c/ _ CjffLrtfC LIBRARY OF THE UNIVERSITY OF CALIFORNIA. , i8g No. ....... 8.2.5.67.. . Class No. AMERICAN TELEPHONE PRACTICE BY KEMPSTER B. MILLER, M. E. THIRD EDITION, REVISED AND ENLARGED NEW YORK AMERICAN ELECTRICIAN COMPANY 120 LIBERTY STREET )f C O library COPYRIGHT, iqoo, BY AMERICAN ELECTRICIAN COMPANY PREFACE. THE intended scope of this book is set forth in its title. To those interested the writer has endeavored to present in as clear a manner as possible the general principles of telephony, the design and construction of commercial apparatus, the circuits connecting such apparatus into operative systems, and the methods used in the construction, operation, and maintenance of these systems. No attempt whatever has been made to treat the subject from its purely mathematical standpoint, that being beyond the scope of this work. The apparatus and methods of both Bell and Independent companies have been given im- partial attention. The writer sincerely thanks his friends, Mr. Wm. H. Donner and Mr. Wm. R. Mackrille, for their many suggestions and untiring labors in proof reading, and also Mr. W. D. Weaver, editor of the Electrical World and Engineer, for his interest and assistance throughout the entire preparation of this book. KEMPSTER B. MILLER. 82567 PREFACE TO THIRD EDITION. THAT a third edition of this work should have been called for within three months after the appearance of the first is indeed gratifying. The time since the second edition was exhausted has been utilized in making many changes in the matter already presented, and in the preparation of much new matter, all of which it is thought will make the work more valuable as a guide and general reference-book in practical telephony. The chapter on Automatic Exchanges has been written because the book seemed incomplete without it. If it is here- after criticised for containing no descriptions of practical appa- ratus, my plea will be that it is mainly the fault of the subject. That very important factor in modern telephony, the storage battery, was certainly very inadequately handled in the first edi- tion. Chapter XXXV. is intended to remedy this. The last chapter, on Specifications, is given as a help rather than as an inflexible guide to those drawing specifications. The specifications given represent modern practice, but they are not meant to serve in the place of common sense or of engineering ability. I desire to thank Mr. Thomas D. Lockwood for his kindly criticisms on the first editions ; Mr. Franz J. Dommerque, who has read the proofs of the chapters on Storage Batteries and Specifications, and made many good suggestions ; and Mr. E. R. Corwin, to whom I am indebted for information concerning lead- burning. KEMPSTER B. MILLER. TABLE OF CONTENTS. CHAPTER I. PAGE HISTORY AND PRINCIPLES OF THE MAGNETO TELEPHONE, i Early Knowledge of Electromagnetism Work of Oersted, "Ampere, Arago and Davy, Sturgeon, Faraday, and Henry Transformation of Electric into Magnetic Energy Transformation of Magnetic into Electric Energy Field of Force Morse's Telegraph Reis' Telephone Sound Waves Bell's Telephone House's Electro- Phonetic Telegraph. CHAPTER II. HISTORY AND PRINCIPLES OF THE BATTERY TRANSMITTER, . . 10 Gray's Variable-Resistance Transmitter Berliner's Transmitter Elec- trodes in Constant Contact Carbon Electrodes Demonstration of Advantages of Loose Contact by Hughes Hughes' Microphone Hunning's Granular Carbon Transmitter Induction Coil with Transmitter Advantages of the Local Circuit. CHAPTER III. THE TELEPHONE RECEIVER, 18 Considerations in Designing Mechanical and Electrical Efficiency Single-Pole Receivers Bipolar Receivers Adjustment between Magnet and Diaphragm Material for Shells Faults of Imitation Hard Rubber Commercial Types of Receivers Receiver Cords Details of Cord- Tip Supports for Receiver Cords. CHAPTER IV. CARBON TRANSMITTERS, v . 32 Action of the Transmitter Single-Contact Transmitters Multiple-Con- tact Transmitters Granular Carbon Transmitters Commercial Types of Transmitters Packing : Its Remedy Unusual Forms of Transmitters Types of Carbon Electrodes. CHAPTER V. INDUCTION COILS, . 53 Advantages of the Induction Coil Primar)' Current Secondary Cur- rent Design of Induction Coils Methods of Making Comparative Tests Results of Comparative Tests Selection of Coil for a Trans- mitter Commercial Coils Varley Method of Winding Mounting of Induction Coils. VI TABLE OF CONTENTS. CHAPTER VI. PAGE BATTERIES .... 62 Simple Cell Direction of Current Positive and Negative Poles Materials Best Suited for Electrodes The LeClanche Cell The Fuller Cell Specifications for Standard Fuller Cell The Gravity Cell Local Action in Batteries Amalgamation of Zinc Storage Batteries The Setting up and Operating of Storage Batteries Determination of Positive and Negative Poles of Charging Circuit Density of Electrolyte Advantages of the Storage Battery. CHAPTER VII. CALLING APPARATUS, 75 Battery Calls The Magneto-Generator Action of Magneto-Generator The Polarized Bell or Ringer Design of Magneto-Generators Construction of Armature Core Winding of Armature Permanent Magnets Form of Current Wave Design of Polarized Bells Iron for Ringer Frames Length of Hammer Rod. CHAPTER VIII. THE AUTOMATIC SHUNT, 86 Necessity for the Automatic Shunt Commercial Types. CHAPTER IX. THE HOOK-SWITCH AND CIRCUITS OF A TELEPHONE, . . . . 90 Functions of the Hook-Switch Simplified Telephone Circuits The Warner Hook-Switch Other Forms of Hook-Switches Circuits of a Series Telephone Circuits of a Bridging Telephone Battery Call Instruments Desk Telephone Wiring of 'Telephones. CHAPTER X. COMMERCIAL CALLING APPARATUS, 104 Types of Generators and Ringers Resistances of Armatures and Ringer Magnets for Different Kinds of Work Constantly Driven Genera- tors Methods of Driving Generators Motor Generators. CHAPTER XI. THE TELEPHONE RELAY OR REPEATER, 118 Simple Relay Circuit Difficulties in Producing a Two- Way Repeater Circuits of Two-Way Repeater The Erdman Repeater The Stone Repeater. TABLE OF CONTENTS. Vll CHAPTER XII. PAGE SELF-INDUCTION AND CAPACITY 124 Ohm's Law Field of Force about Conductor Electromagnetic Induc- tion Action between Turns of the Same Coil Impedance Effects of Self-Induction on Undulatory Currents Charge of Electricity Action between Like and Unlike Charges Electrostatic Induction Condensers Capacity Specific Inductive Capacity of Dielectrics Specific Inductive Capacity in Telephone Cables Effect of Con- denser Bridged across Circuit Effect of Capacity on Varying Cur- rents Trans-Oceanic Telephony. CHAPTER XIII. TELEPHONE LINES, 136 Grounded Circuits Noises on Grounded Circuits Causes of Line Dis- turbances Electromagnetic Induction Electrostatic Induction Carty's Experiments Cross-Talk Elimination of Cross-Talk Transpositions Ground-Return Systems Common-Return Sys- tems Location of Common-Return Wire Size of Common-Return Wire Connection of Metallic and Grounded Circuits The Repeat- ing Coil Elimination of Local Induction. CHAPTER XIV. SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES, 153 Manual and Automatic Switch-Boards Switch-Boards for Grounded or Common-Return Systems Types of Drops and Jacks Switch- Boards for Metallic Circuits Tubular Drops. CHAPTER XV. LISTENING AND RINGING APPARATUS FOR SWITCH-BOARDS, . . . 163 The O'Connell Key The Cook Key The American Key Plug Listen- ing and Ringing Devices Plug-Socket Listening Key. CHAPTER XVI. SELF-RESTORING SWITCH-BOARD DROPS 173 Electrically Restoring Drops Mechanically Restoring Drops Com- mercial Types. CHAPTER XVII. COMPLETE SWITCH-BOARDS FOR SMALL EXCHANGES, .... 183 Arrangement of Switch-Board Parts Description and Operation of Boards in Common Use. CHAPTER XVIII. LAMP-SIGNAL SWITCH-BOARDS, 192 Advantages of Incandescent Lamps for Signals Lamp Directly in Line Circuit Lamp in Local Circuit Controlled by Relay Pilot Lamp Life of Lamp Signals. Vlll TABLE OF CONTENTS. CHAPTER XIX. PAGE THE MULTIPLE SWITCH-BOARD, . 200 Limitations of the Simple Switch-Board General Arrangement of Apparatus in the Multiple Board Tests for Busy Lines The Series Multiple Board: Its Disadvantages The Branch Terminal Multi- ple Board Spring-Jacks for Multiple Boards The Kellogg Divided Multiple Board. CHAPTER XX. TRANSFER SYSTEMS, 216 Complexity of Multiple Switch-Boards The Sabin and Hampton Express System Incoming and Outgoing Trunk Lines Details of Subscribers' Circuits Details of Trunking Circuits The Clearing- out Signals Western Telephone Construction Company's Transfer System The Cook-Beach Transfer System The American Trans- fer System. CHAPTER XXI. COMMON BATTERY SYSTEMS 236 Advantages Series Systems for Grounded and Metallic Circuits The Stone System The Hayes Repeating-Coil System Supplying Cur- rent over Two Line Wires in Multiple Storage Battery at Sub- scriber's Station. Dean's Thermopile System Carty's Multiple- Transmitter Circuit Series-Transmitter Circuit Detailed Descrip- tion of Dean's Common Battery System Of Scribner's Common Battery System Hayes' Systems as Applied to Multiple Board Scribner's System as Applied to Multiple Board The St. Louis Bell Exchange. CHAPTER XXII. HOUSE SYSTEMS, 265 General Plan of House or Intercommunicating Systems Common Bat- tery House Systems The Ness Automatic Switch Circuits of Holtzer-Cabot System. CHAPTER XXIII. PROTECTIVE DEVICES, 272 Static Arresters Fusible Arresters Thermal Arresters Heat Coils Combined Thermal and Static Arresters The Rolfe Arrester. CHAPTER XXIV. DISTRIBUTING BOARDS, 281 Functions of the Distributing Board The Hibbard Board The Ford and Lenfest Board Jumper Wires. TABLE OF CONTENTS. IX CHAPTER XXV. PAGE PARTY LINES NON-SELECTIVE, 294 Classification The Series Party Line Generators and Ringers for Party Lines Carty's Bridged Party Line Generators and Ringers for the Bridging System Induction Coils for Bridged Lines Con- nection of Switch-Board Drops to Party Lines Lock-out Systems. CHAPTER XXVI. PARTY LINES STEP-BY-STEP SELECTIVE SIGNALING, .... 308 General Operation Early Systems of Dickerson, Anders, and Lock- wood Bridging System of Reid and McDonald. CHAPTER XXVII. PARTY LINES SELECTIVE SIGNALING BY STRENGTH AND POLARITY, . 318 Success of this Method in Telegraphy Early work of Anders Sabin and Hampton's Three-Station Line Hibbard's Four-Station Line McBerty's Four-Station Line Dean's Eight-Station Line The Barrett-Whittemore-Craft System. CHAPTER XXVIII. PARTY LINES HARMONIC SYSTEMS OF SELECTIVE SIGNALING, . . . 338 Principles Involved Early Experiments Lighthipe Bridged System Harter System Unsatisfactory Development of Harmonic Systems. . CHAPTER XXIX. WIRE FOR TELEPHONE USE, . . . ... . . . . 347 Iron, Copper, and Aluminum for Conductors Tensile Strength Con- ductivity Resistance Weight per Mile-Ohm Circular Wire Gauge Micrometer Gauge The Brown and Sharpe Gauge Other Wire Gauges Manufacture of Iron Wire Galvanizing Tests for Galvanizing Grades of Iron Wire Specifications for Iron Wire Advantages of Copper for Wire Specifications and Data for Copper Wire. CHAPTER XXX. POLE-LINE CONSTRUCTION, . . . 360 Woods for Poles Treatment of Poles Sizes of Poles Number of Poles per Mile Height of Poles Pole Guards Data Concerning Poles Creosoting and Vulcanizing Poles Cross-Arms Attaching Cross- Arm to Pole Insulators Construction Tools Setting Poles Guy- ing Terminal Poles Anchors Pole Braces Methods of Tying and Splicing Transpositions. X TABLE OF CONTENTS. CHAPTER XXXI. PAGE OVERHEAD CABLE CONSTRUCTION, . . . . . . . 389 Necessity for Cables Comparative Cost of Cable and Bare-Wire Con- struction Rubber Cables Objections to Rubber Cables Saturated Core Paper Cables Dry-Core Paper Cables Lead Sheaths for Cables Data Concerning Cables Supporting Strand for Cables Cable Hangers Method of Suspending Cables Directions for Splicing Cable Terminals Directions for Making Pot-Head Terminals. CHAPTER XXXII. UNDERGROUND CABLE CONSTRUCTION, 409 Requirements for Conduits Economy of Space Wooden Conduits Clay Conduits Cement-Lined Pipe Cement Arch Conduit Methods of Laying Conduits Man-Holes Rodding Drawing in of Cables Steam Power for Drawing in Gas in Man-HolesElec- trolysis Determination of Danger Points Prevention of Elec- trolysis. CHAPTER XXXIII. TESTING, 424 Classification of Tests Rough Tests Magneto Testing Set Tests for Grounds and Crosses with Magneto Receiver Test for Grounds and Cosses Continuity Test Quantitative Measurements Measure- ment of Resistance The Wheatstone Bridge Directions for Operat- ing Bridge The Thomson Galvanometer The D'Arsonval Gal- vanometer Advantages of the D'Arsonval Galvanometer The Gal- vanometer Shunt Taking of Galvanometer Constant Insulation Tests Capacity Tests Location of Faults The Varley Loop Test. CHAPTER XXXIV. AUTOMATIC EXCHANGES, 453 Early Attempts Connolly and McTighe's Apparatus Pioneer Patent on Automatic Exchanges System of Almon B. Strowger Improve- ments by Keith, Lundquist, and Erickson Atlanta, Ga. , Exchange Clark's Automatic System-^The Fault Common to all Automatic Exchanges. CHAPTER XXXV. STORAGE BATTERIES, 471 Fundamental Principles Plante Cell Direction of Current Internal Resistance Chloride Accumulator Construction of Chloride Plates Plant of Bell Telephone Company at Philadelphia The American Cell Construction of Plates Installation, Care, and Main- tenance of Storage Cells Setting Up and Connecting Lead Burn- TABLE OF CONTENTS. XI i PAGE ing Hydrogen Flame for Lead Burning Gas Flame Electrolyte Density of Original Solution Charging Rate of Charge Indi- cations of Full Charge Use of Hydrometer Discharging Rate of Discharge Indications of Discharge Replacing of Electrolyte Faults in Cells Treatment of Defective Cells Color of Plates Sulphating. CHAPTER XXXVI. SPECIFICATIONS, 487 Object of Specifications Specifications for Small Exchange Switch- Board Batteries Telephones Diagram of Circuits Specifications for Large Exchange General Conditions Material and Inspection Risk Time of Completion Equipment of Switch-Board Wire and Wiring Electric Lighting Equipments of Chief Operator's, Monitor's, Wire Chief's, and Trouble Clerk's Desks Distributing Boards Power Plant Power Switch-Board Charging and Ringing Machines Storage Battery Subscribers' Apparatus Wall Sets Desk Sets Apparatus for Subscribers' Sets Cords and Connec- tions Wire Plant Underground Work Conduits Trenches Ob- structions Blasting Manholes Concrete and Mortar Laterals and Pole Connections Paving Pole Lines Depth of Holes Dimensions of Poles Cross- Arms Pins Brackets Braces Hard- ware Guys and Guying Messenger Wire Cable Boxes and Plat- forms Hangers Underground Cables Splices and Joints Aerial Cables Suspension Paper Cable Rubber Cable Bare Wire Joints Tying Dead-Ending Stringing Drop and Service Wires. AMERICAN TELEPHONE PRACTICE. CHAPTER I. HISTORY AND PRINCIPLES OF THE MAGNETO TELEPHONE. THE history of the telephone, from its inception to its present state of perfection, is interesting in the extreme, and affords a striking example of the fact that great inventions are almost in- variably the result of long and careful study on the part of many workers, rather than the sudden inspiration of a single genius. It is of even greater interest from a scientific standpoint, for in no way can one obtain a better idea of the fundamental princi- ples involved in telephony than by following their development, step by step, noting the contributions made by each of the many scientists and inventors whose names are closely connected with electrical progress. These steps were made in logical order, the knowledge con- tributed by each investigator making possible a deeper insight into the subject on the part of his successors. It is best, there- fore, to follow this order in obtaining primary ideas of the subject. The history of the knowledge of electromagnetism begins with July 20, 1820, and with this date very properly begins the history of the electric telephone. On that day Oersted, a professor in the University of Copenhagen, discovered that a magnetic needle tends to place itself at right angles to a wire carrying a current of electricity. Ampere immediately took up the subject, and in a very short time developed the laws upon which present electro- magnetic theory is based. In the following year Arago and Davy discovered that if a current be caused to flow through an insulated wire wrapped about a rod of steel the latter would exhibit magnetic properties. It was William Sturgeon, however, who in 1825 made an electro- magnet as we know it to-day, and called it by that name. To these three men, therefore, belongs the credit of one of the greatest discoveries in the history of science. Joseph Henry also made 2 AMERICAN TELEPHONE PRACTICE. his classic experiments on the electromagnet, and to him must be accredited a large amount of our knowledge regarding it. Henry showed how to build a magnet capable of being operated over a great length of wire, a most important step. In 1831 Faraday and Henry, independently, discovered the con- verse of these laws of electromagnetism that if the intensity of a magnetic field inclosed by a conductor be in any wise changed, a current of electricity will flow in said conductor. This current will flow only while such change is taking place, and its strength will depend directly on the rate of the change. These two laws concerning the transformation of electric energy into magnetic, and its converse, the transformation of magnetic energy into electric, are certainly the most important in the whole realm of electrical science ; as singly or together they form the foundations not only of the telephone and tele- graph, but of electric lighting, electric power transmission, and B Fig. i. Sturgeon-Electromagnet. of every other achievement by which electricity has revolution- ized the methods of life throughout the whole civilized world. As these laws form the very root of all telephone practice, a few illustrations directly in line with the principles of the tele- phone will not be amiss, even though they are very generally understood ; for they will give a clearer understanding of the developments made by subsequent inventors. If, as shown in Fig. i, a coil of wire be wrapped around a rod, R, of iron or steel, and a battery, B, placed in circuit with the coil, the rod becomes a magnet upon the closure of this circuit, and will attract an iron armature, A, in the vicinity of either of its poles. Any variation in the strength of this current will cause corre- sponding variations in the attractive power of the magnet. If the rod be of steel, and permanently magnetized, it will exert an attractive force of its own on the armature, and the current will, according to its direction, increase or diminish this attractive force. About every magnet there exists a field of force ; that is, a region in which any body capable of being magnetized (such as THE MAGNETO TELEPHONE. 3 iron) has exerted on it, by the magnet, an influence of attraction or repulsion. This field of force is usually graphically repre- sented by closed curves, radiating from the poles of the magnet, and the strength of the magnet is commonly measured in terms of the number of such lines radiating from one of its poles. A magnet may be made to map out its own field of force by plac- ing it in a horizontal position and directly over it a sheet of Fig. 2.- Lines of Force of Bar-Magnet. paper or cardboard. If iron filings are then dropped from a height of a few feet, on the paper, they will arrange themselves in the direction of the lines of force. Fig. 2 shows such a map produced by the bar-magnet, N S. If now a galvanometer, G, or other current-indicator (Fig. 3) be placed in circuit with a coil, C, and a magnet, N S, moved in Fig- 3- Faraday and Henry Magneto-Electricity. the vicinity of the coil, or the coil in the vicinity of the magnet in such manner as to change the number of lines of force passing through the coil, a current is generated in the coil and is indi- cated by the galvanometer. This current will flow only while 4 AMERICAN TELEPHONE PRACTICE, the magnet is being so moved. Its direction will depend on the direction of the lines of force threading the coil and on whether their number is being increased or diminished. Its strength will depend on the rate at which their number is changing. If a mass of iron be brought within the field of a magnet, the field becomes distorted by virtue of a larger number of lines find- ing their path through the space occupied by the iron than through the same space when filled with air. Therefore, if a closed coil be placed about a pole of the magnet and a body of iron be moved to and from the pole, the intensity of the field in which the coil lies will vary, and currents of electricity will flow in the coil. In 1837 Professor Page of Salem, Mass., discovered that a rod of iron, suddenly magnetized or demagnetized, would emit certain Fig. 4. Morse Electromagnetic Telegraph. sounds due to a molecular rearrangement caused by the changing magnetic conditions. This phenomenon is known as " Page's effect." Late in the thirties Professor S. F. B. Morse placed at one end of a line Sturgeon's electromagnet, M (Fig. 4), with a pivoted armature, A, and at the other end a battery, B, and a key, K, for making and breaking the circuit. By manually closing and open- ing the key, the core of the magnet became magnetized and de- magnetized, thus alternately attracting and releasing the arma- ture. By this means signals were sent and recorded on a strip of paper, carried on a roller, R, in front of the armature, and thus intelligence was practically conveyed by electrical means be- tween distant points. In 1854 a Frenchman, Charles Bourseul, predicted the trans- mission of speech, and outlined a method correct save in one particular, but for which error one following his directions could have produced a telephone of greater efficiency than that sub- sequently devised by Bell. His words at this date seem almost prophetic: " Suppose a man speaks near a movable disk suffi- THE MAGNETO TELEPHONE. 5 ciently flexible to lose none of the vibrations of the voice, and that this disk alternately makes and breaks the current from a battery ; you may have at a distance another disk which will sim- ultaneously execute the same vibrations." Philip Reis, a German inventor, constructed a telephone in 1861, following very closely the path outlined by Bourseul. He mounted a flexible diaphragm, D (Fig. 5) over an opening in a .- N Fig. 5. Reis' Make-and-Break Telephone. wooden box, and on the center of the diaphragm fastened a small piece of platinum, P. Near this he mounted a heavy brass spring, s, with which the platinum alternately made and broke contact when the diaphragm was caused to vibrate. These contact points formed the terminals of a circuit containing a battery, B, and the receiving instrument. His receiver assumed various forms, prominent among which was a knitting needle, N, wrapped with iL^t Figs. 6. and 7. Reis' Telephone Transmitter and Receiver. silk-insulated copper wire and mounted on a cigar box for a sounding board. Its operation was as follows: The soundwaves set up by the voice struck against the diaphragm of the trans- mitter, causing it to vibrate in unison with them. This made and broke the circuit at the contact points, and allowed intermit- tent currents to flow through the receiver. The currents, 6 AMERICAN TELEPHONE PRACTICE. which exactly synchronized with the sound waves, caused a series of sounds in the knitting needle by virtue of " Page's effect." The sounding board vibrated in unison with the mo- lecular vibrations of the needle, and the sound was thus greatly amplified. Reis' transmitter and one form of his receiver are shown in Figs. 6 and 7 respectively. Reis' telephone could be depended upon to transmit only musical sounds, but it is probable that it did actually transmit articulate speech. The cause of this partial failure will be under- stood from the following facts : A simple musical tone is caused by vibrations of very simple form, while sound waves produced by the voice are very complex in their nature. These two forms of waves are shown graphically in Fig. 8. Sound possesses three qualities : pitch, depending entirely on the frequency of the vibrations; loudness, depending on the Fig. 8. Sound Waves of Voice and Simple Musical Note. amplitude of the vibrations, and timbre or quality, depending on the form of the vibration. The tones of a flute and a violin may be the same as to pitch and loudness and yet be radically different. This difference is in timbre or quality. Reis' transmitter, as he adjusted it, was able only to make and break the circuit, and a movement of the diaphragm barely sufficient to break the circuit produced the same effect as a much greater movement. The current therefore flowed with full strength until the circuit was broken, when it stopped entirely. The intermediate strengths needed for reproducing the delicate modulations of the voice were entirely wanting. This apparatus could therefore exactly reproduce the pitch of a sound, but not its timbre and relative loudness. For the next fifteen years no great advance was made in the art of telephony, although many inventors gave it their careful attention. In 1876 Professor Alexander Graham Bell and Professor Elisha Gray almost simultaneously invented successful speaking THE MAGNETO TELEPHONE, ^ telephones. Bell has, however, apparently reaped the profit, the U. S. Patent Office having awarded priority of invention to him. Bell possessed a greater knowledge of acoustics than of electri- cal science, and it was probably this that led him to appreciate wherein others had failed. His instrument consisted of a per- manent bar-magnet, B (Fig. 9), having on one end a coil of fine wire. In front of the pole carrying the coil a thin diaphragm, D, of soft iron was so mounted as to allow its free vibration close B Fig. 9. Bell-Magneto-Telephone. to the pole. Two of the instruments are shown connected in a circuit in Fig. 9. Two points will be noticed which have heretofore been absent : that no battery is used in the circuit, and that the transmitting and receiving instruments are exactly alike. When the soft-iron diaphragm of the transmitting instrument is spoken to, it vibrates in exact accordance with the sound waves striking against it. The movement of the diaphragm causes changes in the magnetic field in which lies the coil, which changes, as shown above, cause an alternating current to flow in the circuit. This current varies in unison with the movements of the diaphragm. The waves of this current are very complex, and represented graphically are similar to those of the voice shown in Fig. 8. Passing along the line wire, these electrical impulses, so feeble that only the most delicate instruments can detect them, alternately increase and de- Figs. 10 and ii. Bell's Centennial Receiver and Transmitter. crease the strength of the permanent magnet of the receiving in- strument, and thereby cause it to exert a varying pull on its soft- iron diaphragm, which, as a result, takes up the vibrations and reproduces the sound faithfully. Bell's earlier instruments, ex- hibited in 1876 at the Centennial in Philadelphia, are shown in Figs. 10 and n, the former being his receiver, the latter his 8 AMERICAN TELEPHONE PRACTICE. transmitter. The receiver consisted of a tubular magnet com- posed of a coil of wire surrounding a core, and inclosed in an iron tube if inch in diameter and 3 inches long. This tube was closed by a thin iron armature or diaphragm which rested loosely on the upper face of the iron tube, the length of the core being such as not quite to touch the diaphragm when in this position. The transmitter consisted of an electromagnet, in front of which was adjustably mounted a diaphragm of gold- beater's skin, a, carrying a small iron armature, d, at its center. Nearly all books and articles on telephones, that treat of Bell's early receiver at all, show and describe it as having the diaphragm fastened at one edge by a single small screw to the upper face of the iron tube, and sprung away from the tube at its opposite side. Fig. 12. Royal E. House's Electro-Phonetic Telegraph. This mistake occurred in the first two editions of this work, and would have been in this one but for Mr. Thomas D. Lockwood, who was kind enough to call attention to it. The origin of the error is explained in the following interesting extract from a let- ter written by Mr. Lockwood to the writer of this bgok: " This mistake first appeared in the account given by Engineer- ing of Sir William Thomson's address to the British Association in September, 1876, and has been universally copied. . . The origin of the mistake is very odd. The screw of the instrument given to Sir William Thomson, and which he exhibited in Eng- land on his return, was put through a hole in the edge of the diaphragm, and engaged with a threaded hole in the edge of the THE MAGNETO TELEPHONE. 9 tube for the purpose of attaching the diaphragm while in transit, to prevent it from getting lost. No one, however, notified Sir William of this, it probably having been forgotten ; and Sir William seems to have forgotten what the instrument, as he saw it in Philadelphia, looked like. Finally, in knocking about among Sir William's luggage, the free end of the diaphragm was apparently, and without doubt unintentionally, bent upward, as the picture shows. But when so bent, being at the same time rigidly fas- tened at the opposite edge, it would not and could not work; and when Sir William showed it in England he couldn't make it work." Bell's instrument, in a modified form, is the standard of to-day. It is now used as a receiver only, a more efficient transmitter, de- pending upon entirely different principles, having been invented. In speaking of Bell's invention Lord Kelvin has said : " Who can but admire the hardihood of invention which devised such very slight means to realize the mathematical conception that if electricity is to convey all the delicacies of quality which dis- tinguish articulate speech, the strength of its current must vary continuously as nearly as may be in simple proportion to the velocity of a particle of air engaged in constituting the sound?" A very interesting fact, and one which might have changed the entire commercial status of the telephone industry is that in 1868 Royal E. House of Binghamton, N. Y., invented and pat- ented an " electro-phonetic telegraph," which was capable of operating as a magneto-telephone, in the same manner as the instruments subsequently devised by Bell. House knew nothing of its capabilities, however, unfortunately for him. The in- strument is shown in Fig. 12, and is provided with a sound- ing diaphragm of pine wood stiffened with varnish, mounted in one end of a large sound-amplifying chamber so formed as to focus the sound waves at a point near its mouth, where the ear was to be placed to receive them. The electromagnet adapted to be connected in the line circuit had its armature connected by a rod with the center of the wooden diaphragm as shown. By this means any movements imparted to the armature by fluctua- ting currents in the line were transmitted to the diaphragm, causing it to give out corresponding sounds ; and any movements imparted to the diaphragm by sound waves were transmitted to the armature, causing it to induce corresponding currents in the line. Two of these instruments connected in a circuit as shown in Fig. 9 would act alternately as transmitters and receivers in the same manner as Bell's instruments. CHAPTER II. HISTORY AND PRINCIPLES OF THE BATTERY TRANSMITTER. IT has been shown that in order to transmit speech by electric- ity it is necessary to cause an undulatory or alternating current to flov in the circuit over which the transmission is to be effected, andtl it the strength of this current must at all times be in exact accoro^nce with the vibratory movements of the body producing the soi id. Bell c transmitter was used as the generator of this current ; as a dynamo, in fact, the energy for driving which was derived from the sound waves set up by the voice. The amount of energy so derived was, however, necessarily very small and the current correspondingly weak, and for this reason this was not a practi- cal form of transmitter, except for comparatively short lines. Elisha Gray devised a transmitter which, instead of generating the undulatory current itself, simply served to cause variation in the strength of a current generated by some separate source. He accomplished this by mounting on his vibrating diaphragm, a (Fig. 13), a metal needle, a', extend- ing into a fluid of low conductivity, such as water. The needle, a, formed one terminal of the circuit, the other terminal being the metal pin, D, extend- ing up through the bottom of the containing vessel, B. The vibration of the diaphragm caused changes 13^- i n tne resistance of the path through the fluid, and Gray's Vari- corresponding changes in the strength of the current. ance Trans- Bel1 also used a lic l uid transmitter in which a con- mitter. ducting liquid was held in a conducting vessel form- ing one terminal of the circuit. The other terminal was a short metallic needle carried on the diaphragm and projecting slightly into the liquid, so that the area of contact between the liquid and the needle would be varied to better advan- tage by the vibration of the diaphragm than if the needle were immersed a greater distance into the fluid. These instruments, unlike that of Reis, simply caused variations in the resistance of the circuit, and thereby allowed a continuous but undulatory current to pass over the line, the variations in which were able to reproduce all THE BATTERY TRANSMITTER. II the delicate shades of timbre, loudness, and pitch necessary in articulate speech. Gray embodied in his apparatus the main principle upon which all successful battery transmitters are based, but it was not long before a much better means was devised for putting it into practice. In 1877 Emile Berliner of Washington, D. C, applied for a patent on a transmitter depending upon a principle previously pointed out by the French scientist, Du Moncel, that if the pres- sure between two conducting bodies forming part of an electric circuit be increased, the resistance of the path between them will be diminished, and conversely, if the pressure between them be decreased, a corresponding increase of resistance will result. Berliner's transmitter is shown in Fig. 14, which is a reproduc- tion of the principal figure in his now famous patent and in which Fig. 14. Berliner's Transmitter. A is the vibratory diaphragm of metal, against the center of which rests the metal ball, C, carried on a thumb-screw, B, which is mounted in the standard, d. The pressure of the ball, C, against the plate-, A, can be regulated by turning the thumb-screw. The diaphragm and ball form the terminals or electrodes of a circuit, including a battery and receiving instrument. Figs. 15 and 16 show two different views of an exact duplicate of Berliner's original model as filed in the patent office. This was very roughly con- structed as shown. The diaphragm was a circular piece of ordi- nary tin and the contact-piece a common blued-iron wood screw. 12 AMERICAN TELEPHONE PRACTICE. Figs. 15. and 1 6. Berliner's Patent-Office Model. THE BATTERY TRANSMITTER. 13 The action of this instrument is as follows: when the dia- phragm is vibrating, the pressure at the point of contact, a, be- comes greater or less, thus varying the resistance of the contact and causing corresponding undulations in the current flowing. Soon after this Edison devised an instrument using carbon as the medium for varying the resistance of the circuit with changes of pressure. Edison's first type of carbon transmitter consisted simply of a button of compressed plumbago bearing against a small platinum disk secured to the diaphragm. The plumbago button was held against the diaphragm by a spring, the tension of which could be adjusted by a thumb-screw. A form of Edison's transmitter, devised by George M. Phelps in 1878, is shown in Fig. 17. The transmitting device proper is Fig. 17. Phelps-Edison Transmitter. shown in the small cut at the right of this figure, and is in- closed in a cup-shaped case formed of the two pieces, A and B, as shown. Secured to the front of the enlarged head, e, of the adjustment screw, E, is a thin platinum disk, F, against which rests a cylindrical button, G, of compressed lampblack. A plate of glass, /, carrying a hemispherical button, K, has attached to its rear face another platinum disk, H. This second platinum disk rests against the front face of the lampblack disk, G, and the button, K, presses firmly against the center of the diaphragm, D. The plates, F and H, form the terminals of the transmitter, and as the diaphragm, D, vibrates, it causes variations in the pressure, and corresponding changes in the resistance of the cir- cuit, thus producing the desired undulations of current. Professor David B. Hughes made a most valuable contribution tending toward the perfection of the battery transmitter. By a series of interesting experiments, he demonstrated conclusively that a loose contact between the electrodes no matter of what substance they are composed, is far preferable to a firm, strong contact. The apparatus used in one of his earlier experiments, 14 AMERICAN TELEPHONE PRACTICE. made in 1878, is shown in Fig. 18, and consists simply of three wire nails, of which A and B form the terminals of the circuit containing a battery and a receiving instrument. The circuit was completed by a third nail, C, which was laid loosely across the other two. Any vibrations in the air in the vicinity caused vari- ations in the intimacy of contact between the nails, and corre- Fig. 18. Hughes' Nail Microphone. spending variations in the resistance of the circuit. This was a very inefficient form of transmitter, but it demonstrated the prin- ciple of loose contact very cleverly. It was found that carbon was, for various reasons, by far the most desirable substance for electrodes in the loose-contact transmitter, and nothing has ever been found to even approach it in efficiency. Another form of transmitter devised by Hughes, and called by him the microphone, is shown in Fig. 19. This consists of a Fig. 19. Hughes' Carbon Microphone. small pencil of gas carbon, A, pointed at each end, and two blocks, B B, of carbon fastened to a diaphragm or sounding board, C. These blocks are hollowed out, as shown, in such a manner as to loosely hold between them the pencil, A. The blocks, B B, form the terminals of the circuit. This instrument, though crude in form, is of marvelous delicacy and is well termed microphone. The slightest noises in its vicinity, and even those incapable of being heard by the ear alone, produce surprising THE BATTERY TRANSMITTER. effects in the receiving instrument. This particular form of instrument is, in fact, too delicate for ordinary use, as any jar or loud noise will cause the electrodes to break contact and produce deafening noises in the receiver. Nearly all carbon transmitters of to-day are of the loose- contact type, this having entirely su- perseded the first form devised by Edison, which was then sup- posed to depend on the actual resistance of a carbon block being changed under varying pressure. Only one radical improvement now remains to be recorded. In 1 88 1 Henry Runnings devised a transmitter wherein the variable resistance medium consisted of a mass of finely divided carbon granules held between two conducting plates. His transmitter is shown in Fig. 20. Between the metal diaphragm, A, and a parallel conducting plate, B, both of which are securely mounted in a case formed by the block, D, and a mouthpiece, F, is a chamber filled with fine granules of carbon, C. The dia- phragm, A, and the plate, B, form the terminals of the transmitter, and the cur- rent from the battery must therefore flow through the mass of granular carbon, C. When the diaphragm is caused to vibrate by sound waves, it is brought into more or less intimate contact with the carbon granules and causes a varying pressure between them. The resistance offered by them to the current is thus varied, and the desired undulations in the current pro- duced. This transmitter, instead of hav- ing one or more points of variable contact, F }& 20. Running's Gran- _ ' ular Carbon Transmitter. is seen to have a multitude of them. It can carry a larger current without heating, and at the same time produce greater changes in its resistance, than the forms previously devised, and no sound can cause a total break between the elec- trodes. These and other advantages have caused this type in one form or another to largely displace all others. Especially is this true on lines of great length. Up to this time all transmitters, together with the receiver and battery, had been put directly in circuit with the line wire. With this arrangement the changes produced in the resistance by the transmitter were so small in comparison with the total resistance of the circuit, that the changes in current were also very small, and produced but little effect on the receiver. Edi- 16 AMERICAN TELEPHONE PRACTICE. son remedied this difficulty by using an induction coil in connec- tion with the transmitter. The credit of this improvement, however, should be given largely to Gray, for in 1875 he had used an induction coil in connection with his harmonic telegraph transmitter, and Edison merely substituted a telephone transmit- ter in the circuits used by Gray. The induction coil used then and now is made as follows: Around a core formed of a bundle of soft-iron wires is wound a few turns of comparatively heavy insulated copper wire. Out- side of this, and entirely separate from it, is wound another coil, consisting of a great number of turns of fine wire, also of copper and insulated. The inner coil is called the primary, the other the secondary. In telephone work it is now almost universal practice to place the transmitter, together with the battery, in a closed circuit with the primary of the induction coil, and to place the secondary directly in circuit with the line wire and receiving instrument. This is shown in Fig. 21, in which T -p s Fig. 21. Transmitter with Induction Coil. is a transmitter, B a battery, P and 5 the primary and secondary, respectively, of an induction coil, L L' the line wires, and R the receiving instrument. It is well to state here that the usual way of indicating the primary and secondary of an induction coil, in diagraphic representation of electrical circuits, is by an arrange- ment of two adjacent zigzag lines, as shown in Fig. 20. A cur- rent flowing in the primary winding of the induction coil pro- duces a field of force in the surrounding space, and any changes caused by the transmitter in the strength of the current produce changes in the intensity of this field. As the secondary winding lies in this field, these changes will, by the laws of Faraday and Henry, cause currents to flow in the secondary winding and through the line wire to the receiving instrument. In all good induction coils the electromotive forces set up in the secondary coil bear nearly the same ratio to the changes in electromotive THE BATTERY TRANSMITTER. 17 force in the primary coil, as the number of turns in the second- ary bears to the number of turns in the primary. The use of the induction coil with the transmitter accomplishes two very important results : first, it enables the transmitter to operate in a circuit of very low resistance, so that the changes in the resistance produced by the transmitter bear a very large ratio to the total resistance of the circuit. This advantage is well illustrated by contrasting the two following cases : Suppose a transmitter capable of producing a change of resist- ance of one ohm be placed directly in a line circuit whose total resistance is 1000 ohms ; a change in the resistance of the trans- mitter of one ohm will then change the total resistance of the circuit one one-thousandth of its value, and the resulting change in current flowing will be but one one-thousandth of its value. On the other hand, suppose the same transmitter to be placed in a local circuit as above described, the total resistance of which circuit is five ohms ; the change of one ohm in the transmitter will now produce a change of resistance of one-fifth of the total resistance of the circuit and cause a change of one-fifth of the total current flowing. It is thus seen that fluctuations in the current can be produced by a transmitter with the aid of an induction coil which are many times greater than those produced by the same transmitter without the coil. The second advantage is that by virtue of the small number of turns in the primary winding and the large number in the secondary winding of the induction coil, the currents generated in the secondary are of a very high voltage as compared with those in the primary, thus enabling transmission to be effected over much greater length of line and over vastly higher resist- ances than was formerly the case. CHAPTER III. THE TELEPHONE RECEIVER. To construct a receiver capable of reproducing speech is a very simple matter. In fact, nearly any electromagnet, with a com- paratively light iron armature, such as is commonly used in electric bells and telegraph instruments, may be made to repro- duce, with more or less distinctness, sounds uttered in the vicinity of a transmitting apparatus with which it is in circuit. It has proved more difficult, however, to construct a receiving instrument which will reproduce speech well, and at the same time be practically successful in everyday use. The bar-magnet with a thin iron diaphragm in close proximity to one of its poles, used in the early experiments in telephony, has until recently been very generally adhered to throughout this country. The instrument has been made much more sensi- tive than were the early forms, but this result has been accom- plished by better mechanical and electrical designs, and the use of better materials, and not by any departure from the original principles of its action. Aside from actual talking efficiency, many considerations of a purely mechanical nature enter into the design of a good tele- phone receiver. It should be durable and capable of with- standing the rough usage to which it will necessarily be subjected by careless or ignorant users. It should be of such construction that its adjustment will not be changed by mechanical shocks or by changes in temperature. Failure to provide against this latter effect is one of the chief sources of trouble in telephone work. It should be of such external configuration as to enable it to be conveniently placed to the ear. The chamber in which the diaphragm vibrates should be small and of such shape as not to muffle the sound. The binding posts should be so securely fastened in as to prevent their becoming loose and twisting off the wires inside the receiver shell ; and the construction should be so simple as to render the replacing of any damaged part an easy matter. By far the greater number of receivers used in America are of the single-pole type ; although in a few years this statement will THE TELEPHONE RECEIVER. probably not be true. The particular form shown in Fig. 22 has proved efficient, and is now largely used by the American Bell Telephone Company. Its chief merit lies in its simplicity. In Fig. 22, M is a compound bar-magnet, composed of two pairs of separately magnetized steel bars arranged with like poles together. Between the pairs of bars is clamped a soft-iron pole- piece, P, at one end, and a similarly shaped iron block, Q, at the other end. These parts are firmly bound together by the two screws, SS. On the end of the pole-piece is slipped a coil of wire, G. This coil is usually wound with two parallel No. 38 B. & S. silk-insulated copper wires, and has a total resistance of about 75 ohms. The magnet is incased in a shell of hard rubber, composed of two pieces, A and B, which screw to- gether and clamp between them the diaphragm, 2}, of thin sheet iron. The piece, B, is hollowed out as shown, to form a convenient ear- piece. A tailpiece, T, carrying two binding posts, J J, fits over the end of the case opposite the earpiece, , and is held in place by a screw, E. This screw engages a threaded hole in the block, Q, and serves not only to hold the tail-piece in place, but to bind the magnet securely to the shell. Soldered to the binding posts are heavy leading-in wires, W W, which pass along the sides of the magnet and are soldered to the re- spective terminals of the fine wire forming the coil. The diaphragm of this instru- ment is about -j-JV in thickness and 2 eter of the free portion is if. In some single-pole receivers the old style of magnet, consist- ing of a single cylindrical bar of steel, is still used instead of the compound magnet formed of several separately magnetized bars, but with generally inferior results, owing to its weaker and less permanent magnetic field. In bipolar receivers, which are now coming into general use, Fig. 22. Bell Single-Pole Receiver. in diameter. The diam- 20 AMERICAN TELEPHONE PRACTICE. the object is to strengthen the field in which the diaphragm vi- brates, by presenting both magnet poles to the diaphragm. The length of the path of the lines of force through the air is thus greatly shortened, and the field of force is concentrated at the point where it will be most effective. One form of bipolar receiver is shown in Fig. 23, which illustrates the receiver manufactured until recent date by one of the large independent companies. The shell, A, and ear-piece, B, are of a Fig. 23. Bipolar Receiver. material resembling hard rubber, and clamp between them the soft-iron diaphragm, D, as in the instrument described above. The magnet consists of two pairs of separately magnetized steel bars, F F and F'F', the separate bars in each pair being laid with like poles together, so that each pair forms in itself a compound bar-magnet. These two compound bar-magnets are so laid together that the north pole of one is opposite the south pole of the other. The two pairs of bars are held apart at one end by the adjustment block, H, made of the same material as the shell, THE TELEPHONE RECEIVER. 21 and at the other end by the soft-iron block, /. On each side of the block, H, and between it and the pairs of bar-magnets, are the soft-iron pole-pieces, P P, on which are wound the coils G G, having a resistance of 50 ohms each. These coils are wound with No. 36 B. & S. silk-insulated wire and are connected in series, so that the total resistance of the receiver is 100 ohms. The block, H, has two segmental flanges projecting out beyond the sides of the magnet bars. These flanges are screw-threaded on their circumferential surfaces so as to engage a thread, g, on the inner surface of the shell, A. The magnet may thus be adjusted toward or from the diaphragm by turning it in the shell, A. A tail-piece, T, of hard rubber is so shouldered as to fit into the small end of the receiver shell, and is prevented from turning in its place by small lugs fitting into notches in the shell. A screw, E, extends through the tail-piece and clamps the magnet into any position to which it has been adjusted. To the binding posts, JJ, are soldered heavy leading-in wires, W W, which pass through holes in the adjustment block, H, and are soldered to the termi- nals of the fine magnet wire. These heavy wires, W W, are firmly knotted after passing through the block, If, in order to prevent any mechanical strain coming on the hair-like wires of the magnet coils when the tail-piece is removed. Sufficient slack is left in the leading-in wires to allow the removal of the tail-piece a short distance, to give access to the end of the magnet for purposes of adjustment. In many forms of receiving instruments much trouble is experienced in keeping permanent the adjustment between the magnet and the diaphragm. This is due to the fact that steel and hard rubber differ widely as to their amounts of expansion or contraction under changes in temperature. In instruments where the magnet is rigidly secured to the shell only at a point at considerable distance from the diaphragm, the unequal expan- sion or contraction of the magnet and the shell causes the dis- tance between the pole-piece and the diaphragm to vary with every change in temperature. A sudden change will thus often render a receiver inoperative. This defect is seen to exist without any attempt at a remedy in the single-pole receiver shown in Fig. 22. The point of support of the magnet is as far removed from the diaphragm as possible, being at the screw, , and therefore the full benefit (which is of course negative) of all the differences in contraction and expansion between the hard rubber and the steel is obtained. 22 AMERICAN TELEPHONE PRACTICE. In the receiver shown in Fig. 23 an attempt was made to remedy this defect by securing the magnet to the shell at a point close to the diaphragm, so that the differences in expansion and contraction between the shell and magnet will be reduced to a minimum. This, however, in this particular case introduced a defect quite as serious, because the shell was also bound to the magnet by the screw, E. The contraction and expansion thus tended to loosen the screw-thread on the block, H, making fre- quent readjustment necessary. Moreover, a good screw-driver in the hands of an ordinary repair man or of a subscriber often sub- jects the screw-thread on block, ff, to such a strain as to strip the thread, thus rendering the receiver useless. Several important lessons may be and have been learned from the behavior of these two forms of receiver in actual and long- continued service : First : It is poor construction to secure the magnet in the shell at the end farthest from the diaphragm. Second : It is also poor construction to secure it rigidly near the diaphragm and also at the opposite end. Third : It is extremely poor construction to use any of the materials imitating hard rubber in vital portions of the instru- ment. These materials, so far produced, are without exception subject to some or all of the following faults to a greater extent than hard rubber, viz. : They are not sufficiently tough, and are usually very brittle. They absorb moisture. They soften when exposed to heat, and gradually give way under pressure, causing them to retain a permanent set when again cooled. They are capable of having threads molded upon them, and as a rule these molded threads do not fit. Threads in hard rubber are cut, and may therefore be as accurate as desired. They are liable to have seams or " cold shuts " formed in molding which will cause cracks and fractures; and lastly: They are not as good insulating materials as hard rubber. Some of these materials may not possess all of these objections, but all possess some of them. Hard rubber therefore is, so far as materials are at present developed, the only thing to use in the insulating portions of receiver shells. A way of obviating the expansion and contraction difficulty, used largely in European countries and to an increasing extent in this country, is to construct the shell holding the diaphragm of some metal having nearly the same coefficient of expansion as the steel magnets. Fig. 24 shows one of the early forms of bipolar receivers. THE TELEPHONE RECEIVER. 23 This was devised in 1881 by Clement Ader of Paris, France, and is with some modifications largely used in France and other European countries to-day. This embodies the results of one of the few successful attempts at increasing the electrical efficiency of the telephone receiver. The magnet, B, is ring-shaped, and has fastened to its poles two L-shaped pole-pieces carrying coils, C C. The box, R, inclosing the pole-pieces and coils is of brass and is secured to the magnet by screws, E E. It is screw- threaded at G, so as to engage a corresponding screw-thread on the inner surface of the cap, S, which has a flaring portion, H, Fig. 24. Ader Bipolar Receiver. forming an ear-piece. The diaphragm, D, is clamped between the pieces, R and S, as in the American instruments described above. Surrounding the opening, leading from the diaphragm to the ear-piece, is a ring, m, of soft iron, and in this ring lies the chief point of Ader's invention. The additional mass of iron placed near the poles of the magnet affords a more ready path for the lines of force, and their number is thus increased. The dia- phragm, therefore, moves in a stronger field of force, and the power of the receiver is said to be correspondingly augmented. Practice in this country has not, however, shown any perceptible gain of efficiency by the use of this ring. 24 AMERICAN TELEPHONE PRACTICE. Fig. 25 shows the form of receiver now manufactured by the Western Telephone Construction Company of Chicago. In this the shell is of hard rubber, composed of three pieces ; the diaphragm being clamped between the shell and ear-piece in the ordinary manner. The magnet is of horseshoe form and carries a block of brass grooved on each side to partially inclose the magnet limbs. The lower portion of this block is screw-threaded, as shown, so as to engage the corresponding thread turned in the receiver shell. The upper flange on the block rests on a corresponding flange on the interior of the shell when the magnet is screwed home. The pole-pieces are secured to the outside of the magnets by a bolt passing entirely through the brass block, each limb of the magnet, and each pole-piece. This bolt is pro- vided with a nut at each end, so that either pole-piece may be taken off without removing the other. The heads of the magnet Fig. 25. Western Telephone Construction Co.'s Receiver. spools are of brass pressed into position on the pole-pieces. After being insulated, the spools so formed are wound in a machine having a special chuck for holding the pole-pieces. The two coils are for standard work, wound to a resistance of 50 ohms each with No. 36 silk-insulated wire and connected in series, thus making the total resistance of the receiver 100 ohms. The novelty in this receiver is in the method of attaching the receiver cord to the terminals leading from the coils. These terminals, as shown, are composed of heavily insulated wire pass- ing through the brass block into the coil chamber. The other ends of these wires are twisted together and pass through a cen- tral opening, where each is soldered to a connector held in place against the shell by a small machine screw. The cord is provided THE TELEPHONE RECEIVER. with similar connectors, which may be slipped under the screw- heads, thus completing the circuit between the cord and the wires of the receiver. The connection of the cord is, of course, made before the tail-cap is screwed in place. An enlargement in the covering of the cord effectually prevents any strain ever coming on the cord terminals when the receiver is dropped. Another feature secured by this construction is that no metal parts are exposed on the outside of the shell, thus insuring immunity from electric shocks while handling the receiver, this being considered very desirable by some. This receiver, except for the method of connecting the cord, which was designed by the writer, is almost identical with that used by The American Bell Telephone Co., in their long-distance work, and also in most of their common-battery exchanges. In the Bell receiver the tail-cap is not provided, and instead two flanged binding posts are screwed directly to the hard rubber frorr the outside. To these binding posts the ordinary receiver cord is attached in the usual way, and the wires leading from the receiver magnets pass through the shell and are soldered to an extension of the binding posts. These forms of receiver are very efficient, very easily adjusted, and subject to little or no trouble kom the source of expansion and contraction, it being seen that the magnet is sup- ported at a point near the diaphragm without being bound to the shell at any other point. The Stromberg-Carlson receiver is a very powerful one, and has stood the test of time well. It is shown in Fig. 26, in which a is a casing of brass, forming a framework upon which all other parts of the instrument are supported. This is screw-threaded on its outer surface to receive the internally screw-threaded cap, b, and lock-ring, b 3 . One unique feature of this receiver is the method of supporting the diaphragm, which is held in place in the cap, b, by the clamping ring b'. Upon the cap, b, is screwed an ear-piece, b 1 . The lock-ring, b 3 , is adapted to be screwed against the cap, b, to lock it in any adjusted Fig. 26. Stromberg-Carl- son Receiver. 26 AMERICAN TELEPHONE PRACTICE. position. Upon the rear of the casing is provided a projection, a', against the faces of which rest the soft-iron cores, c 1 c 3 , which extend through the bottom of the casing and carry upon their ends the telephone coils, c 3 c\ The ends of the permanent magnet, d, rest upon the cores, c ' c*, and a screw or bolt, e\ passes through the ends of the magnet, the cores, and the projection, to maintain them in position. The ends of the magnet, d, are cut away as shown to permit the cores to be set flush with the inner faces of the magnet. Between the limbs of the magnet, d, is provided a block, d', of Fig. 27. American Electric Telephone Co.'s Receiver. fiber upon which are mounted two binding posts, d", the binding posts being connected to the coils, c 3 c\ by heavy insulated wires, d" d\ To the binding posts, d*, are also attached the ends of the receiver cord. Upon the rear of the casing, , is provided a threaded flange upon which the insulating casing, /, is screwed, this latter being provided with an opening at the end through which the receiver cord passes. The magnet, d, is mounted rigidly upon the casing, a, the cas- ing, /, being entirely independent so that it may be removed by unscrewing. The diaphragm support or cap, b, may be raised or lowered to adjust the diaphragm relatively to the magnet THE TELEPHONE RECEIVER. 27 cores, c' c y , the ring, & a , serving to lock the diaphragm in its ad- justed position. This receiver does away entirely with the troublesome effects due to expansion or contraction. The insulating casing forms a handle and serves as a protection to the cord terminals, but forms no part of the working structure itself. In Fig. 27 is shown the bipolar receiver of the American Electric Telephone Co. The permanent magnet is formed of two pieces which clamp between them, at the end farthest from the diaphragm, a cast-iron block on which is mounted a hard-rubber disk carrying the binding posts. This block, therefore, serves the double purpose of completing the magnetic circuit between the ends of the magnets and of a support for the binding posts and connections. The lower ends of the magnets carry a screw- threaded disk upon which is screwed the metal cup containing the coils and against which the diaphragm rests. Upon this block are also mounted the straight pole-pieces carrying coils similar to those shown in Fig. 23. The cup forming the chamber for the coils is of pressed brass, nickel-plated and screw- threaded to engage the threaded disk carried by the magnet. Over this entire structure is slipped the case of hard rubber, the diaphragm being clamped between the earpiece and the brass cup. In this receiver, adjustment is obtained by turning the cup on the magnet, the screw-threads producing a longitudinal move- ment of the latter in respect to the former, thus moving the pole-pieces toward or from the diaphragm, according to the direction of the rotation. After the desired adjustment has been obtained, the cup may be clamped in the position desired by two screws projecting through flanges carried by the circular disks of the magnets and extending into the interior of the cup. These screws, when set, engage the bottom of the cup in such manner as to hold it from turning. A single-pole receiver, manufactured by the Holtzer-Cabot Electric Co., and embodying features of decided merit, is shown in Fig. 28. In this the magnet is composed of four bars of steel separately magnetized and clamping between them at one end an iron block drilled and tapped to receive the screw passing through the end of the shell. A pole-piece flanged, and screw- threaded as shown, is clamped between the other ends of the magnets. The cup for inclosing the coil and carrying the diaphragm is of brass, having a hole through its center, screw- threaded to engage the threaded portion of the pole-piece. When screwed in position, the flat portion of the cup abuts the 28 AMERICAN TELEPHONE PRACTICE. flange on the pole-piece, thus binding the two rigidly together. The ear-piece is of hard rubber, as usual, and screws to the brass cap, thus holding the diaphragm in position. The inclos- ing shell of hard rubber slips over the magnet as shown, and carries the binding posts, which are connected by heavy leading- in wires to the receiver coil. No form of adjustment is pro- vided for this instrument, and this, by the way, is a feature Fig. 28. Holtzer-Cabot Receiver. which is meeting with considerable favor and is being adopted by several manufacturing companies. Great care is taken by the manufacturers to adjust the instrument properly before it leaves the factory, after which, with an instrument properly con- structed, no need for adjustment should exist. Still another form of receiver, and one of the non-adjustable type, is shown in Fig. 29: this is manufactured by the Erics- son Co. of Sweden, and is being imported into this country to a considerable extent. This is of the bipolar type presenting to the diaphragm two coils and two pole-pieces very similar in shape to those shown in Fig. 23. The magnets are secured to the metal cup by means of two screws shown in the Fig. 29. Ericsson Receiver. figure, each extending transversely through the case and into the magnets. The holes in the case through which these screws project are slotted so that a certain amount of adjustment can be obtained if it is absolutely necessary, although the idea of the manufacturers is to bind it so tightly that no adjustment will ever be needed. The inclosing tube for the magnets is of brass covered by a thin layer of insulating material, usually hard rubber, THE TELEPHONE RECEIVER. but sometimes of leather. This tube is also held in position by the screws before mentioned. A piece of hard rubber projects between the two binding posts of the instrument as shown, the object of this being to prevent the tips of the receiver cords from twisting the posts in their sockets until they touch each other, thus short-circuiting the instrument. This same feature will be noticed in the receiver shown in Fig. 27. This receiver is well made, handsome, and efficient. In the receivers shown in Figs. 26, 27, 28, and 29 the evil effects due to contraction and expansion of the various parts are avoided by the use of metal cups for securing the diaphragm. This method, as before stated, is coming into increasing favor in this country, but has long been used in Europe. The receiver manu- factured by the Kellogg Switchboard and Sup- ply Co. is shown in Fig. 30, the cut at the right- hand portion of this figure representing the magnet and attached parts removed from the shell. The permanent magnet is of the horse- shoe type, and is fast- ened by means of a brass bolt to the pole-pieces and brass bridge-plate, K, as shown. This bridge-plate has two inwardly bent portions which lie be- tween the pole-pieces and form a seat for them when clamped in place by the bolt. On the rear end of the magnet is carried a fiber-disk, D, secured to the magnet by small bent brass pieces clamped to the rear of the magnet by the screw, e. Upon this disk, D, are mounted small terminal cups of brass provided with ears extending down through the disk, and to which the leading- in wires, w, are fastened. To these cups are fastened the ends of the conducting cord, and in order to prevent any strain from coming on the conductors there is a small metal loop fastened to the fiber-disk, D, to which a straining cord woven into the cover of the receiver cord is tied. 3- Kellogg Switchboard and Sup- ply Co.'s Receiver. 3 AMERICAN TELEPHONE PRACTICE. The shell is composed entirely of hard rubber in three parts, a tail-cap screwing into the main body of the shell to cover the cord connections. The magnet, with coils and other parts attached, is fastened to the shell by screws, not shown, passing through the flange of the bridge-piece, K, and into the hard rub- ber. This receiver is provided with no means of adjustment, as having been once adjusted in the factory no further adjustment is ever required. The diaphragm of this receiver is of tinned iron 2|f inches in diameter, this diameter being considerably greater than that of any other receiver in common use. In order to render the receiver sufficiently heavy to actuate an ordinary hook-switch the lead weight, W, is inserted between the limbs of the magnet. The idea of rendering the subscriber less liable to shocks while using the telephone has been well carried out in the receivers shown in Figs. 25 and 30, where not even the binding posts are exposed. The diaphragms used for receivers are made of very soft thin sheet iron ; the ferrotype plate formerly used for tin-types in photography being as good material as can be found for this pur- pose. Some companies, however, are using tinned-iron dia- phragms, which give equally good results. The diaphragms for the various receivers here described vary from 2 to 2|f inches in diameter, the free portions that is, the portions not clamped by the supports ranging from rf- to 2 inches. The usual thickness is from .009 to .01 1 of an inch. The thickness of a diaphragm, to produce the best results with a given receiver, must be obtained by experiment, as it depends on the diameter of the portion free to vibrate, and also on the strength of the magnetic field, due to the permanent magnet. It has been shown that with a very thin diaphragm and a very powerful mag- net the iron in the diaphragm becomes saturated, so that it is not responsive to changes in the strength of the existing field. The question of receiver cords is one of a good deal of impor- tance, as a faulty cord is one of the most prolific sources of trouble of any part of a telephone instrument. If the conductors in a cord are not properly insulated, so that they may come into con- tact, or if a break occurs in one of the conductors, the instrument will be short-circuited in the one case, or the circuit left open in the other. The conductors in receiver cords are usually com- posed of tinsel woven or twisted into strands, and a few strands of fine copper wire are frequently added to give greater strength. These tinsel conductors are then tightly braided, or wrapped with cotton, silk, or linen, sometimes in several layers, and in THE TELEPHONE RECEIVER. some cases both are inclosed in a spiral wrapping of spring- brass wire, the spiral being then braided over with the familiar colored worsted braid. It is probably better, in putting on the first covering over the tinsel, to make it a wrapping instead of a braid, as the former tends to bind the tinsel strands together more securely than the latter, thus preventing any short ends from piercing the covering and short-circuiting the cord. Cord-tips are necessarily subject to rather rough usage, as it frequently happens that a receiver is dropped, thus allowing a heavy strain to come on the cords, which is usually most severe Fig. 31. Details of Receiver Cord Tip. where the tip joins the cord proper. The connection shown in Fig. 31 is one that has become very popular. This is formed by inserting a needle or pin, B, of No. 14 brass wire tapered to a long point, into the hollow of a braided tinsel cord or spiral spring, as the case may be, for one-half inch, when the end is passed out through the conductor and covering and bent backward, forming a hook, as shown at D and A, thus combining the strength of the conductor and covering. Before the pin is put in, the conductor is bared for a short distance and, after the pin is inserted, is wound with fine wire and soldered. The tip is then finished with a spiral of white wire as shown at E, or with a shell as shown at C. In order to prevent an undue strain on the conductors when receiver is dropped, it is best to have the cord provided at each end with an auxiliary loop, A, as in Fig. 32. This loop is usually a continuation of the braiding of the cord, and may be fast- ened to an eyelet in the receiver, or to a small link on one of the binding posts, complishing this same result is by means of a hook sewed to the braiding, just at the fork of the cord, which may be closed by pliers around a screw-eye or one of the binding post screws. Fig. 32. Supporting- Loop and Hook for Receiver Cord. Another way of ac- CHAPTER IV. CARBON TRANSMITTERS. MANY vain attempts have been made to discover a satisfactory substitute for carbon as the variable resistance medium in tele- phone transmitters, the patents on the use of carbon electrodes having, until a few years ago, formed one of the mainstays of the American Bell Telephone Company's great monopoly. The theory of the action of carbon in the transmitter has been the subject of much discussion. As previously pointed out, any motion of the diaphragm increasing the pressure between the electrodes lowers the resistance between them, thus allow- ing the passage of a greater current. A decrease of pressure produces the opposite result. Four different explanations for this action have been put forth, and are as follows : First, that the electrical resistance of the carbon itself is caused to vary by the changes in pressure. Second, that a film of air or gas exists between the electrodes, and that the thickness of this film is varied by the changes in pressure, thus varying the resistance. This theory is apparently still adhered to by Mr. Berliner.* Third, that the peculiar property possessed by carbon of lower- ing its resistance with increased temperature is in the following way accountable for the action, in part at least : that an increase of current (due to increased pressure and diminished resistance between the electrodes) causes a slight heating at the point of contact; that this heating causes a still further diminution of resistance with an additional increase of current ; and that con- versely a momentary decrease of current causes a decrease of temperature with a corresponding additional increase of resist- ance and diminution of current. Fourth, that change in resistance is due to the variation in the area of contact between the electrodes that is, the variation in the number of molecules in actual contact. This change in area is perfectly apparent in the liquid transmitter of Gray, and in the * " Microphonic Telephonic Action," by mile Berliner, American Electrician, March, 1897. 3* CARBON TRANSMITTERS. 33 case of solid electrodes may be well illustrated by the following well-known experiment : If a billiard ball be gently pressed on a plain marble slab coated with graphite, the area of contact of the ball with the slab will be indicated by a small dot of graphite on the ball. If, now, the ball be dropped from a considerable height, it will be noticed that the spot of graphite on the ball is much larger, showing that the ball has flattened out to a considerable extent, owing to the greater pressure exerted. This demonstrates clearly the varia- tion in area of contact between two bodies, due to variations of pressure between them. Of course, if the two bodies are con- ductors of electricity, the resistance between them will vary inversely and the current directly as the area of contact. It seems most probable to the writer that of the above explana- tions, the fourth is the true one, and that none of the others aid in any perceptible degree in producing desirable effects in the microphone. As to the first explanation, that the resistance of the carbon itself changes under pressure, experiments have been made with long carbon rods ; and with measuring instruments of ordinary sensibility no difference whatever could be detected in the resist- ance of a rod when the pressure on it was varied from zero up to the crushing point, care being taken that all contacts in circuit were not subjected to the change in pressure. As to the layer of air theory, Professor Fessenden has thrown some light upon it,* by showing that if the layer of air were in the ordinary gaseous state, its resistance would be almost infinite, while if it existed in some peculiar condensed state of which we know little, but in which air might be conceived to be a conduc- tor, then the law of change of resistance between the electrodes would be different from what it has actually been found to be. On the other hand, the curves plotted with resistances as ordi- nates and with distances as abscissae have been found by Pro- fessor Fessenden and by Messrs. Ross and Dougherty to exactly agree with the form obtained from theoretical considerations on the basis that the change in resistance is due to area of sur- face contact alone. As to the third explanation, it may be said that the very fact that the increase of current is needed to cause the rise of tempera- ture seems to preclude the supposition that the rise of temperature should cause the diminution of resistance with its corresponding " Microphonic Telephonic Action," by Professor R. A. Fessenden, American Electrician, May, 1897. 34 AMERICAN TELEPHONE PRACTICE. increase of current in time to do any good. The heating effects in carbon are comparatively slow, and it would seem that the changes in temperature would lag slightly behind the changes in current producing them, in such a manner as to be detrimental to tele- phone transmission. This property of carbon, of lowering its resistance with in- creased temperature, is, however, important in that when the transmitter becomes warm from constant use its resistance as a whole is decreased. When the transmitter is heated the total resistance of the circuit is lowered, and the changes in resistance produced by the sound waves therefore bear a greater ratio to this total resistance with corresponding increase of efficiency. It is certainly most fortunate that in one substance carbon should be found all of the qualifications which make it particu- larly desirable for microphonic work. It produces the change in resistance with changes in surface contact, all things considered, better than any other known substance, possesses the desirable Fig. 33. The Blake Transmitter. property of lowering its resistance when heated, and is elastic, non-corrosive, non-fusible, cheap, and easily worked. The form of transmitter almost universally used in this country up to within a few years ago, and still largely used, is that de- vised by Francis Blake of Boston. This instrument is shown in Fig. 33, in which B represents a metal ring or frame for holding the mechanism of the instrument. It is screwed to the cover, A', of the box A, and has two diametrically opposed lugs, B \ On. this ring is mounted the diaphragm, C, of rather heavy sh~-*- CARBON TRANSMITTERS. 35 iron, supported in a rubber ring, r, stretched around its edge, and is held in place by two damping springs, D D, each bearing on a small block of soft rubber, a, resting on the diaphragm at a point near its center. The object of these damping springs is to pre- vent too great an amplitude of vibration of the diaphragm, and also to keep it from vibrating in separate parts instead of as a unit. Opposite the center of the diaphragm is the orifice, , in the cover, A', so hollowed out as to form a mouthpiece. The adjust- ing lever, F, is attached to the spring,/, secured to the lug, ', of the ring, B. The lower end of this lever rests upon an adjusting screw, G, in the lug, B*, which is drilled and slotted as shown to prevent the screw from working loose. On the back of the dia- phragm and at its center is placed the front electrode, consisting of a small bar, e, of platinum ; one end of the bar rests against the diaphragm, while the other end is brought to a blunt point and is in contact with the back electrode, e. The electrode, e, is supported independently upon a light spring, c, mounted on the lever, F t but insulated from it. This spring tends to press away from the diaphragm and toward the back electrode. The back electrode is formed of a block of carbon, e , set into a brass block, g, of considerable weight, mounted on a spring, d, supported on the adjusting lever, F. This spring, d, has a tension in the oppo- site direction to that of the spring, c, and being stronger than the latter it keeps the electrode, e, in contact with the diaphragm. It is seen that instead of having one of the electrodes held in fixed position while the other is pressed against it with greater or less force by the vibration of the diaphragm with which it is con- nected, both electrodes are supported in such manner as to move freely with the diaphragm, but the outer electrode is so weighted that its inertia will offer enough resistance to the slight and rapid vibrations of the diaphragm to give a varying pressure between the electrodes and consequent changes of the resistance of the circuit. By this means the initial pressure between the two elec- trodes will not be affected by changes of temperature, and the adjustment will therefore be more nearly permanent. This transmitter is very delicate, and transmits the quality of the voice in a manner unexcelled by others. It is, however, lack- ing in power, especially when compared with instruments of later design. Besides this, it has a tendency to rattle or break contact when acted on by loud noises. Fig. 34 illustrates the Crossley transmitter, introduced into Europe early in 1879. This well illustrates the class very AMERICAN TELEPHONE PRACTICE. appropriately termed "multiple-electrode." Transmitters de- vised by Johnson, Gower, Ader, D'Arsonval, Turnbull, and many others are of this type, and differ merely in the arrange- ment and number of electrodes. They give much more powerful results than the transmitters having a single pair of electrodes, but most of them are subject to the grave defect of breaking the circuit entirely when subjected to loud noises. In this figure, J represents a diaphragm formed of a thin piece of pine board about %" thick and mounted on a supporting ring, Fig. 34. The Crossley Transmitter. K. Fastened to this diaphragm are four carbon blocks, F G H and /, in the relative positions shown. These are hollowed out to receive the conical ends of the carbon pencils, ABC and D, which are supported loosely between them. The blocks, H and G t form the terminals of the transmitter. The current divides at the block, H, and passes through the pencils, A and C, in multi- ple to the blocks, .Fand /, and thence through the pencils, B and D, to the other electrode, G. Vibrations of the diaphragm cause variations in the intimacy of contact between the eight points of Fig. 35. The Turnbull Transmitter. support of the four rods, and thus produce the desired fluctua- tions in resistance. It is seen that this is merely a modification of the Hughes microphone, the principles being the same, but the multiple contact allows a greater current to pass through the transmitter, and at the same time produce greater changes in this current than in the original form, where a single pencil was used. Moreover, the liability of " rattling " is greatly reduced. CARBON TRANSMITTERS. 37 Fig. 35 shows the Turnbull transmitter, which has been used to a considerable extent in this country, even until recently. In this figure, A is the diaphragm of thin wood, on the back of which is mounted the bracket, B. Pivoted on a rod, b, carried by this bracket, are several carbon rods or pendants, a, which rest at their lower end against a carbon rod, c, carried on a bracket, C, also mounted on the diaphragm. The rods, b and C, form the terminals of the transmitter, and the current passes from one of them through the carbon pendants in multiple to the other. The variable resistance contact is mainly between the rod, C, and the pendants, a, although by making the rod, b, of carbon also an additional effect is obtained between it and the pendants. Fig. 36 shows still another form of the multiple-electrode transmitter, using carbon balls instead of pencils or pendants. A represents the vibratory diaphragm of carbon ; B a plate of carbon having a number of cylindrical cavities, t t, upon one side. Fitting loosely in each cavity is a ball of carbon. The t Fig. 36. The Clamond Transmitter. depth of the cavities is a little less than half the diameter of the balls, and the diaphragm is so placed in front of the plate that the balls, following their tendency to roll out of the cavities, will rest against its inner surface and also upon the edges of the cavi- ties. Many other forms of instruments have been devised using one or more balls held in various positions between carbon plates. Some are used to-day, but all the transmitters so far described are being rapidly replaced by the Runnings form of instrument, which, as has already been stated, uses carbon " dust " or granules for the variable resistance medium. Among the earlier forms of the granular transmitter is a very efficient one designed by Emile Berliner, and called the " Ber- liner Universal." In this the diaphragm, D (Fig. 37), is of car- bon, and is mounted horizontally in a case formed of the two pieces, A and B, of hard rubber, a brass ring, R, being clamped above it to insure good electrical contact. Secured to the en- larged head, f, of the screw, s, mounted on the block, B, is a cylindrical block of carbon, on the lower face of which are 3 8 AMERICAN TELEPHONE PRACTICE. turned several concentric V-shaped grooves. The points formed between these grooves almost touch the diaphragm. The finely divided carbon, c, rests on the diaphragm, and is confined in the space between it and the carbon block by a felt ring, F, which Fig. 37. The Berliner Universal Transmitter. surrounds the latter and bears lightly against the diaphragm. To the center of the back plate a soft-rubber tube, r, is fixed which is of sufficient length to make contact with the diaphragm, its function being that of a damper to the vibrations of the dia- phragm. The mouthpiece, M, is so curved as to conduct the sound waves against the center of the diaphragm. This trans- Fig. 38. Details of Solid Back Transmitter. mitter was used to a considerable extent by the American Bell Telephone Company, and has now been entirely replaced for long distance work by the White transmitter. The White, or " solid back," transmitter, as it is called, is CARBON TRANSMITTERS. 39 shown in Figs. 38 and 39, the latter giving a clear idea of the construction of the working parts of the transmitter, the back casing being removed. The upper portion of Fig. 38 shows the section of the complete instrument. The sections of Figs. 38 and 39 are taken on planes at right angles to each other. The separate parts of the " resistance button " of the instrument are shown in the small cut at the bottom of Fig. 38. This instru- ment has proven remarkably successful in practice, it being able to stand a very heavy current without undue heating. Besides this, the tendency of the granules to settle down in a compact mass, commonly called " packing," is greatly diminished. F is of cast brass turned to form the front piece of the trans- mitter case, and is held, as shown, in the hollow shell, C, the two pieces forming a complete metallic casing for the working parts of the instrument. The sound-receiving diaphragm, D, of alu- . 39- Sectional View Solid Back Transmitter. minum, is encased in a soft-rubber ring, e, held in place by two damping springs,/"/, as in the Blake transmitter. Wis a heavy metallic block hollowed out, as shown, to form a casing for the electrodes. The inner circumferential walls of this block are lined with a strip of paper, i. This block is mounted, as shown, on a supporting bracket, P, secured at its ends to the front cast- ing, F. The back electrode, B, of carbon is secured to the face of the metallic piece, a, which is screw-threaded into the block, W. E is the front electrode, also of carbon, carried on the face 40 AMERICAN TELEPHONE PRACTICE. of the metallic piece, b. On the enlarged screw-threaded por- tion, /, of the piece, b t is slipped a mica washer, m, held in place by the nut, u. This washer is of sufficient diameter to completely cover the cavity in the block, W, when the electrode is in place. After the required amount of granular carbon has been put into the cavity, and the front electrode put in position, the cap, c, is screwed in its place on the block, W, as shown, and binds the mica washer, m, firmly against the face of the block, B, thus confining the granules in their place. The electrodes are of somewhat less diameter than the paper-lined interior of the block, W, so that there is a considerable space around the periphery of the former, which is filled with carbon granules. This prevents the binding of the free electrode against the edge of its containing chamber, and also allows room for the granules directly between the electrodes to expand when heated by the passage of current. The screw-threaded portion, /', of the piece, b, passes through a hole in the center of the diaphragm, and is clamped firmly in place by the nuts, / t'. M is the mouthpiece of hard rubber, screw-threaded in an opening in the front block, F. Any vibration of the diaphragm is transmitted directly to the front electrode, E, which is allowed to vibrate by the elas- ticity of the mica washer, m. The back electrode is, of course stationary, being firmly held by the bracket, P. The back electrode is in metallic connection with the frame of the instrument, which forms one terminal. The other terminal, T, is mounted on an insulating block, /, and is connected by a flexible wire with the front electrode, E. This construction is best shown in Fig. 39. This transmitter is now used on all of the long-distance lines of the Bell Company, and ha"s given excellent service. It was formerly always used with three Fuller cells in series, but the tendency is now to use but two. The following data concerning the dimensions and material used in this instrument will, it is believed, be found of much interest : Diaphragm aluminum, 2\" diameter and .022" thick. Rubber band or gasket |' wide, 2-f" double length, very soft and elastic. Front electrode carbon, hard and polished, f fc* diameter, -^ thick. Back electrode carbon, hard and polished, \\" diameter, T V' thick. Mica diaphragm f|" diameter, very thin. CARBON TRANSMITTERS. Back electrode chamber inside diameter, f", depth ^", clear- ance between sides of electrode and walls of chamber ^". Distance between electrodes about .04". Damping spring spring steel, " wide, .010" thick, i^" long; bent at right angles when not in place. The one which rests near center of diaphragm is tipped with soft rubber and also with felt ; the outer spring, with rubber only. Fig. 40 illustrates the Colvin transmitter. Although this is an efficient instrument and extremely unique in design, it is very little used. The shell is formed of two pieces, A, provided with the usual mouthpiece, and B, fitting into a recess in the piece, A. The space in which the diaphragm fits is made large enough to hold the diaphragm very loosely so that it and the cell it carries may vibrate with great freedom under the impact of sound waves. Upon the diaphragm is supported a hollow cylin- drical cell, D, of insulating material (shown in the small cut at the Fig. 40. The Colvin Transmitter. left), carrying two metallic electrodes, E E', insulated from each other. To these electrodes are connected the circuit terminals, G G. The shell, D, is clamped firmly to the diaphragm, 7, by a bolt, F, thus closing the chamber containing the granules. To prevent the access of moisture to the carbon granules the joint between the diaphragm and the edge of the shell is hermetically sealed. The diaphragm is of aluminum, and being loosely mounted is free to vibrate with great amplitude. One of the striking features of this instrument is that the two electrodes, E E', are fixed with relation to each other, the variation in resist- ance being obtained by the variation in pressure between the electrodes and the carbon granules, due to the inertia of the latter, and also to the shaking up of the granules themselves, and the consequent variation of their intimacy of contact with each other. Fig. 41 shows the Sutton transmitter now manufactured by AMERICAN TELEPHONE PRACTICE. the Phoenix Interior Telephone Company. The variable resist- ance parts comprise a pair of carbon buttons, F and G, each surrounded by a sleeve of cloth, // and /, the abutting edges, h and i, of which are frayed out so as to form an intimate but yielding contact. These not only serve to damp the vibrations of the diaphragm, but form with the buttons, F and G, a closed chamber in which the granular carbon is placed. The button, F, is secured to the diaphragm, K, as shown, while the button, G, is Fig. 41. The Sutton Transmitter. rigidly secured to the case of the instrument, and is insulated therefrom. The wire, O, leading from the bolt, L, which secures the button, G, in place, forms one terminal of the instrument, the casing itself the other. The Ericsson transmitter, manufactured in Sweden, is being imported into this country to a considerable extent as a com- panion piece to the Ericsson receiver. This transmitter gives a very clear, soft tone, and requires little battery power. On the Fig. 42. The Ericsson Transmitter. whole it is a very efficient instrument. It is shown in section in Fig. 42, in which a is the sound-receiving diaphragm held against a shoulder in the brass casing, c, by two thin leaf-springs, not shown, each spring having two branches, so as to give in all four points bearing on the diaphragm. CARBON TRANSMITTERS. 43 For preventing moisture, especially the moisture contained in the breath, from entering beyond the diaphragm in the casing in which the diaphragm and other parts of the microphone are situated, a thin disk, b, of silk impregnated with lacquer is placed in front of the diaphragm, a, between it and the mouthpiece. The border of the disk b, as well as that of the diaphragm, a, is close to the wall of the casing, c. The metal plate, d, mounted on the rear side of the diaphragm forms the front electrode, and for that purpose is gold-plated. The backwardly bent rim of the plate, d, surrounds the fore part of a soft ring, , make a sliding con- tact on the casing, B, while the screw, K, turns pivotally under the spring, E. This arrangement effectively remedies the pack- ing difficulty, but much trouble is often caused by poor contacts between the springs and the parts of the transmitter on which they rest. Means have also been devised for automatically turning the transmitter or otherwise agitating the carbon by the removal of the receiver from the hook. These, however, have not generally been found desirable. In Fig. 46 is shown a transmitter recently designed by Mr. T. F. Ahearn, which is interesting as showing one of the many Fig. 46. Ahearn Transmitter. attempts to produce changes in area of contact without changes of pressure. E is a carbon electrode attached to the center of the metal dia- phragm, A, and forms the terminal electrode to which the wire,Z>, is attached. This electrode consists of a plate or plates, of either semicircular or triangular form, as shown. The back electrode, G, is of similar form and is carried on the spring, f, in such manner as to overlap and rest on the front elec- trode, E. The pressure between the two may be regulated by the thumb-screw, as shown. It is claimed that in this no variation in pressure can be caused by the vibration of the diaphragm, but that the electrodes simply slide over each other, the shape of the surfaces in contact ampli- fying the changes in contact area. Fig. 47 shows one of the attempts to increase the efficiency of the microphone, but results so far obtained from this and similar CARBON TRANSMITTERS. 47 experiments have not proved of sufficient value to warrant the additional complexity of parts. This instrument consists of a double Blake transmitter, with a pair of electrodes on each side of the diaphragm. The action of the electrodes, e and i, is the same as that of the electrodes of the regular Blake instrument. The electrodes, d and h, however, being on the side of the diaphragm toward the speaker, serve also to vary the resistance of their point of contact, but an increase in resistance between e and i is accompanied by a decrease in resistance between d and h, and vice versa. The induction coil used with this instrument has two oppositely wound primary coils, M and N. The coil, M, is in circuit with the pair of contacts, d and h, while the coil, JV, is in circuit with the contacts, e and i. As these coils are wound in opposite directions, and as an increase of current flowing from the battery, B, through one of them is always accompanied by a decrease of current through the other, it follows that their induct- ive effects on the secondary coil, S, will be added. A transmitter constructed with the idea of producing actual Fig. 47. Double Transmitter. alternations in the current flowing in the primary has recently been patented by Messrs. G. F. Payne and Wm. D. Gharky of Philadelphia. It is of unique design and produces very power- ful results. It is designed to operate on the principle of a pole- changing switch, and in Fig. 48 its analogy to that familiar form of circuit-changing device is shown. The cuts in the upper por- tion of this figure show the two positions of a pole-changer, and it will be evident that the direction of current through the coil, W, will depend on the positions of the switches as shown by the arrows. In the lower portion of this figure the circuits and elec- trodes of the transmitter are diagrammatically shown. The elec- trodes, K G and J, are stationary, while electrodes, M and N, move in accordance with the vibrations of the diaphragm, being con- nected thereto by a piston-rod. As the movable electrodes are 40 AMERICAN TELEPHONE PRACTICE. by the action of the diaphragm impelled toward the left the resist- ance to the passage of a current between the electrodes, TV and G and M and K, is diminished, while it is increased as between the electrodes, Nand J and J/ and G. Consequently the greater part of the battery current will pass to and through the movable electrode, IV, the stationary electrode, G, upward through the wire connection, F, thence through the wire connection, U, to the stationary electrode, K, and thence through the movable electrode, M y and to the battery. An impulse to the right brings the movable electrodes into the position shown at the right-hand lower cut, and reverses the conditions, producing, as there shown, a downward current through the wire, V, the changes from the one condition to the other being of course gradual and without sensible interruption, and the result being that the greater part of Fig. 48. Diagram of Payne & Gharky Transmitter. the battery current is sent through the coil, W, first in one direc- tion and then in the other, following of course the movement of the diaphragm. The construction of this transmitter is shown in Fig. 49, the lower portion of which shows an enlarged view of the electrodes. Parts B and C form the framework of the instrument, supporting the diaphragm and all working parts. D is a cylindrical box, in which the electrodes are situated, carried on the bracket, C. G is the central stationary electrode, constructed of brass with carbon faces and connected to one terminal of the primary coil, W. The outer stationary electrodes are each formed with stems, as indicated at J and K, which extend through the heads, F F t of the box, and are secured in proper position by the set- screws,//. At the inner end of each rod is a brass disk (indi- CARBON TRANSMITTERS. 49 cated at f and K'\ /' and K*, indicating brass disks screwing on the stems,/ and K, and acting to clamp a light felt washer, L, between themselves and the disks,/', and K. The disks,/' K, are each provided with a carbon facing, H. The movable electrodes, of which one is situated on each side of the central stationary electrode, are made up, as shown, of two brass disks, such as are indicated at M M and N N, a light felt washer, L, being clamped between the disks in each case. The electrode, TV, is secured to the diaphragm by a light metal rod, O, one end, 0\ of which is shown threaded and screwing into the electrode end, while the other end, O', is threaded and screws into the nut, A'. This rod is Fig. 49. Details of Payne & Gharky Transmitter. covered by a non-conducting jacket, P, which in turn is partly inclosed by a conducting-tube, Q, which connects with the elec- trode, M. The two electrodes, Mand N, are thus rigidly bound to each other and to the diaphragm, but are insulated from each other. The battery is connected between them as shown. Granular carbon is placed between each opposite pair of elec- trode-faces, there being thus four separate bodies of granular car- bon, which are prevented from coming into contact with each other by the light felt washers, L. Whether or not the effects produced by the action of these electrodes will be of great enough gain to overcome the objec- tionable complication, time must show; the results obtained, however, are remarkable. 5 AMERICAN TELEPHONE PRACTICE. Figs. 50 and 51 show an oddity in the form of a granular carbon transmitter devised by Mr. W. W. Jacques of the Bell Com- pany. In the ordinary transmitter too much battery power is to be guarded against, as it throws the electrodes into vibration and causes the well-known squealing or sizzling sound. Mr. Jacques claims that when a multitude of electrodes in loose contact are normally kept in a state of rapid and continuous vibration by such a strong current, they are much more sensitive to sound waves falling upon them than they are when at rest. He gives as a probable explanation of this that the resultant normal pres- sure existing between the various pairs of electrodes is less when all of the electrodes are in vibration to and from each other than Figs. 50. and 51. Jacques Transmitter. when they are at rest ; and it is well known that, within certain limits, the sensitiveness of any microphone contact increases as the normal pressure is decreased. He uses a current at a pressure of about 20 volts, which of course sets up a vibration of the granules, thereby maintaining them in the " desired condition of sensitiveness to sound waves." The use of such great battery power also allows the variation of a greater current than where the usual low voltage is used. He proposes to use this only on long lines and claims that " the undulations of current due to the vibrations of the electrodes of the transmitter produced by the normal action of the battery will fade out and disappear at a greater or less distance from the transmitter ; while the undulations of current due to the action CARBON TRANSMITTERS. 51 of sound waves upon the normally vibrating electrodes will per- sist, and the sounds be heard in the telephone at the distant end of the line." In order to stand such a heavy current the transmitter is made practically fireproof by the construction shown in Figs. 50 and 51, in which A is a cup-shaped metallic frame supporting the operative parts of the instrument. C is a metallic cover secured to the frame, A, by screws, s, clamping the diaphragm, D, in the ordinary manner. G is a cylinder made of slate, having a flange,/, and secured to the interior of the cup-shaped frame, A, by a brass ring, r, which rests upon the flange and is there held by screws, /, screwing into the frame. E is the back electrode, being a disk of hard carbon brazed to a brass disk, K, a projection from which lies, as shown, within a hollow projection,/, from frame, A, the two said projections being insulated from each other by a cup-shaped washer of vulcanized fiber. Pis the front or working electrode, being a disk of hard carbon rigidly secured to the diaphragm, D, by a screw and nut. The two electrodes, E and P, fit the cylinder, G, closely. For a variable resistance material between them granulated carbon is employed, the grains being of such size that they will not pass between the peripheries of the electrodes, E and P, and the inner wall of the cylinder, G. These granules are kept in violent vibra- tion by the strength of the battery current, while serving also as the variable resistance medium between the working electrode, />, and the back electrode, E, to take up the vibrations due to vocal waves in the ordinary manner. The back electrode, E, is made adjustable by means of a spring, a, tending to push the brass disk, K, into the cylinder, G, and a brass thumb-screw, b. A flanged washer of vulcanized fiber insulates, frame, A, from the thumb-screw, b. The frame, A, is in metallic connection with the working electrode, P, while the thumb-screw, b, is in metallic connection with the back electrode, E. The noise in the receiving instrument, resulting from the two sets of vibrations in the transmitting instrument at the same end of the line, is not only painful to the ear, but interferes with the proper reception by the ear of sounds coming from the other end of the line. To obviate this difficulty, the receiving telephone is so constructed that the current coming from the transmitting telephone at the same end of the line is divided and passes around the core of the receiving instrument in two directions, while the current from the transmitting telephone at $2 AMERICAN TELEPHONE PRACTICE. the farther end of the line passes around the core always in one direction. The direction of the windings is such that in the former case the two windings neutralize each other and produce Fig. 52. Carbon Electrodes for Transmitter. no noise, while in the latter the effects of the two coils are added, thus giving the receiver its full power. Fig. 52 is interesting as showing some of the standard trans- mitter electrodes and diaphragms used in this country. The cuts of these were loaned by Mr. M. M. Hayden of the Globe Carbon Co. CHAPTER V. INDUCTION COILS. IT has already been pointed out in the chapter on the History and Principles of the Battery Transmitter that the use of the in- duction coil is of decided advantage in that it allows the changes in the transmitter to bear a much larger ratio to the total resistance of the circuit in which these changes occur than would otherwise be the case ; and further, that by virtue of the transformation from a comparatively low to a high voltage, the currents are much better adapted for traversing long lines and higher resistances. It may be further pointed out that with the same battery power the current in the primary circuit is much greater, owing to the lower resistance, than if the battery were placed in the line circuit, and therefore the transmitter is not only able to produce a greater relative change in the current flowing, but to cause these changes to act on a larger current. It should be remembered that the current in the primary cir- cuit is an undulating one and is always in the same direction. The current in the secondary, however, is alternating in character, changing its direction completely with every large fluctuation in the primary current. This latter feature is also productive of better results than would be the case were the current in the line wire of an undulatory character. The size and quality of the iron core, the relation between the number of turns in the primary and secondary windings, and the mechanical construction of the induction coil are matters of the greatest importance, and have not in general received the atten- tion which they merit. A number of attempts have been made to calculate mathematically the best dimensions and resistances of the telephone induction coil, but the matter is of such an extremely complex nature, and all of the quantities are subject to such complex and indeterminate variations, that the results so far produced have been in general unreliable. Only a few series of experiments are on record from reliable sources giving the results of comparative tests between induc- tion coils of various dimensions. It may be said that definite 53 54 AMERICAN TELEPHONE PRACTICE. results from any such series of tests are extremely hard to get, as the quality and loudness of transmission is subject to a very great personal error, even in the case of experienced experimenters. In making comparative tests of any telephone apparatus it is of the greatest importance that all possibility of prejudice on the part of the experimenter be removed, and in order to do this it is essential that he be in ignorance at all times of the particular instrument that he is testing. To illustrate this point, suppose that three transmitters are being compared with respect to determining the general talking qualities of each. If the party at the receiving telephone, who is to judge of the hearing, desires that one of these instruments produces better results than the others, he is very sure, even though he be strictly honest with himself, to conclude at the end that that transmitter is by far the best ; unless, of course, there is a very marked difference between them. For this reason he should be kept in ignorance of the particular transmitter in circuit with the line at any time, and should only be told when changes are made. It is well to have the instruments numbered, the party judging of the merits to be in ignorance of the transmitters to which those numbers refer. It is also a somewhat difficult matter in comparing the clear- ness with which instruments transmit or reproduce speech to select proper subject matter to be transmitted. It is unfair to the first instrument tested to repeat the same sentence or read the same matter in each case, for the reason that the listening- party becomes more or less familiar with the matter to which he is listening, and therefore often catches words at the second or third reading which he fails to grasp at the first. It is therefore better to read a selection from a certain article in the first test, and a continuation of the same matter in each successive test, so that the character of the matter read will be approximately the same in each case. However, where several instruments are apparently of almost the same merit, and where the transmission is so good that all of the matter may be generally understood, it has been found that a better way is to prearrange a number of series of words, a different series to be read into each instrument under test. Care must be taken, in the selection of these words, that each series contains words of the same character. To illustrate : suppose five instruments are to be tested. Five different series of words may be prepared, containing such words as the following : INDUCTION COILS. 55 1st 2d 3d 4th 5th sign rhyme dine fine mine going rowing sewing mowing throwing missile thistle whistle bristle gristle D E G P B etc. etc. etc. etc. etc. Each list should contain about forty words ; and as the words in each series will be seen to differ from the corresponding words in the others to only a slight extent, no instrument can be said to have an easier list than the others. The first list should be read into the first instrument slowly enough for the receiving party to write them down. Then the second list is read to the second instrument, and so on. Such words are difficult to distinguish, especially when there is no context, as is the case in reading an arbitrary list of words. The receiving party should be required to write down the words as he hears them, and the list which is most correct according to his notes will probably represent the work of the best instrument so far as clearness is concerned. It is only by a consideration of such details as these, simple though they be, that an unbiased opinion can be formed as to the relative merits of telephonic apparatus. Many elaborate experiments have been performed for arriving at the comparative merits of similar instruments depending on the quantitative measurements of the amplitude of vibrations, the amount of current, and similar quantities, but in the first place the apparatus and time for such measurements are available to but few, and in the second place it is doubtful if the results obtained are as reliable as those obtained by carefully following the above suggestions in a conscientious manner. Such quantitative experiments are, however, of great impor- tance in adding to our knowledge of the true workings of the telephone, thus greatly aiding in the development of the art in general. A series of experiments, cited by Preece and Stubbs and per- formed by the administration of the Swiss telephone department, is of great interest. In this test a good Blake transmitter was used throughout, the object being to determine the best of a set of ten induction coils. Table i gives complete data concerning the primary and secondary windings of each coil. The results obtained over five different lengths of line are shown in the right-hand portion of the table. In each case the inten- AMERICAN TELEPHONE PRACTICE. TABLE I. PRIMARY WINDING. SECONDARY WINDING. RESULTS FOR VARIOUS LENGTHS OF LINE. '5 . g| 02 a 1 .31 mile 38 miles 49 miles 53 miles 67 miles G .a o A h 3 ^ o 3 M O 11 O a* 1- i- o c jl O o C in ^ in X in' X ^ t/j Ij 8 f. a! to 13 S to '55 B arne 'in C arne &C znc L 1 b r ill B r r l!l .' i rL- VS f AAA-I Fig. 82. Telephone Circuits, Hook Up. firmly and positively made, and surely broken at the proper time. For this reason all the points of contact are preferably provided with platinum tips to prevent corrosion, and, if possible, a slight sliding action at the point of contact should be obtained. A sliding contact tends to clean the points and at the same time prevents particles of dust from keeping the two apart. Too much sliding action is, however, worse than none, as it is sure to cause cutting. The springs for restoring the lever and those serving as contacts should be so arranged that no movement of which the lever is capable will strain them beyond their elastic limit or to such a degree that they will eventually lose their ten- sion or break. It is bad practice to have the same part of a con- tact slide alternately over a conducting material, as of brass, and 94 AMERICAN TELEPHONE PRACTICE. an insulating material, as of hard rubber, as small particles from either surface are sure to be carried upon the other surface, thus forming a partial electrical connection on the insulating surface and a defective connection on the brass or metal surface. Where a sliding contact is used much trouble is often caused by the cutting of the two surfaces. The extent of this cutting, even where the pressure is very light and the movement very limited, is often astonishing. In Fig. 83 is shown the hook switch now almost universally used by the Bell Telephone Company, and known as the " Warner Switch." The hook lever is pivoted to a bracket by a screw as shown, and is provided with a lug,/", and a strip of insulating material, g, on its short arm. On the under side of the lever is an insulating pin, h, and a contact point, /. A spring screwed to the generator box under the lever by the screw, b, Fig. 83. Warner Hook Switch. bears alternately upon the insulating pin, /*, and the contact point, i, and tends to press the lever into its elevated position. Springs, c and d, screwed to the side of the generator box bear alternately upon the insulating piece, g, and the conducting lug, y, according to whether the lever is depressed or elevated. The spring, c, is connected through the secondary winding of the in. duction coil and the receiver to one side of the line. The screw, ^, is connected through the calling apparatus to the same side of the line. The binding screw, a, connected with the lever, *, forms the terminal of the other side of the line. The local circuit terminates on one side in the spring, c, and on the other side in the spring, d. When the hook is depressed, point i is connected through the lower spring with the screw, b, and the THE HOOK SWITCH AND CIRCUITS OF A TELEPHONE. 95 calling circuit is complete. Both the local circuit and the line circuit through the talking apparatus are broken at springs, c and d. When the hook is elevated, the calling circuit is broken at the point, /, and the local and line circuits are completed by the springs, c and d, and the lug,/". In Fig. 84 is shown the hook switch now embodied in all the subscribers' instruments manutactured by the Kellogg Switch- board and Supply Company. The hook lever, L, is of brass, pivoted in a bracket, B, secured at the side of the box. It is normally maintained in its raised position by a strong spring, S, secured to the bottom of the box. Springs, I, 2, 3, 4, and 5, are secured by two screws to a lug on the bracket, B, these springs being insulated from each other by hard rubber blocks and bush- Fig. 84. Kellogg Hook Switch. ings. Spring, 4, is made longer than the others, so as to engage in a slot in the lug, /, cast on the side of the hook lever. The springs are platinum-pointed as shown, the arrangement being such that when the hook is in its raised position the spring, 4, presses 3, 2, and i together, leaving spring, 5, disconnected as shown. When, however, the hook is lowered by the weight of the receiver, the springs, I, 2, and 3, break contact with each other and with spring, 4, the latter making contact with spring, 5. The long spring, 4, is connected with one side of the line, and serves to complete the talking circuits when the lever is raised, and the signaling circuit when the lever is depressed. This switch has a distinct advantage over most other types, in that the lever itself and the restoring spring, S, form no part of the circuit, and, therefore, no provision has to be made to pre- vent loose contacts between them. The contact springs are all platinum-pointed, so that there is small liability of trouble. A strong point in favor of this form of switch is the ease with 9 6 AMERICAN TELEPHONE PRACTICE. which it may be adapted to meet the requirements of almost any circuit, it being very easy to add more springs or to so arrange them that their contacts will be made and broken in a definite order upon the raising or lowering of the hook. Fig. 85 and 86. Long and Short Lever Hook Switches. Figs. 85 and 86 show two hook switches manufactured by the Holtzer-Cabot Electric Company. The former is adapted for mounting in the bottom of the generator box, in front of the generator, and the latter on the side of the box, above the gen- erator.- These hooks have the advantage of being self-con- tained, and depend on a rather heavy sliding contact obtained by the use of long flexible springs. The diagrams of circuits shown in Figs. 79, 80, 81, and 82 were somewhat simplified in order to render clearer the circuits. In Fig. 87 are shown the circuits of a complete telephone set, the connections being arranged as in practice. It is customary to mount the generator, G, polarized bell, jP, and hook switch all in one box. In order to facilitate the Fig. 87. Complete Circuits of Series Telephone. WO rk of making connections ., . . , . between the parts contained m the generator box and the other parts, t. e., the transmitter, induction coil, receiver, and batteries, the terminals of all the circuits in the box are brought out on binding posts on the top THE HOOK SWITCH AND CIRCUITS OF A TELEPHONE. 97 and bottom of the box. The binding posts, I and 2, on top of the box are for attaching the line wires. These form the termi- nals of the instrument as a whole. The center binding post forms the ground terminal of the lightning arrester, and has no .connection within the box. The six binding posts at the bottom of the generator box are in three pairs, r, /, and s. The pair, r, form terminals for attach- ing the receiver cord. The two posts forming pair, /, are for the local circuit, containing between these posts the battery, B, the transmitter, T, and the primary of the induction coil, /. Be- tween the right-hand pair of posts, s, is connected the secondary of the induction coil, /. The bell, P, is mounted on the door of the box, connection to it being made through the hinges. The connections of the automatic shunt are clearly shown in this figure. Normally a short circuit exists around the genera- tor, through the wire, S, spring, b, disk, c, and the frame of the machine. This is broken between the spring, b, and disk, c, when the generator is operated as already described. Fig. 88 is a view of a complete instrument, showing quite clearly the external connections of the generator box. In this, posts, E and C, are the line terminals, and D the ground post for the lightning arrester. The two left-hand binding posts on the bottom of the box form terminals for the receiver cord, as shown. The two center posts,/ and H, correspond with the pair of posts, /, in Fig. 87. The curled wire from /passes through the back board of the instrument and to one terminal of the battery in the battery box below. The post, //, is connected by a similar wire to the binding post, N, on the transmitter base, from which the circuit may be traced through the primary of the in- duction coil, thence to the metallic portion of the transmitter arm and base to the carbon electrode of the transmitter, then through the transmitter and back by a flexible cord along the side of the arm, which connects with the lower post, Z, on the base. This post is then connected by a curled wire passing through the back board to the remaining terminal of the battery. This instrument, which is made by the Western Telephone Construction Company, possesses a unique form of switch hook. The hook lever is journaled in the side of the box, and is provided with a short, downwardly bent lever, carrying on its inner end a small anti-friction shoe. Against this shoe bears the long heavy spring, O, screwed to the inside of the generator box. The strength of this spring, which presses toward the left, is 9 8 AMERICAN TELEPHONE PRACTICE. sufficient to keep the hook elevated while it is not supporting the receiver. The weight of the receiver, however, forces the short arm of the lever toward the right, moving the spring, O, with it. Fig. 88. Complete Telephone Instrument. The changes of circuit are accomplished by the movements of this spring, and not by the lever itself, which forms no part of the circuit. The contact springs with which this main spring engages are long and flexible, and are mounted horizontally on THE HOOK SWITCH AND CIRCUITS OF A TELEPHONE. 99 the inside of the box. These springs are platinum-pointed, as is also the main spring, so that the contacts are made and broken in a reliable manner. The old forms of this hook switch were very faulty. The anti-friction shoe, and the platinum contacts, how- ever, together with the improved design of the springs, have ren- dered it thoroughly reliable. The details of the generator and ringer of this instrument will be described in the next chapter. Another feature of interest in this set is the small coil, A, not shown in the diagram of circuits. This also will be discussed in a subsequent chapter. The circuits of a bridging-bell instrument are shown in Fig. 89. Fig. 89. Complete Circuits of Bridged Telephone. This instrument is especially adapted to use on party lines, but is also used to a large extent in general exchange work. In this instrument the call-bell, P, is permanently bridged across the two sides of the line between the binding posts, I and 2, and its magnets are made of high resistance and retardation. A little consideration will show that the bell circuit is not affected by the position of the hook lever, there being no lower contact to the switch. The generator, G, is in a second bridge circuit, which is normally open, but closed when the generator is operated. The talking circuit, containing the receiver, R, and secondary winding of IOO AMERICAN TELEPHONE PRACTICE. the induction coil, 7, forms a third bridge circuit, which, like the generator circuit, is normally open. The telephone circuit is automatically closed when the receiver is removed from its hook for use, and this operation also closes the local circuit, containing the primary of the induction coil, /, the local battery, B, and the transmitter, 71 The talking circuits are identical with those in Fig. 87. In order that there shall not be an undue leakage of the voice currents through the permanently bridged call-bell circuit, the magnets of these call- bells are wound to a high resistance (usually a thousand ohms) and are also constructed in such manner that they will have a high coefficient of self-induction. The closing of the generator bridge upon the sending of a call may be accomplished manually, as with the key, k. It is usually Fig. 90. Circuits of Battery Call Instruments. done automatically, however, by a device similar to the automatic shunt used in the regular instruments. Thus, if the wire leading from binding post, 2, in Fig. 89, were led to the spring, , instead of to the frame of the generator, and the binding post, i, per- manently connected to the armature spindle pin, it is evident that the inward movement of the disk, c, caused by turning the generator crank, would accomplish the same result as pressure on the key, k, and with the advantage of not requiring the volition of the operator. The specific arrangement of circuits shown in Fig. 89 and their use in multiple on party lines is patented by Mr. John J. Carty, engineer of the New York Telephone Company. The operation THE HOOK SWITCH AA'D CIRCUITS OF A TELEPHONE. IOI and design of this instrument will be considered at greater length under the head of Party Lines. The circuits of a battery call set are shown in Fig. 90. In this i and 2 are the line binding posts, and 3 a post forming one ter- minal of the local battery, the post, I, also serving as a battery terminal. When the hook, Z, is depressed by the receiver the circuit passes from line post, I, through the hook lever, back con- tact of push-button, P, vibrating call-bell, V, and to line post, 2. The instrument is therefore ready to receive a call. To send a call, battery, B, is connected between the line posts by pressing the button, P; the circuit being traced from post, I, through battery, B, and the two upper contacts of the button, to binding post, 2. The talking circuits are closed in the ordinary manner by the raising of the hook lever. Fig. 91 shows a type of telephone set which is becoming Fig. 91. Desk Telephone Set. very popular for business men who do not desire to leave their desks in order to use the telephone. The particular set illus- trated is made by the Holtzer-Cabot Company. The transmitter is of the granular-carbon type mounted on a handsome, nickel- plated and polished wood stand, as shown. One terminal of the transmitter is formed by the frame itself, while the other terminal is carried down the inside of the tube, or standard. The lever of 102 AMERICAN TELEPHONE PRACTICE. the hook switch is pivoted at its end in an enlargement of the standard and actuates a slender rod which passes down into the base, where it engages the switch lever. This lever is pivoted on a separate bar, mounted on a fiber block and acted upon by a spiral spring in such a manner as to press upward against the vertical rod, thus tending to raise the hook. In this position a knife edge, carried on the switch lever in the base, presses against the two springs, which form, respectively, terminals of the pri- mary and secondary talking circuits. When the hook is sub- jected to the weight of the receiver the switch lever is depressed by the rod against the tension of the spiral spring, thus breaking connection with the two springs of the talking circuit and making connection with a spring forming the terminal of the calling cir- cuit. The induction coil is mounted in the base. This apparatus . Q 2 . Circuits of Complete Desk Sets. may be used in connection with a magneto-generator and call- bell, which are usually placed under the desk or at some point where the generator crank is within easy reach of the user, or in connection with a battery-call outfit, in which case the circuits would be similar to those shown in Fig. 90. Fig. 92 shows the circuits of an apparatus similar to this, manu- factured by the Western Telephone Construction Company. Seven binding posts are arranged in this set on the upper side of the magneto-box, to which all terminals from the various pieces of apparatus are run. i and 2 form the line binding posts, and 6 and 7 the battery binding posts, the other terminals being con- THE HOOK SWITCH AND CIRCUITS OF A TELEPHONE. 103 nected as shown. Inasmuch as the induction coil in this set is mounted in the generator box, it becomes necessary for five different conductors to run from the generator box to the desk stand. These conductors are constructed in the same manner as an ordinary receiver cord, there being five strands instead of two. In this latter set the hook switch is of the same type as previously described in connection with Fig. 88, but is so modified as to enable it to be placed in the vertical standard supporting the transmitter. It is platinum-pointed and, as recently modified, should prove reliable. The connections in a telephone cannot be too carefully made. All possibility of two wires coming into accidental contact should be carefully avoided. All joints should be soldered, without the use of acid. Where connection is made under the head of a screw passing through the wood of the box, means must be taken to prevent the loosening of the connection due to shrinkage of the wood. If possible, the connection should be soldered ; if not, a spring washer may be used. When platinum is not used on hook-switch contacts, it is a matter of absolute necessity to have rubbing contacts, and cut- ting may be reduced to a minimum by making the contact sur- faces of dissimilar metals. German-silver springs bearing on brass contacts form as good a combination as can be obtained without the use of platinum. The lever of the hook nearly always forms one branch of the circuit, and in no case should the contact through the pivot screw be depended on for conduc- tivity. A good plan is to form a soldered connection between the lever and base, by means of a short spiral of flexible wire, soldered at one end to the lever and at the other to the base, the connection being made at points where the relative motion between the two parts is a minimum. CHAPTER X. COMMERCIAL CALLING APPARATUS. THE combination of a magneto-generator and a polarized bell or ringer, mounted in a suitable box, is usually termed a magneto- bell. The hook switch, from the fact that it is usually mounted in the generator box, is often counted as a part of a com- plete magneto-bell. Owing to the lively competition between various manufacturers, and also to the increasing demands for good service on the part of the public, great improvement has been made in this line of work during the last few years. This chapter will be devoted to a consideration of some of the more approved forms of this very important part of telephone equipment. The details of the Western Telephone Construction Company's large generator for heavy work are shown in Figs. 93, 94, and 95. Figs. 93 and 94. Details Western Telephone Construction Co. Generator. This instrument is very similar to the one used by the Bell companies. In it the pole-pieces are of cast iron, riveted together by means of the shouldered brass rods, B B. After this they are bored by a special tool to the required internal diameter to re- ceive the armature. The core of the latter is of cast iron and is shown in Fig. 95, being accurately turned to fit between the pole- 104 COMMERCIAL CALLING APPARATUS. 105 pieces. The bearing plates are of cast brass with a shoulder also turned to fit between the pole-pieces so as to be self-centering when secured in place. They are each fastened to the ends of the pole- pieces by four screws, as shown. The gears are cut from heavy cast brass, the large gear being mounted on a shaft journaled in the same bearing plates as the armature itself. The magnets are bent cold from f " x -|" magnet steel, and are secured in place by clamping plates, C C, and screws, 5 S, the latter passing between the magnets and into the pole-pieces. This generator, while it embodies no new or radical features, is well made and generally satisfactory. It would give better results, however, were the armature of smaller diameter. The air gap in machines of this type may be reduced to less than T ^ of an inch without endangering the smooth running of the arma- ture. The automatic shunt already referred to is shown to better advantage in these figures. The polarized bell used with the later types of this instrument is shown in Fig. 88. The two coils, B, are parallel, and attached - 95- Armature Core of Generator. at one end to a soft-iron yoke, the ends of which extend beyond the coils, to receive the two round bar-magnets, which polarize the frame of the bell. On the projecting ends of these two permanent magnets is mounted a second yoke bar of soft iron, on which is adjustably mounted the bracket in which the armature is pivoted. The two magnets have their forward ends of one polarity and their rear ends of the other; and, together with the two yokes, form a rectangle, one of the yokes being of positive polarity, the other negative. In this rectangle are mounted the coils and armature of the ringer, which operate in the usual manner by the alternating ringing currents. The Holtzer-Cabot Electric Company are manufacturing an ex- cellent magneto set, the generator and ringer of which are shown in Figs. 96 and 97. The end plates in which the generator arma- ture is journaled are of cast brass and are screwed directly to the cast-iron pole-pieces, the ends of which are flanged and machined so as to fit accurately. The armature core is of soft sheet-iron io6 AMERICAN TELEPHONE PRACTICE. laminations. The punchings forming the core are clamped together on a steel rod, which therefore serves as the armature shaft. The drive-wheel is mounted in a long bearing, adjustably secured to an extension on the right-hand end plate. This com- pany uses two forms of driving gear, one the regular gear- Fig. 96. Holtzer-Cabot Generator. wheels and the other the chain and sprocket mechanism shown. It seems to prefer the latter, which, it must be said, when provided with a steel chain, runs very easily and noiselessly. This genera- tor is provided with the centrifugal shunt described in Chapter VIII., mounted directly on the armature shaft, the whole forming a very efficient combination. The generators for bridging-bell service manufactured by this company are of a similar type, but provided with four magnets instead of three, and a correspond- Fig. 97. Holtzer-Cabot Ringer. ingly long armature. It is one of the most powerful generators for this purpose ever tested by the writer. The ringer furnished with this generator is of the same general type illustrated in Fig. 72. The gongs are mounted on adjust- able standards pivoted at their upper ends, and each held by a screw engaging a slot in their lower ends. COMMERCIAL CALLING APPARATUS. 107 The Williams-Abbott magneto-bell is shown complete in Fig. 98 and in detail in Figs. 99, 100, and 101. In this the pole- Fig. 98. Williams-Abbott Magneto-Bell Complete. pieces of the generator are stamped from soft sheet iron and fastened to the circumferential edges of the end plates by four screws at each end. The edges of the end plates are of the proper curvature to maintain the inner surfaces of the pole-pieces in their proper relation to the armature. The lower edges of the Figs. 99 and 100. End and Side Views Williams-Abbott Generator. pole-pieces are bent back and up so as to form a seat for the per- manent magnets, which are secured in place by two bolts passing, respectively, between the two outside and the center magnets. The gear-wheels in this instrument are cut with a very wide face to prevent wear. io8 AMERICAN TELEPHONE PRACTICE. The ringer shown in Fig. 101 is unique and embodies several desirable features : The two ends of the U-shaped permanent magnet are of the same polarity say, south and its middle is of the opposite polarity. (Hence the name tripolar, frequently applied to this ringer.) The two coils are mounted on the iron cross bar, extending between the legs of the permanent magnet. The poles of these electromagnets therefore partake of the polarity of the permanent magnet ends, that is, south, while the armature, supported from the center of the permanent magnet, becomes of north polarity. The action of this ringer is usually misunderstood at first sight, the natural supposition being that the limbs of the permanent magnet are of opposite polarity. Besides being a very efficient ringer, this has the advantage of having its working parts inclosed by the rigid permanent magnet, which serves to protect them from mechanical injury. In Figs. 102, 103, 104, 105, and 106 are shown the details Fig. ioi. Williams-Abbott Ringer. of a unique magneto-generator and ringer recently put on the market by the Williams Electric Company of Cleveland. This apparatus is the design of Mr. J. A. Williams, and embodies such radical departures from the details of the ordinary magneto as to warrant a somewhat minute description. The magnets of the generator are formed of heavy bars of steel, f of an inch thick by \\ inch wide. They are placed close together, thus covering the entire length of the pole-pieces and securing a maximum magnetic density across the pole faces. The pole-pieces are punched from sheet iron, and are ac- curately formed to inclose the space in which the armature turns. The end plates, which hold the crank- shaft and armature bear- ings, are punched from heavy sheet brass, and have riveted into them long brass bearings, which is an important feature in secur- ing good wearing qualities. COMMERCIAL CALLING APPARATUS. 109 The end plates and the pole-pieces are secured by screws, as shown in Fig. 104, to horizontal brass plates above and below the armature. The method of affording a path for the magnetic lines from the permanent magnets to the pole-pieces is ingenious. Two sheet-iron contact plates are provided, one for each pole- piece. Each of these has two ears turned inwardly to hold it in position on the pole-piece, and also four ears turned out- wardly to hold the permanent magnets in their proper place on the generator. Three portions of each contact plate are left flat or straight, so as to make a good contact surface on the permanent magnets, and two portions are formed so as to make curves coincident Figs. 102 and 103. Williams Generator and Magneto-Bell. with the outside curvature of the pole-pieces. This arrangement is for the purpose of securing good contact between the perma- nent magnets and pole-pieces, and also for taking the magnetism from the permanent magnets, at points about opposite the center of the armature, thus securing a somewhat longer effective magnet. The gear and pinion are on the left-hand side of the" machine instead of the right, as is usual, the object being to get them as far away as possible from the uneven strain on the shaft due to HO AMERICAN TELEPHONE PRACTICE. the turning of the crank. The feature of the gear, however, is that it is radially corrugated, as shown in Figs. 103 and 104, the object of this being to secure a uniform rate of wear between Fig. 104. Details of Williams Generator. the teeth on the pinion and on the gear. The corrugations on the gear cause its teeth to play over a width of pinion about five times as wide as the face of the gear. This, in view of the fact that the gear has about five times as great a circumference as the pinion, renders the wearing surface on the two about equal. In order to prevent the large gear plowing a rut in the small one by always traveling in the same path, the ratio of the teeth is made uneven, there being 134 teeth on the large gear and 27 on N Figs. 105 and 106. Williams Ringer. the pinion ; thus the armature must make 134 revolutions before a tooth on the gear engages the same tooth on the pinion twice. The pinion is attached to the armature shaft by means of a key and machine screw, and can be easily taken off and replaced COMMERCIAL CALLING APPARATUS. 1 17 without driving, a feature of great convenience in making repairs. The magnets are clamped to the frame of the machine by two brass bolts, these being long enough to pass through the bottom of the generator box, so as to receive nuts for securing the generator in the box. The automatic shunt of this machine has already been described in Chapter VIII. Not less unique than the generator of this machine is the ringer. It has but a single core, which is parallel with the vibrat- ing armature. The core heads or end pieces are formed up of Swedish iron, and are swaged onto the core, forming a very per- fect magnetic joint. , The permanent magnet of the ringer is U-shaped and of the consequent-pole type, the central portion being of one polarity and the extremities of the other. The armature is suspended from the central and consequent pole. The magnetic circuit of this ringer is well shown in Fig. 106 by the iron filings which cling to the poles. It will be noticed that the core of the coil really forms no part of the magnetic circuit of the permanent magnet, as each of its ends is magnet- ized to an equal extent and with the same polarity. The action of this ringer is as follows : The two pole-pieces, which form at the same time the heads of the spool, are mag- netized by the ends of permanent magnet, with the same polarity we will say, north. The armature, attached to the center or south pole, will therefore have the opposite polarity south. When a current traverses the coil, it gives one end of the core and the corresponding head a north polarity and the other end and head a south. This strengthens the magnetism already possessed by the former and weakens that of the latter, thus causing the armature to be attracted by the stronger pole. The next instant the current reverses, and the opposite end of the armature is attracted. The design of this ringer is certainly good. The length of magnetic circuit through which the magnetism set up by the cur- rents in the coil acts is reduced to a minimum. The ends of the core are subjected to a normal magnetic stress, yet the core forms no part of the normal magnetic circuit. It is therefore rendered very sensitive to changes in the magnetizing force due to the coil, because the normal magnetic flux through the core is nil. Fig. 102 shows the complete instrument provided also with a long-lever gravity switch hook, with Craig silver contact-pieces 112 AMERICAN TELEPHONE PRACTICE. attached to a hard-rubber block, which, together with the hook, is mounted in the bell box in such a way as to make the switch hook self-contained. The box is wired throughout with tinned copper wire, which rests between tinned spring washers, secured by entering the binding posts, thus forming spring contacts, all other contacts being soldered. The regular io,ooo-ohm magneto set usually has an armature wound with No. 35 or No. 36 B. & S. silk-covered wire, to resist- ances varying from 400 to 550 ohms. The ringer magnets are usually wound with No. 31 B. & S. wire and have a resistance of from 75 to 100 ohms. The ordinary construction of ringer mag- nets is to drive the fiber heads directly onto the cores, and after insulating the surface of the latter to wind the spools thus formed with silk-insulated wire of the desired size. Fig. 107 shows one of Fig. 107. Varley Ringer Coil. the sectional coils of the Varley Duplex Magnet Company. Their coils are wound separately, with bare wire separated by silk thread. After winding, the coils may be slipped on the core and locked by the end washer or head. The advantages of this construction are in the ease of replacing burned-out or otherwise injured coils, and also in the perfect uniformity of the winding. Bare-wire winding will probably play an important part in telephony of the future. Where many instruments are to be placed in series on a party line the ringer magnets should be made as low as 40 ohms, in order to reduce the amount of self-induction through which it is necessary to talk. This is a practice little followed ; but good, nevertheless, when instruments must be placed in series. For bridging work the conditions are changed. The generator must have greater current capacity, and for that reason a larger wire in the winding is necessary. This necessitates fewer turns for the same winding space and a consequent loss in voltage. In some long lines the voltage must necessarily be maintained, and in order to make up for the loss in this respect, due to fewer COMMERCIAL CALLING APPARATUS. 113 turns, the field magnets may be made stronger and the armature longer. A good generator for bridging work may be wound with No. 33 wire to a resistance of 350 ohms. The ringer magnets for bridging work must possess a very high degree of self-induction. This should be obtained by winding them to a high resistance with a comparatively coarse wire, so as to obtain a large number of turns in the winding. The length of the cores is increased for the double purpose of getting more iron in the magnetic circuit, and therefore a higher retardation, and also for affording a greater amount of room for the winding. The Western Electric Company wind their coils to a resistance of roooohms, using No. 33 single-silk magnet wire. Many other companies use No. 38 wire and wind to a resistance of 1200 or 1600 ohms. This does not give such good results, however, as using the coarser wire and the lower resistance and long cores. Some companies wind, or once wound, their bridging-bell mag- nets partly with German-silver wire, in order to make a high resistance at a low cost. They should learn, however, that resist- ance in itself is not the thing desired, but a great number of turns in the winding, which, of course, incidentally produces a high resistance. CONSTANTLY DRIVEN GENERATORS. In telephone exchanges, constantly driven generators are em- ployed for ringing purposes, in order that the operators may be Fig. 108. Western Power Generator. saved the labor of manually turning a crank every time a subscri- ber is called up. 114 AMERICAN TELEPHONE PRACTICE. Fig. 108 shows one type of machine for this purpose. It is merely a magneto-generator provided with very long and powerful Fig. 109. Holtzer-Cabot Belt-Driven Magneto. permanent magnets. The machine is mounted on a slate base and driven by means of a grooved pulley, mounted on a separate Fig. 1 10. Motor-Generator. shaft connected to the armature shaft by a short flexible rubber coupling. Machines of this type are suitable for small exchanges, and may be driven by any available source of power. They are COMMERCIAL CALLING APPARATUS. 115 frequently placed in an electric light or power plant, where they may be constantly driven. Wires are then run from them to the exchange, from which the current is sent out over the various subscribers' lines as desired. Another convenient method of obtaining power for switch- board generators is by the use of small water motors, run from Fig. in. Motor-Generator for Battery-Charging. city water mains. These are specially built for this purpose, and consume but little water. A more convenient source of power is an electric motor ; and Fig. 109 shows a Holtzer-Cabot magneto belted to a small direct- current motor, also manufactured by that concern. Convenient forms of alternating-current motors are also made for running generators for this purpose. All the machines so far mentioned are suitable for exchanges of 500 subscribers and under. For large exchanges, and also many smaller ones, the motor-generator, types of which are shown in Figs, no, ni,and 112, are being commonly used. The one shown in Fig. no is manufactured by the Holtzer- Cabot Co., and is provided with a double winding, on a single armature core, the same field serving for both windings. The ] Il6 AMERICAN TELEPHONE PRACTICE. side having the commutator is the motor side, and may be wound for any desired direct voltage. The right-hand side is provided with collector rings for supplying alternating current for ringing purposes at a pressure of 75 volts. Fig. 1 1 1 shows a machine of similar type manufactured by the Crecker- Wheeler Co. This furnishes alternating ringing current, and besides is provided with an extra winding for sup- plying low-voltage direct current, for the purpose of charging storage batteries. In Fig. 112 is shown a similar machine for charging storage batteries in telephone work, and provided with an automatic Fig. 112, Motor-Generator with Automatic Circuit-Breaker. circuit-breaker on the dynamo side. Considerable trouble has been experienced in storage-battery work, due to leaving the battery in circuit with the generator while the latter was not running. This allows the battery to discharge itself through the armature of the generator, tending to cause the latter to turn as a motor. In order to prevent this, one of the pole-pieces of the machine is hinged at the base, where it joins the bed plate. This pole-piece is normally held away from the armature by a COMMERCIAL CALLING APPARATUS. 1 17 spring, and in this position the circuit-opening switch, shown on top of the field magnets, is open. When current is supplied to the machine its magnetism causes the pole-piece to draw close to the armature, thus dosing the battery circuit. As soon as the machine stops the magnetism weakens, and the circuit is again opened. CHAPTER XI. THE TELEPHONE RELAY OR REPEATER. ONE of the most attractive fields of research and invention in telephony has been that of the telephone relay or repeater. It has been very natural to suppose that the principle of repeating now used so successfully and extensively on long telegraph lines could be used with equal advantage on long telephone lines. The idea is very simple, and involves merely the placing of a microphone contact in operative relation with the diaphragm of a receiver connected in the first line circuit, and causing the changes produced in the resistance of this contact, when acted upon by the receiver diaphragm, to vary the strength of a cur- rent in a local circuit, which circuit would in turn act inductively on the second line wire with reinforced energy. This method is outlined in Fig. 113, where A is the transmit- ting station, being provided with a transmitter, T, battery, B, and i B m Fig. 113. Simple Relay Circuit. induction coil, /. L is the transmitting line, having connected in its circuit the coil of a receiver, M. D is the vibrating diaphragm of this receiver against the center of which rests a pair of microphone contacts, which may be the same as those in the Blake transmitter, or of any other type. This micro- phone contact must be so arranged with respect to the receiver diaphragm that any vibrations of the latter will be imparted to the former, thus causing them to vary their resistance in exactly the same manner as if acted upon directly by sound waves. The microphone contact, C, serves to vary the resistance of a local circuit containing a battery, B', and the primary of an induction coil, /'. L' is the receiving line, containing the secondary of the induction coil, /', at the relay station, and the receiver, R, at the receiving station, A'. Any changes in current in the local circuit at the station, A, produced by the operator at the trans- 118 THE TELEPHONE RELAY OR REPEATER. 1 19 mitter, T, will induce alternating currents in the line, L, in tne ordinary manner, which will cause the diaphragm,/?, to vibrate as in everyday practice. The vibrations of D will be imparted to the microphone contact, C, which will produce changes in the current flowing in the local circuit at the relay station correspond- ing to those taking place in the local circuit at station, A. These changes will act inductively on the line circuit, L', in the ordinary manner, the receiver, R, finally reproducing the sound. Such an arrangement as this will do its work well, but it is quite evident that the transmission may be effected only in one direction. When it is desired to transmit from station A' to A, a separate circuit would ordinarily have to be used. Much diffi- culty was experienced in making a two-way repeater, for no auto- matic switch could be arranged which would bring about the changes of circuit required when the transmitting station desired to become the receiving. Many attempts were made to associ- .ate two relays with the line circuits in such manner that no in- terference would occur. The difficulties involved in this were, however, great ; and chief among them was the fact that two relays when associated with the same pair of lines would almost invariably set up a singing sound, due to the mutual action be- tween the two ; for instance, a slight vibration of the diaphragm of one relay would produce changes in current in the local cir- cuit, which would act upon the diaphragm of the other relay, producing another change of current, which would in turn react upon the first relay. This action is somewhat analogous to that produced by holding the earpiece of a telephone receiver directly in front of the mouthpiece of a good granular-carbon transmitter ; the singing or shrieking noise set up when a proper adjustment is obtained in this case being due to the fact that the sound waves set up by the receiver diaphragm act upon the transmitter diaphragm, which in turn causes currents to flow through the receiver coil, causing its diaphragm to vibrate still more strongly. This defect, however, was finally overcome, several inventors having produced two-way relays which were successful in so far as they would operate in either direction with equal facility, and with a fair degree of clearness. One of these systems, devised by Edison, is shown in Fig. 1 14, in which A and A' are the telephone stations, each arranged in the ordinary manner. M is the magnet of the relay receiver, the coil of which is included in a local circuit containing the secondary, 3, of an induction coil. The primary winding of this coil is divided into two parts, I and 2, these parts being connected 1*0 AMERICAN TELEPHONE PRACTICE. together in one side of the combined circuit of the two lines, L and L. Between the juncture of these two primary coils and the opposite side of the line is connected the secondary coil, 4, of an ordinary induction coil. The primary coil, 5, of this latter induction coil is connected in a local circuit containing the relay microphone contact, C, and the local battery, B" . Assuming that station, A, is for the time being the transmitting station, currents set up in the line circuit, Z, will divide at the relay station, part passing through the coils, i and 4, and back to the transmitting station, and the other part passing through the primary coils, I and 2, in series and to the receiving station Fig. 114. Two-way Relay Circuit. direct. The current passing through the coil, 4, will, however, under ordinary circumstances, be by far the greater on account of the high resistance of the long line, Z'. The current passing through the coils, i and 2, however, will act inductively upon the coil, 3, thus causing currents to flow through the coil on mag- net, M, and produce changes in the contact resistance of the mi- crophone. These changes will cause fluctuations in the current in the local circuit, which fluctuations will act through the pri- mary coil, 5, upon the secondary coil, 4, and cause currents of considerable comparative strength to flow in the line circuit, L', to the receiving station, A'. It is obvious that as the various circuits at the relay station are symmetrically connected with respect to the two lines, L and Z', the station, A', may in turn serve as the transmitting station. No reactive effect between the relay transmitter and receiver will in this case be produced, and the means for preventing this forms the most interesting portion of this invention. Whatever currents are set up in the coil, 4, by the action of the microphone contact, C, will divide equally between the primary coils, i and 2, passing through them in opposite directions. These coils will therefore act differen- tially upon the coil, 3, and their effects will be neutralized. No current will be caused to flow in the circuit containing coil, 3, and the relay magnet, M, and therefore no reactive effect will be produced upon the transmitter. In other words, any current THE TELEPHONE KELA Y OR REPEA TER. 121 flowing in either line circuit will induce currents in the locat circuit containing the magnet, M, while currents set up in the coil, 4, by virtue of currents flowing through the magnet, M t will produce no effect in turn upon the coil, 3. A great many improvements have been made in the mechani- cal construction of the telephone relay, but with few exceptions they have embodied only the idea of combining an ordinary transmitter with an ordinary receiver. In 1897, however, a relay was devised by Mr. A. W. Erdman, and is shown in Fig. 115, w Fig. 115. Erdman Telephone Relay. and embodies probably the most radical departure in the struc- ture of telephone repeaters of all since the first was produced. In this figure, L is the transmitting line and 1} the receiving line. H is the diaphragm of the receiving instrument and is used to operate the balanced valve, V* y which by its motion to and fro varies the flow of an otherwise constant stream of air flowing through the chamber, C. This chamber is covered by a flexible diaphragm, D, which is caused to vibrate by the changes in pressure within the chamber produced by the motion of the valve, V"*, The diaphragm, D, serves to operate a micro- phone, T, which in this case consists of the variable resistance button of the solid-back transmitter. R is a reservoir containing compressed air, and V a reducing valve by which the amount of air escaping through the chamber may be regulated. In the balanced valve, F 2 , E is a flexible diaphragm and A a movable portion which controls the outlet. The centers of the diaphragm, E, and of the valve plate, A, are connected by the rod, F, to the 122 AMERICAN TELEPHONE PRACTICE. center of the receiver diaphragm, H. The balancing of the valve, F a , renders it extremely sensitive, so that it may be set in motion by the delicate movements of the diaphragm, H. In operation, the vibrations of the diaphragm, H, caused by currents in the transmitting line, L, cause the balanced valve, F a , to vary the opening of the air outlet. This produces changes in pressure within the air chamber under the diaphragm, Z?, which cause that diaphragm to vibrate and thus actuate the microphone in the usual way, thus causing currents to flow in the receiving line, Z a , in the usual way. No reports have been made public con- cerning the results obtained in actual practice with this repeater, Fig. 116. Stone Telephone Relay. but it seems that it may be a step toward the solution of this difficult problem. Instead of employing the mechanical connec- tion commonly used between the diaphragms of the transmitting and receiving mechanisms, Mr. Erdman has, in his current of air or gas, chosen one of the most delicately subtile mediums known. Another relay, devised by Mr. John S. Stone of the American Bell Telephone Co., is shown in Fig. 116. This relay differs in the essentials of its construction from those of the older type only in that its entire working parts are inclosed in a vacuum chamber. The repeater, together with the circuits of the two connected lines, is shown in this figure, in which T is the trans- mitter of the sending station and t the receiver of the receiving station. These are connected with the repeater bylines, L* and L, respectively, the line circuits being associated with the repeater cir- THE TELEPHONE RELAY OR REPEATER. 123 cuits by induction coils, /and P, in the usual manner. B is a polar- ized electromagnet whose poles are in proximity to the diaphragm, D. C is the variable resistance button of a solid-back transmitter, the front electrode of which is rigidly secured to the center of the diaphragm, D, while the back electrode is rigidly secured by means of a cross-piece to the frame, A, which also supports the diaphragm, D, and the electromagnet, B. E is a bell jar closely fitted to the base, b, by an air-tight joint. The air from within the chamber may be withdrawn by the pipe, P t attached to an air pump. It is said that the removal of the air from within the chamber brings about a decided improvement in the operation of the repeater. Concerning the results obtained, Mr. Stone says, "The messages automatically transferred by it from one circuit to another are reproduced in the receiving telephone of the second circuit with a well-defined gain in volume or loudness, and without any sub- stantial distortion or offsetting loss in clearness of articulation." If this claim is borne out in practice, the production of this relay should prove a step of some importance in the matter of long- distance telephony. It has seemed plausible that very feeble currents received at the relay station would, by virtue of the delicate action of the microphone, be able to produce comparatively large changes in resistance of the local relay circuit associated therewith, and that these changes in resistance would produce correspondingly great changes in the current of the local battery at that station, which changes would act inductively on the second line wire with per- haps as much energy as that imparted to the original circuit. As a matter of fact, however, no gain in the volume of transmission has ever been commercially effected by this method. The tele- phone repeater may be made to work perfectly on ordinary lines, but it has not shown its ability to transmit speech between two distant points any better than, or quite as well as could be done by.direct transmission without the use of the relay at all. The amount of energy received by the electromagnet of the relay is so exceedingly small that it cannot be made to produce the desired mechanical effect upon the microphone contact. CHAPTER XII. SELF-INDUCTION AND CAPACITY. SELF-INDUCTION and capacity play such important parts in the question of long-distance telephone transmission, and seem so little understood among the rank and file of telephone workers and users, that this chapter will be devoted to an elementary and non-mathematical discussion of these two phenomena, with a view to explaining their existence and effect in a simple manner, rather than to throw any new light upon the subject. Ohm's law states that for a steady flow of electricity in a given circuit the amount of current in amperes is equal to the electro- motive force expressed in volts, divided by the resistance of the circuit expressed in ohms. In algebraic form this becomes the well-known equation : 7=4 ' R' where 7 represents the current in amperes, E the electromo- tive force in volts, and R the resistance of the circuit in ohms. Knowing any two of the above quantities, the third may be determined from the equation already given, or from the follow- ing, which are derived from it : E = IR, and R = *L These three equations, which are merely different ways of expressing Ohm's law, are the most useful in the entire science of electricity. It is unfortunate for an easy understanding of telephony that these equations in their simple forms hold true for a steady flow only, and that when currents which are rapidly changing in value or in direction are considered, we must face a more complex set of conditions. An electric current flowing through a conductor sets up a field of force about the conductor throughout its entire length. This field of force consists of magnetic lines extending in closed curves about the conductor, and is often termed a magnetic SELF-INDUCTION AND CAPACITY. 12$ whirl. A freely suspended magnetic needle placed within this field of force will tend to assume a direction corresponding to the direction of the lines of force, and therefore at right angles to the conductor. If the current flowing in the conductor is maintained at a con- stant value and in the same direction, the field of force about the conductor will not change. On the other hand, if the current strength fluctuates, the field of force will become more intense and will expand while the current strength is increasing, and will become less intense and will therefore contract while the current strength is decreasing. If the current changes its direction, the field of force existing is entirely destroyed, and is built up in an opposite direction at every such change. Whenever there is such a relative movement between a con- ductor and the lines of force of a magnetic field as to cause the conductor to cut the lines, or the lines to cut the conductor, an electromotive force is set up in the conductor which tends to cause a current to flow. The direction of the electromotive force will depend on the direction of the lines and on the move- ment of the conductor, and its value will depend on the rate of cutting. The 'field of force may be set up either by a magnet or by a conductor carrying a current, and in either case the phe- nomenon just described is called electromagnetic induction. If two wires are formed into parallel coils, each having a large number of turns, then the lines of the field of force set up by the coil carrying the current will cut many of the turns of the other wire, thus inducing an electromotive force in each turn ; the result being that the sum of all the electromotive forces so induced will be added, thus producing a much greater effect than if each wire consisted of but a single turn. Furthermore, if the two coils are wrapped about an iron core, the field of force due to the coil carrying the current will be greatly strengthened, and therefore the electromotive force induced in the second coil will be greatly increased, owing to the greater rate of cutting of lines caused by the changes in the strength of the current. This is due to the fact that a given magnetizing force, or force which tends to set up a field of force, will produce a greater number of lines in iron than in air. It is evident that in a coil of wire carrying a current each turn of the coil is surrounded by a fie'ld of force, and that each turn must therefore lie more or less within the fields of force of all the other turns. Each turn will therefore have an inductive action upon all the other turns when the current through the coil 126 AMERICAN TELEPHONE PRACTICE. is varying. Whenever a diminution of the current occurs the decreasing number of lines of force set up by any one turn will act on each of the other turns to induce an electromotive force tending to cause a current to flow in the same direction. The decreasing field of force around each one of the turns will act in a like manner on all of the other turns, and as all of the electro- motive forces in all of the turns will be in the same direction as the current which is already flowing in the coil, their effects will be added and will tend to prolong the flow of current. On the other hand, an increase in the current will cause an increasing number of lines to surround each turn, and this increase around any one turn will induce electromotive forces in each of the other turns in the opposite direction to that producing the cur- rent already flowing. This phenomenon of induction between the various parts of a single coil of wire each on the other is termed self-induction. In view of the fact that a decreasing current induces an elec- tromotive force tending to produce a current in the same direc- tion as that already flowing, while an increasing current induces an electromotive force tending to produce current in the oppo- .site direction, it follows that the general effect of self-induction in a circuit is to tend to prevent any changes in current from taking place in that circuit. This accounts for the fact that coils of wire, such as those forming electromagnets, tend to so greatly reduce the flow of voice currents through them ; one of the best illustrations being that used in the bridging bell system of party lines, where the ringer magnets are purposely wound with a great number of turns and provided with long, heavy iron cores for the purpose of increasing the self-induction. It is quite evident that in circuits containing self-induction and subject to rapidly fluctuating electromotive forces, the tendency of self-induction to prevent changes in the current will always cause any change in current to lag slightly behind the electro- motive force which produces it. Where the electromotive force impressed upon a circuit varies according to the law of sines, the electromotive force produced by self-induction lags a quarter of a phase or 90 behind the current flowing in the circuit. That this is so may be seen from the fact that the electromotive force of self-induction is a maximum when the current producing it is changing most rapidly, and is zero when the current producing it is not changing at all. The maximum rate of change of the current flowing in a circuit occurs when the current is passing -through zero, and its minimum rate of change occurs at the SELF-INDUCTION AND CAPACITY, 127- crests of the wave, that is, at its maximum points. It therefore follows that the electromotive force of self-induction is a maxi- mum when the current in the circuit is zero, and is zero when, the current is a maximum. This evidently indicates a phase dif- ference of 90, and we have already seen that this phase differ- ence is a lagging rather than a leading one. In circuits containing only non-inductive resistance the electro- motive force impressed upon the circuit has only to overcome the ohmic resistance, and the value of the current may be obtained at any time by a direct application of Ohm's law. Where self-induction, however, is added, the impressed electro- motive force, if it be a varying one, must overcome not only the ohmic resistance, but the electromotive force of self-induction ; and then the current equation becomes Electromotive Force Current = Impedance The word impedance in this equation may be termed the apparent resistance, and the apparent resistance in circuits hav- ing self-induction is always greater than the ohmic resistance. In fact, Z, the impedance, is equal to where f is the frequency of alternations and L is the coeffi- cient of self-induction a term denoting the total number of lines of force set up in a given coil when traversed by current of unit strength. The equation ot the flow of current, /, may then be written /= which is the equivalent of saying that the current flowing is; equal to the electromotive force divided by the apparent resist- ance. The current flowing in a circuit in which self-induction and resistance are present is the resultant of that produced by the impressed electromotive force and the electromotive force of self-induction. The greater the electromotive force of self-induc- tion the greater will be the lag of the current behind the impressed electromotive force. Furthermore, the greater the self-induction the greater will be the apparent resistance or impedance, and consequently the smaller will be the 128 AMERICAN TELEPHONE PRACTICE. current flowing. The above formula applies only to currents varying according to the sine law ; but telephone currents do not vary according to this law, or according to any other definite law, so far as we have been able to determine. This does not, however, destroy the significance of the formula as applied to telephony. Fourier's theorem states that any complex periodic wave motion may be considered as being made up of a number of simple wave motions having i, 2, 3, 4, etc., times the rate of vibration of the complex wave motion. Telephone currents are very complex, and are composed not only of a fundamental tone, but of many overtones ; it is by the various blending of these overtones, with regard to their relative loudness and their relative position in phase with respect to each other, that articulate speech is produced. A consideration of the formula for the flow of current, just given, shows that the effect of self-induction is greater upon currents of high frequency than upon those of low frequency, for as f, the frequency, increases, the value of the impedance or the apparent resistance increases, and, therefore, the value of the current decreases. In other words, self-induc- tion tends to weed out the higher overtones in preference to the lower ones and the fundamental tone, thus rendering speech indistinct, as well as reducing its volume. Every insulated conductor is capable of receiving a certain charge when subjected to an electromotive force; for instance, if a metallic plate insulated from all surrounding bodies is connected to one terminal of a battery the other terminal of which is grounded, a certain amount of electricity will flow into the plate until its potential is raised to that of the battery terminal. The plate is then said to be charged, and the amount of electricity held by it determines its capacity. The charge of electricity on the plate will be considered positive or negative, according to whether the positive or negative terminal of the battery, or other charging source, was connected with it. It is well known that no charge exists by itself there is always an equal and opposite charge induced by it upon neighboring bodies. It is also well known that like charges repel each other, while unlike charges attract; that if an uncharged body be brought near a charged body an equal and opposite charge will be induced on the side of the uncharged body which is toward the charged body, and that similarly a charge on the same sign as that on the charged body will be induced on the opposite side of the uncharged body. If now the body which was originally uncharged is connected with the ground, this latter charge that SELF-INDUCTION AND CAPACITY. 129 is, the one of the same sign as the original charge will be driven to the ground, while the charge of opposite sign will still be attracted by the charge on the first body. The second body will therefore be charged, although it has not been in contact with the first. The action between charges of electricity taking place through an insulating medium is called electrostatic induction. It is found that where two conductors are placed side by side, but insulated from each other, the capacity of each will be greater than if the other were not present. For the purpose of holding charges in this manner the well-known Leyden jars have long been in use. They are usually made by coating a glass jar inside and out with a layer of tin-foil to within a few inches of the top. The outer coating is usually connected with the ground, while the inner coating is connected with a metallic rod approaching it through the mouth of the jar. If the inner coating is connected with a source of electromotive force, a current lasting but an instant will flow into the coating, producing a charge. This charge, which we will say is positive, will attract a nearly equal negative charge to the outer coating, repelling an equal positive charge to the earth, as already described. The amount of charge which the inner coating will receive under these circumstances is very much greater than if the outer coating were not present, and the capacity of the inner coating is therefore much higher than before. A device such as the Leyden jar is called a condenser. The capacity of a condenser is increased as the area of the conducting surface is increased; is increased as the distance between the conductors is diminished, and may be increased or diminished by using different kinds of insulating material between the conductors. The medium separating the conductors is called the dielectric, and, as stated above, upon it depends to a great extent the efficiency of a condenser. Several condensers built exactly alike, so far as size of plates and the distance between them are concerned, and using different materials for dielectrics, will be found to have different capacities. This difference is due to a peculiar property possessed to different degrees by different dielectrics and called specific inductive capacity. The specific inductive capacity of a dielectric is a measure of that quality which enables the dielectric to hold a charge between two conductors, as in a condenser. The specific inductive ca- pacity of air is taken as a standard and is for convenience con- sidered as unity ; it is lower than that of any other known substance excepting, perhaps, hydrogen. If two condensers having plates of equal size and distance apart are constructed 130 AMERICAN TELEPHONE PRACTICE. with dielectrics respectively of air and guttapercha, it will be found that the condenser having the dielectric of guttapercha will receive a charge nearly 2\ times as great as the condenser having the dielectric of air. The actual ratio between the two is 2.462, and for- this reason the specific inductive capacity of guttapercha is said to be 2.462. The following table gives the specific inductive capacities of some of the more important insulators: Air . . . . . . r. oo Glass 3-013 Shellac, 2.74 Sulphur 2.580 Guttapercha, . . . . . . . . 2.462 Ebonite " . ."':.. . . .. .2.284 India-rubber, . . . . . . . '.'".' . 2.220 Paraffin, - .. . 1-994 Carbonic Acid, . 1.00036 Hydrogen, ........... 0.99967 Vacuum, , , . ' . . 0.99941 It is probable that with very rapidly varying electromotive forces, such as are dealt with in telephony, the specific inductive capacities of the various substances would be higher in com- parison with air than those indicated by this table. Specific inductive capacity is a very important consideration in the construction of cables for telephone purposes. In the con- struction of these cables it is desirable, as will be shown later, to reduce the capacity of the wires of the cable to as great an extent as possible, and in order to do this the dielectric is, in the best forms of cables, made to as great an extent as possible of dry air. On the other hand, in the construction of condensers it is desired that the capacity may be as great as possible for a given area of plates, and therefore some material other than air is used. Paper saturated with paraffin is perhaps the most commonly used, paraffin having about twice as great a specific in- ductive capacity as air, and moreover lending itself readily to the purposes of insulation. To sum up, the capacity of a condenser varies in direct proportion as the area of its plates, inversely as the square of the distance between the plates, and directly as the specific inductive capacity of the dielectric. The effect of a condenser bridged across a circuit carrying an alternating current is to absorb a portion of the current as the electromotive force at its terminals increases, and as the electro- motive force decreases, to give this current back to the line. Consider such a circuit when the electromotive force active in driving current through it begins to rise. The electromotive force at the condenser terminals will also rise, and current will there- SELF-INDUCTION AND CAPACITY. 131 fore flow into the condenser. The strength of this current will depend directly upon the rate at which the potential at the terminals of the condenser is changing. When the electromotive force acting in the circuit reaches a maximum, the potential at the condenser terminals will also be a maximum and will for an instant cease to change. At this point the condenser is fully charged, but as the electromotive force of the line is not chang- ing no more current flows into the condenser ; in other words, the condenser current is zero. As the electromotive force in the line decreases, current will flow out of the condenser and into the line, because the condense'r is not capable of holding so much charge at the lower potential. The flow of current out of the condenser reaches a maximum when the electromotive force in the line is changing most rapidly, and this occurs when it is passing through zero. From this it will be seen that the con- denser current is zero when the electromotive force in the line is a maximum, and is a maximum when the electromotive force in the line is zero. This indicates, as in the case of self-induction, a phase difference of 90, or a quarter of a cycle. It is not so easy to say whether this phase difference is lagging or leading, but a consideration of the direction of flow of current throughout the cycle will throw some light upon the subject. At the instant when the current flowing in the line (which is in exact phase with the active electromotive force in the line *) is positive and at a maximum, the condenser current will be zero. As the active electromotive force decreases toward zero the condenser current increases, but in a different direction, negative, because current is now flowing out of the condenser back to the line. As the active electromotive force reaches zero the condenser current is at its maximum negative value, and as the active electromotive force reaches its maximum negative value the condenser current reaches zero. During the next half- cycle the condenser current increases to a positive maximum and decreases to zero, while the active electromotive force passes from a negative maximum to a positive maximum. In other words, while the active electromotive force, and therefore the line current with which it is in phase, decreases from a positive maximum value to a negative maximum value, the condenser current is negative, and while the active electromotive force increases from its negative to its positive maximum value the * The active electromotive force is the resultant of the impressed electromotive force and the condenser electromotive force, and is in phase with the current actually flowing in the line. 132 AMERICAN TELEPHONE PRACTICE. condenser current is positive. The condenser current therefore reaches its zero value, while decreasing, 90 in advance of the same value of the active electromotive force ; its maximum negative value 90 in advance of the maximum negative value of the active electromotive force ; and upon investigation it will be found that every value of the condenser current occurs 90 in advance of the corresponding value of the actual line current. The electromotive force which is in phase with the condenser current is called the condenser electromotive force, and is 90 in advance of the electromotive force which is active in driving current through the line. This latter electromotive force which, as we have said, is in phase with the current flowing in the line, is the resultant of the impressed electromotive force and the condenser electromotive force, and therefore leads the impressed electromotive force by a certain angular distance. The current equation for a circuit containing resistance and capacity is, as before, Electromotive Force Current = ^ -3 Impedance In this case the impedance depends on the ohmic resistance of the circuit and on its capacity, and is equal to the following expression : R + where / is the frequency, as before, R the ohmic resistance, and C the capacity of the condenser in farads. From this the current equation becomes The denominator is the apparent resistance, depending upon the ohmic resistance of the circuit, the capacity, and the frequency of alternations. An inspection of this equation will show that as the frequency, /, is increased the impedance or apparent resist- ance becomes smaller, and this accounts for the fact that a con- denser will readily transmit rapidly fluctuating currents, such as voice currents. Evidently the effect of increasing /reduces the second member in the denominator of the equation, and if SELF-INDUCTION AND CAPACITY. 133 sufficiently great, this may be neglected, and the equation becomes simply Again, increasing the capacity of the condenser also increases the effective current by reducing the impedance. Every telephone line possesses electrostatic capacity, and may be considered in the light of a condenser. In the case of a grounded circuit the line wire takes the part of one plate of the condenser, while the ground and other neighboring conductors act as the other plate. In metallic circuits the one side of the line acts by electrostatic induction upon the other, and the two together upon the ground and neighboring conductors. The capacity of a line, however, differs from that of a condenser in that it is distributed throughout the entire length of a line, while a condenser connected with a line would have all of its capacity at a single point. Capacity such as that of a line is termed dis- tributed capacity in order to distinguish it from that possessed by a condenser, which may be termed local capacity. The effect of distributed capacity upon telephone currents may be more readily grasped by imagining a large number of con- densers bridged across the two sides of a metallic circuit at frequent intervals. When the electromotive force impressed upon one end of the line increases, a current flows from the source over the line wire and into the condensers, charging them all according to the difference of potential across their terminals. The difference of potential across the terminals of all the con- densers will not be the same, because there is a certain drop, due to the ohmic resistance of the line wire. If the current flows in that direction long enough to charge all of the con- densers on the line then a current will reach the distant end of the line, and if it continues in that direction, will attain its full value, in accordance with Ohm's law. It must be remembered, however, that each condenser is capable of taking a certain charge, and in order to receive this charge a certain quantity of electricity must flow over the line wire. The quantity of electricity which flows through a circuit depends upon the strength of the current and upon the time it is allowed to flow. If the time is insufficient it will be im- possible for enough current to flow through the circuit to charge the condensers up to the potential of the source, and therefore 134 AMERICAN TELEPHONE PRACTICE. the current at the distant end of the line will not reach its maxi- mum value, and in fact may not rise practically above zero. If, therefore, the electromotive force of the source is reversed before sufficient current has time to pass through the line to charge all of the condensers, the current will not reach its full value at the distant end of the line. It may begin to build up in the opposite direction, and again be stopped on account of the insufficient time to reach its proper value in that direction. The time in which the current in such a circuit will reach a definite portion of its maximum value at the distant end of the line is called the time constant, and if the time represented by one alternation of the electro- motive force is smaller than the time constant, the current will not reach that value at the distant end of the circuit, and the transmission will be correspondingly impaired. The reduction in the actual volume of current transmitted by the effects of distributed capacity is, however, of less importance than the distortion of the wave form. The higher frequencies of current waves corresponding to the higher overtones are absorbed by the condensers far more readily than the lower frequencies, and therefore the waves corresponding to the higher overtones are reduced to a much greater extent at the distant end of the line than those corresponding to the fundamental and the lower overtones. This weeds out the upper harmonics, thus tending to destroy the clearness. Capacity, however, acts in still another way to alter the form of the wave. The angle of advance for the higher frequencies is greater than that for the lower, and therefore the waves of different frequencies are shifted with respect to their phase relation, thus greatly altering the wave form. It has been shown that the electromotive force of self-induc- tion lags 90 behind the active electromotive force, while the electromotive force due to capacity is 90 in advance of the active electromotive force. It is not difficult to conceive, there- fore, that by properly proportioning the self-induction and capacity of a circuit the electromotive force of self-induction may be made to neutralize the electromotive force of capacity, and this result is readily obtained in experimental work. In this case, even though self-induction and capacity may be present in a circuit to a large degree, the current flowing in the circuit is in exact phase with the impressed electromotive force, and its value is in strict accordance with the ordinary expression of Ohm's law. Unfortunately, however, for long-distance teleph- SELF-INDUCTION AND CAPACITY. 135 ony such a balancing of self-induction against capacity can be obtained only for one particular frequency at a time. To thus tune a circuit for one particular frequency would render that circuit capable of transmitting efficiently one particular frequency of vibration, while the requirements of telephony are that all frequencies within the range of the human voice shall be transmitted with equal facility. Again, and unfortunately, it has been found impossible to neutralize distributed capacity with anything but distributed self-induction, and this has not yet been accomplished in practice. As for trans-oceanic telephony, the high static capacity of the cable has so far proven an insurmountable obstacle. It is im- possible to conceive a transmitter capable of forcing such rapid undulations through our present form of cables. Clearly, then, the solution lies in the betterment of cables, or the substitution of some other transmission medium, rather than the improve- ment of the instruments themselves. There can be little doubt that trans-oceanic telephony will finally be accomplished, but the indications are that our knowl- edge at the present time is not sufficient to cope with the problem. CHAPTER XIII. TELEPHONE LINES. IN the early days of telephony, the fact discovered by Steinheil, that the earth could be used instead of the return wire of an electric circuit, was made use of, and telephone lines were gener- ally constructed accordingly that is, with but a single wire, using the earth as the return. Lines so constructed were, however, soon found to be subject to serious difficulties, chief among which were the strange and unaccountable noises heard in the receiving instruments. There are many causes for such noises, some of which are not entirely understood. The swinging of the wire, in such manner as to cut through the lines of force of the earth's magnetic field, or the sudden shifting of the field itself, causes currents to flow in the line wire which may produce sounds in the receiver. On long grounded lines the variation in the potential of the earth at the ground plates, due to any cause whatever, will cause currents to flow in the line. The passing of clouds or bodies of air charged with electricity will induce charges in the line, and cause currents to flow to or from the earth through the receiving instruments. Electric storms and auroral displays apparently greatly heighten these effects. These noises are of varying character, and Mr. J. J. Carty well describes them in saying: " Sometimes it sounded as though myriads of birds flew twitter- ing by ; again sounds like the rustling of leaves and the croaking of frogs could plainly be heard ; at other times the noises resem- bled the hissing of steam and the boiling of water." The noises due to these natural phenomena, whatever their true cause may be, are chiefly annoying on long lines, short lines being disturbed only during heavy electrical storms. This is not the case, however, with the noises arising from the proximity of other wires carrying varying currents. Telegraphic signals can be plainly heard in a telephone instrument on a line running par- allel with a neighboring telegraph line for a very short distance. The establishment of an electric railway or electric lighting plant in a town using grounded telephone lines will always cause seri- ous noises in the telephones, and if the lighting current is alter- .36 TELEPHONE LINES. 137 nating the use of the telephones is usually out of the question at night time, while the plant is running. Disturbances on telephone lines from neighboring wires may be attributed to one or all of the following three causes: leakage, electromagnetic induction, and electrostatic induction. Leakage may occur through defective insulation between the two circuits ; or even when the insulation of the wires themselves is practically perfect a heavy return current from a grounded circuit, such as of an electric railway, may, upon its arrival at the grounded end of the telephone line, have the choice of two paths, one through the telephone line, and the other a continuation of its path through the ground. This is the greatest source of trouble due to railway work, on grounded telephone lines. A strange fact in connection with this is that the noises in the telephones do not correspond with the fluctuations due to the commutator of the generator armature, as would be supposed, but Fig. 117. Magnetic Lines around a Conductor. to the movements of the armatures on the car motors. The tone in the receiver is an indication of the movements of the car, and variations in speed may be clearly noticed. Electromagnetic induction is due to the fact that the telephone line lies in the field of force set up by the disturbing wire. About every wire carrying a current there is a field of force, or " magnetic whirl," consisting of closed lines of force surrounding the con- ductors. Such a condition is represented in Fig. 117. If the cur- rent is a continuous one, the lines of force will not vary after being once set up, and the telephone wire lying in this field will not be I3 8 AMERICAN TELEPHONE PRACTICE. affected. If the current in the disturbing wire is fluctuating, the number of lines of force in this field will vary; or, by a clearer way of expressing it, the field of force will expand and contract accord- ingly. This expansion and contraction of the field will cause its lines of force to cut the telephone wire, and will by the laws of electromagnetic induction cause currents to flow in the latter. If the current in the disturbing wire is an alternating one, the field of force around it will be established in one direction, de- stroyed and established in the reverse direction, and again destroyed, with every complete cycle of the current. It is easy to see that this will produce a maximum disturbance in the tele- phone wire. Electrostatic induction may be explained by reference to Fig. 1 1 8, where a grounded telephone line is shown running parallel with a disturbing wire, which we will say is carrying an alternat- ing electric current. The disturbing wire will receive from its source of current alternate positive and negative charges of electricity, and its potential will pass from a maximum in one DISTURBING WIRE Fig. 1 1 8. Electrostatic Induction. direction through zero to a maximum in the other, and again through zero to the maximum in the first direction during each cycle. Consider the condition where the potential of the disturb- ing wire is zero. No charge will then be induced on the tele- phone wire, so that its potential will also be zero. The charge on the disturbing wire then becomes, we will say, posi- tive, and this induces a bound negative charge on the side of the telephone wire nearest the disturbing wire, and an equal posi- tive charge on the opposite side. This latter charge is not bound, and flows to earth through the receivers at each end. This flow will be toward the ground, through each receiver, and the current is therefore from the center of the wire in each direction to the ground. The next instant the potential of the disturbing wire becomes zero, thus relieving the bound negative charge on the telephone wire, which flows to earth, or, more properly, is neutralized by a flow of positive electricity from the TELEPHONE LINES. J39 earth. Thus each change in potential of the disturbing wire causes a flow of current through the receivers at each end, this flow always being toward or from the middle point in the length of the wire. These currents produce noises in the receivers at each end in the ordinary way. When two grounded telephone circuits run side by side, each acts inductively on the other, so that a conversation carried on over one circuit may be heard in the telephones on the other. This phenomenon is aptly termed cross-talk, and is usually ex- Ill ll E ^W III fc~ f ( 3 -f-++-t-.--|-.|--|.-i-.|_ C > L w 1 x y z r * ? =! ^ Fig. 119. Electrostatic Induction. plained in text-books and articles on the subject by the sup- position that it is chiefly if not entirely due to electromagnetic induction. In 1889, however, Mr. J. J. Carty, in a paper before the New York Electric Club, and again in 1891, in another paper before the American Institute of Electrical Engineers,* described a series of experiments which show conclusively that cross-talk between lines is due almost entirely to electrostatic induction, electromagnetic induction playing so small a part as not to be noticeable. The arrangement of circuits in one of his experiments is shown in Fig. 119, in which E F and C D are two well-insulated lines, each 200 ft. long, and placed parallel with each other throughout their entire length, at a distance of \ in. apart. E F is the disturbing line and is left open at E. At Fit is connected through the secondary of an induction coil, L, with the ground. In the primary circuit of this coil is a battery, B, and a Blake transmitter, T. A tuning fork vibrating before the transmitter acted on the diaphragm in the usual way, and caused impulses on the line E F of practically the same strength as voice cur- rents. These impulses are, of course, alternately positive and negative, and may be considered in the same light as the impulses on the disturbing line in Fig. 118. Three receivers, * These papers should be read by all interested in this subject. 140 AMERICAN TELEPHONE fKACTJCE. x,y, and z, were placed in the line C D, the receiver, y, being at the middle point in the line. Upon operating the tuning fork, its musical note could be distinctly heard in receivers, x and z, while y remained silent. In explaining the action of static induction in connection with Fig. 1 1 8, it was pointed out that the flow of induced currents would be either toward or from the middle point in the length of the wire. The silence of the receiver, y, in this case bears out that statement, showing the central point to be neutral. If this were electromagnetic induction, the induced current would pass from one end of the line, CD, to the other, returning through the ground, in which case all the receivers would be affected. As it is, however, the induced charges flow in each direction from the receiver, jj/, to the ground at each end, or from the ground at each end to the receiver, y, thus in no case causing its diaphragm to vibrate. The same results were obtained by grounding the point E through an ordinary telephone. The receiver wire still remained silent, while x and z were both affected to an equal degree. It was also found that opening the central point of the line, DISTURBING WFRE B Fig. 1 20. Electromagnetic Disturbances. C D, produced no effect whatever on the existing conditions ; the noises in the receivers, x and z, were plainly heard and of equal loudness. Many other experiments were tried, the results in each case pointing conclusively to the induction from voice currents being of an electrostatic instead of an electromagnetic nature. There is no doubt, however, but that induction from wires carrying heavy currents, such as are used in lighting and power work, is largely due to electromagnetic effects, and this can be easily proven by experiments similar in nature to those described. The one remedy for all the troubles due to disturbing noises from any of the causes is to make the line a complete metallic circuit. Even this will not completely stop noises from most of the causes, and all additional precaution must be taken, by mak- ing the two sides of the circuit alike in all respects and properly TELEPHONE LINES. 141 transposing them at frequent intervals, in order that they may be as nearly symmetrical with respect to the disturbing source or sources as possible. Merely making the line a metallic circuit, as in Fig. 120, does not give complete freedom from inductive troubles from other wires, whether the induction be electromagnetic or electrostatic. Considering the question from the standpoint of electromagnetic induction, a current flowing in the disturbing wire would set up a field of force, the lines of which would cut conductors, A and B. A being closer, however, would be cut by more lines than B, and consequently any currents induced in A by changes in this field will be stronger than those in B. If a current starts to flow in the disturbing wire from right to left, as shown, the induced cur- rents in A and B will each be from left to right, as indicated by the arrows. These currents will partially annul each other, but that in A, being the stronger, will predominate, and the resultant will flow in the circuit in a direction indicated by the small curved arrows. A single transposition in the center of the metallic circuit will completely annul the electromagnetic induction if the disturbing wire is parallel to the two wires throughout its entire length, and if it carries the same current in all its portions. Here an impulse in the direction of the arrow in the disturbing wire (Fig. 121) will DISTURBING WIRE B A Fig. 121. Electromagnetic Disturbances. cause impulses in the opposite direction in both wires, A and B. As the average distances between the disturbing wire and A and B, respectively, are the same, the strengths of the induced currents in A and B will be equal, and they will, therefore, annul each other, producing no sound in the receivers. It is found, however, that a single transposition in the center of the metallic circuit will not free the line from cross-talk, even though the average distance from the two wires and the dis- turbing wire is the same, and the current strength is uniform throughout the entire length of the disturbing wire. Mr. Carty's experiments throw much light on this point. In Fig. 122 is shown a disturbing wire and a metallic telephone cir- cuit composed of two wires, A and B, of which A is nearer the 142 AMERICAN TELEPHONE PRACTICE. disturbing wire than B. At a time when the charge on the dis- turbing wire is positive, as shown, a negative charge will be drawn by it toward the disturbing wire and a positive charge will be repelled from it. The result is that the distribution of charges on the two wires, A and , will be somewhat as shown, a nega- tive charge being held on the wire, A, and a positive charge driven to the wire, B. In order for this rearrangement to have occurred, it is evident that a flow of electricity must have taken place from A to B, and as two paths were afforded from the center point, a, on the wire A, of equal resistance, this flow must have been from that point in each direction as indicated by the arrows, through the receivers and toward the center point, b, on wire, B, where the two currents met. Upon the charge on the disturbing wire becoming zero the potentials on A and B become equal, by a flow of positive elec- tricity from the center point of wire, B, to that of wire, A. The negative charge on the disturbing wire, which follows the positive DISTURBING WlRE B 6 Fig. 122. Electrostatic Disturbances. charge, will cause this latter to flow from 6 to a, to continue until A is positively and B negatively charged. It is evident, therefore, that alternating currents flow through two receivers, and that these currents differ in phase from that in the disturbing wire by 90 degs., which is characteristic of the action of condensers. Further consideration will show that the points a and b are neutral, and experiment bears out this conclu- sion, for by opening the wires at those points the sound in the receivers at the ends still continues. Where receivers are connected in the circuit at a and b no sound is heard on them, although plainly audible in the end receivers. A single transposition in the center of the line, as shown in Fig. 123, will tend to reduce the sound in the end receivers, but will not cause silence. The static charges on the portions of the wires nearest to the disturbing wire now find four paths instead of two to the more remote portions of the circuit, the flow being clearly indicated by the arrows. The center points, a and b, are no longer neutral, and receivers placed in the circuit there will be subject to noises. TELEPHONE LINES. It is evident that if receivers of equal impedance to those at the ends of the line were placed at a and b, the neutral points, c, d, e r and/, would be found at the quarter points on the line ; i. e., mid- way between the transposition and each end. As a matter of fact, however, no instruments are placed at the point of trans- position, and the neutral points are shifted toward the ends of the line, because the impedance of the receivers at those points DISTURBING WIRE 4- + + -f -t- C > ) B A' Fig. 123. Electrostatic Disturbances. makes it easier for most of the current to pass through the trans- position wires. Theoretically, the currents set up in a metallic circuit by electrostatic induction from another circuit can be eliminated only by making an infinite number of transpositions. Practi- cally, however, it is found that on long circuits transpositions every quarter- or half-mile are amply sufficient to render them unnoticeable. The scheme of transposition used by the American Telegraph and Telephone Company on the New York-Chicago telephone line is shown in Fig. 124. It will be seen from this figure that trans- Vfptr cross jlnrv Fig. 124. Diagram of Transpositions. positions are made on this line practically four times in every mile, that is, upon every tenth pole ; and while this involves the placing of transposition insulators on poles a quarter of a mile 144 AMERICAN TELEPHONE PRACTICE. apart, it does not follow that every circuit is transposed at each of these intervals. The reason for this arrangement is that if two lines running side by side were transposed in exactly the same manner throughout their lengths, the desired non-inductive condi- tion would not be secured for the relation between the corre- sponding wires in the two circuits would then be the same as if no transpositions whatever had been made. In order to overcome this difficulty, transpositions on the second circuit should be made twice as often as those on the first. This is the scheme adopted in Fig. 124, where it will be seen that the center pair of wires on each set of cross-arms is transposed every mile, while the pair immediately adjacent to it on each side is transposed twice as often. The outside pairs on each cross-arm are trans- posed only once per mile, but these transpositions are staggered with respect to those on the center pair. The same scheme is followed out on every cross-arm, but the transpositions on the top set of cross-arms are staggered with respect to those on the set immediately below this being the case throughout the entire number of cross arms on a pole ; the 1st, 3d, 5th, 7th, and Qth being transposed according to the scheme shown in the upper part of Fig. 124, while the circuits on arms Nos. 2, 4, 6, 8, and 10 are transposed according to the scheme in the lower part of this figure. A very perfect transposition is effected by twisting two sides of a circuit together, and this idea is followed out in the English pole-line construction, where the two sides of the circuit are not only transposed laterally, but also pass successively over and under each other several times in each mile, thus effectually giving the circuit a number of complete twists. This method, however, involves several disadvantages in the stringing of wires, and increases the liability of crosses between them. It is not, there- fore, adopted to any considerable degree in this country. The twisted pair of insulated wires used so largely in inside wiring, and also in cable work, accomplishes the transposition of circuits very thoroughly, it in fact amounting to a complete transposition for every twist of the wires. This method is now depended upon entirely in the construction of telephone cables, with so great a degree of success as to absolutely prevent all induction between the circuits. This will be discussed at greater length in the chapter on cables. Where a number of lines radiate from a central point to a number of subscribers' stations the cheapest way of arranging the circuits, if expense alone is to be considered, is to make each TELEPHONE LINES. 145 a grounded circuit. This is done by grounding each line at the subscribers' station after it has passed through the telephone there, and also at the central office after it has passed through the coil of the annunciator or signaling device. Such an arrange- ment is shown in Fig. 125, and it may be assumed that the lines there shown run in the same direction on the same poles or in the same cable to the various subscribers whom they serve. In each case D is the line drop at the central office and R repre- sents the entire telephone set at the several subscribers' stations. It is evident that, with such an arrangement, disturbances in the receivers may be produced by any one or all of the causes already considered. An electric light or power wire, carrying a CENTRAL OFFICE Fig. 125. Ground-Return Systems. heavy current, may cause trouble by electrostatic or electro- magnetic induction, or by leakage ; and, moreover, each telephone wire, when in use, may be a disturbing wire to all of the others. As has been pointed out, the proper remedy for these disturb- ances is to make each line a separate metallic circuit, and to prop- erly transpose the two sides of each circuit at frequent intervals where the lines are long. This course is followed in most large telephone exchanges, and many small ones ; but it frequently happens that commercial considerations will not allow it in smaller installations. Where this is the case a system called the common-return or McCluer system is frequently used, with ex- cellent results. The layout is the same as that of the grounded system, with the exception that the return of every circuit is made through a heavy wire common to all of the circuits. A clear conception of the common-return system may be had by considering a heavy wire, Fig. 126, to take the place of the earth in a grounded system ; each line wire being connected to it at or near the subscriber's station after passing through the telephone 146 AMERICAN TELEPHONE PRACTICE. instrument, R, and at the exchange after passing through the switch-board drop, D. It is quite evident that the common-return system will, if properly installed, remove all trouble due to leakage or earth currents ; for the entire system of wiring may be kept highly insulated from the ground and from other conductors. Practice, however, differs to a large extent in this respect, as some com- panies ground the common-return wire at the exchange, and also at several other points along its length, others at the exchange only, while others keep it entirely insulated from earth. Prob- ably the reason for placing several grounds on the return wire is to effect a reduction in the resistance of the return path, but this, if needed, should be brought about in another way. Probably the best practice in most cases is to keep entirely free from grounds, although there are many who claim to have effected a marked improvement by heavily grounding the common return at the central office. At any rate, this is an easy experiment to try. The location of the common-return wire with respect to the fcfl R fcfl R =fl R FT R R COMMON RETURN Fig. 126. Common-Return Systems. other wires on the poles is a question concerning which there is much difference of opinion. Undoubtedly the best way of disposing it, so far as purely electrical considerations are con- cerned, is to place it on brackets between the two middle cross- arms on the poles, for then it bears a more symmetrical position with respect to all of the line wires and is, therefore, better adapted to neutralize induction from outside sources. It is, how- ever, often put on other parts of the pole, sometimes above all the wires, sometimes on a bracket just below the top cross-arm, and frequently below the lowest cross-arm. The latter is prob- TELEPHONE LINES. 147 ably the most convenient place, as wires led off to buildings will usually stand clear of the other wires on the pole. There is even more difference of opinion as to the proper size of the common-return wire than as to its location. An analysis of the inductive action from neighboring wires may perhaps throw some light upon the subject. If we assume that the entire system is insulated from the ground and from other conductors, it will be safe to say that all disturbances in the telephones due to leakage or earth currents will be eliminated. The only way, therefore, by which neighboring wires can affect the telephone wires is by induction, and this may be either electromagnetic or electrostatic, or both. The ideal arrangement of the common- return wire with respect to the line wires would be that in which all were at an equal distance from the disturbing wire. This con- dition can only be roughly approximated in practice, but in Fig. 127 it will be assumed that the disturbing wire, which may be a AL ;E D wv* 'VA/^* I I A C a ladft 8=^ \/V\A C. O. DROP J C.R. 4jn C' DISTURBING WIRE Fig. 127. Induction on Common-Return Lines. trolley or electric light line, is at approximately the same dis- tance from the line wires, i and 2, from the return wire, C R. The lines, i and 2, are connected at central by the cord circuit, C, a clearing-out drop being bridged between the cord and the common return, and the line drops, D D, cut out as in ordinary practice. It is evident that if no sounds are to be produced in the receiver of Line I, the points, A and B, must be of equal potential. Similarly the points, a and b, must be of equal potential for no disturbance to be produced in the receiver of Line 2, and the points, C and C 1 , for no current to flow through the clearing-out drop. It matters not if C, a, and A are all of different poten- tials, the one requisite for silence in the receivers being that the I4 8 AMERICAN TELEPHONE PRACTICE. two points at the terminals of each shall be of the same potential The question, therefore, becomes, what size of wire shall be used for the common return in order to bring about these conditions? Considering the induction from the disturbing wire to be elec- tromagnetic, it is evident that the electromotive force set up in the several wires will be proportional only to their lengths, as their distances from the disturbing wire is assumed to be the same in each case. The sizes of the wire will have nothing to do with the pressure developed, any more than the size of the wire per se affects the electromotive force developed in the armature winding of a dynamo. In the case of the wire, i, the E. M. F. generated in the length, C A, will equal that generated in the length, C l B, of the common-return wire. This means that the points, A and B, have the same potential as have also C and C \ and no current will flow through the receiver of that line or the clearing-out drop. By the same reasoning the pressure devel- oped in length, C a, of Line 2 will equal that in the length, C 1 b, of the common return, and no current will flow through the receiver of Line 2. So far as electromagnetic induction is con- cerned, therefore, the size of the common-return wire is im- material. Considering the question from the standpoint of electro- static induction, it will be seen that for a given charge on the disturbing wire charges of the same potential will be induced on each of the line wires and on the common-return wire. There will, perhaps, be a tendency for the small wires to assume charges of slightly higher potential than the larger wire, on account of their smaller radius of curvature, but this would be so slight as to be negligible under the conditions assumed, and would be eliminated were the common-return wire made of the same size as the line wires. So far as inductive disturbances from outside sources are concerned, it seems that the size of the common-return wire is practically immaterial, with perhaps a slight theoretical advan- tage to be gained by making it of the same size as the line wires. This view seems to be at direct variance with nearly all written statements on the subject. The same reasoning will show that when any one of the tele- phone lines is considered as the disturbing wire, the same conclu- sions are reached. So far no valid reason has been shown for making the com- mon-return larger than the line wires. There is, however, a good reason why this should be done, and this is the fact that TELEPHONE LINES. 149 the return circuit of any one wire is made, not only through the common-return wire, but also through all of the other line wires in multiple. Referring to Fig. 126, it is evident that currents generated in line, B C, may find a return path through the com- mon-return wire, and through all of the other wires in multiple. In fact, all of these return paths will be chosen, the current dividing among the other line wires and common return, in- versely as to their respective impedances. The currents flowing through these other lines would produce cross-talk were they of sufficient magnitude to do so, and the only way of preventing this is to make the common-return wire of such low resistance that practically all of the current will pass through it. The fact that the common-return wire may be made to possess practically no self-induction, and, therefore, only the impedance due to its ohmic resistance ; while the line wires all include in their circuits either the receiver and induction coils or the bell and drop coils, serves to divert nearly all of the current through the common-return wire, where it belongs. On account of the marvelous sensitiveness of even poor receivers, a compara- tively small resistance in the common return will shunt enough current through the various instruments to cause cross-talk to a considerable extent. There is undoubtedly a large amount of copper wasted in common-return wires, and it is probable that a No. 8 B. & S. gauge copper wire will in most cases answer the purpose. It too frequently happens that larger common-return wires are used in the hope of remedying a difficulty which is due to another cause entirely. Cross-talk in switch-boards and office cables is often attributed to the smallness of the return wire, which in this case might be increased indefinitely without improving the service. Of course, the ideal conditions assumed for the disposition of the wires cannot be attained, and in so far as they are not attained, induction is likely to occur. It frequently becomes desirable to connect a grounded line with a metallic line, and for this purpose what is known as the repeating coil forms the most ready solution. A repeating coil is merely a special form of induction coil, usually constructed in the form shown in Fig. 128. The primary and secondary coils are wound upon a heavy core formed of a bundle of soft-iron wires, after which the ends of the cores are bent around the outside of the coil, thus completely inclosing it i,n a casing of wire. The coil is then clamped to the base, usually by metal straps, as I 5 AMERICAN TELEPHONE PRACTICE. shown in the figure. The terminals of the primary are brought out to binding posts at one end of the base, while those of the secondary are similarly brought out to binding posts at the op- posite end. The wire forming the core should be of No. 24 Fig. 128. Repeating Coil. annealed iron formed into a bundle about f- of an inch in diameter. The two coils are usually made equal in resistance and in number of turns, 200 ohms for each coil being perhaps the most common figure. No. 31 B. & S. gauge silk-covered wire is a suitable size for a coil adapted to ordinary work. Of course these figures may be departed from to almost any degree in order to design coils for special work. In Fig. 129 is shown in diagram a metallic circuit line con- nected with a grounded line through a repeating coil. The two METALLIC CIRCUIT /"5f\ GROUNDED LINE G W Fig. 129. Connection of Metallic and Grounded. terminals of the metallic circuit line are merely brought to the two binding posts of one of the coils, while the terminal of the grounded line is brought to one of the binding posts of the other coil ; the remaining binding post is then grounded. Any vary- ing currents set up in one of the circuits will act inductively on the other circuit through the windings of the coil, each of which may thus be called upon to act alternately as a primary and a TELEPHONE LINES. 151 secondary. By the use of the repeating coil in this manner, two lines may be connected for conversation without grounding one side of the metallic circuit, which would be necessary were the repeating coil not used. There is a very common impression among the independent telephone users that a repeating coil is the one panacea for all of the evils connected with grounded lines. It is perhaps well to correct this impression, by saying that no number of repeating coils will render a noisy grounded line quiet. A repeating coil will, however, prevent the unbalancing of a metallic circuit line, and therefore in many cases insure a degree of quietness on two connected lines which would otherwise be unattainable. It sometimes happens that a long grounded line is paralleled throughout a portion of its length only by some disturbing wire, DISTURBING WIRE R \ Fig. 130. Eliminating Local Induction. such, for instance, as an electric-light line. Where it is not pos- sible from commercial considerations to make the entire line a metallic circuit, much relief may sometimes be had by resorting to the plan shown in Fig. 130, which consists in making only that portion of the line a metallic circuit which is within the direct influence of the disturbing wire. The two ends of the grounded line may then be connected with the intermediate metallic portion by means of the repeating coils, R R, as shown. By this arrangement the disturbing wire produces no effect on the metallic circuit between the repeating coils, if proper precau- tions are taken in the way of transposing its two sides. Tele- phonic communciation may be had over the entire length of line, the currents undergoing two transformations at the repeating coils. Much trouble is often had where it is necessary to ring through repeating coils, especially if the lines are very long. It is therefore advisable that repeating coils should always be placed at a central station if possible, and such arrangements 152 AMERICAN TELEPHONE PRACTICE. made that it will not be necessary to ring through them. How- ever, a coil properly constructed with a magnetic circuit com- pletely closed should serve as a very efficient transmitter, even for the slowly alternating currents of a magneto-generator, and good results may be obtained with such coils on good lines even when it becomes necessary to ring through them. UNIVE CHAPTER XIV. SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES. THE object of a telephone exchange is to afford means for placing any telephone user (subscriber) into communication with any other subscriber in the same system. The lines leading to the telephone instruments of the various subscribers radiate from a central point where they terminate in an apparatus known as a switch-board. Switch-boards may be divided into two classes, manual and automatic. In the manual switch-board, operators girls are employed to make the connections called for, while in the au- tomatic, the operation of connecting or disconnecting lines is performed by the subscriber desiring the connection. The manual switch-board only will be considered at present, as it is in almost universal use, the automatic, owing to its great and necessary complexity, having proven successful only in rare cases. The simplest form of switch-board, one typical of the kind used in small exchanges and designed for use on grounded or common-return systems, will first be considered. Each line entering the exchange terminates in what is known as a spring-jack. Spring-jacks are sockets containing or asso- ciated with simple switching devices and are mounted on the face of the board within easy reach of the operator. In order to make a connection with any line, plugs are provided which may be inserted into the jacks, and thus continue the electrical path from the line wire terminating therein, to and through a flexible conducting cord attached to the plug. Fig. 131 shows a simple spring-jack with a connecting plug inserted. The metallic base, a, of the jack, usually of brass, is drilled from its forward end to receive the shank of the plug, P. A forwardly projecting sleeve on this base fits snugly into a hole bored in the front board, A, of the switch-board, to which it is fastened by the shoulder and small wood-screw as shown. Firmly secured to the rear end of the piece, a, is the line spring, c, formed with a rearwardly projecting tongue, to which the wire, /, leading from the line is soldered. The forward end of the spring, c, rests normally against the pin,/, carried by, but in- 154 AMERICAN TELEPHONE PRACTICE. sulated from, the base, a. A wire, g, leads from this pin, and through the coil of the line annunciator or drop to ground. When the plug is inserted in the jack its conducting tip makes contact with the tip of the line spring and at the same time forces it out of engagement with the pin, p. Normally, therefore, Fig. 131. Spring-Jack. the line wire is connected to the ground through the wire, /, spring, c, pin,/, wire, g, and line-drop to the ground connection. When the plug is inserted in the jack, however, the line is dis- connected from the branch leading through the drop, but is con- nected through the medium of the plug to the flexible cord. Fig. 132 shows a common form of switch-board drop. The purpose of the drop is to attract the attention of the operator whenever any subscriber wishes a connection. The coil of the electromagnet is mounted on the back of the front plate, c, of the switch- board, as shown. To the armature, a, pivoted at its upper end, is attached a rod, b, passing forward through a hole in the front plate and provided with a hook on its forward end, adapted to engage the upper portion of a pivoted drop-shutter, s, and to hold it in its raised position. The attraction of the arma- ture due to a current passing through the coil causes the hook to rise, thus releasing the shutter, which falls to a horizontal position and displays to the operator the number by which that line is designated. In order to attract the attention of the operator at night or at such times as she may not be in sight of the board, a night- alarm attachment is provided on each drop, which serves to close the circuit through a battery and vibrating bell whenever the shutter is down. The small cam surface on the lower portion of the shutter, s, forces the light spring, /, into contact with the Fig. 132. Switch- Board Drop. SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES. 155 pin, t', when the shutter is down, thus accomplishing the above result. Fig. 133 shows diagrammatically the circuits of such a switch- board. * But two subscribers' lines with their spring-jacks and n Line Jacks. Line Drops. Subscribers Lines. Clearing-out Drop. Fig. 133. Switch-Board for Grounded Lines. drops are shown. These, it will be noted, enter by the line spring of the jack, and thence when the plug is not inserted their circuits pass through the contact pin of the jack through the electromagnets of their respective drops and to ground at G. In the lower portion of the figure, R represents the operator's receiver, T her transmitter, B the transmitter battery, s and p respectively the secondary and primary windings of the opera- tor's induction coil, Pand P' a pair of plugs, and K, K', and K" keys connected therewith, the purpose of which will be described later. When one of the line drops falls, indicating that the '' desires a connection, the operator takes 156 AMERICAN TELEPHONE PRACTICE. up the plug, P, and inserts it into the jack bearing the cor- responding number, say, No. 20. She then moves the lever of the key, K", into the position shown that is, so that the spring of this key makes contact with the stop below. This movement connects the operator's telephone set with the telephone of the subscriber calling, the circuit being traced from ground at the subscriber's station through his instrument to his line wire, from the line wire to the line spring in the jack, thence to the plug, P, cord, c' y to the lever of the key, K', through the upper contact of this lever to the lever of key, K", thence through the operator's receiver and the secondary of her induction coil to ground, G. She now ascertains from the subscriber the number of the line with which he desires connection, which we will say is No. 63. She thereupon takes up the other plug, /", of the pair and inserts it into jack 63. In order to call subscriber No. 63, she presses the key, K, into contact with its lower stop. This completes connection from the ground at the central office, through the operator's generator, through key, K, cord, c, plug, P, jack No. 63 to subscriber No. 63, and through the ringer magnet of his in- strument to ground. All keys being in their raised position, the two subscribers may converse with each other over the following path : line wire No. 20 to jack No. 20, plug, /", cord, c', key, K' t through the upper contact of this key, through the coil of the clearing-out drop to key, K, thence through cord, c, plug, /*, jack 63 to subscriber 63. In case at any time the operator wishes to " listen in " to ascertain if the parties are through talking, she may do so by depressing key, K" , which throws her telephone into a branch or derived circuit of the circuit between the two subscribers. The key, K', may be used to connect the generator with the line to which the plug, P', is connected. The clearing-out drop is placed in the circuit between the two plugs to indicate to the operator when either of the subscribers turns his generator to ring off. But a single pair of plugs with their corresponding keys and clearing-out drop are shown, for simplicity's sake. It is usual to place ten of such pairs of plugs for each one hundred sub- scribers in the system, it being found that this number is sufficient to meet the requirements at the busiest periods of the day. The drops in a board of this type are usually wound to a resist- ance of about 80 ohms, unless designed for multiple or bridged telephone lines, in which case the resistance of the drops is the SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES. 157 same as that of the ringer coils of the telephone instruments on that line, usually 1000 ohms. This switch-board has not been described because it is a fair example of modern, up-to-date apparatus, but because, stripped of all complicated devices for facilitating the work of the opera- tor, it can be more easily comprehended by the beginner. A large number of such switch-boards are, however, in use, and for small exchanges may give as good service as it is possible to obtain with grounded or common-return lines. It has already been pointed out that in order to avoid induc- tion and other sources of trouble, metallic circuits are rapidly superseding ground circuits in telephone exchanges. The switch- boards in common use for small metallic-circuit exchanges are built on the same general principles as those for grounded circuits just described, differing from them only in such details as to render possible the connections of the two branches of one line with those of another line through. the cord cir- cuits. For this purpose two separate contacts are provided in each jack forming the terminals of the two branches of the line. The plugs also have two separate contact-pieces adapted to register with the contact-pieces in the jack when a connection is made. Each contact on the plug is connected to a similar con- tact on the other plug of a pair through the medium of a double- conductor flexible cord. One form of metallic-circuit jack is shown in Fig. 134. Here the tubular portion, a b, forms a terminal for one side of the line, Fig. 134. Metallic-Circuit Jack. while the flexible spring, d, forms the terminal for the other side of the line. The terminal,,^, connected with the pin upon which the spring, d, normally rests, forms one terminal for the coil of the line-drop. The other terminal of this coil is attached to the terminal, a, so that when the spring, d, is in contact with its pin the circuit is complete from one side of the line to the other through the drop coil. The tubular frame of this jack is made in two pieces, a and b. The front portion, b, is a hollow screw, '58 AMERICAN l^ELEPHONE PRACTICE. threaded to engage a tapped hole in the front of the piece, a. By this arrangement any jack may be readily removed from the board by unscrewing the piece, b, until it disengages the rear portion, a. A slot for receiving a screw-driver is provided on the front of the piece, b, to accomplish this. A metallic-circuit plug in common use is shown in Fig. 135. The tip conductor is formed of a rod of brass slightly enlarged at Fig- 135- Metallic-Circuit Plug. its forward end. This is encased in a bushing, b, of hard rubber, and over this is slid a tube, s, of brass forming the sleeve of the plug. A second bushing, b', covers the rear portion of the sleeve, s, and the rear portion of this latter tube is in turn cov- ered by the tube, b", of hard rubber, forming the handle of the plug. The tube, s, forming the sleeve has a portion which pro- jects rearwardly into the handle and is there provided with a connector, c, to which the terminal of one conductor of the flex- ible cord is attached. The other connector, c', is attached to the rear portion of the tip piece, /, and forms the terminal for the other conductor of the cord. In Fig. 136 is shown a form of jack and plug manufactured by the American Electric Telephone Co. The jack is self- Fig. 136. American Jack and Plug. Fig. 137. Keystone Telephone Co.'s Jack and Plug. contained and is mounted on the board by means of a screw- threaded thimble, in much the same manner as the jack shown in Fig. 134. The two springs are secured rigidly to the frame of the jack, but are insulated from it and from each other by strips of hard rubber and by insulating bushings for the screws. The plug differs in its details from that shown in Fig. 135, but its SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES. '59 contacts perform the same functions. The entire metal portion of the plug, including the tip and sleeve contacts, are screw- threaded into the hard-rubber bushing, forming the handle, and make contact with the terminals of the flexible cord in such manner as to bind it firmly in place without the use of other connectors. The screw-threaded thimble of the jack is provided with a long shank so as to adapt it to fit almost any thickness of panel board. This jack and plug are made with special reference to use upon boards already installed, when it is desired to in- crease their capacity. In Fig. 137 is shown another form of jack and plug, manu- factured by the Keystone Telephone Co. of Pittsburg, Pa. The construction and operation of this are evident from the cut. In Fig. 138 is shown in diagrammatic form the circuits of a switch-board of this class. Here the line wires, I 1 and / 2 , forming K. c.o. Fig. 138. Circuits of Metallic Switch-Board. the two sides of a metallic circuit, enter the spring-jacks, e, e\ and e\ in the manner described in connection with Fig. 133. It will be noticed that while the tip-spring, d, is in its normal position, circuit is traced from the line, /', through the coil of the drop,/, and back to line, / 2 , so that current sent from the subscriber's station will actuate the drop, thus indicating a call. When one of the plugs, P or P', is inserted into the jack spring, d is raised from its normal resting-place and breaks contact with the termi- nal leading to the drop-coil, thus cutting the drop out of the circuit. At the same time, the connection is continued from the two line wires, /' and / 2 , to the two strands of the cord circuit. When an operator notices that a drop has fallen she inserts the answering plug, P, into the jack corresponding to that drop and by pressing the button, K, belonging to that cord circuit, bridges her telephone set, T, across the two strands, I and 2, of the cord circuit. This enables her to communicate with the subscriber call- 160 AMERICAN TELEPHONE PRACTICE. ing, to ascertain his wants. She then inserts the calling plug, /", into the jack of the called subscriber and presses the button, K ', thus connecting the terminal of the generator, G, with the two sides of the line of the subscriber called. It will be noticed that when the key, K', is in its normal posi- tion the conductors from the tip and sleeve of the answering plug to the tip and sleeve of the calling plug are made continuous by the springs of the calling key resting against their inside anvils. When the key is depressed the springs break contact with the inside anvils, thus severing the connection between plugs, P and P', and immediately afterward connect with the outside anvils forming the terminals of the generator, G, thus sending current over the called subscriber's line. The clearing-out drop, C O, is permanently bridged across the cord circuit as shown, in order to indicate to the operator when either subscriber rings off. In order that the efficiency in talk- ing may not be impaired, this drop is made of high resistance and high impedance. The line-drops are usually of the ordinary type described in connection with the grounded-circuit switch-board. The clear- ing-out drops, however, must be made to meet more difficult requirements than the line-drops. As they are always bridged across the circuit of two connected subscribers, it is found that unless special precautions are taken much trouble will be expe- rienced from cross-talk due to induction between two adjacent drops. This difficulty cannot be overcome, as in the line-drops, by cutting them out of the circuit whenever two subscribers are connected, inasmuch as the very purpose for which they exist requires them to be always in such circuits. Neither can it be overcome by placing the drops at such a distance from one another that this induction will not be felt, for the limited space on switch-boards requires that they be put as close together as mechanical conditions will allow. It has thus been found necessary to design drops which would neither affect nor be affected by any similar drop in its imme- diate vicinity. This has been accomplished in several ways, but the best example is that shown in Fig. 139, which illustrates what is known as the " Warner Drop." In this the coil is wound in the ordinary manner on a soft-iron core and is then encased in a tubular shield, c, also of soft iron. The armature, d, is pivoted at points, e, in a bracket, f, mounted directly on the rear portion of the tubular magnet. From this armature a rod, a, extends forward through a notch in the front plate, b, in such manner as SIMPLE SWITCH-BOARDS FOR SMALL EXCHANGES. 161 to engage the upper portion of the shutter and thus hold it in its raised position. A screw, /, passing through the front plate, b, serves not only to hold the magnet in place, but to hold the core in its place within the shell. The terminals of the coil are led out through two small holes in the armature and are con- nected with the terminals, h Amounted on an insulating strip, carried on the bracket,/". These drops should be so nicely made that the armature, d, will fit closely against the end of the tube, c, in such manner as Fig. 139. The Warner Drop. to almost completely close the magnetic circuit in which the coil is placed. The lines of force generated by the passage of a cur- rent through the coil follow almost entirely the path provided for them by the shell and the core of the magnet, thus not only pro- ducing a very efficient electromagnet, but also preventing any of the lines of force from extending beyond the limits of the shell. These drops are usually wound to a resistance of 500 ohms and may be mounted as closely together as desired with- out producing perceptible cross-talk. The impedance due to the great number of turns in the coil, and to the perfect magnetic circuit surrounding the same, is so great that practically no diminution in the strength of speech transmission is felt due to its being bridged across the two sides of the line. 162 AMERICAN TELEPHONE PRACTICE. Another form of tubular drop is shown in Fig. 140. This is manufactured by the American Electric Telephone Co., and needs but slight description. The tubular magnet is mounted on a brass bracket extending from the rear plate of the switch- board, upon the front of which is pivoted the shutter. The armature is pivoted at its lower edge in the brass bracket, and carries on its upper side a forwardly projecting rod which serves as a catch for the shutter. This drop gives excellent service in Figs. 140 and 141. American and Keystone Tubular Drops. practice, but is probably not quite so sensitive as the Warner drop, because the armature in its backward movement must necessarily pull the shutter back slightly before it can release it. This is but a slight objection, however, and does not, as stated above, seriously impair its efficiency. In Fig. 141 is shown the tubular drop of the Keystone Tele- phone Company, the arrangement of the parts being evident from the cut. CHAPTER XV. LISTENING AND RINGING APPARATUS FOR SWITCH-BOARDS. IN order to accomplish the changes of circuit by which the operator is enabled to connect her telephone with the line of any subscriber, and to send calling current to actuate the bells at any subscriber's station, many forms of circuit-changing switches have been devised. One of these, shown in Fig. 142 and known 9*-j Fig. 142. O'Connell Key. as the O'Connell key, has been in use in this country, either in the form shown or with slight modifications, for many years. Six different contact-springs are so mounted and formed as to be acted upon by a wedge, b, of insulating material adapted to slide vertically among them. This wedge is mounted upon a rod, carrying at its upper end a button, by which it may be raised or lowered by the operator. The two springs, b 3 , form the terminals of the two strands of the flexible cord leading respectively to the tip and sleeve of the calling plug. These springs are provided with rollers, b\ in order to reduce friction when acted upon by the wedge, b. The springs, b\ form respectively the terminals for the strands of the cord leading to the tip and sleeve of the answering plug. The two springs, b*, are connected each to one terminal of the operator's set, while the pins, b\ are connected each to one terminal of the calling generator. The normal posi- tion of this apparatus is when the wedge is raised to its highest position. In this position the springs, &*, rest against the smallest portion, b\ of the wedge, b, and are not in engagement with the springs, ft 3 . The springs, &*, however, rest against the springs, b*, thus making complete the connection from the tip and sleeve of the answering plug to the tip and sleeve, respectively, of the call- 163 164 AMERICAN TELEPHONE PRACTICE. ing plug. In this position two subscribers may converse without being heard by the operator. When the button is depressed one notch the springs, b\ ride upon the second portion of the wedge, b, thus forcing them into engagement with the springs, 3 , without causing these latter to break contact with the springs, b\ In this position the circuit between the two plugs is not broken, but the operator's tele- phone set is connected across the two strands of the cord, thus allowing the operator to listen in and to communicate with either of the two subscribers who are talking. In its third position, which is that shown in Fig. 143, the springs, b 3 , break contact Figs. 143 and 144. O'Connell Key. with both springs, b* and 6\ and come into contact with the pins, &\ which are connected with the terminals of the generator. This sends calling current to the called subscriber without affect- ing in any manner the circuits leading to the calling subscriber. When the button is depressed to its utmost extent the springs, 6 s , are pressed outwardly as is shown in Fig. 144, until they not only make contact with pins, < & , but also with pins, b\ These pins, b*, are each connected to pins, 8 , against which the springs, b\ are now resting. This completes a circuit from the generator terminal, 5 , through the springs, 3 , to the pins, b*, thence to the pins, 6 s , and thence through the springs, 4 , to the sleeve and tip of the answering plug and to the line of the calling subscriber. Thus, in this final position of the key, calling current is sent not only to the subscriber to be called, but also to the one who originated the call. Of course, this is necessary only when for some reason the calling subscriber has left his instrument. LISTENING AND RINGING APPARATUS. 165 In later and better keys, arrangement is made whereby either the calling or the called subscriber may be called without dis- turbing the other. The combined listening and ringing key shown in Fig. 145 is the invention of Mr. Frank B. Cook, of the Sterling Electric *? Fig. 145. Cook Key. Co. This key is quite extensively used by some of the licensees of the Bell Company, and also in all of the switch-boards manu- factured by the Sterling Company. The springs in this key are arranged in duplicate sets, the two sets being divided by a hard rubber partition, 18, as shown in the sectional view at the bottom of the figure. The two sets of springs are shown sepa- rated in the upper portion of the figure, and their various circuit connections clearly indicated. The springs are acted upon by the cam, 12, of hard rubber pi voted in the metal frame, and adapted i66 AMERICAN TELEPHONE PRACTICE. to be turned through a small arc by the handle, 15. The springs 23, on each side of the partition bear against the left-hand portion of the cam, and form the terminals of the operator's talking circuit including her receiver and the secondary of her induction coil. On the opposite side of the cam are the two springs, 24,. Fig. 146. American Key. forming the terminals of the clearing-out drop, E, one of them being placed on each side of the partition. The springs, 28, form the terminals of the tip and sleeve strands of the calling plug, while the springs, 29, form the terminals of the two corre- sponding strands of the answering plug. These springs normally rest against the two contact strips, 36, so that the tip of the calling plug is normally connected through one of the strips, 36, to the tip of the answering plug, and similarly the sleeve of the calling plug with the sleeve of the answering plug through the other strip, 36. When the cam is rotated in one direction the listening LISTENING AND RINGING APPARATUS, 167 springs, 23, are pressed into engagement with the two strips, 36, thus bridging the operator's telephone across the two sides of the cord circuit. When the cam is in its opposite position the two clearing-out springs, 24, are pressed into engagement with these strips, thus bridging the clearing-out drop across the cord circuit. This latter position is the normal position of the cam. Two outside terminal strips, 37, are provided, one on each side of the partition, these forming the terminals of the switchboard generator, y. By means of pressure on one of the buttons, 16 or 17, the contact springs, 29 or 28, may be pressed out of engagement with the strips, 36, and into engagement with the generator terminals, 37, thus disconnecting the strands of the cord from the rest of the circuit and at the same time connecting them with the genera* tor terminals. Upon releasing the button the springs resume their normal position, completing the circuit between the two plugs and cutting out the generator. These circuit changers have the advantage of being entirely self-contained, thus rendering the removal of any one of them from the switch-board a comparatively easy matter when repairs are necessary. They are entirely in- closed, and are therefore quite free from dust, which Fig. 147. Section- causes much trouble in the way of poor connec- al View Ameri- tions in many otherwise efficient circuit changers. A new key, just put upon the market by the American Electric Telephone Co., is shown complete in Fig. 146 and in sec- tion in Fig. 147. In this all of the listening and ringing opera- tions are performed by the manipulating of a single lever, no buttons being required to perform the ringing, as is usually the case. As in the Cook key, two sets of springs are provided, being separated by a partition, A, of hard rubber. The springs are mounted in slits cut in hard-rubber blocks, J3, these blocks being clamped between two brass side-plates forming the frame of the circuit changer. The front one of these side-plates is removed in Fig. 147 in order to better show the construction. The circuits of this apparatus are shown in Fig. 148, the two sets of springs being separated in order to render their action clearer. It should be remembered, however, that the cam, C, acts in the same manner on each set of springs, so that the two sets always occupy similar positions. The pair of springs, I, form the terminals of the operator's set, and are adapted to make contact with the springs, 2, when the lever is pressed to the right. The i68 AMERICAN TELEPHONE PRACTICE. springs, 2, normally bear against the springs, 4, which form the terminals of the tip and sleeve strands respectively of the answer- ing plug, P. The springs, 3, make normal contact with the springs, 5, which form the terminals of the tip and sleeve strands of the calling plug, P'. As the springs, 2 and 3, on each side are perma- nently connected together, it follows that in the normal position of the circuit changer the tip strand is made complete through spring, 4, spring, 2, spring, 3, and spring, 5, and the sleeve strand Fig. 148. Circuits American Key. is made complete through the same springs on the other side of the partition. When the cam lever is thrown to the right, the springs, I, are, as before stated, pressed into engagement with the springs, 2, thus bridging the operator's telephone across the combined cord circuit. When the lever is pressed still further to the right, the rubber plate, D, carried upon it, presses the springs, 5, into engagement with the springs, 6, at the same time breaking the contact with the springs, 3. As the springs, 6, form the terminals of the switch-board generator, this sends calling current over the line with which the calling plug, P', is connected. No current is sent to the line with which the other plug is con- nected, because the circuit is broken between springs, 3 and 5, on each side of the partition. In a similar manner a pressure of the lever to the extreme left causes the springs, 4, to break engage- LISTENING AND RINGING APPARATUS. 169 ment with the springs, 2, and to come in contact with the springs, 7, which also form terminals of the calling generator. This sends calling current over the line with which the plug,/", is connected. The combined listening and ringing key of the Western Telephone Construction Co. is operated entirely by one lever, this lever being normally held in its central position by the con- tact springs against which it operates. In order to connect the operator's telephone across the terminals of the cord circuit with this key, the lever is pushed straight down without rocking it at Fig. 149. American Listening Key. all, this action pressing the light operator's springs into contact with the tip- and sleeve-springs of the cord circuit. When the lever is rocked toward the operator, the tip- and sleeve-springs of the calling plug are pressed into engagement with the gener- ator contacts, thus at the same time cutting off the connection with the tip-spring of the answering cord. A rocking motion in the other direction presses the tip- and sleeve-springs of the answering plug into engagement with the generator contacts, at the same time cutting off the tip-springs of the calling cord. Fig. 149 shows a simple key manufactured by the American Electric Telephone Co., and typical of a large number of keys designed for the purpose of either listening or ringing. In this the plunger, A, is of hard rubber, and is normally pressed into 170 AMERICAN TELEPHONE PRACTICE, its upper position by the spiral spring below it. It may be depressed, however, by means of the lever, in an obvious manner. The outside springs form the terminals of the cord circuit, while the inside shorter springs may form the terminals of the operator's telephone set or of the calling generator, according as to whether the key is to be used for listening, or for ringing purposes. When the plunger is depressed, the outside springs fall into the depression on the plunger, while the shorter springs ride upon its enlarged portion, thus pressing the two pairs of springs into contact and bridging the telephone or generator across the cord circuit. To facilitate the manipulation of switch-boards it is, of course, desirable to make the number of motions necessary to effect a connection as few as possible. If, therefore, some act which must necessarily be performed by the operator in inserting a plug in or withdrawing it from a jack can be made use of to bring about some of the other changes of circuit, a decided advantage is gained. In Fig. 150 a device for accomplishing Fig. 150. Plug Listening Device. this is shown. The line springs of a jack are represented by a and b; c and d are two springs arranged adjacent to the line springs and forming terminals of the operator's telephone set. The plugs are formed with alternate depressions and enlargements, which are so spaced that when the plug is partially inserted into a jack the two line springs ride upon the two enlargements, thus pressing the line springs into engage- ment with the operator's terminals, c d. This places the operator into communicative relation with subscriber. After the operator has learned the connections desired, she inserts the plug fully into the jack, thus allowing the two line springs to fall into the recesses of the plug. This maintains the connection between the line and the conductors of the plug, but breaks the connection between the operator's telephone and the line. The apparatus is then in the position shown in Fig. 150. If at anytime the operator wishes to listen in without breaking the connection between two subscribers, she may do so by partially withdrawing the plug. If she finds that they LISTENING AND RINGING APPARATUS. 171 are through talking, the movement is continued and the plug replaced in its seat. Where this device is used an ordinary ringing key is required to connect the generator across the terminals of the calling plug. In Figs. 151 and 152 is shown a device in common use, de- .signed by Mr. W. O. Meissner, which accomplishes the con- c* Fig. 151. Meissner Ringing Device. nection of the calling generator with the line of the subscriber called for by inserting the plug to its utmost extent into the jack. The illustrations show a jack and plug for common-return or grounded lines. In Fig. 151 the plug is shown partially inserted into the jack, in which position the line spring, C, makes contact .at the point, C\ with the conductor, B*, of the plug, thus com- pleting the connection between the line, C y , and the strand, B\ of the corcl. In this position the circuit is continuous between two connected subscribers or between the operator and one sub- scriber, as the case may be. When it is desired to ring out on the line, C*, the plug is pressed to its fullest extent into the jack, Fig. 152. Meissner Ringing Device. as shown in Fig. 152. In this position the spring, C, rides upon the insulated sleeve, B l , of the plug, thus breaking connection be- tween the spring and the contact, B*, and at the same time pressing the tip of the spring into contact with the strip, C 3 , which is connected by wire, C 4 , to one terminal of the generator. Current from the generator thus flows to line until the plug is released, at which time it is forced outward by the action of the spring and again resumes the position shown in Fig. 151. Where this device is used, listening in by the operator is accom- plished by an ordinary listening key. 172 AMERICAN TELEPHONE PRACTICE. In Fig. 153 is shown another device for listening in. Pis the calling plug of any pair, and is shown in its normal socket on the r> \r Fig- 153. Plug Socket Listening Key. key table. By tilting it in its socket until it assumes the posi- tion shown in the dotted lines, the spring, S, is forced from its normal position, and thus presses the springs, q and r, into engagement with terminals, q l and r 1 . As is shown by the diagram, this act connects the operator's telephone set across the two strands of the cord circuit, TT\ The knob, Q, upon the spring, S, may be used to connect the operator's telephone across the cord circuit, in case it is desirable to listen in after the plug, P, has been removed from its socket. Calling is done by pressing the key, K. This affords a very rapid means for connecting the operator's telephone into circuit with any line, for after having inserted an answering plug into the jack of a calling subscriber, she can, by part of the move- ment which withdraws the calling plug, P, from its socket, connect her telephone with the calling subscriber's line. A continuation of this movement completes the connection with the called subscriber, and at the same time cuts the operator's telephone out of circuit. CHAPTER XVI. SELF-RESTORING SWITCH-BOARD DROPS. IT is generally considered of great advantage to have switch- boards so arranged that it will be unnecessary for the operator to manually restore the drops. The reason for this is that every movement on the part of the operator, in establishing a con- nection between two subscribers, requires a certain amount of time, and that in the busier portions of the day an opera- tor is worked almost to the extremity of her endurance, and therefore that the saving of any movements in handling these connections will be a great gain in the rapidity with which the board can be operated. Such saving of the work of the operator not only insures a quicker and therefore a better service, o V / o f^ ]n k a 5 ^? a f iz Fig. 154. Self-Restoring Drop. but also may reduce the cost of the operation of the exchange by enabling fewer operators to handle the system. There are, however, many who contend that the greater part of an operator's time is necessarily taken up in talking or listening to the sub- scriber in order to ascertain his wishes, and that while she is doing this she may restore the drops by hand without loss of time. Notwithstanding this, however, the number of exchanges using self-restoring drops is rapidly increasing, and many inventions have recently been made and put into practice to bring about this result. Brief mention has already been made of the electrically re- storing switch-board drops used to a large extent by the American Bell Telephone Company. The details of such a drop are shown in Figs. 154, 155, and 156. In Fig. 154, a is a tubular electromag- 174 AMERICAN TELEPHONE PRACTICE. Fig. 155. Self-Re- storing Drop. Sec- tional View. net, carrying on its rear end an armature, C, pivoted at c, which armature carries an arm, c 3 , which projects forward and is pro- vided with a catch, c\ on its extremity. So far the arrangement is almost identical with that of the Warner tubular drop already described. A second tubular electromagnet, d, is secured to the front of the plate, b, which also supports the magnet, a. This second magnet has its poles facing the front of the board. An armature, f, is pivoted at its lower side by the pivots, e l and e*, shown in Fig. 1 56. The catch, c\ on the rod, c*, is adapted to engage a lug, e 3 , on the armature and retain it in its vertical position. Pivoted on the bracket, f, which is insulated from the magnet by the strip of insulating material, f, is a light shutter, g. The tendency of the armature,^, when released is to fall outward, and in so doing it presses against the light shutter, g, just below its pivotal point, and forces it into a horizontal position. The coil of the electromagnet, a, is usually termed the line coil, and is included in the circuit of the line wire. The coil of the electromagnet, d, termed the restoring coil, is in a local circuit containing a battery which is closed by the insertion of a plug into the spring-jack of the line belonging to that drop. Various arrangements associating drops of this type with the line circuits and with the local circuits at the exchange have been devised and put into practical operation with almost unqualified success. The ar- rangement in Fig. 157 is typical, and at the same time shows a very interesting improve- ment designed for saving battery power in the exchange. The ordinary arrangement of subscriber's circuit is shown, and it will p. x c6 Self Res'or- be noted that the actuating coil, a, is bridged Drop. Front across the two sides of the line wire. The View, coil, a, of course, is necessarily wound to about 500 ohms resistance to prevent short-circuiting the voice cur- rent. Two sleeves or thimbles, k k\ are shown on each jack of the line, the inner ones, k, of which are shown connected permanently together and grounded through a battery, k 3 . The outer thimbles, k\ are connected together, and are usually connected to the ground directly through the restoring coil, d. When with this ar- rangement a plug is inserted, the two thimbles of the jack, k k\ are short-circuited by the sleeve on the plug, and the circuit through SELF-RESTORING SWITCH-BOARD DROPS. 175 the actuating coil is thus closed through the battery, k\ This pulls the armature, e, back until it engages the catch, c\ and thus allows the shutter to swing into its normal position. Any subsequent currents coming over the line wire will fail to operate the drop, for the coil, d, will not allow its armature, e, to fall against the shutter, g, while the plug is in the jack of that line. This ar- rangement has been found to be sub- ject to one somewhat serious objec- tion, viz., the severe use of the bat- tery, k?, owing to the fact that it is on closed circuit through the low- resistance coil, d, as long as the plug is in its socket. In order to over- come this objection the plan shown in Fig. 157 was devised by Scribner. The wire leading from the thimble, k\ to the restoring coil is run to the pivot of the shutter, g, as is shown, and the side of the restoring coil which is not grounded is run to the armature, e. This connection is very clearly shown in Fig. 157. The shutter, g, is normally insulated from Fig- 157. Circuits of a Self- Restoring Drop. the armature, > > , i > > i of of o 1 u 1 a" a 1 a" a" c r I J . Fig. 174. Simplified Diagram of Series Multiple Board. on every section, and finally to the other terminal of the drop, d. Lines 2 and 3 pass successively through the sections in a similar manner. When a subscriber operates his generator, the current passes over the line wire through all of the contacts, a and c, in series, through the drop-coil, and back over the other side of the line. When an operator inserts a plug into a jack, the spring, a, is lifted from contact with the anvil, c, by the tip of the plug. The sleeve of the plug makes connection with the test-ring, b, and thus the tip and sleeve strands of the plug are connected, respectively, into the metallic circuit of the line, while the circuit through the drop is cut off at the anvil, c. The operator's telephone, T, may be then bridged across the cord circuit in order to enable the operator to converse with the THE MULTIPLE SWITCH-BOARD. 203 .subscriber who has called. Means for connecting the operator's telephone in the circuit in this manner are not shown in Fig. 174, the details of the cord circuit being described later in connection with another figure. This telephone in Fig. 174 is shown connected in a ground branch from the tip side of the cord circuit, in order to better illustrate the principles of testing in this system. The sleeve strand of each cord circuit is grounded through a battery, , and in order that this ground may not produce serious Fig- 175. Cord Circuit of Series Multiple Board. effects in unbalancing or crossing the circuit of two connected lines, an impedance or reactance coil, /, is placed in this circuit. Whenever any plug is inserted into a jack, one side of the test- battery, B y is thrown on to all of the test-rings, b, of the line to which that jack belongs. If now an operator at another board desires to make a connection with that line, she touches the tip of her answering plug to the test-ring, b, of that line. This will connect the test-ring, b, to ground, through her telephone, T, and a click will be Jieard, due to the passage of the current from battery, B. The operator will therefore know that that line is busy, and will refrain from making the connection. In Fig. 1 74 the three lines have their drops located at section 3. It must be remembered that other lines would pass through jacks on the various sections in a similar manner, but would have their drops located on sections I or 2. The operator at any sec- tion will, of course, answer calls on lines terminating or having drops on her section only, but she may be required to connect one of these lines to any other line in the exchange by means of the multiple jack. The details of the cord circuit for this system are shown in 204 AMERICAN TELEPHONE PRACTICE. Fig- 175. 1C and K' are ringing keys for connecting the genera- tor, G, with either of the plugs, P or P. The circuit between the plugs is normally maintained continuous, through the tip and sleeve strands, as can be readily seen. When the listening key, K\ is depressed, the condenser, C, is looped into the tip strand and at the same time the operator's telephone circuit is bridged between the tip and the sleeve strand. The center point of the coil of the operator's receiver, R, is grounded, and the secondary coil is split into two parts, S and S', one part on each side of the receiver. This arrangement is to prevent the unbal- ancing of the line by the ground on the receiver coil. The test is made when the key, K", is depressed, the test circuit then being from the the tip of the plug, /", through the tip strand to the right-hand spring of the key and through its anvil, the part S of the secondary coil, and one-half of the receiver coil to ground. The condenser, C, is for the purpose of preventing disturbances in the line with which the plug, P, is connected from giving a false busy test. The arrangement of circuits here shown is that used in what is termed the series-multiple board, the name series being derived, of course, from the manner in which the line passes through the contact-springs and anvils of the multiple jacks. This system, although once widely used, is subject to grave defects, and is being rapidly replaced by another form of multiple board known as the " bridging " or" branch terminal multiple." In the series- multiple an open circuit may be caused in any one of the jacks by a particle of dust or other foreign insulating matter becoming lodged between the line-spring and its anvil, or by virtue of one of the springs becoming weak and failing to bear upon its anvil. The liability to open circuits, therefore, is very great, especially in large exchanges. Another serious objection to the series board is that when a plug is inserted into a jack, one side of the line is cut off at the anvil of that jack, but the test side is not cut off, and is continu- ous through the drop of that line and back to the anvil of the jack which is plugged. This, in a large exchange, means that to one side of the line is attached an open branch, perhaps several hundred feet long and containing the drop-coil. This destroys to a certain extent the balance of the line, and is liable to pro- duce cross-talk. The branch-terminal system was designed to remedy the defects inherent in the series system, and possesses many advan- tages over it, chief among which are the facts that when a con- THE MULTIPLE SWITCH-BOARD. 205 nection is made with any line the balance of that line is in nowise affected, and that the liability of open contacts in the jacks, which is such a serious defect in the series system, does not exist. The branch-terminal system, moreover, lends itself more readily to the use of self-restoring drops, as will be described later. In Fig. 176 is represented one type of the branch-terminal system, sometimes called the three-wire system. In this figure f r-nrtir 1 - rrrrnrt Fig. 176. Diagram of Branch Terminal Multiple Board. three distict line circuits are shown passing through three sections of board. The wires, k and k ', of each line have branch wires leading off to a jack on each board ; the branches from wire, k, leading to the contact-thimbles,/, and the branches from wire, k', leading to the short springs, c, in the same jacks. Bridged across the two wires of each line is the line coil, ri, of the indi- vidual annunciator belonging to that line. This coil is high- wound in order that it may be left permanently bridged across the line without materially affecting the efficiency of the system in talking. A third wire, f, passes through the board in parallel with each line. From this wire branch wires are run to the test-thimble,^, and to the spring, b ', in each jack belonging to that line. The test wire, /, after passing through all of the boards, runs through a low-resistance coil, a , on the drop of the line to which that particular test wire belongs, and then passes to ground through a battery, o, common to all test wires. These test wires are represented by dotted lines in the figure in order to distinguish 206 AMERICAN TELEPHONE PRACTICE. them more readily from the line wires. The remaining spring, b, in each jack is permanently connected to a ground wire, G, com- mon to all of the jacks. Each plug in this system is provided with two contacts, h and/, which form terminals respectively of the sleeve and tip strands of the cord circuit. The tip, h, registers with the spring, c, when the plug is inserted into the jack (see Fig. 177), and the sleeve,/, JL T^* _, Fig- 177. Three-Wire Plug and Jack. registers with thimble, f. A conducting ring, i, entirely insulated from all other portions of the plug, registers with the springs, b and b , in the jack, and connects them together electrically. Three general results are accomplished by the insertion of a plug into a jack. The tip and sleeve strands of the cord circuit are connected respectively with the sides, k' and k, of the line, thus continuing the line circuit to the cord circuit. The con- necting of springs, b and b ', by the ring, /, completes the circuit of the battery, 0, through the restoring coil, w a , of the annunciator, to the ground wire, G, and thus allows current from this battery to energize the coil, ", and restore the shutter of the annunciator. Lastly, the connecting of springs, b and b ', by the ring, i, connects the test-thimble, g, to ground by a short circuit, so that when an operator at any other board touches the test-thimble of that line with the tip of her plug, a signal will be given denoting the line as busy. In the normal or idle condition of a line, the test-ring,^, is electrified to a difference of potential from the earth by the battery, 0, which finds circuit through the restoring coil, s , of the annunciator of that line to all the different test-rings, g, belong- ing to that line at all of the sections of the board. If when the line is in that condition the tip of the test-plug, which is grounded through the operator's receiver and the same battery, be applied to test ring, no current will flow through the receiver because both the tip and the test-ring are at the same potential. Silence will therefore indicate a free line. When, however, the line has been put into use by the insertion of a plug into the spring-jack thereof, the springs, b and b\ are connected by the contact-ring, z, carried on the plug, whereby all the test-thimbles, g, belonging to that line are connected directly to earth through a short circuit, and therefore no THE MULTIPLE SWITCH-BOARD. 207 difference of potential exists between them and the earth. Thus, when a test is made on a spring-jack of that line there will be a flow of current through the operator's receiver to ground, and a click will be the result. Fig. 176 is stripped of all unnecessary detail in order to enable the general underlying principles to be more readily grasped. In Fig. 178 the same system is shown more in detail as to circuits, Fig. 178. Complete Circuits of Branch Terminal Multiple Board. connections, and apparatus. Fig. 176 will give the reader a better understanding of how the jacks are grouped into sections, and of the relative location of the parts, while Fig. 178 will enable a better study of the circuits. In this figure two subscribers, I and 2, are shown connected by line wires, k and k', with the exchange. Jacks / and /', at sections I and 2 of the board, are shown in connection with the line leading from station I. Jacks r and / 3 are shown connected at the same sections with the line leading from station 2. Across the line leading from station I is bridged the line coil, n', of the annunciator, this annunciator being placed at section 2 of the board. The line coil of the annunciator of line 2 is similarly bridged across the two sides of the line, and is placed at section I of the board. A little study will show that the circuit of the line wires and the test wires are the same in Figs. 176 and 178, 208 AMERICAN TELEPHONE PRACTICE. although represented in an entirely different manner. Like letters correspond to like parts in these two figures. Two pairs of connecting plugs and their accessory appliances are shown complete, one at each section of the switch-board. The tips of the two plugs of a pair are connected together by one of the conductors, q, of the flexible cord, and the sleeves,/, are likewise connected by the conductor, q', of the same cord. Included in circuit between the two plugs of a pair are two call- ing keys, r and r', each adapted to disconnect both contact-pieces of one of the plugs from those of the other, and to connect them to the anvils, s and s', which form the terminals of the calling generator, P, G. A listening key, , is provided for each cord circuit, having con- tact points or anvils connected with the conductors, q and q, as shown, and having its contact-spring, u and *, connected with the terminals of the operator's telephone, w. When the plunger of the listening key, u, is allowed to rise, the operator's telephone is connected in a bridge across the two sides of the cord circuit, as is shown at section i. A wire is connected from the middle point of the coil of the operator's telephone receiver to ground through the battery, a, so that when a test is made of any line, as was described above, a circuit will be completed from the contact- thimble, g, of the jack through the tip strand, q, of the cord cir- cuit, and thence through one-half of the operator's receiver coil to the ground. As this wire leads from the center part of the opera- tor's receiver coil, it maybe left connected permanently, as it does not destroy the balance of the line. A clearing-out annunciator, x, similar in construction to the line annunciator, has its high-resistance coil, x', bridged perma- nently across the two sides of each cord circuit. The restoring coil, x*, is connected in a normally open local circuit, including the battery, o, and terminating in the ground on one side, and in a spring, u 3 , on the other. This spring, u\ is arranged in con- junction with the listening key in such a manner that when the key is raised the spring will make contact with a grounded anvil, d. Thus, whenever the operator listens in on any cord circuit she at the same time restores the clearing-out drop if it happens to be down. In order to give the reader a clearer understanding of the sys- tem so far described, it will be well to follow the operation in connecting one subscriber with another. Suppose Subscriber I desires connection with Subscriber 2. He operates his generator, i, and the current therefrom passes over the line wires, k k', and THE MULTIPLE SWITCH-BOARD. 209 through the coil, ', of the line annunciator, n, at section 2 of the board. The operator, noticing this signal, inserts plug,/, into jack, /'. This completes the circuit from ground, through bat- tery, o, coil, w a , of the line annunciator, thence to spring, b', through the ring, /, on the plug to spring, b, and to ground. The front armature of the annunciator is therefore attracted and the drop restored. The operator then connects her telephone across the cord cir- cuit by raising the key, w, and communicates with Subscriber No. i, in order to ascertain his wishes. Having found that he desires a connection with Subscriber No. 2, she takes up plug, /', of the same pair and tests to find out whether line No. 2 is con- nected to at some other board. If it is busy a current will pass from battery, o, through one-half o f the coil of her receiver, and one part of the secondary to the spring, u\ in the listening key. From this spring it passes to the tip strand, q, of the cord circuit, and to the tip, h, of the testing plug. As the test-thimble to which the plug is applied is grounded by the insertion of a plug at another board, the current will pass through it to ground. This will produce a click, which will indicate to her that the line is busy, and she will not complete the connection called for. If, however, she finds the line to be free she thrusts the plug entirely into the jack, in which position it is shown in the figure, and depresses the key, r ', in order to throw current from generator P, G, upon the line of Subscriber No. 2. The two subscribers are now connected for conversation. When either rings off the current passes through the coil, x\ bridged across the cord circuit, and actuates the clearing-out drop. The operator, noticing this, again listens in, by raising the key, u, in order to find out whether they are through talking, or whether one of them desires another connection. The act of listening in closes spring, u 3 , against anvil, d, and thus restores the shutter of the clearing-out drop. If the subscribers have finished talking, the plugs are removed and placed in their normal resting place. If, while Subscribers I and 2 were connected together at sec- tion 2, as above described, someone at section I had desired con- nection with, say, line No. 2, the operator at section I, in applying the tip of her plug, / 3 , to the test-thimble, g, as shown, would receive a click in her receiver for the reason, as pointed out above, that contact, g, is connected to the ground by a short circuit by the plug inserted in jack, /'. No difference of potential would, therefore, exist between thimble, g, and the ground, and hence a 210 AMERICAN TELEPHONE PRACTICE. current from the battery, o, would pass through the telephone of the operator making the test. Success in practical telephone working can be attained only by the greatest attention to matters of detail. Nowhere is this fact better illustrated than in the design of the various parts which go to make up a multiple board. In the construction of large boards Fig. 179. Plan View of Multiple Jack-Strip. of this type, the possible capacity of the board is limited by the number of spring-jacks that can be placed within the reach of a single operator. It is evident, therefore, that space must be economized to the last degree, and yet the jacks must be sub- stantial, in order to resist the wear and tear of years of service ; must be made easily removable so as to be accessible for repairs ; must perform their electrical functions with absolute certainty, and at the same time be so arranged as to facilitate the orderly Fig. 180. Front and Rear View of Multiple Jack-Strip. and systematic connection of the wires leading from the line cables. Moreover, when we consider that a multiple board with a capacity of 5000 subscribers will have in the neighborhood of 130,- ooo spring-jacks, we can easily realize that the cost of produc- ing these jacks must be seriously considered. It is well to state here, however, that any economy in the construction of a switch- board that will tend to decrease its durability and reliability of action is poor economy indeed. As an illustration of modern spring-jack construction, we will consider the spring-jacks used in the branch-terminal multiple THE MULTIPLE SWITCH-BOARD. 211 board just described. It has become common practice to mount the jacks in strips of twenty, and to so arrange each strip that it may be removed from the board by the removal of two screws, which bind it firmly to the framework. Figs. 179 to 182, inclu- sive, show the details of the construction of one of these jack- strips. The hard-rubber strip, , forms the framework for each strip of twenty jacks. The projections at its ends provide for attach- mirror 5 C) o o 3, Iro o o o) p) !'; i fi a 1 c? | i a i i i 1 y 1 i 1 1 i * i i Q* * t ;; i a [ ;'; ' " Fig. 1 8 1. Bottom View of Multiple Jack-Strip. ment to the switch-board. In this strip are milled, on its upper side, the transverse grooves, a 1 a 1 , and on its lower side similar grooves, a* a*; these being best seen in the right-hand portion of Fig. 182. Perforations are drilled from the front of the strip, one per- foration to each pair of grooves, having its axis centrally located IU1 d I FL Fig. 182. Details of Jack Parts. and parallel with respect to the grooves. A small portion of the hard rubber is removed from between the grooves so as to leave a rectangular opening, a 6 , shown in Figs. 181 and 182, through the strip connecting the two grooves at those ends which are nearer the front of the jack. In the grooves, a 1 , upon the upper surface of the rubber strip are mounted springs, b and c. The spring, b, is the longer of the two, so that its curved extremity is presented close to the end of the perforation through the front portion of the strip, a. The springs are insulated from each other by a strip or tongue, d, of hard rubber, thin and flexible enough not to impede the flection of the two springs. In the 212 AMERICAN TELEPHONE PRACTICE. under groove, a*, is mounted another spring, b\ similar to spring, b, and of equal length. The three springs, ^, c, and b\ are firmly secured to the strip, a, by a bolt, e, passing through them and the body of strip, a. The bolt is insulated from the springs, b l and c, by rubber washers and bushings. In the perforations, a 3 , in front of the strip, are inserted short tubes, f, of brass. Each tube or thimble, f, is pro- vided with a shoulder, which bears against a corresponding ledge in the perforation, a*, so as to prevent the tube from being thrust backward toward the rear of the jack by the insertion of the operators' plugs. The thimble,/", is provided with an exten- sion,/"', to afford electrical connection with it from the rear of Fig. 183. Details of Plug. the jack. This strip,/ 1 , extends through an oblique duct,/"* shown in dotted lines in Fig. 181 and thence through a trans- verse slot or saw-cut, a*, to the rear of the strip. In front of the thimbles,/", in the perforations, a 3 , are placed the test-rings, very short tubes of brass, g. These are forced into place against other ledges in the perforation. The ring,^, is also provided with an extension, g 1 , projecting to the rear of the strip of spring-jacks. These extensions, g l , are of wire and pass through another duct, g 3 , in the front portion of the strip, a, into a saw-cut, a\ thence to the rear of the strip, where they are con- nected with the spring, b. The springs in these jacks are of hard German silver, which has been found the most desirable material for this and similar purposes. The detail of one of the plugs used with these jacks is shown in Fig. 183. The tip, k, of brass is secured by the rod, h\ to the block, h*, also of brass. Insulated from the tip portion by a rubber bushing is the sleeve contact, h* y of the plug, which pro- jects rearwardly and forms the main body of the plug. Over this portion is slipped a shell, //, of hard rubber or fiber, which forms a handle for the plug. Between the tip and sleeve, and insulated from each, is the ring, h\ which, as was described before, is for short-circuiting the springs, b and b 1 , when the plug is inserted in the jack. Screw connectors, h* and If, form convenient terminals for THE MULTIPLE SWITCH-BOARD. 213 attaching the strands of the cord to the tip and sleeve, respect- ively, of the plug. These connectors are always readily acces- sible for inspection or repair by the removal of the sleeve, h*. This brings us to the subject of flexible cords for switch-board use. The matter seems at first thought to be a simple one, but much thought and time have been spent in perfecting this branch of the equipment. Even at this late day the best cord is not good enough. As near a perfect cord as has up to the present time been made is constructed as follows : each conductor in the cord is composed of strands of tinsel and a few fine copper wires, to add strength and conductivity. Around each of these con- ductors are wrapped tightly, in opposite directions, two layers of floss silk. These should be wrappings and not braids, as a wrap- ping serves to keep any broken ends of the tinsel or wire down in the bunch better than a braid. Around these wrappings of silk is a braid of linen, which adds strength to the conductor. If the cord be for a metallic circuit, the two conductors thus formed are then wrapped with two more layers of floss silk, and the whole is then encased in a strong spiral wrapping of hard spring brass wire. The wire in this spiral is stiff enough to retain its shape, but the spiral as a whole is so flexible as to allow free bending of the cord. An outer covering of polished cotton is tightly braided over the spiral, and after the ends are tightly anchored to prevent the conductors sliding back and forth in the spiral, the cord is complete. An extra layer of outside linen braid is often put on the cord for a distance of about one foot back of the plug, to prevent the sharp bending or kinking of the cord by the operator in inserting the plugs into the jacks. For handling very large exchanges Mr. Milo G. Kellogg of Chicago has invented a number of divided multiple-board systems, some of which will probably prove important factors in the telephone industry of the immediate future. In these he divides the lines into four classes and the switch-boards into corresponding divisions, one division for each class of fines. Each line has four polarized drops and four answering jacks, one drop and jack being located on each division of the boards. In addition to this each line has a multiple jack on each section of one of the divisions, the arrangement being the same in this respect as in the ordinary multiple-board. Two of the polarized drqps are of opposite polarity and are connected in series between one side of the line and ground. The other two are also of opposite polarity and are connected in series between the other side of the line and ground. By sending a current of one polarity or the 214 AMERICAN TELEPHONE PRACTICE. other over one or the other of the line wires and ground, a subscriber may thus signal any division of the exchange. In operation, if a subscriber in class A desires to call one in class D he sends, for instance, a negative current over the test side of the line, which operates his drop at division D of the exchange. As the called subscriber belongs to the D class he will have a multiple jack upon each section of the D division of boards, and the operator will answer the call from the A line, by inserting a plug in the answering jack, which is always on the same section as the drop, and complete the connection by Fig. 184. Kellogg Divided Multiple Switch-Board. inserting the other plug of the pair in one of the multiple jacks of the D subscriber called for. The calling of any division of the exchange is done by an arrangement of push-buttons at the subscriber's station, the subscriber always pressing the button bearing the letter of the class to which the subscriber called belongs. In Fig. 184 is shown diagrammatically the circuits of one of the Kellogg divided board systems. The four divisions of the board are represented by A, B, C, and D; A 1 A 2 , B 1 B 2 , etc., representing the various sections of board in each division. THE MULTIPLE SWITCH-BOARD. 215 Each jack, S, is composed of a tip contact, -, a sleeve contact,/, a test contact, d, and two auxiliary contacts, a and b, adapted to be pressed into engagement with each other and with the test contact by an insulated portion carried on the connecting plugs. L and L are two subscribers' lines. Line L is an A line, and therefore has a jack on each section of division A, and one jack on one section only of each of the other divisions. Line L' belongs to class C, and therefore has a jack on each section of the C division and on one section of each of the other divisions. Line cut-off relays, x x, are provided, one for each line. Each of these is included in a local circuit containing the common battery, B , the ground, and the springs, a and b, of each jack belonging to its particular line. These relays when operated cut off both taps to ground, containing the line-drops, w, of which there are four for each line. The insertion of a plug into a jack, therefore, by pressing together contacts, a and b, operates the relay, x, to cut off all the drops from the line. It also connects the test contact, d, with the ground at G through the spring a.; and as all the test contacts, d, of the jacks on one line are per- manently connected together, all are grounded. This ground- ing of all test contacts of the jacks belonging to a busy line is to enable an operator to determine the condition of the line. The tip side of her cord circuit is while making the test grounded through an impedance coil and battery. If the line tested is busy, she obtains a click due to the closed circuit from the tip of her plug to ground, through the test contact and spring, a. If the line is free the test contact is not grounded, the circuit remains open, and no click is obtained. The advantage claimed for this division of multiple-boards is the enormous saving of multiple jacks. The limit to the number of subscribers in a single multiple board is found to be about 6000, a greater number rendering the boards so cumbersome that an operator cannot reach all of the multiple jacks on her section. By thus dividing the exchange into four divisions it becomes possible to place as many as 24,000 subscribers in a single exchange. UNIVERSITY CHAPTER XX. TRANSFER SYSTEMS. No person of intelligence can visit one of the large exchanges of the Bell Company, equipped with a modern multiple switch- board, without being deeply impressed by the magnificence of the equipment, the perfection of the system in its entirety and in its minutest detail. If he is conversant with telephone matters, he must also be impressed by the fact that while the multiple switch-board gives the subscriber what he needs, quick reliable service, it gives it only at a great initial cost to the operating company. There is no question but that at the present stage of telephonic development the multiple-board systems represent the highest type of central-office equipment, and while one form or another of them is in use in nearly all the really large exchanges the world over, there are a few notable exceptions. On account of the apparent success of some of these, and of the expense necessarily entailed in the installation of large multiple boards, it is not to be wondered at that telephone men are constantly seeking a system which, while it may in some degree embody the plan of multiple jacks, will mainly depend for its action on other ideas. In the multiple switch-board one part of the cost of instal- lation increases as the square of the number of the subscribers. When a new section containing, say, 200 spring-jacks, annuncia- tors, and other apparatus, is added to an exchange, the increase does not end there. Two hundred multiple jacks must be added also to each of the sections already existing in the exchange. This is clearly an objection which increases in seriousness as the exchange grows. A multiple board, of the type so far consid- ered, having a capacity for 6000 lines, would probably be divided into 30 sections of 200 lines each. On each section would be placed 6000 multiple jacks, plus the 200 answering jacks belong- ing to the particular 200 lines whose annunciators were located at that section. This would make a total of 6200 spring-jacks on each section, or 186,000 in all. The first cost is, therefore, large; increasing the board is excessively expensive, and cost of main- tenance is necessarily heavy. The enormous multiplying of jacks in the multiple system is for one purpose to enable each operator to have within her 216 TRANSFER SYSTEMS. 217 reach a terminal of every line in the exchange, to the end that she may be able herself to complete any connection called for over any one of the lines under her immediate supervision : that is, that she may be able to answer any call arising at her section, and, without requiring the aid of any other operator, make the connection called for. In other words, in a 6000 ex- change, 186,000 spring-jacks would be used, instead of only 6000, in order to accomplish this result. This appears to be a great cost to pay for such a simple result, but up to the present date practical experience has proved that the end justifies the means. When two or more operators instead of one must handle the connections between two subscribers, as must always be the case where the multiple system is not used, the liability of error is about doubled, and the fact that the attention of both is simul- taneously required on the same connection, necessarily slows down the service. The saving over the multiple system is not as great as it appears at first, for in order to make the co-opera- tion of the operators as effective as possible, a complicated system of automatic signals must be installed, which adds greatly to the complexity of the apparatus and circuits. It is a fact, how- ever, that many medium-sized exchanges and one large one, are being operated successfully without the use of multiple boards. Systems depending tor their operation on tne transfer of a connection from one portion of a board to another are termed transfer systems, and one of the most successful of these is the so- called ''express system" of Messrs. Sabin & Hampton of San Francisco. This system has been used for several years in San Francisco, and has, according to reports, demonstrated its capa- bility of handling with success an exchange having over 6000 subscribers. The system is so radically different from anything so far de- scribed that its consideration in detail should be a matter of much interest. One striking feature in it is that no magneto- generators are used at the subscriber's station, and when it is con- sidered that approximately one-third of the cost of a complete telephone set of the ordinary type is in the magneto, it will be seen that this is a saving of considerable moment. The doing away of the magneto, however, is not an essential feature of the express system, but is one of the advantages incident to its use. Briefly stated, the underlying ideas of the express system are as follows: The boards are divided into two classes, termed for convenience " A " and " B." Similarly, the operators at the re- spective boards are termed "A" operators and "B" operators. 218 AMERICAN TELEPHONE PRACTICE. There is but one line jack for each line in the exchange, and these are divided into groups of one hundred each and are placed only at the " B " board. At the "A" boards, which are entirely removed from the " B " boards (they may even be in another exchange), are placed plugs which form the terminals of other trunk lines leading from the various "B" boards; and also jacks forming the terminals of other trunk lines leading to the "B" boards. The former trunk lines that is, those terminating in plugs at the "A" boards also terminate in plugs at the "B" boards. These are termed "A" trunk lines. The latter trunks that is, those terminating in jacks on the "A" boards terminate in plugs on the "B" boards, and are termed "B" trunk lines. When a call is received it attracts the attention of one of the '" B " operators by displaying an annunciator in the ordinary man- ner. The "B" operator at whose board the call is received pays no further attention to it than to insert one of the plugs of an "A" trunk into the jack of that line, thus transferring the call to an "A" operator, who answers it with a listening key in the ordinary manner. No means whatever are provided for a "B" operator to listen in on a subscriber's circuit, this duty be- ing confined solely to the "A" operators. The " A " operator, having learned that the subscriber calling 'desires to be connected with a certain other subscriber, conveys this information, by means of a special order wire, to the " B " operator at whose board the called-for subscriber's line termi- nates. The " B " operator then tells the " A " operator what " B " trunk line to use, and the "A" operator then inserts the plug of the "A" trunk line used into the jack of the "B" trunk line thus designated. This brings the connection as far as the board of the second " B " operator ; that is, the " B " operator at whose board the called-for subscriber's line terminates. This operator, in order to complete the connection, simply inserts the plug of the " B " trunk used into the jack of the called-for subscriber's line, and presses a ringing key in order to call that subscriber. It will be seen that the connection has really been handled by three different operators, but that the first of these operators does no more than to insert a plug into a jack, giving the matter no further attention. All signaling between the subscribers and the operators and be- tween the various operators, whether it be for establishing or clearing out a connection, is entirely automatic, and therefore not dependent upon the volition of the parties concerned. TRANSFER SYSTEMS. 219 Fig. 185 shows the arrangement of the circuits at the sub- scriber's station, and also the arrangement of the spring-jacks and annunciators on the " B " boards. One side of the line wire at the subscriber's station is normally grounded through the polar- ized ringer, R. This means that calling a subscriber from the central office must be accomplished over one limb of the line I ' VWW/ L II I JJT^" _\ I ^s = ^ < , Fig. 185. Subscribers' Circuits Express System. wire and ground, instead of over a metallic circuit, as in case of talking and other signaling in this system. The other circuits and apparatus at the subscriber's station are of the ordinary ar- rangement and type, the only difference being that the magneto- generator is omitted entirely. The line wires of each subscriber terminate in two springs, a and 6, of their spring-jack, J. These springs normally rest on two anvils, c and d, one of which con- nects through an annunciator, e, with a heavy wire leading to one pole of the calling battery, and the other of which leads to a similar wire connecting with the other pole of this battery. This annunciator, ^, has a shutter which is simply lifted by the attraction of the armature, and again dropped into its normal position when the armature is released. It is, therefore, the simplest type of self-restoring drop. The circuit of the call-bat- 220 AMERICAN TELEPHONE PRACTICE. tery is normally open only at the subscriber's station. It is auto- matically closed through the receiver and secondary winding of the induction coil at the subscriber's station whenever the sub- scriber removes his receiver from its hook. This allows enough current from the calling battery to pass through the drop, e, to raise its shutter, and thus attract the attention of the " B " opera- tor at that board. The shutter remains raised until the operator inserts the plug of one of the " A " trunk lines in order to transfer the call to the " A " operator. The insertion of this plug, however, lifts the springs, a and b, from the anvils, c and d, thus cutting off the battery and allowing the shutter of the annunciator, e, to drop to its normal position, and the " B " operator therefore pays no more attention to it. A single battery is made to serve for actuating the signals of every line in the exchange, no matter how great their number may be. Storage cells are used for this purpose, ten cells being connected in series so as to give a pressure of about twenty volts. It is said that the average flow of current from this battery is about one and one-half ampere, and never exceeds two amperes, in the San Francisco exchange of approximately 6000 subscribers. It will be thus seen that the cost of maintenance of these batteries is trifling. Another good feature of this arrangement is that, should a " B " operator by mistake withdraw a plug from a jack before a subscriber has finished talking, that is, before he has hung up his receiver, she will be at once notified of her mistake by the dis- play of a signal belonging to that line. In Fig. 186 is shown a simplified diagram of the express sys- tem. At the bottom and top of this figure are shown the sub- scribers' lines, leading in each case from the subscriber's telephone apparatus to the drop and jack at the central office. This part of the apparatus is the same as that shown in Fig. 185, but in this figure the details of the local circuit at the subscribers' stations have been omitted for the sake of clearness. The jacks and drops belonging to these lines are, as has already been stated, stationed at the " B " boards of the exchange. The subscriber's indicator battery is represented at 5 / B and the indicators at /. Leading from the section of the " B " board shown at the top of the figure is a trunk line leading to a plug on the second section on the " A " boards. It will be noticed that this trunk line terminates in a plug at each end, and is termed the " A" trunk. An intermediate jack and plug are shown in the TRANSFER SYSTEMS. 221 circuit on this " A " trunk, but these at present need not be considered. Suffice it to say that the plug at the " B " board is connected by a metallic circuit to the plug at the second section of the " A " board. Leading from a certain jack on the second section of the " A " board is a trunk line extending to a plug on another section of the " B " boards. This is termed a " B " Fig. 1 86. Simplified Diagram Express System. trunk. Only one " A " trunk and one " B " trunk are shown, but it must be remembered that a number of "A" trunks lead from each of the " B " boards to the " A " boards, and that from the " A " boards a number of " B " trunks lead back to each of the " B " boards. When a subscriber, as for instance the one shown at the top of the figure, removes his receiver from its hook, his indicator, /, is displaced automatically, and the operator at the particular " B " board at which this indicator is located extends the circuit of his line to one of the "A" boards over an "A" trunk. This 222 AMERICAN TELEPHONE PRACTICE. she does by inserting the plug of an " A " trunk into the jack of the calling subscriber. The operator at the "A " board, having learned the desire of the calling subscriber, extends the circuit to that subscriber's line by means of the "B" trunk, still further on to the particular " B " board at which the jack of the called subscriber is placed. This the " A " operator does by in- serting the plug of the " A " trunk used into the jack of a " B " trunk at her board. The " B " operator at whose board the calling subscriber's jack is placed then completes the connection between the two subscribers by inserting the plug of the " B " trunk used into the jack of the calling subscriber's line. The two subscribers' lines are shown connected in Fig. 186 by the proc- ess and over the circuits just described. In order to facilitate matters it is evidently necessary that a most complete and elaborate set of signals must be provided be- tween operators. The first in this series of signaling operations is to notify the " A " operator that her attention is desired on a certain " A " trunk. This must always occur just after the " B " operator has inserted one of the " A " trunk plugs into the jack of the calling subscriber's line. It will be noticed in Fig. 1 86 that the relay, R, operating signal lamp, L, is bridged across the tip and sleeve strands of the " A" trunk circuit, and this bridge may be traced from the tip strand through the balance coil, B C, thence through the clearing-out indicator battery, C O B, to the battery wire, thence through the coil of the relay at the " A " board, and thence to the sleeve strand of the " A " trunk. Re- membering that the calling subscriber at the top of the figure has removed his receiver from its hook, then the insertion of the plug of the "A" trunk will restore the line drop, /, and at the same time will close the circuit from the clearing-out indicator battery, COB, and the relay, R, through the subscriber's line and telephone instrument. This will operate the relay and cause it to close the circuit of the signal lamp, Z,.thus calling the atten- tion of the " A " operator to the fact that an unanswered call is upon the trunk line to which that lamp belongs. The apparatus of the " A " operator is shown more clearly in Fig. 187. The sleeve and tip strands of the " A " trunk are shown at the extreme left of this figure. When the armature of the re- lay, R, is attracted, as described above, the circuit from the local battery and white lamp is completed at the point, b, of the relay. It will be noticed that this local circuit extends through two of the springs, e and/, normally closed, on the listening key of the " A " operator, and also through a pair of contacts, / and c, TRANSFER SYSTEMS. 22$ held closed by the weight of the plug of the " A " trunk in its socket. The white lamp will therefore remain lighted until one of the following three things happens : until the operator lis- tens in, which causes the local circuit to break at the listening key ; or until she removes the plug of that trunk line from its socket, which would break the local circuit at the point, c; or until the calling subscriber hangs up his receiver, which would cause the relay to let go of its armature, and thus break the circuit at point, b. The white lamp remains lighted, therefore, as long as the call on its " A " trunk is unattended to. The first act of the " A " operator on seeing this light is to throw her lever corresponding to that light into its horizontal position, thus connecting her telephone to the terminals of the "A" trunk in the usual manner. This enables the "A" opera- tor to communicate with the calling subscriber in the ordinary manner. It should be noted that these keys on the " A " opera- tor's boards are the only means afforded to any operators for communicating with subscribers. The operation of this key breaks the circuit of the white lamp at the points, e f. As soon as the listening key is thrown again into its normal position the white lamp is again lighted, thus calling the operator's attention to the plug to be used in making the connection. This precaution is a wise one, for before making the next move in the connection, the "A" operator must communicate with the " B " operator at whose board the called-for subscriber's line terminates. The "A" operator does this by depressing her or- der-wire key, shown in Fig. 186, but omitted from Fig. 187, which act connects the " A " operator's telephone directly with the telephone set of the outgoing " B" operator. The "A" opera- tor then tells the " B " operator the number of the line with which connection is desired, and the " B " operator in return tells the " A " operator the number of the trunk line she is to use in making the connection. Herein lies one of the greatest defects of the system. That is, the necessary waiting by the "A" operator for the reply of the " B" operator. This is a loss of time on the part of the "A " operator, for it often occurs that the " B " operator may have several other connections under way which she could not well leave in order to reply to the " A " operator by designating the number of " B" trunk to be used. It will be noticed that the trunk jacks of the " B " trunks are in reality arranged on the plan of the multiple board. This is for the purpose of placing within the reach of every " A " opera- 224 AMERICAN TELEPHONE PRACTICE. tor a jack belonging to every " B " trunk line. No test system, however, is required on these jacks, as an " A " operator always first learns from an outgoing " B " operator which " B" trunk to use, and of course a " B " operator would never designate any trunk which was already in use, or " busy." It would seem that. a " busy " test, or preferably a visual " busy " signal for the multiple jacks on the "A" boards could be used COS. Fig. 187. Table of " A " Board. with advantage. This would enable the " A " operator to at once select an idle " B " trunk and at the same time inform the proper " B " operator of the connection to be made and the trunk line plug to use in making it. For instance, the " A " operator could simply say: " 1504 on 10," meaning by the first number the number of the subscriber called for, and by the second number the trunk line to be used. We have now carried the connection, or extended the circuit of the calling subscriber's line, as far as the trunk line plug at the outgoing "B" board. The outgoing " B " operator then com- pletes the connection by inserting this plug into the jack of the TRANSFER SYSTEMS. 225 called subscriber. The " B " operator then depresses her ringing key, shown in a simplified form in Fig. 186, which sends calling cur- rent from the generator, G, over the sleeve strand of the plug cord, thence to line and to ground through the polarized call-bell at the subscriber's station. The next feature to consider is that of the automatic clearing- out signals. As the connection between the two subscribers is made by three operators, it is evident that three distinct clear- ing-out signals should be given, one at each of the boards of the operators who help establish the connection. Turning again to Fig. 187, it will be seen that the raising of the "A" trunk plug from its socket changed the circuit of the local battery from the white lamp to the red lamp, by moving the selecting lever, /, from the point, c, to the point, d. Remembering now that as long as the calling subscriber's receiver is off its hook, the circuit from the clearing indicator battery is closed through the relay, R, at the " A " board, thus attracting its armature. As soon, there- fore, as the calling subscriber finishes his conversation, he hangs up his receiver, and thereby breaks the circuit through the relay at the " A " board, thus closing the circuit through the red lamp. This lamp -will therefore be lighted as a notification to the " A " operator that disconnection on that trunk is desired. The replacing of the plug in its socket opens the circuit of the red lamp at the point, d, thus extinguishing the lamp. This apparatus at the " A " board is very ingenious, and deserves special attention. It should be noticed that should an operator by mistake remove one of the " A " plugs, and replace it in its socket before the subscriber connected had hung up his receiver, the white lamp would be relighted, thus calling the attention of the operator to the error. The clearing-out signal is given to the incoming " B" operator in much the same way as that on the " A " board, the clearing indi- cator, or relay, on the " A " trunk of the " B " board being wired in multiple with the relay on the " A " board. The clearing-out signal on the outgoing " B " board is accom- plished by much more complicated means, and will be explained by reference to Fig. 188. In this figure the ringing key,/, is shown in more detail than in Fig. 186. The " B " trunk line jack on the " A " board is represented by a. In the normal position of the key the two strands of the " B " trunk are connected to the tip and sleeve of the corresponding plug on the outgoing " B " board. When, however, the key is depressed, the sleeve strand of the cord is connected with the calling generator, the other terminal of 226 AMERICAN TELEPHONE PRACTICE. which is grounded. When the operator depresses this ringing key, a secondary pair of contacts, i z 1 , are closed, thus actuating the lower magnet, /, of the compound relay, and causing it to attract its armature, /'. When the operator allows the key,/, to rise, the armature, /, falls back, but is caught by the hook, m l , of the upper coil, ;/z, of the relay. The hook, m\ and the tip of the armature a Fig. 188. Table of " B" Board. are platinum-pointed, and their con- tact causes the signal lamp, n, to be lighted. This lamp remains burn- ing until the called subscriber takes his telephone off the hook, which act closes the circuit through the com- bined clearing-out relay and signal, c, in exactly the same manner as the relays on the incoming " A " trunk line were operated. The operation of this relay therefore closes a circuit at the points, ? c 3 , through the upper magnet, m, causing it to raise the hook, m\ and allow the armature, /', to drop back. This extinguishes the lamp, n, and shows the opera- tor that the subscriber has responded. The armature, c\ of the relay,*;, remains attracted until the called subscriber hangs up his receiver, which de-energizes the magnet, c, and allows the signal carried by the armature to resume its normal position. This is the clearing-out signal for the outgoing " B" operator, and she accordingly pulls out the plug. To review the action of the indicators at the outgoing " B " board, the releasing of the key for transmitting a calling signal to the subscriber lights the lamp, n, and shows the operator that this part of her work has been attended to. The response of the subscriber is indicated by the going out of the lamp, and by the raising of the signal, k. The clearing-out signal is given by the lowering of the signal, k. TRANSFER SYSTEMS. 227 We have now traced through the operation of all the signals between the subscribers and the operators, and between the operators themselves, which were necessary to establish a connec- , tion between two subscribers ; and also the subsequent signals between the subscribers and the operators, indicating that a dis- connection is desired. The striking feature of all this elaborate system of signaling is that each signal is automatically given with- out the volition of the operator or subscriber, inasmuch as it is brought about by some action necessary in the actual connection or disconnection. In order to reduce the work of the operators to the last degree, two phonographs are placed in connection with the exchange, one of which is constantly and politely repeating the sentence, " Busy. Please call again," while the other repeats with equal regularity, " Subscriber called for does not reply." Each of these phono- graphs speaks to a transmitter arranged in connection with an induction coil and battery in the ordinary manner. The terminals of the secondary of the induction coil of the " busy " phonograph terminate in a jack on each section of the " A" boards. In like manner the " does not reply " phonograph is connected with a jack on each section of the " B " boards. When, therefore, an "A " operator learns that a line called for is busy, she inserts the plug of the " A " trunk to which the calling subscriber is connected into the phonograph jack, and the familiar but disappointing mes- sage, " Busy. Please call again," is automatically conveyed to the calling subscriber. In a similar manner the outgoing " B " operator may inform the calling subscriber that the subscriber called for does not respond. The use of the phonograph for this purpose may seem at first thought to be carrying the labor-saving idea to an extreme, but it enables an operator to attend to another subscriber while she is telling the first subscriber that his line is busy or that his party does not respond. It moreover insures that the wrath of the calling party will produce no evil effects on the nerves of the operator, which at busy times is no unimportant consideration. The writer is indebted to an able article by Mr. George P. Low, in the Electrical Journal, for much information concerning this very interesting exchange system. Fig. 189 is a schematic representation of the system used 'in the larger exchanges of the Western Telephone Construction Company of Chicago. This system has proved very successful in practice for exchanges up to fifteen hundred subscribers, although with a larger number certain difficulties are met in 228 AMERICAN TELEPHONE PRACTICE. the disposal of the transfer plugs. In this figure I, 2, 3, 4, etc., represent different sections of the board, each section having one hundred combined drops and jacks of the type shown in Figs. 158 and 159 together with a complete operator's equipment. One answering plug, A, together with one calling plug, B, is shown at each section. These are connected together in pairs through clearing-out drops, O, by ordinary flexible cords which contain the 1 a 3 4 S 6 7 S 9 1O 11 2, 1 h ur! trtr LUL tCCC C LCC TTCC i cm :cc c ccc ecu I ILL .[[[ ILL L LIU III 1 1UL LIT ILL I r [ LUC Fig. 189. General Scheme of Western Transfer System. necessary switching apparatus for enabling an operator to listen in and to ring out over either cord as desired. Connected with each of these sets of plugs is a trunk line to which is connected at every third section a transfer plug, C, as shown. Thus, a pair of plugs, A and B, shown at section I, is connected by means of a trunk line to a transfer plug at section 4, another at section 7, another at section 10, and so on. A careful considera- tion of this figure will show that the same is true for each pair of plugs, A and B, at the other boards. Fig. 190 shows in greater detail one pair of plugs, A and B, connected by a trunk line tp the several transfer plugs according to this system. The plugs, A and B, are in this case at section 4, while the transfer plugs, C, C, C, are at sections I, 7, and 10, or in other words, at every third section on each side of section 4. The form of circuit-changing lever L is here shown for convenience only, and serves to illus- TRA NSP'ER S YS TEMS. 229 trate the principle, but not the actual connections in this system. By throwing this lever to the left, its two springs are connected with an operator's circuit through the secondary wind- ing of the induction coil, as shown, while when the lever is thrown to the right the terminals of the generator, G", are con- nected with the plug circuit. The circuit-changer actually used in this cord circuit is the one of the Western Telephone Con- struction Company, described in Chapter XV. The operator ^L 1 " r, ^ B C C o [L 1 F O m 6 ^ I VWV Fig. 190. Transfer and Instruction Circuits. pulls the lever forward to ring the called subscriber, pushes it from her to ring the calling subscriber, and presses it downward to listen in. The system can now be more readily understood by describ- ing its operation. If a subscriber whose line terminates in section 4 calls up, the call is answered by the operator at that board by inserting one of her plugs, A, the insertion of this plug restoring the shutter mechanically. The operator then throws the lever Z, to the left, connecting her telephone set, E, with the line of the subscriber calling. Having learned the want of the subscriber, who we will say is 1001, the operator at 4 depresses the key, K, which connects her telephone set with an instruction circuit, /, terminating in the telephone set of the operator at section 10. The operator at section 4 is thus en- abled to communicate with the operator at section 10 over this circuit, and the former informs the latter of the number desired and of the particular transfer plug, C, she is to use in making this connection. The operator at section 10 then takes up the plug, C r designated and inserts it into the jack of the called sub- 230 AMERICAN TELEPHONE PRACTICE. scriber, the operator at section 4 meanwhile holding the lever, L, of the particular plugs used, into the ringing position. As soon as the connection is completed at section 10 the first operator is informed of the fact by the operation of a buzzer placed in the cord circuit so that she knows that the signal has been properly transmitted to the line of the subscriber 1001. After the conversation is completed, one or both subscribers ring off, which throws the clearing-out drop, O, and informs the operator at section 4 that a disconnection is desired. She there- fore removes her answering plug and places it in the socket, in- forming the operator at section 10 to do likewise. It will be seen that, in addition to the transfer lines extending from the answering and calling plugs at each board to transfer plugs at each third board therefrom, a system of instruction circuits is also provided, each circuit terminating in an operator's set at one board and connected with push-buttons at every third section therefrom, so that an operator is enabled to communicate only with those operators located at every third section from her own board. This peculiar arrangement serves several advantageous purposes, among which is the reduction of plugs necessary for the successful operation of the board and also the reduction of the num- ber of operators talking over any one instruction circuit. It more- over enables any operator to reach, by means of her own calling plug or a transfer plug handled by another operator, any portion of the board. For instance, if a subscriber calling on section 4 desires connection on section 3 or section 5, the operator at section 4 will complete the connection herself by the use of the calling plug, B, as she can readily reach any jack on her own section or on that at her right or left. We have seen how a connection is made between section 4 and some section at which one of the transfer plugs of that section is located. If, how- ever, the subscriber on section 4 had called for a subscriber at section 9 the operator at 4 would have signaled the operator at 10, who would then have completed the connection, using transfer plug, C, with her left hand. If the called-for sub- scriber had been upon section 8, operator No. 4 would have signaled No. 7, who would have used a plug, C, at her section with her right hand to complete the connection. Ten pairs of calling and answering plugs are furnished for each section of 100 drops, each pair being connected by trunk line with transfer plugs distributed through the system as already described. A system of lamp signals for facilitating the work upon these boards has been devised and used in many of the later exchanges. 232 AMERICAN TELEPHONE PRACTICE. In this a white light is so arranged in connection with the night- alarm circuit as to be illuminated, upon each board, whenever a drop is thrown. A similar lamp in series with this is also ar- ranged to be displayed on the chief operator's table, thus serving as a telltale to call the attention of the chief whenever a drop remains unattended on any section. A colored lamp is arranged in connection with each set of transfer plugs and controlled by normally open contact points in the plug seats of the transfer plugs and normally closed contact points in the plug seats of the answering plugs. Two lamps are arrangecl in series in each circuit, one at the set of transfer plugs to which it belongs and the other at the set of answering plugs with which these transfer plugs communicate. Whenever an operator raises an answering plug in order to establish a connection, the lamp circuit is opened at that point by the operation of the contacts in the plug seat. When another operator removes the transfer plug to complete the connection this same lamp circuit is closed at that point by the operation of the contacts in the transfer plug seat. The circuit, however, still remains open at the answering plug seat. When a calling-out signal comes, and the operator removes the answering plug to disestablish the con- nection, the lamp circuit is closed at its only open point, which lights the lamp in front of each operator. This shows the trans- fer operator that a disconnection is desired, and also shows the answering operator that the disconnection has not yet been made. The cycle of events is completed when the transfer operator removes the transfer plug and replaces it in its seat, which act opens the lamp circuit at that point, thus putting out both lamps. The switch-board of the Delmarvia Telephone Company at Wilmington, Del., is shown in Fig. 191. This board embodies all the features mentioned above and is undoubtedly representa- tive of the best exchanges of the Western Telephone Con- struction Co. What is known as the Cook-Beach transfer system has been in long use among some of the Bell exchanges of medium size, and the large switch-boards now manufactured by the Sterling Elec- tric Co., Chicago, are operated upon this plan. The subscriber's lines terminate in drops and jacks on the various sections of the board, no multiple connection whatever being used between them. A set of transfer jacks is also provided on each section, these jacks being connected by trunk lines extending to transfer plugs located at the several sections. When a TRANSFER SYSTEMS. 233 call is received at any section, the operator answers it by inserting one of her answering plugs into the corresponding jack. Having learned the number of the subscriber called for, she inserts the corresponding connecting plug into the transfer jack connected by a trunk line with a plug at the board where the line of the subscriber called for terminates. She then com- municates with the operator at that board, who picks up the transfer plug designated and inserts it into the jack of the called Fig. 192. 8oo-Line Sterling Switch-Board. subscriber. The connection between any two subscribers is thus made complete by the use of three plugs. This style of transfer system has proven its adaptability to good telephone service by long-continued use in both Bell and independent exchanges. A board embodying this plan of operation is shown in Fig. 192. The line-drops may be seen in the upper portion of the panel of each section, and immediately below them the corre- sponding jacks. The drops are of the type already referred to in Chapter XIV., and are provided with the restoring feature by which a whole vertical row of shutters are restored by the pres- sure upon a button below that row. Immediately below the line jacks are shown the transfer jacks, these being of such a nature that when a plug is inserted into any one of them, a signal is 234 AMERICAN TELEPHONE PRACTICE. automatically displayed at the other end of 'the trunk line to which it belongs, notifying the operator at that section that a connection is desired on that line. The listening and ringing keys are of the type shown in Fig. 145. Two trunk lines are provided from each loo-drop section of this board to every other section, this number being found to give an ample number of trunk lines at the busiest portions of the day. The larger exchanges equipped by the American Electric Tele- phone Co. use a transfer system somewhat similar to that of the Cook-Beach type, composed of trunk lines extending be- tween the various sections of the board and terminating in jacks at one end and plugs at the other. Such a board is shown in Fig- 193. There are two such trunk lines extending from each Fig. 193. I2oo-Line American Switch-Board. ioo-drop section to each other section, thus giving four trunk lines between each two sections two outgoing and two in- coming from any position. These trunk lines terminate in jacks at the 'outgoing ends, and plugs at the incoming ends. Each operator has, besides the set of regular listening keys, a set of instruction keys, one for each of the other operators, the de- pression of any one of which connects her telephone set with the set of another operator corresponding with that key. In this way, a call received for a number not within the reach of the answering operator is transmitted to the operator in whose section the line called for terminates, being given by means of the instruction key just mentioned. The calling line is then connected by a pair of cords and plugs to the jack of one of the two transfer lines reaching to the section in which the called- for subscriber's line terminates. The establishment of this con- TRANSFER SYSTEMS. 235 nection causes a lamp to light at the other end of the trunk line and shows the operator there which of the two lines is to be used. The drops used are the same as those illustrated in Figs. 161, 162, and 163, the shutter being restored by the insertion of the plug, and again, after it has been operated as a clearing-out signal, upon the withdrawal of the plug from the jack. Calling is accomplished merely by pressing the calling plug to its fullest extent into the jack of the called subscriber. CHAPTER XXI. COMMON-BATTERY SYSTEMS. IT is an obvious disadvantage to have a separate source of current at every subscriber's station in an exchange ; and it is not to be wondered at that many efforts have been made to centralize not only the transmitter batteries, but the calling cur- rent generators as well. By bringing about such a centralization of the sources of energy many desirable results are attained. The idle capital represented by the local batteries and the calling generators is done away with no small consideration in large exchanges, because the magneto-generator is in itself the most expensive part of an ordinary telephone set. The labor of visit- ing or inspecting the subscribers' apparatus is greatly reduced ; that necessary to repair and renew batteries, together with the expense of material for such renewal, being rendered nil. The subscribers' instruments are made neater and more compact. The electrical efficiency of the plant is greatly increased by having a few large units in operation practically all of the time, instead of a great number of small units in operation but a small portion of the time. Lastly, no freezing of the local batteries occurs; there is no spilling of the acids or other chemicals, and no corrosion of the various parts by fumes therefrom. As indirect advantages attained in the most modern exchanges wherein all sources of energy are centralized, may be mentioned the fact that the labor on the part of the subscriber in obtaining a connection or a disconnection is reduced to a minimum, and the labor on the part of the operator has been so greatly lessened as to enable her to handle with success about twice as many subscribers as with the old system. Most of the advantages enumerated were appreciated by telephone men long ago, and many attempts were made at an early date to realize them in practice. The first attempts involved a return to first principles, doing away with the induction coil and placing the transmitters and receivers of two connected subscribers directly in series in the circuit of the two line wires. In one of these, made in 1881, by George L. Anders, the transmitter batteries were placed in a loop used to connect the circuit of two line wires. In this the 236 COMMON-BATTERY SYSTEMS. 237 switch-board was of the old cross-bar type, and, while it used no cord circuits, the batteries were placed in series in the connecting wire corresponding to the cord circuit in later exchanges. This general method, as applied to a board having plugs and flexible cords, is illustrated in Fig. 194, where A and A represent Fig. 194. Series Common-Battery System Grounded. two subscribers' stations connected at the central office, C, by a pair of plugs, P and P', having a battery, B, included in circuit between them. The transmitter and receiver of each subscriber's station are placed in series in the line wire, and each transmitter when operated serves to vary the resistance of the entire circuit formed by the two connected lines, and to thereby vary the strength of the current flowing from the battery, B, in such manner as to produce the desired effects in the receivers. In Fig. 195 the same principle of operation is applied to Fig. 195- Series Common-Battery System Metallic. metallic-circuit lines, two of which are shown connected at the central office, C, by the pair of metallic-circuit plugs, P and P'. In both of these cases, in which the battery is included in the cord circuit in series with the combined circuit of the two lines, the use of a separate battery for each cord circuit is, under ordinary circumstances, necessary. This is always true of the grounded system shown in Fig. 194, and is also true of the me- tallic-circuit system shown in Fig. 195, unless the battery, B, is made to have a very low internal resistance. This fact was pointed out by Mr. Anthony C. White, who, in 1890, showed that it was possible to supply all of the cord circuits from a single battery by connecting them in the manner shown in Fig. 196. This involves the bunching together of one side of each of the cord circuits, the battery supplying current in multiple to the 238 AMERICAN TELEPHONE PRACTICE. various pairs of lines in use at one time. This figure shows four stations, A, A', A", and A'", connected in pairs by two cord cir- cuits and pairs of plugs. Fluctuations set up by the transmitter in the line of subscriber, A, will circulate in the combined circuit of the lines of subscribers, A and A'. Similar fluctuations set up by the transmitter at A" will flow through the circuit of the Fig. 196. Single Battery Series System. lines, A" and A". The battery, B, and the wire, a and a, in which it is included, are common to both of these line circuits, and if the resistance from the point, a, to the point, a, through the battery is considerable in amount, a part of the fluctuations flowing in the circuit of subscribers, A and A, will be shunted by this resistance through the combined circuits of the subscribers, A" and A'". If, however, the resistance from the point, a, to the point, a' y is made extremely small, practically all of the current changes will flow through the battery instead of being shunted around through the circuit of the subscribers, A" and A'", owing to the comparatively high resistance and impedance of that cir- cuit, with its included instruments. The desired reduction in the resistance between the points, a and a', may be accomplished by making the battery, B, of extremely low resistance and by shortening the wire, a a', which is common to all of the circuits. The former result is accomplished by using a storage battery of rather large capacity, and the latter by joining the various cord circuits directly to the bus-bars with the battery, so as to practi- cally eliminate all resistance in the wire, a a'. The common-battery arrangement shown in Fig. 197 is one which has come into extensive use and was designed by Mr. John S. Stone in 1892 and 1893. In this figure, A and A' are, as before, two subscribers' stations connected by metallic circuit lines with the central office at C. The transmitter and receiver at each station are connected in series in the line circuit. The COMMON-BA TTER Y S Y STEMS. 2 39- battery, B, however, is connected between the two sides of the cord circuit, terminating in the plugs, P and P'. On each side of the battery is placed an impedance coil, / and /', as shown. The action in this case is as follows : the current from the positive pole of the battery, , flows through the impedance coil, /, to the point, a, where it divides, a part passing through Fig. 197. Stone Common-Battery Arrangement. the receiver and transmitter of each of the subscribers' stations. The two parts of the current, after flowing back to the central office through the opposite sides of the lines, unite at the point, a, and flow through the impedance coil, /', to the negative pole of the battery. Inasmuch as the impedance coils are of low ohmic resistance, they offer but little obstruction to the passage of this current. If now the transmitter, T, at station, A, is caused to lower its resistance, the difference of potential between the points, a and a, will be lowered. This will result in a Fig. 198. Stone Common-Battery Arrangement. diminution in the current flowing in the line of subscriber, A'. On the other hand, if the resistance of the transmitter, T, is raised, the difference of potential between a and a' will be raised, thus causing a greater current to flow through the instru- ment of subscriber, A. Every fluctuation in the resistance of the transmitter, caused by sounds at either station, will thus cause corresponding fluctuations in the current flowing through the receiver at the other station, thus causing them to reproduce the sounds. The same battery, B, is used to supply a large 240 AMERICAN TELEPHONE PRACTICE, number of cord circuits, the arrangement being then as shown in Fig. 198, each side of the various cord circuits being con- nected to the poles of the battery through impedance coils, as before. The fluctuations set up in the circuit of the two sub- scribers, A and A', while perfectly free to pass through these two particular lines, cannot find a path to any other lines, as, for in- stance, those of subscribers, A" and A'", without passing through the impedance coils, / and /', and also /", and /'". It is said that by means of this system a direct-current generator can be used in place of the battery, B, the impedance coils serving to effectually weed out all of the fluctuations in the generator cur- rent which have always been found so annoying in telephone work. Notwithstanding this, however, the storage battery is always used in practice in systems embodying these principles. Early in 1892 Mr. Hammond V. Hayes devised the method of supplying current to transmitter batteries shown in Fig. 199, Fig. 199. Hayes Common-Battery Arrangement. this having come into very extended use in the Bell Companies, and it has formed the basis of some of the most successful common- battery systems in the world. The apparatus at the subscribers' stations, A and A, is arranged as in all of the preceding sys- tems. At the central office, K K' are repeating coils each having two windings, k and k'. The two windings of the coil, K, are connected together at the point, a, which is connected with the positive pole of the battery, B. The other ends of these two windings are connected with the upper contacts of the plugs, Pand P, as shown. In an exactly similar manner the two windings of the repeating coil, K, are connected together at the point, a', which is connected with the negative pole of the battery, B, the other two ends of these coils being connected with the lower contacts of the plugs, Pand P'. By this arrange- ment the battery is included in a bridge conductor between the sides of the circuit formed by the two connected lines, and one limb of each line includes one of the windings of one of the re- peating coils. The current from the battery, B, will, when the subscribers' receivers are removed from their hooks, divide at COMMON-BA TTER Y S Y STEMS. 241 the point, a, and pass in multiple through the two windings of the repeating coil, K, thence the two portions of the current will pass through the transmitter and receiver of the two subscribers' stations respectively and back to the repeating coil, K', through the windings of which they pass to the negative terminal of the battery. Any changes in the current in either circuit, produced by the operation of one of the transmitters, will act inductively through the repeating coils upon the other circuit, causing corresponding fluctuations in current to flow through that circuit and actuate its receiver. Thus when the subscriber at station, A, is trans- mitting, the windings, k k, will operate as a primary coil of an induction coil of which the secondary is formed by windings, k k' t When the subscriber, A, is transmitting, this action is re- < < t /V . P I' 1 G Q G Fig. 200. Dean Common-Battery Arrangement. versed, k k serving as a primary and k k as a secondary coil. The transmitter at any station is compelled to vary the resistance of its own line circuit only, and in this way some of the advan- tages of a local circuit are gained. The two helices of each repeat- ing coil are, under ordinary circumstances, of the same resistance and number of turns, and wound side by side on the same core. The resistance of each helix is usually made rather low, being in the neighborhood of five ohms. All of the systems so far described have contained the sub- scriber's talking apparatus directly in series in the line wire. Mr. J. J. Carty is responsible for the broad idea of supplying current to the transmitter of the subscriber's station over the two sides of a metallic line circuit in parallel, using the ground as return. This method, as worked out by Mr. Carty, has been improved upon by Mr. W. W. Dean, who has produced an extended series of inventions embodying this feature. One of them is shown, stripped of details, in Fig. 200, in which A and A are two sub- scribers' stations and C the central office. / is an impedance coil bridged across the two sides of the cord circuit of the plugs, 242 AMERICAN TELEPHONE PRACTICE. P and P. The center point, a, of this coil is grounded through the talking battery, B. The receivers at the subscribers' stations are connected serially with the secondary coil, s, of an induction coil in the metallic circuit formed by the two sides of the line wire. /' is an impedance coil bridged between the two sides of the line circuit at each subscriber's station, the center point,/", of this coil being connected with one side of a primary circuit con- taining the transmitter, T, and the primary coil,/, of the induc- tion coil. The opposite side of this primary circuit is grounded at the point, g. Current from the battery, B, flows to the center point, a, of the impedance coil, /, in the cord circuit ; thence through the two sides of this coil in multiple to the points, b and c, on the opposite sides of the cord circuit. From these points the current flows over the two line wires in multiple to the points, */and e, from which they flow through the two sides of the impe- dance coil, /', at the subscriber's station to the point, f, where they unite. The current then passes to the primary circuit where it again divides, part passing through the transmitter, T, and part through the primary coil,/. It reunites at the point, g, and passes to the ground and back to the battery, B. Variations in the resistance of the transmitter at one of the stations cause more or less of the supply current to be shunted through the primary, /, of the induction coil, and these varying currents through the primary induce corresponding currents in the secondary, s, placed directly in the line circuit with the re- ceiver. These currents flow over the metallic circuit formed by the two connected lines, and are prevented from flowing through the bridge wires, d e and b c, by the presence of the coils, /', and /, contained therein. In a modification of this scheme Mr. Dean uses a transmitter having two variable-resistance buttons, one of which decreases the resistance of its circuit while the other increases the resistance of its circuit. One of these buttons is placed in each of the branches of a primary circuit such as is shown at the subscriber's station in Fig. 200, each side of the circuit also containing the primary of an induction coil. These are so arranged with respect to the secondary that an increase in current flowing through one of them produces the same effect on the secondary as a decrease of current in the other, and there- fore the effects produced by the two variable-resistance buttons of the transmitter are cumulative. The use of secondary batteries at the subscribers' stations supplied by some source of current either at the central office or elsewhere, has been occupying the minds of inventors since the COMMON-BATTERY SYSTEMS. 243 very early days of telephony. Storage batteries are in many respects peculiarly fitted for telephone work. Their extremely low internal resistance, and their ability to maintain a constant E. M. F. for a considerable period, are obvious advantages over the primary battery. Charles E. Buell of Plainfield, N. J., was, in 1881, the pioneer in this line. He was followed by Stearns in 1883, Dyer in 1888, and Dean, Stone, Scribner, McBerty, and others, who have accomplished much in this line of work since 1893. The idea of Dyer in 1888 was to charge the storage battery from the ordinary lighting mains of a city, the battery then acting in a local circuit containing the transmitter and primary of an induction coil, in the same manner as when a pri- mary battery is used. In Fig. 201 is shown one of Stone's Fig. 201. Storage Cell at Subscriber's Station. methods which involves the use of an electrolytic or secondary cell at each of the subscribers' stations. In this, advantage is taken of the fact that if when a storage cell is entirely discharged, a charg- ing current is sent through it, a considerable counter E. M. F. is set up by the cell. The battery, B, at central is grounded through an impedance coil, 7, its other terminal being connected to the center point of a divided repeating coil bridged across the cord circuit of the plugs, PP,' after the manner of the Hayes system. Across the terminals of the line wires at each subscriber's station is connected the secondary, s, of an induction coil, the center point of which is grounded through a secondary cell, S. In circuit with this cell is the primary, /, of the induction coil to- gether with the transmitter, T. The current from the battery, B, passes in multiple over the two line wires, through the trans- mitter and secondary cell in multiple, and returns by ground. When the transmitter is operated variations in current in the local circuit at the sub-station are produced, and these act in- ductively on the line circuit containing the receivers, R, by means of the induction coil. If the cell, 5, is discharged the transmitter may be considered as acting solely by means of the 244 AMERICAN TELEPHONE PRACTICE. battery, B, the counter E. M. F. of the electrolytic cell serving to divert a considerable portion of this current through the transmitter, and thereby accomplishing the same result as if the current originated in the cell, S, itself. If, however, the cell, 5, is fully charged then the transmitter may be considered as work- ing upon the current generated by it, and would so work whether the battery, B, were in circuit or not. The fact that the second- ary cell possesses practically no resistance and no inductance renders it especially advantageous for this work. The use of storage batteries or electrolytic cells at subscribers' stations makes possible a full realization of the advantages of the induction coil, but of course introduces the disadvantages of having fluid cells at points remote from the central office. They have been used in some cases with apparent success, but their use has by no means become general. An electrolytic cell acts in a circuit very much in the same manner as a condenser, and systems have been devised in which condensers were used at the subscribers' stations in place of the ce,lls, S, shown in Fig. 201. If we assume these cells to be re- placed by condensers, the other arrangements of the circuit being left as shown, current from the battery B will pass over the two line wires in multiple, as before, and to ground through the transmitter, T, none of it being allowed to pass through the other branch of the primary circuit, by virtue of the condenser. When, however, the transmitter is caused to vary its resistance, the fluctuations in the current set up by it are readily trans- mitted through the condenser, which offers to them practically no impedance. These fluctuations therefore act inductively upon the secondary coil, s, of the induction coil, thus causing corresponding currents to flow in the metallic circuit in the ordinary manner. Instead of using a storage battery at the subscriber's station, Mr. Dean has proposed the use of a thermal generator, or thermopile, to produce the necessary current. As is well known, If the alternate junctions of a thermopile are heated, the others remaining cooler, an E. M. F. will be set up by the pile. An obvious way of supplying the heat is to wrap the junctures with high-resistance wire, which may be heated by the passage of a current through it. This Mr. Dean does, and his simplest arrangement of circuits and apparatus is represented in Fig. 202, in which the wires of a telephone line are shown leading to the central office of the telephone exchange. The telephone re- ceiver, R, and the secondary, s, of the induction coil are placed in COMMON-BA TTER Y S YS TEMS. 2 45 the line circuit, as in the instruments now in use. This line cir- cuit is normally open, but is closed by the hook-switch when released from the weight of the receiver. The transmitter, T, the thermopile, C, and the primary,/, of the induction coil are placed in series in the local circuit, which is permanently closed. The resistance coil, r, which is here shown in proximity to the thermopile, instead of being wrapped around it, is in a circuit in which is included a generator (either a dynamo or a battery). It is obvious that this generator may be placed at the central station, or that the current may be derived from the street mains of an ordinary electric light circuit. The circuit through this coil, r, is TELEPHONE LINE Fig. 202. Dean Thermopile Method. normally broken at the hook-switch. When, however, the receiver is lifted this circuit is completed, and the coil, r, becom- ing heated, puts the thermopile into action. The thermopile therefore generates the current only as long as the telephone is in use, and the breaking of the primary circuit becomes unneces- sary. The action of the apparatus in talking is precisely the same as if a chemical battery were used. Mr. Dean has worked out a system by which the current is applied to the thermopile over the two wires of the telephone circuit in multiple, the return being made through the ground. Properly arranged retardation coils prevent the short-circuiting of the voice currents, but allow the passage of the comparatively steady battery or dynamo currents. All of the principal methods for supplying current from a cen- tral source to the subscribers' transmitters have now been pointed out ; and in this connection it may be well to show how in large exchanges, not working on the common-battery principle so far as subscribers are concerned, a single battery is made to supply all of the operators" transmitters. The old method was to use a 246 AMERICAN TELEPHONE PRACTICE, separate battery, usually two or three gravity cells, on each operator's transmitter, keeping the circuits entirely separate. Mr. Carty, however, has patented the method shown in Fig. 203, which is largely used, and which gives unqualified satis- faction. In this B and B' are low-resistance storage cells, preferably placed in multiple so as to make their joint resistance still lower. The primary circuits of the operators' sets, each includ- iT P t t t t' Fig. 203. Carty Multiple-Transmitter Circuits. ing a transmitter, T, T', and T", and the primary winding, /, p\ and/*, of their respective induction coils, are connected in mul- tiple between the two heavy bus-bars leading from the termi- nals of the battery. If the resistance of the battery is very low, and the bus-bars are heavy and short enough, no cross- talk will be noticed between the various operators' sets, because the resistance of the battery and bus-bars is so low in com- parison with that of the different transmitter circuits that the drop of potential due to the battery resistance will be inappreci- able, and therefore a fluctuation in the resistance of one of the transmitters will cause no change in the potential at the battery terminals. This is an important phenomenon in common- battery work, and should therefore be thoroughly understood. It may be more readily grasped by a simple application of Ohm's law. Let R t represent the joint resistance of the transmitters; that is, of the path from one bus-bar through the several transmitter circuits in multiple to the other bus-bar. Let y? b represent the resistance of the battery and bus-bars, and R the total resistance of the circuit. Let E represent the total E. M. F. of the battery, and e the difference of potential at the bus-bars. COMMON-BATTERY SYSTEMS. 247 Then R = R b + R . By Ohm's law the current is /- A-.L " - Solving for ^ we obtain e _ER t ER t /? /? I J? For a condition of no interference between the various trans- mitter circuits, it is clear that variations in the resistance of any of the transmitter circuits must not affect the difference of potential at the bus-bars. In other words, e must remain con- stant. From this it follows that the fraction R, must remain constant, since E, the total E. M. F. of the battery, is unchanging. When the resistance of any transmitter is varied, the total resistance. R t , of the transmitter circuits will also be varied, and as R t occurs in both the numerator and de- nominator of the fraction it follows that, in order for this fraction to remain constant, the value of R b must be infinitely small. Of course it is impossible in practice to obtain a battery with no internal resistance, but a single I5o-ampere-hour cell in good condition will give a sufficiently close approximation for practi- cal purposes. In actually installing a system of this kind it is well and almost necessary to run the individual wires of the transmitter circuits directly to the terminals of the storage battery, thus practically eliminating all resistance due to bus-bars. The writer had an intimate acquaintance with a case where serious cross-talk occurred under the following conditions : The battery was a single 2OO-ampere-hour cell of the American type, and there were ten transmitters, each having a resistance of about 10 ohms, the resistance of the primary coils being in each case 0.38 ohm. The bus-bars were each 7 feet long and each composed of two No. 6 B. & S. gauge copper wires in parallel. Serious cross-talk existed 248 AMERICAN TELEPHONE PRACTICE. and was only removed when the bus-bars were made of ooo trolley wire, and shortened down to 18 inches. Another scheme for supplying current from storage batteries to the operators' transmitters, devised by Mr. A. R. Hussey of Chicago, is shown in Fig. 204. In this B, J5, B are storage bat- s v p f s; ;T k& si ^i -|ir j? ^ Fig. 204. Hussey Series-Transmitter Circuit. teries, of one or two cells, connected in series in circuit with a dynamo, D. Looped around each of the batteries is a trans- mitter circuit containing a transmitter, T, and the primary wind- ing, /, of the operator's induction coil. This works well and is used by a few independent exchanges. Mr. Stone has devised a system similar to this by which no batteries are necessary, the current being utilized directly from a dynamo. The circuits are the same as those of Fig. 204, with the exception that condensers replace the storage cells, B. Impedance coils are also placed in the supply circuit on each side of the dynamo. Any changes in the resistance of one of the transmitters vary the potential of the charge of the condenser around which it is shunted, and therefore cause fluctuations in the current through the primary,/. The current actually flowing through that coil may be considered as the resultant of the steady current from the dynamo and an alternating current from the condenser superimposed upon it. In this case the current remains constant in the supply circuit as a whole, while the varia- tions in current set up by the transmitters flow freely through the corresponding local circuit only. The fluctuations of the dynamo current are with this arrangement not heard at all in the telephones. The dynamo used for this purpose by Mr. Stone was a shunt-wound machine having thirty-six commutator bars, running at a speed of 2200 revolutions per minute and generating I2,volts. The impedance coils were wound to have a joint resist- ance of 67 ohms and were provided with a soft-iron core, common COMMON-BA TTER Y SYSTEMS. 249 to both coils, for the purpose of increasing their electromagnetic inertia. The capacity of the condensers was about 6 microfarads. One of the chief advantages of common-battery systems is, as has already been pointed out, the readiness with which they lend themselves to all automatic signaling purposes. The methods of supplying current to the subscribers' transmitters having been described, a few systems embodying these methods will be dis- cussed somewhat in detail in order to show the complete working circuits of the exchanges, not only with respect to the means for transmitting speech between the stations, but also by which the various signaling operations are brought about. The system of Dean, shown in simplified form in Fig. 200, for Fig. 205. Complete Circuits of Dean System. supplying current from the central station over the two sides of the line wire in multiple, is illustrated more in detail in Fig. 205, which represents the circuits as they would be in actual practice. The impedance coil, /, at the central station has two windings, 7 and 8. One terminal of the coil, 7, is connected to the sleeve strand, Awhile one terminal of the coil, 8, is similarly connected with the tip strand, ', the other terminals of these coils are con- nected together at the point, /, which forms one terminal of the common battery, B. In a similar manner the impedance coil, I' t at each subscriber's station is provided with two windings, I and 2, connected respectively with the two sides of the line circuits, a and a', and having their other terminals joined at the point,/. The iron cores of these impedance coils are in the form 25 AMERICAN TELEPHONE PRACTICE. of flattened rings, in order that a complete magnetic circuit may be provided to increase the retardation of the coils as much as possible. The two windings on each coil consist of about 3000 turns of No. 22 silk-covered wire. As a result of this construc- tion, the coils are of very low ohmic resistance, especially when placed in parallel as they are with respect to the battery currents ; but they present a very high impedance to the voice currents flowing in the metallic circuit formed by the two line wires, for it is evident that in order to pass from one side of the circuit to the other these currents would necessarily pass through the two windings of the impedance coil in series. The currents from the battery, , passing through the two windings in parallel, produce no magnetic effect upon the cores of the impedance coils, and therefore these coils are in a condition to offer a maximum amount of retardation. This is due to the fact that a mass of iron when in a neutral magnetic state is more susceptible to a mag- netizing force than when the mass is polarized. At the sub-stations the supply circuit, after being united at the point,/", again divides and passes through the two halves of the primary circuit in multiple ; but in this case two primary coils are provided, one in each side of the primary circuit, so that the changes in each side of the circuit may be utilized in producing an inductive effect upon the secondary coil. Thus at station, A, the circuit divides at the point,/", one part passing through the side, /', of the primary circuit containing the transmitter, ^, and one of the primary coils, represented by a full black line ; and the other half passing through the branch,/ 2 , containing the resist- ance,^', and the other primary coil, represented by an open line. The two branches, f and /', reunite at the point, /", which is grounded through the resistance,^- 2 . The coil, g\ has about the same resistance as the transmitter in its state of rest, so that the supply current will divide equally between the two halves of the primary circuit, and therefore normally produce no magnetiza- tion of the core. A decrease in the resistance of the transmitter will cause a greater current to flow through side,/', of the pri- mary circuit and a correspondingly less current through the side, / 2 . As the two primary coils in this circuit are oppositely wound, a decrease of current in one of them will produce the same inductive effect on the secondary as an increase in the other, and when these two effects take place simultaneously in the pri- mary coils, the inductive effects upon the secondary coil are added. An increase in the transmitter resistance will in the same manner induce a current in the opposite direction in the secondary. COMMON-BATTERY SYSTEMS. 251 The limbs, a and a 1 , of each line circuit terminate in contacts on the hook-switch, H, so that when the hook is raised the con- nection is completed from the line wires through the telephone apparatus already described. When the hook is down, the limb, ', of the line is left open and the limb, a, is closed to ground through a high-resistance polarized bell, C. At the central office the circuits are as already described, with the addition of the line annunciators, K and K', and the clearing- out or supervisory signals, K" and K"'. The operator's talking set is adapted to be bridged across the cord circuit by the listening key, while the generator, h, may be connected between the ground and the tip of the calling plug, P t by the key, L. Assuming the apparatus to be in its normal position, when subscriber, A, desires a connection with subscriber, A', he raises his receiver, R, from its hook,//". This act grounds both sides of his line through his station apparatus. A current from the bat- tery, B, thus flows through the drop, K, to the two sides of the line in multiple, by virtue of the fact that the tip- and sleeve- springs of the jack rest upon the common anvil,/. The current flows through the two sides of the subscriber's circuit in multiple, and to ground, and is of sufficient strength to cause the annun- ciator, K, to raise its target. The operator seeing the signal in- serts the answering plug, P, thus cutting off the circuit through the annunciator, K y and allowing its target to assume its normal position. The circuits are now completed from the battery, B, through the two halves of the impedance coil, and to ground at the sub- scriber's station, as already described. The operator then bridges her telephone set, T, across the cord circuit, and communicates with the subscriber. Learning that subscriber, A', is wanted, she inserts the calling plug,/ 5 ', into the jack of his line and de- presses the key, L, which connects one terminal of the grounded generator, g, with the tip strand of the cord, and therefore with the side, a, of the line. A current flows from the generator to ground at the subscribers' station, and operates the polarized bell, C. That subscriber then removes his receiver from the hook, and the two converse over the metallic circuit formed by the two connected lines. While the subscribers' receivers are removed from their hooks the current from battery, B, flowing through the sleeve strand of the cord circuit energizes the magnets of the clearing-out annunciators, K" and K"', and causes them to lift their targets. As soon as either subscriber hangs up his receiver this current 252 AMERICAN TELEPHONE PRACTICE. ceases to flow, because the line wire, a', with which the sleeve strand is connected is opened at the point, d', on the hook, H. This allows the target of the annunciator, K" or K'", to fall, show- ing that that subscriber has ceased to use his instrument. This represents perhaps the highest development attained in any of the methods for centralizing all energy sources of tele- phone systems wherein the current for the transmitter is sup- plied over the two sides of the line in multiple. Although both calling and talking currents are supplied from the central office, Fig. 206. Scribner Common-Battery System for Small Exchanges. and the apparatus at the sub-stations is greatly simplified, and all signals on the part of the subscribers are automatically sent, and all switch-board drops, both line and clearing-out, are self-restoring, this system has not come into general use, probably on account of the still greater simplicity of the Stone and the Hayes systems. In Fig. 206 is shown the circuits of one of Scribner's common- battery systems used for small exchanges based on the Stone system shown in Fig. 197. This is representative of the most modern practice in this line of work. The line signal is automat- ically operated by the removal of the subscriber's receiver from its hook, and is effaced by the insertion of a plug into the jack, which act opens the signal circuit at the jack. Current from the battery, t, circulates through the impedance coils, 5, and through COMMON-BA TTER Y S Y STEMS. 253 the combined circuit of two connected lines, after the manner of the Stone system, already described. The listening and ringing key is so arranged that when the lever, d, is moved to the right the wedge, d', will be forced between the springs, e and e ', thus connect- ing the operator's telephone across the circuit without breaking its continuity. The springs and the wedge are so formed that the lever will remain in this position until moved by the operator. When pressed in the opposite direction, the wedge is forced be- tween the springs, e* and e 3 , thus connecting the generator with the calling plug, k '. These springs are so formed that the wedge will be forced from between them when the pressure on the lever is released. Arranged in one side of the cord circuit in the ordinary man- ner are the supervisory signals, o and 0', these signals being con- structed as shown in Fig. 207, which also gives a better view of Fig. 207. Supervisory Signals for Scribner System. the construction of the listening and ringing key. The indicators or shutters, a, are pivoted at their edges in cavities formed in the horizontal key-table. Each shutter is provided with a lug, a', upon which bears the free end of a flat spring, b, whose other end is fixed to the frame of a tubular magnet, C, arranged under the key-table. This spring tends to bring the indicator into a horizontal position, as shown in Fig. 207. The armature, c', of the tubular magnet, c, carries an arm, c*, which, when the arma- ture is attracted, is thrown against the spring, b, thus pushing it out of engagement with the lug, a', on the shutter and allowing it to fall from view. The lever, d, of the listening and ringing key is connected by a rod,/, with the springs, b, of the annuncia- tors in such manner that when the lever is pressed into the listening position, as shown in Fig. 206, the springs, b, will be 254 AMERICAN TELEPHONE PRACTICE. withdrawn from the shutters, thus producing the same effect as if the magnets were energized, and allowing the shutters to drop out of sight. With this arrangement the keys are normally left in their lis- tening positions, so that when an operator inserts an answering plug, k, into a jack in response to a call, she is at once placed in communication with the subscriber. Having inserted the calling plug, k', into the jack of the called subscriber, she moves the key to the ringing position, and allows it to spring back to an inter- mediate position in which neither the telephone nor the gen- erator is connected with the cord circuit. This releases the springs, b, from the influence of the rod,/", but signal, o, will not be displayed because current from the battery, /, is passing through the line of the calling subscriber, thus energizing its magnet and preventing its display. As the called subscriber has not yet responded, the signal, o', will be displayed because suffi- cient current cannot pass through the high-resistance bell of the called subscriber to energize its magnet. As soon, however, as the called subscriber responds, current will pass through his line and the signal, o', will be effaced. This condition will be main- tained until one or both of the subscribers hang up their receiv- ers, when the currents through the respective supervisory signal magnets will be cut off, their armatures will be released, and the shutters will be displayed by being forced into a horizontal position. The operator will then withdraw the plugs, and will move the lever into the listening position in anticipation of the next call. This latter act will cause the rod,/", to pull the springs, b, out of engagement with the shutters, thus allowing them to fall. In Fig. 208 is shown a common-battery system, as applied to a multiple switch-board, embodying most of the latest ideas in telephone exchange work. Two subscribers' lines, L and D, are shown extending from the subscribers' stations, A and B, through the spring-jacks, J and J', etc., on the various sections of the switch-board. For clearness the two jacks, J, are shown in separate portions of the diagram, as are also shown the two jacks, J'; but it must be remembered that the jacks, /, are upon the same section of the switch-board, and the jacks, J', upon another section. Of course, in a large exchange a far greater number of jacks than two would be connected with each sub- scriber's line, there being one jack for each line upon each sec- tion. The line wires of each metallic circuit, after passing through the jacks, pass through the contacts, 8 and 9, of a line COMMON-BATTERY SYSTEMS. 2 55 cut-off relay, A, the circuit between them being completed through a battery, T, and a lamp signal relay, B. P and P f represent a pair of plugs located at one section of the board, it being understood that this pair and the apparatus shown asso- ciated with it would be duplicated many times at each section. Across the tip and sleeve conductors, a and b y of the cord circuit is bridged a divided repeating coil and a supply battery, , this arrangement being readily recognized as that of the Hayes sys- T D KG Fig. 208. Hayes System as Applied to Multiple Boards. tern. The other apparatus in connection with the cord cir- cuit will be readily understood when its operation is described, it being only necessary to state that the relays, O, are contained in the sleeve strand of the cord circuit and serve to control the circuits of the relays, N, which in turn serve to control the circuits through the supervisory lamp signals, F. When subscriber, A, removes his receiver from its hook, a current from the battery, T, flows through the line, actuates the line signal relay, B, and causes the illumination of the lamp, C. The operator at that station seeing the signal inserts the plug, P into the jack, /, thus connecting the two sides of the line with the two sides of the cord circuit. This allows current from the battery, E, to flow through the circuit of the subscriber's line, and this current causes the left-hand relay, O, to attract its arma- ture and thus complete the circuit through the relay magnet, N. 25 6 AMERICAN TELEPHONE PRACTICE. The insertion of the plug also completes the circuit from the battery, S, to the conductor,*:, and contact, c', on the plug, thence to test-thimble, 5, of the jack and by wire, 6, through the magnet of the line cut-off relay, A, to ground. The current flowing through this circuit accomplishes three purposes: first, the at- traction of the armature of the relay, IV, thus breaking the circuit through the lamp signal, F ; second, the attraction of the double armature of the relay, A, thus cutting off both branches of the line circuit beyond the jacks and extinguishing the line signal, C ; and, third, the raising of the potential of all of the test-thimbles, 5, connected with that line by an amount equal to the drop in potential through the relay magnet, A; so that any operator at another board attempting to make a connection with this line would be warned, upon touching the tip of her plug to the test thimble, that the line was busy by a click in her head receiver. Upon learning the connection desired, the operator applies the tip of the plug, P', to the jack of the called sub- scriber, and if his line is free she will hear no click, because the test-thimble, 5, will not have been raised to a higher potential than that of the ground, and therefore no current will flow from the tip of the plug through the right-hand upper winding of the repeating coil and to ground by wire, d. Upon the insertion of the plug, P', into the jack of the called subscriber the current from the battery, S, will pass through the right-hand cord lamp, F, through the rearward sleeve of the plug, and by test-thimble to the line cut-off relay of the called subscriber's line to ground. This illuminates the lamp, F, and operates the cut-off relay, as before. The lamp remains lighted until the subscriber, , responds to the call, when, upon the removal of his receiver from its hook, the current from the battery, E, is allowed to flow through his line. This operates the right-hand relay, O, ener- gizes relay, N, and thus extinguishes the lamp, F, at the same time allowing enough current to flow through the magnet, N, to serve for testing purposes and to hold the relay, A, closed. The subscribers now converse by the methods already dis- cussed, and when either of them hangs up his receiver the cir- cuit through that line is broken at the condenser and the cor- responding relay, O, releases its armature. This de-energizes the relay, N, and causes the lamp signal, F, to become lighted as a sign for disconnection. Upon removing the plugs from the jacks all apparatus is automatically restored to its normal position. In circuit with the lamp, F, of the answering cord is a relay magnet, M, controlling the current of a pilot lamp, H, common to COMMON-BA TTER Y SYSTEMS. 257 the group of plugs under any one operator. This lamp is placed either on a conspicuous portion of that switch-board or else upon a chief operator's switch-board, and serves at all times to indicate to the chief whether or not that particular operator is properly attending to her clearing-out signals. This particular arrangement of cord circuit relays was devised by Mr. H. M. Crane of Boston ; but the credit for the system as a whole must be shared by several of the engineers of the Bell and of the Western Electric Company. In Fig. 209 is shown one of Scribner's systems in which the Fig. 209. Scribner Multiple-Board System. signaling is not unlike that of the last system described, but in which the transmitter supply is effected substantially by the Stone system described in connection with Fig. 197. In this the polarized bells at the subscribers' stations, A and A', are high- wound in order to avoid the necessity for the use of a condenser. These bells when so used are wound to a resistance of 5000 ohms, so that the leakage from the main battery, B, is not excessive even when the exchange has a large number of sub- scribers. The reduction in resistance brought about by the raising of the receiver from the hook causes current from the battery, B, to flow through the lamp signal relay, b, and through the circuit of the line wire back to ground at the central office. 25# AMERICAN TELEPHONE PRACTICE. This operates the lamp signal relay, causing it to attract its armature, thus closing the circuit of the battery, D, through the lamp signal, d, and causing the illumination of the lamp as a signal for the operator. Upon the insertion of the answeiing plug the same chain of events is brought about as in the sys- tem just described. The cord circuit is connected with the line of the subscriber, while the connection of the sleeve, ;;/, of the plug with the thimble,/", of the jack allows current to flow from the battery, D, through the magnet of the line cut-off relay, e, which operates its armature to cut off the line beyond the jacks. The presence of the connection with the battery, D, raises the potential of all the thimbles, f, of that line, thus causing it to test busy when any operator at another board applies the tip of her testing plug to it. The battery, B, will be seen to be bridged across the cord circuit, its positive terminal being con- nected with the conductor, 7, of the cord, through the impedance coil, k, and wire, 10, while the negative terminal is connected with the conductor, 6, through the ground, the wires, 8 and 9, and the two impedance coils, K and k~*, in multiple. Inasmuch as the subscriber at station, A, has removed his receiver from its hook, the current from the battery, B, flowing through the cord circuit operates the relay, /, thus short-circuit- ing the supervisory signal, o, and preventing its illumination. The operator communicates with subscriber, A, by operating her listening key, thus connecting her telephone set across the cord circuit ; and at the same time disestablishing the continuity of the cord conductor, 6, except through the two windings, k' and &*, of the impedance coil. If she finds that the line of subscriber, A', is not engaged she inserts the corresponding plug, g' , of the pair into the jack of that line and operates her calling key, h. The insertion of this plug operates the cut-off relay, e, as before. On account of the high-resistance bell at the called subscriber's station being still in circuit, sufficient current does not flow through the relay, /', to cause its operation, and therefore the lamp, o', is illuminated and remains so until subscriber, A', removes his receiver from its hook, which act causes a low-resistance path across the two sides of the line, operates relay, /, and by short- circuiting the lamp, o', extinguishes it. The going out of this lamp informs the operator that subscriber, A\ has responded. The two subscribers then converse by the means outlined in the Stone system in Fig. 197, current being supplied to one side, 7, of the cord circuit through the coil, k, of the impedance coil, and to the other side, 6, of the cord circuit through the impedance coils, r o W g. O AMERICAN TELEPHONE PRACTICE. k' and k*, in multiple, these having their upper ends connected by means of the contact anvil on the listening key. When either subscriber hangs up his receiver the introduction of the high resistance of his bell into the circuit cuts down the cur- rent from battery, B t to such an extent that the corresponding supervisory relay, / or /', releases its armature, thus breaking the short-circuit about the supervisory signal, o or o f , and causing its illumination. The illumination of both of these signals is a sufficient indication for the operator to assume that the con- nection is no longer desired, and she therefore removes both plugs, restoring all of the apparatus automatically to its normal condition. The test in this system is performed by the plug, g', in the ordinary manner. If the test-thimbles,/", of the line tested are raised to a certain potential above the earth by the insertion of a plug into a jack of that line at another board, the current will flow from the thimble through the tip of the plug, to conductor, 6, of the cord circuit, and to ground through the coil, k*. This will act inductively upon the coil, k', so that a current will flow through it and the operator's telephone, the listening key of course being depressed. The general appearance of a modern multiple switch-board, equipped with lamp signals controlled by cut-off relays in a man- ner already described, and operating on the common-battery plan, is shown in Fig. 210, which is taken from a photograph of the new switch-board recently installed by the Bell Telephone Co. of Missouri in their St. Louis exchanges. This board consists of 19 sections, with three operators' positions at each section. It is finished in mahogany, and is about six feet high, four feet wide, with an over-all length of nearly 115 feet. It is at present wired for 4000 subscribers' circuits, and is capable of accommodating an ultimate number of 5600 circuits. Two and one-third sec- tions are reserved for incoming trunks from the various branch exchanges, located in the different districts of the city. A better idea of the construction of the board may be had from Fig. 211. In addition to the 4000 multiple calling jacks shown on the upper panels of each section there are on the lower panels of the switch-board, as illustrated in the view, 260 answer- ing jacks, appearing only in that particular section and represent- ing the set of subscribers' lines over which the three operators at that section receive their calls. On the horizontal keyboard, below the jacks just referred to, is a double row of plugs, the rear set or answering plugs being those o CO 5; cn o c/5 en bo COMMON-BA T TER Y SYS TEMS. 263 first used in the answering jacks in answering a call, and the front set being used for testing and afterward connecting with the line of a subscriber called for, at the multiple jacks above. The listening and ringing keys may be seen directly in front of the plugs. In Fig. 212 a rear view of the switch-board is shown, giving a clear view of the systematic arrangement of the line and relay Fig. 213. Stromberg-Carlson Common-Battery Exchange at Battle Creek, Mich. cables. The following are interesting facts in relation to the wiring of this exchange : there are five million six thousand feet of wire in the straightaway cables, and nine million two hundred and eighteen thousand feet of wires in the relays and other coils. The number of soldered connections between the terminals of cables on the main distributing board and the operators' switch- board is not less than one-half million. In Fig. 213 is shown a view of a common-battery switch-board of the Stromberg-Carlson Telephone Manufacturing Co. of 264 AMERICAN TELEPHONE PRACTICE. Chicago, which company seems to be doing more in the line of common-battery work than the other independent manu- facturing concerns. The circuits of this system present several points of interest, and the writer regrets that he is not at lib- erty to publish them, having been requested by the makers not to do so. CHAPTER XXII. HOUSE SYSTEMS. Two general plans of installing interior telephone systems for giving service between the various departments of a business establishment may be followed : One of these is to install a switch-board at some central point to which all the lines radiate, and at which they are connected as desired by an operator. In following this plan the switch-boards and instruments used may be of any of the types already outlined for use in small exchanges. The second plan involves the use of what is called an intercom- IV COMMON RETURN Fig. 214. Circuits of Ordinary House Systems. municating or house system, in which the instrument at each station is placed on a separate line, the line belonging to each station passing through all of the other stations. By means of a simple switch arranged in connection with each telephone, the party at any station may at will connect his telephone with the line belonging to any other station and call up the party at that station without the intervention of an operator. This involves the necessity of running at least as many wires as there are instruments in the exchange through each one of the stations ; and the simplest way to do this is to run a cable having the requisite number of conduits through each of the stations, all of the conductors in the cable being tapped off to the switch-con- tact points on each telephone. The connections for a system having four stations is shown in Fig. 214. Each of the telephone 265 266 AMERICAN TELEPHONE PRACTICE. sets embraces the ordinary talking and calling apparatus switched alternately into circuit by the ordinary t'onn of hook-switch. These instruments differ in no respect from the ordinary exchange telephone. Connected with one of the binding posts, b, of each instru- ment is the pivot of the lever, Z, which lever is adapted to slide over the buttons, I, 2, 3, and 4, arranged in the arc of a circle beneath. Each button on each telephone is connected with a line wire, I, 2, 3, or 4, bearing the same number as the button. The binding post, b\ on each instrument is connected with the common-return wire which runs through the same cable as the line wires. During the idle periods of each instrument the lever is kept on the button bearing the same number at that station. This button is usually called the home button, and is for con- venience placed at the extreme left of the row of buttons on each instrument. The apparatus as shown represents the condition when station I is about to call station IV. For this purpose the party at station I has moved the lever, L, from its home button to button No. 4, thus connecting the instrument at station I with the line belonging to station IV. When the generator at station IV is operated, the current flows from binding post, b, to the common- return wire to the binding post, b', at station IV, thence through the generator and call-bell at that station to binding post, b, and to lever, L, whence the return is made by line wire, 4, to the lever, Z, and the binding post, b, at station I. When the receivers at both stations are raised the talking apparatus is thrown into the circuit over which the calling current was just sent, and the parties converse over the common-return wire and line wire No. 4. Had station IV called station No. I, then the talking and ring- ing would have been done over the common-return wire and line No. i. This system may be used with battery call instruments, such as is shown in Fig. 90, in which case no generators or polar- ized bells will be required. The great drawback to the system of wiring shown is, however, that the lever at the calling station must always be moved back to the home button when a conversation is finished.. If this is not done the instrument at that station will be left switched upon the wrong line, and will not respond to a call sent over its own line from another party. Moreover, when anyone calls a party on the line to which these two stations are left connected, both bells will ring, thus producing much confusion. To illustrate this : if after station I had called station IV, he had left his switch lever, Z, in the position shown, station II could not call station I HOUSE SYSTEMS. 267 because the instrument at station I would no longer be connected o with line I. Should station Ii attempt to call station IV, the bells at both stations I and IV would ring because both of those in- struments are connected with line No. 4. Frequently instead of using a rotary switch an ordinary plug and cord are used in place of the switch lever, while the buttons are replaced by simple spring-jacks into which the plug may be inserted. In Fig. 215 is shown such a system, where a plug, /*, BATTERY WIRE COMMON RETURNS Fig. 215. Common-Battery House System. in each case takes the place of the lever, L, in Fig. 214. Ten line wires are shown in this figure each connected with ten spring- jacks on each of the telephone instruments; the wiring of but three instruments is shown, this being a sufficient number, inas- much as all are connected to the circuits in the same manner. This system is for common-battery work, a single battery located at any convenient point being used for supplying both trans- mitter and talking current to all of the stations. This battery is connected across the common-return and battery wires, which are common to all of the stations and which are placed in the same cable as the line wires. Connected between the common- return wire and the line wire bearing the same number as its station is an ordinary vibrating bell the circuit through which is broken when the receiver is removed from its hook. By pressure upon the key, k, at any station, circuit may be completed from the common-return wire through the battery to the plug, P, of that station, and therefore if this plug is inserted into the jack belonging to any other station the pressure upon this key will cause the bell to sound at that station. In this way a call may be received or sent. When the hook-switch is raised the trans- mitter of a station is connected between the battery wire and com- 268 AMERICAN TELEPHONE PRACTICE. mon-return wire, so that all of the transmitters at the stations in use take current from the same battery in multiple. In order to reduce cross-talk between two or more pairs of stations which happen to be communicating at the same time, the small impedance coils, c c, are placed in each side of the transmitter circuit at each station. These coils of course cut down the efficiency of the transmission, but they also tend to prevent the fluctuations in current produced in any transmitter from backing up through the battery wire and common-return wire into the local circuits of the other transmitters. Fluctua- tions produced in the local circuit of any transmitter act induct- Fig. 216. Sixty-Point Plug Box and Desk Set. ively through the induction coil, 7, upon the talking circuit containing the receiver, the circuit being completed between two stations by the common-return wire and the wire of the station that has been called. This arrangement necessitates the removal of the plug when through talking, as otherwise both of the stations connected would be rung up when either of the stations was called. As a rule, twenty stations are considered the greatest number that may satisfactorily be served by an intercommunicating sys- tem, and when a greater number of stations is to be installed it is better to use a central office provided with a switch-board, with an operator in attendance. The Stromberg-Carlson Telephone Manufacturing Company of Chicago have, however, recently devel- oped this system so that it is said to satisfactorily serve a greater HOUSE SYSTEMS. 269 number of subscribers. In Fig. 216 is shown one of their desk sets in connection with a sixty-point plug board, this system being arranged for intercommunication between sixty stations, that being probably the largest number ever successfully served in one intercommunicating system. The wiring of this system is so arranged that the difficulty due to the subscriber leaving his plug in the wrong spring-jack is practically overcome. The Holtzer-Cabot Electric Company has overcome the difficulty due to the subscriber calling leaving his switch lever in the wrong position, by the apparatus shown in Fig. 217, this device being Fig. 217. Ness Automatic Switch. the invention of Mr. T. W. Ness. The arrangement is such that when the subscriber hangs up his receiver the switch arm, which is under the influence of a spring, will be automatically released and will fly back to the home position without his volition. In the figure the switch-restoring mechanism is mounted on the inside of the cover of the box, the switch lever itself being mounted on the opposite side. The lever, L, at each station, shown in diagram in Fig. 218, is adapted to slide over the buttons, I, 2, 3, and 4, as in the systems already described. The curved contact-piece, D, is so arranged that the lever will not normally engage it, but by pressure upon the handle of the lever it may be brought into engagement with the contact. Referring 270 AMERICAN TELEPHONE PRACTICE. again to Fig. 217, H is the hook-switch adapted to perform the ordinary functions of connecting the calling and talking ap- paratus alternately in the line circuit. The switch lever is mounted upon the shaft, A, which may be seen passing through the front board of the box and which carries a ratchet-wheel, , of hardened steel. A coiled spring around the shaft tends to rotate it so as to bring the lever always to the home position. F is a sliding pawl normally held in its lower position by a coiled spring surrounding it. This sliding pawl serves to hold the lever, L, in any position to which it has been rotated, by the engagement with the teeth of the ratchet-wheel, E. Upon the Fig. 218. Diagram of Holtzer-Cabot System. short arm of the hook-switch is pivoted a dog, G, adapted, when the receiver is placed upon the hook, to engage a notch in the pawl, F y and lift it out of engagement with the ratchet-wheel. This allows the spiral spring to return the switch lever to its right-hand position in contact with the home button. After raising the pawl out of the notch on the ratchet-wheel the dog slips out of the notch on the pawl, thus allowing the latter to return into contact with the ratchet-wheel, in order to be ready for the next use of the telephone. In order, however, that the pawl may not engage the ratchet before the lever, L, has fully returned to its normal position, a second dog, J, is provided, which is pressed by a spring so as to occupy a position under the pin, p, carried on the pawl, thus holding it out of engagement with the ratchet-wheel until the rotation of the lever is nearly completed. At this point a cam on the under side of the ratchet- wheel pushes the dog, /, out of engagement with the pin, /, and thus allows the pawl to drop into position against the ratchet- wheel. It will be seen that this device accomplishes with cer- HOUSE SYSTEMS. 271 tainty what the memory of the telephone user could not be relied upon to do. This entire mechanism is well constructed, all of the parts subject to wear being of hardened steel. The diagram of circuits given in Fig. 218 shows the system wired for four stations operated with common calling battery, and with local batteries at each instrument for talking purposes. This company also manufactures these instruments arranged for the ordinary magneto system, in which case the wiring may be substan- tially the same as that shown in Fig. 214, but without its dis- advantages. CHAPTER XXIII. PROTECTIVE DEVICES. THE matter of protecting telephone apparatus from the damag- ing effects of currents other than those which properly belong on telephone lines, is not such a simple one as might be at first supposed. The " lightning arresters " found on nearly all telephone instruments are for the purpose of protecting the instruments only from what may be called high-tension currents, such as those produced by lightning. The usual form of this arrester is shown in Fig. 219, in which A and B Fig. 219. Usual Form of Telephone Lightning Arrester. represent the two line plates carrying two binding posts and forming the terminals of the instrument. The plate, C, is con- nected with the ground. These three plates are not in contact, the idea being that a charge of lightning will jump across the air gap to the ground plate before it will pass through the high resistance and impedance of the instrument coils within. They do some good, but are by no means infallible, as lightning has too many freaks to be kept out by any such simple device. The holes, e, f, and g, are for the reception of a metallic plug, which if placed in e short-circuits the instrument, and if in / or g connects either one side or the other of the line to ground. The placing of the plug in the hole, , to melt the solder. This allows the spring, c, which is connected by the wire, 3, to the ground, to push the pin, s, entirely through the coil, so that con- tact is made between spring, c, and flange, 4, as shown in the right-hand cut of Fig. 224. This at once grounds the line with- out leaving any air-gap whatever in the circuit, as in the previous PROTECTIVE DEVICES. 277 arresters. It has been found advisable, however, to use these heat coils in connection with carbon arresters, and also with comparatively heavy fuses, as will be described later. Fig. 225. McBerty Thermal Arrester. Another device somewhat similar to this, but adapted to open the circuit instead of connect it with the ground, is shown in Figs. 225 and 226. This is an invention of Mr. F. R. Mc- Fig. 226. Parts of McBerty Heat Coil. Berty of the Western Electric Company, Chicago. The con- struction of the coil itself is best illustrated in Fig. 226, in which b is a small hollow rivet of conducting material, upon which the Fig. '227. Cook Heat Coil. coil, a, of German-silver wire is wound. One end of this coil is attached to the shank of the rivet, as shown, and the other end to a metallic plug or button,/. The hook, c, is soldered into 278 AMERICAN TELEPHONE PRACTICE. the hollow rivet, b, and the whole is inclosed in a hard-rubber bushing, d, as is clearly shown. A hard-rubber plug, e, is forced into the cavity in the bushing after the rivet has been put in place, and this serves to insulate the head of the rivet from the metallic plug, f. A forked standard, g, is mounted, as shown in Fig. 225, so as to form a support for the heat coil when in place. A leaf-spring, 2, insulated from the support, forms one terminal of the line and presses firmly against the plug,/. The hook, c, of the heat coil is engaged by a spring, /, which is held thereby under a considerable tension. This spring forms the terminal of the instrument wire, so that the circuit from the line passes through the spring, i, the plug,/, the coil, a, the hook, c, and the spring, /, to the instrument. When a current of suffi- 9 Fig. 228. Combined Carbon and Heat Coil Arrester. cient strength to melt the solder passes through the coil, the plug, c, is pulled out and the spring, /, is thus allowed to assume its normal position. This produces a wide opening in the line, so as to prevent arcing across the gap. Fig. 227 shows sectional views of the Cook heat-coil, made by the Sterling Electric Company, and widely used. In this, c, is a hard-rubber cylinder having a tapped hole into one end of which is screwed the flanged brass piece, a, and into the other the brass plug, b. The flanged piece, d, carries a wire rod, e, which extends through a hole in the plug, b, and is soldered therein by a low-fusing solder. In the chamber, / is contained a coil of German-silver wire wrapped about the plug, b; one of its ends terminating in the plug, b, and the other extending through the hole, g, in the hard-rubber block, being soldered to the piece, a. This heat-coil is slipped between springs which it normally holds in such position as to render the line continuous through the coil,/ and to keep the branch circuit to ground open. Heat coils should be placed in each side of the line, and, as economy of space is a very important item in telephone PROTECTIVE DEVICES. 279 exchanges, it becomes necessary to arrange them in as compact a manner as possible. Fig. 228 shows combined carbon arresters and heat coils mounted on long strips of iron for this purpose. The principles of operation are the same as those already de- scribed, although the structural details are somewhat different. The line wire enters at spring, D, and thence passes to spring, F, through the heat coil, B, to the spring, G, and thence to the switch-board wire through terminal, E. The spring, /% rests in a groove of the carbon block, A, which is separated from a similar block by a small strip of mica, shown in detail at the right- hand portion of this figure. This second block rests on a ground plate. An added feature of protection is provided by Fig. 229. Rolfe Arrester. inserting in one of these carbon blocks a small drop of fusible metal. If an arc occurs between the two blocks, this metal will melt, thus establishing a perfect connection between the two, and grounding the line. In case, however, a smaller current comes in over the line, it operates the heat coil and allows the central rod, which is of metal in this case, to press the light spring at- tached to the lower side of F into connection with the ground plate. The other side of the line is connected to the other side of the switch-board coil in the same manner ; the line entering at the terminal, C, passes by means of an insulated bolt through the iron frame on which the apparatus is placed, to and through the corresponding heat coil on the lower side of the plate. It passes to the switch-board wire by the terminal, E. 280 AMERICAN TELEPHONE PRACTICE. Still another type of arrester which is coming into increasing use among the independent companies is that shown in Fig. 229. In this, which is the invention of Mr. C. A. Rolfe of Chicago, the two binding posts which are connected respectively to the clips, D, form the terminals of the line wire, and of the wire lead- ing to the instrument to be protected. On an insulating strip, C, usually of fiber, are provided the metal ends, c, which are adapted to be held firmly between the clips, D. A fine-wire coil, E, of German silver is connected between the metal end pieces, c, its terminals being attached thereto by small screws and washers. The coil, E, is imbedded in a mass, G, of some easily fusible substance resembling plumbers' wax ; the smaller portion of which extends through an eye, ff, on the plate, C. This eye is arranged to support G, and to provide a stop against which the head of the button is normally held by the tension of a spring, /, secured to the upper portion of the plate, C, as shown. A is a coiled spring mounted upon the base, K, and provided with an arm, a, which may be held by a catch, B. The relative positions of the springs, /and A, are such, that if the spring, /, is released, it will strike the arm, a, of the spring, A, and cause it to disen- gage the catch, B. The spring, A, will then, in its attempt to rise, as indicated in the dotted portion of Fig. 229, strike the under side of the plate, C, and lift it entirely out of the clips, D. As the coil, E, forms a part of the circuit, a current in excess of that which it is adapted to carry will develop enough heat to melt the wax, and this will allow the head of the coil to pull through the ring, H. This in itself usually breaks the wire, E, and thereby opens the circuit ; but, as an additional protection, the spring, A, gives a violent kick, which is sufficient to throw the entire plate, C, and its mechanism high into the air. This affords a very effective break between the line ter- minals so much so that is almost impossible for an arc to form. This arrester is sometimes termed the grasshopper cut-out, on account of its peculiar action. Heat coils may be so adjusted as to be operated by extremely small currents, and they show great uniformity in their operation. By varying the length of the resistance wire or its size, they may be made to respond to a given current in almost any length of time desired. These coils are usually adjusted to act when subjected to a current of one-quarter ampere for thirty seconds ; they are, however, sometimes adjusted for currents as low as .15 ampere. In later coils about 30 ins. of No. 31 B. & S. German-silver wire is used. CHAPTER XXIV. DISTRIBUTING BOARDS. IN every central office some means must be provided for dis- tributing the various line wires which enter the exchange to their proper numbers on the switch-board and to enable changes to be made in this distribution as required. If such provision were not made, and the line cables were run directly to the switch-board, the wires in one one-hundred-pair cable, for instance, being led to the No. i section, and those of another to the No. 2 section of the switch-board, and so on, it would be necessary, at any time when a change in a subscriber's number was desired, to open the cable, take out the proper wire and fasten it alongside of one of the other cables leading to the proper section of the board. The changing about of wires from one part of a board to another is a very frequent occurrence, and to do it in the manner above suggested would be entirely impracticable. To do it in any manner without a proper regard to systematic arrangement would lead to endless trouble, by producing a tangle of wires, commonly and well termed a " rat's nest." In order to provide means for the systematic arrangement of the wires, what is called a distributing board or frame is used. These assume a great variety of forms, but the principle on which they are designed is as follows : on one side of the distributing board are placed clips, suitably arranged, in which wires of the line cables may terminate. On the other side of the distributing board is arranged another set of clips or connectors, in which the separate wires of the switch-board cables may terminate. Sup- pose, for convenience, that the cables entering an exchange are twenty in number, each consisting of one hundred pairs of wires. These wires would pass through suitable office cables to the various terminals on the line side of the distributing board. Suppose further that the switch-board was arranged in twenty sections of one hundred drops each. Then twenty switch-board cables, of one hundred pairs each, would lead from the switch- board terminals to the terminals on the switch-board side of the distributing board. This brings connections from the line- cables and also from the switch-board cables, in a permanent manner, to 282 AMERICAN TELEPHONE PRACTICE. the various connectors on the respective sides of the distributing board. The gap between the terminals of any pair on the line side of the distributing board and that of the corresponding pair of wires leading from the switch-board is filled by means of " bridle " or "jumper " wires. Suppose that the line circuit terminating in terminal No. 101 on the line side of the distributing board is to be connected with drop and jack No. 599 on the switch-board ; d' Fig. 230. End View Hibbard Distributing Board. then a bridle wire is run from terminal No. 101 on the line side to terminal No. 599 on the switch-board side, thus completing the circuit of that line between the switch-board drop and the sub- scriber. One side of the distributing board often carries lightning ar- resters through which the various line circuits pass before enter- ing the switch-board. These, however, are sometimes placed on a separate board between the line side of the distributing board and the cable heads. Test clips are also often provided on one side of the distributing board. These are usually simple forms of jacks, normally maintaining the continuity of the lines. They are, however, adapted to receive a test plug so that the testing operator may connect his testing apparatus with the line side of the circuit, leaving the switch-board side open, or with the switch- DISTRIBUTING BOARDS. 283 board side of the circuit, leaving the line side open, or he may merely bridge his testing apparatus between the two sides of the line without breaking its continuity. Inasmuch as there are in a large exchange several thousand of these bridle wires, means are provided for their systematic arrangement as far as possible. The chief object of distributing boards is to bring all of the confusion among the wires leading from the subscribers to the switch-board into one small place, and then to minimize that confusion as much as possible. This r Fig. 231. Side Elevation Hibbard Distributing Board. is well done in the Hibbard distributing board, modifications of which are used to a large extent in the Bell exchanges. This was designed by Mr. Angus S. Hibbard, and is illustrated somewhat in detail in the accompanying figures. The frame is built up entirely of iron pipes, extending in three directions and mounted upon a hollow platform, a, shown in Figs. 230 and 231. These two figures represent respectively the end and side eleva- tions of the complete framework, a plan view being shown in Fig. 232. Vertical pipes serve as supports for the structure, and are intersected at short intervals by transverse pipes, i, and longi- tudinal pipes, M 7l- N a 1 x f T \ ^'jf > ' i< sA W ^ULtllhi i >: i i i i i i L ^ ^= == '' Fig. 261. Lock-out Mechanism. opposite polarity will be formed at h and f. The polarized lever, j, is, therefore, actuated by the seventh and eighth current com- binations and remains unaffected by all others. As shown at the right of Fig. 261, the lever,/, serves not only as a lockout device, but also as a busy signal. The apparatus is shown in its locked or busy position at station, S 2 , of this figure and in its unlocked or free position in station, vS 3 . When the lower portion of the lever is moved to the left it forms a stop to 336 AMERICAN TELEPHONE PRACTICE. , on the hook-switch, z, and thus prevents the latter from rising should the receiver be removed from the hook. At the same time the small target, B, on the other end of the lever is dis- played through a hole in the box, thus showing the party at that station that the line is busy. When in its other position the busy signal is not displayed and the hook-switch is free to rise. Fig. 262. Circuits of Six-Station B. W. C. System. When the operator at central presses the locking key, say key No. 7, all of the locking levers on the line, including that of the party to be called, will be actuated. In order that the party being called may not be thus locked out, the windings, 27 and 28, are provided around the polar extensions, h and /, on each in- strument. This winding has no function except at the station being called. In that station part of the current from the local circuit, which is closed only at that station by the action of the relays, finds path through this winding, and the magnetism so developed serves to unlock the mechanism and to allow the party at that station to use his instrument. PARTY LINES STRENGTH AND POLARITY. 337 In Fig. 262 is shown a six-party line, the equipment at each station being of a similar character to that shown in Fig. 261, but simplified for the purpose of clearer illustration. The two sides of the line terminate in the line springs of a spring-jack, J, which springs normally rest on anvils connected to the windings, 31 and 32, of a differentially wound switch-board drop. These two wind- ings pass around the core of the drop magnet in opposite direc- tions, after which they unite at the point, 60, and pass to ground through a battery, E?. The relative direction of the windings on the drop is such that the current from this battery circulates around the core in opposite directions, and thus does not affect the drop. It then divides equally between the two main con- ductors, A and B, and finally returns by the ground connections, G, at each of the several stations. The current thus flowing to the two conductors from the battery, *, is in a negative direc- tion, and thus tends to maintain the apparatus at the several stations in its unlocked condition. When any subscriber removes his receiver from the hook, the short arm of the hook-lever, Z., makes contact momentarily with the spring, d, which grounds the main line wire,.#, and thus allows a heavy current to pass through the winding, 32, of the drop, /. This throws the drop and attracts the attention of the operator. The operator answers the call in the ordinary way by the inser- tion of one of the plugs, P, with which the ringing keys, /, in Fig. 260 are associated. When a substation is to be signaled, the calling plug, P, is inserted into the spring-jack, which cuts off the annunciator and connects the keys, K, with that particular circuit. Key, l\ which sends the plus current over both mains in parallel, is then operated to lock the apparatus at all stations without ringing any of the bells ; and then the key representing the de- sired station is pressed which results in ringing the bell, and at the same time in releasing the telephone apparatus at that station by the means already described. At the close of any conversa- tion the key, I 6 , sending a negative current over both mains in parallel, is operated to release the apparatus at all stations, re- storing the circuit to its normal condition. CHAPTER XXVIII. PARTY LINES. HARMONIC SYSTEMS OF SELECTIVE SIGNALING, THE third general method of selective signaling on party lines makes use of the fact that every pendulum or elastic reed has a natural period of vibration, and that it can be made to take up this vibration by the action of a succession of impulses of force occurring in the same period as that of the reed or pendulum. A familiar example of this is found in one person pushing another in a swing. The swing has its natural period of vibra- tion, depending on the length of the ropes, and a gentle push applied at proper intervals by the person on the ground will cause the swing to vibrate with a considerable amplitude. If the pushes are applied at intervals not corresponding to the natural period of vibration of the swing, many of them tend to retard rather than help its vibrations, so that a useless bumping results., which produces but little motion. The utilization of this principle has given inventors a very attractive field of work ; but as in the case of the step-by-step systems, the results attained have been of little practical value in telephony, except in so far as they have contributed to the general stock of knowledge on the subject. The idea of selective signaling between different instruments in the same circuit was used in telegraphy before the birth of telephony. A number of currents of different rates of vibration were impressed upon the circuit by as many different transmitters, each particular rate of vibration being capable of operating a reed in one of the receiving instruments, and producing no effect upon the others. By this means each receiving instrument was capable of picking out only those signals sent by the transmitter having the same rate of vibration, and thus all of the transmitters could be used simultaneously in the same circuit, producing a system of multiplex telegraphy. The idea as applied to telephony is shown in Fig. 263, where C is an electromagnet connected with the line wire, A A', in series with similar magnets at all of the other stations. B is an armature of soft iron mounted on the post, b, by a short flat spring, thus forming a reed which it is obvious will have a fixed 338 PARTY LINES HARMONIC SYSTEMS. 339 rate of vibration for any particular adjustment. When a number of current impulses are sent over the line wire having a frequency corresponding to the rate of vibration of the reed, B, the latter will be thrown into vibration. If the frequency of the current impulses does not correspond to the rate of vibration of the reed, then the reed will vibrate but slightly, if at all. D is a Fig. 263. Currier and Rice Harmonic Selector. flexible spring forming a part of a secondary circuit containing an ordinary vibrating bell. When the reed, B, is thrown into a sufficiently wide vibration, this latter circuit is closed at the point, n, and the bell is sounded. The reeds at all of the stations are so adjusted as to have different rates of vibration, and by impressing current impulses of a proper frequency upon the line at the central station, the bell at the desired station can be sounded. This illustration is that of a device invented by Messrs. Currier and Rice in 1880. Fig. 264 shows a signal-receiving instrument designed three years later by Elisha Gray and Frank L. Pope. M is an electro- magnet having polar extensions, m and tri (best shown in the plan view), between which is pivoted the polarized armature, P. This will be attracted toward one or the other of the polar extensions, according to the direction of the current through the coils. O is a vibrating reed having one end rigidly mounted on the post, O'. The rate of vibration of this reed may be varied 340 AMERICAN TELEPHONE PRACTICE. by the sliding weight, 0, which may be clamped in any desired position by the thumbscrew, o '. N is an armature by which the electromagnet, M, may exert its influence on the reed, O. R is a separate lever pivoted as shown and normally making contact with the reed. Three such receiving instruments are shown connected in a line circuit, L, in Fig. 265. At the top portion of this figure is shown the transmitting apparatus at the central office. The three transmitters, B\ B*, B a , have each a vibrating reed, b, play- ing between two pairs of electromagnets, E and e, and main- Fig. 264. Gray and Pope Harmonic Mechanism. tained in constant vibration by the alternate passage through these magnets of currents from the local batteries, F. The reed of each transmitter is attuned to the rate of vibration of the reed of the corresponding receiver, and therefore current impulses sent to line from any transmitter will operate only one of the receivers. A constant current is maintained upon the main line, Z, by means of a main battery, G, at the central office. When the apparatus is at rest, the circuit may be traced as follows: from the earth at the central office by the wires, w and ze/ 1 , to the contact point, v\ thence by the contact-springs, s', to the contact- stop, v 1 , contact-spring, /, stop, v 3 , contact-spring, s 9 , wires, ix? /*, and contact-stop, w\ to the contact-spring, x, and thence by the wire, zv\ to the positive pole of battery, G; thence from the negative pole by the wire, w 6 , to the contact-spring, x l , thence by contact- stop, w 9 , and wires, w 6 w\ to the electromagnet of the signal bell, D, and thence to the line, L, which extends to and through the several stations, and finally to earth at the terminal station. Upon an insulating support, T, is mounted a series of metallic springs, t\ f, and f 3 , carrying buttons, c\ c*, and c 3 , the free ends of which springs project over the free ends of the series of con- PARTY LINES HARMONIC SYSTEMS. 341 tact-springs, s\ s*, s\ The contact-springs, x x\ are also mounted upon the insulating support, T, their free ends being united by a non-conducting bar, X, which passes directly underneath the free ends of the springs, s\ 5 a , and s*. The key springs, t\ t*, 3 , are connected by wires, y l , y* , y , with the respective reeds of the Fig. 265. Gray and Pope Party Line. transmitters, B\ B*, J5 3 , after which they are united to a common wire, z, which is connected directly with the earth. At each substation is placed the receiver already described, a key, ff, a battery, Q, and a vibrating bell, J. The polarized armature, P, of the receiver is held in such a position by the 342 AMERICAN TELEPHONE PRACTICE. normal current from the battery, G, at central as to hold the local circuit open at the point, q\ Besides this, a shunt is normally closed around the bell magnet, K, at each station, by the closure of the contact between the reed, TV, and the arm, R. To call central the party at a substation has only to depress his key, H. This breaks the line circuit and allows the hammer of the central-office bell, D, to strike one blow. When the operator at the central office wishes to transmit a call to one of the substations for example, station 2 she depresses the key, C*. This establishes a connection between the springs, f and s y , and at the same time breaks the contact previously existing between the spring, s*, and the stop, v*. By the same operation the bar, X, is depressed and the springs, x x\ are respectively removed from contact with the terminals, w* and w 9 , and brought into contact with the wires, w' 1 and w". This operation produces the twofold effect of switching the main-line circuit through the appropriate vibrating transmitter reed, *, and of reversing the polarity of the main battery, G, with respect to the line. The change of the polarity of the main-line current causes the polarized armature, P, at every substation to be deflected from its normal position, thus bringing it in contact with the stop, ^% and closing the circuit of the local battery, Q. The closing of the local battery in this manner will, however, in itself produce no effect upon the electromagnet, K, of the bell, as the latter is shunted by the contact between the reed, O, and the bar, R, which rests upon it. The reed, O, at station 2 being adjusted to vibrate in response to the reed of the transmitter, B 1 , will be set in vibration, and this vibration will cause the loosely pivoted bar, R, to hop up and down, interrupt the shunt-circuit, and allow the magnet, K, to become active, thus causing the bell,/, to ring. The bells at all the other substations, being cut out by the action of their respective shunts, will remain quiescent. The bell of any other station is actuated in precisely the same manner, the only difference being that the reed-armature, O, in each instance is adjusted to vibrate in harmony with its corre- sponding transmitter at the central office and to respond only to currents sent to line by it. The device of Currier and Rice depended on the vibrating reed to close the circuit through the call-bell, while in that of Gray and Pope the reed served only to break a shunt around the bell. In Fig. 266 is shown a system designed by J. A. Light- hipe of San Francisco, in which the gongs are struck directly by the reed, without the use of an auxiliary magnet. This will be PARTY LINES HARMONIC SYSTEMS. understood from the diagram without much explanation. The reeds, d*, e 1 , &\ and /", carry hammers adapted to play between the gongs at the substations when acted upon by their magnets, d, e, k, or /. At stations, A and J5, these magnets are bridged directly across the two sides of the metallic line, while at stations, C and D, on another line, they are bridged between one side of the circuit and ground. A condenser, d' or e', is in each bridge wire in the former case, to prevent the leakage of current from .a," a V Fig. 266. Lighthipe Bridged Harmonic System. the signaling battery, b\ when the telephones are not in use. Associated with the cord circuit of a pair of plugs at the central office are the signal transmitters, each having a reed tuned to the rates of vibration of the several reeds of the call receivers. Pressure of the button, k, for instance, closes the circuit of bat- tery, g, through electromagnet, k 9 , over the limb, a, of the tele- phone line through the electromagnets, d and e, at the substations and back by the limb, a', to the opposite pole of the battery. The magnet, /z", is thus excited, and attracts the reed, which in its forward movement completes a short circuit around the battery. The reed vibrates back and forth, sending current 344 AMERICAN TELEPHONE PRACTICE. impulses over the circuit, and as its rate of vibration is the same as that of reed, d*, at station A, these impulses will have the proper frequency to actuate that reed and sound its bells. Call- ing central from the subscribers' stations is performed in precisely the same way as in the Sabin and Hampton and other systems already described. As the apparatus in this system is arranged on the bridging plan, it is of course necessary that the bell magnets should Fig. 267. Harter Harmonic System. possess high impedance in conformity with the requirements of bridged lines. Fig. 267 shows a somewhat elaborate system, invented by Mr. William H. Harter, of Great Falls, Mont., and embodying a lockout mechanism in addition to the signaling devices. In this figure two substations, I and 2, are shown connected by a metallic circuit line, with two transmitting devices at the central office. Instead of this connection being permanently made as shown, it would be brought about in practice by a spring-jack and plugs, the transmitting devices being connected across the cord circuit. The reeds, b and b\ at the substations, I and 2, are tuned to the PARTY LINES HARMONIC SYSTEMS. 345 same pitch as reeds, a and a 1 , of their respective transmitters at central. Upon connecting the cord circuit with the circuit of the line, the battery, C, is thrown across the two sides of the line, and current therefrom passes through each of the locking magnets, v, in multiple, attracting their armatures, s, and locking all of the receiver hooks in their depressed position. Pressure of key No. i (for the purpose of calling station No. i) establishes a local circuit through transmitter magnet,^ 1 , and the back contact of its reed, thus throwing it into rapid vibration. By means of its front contact, 5 72 378 H34 2138 12.42 14-53 17.18 9 148 58 305 915 1720 15-44 18.06 21-35 10 134 47 250 750 1410 18.83 22.04 26.04 ii 12O 38 200 600 1131 23.48 27.48 32-47 12 lOg 165 495 933 28.46 33-3 39-36 13 95 24 125 375 709 37-47 43-85 51-82 14 83 18 96 288 541 49.08 57-44 67.88 15 72 13-7 72 ax6 407 65-23 76.33 90.21 16 65 ii. i 59 177 332 80.03 93.66 110.7 17 58 8.9 47 141 264 100.5 120.4 139- 18 49 6-3 33 99 189 140.8 164.8 193.8 COPPER WIRE. Copper wire is practically indestructible by exposure to ordi- nary climatic influences. After it is first put up it acquires a thin coating of oxide, and after that no change whatever takes place, so far as can be ascertained. The process of manufacturing copper wire is similar to that for iron wire, with the exception that no galvanizing is necessary. The process of drawing copper wire has been so greatly improved recently that the old fault, lack of mechanical strength, has been almost, if not quite, over- come. Copper wire is now drawn so as to possess a breaking strength of 60,000 pounds per square inch, which is quite equal to that of some grades of iron wire. The difference between hard-drawn copper wire and soft wire is due entirely to the fact 356 AMERICAN TELEPHONE PRACTICE. that the hard-drawn wire is not annealed as often between the drawings. The value of the weight per mile-ohm is, for good commercial wire, 682 pounds, the wire having a tensile strength equal to about three times its weight per mile. For hard-drawn wire the percentage of elongation is not nearly so high as that for iron wire, being only about one per cent, before breaking. The value in pounds per mile-ohm of pure annealed copper is 859, this being based on the international ohm. In the following table, taken from Roebling's " Wire in Elec- trical Construction," the weights and resistances of the various B. & S. gauge numbers of copper wire are given : TABLE IV. COPPER WIRE TABLE. 4 ii h 1 a Weights per Resistances per 1000 Feet in International Ohms ffl ii a " . 3 o) E 03 41 cl m 1 1 rt 1 cj Q 3 looo feet. Mile. At 60 F. At 75 F. oooo 4 6o. 211600. 641- 3382. .04811 .04966 ooo 410. 168100. 59- 2687. .06056 .06251 do 365- 133225. 403- 2129. .07642 .07887 o 325- 105625. 320. 1688. 09639 .09948 I 289. 83521. 253- '335- .1219 .1258 2 258. 66564. 202. 1064. 1529 1579 3 229. 52441- 159. 838. .1941 .2004 4 204. 41616. 126. 665. .2446 2525 S 182. 33124- IOO. 529- 3074 .3172 6 162. 26244. 79- 419. 3879 .4004 7 '44- 20736. 63- 331- .491 .5067 8 128. 16384- 262. .6214 .6413 9 114. 12996. 39- 208. .7834 .8085 10 IO2. 10404. 32. 166. 9785 I.OI XI 91. 8281. 25- 132- ' 1.229 1.269 12 81. 6561. 20. 105. 1-552 i. 60 1 3 72- 5184- 15-7 83- 1.964 2.027 64- 4096. W.4 65. 2.485 2.565 15 57- 3249- 9-8 52- 3-133 3-234 16 5'- 2601. 7-9 42- 3-9'4 4.04 17 45- 2025. 6.1 32- 5.028 5.189 ii 40. 1603. 4.8 25-6 6-363 6-567 19 36. 1296. 3-9 20.7 7.855 8.108 20 32- 1024. 16.4 9.942 10.26 M 38-5 812.3 2-5 13- 12.53 12.94 22 25-3 640. 1 i-9 IO.2 15-9 16.41 23 22.6 510.8 i-5 8.2 19.93 20.57 24 20.1 404. 1.2 6-5 25.2 26.01 25 27.9 320.4 97 5.1 31-77 32-79 26 15-9 252.8 77 4- 40.27 41-56 27 14.2 2OI.6 .61 3-2 50.49 52-11 28 12.6 158.8 .48 2-5 64.13 66.18 29 "3 127.7 39 3. 79-73 82.29 3 10. IOO. 3 1.6 101.8 105.1 3* 18.9 79-2 .24 1.27 128.5 132-7 32 8. 64. .19 1.02 159.1 164.2 33 7-1 50-4 15 .81 202. 208.4 34 6-3 39-7 .12 -63 256.5 264.7 35 5-6 095 5 324.6 335-1 36 5- 25- .076 4 407.2 420.3 WIRE FOR TELEPHONE USE, 357 Abbott gives the following specifications governing the require- ments to be made of manufacturers in purchasing copper wire : COPPER WIRE. 1. Finish. Each coil shall be drawn in one length and be exempt from joints or splices. All wire shall be truly cylin- drical and fully up to gauge specified for each size, and must not contain any scale, inequalities, flaws, cold shuts, seams, or other imperfections. 2. Inspection. The purchaser will appoint an inspector, who shall be supplied by the manufacturer with all facilities which may be required for examining the finished product or any of the processes of manufacture. The inspector shall have the privilege of overseeing the packing and shipping of the samples. The inspector will reject any and all wire which does not fully come up to all the specification requirements. The purchaser further reserves the right to reject on reception any or all lots of wire which do not fulfill the specifications, even though they shall previously have been passed or accepted by the inspector. 3. Apparatus. The manufacturer must supply, at the mill, the necessary apparatus for making the examination called for. This apparatus shall consist of a tension-testing machine, a torsion- testing machine, an elongation gauge, an accurate platform scale, and an accurate bridge and battery. Each of these pieces of apparatus may be examined by, and shall be satisfactory, to the inspector. 4. Packing for Shipment. When ready for shipment each coil must be securely tied with not less than four separate pieces of strong twine and shall be protected by a sufficient wrapping of burlap so the wire may not be injured during transportation. The wrappings shall be placed upon the wire bundles, after they have been coiled and secured by the twine. The diameter of the eye of each coil shall be prescribed by the inspector, and all coils shipped shall not vary more than two inches in the diame- ter of the eye. 5. Weight. Each coil shall have its length and weight plainly and indelibly marked upon two brass tags, which shall be secured to the coil, one inside the wrapping and the other outside. 6. Mechanical Properties. All wire shall be fully and truly up to guage standard, as per B. & S. wire guage. The wire shall be cylindrical in every respect. The inspector shall test the size and roundness of the wire by measuring both ends of each coil, 358 AMERICAN TELEPHONE PRACTICE. and also by measuring at least four places in the length of each coil. A variation of not more than \\ mil on either side of the specified wire-guage number will be allowed, and the wire must be truly round within one mil upon opposite diameters at the same point of measurement. The strength of the wire shall be determined by taking a sample from one end of each coil, 30" in length. Of this piece, 18" shall be tested for tension and elonga- tion, by breaking the same in the tension-testing machine. The samples should show a strength in accordance with the following table : TABLE V. BREAKING WEIGHT OF HARD-DRAWN AND ANNEALED COPPER WIRE. Size of Wire, B. & S. Guage. Breaking Weight of Hard-Drawn Pounds. Breaking Weight of Annealed Pounds. oooo 997i 5650 ooo 7907 4480 oo 6271 3553 4973 2818 i 3943 2234 2 3127 1772 3 2480 1405 4 1967 1114 5 1559 883 6 1237 700 7 980 555 8 778 ' 440 9 617 349 10 489 277 ii 388 219 12 307 174 13 244 138 14 193 109 15 153 87 16 133 69 17 97 55 18 77 43 19 61 34 20 48 27 A variation of \\ per cent, on either side of the tabular limits will be accepted by the inspector. The elongation of the wire must be at least three per cent, for all sizes larger than No. i ; \\ per cent, from No. i to No. 10, and i per cent, for sizes less than No. 10, for hard-drawn copper wire. The remainder of the sample selected will be tested for torsion. The torsion sample will be twisted in the torsion-testing machine, to destruction, one foot in length being placed between the jaws of the machine. Under these circumstances hard-drawn copper wire shall show WIRE FOR TELEPHONE USE. 359 not less than 20 twists for sizes over No. i ; from 40 to 90 twists in sizes from No. i to No 10 ; and not less than 100 twists in sizes less than No. 10. Should the sample selected from one end of each coil show failure to come up to the specifications, the inspector may take a second sample from the other end of the coil. If the average of the results from both samples shall be within the specifications, the coil shall be accepted ; if not within the specifications, the coil shall be rejected. The weight per mile shall be determined by carefully weighing 2 per cent, of the number of coils called for in the contract, and the weight thus obtained shall correspond, within 2 per cent, on either side of the result given in the following formulae : CM Weight per mile = Weight per 1000 ft. = CM 330.353 7. Electrical Properties. The electrical properties of the wire shall be determined by the inspector selecting 3 per cent, of the coils, and from them taking lengths of 100 ft., 500 ft., or 1000 ft., at his discretion, and measuring the conductivity of the same with a standard bridge. For soft-drawn copper wire the follow- ing resistance per mil-foot will be assumed : TABLE VI. RESISTANCE OF COPPER WIRE AT VARIOUS TEMPERATURES. a ^. fl 1 fe M 2 i .d ^ w t* u 2 S> a o j~ "*"J rt 2 ^> o 6 If 1 || .52 H ^, '55 e * 04 o 8.96707 60 10.20253 10 9.16413 70 10.42083 20 9.36473 so 10.64268 30 9.56887 90 10.86806 40 9-77655 IOO 11.09698 50 9.98777 For hard-drawn wire the resistance per mil-foot shall be 1.0226 times the foregoing figures. All wire shall be within 98 per cent, of the above figures. CHAPTER XXX. POLE-LINE CONSTRUCTION. THE poles most used in the United States are of Norway pine, chestnut, cedar, and cypress. Southern pine is not as durable as Northern pine, although it is used to a large extent in the South. Canadian cedar, is, however, all things considered, the best wood to use. The average life of the various woods mentioned are, accord- ing to Maver, as follows : Norway Pine, ...... 6 years Chestnut, . . .... . . 15 " Cedar, . . . . v . . . . 12 " Cypress, . . . . . . . . 10 " In choosing the kind of pole to be used, the locality must always be considered, for obviously it would be poor economy to bring cedar poles from Canada for the reason that they would last perhaps a few more years than cypress poles, which could be cut on the ground. Poles should be well seasoned before setting in the ground. This is either accomplished by natural process of drying, or sometimes in a special drying kiln. Before seasoning, however, the pole should be peeled and all knots trimmed. It is easier to do this while the sap is in them than afterwards, and, moreover, the drying takes place in a shorter time if the bark is removed. If the pole is not seasoned before setting or before it is painted, where it is to be painted, the sap is sure to cause a dry rot, which will eventually destroy the pole. The worst feature of this trouble is that the defect is not noticeable on the surface and therefore is likely to- cause trouble when least expected. A pole may have all appearances of being per- fectly sound and yet be a mere shell, so that, when subjected to some heavy storm, it comes down on the line, perhaps bring- ing many other poles with it. Practice differs to some extent concerning the size of poles. Money saved, however, in the purchase of light poles, is usually 360 POLE-LINE CONSTRUCTION. 3 Gl saved at a great cost in the future. Table VII. gives a list of the sizes which meet the demands of the best practice to-day. TABLE VII. Length. Diam. at Top. Diam. 6 ft. from Butt. 25 feet 7 inches 9 inches 30 7 10 35 7 ii 40 7 12 45 7 13 50 7 14 55 7 16 60 7 17 65 7 18 70 7 20 Telephone companies that have been in the field long enough have learned that the days of " fence-post " construc- tion are over, and that in the long run poor construction is much more expensive than good. To be sure, in many of the indepen- dent installations it is a matter of necessity to use a medium con- struction throughout on account of the first expense, and in such case if the dimensions of the poles given in the table above are too expensive, they will at least serve as a standard at which to aim. In many cases poles with 5-inch tops will meet all the de- mands of an exchange, for a few years at least, and it is some- times expedient to use them. The number of wires to be carried on any pole line is also a question that will largely determine the diameter of the poles. On the corners, or where a heavy lead is dead-ended, to make connection perhaps with an underground cable, the poles used should be in many cases much larger than those given. In fact, in such cases the heaviest poles that can be had will be none too large, and it is not uncommon to see a 4O-foot pole with an i8-inch top at critical points on some of the best constructed heavy lines. The question of the number of poles to the mile is one that must be decided to meet the particular conditions of the line to be erected. The greater the number of poles the lower the insulation, but this is a very small disadvantage, and is more than offset by the greater freedom from breakage of wires and consequent decrease in the expense of maintenance when the poles are set closely together. In Europe the common practice 362 AMERICAN TELEPHONE PRACTICE. is to use as few as twenty poles to the mile. In this country, however, the best practice dictates the use of from forty to fifty to the mile, although many lines are successfully operated with thirty, or less. As a rule, the greater the number of wires carried, the closer and heavier the poles should be. The liability of any particular locality to heavy sleet and wind storms is another factor in determining the size and distribution of poles. In the long-distance lines of the American Telegraph and Telephone Company the standard distance between the poles is 130 feet, making approximately forty to the mile. The standard pole is Fig. 270. Pole Equipped with Guards. 35 feet in length, and, while none are shorter than this, many are much longer. Seventy-foot poles are often used, and in some cases the height of 100 feet is reached. In cities poles varying from 40 to 60 feet are, as a rule, used. These are generally of Norway pine, as it is somewhat difficult to get cedar poles of this height. It is usually necessary to use a longer pole in city work, in order that the line may be carried above the city electric light and power circuits, and also that the work of firemen may not be interfered with. It is well to pro- tect poles along the streets of cities from the gnawing of horses hitched to them, and also from the wearing effects of POLE-LINE CONSTRUCTION. 363 wagon-hubs, which often very greatly weaken the poles at a point where they are least able to stand it. Galvanized steel protecting strips are obtainable for the former purpose, and what are termed butt-plates, about 15 inches by 18 inches by T 3 7 inch thick, of the same material, may also be purchased from supply dealers for the latter purpose. A pole thus equipped is shown in Fig. 270. In Table VIII. is given some useful information concerning the weights of poles of various sizes and the number forming a carload. TABLE VIII. WOOD POLES. CEDAR. Length. Top. Weight in Ibs. No. to Carload. 25 feet. 5 inches. 200 1 20 25 6 275 no 30 6 325 IOO 30 7 450 80 35 6 500 1 20 35 7 600 no 40 6 700 IOO 40 7 800 90 45 6 950 82 45 7 IIOO 60 50 6 1250 40 50 7 1450 25 55 6 1500 30 55 7 1800 25 NORWAY PINE. Length. Top. Weight in Ibs. No. to Carload. 40 feet. 7 inches. IIOO 90 45 7 1 200 80 50 7 1350 72 55 7 1500 65 60 7 1700 55 65 7 2OOO 45 70 7 240O 50 75 7 28OO 45 80 7 3400 35 85 ' 7 380O 30 It is not customary, in this country, to treat poles with any preserving process, but it is always well to coat the pole for a distance of six feet from the butt with pitch, before setting it. 3 6 4 AMERICAN TELEPHONE PRACTICE. It is also well to give city poles two coats of good oil paint, and a very neat appearance is added if the lower portions are painted black to a distance of six feet above the ground, while the remaining portion is painted some light color. In Europe a process termed creosoting is meeting with great favor for pre- serving telephone and telegraph poles. It is the cheapest of all known expedients of this kind and consists, briefly, in placing the pole in an iron chamber from which the air may be exhausted. This causes the sap and all other juices from the wood to ooze out from its pores. After this steam, at a pressure of about 100 pounds to the square inch, is admitted to the cyclinder and the poles are subjected to this treatment for about four hours. After this crude petroleum is forced into the cyclinder under a pressure of about 300 pounds to the square inch, and it is found that it penetrates to the very heart of the poles, thus adding very materially to their lasting qualities. Cases are cited where poles treated by this method have been perfectly sound after having been in service for a period of twenty years. Another process, termed vulcanizing, consists in heating the pole in a closed vessel for several hours to a temperature of about 500 F. The principle in this treatment is that the intense heat causes the sap in the wood to coagulate, after which it can produce no evil effects. This would apparently be cheaper even than the creosoting. The cross-arms carrying the insulators are preferably of sawed yellow pine. Two sizes are in general use, the standard being 4i by 3. The lengths vary from 3 to 10 feet, according to the number of pins or insulators to be used. Table IX. shows the lengths of the various standard cross-arms ; also the spacings of the pin-holes. TABLE IX. Spacings. Length. Number of Pins. End. Center. Sides. 3 feet. 2 4 in. 28 in. 4 4 16 12 in. 5 4 18 I? 6 4 22 21 6 6 16 12 8 6 18 I7i 8 8 16 12 10 8 17* 15} 10 10 16 12 POLE-LINE CONSTRUCTION:* The standard size of pin for the above arm has a i|-inch shank, and arms of this size are usually bored accordingly. They are Fig. 271. Four- Pin Cross Arm. also bored as shown in Fig. 271 with two -inch holes for lag- screws used in attaching them to the poles. Another size of cross-arm, called the telephone arm, has come into use to a considerable extent for cheaper installation. The size of this arm is 2-| by 3!, being \ inch smaller in each dimen- Fig. 272. Lag-Screw. sion than the standard. These arms are usually bored for ij- inch pins and the length of a ten-pin arm is only 8 feet. The various dimensions are shown in Table X. TABLE X. Spacings. Length. Number of Pins. End. Center. Sides. 24 in. 2 3 in. 18 in. 30 36 42 62 2 2 4 6 24 30 16 16 10 in. 10 " 82 8 16 10 " IO2 10 16 10 " I2O 12 16 10 " All cross-arms should be given two coats of good metallic paint, usually red, before setting in position. In order to attach them to the pole a gain is cut in the pole of such dimensions as to accurately fit the longest side of the cross-arm. The gain should not be more than one inch deep, however, for the reason that a greater depth is likely to weaken the pole unduly. The gain should be given two coats of good white lead before the cross-arm is put in place. The common way of attaching the 3^6 AMERICAN TELEPHONE PRACTICE. cross-arms to the pole is by two lag-screws of the type shown in Fig. 272. These are of such length as to reach almost through the pole, and their threads are cut in such a manner that they may be driven part of the way home. A better practice now is to attach the cross-arm to the pole by means of a single carriage bolt extending entirely through the arm and pole, being secured by a nut and a washer. This method has an advantage over the use of lag-screws in that the hole for the carriage bolt may be bored perfectly smooth and clean, and of such size as to accurately fit the carriage bolt, so there is little chance for rotting. A slightly better way, perhaps, but one which is not easy to follow on account of the varying sizes in pole tops, is to bore no hole whatever through the pole, but to attach the cross-arm by means of a U-bolt extending through the cross-arm and around the pole and secured from the front by means of two nuts. This means of attaching is often used in the case of sawn poles where the tops are of uniform size. The arm is further braced in any case by the use of wrought- iron or steel strips, commonly termed cross-arm braces. These should consist of straight, flat bars not smaller than ij inch wide by inch thick, and varying in length from 20 to 30 inches. A hole is usually punched in one end for the reception of a -inch or |-inch lag-screw and in the other for a ^-inch carriage bolt. The two braces for each cross-arm are attached by single lag-screws to the pole at a distance varying from 16 to 18 inches from the bottom of the arm. The other ends of the braces are attached by carriage bolts to the cross-arms at points about equal distances from the pole. In all cases suitable washers should be used under carriage bolt nuts and heads, and under lag-screw heads where they are used in attaching an arm to the pole. All hardware to be used on poles, such as bolts, washers, braces, etc., should be thoroughly galvanized and should be made to stand the same test that is required on galvanized iron wire that is, four successive plunges of seventy seconds each in a saturated solution of sulphate of copper without removing all of the zinc coating. The pins most commonly used are of locust or of oak. The former is by far the better, as it is the stronger and more capable of resisting the action of the weather. It is, however, nearly twice as expensive as oak. The pins should be turned from split wood in order that they may not be cross-grained, and all pins should be given two coats of the same kind of paint that is used on cross-arms. In some cases on corners, or in places where excessively heavy POLE-LINE CONSTRUCTION. 367 strain will be brought upon a pin, it is advisable to use a wrought- iron or steel pin, but these must be used with caution, as in many cases they have proven inferior to wooden pins, being so soft that they bend into a horizontal position when subjected to the strain. The insulators used in this country are universally made of glass. Blown glass has been found to be much superior in insu- lating qualities to molded glass, but the latter is so very much cheaper that it is always furnished. Fig. 273 shows a form of Figs. 273 and 274. Pony and Double-Petticoat Glass Insulators. insulator largely used in telephone work, called the " pony " insulator, and Fig. 274 shows another style, termed the " double- petticoat " insulator. It is so termed from the fact that it has two lower flanges, as shown in section, the idea of this being that the path for leakage from the line to the pin is thereby rendered considerably longer, the leakage current having to pass up and down the surfaces of both petticoats in series. Glass is not as suitable a material for insulators as porcelain, which is largely used in Europe. It is more brittle and does not possess such high insulating qualities. A more serious defect is, that it gathers moisture on its surface to a much larger extent than porcelain, thus affording a better path for leakage currents. In an interesting series of experiments described by Abbott it was found that the insulating quality of glass insulators varied largely with the condition of the surface of the insulators. These experiments were conducted over a period of one hundred and fifty days, observations being made once a day. The gen- eral result indicated that the greatest loss in insulation occurred during foggy or misty weather. During heavy rainstorms the insulation was somewhat higher, and after the storm, when the insulators had been dried, the resistance of the line was con- 368 AMERICAN TELEPHONE PRACTICE. siderably higher, owing to the cleaner condition of the surface. In good weather the double-petticoat insulators gave much higher resistance than the single of corresponding size, but dur- ing a rainstorm the double-petticoat form was inferior to the single, although it was found to dry more rapidly after a storm. The determination of the pole-line route is a matter of no small importance. Right of way must be secured, and this usually calls forth all the ingenuity of the party unfortunate enough to be assigned to that duty. Before distributing the poles and other material the route should be thoroughly studied in every detail. Stakes should be driven marking the location of the poles. It should be borne in mind in locating these stakes that bends in the pole line should be avoided wherever possible, that the ground should be of such nature as to form as good a support as possible for the pole, that there will be no inter- ference from trees, houses, or other poles, and lastly that the route shall be as direct as possible. When a turn must be made it should be so located if possible that the guy wire required to hold up the corner will have suitable anchoring ground. Lack of attention to these preliminary details too often brings an endless amount of trouble in the way of rehandling of poles, redigging of holes, and similar useless labor. When the ground is level, or gently undulating, no provision need be made for grading the pole tops. Where, however, the country is hilly it is well to make a survey of the route with a level, placing the instrument between each successive pair of stakes and taking a front and back sight from each position to the adjacent stakes. A record of the data thus obtained will enable one to plat the vertical section of the route. The profile of the pole tops may then be platted, care being taken to smooth out all sharp bends in it. This is accomplished by putting the tallest poles in the hollows and the shortest on the hilltops. The same results may be accomplished, though not so well, without the use of the level, but it requires an experienced eye to do it to best advantage. After having decided on the location of the poles, the length of pole for each point, and all other preliminary details, such as placing of heavy poles at the corners, the poles may be hauled and distributed along the route. They should be laid with the butt near the stakes and pointing downhill if on a grade. The poles are distributed along the route by any available means. If the line runs along a railroad, they may be rolled from the flat car at the proper intervals, and carried to their POLE-LINE CONSTRUCTION. 369 places by carry-hooks (Fig. 275). If the line is a long one, and does not follow the line of a railroad, the poles should be un- ig. 275. Carry Hook. loaded from the cars at convenient points, and hauled to their proper locations by wagons. The cutting of gains and the peaking of the pole may be facili- tated by the use of a template, shown in Fig. 276, by which the Fig. 276. Gaining Template. gains and peak may be marked out. A pole-buck, constructed as shown in Figs. 277 and 278, and used as in Fig. 279,* will also be IRON WASHED Figs. 277 and 278. Pole Buck. of great aid in the work. The spacing between the gains, shown on the template in Fig. 276, makes the distance between the cross-arms 18 inches. Many construction men prefer 20 inches. *For the half-tones and some of the detailed cuts of construction tools in this chap- ter we are indebted to an excellent article in the American Electrician, by Mr. S. H. Dailey, entitled " Erecting a High-Voltage Transmission Line." 37 AMERICAN TELEPHONE PRACTICE. Where a greater number of arms are to be used the distance from the top of the top arm to the peak should be reduced to 10 inches. Poles of medium length may, under ordinary circumstances, be raised with the cross-arms in place, and, as they are much more Fig. 279. Gaining Poles. easily attached on the ground, this should always be done where possible. In digging the pole holes long-handled digging shovels (Fig. 280) and spoon shovels (Fig. 281) having seven- and eight-foot Fig. 280. Long Handle Digging Shovel. handles are used in conjunction with eight-foot steel digging bars, shown in Fig. 282. Sometimes the post-hole auger is used, Fig. 281. Spoon Shovel. but this is only where the conditions are very favorable. Dyna- mite, judiciously applied, is now being used successfully in digging Fig. 282. Digging Bar. holes, even where the soil is of such a nature as not to absolutely require its use. POLE-LINE CONSTRUCTION. 371 No definite rule can be given for the depth at which poles should be set in the ground. The character of the soil, the dis- tance between poles, the number of wires carried, and the sharpness of the turns made in the line must all be considered in determining this question. For average work the data given in Table XI. are believed to be in accordance with the best practice. TABLE XI. 25-foot pole, 5-J- feet in ground. 30 " " 6 " " " ">- " " (\ " " " 40 " " 6 " " " 45 " 6-1 " " 55 " " 6 " " " 60 " " 7 " " " far < -7 " < < 70 " " 7| " " On curves or corners the holes should be dug from six inches to one foot deeper than is specified in this table. After digging the holes the poles are carried or rolled by cant- hooks (Fig. 283), so that its butt is over the hole. A piece of k 4'i>^L ,ra_.. 14 h 5'9M- I Fig. 283. Cant Hook. scantling, or, preferably, a hardwood board in the form of a large paddle, is placed in the hole to serve as a rest for the butt of the pole while it is being raised. The use of this paddle prevents the crumbling of the earth which is sure to result and cause much trouble if this precaution is not taken. The tools required in raising poles of the average length from 30 to 50 feet are five or six pike-poles (Fig. 284), with , -12 Fig. 284. Pike Pole. handles ranging from 12 to 16 feet in length, and two dead men or pole supports, shown in Fig. 285. 372 AMERICAN TELEPHONE PRACTICE. The pole is raised slightly and its end slipped into the hole, resting all the while against the paddle or scantling. The small e o Fig. 285. Dead Man. end of the pole is then raised higher and the dead men placed under it, while the men obtain another hold. The pole is raised 2 X Vi IRON, 1 Fig. 286. Tamping Bar. gradually, the support being each time moved closer to the butt. When too high to be handled directly, the pike-poles are used Fig. 287. Raising Pole. on its upper part (Figs. 287 and 288), and in this way it is readily raised into a vertical position, slipping into the hole while bearing POLE-LINE CONSTRUCTION. 373 against the paddle. It is then braced by the pike-poles, as shown in Fig. 289, and turned by means of cant-hooks, so that the gains or cross-arms, if they were attached before raising the pole, are in proper position ; it being remembered that the cross arms should face each other on every alternate pair of poles. The hole is then filled in with the soil which was removed from it in digging, the soil being thoroughly tamped with tamping bars, shown in Fig. 286, from the bottom up. Great care should Fig. 288. Raising Pole. be taken that the shoveling in is not done so fast that the earth cannot be properly tamped. This is frequently the cause of much trouble, and, while it greatly expedites the erecting of the poles, it causes much loss of time and money later, on account of the poles giving way when placed under strain. If the soil is soft a foot-plate should be placed under the butt of the pole. This can be made by fastening together two 2* x 12" pieces of oak or hard pine, 2 or 2 feet long, at right angles to each other. In case the soil is very soft, as in marshy districts, more elaborate means will have to be taken. The hole should be dug in such places much larger than in ordinary instances, and a larger foot- plate may be inserted. A good plan, under these conditions, is to place in the bottom of the hole a layer, 6 inches deep, of 374 AMERICAN TELEPHONE PRACTICE. concrete, and, after raising the pole, filling in the entire hole to the surface of the ground with the same mixture, thoroughly tamped into place. For this purpose, and for other ca , LEAD 1 "BdANCH CABLE e t 1 ' J .1 f t)- * 1 ; i * ! ! 3 82 AMERICAN TELEPHONE PRACTICE. taking off a branch lead from a main lead. This is shown in the upper left-hand corner of Fig. 297. The use of cable for corner and branch work frequently saves much complexity in difficult places, leaving the work much more open and clean. After about a mile of poles have been set and guyed, and the cross-arms, pins, and insulators put in place, the process of stringing, where but a few wires are to be run, consists in plac- ing the reels on hand barrows, as is shown in Fig. 298, or on a Fig. 298. Hand Barrow. cart, and paying them as they go, drawing the wire up to each pole separately. When, however, a larger number of wires are to be run the method is briefly as follows : The separate coils of wire are placed on spindles at the beginning of the stretch to be strung and each is attached to a hole in a " running board," which is of about the same dimensions and has the same spacing as a cross-arm. To the center of this running board a " running rope " is attached and this is placed on the top of all the cross-arms in the stretch. A team of horses hitched to the other end of the rope then " walk away " with it. A man is stationed on each pole in order to lift the run- ning board over the top of each pole or to properly guide it around. After the wires are all in place each one is separately pulled up to the proper tension, and at a given signal is tied to the insulator at each pole. Two distinct methods are used for securing proper tension. In each case the force is applied by attaching a wire clamp, commonly known as a " come-along," shown in Fig. 299, and pulling it up with a block-and-tackle or by hand. In one method the proper degree of tension is obtained by the use of the dy- namometer, which is merely a form of spring balance. The tension depends on the kind and size of wire, on the distance POLE-LINE CONSTRUCTION. 383 between the poles, and on the temperature at time of the string- ing. The amount of tension put on each wire is usually taken as about one-third the breaking strength of the wire, which may be found from the wire tables. The other method is to allow a certain sag or distance between the center of the span and the straight line between the points of support. Table XII., which Fig. 299. Come-along. is taken from Roebling's handbook on " Wire in Electrical Construction," gives the sag in inches for the various lengths of span at different temperatures, these figures being based on the use of good hard-drawn copper wire. TABLE XII. AMOUNT OF SAG IN SPANS. .s ^ Spans in Feet. ii g&< 75 IOO 115 130 150 200 ill Sag in Inches. -30 IO i ii 2 3 31 3f 4i 5 8 q 10 ij *l 3* 4f 5l 30 3 4 s! 12 60 H 4i si 7 9 5f 80 3? 5f 7 8f nj i8| IOO 4 7 y ii 14 22^ It is the practice of a certain company using forty poles to the mile to allow on either copper or iron wire a three-inch dip or sag at the center of spans in the eastern portion of the United 384 AMERICAN TELEPHONE PRACTICE. States and an eight-inch sag in the western portion. The reason for the difference in the allowable sag in the East and in the West is due to the fact that far greater variations in temperature occur in the West than in the East. Two patterns of climbers are in general use, known respec- tively as the Eastern and the Western climbers. In the East- Fig. 300. " Western " Climber. Fig. 301. " Eastern" Climber. ern the strap-bar passes up the inside of the leg, and in the West- ern it is on the outside. These are shown in Figs. 300 and 301. The tying of wires to the insulators is an important matter, and there are several different methods of doing it. The ordi- nary method, used almost since the beginning of line construc- tion, is shown in Fig. 302. In this the line wire merely passes Fig. 302. Ordinary Tie. along the side of the insulator and should not be bent, being held in the groove by a tie wire, twisted around it in opposite direc- tions at each end as shown. The tie wires are, as a rule, about sixteen inches long, and made of slightly smaller diameter than the line wire itself, especially in case of very heavy wire. Another method, known as the Helvin tie, is shown in Fig. 303. This has been used with considerable success with hard- POLE- LINE CONSTRUCTION. 385 drawn copper wire. In this the tie wire is first wrapped around the insulator and twisted once or twice on itself, after which the ends are twisted around the line wire as before. Still another method of tying the wire to the insulator is shown in Fig. 304. In this, as in the first method, the Fig. 303. Kelvin Tie. line wire is laid in the groove of the insulator and the tie wire is passed entirely around the groove, one end pass- ing down over the line and the other end up under it, the twist being made as shown. This tie is perhaps the best of all where properly made, and is now much used in telephone work. Where a wire is dead-ended it is simply passed once around the insu- Fig. 304. Latest Method of Tying. lator and twisted several times upon itself, the twist beginning at a distance of about two inches from the insulator. In the case where transpositions are to be made the free end of the wire should be left long enough to pass over and make connection with the other side of the circuit. The joining of wires is a matter which has received much at- tention. The old style of joint, and one which gives much satis- faction, is shown in Fig. 305. This is known as the Western 3 86 AMERICAN TELEPHONE PRACTICE. Union joint, and is made by placing the two ends side by side and clamping them with a hand vise or with a heavy pair of pliers. With another pair of pliers, held in the right hand, Fig. 305. Western Union Wire Joint. the free end of each wire is twisted tightly around the other wire, as shown. Another method of joining wires, known as the Mclntire sleeve joint, is shown in Fig. 306. The sleeve for making this joint Fig. 306. Mclntire Sleeve Joint. consists of two copper tubes soldered together and having a bore corresponding to the sizes of the wire to be joined. The ends of the wire are passed in opposite directions through these tubes and are then grasped at each end with a special tool for the pur- pose and given three distinct twists. This joint is now widely used in practice and is very convenient because the use of solder is not required in order to make it perfect. Still another connector, known as the Lillie joint, is shown in Fig. 307. The connector in this consists in a sheet of copper curved longitudinally in opposite directions. The wires are Fig. 307. Lillie Wire Joint. slipped in each curve of the strip and twisted in opposite direc- tions, as in a Mclntire joint. This joint has not come into such extensive use as the Mclntire sleeve, but should prove efficient. Fig. 308 shows how this sleeve may be applied in taking off branch wires, as in the case of attaching bridging telephones to a line. POLE-LINE COXSTR L'CTION. 387 Practice differs somewhat among construction men as to the matter of soldering wire joints, some claiming that the solder Fig. 308. Branch Wires with Lillie Joint. joint gives no better results either as to conductivity or strength than unsoldered ones. The best practice, however, dictates the use of solder on all except the patent sleeve joints. In soldering a Western Union Fig. 309. Transpositions. joint, it is well to apply the heat only at the center of the splice. It is sufficient to solder the joint at its center, and the danger of weakening the line by the annealing effect of the heat is reduced. The method of making transpositions is shown in Fig. 309. AMERICAN TELEPHONE PRACTICE. For this purpose transposition insulators having two grooves may be obtained. In making transpositions a good, though more expensive, way is to use double cross-arms at the transposition poles, dead-ending the wires on each, and bridging across by bridle wires in much the same manner as shown. It is frequently necessary to run a telephone line on the same poles with a high-tension power circuit. Induction from the U 18- f Q- n^- 1 V , A 3A ~ ^ L IB WIRE. '0 i E,. YELLOW p'.NE ^ r\ JllTlK Fig. 310. Telephone Line and Power Circuit. power wires is of course under these conditions very likely to render conversation impossible, especially if the current in the power circuit is alternating. Fig. 310 shows the details of a pole thus equipped, the two insulators on brackets being for the telephone line. The latter should be of No. 12 B. & S. copper, and transposed every three poles. In this way a fairly quiet line may be obtained under the most unfavorable circum- stances. CHAPTER XXXI. OVERHEAD CABLE CONSTRUCTION. THE tendency of good telephone practice in cities is to bunch the line wires following the same route into cables, and it may be added that there is also a strong tendency toward the placing of these cables underground, this latter being due in large measure to the protests of the public against all overhead elec- trical construction. Overhead cables are, however, used to a large extent, and there will always be conditions under which their use will be found advantageous. The overhead cable presents many advantages over the use of bare wires. Besides the fact that in many districts it would be absolutely impossible to handle the required number of wires without the use of cables, on account of lack of space, may be mentioned the following : The lines are rendered far more sightly and offer much less obstruction to firemen in the performance of their duties, for two hundred or more wires, which alone, if bare, would require the use of a pole line carrying at least twenty ten- pin cross-arms, may be crowded into a cylindrical space not over two and a half inches in diameter ; the danger of crosses from high-tension or other wires is greatly reduced ; the liability to injury in heavy wind and snowstorms is lessened, and the cost of construction is in many cases greatly cheapened. In regard to the latter point comparative cost of construc- tion Table XIII., compiled by the Standard Undergound Cable Company, and based upon average prices for material and labor, is of great interest. From this it will be seen that while the bare-wire construction may be somewhat cheaper for lines carrying fifty wires or less, the cables have the advantage in this respect when one hundred or more lines are carried. In the early days of telephony rubber was considered the best insulating material for the wires in cables. A cable so con- structed is still largely used by the British post-office system. It is constructed as follows : The conductors are each composed of three strands of tinned copper wire, having a size corresponding to No. 24 B. & S. gauge. These three together form a single 390 AMERICAN TELEPHONE PRACTICE. TABLE XIII. COMPARATIVE COST PER MILE OF OVERHEAD WIRES AND CABLES. (jj Poles to the Mile.) Overhead Wires, Bare, Materials, etc. 50- Wire Line. 40-Foot Poles. loo-Wire Line. 45-Foot Poles. 2oo-Wire Line. 6o-Foot Poles. Poles, Cedar $111 .2*. $ 166 25 $Ae.e. oo Poles, Setting 28 oo 11 CO Cross- Arms (10 pins) 61 25 122 50 Cross-Arms, attaching to poles 1 7. co 1C. OO 70 oo Braces and Screws 60 I 2O Pins (i j inch Locust) 17 CO 1C OO 2.6O C 2O 10 40 Insulators 21 OO 42 oo 84 oo Insulators, attaching to pins I . CO 3 oo 6 oo No. 14 B. & S. Gauge Hard Drawn Cop- per Wire 4Q7 06 QOC Q2 1987 84 Labor Stringing Wire 2OO.OO 380.00 740 oo Total $Q7Q.l6 $I8I7.C7 $3708 64. LEAD-COVERED AERIAL CABLE. Thirty-five Poles (30 feet) $ 52.50 $ 52.50 $^2.^O Labor, Setting 24.50 24. co 24. 5O One Mile Galvanized Strand 2O. CO 20.9 ej 22 Stringing Same, Including Supports . . One-Mile New Standard Cable and In- stalling Same Complete 52.00 1214.40 52.00 1636.80 52.00 2428.80 Total 3>Il6l.9Q $1786.39 $2609.02 conductor weighing twenty pounds per mile and having a resist- ance of 45 ohms. Each conductor is covered with two coats of non-vulcanized rubber, after which they are taped with rubber- coated cotton and covered with ozokerite. The wires are then twisted together in pairs and the required number laid up into a cable and served with jute and wrapped with tape impregnated with bituminous compound. The whole core is then again coated with the bituminous compound, served with hemp soaked in a compound of gas-tar, and again treated with the bituminous compound. It is then served with an external coating of tape OVERHEAD CABLE CONSTRUCTION. 391 and a coating of silicated compound. This has been found to be a reliable cable and thoroughly water-proof. For short lengths rubber-insulated cable is often used in this country, and under certain conditions is preferable to the lead- covered paper-insulated cable which will be described later. The three-stranded conductor, however, is little used, a single No. 1 8 B. & S. guage tinned wire being used instead. These are double-coated with rubber and separately tested in water for in- sulation. After this they are covered with braid, bunched, and the core so formed covered with tarred jute, over which is placed a heavy braid saturated with so-called weather-proof compound. Table XIV., given below, shows the sizes and weights of the various sizes of this cable as manufactured by a prominent firm : TABLE XIV. AERIAL CABLE RUBBER-COVERED WIRES. - t/i M <*j en O u fe ^ 3 co "o 1| oL o l"a 1* s| 3 o3 fl _cu *J*-4- r ~ l ^ 3 5 10 6 10 20 ii 175 256 452 15 30 i 633 20 4 it 813 25 50 ii 994 Rubber cables are often incased in lead, in which case the rubber is made somewhat thinner and the braid over the indi- vidual wires and much of that over the entire bunch is omitted, because the lead affords protection both from mechanical injury and from the weather. Rubber-covered cables give excellent results as to insulation and durability ; but a serious objection to their use for telephone work is that their electrostatic capacity is very high. This is due to the fact that while rubber is a splendid insulator, its spe- cific inductive capacity is much higher than that of some other insulators. Dry air is the most desirable in this respect, its spe- cific inductive capacity being lower than that of any other known 39 2 AMERICAN TELEPHONE PRACTICE. substance. A great improvement in regard to the electrostatic capacity of cables has been brought about by use of paper insu- lation between the individual conductors. In the earlier forms of cables so constructed the wires were wrapped with paper, which was afterward impregnated with some insulating material, such as paraffin, having a low specific inductive capacity. It has been found by aerating the paraffin thus used with dry carbonic acid gas that the electrostatic capacity between the conductors was reduced as much as 15 per cent. In order to still further reduce the capacity what are known as dry-core cables have been introduced and have come into extensive use. These are usually formed by wrapping the separate conductors with two layers of dry paper loosely laid on. Sometimes only a single wrapping is used. The two wires which are to form a twisted pair are, after being separately wrapped, twisted together, the length of a com- plete twist being about three inches. Another way of forming a twisted pair is to lay the two wires upon opposite sides of a strip of paper and twisting the two together with the paper be- tween them. The pair is afterwards served with a single wrap- ping of paper, forming a complete tube around it. After the twisted pairs are formed, by whatever method, the desired num- ber of them are laid loosely together and covered with a lead sheath, usually one-eighth of an inch in thickness. The saturated-core cable may be formed in the same way, the difference being that the paper is impregnated with some insu- lating material before the lead sheath is put on. The saturated cable has the advantage of not being so suscep- tible to moisture as the dry core, but its electrostatic capacity is usually 15 microfarads per mile, or higher, while in the dry core capacities as low as .05 microfarad are said to have been at- tained. It is doubtful if this latter figure could be reached as an average, and specifications for dry-core cables usually require an average capacity of .080 per mile. So long as the lead covering remains intact no difficulty is experienced with the dry-core cable, but when a puncture is made moisture enters to a sufficient extent to greatly lower the insulation of the cable. If the dam- age is not quickly repaired a considerable length of the cable is apt to be injured, as the moisture finds its way quickly through the dry paper. For this reason, in small telephone exchanges not equipped with the proper means for testing out and repair- ing cables, the saturated core is most desirable. Where the requisite means are at hand for frequent testings the dry core is greatly to be preferred. OVERHEAD CABLE CONSTRUCTION. 393 The locating of faults in cables may be facilitated by specify- ing that one or two small rubber-covered wires be laid through the center of the cables, these wires afterward being reserved as test wires for use in the Varley loop test so often used in locating leaks. The size of wire used in telephone cables varies from No. 18 B. & S. gauge to No. 22, No. 19 being probably the most com- mon. Specifications usually state that the cable sheath shall be composed of an alloy of lead and tin, the amount of the latter be- ing not less than three per cent, of the entire mixture. This re- quirement has been made because it has been found that such an alloy is not so susceptible to chemical action as lead alone, an im- portant consideration in underground work. Much difficulty has been found in manufacturing them, however, to secure an even mixture of the lead and tin. The Standard Underground Cable Company are firm advocates of the use of a pure-lead sheath, afterwards treated with an external coating of pure tin, arguing that the tin when mixed with the lead makes the sheath brittle and that the tin will be most effective if all of it is placed on the outside. Notwithstanding this, it is customary, as stated above, to specify that the sheath shall be composed of the alloy. The use of braiding saturated with a moisture-proof compound placed over the lead sheath is often advocated. Opinions differ as to the advisability of this, but it is probable that its disad- vantages outweigh its advantages in either overhead or under- ground work. The locating of punctures in the sheath is made much more difficult by the use of this braid in overhead cables, for when the sheath is bare they may be often located by mere external inspection ; moreover, the braiding considerably in- creases the expense of the cable, and its only advantage is its prevention of abrasion. This need not occur if the cable is prop- erly supported. In underground work the braiding affords a pro- tection for the sheath during the drawing in process and may afford some protection against chemical action. After it rots, however, the pieces may so thoroughly clog up the conduit as to prevent the withdrawal of the cable, thus not only losing that length of cable, but rendering the duct in the conduit unavailable. Table XV. gives the outside diameter and the weight per 1000 feet of the various sizes of lead-covered paper cable manufac- tured by a prominent firm. The conductors are No. 19 B. & S., each being served with two layers of paper. 394 AMERICAN TELEPHONE PRACTICE. TABLE XV. AERIAL CABLE. Number of Pairs. Outside Diameter, Inches. Weights per 1000 Feet in Pounds. i 214 2 302 3 515 4 i 629 5 i 747 6 \ i 877 7 i 912 10 f 1214 12 ! -1 1375 15 i 1566 18 I"! > 1753 20 T i r: 1940 25 1ft 2232 30 35 40 i 2748 2985 3176 45 1^ 3365 50 i 3678 55 i i 3867 60 i 4055 65 i- 1 4241 70 2 4430 80 2- | 4804 90 2- 5180 100 2 \ 5505 Aerial cables are supported on steel rope stretched tightly be- tween the poles or other supports. This is necessary on account of the fact that the cable does not possess the requisite strength to support its own weight. For the heavier cables the messenger wire, as the supporting strand is called, is usually composed of seven No. 8 steel wires twisted together into a rope. Table XVI. gives the common sizes of messenger wire, together with their weights and breaking strengths : A special strand may be procured, the various sizes of which have about double the breaking strength given for the cor- responding sizes in this table. Table XVTT. is useful in determining the size of cable that any messenger wire can safely carry for any given length of span. By referring to the table giving the weights per 1000 feet of cable and, knowing the length of span, the size of messenger wire is readily determined. OVERHEAD CABLE CONSTRUCTION. 395 TABLE XVI. MESSENGER AND GUY WIRE. 7 Wires. No. Approximate Diam. in Inches. Weight per loo Feet. Tensile Strength in Pounds. 8 ft 52 8320 9 u 42 6720 10 ft 36 572o ii I 29 4640 12 ft 21 3360 13 16 2560 4 w 12 1920 15 1 IO 1600 16 8 1280 17 2 6 960 18 IT 4 T 8 TT 688 19 A 1 U 3 T 8 Tr 528 20 f 2 T% 384 21 A 2 320 TABLE XVII. SUPPORTING CAPACITY OF GALVANIZED STEEL STRANDS. Spans in Feet. 7 Wires. No. 01 100 no 1 20 125 130 140 150 175 200 WEIGHTS IN POUNDS OF IOOO FEET OF CABLE. 8 ft 28l8 2516 2263 2152 2050 1867 1709 1391 "54 9 if 2520 2247 2O2O I92O 1827 1663 1520 1234 1130 IO ft 2O30 1812 1630 1550 1476 1344 1230 IOOI 900 ii 1 1580 1409 1266 I2O4 1146 1043 953 774 640 12 ft mo 899 890 846 805 733 670 544 450 13 A 860 765 680 652 620 563 513 414 340 15 I 585 52i 468 445 423 385 352 280 235 16 A 433 385 346 329 313 284 260 2IO 172 17 ft 337 300 270 257 245 223 204 165 137 The messenger wire may be supported in several ways, one of which is to bolt a piece of angle iron to the pole and suspend one messenger wire from each of its ends. The wire may be sus- pended below the angle-iron cross-arm, or it may rest in a slot in 39 6 AMERICAN TELEPHONE PRACTICE, it. There are also several good forms of supports on the market, one of which is shown in Fig. 311. The cable is supported from the messenger wire in several different ways. Rubber-covered cable is frequently suspended Fig. 311. Messenger Wire Clamp. by binding it to the messenger wire by strong tarred marline. The marline is wrapped around both cable and messenger, usu- ally in two directions, to give greater security. The method now most extensively used in supporting lead-covered cables is by means of metallic clips or hangers, adapted to tightly girdle the cable sheath and provided with a hook to slip over the support- Figs. 312 and 313. Cable Hangers. ing wire. There are several good hangers, two styles of which are shown in Figs. 312 and 313. In attaching the one shown in Fig. 312 the metal strip which passes around the cable is first passed for a distance of one inch through the slot in the lower part of the hook. The other end is then bent around the cable OVERHEAD CABLE CONSTRUCTION. 397 and through the slotted key. The key should then be turned to the left until tight and then locked by driving it endwise until the ears on it engage in the star-shaped hole. The flexible strip in this hanger is made of zinc. The hanger shown in Fig. 313 is of malleable iron and is attached by bending it around the cable with a special tool. It is a good plan to place a piece of sheet lead T ^ inch thick, or a piece of leather or rubber hose, around the cable at the point where the hanger is to be applied ; but if this is to be done the additional thickness must usually be allowed for in ordering the hangers. It is well, in ordering cables, to specify that it shall be placed upon the reels in such manner that both its ends are accessible without unreeling it. Where this is done it is an easy matter to Fig. 314. Running up Cable. make tests for continuity of the conductors and for insula- tion resistance and capacity before the cable is unreeled, and thus any defects which may exist will be known to be the fault of the manufacturer or the transportation company. When the cable arrives both of its ends will be sealed to prevent the entrance of moisture, and, after testing, the ends should be carefully re- sealed in a manner which will be described later. The ordinary method of hanging cables is shown in Fig. 314, which is taken from Roebling's " Handbook on Telephone Cables," as are several of the succeeding cuts illustrating the method of splicing. The end of the supporting strand, after passing over the last clamp or cable cross-arm, D, is firmly secured to a guy-stub driven in the ground at A. The reel on which the cable is coiled is placed in line with the messenger wire, and a few feet beyond the stake, as 39 8 AMERICAN TELEPHONE PRACTICE, shown. One or more grooved pulleys, C C, mounted as shown, are placed between the reel and stake in such manner as to sup- port the cable as it is paid out. A stout rope, or better a small wire cable, is previously hung on pulleys or hooks below the cross- arms of the entire stretch over which the cable is to be drawn. One end of this is attached to the end of the cable, while the distant end is attached to a capstan or other form of windlass. As the cable passes over the rollers, C C, the hangers are at- tached and are placed one by one upon the inclined messenger wire as they reach the point, B. As the cable progresses line- men stationed on each pole lift the hangers over the messenger wire clamp or cross-arm as they pass. In this way the entire length of cable is drawn up to and along the stretch without subjecting any portion of it to an undue strain. The hangers are usually attached at distances of from twenty-four to thirty inches, according to the size of the cable. The work is somewhat expedited if, during the drawing up of the cable, only about every fifth hanger is hooked over the messenger wire. This reduces the labor of the linemen in lifting the hangers over the support. When, however, the forward end of the cable reaches the begin- ning of the last span, the signal should be given to all linemen stationed on the poles to hook on all of the hangers as they pass, and in this way all of the hangers will be secured in place through- out the entire stretch without going out over the line afterwards. This method is subjected to one disadvantage in that the slid- ing of the hanger hooks along the messenger wire tends to loosen them on the cable, sometimes to such an extent that several of them become bunched at one point on the cable. A method for overcoming this disadvantage, and also for expediting the work, has been devised by Mr. F. S. Viele of the Standard Under- ground Cable Company. In this, carriers, each consisting of a small grooved roller with a hook below it for engaging the cable, are placed upon the messenger wire, and serve to support the cable at frequent intervals instead of the hangers during the proc- ess of stringing. At each cross-arm a small switch or side track is placed upon the messenger wire, which serves to displace the car- rier rollers far enough to clear the messenger wire, and then to guide them down under the cross-arm and again up on the messen- ger wire. These side tracks are about three feet long and may be readily attached or detached from the messenger wire. When the forward end of the cable reaches the beginning of the last span of the stretch a man is sent up each pole to place the hang- ers on the messenger wire and remove the carriers as they pass, OVERHEAD CABLE CONSTRUCTION. 399 thus leaving the entire cable permanently suspended when the forward end reaches its destination. It is always well to leave sufficient slack in aerial cables at fre- quent intervals to allow for subsequent splicing in case repairs are needed. This slack, moreover, frequently saves a cable from se- rious injury when the pole line is subjected to some severe strain which the cable, if unable to give, would not be able to bear. At- tention to this point will often prevent the necessity of splicing in a new piece in the middle of a cable, due to insufficient length for making an ordinary splice. Where it becomes necessary to splice a cable the greatest care should be taken that no moisture be allowed to enter while the splice is being made, and that the splice shall be so thoroughly sealed at the end of the operation that there will be no possibility of the subsequent entrance of moisture. A suitable staging should be erected on the pole where the splice is to be made, if Fig- 315. Cable Prepared for Splicing. it is possible to bring the splice within reach of the pole. This can always be provided for in new cable, but sometimes in repair- ing a leak it is necessary to make these splices from a car sus- pended from the messenger wire. When all is ready the lead sheath of each end of the cable to be spliced should be cut away for a distance of about twelve inches, the ends of the cable hav- ing previously been sawed off square. Boiling paraffin, heated in a large pan on a portable furnace, should then be ladled over the ends of the wire to prevent, as far as possible, any moisture from the atmosphere from entering the cable and also to prevent the untwisting of the paper insulation, which, in a dry-core cable, often gives considerable trouble during the operation of splicing. A lead sleeve, consisting of a lead pipe about two feet long, and of a slightly greater internal diameter than the external diameter of the cable sheath, should then be slipped over one end of the cable and back several feet out of the way. A paper sleeve should then be slipped over each wire of each pair. The pairs in paper-covered cables are usually colored red and white, and as a matter of convenience a paper sleeve should be slipped over all of the red wires on one end of the cable and one of the white wires on the other. This brings the cable ends into the condi- 400 AMERICAN TELEPHONE PRACTICE. tion shown in Fig. 315. The corresponding wires of each pair are then skinned for a short distance and twisted together, as shown in Fig. 316, and after a number of pairs are so joined all Figs. 316 and 317. Splicing a Pair. joints should be carefully soldered, particular pains being taken to use no acid flux. Tubular solder provided with a rosin flux inside is convenient for this, or the grease from a tallow candle is perhaps better yet. With a rather large soldering iron these joints may be soldered almost as fast as the iron can be touched to the wire. After soldering, the twists should be bent down as shown in Fig. 317, after which the paper sleeves are slipped over the bare portion of the wires, leaving the completed splice, as shown in Fig. 318. The joints in each pair should not lie opposite Fig. 318. Finished Splice on Pair. each other, and within the space allowed between the ends of the cable sheaths all joints should be staggered as much as possible so as to prevent the formation of a large bunch at any one place. Another point to be remembered is to guard against joining any good wires in one cable to wires known to be bad in the other. Before making these splices all wires should be tested out and the bad ones tagged. Obviously, if a good wire in one length of the cable is attached to a bad one in another, that wire is unavilable for use in either cable. After all of the wires are spliced and Fig. 319. Boiling Out. covered by paper sleeves the cable should be carefully "boiled out," this process being shown in Fig. 319 and consisting in ladling boiling paraffin over the joint until all traces of air bub- bles in the hot paraffin disappear. This portion of the work OVERHEAD CABLE CONSTRUCTION. 401 should never be slighted, as it is one of the most important in the entire operation. After the ''boiling out " process is completed a plain strip of Fig. 320. Finished Cable Splice. white cotton should be wrapped over the splicing, after which the joint should again be boiled out. The joint is now ready for the Fig. 321. Moon Cable Head. services of a plumber, and upon his work much depends. The section of pipe should be slipped over the splice before it has had time to cool, and the sleeve thoroughly wiped to the cable 402 AMERICAN TELEPHONE PRACTICE. sheath at each end by the ordinary method used in joining lead pipes. The finished joint presents the appearance shown in Fig. 320, and when such a joint is properly made that portion of the cable should be practically as good as any other portion. Whenever it is necessary to leave a cable end exposed all moisture should be expelled by boiling out, after which the end of the sheath should be sealed by a wiped solder joint with as much care as if it were to be a permanent affair. If it is suspected that moisture has entered the end of a cable a short length of it should be cut off and dipped into boiling paraffin, when the presence of moisture will be indicated by the rising of bubbles in the hot Fig. 322. Fused Terminal Block for Moon Head. fluid. If there is room to spare the cable should be cut back, a short length at a time, until it gives evidence of being dry, but if this cannot be done the sheath should be heated with a torch, beginning at a point several feet from the end, and working gradu- ally toward the end, so as to expel the moisture. After this the ends should be thoroughly boiled out and a splice made as already described, or, if this is not to be done, the end should be sealed. Where a cable terminates means must be provided for distribut- ing its various wires and connecting them to the wires forming parts of the same circuits. For this purpose what are termed cable terminals or cable heads are used, several forms of which are on the market. These usually consist of iron boxes, inside of which are arranged terminals for the wires in the cable. The lower parts of these boxes are usually provided with a brass tube or sleeve adapted to fit over the cable sheath, after which it is secured thereto by a plumber's wiped joint, thus hermetically sealing both OVERHEAD CABLE CONSTRUCTION. 403 cable head and sheath at that point. The wires of the cable are fanned out to the terminals within the box, which terminals are usually connected through water-tight insulating bushings with their terminals outside of the boxes. After the various connec- tions are made within the box a cast-iron cover is screwed in place, the joints being hermetically sealed by a rubber gasket. On the outside of the boxes are usually provided lightning arresters for Figs. 323 and 324. Cook Pole Top Terminal. each line, the circuit being completed from the inner connectors through these arresters to the outer wires. One of these, known as the Moon cable head, is clearly shown in Fig. 321, in which several wires are shown extending from the tube below the box to one of the terminals within. In Fig. 322 is shown a fused termi- nal for use with the Moon head. The bushing which serves to lead the terminal pin through the iron casing also carries a brass lug between which and the pin is placed the fuse, mounted on a hard-rubber block as clearly shown. Another form of cable head 404 AMERICAN TELEPHOiVE PRACTICE. which is becoming very popular is known as the Cook pole-top terminal, views of which are shown in Figs. 323 and 324. In this, which is the invention of Mr. Frank B. Cook of the Ster- Fig. 325. Pole Equipped with Cook Terminal. ling Electric Company, the cable is led up within the cast-iron box, forming the framework for an entire terminal, the various wires being fanned out to conductors arranged in circular rows around the inside of the box. Connection is made through suit- able air-tight plugs with outside circuits, the line wires being fused at the insulator, as shown at the left-hand lower portion OVERHEAD CABLE CONSTRUCTION. 45 of Fig. 324. Before screwing on the cover several lumps of un- slacked Lime are placed within the box, after which the cover is screwed on, being hermetically sealed by rubber gasket as in the other form described. This lime absorbs any slight mois- ture which may be in the box at the time, and the fact that it has in many cases remained unslacked for years proves conclusively that these terminals may be made perfectly moisture-proof. Af- ter all connections are made a sheet-iron cover is placed over the entire terminal. This terminal, as its name implies, is placed at the top of the pole in a manner shown in Fig. 325. A much less expensive method of terminating cables than any so far described consists in the use of what are termed pot-heads, and while these present a somewhat homely appearance they are very effective, and are used to a large extent by many of the Bell and other companies. There is no doubt but that a pot- head terminal, properly constructed, forms as reliable and service- able a terminal as any, it having the additional advantage of being far cheaper than any of the others. The directions for making these, together with the description of all material, are given in the following specifications, which are those of one of the leading Bell companies. POT-HEAD TERMINALS. Materials. Lead Sleeves of unalloyed lead \ inch thick of the following dimensions : For loo-pair cable ; length, 24 inches, inside diameter, 3 inches. " 50 " " " 20 " " " 2 " .-, - 20 " " " 2 " Drift out the sleeve for one-half its length until its diameter is increased \ of an inch. Okonite Wire, twisted pair, red and black No. 20 B. & S. gauge, ^-inch insulation, without braid or outside covering. Okonite Tape, inch wide. Paper Sleeves, boiled in paraffin just before using. Brass Tubing thin, inch in diameter, length 2\ inches less than that of lead sleeves. Heavy Cotton Twine, or wicking. Wiping Solder, containing 40 per cent. tin. Splicing Compound, as furnished for the purpose by the com- pany. Do not mix the compound with other materials. 40 6 AMERICAN TELEPHONE PRACTICE. Directions. Remove the cable sheath for fifteen inches, slip the lead sleeve over the cable, splice the cable wires to okonite wire in the usual manner, join the colored wire of the cable to the red Fig. 326. Cable Terminal Box with Balcony. okonite of each pair, cover each splice with a sleeve, and keep the splices within a limit of thirteen inches from the end of the cable sheath. Remove all pieces of paper or other debris, bind the cable wires as they leave the sheath tightly with several layers of twine to prevent the compound entering the cable, tape all the okonite wires together for three inches in such a manner that one- half inch of the taped wire will be below the surface of the com- OVERHEAD CABLE CONSTRUCTION. 407 pound. Open up the spliced wires as much as possible to allow free spaces between, bind the brass tube with twine alongside the wires with the lower end even with the end of the sheath of the cable, binding no higher than the wire splices ; draw up the lead sleeve until its lower end laps over the cable sheath i inch, and wipe it to the sheath. Secure the whole in an up- Fig. 327. Interior Cable Heads. right position, warm the lead sleeve until it can barejy be touched with the hand, place a funnel in the brass tube, and slowly pour in the sealing mixture, previously heated to 350 F., until it fills the sleeve to within one-half inch of the top, remove the funnel and allow the compound to settle and cool. Test the compound just before using by putting in a short piece of okonite wire for two minutes. If the okonite is not softened so as to readily come off the wire the compound is not too hot. Protect the open end and the wires leading therefrom against the 48 - AMERICAN TELEPHONE PRACTICE. weather. On the next day fill with hot compound to make up for the settlement. Three days later do the same if necessary. After this, and when thoroughly cold, dress the top of the lead sleeve into contact with the okonite tape wrapping, which at this point should consist of at least four layers. Place a cross on the outside of the lead sleeve at a point opposite the upper end of the brass tube. Caution : Do not boil out the cable end with paraffin. If dampness enters, the cable should not be used until this defect is remedied or the part cut away. Under no circum- stances must paraffin be used on okonite ends. The okonite paired wires shall project above the end of the flexible end for a distance of : For 100 pairs cables 3 feet. Cable heads, whether of the ordinary iron or the pot-head type, when placed on poles are usually inclosed in a wooden box of suitable dimensions for allowing connections with the external circuits. These boxes should be provided with hinged doors and made as nearly water-tight as possible. Directly below these boxes should be erected a balcony upon which the workmen may stand when making connections for testing the various lines. Such construction is well shown in Fig. 326. In Fig. 327 is shown a row of eight cable terminals used in terminating as many one hundred pairs of cables within an ex- change. These particular terminals are the product of the Ster- ling Electric Company, and are provided with combined sneak current and carbon arresters for each line. Where the lightning and sneak-current arresters are placed directly on the distributing-boards, expensive office cable-heads may be entirely done away with by splicing on to the end of each cable leading into the exchange, a length not shorter than thirty feet of lead-covered, wool-insulated cable, the splice being made in the ordinary manner and sealed with a lead sleeve. This is preferably done in the cable vault, whence the wool cable is led directly to the distributing-board and fanned out in the usual manner. Wool cable, tightly compressed into its lead sheath, has been found to be non-hygroscopic to such an extent as to allow no moisture to enter the paper-cable through it, and to retain its own insulation at a high standard when used for inside work. This method is the latest practice of some of the principal Bell companies. CHAPTER XXXII. UNDERGROUND CABLE CONSTRUCTION. THE tendency at present is to place all telephone wires in cities underground, and the primary requisite for this construc- tion is that a suitable conduit shall be provided in which the con- ductors may be laid. It is usually necessary to provide conduits having a suitable number of ducts to meet the requirements for future as well as immediate use, and much judgment should be exercised in this respect in planning the system. Suitable openings are provided for the conduits at frequent intervals, these being in the form of man-holes, from which sections of the cables may be drawn into the ducts and withdrawn when occasion requires for repairs. The principal requirements for a good con- duit may be outlined as follows : The material of which the conduit is made must be durable, and this implies that it must be absolutely proof against decay or corrosion due to moisture, dry rot, gases, or the liquids present In the soil. It should, moreover, be fire-proof if possible, al- though this is a minor consideration. The conduit should possess both tensile shearing and crush- ing strength. Severe vertical strains are frequently imposed upon subway structures, due to the removal of the support from be- neath them, caused by excavations in the streets or by the settling of the ground. Side strains are not so likely to occur, and their effects are usually slight ; therefore, it follows that the conduit should be, if possible, strongest in a vertical direc- tion. If the stress imposed upon the structure is such as to cause a fracture or undue settling, the alignment of the ducts is thereby destroyed, which may interfere with the drawing in or with- drawal of cables. Moreover, the grade of the duct is destroyed, so that the proper drainage cannot be effected. The ducts be- tween man-holes should be straight if possible, and where curves are necessary they should be very gradual and present no sharp cor- ners which would interfere with the drawing in or seriously abrade the cable sheath. Slight turns in conduits are frequently made by joining together short straight sections, but where the nature of the conduit used permits it, it is better to form all bends of 4 10 AMERICAN TELEPHONE PRACTICE. curved sections. It is desirable that the structure should be com- posed of insulating material and be moisture-proof. No depend- ence, however, for insulation of the conductors themselves must be placed on the conduits, as the cables must in all cases provide the means for keeping the conductors thoroughly insulated and free from moisture, even under the most adverse circumstances. Even the most perfectly constructed conduits cannot be kept dry, on account of the sweating of their interior walls. It is very essential that the conduit must contain no chemical agents capable of exerting a deleterious effect on the cable sheath. As an example of this may be mentioned certain forms of wooden conduits, which in the process of decay liberate acetic acid, which in a short time totally destroys the cable sheath, changing it to lead acetate. This difficulty has been experienced with some forms of creosoted wood conduit, but in the later products in this line this difficulty is said to have been com- pletely removed by the use of a better grade of creosote oil and improved methods. Economy of space is often an important item in the selection of conduit to be used, and under crowded conditions that con- duit which will place a given number of ducts within the smallest space is the most desirable, other things being equal. The earliest form of conduit used in this country was the open-box conduit, which consisted merely in a trough made of inch-and-a-half or two-inch lumber and of sufficient size to accommodate enough cables to meet the existing demands, as well as the future growth of the system. These troughs were laid in a trench, the bottom of which was properly graded, the sections of the trough being about fifteen feet in length and butt-jointed that is, laid together end to end. The joints were held in line by boards nailed on the outside and overlapping each end about a foot. The cable was laid in these troughs by driving the reel containing it slowly alongside of the trench, the cable being carefully laid as it was unwound from the reel. After all the cables were in place the trough was filled with hot pitch, when the cover was nailed in position and the trench refilled. This is probably the simplest form of under- ground cable construction, with the exception of a method some- times practiced in Europe, of laying the cable directly in the ground without any conduit whatever. The cheapest and simplest form of conduit which permits the drawing in or withdrawal of the cables is that composed of creo- soted wood tubes, or " pump logs," as they are commonly UNDERGROUND CABLE CONSTRUCTION. 411 and appropriately termed. These are usually made in eight- foot lengths, having a square external section 4^ x 4^ inches, with a 3-inch bore. A tenon joint one-and-one-half inch long is used for securing proper alignment of the joint. Several views of this tube are shown in Fig. 328. The wood is treated with creo- sote or dead oil of coal tar in the following manner: The lum- ber is laid on cars and run into a large steel cylinder six feet in Fig. 328. Creosoted Wood Conduit. diameter, which is closed by a heavy iron door. It is first sub- jected to live steam at a temperature of 250 F. until the timber is heated through and through, the purpose of this being to co- agulate the albumen in the sap. A vacuum pump is next applied to the tank, exhausting all air and steam, the pump maintaining a vacuum of about twenty-six inches. This evaporates practi- cally all of the sap and water from the wood, thus seasoning the timber. The next step in the process is to pump creosote oil pre- viously heated to a temperature of 100 to 125 F. into the tank until it is full. This is then placed under a pressure of about eighty pounds per square inch and the amount of creosote which is forced in after the filling of the tank is carefully measured, this being the amount that is taken up by the pores of the wood. ' Specifications for the treatment require that from eight to twenty pounds of the oil shall be absorbed by each cubic foot of timber. Twelve or fifteen pounds is the average amount re- quired for electrical purposes. As has been stated before, much trouble has existed owing to the liberation of acetic acid from conduit treated with creosote. It is claimed, however, that by using a proper quality of creosote oil, and by using the method of impregnation just described, that this trouble has been entirely eliminated. The life of creosoted wood conduit is, to say the least, problematical, but there seems to be good reason to believe that when properly treated and laid it will last an ordinary life- time, if not longer. In laying this conduit the trench is dug to a 412 AMERICAN TELEPHONE PRACTICE. sufficient depth, and after its bottom is properly graded so as to have a gradual slope either from an intermediate point toward both man-holes or an uninterrupted slope from one man-hole to the other, a creosoted wood plank two inches thick is laid as a foundation. The ducts are then laid on this plank side by side and in as many different layers as are necessary to give the re- quired number. They should be so laid that the separate ducts break joints in order to give strength to the entire struc- ture. Over the upper layer is then laid another creosoted wood plank two inches thick, after which the trench is filled in with earth. The great point in favor of this conduit is its cheapness, this being greatly enhanced by the fact that no concrete is em- ployed for a foundation. Conduits of clay or terra-cotta, burned hard and with vitrified surface, are being extensively used and are giving unqualified satisfaction. These are made up in a number of forms which maybe divided into two classes, namely, multiple duct and single duct. The multiple-duct conduit is made up in a variety of ways, some of which are shown in cross-section in Figs. 329, 330, Fig. 329. Multiple Duct Conduit. and in the upper portion of 331. The sections of conduit shown in Fig. 329 usually have a cross-section 10 x 10 inches and a length of three feet. Similar tiles are frequently used having six or eight ducts, each about 3^x3^ inches square, the tiles varying from three to six feet in length. Frequently the ducts are made large enough to accommodate several cables, but this has a decided disadvantage, owing to the fact that much trouble is experienced in withdrawing cables under these conditions. The successive lengths of these tiles are joined by wrapping them with burlap previously dipped in asphalt. This makes the joint tight, main- tains its alignment, and prevents entrance of dirt during subse- quent operations. The conduit shown in Fig. 330 is valuable only where multi- ples of four ducts are required. Each tile is essentially a trough, open on top and having three intermediate partitions, thus form- ing four ducts, each of which is 3f inches wide by four inches high, the walls being one inch thick. These are made in two-foot lengths, and are laid in concrete as shown, one section being laid directly on top of another until the required number of layers are UNDERGROUND CABLE CONSTRUCTION. 413 formed. The top of the upper section is composed of a sheet of mild steel of No. 22 B. W. G., having its edges bent down so as to lap over the sides of the tile. The tile shown in the upper portion of Fig. 331 was designed by Mr. H. W. Johnston of St. Louis for the purpose of distribut- vS^^^^^^^^ : '^ ;#M & w w* m&- ^mmmmmmsm^ ii c ?/-"> '* V/' (fcVS*^tZ&':8S;:A '^^^il Fig. 330. Four-Duct Terra-Cotta Tile. ing the wires from the main cable, running to a certain section, to the various lateral ducts. The lower central duct is for the main cable running to the man-hole at the center of distribution. From this it branches out through a junction box to the distrib- uting wires, the two four-inch openings on either side being for Fig. 331. Johnston Distributing Duct. these wires or cables, which are led out through holes in the side wall of the duct to the lateral ducts, consisting of three-inch iron pipe. The upper central duct is for rodding and drawing in of additional wires in the side ducts. A runner is drawn through the central upper duct with arms projecting over the side ducts, and by means of these projecting arms new wires may be drawn 414 AMERICAN TELEPHONE PRACTICE. into the side duct without in any way disturbing those already in place. The single-duct class of tiles possesses some advantages over the multiple-duct tiles, chief among which are the greater flexibil- ity and the increased ease of handling. The form shown in Fig. 332 has come into very wide use and has proven its adaptability Fig. 332. Single-Duct Conduit. to meet almost any conditions that may arise. These tiles are 4| inches square by 18 inches long, and have a 3^-inch bore. By it curves are easily made, short curved lengths being provided, or curves of long radius may be made with the regular tiles, the lengths being so short as to form a practically smooth interior surface. This conduit is laid in much the same way as ordinary brick, and in order to insure proper alignment a mandrel (Fig. 333), three inches in diameter and about thirty inches long, is laid in Fig- 333- Mandrel. the duct and pulled along through it by the workmen as each ad- ditional section is laid on. The rear end of this mandrel is pro- vided with a rubber basket a little larger than the diameter of the conduit, which effectually smooths the inner surface and pre- vents the formation of lips which might prove injurious to the cable sheaths in drawing in. On the front end of the mandrel is provided an eye which may be engaged by a hook carried by the workmen in order to move it forward. Fig. 334 is a photograph showing a 48-duct subway in process of construction. In laying vitrified clay tile of any description the process to be used is as follows : The trench is dug to such a depth as to allow at least two feet of earth above the top of the entire structure. Some UNDERGROUND CABLE CONSTRUCTION. 415 specifications call for as great depth as three feet, but this is nec- essary only where there is a probability that new ducts may be added to the conduit in the future. The width of the trench should be at least eight inches in excess of the actual width of the number of ducts which are to be laid side by side. In the bottom of the trench is laid a concrete foundation to a depth of from three to six inches four inches is under ordinary circum- stances sufficient. The tiles are then laid in place in cement mortar, and as each layer is finished the sides of the trench should Fig. 334. Forty-Eight Duct Subway, Cleveland, Ohio. be filled to the top of that layer with the same concrete as that used for the foundation. The space between the tiles in a layer and between the layers should be carefully filled with good cement mortar mixed thin enough to readily fill the interstices. After the required number of layers are in place the top is covered with a mass of concrete not less than four inches in thickness. The concrete used in this work should be composed of one part of hydraulic cement, two parts of clean sharp sand, and five parts of broken stone, screened gravel, or broken brick. The size of the broken stone or brick or gravel should not be larger than one inch in any dimension. The cement and sand should be thoroughly mixed while dry, and then enough water added to form a soft 416 AMERICAN TELEPHONE PRACTICE. mortar, after which the broken stone should be thoroughly mixed in. The mortar should be composed of one part of hydraulic ce- ment and two parts of clean sharp sand, thoroughly mixed to- gether, and then with water as before. It is a matter of greatest importance that the ducts should not be moved while the mor- tar or concrete is setting. After the entire subway is laid from one man-hole to the other it is advisable to draw through it a scraper, thus removing all projec- tions on the inside walls. The ducts may then be washed out with a hose, thus removing all grit and leaving a clean, polished tube. Another style of conduit is the cement line pipe, which has also proven itself to be thoroughly reliable in all respects. This, as usually constructed, consists of a wrought-iron pipe of No. 26 B. W. G., with riveted joints, the rivets being set one and one-half inch apart. This pipe is lined with Rosendale cement, the thickness of the lining being five-eighths of an inch, and the interior of the lining being polished. The standard size of this tube is in eight-foot lengths, with a three-inch bore. It is pro- vided with cast-iron ball and socket joints at the ends in order to insure proper alignment and to provide a certain amount of flex- ibility in making turns. This conduit is laid in concrete in much the same manner as the clay pipe, it being common practice to separate the different pipes in the layer by about one-half of an inch and the various layers themselves by about one inch., Another form of conduit, radically different from all those so far described, is the Sewall cement arch conduit, which is a recent pro- duction, but which- gives great promise of success. The cross- section of a single duct is shown in Fig. 335, while in Fig. 336 is Figs. 335 and 336. Cement-Arch Conduit. shown in perspective the method of joining two lengths. The separate ducts are formed of cement molded around a strip of wire gauze, as shown in Fig. 335. The arch is made of equal parts of Portland cement and sand, being strengthened by the metal gauze, which is made from No. 19 B. & S. gauge iron wire, woven with a mesh three-eighths of an inch square. The UNDERGROUND CABLE CONSTRUCTION. 417 inside measurement of the standard size is three inches wide and three inches from the top of the arch to the base. In lay- ing these arches the trench is dug in the usual manner and a concrete foundation laid. The top of the foundation is made smooth to grade, being troweled to a polish. The arches are then wet and placed on the floor so formed under a templet which gives them accurate alignment. Joints are made by abutting the sec- tions end to end. Over the joint is placed a wire gauge bridge, as shown in Fig. 336, the wire being lined inside with cotton cloth, to which is cemented a similar cloth upon the outside. As the arches are accurately molded, and as the floor is supposed to be perfectly plain, the joints so formed present a smooth interior. As soon as the first tier of arches is in position it is immediately covered with concrete, which is then smoothed down to form a second floor, upon which the second tier of arches is laid. After the required number of ducts are in place, the usual Fig. 337. Twelve-Duct Cement Arch Conduit. layer of concrete is placed over the entire structure. In Fig. 337 is shown a sectional view of a 12-duct conduit of this type. Curved sections of these arches are made for making bends, these curves usually being made in 6-inch lengths. The cross- section of the duct in this conduit possesses some advantage over the round duct. If foreign matter finds its way into the duct after the cable is in place, or if a piece of the external braiding from a cable sheath is torn off, it will not be so likely to bind the cable in drawing it out as in the circular form of duct, because the foreign matter will be likely to sink down in the lower cor- ners of the duct, where considerable room is provided for it. In laying conduit in city streets numerous obstructions are 418 AMERICAN TELEPHONE PRACTICE. met, and must be overcome in the manner best suited to the individual case. It frequently becomes necessary to remove the support from heavy pipe lines for a considerable distance, as, for instance, when such a pipe line lies diagonally across the trench. In all cases suitable supports for these pipes or other structures should be provided until such time as the trench is again filled. Fig- 338. Avoiding Obstacles. The usual means adopted is to place a beam of sufficient strength across the top of the trench and support the pipe therefrom by chains or heavy rope. It is frequently necessary in passing an obstruction to fan out the pipes in one layer so that they occupy the same level as those of another layer. Such a construction, and also a rather crooked piece of conduit work, is shown in Fig. 338, where, on account of obstructions in the street, the two layers of two pipes were formed into one layer of four until the obstructions were passed. This par- UNDERGROUND CABLE CONSTRUCTION. 419 ticular obstruction was a sub-cellar extending out under the street. The man-holes may be built of various forms and dimensions to meet existing requirements. In the best construction the foundation consists of a layer of concrete six inches deep, the concrete being mixed as specified for the laying of tiles, with the exception that the crushed stone may be considerably coarser. The walls of the man-hole are then built of good brick-work of suitable thickness and well plastered on the outside with cement mortar in order to exclude as much dampness as possible. For the ordinary man-hole an eight-inch wall is sufficiently thick, but in building very large underground vaults it sometimes be- comes necessary to double or triple this thickness. Where these very thick walls are required it is good practice to allow about one inch air space between the outer course of brick and the inner in order to render the interior as dry as possible. A common-sized man-hole is five by five by five feet, and smaller sizes down to three by three feet with five feet head room are also common. As a rule a man-hole should provide at least enough room for two men to work in conveniently. Of course, where a great number of ducts enter a man-hole the size must be increased accordingly. After the conduits are laid and the man-holes finished the next step is the drawing in of the cables. In order to accom- plish this a process called rodding is in most cases first neces- sary, in order that a rope may be stretched through the duct, which is afterwards to be used for drawing in the cable itself. For this purpose a large number of wooden rods about three- fourths of an inch in diameter and four feet long, and equipped with screw or bayonet joints at each end, so that they may readily join together, are necessary. A man stationed in one of the man-holes inserts one rod into the duct, and, after join- ing another rod to it, pushes this also into the duct. Suc- cessive rods are joined and pushed through until finally the first rod reaches the next man-hole. A rope is then attached to one end of the series of rods, which is then pulled through, disjoining the rods as they are taken out of the duct. Where the ducts are smooth and comparatively straight this process may be simplified by using a continuous steel wire about one- fourth of an inch in diameter in place of the rods. It is a good plan to attach to the forward end of this wire a lead ball, which will facilitate it in riding over obstructions. The cable reel is then placed near one of the man-holes in such manner that the cable will 420 AMERICAN TELEPHONE PRACTICE. pay out from the top of the reel instead of from the bottom. The end of the cable is then attached to the rope and started into the duct. In the distant man-hole the rope is led over one or more sheaves suitably arranged on upright beams placed within the man-hole to a capstan or other form of windlass by which the cable may be slowly drawn through the duct. A funnel-shaped shield should be placed at the mouth of the duct into which the cable is being fed for protecting the sheath against the sharp corners at the entrance. This shield, however, is not a sufficient protection for the cable, and one or more men should be stationed in the man-hole for guiding the cable into the duct. The best way to attach the rope to the end is by means of clamps especially provided by the cable companies for this purpose. However, if these are not used, a secure grip may be had upon the cable end by winding several strands of stout iron wire in opposite direc- tions about the cable sheath for a distance of two feet from its end. An eye may be formed in this wire opposite the cable end, to which the rope may be attached. Particular attention should be paid to the sealing of the cable end before it is drawn into the Fig. 339. Drawing in by Steam Power. duct, as ducts are always moist, due so sweating of the interior walls. Where a large amount of cable is to be drawn in the method shown in Fig. 339 may be employed. Instead of the hand-oper- ated winch or windlass a three-and-one-half horsepower horizon- tal engine and capstan mounted on a low wagon is used. By suitable gearing the engine causes the capstan to revolve slowly. This method, so far as the writer is aware, has been used in but one case, that being in the recent extensive under- ground construction work in St. Louis by the Bell Telephone Company of Missouri. It is said that with this contrivance a speed of twenty-five feet of cable per minute is easily attained without in any way damaging the cable, and the remarkably UNDERGROUND CABLE CONSTRUCTION. 421 short time in which the enormous amount of cable installed by that company was drawn in testifies further to the practical value of this scheme. Cables, in passing through man-holes, should be laid around the side of the man-hole and supported on hooks provided for that purpose. Shields formed of sheet lead or of heavy felt should be placed under each cable just at the point where it emerges from the duct, in order to prevent injury of the sheath at that point. Workmen should be cautioned against needlessly bending cables while working in ducts, and the use of the cables in place of ladders for climbing in and out of the man-holes should be strictly prohibited. As much slack as possible should be left in the man-hole, in order to allow room for subsequent splicing when necessary. Trouble is frequently experienced, due to the presence of gas in the man-hole, and care should always be exercised before striking a match or taking a torch into a man-hole, to make sure that all gas has been removed. There are several methods of doing this, one of which is to pump the gas out with an inverted umbrella made specially for the purpose. The um- brella is lowered into the man-hole while closed and then suddenly withdrawn, this opening the umbrella and lifting out the gas. Another way of clearing man-holes from gas is to place a cloth screen above the man-hole and on the side opposite to that from which the wind is blowing. The wind on striking the screen is deflected downward, thus causing an eddy which removes the gas from the man-hole. Very serious explosions have been caused by the formation of gas in man-holes, which becomes ig- nited either by an electric spark or by the torch of a workman. One of the most serious difficulties in connection with un- derground cable work is that brought about by electrolysis, due to the action of stray earth currents, usually due to the ground return of electric railways. It is found that the electrolysis oc- curs at points where a current flowing along the cable sheath leaves the sheath and enters the ground. At this point oxy- gen is liberated which, with the chemicals in the earth, rapidly corrodes lead sheaths. Of course, the construction of high-class conduits, composed of insulating material, has done much to- ward the alleviation of this trouble. Frequent tests should be made, however, on all cable systems to determine the polarity of the cable sheaths with respect to surrounding conductors. The tests for this purpose may be made as follows : Two brass rods about six feet long should be provided, each having a steel 422 AMERICAN TELEPHONE PRACTICE. contact point at one end. Between these two rods should be connected by flexible wires a portable voltmeter one reading to five volts will usually be found most suitable. The test should be made at the man-holes, these being the only avail- able points for reaching the cable. One of the steel contact points should then be placed in firm contact with the cable sheath and the other into contact with any water or gas pipes which run thnough the man-hole, and in each case the voltage should be noted, not only in amount but in direction. Read- ing should also be taken between the cable sheaths and the rails of adjacent electric railroads, and to whatever underground structures exist in the immediate vicinity. It is evident that where the cable sheaths are negative to the surrounding con- ductors no danger will exist, as this would indicate that the current tended to flow through the other conductors to the sheath. If, however, the cable is found positive to the sur- rounding conductors, the matter should be carefully followed up by taking readings in successive man-holes. By these means the maximum danger point can be located, it being, as a rule, the point at which the maximum positive difference of poten- tial exists. At this point the cable sheath should be securely bonded by a heavy conductor to the water or gas pipe or to other metallic structures that are in the vicinity. These bonds serve to allow the current to flow from the cable sheath to the other conductors, instead of forcing it to find circuit through the ground or through the walls of the conduit. In some cases the only remedy has been to run separate return circuits from the maximum danger points on a cable directly to the power- house from which the troublesome current emanates. All of the cable sheaths entering a man-hole should be bonded to- gether, the usual method of doing this being to brighten the surface of the lead sheaths and to bend a No. 10 B. & S. cop- per wire around each sheath, afterwards soldering the connec- tion. This assures the fact that all of the cable sheaths will be at an equal potential and that whatever bonds are run for the protection of one sheath will afford protection for all. The method of bonding to a gas pipe usually adopted is as follows ' The surface of the pipe is brightened for a space of about three by eight inches with a coarse file. This surface is then heated by a torch and tinned with ordinary solder. A copper plate about three by seven inches previously tinned is then soldered to the gas pipe, after which the bond wire leading from the cable is wound into a flat coil and soldered to a copper plate. UNDERGROUND CABLE CONSTRUCTION. In bonding to a water pipe it is impossible to heat the pipe sufficiently to make it take solder on account of the water flow- ing within. The method to be followed is to provide a heavy wrought-iron U-shaped band adapted to fit snugly around the pipe. The ends of this band are screw-threaded and pass through a yoke-piece bent to fit the upper portion of the pipe. This yoke-piece is then firmly screwed in place by nuts, the surface of the pipe and the interior of the iron clamp having previously been thoroughly brightened. The bond wire may then be soldered to this yoke-piece and the whole device smeared with asphalt paint. CHAPTER XXXIII. TESTING. TESTS of telephone lines, whether of bare wire on poles or of overhead or underground cables, maybe divided into two general classes : First : Those which are for the determination of the existence of certain conditions, without the necessity of measuring quanti- tatively the extent to which those conditions exist ; in other words, rough tests for the determination of grounds, crosses, or breaks, usually made with instruments such as the magneto-bell, telephone receiver and battery, and a few other such simple but often in the hands of an experienced person most effective instruments. Second : Those for not only determining the existence of cer- tain conditions, but also for their quantitative measurements. These require the use of different and more intricate instruments, and in many cases the operator must be possessed of a fair degree of mathematical training combined with an ingenuity for meeting and mastering unusual problems that arise under differ- ent conditions. The magneto testing set is the most important instrument in making tests under the first class. Such an instrument usually consists of a powerful magneto-generator so wound as to enable it to ring its own bell through a resistance of from 25,000 to 75,000 ohms. A powerful magneto-telephone is carried on the outside of the case in suitable clips, and may be switched in circuit alternately with the generator by a small hand switch. This magneto-telephone serves as both transmitter and receiver, and enables the lineman or other party to communicate from a pole top or man-hole with any other party on the circuit. Fre- quently these sets are made to include microphone transmitter and battery ; but, inasmuch as the instrument is seldom if ever used to talk over very long circuits, the extra weight of these is considered in most cases undesirable. A small, inexpensive gal- vanoscope or current detector will also prove very convenient. In testing for a ground on a wire, whether it be in a cable or bare, and on poles, make sure that the far end of the line is open TESTING. 425 and then connect one terminal of the magneto-bell to the near end of the line and ground the other terminal. The ringing of the bell would seem to indicate that the circuit was complete and the line grounded in this case, but this is not always true, and this test must therefore be relied on only with caution. The static capacity of a long line or of a comparatively short length of cable will often allow enough current to pass to and from the line in charging and discharging to ring the magneto-bell. For testing out local work where there is no room for this capacity effect, the magneto-bell is invaluable. A more reliable means of making tests for grounds or crosses is to connect the current detector in series with several cells of battery and to ground one terminal. Then with the other ter- Fig. 340. Receiver Test for Crosses and Grounds. minal make contact with the near end of the line. A kick of the needle will take place in any event on closing the circuit, due to the current flowing to charge the line, but a permanent deflection will indicate a ground. In testing for a cross, as for instance with some other wire in the line or cable, one terminal of the magneto-bell or the gal- vanoscope and batteries should be connected to the wire under test and the other to all the other wires in the same lead, for which purpose they are bunched. In case it is not convenient to bunch them, however, the test may be made between the sus- pected line and each of the others in succession. Another and perhaps still more simple method for determining a cross or ground is one described in Roebling's pamphlet on Telephone Cables, and illustrated in Fig. 340, as applied to the testing of a cable before it has been unreeled. 426 AMERICAN TELEPHONE PRACTICE. N represents the near end, and F the far end of the wire being tested. B is a battery, of about three cells. T is an ordinary telephone receiver. The wire, N F, is carefully sepa- rated from all the others at each end. At the near end all the wires are stripped of insulation and, except the one under test, are connected together and also with the sheath. The wire, C, connects the sheath to one side of bat- tery, B, and the other side of battery is connected to one side of telephone receiver, T. The testing man rapidly taps with the wire, N Fj the unoccupied binding post of the receiver, T. The first tap will produce in the receiver a distinct click, and if the cable is long there may possibly occur a second faint click, but if the wire, N F, is perfectly insulated, no more sound in the telephone will follow the tapping. If, however, the wire, N F, is crossed with any wire in the cable, or with the sheath, every tap will be followed by a distinct click, and if there is moisture in the paper, making a partial connection, clicking sounds will occur, which are loud or faint, according to the amount of moisture present. The philosophy of this method of testing is very simple, and serves to make the operation more readily understood. When the wire, N F, is first connected to the battery, it be- comes charged. During the process of charging a current flows into the wire and passes through the coil of the receiver and causes the click. If the wire is well insulated, the second tap, immediately following, finds it charged, or nearly so, and there is, therefore, no click, or a very faint one. If, on the contrary, the wire under test is crossed with any of the other wires, or imperfectly insulated from them, or from the sheath, the wire will immediately discharge itself through the cross to the other wires and the sheath, and there will be a flow of current at every tap, and consequently a continuous clicking. If a conductor in a perfectly insulated cable is very long, two or three taps or a long first contact may be necessary to charge it completely. If the cable is in place or if it is a bare aerial line that is being tested this same method may be used. In case of a new cable it is well to test every wire in this manner, and therefore the wire, N F, should be put aside and another slipped out of the bunch and tested in the same way, and so on until all have been gone over. If any of them are found to be in trouble, it is well to care- fully inspect the exposed ends to be sure they are properly cleared from each other and from the sheath. If it is still found to be defective, it should be plainly tagged. TESTING. 427 In the manner just described, twenty-five minutes with two men should be ample time for testing one hundred wires, the testing operator listening and his helper attending to the connec- tion of the different wires at N. For this test as well as many others it is very convenient to use a regular operator's receiver and head band, as it will save the tester a very tired arm at the end of a long test. As a matter of fact, the receiver is little appreciated as a testing instrument. A very convenient set is formed by a watch-case receiver and head band, and two small-sized cells of dry battery, strapped together so as to be carried in the coat pocket. The receiver and battery .are connected in series, the free terminals of the circuit being formed by flexible cords about four feet long. These cords should terminate in convenient clips, or contact points adapted Fi^. 341. Continuity Test. to make contact with the wires to be tested. This arrangement leaves both hands free at all times, and is wonderfully sensitive. The continuity test, or test for broken wires, may be made with the same simple instruments. The wires to be tested should all be grounded or connected to a return wire at the far end. At the near end, one pole of a magneto-bell, or of the battery and galvanoscope, or of the receiver, should be connected to ground or the return wire and the other terminal connected successively to the terminals of the line, which, of course, should all be separated. A ring in the case of the magneto, or a permanent deflection of the needle in the case of the galvanoscope, or a continuous clicking in the receiver, will indicate that the wire is continuous. The same precaution as previously pointed out must, however, be 428 AMERICAN TELEPHONE PRACTICE. observed with the magneto-bell. This same test for continuity is well illustrated in Fig. 341, in which case a vibrating bell in- stead of the receiver or galvanoscope is used. In testing a cable all defective wires should be marked "crossed," "grounded/' or "broken" at the end at which they are tested. The corresponding ends of the tagged wires at the other end of the cable should then be found and similarly marked. If there are not the requisite number of good wires in a new cable it should be rejected. Lead-covered cables, manufactured by reliable firms, are always subjected to a much severer test than it is possible for the pur- chaser to give them before they leave the factory. It is there- fore considered by many as unnecessary to make a test on new cable on reels, purchased from reputable firms, unless some injury in shipment is suspected. It is often desirable to be able to pick out a certain wire at some intermediate point in an open cable, or in a large bunch of insulated wires, in order to establish a branch connection. This is easily done by the foregoing methods if the cable is to be cut, but frequently this is not the case. It may be done without cutting by the following simple method : Ground the wire or wires desired at the distant end, being sure that these wires are free from all the others at both ends. Then having loosened the bunch of wires at the point at which the branch is to be taken off, test each by means of a needle-pointed instrument, con- nected to ground through a bell or receiver and battery. The needle-point can readily pierce the insulation and make good contact with the conductor within. A knowledge of this very simple test will often save an immense amount of trouble. In the second class of tests that is, those requiring quantitative measurements there are three distinct subdivisions, which are as follows : Tests for resistance or conductivity, tests for capacity, and tests for insulation. Tests for the location of faults in lines always depend on the application of one or more of these. There are three principal methods of making resistance tests: First, by the use of a Wheatstone bridge, which is accurate for all resistances except those very large or those very small. Second, the fall of potential method, which is of value in measuring very small resistances, as of a large conductor, such as a trolley-wire or heavy feeder. This method has little use, therefore, in telephone work where all conductors are comparatively small. Third, by the use of a sensitive galvanometer in series with a battery. This method is the most accurate for the determination of TESTING. 429 extremely high resistances and is, therefore, of great use in meas- urements of insulation resistance. For general resistance measurements the Wheatstone bridge is the most suitable, being very accurate and exceedingly simple in manipulation. In order to appreciate the possibilities of this instrument its underlying principles should be understood. In Fig. 342, A, B, R, and ^represent resistances. G is a galvanometer or instrument for detecting the flow of current. The four Fig. 342. Diagram of Wheatstone Bridge. resistances are connected together as shown, the galvanom- eter being connected in the " bridge " between the junctures of A and R, and B and of X. A battery, B ', is connected between the junctures of A and B, and of R and X. Each resistance, A, B, R, and X, forms, what is termed an arm of the bridge. The two fundamental laws upon which the action of the bridge is based may be stated as follows : 1. No current will flow between points of equal potential; and 2. The drop in potential along the various parts of a conductor is proportional respectively to the resistances of those parts. Referring again to the diagram, it is evident that a current from the battery flows to the point, e, where it divides, part flow- ing through A R and part through B X, after which they unite and pass to the negative pole of the battery. But what of the galvanometer? Evidently by Rule I the only time at which no current will pass through it will be at the time when the points, f and h, are at the same potential. By Rule 2 these points will be at the same potential only when A bears the same re- lation to R as B does to X. 43 AMERICAN TELEPHONE PRACTICE. That is A : R :: B : X, or, by alternation, A R B ' X ' A little algebra will render the above evident if not so already. Call a the drop of potential between the points e and i, / that between e and /, and c that between e and h. Then b : a :: A : A + R by Rule 2. '-ara* Similarly B + X a. For a condition of equal potentials at /and h so that no cur- rent will flow through the galvanometer, b must = c. Then A B a = - - - a. A + R whence : AB + AX = AB + BR, and AX = BR. Dividing by BX, we have A. A. B X' which is the equation of the ratios between the resistances of the arms of the bridge, to insure no flow of current through the galvanometer. The resistance to be measured forms the arm X of the bridge, and in order to determine its value the resistances in the various arms are adjusted till no current flows through the galvanom- eter. Then the equation just derived holds good and may be solved for X, thus X = -^- R. The arms A and B are best termed the " ratio arms " of the bridge and arm R the rheostat arm. In commercial forms of the Wheatstone bridge, A and B are usually so arranged that each may be given the values, 10, 100, and 1000 ohms, and in some cases i ohm and 10,000 ohms also. TESTING. 43 r The ratio arms, A and B, may therefore be adjusted to bear any TO T OOO conveinent ratio to each other from - to - , or, in 1000 10 some instances, from - to - - -- . The rheostat arm 10,000 i is in reality a rheostat capable of being adjusted to any value from i to about 11,000 ohms. In some bridges a sealed battery is furnished with and forms a part of the instrument. In those having no battery, suitable binding posts are provided, usually marked BB, between which the battery may be connected. Other binding posts, usually marked XX, are furnished for connecting the terminals of the unknown resistance to be measured. Two keys are usually furnished, one in the battery circuit and the other in the galvanometer circuit. Each keeps its circuit normally open. The operation of the bridge is very simple. First some ratio between the arms A and B is determined upon. The battery is then connected between the proper binding posts, and likewise the resistance to be measured is connected between its binding posts. The battery key is first depressed and then the galvanometer key. A deflection of the galvanometer needle will take place which by its direction will after a few trials show whether the resistance in the rheostat arm is too great or too small. The rheostat is adjusted accordingly until the galvanometer needle shows no deflection upon the operation of the keys. We then know that our equation -=- = - - holds good, -D A. and consequently That is, the unknown resistance is equal to the ratio between B and A multiplied by the resistance in the adjustable arm. Considerable judgment may be exercised in the choosing of the appropriate ratio in the ratio arm to obtain the greatest accuracy. Obviously if a very high resistance is to be measured the ratio should be large, and vice versa. In bridges having resistances of 10, 100, and 1000 ohms in the 43 2 AMERICAN TELEPHONE PRACTICE. ratio arms, the following values in arms A and B will give the best results : Resistance to be measured. A arm. B arm. Under 100 ohms, . 1000 10 100 to 1000 ohms, . . . . . 1000 100 1000 to 10,000 ohms, . ~. " ' .' . 1000 1000 10,000 to 100,000 ohms, . . -. . zoo looo 100,000 to 1,000,000 ohms, ; . . 10 looo As to the accuracy of measurements attainable by the use of the Wheatstone bridge, the following table represents the claim of one reliable manufacturer : .01 of an ohm to an accuracy of I per cent. T H I/ I ohm " " " y, " " 10 ohms " " " \ " " 100 " " " " y% " " looo " " " " ft " " 10,000 " " " " | " " 100,000 " " " " ^ " " 1,000,000 " " " " 5 " " If using the no volt lighting circuit as battery power i meg- ohm may be measured accurate to y 2 per cent. There is no doubt but that with a well-made bridge with a sensitive galvanometer, these results may be equaled if not sur- passed. Great care must be taken in using a voltage as high as no, as there is danger of burning out the coils. Such high volt- age should be used only in measuring very high resistances, and the ratio arms should be adjusted to give as high a multiplying ratio as possible. A particular form of bridge which has come into extensive use in this country and which possesses several unique features is shown complete in Fig. 343 and in plan view in Fig. 344. The various adjustments of the arms are accomplished by placing plugs in the various holes between the brass blocks arranged in rows as shown in the latter figure. Between each successive pair of blocks are arranged resistance coils having the resistances in ohms designated on the plan. Placing a plug in a hole between two blocks short-circuits the resistance connected between those two blocks. The rheostat arm of this bridge is represented by the top and bottom row of blocks, and if all plugs TESTING. 433 are removed the resistance in this arm will amount to n,no ohms. The ratio arms A and B are represented by the left- and right-hand halves respectively of the center row. A galva- ig. 343. Portable Testing Set. nometer and suitable battery, together with battery and galva- nometer keys, are all mounted in a carrying case as shown. The connections of this instrument are indicated in Fig. 344, Fig. 344. Plan of Portable Testing Set. and are ax follows : The top row of blocks is connected to the bottom r->w by a heavy copper bar joining the right-hand blocks. This connection is made very heavy so as to interpose no extra resistance in the rheostat. On the rheostat formed by the 434 AMERICAN TELEPHONE PRACTICE. upper and lower rows of blocks any resistance from i to 1 1, no ohms may be obtained, the resistance being added by leaving out plugs. The lower left-hand block of the rheostat is connected to the lower binding post, D, forming one terminal of the unknown resistance. The upper post, C, forming the other terminal of the unknown resistance, is connected to block, X, which block is also joined to the galvanometer key. The block, R, is connected to the upper left-hand block of the rheostat. The end blocks of the middle row are connected together and to the + terminal of the battery. The terminal of the bat- tery is connected through the battery key to the lower left-hand end of the rheostat. One galvanometer terminal is connected directly to the block, R, the left-hand block of the rheostat, and to the back contact of the galvanometer key. The other galvanometer terminal is connected through the key to the block, X. By carefully following out these connections it will be apparent that the parts as connected form three arms of a Wheatstone bridge, the fourth, of course, being the unknown resistance Fig. 345. Circuits of Portable Testing Set. joined to the line posts. This is shown diagrammatically in Fig. 345, where the corresponding parts are similarly lettered. It will be noticed that this latter figure is practically the same as Fig. 342, with the addition of the center blocks, A, B, X, R, forming a sort of commutator. The object of this arrangement is to make it possible to reverse the connections of arms, A and B, with R and X. Thus with the plugs in the position shown by the black dots, the connection is precisely as shown in Fig. 342, If, however, the plugs are A R and the equation- - = -_ holds true. B X inserted in the holes on the other diagonal, arm, A, will be con- nected to arm, X, and arm, B, to arm, R, and the equation of B R the bridge will be A X TESTING. 435 The bridge arms, A and B, have not the same range of resist- ances in this bridge, A having only i, 10, and 100 ohm coils, while the resistances of .Z?are 10, 100, and 1000 ohms. Therefore, if a ratio of 1000 to I for measuring large resistances is desired, the plugs are inserted in the commutator along the arrow H (Fig. 344) ; while an opposite arrangement of the plugs along the arrow L will give a ratio of I to 1000 for very small resistances. In this bridge the galvanometer key is so arranged as to short- circuit the galvanometer while the key is up. The galvanometers usually furnished with the complete bridges consist of a needle so mounted as to swing freely in a horizontal plane. This needle is given a tendency to point in one direction sometimes by the action of the earth's magnetic field and sometimes by the field of a powerful permanent mag- net. By causing the current through the bridge wire to flow through a coil, either stationary and surrounding the needle, or movable and carried on the needle, the needle is caused to swerve from its normal position and to place itself at right-angles to the lines of force due to the permanent field. The deflection of the needle is great or small according to the strength of the current, and to the right or left according to the direction of the current. In many of the tests to be described later a galvanometer of greater sensitiveness is required, and some form of reflecting in- strument is used. .In these the needle carries a small circular mirror, which reflects a spot of light from a lamp or some other source against a scale. In this arrangement every movement of the needle causes the spot of light to move along the scale, and a little consideration will show that the angle through which the reflected ray of light moves is double that through which the needle travels. Thus this reflected ray of light serves as a needle of any desired length, and has the advantages of magnify- ing the angular movement of the needle to twice its real amount, and of possessing no mass, and therefore no inertia. The two galvanometers used to the greatest extent for quanti- tative measurements in practical work are the Thomson and the D'Arsonval. The Thomson galvanometer is made in a great variety of forms. The needle consists of several very light bar-magnets arranged side by side and with opposing poles together, so that the directive influence of the earth's field shall be very slight. The needle is directly attached to a small silvered glass mirror, and is suspended within the coil or coils by means of a silk or 43 6 AMERICAN TELEPHONE PRACTICE. quartz fiber. The current to be measured is passed through tke coils, and the magnetic field set up thereby causes the needle to swerve from its normal position. The Thomson galvanometer is used in the most delicate tests, and is essentially a laboratory in- strument. It has the disadvantage of being affected to such an extent by external magnetic fields as to render its use impos- sible in many cases. A passing street car or variations in the Fig. 346. D' Arson val Galvanometer. current flowing in a neighboring circuit will cause the needle to swing violently, thus making accurate work out of the question. These disadvantages may be overcome to some extent by in- closing the galvanometer in a heavy iron case such as an old safe but they tend to make it a very undesirable instrument for portable work. Where the instrument can be permanently set up and properly guarded, it is unequaled for delicacy and accuracy. For nearly all practical engineering work, the D'Arsonval gal- vanometer is sensitive enough, and has the advantage of being much more convenient for general work. In this the needle is a coil instead of a permanent magnet, and is suspended within the TESTING. 437 field of a powerful permanent magnet instead of in a coil. The needle carries a mirror, as in the Thomson instrument. The current to be measured is passed through the coil, and as this coil lies in the field of the permanent magnet, a rotation of the coil ensues, the action being identical with that which causes the armature of an electric motor to revolve. In Fig. 346 is shown a much-used form of D'Arsonval gal- vanometer made by Queen & Co., Philadelphia. The field is built up of a number of horizontal permanent magrTets, between the c Fig. 347. Suspension of D'Arsonval Galvanometer. poles of which is suspended the needle. The needle system is shown in detail in Fig. 347. It consists of a coil of wire, W, wound on a boxwood frame, D, and supported by means of the flat phosphor-bronze filament, A, from the torsion pin, E. The current is led in by means of the torsion pin, E, and suspension wire to the coil ; thence to the spiral spring, B, and by means of the bottom contact out to the external circuit. A ring, F, is 43 8 AMERICAN TELEPHONE PRACTICE. joined above the coil frame, and another, G, below the coil frame. These are normally a sufficient distance apart to enable the sys- tem to swing freely, but when packing for transportation the torsion head may be pressed down until the rings above men, tioned firmly clamp the coil. In this condition it will withstand shipment satisfactorily. To the right is shown the coil more clearly. The two points, U and L, have soldered to them the ends of the coil, W. The mirror is shown at C. The great advantage of the D'Arsonval galvanometer is that it is unaffected by variations in the external magnetic field. It may even be used close to dynamo machinery without being sensibly affected. In order to read the deflection produced by a current, in any Fig. 348. Scale and Telescope. form of reflecting galvanometers, two methods may be em- ployed. One is to cause the needle to reflect a spot of light from a stationary source, upon a horizontal scale, and by watch- ing the movement of the spot the number of scale divisions de- flection may be accurately determined. Another and better way is to focus a telescope on the mirror, in such manner that the horizontal scale will be visible in the telescope. The mirror in its movements will reflect different portions of the scale into the telescope, and the deflection may thus be observed with great precision. When this method is used the numbers on the scale should be reversed, in order to appear normal in the tele- scope. Fig. 348 shows a telescope and scale as arranged for this purpose. Complete testing sets, containing reflecting galvanometers, TESTING. 439 bridges, batteries, keys, and other accessories, are frequently mounted in one case, and so arranged as to fold within small compass when not in use. This arrangement is very convenient, but has one disadvantage the manipulation of the keys and plugs jar the box to such an extent as to make the readings on the galvanometer unreliable. The separately mounted gal- vanometer is therefore in general to be preferred. Of course this applies only to reflecting galvanometers. It is frequently found that a current that it is desired to measure is so large that it sends the spot of light completely off the scale, thus rendering the measurement of the deflection im- possible. In order to increase the range of the galvanometer so as to make it available for measuring both large and small cur- rents, certain resistances called shunts may be placed in parallel with the galvanometer coil as in Fig. 349. The resistance of s Fig. 349. Galvanometer and Shunt. the shunt being known, it is easy to calculate the amounts of the currents that pass through the galvanometer coil and the shunt. Calling R g the resistance of the galvanometer, R s that of the shunt, f g the current through the galvanometer, 7 S that through the shunt, and 7 the total current through both, then 7=7 g + 7 s . Also when E is the difference of potential between the com- mon terminals of the galvanometer and shunt, E E T * 1 T ** *(? = -JT anc l * = ~D" r T> E = I v R* L R a . Hence 7, = Substituting this value of 7 a , in the first equation, we have r> I p The quantity - ~= is called the multiplying power of the 440 AMERICAN TELEPHONE PRACTICE. shunt, because it represents the number by which the current through the galvanometer must be multiplied, in order to give the value of the current being measured. Shunt boxes are usually provided for a given galvanometer with a number of coils specially arranged to give such con- venient values of the multiplying powers, as 10, 100, and 1000. For this purpose the various coils of the shunt box have resist- ances of i, -fa, and -yfa of the resistance of the galvanometer. To better show this relation, assume that a multiplying power of looo is desired, then P. + R 1000 i-= R s R g . ""g 1000 i 999 A commercial form of shunt box is shown in Fig. 350, the various multiplying values of the shunt being obtained by plug- ging the block corresponding to the multiplying power desired. Fig. 350. Shunt Box. For moderate deflections, the current traversing the coils of a reflecting galvanometer may, without sensible error, be taken as proportional to the deflection of the spot of light on the scale, or to the deflection read through the telescope. The current is of course inversely proportional to the total resistance of the circuit, and from this it follows that the deflections are inversely proportional to the resistance. This fact enables the galva- nometer to be used for measuring unknown resistances by com- paring the deflection obtained when a given E. M. F. acts through a known resistance with that obtained when the same E. M. F. acts through an unknown resistance. The general method of measuring resistances by the use of a TESTING. 441 galvanometer is to note the deflection obtained with a given battery and a known resistance in the circuit, and from this to compute what is called the working constant. This working constant may be defined as the number of scale divisions deflection that would be obtained by causing the current from the given bat- tery to pass through the galvanometer and a resistance of one megohm. Of course such a deflection as this can exist in our imagination only, but it serves, .nevertheless, as a convenient standard upon which to base our calculations. Having obtained the working constant, a reading is taken of the deflection pro- duced by passing the battery current through the galvanometer in series with the unknown resistance. As the deflections are inversely proportional to the resistances, the unknown resist- ance is then readily computed. If measurements of comparatively low resistance are to be made, then the resistance of the battery and of the galva- nometer must be taken into consideration as well as that of the resistance placed in circuit with them, but as the measurements here considered will be those of very high resistances only, the resistance of the battery and of the galvanometer may be neg- lected. For the purpose of taking the constant, connections are made as shown in Fig. 351, where B is the battery, G the Fig. 351. Circuits for Galvanometer Constant. galvanometer, .S the shunt, and R the known resistance. Usually the value of R is ^ of a megohm, or 100,000 ohms. With the sfa shunt a certain deflection will be obtained when the circuit is closed. Obviously, if the shunt were not present the deflection would be 1000 times as great, because only -nfoy of the current passes through the galvanometer. Therefore the total deflection, if it could be measured, that would be pro- duced through the galvanometer and the 100,000 ohms resist- ance, would be the deflection noted multiplied by 1000. If, now, the resistance, R, had a value of i megohm instead of -fo megohm, the deflection would have been only -j- 1 ^ as great as this. Therefore to find the number of scale divisions deflec- 44 2 AMERICAN TELEPHONE PRACTICE. tions which the galvanometer alone would give with I megohm in circuit, we multiply the deflection noted by 1000 and by -,-V- In general we may say : to find the working constant, multiply the deflection obtained by the multiplying power of the shunt, and by t/ie value of the known resistance in megohms. As a numerical example let us assume that with the 7 | T shunt and the 3 V megohm resistance, we obtain a deflection of 200 scale divisions, then the working constant is 200 X 1000 X = 20,000. 10 In other words, 20,000 would be the number of scale divisions obtained were the entire current from the battery allowed to pass through the galvanometer with one megohm in series. With 50 cells of battery (45 or 50 volts), the constant under ordinary working conditions with a good D'Arsonval galva- nometer, will be from 10,000 to 25,000. With a Thomson instru- ment a much higher constant may be obtained. Mr. George D. Hale of the Western Electric Company's cable-testing de- partment uses a large four-coil Thomson instrument with 600 volts obtained from a motor generator. With this he obtains a constant of 528,000, and by adjusting the suspension for greater delicacy can obtain as high as 2,000,000. Of course this is entirely impracticable for portable instruments, and is, in fact, unnecessary, as good work may be done with a constant of 20,000. In ordinary testing a battery of 50 cells is sufficient. Of course a higher working constant may be obtained with a larger battery, and frequently 100 cells are used. INSULATION TESTS. One of the principal uses of the galvanometer in line testing is in the measurement of insulation resistance. The insulation resistance of any line or conductor is the joint resistance of all the leaks from the line to the ground or to other conductors. On a pole line every insulator forms a leak to earth, and on a line having 40 poles to the mile there would be 40 such leaks in parallel. The insulation resistance of a line as a whole varies inversely as its length, if the insulation is uniform. Evidently, a line two miles long would have one-half as great an insulation resistance as a similar line one mile long, because on the latter there would be only half as many leaks as on the former. In TESTING. 443 general it may be stated that a line n miles long will have only - as great an insulation resistance as a similar line one mile in rl length. In order to obtain a standard of insulation resistance independent of the length of the line, it is convenient to express the insulation resistance as so many megohms per mile. The insulation resistance per mile is found by multiplying the insula- tion of the line as a whole by the length of the line in miles. In order to measure the insulation resistance of a line the constant of the galvanometer is first taken and then the known resistance is cut out of circuit and the line insulation resistance substituted for it. Assuming that the insulation resistance to be measured is that of a wire in a cable, the terminals of the circuit which were connected with resistance, R, in Fig. 351, will be connected one with the wire and the other with the sheath of the cable as shown in Fig. 352. Care must be taken that the Fig. 352. Insulation Resistance of Cable. wire being measured is carefully insulated from the sheath at the other end of the cable. The shunt, 5, is then cut out of circuit in order that the full current may pass through the gal- vanometer. Before completing the circuit with the cable con- ductor and sheath, however, the key, K, should be closed in parallel with the galvanometer, in order to prevent the rush of current that will take place in charging the cable, from causing the needle to give too violent a kick. After a short time the key is opened and all of the current diverted through the galvanometer. The galvanometer then receives only that cur- rent which leaks from the core of the cable to the sheath through the insulation. Under these circumstances a certain deflection will be noted, and by comparing this deflection with the constant already obtained the value of the insulation resist- ance in megohms is readily determined. To illustrate, suppose that a deflection of 75 scale divisions is obtained with the apparatus connected as in Fig. 352. If the constant is 20,000, as already determined, we know that the 444 AMERICAN TELEPHONE PRACTICE. insulation resistance must be 20,000 divided by 75, or 266 meg- ohms, thus indicating that the total insulation resistance of the cable is 266 megohms. That this is true is evident from the fact that the constant, 20,000, represents the number of scale divisions deflection that would be obtained were only one meg- ohm in the circuit. The deflections are inversely proportional to the resistance in the circuit, and therefore the total insulation resistance is equal to the deflection through one megohm divided by the deflection through the insulation resistance, or 20,000 divided by 75. To sum up these operations : 1st. Obtain the galvanometer constant or deflection obtained when the galvanometer in series with one megohm resistance is Fig. 353. Connections for Insulation Test. subjected to the potential of the battery. 2d. Find the deflec- tion obtained when the galvanometer and insulation resistance in series are subjected to the potential of the battery. 36. Divide the constant by the deflection obtained through the insulation resistance, the result being the insulation resistance of the cable expressed in megohms. 4th. To find the insulation resistance per mile, multiply the total insulation resistance by the length of the cable in miles. If the insulation of the cable is low, a shunt must be used in obtaining the deflection through the insulation resistance. If the insulation resistance is high, the deflection will be small and no shunt will be required. The purpose of the shunt is merely to keep the deflections on the scale so that they may be read. In Fig. 353 is shown a convenient arrangement of connections for making insulation tests. In this, B is the battery of say 50 cells, R the -^ megohm box, 5 the shunt box, G the galva- nometer, and V a convenient switch for throwing either the -fa TESTING. 445 megohm box or the line insulation into circuit with the galva- nometer and battery. When the levers of the switch, V, are in the position represented by the dotted line, the circuits are those for taking constant of the galvanometer, and when in the position shown by full lines, the circuits are those for obtaining the deflection through the insulation of the cable. Various forms of keys for changing the direction of the battery current through the galvanometer, and for performing other switching operations with the greatest possible convenience, are obtainable, and form an important part of all testing outfits. The scope of this work will not permit of their detailed description. In making insulation tests the resistance of the lead wires to the cable or line need not be taken into account. It is a matter of the greatest importance, however, that these wires are perfectly insulated from each other. It is a very easy matter in making tests of this nature to measure the wrong quantity. One very important matter in connection with insulation tests has not yet been spoken of. When the reading is being taken, with the cable or line insulation in circuit, it will be noticed that a maximum deflection is obtained at first, and that this gradu- ally diminishes, as though the insulation resistance were increas- ing. This is due to what is called electrification, a phenomenon that is not very thoroughly understood. When the electromo- tive force of the battery is first applied to the cable or line, there is a sudden rush of current, due to the charging of the conductors. The charges, however, apparently soak in to the insulation to a slight extent, thus allowing more current to flow to the conductors. After the first rush due to the first charging of the conductors, there is still a flow of current, due in part to this soaking in, and in part to the actual leakage through the insulation. It is the current due to the latter that we are concerned with in insulation measurements, and therefore we must wait till the soaking in process ceases, when the flow of current will be practically constant, being that through the insulation. In nearly all telephone-testing work, one minute is allowed for electrification, after which the reading is taken of the deflection. When one is thoroughly familiar with his instru- ments he may often, where great accaracy is not required, estimate what the deflection at the end of one minute will be, by watching the deflection for 30 or 40 seconds. This method saves time, but must be used with extreme caution. With a constant of 20,000 a reading taken on a wire in a piece of good new telephone cable, one-quarter mile long, would 446 AMERICAN TELEPHONE PRACTICE. probably show a deflection of 8 or 10 scale divisions upon the closure of the key. This would decrease to about 6 scale divisions in 2 seconds, and to about 2 scale divisions in 30 sec- onds, after which it would remain constant. The reading of 2 divisions at the end of the minute would indicate an insulation , 20,000 resistance of = 10,000 megohms, or 2500 megohms per mile. As examples of deflections on the different wires in various cables the following are given : Dry paper cable, mile long, two years old. Galvanometer constant 22,000: Readings, 12-15-15-10-10-15-15-15-13 scale divisions. Another dry paper cable, 2750 feet long, one year old. Gal- vanometer constant 19000 : Readings, 5-6-5-4-5-5-5-5-6, etc. A piece of jute and ozite cable five years old, 6000 feet long, gave the following with a constant of 20,000: 7500-2500-1000- 1000-600-800-900-800-1000. It was necessary to use the ^ shunt in taking these readings. Another piece of the same kind of cable, 800 feet long, with a constant of 20,000, gave 175-200-250-270-160-110-120-160- 110-125. CAPACITY TESTS. A very important measurement, especially in telephone cables, is the determination of the capacity of the line conductors with respect to all neighboring conductors. The usual method of making capacity tests is to note the deflection produced when a B Fig- 354. Capacity Test. condenser of known capacity, after having been charged to a known potential, is discharged suddenly through the galvanom- eter, and to compare this with the deflection obtained when the cable or conductor being measured, after being charged to the same potential, is discharged through the galvanometer. The TESTING. 447 deflections produced under these circumstances are proportional to the charges, and therefore to the capacities of the standard condenser and the line or cable. The circuits for obtaining the deflection produced by the discharge of the condenser are shown in Fig. 354, where C is the standard condenser, B the battery, and G the galvanometer. When the key is depressed the condenser is charged to the full potential of the battery, B. The key is then suddenly released, thus allowing the charge from the condenser to pass through the galvanometer, thus producing a certain throw of the needle. The connections are then made as shown in Fig. 355, the same battery, .5, being used. JL Fig- 355- Capacity Test. When the key is depressed the cable is charged, and when suddenly released this charge flows through the galvanometer and produces another throw of the needle. By comparing the throw produced by the charge of the condenser with that pro- duced by the charge of the cable, a direct comparison may be made between the capacity of the cable and that of the con- denser. Thus, if with the -^ shunt the discharge from the condenser gave a deflection of 100 scale divisions, the capacity of the condenser being T V microfarad, and if with the same shunt the discharge of the cable produced a deflection twice as great, we would know that the capacity of the cable was 2 X = microfarad. Convenient connections for making capacity tests are shown in Fig. 356, where G is the galvanometer, 5 the shunt, C the standard condenser, KK discharge keys, V the selecting switch, and, B a battery of eight or ten cells. With the switch, V, at the left and both discharge keys depressed, the current from the battery will flow into the condenser, thus charging it. Upon the sudden release of the discharge keys, the condenser will dis- charge through the galvanometer and shunt, giving a deflection which should be noted. With the switch, V, at the right, the 448 AMERICAN TELEPHONE PRACTICE. cable may be charged and discharged in the same manner, and the deflection produced by its discharge noted. About seven cells of battery is usually sufficient for making capacity tests on telephone cables. If a non-adjustable condenser only is avail- able, one having a capacity of ^ microfarad is probably most desirable. For accurate work a subdivided condenser, having its divisions so arranged as to be easily connected in multiple or in series, or in combinations of the two, is very desirable. Then Fig. 356. Circuits for Capacity Test. the condenser capacity may be varied until the throw from the condenser is nearly equal to that from the cable, thus greatly minimizing the liability to error in the results. In making capacity tests the wire under test should be carefully insulated and all the other wires in the cable should be connected together and to the sheath or ground. Fifteen seconds should always be allowed for the charging of the cable. If d is the throw due to the discharge of the condenser, d' that to the discharge of the cable, K the capacity of the condenser in microfarads, and X the capacity of the wire being measured, then X : K :: d' : d d_ d If the throws of the galvanometer are too large to be measured, the shunt must be used. In this case dor d' in the formula, will TESTING. 449 be the actual throws observed multiplied by the multiplying power of the shunt. THE LOCATION OF FAULTS. When a break occurs in a wire in a line or cable, the ends remaining insulated from other wires and the ground, the only recourse is to capacity tests. The capacity of the two parts of the wire will be proportional to their lengths, the wire being uniform in size and in its relation to other wires, throughout its length. We may locate a break of this nature in several ways. Measure the capacity of one end of the broken wire, then go to the other end of the cable and do the same. Calling D the length of the cable in feet, C the capacity of the first portion of the wire, C that of the other, and X the distance in feet to the break from the first end, then : X : D :: C : C + C CD and X = C + C' When a good wire is available, and this is usually the case, set up the instruments for capacity testing, and take a throw, d, on the broken wire, another, d?', on the good wire, and a third, d", on the good wire with the broken wire connected to it at the far end. Evidently the throw on the whole broken wire would be d* - d' + d. Hence where D and X have the same significance as before X : d :: d : d' - d' + d , v (ID 1 d" -d+d' The location of breaks is much complicated by the presence of poor insulation between ruptured portions, and between other wires. The insulation resistances between these parts should always be taken. If less than one megohm, the results obtained by the capacity tests should not be relied on, and other methods too complex for description here may be resorted to. It seldom pays to open a lead-covered telephone cable for the purpose of joining a few broken wires, the expense of making the splice being usually in excess of the value of the wires. 45 AMERICAN TELEPHONE PRACTICE. The location of crosses or grounds is rendered somewhat difficult by the fact that there is nearly always some resistance in the fault itself. If we know the resistance of the defective wire and have no good wire running parallel with it, we may proceed as follows, using a good Wheatstone bridge : Measure the resistance of one end of the defective wire through the fault to ground. Do the same at the other end. Then calling R the total resistance of the wire (either known or calculated from its size and length), R' the measured resistance from the first end, R" that from the other end, X the resistance from the first end to the fault, Fthe resistance from the second end to the fault, and Z the resistance of the fault, we have : R = X + Y. R' = X + Z. R" = Y + Z. Solving these for ^Tand F we have R + R' - R" JL = - 2 _ R - R + R" y --- , 2 which values are independent of the resistance of the fault. Knowing the resistance to the fault, it is easy to compute the distance to it, from the resistance per foot of the conductor. When a good wire is available, the Varley loop test should be used, as it is more accurate than the method just described. X i Y GOOD WIRE Q 1 FAULT ...L._. Fig. 357. Varley Loop Test. For this a Wheatstone bridge is used, and connected as in Fig. 357. The good and bad wires are joined at their distant ends, and one terminal of the battery connected to the point, e, on the bridge, while the other terminal is grounded. It is not TESTING. 45 J difficult to see that the partial ground or fault now bears the same relation to the bridge as the point, z, in the diagram of Fig. 342 ; the rheostat arm now includes the resistance, /?, plus the resistance of the bad wire to the fault, while the unknown arm includes the resistance of the good wire, plus the resistance of the bad wire on the other side of the fault. The equation of the bridge, when balanced, then becomes A - R + X B '-" C + Y' where R is the unplugged resistance of the rheostat, X the resistance to the fault, Y the resistance beyond the fault, and C that of the good wire. Now calling L the resistance of the loop consisting of the good and bad wires, we have L = X+ Y+ C, or C + Y-L-X. Substituting this in the second member of the equation of the bridge, we have A R + X whence X = B ' L- X A L- B R A + B which is independent of the resistance of the fault. When the two ratio arms of the bridge are given equal values we have A = B, and the equation for X becomes : L may be known from records previously made, may be com- puted from the size and length of the wires, or, if only one ground is present on the bad wire, it may be measured directly on the bridge. Sometimes, in ordinary paper cables, a requirement is made that a rubber-covered test wire shall be run through the center of the cable, so that at least one good wire may always be available in testing. Where no good wire is available, a separate wire may be strung to be used as the return in this test. If the lead wires, from the instruments to the faulty wire, have appreciable resistance, this should be measured, and deducted from the value of X. After this the distance to the fault may be readily obtained from the resistance per foot of the conductor. 452 AMERICAN TELEPHONE PRACTICE. There is another loop test not quite so convenient as the Var- ley for those possessing only an ordinary bridge, but available and reliable where one has a standard resistance box and a gal- vanometer. This is known as the Murray loop test and connec- Fig. 358. Murray Loop Test. tions for it should be made as shown in Fig. 358. The point, e, may be a plug inserted into an intermediate hole in a standard resistance box, the points,/" and h, being the end terminals of the box. If two separate resistances are available, the two may be connected in series, the point between them being then repre- sented by e, in Fig. 358. The two resistances are manipulated until a balance is obtained, no current being indicated by the galvanometer, when A X B ' ' c -f y the significance of the various letters being the same as used in describing the Varley test above. From this is obtained A _ X B ' L- X' whence X = AL A +2 in which L is as before the resistance of the loop. of X is independent of the resistance of the fault. This value CHAPTER XXXIV. AUTOMATIC TELEPHONE EXCHANGES. IN the early days of telephony, when the most trifling piece of apparatus furnished a problem for grave consideration and when everything connected with the new art called for designs for which there were no precedents, not only the apparatus, but the very basic ideas underlying the interconnection of subscribers, were so crude as to be positive curiosities. The men who designed most of the early systems, or rather who adapted such telegraphic apparatus as they could use to telephonic purposes, were mostly telegraph operators, men who had received their training in that school which for a very long time produced the only available telephone engineers. In these days, when teleph- ony and telegraphy are differentiated to such a degree that they might as well be totally foreign arts, it seems absurd that the attempt should ever have been made to work telephone exchanges upon telegraphic principles. Yet when the demand arose for means to promptly interconnect and disconnect sub- scribers' lines as occasion required, nothing was immediately available for this purpose but telegraphic apparatus, and those who were accustomed to its use naturally seized upon it as a means of temporarily satisfying conditions until experience had taught the true principles upon which future designs should be based. Even after the lapse of several years from the first announcement that speech could be transmitted, and after designs of various kinds had been produced, the methods of operation remained crude, and the improvement that has steadily progressed from that day to this has been by such slow evolu- tionary stages as to constitute the surest proof of the persistent and patient labor that has been expended upon it. The earliest forms of switch-board were operated upon the divided or trunking principle ; and there was not only a great deal of useless noise and clatter about the exchange room, but there were probably about twice as many attendants as would be considered neces- sary now. Frequently converted telephone magneto call-bells were used instead of drops, and information was communicated between the operators and the switchmen, as they were called, 453 454 AMERICAN TELEPHONE PRACTICE. either by word of mouth or by written slips carried by messen- gers. In view of this confusion and complication, the results of which, moreover, were far from perfect, and the expense account due to the salaries of the large number of operators, it is not surprising that when the subject of automatic teleph- ony was broached it proved a fascinating study. It is with the idea of following briefly, though not completely, the growth of an interesting phase of telephone work, rather than of attempting to chronicle any really practical develop- ments or of giving any hopes of its future practicability that this chapter is written. As early as 1879 Messrs. Connolly and McTighe of Washing- ton conceived the idea of having machines perform the entire work of switching lines, and they worked out a set of apparatus for the purpose that, while somewhat crude in design, neverthe- less embodied practically the same principles that have been incorporated into every scheme for automatic work that has since been devised. The idea was to do away with operators entirely, each subscriber, by means of apparatus controlled from his instrument, connecting his line to any other line in the exchange and to signal over the two lines so connected to call the desired person. This, of course, is just what every automatic sys- tem is intended to accomplish. Connolly and McTighe's appara- tus consisted essentially of a line leading from each subscriber's station to the central office and provided at the substation end with a switch whereby it could be connected either to a make- and-break device or to the telephone set. At the central station each line was connected to three magnets, two of them being in parallel and in series with the third, the wire being continued beyond them to ground. With his switch turned to connect his make-and-break device, any subscriber could cause current impulses to affect his magnets at central, to control certain switching devices, and to continue his line into connection with any other line in the exchange. Fig. 359 shows the arrangement of apparatus and circuits in Connolly and McTighe's system. A and B are subscribers' stations connected by line wires I and 2, respectively, to the central office, C. At each subscriber's station there are pro- vided a battery, M, a switch, S, a telephone, T, a dial telegraph instrument or impulse transmitter, D, a pole-changing switch, P, and a bell or ringer, R. As the apparatus is exactly the same at both stations, it will be described only in connection with station B, wherein the main line, 2, is shown as connected by means of AUTOMATIC TELEPHONE EXCHANGES. 455 to 45 6 AMERICAN TELEPHONE PRACTICE. the switch, S, through the bell, R, to ground. This is the normal condition of the circuits so that the subscriber may receive a call. By turning the switch, S, to the contact, s, the subscriber is enabled to disconnect the line from his bell and con- nect it through the dial transmitter, D, to one arm of the pole- changing switch, the other arm of which is grounded, and the three points of which are connected, two to one and one to the other side of the battery, M. Thus while operating the hand of the transmitter a succession of makes and breaks due to success- ive contacts of the teeth of the wheel of the instrument with its pen, can be sent by the subscriber over the main line to the central office. The pole-changing switch normally rests in the position shown, so that, in signaling, current of a given polarity is normally on the line ; but by throwing over the arms of the pole-changing switch, such polarity can be reversed. By moving the switch arm, 5, around to contact, /, the line circuit can be extended through the secondary winding, /, of the telephone, T, to ground. At the same time an insulated piece, S', carried on the end of the switch arm, by making contact with points, j 2 and s*, will close a circuit including the battery, M, and the primary winding, t', of the telephone, T. This is the condition of the circuits during conversation. At the central station a machine is provided consisting essen- tially of a series of segmental strips arranged side by side and each provided with an arm adapted to sweep over it, the centers of rotation of all the arms lying in a common axis. The axis is lettered E in the figure, and mounted upon it are a series of toothed wheels, E ', each of which carries its own individual arm, E*. Normally the arm, E\ rests in a vertical position, as shown at the right of the figure, being held there by a suitable coiled spring not shown. Each wheel, E', is under the control of a pawl connected to the armature of a magnet, F, to which the main line is connected, and is prevented from making a retro- grade movement during operation by a suitable retaining pawl. From the magnet, F, each line passes by one path to a " dis- abling " magnet, F', and by another path to a polarized relay, H. Overlying and extending along the entire series of segments, G, are an equal number of contact bars, 7, arranged on radii from the axis, E. Each of these bars carries a series of little contact hooks, one hook on each bar overlying each segment, G, and adapted to be touched by the tip of the contact arm, E*, as it sweeps around. The arrangement of the bar and its contact hooks is shown in Fig. 360,, where it will be observed that a AUTOMATIC TELEPHONE EXCHANGES. 457 sleeve, e, is carried on the end of the arm, ?, upon which it has a slight movement. This sleeve is provided with a small finger or trailer which constantly makes contact with the segment, G, and also has an overturned or hooked end adapted to touch and engage with the contact hooks upon the bars, /. Above each bar, 7, is the disabling magnet, F', and as there is a bar for each line, of course there is a disabling magnet for each line also. The armatures of the disabling magnets are polarized, so that Fig. 360. Detail of Mechanism. they will not be attracted by the passage through the magnet windings of the current that is normally used for signaling; and each armature is normally attached to each of the bars, /, so that when it is attracted it will lift said bar, and by reason of the hook construction will also lift therewith the sleeve, e, on any arm, E", which may have been brought around into position for the sleeve hook to engage a bar hook. As previously stated, the magnet, F', is connected on one side to the magnet, F, and on the other side to its own bar, 7, and consequently to all of the contact hooks on said bar. Its connection to the magnet, F, however, is broken at points, x y, whenever the arm, JS\ is moved from its normal position. These points are adapted to be nor- mally bridged by a contact carried on the individual arm, E a , when the latter is in its zero position. One branch of the line, after passing through magnet, F, goes to the polarized relay, If, 45 8 AMERICAN TELEPHONE PRACTICE. and thence to the segment, G. The sole function of this polarized relay is to normally keep a ground on the segment, G. A reversal of current through the relay causes its armature to be retracted or thrown over so as to break the ground circuit from the segment. The operation of this system is as follows : Suppose subscriber A desires to converse with subscriber B. He first throws up his switch, S, so that the line is connected to the transmitter, D, and then proceeds to move the hand of the latter around until it reaches the number of subscriber B, which we will suppose is 3. As soon as the switch, S, was thrown up, current commenced to pass from battery, M, over the line to and through magnets, F, the relay, H, to segment, G, and to ground and back to the battery. As the transmitter is operated the teeth pass under the pen and make and break the circuit as many times as may be necessary to reach the particular number desired. These successive makes and breaks cause the magnet, F, to alternately release and attract its armature, and each successive make or attraction advances the wheel, E, and consequently the arm, E*, one step forward. The first step causes the arm to move away from the contacts, x and y, and successive steps bring its sleeve hook, one after another, into contact with the contact hooks of the bars, /. When it has been stepped around to number 3, which in the present case is the desired line, it is left there with the hook of its sleeve in engagement with a hook on the bar, /, of that line. A circuit is then complete from the calling station, A, by line wire, I, through magnet, F, to and through the magnet, H, to the contact segment, G, through the finger to sleeve, e, by contact hook to the bar, /, to the magnet, F', and thence by way of the magnet, F, of the line called to its line wire and station. Before conversation proceeds, it is necessary to remove the ground which is normally connected to the segment, G, and also to disable the line connection so that nobody else can call either of the connected lines during the conversation. It will be observed that by making connection with the bar, /, of the wanted line, the calling subscriber has obtained a circuit through the disabling magnet, F', of that line. In order to work the dis- abling magnet the calling subscriber throws over his pole-changing switch, P, with the result that current of opposite polarity will immediately start to flow through the united circuits. This will not affect magnet, F, but it will affect the relay, H, whose arma- ture is thrown over and cuts off the ground, and also the magnet, F', which immediately acts to draw up toward itself the entire AUTOMATIC TELEPHONE EXCHANGES. 459 bar, /, with all of its hooks. These hooks are thus lifted beyond the reach of the sleeve, on the armature lever, H, acts upon the smaller toothed wheel, E', to turn the arm past a group of contacts at each step, this group being conveniently a decimal group, or ten. The pawl, t, on the armature lever, 7, on the other hand, acts upon the toothed wheel, E, to turn the arm from one single contact to the next at each step. Thus, if each cir- cular row contains one hundred contacts, the pawl, g, may be AUTOMATIC TELEPHONE EXCHANGES. 461 used to step up the arm to the proper hundreds row, the pawl, h, will then be used to step the arm rapidly around to the proper tens group in that row, and lastly the pawl, z, steps the arm a certain number of units farther to make contact with the partic- Fig. 362. Central-Office Mechanism, Strowger System. ular unit desired. The three magnets, K, which control these pawls are included respectively in the three circuits, g' y h', and 2', which extend to the subscriber's station, X, and are there placed under control of the push-buttons, G', H', and /'. Each pawl is connected by a link, P, to the armature of a restoring-magnet, K', and all of the restoring-magnets, K', are included in series in the circuit,/', which also extends to the subscriber's station, and is there placed under control of the fourth push-button, P'. N is the telephone wire, which is used for talking only. Thus there are five wires extending from each subscriber's station to the central station ; and for each station there must be at the central office one of the cylinders, A, containing a contact for every other line in the exchange, and a complete set of magnets, KK ' . The operating battery is placed at the subscriber's station, and the telephone wire, N, is connected through the shaft, D, to the arm, ", and is also multipled off to its appro- priate contact on every other cylinder. The defects in this system are perfectly obvious. The heavy battery required at each subscriber's station, the multiplicity of wires extending therefrom to the central office, and the compli- cated system of operating magnets, are sufficient to condemn it ; but when it is considered that the number of contacts on the 462 AMERICAN TELEPHONE PRACTICE. cylinders, A, in the exchange increases directly as the square of the number of lines, the total impracticability of the scheme is apparent. Thus, in an exchange having five hundred lines there would be two hundred and fifty thousand contacts. This system formed the subject-matter of the original Strowger patent, which was dated March 10, 1891. In 1892 and 1893 two other patents were granted to the same inventor for apparatus to be used in the same sort of a system. This apparatus is very complicated, and it is not thought necessary to describe it, inasmuch as it is not used, and, commercially speak- ing, is absolutely impracticable. In 1894 a system was produced by Keith, Lundquist, and Erickson, which was acquired by the Strowger Company, and which gave promise of being at least a starter toward practicable designs. Fig. 363 gives a general idea of the circuits in this system. A series of wires, 8, are arranged parallel to each other, one for each line coming into the exchange. A series of shafts, 2, having both rotary and longitudinal motion, are arranged transversely to the wires. Each shaft carries a series of arms, 3, 4, 5, etc., set upon it spirally that is, occupying different angular positions around it. Step-by-step mechanism is utilized for both motions of the shafts, and a common battery is utilized to supply the energy. The connecting wires at the central office are preferably divided as shown into tens groups, and each shaft has an arm for each group. The mechanism for this system is shown in plan and side ele- vation in Fig. 364 ; this figure also showing the wires, 8, divided into groups A, B, C, D, and E. Shaft, 2, passes through a miter-wheel, 9, which is mounted in the top of a bearing, 10, to permit rotation only, the shaft being capable of rotation with the miter wheel and also of a longitudi- nal movement therethrough, being held to revolve with the wheel by the long spline, II. The shaft is caused to rotate, step by step, by virtue of electromagnet, 12, operating bell-crank (ever, 13, whose pawl, 14, engages the teeth of ratchet-wheel, 15, which ratchet-wheel is mounted to rotate on a stud-pin, 16, with miter-wheel, 17, the two miter-wheels being in engagement as shown, so that any rotary movement given to ratchet- wheel, 15, is transmitted to the shaft. A helical spring, 19, revolves the ratchet-wheel and the bevel-wheels in an opposite direction to that imparted by the action of pawl, 14. A stop-pin, 20, projects outwardly from ratchet-wheel, 15, which strikes against a bent wire, 21, and limits the motion imparted to it by AUTOMATIC TELEPHONE EXCHANGES. 463 spring, 19. A spring, 50, attached to, but insulated from, the base plate, is adapted to be engaged by the pin, 20, when the Fig. 363. General Plan Strowger System. wheel, 15, has been rotated one step from its normal position, and to be disengaged when the wheel has been rotated farther. A projection, 22, from standard, 23, against which the stop-pin 464 AMERICAN TELEPHONE PRACTICE. strikes, serves to limit the motion imparted by the pawl, 14, the ratchet-wheel never completing a whole revolution. Detent, 24, when in engagement with the teeth of the ratchet-wheel serves to hold it after the thrusts of pawl, 14. At the inside end of shaft, 2, is an enlargement, 25, which is fitted with a series of circumferential grooves which are engaged Fig. 364. Plan and Elevation of Single Strowger Machine. by a cog-wheel, 26, mounted together with ratchet-wheels, 27 and 28, upon a stud-pin, 29, which is secured at one end to a stout standard, 30, projecting upward from base plate, I. Cog-wheel, 26, is made to rotate step by step in one direction by virtue of electromagnet, 31, operating bell-crank lever, 32, when pawl, 33, is in engagement with ratchet-wheel, 27, ratchet- wheel, 28, serving as a stop-wheel when engaged by the outer end, 32', of bell-crank lever, 32. Helical spring, 34, serves to operate cog-wheel, 26, in the opposite direction to that just described. The electrical connections of the exchange and of the sub- scribers' stations, so far as the manipulating keys are concerned, AUTOMATIC TELEPHONE EXCHANGES. 4&5 are shown in Fig. 363. Each of the magnets, 12 and 31, is con- nected on one side to one pole of the common battery, the other terminal of which is grounded. The other side of magnet, 12, is connected to one line wire, P, while the other terminal of mag- net, 3 1, is connected through wire, Q, to spring, 50. The line wire, R, is permanently connected to the shaft, 2, of its particular machine through the frame, i. The bent arm, 21, is permanently connected with one of the wires, 8, by the wire, /, there being thus one machine and one wire, 8, for every metallic-circuit line leading to a telephone instrument. The arm, 21, and therefore its corresponding wire, 8, is normally in connection with the shaft, 2, through the pin, 20, but as soon as the wheel, 15, has rotated one step this connection is broken. It will thus be seen that the first step of the wheel, inbreaks the circuit between the shaft and its wire, 8, thereby preventing outside interference. At the same time a circuit is established through the magnet, 31, whereby the shaft may be moved longitudinally if desired. It will be noticed that in Fig. 363 the wires, 8, are arranged in three groups, numbered from 101 to 191, 102 to 192, and 103 to 193, respectively. Each shaft, 2, is provided with three contact- arms which by rotary motion of the shaft may be brought, one at a time, into contact with the three wires over which they lie. By the longitudinal motion of the shaft the three contact-arms may be brought over any of the other wires in their respective groups. Four keys are placed on each subscriber's instrument marked " H " (hundreds), " T " (ten), " U " (units), and " R " (release). In order that button H may always be pressed at least once, sub- scribers are numbered from 100 up. The operation of the key H will close the circuit through wire, P, magnet, 12, and wire, JV, and will rotate the shaft one step, and cause pin, 20, on ratchet- wheel, 15, to make contact with spring, 50, after which each opera- tion of the key T will close the circuit through the wire, R, and the frame and mechanism to pin, 20, spring, 50, magnet, 31, and wire, N, and will move the shaft longitudinally one step. By this means the contacts on the arms may be brought directly over the set of wires in which the one desired belongs. The unit key may then be operated as many times as is necessary to cause the proper switch-arm to engage with the desired wire. For instance, if a subscriber wants to call 143, he first presses the key H which indicates the hundreds and also rotates the shaft one step. He then operates the key T four times, which carries the shaft four steps, after which he operates the U key 466 AMERICAN TELEPHONE PRACTICE. three times, which makes the third switch-arm contact with its wire. To release the mechanism, the key R is pressed down, which will close the circuits through both wires and magnets and the wire N and will permit all the parts to assume their normal positions. Lever, 13, being held in the operated position, causes pawl, 33, to be forced back under projections, 39, by arm, 37. When lever, 32, is operated, the detent, 38, and the attached detent, 24, are lifted from their ratchets, and the springs, 19 and 34, cause a backward movement of the operative parts to the initial position. This system as described could not accommodate more than 100 subscribers, there being no means for selecting between various hundreds groups. With improvements made later by Messrs. Strowger, and the others mentioned, this general system has been put into operation in a few small exchanges. In these later developments, the shafts, 2, were provided with a number of insulated sections, each carrying a set of contact-arms or wipers, each set adapted to be rotated over a separate set of connecting wires, 8. The movement of the hundreds key was made to select the particular section of shaft which operated over the bank of connecting wires in which the wire of the desired subscriber belonged, and connected that section with the line of the calling instrument, after which the longitudinal and rotative movement of the shaft selected the particular wire in that group. Further developments have been made by the same parties in matters of detail ; but no matter how many improvements may be effected in this type of apparatus, the enormous number of contacts required in an exchange of any size must ever be appalling to engineers who have spent much of their time in endeavoring to reduce, by perhaps a few hundred, those ordinary contacts in a manual exchange which are productive of the greatest amount of trouble. The latest type of Strowger apparatus, and that which is in use in some established exchanges, has semi-cylindrical plates with the contact-arms mounted to revolve in front of them, and the line contacts imbedded in them and protruding from their surfaces. Unless the entire exchange be inclosed in glass, these contacts are well calculated for the free deposition of moisture and dust, and for oxidation. One of the largest exchanges operating on the Strowger system is that at Augusta, Ga., which is arranged for an ultimate capacity of 1000 lines, and which is now equipped with machines for AUTOMATIC TELEPHONE EXCHANGES. 467 about 500 lines. In Fig. 365 is shown the switch-board with its banks of machines, and in Fig. 366 one of the machines is shown detached. The type of telephone used in the Augusta exchange is shown in Fig. 367. In this a dial is used for obtaining the desired connection, instead of the four keys already described. In order to call No. 653, the subscriber first takes down his Fig. 365. View of Strovvger Exchange. receiver, then places his finger in the slot numbered 6, and turns the dial until his finger strikes the stop on the lower edge of the dial, then he lets go and the dial returns to normal position. He repeats this operation with the slots numbered 5 and 3, and he is in direct connection with the desired telephone. He then turns the magneto-crank on his telephone, and this rings his own bell and that of the telephone 468 AMERICAN TELEPHONE PRACTICE. called. If his bell does not ring, he knows that the telephone he is trying to call is already in use, and he hangs up his receiver and waits a reasonable time before attempting to make another call. It is reported that the average number of calls per subscriber Fig. 366. View of Single Machine. per day in the exchange is 12, which indicates a busy exchange, especially for a Southern town and for one of its size. Fig- 368 shows diagrammatically the circuits of a system devised by Mr. E. A. Clark in 1892 ; and as it has been put into actual service to some extent it will be described. The under- lying idea is very similar to that of the original Connolly and McTighe system, the main difference being that Mr. Clark AUTOMATIC TELEPHONE EXCHANGES. 469 employs separate and distinct wires for talking and for calling. In the figure A and B are supposed to be two subscriber's stations, each equipped with a telephonic outfit, T, and an impulse transmitter, D. This latter consists of a toothed wheel carried on a shaft which also carries a gear-wheel, d, with which meshes a pinion, d', to which a crank is fitted. The main shaft Fig. 367. Strowger Telephone. also carries an indicating hand which moves over a dial so that the desired number of impulses may be sent without mistake. Each impulse transmitter has a pen,/, connected on one side by wire, I, to the ground and on the other through the tele- phone instrument to line. At the central office each line is provided with a step-by-step device, S, in which a ratchet-wheel carrying a contact-arm is moved around by a double pawl of the anchor type, carried by an armature lever, s, and operated by an 470 AMERICAN TELEPHONE PRACTICE. electromagnet, s ' . Each device, 5, has a series of contacts over which the arm is adapted to sweep, and to which branches from all the line wires coming into the exchange are connected. Suppose now that station A desires connection with station B. The subscriber moves his arm around the dial of the trans- mitter, D, until he has sent tkree impulses, 3 being the number of subscriber B, Current from battery, M, will then have energized the magnet, s', three times, and the wheel will have been rotated through the space of three teeth, so that the arm rests as shown upon the third contact. A circuit will then be ajr rP 'Jo J? 5- L 4 c' 4 " ^5 SL * t. K-rrnTx ^P> SM I Fig. 368. Circuits Clark Automatic System. complete fr.om ground through the wires, i and 2, to the tele- phone, C, by line wire, 3, and branch, 4, to the shaft of the stepping device, S, by the arm to the third contact, and thence by wires, 5 and 6, to the telephone set at station, B, and thence by wires, 7 and 8, to ground. Subscriber A can then, by operating his magneto-generator, ring the bell at station B. Some improvements in this system have been made by Mr. Clark and by others, but its principal fault is that common to all automatic exchanges a multiplicity of contacts which increases at a ratio far greater than that of direct proportion to the number of subscribers. CHAPTER XXXV. STORAGE BATTERIES. IF two plates of lead are immersed in a weak solution of sul- phuric acid, no difference of potential will be established between them, because the acid, if it acts on them at all, does so to an equal extent on each plate. If now an electric current, as from a battery or a direct-current dynamo,, is sent through the two plates and the solution between them, a redistribution of ma- terials will take place in the cell. The electrolyte will be decom- posed, the oxygen in it forming, with the plate to which the positive terminal of the charging source is connected, lead per- oxide ; while hydrogen is liberated at the plate to which the negative terminal is connected. On disconnecting the source of o <- current, the cell, which was before incapable of producing a difference of potential, is found able to drive a current through a circuit formed by connecting its poles together by a wire or any other conductor. The combination has become voltaic couple. The cell, consisting of two lead plates in a solution of sulphuric acid, was devised by Gaston Plante, and is the prototype of all modern storage batteries or accumulators. Nearly all commer- cial cells, of which there are many, have the plates coated with, or in close mechanical contact with, some compound of lead, rich in oxygen. This is changed by the charging current into lead per- oxide on the positive plate, and to spongy lead on the negative. In this condition the cells will give an electromotive force of slightly over 2 volts, the pressure remaining nearly constant during the greater portion of the time while the cell is being dis- charged through some external circuit. The direction of the current flowing from the cell while discharging is always opposite to that of the charging current. A charged storage cell behaves exactly like a primary battery, but it has the advantage that after being discharged it can be again made useful without the addi- tion of any material whatever, by merely sending a current through it in the proper direction. In all but the smallest storage cells more than two plates are used, all the positive plates being connected by a heavy strip of lead, and likewise all the negative plates by another similar strip. 47 2 AMERICAN TELEPHONE PRACTICE. There is usually one more of the negative than of the positive plates, the arrangement being such that the plates are alternately positive and negative. The extremely low internal resistance of storage batteries, and the fact that their voltage is high (2 + volts per cell) and constant and that they are not subject to polarization, make them, all things considered, the ideal source of current for telephone work. They are much more economical in operation than any form of primary cell, inasmuch as there is practically no consumption whatever of the materials in the cell itself, it depending of course for its energy on some outside source. Their ease of manipula- tion and general cleanliness and reliability are also strong points in their favor. They have long been used for supplying the operators' transmitters in large central offices, but the recent de- velopments leading to the almost universal adoption of the vari- ous common-battery systems have vastly increased their field of usefulness, since they are now called upon to furnish current for subscribers as well as operators. Among the several good storage batteries on the market, the chloride accumulator made by the Electric Storage Battery Company of Philadelphia may be mentioned first on account of its being more widely used than any other. In this the negative plate is composed of a number of small square blocks of spongy lead held together by a grid of lead with a small amount of antimony added for hardness. The blocks are made by fusing together zinc chloride and lead chloride, after which they are placed in a mold and the grid cast around them under pressure. Afterwards by an electro-chemical process all traces of zinc are removed, leaving the composition of the blocks pure spongy lead. The positive plate consists of a lead-antimony grid in which circular holes are molded. These holes are filled with buttons made by rolling up a crimped lead ribbon in the form of a spiral, of a size to fit tightly in the holes. The plates are then treated electro-chemically in order to form the proper oxide from this ribbon. The positive and negative plates so formed present a large sur- face to the electrolyte for it to act upon, the negative plates on account of the porous spongy-lead blocks, and the positive plates on account of the crimped lead ribbon. The plates are held apart by long hard-rubber washers hung over each end of each negative element. These are placed in a vertical position to prevent forming a shelf upon which loose particles from the plates might lodge and form short-circuit. These cells have STORA GE BA TTERIES. 473 amply demonstrated their adaptability to telephone use by long service in this and other fields. The electrolyte for these cells is a mixture of sulphuric acid and water in the proportion of about five parts of water to one of acid. The proper specific gravity of this mixture, as specified by Fig. 369. Eleven-Plate Chloride Cell. the manufacturers, is from 1180 to 1190, as indicated on an or- dinary hydrometer. In Fig. 369 the general appearance of one of these cells having six negative and five positive plates is sen. Fig. 370 shows a larger cell having ten negative and nine positive plates mounted in a lead-lined wooden tank. Lead-lined wooden tanks are preferable to glass jars in large batteries, on account of the liability of glass to breakage. In Fig. 371 is shown two batteries, of four cells each, of chloride accumulators, as used by the Bell Telephone Company at Philadelphia. These batteries are for operating the 474 AMERICAN TELEPHONE PRACTICE. cut-off relays and supervisory lamps. This cut also gives a good idea of the modern practice in regard to the arrangement of charg- ing and ringing machines and of the power switch-board. The ma- chines are mounted on a firm table, usually having a framework of structural iron, and placed near the switch-board. The switch- board is of slate or marble usually the latter mounted in an Fig. 370. Nineteen-Plate Chloride Cell with Wooden Tank. angle-iron framework and bearing all meters, circuit-breakers, rheostats, starting boxes, and switches for controlling all of the power circuits. Another type of storage battery largely used by both Bell and independent companies is that made by the American Battery Company of Chicago. In this battery the plate is made from a solid sheet of pure rolled lead deeply grooved oh both sides so as to leave projecting ribs, one-twentieth of an inch apart, affording a very large surface for the electrolyte to act upon. The form of a single plate is quite clearly shown in Fig. 372. The ribs are STORAGE BATTERIES. 475 slightly upturned in order to better retain the active material carried in the grooves between them. The active material in the plate is electro-chemically formed in a strongly oxidizing solu- tion, filling the grooves and covering the surface of the ribs with an adherent peroxide of lead coating. The positive and nega- tive plates are alike in construction, the active element, spongy lead, in the negative, being reduced from the peroxide of lead of the positive. Hard-rubber insulators, shown in Fig. 373, serve Fig- 3/1. Storage Battery and Power Plant, Bell Exchange, Philadelphia. to separate the plates as well as to hold them clear of the bottom of containing jars. The insulators are held rigidly in place, being firmly keyed to the plates and surrounded by heavy rubber bands as shown in Figs. 374 and 375. These cells have a very low internal resistance, are electrically efficient, and have long life even in severe service. The electro- chemically formed active material and the division of it into small units, so that nowhere in the plates is the adherent coating more 476 AMERICAN TELEPHONE PRACTICE. than one-fortieth of an inch from its leaden support, which serves also as a conductor, are among strong points claimed for this cell. The excellent mechanical construction of the " American " plate itself, wherein a sufficient quantity of lead is employed to insure long life ; the absence of " local action," due to the fact that pure lead only is used ; and the substantial manner in which the complete cell is built up, render them desirable for telephone service, in which work they are now being extensively used. INSTRUCTIONS FOR INSTALLATION, CARE, AND MAINTE- NANCE OF STORAGE BATTERIES. Setting Up and Connecting. The elements, before being placed in the glass jars, should be examined carefully, and any foreign substances which may have Fig. 372. Single Plate of American Cell. Fig. 373. Hard Rubber Separator. lodged between the plates should be removed. See that the hard-rubber insulators are in position and that the positive plates do not in any way touch the negatives. After the elements are placed in the jars, connect the terminals the positive of one element to the negative of the next and so on. The best method of connecting the cells permanently is to burn the terminal strips together, or when it is not practicable to do this solder them together, using a hot iron or soldering torch, first scraping bright the surface to be soldered. For a soldering flux it is best to use ordinary pure tallow. Under no circum- stances should muriatic acid or soldering salts be used. For temporary connections the connectors usually sent with the ele- ments will answer. To prevent corrosion, these must be painted STORAGE BATTERIES. 477 with some protective paint and well covered with okonite tape. The best method to thoroughly insulate the cells is to place each one on a separate wooden base painted with good in- sulating paint, this base resting on glass or porcelain insulators; or to place them on shelves or tables covered with sheet glass. Care should be taken to place cells in a cool, well-ventilated place, where each cell is readily accessible, so that elements can be removed when necessary with the least possible trouble. Lead- Burn ing. As stated under the preceding heading, the best way to per- Fig. 374. American Plates Assembled. Fig- 375- Complete Nine-Plate American Cell. manently connect the lugs or terminal strips of the separate cells is to burn them together. This process is not difficult to carry out if well understood, but of course the beginner should first experiment on lead strips that do not form the terminals of storage cells. The best flame to use for lead-burning is the hydrogen flame, as this can be used without flux and in general produces cleaner work. A hydrogen generator is necessary for this, this usually consisting of two lead chambers, one above the other and con- nected by a lead pipe. The pipe leads from a point near the bottom of the lower chamber, out at the top of this chamber, and into the bottom of the upper one. Equal parts of zinc and sul- 478 AMERICAN TELEPHONE PRACTICE. phuric acid are put in the bottom chamber. The acid in attack- ing the zinc forms zinc sulphate and liberates hydrogen gas. As the gas is generated the acid solution is forced up into the upper chamber, thus maintaining a fairly constant pressure, which is usually from 6 to 8 pounds per square inch. A rubber tube serves to lead the gas from the top of the lower chamber to the burner tip. The acid used for this purpose should be free from arsenic and most other impurities. Arsenic will make a white deposit on the lead, and thus may cause a poor connection. The burner used 'for this work is usually of a form shown in Fig. 376, having two leading-in tubes, one for air and one for gas. These are provided with several tips having different-sized holes LARGE FLAME TIP Fig. 376. Burner for Lead-Burning. for obtaining different sizes of flames. Air must be supplied to the burner by an ordinary form of foot-bellows or by any other available means. Lead-burning may often be successfully done by using ordinary illuminating gas instead of hydrogen, but this depends largely on the quality of the gas. Natural gas is sometimes used, but this is likely to prove unsuccessful. In any case the work is somewhat more difficult to perform than when hydrogen is used. The method of procedure will depend somewhat on the form of terminals to be joined. On comparatively small cells the terminals are usually of the form shown in Figs. 369 and 375. With these, the terminals should be bent down and beveled off as shown in Fig. 377, and the surfaces to be joined should be scraped perfectly clean and bright. A scraper made by securing a triangular piece of steel with sharp edges to a suitable handle, STORAGE BATTERIES. 47 as shown in Fig. 378, is very convenient for this purpose. A small sheet-iron trough, as shown in the lower portion of Fig. 377, and having an anterior cross-section equal to the cross-section of the terminals, should be slipped over the two lugs from beneath and secured in place by a wooden clamp or in any other con- venient manner. A chamber is thus formed into which the melted solder may be run as described later. If the battery is of large size, it will usually have a number of lugs, which will be joined to a cross bus-bar when two cells are con- nected. This form is clearly shown in Figs. 370 and 371. When burning such lugs and bus-bars together, a pair of lead-burning tongs made of iron, and of a form shown in Fig. 379, should be used. The front surfaces of these are beveled to an angle accur- ately corresponding to that of the bus-bar, so that they will fit SHEET IRON TROUGH. Fi.-v 377- Method of Burning Small Terminals. snugly against the bus-bar when in place. The inner surfaces of these tongs should be parallel when opened just wide enough to fit the lug on the cell. The lug after being properly beveled and scraped is brought into position almost touching the bus-bar, the surface of which has also been scraped, after which the tongs are applied in such manner as to form a chamber in which to run the melted solder. Having proceeded thus far according to either of the methods described, the blow-torch, using either hydrogen or illuminating gas, may be brought into play. The solder should be pure lead. No flux will be needed with the hydrogen^ but tallow will be re- quired with the other gas. The solder should be melted off the stick and allowed to drop into the chamber, and at the same time the surfaces to be joined should be kept just at the melting-point by a judicious application of the flame. The chamber may thus 480 AMERICAN TELEPHONE PRACTICE. gradually be filled, and if all goes well the solder will unite per- fectly with the lead surfaces, thus making a continuous piece of metal. The melting-point of lead is not far from 610 F., and in burning the lugs to a bus-bar of a large cell great care must be taken not to entirely melt the lugs before the bus-bar is suffi- ciently hot it requiring, of course, more heat than the lugs. In the flame of the burner there will be noticed an outer bluish-red and rather scattering flame, and an inner flame blue and well de- fined. The best results are usually obtained by holding the point of this inner blue flame on the surface to be burned. In burning lugs it is usually better to use a comparatively large flame, as shown in the upper portion of Fig. 376. In burning seams, SHARP EDGE, Fig. 378. Scraper for Lead Terminals. however, the small flame shown in the lower portion of that figure is more desirable. After the metal has thoroughly set, the tongs or clamps may be removed and the joint trimmed up a little if necessary for appearance. It is always well to leave the joint of slightly larger cross-section than the lugs. Electrolyte. After the cells are connected up, and the terminals of the bat- tery led to the switch-board so that the charging current is ready to be turned on, the electrolyte may be added. It is important that this should not be done before. The electrolyte, consisting of a mixture of pure sulphuric acid and water, preferably distilled, should indicate a specific gravity of 1190 on the ordinary specific-gravity hydrometer or 23 Baume scale. This solution should be mixed in stone jars in about the proportion of five parts water to one of concentrated sulphuric acid, by volume, pouring the acid slowly into the water. It is very dangerous to pour the water into the acid, and one cannot be too careful on this point. The electrolyte becomes STORAGE BATTERIES. 481 hot after mixing, and it should be allowed to cool for at least four hours before using. Under no circumstances allow the glass jars to be used for mixing purposes. River and well water usually contain impurities and should be avoided, as the least quantity of chlorine or ammoniacal salts present in the electrolyte will seriously affect the life of the plates. Charging. Great care must in all cases be taken that the batteries are so connected that the charging current passes through them in the proper direction. The positive pole of the battery must be con- nected to the positive pole of the generator. The positive pole Fig- 379- Method of Burning Lugs to Bus-Bars. of the generator leads may be determined by the use of a direct current volt- or ammeter. A simple test, and the most reliable one, for determining which is the positive pole of any source of current is to dip wires leading from both terminals into a small vessel containing slightly acidulated water. Bubbles of gas will be given off from each wire, but at a very much higher rate from the wire leading to the negative pole than from that leading to the positive. The poles of the charging dynamo should always be determined with absolute certainty before connection is made to the terminals of the storage battery, for a reversal in the con- nections is very likely to ruin the battery. As soon as possible after the electrolyte has been poured into the glass jars, seeing that the plates are covered with at least 482 AMERICAN TELEPHONE PRACTICE. o:ie-half inch of the fluid, the elements should receive their first charge. It is well to begin at about half the normal rate of charg- ing, but after making sure that all is going well the charging may proceed at the normal rate, which is always given by the manufacturers, but which may be found by dividing the normal rated capacity of the cells in ampere hours by eight. Thus for a cell the rated capacity of which is 400 ampere hours the normal charging rate would be 50 amperes. The manufacturers of the American cell advocate the continuance of the first charge for 30 hours, while the chloride people advocate 20 hours. During the last hours of the first charge the solution will ap- pear to boil. The specific gravity of the electrolyte which fell much below 1200 shortly after it was first poured into the cells should have risen to 1225 by this time. If it is higher than this, water should be added until the solution indicates uniformly 1225 ; on the other hand, add dilute sulphuric acid if specific gravity does not show 1225. A lower charging current than the normal may be used if it is not convenient to obtain that rate, but the time of charging must be continued proportionately longer. After the first charge in telephone work, when the amount of current drawn from the cells each day only partially discharges them, use the lowest charging rate consistent with the time available for charging. A 10 per cent, or even 20 per cent, overcharge will not injure the elements in any way ; excessive overcharging, however, if practiced often, seriously shortens their life. Even though for any reason the battery is only slightly used, it should, nevertheless, be charged fully about once in two weeks. Determination of Amount of Charge. By means of an accurate low-reading voltmeter the state of charge may be ascertained quite closely, providing that the same rate of charge be employed regularly. In charging at the normal rats when the pressure at the cell terminals indicates 2.5 volts it may be assumed that the cell is charged within 90 per cent, of its rated capacity, so that when this point is reached the charging operation may be discontinued. On the other hand, when the voltage falls as low as 1.8 or 1.9 the cell may be considered dis- charged, and recharging should begin as soon as possible. The hydrometer may be used with a fair degree of accuracy to show the condition of a cell, inasmuch as the density of the solution varies between full charge and discharge. As this varia- tion is a function of the ratio of the area of the plate surface STORAGE BATTERIES. 4 8 3 to the volume of electrolyte, it is difficult to state a definite rule, inasmuch as there is no constant relation between plate sur- face and electrolyte in the various sizes of cells. The specific gravity of the electrolyte may be noted at full charge, i. e., when the voltage is 2.5, and that indication made a basis for the deter- mination of future full charge. Another reading of the specific gravity made when the voltage is 1.9 per cell may be made a basis for future determination of the discharged condition of the cell. A little careful observation will soon enable the one in charge of battery to determine approximately its condition by means of the hydrometer alone. The charging rate may exceed the normal when proper atten- tion is paid to duration of charge. For instance, it may be con- venient to charge cells of 4O-ampere-hour capacity, which would have a normal charging rate of 5 amperes, with an arc-light cur- rent of 10 amperes ; in this case the current should not be left on more than 4 hours under any circumstances, and in most cases 2 hours will be sufficient. The cells may be connected in the cir- cuit the same as an arc lamp, but in all cases a dead cut-out switch must be used, as well as an automatic circuit-breaker to prevent current from backing over the line should the dynamo be stopped for any cause. Batteries may be charged from a direct-current, incandescent lighting or power circuit, but in this case some resistance must be connected in circuit, depending on the charg- ing current wanted. The resistance used may consist of lamps in parallel or iron-wire coils conveniently arranged as in the ordinary motor-regulating rheostat. Where the best efficiency is desired it is advisable to use a motor generator, the generator end being wound to suit the number of cells to be charged. This method is the only practical one for 5oo-volt direct- or alternating-current circuits. In charging from any source of electricity, an automatic under- and overload circuit-breaker should be used, so as to prevent current from the battery passing back over the line in the event of a shut-down at the power house. Discharging. At least 75 per cent, of the energy in a good storage cell is ob- tained before the voltage falls below 1.9 volt. It is advisable not to discharge beyond this point ordinarily, although a discharge to 1.8 is allowable. A cell may be considered discharged at 1.8 volt. Never under any circumstances completely exhaust the battery. 484 AMERICAN TELEPHONE PRACTICE. If a battery is allowed to stand discharged for a period exceeding two or three days, the capacity of the cells may be found on subsequent discharge to be materially lessened, due to conver- sion of the lead oxides into inert sulphate of lead. Replacing of Electrolyte. The boiling of the electrolyte when charging causes a fine spray to arise, by which some of the fluid is carried off, so that diluted acid must sometimes be added to replace it. All spilled solution should be replaced by solution of like density. Evapo- ration loss should be replaced by the addition of water only. The electrolyte must be maintained at all times over the tops of the plates and each cell individually inspected at least once a week, when the strength of the electrolyte should be tested, and if the density is below 1200 when other indications point to the full charge of the cell, dilute acid should be added, until the density reaches that figure. Never under any circumstances pour pure acid into a cell in order to bring the electrolyte up to the proper density. A portion of the old fluid should be siphoned off and fresh acid in a diluted form added. When water is added to re- place loss due to evaporation, it should be added at the bottom of the cell either by a glass or rubber tube or syringe. Defective Cells. If for any reason one or more of the cells should act strangely, that is, show a marked difference in color, voltage, or specific gravity indication from the others, they should be examined at once and the cause of the trouble ascertained. Should a nail or other foreign subtance, or any material scaling from the plates themselves, lodge between the plates, it will cause a short-circuit, and this will be indicated by low voltage and low specific gravity, and should be at once removed. Its most probable cause is the lodging between the plates of some foreign article, but it may also be due to the depth of sediment in the bottom of the cells reaching the bottom of the plates. If the short-circuit is due to a foreign body, it should be removed ; if to a loosened por- tion of the plates, it may be forced to the bottom of the cell ; if to sediment, the cell should be cleaned out. A strip of hard rubber that will go between the plates is convenient for use in this work. If the short-circuit is removed promptly, no harm will result ; on the other hand, if the cause of a short-circuit is allowed to remain, the plates may be seriously affected, injured possibly beyond repair. STORAGE BATTERIES. 485 It is important to use only glass, rubber, or wood in reaching into the cells. Metals will be attacked by the electrolyte, and thus impair its purity. Moreover, they are likely to short-circuit the cells and thus damage the plates. Treatment of Short-Circuited Cell. A cell that has suffered a short-circuit will need more than its usual amount of charge after the trouble has been removed. This may be obtained in various ways. First, by overcharging the whole battery, a bad practice if done too frequently, but it may be occasionally resorted to without evil effects. Second, by cutting out of circuit the cell in trouble on one or two discharges and replacing it on the charges. This can be most conveniently done on small cells with bolted connections, but not very well with cells permanently connected. Third, by giving the cell in trouble an individual charge during the discharge of the battery. Current for this can be obtained from either a small dynamo or another cell that is fully charged. The third method is the best one, and is easily carried out. The regular charging dynamo may be con- nected around the defective cell alone, and sufficient resistance introduced into its circuit to reduce the current to the proper normal charging rate. Color of Plates. The color of the plates is a valuable indication as to the condi- tion of the cells. The positive plate should have a dark brown, velvety appear- ance. Any lightness in color indicates insufficient charging. The negatives should have a clear bluish-lead or light slate color. Cells that have not been sufficiently charged for a continued period, or that have been left standing uncharged too long, are liable to what is termed sulphating. This is indicated by a white deposit on the plates due to the formation of lead sulphate which is insoluble in the electrolyte, and is deleterious to the action of the cells. When it occurs between the lead of the plate and the active material it is liable to loosen the latter, causing it to drop off, or, if it remains on the plate, to increase the resistance of the cell. The remedy for sulphating is to continue the charging current at about half normal value for several hours after the cells give indication of being fully charged. A whitish deposit sometimes appears on the surface of the plates in spots, even when the cells have not been subject to un- 486 AMERICAN TELEPHONE PRACTICE. der-charging. This, the manufacturers claim, need not give alarm. It usually disappears after the battery has been in service some months. It is probably a mild form of sulphating, oc- curring only on the outer surface of the active material. Taking a Battery out of Service. Charge the battery fully at a rate not higher than the normal. Siphon off the acid (which may be used again) into convenient receptacles, preferably thoroughly clean carboys, and immedi- ately refill each cell with water. Then discharge the battery at the normal rate down to less than one volt per cell. The elements should then be removed from the water and allowed to dry. They may then be replaced in the empty tanks or jars, previously dried and cleaned, or stored in any dry place. CHAPTER XXXVI. SPECIFICATIONS. IT is not the object of these specifications to enable telephone companies to dispense with the services of competent engineers and competent lawyers in installing important exchanges. Such, indeed, would be poor policy. As a reference, however, to all interested or engaged in the installation of telephone plants, it is thought they will be of value. No set of specifications can be drawn to meet the requirements of all telephone exchanges alike, for to cover all the vary- ing conditions that might arise they would necessarily be so broad as to be useless. Two sets of specifications are presented in this chapter one covering the apparatus for a very small interior exchange, and the other the apparatus, material, and labor for the complete equipment of a large exchange. Both of these represent good practice in general, but each must perhaps be modified to meet any local conditions that may impose peculiar or special requirements. SPECIFICATIONS OF APPARATUS FOR SMALL INTERIOR TELE- PHONE EXCHANGE. GENERAL CONDITIONS. This exchange shall consist of seventeen telephones, a central switch-board with operator's equipment, and all accessory appara- tus necessary to equip the system complete for operation. All sources of current for both talking and calling purposes shall be centrally located. The manufacturer is given the option of using either battery or generator current for calling purposes, and bids are invited on either or both of these systems. The system must be absolutely free from cross-talk or other inductive troubles. No instrument will be at a greater distance from the switch- board than four hundred feet. All wiring will be of No. 20 B. & S. gauge rubber-covered " fixture " wire placed in interior conduits, the latter now in place. 487 488 AMERICAN TELEPHONE PRACTICE. SWITCH-BOARD. The switch-board shall have an ultimate capacity of fifty metallic circuit lines, and shall have twenty drops and jacks installed. The drop magnets shall have annealed iron cores and armatures, and shall be wound with a silk-covered wire to a suitable resistance, shall be readily removable, and shall be absolutely free from "sticking" or " freezing." If battery cur- rent is to be used for throwing the shutters, the drops must be capable of being operated by one-fourth the normal battery pressure, and, with the same adjustment, of standing double the normal pressure without " freezing." The jacks must be firmly constructed with springs of sufficient length and strength to insure the proper making of the electri- cal contacts. All springs and contacts in the jacks must be thoroughly insulated from each other, and all parts must be readily accessible for repairs. The switch-board shall be mounted on a handsome desk or table of a design acceptable to the company. This desk shall be of dark, selected, quarter-sawed oak, with cabinet work first-class in every respect. Four pairs of plugs and cords, with clearing-out drops and listening and ringing devices, shall be provided. All switch con- tacts included in the talking circuit shall be platinum-pointed. The plugs shall be of the metallic-circuit type, having two concentric contact pieces, securely fitted and insulated, and provided with suitable connectors for the strands of the cord ; and, also, with means for securely fastening the body of the cord in the shank of the plug in such manner as to remove all strain from the conductors within. The cords shall have two conductors, each formed of twisted strands of tinsel, insulated with silk and braided over with linen. The conductors shall be armored with a spiral wrapping of spring-brass wire, and braided over with heavy linen or polished cotton. The operator's equipment shall consist of a watch-case receiver and nickel-plated head-band, and shall be provided with a flexible cord terminating in a plug adapted to fit in an operator's jack to make connection with the operator's talking circuits. The transmitter shall be of the type hereinafter specified and shall be adjustably mounted in front of the board. The switch-board wiring shall be neatly done with tinned- copper conductors, provided with a double insulation of silk and SPE&IFICA TIOMS. cotton. All joints shall be neatly soldered without the use of acid. BATTERIES. The current for all transmitters in the system shall be supplied by storage batteries of sufficient voltage to properly actuate the various instruments. All batteries shall be in duplicate, with suitable two-point, double-throw knife-switches for connecting either with the charging circuit. MOTOR-GENERATOR. A motor-generator is to be provided of the Crocker-Wheeler or Holtzer-Cabot type. This is to be wound on the primary side for 230 volts direct current [or state what current is avail- able] ; on the secondary side, so as to generate a direct current of suitable voltage for charging the storage batteries. If alternating currents are to be used for calling purposes, the motor-generator shall also be provided with a separate secondary winding for generating alternating currents at a suitable voltage for actuating the call-bells and drops. An automatic circuit-breaker is to be provided in the battery-charging circuit for the purpose of open- ing this circuit whenever the generator is at rest. PROTECTIVE DEVICES. A strong current arrester-board with arresters for twenty-five metallic circuit lines and adapted to be mounted near the switch- board shall be provided. TELEPHONES. Seventeen telephones besides the operator's equipment shall be furnished. Four of these shall be wall sets, and thirteen desk sets, all exposed metallic parts to be finely nickel-plated and polished. The transmitters shall be of the granular-carbon type in substantial brass or iron casings. If carbon diaphragms are used, a wire gauze shall be inserted in each transmitter between the diaphragm and the mouthpiece to prevent breakage. All carbons shall be of uniformly good quality. The receivers shall be of the bipolar type, substantially made ; the shell to be of hard rubber, with the magnets so mounted therein that the adjustment of the instrument will not be affected by changes of temperature. The hook-switches shall be automatic in action, with all con- tacts platinum-pointed. 49 AMERICAN TELEPHONE PKAC'llCE. The call-bells must be first-class in all particulars. If for battery currents, they are to be provided with platinum contact points, and if for alternating currents must be polarized and contain no cast iron in the variable magnetic circuit. The woodwork in all telephones shall be of dark, selected, quarter-sawed oak, finely finished and well jointed. The receiver cords on all instruments shall be silk-covered, with terminals securely fastened to the conductors in such manner that all strain will be taken by the braiding of the cord. The flexible cords for the desk sets shall be silk-covered, and not less than six feet in length ; they shall be provided with suitable terminal blocks for attaching the line wires. DIAGRAM OF CIRCUITS. With each bid a complete diagram of all electrical circuits of the system must be provided. The contract will, positively, not be awarded to any company which does not comply with this condition, in full. SPECIFICATIONS FOR A COMPLETE TELEPHONE EXCHANGE SYSTEM FOR THE CITY OF , . Contracts for this exchange system will be let under three separate heads, Central Office Equipment, Subscribers Appa- ratus, and Wire Plant. GENERAL CONDITIONS. All material, labor, and tools necessary to properly complete the equipment and installation of this exchange system, whether specifically mentioned or not, shall be furnished and installed by the contractor working under these specifications. After com- pletion of the work the contractor shall remove all debris, disposing of it in a manner suitable to the telephone company and to the city authorities. The work shall not be deemed completed until the equipment called for by the part of the specifications under which the contractor is acting is ready for immediate and continued service, all material and workmanship having been duly approved by the engineers of the telephone company. Extra Work. No claim for extra work will be allowed unless the work shall have been performed upon the authority of a written order from the telephone company. The telephone company may make any reasonable alterations in the plan or SPECIF 1C A TIONS. 49 * extent of the work during construction. If such alterations diminish or increase the quantity of the work to be done, an amount shall be deducted from or added to the sum to be paid, according to quantity actually done at the price established for such work by contract. Material and Inspection. All materials used in this work shall be first-class of their kind, subject to the inspection of the engi- neers of the telephone company. All work shall be done to the satisfaction of the company and be subject to the inspection of its engineers as the work progresses, but such inspection is not to relieve the contractor of any of his obligations to deliver sound and reliable work. The contractor shall furnish all facilities for making any tests and shall permit access of the company's engineers or agents to the work or any part thereof wherever located. The engineers of the telephone company shall have the right to reject any material or workmanship which they deem after care- ful inspection not fit for acceptance or not in full compliance with these specifications. The telephone company shall have the right to suspend the work or annul the contract upon a continuance of the contractor to use faulty material or workman- ship, but such annulment or cancellation shall not prevent the company from collecting damages caused thereby. Risk. Any damage or loss occurring to any of the property necessary for, or used in, the installation of the plant, whether caused by the fault or carelessness of the contractor or his employees, by theft, fire, water, or in other way, before the entire work is formally accepted by the telephone company, shall be made good at his own expense and in a manner satis- factory to the engineers of the company. To insure the telephone company against any such loss or damage to property, or against any loss or damage resulting from any accident due to the carelessness or neglect of the contractor or any of his employees, the contractor will be required to furnish a satisfactory bond. Time of Completion. Time shall be the essence of any contract based upon these specifications. The contractor shall name in his proposal a time in which he will agree to have the work ready to turn over to the telephone company, completed in every detail and ready for immediate and continued service. He shall understand and agree that for each and every day beyond the stipulated time, until the plant is so completed and accepted by the engineers of the telephone company, a pen- 49 2 AMERICAN TELEPHONE PRACTICE. alty of $ shall be paid as indemnity to the telephone com- pany for losses incurred through or due to such delay. CENTRAL-OFFICE EQUIPMENT. GENERAL DESCRIPTION. The central-office equipment shall be understood to embrace all material, apparatus, wires, and cables necessary to render the system complete within the central office and extending on the ines to the office cable heads. The system shall be operated on the common-battery or centralized-energy plan, all sources of electrical energy being located at the central office. All signals from the subscriber shall be sent automatically by the removal of the receiver from its hook. All lines will be metallic circuits, and no provision need be made for the accommodation of grounded or common-return lines, as none will be used. The present capacity of the central office is to be for 3120 lines, but an ultimate capacity of 6000 lines is to be provided for, and the system must be capable of being readily extended as occasion demands, to accommodate that number. SWITCHBOARD. Jack and Drop Equipment. The switchboard is to have a present capacity of 3120 subscribers' lines, and an ultimate capac- ity of 6000 lines, and is to consist at present of twelve regular multiple sections and two additional sections for incoming trunk lines. Each regular section is to be provided with 260 answer- ing jacks and drops or signals appearing on that section only, and with 3120 multiple jacks. Provision is also to be made for 200 outgoing trunk lines. Each incoming trunk section is to contain 200 incoming trunk jacks and signals, and 3120 regular multiple jacks, multipled from the regular section. Cord Equipment. Each section is to be provided with 36 pairs of plugs and cords, with listening and ringing keys, and two supervisory signals for each pair. Operators Positions. Each section is to have three operators' positions, each provided with a complete operator's equipment as hereafter specified. Frameivork is to be of structural iron securely riveted, screwed, or bolted together. The entire framework must be heavy enough to be rigid under all strains which may come upon SPECIF1CA TIONS. 493 it, and must be so accurately constructed as to properly line up when in place on the floor. The entire framework must be given a coat of good metallic paint after assembling. All parts on the front of the board not covered by the woodwork are to be covered by finishing strips of wood. Woodwork. All woodwork on the front of the board shall be of mahogany, finely finished and joined in a workmanlike man- ner. The key shelves shall be hinged and built up of five layers of mahogany glued together, the grain of the two outside and the center layers running lengthwise, and that of the other two layers running crosswise. The plug shelves are to be covered with best belt leather at least -J- inch thick. The entire woodwork is to be of a design approved by the engineers of the telephone company and of uniform shade throughout. Drops. The drops shall be readily removable and shall be free from sticking or freezing. If battery current is to be used for throwing the shutters, the drops must be capable of being oper- ated on at least one-fifth of the normal battery pressure, and with the same adjustment of standing double that pressure with- out freezing. The coils shall be wound to a suitable and nearly uniform resistance ; the winding shall be of silk-insulated copper wire, over cores of soft Swedish iron carefully insulated before winding ; free from short circuits or grounds, and so constructed or connected as to prevent cross-talk. If the bridged system is used, the coils must be wound to a resistance of at least 400 ohms, and must be ifon-clad. A suitable night-bell attachment shall also be provided. Jacks. The jacks shall be firmly constructed with springs of German silver and of sufficient length and strength to insure the proper making and breaking of the electrical contacts. All springs and contacts in the jack shall be thoroughly insulated from each other by hard rubber, and the entire mounting shall be composed of the very best quality of hard rubber. The jacks shall be mounted twenty per strip, on half-inch centers each way. Ringing and Listening Keys. Three sets of twelve each ringing and listening keys shall be placed upon each section of the board. Each listening key to be so constructed as to remain in the listening position until released by the operator. The ringing keys are to automatically return to the normal position after ringing. The ringing and listening keys may be combined into one piece of apparatus or may be separately constructed. The springs are to be of German silver and of sufficient length and 494 AMERICAN TELEPHONE PRACTICE. thickness to insure strength, elasticity, and durability. They must be platinum-pointed throughout. Plugs shall be of the metallic-circuit type, having two concentric contact pieces securely fitted and insulated, provided with con- nectors for the two strands of the cord, and with means for securely fastening the body of the cord in the shank of the plug to avoid the danger of breaking the conductors. A fiber sleeve is to be slipped over the plug after cord is attached, serving as handle for plug and protector for cord-connectors. Cords shall have two conductors, each formed of twisted strands of tinsel and copper wire, insulated with wrapped silk, braided over with linen, armored with a spiral wrapping of brass wire, and covered over with a heavy linen braid. The cosds shall be of sufficient length to reach the furthest jack on each of the adjacent positions of switch-board, without strain. Cord Weights. All cord weights shall be provided with smoothly finished boxwood or metallic pulleys, and shall be of sufficient weight to keep the cord properly taut. The weights shall be of such configuration as to minimize the liability of interference with each other and the tangling of the cords. Line Relays. If used in the system proposed by the bidder, these are to be readily accessible and removable from their mount- ing. If operated over the line circuit they shall be capable of operating on short circuit with one-fifth of the normal battery pressure, and if a over a local circuit shall be capable of operat- ing with three-quarters of the normal pressure. In any case they must stand twice the normal pressure without sticking or freezing. The coils shall be wound to a suitable and nearly uniform resistance; the winding shall be of silk-insulated copper wire, over cores of soft Swedish iron carefully insulated before wind- ing; free from short-circuits or grounds, and so constructed or connected as to prevent cross-talk. All contact springs shall be of German silver, platinum- pointed. All contacts shall be firmly made and positively broken. Line relays if used may be mounted on the back of the switch- board legs if ample space is present, so that other apparatus and wiring will not be unduly crowded, otherwise they shall be mounted on a separate iron relay frame, in a separate room. Repeating Coils. If repeating coils are to be used in the system proposed, they shall be wound to suitable and nearly uniform resistance, the several windings being well insulated SPECIFICA TIONS. 495 from each other by paraffined paper. The terminals are to be brought out in such manner as to secure a minimum liability to crosses or open coils. The windings are to be of silk-covered copper wire on a core of annealed soft-iron wire. The magnetic circuit shall be completed either by bending the ends of the core out and around the coil, or by inclosing the whole in a soft-iron shell. Supervisory Relays. There shall be two supervisory relays to each pair of plugs. These shall be iron-clad, wound to a suitable and nearly uniform resistance, and shall be sufficiently sensitive to operate positively over the longest line, and with the same adjustment shall not stick or freeze when subjected to twice the normal voltage on a short circuit. These relays shall be mounted on iron strips, on the back of the switch-board legs. Supervisory Lamps. These shall be of \ candlepower and so fitted as to slide rather than screw into their bases or jacks. The lamp-jacks shall be so mounted as to be readily accessible. Suitable opalescent glass jewels shall be mounted in brass caps in front of each lamp-jack, and these jewels must be protected in some suitable way from breakage due to impact of the plugs. Operator's Equipment. Each operator's set shall consist of a granular-carbon transmitter, suspended in front of the operator, and in such a manner as to permit adjustment, connected with an induction coil, if used, on the back of the section, by flexible cords ; and a watch-case receiver with insulated head-band ; the receiver cord to terminate in a cut-in plug to be inserted, while in use, into a cut-in jack conveniently located upon the board. The materials used in the operator's telephone shall be equal in qual- ity to those hereinafter specified for like parts in subscribers' instruments. Night-Circuit Equipment. A night-circuit lamp shall be pro- vided on each operator's position, and so wired as to be illu- minated whenever and as long as any line signal on that position is displayed. These lamps shall be of such candlepower and so located as to be visible from any point in front of the switch- board. Wire and Wiring. All switch-board cables shall be made up of No. 22 B. & S. gauge tinned annealed copper-wire with silk and cotton insulation. One extra pair is to be left in each cable. The outside layer of cotton is to be colored in accordance with a predetermined color code for facilitating the rapid handling of the wire. The dye in this cable shall not be of such a nature as to seriously impair the insulation of the wires. Where standard 49 6 AMERICAN TELEPHONE PRACTICE. cables are used a heavy wrapping of paper and outside braid of cotton will be required, the whole to be dipped in beeswax. All cable ends shall be properly butted and formed and dipped in beeswax. All special cables requiring hand forming shall be laced with waxed linen thread and afterward boiled out in bees- wax. All wires in the talking circuits shall be in twisted pail's. All leads are to be properly fused if possible on each side of the apparatus to be protected. Operator's battery leads, generator taps, and common wires in the individual sections are to be of No. 18 B. & S. gauge -g^-inch okonite wire twisted in pairs when forming a part of talking circuits. All joints are to be soldered with a resin flux; no acid flux whatever will allowed on any part of the work. All wiring shall conform to the requirements of the National Board of Fire Underwriters. Electric Lighting. Both the front and back of the switch-board shall be wired for a sufficient number of i6-candlepower incandes- cent lamps to thoroughly illuminate the switch-board for both operating and repair work. Rubber-covered wire of suitable size is to be used for the lighting circuits. All cut-outs, switches, lamp-sockets, lamp-brackets, and other necessary apparatus must be of standard design, and, with the wiring, shall be of such nature as to conform to the requirements of the National Board of Fire Underwriters. DESK EQUIPMENTS. A desk specially equipped is to be provided for the chief operator, monitor, wire chief, and trouble clerk. The woodwork of these desks is to be of mahogany of a finish to match that of the switch-board, and of a design to be approved by the engineers of the telephone company. The apparatus to be installed in these desks will be subject to the same specifications as that of the switch-board. Chief Operator s Desk shall be provided with terminals for three regular exchange lines, and for one line each to the wire chief's desk, the monitor's desk, the trouble clerk's desk, and to the exchange manager's desk, together with necessary plugs, cords, keys, and complete talking apparatus. Monitor s Desk. A tap from each operator's head telephone circuit is to lead to the monitor's desk, each tap terminating in a listening key. This desk is also to be provided with terminals SPECIFICA TIONS. 497 for one line each to the chief operator's desk, the wire chief's desk, and the trouble clerk's desk, together with necessary plugs, cords, and keys, and complete talking apparatus. A monitor lamp shall be provided on this desk for each operator's position on the switch-board, these lamps to be wired in such manner that each , will be lighted as long as any drop or signal on its corresponding operator's position remains displayed. Wire Chief 's Desk shall be provided with terminals for two regular exchange lines, two lines to the trouble clerk's desk, and one line each to the chief operator's desk and the monitor's desk, together with plugs, cords, and keys, and complete talking apparatus. Trouble Clerk's Desk. This shall be equipped in the same manner as the wire chief's desk. In addition to this, it shall be provided with complete instruments and facilities for testing any line in the exchange. DISTRIBUTING BOARDS. Main Board. This shall be installed with a present capacity of 4000 lines and shall have an ultimate capacity of 6000 lines. The frame shall be of structural iron given one coat of good metallic paint after assembling. It shall provide permanent connectors for the cables leading from the cable-heads, and for those leading from the switch-board or relay-rack. It shall be so designed as to permit a universal distribution, any change in distribution being effected without changing the permanent connections of the cables. It shall also provide for the ready withdrawal of dead jumper wires; The wiring and cross-connecting shall be made complete and ready for immediate and continued use by the contractor. Jumper wires shall be in twisted pairs, red and white No. 22 B. & S. tinned annealed copper wire, having one wrap of silk and one braid of cotton. Intermediate Board. This shall conform to the general speci- fications for the main distributing board. All lines shall pass to the intermediate board, which shall provide means whereby any answering-jack and drop maybe wired to any operator's position. POWER PLANT. A power plant complete in all respects shall be provided, hav- ing ample capacity to meet the maximum requirements of the exchange when 6000 subscribers' lines shall have been connected. 49 8 AMERICAN TELEPHONE PRACTICE, It shall consist of a power switch-board, charging and ringing machines, a battery plant, fuse boards, and all other necessary apparatus. Power Switch-board. This shall be of polished Italian marble, mounted in an angle-iron framework. It shall be provided with all-copper switches of sufficient capacity and of such arrangement as to enable all desirable changes in the power circuit to be conveniently and quickly made. Voltmeters and ammeters of the Weston or other equally good pattern, and capable of meas- uring all voltage and current used in the power circuits, shall be mounted on the face of the board. Under-and over-load circuit- breakers of suitable capacity shall be mounted on the lower portion of the board. Suitable rheostats and starting boxes for the charging and ringing machines shall also be mounted in con- venient position on the front of the board. All wiring shall be done and bus-bars be mounted on the back of the board in a neat and workmanlike manner. Charging and Ringing Machines shall be provided in dupli- cate. The machines shall be wound to run on I ro-volt direct- current mains [or specify whatever other source of current is available]. The charging machines shall generate sufficient current at a suitable pressure to meet the maximum demands of the storage battery, and shall run quietly and efficiently without undue heating. The ringing machines shall deliver alternating [or pulsating, or both] current at a pressure of 65 volts, and at a frequency of approximately 950 cycles per minute. The ringing and charging machines shall be mounted on a suitable table with structural iron framework, and so secured as to run without undue noise or vibration. Storage Battery. A storage battery or batteries of suitable size and capacity shall be provided for meeting the maximum requirements of the ultimate capacity of the exchange, without danger of injury. All battery leads and bus-bars shall be of copper and of sufficient size to insure freedom from cross-talk. The lugs of the several cells shall be burned together and to the bus-bars in a workmanlike and thorough manner. All distri- bution- and fuse-bars at the point of distribution to the switch- board leads shall be of copper or brass and mounted on a marble slab. SUBSCRIBERS' APPARATUS. Each subscriber's apparatus shall be a complete telephone instrument provided with a granular-carbon transmitter on a SPECIFIC A TIONS. 499 suitable arm, a bipolar receiver and cord, an automatic hook- switch, a polarized call-bell, and whatever other apparatus, such as condensers, induction coils, etc., is needed to make the instru- ment properly co-operate with the central office equipment here- inbefore specified ; all mounted and assembled as hereinafter specified. There shall be 3120 complete subscribers' instruments, of which 2500 shall be wall sets, 600 portable desk sets, and 20 cabinet desk sets. All telephones shall be of good material, thoroughly well made, first-class in every particular, and shall be approved by the engineers of the company. Wall Sets, In these the above-mentioned parts, except the transmitter and receiver, are to be contained in a suitable box of oak or walnut, which together with the transmitter is to be mounted on a back-board of the same material as the box. The whole to be of a design and finish approved by the engineers of the telephone company. Portable Desk Sets. In these the transmitter and hook-switch carrying the receiver are to be mounted on a metallic stand suit- able for placing on the top of a desk, and of such height as to allow the lid of an ordinary roll-top desk to be closed over it. The metallic parts exposed shall not be connected with the circuits in such manner as to render the subscriber liable to electric shocks, no matter how slight. The call-bell, together with induction coil and condenser, if such there are, shall be mounted in a neat box of oak or walnut which may be mounted in any convenient place near by. The flexible cords of the desk stands shall terminate in suitable terminal blocks, and all wires or conductors from line, bell-box, and desk stand shall be provided with terminals, properly marked on these blocks. All portable desk sets shall be capable of serv- ing as extension instruments to the wall sets. Cabinet Desk Sets. These shall embrace the same apparatus as the wall sets, all mounted in a neat desk of quarter-sawed oak. These desks are to be finely finished, and so arranged that the apparatus will be easily accessible. The design must be approved by the engineers of the telephone company. Transmitters. Shall be of the granular-carbon type. The carbon backs and diaphragms shall be accurately made. The granules shall be of uniform size and polished ; the whole incased in a brass or iron cup of neat design and mounted upon an exten- sion or rocker arm, either nickel-plated or japanned, provided with hinged connection between the arm and the base, to admit $00 AMERICAN TELEPHONE PRACTICE. of vertical adjustment. The electrodes shall be so mounted as to insure a maximum amount of radiation of heat from the buttons and granules. Receivers. Shall have a compound permanent magnet of the best quality of magnet steel, the coils to be wound with silk- insulated copper wire to a suitable and nearly uniform resistance, the whole incased in a rubber shell and supported in the shell at a point near the diaphragm. Imitation hard rubber will not be accepted. The adjustment, if any, must be provided with a lock-nut or other suitable device to insure permanency. Call-bells. Shall have cores of annealed soft iron properly insulated and wound with single silk-covered magnet wire to a suitable and nearly uniform resistance. All parts of the magnetic circuk affected by the varying currents shall be of annealed soft iron. The permanent magnet shall be of magnet steel adapted to retain permanently its magnetism. Switch-Hooks. These shall be of the long-lever type, having German-silver springs platinum-pointed throughout, and having length enough to insure resiliency. Induction Coils. If used these shall have a core of annealed soft-iron wire insulated with paper and wound with silk-insulated copper wire of proper size and to a proper resistance. Terminals of both primary and secondary shall be brought out to brass clips mounted on the coil heads. Condensers. If used these shall be built up of layers of tin- foil and thin bond paper, two layers of paper being between each two layers of tinfoil. They shall be thoroughly boiled out in paraffin until all moisture is expelled. They shall be able to stand without breaking a constant potential of 500 volts for a period of one hour. They shall be sealed in an air-tight tin case. If the condensers are to form a part of the ringing circuit they shall have a capacity of at least two microfarads. If in the talk- ing circuit alone, their capacity must be that most suitable to the particular system proposed. Cords. Receiver cords shall be green-silk covered, with ter- minals securely fastened to the conductors in such manner that all strain will be taken by the braiding of the cord. Flexible cords for desk stands shall be green-silk covered and not less than six feet in length ; they shall be provided with suitable terminals at each end. All Connections. The telephones shall be wired with No. 18 B. & S. gauge tinned-copper wire, and all connections shall be soldered without the use of acid. The wiring must be done in SPE CIFICA TIONS. 5 o I a neat and workmanlike manner, and no wires shall be left in undue proximity, and no slack shall be left in any connecting wire. WIRE PLANT. Under the term "wire plant" is included all conduits, pole lines, cables and wires, and all auxiliary apparatus and appliances, including the office cable-heads necessary for completing the system between the office cable-heads and the various subscribers' stations. UNDERGROUND WORK. Underground District. All wires are to be placed underground in the district bounded by , , , and Streets, and adjacent thereto, laid in conduits connected with manholes and cable poles in the form, plan, and location as, designated by the company. The Conduits shall be built up of sections of well-burned vitrified clay, cement, or iron, and shall have an opening longi- tudinally through them, the area of which shall be uniform and shall be capable of containing an inscribed circle not less than 3| inches in diameter. The sections shall be straight (except on curves in the line of conduit), the ends cut square or provided with ball-and-socket joints. They shall be laid in concrete, well set and bedded, and all joints shall be covered with mortar and all spaces between the ducts filled in with mortar or concrete. The distance between centers of ducts in both horizontal and vertical directions shall be approximately 4^ inches. The section shall be laid with a mandrel and gasket for securing proper alignment and for cleaning the ducts, which shall present an even surface on the inside, free from all offsets or projections. The ducts shall be laid so as to break joints and to an even grade between manholes, always so as to drain to one or both of the manholes between which they lie. In no case will sags or pockets preventing drainage be permitted. All ducts shall be straight between manholes, except where slight bends are neces- sary to avoid obstructions. The open ends of all ducts shall be protected with wooden stoppers, whenever, work is left off or suspended. The process of laying shall be approved by the company. Trenches. Shall be of sufficient depth to allow a covering of at least 2\ feet over the top tier of ducts, and of a greater depth 502 AMERICAN TELEPHONE PRACTICE. whenever deemed advisable by the engineers of the telephone company. They shall be dug at least 8 inches wider than the width of the layer having the greatest number of ducts placed therein. The bottom of all trenches must be leveled to an even and accurate grade, sloping slightly to one or both of the man- holes at the ends of sections and covered with 4 inches of con- crete, as a foundation for the bottom layer of ducts. The sides of the trenches, where necessary to prevent caving in, are to be supported by suitable planks and shoring, and the trenches are to be kept free from water by draining, pumping, or bailing. All excavations below the bottom line of the conduit shall be refilled with good material, solidly rammed into place. After the ducts have been laid, and the spaces between them filled with cement mortar, or concrete, they shall be covered over the top and sides with 4 inches of concrete. The work shall be conducted so as to provide ample time for the mortar to set. The remaining portion of the trenches shall be filled with earth in layers not exceeding 4 inches, flushed and thoroughly rammed with a heavy block rammer. The earth remaining shall be carted away and disposed of by the contractor to the satisfaction of the proper municipal authorities. Obstructions. Gas, water, and other pipes, conduits, subways, or fixtures of any kind, in the line of work, must not be disturbed except by written permission of the proper authorities. They must be securely supported by braces or chains until refilling is rammed firmly under and around them. Any damage done to pipes, conduits, subways, or fixtures, in whatever manner, or caused by neglect, must be paid for by the contractor. Where- ever it is necessary to cross or interfere with the tracks of any street railway or steam road, due notice shall be given the com- pany owning or controlling the same, and where needful an agreement shall be made with said company for shoring up and protecting its track and traffic. The telephone company is to be put to no extra expense by or on account of any such interference. Blasting. Where blasting is required moderate charges of explosives are to be used, and the blast is to be so covered as to protect life and property. Manholes. The manholes shall be built of such form and dimensions, and at such locations, as the telephone company may from time to time direct. They shall be for all straight-away work of a nearly elliptical shape, the long axis of the ellipse being in line with the line of ducts. The foundation will consist SPE CIFICA TIONS. 503 of a layer of concrete 6 inches thick. The walls are to rest on the concrete foundation and shall be of hard-pressed brick at least 8 inches thick and plastered on the outside with good cement mortar. The walls shall be arched to an opening closely fitting the iron thimble for the manhole cover; or, when deemed desirable by the engineer of the telephone company, the man- hole cover shall be supported upon steel I beams resting on the brick walls. The ducts are to be brought through the walls within 2 inches of their inside faces, and the edges of the open- ing into the duct shall be beveled off fora distance of two inches and troweled smooth with cement to prevent cutting or kinking the cables. The floor of the manholes shall be graded to the center or one corner, and connected with a 4-inch-tile sewer pipe, leading through a J-S trap to the nearest sewer. The ends of the drain pipe shall be protected by screens. Each side of the manholes is to be furnished with iron brackets for cable-sup- porting hooks. These are to be of sufficient length to provide space for hooks, placed 6 inches apart, for the support Of cables to the full capacity of the ducts leading into or through the manhole. Concrete and Mortar. All concrete used in this work shall be composed of one part of hydraulic cement, three parts of clean sharp sand, and five parts of broken stone or broken brick. The cement and sand shall be first mixed dry and then a sufficient quantity of water added to form a soft mortar. The broken stone or brick shall be then added and thoroughly mixed with it. The broken stone or brick for concrete used in laying conduit shall not exceed I inch in diameter in its largest dimension, while that for concrete used for manhole construction shall not be larger than 2 inches in its greatest dimension. All mortar shall be composed of one part of hydraulic cement and two parts of sharp clean sand mixed dry and afterwards wet with water to a soft mortar. All cement shall be of best quality, fine ground and fresh. It shall be protected from moisture until used, and shall at any time be subject to test by the engineers of the telephone company. Laterals and Cable-Pole Connections. Each lateral shall termi- nate in a 3-inch iron pipe, galvanized or covered with asphaltum paint, bent at an angle so that it may be carried upward parallel with the cable pole, to a point at least 20 feet above the surface of the ground, and securely fastened to the pole by means of cleats or staples. The end joining the duct shall be 54 AMERICAN TELEPHONE PRACTICE. flanged to at least the diameter of the opening and cemented into place. Street Pavements. These are to be replaced in kind wherever disturbed. The pavements are to be left in as good condition as before the work commenced, and to be made to the entire satis- faction of the municipal authorities. Protection. Trenches, manholes, and obstructions shall be guarded by the contractor with barriers, lights, and watchmen, in accordance with the regulations and laws of the city. Trenches shall be provided with bridges to admit the passage of teams and pedestrians at alley and street intersections. ^POLE LINES. Poles. All poles shall be of live sound white cedar, straight and evenly tapered, peeled and shaved to a smooth surface. The dimensions of poles of various heights, and the distance to which they shall be sunk in the ground under ordinary circum- stances, shall be as shown in the following table : LENGTH DIAMETER AT TOP DIAMETER SIX FEET FROM BUTT DEPTH OF HOLE 25 feet 7 inches 9 inches 5t feet 30 7 10 6 35 7 ii 6 40 7 12 6 45 7 13 64 50 7 14 . tf 55 7 16 <4 60 7 17 7 65 7 18 7 70 7 20 7* On corners and sharp turns the poles may be required heavier and sunk deeper in the ground, as deemed necessary by the engineers of the telephone company. Each pole shall be roofed to an angle of 45 on each side of the pole center. The roof shall be given three coats of good white lead. The poles shall be gained for cross-arms to carry 50 per cent, more wires than those at present to be installed. All gains shall be accurately cut to fit the cross-arm and be given two coats of white-lead paint, before securing the cross-arm in place. The butts of the poles shall be squared before setting. The poles shall be erected upon the routes, lines, and locations designated from time to time by the company. They shall be in SPECIFICA TIONS. 505 good alignment, well braced, guyed, and strengthened, each according to the requirements of its service. They shall be painted with two coats of white lead and oil paint from the top to 9 feet above ground ; the second coat on the bottom 9 feet shall be black in color. The holes in which the poles are set shall be of the depths specified, 120 feet apart, except where necessary to shorten the distance between poles. Each hole shall have a diameter across the bottom at least six inches greater than that of the butt of pole to be set therein. They shall be refilled with earth, flushed and well tamped. All surplus earth shall be carted away and disposed of, and pavements shall be replaced and repaired to the satisfaction of the city authorities. Cross-Arms. Are to be sawed from sound, green white or Norway pine, painted with two coats of red metallic paint, finished to a size 3^ inches by 4^ inches, bored for pins i\ inch in diameter. Two pin arms to be 36 inches long, 28 inches between holes. Four pin arms to be 48 inches long. Six pin arms to be 72 inches long. Eight pin arms to be 96 inches long. Ten pin arms to be 120 inches long. All arms bored for a greater num- ber than two pins shall have 1 6 inches of space between center holes, 12 inches between side holes, and 4 inches from outer hole to end. Each arm to be set in a gain not to exceed I inch in depth and secured to its pole by one f-inch galvanized bolt extending entirely through the pole and secured with nut and washer. Each pin hole in each arm shall be filled with a i^-inch locust pin. The distance between centers of cross-arms on poles shall be 20 inches. All cable poles and poles on corners and sharp turns shall be double-armed, in which case the arms shall be placed on a level with each other and on opposite sides of the pole. Blocks 3 by 4^ inches, and of a length equal to the distance between the two arms on the same level, shall be inserted and secured by a carriage-bolt passing through the two arms and the block, between the second and third pins ; also between the fourth and fifth pins each side of pole. Pins. Are to be made from sound, split, green locust timber 1 1 inch in diameter below the shoulder, with thread cut to fit glass insulators. They shall be given two coats of metallic paint and nailed in the arm each with one sixpenny wire nail. Brackets. To be good sound oak, pony size, 12 inches in length by \\ inch diameter of thread, to be painted with two coats of metallic paint. Cross- Arm Braces. Shall be of galvanized iron 28 by i^ by J- 506 AMERICAN TELEPHONE PRACTICE. inches, punched at one end with |- and at the other with f-inch holes. Two of them shall be used in the form of a V on all cross-arms, attached to the pole by a lag-screw 5 by inches, passing through both braces into the pole at a point about 17 inches below the cross-arm, and to the cross-arm on each side by a carriage-bolt f inch by 4 inches passing through brace and arm and secured by a nut and washer. Pole Steps. Shall be of galvanized iron 9 inches by T 9 F stag- gered 3 feet apart on each side of each pole, the lower step to be 12 feet above the surface of the ground. Lag-Screivs and Bolts shall be of galvanized wrought iron of good quality. Lag-screws shall always be provided with a galvanized iron washer under the head, and carriage-bolts with a similar washer under both head and nut. The bolt for securing the cross-arm to the pole shall be f inch in diameter and pro- vided with two galvanized iron washers 2\ inches in diameter. All bolts shall be of sufficient length to extend entirely through the work and two washers, projecting enough to engage all threads in the nut. Galvanizing. All galvanized-iron parts shall be able to with- stand successfully the following test : they are to be plunged four times into a saturated solution of sulphate of copper, for a period of seventy seconds each time, and wiped dry after each immersion. If at the end of the fourth immersion the sample appears black the galvanizing has been properly done. Insulators. Double petticoat white-glass insulators shall be placed upon all pins or brackets supporting wires. These shall be free from cracks, burrs, or sharp edges, and shall accurately fit the pins. Porcelain knobs i|- by I inch may be used for the support of rubber-covered wires at their entrance to buildings and other places where the use of cross-arms and brackets may be impossible or inconvenient. Guy-Rods shall be of the best grade of galvanized wrought iron from 6 to 10 feet in length and of suitable diameter to withstand the strain. They shall be threaded at their lower ends and have eyes forged in their upper ends for the reception of the guy- wire. A galvanized nut and washer shall be provided for each guy-rod, the washer to be at least \ inch thick and 3 inches in diameter. Guy and Messenger Wires. The guys on all cable-terminal poles and poles at sharp corners, carrying cables or six or more cross-arms, shall be of one or more galvanized steel cable \ inch in diameter. Guys resisting lesser strains may be of -^ or f SPECIF1CA TIONS. S7 inch in diameter, as needed. All messenger wire, for the support of aerial cables of any size, shall be | inch in diameter. The messenger wires for the support of cables shall begin and end in the anchors at both terminals, and shall be supported on suitable iron brackets or cross-arms, bolted firmly to the pole. The messenger wire must in all cases be pulled taut, so that when loaded with its cable the sag will not exceed one per cent, of the span. Guy-Stubs and Anchor-Logs. The quality of the wood shall conform to the requirements for the poles. Guy-stubs shall have a diameter of 8 inches at the top. Anchor-logs shall have a diameter of not less than 10 inches and a length of not less than 5 feet. These shall be sunk at least 6 feet in the ground. Where heavier strains are to be met, larger anchor-logs may be used, or an anchor formed of two or more logs bolted together. In all cases the hole shall be filled level with the top of the timbers with earth, flushed and well rammed, and covered with 2-inch oak lumber; the remaining portion of the opening to be filled with earth, flushed and well rammed in layers not exceeding 4 inches. Heavy rocks or iron may be used as filling next above the timbers. Anchors of equal resist- ing power may be substituted by the contractor upon approval by the engineers of the telephone company. Cable Boxes and Platforms. Suitable coverings of iron or wood shall be provided for all cable terminals and arresters upon poles. Platforms, either square or circular in form, protected by rail- ings, shall be suspended below each box, in such a manner and at such distance as will provide convenient and safe standing- room for workman. Cable Hangers. Shall be formed of malleable iron made up in the form of a band, ending in a hook, for suspension to the mes- senger wire. The band, when closed, shall have an inside diame- ter equal to the diameter of the cable on which used, and shall be of such a nature as not to cut or mar the sheath. They shall be firmly clamped to the lead sheath at intervals of 18 inches. UNDERGROUND CABLES. Core. All cables for use underground shall be made up of copper wire having a conductivity equal to 98 per cent, of that of pure copper and shall have a diameter of 35.89 mils, equal to that of No. 19 B. & S. gauge. Each conductor to be insulated with 2 wraps of dry paper wound spirally in opposite directions 508 AMERICAN TELEPHONE PRACTICE. or folded over the wire. The insulated conductors shall be twisted in pairs, length of twists not exceeding 3 inches. The twisted pairs shall then be formed into a core, arranged in reverse layers, and the core so formed shall be covered over with jute or paper. Sheath. The sheath shall either be composed of an even mixture of 97 per cent, of pure lead with 3 per cent, of pure tin, or consist of pure lead with a thin, even coating of pure tin on the outside. The pipe shall be -| inch in thickness, formed around the core, and shall be free from holes or other defects' and of uniform thickness and composition. Capacity. The average electrostatic capacity of the wires, each wire measured against all the rest, the sheath, and ground, and after the cable has been spliced and connected with its terminal heads, shall not exceed .080 microfarad per mile. The capacity of any wire so measured shall not exceed .085 micro- farad per mile. Insulation Resistance. Each conductor, after being spliced and connected with its terminal ready for use, must show an in- sulation of 500 megohms per mile at a temperature of 68 F.; each wire being measured against all the rest and the grounded sheath. Conductor Resistance. Each conductor shall have a resistance of not more than 45 international ohms at 68 F. per mile, after the cable is laid and connected to its terminals. Route of Cables. The cables shall run from the cable-room at the central station, through the cable vault and the underground conduits to the several points of distribution, as established by the company. The ducts in the lower tier of conduits shall be first used, the several cables following in regularity the lines of duct in which they start from or leave the cable vault, to their several distributing poles. The cables shall be conducted through the manholes, bent around the sides, and supported on brackets there provided. Terminals. Two feet of the paper insulation of each end of each section shall be saturated with a good insulating and moisture-resisting compound. Each end of each cable shall be protected by a terminal head, to be approved by the engineers of the company, of sufficient capacity to accommodate all of its conductors. Corresponding binding posts or connectors in each head shall be joined to the same pair of conductors. Each pole terminal shall be incased in a box of suitable size and design, and shall be equipped with arresters and fuses. Splices. The ends of the cable before splicing shall be thor- SPECIFICA TIONS. 509 oughly boiled out, and the joints in the wire staggered as much as possible and soldered with resin or tallow flux. A dry paper sleeve at least 3 inches long, previously boiled out, shall be slipped over each wire joint. The splice shall be then boiled out and wrapped with several layers of white cotton cloth and again boiled out. The splice shall then be covered with a lead sleeve wiped to the lead sheath of the two sections joined. Care must be taken to thoroughly insulate all conductors, to exclude all moisture, and to render the sheath air- and water-tight. Unfinished Joints. No unfinished splice or open cable end shall ever be left without first sealing the end by a wiped lead joint impervious to air or moisture. AERIAL CABLES. All cables for use above ground shall be made up of copper wire having a conductivity equal to 98 per cent, of that of pure copper, and shall have a diameter of 31.96 mils, equal to that of No. 20 B. & S. gauge. Each conductor shall be insulated with two wraps of dry paper wound spirally in opposite directions or folded over the wire. The insulated conductors shall be twisted in pairs ; the lengths of twists "not exceeding 3 inches. The twisted pairs shall then be formed into a core, arranged in reverse layers, and the core shall be covered over with jute or paper. Sheath. The sheath shall be composed of pure lead. The pipe shall be formed around the core and shall have a uniform density and thickness of ^ inch, free from holes or other defects. Capacity. The average electrostatic capacity of the wires, each wire measured against all the rest, the sheath, and ground, and after the cable has been spliced and connected with its terminal heads, shall not exceed .080 microfarad per mile. The capacity of any wire so measured shall not exceed .085 microfarad per mile. Insulation Resistance. Each conductor, after being spliced and connected with its terminal, must show an insulation of 500 megohms per mile at a temperature of 68 F., each wire being measured against all the rest and the grounded sheath. Conductor Resistance. Each conductor shall have a resistance of not more than 57 international ohms per mile at 68 F., after the cable is strung and connected to its terminals. Suspension. The cables shall be suspended from the messenger wires along the routes designated by the engineers of the tele- 510 AMERICAN TELEPHONE PRACTICE. phone company. The messenger wires, cable hangers, cable boxes, and platforms shall be in accordance with the specifica- tions under Pole Lines. Terminals. Two feet of the paper insulation of each con- ductor, at each end of each section, shall be saturated with a good insulating and moisture-resisting compound. Each end of each cable shall be protected by a terminal head, to be approved by the company, of sufficient capacity to accommodate each of its conductors. Corresponding binding posts or connectors in -each head shall be joined to the same pair of conductors. Each terminal shall be incased in a cable-box of suitable size, and shall be equipped with arresters and fuse. Splices. The ends of the cable before splicing shall be thoroughly boiled out, and the joints in the wire staggered as much as possible and soldered with a resin or tallow flux. A dry paper sleeve at least 3 inches long, previously boiled out, shall be slipped over each wire joint. The splices shall be then boiled out and wrapped with several layers of white cotton cloth and again boiled out. The splice shall then be covered with a lead sleeve wiped to the lead sheath of the two sections joined. Care must be taken to thoroughly insulate all conductors, to exclude .all moisture and to render the sheath air- and water-tight. Unfinished Joints. No unfinished splice or open cable end shall ever be left without first sealing the end by a wiped lead joint impervious to air or moisture. Rubber Cable. This may be used for short stretches through trees and at crossings of electric light, street railway, or other high-tension wires, but only when such use is approved by the engineers of the telephone company. This cable shall have a core made of No. 18 B. & S. gauge copper wire, 98 per cent, conductivity, with ^ 2 -inch insulation consisting of two coats of best rubber. After forming the core shall be double-taped and covered with tarred jute, over which shall be placed a braid of heavy cotton, saturated with weatherproof compound. Each wire shall have an insulation resistance of not less than 300 meg- ohms per mile at 68 F., after being immersed in water twenty- four hours. This test shall be made on the core after being formed, but before the outside coverings are put on. BARE WIRE. Line Wires. Bare line wire shall be of hard-drawn copper No. 12 B. & S. gauge. [Here should follow in substance the SPECIFIC A TIONS. 5 1 1 specifications given on pages 357, 358, and 359, or such parts o them as may be deemed necessary.] Joints. All joints shall be made with Mclntire sleeves, or with some equally good sleeve joint approved by the engineers of the telephone company. All joints shall be given at least three complete twists. Where two wires are joined without the use of copper cylindrical connectors, their points of contact shall be soldered. Tying. Line wires shall be tied to the insulators in a manner approved by the engineers of the telephone company. The tie wires shall be of hard-drawn copper not smaller than No. 14, B. & S. gauge, and shall be long enough for each end to be wrapped six times about the line wire. The line wire shall pass straight through the tie past the insulator. Dead-Ending. When a wire is dead-ended on an insulator it shall be wrapped once around the insulator and given six twists about itself, the twists beginning not closer than 2 inches from the insulator. In dead-ending for transpositions the free end of the wire should be left long enough to pass over or under the cross-arm to make connection with the other wire. Stringing. Great care must be exercised, in stringing wire, ta prevent injury from kinks and nicks. It must be evenly drawn taut, so that the dip or sag in inches will not in summer exceed fa of the span in feet; and in winter ^ of the span in feet. Each pair of wires must occupy relatively the same pair of pins on each pole as that from which it leaves the cable pole. Transpositions will be made on every tenth pole and in accordance with a scheme approved by the engineers of the telephone company. Drop or Service Wires. Drop or service wires from the pole nearest the subscriber's building shall be of No. 14 B. &S. gauge duplex rubber-covered copper wire with a braiding over each wire. Between the drop or service wire and the telephone binding posts No. 20 B. & S. gauge copper wire with ^ inch rubber in- sulation twisted in pairs shall be used. INDEX. Abbott, A. V., 199, 367 Accumulators, chloride, 472 Ader, Clement, 23, 36 Ahearn, T. F., 46 American transfer system, 334 storage batteries, 474 Ampere, work of, I Anchor for guy, 379 pole, 377 Anders, George L., 236, 312, 318 Anders' step-by-step system, 312 Arago and Davy, work of, I Arrester carbon, 273 combined carbon and heat coil, 278 . , Cook, 277 , Hayes, 275 , McBerty, 277 , Rolfe, 280 Augusta, Ga., Automatic Exchange, 468 Automatic Exchanges, 453 , Clark, 470 , Connolly and McTighe, 455 , Keith, Lundquist, and Erickson, 463 , Strowger, 460 Automatic shunt, 86 B arrett, Whittemore, and Craft, 330 Battery call instruments, circuits of, 101 charging, motor-generator, 116 Batteries, Fuller, 66 gravity, 71 Hayden, 64 Gordon, 73 Le Clanche, 63 primary, 62 storage, 471 specifications, 489, 498 Bell, Alexander Graham, 6 Bell's early instruments, 7 magneto telephone, 7 Belt-driven magneto, 114 Berliner, Emile, II Birmingham wire gauge, 350 Blasting, 502 Boiling out cable, 400 Bonding of cable sheaths, 422 Bourseul, work of, 5 Brackets, 505 Braiding on cables, 393 Branch terminal multiple switch-board, 204 Breaking strength, copper wire, 358 strength of wire, 347 Breaks in line, location of, 449 Bridging bell system, Carty, 297 telephone, circuits of, 99 Brown and Sharpe gauge, 350 Buell, Charles E., 243 Busy signal on party lines, 306 test on multiple switch-boards, 201, 203, 206, 260 Cable, boiling out of, 400 boxes and platforms, 507 construction, underground, 409 hangers, 396, 507 head, Cook, 403 head, interior, 408 head, Moon, 401 , capacity of, 391, 508, 509 , comparative cost, 389 , drawing in, 419 , dry-core, 392 , lead-covered, sizes of, 394 , overhead, 389, 509 , rubber-covered, 389, 510 , rubber-covered, data concerning, 391 , saturated-core, 392 , sheaths of. 393, 508, 509 , electrolysis on, 421 , sizes of wires in, 393 , splicing of, 399, 508, 510 , stringing of, 397 , terminals, pot-head. 405 , testing with receiver, 425 , underground, 409, 507 , unfinished joints in, 509, 510 Calling apparatus, 75 apparatus battery, 75 apparatus, commercial, 104 Cant-hook, 371 Capacity, 128 of cables, 508, 509 of telephone lines, 133 tests, 446 Carbon transmitter, 13 transmitter, action of, 32 Carry-hook, 369 Carty, J. J., 100, 136, 139, 241, 246, 297 bridging bell system, 297 Carty's experiments in line induction, 139 Cement arch conduit, 416 513 5'4 INDEX. Charge, 128 Charging storage batteries, 481 Chloride accumulator, 472 Chief operator's desk, 496 Circuits of telephone, qo Clamp for guy wire, 381 for messenger wire, 396 Clay conduits, 412 Clearing-out drop, 156 Climbers, Eastern, 384 . Wotern, 384 Color of plates in storage cells, 485 Colvin, F. R., 41 Come-along, 383 Common battery systems, 236 Dean, 241, 249 Hayes, 240, 254 house, 267 multiple board, 257 repeating coil, 240 Scribner, 252 series, 237 Stone, 238 storage cell at subscriber's station, 242 Stromberg-Carlson, 263 Common return lines, induction on, 147 return systems, 144 return wire, position of, 144 return, size of, 146 Concrete, 374 for conduit, 415, 503 Condenser, capacity of, 129 Condensers, 500 Conduit, cement-arch, 416 , clay, 412 , concrete for. 415 , creosotecl wood, 411 , Johnston distributing, 413 , mandrel for. 414 , mortar for, 416 , multiple duct, 412 , obstructions in laying, 418, 502 , open box, 410 , pump log, 410 , requirements of, 409 , single duct, clay, 414 specifications, 501 , trench for, 414 , vitrified clay, 412 Connolly and McTighe, 455 Constant of galvanometer, 441 Construction tools, 370 Continuity tests, 427 Cook, F. K., 88, 165 Cook-Beach transfer system, 232 Copper wire, 355 wire, breaking strength of, 358 wire, data concerning, 356 wire specifications, 357 Cords, 30, 213, 488, 490, 493, 500 Cord weights, 494 Creosoted wood conduit, 411 Cresoting of poles, 304 Cross-arms, attachment to poles, 365 , sizes of, 364 , spacing of holes, 364 specifications, 505 Cross-talk, 139 D'Arsonval galvanometer, 435 Dead-ending, 385, 511 Dead-man, 372 Dean thermopile system, 245 Dean, W. W., 241, 243, 244, 249, 327 Density of electrolyte, 480, 482, 483, 484 Desk set, 101 , circuits of, 102 , cabinet, 499 , portable, 499 Diaphragms, receiver, 29 Dickerson, E. N., 308 Dielectric, 129 Digging J>ar, 370 Distributing boards, 281, 497 , Ford & Lenfest, 286 . llibbnrd, 283 Distui bances in telephone lines, 136 Drawing in of cables, 419 Drop, American, 162 and jack combined, American, 179 and jack combined, Western, 177 , Keystone, 162 , Warner, 160 , electrically self-restoring. 173 , mechanically self-restoring, 176 Drops, specifications for, 488, 493 Dry-core cables, 392 Du Moncel, n Dynamometer, 382 T^dison, Thomas A., 13, 15, 119 *-^ Edison's transmitter, 13 Electrification, 445 Electrolysis, 421 Electromagnetic induction, 125 induction, disturbances due to, 137 Electromagnetism, 2 Electromotive force, active, 131 Electrolyte, 62, 63, 66, 480 Electrolytic cell, 244 Electropoin fluid, 67 Electrostatic induction, 129 induction, disturbances due to, 138 Erdman, A. W. , 121 relay, 121 Ericsson, L. M., 42 Explosions in manholes, 421 Express system, 217 Extra work, 490 Faraday, 2 Faults, location of, 449 Fessenden, Professor R. A., 33 INDEX. Field of force, 125 Ford and Lenfest, 286 distributing board, 286 Framework for switch-boards, 492 Fuller cell, Standard, 66 Gaining template, 369 Gains in poles, 367 Galvanized steel strands, 395 Galvanizing of iron wire, 352, 506 Galvanometer, 435 , constant of, 44 1 shunt, 439 Gas in manholes, 421 Gauge, circular wire, 349 , micrometer, 349 Generator, Holtzer-Cabot, 105 t Western Telephone Construction Co., 104 1 Williams, 108 , Williams-Abbott, 107 , constantly driven, 113 Gharky, W. D., 47 Gordon battery, 73 Gravity cell, 71 Gray, Elisha, 6, 339 Gray's transmitter, 10 Ground return systems, 144 Grounded to metallic circuits, connection of, 149 Grounds or crosses, location of, 450 Guy, anchor, 379, 507 clamp, 381 Guy-rods, 379, 506 Guying, head, 375 , side, 375 Guy, Y-, 378 Hampton. 217, 320-21 Hand barrow, 382 Hard rubber, imitation, 22 Harmonic signaling, 338 Hayes, H. V., 240, 255, 275 Head guys, 375 ^eat-coils, 275 . , Cook, 278 , Hayes, 276 , McBerty, 277 , Rolfe, 279 Henry, Joseph, I Hibbard, Angus S., 281, 322 r. distributing board, 283 History of the telephone, I Holtzer-Cabot house system, 269 Hook-switch, 90 , Kellogg, 95 , Warner, 94 House, Royal E., 8 House systems, 265 Hughes' microphone, 14 . . David E., 13 Running, Henry, 15 ^Hydrqmeter, 480, 482 Impedance, 127, 132 Imitation hard rubber, 22 Incandescent lamps as signals, 192 Induction, electromagnetic, 125 Induction coil, action of, 16, 53 coil in base of transmitter arm, 60 coil, commercial types, 58 coils, comparison of, 5 coils, selection of, 57 coils, Varley, 59 Inspection, 491 Insulation resistance, measurement of, 442 Insulators, test of, 367 Insulator specifications, 506 Iron wire, 352 wire, data concerning, 355 wire, grades of, 353 wire, specifications for, 354 Jacks, specifications for, 488, 493 ' Jack-strip, 210 Jacques, W. W., 50 Johnston, H. W., 413 Joint, Lillie, 386 , Mclntire, 386 , Western Union, 386 specifications, 511 Jumper wires, 293 Kellogg, Milo G., 213 divided multiple switch- board, 214 Knudsen, A. M., 178 Lag screws, 365 Lamp signal controlled by relay, 195 directly in line circuit, 193 , life of, 199 switch-boards, 192 Leakage, disturbances due to, 137 Lead-burning, 477 Le Clanche batteries, 63 Lenfest and Ford, 286 Lightipe, J. A., 342 Lighting of switch-boards, 496 Lightning arrester, American, 273 arrester, ordinary form, 272 arrester. Western, 274 Lillie wire joint, 386 Line disturbances, remedy for, 140 Lines of force, 3 Listening and ringing apparatus, 163 and ringing key, American, 167 and ringing key, Cook, 165 and ringing key, Western Tele- phone Construction Co., 169 key, American, 169 key plug socket, 172 Location of breaks in line, 449 of crosses in grounds, 450 of faults, 449 Lockout systems on party lines, 301 INDEX. Lockwood, Thomas D., 8, 312 Loop test, Varley, 450 , Murray, 452 Low, George P., 227 M agneto generator, 75 generator, voltage of, 83 testing set, 424 testing set, errors from use of, 425 Magnets, permanent, 81 Make-and-break telephone, 5 Manholes, 419, 502 explosions, 421 McBerty, F. R., 243, 277, 325 McCluer system, 146 McDonnell, J. L., 315 Mclntire sleeve joint, 386 McTighe and Connolly, 455 Measurement of capacity, 446 of insulation resistance, 442 of resistance with Wheatstone bridge, 431 Meissner ringing device, 171 Messenger wire, 394 wire clamp, 396 Micrometer guage, 349 Mile-ohm, 348 Monitor's desk, 496 Morse telegraph, 4 Mortar for conduit, 416, 503 Motor-generator, 115, 489, 498 for battery charging, 116, 489, 498 Multiple duct conduits, 412 switch-board, 200 switch-board branch terminal, 204 switch-board, series, 2OI transmitter circuits, 246 Murray loop test, 452 N ess automatic switch, 269 O'Connell, J. J., 163, 192 O'Connell key, 163 Oersted, work of, I Ohm's law, 124 Overhead cable construction, 389 Packing, 45 Party line, Anders* strength and pol- arity, 318 , Barrett, Whittemore, and Craft, 330 bridged grounded, 297 busy signal on, 306 classification of, 294 Currier and Rice, 339 Dean, 327 Dickerson, 308 Party line, Harter, 344 Hibbard, 322 Lightipe, 342 Lockwood, 312 McBerty, 325 non-selective, 294 Reid and McDonnell, 313 Sabin and Hampton, 320 secrecy on, 301 series grounded, 295 step-by-step selective signaling, 308 , Gray and Pope, 339 , harmonic selective signaling, 338 strength and polarity, 318 Wood, F. B., 315 Payne, George F., 47 Permanent magnets, 81 Permeability, 80 Phelps, George M., 13 Phonograph, busy test, 227 Pike-poles, 371 Pilot lamp, 198 Pins, 365, 505 Plante, Gaston, 471 Plates, color of, 485 Plug for branch terminal system, 212 for metallic circuit system, 158 listening device, 170 Plugs, specifications for, 488, 494 Polarized bell, 78 , construction of, 84 specifications, 500 , Holtzer-Cabot, 106 , Williams, III , Williams-Abbott, 108 Pole, anchor, 377 brace, 381 buck, 369 guards, 362 holes, depths of, 371 line construction, 360 line, route of, 368 top terminal, 403 tops, grading of, 368 Poles, creosoting of, 364 heights of, 362 life of, 360 number to the carload, 363 numbers to the mile, 361 specifications, 504 sizes of, 360 vulcanizing of, 364 weight of, 363 wood for, 360 Pope, Frank L. , 339 Pot-head terminals, 405 Power circuit and telephone line, 388 Power generators, 113 Power plant, storage batteries in, 473 Power switch-board, 473, 498 Preece and Stubbs, 55 Protection of cable sheaths from electrol- ysis, 44 Protective devices, 272, 489 Pump-log conduits, 410 INDEX. 517 Raising poles, 372 Receiver, Ader, 23 . American Electric Telephone Co., 27 , Bell double-pole, 25 , Bell single-pole, 19 cords, 30 diaphragm, 29 , efficiency of, 18 , Ericsson, 28 , faults in, 22 , Holtzer-Cabot Electric Co., 27 , Kellogg, 29 specifications, 500 , Stromberg-Carlson, 25 , Western Telephone Construc- tion Co., 24 Reid, R. T., 315 Reis, Philip, 5 telephone instruments, 5 Relay circuits, simple, 118 circuits, two-way, 113 , Erdman, 121 lamp system, 195 , Stone, 122 telephone, 118 specifications, 494, 495 Repeater telephone, 118 Repeating coils, 149, 495 Resistance measurement with Wheat- stone bridge, 431 Ringer, 78 Ringing and listening keys, 163, 493 Risk, 491 Rodding, 419 Rolfe, C. A., 280 Rough tests, 424 Route of pole line, 368 Sabin, John I., 217, 315, 320 Sag in wires, 363 Saturated core cables, 392 Scribner, Charles E., 195, 198, 243, 252, 257, 302 Secrecy on party lines, 301 Self-induction, 124 , effect of, 126 Self-restoring switch-board drops, 173 Series multiple switch-board, 201 telephone, circuits of, 96 transmitter circuit, 248 Sewall cement-arch conduit, 416 Shovels, 370 Shunt, automatic, 86 centrifugal, 87 Cook, 88 for galvanometers, 439 Holtzer-Cabot, 88 Western Electric, 86 Western Telephone Construc- tion Co., 87 , Williams, 89 Specifications for small exchange, 487 for large exchange, 491 for subscriber's apparatus, 489, 498 for construction work, 501 for copper wire, 357 for iron wire, 354 Specific inductive capacity, 129 Splicing of cable, 399, 508, 510 Spring jack, 153, 488, 493 , American, 158 , Keystone, 158 , metallic circuit, 157 for multiple switch-boards, 2IO Steam power for drawing in cables, 420 Steel strands, 395 Step-by-step mechanisms, 308 Sterling Company cable terminals, 408 Electric Company's switch-board, 187 St. Louis bell exchange, 260 Stone, JohnS., 122, 238, 243 relay, 122 Storage batteries, 471 , American, 474 , amount of charge, 482 , charging, 481 , chloride, 472 , color of plates, 485 , connecting up, 476 , defective cells, 484 , density of electrolyte, 480, 482, 483, 484 , discharging, 483 , electrolyte, 480 , internal resistance, 472 , Plante cell, 471 , plates, color of, 485 , replacing electrolyte, 484 , setting up, 476 , short-circuited cell, 485 taking out of service, 486 Strains on pole lines, 375 Strength and polarity signaling on party lines, 318 Stringing of wires, 382 Strowger, Almon B., 460 Strowger automatic exchange, 460 Sturgeon, William, I Supervisory lamp, 495 Supervisory relay, 495 Supporting strand for aerial cable, 395 Sutton transmitter, 42 Switch-board drop, 154 for grounded lines, circuits of, 155 488 , metallic circuit, 159 for small exchanges, 153, specifications, 488, 492 183, Side guys, 375 Sound waves, 6 Tamping bar, 372 Telephone lines, 136 Telephone specifications, 489, 498 INDEX. Telescope for galvanometer, 438 Tensile strength of wire, 347 Tension in wires, 383 Terminal pole, 376 , pole top, 403 Testing, 424 set, magneto, 424 Tests for capacity, 446 for continuity, 427 for crosses and grounds, 4 2 5 for electrolysis, 421 with receiver, 425 Thermal arrester, Hayes, 275 , Cook, 277 , McBerty, 277 Thermopile system, Dean, 245 Thomson galvanometer, 435 Thomson, Sir William, 8 Tie, Helvin, 384 , latest method, 385 , ordinary, 384 Tin in cable sheaths, 393 Transfer systems, 216 , Western Telephone Construction Co., 227 Transpositions, 143 , method of making, 387 Trench for conduits, 414, 501 Transmitter, Ader, 36 , Ahearn, 46 , Berliner's, II , Universal, 37 , Blake, 34 , carbon, 13 , Clamond, 37 , Colvin, 41 , Crossley, 35 , D'Arsonval, 36 , Ericsson, 42 , Gower, 36 , Hunnings', 15 , Jacques, 50 , Johnson, 36 , Kellogg, 44 , multiple-electrode, 36 , Payne and Gharky, 47 specifications, 499 , " solid back," 38 , Sutton, 41 , Turnbull, 36 , variable resistance, IO -, Western Telephone Construction Transmitter, White, 38 Trouble clerk's desk, 499 Tubular drops, 160 Tying, 384. 5H TTnderground cable construction, 409 Variable resistance transmitter, 10 Varley coils, 112 loop test, 450 Viele, V. S., 398 Vitrified clay conduit, 412 Vulcanizing of poles, 364 W all telephone set, 499 Warner, 94, 160 - drop, 160 hook-switch, 94 Waves of sound, 6 Weight per mile-ohm, 348 Western switch-board, 183 Union wire joint, 386 Wheatstone bridge, 424 White, A. C., 38, 237 Williams, J. A., 89, 108, HI Wilmington, Del., switch-board, 232 Wire, breaking strength of, 347 Wire, chief's desk, 497 , conductivity of, 347 , copper, 355 , copper, specifications, 357, 510 for guying, 381 for telephone use, 347 gauges, 348 gauge, Birmingham, 350 gauge. Brown and Sharpe, 350 gauges, table of, 351 iron, 352 . iron, grades of, 353 iron, specifications for, 354 Wire plant specifications, SOT resistance of, 348 tensile strength of, 347 tension in, 383 Wiring of switch-boards, 495 Wood, F. B., 315 Woodwork, 493 Co., 43 y-Guy, 378 THE END. UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY Return to desk from which borrowed. This book is DUE on the last date stamped below. ENGINEERING / 24 195( LIBRARY LD 21-100m-9,'48(B399sl6)476