the same strain giving a uniform as ten wires can be pulled over at in the line wire means of the once. All joints are made by Mclntire sleeve. * The dip and pull for a given span can be readily calculated bj the formulae given below : Copper wire, 1 74 pounds per mile. 870 40 poles to the mile, dip (inches) 43 poles to the mile, dip (inches) = 45 poles to the mile, dip (inches) = 53 poles to the mile, dip (inches) = Pull (lbs.) 756 Pull (lbs.) 684 Pull (lbs.) 500 Pull (Ibs.)^ ■ Approximate. Telephone Lines and Their Properties. 55 The location of the wires on the cross-arm is shown in the diagrams. On the straight line, all wires are placed on the inner side of the pins, ex- X = one-half the span (in feet). y = dip (in feet). S = one-half the length of vdre (feet). jFf = horizontal pull (in pounds). ■U) = weight of wire per foot (in pounds). m/ '- y — —le^ + e —le" — e "1 + -rr + X ■— + m '■\L This is the equation of the Catenary, which is the curve in which the wire hangs. L Where « = — ■w-i ml X = -( IH 2\ m + I- x^ \ ■ — . . . .\— m. m'\2 ) ml x^ \ Terms of degree higher than the second m4y be neglected when the conditions are such as are met in practice. 2m 2H For copper wire 0.104 inch diam., weighing 174 pounds per mile : y = .0165 — ; y (inches) = 0.2 --. 56 Teleplwne Lines and Their Properties. cepting the two inner wires, which are placed outside for greater clearance of the pole. 1 c « e c P (*ns)e I D 3 n 1 m^ On curves, the wires are so placed that the strain forces them against the insulator, and no unusual duty is required of the tie wire. Where a wire is attached to the last insulator on the line — a " dead end," as it is called — for the pur- pose of connecting ^ssm to a cable or to a subscriber's instru- ment, the attach- ment is made in the manner shown. A single loop is made about the in- sulator, the end given eight close wraps about the line wire, and eight inches left pro- jecting downward for making the connection. Telephone Lines and Their Properties. 57 The older method of making transpositions is shown in the diagram, which gives the simplest case. The line wires are terminated at their insulators and the crosses made with smaller insulated copper wire, as shown. The modern practice is a great improvement on this. Mclntire sleeves only are used for connections. '*^ The w ires insulators as the cross- with regular s u lat i o n. are looped about their in dead - ending, and connections are made line wire, without in- The standard transpo- *<3E>- sition is shown above. Half-sleeves are used for the short connections. For transposition of the wires next the pole, the plan shown above will not do ; for the cross-connec- tions would pass through the pole. The plan is there- fore modified, the ends being bent around outside the insulators and the connecting wires crossed behind the pole. 58 Telephone Lines and Their Properties. When long distance lines enter a city, the pole lines are usually terminated near the city limits, the connections being made to underground cables in the manner already described.* * See page 28 et seq. CHAPTER IV. WIRE. The wire used in the construction of long distance lines is of two kinds, viz., hard-drawn copper for the lines themselves, and steel for the guys. In the manufacture, the treatment of the metal, whether iron or copper, differs but little in the early stages of the process. The " bloom " or lump of metal taken from the furnace, is in each case about two feet long, and has a section of about sixteen square inches. The bloom of iron weighs about 135 pounds, and that of copper from 150 to 200 pounds. More than about 200 pounds cannot be advantage- ously handled in one piece. Having been heated in a gas furnace to the proper temperature, it is " roughed " in the first train of rolls, which reduces it to a rod about one square inch in section. It is then passed automatically through guides into the " inter- mediate train," which reduces it still further, with a corresponding increase in length. It is then passed into the finishing train, which rolls it alternately square and oval, and leaves it a cylindrical rod about O. 2 inch diameter. The finishing train of rolls is usually of the type known as the " Belgian," which has all the rolls on 6o Telephone Lines and Tluir Properties. one shaft. This avoids gearing and its accompany- ing waste of power, which in such work is always great. A disadvantage attaches to this method, how- ever, which in some measure compensates for its ad- vantages. A greater length of wire is necessarily exposed between the passes than where the rolls are successive and connected by gears, and consequently the "scaling" is greater with the Belgian rolls than in the older method. The wire left by the finishing train is drawn down through dies to the required size, the dies for the larger sizes being of chilled iron, and for the smaller of crucible steel. The iron dies, when slightly worn, are reamed out to the next larger size. The steel dies are hammered on the face, thus contracting the hole, and then reamed back to the same size. Another method of wire-making, which has not yet been very extensively used, is to roll the rods cold between successive flat rolls so placed as to give the wire a polygonal cross-section. By a proper adjust- ment of the rolls, however, the sides of the bound- ing polygon are made so many, and each so short, that the wire is practically cylindrical. Although the breaking strength of such wire is about the same as that of uninjured hard-drawn copper, it is more homogeneous in structure, and less liable to loss of strength from an injury to its surface than the hard- drawn wire, which loses very greatly in strength if scratched or nicked. It is characteristic of the hard- Telephone Lines and Their Properties, 6i drawn wire, also, that if bent slowly at a sharp angle a point is soon reached at which the strength of the outer skin is exceeded, and the wire suddenly gives way. This appears not to be the case with the cold rolled wire, which presents an approximately uni- form resistance to bending through any angle. This wire is very free from splinters. The hard-drawn copper wire now used is 0.104 inch in diameter. Smaller wire was first used, but it proved less satisfactory, and a wire o. 104 inch in diameter was determined upon as the most advantageous size to use. Tests of this wire, with regard to both its mechanical and its electrical properties, are made by the manufacturers during the process of manufacture as well as after its completion. In addition to these tests, however, the wire is tested by the telephone companies before acceptance, as it must test in ac- cordance with certain specifications which are in part as follows : Conductivity 97 per cent, that of pure copper. Tensile strength, 550 pounds. Elongation in 5 feet, i per cent. The finished wire is made up into rolls, or coils, each consisting of about three-fourths of a mile of wire, drawn in one continuous length and without factory joints. A wire inspector, appointed by the telephone company, visits the factory as each lot of wire is completed, and carefully examines these coils, making sure that the wire is free from rough places 62 Telephone Lines and Their Properties. and splinters.* If the wire is imperfect in this respect, or does not come up to the requirements named above, the lot is rejected. The coils are then pulled over, and from each coil that the inspector designates, a sample length is cut, about forty feet in length, to be tested for its mechanical and electrical properties. It is usual to take a specimen from one coil out of every ten, and on each specimen three sets of meas- urements are made, viz. : the maximum load (usu- ally called breaking-weight), weight, and conduc- tivity. From thcpC measurements all the other prop- erties of the wire are obtained by calculation. In the test for breaking-weight a length of about five feet is clamped at the ends, and the strain on the wire slowly increased to the breaking-point, the measurements being made by weights. The elonga- tion of the wire before breaking is also obtained from this test in the following way : An initial strain of fifty pounds is put on the wire, to straighten it out, and the length of the wire carefully measured. The wire is then broken, as described, and the dis- tance between the end clamps again measured. The difference between the two measurements is the amount of elongation. In practice this measure- ment is made automatically, so that all error arising from the play of the clamps is eliminated, and the distance between clamps is known at any instant. * It is not safe to handle wire which is full of splinters, as they tear the hands of the lineman, and the copper is poisonous. Telephone Lines and Their Properties. &i The strength of the wire for torsion is measured by twisting a length of six inches and counting the number of turns before the wire breaks. These tests are made by power on machines especially con- structed for the purpose, and can be performed very accurately and rapidly. The diameter of the wire is obtained from a knowledge of the specific gravity, by weighing a given length. The length used is, for convenience, generally 17.6 feet, or ^^ of a mile, and this meas- urement gives also the weight per mile. The method of testing for conductivity and resist- ance per mile is not new and is well known, but the apparatus * now used for this purpose is not well known, and is so simple as to merit description. --® Galvanometer "sT .^"WmE Under Test "IOC 1. 5 is a standard wire, whose conductivity has been carefully determined, once for all, and is exactly 17.6 feet long between contacts. This standard wire is left in position permanently, and never disturbed. B is a heavy bar of metal into which are set a num- ber of massive binding posts, with corresponding posts in brass blocks at C C C", etc. The blocks C C C" etc., are provided with plug connections with a metal bar in connection with a battery. 1/ is a * Devised by Mr. A. C. White, of the Amer. Bell Tel. Co. 64 Telcplwne Lines and Their Properties. strip of wood, and has fastened permanently upon it a metal scale graduated in distances (feet and tenths, etc.) from the edge of the binding posts on B. //can be moved vertically between guides and is provided with a horizontally sliding contact having a pointer which moves over the graduated scale. R and R' are re- sistances, in this case about 1,000 ohms each, and equal. The wires whose conductivities are to be meas- ured are drawn tight between the binding posts, C C C", etc., and the posts in B. Each is in turn plugged into connection with the battery, H moved under the wire, and the sliding contact adjusted until no deflection is obtained on the galvanometer. The reading of the scale is then taken, and from that and the weight of the wire its conductivity can be calculated. In practice, tables or curves are used, rendering the operation short and simple.* As the * The necessary formulae for this calculation and for the construc- tion of the curves and tables referred to are given lielow : To determine the conductivity of the standard yi'we (or any vine), r = resistance in B. A. ohms of 17.60 feet or -uiu mile, corrected to a temperature of 20° C. or 68° F. w = weight of 17.60 feet of wire in grammes. -^>^ ^ Dro^ one side of the line, in series, the drop is last, as shown, and the rings come into the other side of the Telephone Lines and Their Properties. 87 line. The drop is thus cut out whenever a connec- tion is made, and for making connections a cord is provided, double, with a plug at each end. In this METALLrC Circuit to Subscriber Operator Complete SET(-DlfTEnENTIAL Winding. RCTARDATION COILo Metallic Circuit Board. (Line and Test Twisted.) board, however, a leg was left open beyond the jack in use ; and it is now considered better to use a form of board where each side of the line passes through jacks, and in which a double cord with twin plugs is used. This insures good rubbing contact for each side of the line, is about as easy to handle, and all beyond the jacks is cut out. In a metallic circuit multiple board the wires lead- ing from the distributing board must be twisted in pairs, to avoid cross-talk; and indeed this should be done also in the case of the multiple board for 88 Telcplionc Lines and Their Properties. grounded circuits. Both wires of the pair should be used, the second wire never beiiig left open, but grounded at the farther and of the pair, even if this should be at the end ofta long cable. In this way cross-talk and other disturbances, both in the board and outside of it, are completely avoided, or very much diminished. It is now recommended that in the larger ex- changes a multiple switchboard should be placed, adapted for metallic circuits. This should be, if ^ _ _ _ _ It *"—"■• V V Y V^ er q cosh — bei q sin St). /■2.N q = 2t!'A/ a = 27r ATi. berq= i - -^ -^ -I- 2« ■ 42 ■ 6= bn q = ~^- 2' 2' • 4' ■ b" When r ■= a, p ■= q. R{N) I be rp ■ bei' p — bet p • ber' p 'R\S) ~ 2^ ■ [per' pf + (bei pf Accents denote differential co-efficients. In cable, where the dis- tance between wires is not great in comparison with the diameter of wire, this formula does not hold strictly. Ii8 Telephone Lines and Their Properties. Some experiments in this direction were tried by the author, which go far to prove that the sharpness of the wave is not impaired by simple resistance, and that sound may be transmitted electrically through immensely high resistance, provided no other hurtful factors are introduced. In these experiments sound was transmitted, per- fectly sharp and clear, through a resistance measur- ing three megohms. This resistance consisted of a pencil mark upon paper, and possessed therefore, practically, no self-induction. As a source of sound, both a tuning-fork and the voice were used ; and it was abundantly proved by further experiment that the observed effect was due to real transmission, and not to electrostatic action. Smaller resistances were also tried ; and there is scarcely rooin for doubt that speech can be clearly transinitted and readily under- stood through several hundred thousand ohms of simple resistance* without self-induction. • In order to avoid confusion of terms, the effective resistance, or apparent resistance, offered by a conductor to tlie passage of a current of a given period is called its impedance for that period. The term conductance, meaning the reciprocal of the resistance ( -V will also be used. Conductivity has heretofore had \vio meanings ; first, the same as conductance ; second, the ability of any given conductor to conduct electric currents, as compared vifith some standard material, such as pure copper. Resistance., as used hereafter, will mean only what it has usually meant — the obstruction offered by a conductor to the passage of a steady current ; conductance is the reciprocal of the resistance ; impe- dance is the obstruction offered to the passage of the current under Telephone Lines and Their Properties. iig SELF-INDUCTION. In order to consider understanding!/ the effects of self-induction we must investigate tiie actions wliich take place when a current flows in a wire. When a current is flowing in a wire, there exist always, in the space about the wire, rings or loops of magnetic induction. When the current increases, these rings or loops of induction are expanded, fresh loops being " shed off," so to speak, from the wire ; and when the current decreases, the loops of induction are con- tracted in upon the wire, and vanish at the centre. If, therefore, an alternating current is flowing in the wire, the rings contract to the centre, one after an- other, then expand from the centre with the direc- tion reversed. We may consider, in a sense, that at the centre they are turned inside out, and expanded again. As stated by Faraday, " If the magnetic induction through any circuit be varied by any means, an elec- tro-motive force is set up in that circuit proportional at any instant to the rate of change of the magnetic induction at that instant." * consideration ; and conductivity is the ability, as compared with that of a standard material, to conduct steady currents. * If TV is the number of lines of induction passing through a circuit, any small movement for time dt, which changes M by dN, will start in dN motion a quantity dg. If R is the resistance of the circuit, — = dq. I20 Telephone Lines and Their Properties. This electro-motive force of induction is opposed in direction to the electro-motive force impressed on the circuit. The current, in increasing, causes more lines of magnetic induction to be inserted in its cir- cuit, and thus creates, during the period of increase, an electro-motive force equal, at any instant, to its own rate of increase, and opposed in direction to the electro-motive force which is impelling the current. Any circuit traversed by a current encloses within the space bounded by the conductor a certain num- ber of lines of induction, the number depending upon the strength of the current ; and if we assume the circuit to be wholly removed from all other cur- rents and magnets, the total inductance (number of lines of induction) through the circuit, for a unit cur- rent flowing in it, is called the co-efficient of self-in- duction. The lines of induction through any circuit in which current is flowing, are always closed upon themselves, and form rings or loops. The exact form of the loop depends upon what is going on in the space outside the circuit, and upon the magnetic If E is Ihe average electro-motive force during the movement, and C the average current during dt, C dt = dq. Substituting, By Ohm's law, Cdt = dN CR = ^E. ■. Edt = dN. dN _ dt ~ --E. Telephone Lines and Their Properties. 121 properties of the medium through which it passes; but every loop of induction must surround the cur- rent which caused its existence. If, then, we imag- ine a second circuit placed near the first, some of these loops, in passing around through space, will be threaded through the area enclosed by the second circuit. Hence, if any change takes place in the cur- rent flowing in the first circuit, it will cause contrac- tion or expansion of the loops of induction, and therefore a change in the number of lines of induc- tion passing through the second circuit. This will give rise to an electro-motive force of induction in the second circuit, opposed in direction to the elec- tro-motive force in the first circuit, and equal to the rate of change in the number of lines of induction passing through the second ; and this will give rise to a current in the second circuit. This phenomenon is called mutual induction. If, now, we go a step farther, and imagine that an electro-motive force is originally impressed on the first circuit, and another electro-motive force is im- pressed on the second circuit, this condition of affairs will exist : Let us call the first and second circuits A and B, respectively. The circuit A will have its sys- tem of loops of induction, some of which pass through the circuit B, and the circuit B will have its system of loops of induction, some of which pass through the circuit A. There will then be a bundle of loops of induction which are linked with both A and B ; and 122 Telephone Lines and TJieir Properties. the number of lines of induction so linked will de- pend on the current flowing in each circuit, and the position of one circuit with respect to the other. If we assume the two circuits to be wholly removed from all other currents and magnetic material, and unit current passing in each, the number of loops of induction linked with both circuits is the co-efficient of mutual induction for those circuits in their given relative positions.* Evidently, the co-efificient of mutual induction will change with every change in the relative positions of the two circuits. If one circuit is in the plane con- taining a loop of induction due to the other circuit, then no lines belonging to the A system will pass through B, and no lines of the B system will pass through A. In this case the co-efificient of mutual induction is zero, and a change in the current in one circuit will have no effect upon the other. We thus see how we can represent the effect of one current upon another, by constructing the system of lines of induction due to each, and seeing what lines pass through both circuits. If we add a third circuit, C, with its system of lines of induction, some •We use the terms "lines of induction" and "loops of induc- tion " merely as a help to the imagination in forming conceptions of the state of things we are considering. It must not be understood that they are actual lines ; but, by speaking of the number of lines through a given surface, we are able to see a clear and actual image of the intensity of the inductance through that surface. In a similar sense we speak of lines of force of any other kind. Telephone Lines and Their Properties. 123 of the lines of the C system will pass through A and some through B ; some of the lines of the B system will pass through A and some through C; and some of the lines of the A system will pass through B and some through C. We have therefore a new relation depending upon the positions of A, B, and C with respect to each other, and upon the current flowing in each circuit. In the same way, we could make up a resultant system of as many circuits as we chose. Let us assume that we have a large num- ber of such circuits, all alike in form and dimensions, but a considerable distance apart. Then the number of lines of induction of one system, which is linked with each of the other circuits, will be very small. If we now move the circuits into new positions, nearer together, the number of lines linking the dif- ferent circuits together will be increased. We may thus approach the circuits nearer and nearer, the strength of the links continually increasing, until the different circuits come together and form one circuit, the conductor having a cross-sectional area equal to the sum of the cross-sectional areas of the elemental circuits of which it is composed, and the current flowing in it being equal to the sum of the elemental currents. Evidently, in this case, all the lines of each elemental system will be linked with every other system, and we see, therefore, that the self- induction of a circuit may be regarded as the mutual induction of its elements upon each other. This 124 Telephone Lines and Their Properties. condition is very nearly, if not quite, attained in practice in induction coils having the primary and secondary wound side by side throughout, as in some forms of repeating coils. The subject of induction becomes much compli- cated when the medium through which the lines of magnetic induction pass is of magnetic material ; but, as we shall consider the case of circuits of cop- per only, and shall not deal with apparatus in which magnetic material is used, that portion of the subject need be considered here but very briefly. In circuits consisting entirely of non-magnetic ma- terial, and surrounded by material of constant mag- netic permeability, the inductance (the number of lines of induction passing through the circuit) is a constant depending only on the form and the size of the circuit. If, however, the medium through which the lines of magnetic induction pass is wholly or in part of magnetic material, the inductance is no lon- ger constant. The value of the inductance in this case depends largely on the magnetic history of the material, and will be different for different directions of the magnetizing force — that is, it will depend on whether the magnetization is increasing or diminish- ing. With a periodic electro-motive force, the co- efficient of induction will not only be variable, but will have two values for a given value of current. If we magnetize a piece of iron to what is called saturation, and then demagnetize it by a gradual Telephone Lines and Their Properties. 125 withdrawal of the magnetizing force, the value of the magnetization for a given value of the magnetiz- ing force is greater during the withdrawal of the force than during its increase; and if the piece of iron is carried through a complete cycle of magnet- ization, from any degree of magnetization to any other degree, and then back to the starting-point again, it is found that the curve representing the increase of magnetization encloses, with the curve representing its decrease, an area which represents work done upon the iron. The curve of decrease lags behind the curve of increase, and this lagging behind is called magnetic hysteresis. Energy is actu- ally expended in performing the cycle of magnetiza- tion, and this energy is dissipated as heat in the iron. This is something quite apart from the pro- duction of heat by eddy currents, and would take place in iron so perfectly divided that eddy currents could not occur. It is due to a sort of magnetic friction — a resistance of the molecules to change of arrangement, and is diminished by mechanical vibration. There is also a magnetic lag due to the fact that time is required for a given magnetizing force to produce its effect. This is most noticeable in the softest iron and under feeble magnetizing forces. The electro-motive force impressed on any circuit, then, maybe resolved into two portions, one of which serves to overcome the opposing electro-motive force 126 Telephone Lines and Their Properties. of induction, and the remainder drives the current. That is, Impressed electro-motive force = effective electro- motive force + counter electro-motive force of induc- tion. The effective electro-motive force and the counter electro-motive force differ by a quarter of a period, and the effective electro-motive force is in the same phase as the current. The mathematical investiga- tion of the flow of simple periodic currents,* shows us that tiie phase of the current is retarded behind that of the impressed electromotive force by an angle S, such that tan ^ = 27r « -^i K where n is the number of vibrations per second, L is the co-efificient of self-induction, and R the resist- ance. We see also that the maximum value of the current would be obtained by dividing the maximum electro-motive force by Vi?' -|- {t.'u- n Lf ; and that the form of the current curve has not been changed from the simple harmonic, or sine curve, by its re- tardation of phase. The quantity \/ 1? -t- (27r n Lf is the impedance, and, as we shall see, in dealing with the properties of telephone lines it is the impedance and the angle of * See Fleming, Alt. Current Transformers, p. 115 et seq. Telephone Lines and Their Properties. 137 lag, .5, which concern us immediately. As the cur- E rent = f r > impedance takes the same place. Impedance for alternating currents, that the resistance has for steady currents. The impedance can therefore be measured in ohms. Having obtained the impe- dance in ohms and R, in ohms, and knowing n, in seconds, we can obtain the self-induction, in " sec- ohms," and the value of ^.* The relations can be illustrated by the figure below. As the resultant curve representing the waves of spoken sound is made up of a fundamental sine curve with the sine curves of the different overtones super- posed upon it, the effect of self-induction, in retard- ing the phase of each component curve, is the same • The practical unit of self-induction is 99777 x lo'' centimetres ; it is not exactly the earth's quadrant, in consequence of the legal ohm dc not being exactly the intended or true ohm. r C + L— = JS, where r is in ohms, C in amperes, and E in volts. L must therefore be expressed in terms of a unit which is 99777 x lo* cm., or about 6,200 miles. 128 Telephone Lines and Their Properties. as if each of the component sets of vibrations were taken by itself. Self-induction, then, causes an ap- parent increase of resistance — an actual increase of impedance — and a retardation of phase. That is, the waves are, in effect, hehl back through a certain portion of a cycle. The amount of this retardation depends upon the period of the alternations and the nature and form of the circuit. For a simple vibra- tion, therefore, consisting of waves of one period only, this effect would be entirely immaterial; for the waves would be simply shifted in position, their form remaining unchanged, and the effect at the receiving end would be precisely the same as though no change had taken place. Or, if the retardation of phase in any given circuit were the same for all periods and amplitudes, the practical effect would be nil. Unfortunately, however, this is not the case. The waves which we wish to transmit are very com- plex, consisting of simple harmonic waves of all fre- quencies, from about two hundred per second up to fifteen hundred per second, or perhaps even higher; and the phase of waves of high frequencies is retarded more than the phase of waves of low frequencies. This will evidently result in the displacement of the overtone waves with regard to the fundamental and to each other ; and the resultant wave at the receiv- ing end will be different in form from the wave at the transmitting end. We have necessarily, then, so far as we have con- Telephone Lines and Their Properties. 129 sidered the transmission of speech by electric cur- rents, two harmful effects : 1. The overtone waves of shorter period are re- duced in amplitude to a greater extent than are the waves of longer period ; and 2. The waves of different periods are displaced with regard to each other, those of the shorter pe- riods being retarded to the greater extent. The first efTect is, as we have seen, usually not no- ticeable ; but the second effect is often considerable, producing confusion of the component waves among themselves, and hence indistinctness of speech. There is one other property possessed by every material circuit, which has an effect upon an alter- nating current flowing in that circuit. That is elec- trostatic capacity. The subject of the transmission of alternating cur- rents over lines having capacity, and the phenomena produced by the existence of capacity, has been very ably treated by Mr. Thomas H. Blakesley, according to geometrical methods.* Without going into the demonstrations, either geometrical or mathematical, we may consider the results and conclusions reached. If a simple periodic current is flowing in a circuit having a condenser bridged across at some one point, the variation in the supply of the condenser will evi- * For the full treatment and develbpment of the subject the reader is referred to Mr. Blakesley' s book, "Alternating Currents of Elec- tricity." 9 130 TLicphone Lines and Their Properties. dently be harmonic. As the maximum difference of potential in the condenser occurs when the current is passing the zero point, however, the curve of poten- tial differences in the condenser, although similar to the curve of current, will be retarded in phase by one-quarter period behind the phase of effective elec- tro-motive force. The value of the current between the condenser and the source of electro-motive force is augmented, and its value beyond the condenser decreased. With a simple harmonic current, indeed, the quantities concerned may be so adjusted that the effect of self-induction is annulled. As, however, we never have a simple harmonic current in practical telephonic transmission, but a resultant curve made up of components of widely differing periods, this fact can be of no practical service to us, and it would be fruitless to develop the conditions necessary to its accomplishment. If, now, we insert several condensers at intervals along the line, the effects will be twofold. The cur- rent in the sections nearer the source will be in- creased, and in the more distant sections it will be diminished. We shall find, also, a continual delay in the phase of the current as we recede from the generating source. Thus the current in each section is slightly less than in the section next preceding, and slightly later in reaching the same phase, so that the current in each section differs from that in every other section in both respects. Even with a simple Telephone Lines and Their Properties. 131 periodic current, therefore, there will be a continual decay in passing through a long line or cable hav- ing capacity. Self-induction, if the conductor has capacity also, does not necessarily diminish the strength of the current, but may, up to a certain point, be actually beneficial. If the line has no ca- pacity, however, self-induction always diminishes the current. Any wire or cable may be looked upon as a con- ductor having capacity, distributed more or less uni- formly along its length, not concentrated at any defi- nite points ; and in such a case we see, from our consideration of the subject thus far, that at no two points in its length will the current be the same in value or in phase, even when the current is simply periodic. When the resultant periodic current is made up of many simple currents, of different pe- riods, the effects are much more disastrous. For the amount of falling off in current depends upon the period, being greater as the period is smaller. Thus the component waves will have their relative values changed, and will be displaced with regard to each other and to the fundamental, as in the case of self- induction. We can see, too, that if the circuit is di- vided between any two points, the arms having dif- ferent values for capacity and inductance, the wave will be still further confused. In actual lines it is probable that the effects of capacity greatly preponderate over those of self-in- 133 Telephone Lines and Their Properties. duction ; while in all apparatus the effects of self-in- duction are the greater. CROSS-TALK. We have, so far, considered only the effects due to the flow of an alternating current in a circuit wholly removed from all other conductors. Let us now in- vestigate the effects upon the current of the proxim- ity of other conductors. When an alternating current traverses a conductor, the conductor is given static charges of alternately opposite sign. Lot us assume that it is first charged positively. Then displacement currents are pro- duced from neighboring conductors into the dielec- tric, and a corresponding negative charge on these conductors results. When the next reversal of the current occurs, the negative charge in the surround- ing conductors is reversed also; and this reversal of charges on surrounding conductors is caused by the current twice in every complete vibration. In the case of the telephone current, therefore, the com- plete reversal of charges on surrounding bodies is caused from four hundred to one thousand times a second, and the induced charge is varied in amount by the overtone waves, in some cases as often as three thousand times a second. The performance of this work by the current causes in general a consid- erable loss of sharpness of the waves of the tele- Telephone Lines and Their Properties. 133 phone current, the tendency being to reduce the am- plitude of all the component waves, thus " smooth- ing off" the current wave and changing the character as well as lessening the volume. In these surrounding conductors the changes in static charge just described will evidently result in the production of a current corresponding to the in- ducing current, but less in amplitude. And if such a wire forms part of a second telephone circuit, the speech transmitted over the first circuit will be over- heard in the telephones of the second circuit, but fainter. This phenomenon, which has always been common and very marked, is called "cross-talk." In the earlier days of telephony, and in fact up to a comparatively recent time, it was generally supposed that the phenomenon of " cross-talk " was due to electro-magnetic induction, that is, to mutual induc- tion, as we have already developed it. The effects of electro-magnetic induction and of electro-static in- duction are in this respect the same; and whether the phenomenon is due to the one or to the other is determined by the strength of the inducing current and the physical relations of the circuits. It has now become generally recognized that, with the usual relations of circuits to each other and the usual strength of the telephone current, the effects of mutual induction are very feeble — in fact inappre- ciable — and cross-talk is due to electro-static induc- tion alone ; although in induction coils and such ap- 134 Telephone Lines and Their Properties. paratus, electro-magnetic induction is the chief and only important cause. Some very interesting experiments on this point have been performed by Mr. J. J. Carty, and were described by him in a paper read before the New York Electric Club in November, 1889. These ex- periments show conclusively that cross-talk on tele- phone lines is due to electro-static induction alone; and I give Mr. Carty's own description of the ex- periments, only emphasizing the fact that the volume of cross-talk depends upon the specific inductive ca- pacity of the insulation, as well as upon the distance apart of the conductors. This would not be the case were the effect due to electro-magnetic induction. " While a study of the strange noises heard in the telephone might be of interest, I shall in this paper limit myself to the consideration of ' cross-talk,' and to the action which takes place when wires are bunched in cables. In the simple case of inductive ' cross-talk ' first cited, in which telephone wires are strung parallel on the same cross-arm, the presence of ' cross-talk ' is .said to be due to dynamic or current induction ; that is, if a current commences to flow in one of the wires from north to south, it will at that instant cause an induced current to flow in the other wire in the opposite direction, from south to north. As the telephone current is constantly changing its direction and strength, this explanation seems to apply and is the one given in the text-books. Telephone Lines and Their Properties. 135 " This is the kind of induction referred to in the law of Lenz, and applies to induction coils and to parallel wires when the current is of sufficient strength. I shall speak of this hereafter as electro- magnetic induction. To-night I shall describe some experiments which seem to prove that the induction between telephone wires is due to electro-static rather than to electro-magnetic action. " I will first show a case of electro-static induc- tion * between telephone wires, in which there is a neutral point at the centre of the secondary wire, at which point there is no induction, while at the ends marked inductive effects are noticeable, fi H -^ ________ _ _ _ - . L A _ ^_. B ri + ' + + ; -f -f + Id " In Fig. I, E F and C D are two well-insulated parallel telephone wires, each 200 feet long and placed one-eighth of an inch apart. E F is open at one end and connected to ground at the other through a Blake transmitter, L, in the ordinary * In explaining an experiment of Mr. CuUey before the Society of Telegraph Engineers in 1875, Mr. Preece pointed out that in a certain telegraph line, subjected to induction from a neighboring telegraph line, there was a neutral point. I can find, however, no reference to this in Mr. Preece's book on "The Telephone," and its practical application to telephone induction seems to have been lost sight of. 13^ Telephone Lines and Their Properties. manner. In front of the Blake transmitter I place a vibrating tuning-fork, which acts on the transmitter in tlie same manner as the voice, and which produces impulses on the line E F of the same strength as voice currents. At the centre of the line C D we have the telephone Y, and at the extreme ends the telephones X and Z. With the tuning-fork at L in operation, tones are heard at X and Z, but the middle telephone, Y, is silent. A study of the changes of potential produced in the wire E F by the transmitter, will give us an explanation of this phenomenon. As is well known, the telephone cur- rent is an alternating one, and the potential of the line E F varies constantly and is changed from posi- tive to negative many times per second. The wire E F, being open at E, would be at tlie same potential throughout. It is assumed that at a given instant, the height of potential at F would be represented by the dotted line F H; then the potential at E would be represented by the dotted line E G, and the total charge on E F would be represented by the rec- tangle E G H F. We will assume that this charge is of the minus sign. The existence of this charge on E F presupposes the presence of a charge of op- posite sign on C D, which would be represented by the rectangle A C D B. " Now suppose the potential on E F becomes zero, owing to the operation of the transmitter, then the whole charge on E F gets to earth at the grounded Telephone Lines and Their Properties. 137 end of E F, but the charge on C D has two paths to earth, one at C and the other at D. This results in two currents, as shown by the arrows, one flowing to earth through the telephone X, and producing sound at X, and the other flowing to earth through the telephone Z, and producing sound at Z. No current flows through the telephone Y, and conse- quently no sound is produced therein. Again, changing the potential of E F causes a correspond- ing set of currents, but in opposite directions to those first described, meeting in the centre and pro- ducing no sound in the centre telephone, but caus- ing the end telephones to give out sounds the same as in the first instance. Inasmuch as the line E F is opened at one end, and therefore has almost an in- finite resistance, it is clear that this phenomenon is purely an electro-static one. " In this and the succeeding experiments I have not attempted to give the exact shape of the induced charge, as it would unnecessarily complicate the sub- ject and would not in any way affect the result. As a matter of fact the dotted line A B should slope off from the centre toward the ends. " With the line E F grounded through an ordi- nary subscriber's line and instrument, the effect is the same as when the line is open, and the neutral point is still found. This is because the telephone current, even when flowing in a closed circuit, is so weak that it is not capable of producing a magnetic field of 138 Telephone Lines and Their Properties. sufficient strength to affect the neighboring wire, or the magnetic effect is so small that it is obliterated by the movement of the static charge. For conven- ience in some of the succeeding experiments, the dis- turbing wire will be shown open at one end. " If there is no current flowing at the neutral point, opening the wire at that point should have no effect on the telephones located at the ends. In Fig. 3 A B is the disturbing wire with the transmitter, L, K :^i^.j. arranged as before ; K is a key located at the neutral point of C D. With the transmitter, L, in operation, no change is produced in the tones heard at the tele- phones X and Y by opening and closing the key, K. If this induction were electro-magnetic, opening the line C D would prevent current from flowing in any part of the circuit. In Fig. 2 we have another proof of the electro-static nature of telephone induc- ^^£. tion. A B is the usual disturbing wire with its transmitter, L. C D is the secondary wire with the Telephone Lines and Their Properties. 139 telephones X and Y, located at the ends, as in the previous experiment. By means of the key, K, the telephone X may be cut in and out of the circuit. With the key open, the usual tones are heard at X and Y. Now, if the induced current flowing in the circuit C D is due to electro-magnetic induction, upon short-circuiting the telephone X, and thereby reducing the resistance of the circuit C D, the strength of the induced current should be increased and the tone at the telephone Y should be corre- spondingly louder. But this is not the case, as on closing the key the sound at Y, instead of being increased, entirely disappears. This is because the charge on the wire C D finds an easy path to earth through the key, and such a small portion of the charge goes to earth through the telephone Y that no audible effect is produced therein. B.L " Fig. 4 shows the disturbing wire, A B, arranged as before, but the secondary wire, instead of being put to earth at both ends, has its circuit completed by a second wire placed outside of the field of A B. With this arrangement, neutral points are found at T and T', while the usual disturbances are heard at I40 Telephone Lities and Their Properties. telephones R and R', located at the ends. In this case a movement of the static cliarge takes place in the metallic circuit, causing at one moment a set of currents starting from T' in both directions through the end telephones and meeting at T, and at the next instant the reverse takes place. " I will now show some of the effects of electro- magnetic induction, at the same time suppressing electro-static induction. " In Fig. 5, A B is the disturbing wire as before, but grounded through a short thick wire at A. In- stead of placing the tuning-fork in front of the trans- mitter, L, and acting on it through the air, producing delicate currents like voice currents, an automatic circuit breaker and five cells of Leclanche battery were connected in the primary circuit of the trans- mitter, thus producing in the line A B alternating currents of great strength. The secondary wire con- taining the telephones X, Y, and Z is of the same length and at the same distance from the disturbing wire, as in the previous experiments. With the cir- cuit breaker at L in operation, loud musical tones are heard at X, Y, and Z, and the tone is the same Telephone Lines and Their Properties. 141 in all three telephones, the neutral point in this case having disappeared. This is a true case of electro- magnetic induction, because short-circuiting the tele- phone Y increases the sound in the remaining tele- phones, and leaving the short circuit on Y, and short-circuiting the telephone X still further in- creases the sound at Z. This latter experiment fur- nishes a most striking contrast to the result obtained in Fig. 2, where by short-circuiting the telephone X the sound was completely removed from the telephone at the other end of the line. " Another proof that this is electro-magnetic in- duction is found in the fact that if the line be opened at Y, the sound disappears from all the telephones. If the line A B were opened at A, the potential along the line would be constant, and the charge which A B would take would be represented by the rectangle A G H D ; but when the line is grounded at A through a low resistance, there is, of course, a fall of potential along A B ; the principal drop, how- ever, occurring in the secondary coil of the transmit- ter L, the line B E representing the height of poten- tial at the terminal of the coil, and the electro-static effect which the line A B would have is represented by the triangle A E B. This triangle, although ex- aggerated in the diagram, is still much smaller than the rectangle A G H D, and explains the absence of electro-static induction between the wires ; while the powerful current generated in L, and the com- 143 Telephone Lines and Their Properties. paratively low resistance through which it has to flow, account fully for the electro-magnetic effects observed. " I have made a large number of laboratory ex- periments and observations on actual telephone lines, all of which point strongly to the conclusion that electro-magnetic induction does not exist in tele- phone lines outside of the telephone and transmitter. This view of the subject, applied to the theory of transpositions of metallic circuits and to the action of wires in cables, gives the only satisfactory explana- tion of observed phenomena." * From this view of the cause of " cross-talk " we can form much more accurate ideas as to the actual condition of things on a telephone circuit. When a neighboring conductor forms a part of the primary telephone circuit, the harmful effects of in- duction, both internal and external to the circuit, are very much reduced. In this case the positive charge ^ ^ ^ ^ I sent out in one direction, and the negative charge in the other direction, assist each other. The work which must be done in the direct reversal of charges upon A and B by the impressed electro-motive force at E, is lessened by an amount due to the electro- * Another series of experiments by Mr. Carty on the same subject was described in a paper read before the American Institute of Elec- trical Engineers in March, 1891. This is given in full in Appendix R. Telephone Lines and Their Properties. 143 static, induction from A to B and vice versa. The induced static charges on other bodies at a greater distance, being the resultant charge due to both conductors, are in general very slight, and the loss of sharpness from static induction upon external conductors is usually, in the case of metallic circuits, inappreciable. Let us now recapitulate, briefly, the characteristics of spoken sound and of the telephone current. The properties of spoken sound are : 1. Loudness, or volume. 2. Clearness. 3. Quality. It is, of course, desirable that the sound received should be of a good volume. But speech, however loud, is utterly unintelligible if it is not clear. Quality it is desirable to retain, but a change in quality, so far as it is separable from clearness, is objectionable only in that it may disguise the voice of the speaker or become disagreeable to the ear. Clearness it is absolutely necessary to preserve to the utmost extent possible. Volume is affected by any conditions which alter the amplitude of the wave. Clearness is affected by any conditions which alter the positions of the waves (of all periods) with rela- tion to each other. Quality is affected by any conditions which alter the form of the wave. 144 Telephone Lines and Their Properties. Therefore, 1. Volume is reduced by resistance, by leakage, by static induction, and by self-induction ; for the effect of all these properties is to reduce the amplitude of the waves. 2. Clearness is reduced by static induction and by self-induction ; for the effect of both these properties is to alter the inter-relations of the waves. Static induction causes a rounding off of the crest of the wave, thereby involving a loss of sharpness ; and both static induction and self-induction produce an unequal retardation of phase for vibrations of differ- ent periods, thus causing interference and a resultant deformed wave. 3. Quality is changed by all the properties which reduce the clearness, and by self-induction in an- other sense as well. For one effect of self-induc- tion is to reduce the amplitude of the overtone waves to a greater extent than that of waves of longer period. To successfully accomplish good telephonic trans- mission of speech, therefore, we must make the self- induction and electro-static capacity of our line and apparatus as low as possible. Resistance and leak- age are comparatively unimportant, although it is of course desirable to keep the resistance low, and the leakage within reasonable limits. A slight and well distributed leakage, however, is often an advan- tage. It allows the static charges to escape, and Telephone Lines and Their Properties. 145 thus neutralizes to some extent the disfiguring effect of capacity. When iron or steel wires are used for the trans- mission of telephone currents, there is, in addition to the effects which have already been described, a fur- ther deformation of the waves and decrease in ampli- tude, beyond what would be caused by the greater resistance. This is due to the fact that the wire is to some extent circularly magnetized, and that this magnetism has to be reversed twice in every vibra- tion. In viev/ of the distribution of alternating cur- rents on or near the surface, it has been contended by many, with some reason, that no magnetization could exist ; but the great alteration in form of the waves cannot be accounted for by the greater resist- ance of iron. There must be also some considerable increase in self-induction, due to the magnetic prop- erties of the metal. ELECTRO-MOTIVE FORCE AND VOLUME OF THE TELEPHONE CURRENT. The electro-motive force of the telephone current, as generally used, has never, to my knowledge, been measured ; and we can only arrive at a rough ap- proximation to its value by calculation. The elec- tro-motive force will depend, evidently, upon the transmitter used, the condition of the battery if the transmitter is a microphone, and upon the induction coil. With the best forms of transmitter now in 146 Telephone Lines and Their Properties. commercial use, and a battery of three cells in good condition, there is a change in potential at the ter- minals of the primary coil of one-half a volt, more or less. The electro-motive force which this will in- duce in the secondary depends, of course, on the winding of the coil, and can be determined approxi- mately from a knowledge of the relative number of turns in the primary and in the secondary. Owing to the nature of the telephone current, the measurement of its volume involves many difficulties which cannot readily be overcome, except in the best appointed laboratories where the most delicate ap- paratus is at hand and can be used, and where suffi- cient time can be devoted to thorough and careful investigation. The Massachusetts Institute of Tech- nology took up the subject of telephone currents for investigation some years ago, and careful experi- ments on this subject have been carried on during each year since. Owing chiefly to these efforts, we have now some definite knowledge of the volume of the telephone current, and the effects due to differ- ent forms of transmitter. With the single contact microphone transmitter, of which the Blake is the typical form, the volume of current produced in the secondary is usually somewhere between .0001 ampere and .0007 ampere ; the exact value depending, as has been pointed out above, upon the conditions of battery, induction coil, and initial loudness of sound. Telephone Lines and Their Properties. 147 With the granular instruments, or multiple con- tact microphones, of the type of the long-distance transmitter, the current in the secondary will have a volume of .0002 to .001 ampfere, the exact value, as before, depending upon the conditions of battery, in- duction coil, and initial loudness of sound. The lat- est and most improved forms of granular transmitter have never yet been tested in this way ; but as they are far superior to the older forms, which have been tested, it seems probable that it will be found that they produce in the secondary a current of as great a volume as .01 ampfere. The figures given are for the mean value of the current. This, in a simple sine curve, is 0.6369 of the maximum value. In considering the question of loudness, it must be borne in mind that the different vowel sounds pro- duce currents differing widely in volume. The vol- ume of current produced by the sound of the vowel e, for instance, is much less than that produced by the sound of the vowels a or 0. CHAPTER X. MEASUREMENT. In order that we may work intelligently it is most necessary that we should be able to obtain values for the quantities with which we have to deal. We must be able to measure impedance, capacity, resist- ance, and it is highly desirable that we should be able to measure volume or current strength also. The methods of obtaining the values of capacity and resistance are too well known to need any descrip- tion ; but easy methods of measuring impedance and volume are not so well known. We may find the impedance of any circuit by obtaining its co-efficient of self-induction, according to methods given in many text-books, and by cal- culation. As, however, the methods referred to are rather difficult and tedious, and as the co-efficient of self-induction itself is practically of less importance than the impedance, we may measure the impedance directly in the way shown in the cut on the follow- ing page. C is a commutator mounted on a shaft which can be revolved by any desired means. It may advan- tageously be done through a pair of cone pulleys Telephone Lines and Their Properties. 1^9 belted on to the pulley P, and driven from any source of constant speed. Then, by shifting the belt (3= 'C =€ 3 :b along the cone pulleys, any desired speed may be obtained at the commutator. The speed of the com- mutator shaft may be measured by the use of a mer- cury-box and column, as used by Professors Ayrton and Perry, or by a ball governor whose height can be read off a graduated scale previously calibrated. Knowing this speed and the number of segments in the commutator, the period of alternation, n, may be obtained. B, B, B, are brushes bearing on the com- 150 Tilcplione Lines and Their Properties. mutator; and if the commutator and brushes are made and mounted according to the best practice in dynamo work, this arrangement will give little or no trouble. By an inspection of the diagram it will be seen that the battery and the galvanometer are re- versed at the same instant. An ordinary Wheat- stone bridge forms the remainder of the apparatus. It is most desirable that the known resistances in the bridge should have no self-induction and no capac- ity ; but even with the usual double-wound resist- ance coils, results may be obtained that will be suffi- ciently approximate for most practical work. The resistance at the contacts between the brushes and the commutator will evidently not enter into the measurements at all. If desired, an alternating cur- rent ammeter may be inserted in the arm of the bridge containing the unknown impedance, but cor- rection must be made for this in the results. If a telephone is used instead of the galvanometer, a variable self-induction must also be used to obtain silence, making the apparatus a Hughes induction balance. In using this apparatus, the resistance should first be measured with the commutator at rest. The commutator should then be run at such a speed as to give n the desired value, and a balance again ob- tained in the bridge. We have then found R and impedance, in ohms, and know n, in fractions of a second, and can calculate ^, the angle of retardation, Telephone Lines and Their Properties. 151 from the formulae tan ^ = — = — and impedance = VR' + (27r n L)\ We may be able to find L from these same meas- urements, but not necessarily. For, if n is very small and L is large, the currents do not at any time attain their steady value, and the value obtained for L by calculation from our observations would be smaller than its real value. If we wish to find L we must make n so large that two values of L, obtained from two sets of observations, with different values of n, are found to agree. In practical work, however, as we have seen, it is the impedance we want to find, and we do not care what the value of L may be. The values of the angle ^, found with different values of n, will give an idea of the deformation of the re- sultant wave experienced in traversing the circuit under consideration ; and different circuits or pieces of apparatus may be compared in this way with highly instructive results. It has been advanced as an objection to this method that the commutation of a battery current does not give a true sine curve. The approximation, however, is probably sufficiently close to serve the purposes of comparison, although the method could not be used, without modification, to obtain absolute values. It has the advantages of ease and quickness in operation. Volume, or current, may be measured most accu- rately by means of a dynamometer. But this method 152 Telephone Lines and Their Properties. also requires great care and conditions often difficult to attain. A method of easily obtaining the com- parative volumes of different sounds is by the use of the apparatus described below : This apparatus, as originally designed,* depended for its operation on the finding of the point of dis- appearance of the sound in a secondary coil as it was moved away from the coil carrying the current to be measured. Through a fixed coil, C, at one I I I I I I I I I I I I I I I I I I I I I I ured was passed was then moved away was reached at which end of the balance, the current to be meas- The movable coil, D, from C, until the point the sound just disap- peared ; and this position was read off on the graduated scale at the side. There are, obviously, many difficulties in the way of the practical use of such an instrument. The point of apparent disap- pearance of the sound will depend on the acute- ness of hearing of the observer, on the degree of quiet that could be obtained in the room, and there is a considerable range in which it is difficult to tell whether any sound is heard or not. In prac- tice, these difficulties were found to be so great that the instrument could not be used for accurate measure- * By Dr. H. V. Hayes, of the Am. Bell Tel. Co. Telephone Lines, and Their Properties. 153 merits. The apparatus was therefore modified by the writer, as follows : Rigidly attached to the coil D, at a fixed dis- tance, was placed a |^ third coil E. D and E then moved together, and their position could be read off on the scale as before. The current to be measured was then passed through^, and the current from a standard source of the same pitch was passed through C. The switch 5' enabled the observer to change rapidly from one circuit to the other, so that he heard in the telephone, alternately, the sound pro- duced by the current in C and that produced by the current in E. It was then possible to so adjust the position of the movable coils that the sound from either source had the same volume. This could be done with considerable accuracy, and an arbitrary value obtained for the sound in E from a previous calibration of the instrument ; and by means of these arbitrary values different sounds could be compared with each other, although the actual value of the currents producing them might be unknown. Such an instrument can best be used to compare the trans- mitting qualities of different lines or apparatus, by using a pair of vibrating tuning-forks working 154 Telephone Lines and TJieir Properties. through induction coils. The forks must, of course, be of the same pitch, and the coils alike, and equal battery power must be used with each. Then if one fork is set at work at the distant end of the line, and the other fork worked through the coil C, an approxi- mately accurate value can be found for the trans- mitting power of the line, in terms of volume of sound. The chief difficulty in the use of the instru- ment consists in the change in quality of the sound from the distant source, as heard in the telephone. The quality was alwaj's found to be much altered by the properties of a line, of any considerable length, being made round and full and unctuous in character, as compared with the sharp and somewhat harsh note from the near fork. CHAPTER XI. PROPERTIES OF CITY LINES. With our knowledge of the telephone current and its properties, we are now in a position to determine the proper disposition of wires for telephone lines and the conditions to be observed for efficient com- munication over them. In dealing with city lines we will at first confine ourselves to the consideration of grounded circuits. For, although with the con- stant and rapid growth of the long-distance service metallic circuits in cities are becoming of continually increasing importance, by far the greater part of city exchange lines are still made up of grounded circuits; and the treatment of metallic circuits, as developed later, may properly be applied to overhead metallic circuits in cities, as well as to trunk lines. Under- ground cables, although used exclusively in cities, and forming now to some extent a part of almost every exchange system, are considered by themselves. The properties of any telephone line are : 1. Resistance. 2. Capacity.* • It has been customary to use the term "retardation," meaning anything or everything that produced an interference or confusion or 1S6 Telephone Lines and Their Properties. 3. Insulation. 4. Self-induction. In city lines, considered by themselves, the length of line is comparatively slight (the average length is, perhaps, two miles), so that, as they are usually con- structed, none of the properties named will have so great an effect as to make conversation very diffi- cult. The effect of resistance is not important, even when iron or steel wire is used ; but on account of the magnetic character of iron and steel, these metals are not suitable for telephone lines, and should not be employed. Copper wire, hard drawn, of a diame- ter not less than .080 inch, should be used when pos- sible, on account of both its mechanical and its elec- trical properties. The resistance of the average line, if constructed of this wire, will then be about 17 ohms, and its impedance but little greater. The impedance of iron wire, of considerably greater diam- eter, is much higher. The impedance offered by the instruments which it is necessary to have in circuit, is, however, so great, as compared with that offered by the line, that for local exchange work, even iron wire, properly placed, will prove fairly efficient. muffling of sound ; and very few have seemed to know just what it did mean. " Retardation" is a convenient term if its meaning is clearly understood. It includes, properly, some of the effects of self-induc- tion, and of static capacity of the conductor under consideration. It includes all conditions which produce an unequal retardation, or "lag," of phase for vibrations of different periods. Retardation may, therefore, be considered a property of a line, but not as sepa- rate from the other properties enumerated above. Telephone Lines and Their Properties. 157 Electro-static capacity is the property probably the most injurious in its effects, as it acts in a double sense : First, as cause of retardation, mufifling and confusing speech, while at the same time it reduces the volume ; and second, as a cause of cross-talk. Its effect is independent of the material of the wire, and depends solely on its size and its position with relation to the earth and to other wires. Evidently, therefore, so far as capacity alone is concerned, it is advantageous to have as small a wire as possible ; and it is in every respect advantageous to place the wire as far as possible from all other bodies. Here we see an advantage in the use of copper wire. For the same impedance, a wire of copper is much smaller than a wire of iron ; and by reason of its less surface, its capacity is correspondingly less than that of the iron wire. As regards distance from other bodies, it is, of course, practically impossible to exceed certain limits, but within these limits there are many degrees of choice. Pole-lines have, in general, less capacity than house-top lines, as the lines in the former case are at a greater distance from the earth than they are in the latter case from the roofs ; and electri- cally, the roofs are practically the same as earth, es- pecially when covered with tin, as many roofs in cities are. Assuming, then, that we have a pole-line of copper wire, it should be our next object to sepa- rate the individual wires as widely as may be. Here again, however, we have limits set by considerations 158 Telephone Lines and Their Properties. of convenience and available space. The wires can- not be placed very far apart, vertically, without using poles of great and absurd height ; and they cannot be very widely separated, horizontally, without using unduly long cross-arms. Considering carefully all the points that have been named, the desirability of a great separation of all the wires from each other and from the earth, and the limitations in space imposed upon us in practice, we are led to adopt, as on the whole the best, the dimensions usually employed in long-distance con- struction. The wires are placed one foot apart, hori- zontally, and about two feet apart, vertically ; and the poles should be not less than thirty-five feet high, preferably forty feet. The wires should be of cop- per, not less than .080 inch in diameter. A line so constructed will unite efficiency in transmission with ease of inspection and maintenance. INSULATION. From the facts which have already been consid- ered, we see that it is not necessary that the insula- tion on short lines should be kept very high. And, in fact, it would be difficult to keep it high. The smoke, dust, dampness, and acids, which are abun- dant in the atmosphere of the busy portions of cities, are fatal to high insulation. It is the usual rule, in exchanges, that the insulation of the lines shall not Telephone Lines and Their Properties. 159 be allowed to fall below one megohm per mile, and although the insulation should preferably be consid- erably higher than that, and generally is higher, the loss of volume due to a leakage of one-half a meg- ohm (for a line two miles long) is not sufficient to seriously interfere with transmission. Moreover, as has been pointed out, a well-distributed leakage is an assistance to clearness, as the static charges are allowed to escape, and the disfiguring effect of capac- ity upon the telephone current is thereby avoided. It is much easier to keep up the insulation of a pole-line than of a line run over house-tops ; for the smoke and gases from the chimneys readily cause a deposit upon insulators placed near them. Pole- lines, being usually below the level of the chimneys do not so easily accumulate this deposit on the in- sulators. It is especially difficult to maintain high insulation in cities situated on the sea coast. When the wind blows from the salt water, and indeed, at all times to some extent, a thin film of salt is formed all over the insulator. This is not entirely washed off by rain, and readily absorbs moisture, so that the insulator may never be quite dry. For good insulation in bad weather, an insulator should have a narrow opening and long petticoat.* This keeps a considerable length dry during rain, and seems to dry out practically as rapidly as one with a wider opening. Moreover, it does not so easily * See tests given earlier. i6o Telephone Lines and Their Properties. become coated with a film of moisture during fog or mist. It is not necessary, in this country, that the insulators should be so large as those in common use in England. The element of conspicuousness is, therefore, not an important one with us, and porce- iain may be used without fear that the insulators will be mischievously broken. Porcelain is in many re- spects superior to glass, notably in toughness. RETARDATION. If, as has been recommended above, copper wire were used for exchange lines, the retardation in the line itself would be inappreciable. As most of the existing city exchange lines, however (and many trunk lines also), are of iron wire, the retardation even in a short line may be considerable. The effect of retardation is to confuse and mufifle speech, and is much more injurious, so far as successful inter- communication is concerned, than the effect of any other property that the line possesses. In this case retardation can be reduced only by reducing the electro-static capacity, and by using non-magnetic metal for line wire. DISTURBANCES. On grounded telephone circuits, much more trouble is caused by interfering currents, from vari- ous sources, than by a lack of efficiency due to the Telephone Lines and Their Properties. i6i properties of the line itself. These disturbances come in from the earth, from the air, and from other wires ; from telegraph wires, electric light and power currents, and electric railways ; and unless great precautions are taken, as much disturbance is caused by the weak telephone current itself, as by the powerful current used to propel the cars or light the streets. CROSS-TALK. This interference with a telephone current by an- other telephone current is called cross-talk, and has already been explained. It is due to electro-static induction, as has been conclusively shown by Mr. Carty's experiments, and to guard against its occur- rence, therefore, we have to so construct our lines as to make the electro-static capacity of one wire to an- other as small as possible. This we can do in three ways : By using a dielectric medium of minimum specific inductive capacity ; by using a wire of mini- mum surface ; and by placing the wires at a maxi- mum distance apart. The limits in regard to sur- face and distance have already been settled so far as outside lines are concerned, by practical considera- tions of convenience and available space ; and as we are striving, in the cases of both impedance and capac- ity, for the same end, we must accept these limits as the best we can attain. As a dielectric medium, dry air has as low a specific inductive capacity as any that 1 62 Telephone Lines and Their Properties. could be used, and no improvement can be looked for in that direction over what nature provides us with. The method of preventing induction by neu- tralizing the effect in opposite sides of the circuit, as practised in metallic circuits, it is, of course, impossi- ble to use with grounded circuits. In the older forms of switch-boards the wires were led in to the back of the board indiscriminately- bunched together. This practice, of course, brought the wires of different circuits very near together for some distance, and resulted in an excessive amount of cross-talk, which was always a great source of an- noyance, and often caused serious difficulty in carry- ing on conversation. The only way of avoiding this difficulty was to change the method of wiring the switch-board : to lead the wires in regular cables properly laid up, each circuit being complete, and the two wires of the circuit twisted about each other. If, then, the second wire of the circuit were grounded outside the board, much of the induction in the switch-board would be neutralized, and but very little would be heard on the lines. This plan has been followed in all the modern forms of switch- board, and but little annoyance is now felt from what was once the chief source of cross-talk. In city exchange lines the length of line is not great ; the outside wires do not run parallel for any great distance, and within that distance are favorably situated with regard to each other ; and the trans- Telephone Lines and Their Properties. i6 J mitters employed are too weak to give the wires any considerable charge. The volume of cross-talk due to induction on the outside wires is therefore usually very slight. As the length of two parallel wires is increased, however, the amount of induction is in- creased. Assuming the source of electro-motive force in the transmitter to be constant, as is practically the case, the volume of the inducing current would decrease with the length of wire, owing to the in- creased resistance ; while, on the other hand, with the increased distance of parallelism the amount of the induced current would increase. To determine just how much an increase in length of two parallel wires would affect the volume of cross-talk, the fol- lowing experiment was performed : A twisted pair of wires was so arranged that the length used could be varied at pleasure from twelve feet up to about fifteen hundred feet. The twisted wires were fastened to a wooden wall back and forth in straight lines, to avoid electro-magnetic induction, and were carefully tested to avoid leakage from one to the other. A weak alternating current was then sent through one wire, and the current induced on the other wire was measured by comparison with a standard sound, using the balance already described. The length of wire carrying the inducing current was in each case the same as the length of wire carrying the induced current. It was found that the volume of the in- duced current, or cross-talk, was approximately pro- 1 64 Telephone Lines and Their Properties. portional to the square root of the length of line, or C=K\/1. in which ^ is a constant quantity depending on the capacity of one wire to the other. In some exchanges where an underground cable system is being established, it is the practice to use the two wires of a twisted pair for separate circuits instead of allowing one wire of the pair to serve as a return for the other, and grounding it at the end of the cable. The result of such a method is, of course, to cause a great deal of cross-talk in the cable, especially between the two twisted wires,* and it is very much better to use a pair of wires in a cable for one circuit only. * This method is a bad one for another reason. When it becomes necessary to change the circuits from grounded to metallic, an entire rearrangement of connections will be necessary. CHAPTER XII, INTERFERENCES FROM OUTSIDE SOURCES. With the earliest use of grounded circuits for tele- phones, serious disturbances were felt from several outside sources. Electrical disturbances in the earth, so small as to have been entirely disregarded before, were picked up by the telephone wire, and caused sometimes almost deafening noises ; disturbances in the air, up to that time unheeded, were sufificiently powerful to seriously interfere with speech. The messages on the telegraph wires, when the telephone wires ran near them, could be easily read in the tele- phones. With the growth of both telephony and electric lighting, the arc-lighting currents became one of the most powerful sources of disturbances; alternating 'Currents, in their turn, were far worse, and indeed still hold the palm as a source of disturb- ance; and last, electric railways have proved a source of more or less serious annoyance to telephone users. AIR AND EARTH CURRENTS. Air and earth currents do not cause any consider- able interference on short lines. They are powerful 1 66 Tclcpltone Lines and Their Properties. only during electric storms, and at such times the telephone is not used. On long lines they have proved to be a source of such very serious interfer- ence as to render the use of metallic circuits a neces- sity for successful long-distance work. In this connection, Mr. J. J. Carty says, in a paper* which has already been extensively quoted : " One of the most peculiar developments connected with the introduction of the telephone was the pres- ence of remarkable sounds which were heard when the telephone line was of any considerable length. Sometimes it sounded as though myriads of birds flew twittering by; again, sounds like the rustling of leaves and the croaking of frogs could plainly be heard; at other times the noise resembled the hiss- ing of steam and the boiling of water. Even a dis- play of aurora borealis caused powerful currents in the telephone. "At one time, I think it was in 1882, during the prevalence of sun spots and after the appearance of a comet, the auroral current became so strong on a line from Boston to Brockton as to operate a miniature arc-light which I improvised out of a pair of lead- pencil carbons and connected into the line. " Some of these disturbances have been more or less satisfactorily accounted for by assuming differ- ences of potential at the two ends of the line ; by the sudden heating or cooling of the line; and by the passage of electrified bodies of air or clouds." Air and earth currents are still often a cause of much trouble in the use of measuring instruments of any * A New View of Telephone Induction. Telephone Lines and Their Properties. 167 great degree of delicacy, in connection with long lines, even although the lines are metallic circuits. TELEGRAPH INDUCTION. Induction from telegraph wires is now avoided, as telephone lines are seldom run near to telegraph lines. The telegraph lines run along the railroads, while the telephone lines take the highways. There is an occasional instance of induction from telegraph lines upon telephone lines; but such instances are rare, and are of slight importance. INDUCTION FROM ELECTRIC-LIGHTING CIRCUITS. The serious nature of the disturbances due to elec- tric lighting currents has been forced upon the atten- tion of telephone companies more and more strongly for some years. Electric railways have still more re- cently been added to the list of disturbing causes, and it has become imperative that something should be done to prevent or to cure the interference. Electric currents produced by dynamo-electric machines are, of course, not absolutely steady. The current which is sent out is made up of a number of overlapping single waves, each single wave being produced by a single coil of wire on the armature of the machine. The resultant wave will therefore have an undulating crest, the depth of the undula- tions — or the variations in electro-motive force — de- i68 Telepho7ic Lines and Their Pf'opcrties. pending on the number of coils with which the arma- ture is wound. Obviously, therefore, all dynamo machines are not alike in the effect that is produced in the telephone. In general, the machines for arc- lighting have fewer coils in the armature than the machines for incandescent lighting, and arc-currents, therefore, produce greater disturbance than any other so-called steady currents. Machines wliich produce an alternating current will, of course, cause a far greater noise in the telephone, as in this case the full poten- tial of the machine is operative in inducing current on the telephone line. Moreover, the vibrations are very rapid, and the sound produced is of about the same pitch as some of the notes in the human voice. Even if the noise is not overpowering, therefore, it is much more difficult for the ear to distinguish the speech from the disturbance, and to understand con- versation, than in the case of slower vibrations, where the pitch of the disturbing noise is low. All circuits carrying currents for lighting are now insulated from the ground, or nominally so ; but it is a difficult matter to keep this insulation high, and of course impossible to make it perfect. There is there- fore more or less leakage, at all times, from the lighting circuits to the poles and to the earth. In many places, also, telephone wires and electric light wires run upon the same poles. Therefore, although induction is probably in most cases the chief cause of disturbance from electric light wires, Telephone Lines and Their Properties. 169 leakage is often an important factor, and must always be considered. The disturbances arising from these sources have proved a very serious inconvenience in operating tele- phone exchanges ; and although it has always been recognized that the use of metallic circuits by the telephone companies, or the removal or entire read- justment of the disturbing circuits would prevent interference, it has been in most cases impossible to successfully accomplish the necessary changes in the disturbing circuits, and the expense of an immediate and wholesale installation of metallic circuits is enor- J TUe^honr wire Alt. eorrtnl. circuit P ■ , mous. Every plan and device that could be thought of as a remedy for these disturbances was therefore carefully and thoroughly tested. Most of these plans involved the insertion of apparatus in connec- tion with the telephone instruments ; but it has been found that all devices of this nature cause a diminution of the telephone current in about the same proportion that the disturbing noise is reduced, so that nothing would be gained by any such ar- rangement. One remedy, however, to be used in the case of alternating currents, was more successful. The ar- rangement is shown in the diagram. J70 Telephone Lines and Their Properties. When two coils, / /', connected as shown, were properly adjusted as to direction of current and rela- tive position, the current induced in the coils neu- tralized the current induced in the wire in the oppo- site direction. The objection to this arrangement is the necessity of frequent readjustment of the rela- tions between the coils, and the retarding effect of the coils on both the lighting current and the tele- phone current. It seems, therefore, that the idea of correcting these evils and obliterating disturbances after per- mitting them to be created, must be abandoned. The only way to avoid disturbances is to prevent their existence by a proper arrangement of circuits. Several plans which have been advocated as a cure for the disturbances from electric railways, are also, to some extent, a protection from interference from electric lighting currents ; but these plans are more especially applicable in the case of disturbance from electric railways, and are described under that head. In the arrangement of electric lighting and power circuits with regard to telephone circuits, the first and most iinportant requisite for the prevention of disturbance is distance. If the electric light wires and the telephone wires are only sufficiently far apart, it does not matter what is the arrangement of the lighting circuits. The maximum distance at which induction will be noticed depends upon the system of lighting ; that is, upon the rate of variation of the Telephone Lines and Their Properties. 171 current, and hence upon the winding of the dynamo. This maximum distance, however, is usually not ex- cessively great if the two wires of the electric light circuit are parallel and near together, as is now gen- erally the case. Electric light and telephone wires should not be run on the same side of the street if it can be avoided, and under no circumstances should they be strung on the same poles. Even " steady " currents supplying incandescent lamps may cause disturbance in this case, although, in general, little or no trouble is experienced from this class of currents. In ceises where the two systems are already run parallel and near each other, and where it is impos- sible to sufficiently increase the distance between them, the disturbances can be reduced to a minimum by a proper arrangement of the lighting circuits. To accomplish this, it is necessary, first, that a return wire be used for the lighting circuit, and that it be placed near and parallel to the outgoing wire. This will not in itself be sufficient to prevent disturbance, if the telephone and electric light circuits are very near together ; for both the wires of the electric light circuit cannot be at the same distance from all the telephone wires, and transpositions of the two wires of the former, as practised in the long-distance tele- phone lines, are manifestly impracticable. A slight lack of balance on the lighting circuit is enough, in such cases, to produce considerable disturbance on telephone wires. This lack of balance is due to an 1J2 Telephone Lines and Their Properties. unequal distribution of lamps over the circuit, and if the inequality is great, will result in considerable trouble, however carefully the wires may be arranged in other respects. Both the following conditions, therefore, must, if possible, be strictly observed : The wires of the lighting circuit must be carried side by side and near together; and the distribution of lamps must be symmetrical. The beneficial results to be expected from a proper arrangement of outside circuits and lamps are often lost, and good intentions frustrated, by the method of connecting in circuits at central stations. It is not infrequently the case that circuits, over which the lamps are properly distributed, are so connected at the station as to make a resultant circuit which is de- cidedly unbalanced with regard to the telephone wires. An example of this is shown in the diagram, which represents an actual case. Here A represents a lamp circuit over which the distribution of lamps is proper. "Jiffhhone wirt B represents a second lamp circuit over which there is likewise a proper distribution of lamps; and the line of telephone wires runs on the same poles as circuit B. When, therefore, circuit B is operated alone, there is little or no trouble on the telephone lines ; but when, as sometimes happens, circuit A is Telephone Lines and Their Properties. 173 connected in series with circuit B, a decidedly unbal- anced circuit results, and a considerable disturbance is caused on the telephone wires. The action of unbalanced circuits in causing induc- tion may be made clear by the following explana- tion : Tefe/tftone wtrf +a A- Suppose the telephone wire, in this case, to be equidistant from the two wires of the dynamo circuit, and let R represent a resistance or other unbalancing cause in one side of the dynamo circuit. (In practice R would be the excess of lamps or transformers on one side over those on the other side.) Let us consider now a single impulse only. A positive and a negative charge are sent out from the dynamo in opposite directions simultaneously. At a given instant the negative charge has reached the point A, and induces at A' a corresponding positive charge. If, now, the positive charge (equal to the negative charge at A^ could reach A" at that same instant, the corresponding negative charge would be induced at A' ; the two induced charges at^' would be equal and opposite, and the result on the telephone wire would be nil. Owing, however, to the excess of resistance R, the positive charge cannot reach A" at the instant that the negative charge reaches A. The 174 Telephone Lines and Their Properties. positive charge is, say, at B when the negative charge is at A, and therefore the induced charges do not neutralize each other, and a current results on the telephone wire.* BELT CIRCUITS. In the early days of electric lighting, far less care was exercised in running the wires than is now used. No especial pains were taken to run the two sides of a circuit either parallel or near together ; and there was one arrangement in particular, known as a " belt circuit " which seems to have been a favorite. Hap- pily for the telephone, this arrangement is rapidly disappearing, but there are still some such lines in use. The method of running belt lines is shown in the diagram, each circuit running perhaps almost Jt^iot completely around a town, and the lamps being cut in wherever they were located. Any telephone wires running parallel with these belt lines are subjected to the full disturbing influence of the lighting current, * The explanation by principles of electro-magnetic induction would be exactly similar to that given above for electro-static induction ; considering elements of current instead of elements of static charge. That is, the phase of the dynamo current would be delayed more in the B side of the circuit than in the A side, and an induced current would result on the telephone wire. Telephone Lines and Their Properties. 175 with no compensating effect from the return wire. It has been claimed by some electric lighting com- panies that, where a number of belt lines take ap- proximately the same course, the effect from each tends to neutralize the effects from the others. This is, however, rarely the case, the effect from the aggre- gation of circuits being usually much greater and more annoying than that from one ; for it is practi- cally impossible to run the dynamo machines with such regularity that the phases of the current from one shall be exactly, or even approximately, the op- posite of the phases from another. The disturbance will rise and fall in volume, in beats, according to the relative speeds of rotation of the armatures. When there is an even number of these belt cir- cuits running on the same poles, it would be a simple and inexpensive matter to rearrange them into proper metallic circuits, by connecting them up in Staitan pairs in the middle, as shown. The same object could be accomplished also by connecting in pairs at one end, but this would double the number of lamps on one circuit, and would therefore make it neces- sary to double the electro-motive force at the sta- tion, which would be a disadvantage. 1/6 Telephone Lines and Their Properties. There are, therefore, but two ways of avoiding dis- turbance from electric light circuits. 1st. To use metallic circuits for the telephone lines. 2d. To arrange the electric light circuits accord- ing to the following rules : a. Circuits carrying currents for electric lighting must not be run near telephone wires. b. Electric light circuits already near enough to cause disturbance, which cannot be removed, must be made up of two wires, the outgoing wire and the return wire. c. The outgoing and return wires must be placed side by side, and as near together as possible. d. The distribution of lamps on such circuits must be symmetrical, or approximately so. e. No branch circuit should be taken off one side of a main circuit without properly compensating for it on the other side. Obviously, it will seldom be possible for telephone companies to insist on the observance of the rules just given, and the only entirely feasible way of avoiding all these difficulties seems to be to have all telephone lines made up of metallic circuits. INTERFERENCE FROM ELECTRIC RAILWAYS. The electric railway is a comparatively recent de- velopment. Many systems of operation have been tried, but the only system that is now extensively Telephone Lines and Their Properties. 177 ill use, or is likely to be for some time, is that using the rails as the return circuit, and known as the "single trolley system." In order that the effect upon the telephone of the operation of such a system may be clearly understood, it is necessary to describe it more fully. In the single trolley system, a wire of large diame- ter (from o".20 to o".32) is suspended over the centre of the track, wherever the cars are to run, and the rails are carefully connected together, electrically, by cop- per wires riveted into the ends of each rail, bridging across the joints. It has been usual, also, to lay a bare copper wire in the ground near the track, and to connect every rail to it. The object of these rail connections is, of course, to reduce the resistance of the rail circuit, and thus avoid too great a loss of po- tential ; but it is incidentally advantageous to the telephone that the rail circuit shall have a minimum resistance. The positive pole of the generator is usually connected to the overhead line, and the neg- ative pole to the track. It was the practice, in the earlier roads, to carefully ground both the rails and that side of the generator which was connected to them, but this is not always done now. The cars are in multiple, contact being made with the over- head wire by means of a long arm, known as the trolley arm, on the top of the car, and a metal wheel (trolley wheel) running along the under side of the wire. Contact with the rails is made through the 178 TclcpJwne Lines and Their Properties. car wheels. A constant potential is employed, and it is now the universal custom, on single trolley roads, to maintain the potential at or near 500 volts. The diagram shows the connections. € ixiT^ ::x: Even if no pains are taken to make a good connec- tion between the rails and earth, there is at all times a more or less perfect contact. A portion of the railway current, therefore, as it enters the rail from the car wheel, passes into the earth, and through the earth by various paths to the generator. This por- tion of the current which passes ofl into the earth is probably comparatively slight ; but if it encounters the ground wire of a telephone instrument, it is suffi- cient to produce in that instrument, and in others connected with it, a considerable disturbance of a rather disagreeable character. The noise produced by the diverted railway current is a harsh hissing sound, entirely different from the sounds produced by induction, and very difficult to talk over. If telephone wires run parallel with the wires of the railway for any considerable distance, and are sufficiently near, a current will be produced by in- duction upon the telephone wire. This results in a sound in the telephone similar in character to that produced by arc or alternating currents, but of a Telephone Lines and Their Properties. i/g different pitch. The pitch of this sound does not seem to correspond to the speed of the generator, nor, in fact, to any sound usually caused by the gen- erator current. Induction from railway current is due to the counter electro-motive force of the motors, and the pitch of the sound produced corresponds to the speed of the motors ; the generator current appar- ently being equalized by the conditions of the circuit. The sound caused by induction is a clear, shrill, ring- ing sound, not particularly disagreeable to the ear, nor troublesome to conversation, unless very loud. These disturbances to the telephone from electric railways have led to many controversies between tel- ephone and railway companies, and many devices have been proposed as means of preventing or remov- ing the difificulty. All devices designed to remove the disturbance after it has occurred, are open to the same objection as in the case of disturbance from electric lighting circuits. The most important of the plans for preventing the interference are : 1st. Change of the railway system to double trol- ley, or complete metallic circuit without ground con- nection. 2d. Change of the telephone system to complete metallic circuit. 3rd. Common return wire for the telephone sys- tem, or " McCluer system." ' 4th. Change of the telephone system, in part, to fnetallic circuits, with repeating coils. i8o Telephone Lines and Their Properties. 5th. Connecting all telephone " ground " terminals together, and removing the earth connections — Wiley-Smith system. 6th. Insertion of a condenser between each tele- phone instrument and the earth. THE DOUBLE TROLLEY SYSTEM. I. In the double trolley system the arrangement is the same as in the single trolley, except that there are two wires overhead, two trolleys, each making contact with one of these wires, and the track is not used as a portion of the circuit. Undoubtedly, the use of the double trolley system would prevent any material disturbance to the telephone, as there is no connection of the railway circuit with earth, except by accident, and the two wires run side by side and near together, thus avoiding any appreciable amount of induction. There is, however, but one city in which the double trolley system just described is running successfully. The leading railway construct- ing companies refuse to adopt the system, saying that it is not practicable except in very simple cases, and that in the case referred to, in Cincinnati, the road presents such simple conditions of engineering that for that reason only is it possible to operate the system successfully there. It has been found impos- sible to compel the railway companies to adopt the system ; and although this method would be a solu- tion of the problem very satisfactory to telephone Telephone Lines and Their Properties. i8i interests, the plan cannot be considered one which it is in the power of telephone companies to apply. 2. The use of metallic circuits for all telephone lines would prove as perfect a means of preventing disturbances from railway currents as the use of the double trolley system by the railways. The argu- ments already advanced in the case of electric light disturbances apply with equal force to this case, and the only practical objection to the adoption of this system is the cost of installation. It is earnestly to be hoped that metallic circuits will ultimately be used for all telephone lines ; for this would not only free the telephone lines from disturbances of all kinds from foreign sources, but in addition to that the lines would be, in some respects, more efficient than grounded circuits. The two methods already described are practically perfect in preventing disturbances to telephones from railway currents. The methods yet to be described furnish but partial protection. 3. The " common return system," or " McCluer system," so called, consists essentially of a wire, of proper size and resistance, strung on the same poles with the telephone wires for which it forms the re- turn. The " ground " wire of each instrument is re- moved from its earth connection, and connected to this return wire, which is common to all the instru- ments in the system, and in this respect exactly ful- fils the function of the earth. 1 82 Telephone Lines and Their Properties. Obviously, if all connection is broken between the earth and the telephone system, no disturbance can take place from conduction of the railway current in its passage from the rails to the power-station, what- ever the size or disposition of the common return wire ; but somewhat more than this can be accom- plished by a proper determination of the size and location of this wire. Assume the resistance of the wire and its location to be such, at each point, that the current induced upon it is always equal to the sum of the currents induced upon all the wires con- nected with it, then the resultant current in each telephone will be zero, and all disturbance from in- duction, as well as that from conduction of railway current, will have been eliminated. This happy state of affairs cannot be attained in practice, for reasons which will soon appear, but it can be approached by careful consideration of the conditions met with in each case, and the system can be so constructed as to eliminate a part, at least, of the disturbance arising from induction. When several telephone wires run together upon a line of poles, it is impossible to place a single wire serving as a return for all, so that it shall be at the same distance as each of the telephone wires from a disturbing wire. If, however, the return wire be run on the same poles as the telephone wires, and at, as nearly as possible, the centre of the group, the varia- tion from actual equidistance will be very slight. Telephone Lines and Their Properties. 183 The real difficulty lies in another direction. Sup- pose I, 2, 3, 4, 5, to represent a line of telephone -_o T\ _ _ _ _ °j '~i c wires, and c the common return for them all ; the whole line being subject to induction from a disturb- ing current of any kind. If c be so designed, in size, material, and location, that the current induced upon it is equal to the sum of the currents induced upon I, 2, 3, 4, 5 ; and if the currents induced upon i, 2, 3, 4, 5, be exactly equal to each other; then no dis- turbance will result in the telephones. Usually, however, the currents induced upon i, 2, 3, 4, 5, are unequal. Wire No. i, say, is the longest, has the greatest exposure, and therefore the most disturb- ance. For similar reasons wire No. 2 has greater disturbance than No. 3, and so on. # 1-0- d> 2 ^ Obviously, then, if c be of the same size through- out, the induction from e to ^ will more than balance the induction on l, and a disturbance will result, in ;,. For similar reasons, there will be a disturbance to a less and less extent, in t^, t,, and t^, but none in 4, if c is properly designed for the five wires. For 1 84 Tclcplw7ie Lines and Their Properties. practical reasons it is not convenient, nor is it neces- sary, to diminish the size of c by exactly the proper amount whenever a wire leaves the pole-line. A reasonable approximation to exactness will insure a freedom from disturbance, in all the instruments, suf- ficient to permit easy conversation. But the proper dimensions of the return wire must be determined with much greater care than has ever yet been used in such a ceise. It is an engineering problem, and it will not do to put up a return wire by guess-work if good results are to be obtained. Let us look at it from another point of view. Suppose the same wires, i, 2, 3, 4, 5, to be provided -^ each with I? its individual ;J return wire, prop- < p 9 erly balanced and trans- posed, making a system of complete metallic circuits. It will not be neces- sary to demonstrate that in this case there will be no disturbance from an outside source, for this fact is well known to all telephone engineers. Having such a system, suppose we connect all the individual return wires together. There should be, in this case, no disturbance whatever. And how does such a system differ from a properly designed common re- turn system ? Simply in the location of the return Telephone Lines and Their Properties. 185 with respect to the outgoing wires. We are led, therefore, to the proposition that, a^ properly designed common return wire is practically equivalent, in every respect, to an individual return wire similarly placed. Its use, therefore, will not only prevent dis- turbance due to earth currents, but the disturbances due to induction ^ ? I alone will be pro- 9 J portionately reduced as the outgoing and com- mon return wires are more nearly equidistant from the disturbing source. The proper size to be given to the return wire will be different for different telephone systems. In the closed circuit system, which is the one in most com- mon use, the return wire must be larger than in the Law system, in which the circuits are normally open. In the Law system, the common return wire must be designed for the number of telephone wires likely to be in use simultaneously, which is but a compara- tively small portion of the whole number of telephone lines. It is therefore, to some extent, a matter of guesswork to determine the proper size of the return wire, and the common return system cannot, in this case, be made so perfect a protection against disturb- ance as in the telephone systems using closed cir- 1 86 Telephone Lines and Their Properties. cuits, where the number of wires to be provided for is definitely known and is constant. 4. Change of telephone system, in part, to me- tallic circuits, with repeating coils. This arrangement is, of course, only a makeshift, and is not to be recommended in cases where it is feasible to have a complete metallic system, or where a large portion of the lines in an exchange are sub- ject to disturbance. Where comparatively few lines are disturbed, however, it can be used to advantage, and satisfactory service obtained by using a combina- tion switch-board. In such cases the disturbed lines are usually all in the same district ; and if the daily business in that district is not excessively great, it will not be necessary to provide a great number of repeating coils — perhaps eight or ten. It is perfectly feasible to operate a combination board for such a purpose, the only objection being the slight delay in making connections. After connection is made the service will be as satisfactory as if the railway were not in operation. The cost of this arrangement depends, of course, on local conditions. When the disturbed lines con- stitute as much as half the exchange, it will be found cheaper in the end, and more satisfactory in every way, to install the metallic circuit system throughout. It is unnecessary to describe this arrangement more in detail, as it is exactly similar to that used in Telephone Lines and Their Properties. 187 cases where metallic circuit trunk lines come into ex- changes operating grounded circuits. 5th. Connecting telephone " ground " wires to- gether and disconnecting them from the earth — " Wiley-Smith system." This system is the cheapest of any that have been named, and has been used with fair success in several exchanges in the West. Its use will obviate all the trouble from conduction of railway currents through the earth, but so far as induction disturbances are concerned, little, if any, relief can be expected from it. It consists in connecting together all the sub- scribers' " ground " wires, and removing the earth connection ; so that the telephone current passing out over a subscriber's line is returned through all the other wires together. It has not been found that the use of this method causes any interference of telephone currents with each other. 6th. Insertion of a condenser in the telephone line, between the subscriber's instrument and the earth. This is a method which can hardly be recom- mended generally, although it might be of advantage in isolated cases. The result would be comparative freedom from disturbance due to conduction of rail- way current, and possibly a slight modification of the telephone current, producing a sharper and harsher sound. I know of no instance of the use of this method as proposed. Whenever, then, serious disturbance is caused upon 1 88 Telephone Lines and Their Properties. telephone lines by the operation of any circuits car- rying heavy currents, it is necessary to adopt some means of preventing the disturbance. If it is possi- ble to so re-arrange the power circuits that they will cause no disturbance, that should be done. If, how- ever, this is not possible, as it generally is not, one of the methods just described must be used ; and a careful consideration of the conditions existing in each case will determine just which method it is best and most economical to apply. In some cases iron rods driven at frequent intervals along the railway track, and connected to the rails, give good results in preventing leakage of railway current. The efficacy of this system, however, must in general depend on the character of the soil. In rocky soil it can be of but little service. It may be instructive to close our consideration of this subject with an account of an actual case that came under the observation of the author. The sit- uation is roughly indicated in the diagram opposite. The electric road ran from a distant town, not shown, through the village A, over the creek into the town B, continuing by the straight track to the outer edge of the town. In returning, the cars ran around the loop in B, past the telephone exchange E, and back through A by the straight track. The greatest trouble was experienced in the B ex- change. Here, although the noise on the lines was not troublesome except in rare cases, many of the Telephone Lines and Their Properties. 189 drops were frequently thrown and held down for some time. Moreover, the conditions were some- what peculiar and the results unexpected, as it was not the drops of the B lines which were thrown where the cars were running, but the C drops, con- necting with lines grounded at a very considerable distance away from the moving car. Broad St.^ from A Telephone Main St. Creek Profile of Road into B and on Main Street. It was noted that almost invariably, when cars go- ing toward B were mounting the short grade M, in I go Telephone Lines and Their Properties. A, and cars from B were on the grade N, many of the C drops were thrown ; and it had been ascer- tained by experiment that the direction of the cur- rent on the affected telephone lines was from C to the Exchange. When the car was going up Broad Street, a few of the drops in the distant town were thrown, as would be expected, and when the car was on Main Street, passing the Exchange, another set of drops fell. The reason for the throwing of the C drops by the passage of a car over the grade in A, was probably this : A and C have the same water system. The creek is at the bottom of a deep cut with steep and rocky sides, crossed by a bridge laid with centre-bear- ing rails. When a car was passing over either of the grades in A, there was no car in B ; and owing partly to this fact, partly to the rocky character of the soil, and partly, perhaps, to the fact that the track circuit over the bridge was imperfect, the potential of the track and ground at the hill in A was raised by the passage of a car somewhat above that of the track in B. A current passed, therefore, over the water-pipes to C, upon the telephone grounds, over the lines to the Exchange, and threw the drops, passing out through the central office ground and the grounds of neighboring subscribers. The same effects were ob- served when the central office ground was removed. This must necessarily have been the case so long as subscribers' grounds existed in the neighborhood, un- Telephone Lines and Their Properties. 191 less the lines were separated at the switch-board be- yond the drops. Several experiments were made on the effect of moving the Exchange ground to different points. When the Exchange ground was moved to a distance of two or three miles into the country, away from the electric road and from C, the trouble with the C drops was entirely relieved, but a number of the B drops fell instead. The Exchange ground was then moved to C, and it was found that only one or two of the drops fell often enough to cause any trouble. ■ It was proposed by the telephone company that the road should put in the Sabold system of ground rods to avoid the difiSculty. In many cases this sys- tem would undoubtedly greatly help matters, but the circumstances of each individual case must be considered. In this case the whole country is very hilly, the slopes usually steep ; it is underlaid by beds of slate and limestone, and " sink-holes," so-called, are very common. These sink-holes serve to drain off all the surface water which succeeds in reaching the rock, and in fact they do this so thoroughly that the sewage is disposed of by means of sink-holes. It is improbable, therefore, that any particular benefit would have been obtained by the use of the system of ground rods, even if it had been possible to drive them. In many cases this would not have been pos- sible, and in any case probability of reaching a level 192 Telephone Lines and Their Properties. of permanent moisture in such ground, at any reas- onable depth, would have been very small. MAPPING OUT GROUND POTENTIAL. In any telephone exchange system, the distribution of potential over the area of ground covered by the system may be very readily found by the use of a direct-reading volt-meter. This practice was in- augurated by the electric railway companies for the purpose of finding the loss in the track circuit ; but it was found to give such useful information that it has been adopted by some of the telephone ex- changes. To find the potential over a system by this method, connect an ordinary switch-board cord to each terminal of the volt-meter, plug one — usually the negative terminal — into the central office ground, and plug the other successively into the subscribers' lines. The needle of the volt-meter will indicate usually a varying potential ; but the average can be estimated with fair accuracy, and the maximum and minimum noted if desired. Having obtained read- ings from all the lines in the exchange, equipoten- tial lines may then be plotted on a map of the city. As the resistance of the volt-meter is usually high, it is generally unnecessary to correct for the resist- ance of subscribers' instruments, and a very good approximation is obtained in each case to the differ- ence of potential between the central ofifice ground and the subscribers' ground. Telephone Lines and Their Properties. 193 The results obtained in different cases vary very greatly, and depend to a large extent on the nature of the soil. In cities having an electric road with an imperfect track, circuit, the difference of potential may rise at times, and on certain lines, to 50 or 60 volts. Where the electric road has a good track cir- cuit and a network of tracks, this difference is usually only a fraction of a volt. 13 CHAPTER XIII. PROPERTIES OF METALLIC CIRCUITS. The most obvious advantage of metallic circuits over grounded circuits, and the point first looked to in their adoption, lies in the fact that metallic circuits are, and must always be, free from those currents arising from various accidental causes existing out- side the circuit. Such currents are continually creeping in on grounded circuits, even when the lines are short ; and on long lines a grounded wire is never free from them. It was a necessity, therefore, that metallic circuits should be used if it was expected to successfully transmit conversation over long lines. The other advantages to be gained by the use of metallic circuits were then unknown, or at least not fully appreciated, and were only fully developed after metallic circuits of copper had come into use for the long-distance lines. In discussing the properties of long distance lines, so called, I shall confine myself to a considera- tion of trunk lines consisting of metallic circuits of copper wire, as described in a previous chapter, al- though the discussion will apply, to a great extent, to any metallic circuits of copper, whether trunk lines Telephone Lines and Their Properties. 195 or not. The mechanical properties of the long distance lines should be well understood from the description already given of the construction of these lines. The electrical properties of any telephone line are : 1. Resistance. 2. Capacity (static). 3. Insulation. 4. Self-induction (retardation). Retardation may also be considered a property of a telephone line, as it includes some of the effects of both capacity and self-induction ; and as it is impos- sible to separate these effects, we will consider them together as retardation. Let us now investigate these properties in the case of long distance lines, and their effect upon the telephone current. DESIGN. It was in connection with the proposed construc- tion of long distance lines that the question of de- signing the proportions of a line first arose as an electrical problem. If it was desired to transmit speech over a given distance sufificiently well for business purposes, of what size and material must the wire be ? What must be the distance of the wires from each other, and how far must they be from the earth ? In a word, what values must we give to the proper- ties of the line ? As an answer to these questions the empirical rule was deduced from experiment, and 196 Telephone Lines and Their Properties. supported by some eminent electricians, that the product of the resistance and the capacity must be constant ; that is, CR = K where K was given different values according to the transmitter used. R was to be taken without regard to the magnetic properties of the metal used, and C was the capacity measured to "earth." If the line was a metallic circuit, the equation was to be solved for a grounded circuit, and a wire similar to that thus determined was to be run as a return. RESISTANCE AND IMPEDANCE. From the consideration we have already given to the telephone current, we see that the impedance of- fered by a wire to the passage of a telephone current does not necessarily bear any definite relation to its resistance ; and we shall see later that the capacity of a metallic circuit does not necessarily bear any de- finite relation to the capacity of either wire to the earth. Although the formula cited may, as tele- phone lines are usually constructed, give a very rough approximation to truth, it cannot be considered by any means a basis for accurate calculation. As the impedance offered by a given wire to the passage of an alternating current depends upon the frequency of alternations, the resistance of the wire becomes of continually lessening importance as the frequency of alternation increases. Hence the resist- Telephone Lines and Their Properties. 197 ance of the wire does not come in directly as a factor in determining the volume. Any formula, therefore, in which the first power of R appears alone is math- ematically incorrect. The wire used for long distance lines is hard drawn copper 0.104" in diameter. This wire has a resist- ance of about 5.2 to 5.3 ohms per mile ; and an im- pedance for currents of 1,500 alternations per second, only about 1.4 per cent, greater. The average im- pedance offered by this wire to the passage of tele- phone currents in which the vibrations range from 200 to 1,500 per second, is practically the same as the resistance, and the difference in impedance for the different rates of vibration is so slight as to be negli- gible. If the size of the wire is increased, this differ- ence in impedance increases, and in view of the slight effect of pure resistance upon transmission there might be but little gained electrically by lessening the re- sistance by an increase in the size of wire. CAPACITY. The electro-static capacity is the most important property of a circuit, so far as transmission is concerned. In considering the question of capacity of a metallic circuit, we must always bear in mind that what is meant is the capacity of one wire of the circuit to the other wire. The capacity to earth may be any quantity whatever, so long as it is the same for each wire, without affecting the condition of the two wires 198 Telephone Lines and Tlicir Properties. as a circuit. For a metallic circuit constitutes a con- denser of which each wire of the circuit is one of the plates. Now, the capacity of a condenser depends upon the surface of the plates and their distance apart; and it is unchanged by the proximity of a third plate, provided the third plate is at the same distance from each plate of the condenser. If the third plate be inserted between the plates of the con- denser, at the same distance from each plate, the capacity of the condenser is unchanged if the thick- ness of the third plate be infinitely small. In this case it is necessary to take account of the thickness of the plate only because the distance between the plates of the condenser is virtually diminished by an amount equal to the thickness of the third plate. Thus in the diagram, if A and B are two wires consti- tuting a metallic circuit, any other wire may be intro- j. 0_. duced on the line C D with- out affecting transmission over A B, except when the position of the third wire results in virtually diminish- ing the distance between A and B. Even then, the maximum effect is that due to the diameter of the wire, which is small in comparison with- the dis- tance A B. Evidently, where the circuit A B is strung on A -e- — Telephone Lines and Their Properties. 199 poles, the earth is practically at the same distance from each, and its proximity cannot affect the capac- ity of the circuit. The proposition may be generally stated as follows : Given, a pair of wires constituting a metallic circuit. The proximity of any other conductor or conductors symmetrically disposed relatively to it and equidis- tant from its wires, will not affect its capacity in any way, unless it is so placed, and of such dimensions, as to virtually diminish the distance between the given pair of wires.* , In the case of metallic i_ C circuits the presence of any ■ wire lying in the plane C 4~ D, will have no effect on A the current in A and B. ""'Q For the potential at C D is always zero, when A and B are charged with equal and opposite quantities. Hence there is no change of potential at C D, due to change of potentials at A and B, so long as the charges on A and B remain equal and opposite. No work is done on C D by the change of current in A * " Since this spherical surface [in this case of radius = oo , hence a plane midway between the wires and normal to the Ime joining their centres,] is at potential zero, if we suppose it constructed of thin metal and connected with the earth, there will be no alteration of the potential at any point either outside or inside, but the electrical action will remain that due to the points A and B." — Maxwell, § 156. 200 Tcleplwne Lines and Their Properties. and B. Therefore the presence of conductors in the plane C D can cause no retardation in the current in A and B. If the charges on A and B are not equal and op- posite — that is, if the metallic circuit is unbal- anced, the effect will be to move the plane C D to one side or the other. This will necessitate work by the current in A and B, and will cause retardation in that current. In the case of metallic circuits upon poles, the dis- tance between the two wires of a circuit is usually great in comparison with the diameter of the wire.* The error involved is therefore very small, if we ap- ply the formula which expresses the capacity of a condenser with flat plates. ^ir t where 5 is the surface of the wires, and t the dis- tance between them.f As we have seen, in any circuit traversed by tele- phone currents, the electro-static charges must be rapidly reversed ; and this produces an unequal re- tardation of phase which impairs and destroys the clearness of signals. Another effect of capacity is to reduce the volume, producing interferences of waves which reduce their amplitude. The electro-static capacity of a circuit is the most powerful factor in • About one hundred and twenty times. f When the distance between wires is not great in comparison with diameter, this formula does not apply. Telepho7ie Lines and Their Properties. 201 reducing the efficiency of transmission ; and if the capacity can be diminished the efficiency of the h'ne will be increased. We can diminish this retarding influence in two ways : The size of the wire can be reduced, thus lessening the surface ; or the distance between wires can be increased. This increase of distance can be accomplished either by increasing the length (or number) of cross-arms — which is not advisable for mechanical reasons — or by using as a circuit the two wires farthest apart in the set. This method is of material advantage, and is often employed on the longest lines, when talking is very difficult. In ex- perimenting on the line between New York and Boston, using a circuit about two hundred and fifty miles long, it has been found that the circuit includ- ing the pair of wires farthest apart on the poles has an electro-static capacity from twelve per cent, to twenty per cent, less than that of the circuit includ- ing a pair of adjacent wires. Both these circuits in- cluded also a considerable length of cable whose capacity forms a large percentage of the total capac- ity of the line ; and this portion was, of course, unchanged. As the length of pole-line in use is greater, its capacity is a greater percentage of the total capacity of the line, and the advantage to be gained by this method of increasing the distance be- tween wires becomes proportionately greater. Retardation, due to electro-static capacity, is less 202 Telephone Lines and Their Properties. in the case of metallic circuits than on grounded cir- cuits, for two reasons : The capacity of the metallic circuit is usually less than that of the grounded cir- cuit of the same dimensions ; and the telephone cur- rent is assisted in its work of reversing static charges by the effects of static induction from one wire to the other. INSULATION. The insulation of the long distance lines is usually very high. Although the insulator in most common use is a small one, with a comparatively slight length of leakage surface, the insulation frequently measures, in dry weather, as high as 3,000 megohms per mile. In rainy weather it drops to a few hundred thousand ohms per mile — it is generally too unsteady to measure at such times — but, except on very long lines, it is never so low that the leakage seriously in- terferes with conversation. Although much experi- menting has been done in the direction of improving the insulation, the results of experience seem to show that exceedingly high insulation is undesirable on telephone lines. The effect of leakage is to allow a portion of the current to escape, and therefore to diminish the volume. But a slight and well-dis- tributed leakage also allows the static charges to escape, thus clearing the line and diminishing the retardation. The slight loss of volume due to low insulation is more than counterbalanced by the gain Telephone Lines and Their Properties. 203 in clearness, provided only that the leakage is fairly well distributed over the whole length of line, as is usually the case, A line having an insulation of 4,000 or 5,000 megohms per mile, for instance, will probably not " talk " so well as when its insulation is only four or five megohms per mile, or perhaps even lower. SELF-INDUCTION. The self-induction of a circuit made up of copper wire not larger than 0.104" diameter is very slight. In comparison with the effects of static capacity, the effects of self-induction in the line wires themselves are practically negligible. The self-induction of ap- paratus which always forms a portion of any circuit traversed by telephone currents is often considerable, and plays a very important part in changing the characteristics of the current ; but it cannot be con- sidered here. We are now considering the line it- self, without apparatus.* * The self-induction of aerial wires has not as yet been accurately measured ; but the table below gives the results of calculations of the inductance of single wires of copper (calculated by Mr. Kennelly from formula given by Maxwell). Diameter. Elevation above ground, 400 cm., 13.1 ft. Elevation above ground, 700 cm., 23 ft. Elevation above ground, 1,000 cm., 32.8 ft. Elevation above ground, 1,300 cm., 42.7 ft. Cm. o.io 0.20 0.30 Inch. 0039 0079 0.I18 Per kilom. 1.986 1.848 1.766 Per mile. 3.196 2.974 2.842 Per kilom. 2.109 1.960 1.878 Per mile. 3 393 3154 3 022 Per kilom. 2.170 2 031 I -950 Per mile. 3-493 3.268 3-138 Per kilom. 2.222 2-083 2.002 Per mile. 3-S76 3-3S2 3.222 These results are given in myriametres. 204 Telephone Lines a7id Their Properties. MUTUAL INDUCTION. Electro-magnetic induction — what has been called " dynamic " or " kinetic " induction — more properly mutual induction — has no retarding influence in a metallic circuit. Although it is very small, its effect is distinctly beneficial. In respect of retardation there is, therefore, a slight gain in the use of metallic circuits, in addition to the freedom from external noise. In metallic circuits of copper, then, the effect of self-induction in producing retardation is very slight, and is to some extent counterbalanced by the effect of mutual induction in lessening retardation. The chief cause of retardation, so far as the line wires alone are concerned, is electro-static capacity. In the use of metallic circuits of iron there is a very considerable retarding effect due to the magnetic properties of the metal. Iron wire is in no case a suitable material for telephone lines. CROSS-TALK. Long-distance lines would be, on account of their length, particularly liable to disturbance from cross- talk, were not some means found of preventing it. On circuits one hundred miles or more in length, the effect of static induction, even by so slight a poten- tial as that of the telephone current, would be very Telephone Lines and Their Properties. 205 considerable ; and when from ten to fifty circuits on the same line of poles were working at the same time, the sound produced by induction upon any one circuit would be so confused and so loud that it would be impossible to carry on any conversation at all. There are two cases in which cross-talk will not be produced on a circuit by the passage of alternating or varying currents on a neighboring wire. The first case is when the neighboring wire is at an equal dis- tance from each of the wires of the circuit in ques- tion, and may be represented thus D^' "+ ^ + + + * + B -^g—^ ^^-± ± — *-±- =fl A A is the metallic circuit, and B oA the third wire. B may be anywhere o on the centre line between the two " "" —©-. wires. Suppose B to be first charged negatively. Then the induced charges oA on A A will be distributed somewhat as shown, the positive charge being on the side of the wires A A nearest to B, and the negative charge on the opposite side. When the charge on B is reversed, the induced charges on A A will be reversed also, and a tempo- rary current will flow across the wire A. An alter- nating or varying current in B, therefore, will produce a corresponding alternating current across the wire 206 Telephone Lines and Their Properties. A, but will produce no effect in the wire A longi- tudinally, and no sound will be heard in the tele- phones at the ends of the line. TRANSPOSITIONS. If it were possible to so string the wires in a line that the condition of equidistance would be fulfilled, there never would be trouble from cross-talk. A short consideration, however, will show that this relation Qg can be maintained for four wires, or two cir- Ao oA cults only, as A A and B B. It is therefore OR necessary to resort to the second method to prevent cross-talk. This case is represented in the diagram. [^ "A 4A_ b'A ,0A =0 B Here B, the disturbing wire, is nearer to one of the wires of the circuit A A than it is to the other, and normally there would be an induced current through the telephones, depending upon the length of the circuit. If, however, the wires A A be crossed or transposed, as shown, the currents will be dimin- ished. In this case the currents may be made as small as desired by increasing the number, and so decreasing the length of the transpositions ; and in Telephone Lines and Their Properties. 207 practice the transpositions are made so short that the current resulting in the telephones is inappre- ciable. Suppose B to be charged negatively. The in- duced charges upon the wires A A are distributed as shown. When the charge on B is reversed, the in- duced charges flow away in both directions, produc- ing currents as indicated by the arrows. Moreover, as the charges flow away in both directions, there must be, in each transposition, corresponding neutral points, o o' o" o'", etc., at which points there is no current. In the case of simple wires with no appara- tus, these neutral points would be at the longitudinal centre of each transposition ; but the introduction of apparatus at the ends of the line causes the neutral points of the end transpositions to move nearer the apparatus. For the retardation of the apparatus tends to prevent the flow of current through it, so that the greater part flows in the other direction through the wires whose retardation is small. The existence of retardation in the apparatus at the end of the line, therefore, makes it possible to use longer transpositions than could be used with simple wires, or with apparatus having no retardation ; and the greater the retardation of the apparatus, the longer the transpositions may be without producing any appreciable cross-talk. There is a third method of preventing cross-talk, more strictly applicable to cables. Jhis consists in 2o8 Telephone Lines and Their Properties. twisting the two wires of the circuit about each other. It will be evident, on consideration, that this Q^ method combines the features of the two methods just described. In por- — - -o- tions of the twisted circuit the rela- tions will be as in the first method ; oA and in other portions the effect will be the same as in the case of transposition.* The method of transpositions is the one univer- sally used for pole-lines ; and it would seem, from the development of it so far, to be a very simple problem. As the number of circuits on a line of poles increases, however, the difificulty in planning our transposi- tions increases also. For instance, if we have two circuits it is very easy to so transpose one of them that there shall be no cross-talk. If, however, we have a third circuit and transpose it as we did the second, there will be cross-talk from the second to the third, because their relations to each other are the same as if there had been no transpositions at all. To get over this difficulty we must transpose the third circuit twice as often as we did the second. 3- 2 ^ ^ A fourth circuit may be transposed at the middle points of the last transpositions, and so on. * See Appendix B, for Mr. Carty's experiments on this subject. Telephone Lines and Their Properties. 2og ii u ¥Mr h ■X- ■X- Mr X X -X-'« ->4 -X- U3 -X- -K>4- ->f ■«'-x- 4o4- ■ X X -X"!- X X -X- i yx- -''■X- X X u -X- X- X"?- X-H- Fig. 4. each end. In this case, if we could neglect the resist- ance of the telephones, the current going through the ends would be reduced to one-fourth of its original proportions, and eight neutral points would be pro- duced, one in each wire at the centre of each trans- position. In practice the impedance of the telephones must be taken into consideration, so that, if in Fig. 3 the middle telephones x and y were omitted, the neutral points would be found to move toward the ends, and Appendix. 253 the currents flowing through the end telephones would be correspondingly reduced. According to this theory of transpositions, if the instruments and distances between wires on a given circuit remain constant for a given period of alterna- tions in the disturbing wire, the number of transposi- tions necessary to obtain silence will depend on the E. M. F. of the disturbing wire, and the specific induc- tive capacity of the dielectric. An increase in the value of either of these factors will, if silence is to be maintained, require additional transpositions, their number depending upon the value of the change which is made. Where the disturbing wire is placed at an equal distance from both sides of a metallic circuit, no noise is produced in telephones located in that cir- cuit, and a balance once being obtained, it is inde- pendent of both the E. M. F. of the disturbing wire and the specific inductive capacity of the dielectric. Fig. 5 shows such an arrangement of circuits — l" and '"^ ,.; . t I ! 11^ 1 4 1 I 1 >" _ Fig. s. l' are the two wires composing the metallic circuit placed the same distance apart as before, and the disturbing wire is at an equal distance from both. 254 Appendix. When the disturbing wire is in operation, no sound is heard at the end telephones, or at telephones lo- cated at the centres. This may be accounted for by assuming that at a given instant a negative charge is on the disturbing wire, which produces a positive charge on the inside of l" and l' and a negative charge on the outside of those wires ; and that when the charge is removed from the disturbing wire, a set of currents is set up in the wires L^ and L' in a direc- tion at right angles to their axes, as shown by the arrows. In this case the flow is lateral, and no cur- rent passes through the end telephones or through telephones located at the centres. It will thus be seen that this method of arranging wires differs es- sentially in its action from the plan of using trans- positions. Unfortunately, however, its practical ap- plication is limited to two circuits. Where the disturbing wire occupies the position shown in Fig. 5, the flow in the conductors is lateral only when the wires l' and l' are insulated from the earth. If a ground be attached to the centre of l", as shown in Fig. 6, the flow of current becomes lon- gitudinal, and the telephones a and b are found to be affected by loud disturbances, while the telephone x, at the centre of l', is found to be silent. This is be- cause the disturbing wire, which we will say is nega- tively charged, induces a positive charge upon l' and l' and a negative charge upon the earth. The discharge in this case is effected by two currents Appendix. 255 starting from x, which thus becomes a neutral point, and passing through the end telephones to the C^^ -- .. -- — -— ^j: '^ ^ ^Y ' ' Fig. 6. ground, as shown by the arrows. If the ground be removed from the point y toward the telephone a, the neutral point will be found to move toward tele- phone b, and if the ground be put at the centre of resistance of the telephone a, the neutral point will be found to be at the centre of the telephone b. This is well illustrated in Fig. 7, where a and b are telephones of special construction, admitting of the attachment of grounding keys K' and K' at their re- spective centres. In this instance, when the disturb- ing wire is in operation and both keys open, no sound is heard at any of the telephones, the flow of current being lateral. If the key K' be closed, sound is immediately heard at telephones x and y located at the centres of L" and L°, but the telephones a and b are still silent. This is because the charge and discharge take place along the conductors L° and L' to and from the earth at K', thus passing through x and J/, .« being silent because the currents go through it differentially, and 5 is silent because it is located 256 Appendix. at a neutral point. To prove that current flows through telephone a, another telephone may be in- serted in the ground branch at k', and it will be found to be loudly affected. If both keys K' and K' are closed, silence is again obtained in the four tele- Fig. 7. phones a, b, x, y. In this case the charge and dis- charge from the wires l" and L' divide at the centre and flow back and forth at both ends, x and y being silent because they are at neutral points, and a and b are not affected by the currents which flow through them because of the differential action referred to. Two systems have now been described, one in which the induced current is lateral, and the other in which it is longitudinal. I think it follows from the foregoing experiments that where wires are twisted about each other, as shown in Fig. i, both of these actions are combined. At the left hand of Fig. 8 a cross-section of the three wires l', l', and l' [Fig. i] is shown. In this position the wires occupy Appendix. 257 a place with reference to each other exactly as in •^'g' 2, and the tendency of the disturbing wire is to cause a longitudinal flow in l' and l'. If repeated cross-sections of these wires are made, a point will be reached at which the three wires are disposed as shown at the right hand of Fig. 8, where it is seen that the disturbing wire L' is at an equal distance from \I and l', and the tendency is to produce a lat- eral flow. The actual currents produced must be the resultant of these two actions. Fig. 9 shows a plan quite different in principle from anything heretofore employed. It is of interest not so much on account of any practical application which it may have at present, but because it is a very striking proof of the electro-static nature of in- ductive cross-talk between telephone circuits, l', t-'/^^ i ? 1 i,»^-^ 1 5 S ■f- I> Fig. 9, L°, and l' are the same wires as used in the previous cases, L' being half an inch from L^ with the addi- tion of an extra wire, L*, placed half an inch from L' and joined by a conductor w with the disturbing wire l'. l' and L' are three feet apart. When in this condition the transmitter is operated, no dis- t rbance whatever is heard in the end telephones ; if 17 258 Appendix. the wire w be disconnected, the usual noise is heard, but is found to disappear as often as L' and L* are joined together. This action is explained by the fact that l' is at the same potential as L", on account of being joined to it by the wire ic, and acts with the force on L' that L' does on l'. The flow in this case is lateral, as indicated by the arrows ; and the telephones are silent. Neutral points may be produced in a circuit by the use of shunts. Fig. 10 shows the usual arrangement of circuits with the telephones a and b at the ends, and another telephone x, of equal impedance, branched between the two wires at the centre. In this case four neutral points are found, two in each wire. The Fig. 10, currents produced by the discharge are indicated by the arrows. Thus, by the addition of one shunt, the disturbing currents in the end telephones have been reduced one-half. If similar shunts were placed at the quarters, the currents at the end telephones would be still further reduced to one-quarter of their original strength. This plan is not a practicable one, because of its shunting effect on the telephone current, but is of value as showing one of the actions which occur Appendix. 259 when instruments are bridged into metallic circuits. It is interesting to note that in Fig. 10 the telephone X is affected by a current twice as great as that which flows through either of the end telephones. Before closing I shall describe one more experi- ment. In Fig. II, l' is the disturbing wire and l' is a grounded telephone circuit placed half an inch from l'. At the centre of l" there is an ordinary T^ N Fig. telephone repeating coil or transformer, c, containing two windings, e and /, of copper wire, each having a resistance of 160 ohms. One end of each winding is grounded, and the other end is connected to the line as shown. Assuming that the impedance of each telephone is equal to that of each coil of the trans- former, a neutral point will be found at the centre of each half of l', and the disturbing currents flowing through the end telephones will be only half as strong as though the transformer were omitted. If, now, the connections of the transformer be reversed so that the discharges from the two sections of line pass through it in opposite directions, no magnetism will be produced in the core k, and consequently the transformer coils offer an easier path to the discharge. 26o Appendix. This causes the neutral points to move toward the end telephones, and consequently reduces the disturb- ance still further. I have not had an opportunity of trying this ex- periment with a transformer whose coils contained a lower copper resistance and a high inductance, but according to theory we might expect that such a transformer having its coils connected differentially, should free a grounded line of considerable length from cross-talk and other electro-static disturbances. The number of such coils which can be worked in a given line is of course limited, but with properly de- signed apparatus a large number of them might be used. This arrangement is also interesting when considered with reference to electro-magnetic induc- tion, and brings to mind a question as to whether we may not at some future time abandon the use of metallic circuits and again make use of Steinheil's discovery. In the discussion on this paper, Mr. Thomas D. Lockwood, although acknowledging that Mr. Carty was correct in attributing a large share of the dis- turbance to electro-static influences, yet maintained that insufficient importance was given to the effect of electro-magnetic induction. He said, also, that while the twisted metallic circuit " is indeed a speci- fic against electro-magnetic inductive disturbance, it also tends to increase the retarding effect of electro- Appendix. 261 static induction exercised between the two wires of the circuit." The correctness of this view may be very seriously questioned. Mr. Lockwood further calls attention to the fact — which had long been recognized, and which was mentioned by Mr. Carty in a previous paper — that an electro-static charge, while it is changing, is an elec- tric current. The action must therefore be a com- pound one, and electro-magnetic induction must be a factor in causing disturbances. This same point was mentioned by several others who took part in the discussion. It is not denied by Mr. Carty, and in fact he has in several places dis- tinctly stated that electro-magnetically induced cur- rents must exist. His experiments, however, show quite clearly that in comparison with electro-static influences, under the conditions usually met in prac- tice, the electro-magnetic induction is negligibly small. His account of further experiment sustain- ing him in this view was as follows : " I will now show what I consider the most favor- able condition for creating electro-magnetic induction iJ ii?" b Fig. 12. between two telephone wires. That [indicating T, Fig. 12] represents the coil of a long-distance trans- 262 Appendix. mitter sending the most powerful telephone current that it is now possible to generate. L' represents the wire connected to earth. It may be, we will say, 200 feet long. Parallel to it, and an eighth of an inch away, we will place another telephone circuit. These wires may have an insulation of io,ooo meg- ohms, and the resistance might be considered as one ohm. Now, a current resulting from a given note in the transmitter through the wire l', produces a series of changes in the magnetic field surrounding it, and that action is roughly explained by assuming that the current starting in l' induces a current in l" in the opposite direction, and that the induced current will flow through both telephones a and b. Now, it is a fact that under those conditions absolutely no sound is heard in the telephones a or b. " That represents the strongest current that it is possible to produce by that transmitter — the strong- est changes — and consequently we should have the greatest fluctuation in the magnetic field surrounding the wire L' ; but under those circumstances there is absolutely no disturbance whatever effected in the telephones a ox b located in l'. That, I think, is a crucial test. " Now, I will describe an experiment which I made with those same wires. We will open the wire l' (Fig. 13) at the far end. Then, when the transmit- ter is operated, noise is immediately found at the telephones a and b, located at the ends of the second Appendix. 263 ary wire, and if a telephone, c,be located at the exact centre of impedance it will be found to be silent. When the wire l' is opened at the far end we have the maximum electro-static and the minimum elec- ^ s % Fig. 13. tro-magnetic action ; when it is closed we have the maximum electro-magnetic action and the minimum electro-static action. The reason that the electro- static action is slight in Fig. 12 can be seen from Fig. 14. Assuming that m n represents the height of po- n ,-1 o 0/ 'WAAWV^ Fig. 14. tential of the transmitter, the resistance of the cir- cuit is mostly in the transmitter, and the fall of po- tential therein is very sudden. At any point along the wire l' the potential would be practically zero, as shown by the dotted line op. " I wish to show another experiment in proof of the 264 Appendix. fact, or what I think is a fact, that electro-magnetic induction is negligible when we consider the action that goes on between two telephone circuits. I ex- pressly limit the statement to the action between telephone circuits, and I am not discussing disturb- ances in general, but merely inductive cross-talk. Now, I will draw a line — l" — (Fig. 15), similar to the ** VWWVv- ^^ ^ Fig. is. one I used before, and we will assume that the cir- cuit is completed by a return wire of no resistance, or of very low resistance, entirely outside of the field of disturbance, so as to eliminate all questions of leakage through the earth. We will say that l' is grounded on a gas-pipe. We then have the disturb- ing wire as before, and when the transmitter is oper- ated, loud tones are heard at the end telephones, a and b, thus : If this were entirely due to the creation of an electro-magnetic field around the disturbing wire, the effect on the secondary wire would be in- creased by short-circuiting one of the telephones by the key k. Now, it is found that when this is done, instead of increasing the sound in the other tele- Appendix. 265 phone, it absolutely removes it. Now, the case just described is another condition where you should get an increase of electro-magnetic action, if the disturb- ance is due to the magnetic field ; but the noise, in- stead of increasing, disappears entirely. Now, you will see how beautifully the thing may be explained by assuming that the disturbance is due to direct electro-static action. In this case we will say that the wire l\ at a given instant, has a plus charge upon it which induces a minus charge on \?. Now, when the inducing charge is removed, the induced charge divides and flows away at both ends. But as there is a great deal of resistance in telephone b, and prac- tically no resistance in key k, the charge and dis- charge of L° takes place up and down through the key end, and none of it goes through the telephone b. Now, I think that is a complete refutation of the statement that any portion of the noise produced in the telephone is due to electro-magnetic action. There is no doubt about it that when the current is going through L', in the act of charging it, that there is a magnetic field surrounding that current, but I have shown that it is so feeble that it does not pro- duce any effect in the telephone ; that is, the action that is due to the electro-magnetic field is so feeble that the ear is not able to tell whether it is present or not." In reply to a question by Mr. George B. Prescott, Jr., as to whether any experiment had been tried, 265 Appendix. using a powerful current as a disturbing source, Mr. Carty says : " Now, in this case, T, Fig. 12, represents a vibrator connected with a large battery. We had a very powerful current flowing through that wire, L". The current was constantly vibrating, and it was a very strong current, indeed. Now, in that case 1 used the same secondary wire, and a telephone in the centre, and then we found a noise in all three tele- phones, just exactly what you should expect in deal- ing with electro-magnetic induction. You would expect to find a current at a given instant, with very slight differences, constant in all parts of the circuit ; that is, you would expect to find as much noise at the centre telephone as at the end telephone. You would expect that if this middle telephone was short-circuited the noise would be increased at the end telephones, and then you would further expect that if one end telephone were short-circuited, that the remaining one would be still louder. That was the case. Now, consider the circuit with the middle telephone cut out and one end telephone cut out, and the other end telephone giving a loud noise, with a strong current in L' ; but with the strongest tele- phone current we could produce in l' there was no noise there at all. And further than that, the meth- ods of every-day practice have been changed to meet these views, and our predictions and calculations, based on this way of working, are invariably correct. Appendix. 267 All of our cables, which are twisted in pairs, are sub- jected to a very rigid cross-talk test, and some of the refinements which have to be taken into considera- tion in making those tests are certainly most sur- prising." From data furnished by Mr. Carty, Mr. A. E. Kennelly made some calculations " on the relative degree of disturbance caused by electro-static and electro-magnetic induction between certain simple ar- rangements of telephone circuits." He found that with the arrangement shown in Fig. 2 of Mr. Carty's i '■"'- '-'E"''- "— Fig. 2. paper, the ratio of static to magnetic disturbance is !— ^ where k = the mutual electro-static capac- 2 /i ity of I-, and L, per centimetre of length ; ^ = mu- tual induction of l,, and L^ per centimetre of length ; «», = impedance in the circuit of L, ; <», = imped- ance in the circuit of L^. " Provided, then, that the impedance of the pri- mary circuit at the receiving end is always large, the ratio of static to magnetic disturbance will be ap- proximately expressed by this equation." 268 Appendix. Giving the quantities in this expression the proper values for the case shown in Fig. 2, the impedance o), is found to be nearly twenty times greater than that needed to produce equality between the two kinds of disturbance, " and, roughly, the static dis- turbance in Fig. 2 may be estimated as twenty times greater than the magnetic." INDEX. Altbenating currents, 114. Aluminium bronze, 66. Amplitude, 98. Anchor-rod, 47. Annunciators, 85. Answering-jacks, 85, 86. Apparatus for testing wire, 63. Arrangement of cables in exchanges, 79. of cables in man-holes, 28. Artificial foundations, 42, 43. Atom, 95. Atomic theory, 95. Automatic exchanges, 94. Ban]0, S3- Battery room, 78. Belgian rolls, 59. Belt circuits, 174. Bloom, 59. Bolt, fetter-drive, 40. Brace-straps, S, 40. Bracing of cross-arms, 5. Branch office, 88. Breaking, poles, 45. Breaking strength, hard-drawn copper, 65. . Breaking strength, tests, 62. Bridging-bell system, 214. Bridle wires, 30. Bronze wire, 66. Cable-box, 28, 29. 31. Cable capacity, 222, 236. dimensions, 220. head, 30, 80. impedance, 222. insulation, 217. shaft, 78. sheath, 218, 219. size of, 27. Cables, 32, 217. specifications, 221. Cabling, 218. CaUing wire, 93. Capacity, city lines, 157. concentric cables. 235. effect of, on current, 129. measurement of, 148. metallic circuits, 197. metallic circuits, formula, 200. the " K.R " formula, 196. Carty's experiments, 134, 246. Catenary, S5- Cedar poles, 4, 5. Cement conduits, 15, 22. Cement lined iron-pipe, 18. Character of sound, 113, Chenoweth conduit, 20. Chief inspector, &i. Chief operator, 81. City lines, insulation, 158. City lines, properties of, 155. Clearing obstacles, 39. Clearing-out drops, 85. Clearness, 113, 143. Coefficient of self-mduction, 120. Cold-rolled wire, 60. Combination board, 89. loop, 89. Concentric cable, 234. Concrete, 15. Condensers, use of, 180, 187. Conductivity of wire, 6^. Conductivity of wire, hard-drawn copper, 61, 65. Conduits, 14. " Conference standard " cables, 34 Connections in cable-box, 30. Construction of cables, 35. Copper wire, 9. Corners,.?, 50. Corrosion, bronze wire, 67. copper wire. 68. Cost of so-wire line, 38. Cross-arms, 5, 40. 2/0 Index. Cross-arms, setting, $. Cross-country line, 39, 44. Cross-talk, 132, 232. Cross-talk, city lines, 161. formula, 164. metallic circuits, 204. Dead-ending, 56. Design, metallic currents, 193. Destruction of cable-sheath, 21. Deterioration of wire, 67. Diameter, measurement, 63. Dimensions, twisted pair cables, 34. Dimensions, poles, 38. Dip, 10. Dip and pull, formula, 55. Distributed capacity, effect of, 130. Distributing board, 80, 85. Distributing room, 79. pole, 29. wires, 80. Distribution, 28. Distribution, city, 3. Disturbanceo, city lines, 160. Dorsett conduit, 18. Double petticoat insulator, 71, 74. Double trolley system, 179, 180. Drawing in cables, 25, 26. Drops, 85. Earth currents, 165. Earthenware, 69. Ebonite, 70. Effect of moisture on insulation, Efficiency of cables, 228. Electric currents, '106. lighting circuits, 171. railways, interference, 176. Electro-magnetic disturbance. 103. Electro-motive force of telephone current, 145. Electrostatic action, 104. Elongation of copper wire, 62. Energy, 95, 100, 107, 109. Entrance of cables into exchanges, 79. Ether. 96, 102. Exchanges, 77. Explosions in conduits, 25. Facing cross-arms. 6, *' Faraday " cable, 218. Fetter drive bolt, 40. Fibre, ■^'^, 217. Finishing train, 60. Flow of current in a conductor, 115. Flow of energy, 100, 107, 109. Form of insulator, 74. Formulae for conductivity, 64. Formulae for resistance per mile, 65. Formulae for weight per mile, 65. Fusible arresters, 80. Galvanizing, 40. Gas, in conduits, 22. in man-holes, 24. Glass, 69. Ground potential, mapping, 192. Ground rods, 188. Grouping system, 82. Grouting, 42. Guy, 45. Guy clamp, 47. rope, 45, 46. stub, 12, 48. wire, 12. Guying corner poles, 8. straight hues, 51. Hard-drawn wire, 61. Harmonic vibration, 103. Head-guy, 47. Height of poles, 5, 39. Hertz's experiments, 105. House-top lines, 2. Hysteresis; 125. Impedance, 118, 126. of cables, 222. formulce, 116, 127. for high frequencies, 245. measurement, 148. metallic circuits, 196. Inchnation of poles, 45. Inductance, 120, 124. Induction coil, action of. 108. Induction from lighting circuits, 167. in magnetic medium. 124. disturbances, remedies, 169. telegraph, 167. Inspection of wire, 61, Insulation in cables, '^■^, 217, 231. in city lines, 158. in metallic circuits, 202. Insulators, 69. Insulator tests, 71. Index, 271 Interference from electric railways, 176. from outside sources, 165. Iron conduits, 15, 16, 17, 22. wire, 8, 66. Jacks, 84, 85, 87. Johnstone conduit, 16. Joint, American, 11, in cable, 27. in line, 54. in iron conduit, 17. Lag, magnetic, 125. Lake conduit, 19. Law system. 93. Laying concrete, 17. Lead, corrosion of, 21. Length of waves, 103, 104. Life of wire, 9, 10. Lightning arresters, 30, 32. conductors, 243. discharges, 242. Line drops, 85. Lines of induction, 120, Locust, 5. Long distance insulator, j^, lines, 3/. Longitudinal vibrations, 98. Long lines, 212. *' Looping in," 213. Loss of volume in transmission, 211. Loudness, 98, 143. Man-holes, 23. construction, 24. cover, 24. Manufacture of cables, 218. of wire, 59. Maximum load tests, 62. McCluer system, 179, 181. Mclntire sleeve, 11. Measurement, 148. Metallic circuit boards, 86, 88. Metallic circuit, properties of, 194. Moisture, on insulators, 69, 73. in cables, 230. Molecule, 95. Multiple switchboard, 83, switchboard connections, 85. Mutual induction, 121. _ induction, metallic circuits, 204. Nicks in hard-drawn wire, 66. Office building, 77, Operating room, 78. Operator, 83, 84. Oscillations, 241. Overhead city lines, j,. Overtones, 102. Paper cables, 219. conduit, 16. Paraffine, 218, 227. Party'lines, 4. lines, metallic circuits, 92, 213. Piling, 43. Pine, Norway, 5. Pins, 5, 40. Pitch of sound, 98, 113. Placing of wires, 56. Plugs, 87, 91. Pole brace, 49, Pole lines, capacity of, 157. Pole, preparation of, 39. setting, 5, 42. transposition, 58. Poles, 4, 38. Porcelain, 69, 70. Potential, ground, mapping, 192. Propagation of energy, 95. Properties of city lines, 145. of lines, 114. of metallic circuits, 194, Pulling up wires, 53, 54. Quality, 113, 143. Remedies for interference, elec- tric railway, 179. Repeating coils, 91, 179, 186. Resistance, alternating currents, 116. city lines, 156, effect of, on overtones, 117. measurement, 148. metallic circuits, 196, per mile, tests, 63. Retardation, 118, 126. cables, 231. city lines, 160. metallic circuits, 195, 200. Road crossing, 50. Rodding, 26. Rubber insulation, ^^, 217. Running board, 52. Running rope, 52. Sag, 10. Scaling of wire, 60. 272 Index. Seasoning, 38. Self-induction, 119. cables, 231. metallic circuits, 203. Setting cross-arms, 6, 41, 40. Setting pins, 41. Setting poles, 5, 42. Side guy, 48, 49. Silicon bronze, 66. Single petticoat insulator, 71, 74. Single trolley system, 177. Sound, 98, III. Spacing poles, 44. Specific inductive capacity of fibre, 217, 227. rubber, 34, 217, 227. table, 227. Specifications for cables, 221. for cable sheath, •^^. for guy-rope. 46. for hard-drawn copper, 61. Spiralled fours, 238. pairs, 236. " Standard " cable, 219, Standard long distance pole, 41. Steam-heating pipes, 23. Steel wire, 66. Stoneware, 70. conduits, 22. Stringing, 10, 52. Sub-exchanges. 3, 4. Switchboards, 82. System, design of, 2. Telephone current, iii. Terminal poles, 50. Terra-cotta conduit, 19, 23. Testing cables. 27. 80. insulation and capacity, ex- change, 81. insulators, 71. wire, 62. Thomson bridge method of test- ing. 63. Thomson's, Sir William, formula for resistance, 116. Tie, 10, II, 54- Torsion tests, 63. 'Iransformations of energy, 96, 100, 107. Transmission of speech, 113. through high resistance, 118. Transmitters, 145. Transposition in cables, 232. insulator, 76, Transpositions, 42, 57, 58, 206, 209. Transverse vibrations, 99. Twin plugs, 87, 91. Twisted pairs, 226, 232. Unbalanced lighting circuits, 172. Underground lines, 14, 220. Valentinr conduit, 15. Vegetable material, conduits of, IS- Velocity of propagation of wave, 99. 103- Ventilation of conduits, 25. Voice. 112. Volume, measurement of, 148, 151. of telephone current, 145. Vortex theory, no. Water in conduits, 22, 24. Wave-length, 98. Waves, 97. " Western Electric " cable, 219. " Wiley-Smith " system, 180, 187. Wire, 8, 59. Wire clamp, 53. Wood conduits, 15, 20, creosoted, 15, 20, 21. insulators, 71. WorI<, 109. Wyckoff conduit, 16. Y-GtJY, 8, 48. Zinc tube conduit, 20.