Reference / Paper · 1913
Telephony: A Comprehensive and Detailed Exposition
Large reference including automatic systems.
Reference / Paper · 1913
Large reference including automatic systems.
TELEPHONY A COMPREHENSIVE AND DETAILED EXPOSITION OF PRACTICE OF TELEPHONE ART THE By SAMUEL THE THEORY AND G. McMEEN Member, American Institute of Electrical Engineers, and Western Society of Engineers and KEMPSTER B. MILLER Member, American Institute of Electrical Engineers, and Western Society of Engineers Author ol "American Telephone Practice" ILLUSTRATED AMERICAN CHICAGO SCHOOL OF CORRESPONDENCE 1912 t Engineering library M COPYRIGHT 1912 BY AMERICAN SCHOOL OP CORRESPONDENCE Entered at Stationers' Hall, London All Rights Reserved I Preface the very opening of this work we have endeavored to give a general view, a mere sketch, of the telephone art as a whole, so that the reader, if a beginner, may be disabused of the idea that telephony is fairly represented by the instrument on his wall or desk. Also, before plunging into A' the maze of details, we have endeavored to set forth the general principles involved in the transmission of speech and of signals, and to discuss the electrical properties with which the art has to deal. It has been our hope that this would give the student a greater interest and a broad enough view to enable him to correlate individual facts with other phases of the art. C. A similar course has been followed in the preparation of the circuit diagrams. The telephone man has a sign language of his own, of which there are many dialects. We have, in the preliminary discussion of each piece of apparatus, shown one or more appropriate conventional symbols representing it, and these symbols have been adhered to in the preparation of all the drawings. Where, as in automatic systems, intricate mechanical functions are involved, we have made the circuit drawings as suggestive as possible of the mechanical as well as the electrical working. Thus, the drawings in large measure tell their own stories, always in the same language, avoiding the necessity of employing the bewildering mass of reference letters and figures so often found. Every drawing used has been prepared especially for this work. The photographs have been collected with care, and, like the drawings, they have been used to illustrate the text rather than as a medium around which to build the text. 238200 C. No attempt has been made to make the work historically complete, and except in rare occasions, where a distinct lesson could be drawn from obsolete types or methods, we have confined the discussion to modern practice — to the surviving works of the first generation of telephone men. C. It is fitting that we should express our appreciation of the intelligent, painstaking, and faithful work of those who have aided us. Our first acknowledgment is tc Mr. Leigh S. Keith, whose fund of information, thoughtful criticism, and unlimited patience have contributed in no small degree to whatever merit and accuracy this work may possess. It also gives us pleasure to commend the work, and the spirit in which it has been done, of Miss Leona Ekstrom, of Chicago, and Miss Gertrude Cohen, of San Francisco. These young ladies have carefully refrained from mutilating our thoughts, always returning them to us in form as good or better than when received. C. Acknowledgment is due to the various manufacturing companies, who have freely placed at our disposal illustrations and information concerning their products. When compared with the attitude existing some years ago, the willingness on the part of some to make such matters public is, we think, one of the hopeful signs of the times. C, Nor must the American School of Correspondence, the publisher of this work, be forgotten in our acknowledgment. It has co-operated in every way in which a publisher may cooperate with authors. The physical form in which the work is to appear will bear evidence of this. SAMUEL G. MCMEEN KEMPSTER B. MILLER January 1, 1912. TABLE OF CONTENTS INTRODUCTION Page HISTORY AND DEVELOPMENT 1 The First Telephone— The Work of Alexander Graham Bell, Sir William Thomson, and Lord Kelvin — Public Use of the Telephone — Telephone System — Telephone Exchange — Operation of the System — Switchboards — Manual Apparatus — Automatic Telephony— Progress in the Telephone Field — Long- Distance Transmis ion— The Bell Organization — Independent Companies CHAPTER I ACOUSTICS 9 The Art of Telephony— Propagation of Sound— Characteristics of Sound— Loudness — Pitch — Timbre Harmonics — Human Voice — Human Ear CHAPTER II ELECTRICAL REPRODUCTION OF SPEECH .... .11 Limitations of the Present Art of Telephony — Complete Telephonic Cycle — Magneto Telephones — Early Conceptions — Electrostatic Telephone — Variation of Electrical Pressure— Variation of Resistance — Carbon — Carbon Transmitter — Induction Coil— Capacity — The Condenser — Measurement of Telephone Currents CHAPTER III ELECTRICAL SIGNALS 29 Audible Signals— Telephone Sounder — Vibrating Bell— Magneto Bell — Polarized Ringer — Telephone Receiver— Visible Signals — Electromagnetic-Signal — Electric Lamp Signal— Signal Circuits CHAPTER IV TELEPHONE LINES 37 Telephone Line Circuit — Underground, Submarine, and Aerial Cables — Materials — Conductivity — Ohm's Law — Electrostatic Capacity — Units — Inductance—Reactance— Insulation of Conductors — Inductance vs. Capacity — Transmission Distances— Pupin or Loading Coils — Improving Transmission CHAPTER V TRANSMITTERS 53 Variable- Resistance Transmitter — Single-Contact Transmitter — Multiple-Contact Transmitter — Granular Carbon — Western Electric Solid-Back Transmitter — New Western Electric Transmitter — Kellogg Transmitter — Automatic Electric Company Transmitter — Monarch Transmitter — Electrodes — Packing — Acousticon Transmitter— Switchboard Transmitter CHAPTER VI RECEIVERS Early Types — Modern Receivers — Western Electric Receiver — Kellogg Receiver —Direct-Current Receiver — Automatic Electric Company Receiver— Monarch Receiver — Dean Receiver — Operator's Receiver 70 vi TABLE OF CONTENTS CHAPTER VII PRIMARY CELLS . 82 Work of Galvani and Volta— Simple Voltaic Cell— Potential Differences — Electrolytes— Polarization — Local Action — Amalgamation — Series and Multiple Connections—Types ofPrimary Cells— Open-Ci rcuit Cells— Wet Cells— Dry Cells— ClosedCircuit Cells — Types and Construction — Standard Cell CHAPTER MAGNETO SIGNALING APPARATUS VIII 105 Method of Signaling — Battery Bell — Magneto Bell — Magneto Generator — Armature— Automatic Shunt — Pulsating Current— Polarized Ringer — Types — Biased Bell CHAPTER IX THE HOOK SWITCH 122 Automatic Operation — Design — Kellogg, Western Electric, and Dean Wall-Telephone Hooks — Western Electric and Kellogg Desk-Stand Hooks CHAPTER ELECTROMAGNETS AND INDUCTIVE COILS X 133 The Electromagnet — Permeability — Magnetization Curves — Reluctance — LowReluctance Circuits — Bar, Horseshoe, and Ironclad Electromagnets — Armature Motion — Differential Electromagnet — Magnet Wire — Wire Table — Insulation — Winding Impedance Coils — Types — Induction Coil — Design — Current and Voltage Ratios— Repeating Coil— Types CHAPTER NON-INDUCTIVE RESISTANCE XI DEVICES 162 Temperature Coefficient — Wire Tables— Inductive Neutrality— Heating— Types — Mica-Card Unit— Differentially-Wound Unit— Lamp Filament— Iron Wire Ballast CHAPTER XII CONDENSERS 168 Charge — Capacity — Units — Dielectrics — Inductive Capacity — Condenser Sizes — Functions— Assorting Currents CHAPTER CURRENT SUPPLY TO TRANSMITTERS XIII 176 Local Battery — Common Battery — Advantages of the Common Battery — Series Battery— Bridging Battery with Repeating Coil — Bell Substation ArrangementBridging Battery with Impedance Coils — Double Battery with Impedance Coils — Kellogg, Dean, Stromberg-Carlson, and North Substation Arrangements — Common Source Supply — Repeating Coils — Retardation Coils CHAPTER XIV THE TELEPHONE SET 197 Classification — Magneto, Common- Battery, Wall, and Desk Telephones — Circuits of each Class — Micro-Telephone Set CHAPTER NON-SELECTIVE PARTY-LINE SYSTEMS XV Party-Lines—Private Lines — Party-Line Signaling — Series Systems — Bridging Systems— Signal Code 217 TABLE OF CONTENTS CHAPTER vii XVI SELECTIVE PARTY-LINE SYSTEMS 228 Problem of Selection — Classification: Polarity, Harmonic, Step-by-Step, and Broken-Line Systems — Polarity Method — Two and Four-Party Circuits — Harmonic Method — Tuning — Under-Tuned System — Under-Tuned and In-Tuned Ringers — Harmonic System Circuits — Step-by-Step Method — Broken-Line Method CHAPTER XVII LOCK-OUT PARTY-LINE SYSTEMS 253 Rural Party-Line Problems— Methods— Poole System — Step-by-Step System— Broken-Line System — Operation CHAPTER XVIII ELECTRICAL HAZARDS 277 Hazards, Exposed and Unexposed — Lightning Discharges — Power Circuits — Classification — Electrical Actions — Design of Apparatus to Resist Electrical Hazards — Self -Protecting Devices — Electrolysis CHAPTER XIX PROTECTIVE MEANS 284 Protection Against High Potentials— Air-Gap Arrester— Gap Discharges— Carbon — Commercial Types of Arresters — Arcs — Protection Against Strong Currents — Fuses — Types — Fuse Banks — Protection Against Sneak Currents — Sneak-Current Arresters — Heat Coil — Complete Line Protection — Exposed and Unexposed Wiring — Location of Fuses — Central Office Protectors — Self -Soldering Heat Coils — Cook Arrester — Subscribers' Station Protectors — City Exchange Requirements — Drainage Coils CHAPTER XX GENERAL FEATURES OF THE TELEPHONE EXCHANGE . . .307 Subscribers' Lines — Trunk Lines — Toll Lines — Districts — Switchboards — Manual Boards — Magneto and Common Battery Switchboards — Multiple and Non-Multiple Switchboards— Transfer Systems — Trunking Systems— Call Circuits— Toll Boards CHAPTER THE SIMPLE MAGNETO SWITCHBOARD XXI 314 Operation — Component Parts— Line Signals — Jacks and Plugs — Keys — Line and Cord Equipments — Operation of the Magneto Switchboard — Circuits — Commercial Types of Drops and Jacks — Night Alarm — Restoration — Code Signaling — Switchboard Plugs — Cord Attachment — Switchboard Cords — ConductorsOperator's Telephone Equipment — Complete Switchboard Circuits — Lines — Magneto Cord-Circuit Connections — Convertible Cord Circuits — Through-RingingCabinets—Target Signals— Sectional Switchboards CHAPTER THE SIMPLE COMMON-BATTERY XXII SWITCHBOARD . . . .377 Advantage of Common- Battery Operation — Line Lamps — Pilot Lamp — Cord Circuits— Supervisory Signals— Common- Battery Switchboard Cycle of Operations— Mechanical Signals — Western Electric, Kellogg, and Monarch Types — Relays — Jacks CHAPTER XXIII TRANSFER SWITCHBOARD Limitations of Simple Switchboard—Transfer Lines— Types of Trunks— PlugSeat Switch — Handling Transfers — Limitations of Transfer System — Obsolete Systems — Modern Systems 400 viii TABLE OF CONTENTS CHAPTER XXIV PRINCIPLES OF THE MULTIPLE SWITCHBOARD 409 The Multiple Feature— Line Signals— Cord Circuits— Double Connections — Busy Test— Field of Each Operator— Influence of Traffic CHAPTER XXV THE MAGNETO MULTIPLE SWITCHBOARD 419 Series- Multiple Board— Circuits — Operation — Branch-Terminal Multiple Board— Operation—Modern Magneto Multiple Boards— Tests— Construction CHAPTER THE COMMON-BATTERY XXVI MULTIPLE SWITCHBOARD ; . .435 Types — Western Electric Number 1 Relay Board — Circuits and Operation— Testing—Distributing Frames— Signals— Relays— The Western Electric Number 10 Board — Operation — Kellogg Two- Wire Multiple Board — Circuits — Signals — Battery Feed— Busy Test— Wiring of Line Circuit— Dean Multiple Board— Keys —Tests — Stromberg-Carlson Multiple Board— Signals— Tests— Multiple Switchboard Apparatus— Jacks — Lamp Jacks— Relays— Key and Jack Arrangement CHAPTER XXVII TRUNKING IN MULTI-OFFICE SYSTEMS 475 Multi-Office Exchanges— Trunk:ng between Exchanges — Two- Way Trunks— One-Way Trunks— Trunk Operation — Western Electric Trunk Circuits— Trunk Relays — Kellogg Trunk Circuits CHAPTER FUNDAMENTAL XXVIII CONSIDERATIONS OF AUTOMATIC SYSTEMS . . 501 Complexity— Expense— Flexibility— Attitude of the Public— Subscriber's Station Equipment — Comparative Costs — Operation — Strowger or Automatic Electric Company System— Lorimer System— Switches— Grouping of Subscribers— Trunking— Testing CHAPTER XXIX THE AUTOMATIC ELECTRIC COMPANY'S SYSTEM . . . .515 Selecting Switch— Up-and- Around Movement— Line Switch— Trunking— Selector — Connector — Systems — Subscriber's Station Apparatus — Operation — LineSwitch Units— Master Switch— Locking— Cut-Off—Guarding— Selecting Switches — Side Switch — First Selector — Busy Signals— Battery Supply— Release— MultiOffice System— Automatic Sub-Offices — Party Lines CHAPTER THE LORIMER AUTOMATIC SYSTEM . XXX . . . . .571 Subscriber's Station Equipment — Central-Office Apparatus — The Section — The Connective Division — Switches — Signal-Transmitter Controller — Selector — Connectors CHAPTER THE AUTOMANUAL XXXI SYSTEM Operation — Subscriber's Apparatus — Operator's Equipment — Automatic Switching Equipment — Distribution of Calls — Connecting— Speed 584 TABLE OF CHAPTER CONTENTS ix XXXII POWER PLANTS 593 Direct Current— Alternating Current— Types of Power Plants — Operator's Transmitter Supply — Ringing-Current Supply — Pole Changers — Ringing Dynamos — Auxiliary Signaling Currents— Primary Sources of Power— Rotary Converters — Mercury-Arc Rectifiers — Provision Against Breakdown — Storage Battery — Care and Management — Power Switchboard— Circuits CHAPTER XXXIII HOUSING CENTRAL-OFFICE EQUIPMENT 615 FireHazard — Building — Provisions for Employes »nd Apparatus — Arrangement of Apparatus in Manual Offices— Automatic Offices CHAPTER XXXIV PRIVATE BRANCH EXCHANGES 637 Functions of the Private Branch- Exchange Operator — Switchboards — Types — Supervision— Connection with Automatic Offices— Battery Supply — Ringing Current—Desirable Features CHAPTER XXXV INTERCOMMUNICATING SYSTEMS 648 Limitations of the System — Types — Simple Magneto System — Common -Battery Systems— Kellogg, Western Electric, and Monarch Systems — Intercommunicating System for Private Branch Exchanges CHAPTER LONG-DISTANCE XXXVI SWITCHING 659 Repeating Coils — Switching through Local Board — Operator's Orders — TwoNumber Calls— Particular-Party-Calls — Trunking — Ticket Passing — Way Stations — Center Checking CHAPTER XXXVI. TELEPHONE TRAFFIC 664 Unit of Traffic— Traffic Variations— Rates— Traffic Study— Data— Loads— Trunking Factor — Trunk Efficiency — Toll Traffic — Quality of Service — Time Consumed — Busy and Don't Answer Calls — Checks CHAPTER XXXVIII MEASURED SERVICE 676 Paying— Rates— Units— ^oll Service— Long Haul— Short Haul— Timing Toll Collections—Types ofLocal Service Methods— Ticket Method— Meter MethodPrepayment Method CHAPTER XXXIX PHANTOM, SIMPLEX, AND COMPOSITE CIRCUITS .... Phantom Circuits — Transpositions — Transmission over Phantom Circuits — Simplex Circuits — Composite Circuits — Ringing — Phantoms from Simplex and Composite Circuits— Railway Composite Circuit and Equipment 687 x TABLE OF CONTENTS CHAPTER TELEPHONE TRAIN DISPATCHING XL 699 Growth — Its Introduction — Transmitting Orders — Western Electric Selector — Gill Selector— Cummings-Wray Selector — Telephone Equipment— Dispatcher's Transmitter — Waystation and Siding Telephones — Circuits — Voltage — Simultaneous Talking and Signaling— Test Boards— Locking Sets— Dispatching on Electric Railways CHAPTER XLI TYPES OF TELEPHONE LINES 725 Underground eral Practice and Overhead Lines— Insulation Poles— Resistance— Cables— Gen- CHAPTER XLII OPEN WIRES ... 728 Materials — Iron— Galvanizing— Copper— Copper-Clad Steel— Insulated Open Wire —Wire Tables CHAPTER XLIII CABLES 736 Types— Insulating Material— Pairs— Core— Lead Sheath — Capacity— Corrective Factors— Diameters and Weights— Submarine Cable— Loading CHAPTER XLIV POLES AND POLE FITTINGS 745 Equipment — Timber — Cutting— Sizes — Trimming — Treating — Roofing — Cross Arms — Pins — Steps — Hardware Requirements — Pole Setting — Toll Line Construction— Guys — Braces — Stringing Wire — Transpositions — Tying — Sag — Dead-Ending— Test Points— Rural Lines— City Exchange Lines— Locating— CrossingGrading — Strain — Splices— Cable Erection— Terminals— Pole Seats CHAPTER XLV UNDERGROUND CONSTRUCTION 799 Early Practice— Buried Cable— Underground Conduit— Materials— Types of Conduit—Laying— Manholes— Grading— Curves— Lateral Runs— Risers— Cable Supports— Installing Underground Cables — Drawing In — Vaults CHAPTER CABLE SPLICING . XLVI . 828 Dryness— Methods— Straight Splice— Tap Splice — Y-Splices — Lead Sleeves — Pot- Heads— Types CHAPTER XLVII OFFICE TERMINAL CABLES 838 Aerial Cable Entrance— Underground Cable Entrance for Small Plants — Underground Entrances for Large Plants— Treatment of Cable Ends— Cable Runs and Shafts CHAPTER SERVICE CONNECTIONS XLVIII ... .848 Connections from Bare Wire Lines— Connections from Cable Lines — Drop Wire Distribution— Ties— Rear Wall or Fence Distribution— Distribution from Underground Terminals in Buildings TABLE OF CONTENTS CHAPTER XLIX SUBSCRIBER'S STATION WIRING CHAPTER ELECTROLYSIS OF UNDERGROUND xi 862 L CABLES 878 Results — Early Controversy — Court Decisions — McCluer System — Metallic Circuits and Cables — Troubles — Causes— Conditions — Remedies — Insulation Joints — Moisture- Proof Conduits — Bonding CHAPTER DEVELOPMENT LI STUDIES 887 Long-Distance or Toll Line — Exchange Development Study — Ratio of Telephones to Population — House Count — Ratio of Telephones to Buildings — Comparison of Estimates— Office Districts — Central-Office Locations— Conduit and Pole-Line Routes— Ultimate Sizes — Subdivision CHAPTER LII CA*E OF PLANT 895 Maintenance and Depreciation — The Outside Plant — Supports, Conduits, Open Wire and Cables — Subscriber's Equipment — Manual Office Equipment — Troubles — Remedies — Soldering CHAPTER LIII TESTING Implements— Faults — Wire Chief's Tests — Galvanometer Tests — Identification — Cable Testing — -Instruments — Testing Sets — Loss-of-Charge Tests — Varley Loop Tests — Murray Loop Tests — Capacity Tests for Opens — Listening Tests 904 ALEXANDER GRAHAM BELL The Inventor of the Telephone, TELEPHONY INTRODUCTION The telephone was invented in 1875 by Alexander Graham Bell, a resident of the United States, a native of Scotland, and by profession ateacher of deaf mutes in the art of vocal speech. In that year, Professor Bell was engaged in the experimental development of a system of multiplex telegraphy, based on the use of rapidly varying currents. During those experiments, he observed an iron reed to vibrate before an electromagnet as a result of another iron reed vibrating before a distant electromagnet connected to the nearer one by wires. The telephone resulted from this observation with great promptness. In the instrument first made, sound vibrated a membrane diaphragm supporting a bit of iron near an electromagnet; a line joined this simple device of three elements to another like it; a battery in the line magnetized both electromagnet cores; the vibration of the iron in the sending device caused the current in the line to undulate and to vary the magnetism of the receiving device. The diaphragm of the latter was vibrated in consequence of the varying pull upon its bit of iron, and these vibrations reproduced the sound that vibrated the sending diaphragm. The first public use of the electric telephone was at the Centennial Exposition in Philadelphia in 1876. It was there tested by many interested observers, among them Sir William Thomson, later Lord Kelvin, the eminent Scotch authority on matters of electrical communication. It was he who contributed so largely to the success of the early telegraph cable system between England and 2 America. lows: TELEPHONY Two of his comments which are characteristic are as fol- To-day I have seen that which yesterday I should have deemed impossible. Soon lovers will whisper their secrets over an electric wire. Who can but admire the hardihood of invention which devised such slight means to realize the mathematical conception that if electricity is to convey all the delicacies of sound which distinguish 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 the air engaged in constituting the sound. Contrary to usual methods of improving a new art, the earliest improvement of the telephone simplified it. The diaphragms became thin iron disks, instead of membranes carrying iron; the electromagnet cores were made of permanently magnetized steel instead of temporarily magnetized soft iron, and the battery was omitted from the line. The undulatory current in a system of two such telephones joined by a line is produced in the sending telephone by the vibration of the 'iron diaphragm. The vibration of the diaphragm in the receiving telephone is produced by the undulatory current. Sound is produced by the vibration of the diaphragm of the receiving telephone. Such a telephone is at once the simplest known form of electric generator or motor for alternating currents. It is capable of translating motion into current or current into motion through a wide range of frequencies. It is not known that there is any frequency of alternating current which it is not capable of producing and translating. Itcan produce and translate currents of greater complexity than any other existing electrical machine. Though possessing these admirable qualities as an electrical machine, the simple electromagnetic telephone had not the ability to transmit speech loudly enough for all practical uses. Transmitters producing stronger telephonic currents were developed soon after the fundamental invention. Some forms of these were invented by Professor Bell himself. Other inventors contributed devices embodying the use of carbon as a resistance to be varied by the motions of the diaphragm. This general form of transmitting telephone has prevailed and at present is the standard type. It is interesting to note that the earliest incandescent lamps, as INTRODUCTION 3 invented by Mr. Edison, had a resistance material composed of carbon, and that such a lamp retained its position as the most efficient small electric illuminant until the recent introduction of metal filament lamps. It is possible that some form of metal may be introduced as the resistance medium for telephone transmitters, and that such a change as has taken place in incandescent lamps may increase the efficiency of telephone transmitting devices. At the time of the invention of the telephone, there were in existence two distinct types of telegraph, working in regular commercial service. In the more general type, many telegraph stations were connected to a line and whatever was telegraphed between two stations could be read by all the stations of that line. In the other and less general type, many lines, each having a single telegraph station, were centered in an office or "exchange," and at the desire of a user his line could be connected to another and later disconnected from it. Both of these types of telegraph service were imitated at once in telephone practice. Lines carrying many telephones each, were established with great rapidity. Telephones actually displaced telegraphic apparatus in the exchange method of working in America. The fundamental principle on which telegraph or telephone exchanges operate, being that of placing any line in communication with any other in the system, gave to each line an ultimate scope so great as to make this form of communication more popular than any arrangement of telephones on a single line. Beginning in 1877, telephone exchanges were developed with great rapidity in all of the larger communities of the United States. Telegraph switching devices were utilized at the outset or were modified in such minor particulars as were necessary to fit them to the new task. In its simplest form, a telephone system is, of course, a single line permanently joining two telephones. In its next simplest form, it is a line permanently joining more than two telephones. In its most useful form, it is a line joining a telephone to some means of connecting it at will to another. A telephone exchange central office contains means for connecting lines at will in that useful way. The least complicated machine for that purpose is a switchboard to be operated by hand, having some way of letting the operator know that a connection is 4 TELEPHONY wished and a way of making it. The customary way of connecting the lines always has been by means of flexible conductors fitted with plugs to be inserted in sockets. If the switchboard be small enough so that all the lines are within arm's reach of the operator, the whole process is individual, and may be said to be at its best and simplest. There are but few communities, however, in .which the number of lines to be served and calls to be answered is small enough so that the entire traffic of the exchange can be handled by a single person. An obvious way, therefore, is to provide as many operators in a central office as may be required by the number of calls to be answered, and to terminate before each of the operators enough of the lines to bring enough work to keep that operator economically occupied. This presents the additional problem, how to connect a line terminating before one operator to a line normally terminating before another operator. The obvious answer is to provide lines from each operator's place of work to each other operator's place, connecting a calling line to some one of these lines which are local within the central office, and, in turn, connecting that chosen local line to the line which is called. Such lines between operators have come to be known as trunk lines, because of the obvious analogy to trunk lines of railways between common centers, and such a system of telephone lines may be called a trunking system. Very good service has been given and can be given by such an arrangement of local trunks, but the growth in lines and in traffic has developed in most instances certain weaknesses which make it advisable to find speedier, more accurate, and more reliable means. For the serving of a large traffic from a large number of lines, as is required in practically every city of the world, a very great contribution to the practical art was made by the development of the multiple switchboard. Such a switchboard is merely such a device as has been described for the simpler cases, with the further refinement that within reach of each operator in the central office appears every line which enters that office, and this without regard to what point in the switchboard the lines may terminate for the answering of calls. In other words, while each operator answers a certain subordinate group of the total number of lines, each operator may reach, for calling purposes, every line which enters that office. It INTRODUCTION 5 is probable that the invention and development of the multiple switchboard was the first great impetus toward the wide-spread use of telephone service. Coincident with the development of the multiple switchboard for manually operated, central -office mechanisms was the beginning of the development of automatic apparatus under the control of the calling subscriber for finding and connecting with a called line. It is interesting to note the general trend of the early development of automatic apparatus in comparison with the development, to that time, of manual telephone apparatus. While the manual apparatus on the one hand attempted to meet its problem by providing local trunks between the various operators of a central office, and failing of success in that, finally developed a means which placed all the lines of a central office within connecting reach of each operator, automatic telephony, beginning at that point, failed of success in attempting to bring each line in the central office within connecting reach of each connecting mechanism. In other terms, the first automatic switching equipment consisted of a machine for each line, which machine was so organized as to be able to find and connect its calling line with any called line of the entire central-office group. It may be said that an attempt to develop this plan was the fundamental reason for the repeated failure of automatic apparatus to solve the problem it attacked. All that the earlier automatic system did was to prove more or less successfully that automatic apparatus had a right to exist, and that to demand of the subscriber that he manipulate from his station a distant machine to make the connection without human aid was not fallacious. When it had been recognized that the entire multiple switchboard idea could not be carried into automatic telephony with success, the first dawn of hope in that art may be said to have come. Success in automatic telephony did come by the re-adoption of the trunking method. As adopted for automatic telephony, the method contemplates that the calling line shall be extended, link by link, until it finds itself lengthened and directed so as to be able to seize the called line in a very much smaller multiple than the total group of one office of the exchange. A similar curious reversion has taken place in the development of telephone lines. The earliest telephone lines were merely tele- 6 TELEPHONY graph lines equippeJ with telephone instruments, and the earliest telegraph lines were planned by Professor Morse to be insulated wires laid in the earth. A lack of skill in preparing the wires for putting in the earth caused these early underground lines to be failures. At the urging of one of his associates, Professor Morse consented to place his earliest telegraph lines on poles in the air. Each such line originally consisted of two wires, one for the going and one for the returning current, as was then considered the action. Upon its being discovered that, a single wire, using the earth as a return, would serve as a satisfactory telegraph line, such practice became universal. Upon the arrival of the telephone, all lines obviously were built in the same way, and until force of newer circumstances compelled it, the present metallic circuit without an earth connection did not come into general use. The extraordinary growth of the number of telephone lines in a community and the development of other methods of electrical utilization, as well as the growth in the knowledge of telephony itself, ultimately forced the wires underground again. At the same time and for the same causes, a telephone line became one of two wires, so that it becomes again the counterpart of the earliest telegraph line of Professor Morse. Another curious and interesting example of this reversion to type exists in the simple telephone receiver. An early improvement in telephone receivers after Professor Bell's original invention was to provide the necessary magnetism of the receiver core by making it of steel and permanently magnetizing it, whereas Professor Bell's instrument provided its magnetism by means of direct current flowing in the line. In later days 'the telephone receiver has returned almost to the original form in which Professor Bell produced itand this change has simplified other elements of telephoneexchange apparatus in a very interesting and gratifying way. By reason of improvements in methods of line construction and apparatus arrangement, the radius of communication steadily has increased. Commercial speech now is possible between points several thousand miles apart, and there is no theoretical reason why communication might not be established between any two points on the earth's surface. The practical reasons of demand and cost may prevent so great an accomplishment as talking half around the earth. INTRODUCTION 7 So far as science is concerned there would seem to be no reason why this might not be done today, by the careful application of what already is known. In the United States, telephone service from its beginning has been supplied to users by private enterprise. In other countries, it is supplied by means of governmentally-owned equipment. In general, it may be said that the adequacy and the amount, as well as the quality of telephone service, is best in countries where the service is provided by private enterprise. Telephone systems in the United States were under the control of the Bell Telephone Company from the invention of the device in 1876 until 1893. The fundamental telephone patent expired in 1893. This opened the telephone art to the general public, because itno longer was necessary to secure telephones solely from the patent-holding company nor to pay royalty for the right to use them, if secured at all. Manufacturers of electrical apparatus generally then began to make and sell telephones and telephone apparatus, and operating companies, also independent of the Bell organization, began to install and use telephones. At the end of seventeen years of patent monopoly in the United States, there were in operation a little over 250,000 telephones. In the seventeen years since the expiration of the fundamental patent, independent telephone companies throughout the United States have installed and now have in daily successful use over 3,911,400 telephones. In other words, since its first beginnings, independent telephony has brought into continuous daily use nearly sixteen times as many telephones as were brought into use in the equal time of the complete monopoly of the Bell organization. At the beginning of 1910, there were in service by the Bell organization about 3,633,900 telephones. These with the 3,911,400 independent telephones, make a total of 7,545,300, or about one-twelfth as many telephones as there are inhabitants of the United States. The influence of this development upon the lives of the people has been profound. Whether the influence has been wholly for good may not be so conclusively apparent. Lord Bacon has declared that, excepting only the alphabet and the art of printing, those inventions abridging distance are of the greatest service to mankind. If this be true, it may be. said that 8 TELEPHONY the invention of telephony deserves high place winong the civilizing influences. There is no industrial art in which the advancement of the times has been followed more closely by practical application than in telephony. Commercial speech by telephone is possible by means of currents which so far are practically unmeasurable. In other words, it is possible to speak clearly and satisfactorily over a line by means of currents which cannot be read, with certainty as to their amount, by any electrical measuring device so far known. In this regard, telephony is less well fortified than are any of the arts utilizing electrical power in larger quantities. The real wonder is that with so little knowledge of what takes place, particularly as to amount, those working in the art have been able to do as well as they have. When an exact knowledge of quantity is easily obtainable, very striking advances may be looked for. The student of these phases of physical science and industrial art will do well to combine three processes: study of the words of others; personal experimentation; and digestive thought. The last mentioned is the process of profoundest value. On it finally depends mastery. It is not of so much importance how soon the concept shall finally be gained as that it is gained. A statement by another may seem lifeless and inert and the meaning of an observation may be obscure. Digestive thought is the only assimilative process. The whole art of telephony hangs on taking thought of things. Judge R. F. Taylor of Indiana said of Professor Bell, "It has been said that no man by taking thought may add a cubit to his stature, yet here is a man who, by taking thought, has added not cubits but miles to the lengths of men's tongues and ears." In observations of many students, it is found that the notion of each must pass through a certain period of incubation before his private and personal knowledge of Ohm's law is hatched. Once hatched, however, it is his. By just such a process must come each principal addition to his stock of concepts. The periods may vary and practice in the uses of the mind may train it in alertness in its work. If time is required, time should be given, the object always being to keep thinking or re-reading or re-trying until the thought is wholly encompassed and possessed. CHAPTER I ACOUSTICS Telephony is the art of reproducing at a distant point, usually by the agency of electricity, sounds produced at a sending point. In this art the elements of two general divisions of physical science are concerned, sound and electricity. Sound is the effect of vibrations of matter upon the ear. The vibrations may be .those of air or other matter. Various forms of matter transmit sound vibrations in varying degrees, at different specific speeds, and with different effects upon the vibrations. Any form of matter may serve as a transmitting medium for sound vibrations. Sound itself is an effect of .sound vibrations upon the ear. Propagation of Sound. Since human beings communicate with each other by means of speech and hearing through the air, it is with air that the acoustics of telephony principally is concerned. In air> sound vibrations consist of successive condensations and rarefactions tending to proceed outwardly from the source in all directions. The source is the center of a sphere of sound vibrations. Whatever may be the nature of the sounds or of the medium transmitting them, they consist of waves emitted by the source and observed bythe ear. A sound wave is one complete condensation and rarefaction of the transmitting medium. It is produced by one complete vibration of the sound-producing thing. Sound waves in air travel at a rate of about 1,090 feet per second. The rate of propagation of sound waves in other materials varies with the density of the material. For example, the speed of transmission ismuch greater in water than in air, and is much less in highly rarefied air than in air at ordinary density. The propagation of sound waves in a vacuum may be said not to take place at all. Characteristics of Sound. Three qualities distinguish soundioudness, pitch, and timbre. 10 TELEPHONY Loudness. Loudness depends upon the violence of the effect upon the ear; sounds may be alike in their other qualities and differ in loudness, the louder sounds being produced by the stronger vibrations ofthe air or other medium at the ear. Other things being equal, the louder sound is produced by the source radiating the greater energy and so producing the greater degree of condensation and rarefaction of the medium. Pitch. Pitch depends upon the frequency at which the sound waves strike the ear. Pitches are referred to as high or low as the frequency of waves reaching the ear are greater or fewer. Familiar low pitches are the left-hand strings of a piano; the larger ones of stringed instruments generally; bass voices; and large bells. Familiar high pitches are right-hand piano strings; smaller ones of other stringed instruments; soprano voices; small bells; and the voices of most birds and insects. Doppler's Principle: — As pitch depends upon the frequency at which sound waves strike the ear, an object may emit sound waves at a constant frequency, yet may produce different pitches in ears differently situated. Such a case is not usual, but an example of it will serve a useful purpose in fixing certain facts as to pitch. Conceive two railroad trains to pass each other, running in opposite directions, the engine bells of both trains ringing. Passengers on eac