Reference / Paper · 1905
American Telephone Practice (4th ed.)
Standard reference including the early automatic exchange chapters.
Reference / Paper · 1905
Standard reference including the early automatic exchange chapters.
Digitized by the Internet Archive in 2011 with funding from Boston Library Consortium IVIember Libraries http://www.archive.org/details/americantelephonOOmill pjjini»inniuiipiuiinii»HiMiiiiiiHii»HiiiiHiiiniiHiiiniiiiiiniiiiiiiiiiiiui^ "Ms Qraw'3Jill Bock Gx Im PUBLISHERS OF BOOKS F O B^ Coal Age ^ Electric Railway Journal Electrical World "^ Engineering News-Record American Machinist ^ Ingenieria Internacional Engineering g Mining Journal ^ Power Chemical S Metallurgical Engineering Electrical Merchandising ^iiiiiiiiiii1iiiiiiiiiliNriiimiiiiii1iiiiiiiniiiiiinYiiiiiiiliiiiiiii1niiiiiiiiiiiiiriniiinin»^^^^^^^^ 1 I\ AMERICAN TELEPHONE PRACTICE BY KEMPSTER Fourth Enlarged McGRAW-HILL YORK: LONDON: Edition and Entirely Tenth NEW : B. MILLER Impression BOOK 239 Rewritten COMPANY, WEST 6 & 8 BOUVERIE 1905 39TH Inc. STREET ST., E. C. 4 ©10/ Copyrighted, 1900, AMERICAN by the ELECTRICIAN COMPANY, and CoryRiGHTED, McGRAW by the 1905, PUBLISHING New York. COMPANY, M o to the memory of my father^ joseph kempster miller, this book is dedicated. it was through his influence that the work was begun. Kempster B. Miller. PREFACE TO FIRST EDITION. The intended scope of this book is set forth in its title. To those interested the writer has endeavored to present m 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 impartial 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. 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 referencebook in practical telephony. The chapter on Automatic Exchanges has been written because the book seemed incomplete without it. If it is hereafter criticised for containing no descriptions of practical apparatus, 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 edition. 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. PREFACE TO FOURTH EDITION. The revision for this edition has resulted in an almost complete rewriting, upon which far more labor has been expended than in the original writing of the early editions. During the past year and a half in which the work has been done, the magnitude of the intended task has steadily grown, owing to the development of the telephone field, and to the wider view of this field made possible by my own changed environment. The work is now believed to cover telephone practice of to-day much more comprehensively and accurately than was true of any of the earlier editions with respect to the practiced art at the times of their publication. Obsolete methods and equipment are not dealt with, except where of distinct educational or historic value ; and much of the historic matter of former editions has been cut out in order to make room for modern methods and things. The classification according to chapters has been materially altered, many of the old chapters having been combined with others or subdivided, in order to afford a more logical arrangement. The chapter on The Telephone Exchange in General has been added as an introduction to the subsequent chapters, which deal with the various parts of the exchange, particularly with the switch-board systems, appliances and auxiliaries. When former editions were written, the common battery or central energy systems were comparatively new and little information concerning them was available. The resulting lack of full information on this subject, for which the early editions might have been criticised with justice, is now thought to have been removed. Chapters XVT to XXXV., inclusive, are almost entirely new, although they contain some matter that was barely touched upon in previous editions. Such subjects as trunking between common battery offices, private branch exchange service, measured service, toll switch-board systems and power plants are now here treated for the first time. There has been rapid recent development in automatic switchboards and the complete description of the modern Strowger system, vii viii PREFACE TO FOURTH EDITION. contained in Chapter XXXV., is indicative of the increasing importance of this branch. This work is intended not only as a guide to the student of practical telephony whose experience has not been sufficient to make him conversant with all branches of the subject, but also as an aid to the more experienced telephone engineer and operator, who may find it of value as a general reference work. To my partner, Mr. Samuel G. McMeen, and to my former associate, Mr. Charles S. Winston, now Chief Engineer of the Kellogg Switchboard and Supply Company, are due my first acknowledgements for valuable assistance. I have drawn freely on their funds of knowledge, judgment and good nature, and on their much more limited funds of time. I feel some pride in the illustrations, most of which are quite, new. This is true particularly of the diagrams of complicated circuits, which are thought to be more complete than any thing heretofore attempted in this line. Much of the credit for the appearance and arrangement of these is due to the skill of Mr. Howard M. Post, who drew most of them. To Mr. R. H. Burfiend credit is given for the appearance of many of the apparatus drawings and some of the circuit work. The various telephone manufacturing companies have co-operated by loaning photographs and half-tones illustrative of modern apparatus and work. Especially is this true of the Stromberg-Carlson Telephone Manufacturing Company, the Kellogg Switchboard and Supply Company, the Holtzer Cabot Electric Company, and the Automatic Electric Company. In the preparation of the manuscript for a work of this nature the amanuensis may be a boon or a "calamity." I tender sincere thanks to Miss B. R. Werden and Miss Eva A. Garlock for their unusually painstaking and intelligent work in preparing all the manuscript from my dictation and notes, and in the proof-reading. I feel that the public will, as the writer does, thank the makers of this book, the McGraw Publishing Company, for the manner in which they have done their work. Their patience is also to be commended. The kindly reception of the earlier editions is keenly appreciated, and it is my hope that the work is now more nearly worthy. Kempster B. Miller. TABLE OF CONTENTS. CHAPTER I. PAGE History and Principles of the Magneto Telephone, . . . i . 13 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 ElectroPhonetic Telegraph. CHAPTER H. History and Principles of the Variable Resistance Transmitter, Gray's Variable Resistance Transmitter — Bell's Liquid Transmitter — Berliner's Transmitter — Electrodes in Constant Contact — Carbon Electrodes — Demonstration of Advantages of Loose Contact by Hughes — Hughes' Microphone — Hunning's Granular Carbon Transmitter— Induction Coil with Transmitter. CHAPTER Electromagnetic and Electrostatic HI. Induction, .... 23 Ohm's Law — Field of Force about Conductor — Electromagnetic Induction— Action between Turns of the Same Coil — Impedance — Efifects of Self-induction on Undulatory Currents — Charge of Electricity — Action between Like and Unlike Charges — Electrostatic Induction — Condensers — Capacity — Specific Inductive Capacity of DielectricsSpecific Inductive Capacity in Telephone Cables — Effect of Condenser Bridged across Circuit — Effect of Capacity on Carrying Currents — Trans-Oceanic Telephony. CHAPTER The Telephone IV. Receiver, 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. 34 X TABLE OF CONTENTS. CHAPTER V. PAGE The Carbon Transmitter^ .53 Action of the Transmitter — Single-Contact Transmitters — Multiple-Contact Transmitters — Granular Carbon Transmitters — Commercial Types of Transmitters — Packing: Its Remedy — Unusual Forms of Transmitters. CHAPTER VI. Induction Coils for Local Battery Telephones, . . -. - 7Z Advantages of the Induction Coil — Primary Current — Secondary Current— Design of Induction Coils — Commercial Coils — Varley Method of Winding — Mounting of Induction Coils — Results of Comparative Tests — Methods of Making Comparative Tests. CHAPTER Primary VII. Batteries, 85 Simple Cell — Direction of Current — Positive and Negative Poles — Materials Best Suited for Electrodes — The LeClanche Cell — The Fuller Cell — Specification for Standard Fuller Cell — The Gravity Cell — The Gordon Cell — The Dry Cell — Comparative Battery Tests. CHAPTER Magneto Calling Apparatus, VIII. ........ 104 Battery Calls — Magneto Generator — Its Action — The Magneto Bell — Factors Governing the Output of Generators — Design of Generators — WaVe-Form of Magneto Generators — Points in Design of Magneto Bell — Varley Windings — The Manually Operated Shunt — The Automatic Shunt — Commercial Forms of Generators and Ringers — The Biased Bell. CHAPTER Local Battery Sub-Station IX. Equipments, 131 The Term Sub-Station — Series Equipments — Bridging Equipments — The Hand-Switch — The Automatic Hook-Switch — Circuits of Series Sub-Station Equipment — Types of Bridging Circuits — Complete Telephones — Circuits of Complete Telephone — Portable Desk Stands — Types of Desk Stand Circuits — Commercial Types ' of Switch Hooks. CHAPTER X. Telephone Lines, The Grounded Line — Work of J. J. Carty — Electromagnetic Disturbances on Telephone Lines — Electrostatic Disturbances — Transposition of Telephone Lines — The Repeating Coil — The Uses of the Repeating Coil. 158 TABLE OF CONTENTS. CHAPTER The Telephone Exchange xi XL PAGE in General, 170 The Functions of the Telephone Exchange — Definition of Telephone Office — Definition of Telephone Exchange — Single-Office Exchanges — Two-Office Exchanges — Multi-Office Exchanges — Manually Operated Exchanges — Automatic Exchanges — Early Idea of Central Office Exchange Work — The Law System — The Prototype of the Modern Switch-Board. CHAPTER The Magneto Switch-Board Xn. for Small Exchanges, .... 176 Simplest Switch-Board for Grounded Lines — The Spring Jack — The Switch-Board Drop — Details of Circuits for Simple Switch-Boards — The Metallic Circuit Jack — The Plug and Cord — Types of Commercial Drops and Jacks — Ringing and Listening Keys — Cord Circuits — Self-Restoring Drops — Combined Drops and Tacks — Complete Magneto Switch-Boards — Audible Line Signals — Toll Cut-in Stations— The Wiring of Switch-Boards. CHAPTER XIIL The Theory of the Multiple Switch-Board, ..... 219 Necessity for Multiple Boards — Fundamental Object of Multiple Boards — Answering Jacks — Multiple Jacks — Sections of Switch-Board — Operators' Positions — The Busy Test — Limiting Factors in SwitchBoard Capacity — End Positions. CHAPTER The Magneto Multiple XIV. Switch-Board, 225 Switch-Board for Grounded Lines — The Series Multiple Board — The Courtlandt Street Switch-Board in New York — The Branch Terminal Multiple Board — Spring Jacks for Multiple Boards — The Paris Switch-Board. CHAPTER Transfer XV. Systems, Apparent Desirability of Transfer Systems — Saving in Jacks — The Sabin & Hampton Express System — A Boards — B Boards — Arrangement of Circuits in the Sabin & Hampton System — Automatic Calling — Automatic Clearing-Out — The Multiple Plug Transfer System — SwitchBoard at Wilmington, Delaware — Old Transfer System at Grand Rapids, Michigan — Transfer System for Small Switch-Board. 245 xii TABLE OF CONTENTS. CHAPTER XVI. PAGE Systems of Transmission in Common Battery Exchanges, . . 265 Advantages of Locating Sources of Energy at the Central Office — Various Methods of Supplying Transmitter Current from Central Office — The Stone System — The Hayes System — Kellogg Two-Battery System — The Dean System — Electrolytic Cells at Subscribers' Stations — Thermopile at Subscribers' Stations. CHAPTER Signaling in Common XVII. Battery Systems, 277 Fundamental Requirements in Automatic Signaling — Difference between Common Battery and Magneto Signaling — Mechanical Signals — Lamp Signals — Elements of Sub-Station Circuits — Signaling without Relay — Signaling with Relay — The Cut-off Jack — The Cut-off Relay — Supervisory Signals — Various Methods of Controlling Supervisory Signals — Types of Switch-Board Lamps — Types of Mechanical Signals. CHAPTER Common Battery Switch-Boards XVIII. in Small Exchanges, . . . 293 Typical Circuits — Operation of Complete System — System Using Direct Lamps — Cabinet for Small Switch-Boards. CHAPTER XIX. Common Battery Sub-Station Equipment, Simple Sub-Station Circuit — Circuit with Condenser — Western Electric Sub-Station Circuit — Stromberg-Carlson Sub-Station Circuit — Kellogg Sub-Station Circuit — Common Battery Telephone Sets. CHAPTER 304 XX. The Common Battery Multiple Switch-Board, .... Western Electric System — Details of Operation of System — Wiring Circuit of Line and Pilot Signals — The Kellogg Two-Wire System — Three-Wire vs. Two-Wire System — The Stromberg-Carlson TwoWire System — The Stromberg-Carlson Three-Wire System — The Sterling Multiple Board — The North System — The International 313 System — The American Electric Telephone Company's System — The Bell Switch-Board at St. Louis. CHAPTER Trunking System Between Common XXL Battery Offices, . . . 354 Necessity of Trunking — Outgoing and Incoming Trunk Lines — A and B Positions — Approved Method of Trunking^-Western Electric Trunk Circuit — Its Operation — Automatic Ringing — The Kellogg Trunk Circuit — The Stromberg-Carlson Trunk Circuit — Guarding Against Mistakes in the Operation of Trunk Circuits. TABLE OF CONTENTS. CHAPTER xiii XXII. PAGE The Divided Multiple System, ....... 378 Limitation of Ordinary Multiple Switch-Board— A Method of Increasing the Possible Size of Multiple Switch-Boards — Capacity of the Divided Multiple System — Schematic Treatment of Divided Multiple System — Method of Signaling — The System at St. Louis and Cleveland— The Divided Multiple for Common Battery — The Four-Division Board at Cleveland, Ohio. Private Branch Exchange CHAPTER XXIII. Service^ ....... 396 Definition of Private Branch Exchange — Local Connection — Single Track vs. Double Track System — Division of Work between Operators— Methods of Operation in Private" Branch Exchanges — Private Branch Exchange Circuits — Complete Private Branch Exchange Switch-Boards. CHAPTER XXIV. Party Line System s^ 423 General Classification of Party Lines — The Carty Bridging Bell System — Early Types of Party-Line Signaling — The Hibbard System — The McBerty System Using a Jack for Each Station — The Thompson & Robes System — The Dean Harmonic System — The Leich System. CHAPTER Measured XXV. Service, 460 Methods of Charging for Telephone Service — "Deadhead" Business — The Measuring of Telephone Service — Coin-Collecting Devices — The Baird Pay-Station — The Gray Station — The Scribner Coin-Collecting Device — The Stroud Coin-Collecting Device — Circuits of Various Coin-Collecting Systems — Telephone Meters or Counters — Counters at the Subscribers' Stations — Counters at the Central Office — The Western Electric Counter or Meter. CHAPTER Toll Switch-Board XXVI. Systems, Points in the Operation of Toll-Boards — Desirability of giving Regular Operators no Work of Special Nature — Toll-Line Operators — Toll Recording Operators — Three Methods of Handling Toll Service — Toll-Board Sections. 485 xiv TABLE OF CONTENTS. CHAPTER XXVII. Switch-Board Apparatus, PACE Details of Multiple .... 513 Three-Wire Jacks — Two-Wire Jacks — Grouping of Jacks in Strips or Banks — Numbering of Multiple Jacks — Lamp Jacks — Arrangement of Answering and Multiple Jacks — Details of Switch-Board Frame — Lamp Mounting — Cords and Plugs — Cord Weights — Ringing and Listening Keys — Order Wire Keys — Cut-in Jack and Plug — Operators' Transmitting Mounting — Types of Relays. CHAPTER Power Plants in Common XXVIII. Battery Systems, 544 Old Types of Power Plants— The Necessity of Reliable Primary Power — The Circuits of a Modern Power System — Power Switch-Boards — Methods of Wiring Power Switch-Boards — Impedance Coils in Power Plants — Magneto Generators — Various Types of Charging and Ringing Machines — Busy-Back Attachments — Power Tables — Rheostats. Storage Batteries, CHAPTER XXIX. .......... 572 Simple Storage Cell — The Plante Cell — The Chloride Accumulator — Glass and Wooden Tanks — The American Battery — Lead Burning — The Electrolyte — The Charging of Batteries — Determination of Amount of Charge — Replacing of Electrolyte — Treatment of Injured Cells — Color of Plates — Taking Batteries out of Service. CHAPTER Protective Devices, XXX. .......... 587 Problems Involved in Adequate Protection — Elements Against which Protection Must Be Made — The Saw-Tooth Arrester — The Carbon Block Arrester — Fusible Cut-Outs — The Mica-Fuse — Heat Coils — Complete Protection for Telephone Lines — The Enclosed Fuse — Methods of Mounting Fuses — Types of Complete Protectors. Distributing Frames, CHAPTER . . . XXXI. . .. . . .612 The Object of the Distributing Frame — Old St. Louis Frame — The HiJbbard Frame — The Ford and Lenfest Frame — The Cook Frame — The Intermediate Distributing Frame — Methods of Wiring Through Distributing Frame. CHAPTER XXXIL Chief Operator's and Monitor's Equipments, . v • • • • The Duty of Chief Operators, Monitors and Supervisors — Lines for Observing Service — Monitor's Taps — Lines to Local Board — Functions of Various Circuits. 628 TABLE OF CHAPTER CONTENTS. xv XXXIII. PAGE Wire Chief's Equipment, 635 Duty of the Wire Chief — Testing Trunks — Details of Testing Trunk Circuits— Tests to Outside Lines — Tests to Switch-Board Lines — Modern Testing Circuits — The Use of the Voltmeter in Testing — Directions for Making Various Tests. CHAPTER The Lay-Out XXXIV. and Wiring of Central Office Equipments, . . 651 Necessity for Good Workmanship — Details of Western Electric Wiring System — The Use of Cables in Exchange Wiring — Method of Fanning out Cables — Multiple-Jack Wiring — The Termination of Line Cables at the Central Office — Pot-Heads vs. Iron Box Terminals — Arrangement of Frames in Kellogg System — The Wiring of the Courtlandt Street Office — Arrangement of Multiple Cables — The Plaza Office in New York — Floor Plans of Several Modern Offices — Operators' Quarters. CHAPTER Automatic Switch-Board XXXV. Systems, 691 Early Efforts towards Producing Automatic Switch-Boards — The Connolly-McTighe System — The Early Strowger System — The Vertical and Rotary Switch — The Operation of Modern Strowger System — The Sub-Station Equipment — Circuit of the Sub-Station Equipment — Vertical and Rotary Wipers — Circuits of the First Selector — The Side-Switch — Circuits of Second Selector — Circuits of Connector — Various Steps in the Connection between Two Subscribers — The Systems at Fall River and Grand Rapids — Methods of Handling Toll Service — Common Battery Automatic Systems — The Work of BuUard and Rorty — Problem : The Automatic vs. The Manual Exchange. CHAPTER Intercommunicating Systems, XXXVI. ........ 736 Interior Installations — The "House System" in General — Circuits of Magneto House Systems — Common Battery House Systems — The Ness Automatic Switch — The Holtzer-Cabot Systems — Desk Stands for House Systems — Metallic Circuit Systems. CHAPTER The Telephone Relay or Repeater, XXXVII. ....... The General Idea of the Repeater — Simplest Form of Repeater — TwoWay Repeater — The Erdman Repeater — The Stone Repeater — The Cooper Hewitt Mercury Vapor Repeater. 745 xvi TABLE OF CHAPTER Wire for Telephone CONTENTS. XXXVIII. Use, ^^^^ 752 Materials for Telephone Wire — Tensile Strength of Wire — Conductivity of Wire — The Mile-Ohm — Wire Gauges — Properties of Iron Wire — Galvanizing of Iron Wire — Grades of Iron Wire — Steel Wire — Properties of Copper Wire — Specilications for Copper Wire — Insulated Wire — Magnet Wire — Rubber-Covered Telephone Wire — Specifications for Rubber-Covered Wire. CHAPTER Pole-Line XXXIX. Construction^ 768 Kinds of Wood for Telephone Poles — Life of Telephone Poles — Sizes of Poles — Spacing of Poles — Butt Plates — Data Concerning Loading of Poles on Cars — Pole Preserving Processes — Creosoting — The Chloride of Zinc Process — Vulcanizing — Structural Iron Poles — Cross Arms — Pins — Pole Hardware — Insulators — Construction Tools — Method of Raising and Setting Poles — Derrick Wagon — Method of Guying and Anchoring Messenger Wire — Clamps for Messenger Wire — Corner Work — Tying and Splicing of Wires — Sag in Wire — Telephone Circuit and Power Line. CHAPTER Aerial Cable Construction, XL. ........ 805 Present Tendency Toward Cable Work — Electrical and Mechanical Problems Involved — Rubber-Covered Cable — Dry Core Paper Cable — Properties of Paper Cable — Two Methods of Measuring Capacity — "A New Danger to Lead-Covered Cables" — The Work of John Hesketh — The K. R. Law — Effect of K. R. Value on -Transmission — : Messenger Wire — Messenger Wire Supports — Cable Hangers — Methods of Stringing Cable — Cable Splicing — Pot-Head Terminals — Iron-Box Heads — Multiple Cable Taps — Method of Making Y-Splice — Method of Distributing from Terminal Poles, Underground CHAPTER Cable Construction, XLL ■ Advantage of Placing Telephone Wire Underground — Requirements of Conduit — Different Kinds of Conduit — Methods of Laying Conduit — Bends to Avoid Obstacles — Rodding — The Drawing in of Cable — Steam Winch — Electric Winch — Cable Reel Truck — Data Concerning Cost of Conduit. CHAPTER XLII. Testing, Classification of Tests — Tests with Magneto Set — Tests with Telephone Receiver — The Wheatstone Bridge — Commercial Forms of Bridges — The Galvanometer — Types of Galvanometers — Methods of Using Shunt — Obtaining Constant of Galvanometer — Measurement of Insulation Resistance — Capacity Tests — Location of Faults — The Varley Loop Test— The Murray Loop Test— Voltmeter Tests. 835 851 AMERICAN TELEPHONE CHAPTER HISTORY AND PRINCIPLES OF PRACTICE. I. 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 invariably 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 principles 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 contributed byeach investigator making possible a deeper insight into the subject on the part of his successors. It is best, therefore, 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 disclosed the laws upon which present electroma^neti : 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 electromagnet 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 his classic experiments on the electromagnet, and to him must be accredited a 1 2 AMERICAN TELEPHONE PRACTICE. 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 183 1 Faraday and Henry, independently, discovered the converse of these laws of electromagnetism — that if the intensity of a magnetic field inclosed by a conductor be in anywise changed, a current of electricity will flow in the conductor. This current will How 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 npt only of the telephone and telegraph, but of electric II FIG. 1.— STURGEON ELECTROMAGNET. lighting, electric power transmission, and of every other achievement by which electricity has revolutionized 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 telephone 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 t)f either of its poles. Any variation in the strength of this current will cause corresponding variations in the attractive power of the magnet. If the rod be of steel, and permanently magnetized, it will exert an attract- THE MAGNETO TELEPHONE. 3 ive 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 FIG. 2.-LINES OF FORCE OF BAR-MAGNET. iron) has exerted on it, by the magnet, an influence of attraction or repulsion. This field of force is usually graphically represented by closed curves, radiating from the poles of the magnet, and the strength of the magnet is commonly measured in terms of FIG. 3.— FARADAY AND HENRY MAGNETO ELECTRICITY. 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 placing it in a horizontal position and directly over it a sheet of paper or cardboard. Ifiron filings are then dropped from a height of a few feet, 4 AMERICAN TELEPHONE PRACTICE. . on the paper, they will arrange themselves in the direction of the lines of force. Fig. 2 shows such a map produced by the barA'' 5". magnet, If now a galvanometer, G, or other current-indicator (Fig, 3) be placed in circuit with a cOil, C, and a magnet A'^ S, moved in the vicinity of the coil, or the coil in the vicinity of the magnet, in such manner as to change the number of the lines of force passing through the coil, a current is generated in the coil and is indicated by the galvanometer. This current will flow only while such movement is taking place. 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 finding 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 ^p t \M K J^ FIG. 4.— MORSE ELECTROMAGNETIC TELEGRAPH. about a pole of the magnet and the 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 fiow in the coil. In 1837 Professor Page of Salem, Mass., discovered that a rod of iron, suddenly magnetized or demagnetized, would emit certain 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 THE MAGNETO TELEPHONE. 5 and breaking the circuit. By manually closing and opening the key, the core of the magnet became magnetized and demagnetized, thus alternately attracting and releasing the armature. 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 between distant points. In 1854 a Frenchman, Charles Bourseul, predicted* the transmission of speech, and outlined a method correct save in one particular, but for which error one following his directions could have produced a speaking telephone. His words at this date seem almost prophetic: "I have asked myself, for example, if the spoken word itself could not be transmitted by electricity; in a word, if what was spoken in Vienna may not be heard in Paris ? The thing is practicable in this way: "Suppose that a man speaks near a movable disc, sufficiently flex- // ^ FIG. 5.— REIS' MAKE-AND-BREAK TELEPHONE. ible to lose none of the vibrations of the voice ; that this disc alternately makes and breaks the connection from a battery : you may have at a distance another disc which will simultaneously execute the same vibrations." The words "makes and breaks" in Bourseul's quotation have been italicized by the present writer. They form the keynote of the failures of those who subsequently followed Bourseul's directions literally. Philip Reis, a German inventor, constructed what he called a telephone in 1861, following implicitly the path outlined by Bourseul. He mounted a flexible diaphragm, D (Fig. 5), over an opening in a wooden box, and on the center of the diaphragm fastened a small • Vol. XXIV., "L'lllustration," Paris, Aug. 26, 1854. AMERICAN 6 TELEPHONE PRACTICE. piece of platinum, P. Near this he mounted' a heav^ 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, A^, wrapped with silk-insulated copper FIGS. 6 AND 7.— REIS' TELEPHONE TRANSMITTER AND RECEIVER. wire and mounted on a cigar box for a sounding board. Its operation was as follows: The sound waves set up in the air struck against the diaphragm of the transmitter, causing it to vibrate in unison with them. This caused the alternate making and breaking of the circuit at the point of contact between the platinum and the spring, and allowed intermittent currents to flow through the receiver. These fIG. 8.— SOUND WAVES OF VOICE AND SIMPLE MUSICAL NOTE. caused a series of sounds in the knitting needle by virtue of "Page's effect." The sounding board vibrated in unison with the molecular 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 sovinds. The question as to whether it actually did transmit speech THE MAGNETO TELEPEIONE. 7 has been the subject of much discussion, but if it did this at all it was very imperfectly! The cause of its failure to successfully transmit speech will be understood from the following facts : A simple musical tone is caused by vibrations of very simple forms, while sound waves produced by the voice in speaking, are very complex in their nature. These two forms of waves may be graphically represented as in Fig. 8. Sound possesses three qualities : pitch, depending entirely |On the frequency of the vibrations ; loudness, depending on the 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 n FIG. 9.— BELL MAGNETO TELEPHONE. 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 lacking. This apparatus could therefore exactly reproduce the pitch of a sound, but not its timbre and relative loudness. For the next fifteen years no apparent advance was made in the art of telephony, although several inventors gave it their attention. In 1876 Professor Alexander Graham Bell and Professor Elisha Gray almost simultaneously invented successful speaking telephones. Gray has been one of the principal claimants for the honor of being the first inventor of the telephone, but Bell has apparently established his right to it, and has also reaped the profit, for, after long litigation, the United States Patent Office and' the courts have •awarded the priority to him as against Gray and many others. AMERICAN TELEPHONE PRACTICE. Bell possessed a greater knowledge of acoustics than of electrical science, and it was probably this that led him to appreciate wherein others had failed. His instrument consisted of a permanent barmagnet, 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 FIG. 10.— BELL'S CENTENNIAL FIG. IL— BELL'S CENTENNIAL RECEIVER. TRANSMITTER. movinted as to allow its free vibration close 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 THE MAGNETO TELEPHONE. 9 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 already pointed out, cause currents to flow in the circuit. These currents flow first in one direction, and then in the other, varying in unison with the movements of the diaphragm, the waves being very complex, and represented graphically, 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 decrease the strength of the permanent magnet of the receiving instrument, 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, exhibited in 1876 at the Centennial in Philadelphia, are shown in Figs. 10 and 11, the former being his receiver, the latter his transmitter. The receiver consisted of a tubular magnet, composed of a coil of wire, H, surrounding a core, C, and inclosed in an iron tube, E, which was about if inches in diameter and 3 inches long. This tube was closed by a thin iron armature or diaphragm, D, 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 whole was mounted on a base, F, as shown, arrangements being made to adjust the air gap between the pole of the core and the diaphragm by means of a thumb-screw. The transmitter, Fig. 11, consisted of an electromagnet, H, in front of the core, C, of which was adjustably mounted a diaphragm of goldbeater's skin, D, carrying a small iron armature, A, at its center. A long mouthpiece, M, into which the sounds to be transmitted were spoken, served to convey the sound waves more directly to the diaphragm. 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. 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 letter written by Mr. Lockwood to the writer of this book : 10 AMERICAN TELEPHONE PRACTICE. "This mistake first appeared in the account given by Engineering of Sir William Thomson's address to the British Association in September, 1876, and has been universally copied. * * * xhe origin of the mistake is very odd. The screw of the instrument given to Sir William Thomson, and which he exhibited in England 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 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