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+The Project Gutenberg EBook of Cyclopedia of Telephony & Telegraphy Vol. 1
+by Kempster Miller, George Patterson, Charles Thom, Robert Millikan,
+Samuel McMeen
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Cyclopedia of Telephony & Telegraphy Vol. 1
+       A General Reference Work on Telephony, etc. etc.
+
+Author: Kempster Miller
+        George Patterson
+        Charles Thom
+        Robert Millikan
+        Samuel McMeen
+
+Release Date: April 14, 2005 [EBook #15617]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CYCLOPEDIA OF TELEPHONY 1 ***
+
+
+
+
+Produced by Ronald Holder and the Online Distributed
+Proofreading Team at https://www.pgdp.net.
+
+
+
+
+
+[Transcriber's Note: References to page numbers in table of contents
+and index removed, as well as the numbers themselves.]
+
+[Illustration: ALEXANDER GRAHAM BELL The Inventor of the Telephone.]
+
+
+Cyclopedia
+
+of
+
+Telephony and Telegraphy
+
+_A General Reference Work on_
+
+TELEPHONY, SUBSTATIONS, PARTY-LINE SYSTEMS, PROTECTION, MANUAL
+SWITCHBOARDS, AUTOMATIC SYSTEMS, POWER PLANTS, SPECIAL
+SERVICE FEATURES, CONSTRUCTION,  ENGINEERING,
+OPERATION, MAINTENANCE, TELEGRAPHY, WIRELESS
+TELEGRAPHY AND TELEPHONY, ETC.
+
+_Prepared by a Corps of_
+
+TELEPHONE AND TELEGRAPH EXPERTS, AND ELECTRICAL ENGINEERS OF
+THE HIGHEST PROFESSIONAL STANDING
+
+_Illustrated with over Two Thousand Engravings_
+
+FOUR VOLUMES
+
+CHICAGO
+
+AMERICAN SCHOOL OF CORRESPONDENCE
+
+1919
+
+
+
+
+
+Authors and Collaborators
+
+       *       *       *       *       *
+
+KEMPSTER B. MILLER. M.E.
+Consulting Engineer and Telephone Expert Of the Firm of McMeen and
+Miller, Electrical Engineers and Patent Experts, Chicago
+American Institute of Electrical Engineers
+Western Society of Engineers
+
+       *       *       *       *       *
+
+GEORGE W. PATTERSON, S.B., Ph.D.
+Head, Department of Electrical Engineering, University of Michigan
+
+       *       *       *       *       *
+
+CHARLES THOM
+Chief of Quadruplex Department, Western Union Main Office, New York City
+
+       *       *       *       *       *
+
+ROBERT ANDREWS MILLIKAN, Ph.D.
+Associate Professor of Physics, University of Chicago
+Member, Executive Council, American Physical Society
+
+       *       *       *       *       *
+
+SAMUEL G. McMEEN
+Consulting Engineer and Telephone Expert Of the Firm of McMeen and
+Miller, Electrical Engineers and Patent Experts, Chicago
+American Institute of Electrical Engineers
+Western Society of Engineers
+
+       *       *       *       *       *
+
+LAWRENCE K. SAGER, S.B., M.P.L.
+Patent Attorney and Electrical Expert
+Formerly Assistant Examiner, U.S. Patent Office
+
+       *       *       *       *       *
+
+GLENN M. HOBBS, Ph.D.
+Secretary, American School of Correspondence
+Formerly Instructor in Physics, University of Chicago
+American Physical Society
+
+       *       *       *       *       *
+
+CHARLES G. ASHLEY
+Electrical Engineer and Expert in Wireless Telegraphy and Telephony
+
+       *       *       *       *       *
+
+A. FREDERICK COLLINS
+Editor, _Collins Wireless Bulletin_
+Author of "Wireless Telegraphy, Its History, Theory, and Practice"
+
+       *       *       *       *       *
+
+FRANCIS B. CROCKER, E.M., Ph.D.
+Head, Department of Electrical Engineering, Columbia University
+Past-President, American Institute of Electrical Engineers
+
+       *       *       *       *       *
+
+MORTON ARENDT, E.E.
+Instructor in Electrical Engineering, Columbia University, New York
+
+       *       *       *       *       *
+
+EDWARD B. WAITE
+Head, Instruction Department, American School of Correspondence
+American Society of Mechanical Engineers
+Western Society of Engineers
+
+       *       *       *       *       *
+
+DAVID P. MORETON, B.S., E.E.
+Associate Professor of Electrical Engineering, Armour Institute of
+Technology
+American Institute of Electrical Engineers,
+
+       *       *       *       *       *
+
+LEIGH S. KEITH, B.S.
+Managing Engineer with McMeen and Miller, Electrical Engineers and
+Patent Experts Chicago
+Associate Member, American Institute of Electrical Engineers
+
+       *       *       *       *       *
+
+JESSIE M. SHEPHERD, A.B.
+Associate Editor, Textbook Department, American School of Correspondence
+
+       *       *       *       *       *
+
+ERNEST L. WALLACE, B.S.
+Assistant Examiner, United States Patent Office, Washington, D. C.
+
+       *       *       *       *       *
+
+GEORGE R. METCALFE, M.E.
+Editor, _American Institute of Electrical Engineers_
+Formerly Head of Publication Department, Westinghouse Elec. & Mfg. Co.
+
+       *       *       *       *       *
+
+J.P. SCHROETER
+Graduate, Munich Technical School
+Instructor in Electrical Engineering, American School of Correspondence
+
+       *       *       *       *       *
+
+JAMES DIXON, E.E.
+American Institute of Electrical Engineers
+
+       *       *       *       *       *
+
+HARRIS C. TROW, S.B., _Managing Editor_
+Editor-in-Chief, Textbook Department, American School of Correspondence
+
+
+Authorities Consulted
+
+
+The editors have freely consulted the standard technical literature of
+America and Europe in the preparation of these volumes. They desire to
+express their indebtedness particularly to the following eminent
+authorities, whose well-known works should be in the library of every
+telephone and telegraph engineer.
+
+Grateful acknowledgment is here made also for the invaluable
+co-operation of the foremost engineering firms and manufacturers in
+making these volumes thoroughly representative of the very best and
+latest practice in the transmission of intelligence, also for the
+valuable drawings, data, suggestions, criticisms, and other courtesies.
+
+       *       *       *       *       *
+
+ARTHUR E. KENNELY, D.Sc.
+Professor of Electrical Engineering, Harvard University.
+Joint Author of "The Electric Telephone." "The Electric Telegraph,"
+"Alternating Currents," "Arc Lighting," "Electric Heating,"
+"Electric Motors," "Electric Railways," "Incandescent Lighting," etc.
+
+       *       *       *       *       *
+
+HENRY SMITH CARHART, A.M., LL.D.
+Professor of Physics and Director of the Physical Laboratory,
+University of Michigan.
+Author of "Primary Batteries," "Elements of Physics," "University
+Physics," "Electrical Measurements," "High School Physics," etc.
+
+       *       *       *       *       *
+
+FRANCIS B. CROCKER, M.E., Ph.D.
+Head of Department of Electrical Engineering, Columbia University,
+New York; Past-President, American Institute of Electrical Engineers.
+Author of "Electric Lighting;" Joint Author of "Management of
+Electrical Machinery."
+
+       *       *       *       *       *
+
+HORATIO A. FOSTER
+Consulting Engineer; Member of American Institute of Electrical
+Engineers; Member of American Society of Mechanical Engineers.
+Author of "Electrical Engineer's Pocket-Book."
+
+       *       *       *       *       *
+
+WILLIAM S. FRANKLIN, M.S., D.Sc.
+Professor of Physics, Lehigh University.
+Joint Author of "The Elements of Electrical Engineering,"
+"The Elements of Alternating Currents."
+
+       *       *       *       *       *
+
+LAMAR LYNDON, B.E., M.E.
+Consulting Electrical Engineer; Associate Member of American Institute
+of Electrical Engineers; Member, American Electro-Chemical Society.
+Author of "Storage Battery Engineering."
+
+       *       *       *       *       *
+
+ROBERT ANDREWS MILLIKAN, Ph.D.
+Professor of Physics, University of Chicago.
+Joint Author of "A First Course in Physics," "Electricity, Sound and
+Light," etc.
+
+       *       *       *       *       *
+
+KEMPSTER B. MILLER, M.E.
+Consulting Engineer and Telephone Expert; of the Firm of McMeen and
+Miller, Electrical Engineers and Patent Experts, Chicago.
+Author of "American Telephone Practice."
+
+       *       *       *       *       *
+
+WILLIAM H. PREECE
+Chief of the British Postal Telegraph.
+Joint Author of "Telegraphy," "A Manual of Telephony," etc.--
+
+       *       *       *       *       *
+
+LOUIS BELL, Ph.D.
+Consulting Electrical Engineer; Lecturer on Power Transmission,
+Massachusetts Institute of Technology.
+Author of "Electric Power Transmission," "Power Distribution for Electric
+Railways," "The Art of Illumination," "Wireless Telephony," etc.
+
+       *       *       *       *       *
+
+OLIVER HEAVISIDE, F.R.S.
+Author of "Electro-Magnetic Theory," "Electrical Papers," etc.
+
+       *       *       *       *       *
+
+SILVANUS P. THOMPSON, D.Sc, B.A., F.R.S., F.R.A.S.
+Principal and Professor of Physics in the City and Guilds of London
+Technical College.
+Author of "Electricity and Magnetism," "Dynamo-Electric Machinery,"
+"Polyphase Electric Currents and Alternate-Current Motors,"
+"The Electromagnet," etc.
+
+       *       *       *       *       *
+
+ANDREW GRAY, M.A., F.R.S.E.
+Author of "Absolute Measurements in Electricity and Magnetism."
+
+       *       *       *       *       *
+
+ALBERT CUSHING CREHORE, A.B., Ph.D.
+Electrical Engineer; Assistant Professor of Physics, Dartmouth College;
+Formerly instructor in Physics, Cornell University.
+Author of "Synchronous and Other Multiple Telegraphs;" Joint Author of
+"Alternating Currents."
+
+       *       *       *       *       *
+
+J. J. THOMSON, D.Sc, LL.D., Ph.D., F.R.S.
+Fellow of Trinity College, Cambridge University; Cavendish Professor of
+Experimental Physics, Cambridge University.
+Author of "The Conduction of Electricity through Gases," "Electricity
+and Matter."
+
+       *       *       *       *       *
+
+FREDERICK BEDELL, Ph. D.
+Professor of Applied Electricity, Cornell University.
+Author of "The Principles of the Transformer;" Joint Author of
+"Alternating Currents."
+
+       *       *       *       *       *
+
+DUGALD C. JACKSON, C.E.
+Head of Department of Electrical Engineering, Massachusetts Institute of
+Technology; Member, American Institute of Electrical Engineers, etc.
+Author of "A Textbook on Electromagnetism and the Construction of Dynamos;"
+Joint Author of "Alternating Currents and Alternating-Current Machinery."
+
+       *       *       *       *       *
+
+MICHAEL IDVORSKY PUPIN, A.B., Sc.D., Ph.D.
+Professor of Electro-Mechanics, Columbia University, New York.
+Author of "Propagation of Long Electric Waves," and "Wave-Transmission
+over Non-Uniform Cables and Long-Distance Air Lines."
+
+       *       *       *       *       *
+
+FRANK BALDWIN JEWETT, A.B., Ph.D.
+Transmission and Protection Engineer, with American Telephone &
+Telegraph Co.
+Author of "Modern Telephone Cable," "Effect of Pressure on Insulation
+Resistance."
+
+       *       *       *       *       *
+
+ARTHUR CROTCH
+Formerly Lecturer on Telegraphy and Telephony at the Municipal Technical
+Schools, Norwich, Eng.
+Author of "Telegraphy and Telephony."
+
+       *       *       *       *       *
+
+JAMES ERSKINE-MURRAY, D.Sc.
+Fellow of the Royal Society of Edinburgh; Member of the Institution of
+Electrical Engineers.
+Author of "A Handbook of Wireless Telegraphy."
+
+       *       *       *       *       *
+
+A.H. MCMILLAN, A.B., LL.B.
+Author of "Telephone Law, A Manual on the Organization and Operation of
+Telephone Companies."
+
+       *       *       *       *       *
+
+WILLIAM ESTY, S.B., M.A.
+Head of Department of Electrical Engineering, Lehigh University.
+Joint Author of "The Elements of Electrical Engineering."
+
+       *       *       *       *       *
+
+GEORGE W. WILDER, Ph.D.
+Formerly Professor of Telephone Engineering, Armour Institute of
+Technology.
+Author of "Telephone Principles and Practice," "Simultaneous Telegraphy
+and Telephony," etc.
+
+       *       *       *       *       *
+
+WILLIAM L. HOOPER, Ph.D.
+Head of Department of Electrical Engineering, Tufts College.
+Joint Author of "Electrical Problems for Engineering Students."
+
+       *       *       *       *       *
+
+DAVID S. HULFISH
+Technical Editor, _The Nickelodeon_; Telephone and Motion-Picture Expert;
+Solicitor of Patents.
+Author of "How to Read Telephone Circuit Diagrams."
+
+       *       *       *       *       *
+
+J.A. FLEMING, M.A., D.Sc. (Lond.), F.R.S.
+Professor of Electrical Engineering in University College, London; Late
+Fellow and Scholar of St. John's College, Cambridge; Fellow of
+University College, London.
+Author of "The Alternate-Current Transformer," "Radiotelegraphy and
+Radiotelephony," "Principles of Electric Wave Telegraphy," "Cantor
+Lectures on Electrical Oscillations and Electric Waves," "Hertzian Wave
+Wireless Telegraphy," etc.
+
+       *       *       *       *       *
+
+F.A.C. PERRINE, A.M., D.Sc.
+Consulting Engineer: Formerly President, Stanley Electric Manufacturing
+Company; Formerly Professor of Electrical Engineering, Leland Stanford,
+Jr. University.
+Author of "Conductors for Electrical Distribution."
+
+       *       *       *       *       *
+
+A. FREDERICK COLLINS
+Editor, _Collins Wireless Bulletin_.
+Author of "Wireless Telegraphy, Its History, Theory and Practice,"
+"Manual of Wireless Telegraphy," "Design and Construction of Induction
+Coils," etc.
+
+       *       *       *       *       *
+
+SCHUYLER S. WHEELER, D.Sc.
+President, Crocker-Wheeler Co.; Past-President, American Institute of
+Electrical Engineers.
+Joint Author of "Management of Electrical Machinery."
+
+       *       *       *       *       *
+
+CHARLES PROTEUS STEINMETZ
+Consulting Engineer, with the General Electric Co.; Professor of
+Electrical Engineering, Union College.
+Author of "The Theory and Calculation of Alternating-Current Phenomena,"
+"Theoretical Elements of Electrical Engineering", etc.
+
+       *       *       *       *       *
+
+GEORGE W. PATTERSON, S.B., Ph.D.
+Head of Department of Electrical Engineering, University of Michigan.
+Joint Author of "Electrical Measurements."
+
+       *       *       *       *       *
+
+WILLIAM MAVER, JR.
+Ex-Electrician Baltimore and Ohio Telegraph Company; Member of the
+American Institute of Electrical Engineers.
+Author of "American Telegraphy and Encyclopedia of the Telegraph,"
+"Wireless Telegraphy."
+
+       *       *       *       *       *
+
+JOHN PRICE JACKSON, M.E.
+Professor of Electrical Engineering, Pennsylvania State College.
+Joint Author of "Alternating Currents and Alternating-Current Machinery."
+
+       *       *       *       *       *
+
+AUGUSTUS TREADWELL, JR., E.E.
+Associate Member, American Institute of Electrical Engineers.
+Author of "The Storage Battery, A Practical Treatise on Secondary
+Batteries."
+
+       *       *       *       *       *
+
+EDWIN J. HOUSTON, Ph.D.
+Professor of Physics, Franklin Institute, Pennsylvania; Joint Inventor
+of Thomson-Houston System of Arc Lighting; Electrical Expert and
+Consulting Engineer.
+Joint Author of "The Electric Telephone," "The Electric Telegraph,"
+"Alternating Currents," "Arc Lighting," "Electric Heating," "Electric
+Motors," "Electric Railways," "Incandescent Lighting," etc.
+
+       *       *       *       *       *
+
+WILLIAM J. HOPKINS
+Professor of Physics in the Drexel Institute of Art, Science, and Industry,
+Philadelphia.
+Author of "Telephone Lines and their Properties."
+
+[Illustration: A TYPICAL SMALL MAGNETO SWITCHBOARD INSTALLATION]
+
+[Illustration: A TYPICAL CENTRAL OFFICE FOR RURAL EXCHANGE Line
+Protectors on Wall at Left.]
+
+
+
+
+Foreword
+
+
+The present day development of the "talking wire" has annihilated both
+time and space, and has enabled men thousands of miles apart to get
+into almost instant communication. The user of the telephone and the
+telegraph forgets the tremendousness of the feat in the simplicity of
+its accomplishment; but the man who has made the feat possible knows
+that its very simplicity is due to the complexity of the principles
+and appliances involved; and he realizes his need of a practical,
+working understanding of each principle and its application. The
+Cyclopedia of Telephony and Telegraphy presents a comprehensive and
+authoritative treatment of the whole art of the electrical
+transmission of intelligence.
+
+The communication engineer--if so he may be called--requires a
+knowledge both of the mechanism of his instruments and of the vagaries
+of the current that makes them talk. He requires as well a knowledge
+of plants and buildings, of office equipment, of poles and wires and
+conduits, of office system and time-saving methods, for the
+transmission of intelligence is a business as well as an art. And to
+each of these subjects, and to all others pertinent, the Cyclopedia
+gives proper space and treatment.
+
+The sections on Telephony cover the installation, maintenance, and
+operation of all standard types of telephone systems; they present
+without prejudice the respective merits of manual and automatic
+exchanges; and they give special attention to the prevention and
+handling of operating "troubles." The sections on Telegraphy cover
+both commercial service and train dispatching. Practical methods of
+wireless communication--both by telephone and by telegraph--are
+thoroughly treated.
+
+The drawings, diagrams, and photographs incorporated into the
+Cyclopedia have been prepared especially for this work; and their
+instructive value is as great as that of the text itself. They have
+been used to illustrate and illuminate the text, and not as a medium
+around which to build the text. Both drawings and diagrams have been
+simplified so far as is compatible with their correctness, with the
+result that they tell their own story and always in the same language.
+
+The Cyclopedia is a compilation of many of the most valuable
+Instruction Papers of the American School of Correspondence, and the
+method adopted in its preparation is that which this School has
+developed and employed so successfully for many years. This method is
+not an experiment, but has stood the severest of all tests--that of
+practical use--which has demonstrated it to be the best yet devised
+for the education of the busy, practical man.
+
+In conclusion, grateful acknowledgment is due to the staff of authors
+and collaborators, without whose hearty co-operation this work would
+have been impossible.
+
+
+
+
+Table of Contents
+
+VOLUME I
+
+
+FUNDAMENTAL PRINCIPLES _By K. B. Miller and S. G. McMeen_[A]
+
+Acoustics--Characteristics of Sound--Loudness--Pitch--Vibration of
+Diaphragms--Timbre--Human Voice--Human Ear--Speech--Magneto
+Telephones--Loose-Contact Principle--Induction Coils--Simple Telephone
+Circuit--Capacity--Telephone Currents--Audible and Visible
+Signals--Telephone Lines--Conductors--Inductance--Insulation
+
+
+SUBSTATION EQUIPMENT _By K. B. Miller and S. G. McMeen_
+
+Transmitters--Variable Resistance--Materials--Single and Multiple
+Electrodes--Solid-Back Transmitter--Types of
+Transmitters--Electrodes--Packing--Acousticon Transmitter--Switchboard
+Transmitter--Receivers--Types of Receivers--Operator's
+Receiver--Primary Cells--Series and Multiple Connections--Types of
+Primary Cells--Magneto Signaling Apparatus--Battery Bell--Magneto
+Bell--Magneto Generator--Armature--Automatic Shunt--Polarized
+Ringer--Hook Switch--Electromagnets--Impedance, Induction, and
+Repeating Coils--Non-Inductive Resistance
+Devices--Differentially-Wound Unit--Condensers--Materials--Current
+Supply to Transmitters--Local Battery--Common Battery--Diagrams of
+Common-Battery Systems--Telephone Sets: Magneto, Series and Bridging,
+Common-Battery
+
+PARTY-LINE SYSTEMS _By K. B. Miller and S. G. McMeen_
+
+Non-Selective Party-Line Systems--Series and Bridging--Signal
+Code--Selective Party-Line Systems: Polarity, Harmonic, Step-by-Step,
+and Broken-Line--Lock-Out Party-Line Systems: Poole, Step-by-Step, and
+Broken-Line
+
+PROTECTION _By K. B. Miller and S. G. McMeen_
+
+Electrical Hazards--High Potentials--Air-Gap Arrester--Discharge
+across Gaps--Types of Arrester--Vacuum Arrester--Strong
+Currents--Fuses--Sneak Currents--Line Protection--Central-Office and
+Subscribers' Station Protectors--City Exchange
+Requirements--Electrolysis
+
+MANUAL SWITCHBOARDS _By K. B. Miller and S. G. McMeen_
+
+The Telephone Exchange--Subscribers', Trunk, and Toll
+Lines--Districts--Switchboards--Simple Magneto
+Switchboard--Operation--Commercial Types of Drops and Jacks--Manual
+vs. Automatic Restoration--Switchboard Plugs and Cords--Ringing and
+Listening Keys--Operator's Telephone Equipment--Circuits of Complete
+Switchboard--Night-Alarm Circuits--Grounded and Metallic Circuit
+Line--Cord Circuit--Switchboard Assembly
+
+REVIEW QUESTIONS
+
+INDEX
+
+[Footnote A: For professional standing of authors, see list of Authors
+and Collaborators at front of volume.]
+
+[Illustration: OLD BRANCH-TERMINAL MULTIPLE BOARD, PARIS, FRANCE]
+
+
+
+
+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
+a teacher 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 America. Two of
+his comments which are characteristic are as follows:
+
+     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. It can 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
+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 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
+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 telegraph
+lines equipped 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 it and this
+change has simplified other elements of telephone-exchange 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. 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 it no 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 the invention of telephony deserves high
+place among 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 by the 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 is
+much 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 sound:
+loudness, pitch, and timbre.
+
+_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
+of the 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
+each train will hear the bell of the other, first as a _rising_ pitch,
+then as a _falling_ one. Passengers on each train will hear the bell
+of their own train at a _constant_ pitch.
+
+The difference in the observations in such a case is due to relative
+positions between the ear and the source of the sound. As to the bell
+of their own train, the passengers are a fixed distance from it,
+whether the train moves or stands; as to the bell of the other train,
+the passengers first rapidly approach it, then pass it, then recede
+from it. The distances at which it is heard vary as the secants of a
+circle, the radius in this case being a length which is the closest
+approach of the ear to the bell.
+
+If the bell have a constant intrinsic fundamental pitch of 200 waves
+per second (a wave-length of about 5.5 feet), it first will be heard
+at a pitch of about 200 waves per second. But this pitch rises
+rapidly, as if the bell were changing its own pitch, which bells do
+not do. The rising pitch is heard because the ear is rushing down the
+wave-train, every instant nearer to the source. At a speed of 45 miles
+an hour, the pitch rises rapidly, about 12 vibrations per second. If
+the _rate of approach_ between the ear and the bell were constant, the
+pitch of the bell would be heard at 212 waves per second. But suddenly
+the ear passes the bell, hears the pitch stop rising and begin to
+fall; and the tone drops 12 waves per second as it had risen. Such a
+circumflex is an excellent example of the bearing of wavelengths and
+frequencies upon pitch.
+
+Vibration of Diaphragms:--Sound waves in air have the power to move
+other diaphragms than that of the ear. Sound waves constantly vibrate
+such diaphragms as panes of windows and the walls of houses. The
+recording diaphragm of a phonograph is a window pane bearing a stylus
+adapted to engrave a groove in a record blank. In the cylinder form of
+record, the groove varies in depth with the vibrations of the
+diaphragm. In the disk type of phonograph, the groove varies sidewise
+from its normal true spiral.
+
+If the disk record be dusted with talcum powder, wiped, and examined
+with a magnifying glass, the waving spiral line may be seen. Its
+variations are the result of the blows struck upon the diaphragm by a
+train of sound waves.
+
+In reproducing a phonograph record, increasing the speed of the record
+rotation causes the pitch to rise, because the blows upon the air are
+increased in frequency and the wave-lengths shortened. A transitory
+decrease in speed in recording will cause a transitory rise in pitch
+when that record is reproduced at uniform speed.
+
+_Timbre._ Character of sound denotes that difference of effect
+produced upon the ear by sounds otherwise alike in pitch and loudness.
+This characteristic is called timbre. It is extraordinarily useful in
+human affairs, human voices being distinguished from each other by it,
+and a great part of the joy of music lying in it.
+
+A bell, a stretched string, a reed, or other sound-producing body,
+emits a certain lowest possible tone when vibrated. This is called its
+_fundamental tone_. The pitch, loudness, and timbre of this tone
+depend upon various controlling causes. Usually this fundamental tone
+is accompanied by a number of others of higher pitch, blending with it
+to form the general tone of that object. These higher tones are called
+_harmonics_. The Germans call them _overtones_. They are always of a
+frequency which is some multiple of the fundamental frequency. That
+is, the rate of vibration of a harmonic is 2, 3, 4, 5, or some other
+integral number, times as great as the fundamental itself. A tone
+having no harmonics is rare in nature and is not an attractive one.
+The tones of the human voice are rich in harmonics.
+
+In any tone having a fundamental and harmonics (multiples), the
+wave-train consists of a complex series of condensations and
+rarefactions of the air or other transmitting medium. In the case of
+mere noises the train of vibrations is irregular and follows no
+definite order. This is the difference between vowel sounds and other
+musical tones on the one hand and all unmusical sounds (or noises) on
+the other.
+
+Human Voice. Human beings communicate with each other in various
+ways. The chief method is by speech. Voice is sound vibration produced
+by the vocal cords, these being two ligaments in the larynx. The vocal
+cords in man are actuated by the air from the lungs. The size and
+tension of the vocal cords and the volume and the velocity of the air
+from the lungs control the tones of the voice. The more tightly the
+vocal cords be drawn, other things being equal, the higher will be the
+pitch of the sound; that is, the higher the frequency of vibration
+produced by the voice. The pitches of the human voice lie, in general,
+between the frequencies of 87 and 768 per second. These are the
+extremes of pitch, and it is not to be understood that any such range
+of pitch is utilized in ordinary speech. An average man speaks mostly
+between the fundamental frequencies of 85 and 160 per second. Many
+female speaking voices use fundamental frequencies between 150 and 320
+vibrations per second. It is obvious from what has been said that in
+all cases these speaking fundamentals are accompanied by their
+multiples, giving complexity to the resulting wave-trains and
+character to the speaking voice.
+
+Speech-sounds result from shocks given to the air by the organs of
+speech; these organs are principally the mouth cavity, the tongue, and
+the teeth. The vocal cords are _voice-organs_; that is, man only truly
+speaks, yet the lower animals have voice. Speech may be whispered,
+using no voice. Note the distinction between speech and voice, and the
+organs of both.
+
+The speech of adults has a mean pitch lower than that of children; of
+adult males, lower than that of females.
+
+There is no close analogue for the voice-organ in artificial
+mechanism, but the use of the lips in playing a bugle, trumpet,
+cornet, or trombone is a fairly close one. Here the lips, in contact
+with each other, are stretched across one end of a tube (the
+mouthpiece) while the air is blown between the lips by the lungs. A
+musical tone results; if the instrument be a bugle or a trumpet of
+fixed tube length, the pitch will be some one of several certain
+tones, depending on the tension on the lips. The loudness depends on
+the force of the blast of air; the character depends largely on the
+bugle.
+
+Human Ear. The human ear, the organ of hearing in man, is a complex
+mechanism of three general parts, relative to sound waves: a
+wave-collecting part; a wave-observing part, and a wave-interpreting
+part.
+
+The outer ear collects and reflects the waves inwardly to beat upon
+the tympanum, or ear drum, a membrane diaphragm. The uses of the rolls
+or convolutions of the outer ear are not conclusively known, but it is
+observed that when they are filled up evenly with a wax or its
+equivalent, the sense of direction of sound is impaired, and usually
+of loudness also.
+
+The diaphragm of the ear vibrates when struck by sound waves, as does
+any other diaphragm. By means of bone and nerve mechanism, the
+vibration of the diaphragm finally is made known to the brain and is
+interpretable therein.
+
+The human ear can appreciate and interpret sound waves at frequencies
+from 32 to about 32,000 vibrations per second. Below the
+lesser-number, the tendency is to appreciate the separate vibrations
+as separate sounds. Above the higher number, the vibrations are
+inaudible to the human ear. The most acute perception of sound
+differences lies at about 3,000 vibrations per second. It may be that
+the range of hearing of organisms other than man lies far above the
+range with which human beings are familiar. Some trained musicians are
+able to discriminate between two sounds as differing one from the
+other when the difference in frequency is less than one-thousandth of
+either number. Other ears are unable to detect a difference in two
+sounds when they differ by as much as one full step of the chromatic
+scale. Whatever faculty an individual may possess as to tone
+discrimination, it can be improved by training and practice.
+
+
+
+
+CHAPTER II
+
+ELECTRICAL REPRODUCTION OF SPEECH
+
+
+The art of telephony in its present form has for its problem so to
+relate two diaphragms and an electrical system that one diaphragm will
+respond to all the fundamental and harmonic vibrations beating upon it
+and cause the other to vibrate in exact consonance, producing just
+such vibrations, which beat upon an ear.
+
+The art does not do all this today; it falls short of it in every
+phase. Many of the harmonics are lost in one or another stage of the
+process; new harmonics are inserted by the operations of the system
+itself and much of the volume originally available fails to reappear.
+The art, however, has been able to change commercial and social
+affairs in a profound degree.
+
+Conversion from Sound Waves to Vibration of Diaphragm. However
+produced, by the voice or otherwise, sounds to be transmitted by
+telephone consist of vibrations of the air. These vibrations, upon
+reaching a diaphragm, cause it to move. The greatest amplitude of
+motion of a diaphragm is, or is wished to be, at its center, and its
+edge ordinarily is fixed. The diaphragm thus serves as a translating
+device, changing the energy carried by the molecules of the air into
+localized oscillations of the matter of the diaphragm. The waves of
+sound in the air advance; the vibrations of the molecules are
+localized. The agency of the air as a medium for sound transmission
+should be understood to be one in which its general volume has no need
+to move from place to place. What occurs is that the vibrations of the
+sound-producer cause alternate condensations and rarefactions of the
+air. Each molecule of the air concerned merely oscillates through a
+small amplitude, producing, by joint action, shells of waves, each
+traveling outward from the sound-producing center like rapidly growing
+coverings of a ball.
+
+Conversion from Vibration to Voice Currents. Fig. 1 illustrates a
+simple machine adapted to translate motion of a diaphragm into an
+alternating electrical current. The device is merely one form of
+magneto telephone chosen to illustrate the point of immediate
+conversion. _1_ is a diaphragm adapted to vibrate in response to the
+sounds reaching it. _2_ is a permanent magnet and _3_ is its armature.
+The armature is in contact with one pole of the permanent magnet and
+nearly in contact with the other. The effort of the armature to touch
+the pole it nearly touches places the diaphragm under tension. The
+free arm of the magnet is surrounded by a coil _4_, whose ends extend
+to form the line.
+
+[Illustration: Fig. 1. Type of Magneto Telephone]
+
+When sound vibrates the diaphragm, it vibrates the armature also,
+increasing and decreasing the distance from the free pole of the
+magnet. The lines of force threading the coil _4_ are varied as the
+gap between the magnet and the armature is varied.
+
+The result of varying the lines of force through the turns of the coil
+is to produce an electromotive force in them, and if a closed path is
+provided by the line, a current will flow. This current is an
+alternating one having a frequency the same as the sound causing it.
+As in speech the frequencies vary constantly, many pitches
+constituting even a single spoken word, so the alternating voice
+currents are of great varying complexity, and every fundamental
+frequency has its harmonics superposed.
+
+Conversion from Voice Currents to Vibration. The best knowledge of
+the action of such a telephone as is shown in Fig. 1 leads to the
+conclusion that a half-cycle of alternating current is produced by an
+inward stroke of the diaphragm and a second half-cycle of alternating
+current by the succeeding outward stroke, these half-cycles flowing in
+opposite directions. Assume one complete cycle of current to pass
+through the line and also through another such device as in Fig. 1 and
+that the first half-cycle is of such direction as to increase the
+permanent magnetism of the core. The effort of this increase is to
+narrow the gap between the armature and pole piece. The diaphragm will
+throb inward during the half-cycle of current. The succeeding
+half-cycle being of opposite direction will tend to oppose the
+magnetism of the core. In practice, the flow of opposing current never
+would be great enough wholly to nullify and reverse the magnetism of
+the core, so that the opposition results in a mere decrease, causing
+the armature's gap to increase and the diaphragm to respond by an
+outward blow.
+
+Complete Cycle of Conversion. The cycle of actions thus is complete;
+one complete sound-wave in air has produced a cycle of motion in a
+diaphragm, a cycle of current in a line, a cycle of magnetic change in
+a core, a cycle of motion in another diaphragm, and a resulting wave
+of sound. It is to be observed that the chain of operation involves
+the expenditure of energy only by the speaker, the only function of
+any of the parts being that of _translating_ this energy from one form
+to another. In every stage of these translations, there are losses;
+the devising of means of limiting these losses as greatly as possible
+is a problem of telephone engineering.
+
+[Illustration: Fig. 2. Magneto Telephones and Line]
+
+Magneto Telephones. The device in Fig. 1 is a practical magneto
+receiver and transmitter. It is chosen as best picturing the idea to
+be proposed. Fig. 2 illustrates a pair of magneto telephones of the
+early Bell type; _1-1_ are diaphragms; _2-2_ are permanent magnets
+with a free end of each brought as near as possible, without touching,
+to the diaphragm. Each magnet bears on its end nearest the diaphragm a
+winding of fine wire, the two ends of each of these windings being
+joined by means of a two-wire line. All that has been said concerning
+Fig. 1 is true also of the electrical and magnetic actions of the
+devices of Fig. 2. In the latter, the flux which threads the fine wire
+winding is disturbed by motions of the transmitting diaphragm. This
+disturbance of the flux creates electromotive forces in those
+windings. Similarly, a variation of the electromotive forces in the
+windings varies the pull of the permanent magnet of the receiving
+instrument upon its diaphragm.
+
+[Illustration: No. 10 SERIES MULTIPLE SWITCHBOARD _Monarch Telephone
+Mfg. Co._]
+
+[Illustration: Fig. 3. Magneto Telephones without Permanent Magnets]
+
+Fig. 3 illustrates a similar arrangement, but it is to be understood
+that the cores about which the windings are carried in this case are
+of soft iron and not of hard magnetized steel. The necessary magnetism
+which constantly enables the cores to exert a pull upon the diaphragm
+is provided by the battery which is inserted serially in the line.
+Such an arrangement in action differs in no particular from that of
+Fig. 2, for the reason that it matters not at all whether the
+magnetism of the core be produced by electromagnetic or by permanently
+magnetic conditions. The arrangement of Fig. 3 is a fundamental
+counterpart of the original telephone of Professor Bell, and it is of
+particular interest in the present stage of the art for the reason
+that a tendency lately is shown to revert to the early type,
+abandoning the use of the permanent magnet.
+
+The modifications which have been made in the original magneto
+telephone, practically as shown in Fig. 2, have been many. Thirty-five
+years' experimentation upon and daily use of the instrument has
+resulted in its refinement to a point where it is a most successful
+receiver and a most unsuccessful transmitter. Its use for the latter
+purpose may be said to be nothing. As a receiver, it is not only
+wholly satisfactory for commercial use in its regular function, but it
+is, in addition, one of the most sensitive electrical detecting
+devices known to the art.
+
+Loose Contact Principle. Early experimenters upon Bell's device, all
+using in their first work the arrangement utilizing current from a
+battery in series with the line, noticed that sound was given out by
+disturbing loose contacts in the line circuit. This observation led to
+the arrangement of circuits in such a way that some imperfect contacts
+could be shaken by means of the diaphragm, and the resistance of the
+line circuit varied in this manner. An early and interesting form of
+such imperfect contact transmitter device consisted merely of metal
+conductors laid loosely in contact. A simple example is that of three
+wire nails, the third lying across the other two, the two loose
+contacts thus formed being arranged in series with a battery, the
+line, and the receiving instrument. Such a device when slightly
+jarred, by the voice or other means, causes abrupt variation in the
+resistance of the line, and will transmit speech.
+
+Early Conceptions. The conception of the possibility and
+desirability of transmitting speech by electricity may have occurred
+to many, long prior to its accomplishment. It is certain that one
+person, at least, had a clear idea of the general problem. In 1854,
+Charles Bourseul, a Frenchman, wrote: "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 might not be
+heard in Paris? The thing is practicable in this way:
+
+[Illustration: Fig. 4. Reis Transmitter]
+
+"Suppose that a man speaks near a movable disk sufficiently flexible
+to lose none of the vibrations of the voice; that this disk
+_alternately makes and breaks_ the connection from a battery; you may
+have at a distance another disk which will simultaneously execute the
+same vibrations." The idea so expressed is weak in only one
+particular. This particular is shown by the words italicized by
+ourselves. It is impossible to transmit a complex series of waves by
+any simple series of makes and breaks. Philipp Reis, a German, devised
+the arrangement shown in Fig. 4 for the transmission of sound, letting
+the make and break of the contact between the diaphragm _1_ and the
+point _2_ interrupt the line circuit. His receiver took several forms,
+all electromagnetic. His success was limited to the transmission of
+musical sounds, and it is not believed that articulate speech ever was
+transmitted by such an arrangement.
+
+It must be remembered that the art of telegraphy, particularly in
+America, was well established long before the invention of the
+telephone, and that an arrangement of keys, relays, and a battery, as
+shown in Fig. 5, was well known to a great many persons. Attaching the
+armatures of the relays of such a line to diaphragms, as in Fig. 6, at
+any time after 1838, would have produced the telephone. "The hardihood
+of invention" to conceive such a change was the quality required.
+
+[Illustration: Fig. 5. Typical Telegraph Line]
+
+Limitations of Magneto Transmitter. For reasons not finally
+established, the ability of the magneto telephone to produce large
+currents from large sounds is not equal to its ability to produce
+large sounds from large currents. As a receiving device, it is
+unexcelled, and but slight improvement has been made since its first
+invention. It is inadequate as a transmitter, and as early as 1876,
+Professor Bell exhibited other means than electromagnetic action for
+producing the varying currents as a consequence of diaphragm motion.
+Much other inventive effort was addressed to this problem, the aim of
+all being to send out more robust voice currents.
+
+[Illustration: Fig. 6. Telegraph Equipment Converted into Telephone
+Equipment]
+
+Other Methods of Producing Voice Currents. Some of these means are
+the variation of resistance in the path of direct current, variation
+in the pressure of the source of that current, and variation in the
+electrostatic capacity of some part of the circuit.
+
+_Electrostatic Telephone._ The latter method is principally that of
+Dolbear and Edison. Dolbear's thought is illustrated in Fig. 7. Two
+conducting plates are brought close together. One is free to vibrate
+as a diaphragm, while the other is fixed. The element _1_ in Fig. 7 is
+merely a stud to hold rigid the plate it bears against. Each of two
+instruments connected by a line contains such a pair of plates, and a
+battery in the line keeps them charged to its potential. The two
+diaphragms of each instrument are kept drawn towards each other
+because their unlike charges attract each other. The vibration of one
+of the diaphragms changes the potential of the other pair; the degree
+of attraction thus is varied, so that vibration of the diaphragm and
+sound waves result.
+
+Examples of this method of telephone transmission are more familiar to
+later practice in the form of condenser receivers. A condenser, in
+usual present practice, being a pair of closely adjacent conductors of
+considerable surface insulated from each other, a rapidly varying
+current actually may move one or both of the conductors. Ordinarily
+these are of thin sheet metal (foil) interleaved with an insulating
+material, such as paper or mica. Voice currents can vibrate the metal
+sheets in a degree to cause the condenser to speak. These condenser
+methods of telephony have not become commercial.
+
+[Illustration: Fig. 7. Electrostatic Telephone]
+
+_Variation of Electrical Pressure._ Variation of the pressure of the
+source is a conceivable way of transmitting speech. To utilize it,
+would require that the vibrations of the diaphragm cause the
+electromotive force of a battery or machine to vary in harmony with
+the sound waves. So far as we are informed this method never has come
+into practical use.
+
+_Variation of Resistance._ Variation of resistance proportional to the
+vibrations of the diaphragm is the method which has produced the
+present prevailing form of transmission. Professor Bell's Centennial
+exhibit contained a water-resistance transmitter. Dr. Elisha Gray
+also devised one. In both, the diaphragm acted to increase and
+diminish the distance between two conductors immersed in water,
+lowering and raising the resistance of the line. It later was
+discovered by Edison that carbon possesses a peculiarly great property
+of varying its resistance under pressure. Professor David E. Hughes
+discovered that two conducting bodies, preferably of rather poor
+conductivity, when laid together so as to form a _loose contact_
+between them, possessed, in remarkable degree, the ability to vary the
+resistance of the path through them when subject to such vibrations as
+would alter the _intimacy of contact_. He thus discovered and
+formulated the principles of _loose contact_ upon which the operation
+of all modern transmitters rests. Hughes' device was named by him a
+"microphone," indicating a magnification of sound or an ability to
+respond to and make audible minute sounds. It is shown in Fig. 8.
+Firmly attached to a board are two carbon blocks, shown in section in
+the figure. A rod of carbon with cone-shaped ends is supported loosely
+between the two blocks, conical depressions in the blocks receiving
+the ends of the rod. A battery and magneto receiver are connected in
+series with the device. Under certain conditions of contact, the
+arrangement is extraordinarily sensitive to small sounds and
+approaches an ability indicated by its name. Its practical usefulness
+has been not as a serviceable speech transmitter, but as a stimulus to
+the devising of transmitters using carbon in other ways. Variation of
+the resistance of metal conductors and of contact between metals has
+served to transmit voice currents, but no material approaches carbon
+in this property.
+
+[Illustration: Fig. 8. Hughes' Microphone]
+
+Carbon. _Adaptability._ The application of carbon to use in
+transmitters has taken many forms. They may be classified as those
+having a single contact and those having a plurality of contacts; in
+all cases, the _intimacy of contact_ is varied by the diaphragm
+excursions. An example of the single-contact type is the Blake
+transmitter, long familiar in America. An example of the
+multiple-contact type is the loose-carbon type universal now. Other
+types popular at other times and in particular places use solid rods
+or blocks of carbon having many points of contact, though not in a
+powdered or granular form. Fig. 9 shows an example of each of the
+general forms of transmitters.
+
+The use of granular carbon as a transmitter material has extended
+greatly the radius of speech, and has been a principal contributing
+cause for the great spread of the telephone industry.
+
+[Illustration: Fig. 9. General Types of Transmitters]
+
+_Superiority._ The superiority of carbon over other resistance-varying
+materials for transmitters is well recognized, but the reason for it
+is not well known. Various theories have been proposed to explain why,
+for example, the resistance of a mass of carbon granules varies with
+the vibrations or compressions to which they are subjected.
+
+Four principal theories respectively allege:
+
+     First, that change in pressure actually changes the specific
+     resistance of carbon.
+
+     Second, that upon the surface of carbon bodies exists some gas in
+     some form of attachment or combination, variations of pressure
+     causing variations of resistance merely by reducing the thickness
+     of this intervening gas.
+
+     Third, that the change of resistance is caused by variations in
+     the length of electrical arcs between the particles.
+
+     Fourth, that change of pressure changes the area of contact, as
+     is true of solids generally.
+
+One may take his choice. A solid carbon block or rod is not found to
+decrease its resistance by being subjected to pressure. The gas theory
+lacks experimental proof also. The existence of arcs between the
+granules never has been seen or otherwise observed under normal
+working conditions of a transmitter; when arcs surely are
+experimentally established between the granules the usefulness of the
+transmitter ceases. The final theory, that change of pressure changes
+area of surface contact, does not explain why other conductors than
+carbon are not good materials for transmitters. This, it may be
+noticed, is just what the theories set out to make clear.
+
+There are many who feel that more experimental data is required before
+a conclusive and satisfactory theory can be set up. There is need of
+one, for a proper theory often points the way for effective advance in
+practice.
+
+Carbon and magneto transmitters differ wholly in their methods of
+action. The magneto transmitter _produces_ current; the carbon
+transmitter _controls_ current. The former is an alternating-current
+generator; the latter is a rheostat. The magneto transmitter produces
+alternating current without input of any electricity at all; the
+carbon transmitter merely controls a direct current, supplied by an
+external source, and varies its amount without changing its direction.
+
+The carbon transmitter, however, may be associated with other devices
+in a circuit in such a way as to _transform_ direct currents into
+alternating ones, or it may be used merely to change constant direct
+currents into _undulating_ ones, which _never_ reverse direction, as
+alternating currents _always_ do. These distinctions are important.
+
+[Illustration: Fig. 10. Battery in Line Circuit]
+
+_Limitations._ A carbon transmitter being merely a resistance-varying
+device, its usefulness depends on how much its resistance can vary in
+response to motions of air molecules. A granular-carbon transmitter
+may vary between resistances of 5 to 50 ohms while transmitting a
+particular tone, having the lower resistance when its diaphragm is
+driven inward. Conceive this transmitter to be in a line as shown in
+Fig. 10, the line, distant receiver, and battery together having a
+resistance of 1,000 ohms. The minimum resistance then is 1,005 ohms
+and the maximum 1,050 ohms. The variation is limited to about 4.5 per
+cent. The greater the resistance of the line and other elements than
+the transmitter, the less relative change the transmitter can produce,
+and the less loudly the distant receiver can speak.
+
+[Illustration: Fig 11. Battery in Local Circuit]
+
+Induction Coil. Mr. Edison realized this limitation to the use of
+the carbon transmitter direct in the line, and contributed the means
+of removing it. His method is to introduce an induction coil between
+the line and the transmitter, its function being to translate the
+variation of the direct current controlled by the transmitter into
+true alternating currents.
+
+An induction coil is merely a transformer, and for the use under
+discussion consists of two insulated wires wound around an iron core.
+Change in the current carried by one of the windings _produces_ a
+current in the other. If direct current be flowing in one of the
+windings, and remains constant, no current whatever is produced in the
+other. It is important to note that it is change, and change only,
+which produces that alternating current.
+
+Fig. 11 shows an induction coil related to a carbon transmitter, a
+battery, and a receiver. Fig. 12 shows exactly the same arrangement,
+using conventional signs. The winding of the induction coil which is
+in series with the transmitter and the battery is called the primary
+winding; the other is called the secondary winding. In the arrangement
+of Figs. 11 and 12 the battery has no metallic connection with the
+line, so that it is called a _local battery_. The circuit containing
+the battery, transmitter, and primary winding of the induction coil is
+called the _local circuit_.
+
+Let us observe what is the advantage of this arrangement over the
+case of Fig. 10. Using the same values of resistance in the
+transmitter and line, assume the local circuit apart from the
+transmitter to have a fixed resistance of 5 ohms. The limits of
+variations in the local circuit, therefore, are 10 and 55 ohms, thus
+making the maximum 5.5 times the minimum, or an increase of 450 per
+cent as against 4.5 per cent in the case of Fig. 10. The changes,
+therefore, are 100 times as great.
+
+[Illustration: Fig. 12. Conventional Diagram of Talking Circuit]
+
+The relation between the windings of the induction coil in this
+practice are such that the secondary winding contains many more turns
+than the primary winding. Changes in the circuit of the primary
+winding produce potentials in the secondary winding correspondingly
+higher than the potentials producing them. These secondary potentials
+depend upon the _ratio_ of turns in the two windings and therefore,
+within close limits, may be chosen as wished. High potentials in the
+secondary winding are admirably adapted to transmit currents in a
+high-resistance line, for exactly the same reason that long-distance
+power transmission meets with but one-quarter of one kind of loss when
+the sending potential is doubled, one-hundredth of that loss when it
+is raised tenfold, and similarly. The induction coil, therefore,
+serves the double purpose of a step-up transformer to limit line
+losses and a device for vastly increasing the range of change in the
+transmitter circuit.
+
+Fig. 13 is offered to remind the student of the action of an induction
+coil or transformer in whose primary circuit a direct current is
+increased and decreased. An increase of current in the local winding
+produces an impulse of _opposite_ direction in the turns of the
+secondary winding; a decrease of current in the local winding produces
+an impulse of _the same_ direction in the turns of the secondary
+winding. The key of Fig. 13 being closed, current flows upward in the
+primary winding as drawn in the figure, inducing a downward impulse of
+current in the secondary winding and its circuit as noted at the right
+of the figure. On the key being opened, current ceases in the primary
+circuit, inducing an upward impulse of current in the secondary
+winding and circuit as shown. During other than instants of opening
+and closing (changing) the local circuit, no current whatever flows in
+the secondary circuit.
+
+[Illustration: Fig. 13. Induction-Coil Action]
+
+It is by these means that telephone transmitters draw direct current
+from primary batteries and send high-potential alternating currents
+over lines; the same process produces what in Therapeutics are called
+"Faradic currents," and enables also a simple vibrating contact-maker
+to produce alternating currents for operating polarized ringers of
+telephone sets.
+
+Detrimental Effects of Capacity. Electrostatic capacity plays an
+important part in the transmission of speech. Its presence between the
+wires of a line and between them and the earth causes one of the
+losses from which long-distance telephony suffers. Its presence in
+condensers assists in the solution of many circuit and apparatus
+problems.
+
+A condenser is a device composed of two or more conductors insulated
+from each other by a medium called the _dielectric_. A pair of metal
+plates separated by glass, a pair of wires separated by air, or a pair
+of sheets of foil separated by paper or mica may constitute a
+condenser. The use of condensers as pieces of apparatus and the
+problems presented by electrostatic capacity in lines are discussed in
+other chapters.
+
+Measurements of Telephone Currents. It has been recognized in all
+branches of engineering that a definite advance is possible only when
+quantitative data exists. The lack of reliable means of measuring
+telephone currents has been a principal cause of the difficulty in
+solving many of its problems. It is only in very recent times that
+accurate and reliable means have been worked out for measuring the
+small currents which flow in telephone lines. These ways are of two
+general kinds: by thermal and by electromagnetic means.
+
+_Thermal Method_. The thermal methods simply measure, in some way, the
+amount of heat which is produced by a received telephone current. When
+this current is allowed to pass through a conductor the effect of the
+heat generated in that conductor, is observed in one of three ways: by
+the expansion of the conductor, by its change in resistance, or by the
+production of an electromotive force in a thermo-electric couple
+heated by the conductor. Any one of these three ways can be used to
+get some idea of the amount of current which is received. None of them
+gives an accurate knowledge of the forms of the waves which cause the
+reproduction of speech in the telephone receiver.
+
+[Illustration: Fig. 14. Oscillogram of Telephone Currents]
+
+_Electromagnetic Method_. An electromagnetic device adapted to tell
+something of the magnitude of the telephone current and also something
+of its form, _i.e._, something of its various increases and decreases
+and also of its changes in direction is the oscillograph. An
+oscillograph is composed of a magnetic field formed by direct currents
+or by a permanent magnet, a turn of wire under mechanical tension in
+that field, and a mirror borne by the turn of wire, adapted to reflect
+a beam of light to a photographic film or to a rotating mirror.
+
+When a current is to be measured by the oscillograph, it is passed
+through the turn of wire in the magnetic field. While no current is
+passing, the wire does not move in the magnetic field and its mirror
+reflects a stationary beam of light. A photographic film moved in a
+direction normal to the axis of the turn of wire will have drawn upon
+it a straight line by the beam of light. If the beam of light,
+however, is moved by a current, from side to side at right angles to
+this axis, it will draw a wavy line on the photographic film and this
+wavy line will picture the alternations of that current and the
+oscillations of the molecules of air which carried the originating
+sound. Fig. 14 is a photograph of nine different vowel sounds which
+have caused the oscillograph to take their pictures. They are copies
+of records made by Mr. Bela Gati, assisted by Mr. Tolnai. The
+measuring instrument consisted of an oscillograph of the type
+described, the transmitter being of the carbon type actuated by a
+2-volt battery. The primary current was transformed by an induction
+coil of the ordinary type and the transformed current was sent through
+a non-inductive resistance of 3,000 ohms. No condensers were placed in
+the circuit. It will be seen that the integral values of the curves,
+starting from zero, are variable. The positive and the negative
+portions of the curves are not equal, so that the solution of the
+individual harmonic motion is difficult and laborious.
+
+These photographs point out several facts very clearly. One is that
+the alternations of currents in the telephone line, like the motions
+of the molecules of air of the original sound, are highly complex and
+are not, as musical tones are, regular recurrences of equal
+vibrations. They show also that any vowel sound may be considered to
+be a regular recurrence of certain groups of vibrations of different
+amplitudes and of different frequencies.
+
+
+
+
+CHAPTER III
+
+ELECTRICAL SIGNALS
+
+
+Electric calls or signals are of two kinds: audible and visible.
+
+[Illustration: Fig. 15. Telegraph Sounder and Key]
+
+[Illustration: Fig. 16. Vibrating Bell]
+
+Audible Signals. _Telegraph Sounder._ The earliest electric signal
+was an audible one, being the telegraph sounder, or the Morse register
+considered apart from its registering function. Each telegraph sounder
+serves as an audible electric signal and is capable of signifying more
+than that the call is being made. Such a signal is operated by the
+making and breaking of current from a battery. An arrangement of this
+kind is shown in Fig. 15, in which pressure upon the key causes the
+current from the battery to energize the sounder and give one sharp
+audible rap of the lever upon the striking post.
+
+_Vibrating Bell_. The vibrating bell, so widely used as a door bell,
+is a device consequent to the telegraph. Its action is to give a
+series of blows on its gong when its key or push button closes the
+battery circuit. At the risk of describing a trite though not trivial
+thing, it may be said that when the contact _1_ of Fig. 16 is closed,
+current from the battery energizes the armature _2_, causing the
+latter to strike a blow on the gong and to break the line circuit as
+well, by opening the contact back of the armature. So de-energized,
+the armature falls back and the cycle is repeated until the button
+contact is released. A comparison of this action with that of the
+polarized ringer (to be described later) will be found of interest.
+
+[Illustration: Fig. 17. Elemental Magneto-Generator]
+
+_Magneto-Bell._ The magneto-bell came into wide use with the spread of
+telephone service. Its two fundamental parts are an
+alternating-current generator and a polarized bell-ringing device.
+Each had its counterpart long before the invention of the telephone,
+though made familiar by the latter. The alternating-current generator
+of the magneto-bell consists of a rotatable armature composed of a
+coil of insulated wire and usually a core of soft iron, its rotation
+taking place in a magnetic field. This field is usually provided by a
+permanent magnet, hence the name "magneto-generator." The purist in
+terms may well say, however, that every form whatever of the
+dynamo-electric generator is a magneto-generator, as magnetism is one
+link in every such conversion of mechanical power into electricity.
+The terms magneto-electric, magneto-generator, etc., involving the
+term "magneto," have come to imply the presence of _permanently_
+magnetized steel as an element of the construction.
+
+In its early form, the magneto-generator consisted of the arrangement
+shown in Fig. 17, wherein a permanent magnet can rotate on an axis
+before an electromagnet having soft iron cores and a winding.
+Reversals of magnetism produce current in alternately reversing
+half-cycles, one complete rotation of the magnet producing one such
+cycle. Obviously the result would be the same if the magnet were
+stationary and the coils should rotate, which is the construction of
+more modern devices. The turning of the crank of a magneto-bell
+rotates the armature in the magnetic field by some form of gearing at
+a rate usually of the order of twenty turns per second, producing an
+alternating current of that frequency. This current is caused by an
+effective electromotive force which may be as great as 100 volts,
+produced immediately by the energy of the user. In an equipment using
+a magneto-telephone as both receiver and transmitter and a
+magneto-bell as its signal-sending machine, as was usual in 1877, it
+is interesting to note that the entire motive power for signals and
+speech transmission was supplied by the muscular tissues of the
+user--a case of working one's passage.
+
+[Illustration: Fig. 18. Extension of a Permanent Magnet]
+
+The alternating current from the generator is received and converted
+into sound by means of the _polarized ringer_, a device which is
+interesting as depending upon several of the electrical, mechanical,
+and magnetic actions which are the foundations of telephone
+engineering.
+
+[Illustration: Fig. 19. Extension of a Permanent Magnet]
+
+"Why the ringer rings" may be gathered from a study of Figs. 18 to 21.
+A permanent magnet will impart temporary magnetism to pieces of iron
+near it. In Fig. 18 two pieces of iron are so energized. The ends of
+these pieces which are nearest to the permanent magnet _1_ are of the
+opposite polarity to the end they approach, the free ends being of
+opposite polarity. In the figure, these free ends are marked _N_,
+meaning they are of a polarity to point north if free to point at all.
+English-speaking persons call this _north polarity_. Similarly, as in
+Fig. 19, any arrangement of iron near a permanent magnet always will
+have free poles of the same polarity as the end of the permanent
+magnet nearest them.
+
+A permanent magnet so related to iron forms part of a polarized
+ringer. So does an electromagnet composed of windings and iron cores.
+Fig. 20 reminds us of the law of electromagnets. If current flows from
+the plus towards the minus side, with the windings as drawn,
+polarities will be induced as marked.
+
+[Illustration: Fig. 20. Electromagnet]
+
+[Illustration: Fig. 21. Polarized Ringer]
+
+If, now, such an electromagnet, a permanent magnet, and a pivoted
+armature be related to a pair of gongs as shown in Fig. 21, a
+polarized ringer results. It should be noted that a permanent magnet
+has both its poles presented (though one of the poles is not actually
+attached) to two parts of the iron of the _electro_-magnet. The result
+is that the ends of the armature are of south polarity and those of
+the core are of north polarity. All the markings of Fig. 21 relate to
+the polarity produced by the permanent magnet. If, now, a current flow
+in the ringer winding from plus to minus, obviously the right-hand
+pole will be additively magnetized, the current tending to produce
+north magnetism there; also the left-hand pole will be subtractively
+magnetized, the current tending to produce south magnetism there. If
+the current be of a certain strength, relative to the certain ringer
+under study, magnetism in the left pole will be neutralized and that
+in the right pole doubled. Hence the armature will be attracted more
+by the right pole than by the left and will strike the right-hand
+gong. A reversal of current produces an opposite action, the left-hand
+gong being struck. The current ceasing, the armature remains where
+last thrown.
+
+[Illustration: OPERATOR'S EQUIPMENT
+Clement Automanual System.]
+
+It is important to note that the strength of action depends upon the
+strength of the current up to a certain point only. That depends
+upon the strength of the permanent magnet. Whenever the current is
+great enough just to neutralize the normal magnetism of one pole and
+to double that of the other, no increase in current will cause the
+device to ring any louder. This makes obvious the importance of a
+proper permanent magnetism and displays the fallacy of some effort to
+increase the output of various devices depending upon these
+principles. This discussion of magneto-electric signaling is
+introduced here because of a belief in its being fundamental. Chapter
+VIII treats of such a signaling in further detail.
+
+_Telephone Receiver._ The telephone receiver itself serves a useful
+purpose as an audible signal. An interrupted or alternating current of
+proper frequency and amount will produce in it a musical tone which
+can be heard throughout a large room. This fact enables a telephone
+central office to signal a subscriber who has left his receiver off
+the switch hook, so that normal conditions may be restored.
+
+Visible Signals. _Electromagnetic Signal._ Practical visual signals
+are of two general kinds: electromagnetic devices for moving a target
+or pointer, and incandescent lamps. The earliest and most widely used
+visible signal in telephone practice was the annunciator, having a
+shutter adapted to fall when the magnet is energized. Fig. 22 is such
+a signal. Shutter _1_ is held by the catch _2_ from dropping to the
+right by its own gravity. The name "gravity-drop" is thus obvious.
+Current energizing the core attracts the armature _3_, lifts the catch
+_2_, and the shutter falls. A simple modification of the gravity-drop
+produces the visible signal shown in Fig. 23. Energizing the core
+lifts a target so as to render it visible through an opening in the
+plate _1_. A contrast of color between the plate and the target
+heightens the effect.
+
+[Illustration: Fig. 22. Gravity-Drop]
+
+The gravity-drop is principally adapted to the magneto-bell system of
+signaling, where an alternating current is sent over the line to a
+central office by the operation of a bell crank at the subscriber's
+station, this current, lasting only as long as the crank is turned,
+energizes the drop, which may be restored by hand or otherwise and
+will remain latched. The visible signal is better adapted to lines in
+which the signaling is done by means of direct current, as, for
+example, in systems where the removal of the receiver from the hook at
+the subscriber's station closes the line circuit, causing current to
+flow through the winding of the visible signal and so displaying it
+until the receiver has been hung upon the hook or the circuit opened
+by some operation at the central office. Visible signals of the
+magnetic type of Fig. 23 have been widely used in connection with
+common-battery systems, both for line signals and for supervisory
+purposes, indicating the state and the progress of the connection and
+conversation.
+
+[Illustration: Fig. 23. Electromagnetic Visible Signal]
+
+[Illustration: Fig. 24. Lamp Signal and Lens]
+
+_Electric-Lamp Signal._ Incandescent electric lamps appeared in
+telephony as a considerable element about 1890. They are better than
+either form of mechanical visible signals because of three principal
+qualities: simplicity and ease of restoring them to normal as compared
+with drops; their compactness; and their greater prominence when
+displayed. Of the latter quality, one may say that they are more
+_insistent_, as they give out light instead of reflecting it, as do
+all other visible signals. In its best form, the lamp signal is
+mounted behind a hemispherical lens, either slightly clouded or cut in
+facets. This lens serves to distribute the rays of light from the
+lamp, with the result that the signal may be seen from a wide angle
+with the axis of the lens, as shown in Fig. 24. This is of particular
+advantage in connection with manual-switchboard connecting cords, as
+it enables the signals to be mounted close to and even among the
+cords, their great visible prominence when shining saving them from
+being hidden.
+
+The influence of the lamp signal was one of the potent ones in the
+development of the type of multiple switchboard which is now universal
+as the mechanism of large manual exchanges. The first large trial of
+such an equipment was in 1896 in Worcester, Mass. No large and
+successful multiple switchboard with any other type of signal has been
+built since that time.
+
+Any electric signal has upper and lower limits of current between
+which it is to be actuated. It must receive current enough to operate
+but not enough to become damaged by overheating. The magnetic types of
+visible signals have a wider range between these limits than have lamp
+signals. If current in a lamp is too little, its filament either will
+not glow at all or merely at a dull red, insufficient for a proper
+signal. If the current is too great, the filament is heated beyond its
+strength and parts at the weakest place.
+
+This range between current limits in magnetic visible signals is great
+enough to enable them to be used direct in telephone lines, the
+operating current through the line and signal in series with a fixed
+voltage at the central office being not harmfully great when the
+entire line resistance is shunted out at or near the central office.
+The increase of current may be as great as ten times without damage to
+the winding of such a signal. In lamps, the safe margin is much less.
+The current which just gives a sufficient lighting of the signal may
+be about doubled with safety to the filament of the lamp. Consequently
+it is not feasible to place the lamp directly in series with long
+exposed lines. A short circuit of such a line near the central office
+will burn it out.
+
+[Illustration: Fig. 25. Lamp Signal Controlled by Relay]
+
+The qualities of electromagnets and lamps in these respects are used
+to advantage by the lamp signal arrangement shown in Fig. 25. A relay
+is in series with the line and provides a large range of sensibility.
+It is able to carry any current the central-office current source can
+pass through it. The local circuit of the relay includes the lamp.
+Energizing the relay lights the lamp, and the reverse; the lamp is
+thus isolated from danger and receives the current best adapted to its
+needs.
+
+All lines are not long and when enclosed in cable or in well-insulated
+interior wire, may be only remotely in danger of being
+short-circuited. Such conditions exist in private-branch exchanges,
+which are groups of telephones, usually local to limited premises,
+connected to a switchboard on those premises. Such a situation
+permits the omission of the line relay, the lamp being directly in the
+line. Fig. 26 shows the extreme simplicity of the arrangement,
+containing no moving parts or costly elements. Lamps for such service
+have improved greatly since the demand began to grow. The small bulk
+permitted by the need of compactness, the high filament resistance
+required for simplicity of the general power scheme of the system, and
+the need of considerable sturdiness in the completed thing have made
+the task a hard one. The practical result, however, is a signal lamp
+which is highly satisfactory.
+
+[Illustration: Fig. 26. Lamp Signal Directly in Line]
+
+[Illustration: Fig. 27. Lamp Signal and Ballast]
+
+The nature of carbon and certain earths being that their conductivity
+_rises_ with the temperature and that of metals being that their
+conductivity _falls_ with the temperature, has enabled the Nernst lamp
+to be successful. The same relation of properties has enabled
+incandescent-lamp signals to be connected direct to lines without
+relays, but compensated against too great a current by causing the
+resistance in series with the lamp to be increased inversely as the
+resistance of the filament. Employment of a "ballast" resistance in
+this way is referred to in Chapter XI. In Fig. 27 is shown its
+relation to a signal lamp directly in the line. _1_ is the
+carbon-filament lamp; _2_ is the ballast. The latter's conductor is
+fine iron wire in a vacuum. The resistance of the lamp falls as that
+of the ballast rises. Within certain limits, these changes balance
+each other, widening the range of allowable change in the total
+resistance of the line.
+
+
+
+
+CHAPTER IV
+
+TELEPHONE LINES
+
+
+_The line is a path over which the telephone current passes from
+telephone to telephone._ The term "telephone line circuit" is
+equivalent. "Line" and "line circuit" mean slightly different things
+to some persons, "line" meaning the out-of-doors portion of the line
+and "line circuit" meaning the indoor portion, composed of apparatus
+and associated wiring. Such shades of meaning are inevitable and serve
+useful purposes. The opening definition hereof is accurate.
+
+A telephone line consists of two conductors. One of these conductors
+may be the earth; the other always is some conducting material other
+than the earth--almost universally it is of metal and in the form of a
+wire. A line using one wire and the earth as its pair of conductors
+has several defects, to be discussed later herein. Both conductors of
+a line may be wires, the earth serving as no part of the circuit, and
+this is the best practice. A line composed of one wire and the earth
+is called a _grounded line_; a line composed of two wires not needing
+the earth as a conductor is called a _metallic circuit_.
+
+In the earliest telephone practice, all lines were grounded ones. The
+wires were of iron, supported by poles and insulated from them by
+glass, earthenware, or rubber insulators. For certain uses, such lines
+still represent good practice. For telegraph service, they represent
+the present standard practice.
+
+Copper is a better conductor than iron, does not rust, and when drawn
+into wire in such a way as to have a sufficient tensile strength to
+support itself is the best available conductor for telephone lines.
+Only one metal surpasses it in any quality for the purpose: silver is
+a better conductor by 1 or 2 per cent. Copper is better than silver in
+strength and price.
+
+In the open country, telephone lines consist of bare wires of copper,
+of iron, of steel, or of copper-covered steel supported on insulators
+borne by poles. If the wires on the poles be many, cross-arms carry
+four to ten wires each and the insulators are mounted on pins in the
+cross-arms. If the wires on the poles be few, the insulators are
+mounted on brackets nailed to the poles. Wires so carried are called
+_open wires_.
+
+In towns and cities where many wires are to be carried along the same
+route, the wires are reduced in size, insulated by a covering over
+each, and assembled into a group. Such a bundle of insulated wires is
+called a _cable_. It may be drawn into a duct in the earth and be
+called an _underground cable_; it may be laid on the bottom of the sea
+or other water and be called a _submarine cable_; or it may be
+suspended on poles and be called an _aerial cable_. In the most
+general practice each wire is insulated from all others by a wrapping
+of paper ribbon, which covering is only adequate when very dry. Cables
+formed of paper-insulated wires, therefore, are covered by a seamless,
+continuous lead sheath, no part of the paper insulation of the wires
+being exposed to the atmosphere during the cable's entire life in
+service. Telephone cables for certain uses are formed of wires
+insulated with such materials as soft rubber, gutta-percha, and cotton
+or jute saturated with mineral compounds. When insulated with rubber
+or gutta-percha, no continuous lead sheath is essential for
+insulation, as those materials, if continuous upon the wire, insulate
+even when the cable is immersed in water. Sheaths and other armors can
+assist in protecting these insulating materials from mechanical
+injury, and often are used for that purpose. The uses to which such
+cables are suitable in telephony are not many, as will be shown.
+
+A wire supported on poles requires that it be large enough to support
+its own weight. The smaller the wire, the weaker it is, and with poles
+a given distance apart, the strength of the wire must be above a
+certain minimum. In regions where freezing occurs, wires in the open
+air can collect ice in winter and everywhere open wires are subject to
+wind pressure; for these reasons additional strength is required.
+Speaking generally, the practical and economical spacing of poles
+requires that wires, to be strong enough to meet the above conditions,
+shall have a diameter not less than .08 inch, if of hard-drawn copper,
+and .064 inch, if of iron or steel. The honor of developing ways of
+drawing copper wire with sufficient tensile strength for open-air uses
+belongs to Mr. Thomas B. Doolittle of Massachusetts.
+
+Lines whose lengths are limited to a few miles do not require a
+conductivity as great as that of copper wire of .08-inch diameter. A
+wire of that size weighs approximately 100 pounds per mile. Less than
+100 pounds of copper per mile of wire will not give strength enough
+for use on poles; but as little as 10 pounds per mile of wire gives
+the necessary conductivity for the lines of the thousands of telephone
+stations in towns and cities.
+
+Open wires, being exposed to the elements, suffer damage from storms;
+their insulation is injured by contact with trees; they may make
+contact with electric power circuits, perhaps injuring apparatus,
+themselves, and persons; they endanger life and property by the
+possibility of falling; they and their cross-arm supports are less
+sightly than a more compact arrangement.
+
+Grouping small wires of telephone lines into cables has, therefore,
+the advantage of allowing less copper to be used, of reducing the
+space required, of improving appearance, and of increasing safety. On
+the other hand, this same grouping introduces negative advantages as
+well as the foregoing positive ones. It is not possible to talk as far
+or as well over a line in an ordinary cable as over a line of two open
+wires. Long-distance telephone circuits, therefore, have not yet been
+placed in cables for lengths greater than 200 or 300 miles, and
+special treatment of cable circuits is required to talk through them
+for even 100 miles. One may talk 2,000 miles over open wires. The
+reasons for the superiority of the open wires have to do with position
+rather than material. Obviously it is possible to insulate and bury
+any wire which can be carried in the air. The differences in the
+properties of lines whose wires are differently situated with
+reference to each other and surrounding things are interesting and
+important.
+
+A telephone line composed of two conductors always possesses four
+principal properties in some amount: (1) conductivity of the
+conductors; (2) electrostatic capacity between the conductors; (3)
+inductance of the circuit; (4) insulation of each conductor from other
+things.
+
+Conductivity of Conductors. The conductivity of a wire depends upon
+its material, its cross-section, its length, and its temperature.
+Conductivity of a copper wire, for example, increases in direct ratio
+to its weight, in inverse ratio to its length, and its conductivity
+falls as the temperature rises. Resistance is the reciprocal of
+conductivity and the properties, conductivity and resistance, are more
+often expressed in terms of resistance. The unit of the latter is the
+_ohm_; of the former the _mho_. A conductor having a resistance of 100
+ohms has a conductivity of .01 mho. The exact correlative terms are
+_resistance_ and _conductance_, _resistivity_ and _conductivity_. The
+use of the terms as in the foregoing is in accordance with colloquial
+practice.
+
+Current in a circuit having resistance only, varies inversely as the
+resistance. Electromotive force being a cause, and resistance a state,
+current is the result. The formula of this relation, Ohm's law, is
+
+C = E/R
+
+_C_ being the current which results from _E_, the electromotive force,
+acting upon _R_, the resistance. The units are: of current, the
+ampere; of electromotive force, the volt; of resistance, the ohm.
+
+As the conductivity or resistance of a line is the property of
+controlling importance in telegraphy, a similar relation was expected
+in early telephony. As the current in the telephone line varies
+rapidly, certain other properties of the line assume an importance
+they do not have in telegraphy in any such degree.
+
+The importance that these properties assume is, that if they did not
+act and the resistance of the conductors alone limited speech,
+transmission would be possible direct from Europe to America over a
+pair of wires weighing 200 pounds per mile of wire, which is less than
+half the weight of the wire of the best long-distance land lines now
+in service. The distance from Europe to America is about twice as
+great as the present commercial radius by land lines of 435-pound
+wire. In other words, good speech is possible through a mere
+resistance twenty times greater than the resistance of the longest
+actual open-wire line it is possible to talk through. The talking
+ratio between a mere resistance and the resistance of a regular
+telephone cable is still greater.
+
+Electrostatic Capacity. It is the possession of electrostatic
+capacity which enables the condenser, of which the Leyden jar is a
+good example, to be useful in a telephone line. The simplest form of a
+condenser is illustrated in Fig. 28, in which two conducting surfaces
+are separated by an insulating material. The larger the surfaces, the
+closer they are together; and the higher the specific inductive
+capacity of the insulator, the greater the capacity of the device. An
+insulator used in this relation to two conducting surfaces is called
+the _dielectric_.
+
+[Illustration: Fig. 28. Simple Condenser]
+
+[Illustration: Fig. 29. Condenser Symbols]
+
+Two conventional signs are used to illustrate condensers, the upper
+one of Fig. 29 growing out of the original condenser of two metal
+plates, the lower one suggesting the thought of interleaved conductors
+of tin foil, as for many years was the practice in condenser
+construction.
+
+With relation to this property, a telephone line is just as truly a
+condenser as is any other arrangement of conductors and insulators.
+Assume such a line to be open at the distant end and its wires to be
+well insulated from each other and the earth. Telegraphy through such
+a line by ordinary means would be impossible. All that the battery or
+other source could do would be to cause current to flow into the line
+for an infinitesimal time, raising the wires to its potential, after
+which no current would flow. But, by virtue of electrostatic capacity,
+the condition is much as shown in Fig. 30. The condensers which that
+figure shows bridged across the line from wire to wire are intended
+merely to fix in the mind that there is a path for the transfer of
+electrical energy from wire to wire.
+
+[Illustration: Fig. 30. Line with Shunt Capacity]
+
+A simple test will enable two of the results of a short-circuiting
+capacity to be appreciated. Conceive a very short line of two wires to
+connect two local battery telephones. Such a line possesses
+negligible resistance, inductance, and shunt capacity. Its insulation
+is practically infinite. Let condensers be bridged across the line,
+one by one, while conversation goes on. The listening observer will
+notice that the sounds reaching his ear steadily grow less loud as the
+capacity across the line increases. The speaking observer will notice
+that the sounds he hears through the receiver in series with the line
+steadily grow louder as the capacity across the line increases. Fig.
+31 illustrates the test.
+
+The speaker's observation in this test shows that increasing the
+capacity across the line increased the amount of current entering it.
+The hearer's observation in this test shows that increasing the
+capacity across the line decreased the amount of energy turned into
+sound at his receiver.
+
+[Illustration: Fig. 31. Test of Line with Varying Shunt Capacity]
+
+The unit of electrostatic capacity is the _farad_. As this unit is
+inconveniently large, for practical applications the unit
+_microfarad_--millionth of a farad--is employed. If quantities are
+known in microfarads and are to be used in calculations in which the
+values of the capacity require to be farads, care should be taken to
+introduce the proper corrective factor.
+
+The electrostatic capacity between the conductors of a telephone line
+depends upon their surface area, their length, their position, and the
+nature of the materials separating them from each other and from other
+things. For instance, in an open wire line of two wires, the
+electrostatic capacity depends upon the diameter of the wires, upon
+the length of the line, upon their distance apart, upon their distance
+above the earth, and upon the specific inductive capacity of the air.
+Air being so common an insulating medium, it is taken as a convenient
+material whose specific inductive capacity may be used as a basis of
+reference. Therefore, the specific inductive capacity of air is taken
+as unity. All solid matter has higher specific inductive capacity than
+air.
+
+The electrostatic capacity of two open wires .165 inch diameter, 1
+ft. apart, and 30 ft. above the earth, is of the order of .009
+microfarads per mile. This quantity would be higher if the wires were
+closer together; or nearer the earth; or if they were surrounded by a
+gas other than the air or hydrogen; or if the wires were insulated not
+by a gas but by any solid covering. As another example, a line
+composed of two wires of a diameter of .036 inch, if wrapped with
+paper and twisted into a pair as a part of a telephone-cable, has a
+mutual electrostatic capacity of approximately .08 microfarads per
+mile, this quantity being greater if the cable be more tightly
+compressed.
+
+The use of paper as an insulator for wires in telephone cables is due
+to its low specific inductive capacity. This is because the insulation
+of the wires is so largely dry air. Rubber and similar insulating
+materials give capacities as great as twice that of dry paper.
+
+The condenser or other capacity acts as an effective barrier to the
+steady flow of direct currents. Applying a fixed potential causes a
+mere rush of current to charge its surface to a definite degree,
+dependent upon the particular conditions. The condenser does not act
+as such a barrier to alternating currents, for it is possible to talk
+through a condenser by means of the alternating voice currents of
+telephony, or to pass through it alternating currents of much lower
+frequency. A condenser is used in series with a polarized ringer for
+the purpose of letting through alternating current for ringing the
+bell, and of preventing the flow of direct current.
+
+The degree to which the condenser allows alternating currents to pass
+while stopping direct currents, depends on the capacity of the
+condenser and on the frequencies of alternating current. The larger
+the condenser capacity or the higher the frequency of the
+alternations, the greater will be the current passing through the
+circuit. The degree to which the current is opposed by the capacity is
+the reactance of that capacity for that frequency. The formula is
+
+Capacity reactance = 1 /_C_[omega]
+
+wherein _C_ is the capacity in farads and [omega] is 2[pi]_n_, or
+twice 3.1416 times the frequency.
+
+All the foregoing leads to the generalization that the higher the
+frequency, the less the opposition of a capacity to an alternating
+current. If the frequency be zero, the reactance is infinite, _i.e._,
+the circuit is open to direct current. If the frequency be infinite,
+the reactance is zero, _i.e._, the circuit is as if the condenser were
+replaced by a solid conductor of no resistance. Compare this statement
+with the correlative generalization which follows the next thought
+upon inductance.
+
+Inductance of the Circuit. Inductance is the property of a circuit
+by which change of current in it tends to produce in itself and other
+conductors an electromotive force other than that which causes the
+current. Its unit is the _henry_. The inductance of a circuit is one
+henry when a change of one ampere per second produces an electromotive
+force of one volt. Induction _between_ circuits occurs because the
+circuits possess inductance; it is called _mutual induction_.
+Induction _within_ a circuit occurs because the circuit possesses
+inductance; it is called _self-induction_. Lenz' law says: _In all
+cases of electromagnetic induction, the induced currents have such a
+direction that their reaction tends to stop the motion which produced
+them_.
+
+[Illustration: Fig. 32. Spiral of Wire]
+
+[Illustration: Fig. 33. Spiral of Wire Around Iron Core]
+
+All conductors possess inductance, but straight wires used in lines
+have negligible inductance in most actual cases. All wires which are
+wound into coils, such as electromagnets, possess inductance in a
+greatly increased degree. A wire wound into a spiral, as indicated in
+Fig. 32, possesses much greater inductance than when drawn out
+straight. If iron be inserted into the spiral, as shown in Fig. 33,
+the inductance is still further increased. It is for the purpose of
+eliminating inductance that resistance coils are wound with double
+wires, so that current passing through such coils turns in one
+direction half the way and in the other direction the other half.
+
+A simple test will enable the results of a series inductance in a line
+to be appreciated. Conceive a very short line of two wires to connect
+two local battery telephones. Such a line possesses negligible
+resistance, inductance, and shunt capacity. Its insulation is
+practically infinite. Let inductive coils such as electromagnets be
+inserted serially in the wires of the line one by one, while
+conversation goes on. The listening observer will notice that the
+sounds reaching his ear steadily grow faint as the inductance in the
+line increases and the speaking observer will notice the same thing
+through the receiver in series with the line.
+
+Both observations in this test show that the amount of current
+entering and emerging from the line decreased as the inductance
+increased. Compare this with the test with bridged capacity and the
+loading of lines described later herein, observing the curious
+beneficial result when both hurtful properties are present in a line.
+The test is illustrated in Fig. 34.
+
+The degree in which any current is opposed by inductance is termed the
+reactance of that inductance. Its formula is
+
+Inductive reactance = _L_[omega]
+
+wherein _L_ is the inductance in henrys and [omega] is _2_[pi]_n_, or
+twice 3.1416 times the frequency. To distinguish the two kinds of
+reactance, that due to the capacity is called _capacity reactance_ and
+that due to inductance is called _inductive reactance_.
+
+All the foregoing leads to the generalization that the higher the
+frequency, the greater the opposition of an inductance to an
+alternating current. If the frequency be zero, the reactance is zero,
+_i.e._, the circuit conducts direct current as mere resistance. If the
+frequency be infinite, the reactance is infinite, _i.e._, the circuit
+is "open" to the alternating current and that current cannot pass
+through it. Compare this with the correlative generalization following
+the preceding thought upon capacity.
+
+[Illustration: Fig. 34. Test of Line with Varying Serial Inductance]
+
+Capacity and inductance depend only on states of matter. Their
+reactances depend on states of matter and actions of energy.
+
+In circuits having both resistance and capacity or resistance and
+inductance, both properties affect the passage of current. The joint
+reaction is expressed in ohms and is called _impedance_. Its value is
+the square root of the sum of the squares of the resistance and
+reactance, or, Z being impedance,
+
+       -------------------------
+      /                1
+Z =  /  R^{2} + ----------------
+   \/           C^{2}[omega]^{2}
+
+
+and
+
+
+      --------------------------
+Z =  /  R^{2} + L^{2}[omega]^{2}
+   \/
+
+
+the symbols meaning as before.
+
+In words, these formulas mean that, knowing the frequency of the
+current and the capacity of a condenser, or the frequency of the
+current and the inductance of a circuit (a line or piece of
+apparatus), and in either case the resistance of the circuit, one may
+learn the impedance by calculation.
+
+Insulation of Conductors. The fourth property of telephone lines,
+insulation of the conductors, usually is expressed in ohms as an
+insulation resistance. In practice, this property needs to be
+intrinsically high, and usually is measured by millions of ohms
+resistance from the wire of a line to its mate or to the earth. It is
+a convenience to employ a large unit. A million ohms, therefore, is
+called a _megohm_. In telephone cables, an insulation resistance of
+500 megohms per mile at 60 deg. Fahrenheit is the usual specification. So
+high an insulation resistance in a paper-insulated conductor is only
+attained by applying the lead sheath to the cable when its core is
+made practically anhydrous and kept so during the splicing and
+terminating of the cable.
+
+Insulation resistance varies inversely as the length of the conductor.
+If a piece of cable 528 feet long has an insulation resistance of
+6,750 megohms, a mile (ten times as much) of such cable, will have an
+insulation resistance of 675 megohms, or one-tenth as great.
+
+Inductance vs. Capacity. The mutual capacity of a telephone line is
+greater as its wires are closer together. The self-induction of a
+telephone line is smaller as its wires are closer together. The
+electromotive force induced by the capacity of a line leads the
+impressed electromotive force by 90 degrees. The inductive
+electromotive force lags 90 degrees behind the impressed electromotive
+force. And so, in general, the natures of these two properties are
+opposite. In a cable, the wires are so close together that their
+induction is negligible, while their capacity is so great as to limit
+commercial transmission through a cable having .06 microfarads per
+mile capacity and 94 ohms loop resistance per mile, to a distance of
+about 30 miles. In the case of open wires spaced 12 inches apart, the
+limit of commercial transmission is greater, not only because the
+wires are larger, but because the capacity is lower and the inductance
+higher.
+
+Table I shows-the practical limiting conversation distance over
+uniform lines with present standard telephone apparatus.
+
+TABLE I
+
+Limiting Transmission Distances
+
++-----------------------------+----------------------+
+|   SIZE AND GAUGE OF WIRE    |  LIMITING  DISTANCE  |
++-----------------------------+----------------------+
+| No.  8 B. W. G. copper      |      900 miles       |
+|     10 B. W. G. copper      |      700 miles       |
+|     10 B. &  S. copper      |      400 miles       |
+|     12 N. B. S. copper      |      400 miles       |
+|     12 B. &  S. copper      |      240 miles       |
+|     14 N. B. S. copper      |      240 miles       |
+|      8 B. W. G. iron        |      135 miles       |
+|     10 B. W. G. iron        |      120 miles       |
+|     12 B. W. G. iron        |       90 miles       |
+|  16 B. &  S. cable, copper  |       40 miles       |
+|  19 B. &  S. cable, copper  |       30 miles       |
+|  22 B. &  S. cable, copper  |       20 miles       |
++-----------------------------+----------------------+
+
+In 1893, Oliver Heaviside proposed that the inductance of telephone
+lines be increased above the amount natural for the inter-axial
+spacing, with a view to counteracting the hurtful effects of the
+capacity. His meaning was that the increased inductance--a harmful
+quality in a circuit not having also a harmfully great capacity--would
+act oppositely to the capacity, and if properly chosen and applied,
+should decrease or eliminate distortion by making the line's effect on
+fundamentals and harmonics more nearly uniform, and as well should
+reduce the attenuation by neutralizing the action of the capacity in
+dissipating energy.
+
+There are two ways in which inductance might be introduced into a
+telephone line. As the capacity whose effects are to be neutralized
+is distributed uniformly throughout the line, the counteracting
+inductance must also be distributed throughout the line. Mere increase
+of distance between two wires of the line very happily acts both to
+increase the inductance and to lower the capacity; unhappily for
+practical results, the increase of separation to bring the qualities
+into useful neutralizing relation is beyond practical limits. The
+wires would need to be so far above the earth and so far apart as to
+make the arrangement commercially impossible.
+
+Practical results have been secured in increasing the distributed
+inductance by wrapping fine iron wire over each conductor of the line.
+Such a treatment increases the inductance and improves transmission.
+
+The most marked success has come as a result of the studies of
+Professor Michael Idvorsky Pupin. He inserts inductances in series
+with the wires of the line, so adapting them to the constants of the
+circuit that attenuation and distortion are diminished in a gratifying
+degree. This method of counteracting the effects of a distributed
+capacity by the insertion of localized inductance requires not only
+that the requisite total amount of inductance be known, but that the
+proper subdivision and spacing of the local portions of that
+inductance be known. Professor Pupin's method is described in a paper
+entitled "Wave Transmission Over Non-uniform Cables and Long-Distance
+Air Lines," read by him at a meeting of the American Institute of
+Electrical Engineers in Philadelphia, May 19, 1900.
+
+     NOTE. United States Letters Patent were issued to Professor Pupin
+     on June 19, 1900, upon his practical method of reducing
+     attenuation of electrical waves. A paper upon "Propagation of
+     Long Electric Waves" was read by Professor Pupin before the
+     American Institute of Electrical Engineers on March 22, 1899, and
+     appears in Vol. 15 of the Transactions of that society. The
+     student will find these documents useful in his studies on the
+     subject. He is referred also to "Electrical Papers" and
+     "Electromagnetic Theory" of Oliver Heaviside.
+
+Professor Pupin likens the transmission of electric waves over
+long-distance circuits to the transmission of mechanical waves over a
+string. Conceive an ordinary light string to be fixed at one end and
+shaken by the hand at the other; waves will pass over the string from
+the shaken to the fixed end. Certain reflections will occur from the
+fixed end. The amount of energy which can be sent in
+this case from the shaken to the fixed point is small, but if the
+string be loaded by attaching bullets to it, uniformly throughout its
+length, it now may transmit much more energy to the fixed end.
+
+[Illustration: MAIN ENTRANCE AND PUBLIC OFFICE, SAN FRANCISCO HOME
+TELEPHONE COMPANY Contract Department on Left. Accounting Department
+on Right.]
+
+The addition of inductance to a telephone line is analogous to the
+addition of bullets to the string, so that a telephone line is said to
+be _loaded_ when inductances are inserted in it, and the inductances
+themselves are known as _loading coils_.
+
+Fig. 35 shows the general relation of Pupin loading coils to the
+capacity of the line. The condensers of the figure are merely
+conventionals to represent the condenser which the line itself forms.
+The inductances of the figure are the actual loading coils.
+
+[Illustration: Fig. 35. Loaded Line]
+
+The loading of open wires is not as successful in practice as is that
+of cables. The fundamental reason lies in the fact that two of the
+properties of open wires--insulation and capacity--vary with
+atmospheric change. The inserted inductance remaining constant, its
+benefits may become detriments when the other two "constants" change.
+
+The loading of cable circuits is not subject to these defects. Such
+loading improves transmission; saves copper; permits the use of longer
+underground cables than are usable when not loaded; lowers maintenance
+costs by placing interurban cables underground; and permits submarine
+telephone cables to join places not otherwise able to speak with each
+other.
+
+Underground long-distance lines now join or are joining Boston and New
+York, Philadelphia and New York, Milwaukee and Chicago. England and
+France are connected by a loaded submarine cable. There is no
+theoretical reason why Europe and America should not speak to each
+other.
+
+The student wishing to determine for himself what are the effects of
+the properties of lines upon open or cable circuits will find most of
+the subject in the following equation. It tells the value of _a_ in
+terms of the four properties, _a_ being the attenuation constant of
+the line.
+
+That is, the larger _a_ is, the more the voice current is reduced in
+passing over the line. The equation is
+
+      -----------------------------------------------------------------------
+     /   -----------------------------------------------
+a=  /1/2  /(R^{2}+L^{2}[omega]^{2})(S^{2}+C^{2}[omega]^{2} + 1/2(RS-LC[omega]^{2}
+  \/  \/
+
+
+The quantities are
+
+R = Resistance in ohms
+L = Inductance in henrys
+C = Mutual (shunt) capacity in farads
+[omega] = 2[pi]_n_ = 6.2832 times the frequency
+S = Shunt leakage in mhos
+
+The quantity _S_ is a measure of the combined direct-current
+conductance (reciprocal of insulation resistance) and the apparent
+conductance due to dielectric hysteresis.
+
+     NOTE. An excellent paper, assisting such study, and of immediate
+     practical value as helping the understanding of cables and their
+     reasons, is that of Mr. Frank B. Jewett, presented at the
+     Thousand Islands Convention of the American Institute of
+     Electrical Engineers, July 1, 1909.
+
+     Chapter 43 treats cables in further detail. They form a most
+     important part of telephone wire-plant practice, and their uses
+     are becoming wider and more valuable.
+
+Possible Ways of Improving Transmission. Practical ways of improving
+telephone transmission are of two kinds: to improve the lines and to
+improve the apparatus. The foregoing shows what are the qualities of
+lines and the ways they require to be treated. Apparatus treatment, in
+the present state of the art, is addressed largely to the reduction of
+losses. Theoretical considerations seem to show, however, that great
+advance in apparatus effectiveness still is possible. More powerful
+transmitters--and more _faithful_ ones--more sensitive and accurate
+receivers, and more efficient translating devices surely are possible.
+Discovery may need to intervene, to enable invention to restimulate.
+
+In both telegraphy and telephony, the longer the line the weaker the
+current which is received at the distant end. In both telegraphy and
+telephony, there is a length of line with a given kind and size of
+wire and method of construction over which it is just possible to send
+intelligible speech or intelligible signals. A repeater, in
+telegraphy, is a device in the form of a relay which is adapted to
+receive these highly attenuated signal impulses and to re-transmit
+them with fresh power over a new length of line. An arrangement of two
+such relays makes it possible to telegraph both ways over a pair of
+lines united by such a repeater. It is practically possible to join up
+several such links of lines to repeating devices and, if need be, even
+submarine cables can be joined to land lines within practical limits.
+If it were necessary, it probably would be possible to telegraph
+around the world in this way.
+
+If it were possible to imitate the telegraph repeater in telephony,
+attenuated voice currents might be caused to actuate it so as to send
+on those voice currents with renewed power over a length of line,
+section by section. Such a device has been sought for many years, and
+it once was quoted in the public press that a reward of one million
+dollars had been offered by Charles J. Glidden for a successful device
+of that kind. The records of the patent offices of the world show what
+effort has been made in that direction and many more devices have been
+invented than have been patented in all the countries together.
+
+Like some other problems in telephony, this one seems simpler at first
+sight than it proves to be after more exhaustive study. It is possible
+for any amateur to produce at once a repeating device which will relay
+telephone circuits in one direction. It is required, however, that in
+practice the voice currents be relayed in both directions, and
+further, that the relay actually augment the energy which passes
+through it; that is, that it will send on a more powerful current than
+it receives. Most of the devices so far invented fail in one or the
+other of these particulars. Several ways have been shown of assembling
+repeating devices which will talk both ways, but not many assembling
+repeating devices have been shown that will talk both ways and augment
+in both directions.
+
+[Illustration: Fig. 36. Shreeve Repeater and Circuit]
+
+Practical repeaters have been produced, however, and at least one type
+is in daily successful use. It is not conclusively shown even of it
+that it augments in the same degree all of the voice waves which reach
+it, or even that it augments some of them at all. Its action, however,
+is distinctly an improvement in commercial practice. It is the
+invention of Mr. Herbert E. Shreeve and is shown in Fig. 39.
+Primarily it consists of a telephone receiver, of a particular type
+devised by Gundlach, associated with a granular carbon transmitter
+button. It is further associated with an arrangement of induction
+coils or repeating coils, the object of these being to accomplish the
+two-way action, that is, of speaking in both directions and of
+preventing reactive interference between the receiving and
+transmitting elements. The battery _1_ energizes the field of the
+receiving element; the received line current varies that field; the
+resulting motion varies the resistance of the carbon button and
+transforms current from battery _2_ into a new alternating line
+current.
+
+By reactive interference is meant action whereby the transmitter
+element, in emitting a wave, affects its own controlling receiver
+element, thus setting up an action similar to that which occurs when
+the receiver of a telephone is held close to its transmitter and
+humming or singing ensues. No repeater is successful unless it is free
+from this reactive interference.
+
+[Illustration: Fig. 37. Mercury-Arc Telephone Relay]
+
+Enough has been accomplished by practical tests of the Shreeve device
+and others like it to show that the search for a method of relaying
+telephone voice currents is not looking for a pot of gold at the end
+of the rainbow. The most remarkable truth established by the success
+of repeaters of the Shreeve type is that a device embodying so large
+inertia of moving parts can succeed at all. If this mean anything, it
+is that a device in which inertia is absolutely eliminated might do
+very much better. Many of the methods already proposed by inventors
+attack the problem in this way and one of the most recent and most
+promising ways is that of Mr. J.B. Taylor, the circuit of whose
+telephone-relay patent is shown in Fig. 37. In it, _1_ is an
+electromagnet energized by voice currents; its varying field varies an
+arc between the electrodes _2-2_ and _3_ in a vacuum tube. These
+fluctuations are transformed into line currents by the coil _4_.
+
+
+
+
+CHAPTER V
+
+TRANSMITTERS
+
+
+Variable Resistance. As already pointed out in Chapter II, the
+variable-resistance method of producing current waves, corresponding
+to sound waves for telephonic transmission, is the one that lends
+itself most readily to practical purposes. Practically all telephone
+transmitters of today employ this variable-resistance principle. The
+reason for the adoption of this method instead of the other possible
+ones is that the devices acting on this principle are capable, with
+great simplicity of construction, of producing much more powerful
+results than the others. Their simplicity is such as to make them
+capable of being manufactured at low cost and of being used
+successfully by unskilled persons.
+
+Materials. Of all the materials available for the
+variable-resistance element in telephone transmitters, carbon is by
+far the most suitable, and its use is well nigh universal. Sometimes
+one of the rarer metals, such as platinum or gold, is to be found in
+commercial transmitters as part of the resistance-varying device, but,
+even when this is so, it is always used in combination with carbon in
+some form or other. Most of the transmitters in use, however, depend
+solely upon carbon as the conductive material of the
+variable-resistance element.
+
+Arrangement of Electrodes. Following the principles pointed out by
+Hughes, the transmitters of today always employ as their
+variable-resistance elements one or more loose contacts between one or
+more pairs of electrodes, which electrodes, as just stated, are
+usually of carbon. Always the arrangement is such that the sound waves
+will vary the intimacy of contact between the electrodes and,
+therefore, the resistance of the path through the electrodes.
+
+A multitude of arrangements have been proposed and tried. Sometimes a
+single pair of electrodes has been employed having a single point of
+loose contact between them. These may be termed single-contact
+transmitters. Sometimes the variable-resistance element has included a
+greater number of electrodes arranged in multiple, or in series, or in
+series-multiple, and these have been termed multiple-electrode
+transmitters, signifying a plurality of electrodes. A later
+development, an outgrowth of the multiple-electrode transmitter, makes
+use of a pair of principal electrodes, between which is included a
+mass of finely divided carbon in the form of granules or small spheres
+or pellets. These, regardless of the exact form of the carbon
+particles, are called granular-carbon transmitters.
+
+[Illustration: Fig. 38. Blake Transmitter]
+
+Single Electrode. _Blake_. The most notable example of the
+single-contact transmitter is the once familiar Blake instrument. At
+one time this formed a part of the standard equipment of almost every
+telephone in the United States, and it was also largely used abroad.
+Probably no transmitter has ever exceeded it in clearness of
+articulation, but it was decidedly deficient in power in comparison
+with the modern transmitters. In this instrument, which is shown in
+Fig. 38, the variable-resistance contact was that between a carbon and
+a platinum electrode. The diaphragm _1_ was of sheet iron mounted, as
+usual in later transmitters, in a soft rubber gasket _2_. The whole
+diaphragm was mounted in a cast-iron ring _3_, supported on the inside
+of the box containing the entire instrument. The front electrode _4_
+was mounted on a light spring _5_, the upper end of which was
+supported by a movable bar or lever _6_, flexibly supported on a
+spring _7_ secured to the casting which supported the diaphragm. The
+tension of this spring _5_ was such as to cause the platinum point to
+press lightly away from the center of the diaphragm. The rear
+electrode was of carbon in the form of a small block _9_, secured in a
+heavy brass button _10_. The entire rear electrode structure was
+supported on a heavier spring _11_ carried on the same lever as the
+spring _5_. The tension of this latter spring was such as to press
+against the front electrode and, by its greater strength, press this
+against the center of the diaphragm. The adjustment of the instrument
+was secured by means of the screw _12_, carried in a lug extending
+rearwardly from the diaphragm supporting casting, this screw, by its
+position, determining the strength with which the rear electrode
+pressed against the front electrode and that against the diaphragm.
+This instrument was ordinarily mounted in a wooden box together with
+the induction coil, which is shown in the upper portion of the figure.
+
+The Blake transmitter has passed almost entirely out of use in this
+country, being superseded by the various forms of granular
+instruments, which, while much more powerful, are not perhaps capable
+of producing quite such clear and distinct articulation.
+
+The great trouble with the single-contact transmitters, such as the
+Blake, was that it was impossible to pass enough current through the
+single point of contact to secure the desired power of transmission
+without overheating the contact. If too much current is sent through
+such transmitters, an undue amount of heat is generated at the point
+of contact and a vibration is set up which causes a peculiar humming
+or squealing sound which interferes with the transmission of other
+sounds.
+
+Multiple Electrode. To remedy this difficulty the so-called
+multiple-electrode transmitter was brought out. This took a very great
+number of forms, of which the one shown in Fig. 39 is typical. The
+diaphragm shown at _1_, in this particular form, was made of thin pine
+wood. On the rear side of this, suspended from a rod _3_ carried in a
+bracket _4_, were a number of carbon rods or pendants _5_, loosely
+resting against a rod _2_, carried on a bracket _6_ also mounted on
+the rear of the diaphragm. The pivotal rod _3_ and the rod _2_,
+against which the pendants rested, were sometimes, like the pendant
+rods, made of carbon and sometimes of metal, such as brass. When the
+diaphragm vibrated, the intimacy of contact between the pendant rod
+_5_ and the rod _2_ was altered, and thus the resistance of the path
+through all of the pendant rods in multiple was changed.
+
+[Illustration: Fig. 39. Multiple-Electrode Transmitter]
+
+A multitude of forms of such transmitters came into use in the early
+eighties, and while they in some measure remedied the difficulty
+encountered with the Blake transmitter, _i.e._, of not being able to
+carry a sufficiently large current, they were all subject to the
+effects of extreme sensitiveness, and would rattle or break when
+called upon to transmit sounds of more than ordinary loudness.
+Furthermore, the presence of such large masses of material, which it
+was necessary to throw into vibration by the sound waves, was
+distinctly against this form of transmitter. The inertia of the moving
+parts was so great that clearness of articulation was interfered with.
+
+Granular Carbon. The idea of employing a mass of granular carbon,
+supported between two electrodes, one of which vibrated with the sound
+waves and the other was stationary, was proposed by Henry Hunnings in
+the early eighties. While this idea forms the basis of all modern
+telephone transmitters, yet it did not prevent the almost universal
+adoption of the single-contact form of instrument during the next
+decade.
+
+Western Electric Solid-Back Transmitter. In the early nineties,
+however, the granular-carbon transmitter came into its own with the
+advent and wide adoption of the transmitter designed by Anthony C.
+White, known as the _White_, or _solid-back_, transmitter. This has
+for many years been the standard instrument of the Bell companies
+operating throughout the United States, and has found large use
+abroad. A horizontal cross-section of this instrument is shown in Fig.
+40, and a rear view of the working parts in Fig. 41. The working parts
+are all mounted on the front casting _1_. This is supported in a cup
+_2_, in turn supported on the lug _3_, which is pivoted on the
+transmitter arm or other support. The front and rear electrodes of
+this instrument are formed of thin carbon disks shown in solid black.
+The rear electrode, the larger one of these disks, is securely
+attached by solder to the face of a brass disk having a rearwardly
+projecting screw-threaded shank, which serves to hold it and the rear
+electrode in place in the bottom of a heavy brass cup _4_. The front
+electrode is mounted on the rear face of a stud. Clamped against the
+head of this stud, by a screw-threaded clamping ring _7_, is a mica
+washer, or disk _6_. The center portion of this mica washer is
+therefore rigid with respect to the front electrode and partakes of
+its movements. The outer edge of this mica washer is similarly clamped
+against the front edge of the cup _4_, a screw-threaded ring _9_
+serving to hold the edge of the mica rigidly against the front of the
+cup. The outer edge of this washer is, therefore, rigid with respect
+to the rear electrode, which is fixed. Whatever relative movement
+there is between the two electrodes must, therefore, be permitted by
+the flexing of the mica washer. This mica washer not only serves to
+maintain the electrodes in their normal relative positions, but also
+serves to close the chamber which contains the electrodes, and,
+therefore, to prevent the granular carbon, with which the space
+between the electrodes is filled, from falling out.
+
+[Illustration: Fig. 40. White Solid-Back Transmitter]
+
+The cup _4_, containing the electrode chamber, is rigidly fastened
+with respect to the body of the transmitter by a rearwardly projecting
+shank held in a bridge piece _8_ which is secured at its ends to the
+front block. The needed rigidity of the rear electrode is thus
+obtained and this is probably the reason for calling the instrument
+the _solid-back_. The front electrode, on the other hand, is fastened
+to the center of the diaphragm by means of a shank on the stud, which
+passes through a hole in the diaphragm and is clamped thereto by two
+small nuts. Against the rear face of the diaphragm of this transmitter
+there rest two damping springs. These are not shown in Fig. 40 but are
+in Fig. 41. They are secured at one end to the rear flange of the
+front casting _1_, and bear with their other or free ends against the
+rear face of the diaphragm. The damping springs are prevented from
+coming into actual contact with the diaphragm by small insulating
+pads. The purpose of the damping springs is to reduce the
+sensitiveness of the diaphragm to extraneous sounds. As a result, the
+White transmitter does not pick up all of the sounds in its vicinity
+as readily as do the more sensitive transmitters, and thus the
+transmission is not interfered with by extraneous noises. On the other
+hand, the provision of these heavy damping springs makes it necessary
+that this transmitter shall be spoken into directly by the user.
+
+[Illustration: Fig. 41. White Solid-Back Transmitter]
+
+The action of this transmitter is as follows: Sound waves are
+concentrated against the center of the diaphragm by the mouth-piece,
+which is of the familiar form. These waves impinge against the
+diaphragm, causing it to vibrate, and this, in turn, produces similar
+vibrations in the front electrode. The vibrations of the front
+electrode are permitted by the elasticity of the mica washer _6_. The
+rear electrode is, however, held stationary within the heavy chambered
+block _4_ and which in turn is held immovable by its rigid mounting.
+As a result, the front electrode approaches and recedes from the rear
+electrode, thus compressing and decompressing the mass of granular
+carbon between them. As a result, the intimacy of contact between the
+electrode plates and the granules and also between the granules
+themselves is altered, and the resistance of the path from one
+electrode to the other through the mass of granules is varied.
+
+New Western Electric Transmitter. The White transmitter was the
+prototype of a large number of others embodying the same features of
+having the rear electrode mounted in a stationary cup or chamber and
+the front electrode movable with the diaphragm, a washer of mica or
+other flexible insulating material serving to close the front of the
+electrode chamber and at the same time to permit the necessary
+vibration of the front electrode with the diaphragm.
+
+[Illustration: Fig. 42. New Western Electric Transmitter]
+
+One of these transmitters, embodying these same features but with
+modified details, is shown in Fig. 42, this being the new transmitter
+manufactured by the Western Electric Company. In this the bridge of
+the original White transmitter is dispensed with, the electrode
+chamber being supported by a pressed metal cup _1_, which supports the
+chamber as a whole. The electrode cup, instead of being made of a
+solid block as in the White instrument, is composed of two portions, a
+cylindrical or tubular portion _2_ and a back _3_. The cylindrical
+portion is externally screw-threaded so as to engage an internal screw
+thread in a flanged opening in the center of the cup _1_. By this
+means the electrode chamber is held in place in the cup _1_, and by
+the same means the mica washer _4_ is clamped between the flange in
+this opening and the tubular portion _2_ of the electrode chamber. The
+front electrode is carried, as in the White transmitter, on the mica
+washer and is rigidly attached to the center of the diaphragm so as to
+partake of the movement thereof. It will be seen, therefore, that this
+is essentially a White transmitter, but with a modified mounting for
+the electrode chamber.
+
+A feature in this transmitter that is not found in the White
+transmitter is that both the front and the rear electrodes, in fact,
+the entire working portions of the transmitter, are insulated from the
+exposed metal parts of the instrument. This is accomplished by
+insulating the diaphragm and the supporting cup _1_ from the
+transmitter front. The terminal _5_ on the cup _1_ forms the
+electrical connection for the rear electrode, while the terminal _6_,
+which is mounted _on_ but insulated _from_ the cup _1_ and is
+connected with the front electrode by a thin flexible connecting
+strip, forms the electrical connection for the front electrode.
+
+Kellogg Transmitter. The transmitter of the Kellogg Switchboard and
+Supply Company, originally developed by Mr. W.W. Dean and modified by
+his successors in the Kellogg Company, is shown in Fig. 43. In this,
+the electrode chamber, instead of being mounted in a stationary and
+rigid position, as in the case of the White instrument, is mounted on,
+and, in fact, forms a part of the diaphragm. The electrode which is
+associated with the mica washer instead of moving with the diaphragm,
+as in the White instrument, is rigidly connected to a bridge so as to
+be as free as possible from all vibrations.
+
+Referring to Fig. 43, which is a horizontal cross-section of the
+instrument, _1_ indicates the diaphragm. This is of aluminum and it
+has in its center a forwardly deflected portion forming a chamber for
+the electrodes. The front electrode _2_ of carbon is backed by a disk
+of brass and rigidly secured in the front of this chamber, as clearly
+indicated. The rear electrode _3_, also of carbon, is backed by a disk
+of brass, and is clamped against the central portion of a mica disk by
+means of the enlarged head of stud _6_. A nut _7_, engaging the end of
+a screw-threaded shank from the back of the rear electrode, serves to
+bind these two parts together securely, clamping the mica washer
+between them. The outer edge of the mica washer is clamped to the
+main diaphragm _1_ by an aluminum ring and rivets, as clearly
+indicated. It is seen, therefore, that the diaphragm itself contains
+the electrode chamber as an integral part thereof. The entire
+structure of the diaphragm, the front and back electrodes, and the
+granular carbon within are permanently assembled in the factory and
+cannot be dissociated without destroying some of the parts. The rear
+electrode is held rigidly in place by the bridge _5_ and the stud _6_,
+this stud passing through a block _9_ mounted on the bridge but
+insulated from it. The stud _6_ is clamped in the block _9_ by means
+of the set screw _8_, so as to hold the rear electrode in proper
+position after this position has been determined.
+
+[Illustration: Fig. 43. Kellogg Transmitter]
+
+In this transmitter, as in the transmitter shown in Fig. 42, all of
+the working parts are insulated from the exposed metal casing. The
+diaphragm is insulated from the front of the instrument by means of a
+washer _4_ of impregnated cloth, as indicated. The rear electrode is
+insulated from the other portions of the instrument by means of the
+mica washer and by means of the insulation between the block _9_ and
+the bridge _5_. The terminal for the rear electrode is mounted on the
+block _9_, while the terminal for the front electrode, shown at _10_,
+is mounted on, but insulated from, the bridge. This terminal _10_ is
+connected with the diaphragm and therefore with the front electrode by
+means of a thin, flexible metallic connection. This transmitter is
+provided with damping springs similar to those of the White
+instrument.
+
+It is claimed by advocates of this type of instrument that, in
+addition to the ordinary action due to the compression and
+decompression of the granular carbon between the electrodes, there
+exists another action due to the agitation of the granules as the
+chamber is caused to vibrate by the sound waves. In other words, in
+addition to the ordinary action, which may be termed _the piston
+action between the electrodes_, it is claimed that the general
+shaking-up effect of the granules when the chamber vibrates produces
+an added effect. Certain it is, however, that transmitters of this
+general type are very efficient and have proven their capability of
+giving satisfactory service through long periods of time.
+
+Another interesting feature of this instrument as it is now
+manufactured is the use of a transmitter front that is struck up from
+sheet metal rather than the employment of a casting as has ordinarily
+been the practice. The formation of the supporting lug for the
+transmitter from the sheet metal which forms the rear casing or shell
+of the instrument is also an interesting feature.
+
+Automatic Electric Company Transmitter. The transmitter of the
+Automatic Electric Company, of Chicago, shown in Fig. 44, is of the
+same general type as the one just discussed, in that the electrode
+chamber is mounted on and vibrates with the diaphragm instead of being
+rigidly supported on the bridge as in the case of the White or
+solid-back type of instrument. In this instrument the transmitter
+front _1_ is struck up from sheet metal and contains a rearwardly
+projecting flange, carrying an internal screw thread. A heavy inner
+cup _2_, together with the diaphragm _3_, form an enclosure containing
+the electrode chamber. The diaphragm is, in this case, permanently
+secured at its edge to the periphery of the inner cup _2_ by a band of
+metal _4_ so formed as to embrace the edges of both the cup and the
+diaphragm and permanently lock them together. This inner chamber is
+held in place in the transmitter front _1_ by means of a lock ring _5_
+externally screw-threaded to engage the internal screw-thread on the
+flange on the front. The electrode chamber proper is made in the form
+of a cup, rigidly secured to the diaphragm so as to move therewith, as
+clearly indicated. The rear electrode is mounted on a screw-threaded
+stud carried in a block which is fitted to a close central opening in
+the cup _2_.
+
+This transmitter does not make use of a mica washer or diaphragm, but
+employs a felt washer which surrounds the shank of the rear electrode
+and serves to close and seal the carbon containing cup. By this means
+the granular carbon is retained in the chamber and the necessary
+flexibility or freedom of motion is permitted between the front and
+the rear electrodes. As in the Kellogg and the later Bell instruments,
+the entire working parts of this transmitter are insulated from the
+metal containing case, the inner chamber, formed by the cup _2_ and
+the diaphragm _3_, being insulated from the transmitter front and its
+locking ring by means of insulating washers, as shown.
+
+Fig. 44. Automatic Electric Company Transmitter
+
+Monarch Transmitter. The transmitter of the Monarch Telephone
+Manufacturing Company, shown in Fig. 45, differs from both the
+stationary-cup and the vibrating-cup types, although it has the
+characteristics of both. It might be said that it differs from each
+of these two types of transmitters in that it has the characteristics
+of both.
+
+This transmitter, it will be seen, has two flexible mica washers
+between the electrodes and the walls of the electrode cup. The front
+and the back electrodes are attached to the diaphragm and the bridge,
+respectively, by a method similar to that employed in the solid-back
+transmitters, while the carbon chamber itself is free to vibrate with
+the diaphragm as is characteristic of the Kellogg transmitter.
+
+[Illustration: Fig. 45. Monarch Transmitter]
+
+An aluminum diaphragm is employed, the circumferential edge of which
+is forwardly deflected to form a seat. The edge of the diaphragm rests
+_against_ and is separated _from_ the brass front by means of a
+one-piece gasket of specially treated linen. This forms an insulator
+which is not affected by heat or moisture. As in the transmitters
+previously described, the electrodes are firmly soldered to brass
+disks which have solid studs extending from their centers. In the case
+of both the front and the rear electrodes, a mica disk is placed over
+the supporting stud and held in place by a brass hub which has a
+base of the same size as the electrode. The carbon-chamber wall
+consists of a brass ring to which are fastened the mica disks of the
+front and the back electrodes by means of brass collars clamped over
+the edge of the mica and around the rim of the brass ring forming the
+chamber.
+
+[Illustration: MAIN OFFICE BUILDING, BERKELEY, CALIFORNIA
+Containing Automatic Equipment, Forming Part of Larger System
+Operating in San Francisco and Vicinity.
+Bay Cities Home Telephone Company.]
+
+Electrodes. The electrode plates of nearly all modern transmitters
+are of specially treated carbon. These are first copper-plated and
+soldered to their brass supporting disks. After this they are turned
+and ground so as to be truly circular in form and to present
+absolutely flat faces toward each other. These faces are then highly
+polished and the utmost effort is made to keep them absolutely clean.
+Great pains are taken to remove from the pores of the carbon, as well
+as from the surface, all of the acids or other chemicals that may have
+entered them during the process of electroplating them or of soldering
+them to the brass supporting disk. That the two electrodes, when
+mounted in a transmitter, should be parallel with each other, is an
+item of great importance as will be pointed out later.
+
+In a few cases, as previously stated, gold or platinum has been
+substituted for the carbon electrodes in transmitters. These are
+capable of giving good results when used in connection with the proper
+form of granular carbon, but, on the whole, the tendency has been to
+abandon all forms of electrode material except carbon, and its use is
+now well nigh universal.
+
+_Preparation of Carbon_. The granular carbon is prepared from
+carefully selected anthracite coal, which is specially treated by
+roasting or "re-carbonizing" and is then crushed to approximately the
+proper fineness. The crushed carbon is then screened with extreme care
+to eliminate all dust and to retain only granules of uniform size.
+
+Packing. In the earlier forms of granular-carbon transmitters a
+great deal of trouble was experienced due to the so-called packing of
+the instrument. This, as the term indicates, was a trouble due to the
+tendency of the carbon granules to settle into a compact mass and thus
+not respond to the variable pressure. This was sometimes due to the
+presence of moisture in the electrode chamber; sometimes to the
+employment of granules of varying sizes, so that they would finally
+arrange themselves under the vibration of the diaphragm into a fairly
+compact mass; or sometimes, and more frequently, to the granules in
+some way wedging the two electrodes apart and holding them at a
+greater distance from each other than their normal distance. The
+trouble due to moisture has been entirely eliminated by so sealing the
+granule chambers as to prevent the entrance of moisture. The trouble
+due to the lack of uniformity in size of the granules has been
+entirely eliminated by making them all of one size and by making them
+of sufficient hardness so that they would not break up into granules
+of smaller size. The trouble due to the settling of the granules and
+wedging the electrodes apart has been practically eliminated in
+well-designed instruments, by great mechanical nicety in manufacture.
+
+Almost any transmitter may be packed by drawing the diaphragm forward
+so as to widely separate the electrodes. This allows the granules to
+settle to a lower level than they normally occupy and when the
+diaphragm is released and attempts to resume its normal position it is
+prevented from doing so by the mass of granules between. Transmitters
+of the early types could be packed by placing the lips against the
+mouthpiece and drawing in the breath. The slots now provided at the
+base of standard mouthpieces effectually prevent this.
+
+In general it may be said that the packing difficulty has been almost
+entirely eliminated, not by the employment of remedial devices, such
+as those often proposed for stirring up the carbon, but by preventing
+the trouble by the design and manufacture of the instruments in such
+forms that they will not be subject to the evil.
+
+Carrying Capacity. Obviously, the power of a transmitter is
+dependent on the amount of current that it may carry, as well as on
+the amount of variation that it may make in the resistance of the path
+through it. Granular carbon transmitters are capable of carrying much
+heavier current than the old Blake or other single or multiple
+electrode types. If forced to carry too much current, however, the
+same frying or sizzling sound is noticeable as in the earlier types.
+This is due to the heating of the electrodes and to small arcs that
+occur between the electrodes and the granules.
+
+One way to increase the current-carrying capacity of a transmitter is
+to increase the area of its electrodes, but a limit is soon reached in
+this direction owing to the increased inertia of the moving electrode,
+which necessarily comes with its larger size.
+
+The carrying capacity of transmitters may also be increased by
+providing special means for carrying away the heat generated in the
+variable-resistance medium. Several schemes have been proposed for
+this. One is to employ unusually heavy metal for the electrode
+chamber, and this practice is best exemplified in the White solid-back
+instrument. It has also been proposed by others to water-jacket the
+electrode chamber, and also to keep it cool by placing it in close
+proximity to the relatively cool joints of a thermopile. Neither of
+these two latter schemes seems to be warranted in ordinary commercial
+practice.
+
+Sensitiveness. In all the transmitters so far discussed damping
+springs of one form or another have been employed to reduce the
+sensitiveness of the instrument. For ordinary commercial use too great
+a degree of sensitiveness is a fault, as has already been pointed out.
+There are, however, certain adaptations of the telephone transmitter
+which make a maximum degree of sensitiveness desirable. One of these
+adaptations is found in the telephone equipments for assisting
+partially deaf people to hear. In these the transmitter is carried on
+some portion of the body of the deaf person, the receiver is strapped
+or otherwise held at his ear, and a battery for furnishing the current
+is carried in his pocket. It is not feasible, for this sort of use,
+that the sound which this transmitter is to reproduce shall always
+occur immediately in front of the transmitter. It more often occurs at
+a distance of several feet. For this reason the transmitter is made as
+sensitive as possible, and yet is so constructed that it will not be
+caused to produce too loud or unduly harsh sounds in response to a
+loud sound taking place immediately in front of it. Another adaptation
+of such highly sensitive transmitters is found in the special
+intercommunicating telephone systems for use between the various
+departments or desks in business offices. In these it is desirable
+that the transmitter shall be able to respond adequately to sounds
+occurring anywhere in a small-sized room, for instance.
+
+Acousticon Transmitter. In Fig. 46 is shown a transmitter adapted
+for such use. This has been termed by its makers the _acousticon
+transmitter_. Like all the transmitters previously discussed, this is
+of the variable-resistance type, but it differs from them all in that
+it has no damping springs; in that carbon balls are substituted for
+carbon granules; and in that the diaphragm itself serves as the front
+electrode.
+
+This transmitter consists of a cup _1_, into which is set a
+cylindrical block _2_, in one face of which are a number of
+hemispherical recesses. The diaphragm _3_ is made of thin carbon and
+is so placed in the transmitter as to cover the openings of the
+recesses in the carbon block, and lie close enough to the carbon
+block, without engaging it, to prevent the carbon particles from
+falling out. The diaphragm thus serves as the front electrode and the
+carbon block as the rear electrode. The recesses in the carbon block
+are about two-thirds filled with small carbon balls, which are about
+the size of fine sand. The front piece _4_ of the transmitter is of
+sheet metal and serves to hold the diaphragm in place. To admit the
+sound waves it is provided with a circular opening opposite to and
+about the size of the rear electrode block. On this front piece are
+mounted the two terminals of the transmitter, connected respectively
+to the two electrodes, terminal _5_ being insulated from the front
+piece and connected by a thin metal strip with the diaphragm, while
+terminal _6_ is mounted directly on the front piece and connected
+through the cup _1_ with the carbon block _2_, or back electrode of
+the transmitter.
+
+[Illustration: Fig 46. Acousticon Transmitter]
+
+When this transmitter is used in connection with outfits for the deaf,
+it is placed in a hard rubber containing case, consisting of a hollow
+cylindrical piece _7_, which has fastened to it a cover _8_. This
+cover has a circular row of openings or holes near its outer edge, as
+shown at _9_, through which the sound waves may pass to the chamber
+within, and thence find their way through the round hole in the center
+of the front plate _4_ to the diaphragm _3_. It is probable also that
+the front face of the cover _8_ of the outer case vibrates, and in
+this way also causes sound waves to impinge against the diaphragm.
+This arrangement provides a large receiving surface for the sound
+waves, but, owing to the fact that the openings in the containing case
+are not opposite the opening in the transmitter proper, the sound
+waves do not impinge directly against the diaphragm. This peculiar
+arrangement is probably the result of an endeavor to prevent the
+transmitter from being too strongly actuated by violent sounds close
+to it. Instruments of this kind are very sensitive and under proper
+conditions are readily responsive to words spoken in an ordinary tone
+ten feet away.
+
+[Illustration: Fig. 47. Switchboard Transmitter]
+
+Switchboard Transmitter. Another special adaptation of the telephone
+transmitter is that for use of telephone operators at central-office
+switchboards. The requirements in this case are such that the operator
+must always be able to speak into the transmitter while seated before
+the switchboard, and yet allow both of her hands to be free for use.
+This was formerly accomplished by suspending an ordinary
+granular-carbon transmitter in front of the operator, but a later
+development has resulted in the adoption of the so-called breast
+transmitter, shown in Fig. 47. This is merely an ordinary
+granular-carbon transmitter mounted on a plate which is strapped on
+the breast of the operator, the transmitter being provided with a long
+curved mouthpiece which projects in such a manner as to lie just in
+front of the operator's lips. This device has the advantage of
+automatically following the operator in her movements. The breast
+transmitter shown in Fig. 47, is that of the Dean Electric Company.
+
+[Illustration: Fig. 48. Transmitter Symbols]
+
+Conventional Diagram. There are several common ways of illustrating
+transmitters in diagrams of circuits in which they are employed. The
+three most common ways are shown in Fig. 48. The one at the left is
+supposed to be a side view of an ordinary instrument, the one in the
+center a front view, and the one at the right to be merely a
+suggestive arrangement of the diaphragm and the rear electrode. The
+one at the right is best and perhaps most common; the center one is
+the poorest and least used.
+
+
+
+
+CHAPTER VI
+
+RECEIVERS
+
+
+The telephone receiver is the device which translates the energy of
+the voice currents into the energy of corresponding sound waves. All
+telephone receivers today are of the electromagnetic type, the voice
+currents causing a varying magnetic pull on an armature or diaphragm,
+which in turn produces the sound waves corresponding to the
+undulations of the voice currents.
+
+Early Receivers. The early forms of telephone receivers were of the
+_single-pole_ type; that is, the type wherein but one pole of the
+electromagnet was presented to the diaphragm. The single-pole receiver
+that formed the companion piece to the old Blake transmitter and that
+was the standard of the Bell companies for many years, is shown in
+Fig. 49. While this has almost completely passed out of use, it may be
+profitably studied in order that a comparison may be made between
+certain features of its construction and those of the later forms of
+receivers.
+
+The coil of this receiver was wound on a round iron core _2_,
+flattened at one end to afford means for attaching the permanent
+magnet. The permanent magnet was of laminated construction, consisting
+of four hard steel bars _1_, extending nearly the entire length of the
+receiver shell. These steel bars were all magnetized separately and
+placed with like poles together so as to form a single bar magnet.
+They were laid together in pairs so as to include between the pairs
+the flattened end of the pole piece _2_ at one end and the flattened
+portion of the tail piece _3_ at the other end. This whole magnet
+structure, including the core, the tail piece, and the permanently
+magnetized steel bars, was clamped together by screws as shown. The
+containing shell was of hard rubber consisting of three pieces, the
+barrel _4_, the ear-piece _5_, and the tail cap _6_. The barrel and
+the ear piece engaged each other by means of a screw thread and served
+to clamp the diaphragm between them. The compound bar magnet was held
+in place within the shell by means of a screw _7_ passing through the
+hard rubber tail cap _6_ and into the tail block _3_ of the magnet.
+External binding posts mounted on the tail cap, as shown, were
+connected by heavy leading-in wires to the terminals of the
+electromagnet.
+
+A casual consideration of the magnetic circuit of this instrument will
+show that it was inefficient, since the return path for the lines of
+force set up by the bar magnet was necessarily through a very long air
+path. Notwithstanding this, these receivers were capable of giving
+excellent articulation and were of marvelous delicacy of action. A
+very grave fault was that the magnet was supported in the shell at the
+end farthest removed from the diaphragm. As a result it was difficult
+to maintain a permanent adjustment between the pole piece and the
+diaphragm. One reason for this was that hard rubber and steel contract
+and expand under changes of temperature at very different rates, and
+therefore the distance between the pole piece and the diaphragm
+changed with changes of temperature. Another grave defect, brought
+about by this tying together of the permanent magnet and the shell
+which supported the diaphragm at the end farthest from the diaphragm,
+was that any mechanical shocks were thus given a good chance to alter
+the adjustment.
+
+[Illustration: Fig. 49. Single-Pole Receiver]
+
+Modern Receivers. Receivers of today differ from this old
+single-pole receiver in two radical respects. In the first place, the
+modern receiver is of the bi-polar type, consisting essentially of a
+horseshoe magnet presenting both of its poles to the diaphragm. In the
+second place, the modern practice is to either support all of the
+working parts of the receiver, _i.e._, the magnet, the coils, and the
+diaphragm, by an inner metallic frame entirely independent of the
+shell; or, if the shell is used as a part of the structure, to rigidly
+fasten the several parts close to the diaphragm rather than at the end
+farthest removed from the diaphragm.
+
+Western Electric Receiver. The standard bi-polar receiver of the
+Western Electric Company, in use by practically all of the Bell
+operating companies throughout this country and in large use abroad,
+is shown in Fig. 50. In this the shell is of three pieces, consisting
+of the barrel _1_, the ear cap _2_, and the tail cap _3_. The tail cap
+and the barrel are permanently fastened together to form substantially
+a single piece. Two permanently magnetized bar magnets _4-4_ are
+employed, these being clamped together at their upper ends, as shown,
+so as to include the soft iron block _5_ between them. The north pole
+of one of these magnets is clamped to the south pole of the other, so
+that in reality a horseshoe magnet is formed. At their lower ends,
+these two permanent magnets are clamped against the soft iron pole
+pieces _6-6_, a threaded block _7_ also being clamped rigidly between
+these pole pieces at this point. On the ends of the pole pieces the
+bobbins are wound. The whole magnet structure is secured within the
+shell _1_ by means of a screw thread on the block _7_ which engages a
+corresponding internal screw thread in the shell _1_. As a result of
+this construction the whole magnet structure is bound rigidly to the
+shell structure at a point close to the diaphragm, comparatively
+speaking, and as a result of this close coupling, the relation between
+the diaphragm and the pole piece is very much more rigid and
+substantial than in the case where the magnet structure and the shell
+were secured together at the end farthest removed from the diaphragm.
+
+[Illustration: Fig. 50. Western Electric Receiver]
+
+Although this receiver shown in Fig. 50 is the standard in use by the
+Bell companies throughout this country, its numbers running well into
+the millions, it cannot be said to be a strictly modern receiver,
+because of at least one rather antiquated feature. The binding posts,
+by which the circuit conductors are led to the coils of this
+instrument, are mounted on the outside of the receiver shell, as
+indicated, and are thus subject to danger of mechanical injury and
+they are also exposed to the touch of the user, so that he may, in
+case of the wires being charged to an abnormal potential, receive a
+shock. Probably a more serious feature than either one of these is
+that the terminals of the flexible cords which attach to these binding
+posts are attached outside of the receiver shell, and are therefore
+exposed to the wear and tear of use, rather than being protected as
+they should be within the shell. Notwithstanding this undesirable
+feature, this receiver is a very efficient one and is excellently
+constructed.
+
+[Illustration: Fig. 51. Kellogg Receiver]
+
+Kellogg Receiver. In Fig. 51 is shown a bi-polar receiver with
+internal or concealed binding posts. This particular receiver is
+typical of a large number of similar kinds and is manufactured by the
+Kellogg Switchboard and Supply Company. Two straight permanently
+magnetized bar magnets _1-1_ are clamped together at their opposite
+ends so as to form a horseshoe magnet. At the end opposite the
+diaphragm these bars clamp between them a cylindrical piece of iron
+_2_, so as to complete the magnetic circuit at the end. At the end
+nearest the diaphragm they clamp between them the ends of the soft
+iron pole pieces _3-3_, and also a block of composite metal _4_ having
+a large circular flange _4'_ which serves as a means for supporting
+the magnet structure within the shell. The screws by means of which
+the disk _4'_ is clamped to the shouldered seat in the shell do not
+enter the shell directly, but rather enter screw-threaded brass blocks
+which are moulded into the structure of the shell. It is seen from
+this construction that the diaphragm and the pole pieces and the
+magnet structure itself are all rigidly secured together through the
+medium of the shell at a point as close as possible to the diaphragm.
+
+Between the magnets _1-1_ there is clamped an insulating block _5_, to
+which are fastened the terminal plates _6_, one on each side of the
+receiver. These terminal plates are thoroughly insulated from the
+magnets themselves and from all other metallic parts by means of
+sheets of fiber, as indicated by the heavy black lines. On these
+plates _6_ are carried the binding posts for the receiver cord
+terminals. A long tongue extends from each of the plates _6_ through a
+hole in the disk _4'_, into the coil chamber of the receiver, at which
+point the terminal of the magnet winding is secured to it. This tongue
+is insulated from the disk _4'_, where it passes through it, by means
+of insulating bushing, as shown. The other terminal of the magnet
+coils is brought out to the other plate _6_ by means of a similar
+tongue on the other side.
+
+In order that the receiver terminals proper may not be subjected to
+any strain in case the receiver is dropped and its weight caught on
+the receiver cord, a strain loop is formed as a continuation of the
+braided covering of the receiver cord, and this is tied to the
+permanent magnet structure, as shown. By making this strain loop
+short, it is obvious that whatever pull the cord receives will not be
+taken by the cord conductors leading to the binding posts or by the
+binding posts or the cord terminals themselves.
+
+A number of other manufacturers have gone even a step further than
+this in securing permanency of adjustment between the receiver
+diaphragm and pole pieces. They have done this by not depending at all
+on the hard rubber shell as a part of the structure, but by enclosing
+the magnet coil in a cup of metal upon which the diaphragm is mounted,
+so that the permanency of relation between the diaphragm and the pole
+pieces is dependent only upon the metallic structure and not at all
+upon the less durable shell.
+
+Direct-Current Receiver. Until about the middle of the year 1909, it
+was the universal practice to employ permanent magnets for giving the
+initial polarization to the magnet cores of telephone receivers. This
+is still done, and necessarily so, in receivers employed in connection
+with magneto telephones. In common-battery systems, however, where the
+direct transmitter current is fed from the central office to the local
+stations, it has been found that this current which must flow at any
+rate through the line may be made to serve the additional purpose of
+energizing the receiver magnets so as to give them the necessary
+initial polarity. A type of receiver has come into wide use as a
+result, which is commonly called the _direct-current receiver_,
+deriving its name from the fact that it employs the direct current
+that is flowing in the common-battery line to magnetize the receiver
+cores. The Automatic Electric Company, of Chicago, was probably the
+first company to adopt this form of receiver as its standard type.
+Their receiver is shown in cross-section in Fig. 52, and a photograph
+of the same instrument partially disassembled is given in Fig. 53. The
+most noticeable thing about the construction of this receiver is the
+absence of permanent magnets. The entire working parts are contained
+within the brass cup _1_, which serves not only as a container for the
+magnet, but also as a seat for the diaphragm. This receiver is
+therefore illustrative of the type mentioned above, wherein the
+relation between the diaphragm and the pole pieces is not dependent
+upon any connection through the shell.
+
+[Illustration: Fig. 52. Automatic Electric Company Direct-Current
+Receiver]
+
+[Illustration: Fig. 53. Automatic Electric Company Direct-Current
+Receiver]
+
+The coil of this instrument consists of a single cylindrical spool
+_2_, mounted on a cylindrical core. This bobbin lies within a soft
+iron-punching _3_, the form of which is most clearly shown in Fig. 53,
+and this punching affords a return path to the diaphragm for the
+lines of force set up in the magnet core. Obviously a magnetizing
+current passing through the winding of the coil will cause the end of
+the core toward the diaphragm to be polarized, say positively, while
+the end of the enclosing shell will be polarized in the other
+polarity, negatively. Both poles of the magnet are therefore presented
+to the diaphragm and the only air gap in the magnetic circuit is that
+between the diaphragm and these poles. The magnetic circuit is
+therefore one of great efficiency, since it consists almost entirely
+of iron, the only air gap being that across which the attraction of
+the diaphragm is to take place.
+
+The action of this receiver will be understood when it is stated that
+in common-battery practice, as will be shown in later chapters, a
+steady current flows over the line for energizing the transmitter. On
+this current is superposed the incoming voice currents from a distant
+station. The steady current flowing in the line will, in the case of
+this receiver, pass through the magnet winding and establish a normal
+magnetic field in the same way as if a permanent magnet were employed.
+The superposed incoming voice currents will then be able to vary this
+magnetic field in exactly the same way as in the ordinary receiver.
+
+An astonishing feature of this recent development of the so-called
+direct-current receiver is that it did not come into use until after
+about twenty years of common-battery practice. There is nothing new in
+the principles involved, as all of them were already understood and
+some of them were employed by Bell in his original telephone; in fact,
+the idea had been advanced time and again, and thrown aside as not
+being worth consideration. This is an illustration of a frequent
+occurrence in the development of almost any rapidly growing art. Ideas
+that are discarded as worthless in the early stages of the art are
+finally picked up and made use of. The reason for this is that in some
+cases the ideas come in advance of the art, or they are proposed
+before the art is ready to use them. In other cases the idea as
+originally proposed lacked some small but essential detail, or, as is
+more often the case, the experimenter in the early days did not have
+sufficient skill or knowledge to make it fit the requirements as he
+saw them.
+
+Monarch Receiver. The receiver of the Automatic Electric Company
+just discussed employs but a single electromagnet by which the initial
+magnetization of the cores and also the variable magnetization
+necessary for speech reproduction is secured. The problem of the
+direct-current receiver has been attacked in another way by Ernest E.
+Yaxley, of the Monarch Telephone Manufacturing Company, with the
+result shown in Fig. 54. The construction in this case is not unlike
+that of an ordinary permanent-magnet receiver, except that in the
+place of the permanent magnets two soft iron cores _1-1_ are employed.
+On these are wound two long bobbins of insulated wire so that the
+direct current flowing over the telephone line will pass through these
+and magnetize the cores to the same degree and for the same purpose as
+in the case of permanent magnets. These soft iron magnet cores _1-1_
+continue to a point near the coil chamber, where they join the two
+soft iron pole pieces _2-2_, upon which the ordinary voice-current
+coils are wound. The two long coils _4-4_, which may be termed the
+direct-current coils, are of somewhat lower resistance than the two
+voice-current coils _3-3_. They are, however, by virtue of their
+greater number of turns and the greater amount of iron that is
+included in their cores, of much higher impedance than the
+voice-current coils _3-3_. These two sets of coils _4-4_ and _3-3_ are
+connected in multiple. As a result of their lower ohmic resistance the
+coils _4-4_ will take a greater amount of the steady current which
+comes over the line, and therefore the greater proportion of the
+steady current will be employed in magnetizing the bar magnets. On
+account of their higher impedance to alternating currents, however,
+nearly all of the voice currents which are superposed on the steady
+currents, flowing in the line will pass through the voice-current
+coils _3-3_, and, being near the diaphragm, these currents will so
+vary the steady magnetism in the cores _2-2_ as to produce the
+necessary vibration of the diaphragm.
+
+[Illustration: Fig. 54. Monarch Direct-Current Receiver]
+
+This receiver, like the one of the Automatic Electric Company, does
+not rely on the shell in any respect to maintain the permanency of
+relation between the pole pieces and the diaphragm. The cup _5_, which
+is of pressed brass, contains the voice-current coils and also acts as
+a seat for the diaphragm. The entire working parts of this receiver
+may be removed by merely unscrewing the ear piece from the hard rubber
+shell, thus permitting the whole works to be withdrawn in an obvious
+manner.
+
+Dean Receiver. Of such decided novelty as to be almost revolutionary
+in character is the receiver recently put on the market by the Dean
+Electric Company and shown in Fig. 55. This receiver is of the
+direct-current type and employs but a single cylindrical bobbin of
+wire. The core of this bobbin and the return path for the magnetic
+lines of force set up in it are composed of soft iron punchings of
+substantially =E= shape. These punchings are laid together so as to
+form a laminated soft-iron field, the limbs of which are about square
+in cross-section. The coil is wound on the center portion of this _E_
+as a core, the core being, as stated, approximately square in
+cross-section. The general form of magnetic circuit in this instrument
+is therefore similar to that of the Automatic Electric Company's
+receiver, shown in Figs. 52 and 53, but the core is laminated instead
+of being solid as in that instrument.
+
+[Illustration: Fig. 55. Dean Steel Shell Receiver]
+
+The most unusual feature of this Dean receiver is that the use of hard
+rubber or composition does not enter into the formation of the shell,
+but instead a shell composed entirely of steel stampings has been
+substituted therefor. The main portion of this shell is the barrel
+_1_. Great skill has evidently been exercised in the forming of this
+by the cold-drawn process, it presenting neither seams nor welds. The
+ear piece _2_ is also formed of steel of about the same gauge as the
+barrel _1_. Instead of screw-threading the steel parts, so that they
+would directly engage each other, the ingenious device has been
+employed of swaging a brass ring _3_ in the barrel portion and a
+similar brass ring _4_ in the ear cap portion, these two being slotted
+and keyed, as shown at _8_, so as to prevent their turning in their
+respective seats. The ring _3_ is provided with an external screw
+thread and the ring _4_ with an internal screw thread, so that the
+receiver cap is screwed on to the barrel in the same way as in the
+ordinary rubber shell. By the employment of these brass screw-threaded
+rings, the rusting together of the parts so that they could not be
+separated when required--a difficulty heretofore encountered in steel
+construction of similar parts--has been remedied.
+
+[Illustration: Fig. 56. Working Parts of Dean Receiver]
+
+The entire working parts of this receiver are contained within the cup
+_5_, the edge of which is flanged outwardly to afford a seat for the
+diaphragm. The diaphragm is locked in place on the shell by a
+screw-threaded ring _6_, as is clearly indicated. A ring _7_ of
+insulating material is seated within the enlarged portion of the
+barrel _1_, and against this the flange of the cup _5_ rests and is
+held in place by the cap _2_ when it is screwed home. The working
+parts of this receiver partially disassembled are shown in Fig. 56,
+which gives a clear idea of some of the features not clearly
+illustrated in Fig. 55.
+
+It cannot be denied that one of the principal items of maintenance of
+subscribers' station equipment has been due to the breakage of
+receiver shells. The users frequently allow their receiver to fall and
+strike heavily against the wall or floor, thus not only subjecting the
+cords to great strain, but sometimes cracking or entirely breaking the
+receiver shell. The innovation thus proposed by the Dean Company of
+making the entire receiver shell of steel is of great interest. The
+shell, as will be seen, is entirely insulated from the circuit of the
+receiver so that no contact exists by which a user could receive a
+shock. The shell is enameled inside and out with a heavy black
+insulating enamel baked on, and said to be of great durability. How
+this enamel will wear remains to be seen. The insulation of the
+interior portions of the receiver is further guarded by providing a
+lining of fiber within the shell at all points where it seems possible
+that a cross could occur between some of the working parts and the
+metal of the shell. This type of receiver has not been on the market
+long enough to draw definite conclusions, based on experience in use,
+as to what its permanent performance will be.
+
+Thus far in this chapter only those receivers which are commonly
+called _hand receivers_ have been discussed. These are the receivers
+that are ordinarily employed by the general public.
+
+[Illustration: Fig. 57. Operator's Receiver]
+
+Operator's Receiver. At the central office in telephone exchanges
+the operators are provided with receivers in order that they may
+communicate with the subscribers or with other operators. In order
+that they may have both of their hands free to set up and take down
+the connections and to perform all of the switching operations
+required, a special form of receiver is employed for this purpose,
+which is worn as a part of a head-gear and is commonly termed a _head
+receiver_. These are necessarily of very light construction, in
+order not to be burdensome to the operators, and obviously they
+must be efficient. They are ordinarily held in place at the ear by a
+metallic head band fitting over the head of the operator.
+
+[Illustration: GRANT AVENUE OFFICE OF HOME TELEPHONE COMPANY, SAN
+FRANCISCO, CAL. A Type of Central-Office Buildings in Down-Town
+Districts of Large Cities.]
+
+Such a receiver is shown in cross-section in Fig. 57, and completely
+assembled with its head band in Fig. 58. Referring to Fig. 57 the
+shell _1_ of the receiver is of aluminum and the magnets are formed of
+steel rings _2_, cross-magnetized so as to present a north pole on one
+side of the ring and a south pole on the other. The two L-shaped pole
+pieces _3_ are secured by screws to the poles of these ring magnets,
+and these pole pieces carry the magnet coils, as is clearly indicated.
+These poles are presented to a soft iron diaphragm in exactly the same
+way as in the larger hand receivers, the diaphragm being clamped in
+place by a hard rubber ear piece, as shown. The head bands are
+frequently of steel covered with leather. They have assumed numerous
+forms, but the general form shown in Fig. 58 is the one commonly
+adopted.
+
+[Illustration: Fig. 58. Operator's Receiver and Cord]
+
+[Illustration: Fig. 59. Receiver Symbols]
+
+Conventional Symbols. The usual diagrammatic symbols for hand and
+head receivers are shown in Fig. 59. They are self-explanatory. The
+symbol at the left in this figure, showing the general outline of the
+receiver, is the one most commonly used where any sort of a receiver
+is to be indicated in a circuit diagram, but where it becomes
+desirable to indicate in the diagram the actual connections with the
+coil or coils of the receiver, the symbol shown at the right is to be
+preferred, and obviously it may be modified as to number of windings
+and form of core as desired.
+
+
+
+
+CHAPTER VII
+
+PRIMARY CELLS
+
+
+Galvani, an Italian physician, discovered, in 1786, that a current of
+electricity could be produced by chemical action. In 1800, Volta, a
+physicist, also an Italian, threw further light on Galvani's discovery
+and produced what we know as the _voltaic_, or _galvanic_, cell. In
+honor of these two discoverers we have the words volt, galvanic, and
+the various words and terms derived therefrom.
+
+Simple Voltaic Cell. A very simple voltaic cell may be made by
+placing two plates, one of copper and one of zinc, in a glass vessel
+partly filled with dilute sulphuric acid, as shown in Fig. 60. When
+the two plates are not connected by a wire or other conductor,
+experiment shows that the copper plate bears a positive charge with
+respect to the zinc plate, and the zinc plate bears a negative charge
+with respect to the copper. When the two plates are connected by a
+wire, a current flows from the copper to the zinc plate through the
+metallic path of the wire, just as is to be expected when any
+conductor of relatively high electrical potential is joined to one of
+relatively low electrical potential. Ordinarily, when one charged body
+is connected to another of different potential, the resulting current
+is of but momentary duration, due to the redistribution of the charges
+and consequent equalization of potential. In the case of the simple
+cell, however, the current is continuous, showing that some action is
+maintaining the charges on the two plates and therefore maintaining
+the difference of potential between them. The energy of this current
+is derived from the chemical action of the acid on the zinc. The cell
+is in reality a sort of a zinc-burning furnace.
+
+In the action of the cell, when the two plates are joined by a wire,
+it may be noticed that the zinc plate is consumed and that bubbles of
+hydrogen gas are formed on the surface of the copper plate.
+
+_Theory_. Just why or how chemical action in a voltaic cell results
+in the production of a negative charge on the consumed plate is not
+known. Modern theory has it that when an acid is diluted in water the
+molecules of the acid are split up or _dissociated_ into two
+oppositely charged atoms, or groups of atoms, one bearing a positive
+charge and the other a negative charge of electricity. Such charged
+atoms or groups of atoms are called _ions_. This separation of the
+molecules of a chemical compound into positively and negatively
+charged ions is called _dissociation_.
+
+Thus, in the simple cell under consideration the sulphuric acid, by
+dissociation, splits up into hydrogen ions bearing positive charges,
+and SO_{4} ions bearing negative charges. The solution as a whole is
+neutral in potential, having an equal number of equal and opposite
+charges.
+
+[Illustration: Fig. 60. Simple Voltaic Cell]
+
+It is known that when a metal is being dissolved by an acid, each atom
+of the metal which is torn off by the solution leaves the metal as a
+positively charged ion. The carrying away of positive charges from a
+hitherto neutral body leaves that body with a negative charge. Hence
+the zinc, or _consumed_ plate, becomes negatively charged.
+
+In the chemical attack of the sulphuric acid on the zinc, the positive
+hydrogen ions are liberated, due to the affinity of the negative
+SO_{4} ions for the positive zinc ions, this resulting in the
+formation of zinc sulphate in the solution. Now the solution itself
+becomes positively charged, due to the positive charges leaving the
+zinc plate with the zinc ions, and the free positively charged
+hydrogen ions liberated in the solution as just described are repelled
+to the copper plate, carrying their positive charges thereto. Hence
+the copper plate, or the _unconsumed_ plate, becomes positively
+charged and also coated with hydrogen bubbles.
+
+The plates or electrodes of a voltaic cell need not consist of zinc
+and copper, nor need the fluid, called the _electrolyte_, be of
+sulphuric acid; any two dissimilar elements immersed in an electrolyte
+that attacks one of them more readily than the other will form a
+voltaic cell. In every such cell it will be found that one of the
+plates is consumed, and that on the other plate some element is
+deposited, this element being sometimes a gas and sometimes a solid.
+The plate which is consumed is always the negative plate, and the one
+on which the element is deposited is always the positive, the current
+through the connecting wire always being, therefore, from the
+unconsumed to the consumed plate. Thus, in the simple copper-zinc cell
+just considered, the zinc is consumed, the element hydrogen is
+deposited on the copper, and the current flow through the external
+circuit is from the copper to the zinc.
+
+The positive charges, leaving the zinc, or consumed, plate, and
+passing through the electrolyte to the copper, or unconsumed, plate,
+constitute in effect a current of electricity flowing within the
+electrolyte. The current within the cell passes, therefore, from the
+zinc plate to the copper plate. The zinc is, therefore, said to be
+positive with respect to the copper.
+
+_Difference of Potential._ The amount of electromotive force, that is
+generated between two dissimilar elements immersed in an electrolyte
+is different for different pairs of elements and for different
+electrolytes. For a given electrolyte each element bears a certain
+relation to another; _i.e._, they are either electro-positive or
+electro-negative relative to each other. In the following list a group
+of elements are arranged with respect to the potentials which they
+assume with respect to each other with dilute sulphuric acid as the
+electrolyte. The most electro-positive elements are at the top and the
+most electro-negative at the bottom.
+
++Sodium       Lead           Copper
+ Magnesium    Iron           Silver
+ Zinc         Nickel         Gold
+ Cadmium      Bismuth        Platinum
+ Tin          Antimony      -Graphite (Carbon)
+
+Any two elements selected from this list and immersed in dilute
+sulphuric acid will form a voltaic cell, the amount of difference of
+potential, or electromotive force, depending on the distance apart in
+this series of the two elements chosen. The current within the cell
+will always flow from the one nearest the top of the list to the one
+nearest the bottom, _i.e._, from the most electro-positive to the most
+electro-negative; and, therefore, the current in the wire joining the
+two plates will flow from the one lowest down in the list to the one
+highest up.
+
+From this series it is easy to see why zinc and copper, and also zinc
+and carbon, are often chosen as elements of voltaic cells. They are
+widely separated in the series and comparatively cheap.
+
+This series may not be taken as correct for all electrolytes, for
+different electrolytes alter somewhat the order of the elements in the
+series. Thus, if two plates, one of iron and the other of copper, are
+immersed in dilute sulphuric acid, a current is set up which proceeds
+through the liquid from the iron to the copper; but, if the plates
+after being carefully washed are placed in a solution of potassium
+sulphide, a current is produced in the opposite direction. The copper
+is now the positive element.
+
+Table II shows the electrical deportment of the principal metals in
+three different liquids. It is arranged like the preceding one, each
+metal being electro-positive to any one lower in the list.
+
+TABLE II
+
+Behavior of Metals in Different Electrolytes
+
++------------------+-------------------+--------------------+
+|  CAUSTIC POTASH  | HYDROCHLORIC ACID | POTASSIUM SULPHIDE |
++------------------+-------------------+--------------------+
+|    + Zinc        |    + Zinc         |     + Zinc         |
+|      Tin         |      Cadmium      |       Copper       |
+|      Cadmium     |      Tin          |       Cadmium      |
+|      Antimony    |      Lead         |       Tin          |
+|      Lead        |      Iron         |       Silver       |
+|      Bismuth     |      Copper       |       Antimony     |
+|      Iron        |      Bismuth      |       Lead         |
+|      Copper      |      Nickel       |       Bismuth      |
+|      Nickel      |      Silver       |       Nickel       |
+|    - Silver      |    - Antimony     |     - Iron         |
++------------------+-------------------+--------------------+
+
+It is important to remember that in all cells, no matter what elements
+or what electrolyte are used, the electrode _which is consumed_ is the
+one that becomes _negatively charged_ and its terminal, therefore,
+becomes the _negative terminal_ or _pole_, while the electrode _which
+is not consumed_ is the one that becomes _positively charged_, and its
+terminal is, therefore, the _positive terminal_ or _pole of the cell_.
+However, because the current in the electrolyte flows from the
+_consumed_ plate to the _unconsumed_ plate, the consumed plate is
+called the _positive_ plate and the unconsumed, the _negative_. This
+is likely to become confusing, but if one remembers that the _active_
+plate is the _positive_ plate, because it sends forth _positive_ ions
+in the electrolyte, and, therefore, itself becomes _negatively_
+charged, one will have the proper basis always to determine the
+direction of the current flow, which is the important thing.
+
+_Polarization._ If the simple cell already described have its
+terminals connected by a wire for some time, it will be found that the
+current rapidly weakens until it ceases to be manifest. This weakening
+results from two causes: first, the hydrogen gas which is liberated in
+the action of the cell is deposited in a layer on the copper plate,
+thereby covering the plate and reducing the area of contact with the
+liquid. This increases the internal resistance of the cell, since
+hydrogen is a non-conductor. Second, the plate so covered becomes in
+effect a hydrogen electrode, and hydrogen stands high as an
+electro-positive element. There is, therefore, actual reduction in the
+electromotive force of the cell, as well as an increase in internal
+resistance. This phenomenon is known as polarization, and in
+commercial cells means must be taken to prevent such action as far as
+possible.
+
+The means by which polarization of cells is prevented or reduced in
+practice may be divided into three general classes:
+
+     First--_mechanical means_. If the hydrogen bubbles be simply
+     brushed away from the surface of the electrode the resistance and
+     the counter polarity which they cause will be diminished. The
+     same result may be secured if air be blown into the solution
+     through a tube, or if the liquid be kept agitated. If the surface
+     of the electrode be roughened or covered with points, the bubbles
+     collect more freely at the points and are more quickly carried
+     away to the surface of the liquid. These means are, however,
+     hardly practical except in cells for laboratory use.
+
+     Second--_chemical means_. If a highly oxidizing substance be
+     added to the electrolyte, it will destroy the hydrogen bubbles by
+     combining with them while they are in a nascent state, and this
+     will prevent the increase in internal resistance and the opposing
+     electromotive force. Such substances are bichromate of potash,
+     nitric acid, and chlorine, and are largely used.
+
+     Third--_electro-chemical means_. Double cells, arranged to
+     separate the elements and liquids by means of porous partitions
+     or by gravity, may be so arranged that solid copper is liberated
+     instead of hydrogen at a point where the current leaves the
+     liquid, thereby entirely obviating polarization. This method also
+     is largely used.
+
+_Local Action._ When a simple cell stands idle, _i.e._, with its
+circuit open, small hydrogen bubbles may be noticed rising from the
+zinc electrode instead of from copper, as is the case where the
+circuit is closed. This is due to impurities in the zinc plate, such
+as particles of iron, tin, arsenic, carbon, etc. Each of these
+particles acts with the surrounding zinc just as might be expected of
+any pair of dissimilar elements opposed to each other in an
+electrolyte; in other words, they constitute small voltaic cells.
+Local currents, therefore, are generated, circulating between the two
+adjacent metals, and, as a result, the zinc plate and the electrolyte
+are needlessly wasted and the general condition of the cell is
+impaired. This is called _local action_.
+
+_Amalgamated Zincs._ Local action might be prevented by the use of
+chemically pure zinc, but this, on account of its expense, cannot be
+employed commercially. Local action, however, may be overcome to a
+great extent by amalgamating the zinc, _i.e._, coating it with
+mercury. The iron particles or other impurities do not dissolve in the
+mercury, as does the zinc, but they float to the surface, whence the
+hydrogen bubbles which may form speedily carry them off, and, in other
+cases, the impurities fall to the bottom of the cell. As the zinc in
+the pasty amalgam dissolves in the acid, the film of mercury unites
+with fresh zinc, and so always presents a clear, bright, homogeneous
+surface to the action of the electrolyte.
+
+The process of amalgamating the zinc may be performed by dipping it in
+a solution composed of
+
+        Nitric Acid        1 lb.
+        Muriatic Acid      2 lbs.
+        Mercury            8 oz.
+
+The acids should be first mixed and then the mercury slowly added
+until dissolved. Clean the zinc with lye and then dip it in the
+solution for a second or two. Rinse in clean water and rub with a
+brush.
+
+Another method of amalgamating zincs is to clean them by dipping them
+in dilute sulphuric acid and then in mercury, allowing the surplus to
+drain off.
+
+Commercial zincs, for use in voltaic cells as now manufactured,
+usually have about 4 per cent of mercury added to the molten zinc
+before casting into the form of plates or rods.
+
+Series and Multiple Connections. When a number of voltaic cells are
+joined in series, the positive pole of one being connected to the
+negative pole of the next one, and so on throughout the series, the
+_electromotive forces_ of all the cells are added, and the
+electromotive force of the group, therefore, becomes the sum of the
+electromotive forces of the component cells. The currents through all
+the cells in this case will be equal to that of one cell.
+
+If the cells be joined in multiple, the positive poles all being
+connected by one wire and the negative poles by another, then the
+_currents_ of all the cells will be added while the electromotive
+force of the combination remains the same as that of a single cell,
+assuming all the cells to be alike in electromotive force.
+
+Obviously combinations of these two arrangements may be made, as by
+forming strings of cells connected in series, and connecting the
+strings in multiple or parallel.
+
+The term battery is frequently applied to a single voltaic cell, but
+this term is more properly used to designate a plurality of cells
+joined together in series, or in multiple, or in series multiple so as
+to combine their actions in causing current to flow through an
+external circuit. We may therefore refer to a battery of so many
+cells. It has, however, become common, though technically improper, to
+refer to a single cell as a battery, so that the term battery, as
+indicating necessarily more than one cell, has largely lost its
+significance.
+
+Cells may be of two types, primary and secondary.
+
+Primary cells are those consisting of electrodes of dissimilar
+elements which, when placed in an electrolyte, become immediately
+ready for action.
+
+Secondary cells, commonly called _storage cells_ and _accumulators_,
+consist always of two inert plates of metal, or metallic oxide,
+immersed in an electrolyte which is incapable of acting on either of
+them until a current has first been passed through the electrolyte
+from one plate to the other. On the passage of a current in this way,
+the decomposition of the electrolyte is effected and the composition
+of the plates is so changed that one of them becomes electro-positive
+and the other electro-negative. The cell is then, when the _charging_
+current ceases, capable of acting as a voltaic cell.
+
+This chapter is devoted to the primary cell or battery alone.
+
+Types of Primary Cells. Primary cells may be divided into two
+general classes: first, those adapted to furnish constant current; and
+second, those adapted to furnish only intermittent currents. The
+difference between cells in this respect rests largely in the means
+employed for preventing or lessening polarization. Obviously in a cell
+in which polarization is entirely prevented the current may be allowed
+to flow constantly until the cell is completely exhausted; that is,
+until the zinc is all eaten up or until the hydrogen is exhausted from
+the electrolyte or both. On the other hand some cells are so
+constituted that polarization takes place faster than the means
+intended to prevent it can act. In other words, the polarization
+gradually gains on the preventive means and so gradually reduces the
+current by increasing the resistance of the cell and lowering its
+electromotive force. In cells of this kind, however, the arrangement
+is such that if the cell is allowed to rest, that is, if the external
+circuit is opened, the depolarizing agency will gradually act to
+remove the hydrogen from the unattacked electrode and thus place the
+cell in good condition for use again.
+
+Of these two types of primary cells the intermittent-current cell is
+of far greater use in telephony than the constant-current cell. This
+is because the use of primary batteries in telephony is, in the great
+majority of cases, intermittent, and for that reason a cell which will
+give a strong current for a few minutes and which after such use will
+regain practically all of its initial strength and be ready for use
+again, is more desirable than one which will give a weaker current
+continuously throughout a long period of time.
+
+Since the cells which are adapted to give constant current are
+commonly used in connection with circuits that are continuously
+closed, they are called _closed-circuit cells_. The other cells, which
+are better adapted for intermittent current, are commonly used on
+circuits which stand open most of the time and are closed only
+occasionally when their current is desired. For this reason these are
+termed _open-circuit cells_.
+
+_Open-Circuit Cells_. LeClanche Cell:--By far the most important
+primary cell for telephone work is the so-called LeClanche cell. This
+assumes a large variety of forms, but always employs zinc as the
+negatively charged element, carbon as the positively charged element,
+and a solution of sal ammoniac as the electrolyte. This cell employs a
+chemical method of taking care of polarization, the depolarizing agent
+being peroxide of manganese, which is closely associated with the
+carbon element.
+
+The original form of the LeClanche cell, a form in which it was very
+largely used up to within a short time ago, is shown in Fig. 61. In
+this the carbon element is placed within a cylindrical jar of porous
+clay, the walls of this jar being of such consistency as to allow
+moisture slowly to permeate through it. Within this porous cup, as it
+is called, a plate or disk of carbon is placed, and around this the
+depolarizing agent, consisting of black oxide of manganese. This is
+usually mixed with, broken carbon, so as to increase the effective
+area of the carbon element in contact with the depolarizing agent, and
+also to reduce the total internal resistance of the cell. The zinc
+electrode usually consisted merely in a rod of zinc, as shown, with a
+suitable terminal at its upper end.
+
+[Illustration: Fig. 61. LeClanche Cell]
+
+The chemical action taking place within the LeClanche cell is,
+briefly, as follows: Sal ammoniac is chemically known as chloride of
+ammonium and is a combination of chlorine and ammonia. In the action
+which is assumed to accompany the passage of current in this cell, the
+sal ammoniac is decomposed, the chlorine leaving the ammonia to unite
+with an atom of the zinc plate, forming chloride of zinc and setting
+free ammonia and hydrogen. The ammonia is immediately dissolved in the
+water of the cell, and the hydrogen enters the porous cup and would
+speedily polarize the cell by adhering to the carbon plate but for the
+fact that it encounters the peroxide of manganese. This material is
+exceedingly rich in oxygen and it therefore readily gives up a part of
+its oxygen, which forms water by combination with the already
+liberated hydrogen and leaves what is termed a _sesquioxide_ of
+manganese. This absorption or combination of the hydrogen prevents
+immediate polarization, but hydrogen is evolved during the operation
+of the cell more rapidly than it can combine with[typo was 'wth'] the
+oxygen of the manganese, thereby leading to polarization more rapidly
+than the depolarizer can prevent it when the cell is heavily worked.
+When, however, the cell is left with its external circuit open for a
+time, depolarization ensues by the gradual combination of the hydrogen
+with the oxygen of the peroxide of manganese, and as a result the cell
+recuperates and in a short time attains its normal electromotive
+force.
+
+The electromotive force of this cell when new is about 1.47 volts. The
+internal resistance of the cell of the type shown in Fig. 61 is
+approximately 1 ohm, ordinarily less rather than more.
+
+A more recent form of LeClanche cell is shown in cross-section in Fig.
+62. This uses practically the same materials and has the same chemical
+action as the old disk LeClanche cell shown in Fig. 61. It dispenses,
+however, with the porous cup and instead employs a carbon electrode,
+which in itself forms a cup for the depolarizing agent.
+
+[Illustration: Fig. 62. Carbon Cylinder LeClanche Cell]
+
+The carbon electrode is in the form of a corrugated hollow cylinder
+which engages by means of an internal screw thread a corresponding
+screw thread on the outer side of the carbon cover. Within this
+cylinder is contained a mixture of broken carbon and peroxide of
+manganese. The zinc electrode is in the form of a hollow cylinder
+almost surrounding the carbon electrode and separated therefrom by
+means of heavy rubber bands stretched around the carbon. The rod,
+forming the terminal of the zinc, passes through a porcelain bushing
+on the cover plate to obviate short circuits. This type of cell has an
+electromotive force of about 1.55 volts and recuperates very quickly
+after severe use. It also has considerably lower internal resistance
+than the type of LeClanche cell employing a porous cup, and,
+therefore, is capable of generating a considerably larger current.
+
+Cells of this general type have assumed a variety of forms. In some
+the carbon electrode, together with the broken carbon and peroxide of
+manganese, were packed into a canvas bag which was suspended in the
+electrolyte and usually surrounded by the zinc electrode. In other
+forms the carbon electrode has moulded with it the manganese
+depolarizer.
+
+In order to prevent the salts within the cell from creeping over the
+edge of the containing glass jar and also over the upper portion of
+the carbon electrode, it is common practice to immerse the upper end
+of the carbon element and also the upper edge of the glass jar in hot
+paraffin.
+
+In setting up the LeClanche cell, place not more than four ounces of
+white sal ammoniac in the jar, fill the jar one-third full of water,
+and stir until the sal ammoniac is all dissolved. Then put the carbon
+and zinc elements in place. A little water poured in the vent hole of
+the porous jar or carbon cylinder will tend to hasten the action.
+
+An excess of sal ammoniac should not be used, as a saturated solution
+tends to deposit crystals on the zinc; on the other hand, the solution
+should not be allowed to become too weak, as in that case the chloride
+of zinc will form on the zinc. Both of these causes materially
+increase the resistance of the cell.
+
+A great advantage of the LeClanche cell is that when not in use there
+is but little material waste. It contains no highly corrosive
+chemicals. Such cells require little attention, and the addition of
+water now and then to replace the loss due to evaporation is about all
+that is required until the elements become exhausted. They give a
+relatively high electromotive force and have a moderately low internal
+resistance, so that they are capable of giving rather large currents
+for short intervals of time. If properly made, they recuperate quickly
+after polarization due to heavy use.
+
+_Dry Cell_. All the forms of cells so far considered may be quite
+properly termed _wet cells_ because of the fact that a free liquid
+electrolyte is used. This term is employed in contradistinction to the
+later developed cell, commonly termed the _dry cell_. This term "dry
+cell" is in some respects a misnomer, since it is not dry and if it
+were dry it would not work. It is essential to the operation of these
+cells that they shall be moist within, and when such moisture is
+dissipated the cell is no longer usable, as there is no further useful
+chemical action.
+
+The dry cells are all of the LeClanche type, the liquid electrolyte
+of that type being replaced by a semi-solid substance that is capable
+of retaining moisture for a considerable period.
+
+As in the ordinary wet LeClanche cell, the electrodes are of carbon
+and zinc, the zinc element being in the form of a cylindrical cup and
+forming the retaining vessel of the cell, while the carbon element is
+in the form of a rod or plate and occupies a central position with
+regard to the zinc, being held out of contact with the zinc, however,
+at all points.
+
+A cross-section of an excellent form of dry cell is shown in Fig. 63.
+The outer casing is of zinc, formed in the shape of a cylindrical cup,
+and serves not only as the retaining vessel, but as the negatively
+charged electrode. The outer surface of the zinc is completely covered
+on its sides and bottom with heavy pasteboard so as to insulate it
+from bodies with which it may come in contact, and particularly from
+the zinc cups of other cells used in the same battery. The positively
+charged electrode is a carbon rod corrugated longitudinally, as shown,
+in order to obtain greater surface. This rod is held in the center of
+the zinc cup out of contact therewith, and the intervening space is
+filled with a mixture of peroxide of manganese, powdered carbon, and
+sal ammoniac. Several thicknesses of blotting paper constitute a
+lining for the inner portion of the zinc electrode and serve to
+prevent the manganese mixture from coming directly into contact
+therewith. The cell is sealed with pitch, which is placed on a layer
+of sand and sawdust mixed in about equal parts.
+
+[Illustration: Fig. 63. Dry Cell]
+
+The electrolyte in such cells varies largely as to quantities and
+proportions of the materials employed in various types of cells, and
+also varies in the method in which the elements are introduced into
+the container.
+
+The following list and approximate proportions of material will serve
+as a fair example of the filling mixture in well-known types of cells.
+
+        Manganese dioxide             45 per cent
+        Carbon or graphite, or both   45 per cent
+        Sal ammoniac                   7 per cent
+        Zinc chloride                  3 per cent
+
+Water is added to the above and a sufficient amount of mixture is
+taken for each cell to fill the zinc cup about seven-eighths full when
+the carbon is in place. The most suitable quantity of water depends
+upon the original dryness and fineness of material and upon the
+quality of the paper lining.
+
+In some forms of dry batteries, starch or other paste is added to
+improve the contact of the electrolyte with the zinc and promote a
+more even distribution of action throughout the electrolyte. Mercury,
+too, is often added to effect amalgamation of the zinc.
+
+As in the ordinary wet type of LeClanche cell, the purpose of the
+manganese is to act as a depolarizer; the carbon or graphite being
+added to give conductivity to the manganese and to form a large
+electrode surface. It is important that the sal ammoniac, which is the
+active agent of the cell, should be free from lumps in order to mix
+properly with the manganese and carbon.
+
+A small local action takes place in the dry cell, caused by the
+dissimilar metals necessarily employed in soldering up the zinc cup
+and in soldering the terminal rod of zinc to the zinc cup proper. This
+action, however, is slight in the better grades of cells. As a result
+of this, and also of the gradual drying out of the moisture within the
+cell, these cells gradually deteriorate even when not in use--this is
+commonly called _shelf-wear_. Shelf-wear is much more serious in the
+very small sizes of dry cells than in the larger ones.
+
+Dry cells are made in a large number of shapes and sizes. The most
+useful form, however, is the ordinary cylindrical type. These are made
+in sizes varying from one and one-half inches high and three-quarters
+inch in diameter to eight inches high and three and three-quarters
+inches in diameter. The most used and standard size of dry cell is of
+cylindrical form six inches high and two and three-quarters inches in
+diameter. The dry cell when new and in good condition has an
+open-circuit voltage of from 1.5 to 1.6 volts. Perhaps 1.55 represents
+the usual average.
+
+A cell of the two and three-quarters by six-inch size will give
+throughout its useful life probably thirty ampere hours as a maximum,
+but this varies greatly with the condition of use and the make of
+cell. Its effective voltage during its useful life averages about one
+volt, and if during this life it gives a total discharge of thirty
+ampere hours, the fair energy rating of the cell will be thirty
+watt-hours. This may not be taken as an accurate figure, however, as
+the watt-hour capacity of a cell depends very largely, not only on the
+make of the cell, but on the rate of its discharge.
+
+An examination of Fig. 63 shows that the dry cell has all of the
+essential elements of the LeClanche cell. The materials of which the
+electrodes are made are the same and the porous cup of the disk
+LeClanche cell is represented in the dry cell by the blotting-paper
+cylinder, which separates the zinc from the carbon electrode. The
+positively charged electrode must not be considered as merely the
+carbon plate or rod alone, but rather the carbon rod with its
+surrounding mixture of peroxide of manganese and broken carbon. Such
+being the case, it is obvious that the separation between the
+electrodes is very small, while the surface presented by both
+electrodes is very large. As a result, the internal resistance of the
+cell is small and the current which it will give on a short circuit is
+correspondingly large. A good cell of the two and three-quarters by
+six-inch size will give eighteen or twenty amperes on short-circuit,
+when new.
+
+As the action of the cell proceeds, zinc chloride and ammonia are
+formed, and there being insufficient water to dissolve the ammonia,
+there results the formation of double chlorides of zinc and ammonium.
+These double chlorides are less soluble than the chlorides and finally
+occupy the pores of the paper lining between the electrolyte and the
+zinc and greatly increase the internal resistance of the cell. This
+increase of resistance is further contributed to by the gradual drying
+out of the cell as its age increases.
+
+Within the last few years dry batteries have been so perfected
+mechanically, chemically, and electrically that they have far greater
+outputs and better recuperative power than any of the other types of
+LeClanche batteries, while in point of convenience and economy,
+resulting from their small size and non-breakable, non-spillable
+features and low cost, they leave no room for comparison.
+
+_Closed-Circuit Cells_. Gravity-Cell:--Coming now to the consideration
+of closed-circuit or constant-current cells, the most important is the
+well-known gravity, or blue-stone, cell, devised by Daniell. It is
+largely used in telegraphy, and often in telephony in such cases as
+require a constantly flowing current of small quantity. Such a cell is
+shown in Fig. 64.
+
+The elements of the gravity cell are electrodes of copper and zinc.
+The solution in which the copper plate is immersed is primarily a
+solution of copper sulphate, commonly known as blue-stone, in water.
+The zinc plate after the cell is in action is immersed in a solution
+of sulphate of zinc which is formed around it.
+
+The glass jar is usually cylindrical, the standard sizes being 5
+inches diameter and 7 inches deep; and also 6 inches diameter and 8
+inches deep. The copper electrode is of sheet copper of the form
+shown, and it is partly covered with crystals of blue-stone or copper
+sulphate. Frequently, in later forms of cells, the copper electrode
+consists merely of a straight, thick, rectangular bar of copper laid
+horizontally, directly on top of the blue-stone crystals. In all cases
+a rubber-insulated wire is attached by riveting to the copper
+electrode, and passes up through the electrolyte to form the positive
+terminal.
+
+[Illustration: Fig. 64. Gravity Cell]
+
+The zinc is, as a rule, of crowfoot form, as shown, whence this cell
+derives the commonly applied name of _crowfoot cell_. This is
+essentially a two-fluid cell, for in its action zinc sulphate is
+formed, and this being lighter than copper sulphate rises to the top
+of the jar and surrounds the zinc. Gravity, therefore, serves to keep
+the two fluids separate.
+
+[Illustration: INTERIOR OF WAREHOUSE FOR TELEPHONE CONSTRUCTION
+MATERIAL]
+
+In the action of the cell, when the external circuit is closed,
+sulphuric acid is formed which attacks the zinc to form sulphate of
+zinc and to liberate hydrogen, which follows its tendency to attach
+itself to the copper plate. But in so doing the hydrogen necessarily
+passes through the solution of sulphate of copper surrounding the
+copper plate. The hydrogen immediately combines with the SO_{4}
+radical, forming therewith sulphuric acid, and liberating metallic
+copper. This sulphuric acid, being lighter than the copper sulphate,
+rises to the surface of the zinc and attacks the zinc, thus forming
+more sulphate of zinc. The metallic copper so formed is deposited on
+the copper plate, thereby keeping the surface bright and clean. Since
+hydrogen is thus diverted from the copper plate, polarization does not
+ensue.
+
+The zinc sulphate being colorless, while the copper sulphate is of a
+dark blue color, the separating line of the two liquids is easily
+distinguishable. This line is called the _blue line_ and care should
+be taken that it does not reach the zinc and cause a deposit of copper
+to be placed thereon.
+
+As has been stated, these two liquids do not mix readily, but they
+will eventually mingle unless the action of the cell is sufficient to
+use up the copper sulphate as speedily as it is dissolved. Thus it
+will be seen that while the cell is free from polarization and local
+action, there is, nevertheless, a deteriorating effect if the cell is
+allowed to remain long on open circuit. Therefore, it should be used
+when a constant current is required.
+
+Prevention of Creeping:--Much trouble has been experienced in gravity
+cells due to the creeping of the salts over the edge of the jar.
+Frequently the upper edges of the jars are coated by dipping in hot
+paraffin wax in the hope of preventing this. Sometimes oil is poured
+on top of the fluid in the jar to prevent the creeping of the salts
+and the evaporation of the electrolyte. The following account of
+experiments performed by Mr. William Reid, of Chicago, throws light on
+the relative advantages of these and other methods of preventing
+creeping.
+
+     The experiment was made with gravity cells having 5-inch by
+     7-inch glass jars. Four cells were made up and operated in a
+     rather dry, warm place, although perhaps under no more severe
+     local conditions than would be found in most telephone exchanges.
+     Cell No. 1 was a plain cell as ordinarily used. Cell No. 2 had
+     the top of the rim of the jar treated with paraffin wax by
+     dipping the rim to about one inch in depth in melted paraffin
+     wax. Cell No. 3 had melted paraffin wax poured over the surface
+     of the liquid forming a seal about 3/16 inch in thickness. After
+     cooling, a few small holes were bored through the seal to let
+     gases escape. Cell No. 4 had a layer of heavy paraffin oil nearly
+     1/2 inch in thickness (about 6 oz. being used) on top of the
+     solutions.
+
+     These cells were all run on a load of .22 to .29 amperes for
+     15-1/2 hours per day for thirty days, after which the following
+     results were noted:
+
+     (_a_) The plain cell, or cell No. 1, had to have 26 ounces of
+     water added to it to replace that which had evaporated. The
+     creeping of zinc sulphate salts was very bad.
+
+     (_b_) The waxed rim cell, or cell No. 2, evaporated 26 ounces of
+     water and the creeping of zinc sulphate salts was not prevented
+     by the waxed rim. The wax proved of no value.
+
+     (_c_) The wax sealed cell, or cell No. 3, showed practically no
+     evaporation and only very slight creeping of zinc sulphate salts.
+     The creeping of salts that took place was only around spots where
+     the edges of the seal were loose from the jar.
+
+     (_d_) The paraffin oil sealed cell, or cell No. 4, showed no
+     evaporation and no creeping of salts.
+
+It was concluded by Mr. Reid from the above experiments that the wax
+applied to the rim of the jar is totally ineffective and has no
+merits. The wax seal loosens around the edges and does not totally
+prevent creeping of the zinc sulphate salts, although nearly so. The
+wax-sealed jar must have holes drilled in it to allow the gases to
+escape. The method is hardly commercial, as it is difficult to make a
+neat appearing cell, besides making it almost impossible to manipulate
+its contents. A coat of paraffin oil approximately 1/2 inch in
+thickness (about 6 ounces) gives perfect protection against
+evaporation and creeping of the zinc sulphate salts. The cell, having
+the paraffin-oil seal, had a very neat, clean appearance as compared
+with cells No. 1 and No. 2. It was found that the zinc could be drawn
+out through the oil, cleaned, and replaced with no appreciable effect
+on voltage or current.
+
+Setting Up:--In setting up the battery the copper electrode is first
+unfolded to form a cross and placed in the bottom of the jar. Enough
+copper sulphate, or blue-stone crystals, is then dropped into the jar
+to almost cover the copper. The zinc crowfoot is then hung in place,
+occupying a position about 4 inches above the top of the copper. Clear
+water is then poured in sufficient to fill the jar within about an
+inch of the top.
+
+If it is not required to use the cell at once, it may be placed on
+short circuit for a time and allowed to form its own zinc sulphate.
+The cell may, however, be made immediately available for use by
+drawing about one-half pint of a solution of zinc sulphate from a
+cell already in use and pouring it into the jar, or, when this is not
+convenient, by putting into the liquid four or five ounces of
+pulverized sulphate of zinc, or by adding about ten drops of sulphuric
+acid. When the cell is in proper working condition, one-half inch in
+thickness of heavy paraffin oil of good quality may be added.
+
+If the blue line gets too low, and if there is in the bottom of the
+cell a sufficient quantity of sulphate of copper, it may be raised by
+drawing off a portion of the zinc sulphate with a battery syringe and
+replacing this with water. If the blue line gets too high, it may be
+lowered by short-circuiting the cell for a time, or by the addition of
+more sulphate of zinc solution from another battery. If the copper
+sulphate becomes exhausted, it should be replenished by dropping in
+more crystals.
+
+Care should be taken in cold weather to maintain the temperature of
+the battery above 65 deg. or 70 deg. Fahrenheit. If below this temperature,
+the internal resistance of a cell increases very rapidly, so much so
+that even at 50 deg. Fahrenheit the action becomes very much impaired.
+This follows from the facts that the resistance of a liquid decreases
+as its temperature rises, and that chemical action is much slower at
+lower temperatures.
+
+The gravity cell has a practically constant voltage of 1.08 volts. Its
+internal resistance is comparatively high, seldom falling below 1 ohm
+and often rising to 6 ohms. At best, therefore, it is only capable of
+producing about 1 ampere. The gravity cell is perhaps the most common
+type of cell wherein depolarization is affected by electro-chemical
+means.
+
+Fuller Cell:--A form of cell that is adapted to very heavy
+open-circuit work and also closed-circuit work where heavier currents
+are required than can be supplied by the gravity battery is the
+Fuller. In this the electrodes are of zinc and carbon, respectively,
+the zinc usually being in the form of a heavy cone and placed within a
+porous cup. The electrolyte of the Fuller cell is known as
+_electropoion fluid_, and consists of a mixture of sodium or potassium
+bichromate, sulphuric acid, and water.
+
+The various parts of the standard Fuller cell, as once largely
+employed by the various Bell operating companies, are shown in Fig.
+65. In this the jar was made of flint glass, cylindrical in form, six
+inches in diameter and eight inches deep. It is important that a good
+grade of glass be used for the jar in this cell, because, on account
+of the nature of the electrolyte, breakage is disastrous in the
+effects it may produce on adjacent property. The carbon plate is
+rectangular in form, about four inches wide, eight and three-quarters
+inches long, and one-quarter inch thick. The metal terminal at the top
+of the carbon block is of bronze, both it and the lock nuts and bolts
+being nickel-plated to minimize corrosion. The upper end of the carbon
+block is soaked in paraffin so hot as to drive all of the moisture out
+of the paraffin and out of the pores of the block itself.
+
+The zinc, as is noted from the cut, is in the form of a truncated
+cone. It is about two and one-eighth inches in diameter at the base
+and two and one-half inches high. Cast into the zinc is a soft copper
+wire about No. 12 B. & S. gauge. This wire extends above the top of
+the jar so as to form a convenient terminal for the cell.
+
+The porous cup is cylindrical in form, about three inches in diameter
+and seven inches deep. The wooden cover is of kiln-dried white wood
+thoroughly coated with two coats of asphalt paint. It is provided with
+a slot for the carbon and a hole for the copper wire extending to the
+zinc.
+
+The electrolyte for this cell is made as follows:
+
+        Sodium bichromate    6 oz.
+        Sulphuric acid      17 oz.
+        Soft water          56 oz.
+
+This solution is mixed by dissolving the bichromate of sodium in the
+water and then adding slowly the sulphuric acid. Potassium bichromate
+may be substituted for the sodium bichromate.
+
+In setting up this cell, the amalgamated zinc is placed within the
+porous cup, in the bottom of which are about two teaspoonfuls of
+mercury, the latter serving to keep the zinc well amalgamated. The
+porous cup is then placed in the glass jar and a sufficient quantity
+of the electrolyte is placed in the outer jar to come within about one
+and one-half inches of the top of the porous cup. About two
+teaspoonfuls of salt are then placed in the porous cup and sufficient
+soft water added to bring the level of the liquid within the porous
+cup even with the level of the electrolyte in the jar surrounding the
+cup. The carbon is then placed through the slot in the cover, and the
+wire from the zinc is passed through the hole in the cover provided
+for it, and the cover is allowed to fall in place. The cell is now
+ready for immediate use.
+
+The action of this cell is as follows: The sulphuric acid attacks the
+zinc and forms zinc sulphate, liberating hydrogen. The hydrogen
+attempts to pass to the carbon plate as usual, but in so doing it
+meets with the oxygen of the chromic acid and forms water therewith.
+The remainder of the chromic acid combines with the sulphuric acid to
+form chromium sulphate.
+
+[Illustration: Fig 65. Fuller Cell]
+
+The mercury placed in the bottom of the porous cup with the zinc keeps
+the zinc in a state of perpetual amalgamation. This it does by
+capillary action, as the mercury spreads over the entire surface of
+the zinc. The initial amalgamation, while not absolutely essential,
+helps in a measure this capillary action.
+
+In another well-known type of the Fuller battery the carbon is a
+hollow cylinder, surrounding the porous cup. In this type the zinc
+usually took the form of a long bar having a cross-shaped section, the
+length of this bar being sufficient to extend the entire depth of the
+porous cup. This type of cell has the advantage of a somewhat lower
+internal resistance than the standard form just described.
+
+Should the electrolyte become supersaturated by virtue of the battery
+being neglected or too heavily overworked, a set of secondary
+reactions will occur in the cell, resulting in the formation of the
+yellow crystals upon the carbon. This seriously affects the e.m.f. of
+the cell and also its internal resistance. Should this occur, some of
+the solution should be withdrawn and dilute sulphuric acid inserted in
+its place and the crystals which have formed on the carbon should be
+carefully washed off. Should the solution lose its orange tint and
+turn blue, it indicates that more bichromate of potash or bichromate
+of sodium is needed. This cell gives an electromotive force of 2.1
+volts and a very large current when it is in good condition, since its
+internal resistance is low.
+
+The Fuller cell was once largely used for supplying current to
+telephone transmitters at subscribers' stations, where very heavy
+service was demanded, but the advent of the so-called common-battery
+systems, in some cases, and of the high-resistance transmitter, in
+other cases, has caused a great lessening in its use. This is
+fortunate as the cell is a "dirty" one to handle and is expensive to
+maintain.
+
+The Fuller cell still warrants attention, however, as an available
+source of current, which may be found useful in certain cases of
+emergency work, and in supplying special but temporary needs for
+heavier current than the LeClanche or gravity cell can furnish.
+
+Lalande Cell:--A type of cell, specially adapted to constant-current
+work, and sometimes used as a central source of current in very small
+common-battery exchanges is the so-called _copper oxide_, or _Lalande
+cell_, of which the Edison and the Gordon are types. In all of these
+the negatively charged element is of zinc, the positively charged
+element a mass of copper oxide, and the electrolyte a solution of
+caustic potash in water. In the Edison cell the copper oxide is in the
+form of a compressed slab which with its connecting copper support
+forms the electrode. In the Gordon and other cells of this type the
+copper oxide is contained loosely in a perforated cylinder of sheet
+copper. The copper oxide serves not only as an electrode, but also as
+a depolarizing agent, the liberated hydrogen in the electrolyte
+uniting with the oxygen of the copper oxide to form water, and leaving
+free metallic copper.
+
+On open circuit the elements are not attacked, therefore there is no
+waste of material while the cell is not in use. This important
+feature, and the fact that the internal resistance is low, make this
+cell well adapted for all forms of heavy open-circuit work. The fact
+that there is no polarizing action within the cell makes it further
+adaptable to heavy closed-circuit service.
+
+These cells are intended to be so proportioned that all of their parts
+become exhausted at once so that when the cell fails, complete
+renewals are necessary. Therefore, there is never a question as to
+which of the elements should be renewed.
+
+After the elements and solution are in place about one-fourth of an
+inch of heavy paraffin oil is poured upon the surface of the solution
+in order to prevent evaporation. This cell requires little attention
+and will maintain a constant e.m.f. of about two-thirds of a volt
+until completely exhausted. It is non-freezable at all ordinary
+temperatures. Its low voltage is its principal disadvantage.
+
+_Standard Cell_. Chloride of Silver Cell:--The chloride of silver cell
+is largely used as a standard for testing purposes. Its compactness
+and portability and its freedom from local action make it particularly
+adaptable to use in portable testing outfits where constant
+electromotive force and very small currents are required.
+
+[Illustration: Fig. 66. Chloride of Silver Cell]
+
+A cross-section of one form of the cell is shown in Fig. 66. Its
+elements are a rod of chemically-pure zinc and a rod of chloride of
+silver immersed in a water solution of sal ammoniac. As ordinarily
+constructed, the glass jar or tube is usually about 2-1/2 inches long
+by 1 inch in diameter. After the solution is poured in and the
+elements are in place the glass tube is hermetically sealed with a
+plug of paraffin wax.
+
+The e.m.f. of a cell of this type is 1.03 volts and the external
+resistance varies with the age of the cell, being about 4 ohms at
+first. Care should be taken not to short-circuit these cells, or use
+them in any but high-resistance circuits, as they have but little
+energy and become quickly exhausted if compelled to work in
+low-resistance circuits.
+
+Conventional Symbol. The conventional symbol for a cell, either of
+the primary or the secondary type, consists of a long thin line and a
+short heavy line side by side and parallel. A battery is represented
+by a number of pairs of such lines, as in Fig. 67. The two lines of
+each pair are supposed to represent the two electrodes of a cell.
+Where any significance is to be placed on the polarity of the cell or
+battery the long thin line is supposed to represent the positively
+charged plate and the short thick line the negatively charged plate.
+The number of pairs may indicate the number of cells in the battery.
+Frequently, however, a few pairs of such lines are employed merely for
+the purpose of indicating a battery without regard to its polarity or
+its number of cells.
+
+[Illustration: Fig. 67. Battery Symbols]
+
+In Fig. 67 the representation at _A_ is that of a battery of a number
+of cells connected in parallel; that at _B_ of a battery with the
+cells connected in series; and that at _C_ of a battery with one of
+its poles grounded.
+
+
+
+
+CHAPTER VIII
+
+MAGNETO SIGNALING APPARATUS
+
+
+Method of Signaling. The ordinary apparatus, by which speech is
+received telephonically, is not capable of making sufficiently loud
+sounds to attract the attention of people at a distance from the
+instrument. For this reason it is necessary to employ auxiliary
+apparatus for the purpose of signaling between stations. In central
+offices where an attendant is always on hand, the sense of sight is
+usually appealed to by the use of signals which give a visual
+indication, but in the case of telephone instruments for use by the
+public, the sense of hearing is appealed to by employing an audible
+rather than a visual signal.
+
+Battery Bell. The ordinary vibrating or battery bell, such as is
+employed for door bells, is sometimes, though not often, employed in
+telephony. It derives its current from primary batteries or from any
+direct-current source. The reason why they are not employed to a
+greater extent in telephony is that telephone signals usually have to
+be sent over lines of considerable length and the voltage that would
+be required to furnish current to operate such bells over such lengths
+of line is higher than would ordinarily be found in the batteries
+commonly employed in telephone work. Besides this the make-and-break
+contacts on which the, ordinary battery bell depends for its operation
+are an objectionable feature from the standpoint of maintenance.
+
+Magneto Bell. Fortunately, however, there has been developed a
+simpler type of electric bell, which operates on smaller currents, and
+which requires no make-and-break contacts whatever. This simpler form
+of bell is commonly known as the _polarized_, or _magneto_, bell or
+_ringer_. It requires for its operation, in its ordinary form, an
+alternating current, though in its modified forms it may be used with
+pulsating currents, that is, with periodically recurring impulses of
+current always in the same direction.
+
+Magneto Generator. In the early days of telephony there was nearly
+always associated with each polarized bell a magneto generator for
+furnishing the proper kind of current to ring such bells. Each
+telephone was therefore equipped, in addition to the transmitter and
+receiver, with a signal-receiving device in the form of a polarized
+bell, and with a current generator by which the user was enabled to
+develop his own currents of suitable kind and voltage for ringing the
+bells of other stations.
+
+Considering the signaling apparatus of the telephones alone,
+therefore, each telephone was equipped with a power plant for
+generating currents used by that station in signaling other stations,
+the prime mover being the muscles of the user applied to the turning
+of a crank on the side of the instrument; and also with a
+current-consuming device in the form of a polarized electromagnetic
+bell adapted to receive the currents generated at other stations and
+to convert a portion of their energy into audible signals.
+
+The magneto generator is about the simplest type of dynamo-electric
+machine, and it depends upon the same principles of operation as the
+much larger generators, employed in electric-lighting and
+street-railway power plants, for instance. Instead of developing the
+necessary magnetic field by means of electromagnets, as in the case of
+the ordinary dynamo, the field of the magneto generator is developed
+by permanent magnets, usually of the horseshoe form. Hence the name
+_magneto_.
+
+[Illustration: Fig. 68. Principles of Magneto Generator]
+
+In order to concentrate the magnetic field within the space in which
+the armature revolves, pole pieces of iron are so arranged in
+connection with the poles of the permanent magnet as to afford a
+substantially cylindrical space in which the armature conductors may
+revolve and through which practically all the magnetic lines of force
+set up by the permanent magnets will pass. In Fig. 68 there is shown,
+diagrammatically, a horseshoe magnet with such a pair of pole pieces,
+between which a loop of wire is adapted to rotate. The magnet _1_ is
+of hardened steel and permanently magnetized. The pole pieces are
+shown at _2_ and _3_, each being of soft iron adapted to make good
+magnetic contact on its flat side with the inner flat surface of the
+bar magnet, and being bored out so as to form a cylindrical recess
+between them as indicated. The direction of the magnetic lines of
+force set up by the bar magnet through the interpolar space is
+indicated by the long horizontal arrows, this flow being from the
+north pole (N) to the south pole (S) of the magnet. At _4_ there is
+shown a loop of wire supposed to revolve in the magnetic field of
+force on the axis _5-5_.
+
+Theory. In order to understand how currents will be generated in
+this loop of wire _4_, it is only necessary to remember that if a
+conductor is so moved as to cut across magnetic lines of force, an
+electromotive force will be set up in the conductor which will tend to
+make the current flow through it. The magnitude of the electromotive
+force will depend on the rate at which the conductor cuts through the
+lines of force, or, in other words, on the number of lines of force
+that are cut through by the conductor in a given unit of time. Again,
+the direction of the electromotive force depends on the direction of
+the cutting, so that if the conductor be moved in one direction across
+the lines of force, the electromotive force and the current will be in
+one direction; while if it moves in the opposite direction across the
+lines of force, the electromotive force and the current will be in the
+reverse direction.
+
+It is, evident that as the loop of wire _4_ revolves in the field of
+force about the axis _5-5_, the portions of the conductor parallel to
+the axis will cut through the lines of force, first in one direction
+and then in the other, thus producing electromotive forces therein,
+first in one direction and then in the other.
+
+Referring now to Fig. 68, and supposing that the loop _4_ is revolving
+in the direction of the curved arrow shown between the upper edges of
+the pole pieces, it will be evident that just as the loop stands in
+the vertical position, its horizontal members will be moving in a
+horizontal direction, parallel with the lines of force and, therefore,
+not cutting them at all. The electromotive force and the current will,
+therefore, be zero at this time.
+
+As the loop advances toward the position shown in dotted lines, the
+upper portion of the loop that is parallel with the axis will begin to
+cut downwardly through the lines of force, and likewise the lower
+portion of the loop that is parallel with the axis will begin to cut
+upwardly through the lines of force. This will cause electromotive
+forces in opposite directions to be generated in these portions of the
+loop, and these will tend to aid each other in causing a current to
+circulate in the loop in the direction shown by the arrows associated
+with the dotted representation of the loop. It is evident that as the
+motion of the loop progresses, the rate of cutting the lines of force
+will increase and will be a maximum when the loop reaches a horizontal
+position, or at that time the two portions of the loop that are
+parallel with the axis will be traveling at right angles to the lines
+of force. At this point, therefore, the electromotive force and the
+current will be a maximum.
+
+From this point until the loop again assumes a vertical position, the
+cutting of the lines of force will still be in the same direction, but
+at a constantly decreasing rate, until, finally, when the loop is
+vertical the movement of the parts of the loop that are parallel with
+the axis will be in the direction of the lines of force and,
+therefore, no cutting will take place. At this point, therefore, the
+electromotive force and the current in the loop again will be zero. We
+have seen, therefore, that in this half revolution of the loop from
+the time when it was in a vertical position to a time when it was
+again in a vertical position but upside down, the electromotive force
+varied from zero to a maximum and back to zero, and the current did
+the same.
+
+It is easy to see that, as the loop moves through the next half
+revolution, an exactly similar rise and fall of electromotive force
+and current will take place; but this will be in the opposite
+direction, since that portion of the loop which was going down through
+the lines of force is now going up, and the portion which was
+previously going up is now going down.
+
+The law concerning the generation of electromotive force and current
+in a conductor that is cutting through lines of magnetic force, may be
+stated in another way, when the conductor is bent into the form of a
+loop, as in the case under consideration: Thus, _if the number of
+lines of force which pass through a conducting loop be varied,
+electromotive forces will be generated in the loop_. This will be true
+whether the number of lines passing through the loop be varied by
+moving the loop within the field of force or by varying the field of
+force itself. In any case, _if the number of lines of force be
+increased, the current will flow in one way, and if it be diminished
+the current will flow in the other way_. The amount of the current
+will depend, other things being equal, on the rate at which the lines
+of force through the loop are being varied, regardless of the method
+by which the variation is made to take place. One revolution of the
+loop, therefore, results in a complete cycle of alternating current
+consisting of one positive followed by one negative impulse.
+
+The diagram of Fig. 68 is merely intended to illustrate the principle
+involved. In the practical construction of magneto generators more
+than one bar magnet is used, and, in addition, the conductors in the
+armature are so arranged as to include a great many loops of wire.
+Furthermore, the conductors in the armature are wound around an iron
+core so that the path through the armature loops or turns, may present
+such low reluctance to the passage of lines of force as to greatly
+increase the number of such lines and also to cause practically all of
+them to go through the loops in the armature conductor.
+
+Armature. The iron upon which the armature conductors are wound is
+called the _core_. The core of an ordinary armature is shown in Fig.
+69. This is usually made of soft gray cast iron, turned so as to form
+bearing surfaces at _1_ and _2_, upon which the entire armature may
+rotate, and also turned so that the surfaces _3_ will be truly
+cylindrical with respect to the axis through the center of the shaft.
+The armature conductors are put on by winding the space between the
+two parallel faces _4_ as full of insulated wire as space will admit.
+One end of the armature winding is soldered to the pin _5_ and,
+therefore, makes contact with the frame of the generator, while the
+other end of the winding is soldered to the pin _6_, which engages the
+stud _7_, carried in an insulating bushing in a longitudinal hole in
+the end of the armature shaft. It is thus seen that the frame of the
+machine will form one terminal of the armature winding, while the
+insulated stud _7_ will form the other terminal.
+
+[Illustration: Fig. 69. Generator Armature]
+
+Another form of armature largely employed in recent magneto
+generators is illustrated in Fig. 70. In this the shaft on which the
+armature revolves does not form an integral part of the armature core
+but consists of two cylindrical studs _2_ and _3_ projecting from the
+centers of disks _4_ and _5_, which are screwed to the ends of the
+core _1_. This =H= type of armature core, as it is called, while
+containing somewhat more parts than the simpler type shown in Fig. 69,
+possesses distinct advantages in the matter of winding. By virtue of
+its simpler form of winding space, it is easier to insulate and easier
+to wind, and furthermore, since the shaft does not run through the
+winding space, it is capable of holding a considerably greater number
+of turns of wire. The ends of the armature winding are connected, one
+directly to the frame and the other to an insulated pin, as is shown
+in the illustration.
+
+[Illustration: Fig. 70. Generator Armature]
+
+[Illustration: Fig. 71. Generator Field and Armature]
+
+The method commonly employed of associating the pole pieces with each
+other and with the permanent magnets is shown in Fig. 71. It is very
+important that the space in which the armature revolves shall be truly
+cylindrical, and that the bearings for the armature shall be so
+aligned as to make the axis of rotation of the armature coincide with
+the axis of the cylindrical surface of the pole pieces. A rigid
+structure is, therefore, required and this is frequently secured, as
+shown in Fig. 71, by joining the two pole pieces _1_ and _2_ together
+by means of heavy brass rods _3_ and _4_, the rods being shouldered
+and their reduced ends passed through holes in flanges extending from
+the pole pieces, and riveted. The bearing plates in which the armature
+is journaled are then secured to the ends of these pole pieces, as
+will be shown in subsequent illustrations. This assures proper
+rigidity between the pole pieces and also between the pole pieces and
+the armature bearings.
+
+The reason why this degree of rigidity is required is that it is
+necessary to work with very small air gaps between the armature core
+and its pole pieces and unless these generators are mechanically well
+made they are likely to alter their adjustment and thus allow the
+armature faces to scrape or rub against the pole pieces. In Fig. 71
+one of the permanent horseshoe magnets is shown, its ends resting in
+grooves on the outer faces of the pole pieces and usually clamped
+thereto by means of heavy iron machine screws.
+
+With this structure in mind, the theory of the magneto generator
+developed in connection with Fig. 68 may be carried a little further.
+When the armature lies in the position shown at the left of Fig. 71,
+so that the center position of the core is horizontal, a good path is
+afforded for the lines of force passing from one pole to the other.
+Practically all of these lines will pass through the iron of the core
+rather than through the air, and, therefore, practically all of them
+will pass through the convolutions of the armature winding.
+
+When the armature has advanced, say 45 degrees, in its rotation in the
+direction of the curved arrow, the lower right-hand portion of the
+armature flange will still lie opposite the lower face of the
+right-hand pole piece and the upper left-hand portion of the armature
+flange will still lie opposite the upper face of the left-hand pole
+piece. As a result there will still be a good path for the lines of
+force through the iron of the core and comparatively little change in
+the number of lines passing through the armature winding. As the
+corners of the armature flange pass away from the corners of the pole
+pieces, however, there is a sudden change in condition which may be
+best understood by reference to the right-hand portion of Fig. 71. The
+lines of force now no longer find path through the center portion of
+the armature core--that lying at right angles to their direction of
+flow. Two other paths are at this time provided through the now
+horizontal armature flanges which serve almost to connect the two pole
+pieces. The lines of force are thus shunted out of the path through
+the armature coils and there is a sudden decrease from a large number
+of lines through the turns of the winding to almost none. As the
+armature continues in its rotation the two paths through the flanges
+are broken, and the path through the center of the armature core and,
+therefore, through the coils themselves, is reestablished.
+
+As a result of this consideration it will be seen that in actual
+practice the change in the number of lines passing through the
+armature winding is not of the gradual nature that would be indicated
+by a consideration of Fig. 68 alone, but rather, is abrupt, as the
+corners of the armature flanges leave the corners of the pole pieces.
+This abrupt change produces a sudden rise in electromotive force just
+at these points in the rotation, and, therefore, the electromotive
+force and the current curves of these magneto generators is not
+usually of the smooth sine-wave type but rather of a form resembling
+the sine wave with distinct humps added to each half cycle.
+
+[Illustration: Fig. 72. Generator with Magnets Removed]
+
+As is to be expected from any two-pole alternating generator, there is
+one cycle of current for each revolution of the armature. Under
+ordinary conditions a person is able to turn the generator handle at
+the rate of about two hundred revolutions a minute, and as the ratio
+of gearing is about five to one, this results in about one thousand
+revolutions per minute of the generator, and, therefore, in a
+current of about one thousand cycles per minute, this varying
+widely according to the person who is doing the turning.
+
+[Illustration: HOWARD OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO
+An All-Concrete Building Serving the District South of
+Market Street.]
+
+The end plates which support the bearings for the armature are usually
+extended upwardly, as shown in Fig. 72, so as to afford bearings for
+the crank shaft. The crank shaft carries a large spur gear which
+meshes with a pinion in the end of the armature shaft, so that the
+user may cause the armature to revolve rapidly. The construction shown
+in Fig. 72 is typical of that of a modern magneto generator, it being
+understood that the permanent magnets are removed for clearness of
+illustration.
+
+Fig. 73 is a view of a completely assembled generator such as is used
+for service requiring a comparatively heavy output. Other types of
+generators having two, three, or four permanent magnets instead of
+five, as shown in this figure, are also standard.
+
+[Illustration: Fig. 73. Five-Bar Generator]
+
+Referring again to Fig. 69, it will be remembered that one end of the
+armature winding shown diagrammatically in that figure, is terminated
+in the pin _5_, while the other terminates in the pin _7_. When the
+armature is assembled in the frame of the generator it is evident that
+the frame itself is in metallic connection with one end of the
+armature winding, since the pin _5_ is in metallic contact with the
+armature casting and this is in contact with the frame of the
+generator through the bearings. The frame of the machine is,
+therefore, one terminal of the generator. When the generator is
+assembled a spring of one form or another always rests against the
+terminal pin _7_ of the armature so as to form a terminal for the
+armature winding of such a nature as to permit the armature to rotate
+freely. Such spring, therefore, forms the other terminal of the
+generator.
+
+Automatic Shunt. Under nearly all conditions of practice it is
+desirable to have the generator automatically perform some switching
+function when it is operated. As an example, when the generator is
+connected so that its armature is in series in a telephone line, it is
+quite obvious that the presence of the resistance and the impedance of
+the armature winding would be objectionable if left in the circuit
+through which the voice currents had to pass. For this reason, what is
+termed an _automatic shunt_ is employed on generators designed for
+series work; this shunt is so arranged that it will automatically
+shunt or short-circuit the armature winding when it is at rest and
+also break this shunt when the generator is operated, so as to allow
+the current to pass to line.
+
+[Illustration: Fig 74. Generator Shunt Switch]
+
+A simple and much-used arrangement for this purpose is shown in Fig.
+74, where _1_ is the armature; _2_ is a wire leading from the frame of
+the generator and forming one terminal of the generator circuit; and
+_3_ is a wire forming the other terminal of the generator circuit,
+this wire being attached to the spring _4_, which rests against the
+center pin of the armature so as to make contact with the opposite end
+of the armature winding to that which is connected with the frame. The
+circuit through the armature may be traced from the terminal wire _2_
+through the frame; thence through the bearings to the armature _1_ and
+through the pin to the right-hand side of the armature winding.
+Continuing the circuit through the winding itself, it passes to the
+center pin projecting from the left-hand end of the armature shaft;
+thence to the spring _4_ which rests against this pin; and thence to
+the terminal wire _3_.
+
+Normally, this path is shunted by what is practically a short circuit,
+which may be traced from the terminal _2_ through the frame of the
+generator to the crank shaft _5_; thence to the upper end of the
+spring _4_ and out by the terminal wire _3_. This is the condition
+which ordinarily exists and which results in the removal of the
+resistance and the impedance on the armature winding from any circuit
+in which the generator is placed, as long as the generator is not
+operated.
+
+An arrangement is provided, however, whereby the crank shaft _5_ will
+be withdrawn automatically from engaging with the upper end of the
+spring _4_, thus breaking the shunt around the armature circuit,
+whenever the generator crank is turned. In order to accomplish this
+the crank shaft _5_ is capable of partial rotation and of slight
+longitudinal movement within the hub of the large gear wheel. A spring
+7 usually presses the crank shaft toward the left and into engagement
+with the spring _4_. A pin _8_ carried by the crank shaft, rests in a
+V-shaped notch in the end of the hub _6_ and as a result, when the
+crank is turned the pin rides on the surface of this notch before the
+large gear wheel starts to turn, and thus moves the crank shaft _5_ to
+the right and breaks the contact between it and the spring _4_. Thus,
+as long as the generator is being operated, its armature is connected
+in the circuit of the line, but as soon as it becomes idle the
+armature is automatically short-circuited. Such devices as this are
+termed _automatic shunts_.
+
+In still other cases it is desirable to have the generator circuit
+normally open so that it will not affect in any way the electrical
+characteristics of the line while the line is being used for talking.
+In this case the arrangement is made so that the generator will
+automatically be placed in proper circuit relation with the line when
+it is operated.
+
+[Illustration: Fig. 75. Generator Cut-in Switch]
+
+A common arrangement for doing this is shown in Fig. 75, wherein the
+spring _1_ normally rests against the contact pin of the armature and
+forms one terminal of the armature circuit. The spring _2_ is adapted
+to form the other terminal of the armature circuit but it is normally
+insulated from everything. The circuit of the generator is, therefore,
+open between the spring _2_ and the shaft _3_, but as soon as the
+generator is operated the crank shaft is bodily moved to the left by
+means of the =V=-shaped notch in the driving collar _4_ and is thus
+made to engage the spring _2_. The circuit of the generator is then
+completed from the spring _1_ through the armature pin to the armature
+winding; thence to the frame of the machine and through shaft _3_ to
+the spring _2_. Such devices as this are largely used in connection
+with so-called "bridging" telephones in which the generators and bells
+are adapted to be connected in multiple across the line.
+
+A better arrangement for accomplishing the automatic switching on the
+part of the generator is to make no use of the crank shaft as a part
+of the conducting path as is the case in both Figs. 74 and 75, but to
+make the crank shaft, by its longitudinal movement, impart the
+necessary motion to a switch spring which, in turn, is made to engage
+or disengage a corresponding contact spring. An arrangement of this
+kind that is in common use is shown in Fig. 76. This needs no further
+explanation than to say that the crank shaft is provided on its end
+with an insulating stud _1_, against which a switching spring _2_
+bears. This spring normally rests against another switch spring _3_,
+but when the generator crank shaft moves to the right upon the turning
+of the crank, the spring _2_ disengages spring _3_ and engages spring
+_4_, thus completing the circuit of the generator armature. It is seen
+that this operation accomplishes the breaking of one circuit and the
+making of another, a function that will be referred to later on in
+this work.
+
+[Illustration: Fig. 76. Generator Cut-in Switch]
+
+Pulsating Current. Sometimes it is desirable to have a generator
+capable of developing a pulsating current instead of an alternating
+current; that is, a current which will consist of impulses all in one
+direction rather than of impulses alternating in direction. It is
+obvious that this may be accomplished if the circuit of the generator
+be broken during each half revolution so that its circuit is completed
+only when current is being generated in one direction.
+
+Such an arrangement is indicated diagrammatically in Fig. 77. Instead
+of having one terminal of the armature winding brought out through the
+frame of the generator as is ordinarily done, both terminals are
+brought out to a commuting device carried on the end of the armature
+shaft. Thus, one end of the loop representing the armature winding is
+shown connected directly to the armature pin _1_, against which bears
+a spring _2_, in the usual manner. The other end of the armature
+winding is carried directly to a disk _3_, mounted _on_ but insulated
+_from_ the shaft and revolving therewith. One-half of the
+circumferential surface of this disk is of insulating material _4_ and
+a spring _5_ rests against this disk and bears alternately upon the
+conducting portion _3_ or the insulating portion _4_, according to the
+position of the armature in its revolution. It is obvious that when
+the generator armature is in the position shown the circuit through it
+is from the spring _2_ to the pin _1_; thence to one terminal of the
+armature loop; thence through the loop and back to the disk _3_ and
+out by the spring _5_. If, however, the armature were turned slightly,
+the spring _5_ would rest on the insulating portion _4_ and the
+circuit would be broken.
+
+[Illustration: Fig. 77. Pulsating-Current Commutator]
+
+[Illustration: Fig. 78. Generator Symbols]
+
+It is obvious that if the brush _5_ is so disposed as to make contact
+with the disk _3_ only during that portion of the revolution while
+positive current is being generated, the generator will produce
+positive pulsations of current, all the negative ones being cut out.
+If, on the other hand, the spring _5_ may be made to bear on the
+opposite side of the disk, then it is evident that the positive
+impulses would all be cut out and the generator would develop only
+negative impulses. Such a generator is termed a "direct-current"
+generator or a "pulsating-current" generator.
+
+The symbols for magneto or hand generators usually embody a simplified
+side view, showing the crank and the gears on one side and the
+shunting or other switching device on the other. Thus in Fig. 78 are
+shown three such symbols, differing from each other only in the
+details of the switching device. The one at the left shows the simple
+shunt, adapted to short-circuit the generator at all times save when
+it is in operation. The one in the center shows the cut-in, of which
+another form is described in connection with Fig. 75; while the symbol
+at the right of Fig. 78 is of the make-and-break device, discussed in
+connection with Fig. 76. In such diagrammatic representations of
+generators it is usual to somewhat exaggerate the size of the
+switching springs, in order to make clear their action in respect to
+the circuit connections in which the generator is used.
+
+Polarized Ringer. The polarized bell or ringer is, as has been
+stated, the device which is adapted to respond to the currents sent
+out by the magneto generator. In order that the alternately opposite
+currents may cause the armature to move alternately in opposite
+directions, these bells are polarized, _i.e._, given a definite
+magnetic set, so to speak; so the effect of the currents in the coils
+is not to create magnetism in normally neutral iron, but rather to
+alter the magnetism in iron already magnetized.
+
+_Western Electric Ringer._ A typical form of polarized bell is shown
+in Fig. 79, this being the standard bell or ringer of the Western
+Electric Company. The two electromagnets are mounted side by side, as
+shown, by attaching their cores to a yoke piece _1_ of soft iron. This
+yoke piece also carries the standards _2_ upon which the gongs are
+mounted. The method of mounting is such that the standards may be
+adjusted slightly so as to bring the gongs closer _to_ or farther
+_from_, the tapper.
+
+The soft iron yoke piece _1_ also carries two brass posts _3_ which,
+in turn, carry another yoke _4_ of brass. In this yoke _4_ is pivoted,
+by means of trunnion screws, the armature _5_, this extending on each
+side of the pivot so that its ends lie opposite the free poles of the
+electromagnets. From the center of the armature projects the tapper
+rod carrying the ball or striker which plays between the two gongs.
+
+In order that the armature and cores may be normally polarized, a
+permanent magnet _6_ is secured to the center of the yoke piece _1_.
+This bends around back of the electromagnets and comes into close
+proximity to the armature _5_. By this means one end of each of the
+electromagnet cores is given one polarity--say north--while the
+armature is given the other polarity--say south. The two coils of the
+electromagnet are connected together in series in such a way that
+current in a given direction will act to produce a north pole in one
+of the free poles and a south pole in the other. If it be assumed that
+the permanent magnet maintains the armature normally of south polarity
+and that the current through the coils is of such direction as to make
+the left-hand core north and the right-hand core south, then it is
+evident that the left-hand end of the armature will be attracted and
+the right-hand end repelled. This will throw the tapper rod to the
+right and sound the right-hand bell. A reversal in current will
+obviously produce the opposite effect and cause the tapper to strike
+the left-hand bell.
+
+An important feature in polarized bells is the adjustment between the
+armature and the pole pieces. This is secured in the Western Electric
+bell by means of the nuts _7_, by which the yoke _4_ is secured to the
+standards _3_. By moving these nuts up or down on the standards the
+armature may be brought closer _to_ or farther _from_ the poles, and
+the device affords ready means for clamping the parts into any
+position to which they may have been adjusted.
+
+[Illustration: Fig. 79. Polarized Bell]
+
+_Kellogg Ringer._ Another typical ringer is that of the Kellogg
+Switchboard and Supply Company, shown in Fig. 80. This differs from
+that of the Western Electric Company mainly in the details by which
+the armature adjustment is obtained. The armature supporting yoke _1_
+is attached directly to the cores of the magnets, no supporting side
+rods being employed. Instead of providing means whereby the armature
+may be adjusted toward or from the poles, the reverse practice is
+employed, that is, of making the poles themselves extensible. This is
+done by means of the iron screws _2_ which form extensions of the
+cores and which may be made to approach or recede from the armature by
+turning them in such direction as to screw them in or out of the core
+ends.
+
+[Illustration: Fig. 80. Polarized Bell]
+
+[Illustration: Fig. 81. Biased Bell]
+
+_Biased Bell._ The pulsating-current generator has already been
+discussed and its principle of operation pointed out in connection
+with Fig. 77. The companion piece to this generator is the so-called
+biased ringer. This is really nothing but a common alternating-current
+polarized ringer with a light spring so arranged as to hold the
+armature normally in one of its extreme positions so that the tapper
+will rest against one of the gongs. Such a ringer is shown in Fig. 81
+and needs no further explanation. It is obvious that if a current
+flows in the coils of such a ringer in a direction tending to move the
+tapper toward the left, then no sound will result because the tapper
+is already moved as far as it can be in that direction. If, however,
+currents in the opposite direction are caused to flow through the
+windings, then the electromagnetic attraction on the armature will
+overcome the pull of the spring and the tapper will move over and
+strike the right-hand gong. A cessation of the current will allow the
+spring to exert itself and throw the tapper back into engagement with
+the left-hand gong. A series of such pulsations in the proper
+direction will, therefore, cause the tapper to play between the two
+gongs and ring the bell as usual. A series of currents in a wrong
+direction will, however, produce no effect.
+
+Conventional Symbols. In Fig. 82 are shown six conventional symbols
+of polarized bells. The three at the top, consisting merely of two
+circles representing the magnets in plan view, are perhaps to be
+preferred as they are well standardized, easy to draw, and rather
+suggestive. The three at the bottom, showing the ringer as a whole in
+side elevation, are somewhat more specific, but are objectionable in
+that they take more space and are not so easily drawn.
+
+[Illustration: Fig. 82. Ringer Symbols]
+
+Symbols _A_ or _B_ may be used for designating any ordinary polarized
+ringer. Symbols _C_ and _D_ are interchangeably used to indicate a
+biased ringer. If the bell is designed to operate only on positive
+impulses, then the plus sign is placed opposite the symbol, while a
+minus sign so placed indicates that the bell is to be operated only by
+negative impulses.
+
+Some specific types of ringers are designed to operate only on a given
+frequency of current. That is, they are so designed as to be
+responsive to currents having a frequency of sixty cycles per second,
+for instance, and to be unresponsive to currents of any other
+frequency. Either symbols _E_ or _F_ may be used to designate such
+ringers, and if it is desired to indicate the particular frequency of
+the ringer this is done by adding the proper numeral followed by a
+short reversed curve sign indicating frequency. Thus 50~ would
+indicate a frequency of fifty cycles per second.
+
+
+
+
+CHAPTER IX
+
+THE HOOK SWITCH
+
+
+Purpose. In complete telephone instruments, comprising both talking
+and signaling apparatus, it is obviously desirable that the two sets
+of apparatus, for talking and signaling respectively, shall not be
+connected with the line at the same time. A certain switching device
+is, therefore, necessary in order that the signaling apparatus alone
+may be left operatively connected with the line while the instrument
+is not being used in the transmission of speech, and in order that the
+signaling apparatus may be cut out when the talking apparatus is
+brought into play.
+
+In instruments employing batteries for the supply of transmitter
+current, another switching function is the closing of the battery
+circuit through the transmitter and the induction coil when the
+instrument is in use for talking, since to leave the battery circuit
+closed all the time would be an obvious waste of battery energy.
+
+In the early forms of telephones these switching operations were
+performed by a manually operated switch, the position of which the
+user was obliged to change before and after each use of the telephone.
+The objection to this was not so much in the manual labor imposed on
+the user as in the tax on his memory. It was found to be practically a
+necessity to make this switching function automatic, principally
+because of the liability of the user to forget to move the switch to
+the proper position after using the telephone, resulting not only in
+the rapid waste of the battery elements but also in the inoperative
+condition of the signal-receiving bell. The solution of this problem,
+a vexing one at first, was found in the so-called automatic hook
+switch or switch hook, by which the circuits of the instrument were
+made automatically to assume their proper conditions by the mere act,
+on the part of the user, of removing the receiver from, or placing it
+upon, a conveniently arranged hook or fork projecting from the side of
+the telephone casing.
+
+Automatic Operation. It may be taken as a fundamental principle in
+the design of any piece of telephone apparatus that is to be generally
+used by the public, that the necessary acts which a person must
+perform in order to use the device must, as far as possible, follow as
+a natural result from some other act which it is perfectly obvious to
+the user that he must perform. So in the case of the switch hook, the
+user of a telephone knows that he must take the receiver from its
+normal support and hold it to his ear; and likewise, when he is
+through with it, that he must dispose of it by hanging it upon a
+support obviously provided for that purpose.
+
+In its usual form a forked hook is provided for supporting the
+receiver in a convenient place. This hook is at the free end of a
+pivoted lever, which is normally pressed upward by a spring when the
+receiver is not supported on it. When, however, the receiver is
+supported on it, the lever is depressed by its weight. The motion of
+the lever is mechanically imparted to the members of the switch
+proper, the contacts of which are usually enclosed so as to be out of
+reach of the user. This switch is so arranged that when the hook is
+depressed the circuits are held in such condition that the talking
+apparatus will be cut out, the battery circuit opened, and the
+signaling apparatus connected with the line. On the other hand, when
+the hook is in its raised position, the signaling apparatus is cut
+out, the talking apparatus switched into proper working relation with
+the line, and the battery circuit closed through the transmitter.
+
+In the so-called common-battery telephones, where no magneto generator
+or local battery is included in the equipment at the subscriber's
+station, the mere raising of the hook serves another important
+function. It acts, not only to complete the circuit through the
+substation talking apparatus, but, by virtue of the closure of the
+line circuit, permits a current to flow over the line from the
+central-office battery which energizes a signal associated with the
+line at the central office. This use of the hook switch in the case of
+the common-battery telephone is a good illustration of the principle
+just laid down as to making all the functions which the subscriber has
+to perform depend, as far as possible, on acts which his common sense
+alone tells him he must do. Thus, in the common-battery telephone the
+subscriber has only to place the receiver at his ear and ask for what
+he wants. This operation automatically displays a signal at the
+central office and he does nothing further until the operator
+inquires for the number that he wants. He has then nothing to do but
+wait until the called-for party responds, and after the conversation
+his own personal convenience demands that he shall dispose of the
+receiver in some way, so he hangs it up on the most convenient object,
+the hook switch, and thereby not only places the apparatus at his
+telephone in proper condition to receive another call, but also
+conveys to the central office the signal for disconnection.
+
+Likewise in the case of telephones operating in connection with
+automatic exchanges, the hook switch performs a number of functions
+automatically, of which the subscriber has no conception; and while,
+in automatic telephones, there are more acts required of the user than
+in the manual, yet a study of these acts will show that they all
+follow in a way naturally suggested to the user, so that he need have
+but the barest fundamental knowledge in order to properly make use of
+the instrument. In all cases, in properly designed apparatus, the
+arrangement is such that the failure of the subscriber to do a certain
+required act will do no damage to the apparatus or to the system, and,
+therefore, will inconvenience only himself.
+
+Design. The hook switch is in reality a two-position switch, and
+while at present it is a simple affair, yet its development to its
+high state of perfection has been slow, and its imperfections in the
+past have been the cause of much annoyance.
+
+Several important points must be borne in mind in the design of the
+hook switch. The spring provided to lift the hook must be sufficiently
+strong to accomplish this purpose and yet must not be strong enough to
+prevent the weight of the receiver from moving the switch to its other
+position. The movement of this spring must be somewhat limited in
+order that it will not break when used a great many times, and also it
+must be of such material and shape that it will not lose its
+elasticity with use. The shape and material of the restoring spring
+are, of course, determined to a considerable extent by the length of
+the lever arm which acts on the spring, and on the space which is
+available for the spring.
+
+The various contacts by which the circuit changes are brought about
+upon the movement of the hook-switch lever usually take the form of
+springs of German silver or phosphor-bronze, hard rolled so as to have
+the necessary resiliency, and these are usually tipped with platinum
+at the points of contact so as to assure the necessary character of
+surface at the points where the electric circuits are made or broken.
+A slight sliding movement between each pair of contacts as they are
+brought together is considered desirable, in that it tends to rub off
+any dirt that may have accumulated, yet this sliding movement should
+not be great, as the surfaces will then cut each other and, therefore,
+reduce the life of the switch.
+
+Contact Material. On account of the high cost of platinum, much
+experimental work has been done to find a substitute metal suitable
+for the contact points in hook switches and similar uses in the
+manufacture of telephone apparatus. Platinum is unquestionably the
+best known material, on account of its non-corrosive and
+heat-resisting qualities. Hard silver is the next best and is found in
+some first-class apparatus. The various cheap alloys intended as
+substitutes for platinum or silver in contact points may be dismissed
+as worthless, so far as the writers' somewhat extensive investigations
+have shown.
+
+In the more recent forms of hook switches, the switch lever itself
+does not form a part of the electrical circuit, but serves merely as
+the means by which the springs that are concerned in the switching
+functions are moved into their alternate cooperative relations. One
+advantage in thus insulating the switch lever from the
+current-carrying portions of the apparatus and circuits is that, since
+it necessarily projects from the box or cabinet, it is thus liable to
+come in contact with the person of the user. By insulating it, all
+liability of the user receiving shocks by contact with it is
+eliminated.
+
+Wall Telephone Hooks. _Kellogg._ A typical form of hook switch, as
+employed in the ordinary wall telephone sets, is shown in Fig. 83,
+this being the standard hook of the Kellogg Switchboard and Supply
+Company. In this the lever _1_ is pivoted at the point _3_ in a
+bracket _5_ that forms the base of all the working parts and the means
+of securing the entire hook switch to the box or framework of the
+telephone. This switch lever is normally pressed upward by a spring
+_2_, mounted on the bracket _5_, and engaging the under side of the
+hook lever at the point _4_. Attached to the lever arm _1_ is an
+insulated pin _6_. The contact springs by which the various electrical
+circuits are made and broken are shown at _7_, _8_, _9_, _10_, and
+_11_, these being mounted in one group with insulated bushings between
+them; the entire group is secured by machine screws to a lug
+projecting horizontally from the bracket _5_. The center spring _9_
+is provided with a forked extension which embraces the pin _6_ on the
+hook lever. It is obvious that an up-and-down motion of the hook lever
+will move the long spring _9_ in such manner as to cause electrical
+contact either between it and the two upper springs _7_ and _8_, or
+between it and the two lower springs _10_ and _11_. The hook is shown
+in its raised position, which is the position required for talking.
+When lowered the two springs _7_ and _8_ are disengaged from the long
+spring _9_ and from each other, and the three springs _9_, _10_, and
+_11_ are brought into electrical engagement, thus establishing the
+necessary signaling conditions.
+
+[Illustration: Fig. 83. Long Lever Hook Switch]
+
+The right-hand ends of the contact springs are shown projecting beyond
+the insulating supports. This is for the purpose of facilitating
+making electrical joints between these springs and the various wires
+which lead from them. These projecting ends are commonly referred to
+as ears, and are usually provided with holes or notches into which the
+connecting wire is fastened by soldering.
+
+_Western Electric._ Fig. 84 shows the type of hook switch quite
+extensively employed by the Western Electric Company in wall telephone
+sets where the space is somewhat limited and a compact arrangement is
+desired. It will readily be seen that the principle on which this hook
+switch operates is similar to that employed in Fig. 83, although the
+mechanical arrangement of the parts differs radically. The hook lever
+_1_ is pivoted at _3_ on a bracket _2_, which serves to support all
+the other parts of the switch. The contact springs are shown at _4_,
+_5_, and _6_, and this latter spring _6_ is so designed as to make it
+serve as an actuating spring for the hook. This is accomplished by
+having the curved end of this spring press against the lug _7_ of the
+hook and thus tend to raise the hook when it is relieved of the weight
+of the receiver. The two shorter springs _8_ and _9_ have no
+electrical function but merely serve as supports against which the
+springs _4_ and _5_ may rest, when the receiver is on the hook, these
+springs _4_ and _5_ being given a light normal tension toward the stop
+springs _8_ and _9_. It is obvious that in the particular arrangement
+of the springs in this switch no contacts are closed when the receiver
+is on the hook.
+
+[Illustration: Fig. 84. Short Lever Hook Switch]
+
+Concerning this latter feature, it will be noted that the particular
+form of Kellogg hook switch, shown in Fig. 83, makes two contacts and
+breaks two when it is raised. Similarly the Western Electric Company's
+makes two contacts but does not break any when raised. From such
+considerations it is customary to speak of a hook such as that shown
+in Fig. 83 as having two make and two break contacts, and such a hook
+as that shown in Fig. 84 as having two make contacts.
+
+It will be seen from either of these switches that the modification of
+the spring arrangement, so as to make them include a varying number of
+make-and-break contacts, is a simple matter, and switches of almost
+any type are readily modified in this respect.
+
+[Illustration: Fig. 85. Removable Lever Hook Switch]
+
+_Dean_. In Fig. 85 is shown a decidedly unique hook switch for wall
+telephone sets which forms the standard equipment of the Dean Electric
+Company. The hook lever _1_ is pivoted at _2_, an auxiliary lever _3_
+also being pivoted at the same point. The auxiliary lever _3_ carries
+at its rear end a slotted lug _4_, which engages the long contact
+spring _5_, and serves to move it up and down so as to engage and
+disengage the spring _6_, these two springs being mounted on a base
+lug extending from the base plate _7_, upon which the entire
+hook-switch mechanism is mounted. The curved spring _8_, also mounted
+on this same base, engages the auxiliary lever _3_ at the point _9_
+and normally serves to press this up so as to maintain the contact
+springs _5_ in engagement with contact spring _6_. The switch springs
+are moved entirely by the auxiliary lever _3_, but in order that this
+lever _3_ may be moved as required by the hook lever _1_, this lever
+is provided with a notched lug _10_ on its lower side, which notch is
+engaged by a forwardly projecting lug _11_ that is integral with the
+auxiliary lever _3_. The switch lever may be bodily removed from the
+remaining parts of the hook switch by depressing the lug _11_ with the
+finger, so that it disengages the notch in lug _10_, and then drawing
+the hook lever out of engagement with the pivot stud _2_, as shown in
+the lower portion of the figure. It will be noted that the pivotal end
+of the hook lever is made with a slot instead of a hole as is the
+customary practice.
+
+The advantage of being able to remove the hook switch bodily from the
+other portions arises mainly in connection with the shipment or
+transportation of instruments. The projecting hooks cause the
+instruments to take up more room and thus make larger packing boxes
+necessary than would otherwise be used. Moreover, in handling the
+telephones in store houses or transporting them to the places where
+they are to be used, the projecting hook switch is particularly liable
+to become damaged. It is for convenience under such conditions that
+the Dean hook switch is made so that the switch lever may be removed
+bodily and placed, for instance, inside the telephone box for
+transportation.
+
+Desk-Stand Hooks. The problem of hook-switch design for portable
+desk telephones, while presenting the same general characteristics,
+differs in the details of construction on account of the necessarily
+restricted space available for the switch contacts in the desk
+telephone.
+
+[Illustration: WEST OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO
+Serving the General Western Business and Residence Districts.]
+
+_Western Electric._ In Fig. 86 is shown an excellent example of
+hook-switch design as applied to the requirements of the ordinary
+portable desk set. This figure is a cross-sectional view of the
+base and standard of a familiar type of desk telephone. The base
+itself is of stamped metal construction, as indicated, and the
+standard which supports the transmitter and the switch hook for the
+receiver is composed of a black enameled or nickel-plated brass tube
+_1_, attached to the base by a screw-threaded joint, as shown. The
+switch lever _2_ is pivoted at _3_ in a brass plug _4_, closing the
+upper end of the tube forming the standard. This brass plug supports
+also the transmitter, which is not shown in this figure. Attached to
+the plug _4_ by the screw _5_ is a heavy strip _6_, which reaches down
+through the tube to the base plate of the standard and is held therein
+by a screw _7_. The plug _4_, carrying with it the switch-hook lever
+_2_ and the brass strip _6_, may be lifted bodily out of the standard
+_1_ by taking out the screw _7_ which holds the strip _6_ in place, as
+is clearly indicated. On the strip _6_ there is mounted the group of
+switch springs by which the circuit changes of the instrument are
+brought about when the hook is raised or lowered. The spring _8_ is
+longer than the others, and projects upwardly far enough to engage the
+lug on the switch-hook lever _2_. This spring, which is so bent as to
+close the contacts at the right when not prevented by the switch
+lever, also serves as an actuating spring to raise the lever _2_ when
+the receiver is removed from it. This spring, when the receiver is
+removed from the hook, engages the two springs at the right, as shown,
+or when the receiver is placed on the hook, breaks contact with the
+two right-hand springs and makes contact respectively with the
+left-hand spring and also with the contact _9_ which forms the
+transmitter terminal.
+
+[Illustration: Fig. 86. Desk-Stand Hook Switch]
+
+It is seen from an inspection of this switch hook that it has two make
+and two break contacts. The various contact springs are connected with
+the several binding posts shown, these forming the connectors for the
+flexible cord conductors leading into the base and up through the
+standard of the desk stand. By means of the conductors in this cord
+the circuits are led to the other parts of the instrument, such as the
+induction coil, call bell, and generator, if there is one, which, in
+the case of the Western Electric Company's desk set, are all mounted
+separately from the portable desk stand proper.
+
+This hook switch is accessible in an easy manner and yet not subject
+to the tampering of idle or mischievous persons. By taking out the
+screw _7_ the entire hook switch may be lifted out of the tube forming
+the standard, the cords leading to the various binding posts being
+slid along through the tube. By this means the connections to the hook
+switch, as well as the contact of the switch itself, are readily
+inspected or repaired by those whose duty it is to perform such
+operations.
+
+_Kellogg._ In Fig. 87 is shown a sectional view of the desk-stand hook
+switch of the Kellogg Switchboard and Supply Company. In this it will
+be seen that instead of placing the switch-hook springs within the
+standard or tube, as in the case of the Western Electric Company, they
+are mounted in the base where they are readily accessible by merely
+taking off the base plate from the bottom of the stand. The hook lever
+operates on the long spring of the group of switch springs by means of
+a toggle joint in an obvious manner. This switch spring itself serves
+by its own strength to raise the hook lever when released from the
+weight of the receiver.
+
+[Illustration: Fig. 87. Desk-Stand Hook Switch]
+
+In this switch, the hook lever, and in fact the entire exposed metal
+portions of the instrument, are insulated from all of the contact
+springs and, therefore, there is little liability of shocks on the
+part of the person using the instrument.
+
+Conventional Symbols. The hook switch plays a very important part
+in the operation of telephone circuits; for this reason readily
+understood conventional symbols, by which they may be conveniently
+represented in drawings of circuits, are desirable. In Fig. 88 are
+shown several symbols such as would apply to almost any circuit,
+regardless of the actual mechanical details of the particular hook
+switch which happened to be employed. Thus diagram _A_ in Fig. 88
+shows a hook switch having a single make contact and this diagram
+might be used to refer to the hook switch of the Dean Electric Company
+shown in Fig. 85, in which only a single contact is made when the
+receiver is removed, and none is made when it is on the hook.
+Similarly, diagram _B_ might be used to represent the hook switch of
+the Kellogg Company, shown in Fig. 83, the arrangement being for two
+make and two break contacts. Likewise diagram _C_ might be used to
+represent the hook switch of the Western Electric Company, shown in
+Fig. 84, which, as before stated, has two make contacts only. Diagram
+_D_ shows another modification in which contacts made by the hook
+switch, when the receiver is removed, control two separate circuits.
+Assuming that the solid black portion represents insulation, it is
+obvious that the contacts are divided into two groups, one insulated
+from the other.
+
+[Illustration: Fig. 88. Hook Switch Symbols]
+
+[Illustration: COMPRESSED AIR WAGON FOR PNEUMATIC DRILLING AND
+CHIPPING IN MANHOLES]
+
+
+
+
+CHAPTER X
+
+ELECTROMAGNETS AND INDUCTIVE COILS
+
+
+Electromagnet. The physical thing which we call an electromagnet,
+consisting of a coil or helix of wire, the turns of which are
+insulated from each other, and within which is usually included an
+iron core, is by far the most useful of all the so-called translating
+devices employed in telephony. In performing the ordinary functions of
+an electromagnet it translates the energy of an electrical current
+into the energy of mechanical motion. An almost equally important
+function is the converse of this, that is, the translation of the
+energy of mechanical motion into that of an electrical current. In
+addition to these primary functions which underlie the art of
+telephony, the electromagnetic coil or helix serves a wide field of
+usefulness in cases where no mechanical motion is involved. As
+impedance coils, they serve to exert important influences on the flow
+of currents in circuits, and as induction coils, they serve to
+translate the energy of a current flowing in one circuit into the
+energy of a current flowing in another circuit, the translation
+usually, but not always, being accompanied by a change in voltage.
+
+When a current flows through the convolutions of an ordinary helix,
+the helix will exhibit the properties of a magnet even though the
+substance forming the core of the helix is of non-magnetic material,
+such as air, or wood, or brass. If, however, a mass of iron, such as a
+rod or a bundle of soft iron wires, for instance, is substituted as a
+core, the magnetic properties will be enormously increased. The reason
+for this is, that a given magnetizing force will set up in iron a
+vastly greater number of lines of magnetic force than in air or in any
+other non-magnetic material.
+
+Magnetizing Force. The magnetizing force of a given helix is that
+force which tends to drive magnetic lines of force through the
+magnetic circuit interlinked with the helix. It is called
+_magnetomotive force_ and is analogous to electromotive force, that
+is, the force which tends to drive an electric current through a
+circuit.
+
+The magnetizing force of a given helix depends on the product of the
+current strength and the number of turns of wire in the helix. Thus,
+when the current strength is measured in amperes, this magnetizing
+force is expressed as ampere-turns, being the product of the number of
+amperes flowing by the number of turns. The magnetizing force exerted
+by a given current, therefore, is independent of anything except the
+number of turns, and the material within the core or the shape of the
+core has no effect upon it.
+
+Magnetic Flux. The total magnetization resulting from a magnetizing
+force is called the magnetic flux, and is analogous to current. The
+intensity of a magnetic flux is expressed by the number of magnetic
+lines of force in a square centimeter or square inch.
+
+While the magnetomotive force or magnetizing force of a given helix is
+independent of the material of the core, the flux which it sets up is
+largely dependent on the material and shape of the core--not only upon
+this but on the material that lies in the return path for the flux
+outside of the core. We may say, therefore, that the amount of flux
+set up by a given current in a given coil or helix is dependent on the
+material in the magnetic path or magnetic circuit, and on the shape
+and length of that circuit. If the magnetic circuit be of air or brass
+or wood or any other non-magnetic material, the amount of flux set up
+by a given magnetizing force will be relatively small, while it will
+be very much greater if the magnetic circuit be composed in part or
+wholly of iron or steel, which are highly magnetic substances.
+
+Permeability. The quality of material, which permits of a given
+magnetizing force setting up a greater or less number of lines of
+force within it, is called its permeability. More accurately, the
+permeability is the ratio existing between the amount of magnetization
+and the magnetizing force which produces such magnetization.
+
+The permeability of a substance is usually represented by the Greek
+letter  mu (pronounced _mu_). The intensity of the magnetizing force
+is commonly symbolized by H, and since the permeability of air is
+always taken as unity, we may express the intensity of magnetizing
+force by the number of lines of force per square centimeter which it
+sets up in air.
+
+Now, if the space on which the given magnetizing force H were acting
+were filled with iron instead of air, then, owing to the greater
+permeability of iron, there would be set up a very much greater number
+of lines of force per square centimeter, and this number of lines of
+force per square centimeter in the iron is the measure of the
+magnetization produced and is commonly expressed by the letter =B=.
+
+From this we have
+
+ mu = B/H
+
+Thus, when we say that the permeability of a given specimen of wrought
+iron under given conditions is 2,000, we mean that 2,000 times as many
+lines of force would be induced in a unit cross-section of this sample
+as would be induced by the same magnetizing force in a corresponding
+unit cross-section of air. Evidently for air B = H, hence  mu becomes
+unity.
+
+The permeability of air is always a constant. This means that whether
+the magnetic density of the lines of force through the air be great or
+small the number of lines will always be proportional to the
+magnetizing force. Unfortunately for easy calculations in
+electromagnetic work, however, this is not true of the permeability of
+iron. For small magnetic densities the permeability is very great, but
+for large densities, that is, under conditions where the number of
+lines of force existing in the iron is great, the permeability becomes
+smaller, and an increase in the magnetizing force does not produce a
+corresponding increase in the total flux through the iron.
+
+Magnetization Curves. This quality of iron is best shown by the curves
+of Fig. 89, which illustrate the degree of magnetization set up in
+various kinds of iron by different magnetizing forces. In these curves
+the ordinates represent the total magnetization =B=, while the abscissas
+represent the magnetizing force =H=. It is seen from an inspection of
+these curves that as the magnetizing force =H= increases, the intensity
+of flux also increases, but at a gradually lessening rate, indicating a
+reduction in permeability at the higher densities. These curves are also
+instructive as showing the great differences that exist between the
+permeability of the different kinds of iron; and also as showing how,
+when the magnetizing force becomes very great, the iron approaches what
+is called _saturation_, that is, a point at which the further increase
+in magnetizing force will result in no further magnetization of the
+core.
+
+From the data of the curves of Fig. 89, which are commonly called
+_magnetization curves_, it is easy to determine other data from which
+so-called permeability curves may be plotted. In permeability curves
+the total magnetization of the given pieces of iron are plotted as
+abscissas, while the corresponding permeabilities are plotted as
+ordinates.
+
+[Illustration: Fig. 89. Magnetization Curve]
+
+Direction of Lines of Force. The lines of force set up within the
+core of a helix always have a certain direction. This direction always
+depends upon the direction of the flow of current around the core. An
+easy way to remember the direction is to consider the helix as grasped
+in the right hand with the fingers partially encircling it and the
+thumb pointing along its axis. Then, if the current through the
+convolutions of the helix be in the direction in which the fingers of
+the hand are pointed around the helix, the magnetic lines of force
+will proceed through the core of the helix along the direction in
+which the thumb is pointed.
+
+In the case of a simple bar electromagnet, such as is shown in Fig.
+90, the lines of force emerging from one end of the bar must pass back
+through the air to the other end of the bar, as indicated by dotted
+lines and arrows. The path followed by the magnetic lines of force is
+called the _magnetic circuit_, and, therefore, the magnetic circuit of
+the magnet shown in Fig. 90 is composed partly of iron and partly of
+air. From what has been said concerning the relative permeability of
+air and of iron, it will be obvious that the presence of such a long
+air path in the magnetic circuit will greatly reduce the number of
+lines of force that a given magnetizing force can set up. The presence
+of an air gap in a magnetic circuit has much the same effect on the
+total flow of lines of force as the presence of a piece of bad
+conductor in a circuit composed otherwise of good conductor, in the
+case of the flow of electric current.
+
+Reluctance. As the property which opposes the flow of electric
+current in an electrical circuit is called _resistance_, so the
+property which opposes the flow of magnetic lines of force in a
+magnetic circuit is called _reluctance_. In the case of the electric
+circuit, the resistance is the reciprocal of the conductivity; in the
+case of the magnetic circuit, the reluctance is the reciprocal of the
+permeability. As in the case of an electrical circuit, the amount of
+flow of current is equal to the electromotive force divided by the
+resistance; so in a magnetic circuit, the magnetic flux is equal to
+the magnetizing force or magnetomotive force divided by the
+reluctance.
+
+[Illustration: Fig. 90. Bar Electromagnet]
+
+Types of Low-Reluctance Circuits. As the pull of an electromagnet
+upon its armature depends on the total number of lines of force
+passing from the core to the armature--that is, on the total flux--and
+as the total flux depends for a given magnetizing force on the
+reluctance of the magnetic circuit, it is obvious that the design of
+the electromagnetic circuit is of great importance in influencing the
+action of the magnet. Obviously, anything that will reduce the amount
+of air or other non-magnetic material that is in the magnetic circuit
+will tend to reduce the reluctance, and, therefore, to increase the
+total magnetization resulting from a given magnetizing force.
+
+_Horseshoe Form._ One of the easiest and most common ways of reducing
+reluctance in a circuit is to bend the ordinary bar electromagnet
+into horseshoe form. In order to make clear the direction of current
+flow, attention is called to Fig. 91. This is intended to represent a
+simple bar of iron with a winding of one direction throughout its
+length. The gap in the middle of the bar, which divides the winding
+into two parts, is intended merely to mark the fact that the winding
+need not cover the whole length of the bar and still will be able to
+magnetize the bar when the current passes through it. In Fig. 92 a
+similar bar is shown with similar winding upon it, but bent into
+=U=-form, exactly as if it had been grasped in the hand and bent
+without further change. The magnetic polarity of the two ends of the
+bar remain the same as before for the same direction of current, and
+it is obvious that the portion of the magnetic circuit which extends
+through air has been very greatly shortened by the bending. As a
+result, the magnetic reluctance of the circuit has been greatly
+decreased and the strength of the magnet correspondingly increased.
+
+[Illustration: Fig. 91. Bar Electromagnet]
+
+[Illustration: Fig. 92. Horseshoe Electromagnet]
+
+[Illustration: Fig. 93. Horseshoe Electromagnet]
+
+If the armature of the electromagnet shown in Fig. 92 is long enough
+to extend entirely across the air gap from the south to the north
+pole, then the air gap in the magnetic circuit is still further
+shortened, and is now represented only by the small gap between the
+ends of the armature and the ends of the core. Such a magnet, with an
+armature closely approaching the poles, is called a _closed-circuit
+magnet_, since the only gap in the iron of the magnetic circuit is
+that across which the magnet pulls in attracting its armature.
+
+In Fig. 93 is shown the electrical and magnetic counterpart of Fig.
+92. The fact that the magnetic circuit is not a single iron bar but is
+made up of two cores and one backpiece rigidly secured together, has
+no bearing upon the principle, but only shows that a modification of
+construction is possible. In the construction of Fig. 93 the armature
+_1_ is shown as being pulled directly against the two cores _2_ and
+_3_, these two cores being joined by a yoke _4_, which, like the
+armature and the core, is of magnetic material. The path of the lines
+of force is indicated by dotted lines. This is a very important form
+of electromagnet and is largely used in telephony.
+
+_Iron-Clad Form_. Another way of forming a closed-circuit magnet that
+is widely used in telephony is to enclose the helix or winding in a
+shell of magnetic material which joins the core at one end. This
+construction results in what is known as the _tubular_ or _iron-clad_
+electromagnet, which is shown in section and in end view in Fig. 94.
+In this the core _1_ is a straight bar of iron and it lies centrally
+within a cylindrical shell _2_, also of iron. The bar is usually held
+in place within the shell by a screw, as shown. The lines of force set
+up in the core by the current flowing through the coil, pass to the
+center of the bottom of the iron shell and thence return through the
+metal of the shell, through the air gap between the edges of the shell
+and the armature, and then concentrate at the center of the armature
+and pass back to the end of the core. This is a highly efficient form
+of closed-circuit magnet, since the magnetic circuit is of low
+reluctance.
+
+[Illustration: Fig. 94. Iron-Clad Electromagnet]
+
+Such forms of magnets are frequently used where it is necessary to
+mount a large number of them closely together and where it is desired
+that the current flowing in one magnet shall produce no inductive
+effect in the coils of the adjacent magnets. The reason why mutual
+induction between adjacent magnets is obviated in the case of the
+iron-clad or tubular magnet is that practically all stray field is
+eliminated, since the return path for the magnetic lines is so
+completely provided for by the presence of the iron shell.
+
+_Special Horseshoe Form._ In Fig. 95 is shown a type of relay commonly
+employed in telephone circuits. The purpose of illustrating it in this
+chapter is not to discuss relays, but rather to show an adaptation of an
+electromagnet wherein low reluctance of the magnetic circuit is secured
+by providing a return leg for the magnetic lines developed in the core,
+thus forming in effect a horseshoe magnet with a winding on one of its
+limbs only. To the end of the core _1_ there is secured an =L=-shaped
+piece of soft iron _2_. This extends upwardly and then forwardly
+throughout the entire length of the magnet core. An =L=-shaped armature
+_3_ rests on the front edge of the piece _2_ so that a slight rocking
+motion will be permitted on the "knife-edge" bearing thus afforded. It
+is seen from the dotted lines that the magnetic circuit is almost a
+closed one. The only gap is that between the lower end of the armature
+_3_ and the front end of the core. When the coil is energized, this gap
+is closed by the attraction of the armature. As a result, the rearwardly
+projecting end of the armature _3_ is raised and this raises the spring
+_4_ and causes it to break the normally existing contact with the spring
+_5_ and to establish another contact with the spring _6_. Thus the
+energy developed within the coil of the magnet is made to move certain
+parts which in turn operate the switching devices to produce changes in
+electrical circuits. These relays and other adaptations of the
+electromagnet will be discussed more fully later on.
+
+[Illustration: Fig. 95. Electromagnet of Relay]
+
+There are almost numberless forms of electromagnets, but we have
+illustrated here examples of the principal types employed in
+telephony, and the modifications of these types will be readily
+understood in view of the general principles laid down.
+
+Direction of Armature Motion. It may be said in general that the
+armature of an electromagnet always moves or tends to move, when the
+coil is energized, in such a way as to reduce the reluctance of the
+magnetic circuit through the coil. Thus, in all of the forms of
+electromagnets discussed, the armature, when attracted, moves in such a
+direction as to shorten the air gap and to introduce the iron of the
+armature as much as possible into the path of the magnetic lines, thus
+reducing the reluctance. In the case of a solenoid type of
+electromagnet, or the coil and plunger type, which is a better name than
+solenoid, the coil, when energized, acts in effect to suck the iron core
+or plunger within itself so as to include more and more of the iron
+within the most densely occupied portion of the magnetic circuit.
+
+[Illustration: Fig. 96. Parallel Differential Electromagnet]
+
+
+Differential Electromagnet. Frequently in telephony, the
+electromagnets are provided with more than one winding. One purpose of
+the double-wound electromagnet is to produce the so-called
+differential action between the two windings, _i.e._, making one of
+the windings develop magnetization in the opposite direction from that
+of the other, so that the two will neutralize each other, or at least
+exert different and opposite influences. The principle of the
+differential electromagnet may be illustrated in connection with Fig.
+96. Here two wires _1_ and _2_ are shown wrapped in the same direction
+about an iron core, the ends of the wire being joined together at _3_.
+Obviously, if one of these windings only is employed and a current
+sent through it, as by connecting the terminals of a battery with the
+points _4_ and _3_, for instance, the core will be magnetized as in an
+ordinary magnet. Likewise, the core will be energized if a current be
+sent from _5_ to _3_. Assuming that the two windings are of equal
+resistance and number of turns, the effects so produced, when either
+the coil _1_ or the coil _2_ is energized, will be equal. If the
+battery be connected between the terminals _4_ and _5_ with the
+positive pole, say, at _5_, then the current will proceed through the
+winding _2_ and tend to generate magnetism in the core in the
+direction of the arrow. After traversing the winding _2_, however, it
+will then begin to traverse the other winding _1_ and will pass around
+the core in the opposite direction throughout the length of that
+winding. This will tend to set up magnetism in the core in the
+opposite direction to that indicated by the arrow. Since the two
+currents are equal and also the number of turns in each winding, it is
+obvious that the two magnetizing influences will be exactly equal and
+opposite and no magnetic effect will be produced. Such a winding, as
+is shown in Fig. 96, where the two wires are laid on side by side, is
+called a _parallel differential winding_.
+
+Another way of winding magnets differentially is to put one winding on
+one end of the core and the other winding on the other end of the core
+and connect these so as to cause the currents through them to flow
+around the core in opposite directions. Such a construction is shown
+in Fig. 97 and is called a _tandem differential winding_. The tandem
+arrangement, while often good enough for practical purposes, cannot
+result in the complete neutralization of magnetic effect. This is true
+because of the leakage of some of the lines of force from intermediate
+points in the length of the core through the air, resulting in some of
+the lines passing through more of the turns of one coil than of the
+other. Complete neutralization can only be attained by first twisting
+the two wires together with a uniform lay and then winding them
+simultaneously on the core.
+
+[Illustration: Fig. 97. Tandem Differential Electromagnet]
+
+Mechanical Details. We will now consider the actual mechanical
+construction of the electromagnet. This is a very important feature of
+telephone work, because, not only must the proper electrical and
+magnetic effects be produced, but also the whole structure of the
+magnet must be such that it will not easily get out of order and not
+be affected by moisture, heat, careless handling, or other adverse
+conditions.
+
+The most usual form of magnet construction employed in telephony is
+shown in Fig. 98. On the core, which is of soft Norway iron, usually
+cylindrical in form, are forced two washers of either fiber or hard
+rubber. Fiber is ordinarily to be preferred because it is tougher and
+less liable to breakage. Around the core, between the two heads, are
+then wrapped several layers of paper or specially prepared cloth in
+order that the wire forming the winding may be thoroughly insulated
+from the core. One end of the wire is then passed through a hole in
+one of the spool heads or washers, near the core, and the wire is then
+wound on in layers. Sometimes a thickness of paper is placed around
+each layer of wire in order to further guard against the breaking down
+of the insulation between layers. When the last layer is wound on, the
+end of the wire is passed out through a hole in the head, thus leaving
+both ends projecting.
+
+[Illustration: Fig. 98 Construction of Electromagnet]
+
+Magnet Wire. The wire used in winding magnets is, of course, an
+important part of the electromagnet. It is always necessary that the
+adjacent turns of the wire be insulated from each other so that the
+current shall be forced to pass around the core through all the length
+of wire in each turn rather than allowing it to take the shorter and
+easier path from one turn to the next, as would be the case if the
+turns were not insulated. For this purpose the wire is usually covered
+with a coating of some insulating material. There are, however,
+methods of winding magnet coils with bare wire and taking care of the
+insulation between the turns in another way, as will be pointed out.
+
+Insulated wire for the purpose of winding magnet coils is termed
+_magnet wire_. Copper is the material almost universally employed for
+the conductor. Its high conductivity, great ductility, and low cost
+are the factors which make it superior to all other metals. However,
+in special cases, where exceedingly high conductivity is required with
+a limited winding space, silver wire is sometimes employed, and on the
+other hand, where very high resistance is desired within a limited
+winding space, either iron or German silver or some other
+high-resistance alloy is used.
+
+_Wire Gauges_. Wire for electrical purposes is drawn to a number of
+different standard gauges. Each of the so-called wire gauges consists
+of a series of graded sizes of wire, ranging from approximately
+one-half an inch in diameter down to about the fineness of a lady's
+hair. In certain branches of telephone work, such as line
+construction, the existence of the several wire gauges or standards is
+very likely to lead to confusion. Fortunately, however, so far as
+magnet wire is concerned, the so-called Brown and Sharpe, or American,
+wire gauge is almost universally employed in this country. The
+abbreviations for this gauge are B.&S. or A.W.G.
+
+
+TABLE III
+
+Copper Wire Table
+
+Giving weights, lengths, and resistances of wire @ 68 deg. F., of Matthiessen's
+Standard Conductivity.
+
++-------+----------+----------+-----------------------+--------------------+-----------------------+
+|       |          |          |       RESISTANCE      |       LENGTH       |       WEIGHT          |
+| A.W.G.| DIAMETER |   AREA   +-----------------------+--------------------+-----------------------+
+| B.&S. |   MILS   | CIRCULAR | OHMS PER  | OHMS PER  | FEET PER | FEET PER| POUNDS PER |POUNDS PER|
+|       |          |   MILS   |   POUND   |   FOOT    |  POUND   |   OHM   |    FOOT    |   OHM    |
++-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
+| 0000  |  460.    | 211,600. |0.00007639 | 0.0000489 |    1.561 | 20,440. | 0.6405     | 13,090.  |
+|  000  |  409.6   | 167,800. |0.0001215  | 0.0000617 |    1.969 | 16,210. | 0.5080     |  8,232.  |
+|   00  |  364.8   | 133,100. |0.0001931  | 0.0000778 |    2.482 | 12,850. | 0.4028     |  5,177.  |
+|    0  |  324.9   | 105,500. |0.0003071  | 0.0000981 |    3.130 | 10,190. | 0.3195     |  3,256.  |
++-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
+|    1  |  289.3   |  83,690. | 0.0004883 | 0.0001237 |    3.947 |  8,083. | 0.2533     | 2,048.   |
+|    2  |  257.6   |  66,370. | 0.0007765 | 0.0001560 |    4.977 |  6,410. | 0.2009     | 1,288.   |
+|    3  |  229.4   |  52,630. | 0.001235  | 0.0001967 |    6.276 |  5,084. | 0.1593     |  810.0   |
+|    4  |  204.3   |  41,740. | 0.001963  | 0.0002480 |    7.914 |  4,031. | 0.1264     |  509.4   |
+|    5  |  181.9   |  33,100. | 0.003122  | 0.0003128 |    9.980 |  3,197. | 0.1002     |  320.4   |
+|    6  |  162.0   |  26,250. | 0.004963  | 0.0003944 |   12.58  |  2,535. | 0.07946    |  201.5   |
+|    7  |  144.3   |  20,820. | 0.007892  | 0.0004973 |   15.87  |  2,011. | 0.06302    |  126.7   |
+|    8  |  128.5   |  16,510. | 0.01255   | 0.0006271 |   20.01  |  1,595. | 0.04998    |   79.69  |
+|    9  |  114.4   |  13,090. | 0.01995   | 0.0007908 |   25.23  |  1,265. | 0.03963    |   50.12  |
+|   10  |  101.9   |  10,380. | 0.03173   | 0.0009273 |   31.82  |  1,003. | 0.03143    |   31.52  |
++-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
+|   11  |   90.74  |  8,234.  |  0.05045  | 0.001257  |    40.12 |  795.3  | 0.02493    | 19.82    |
+|   12  |   80.81  |  6,530.  |  0.08022  | 0.001586  |    50.59 |  630.7  | 0.01977    | 12.47    |
+|   13  |   71.96  |  5,178.  |  0.1276   | 0.001999  |    63.79 |  500.1  | 0.01568    |  7.840   |
+|   14  |   64.08  |  4,107.  |  0.2028   | 0.002521  |    80.44 |  396.6  | 0.01243    |  4.931   |
+|   15  |   57.07  |  3,257.  |  0.3225   | 0.003179  |   101.4  |  314.5  | 0.009858   |  3.101   |
+|   16  |   50.82  |  2,583.  |  0.5128   | 0.004009  |   127.9  |  249.4  | 0.007818   |  1.950   |
+|   17  |   45.26  |  2,048.  |  0.8153   | 0.005055  |   161.3  |  197.8  | 0.006200   |  1.226   |
+|   18  |   40.30  |  1,624.  |  1.296    | 0.006374  |   203.4  |  156.9  | 0.004917   |  0.7713  |
+|   19  |   35.89  |  1,288.  |  2.061    | 0.008038  |   256.5  |  124.4  | 0.003899   |  0.4851  |
+|   20  |   31.96  |  1,022.  |  3.278    | 0.01014   |   323.4  |   98.66 | 0.003092   |  0.3051  |
++-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
+|   21  |   28.46  |   810.1  |   5.212   | 0.01278   |   407.8  |  78.24  | 0.002452   | 0.1919   |
+|   22  |   25.35  |   642.4  |   8.287   | 0.01612   |   514.2  |  62.05  | 0.001945   | 0.1207   |
+|   23  |   22.57  |   509.5  |  13.18    | 0.02032   |   648.4  |  49.21  | 0.001542   | 0.07589  |
+|   24  |   20.10  |   404.0  |  20.95    | 0.02563   |   817.6  |  39.02  | 0.001223   | 0.04773  |
+|   25  |   17.90  |   320.4  |  33.32    | 0.03231   | 1,031.   |  30.95  | 0.0009699  | 0.03002  |
+|   26  |   15.94  |   254.1  |  52.97    | 0.04075   | 1,300.   |  24.54  | 0.0007692  | 0.1187   |
+|   27  |   14.2   |   201.5  |  84.23    | 0.05138   | 1,639.   |  19.46  | 0.0006100  | 0.01888  |
+|   28  |   12.64  |   159.8  | 133.9     | 0.06479   | 2,067.   |  15.43  | 0.0004837  | 0.007466 |
+|   29  |   11.26  |   126.7  | 213.0     | 0.08170   | 2,607.   |  12.24  | 0.0003836  | 0.004696 |
+|   30  |   10.03  |   100.5  | 338.6     | 0.1030    | 3,287.   |   9.707 | 0.0003042  | 0.002953 |
++-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
+|   31  |    8.928 |   79.70  |   538.4   | 0.1299    |  4,145.  |  7.698  | 0.0002413  |0.001857  |
+|   32  |    7.950 |   63.21  |   856.2   | 0.1638    |  5,227.  |  6.105  | 0.0001913  |0.001168  |
+|   33  |    7.080 |   50.13  |  1,361.   | 0.2066    |  6,591.  |  4.841  | 0.0001517  |0.0007346 |
+|   34  |    6.305 |   39.75  |  2,165.   | 0.2605    |  8,311.  |  3.839  | 0.0001203  |0.0004620 |
+|   35  |    5.615 |   31.52  |  3,441.   | 0.3284    | 10,480.  |  3.045  | 0.00009543 |0.0002905 |
+|   36  |    5.0   |   25.0   |  5,473.   | 0.4142    | 13,210.  |  2.414  | 0.00007568 |0.0001827 |
+|   37  |    4.453 |   19.83  |  8,702.   | 0.5222    | 16,660.  |  1.915  | 0.00006001 |0.0001149 |
+|   38  |    3.965 |   15.72  | 13,870.   | 0.6585    | 21,010.  |  1.519  | 0.00004759 |0.00007210|
+|   39  |    3.531 |   12.47  | 22,000.   | 0.8304    | 26,500.  |  1.204  | 0.00003774 |0.00004545|
+|   40  |    3.145 |    9.888 | 34,980.   | 1.047     | 33,410.  |  0.9550 | 0.00002993 |0.00002858|
++-------+----------+----------+-----------+-----------+----------+---------+------------+----------+
+
+[Illustration: SOUTH OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO]
+
+In the Brown and Sharpe gauge the sizes, beginning with the largest,
+are numbered 0000, 000, 00, 0, 1, 2, and so on up to 40. Sizes larger
+than about No. 16 B.&S. gauge are seldom used as magnet wire in
+telephony, but for the purpose of making the list complete, Table III
+is given, including all of the sizes of the B.&S. gauge.
+
+In Table III there is given for each gauge number the diameter of the
+wire in mils (thousandths of an inch); the cross-sectional area in
+circular mils (a unit area equal to that of a circle having a diameter
+of one one-thousandth of an inch); the resistance of the wire in
+various units of length and weight; the length of the wire in terms of
+resistance and of weight; and the weight of the wire in terms of its
+length and resistance.
+
+It is to be understood that in Table III the wire referred to is bare
+wire and is of pure copper. It is not commercially practicable to use
+absolutely pure copper, and the ordinary magnet wire has a
+conductivity equal to about 98 per cent of that of pure copper. The
+figures given in this table are sufficiently accurate for all ordinary
+practical purposes.
+
+_Silk and Cotton Insulation_. The insulating material usually employed
+for covering magnet wire is of silk or cotton. Of these, silk is by
+far the better material for all ordinary purposes, since it has a much
+higher insulating property than cotton, and is very much thinner.
+Cotton, however, is largely employed, particularly in the larger sizes
+of magnet wire. Both of these materials possess the disadvantage of
+being hygroscopic, that is, of readily absorbing moisture. This
+disadvantage is overcome in many cases by saturating the coil after it
+is wound in some melted insulating compound, such as wax or varnish or
+asphaltum, which will solidify on cooling. Where the coils are to be
+so saturated the best practice is to place them in a vacuum chamber
+and exhaust the air, after which the hot insulating compound is
+admitted and is thus drawn into the innermost recesses of the winding
+space.
+
+Silk-insulated wire, as regularly produced, has either one or two
+layers of silk. This is referred to commercially as single silk wire
+or as double silk wire. The single silk has a single layer of silk
+fibers wrapped about it, while the double silk has a double layer, the
+two layers being put on in reverse direction. The same holds true of
+cotton insulated wire. Frequently, also, there is a combination of the
+two, consisting of a single or a double wrapping of silk next to the
+wire with an outer wrapping of cotton. Where this is done the cotton
+serves principally as a mechanical protection for the silk, the
+principal insulating properties residing in the silk.
+
+_Enamel_. A later development in the insulation of magnet wire has
+resulted in the so-called enamel wire. In this, instead of coating the
+wire with some fibrous material such as silk or cotton, the wire is
+heated and run through a bath of fluid insulating material or liquid
+enamel, which adheres to the wire in a very thin coating. The wire is
+then run through baking ovens, so that the enamel is baked on. This
+process is repeated several times so that a number of these thin
+layers of the enamel are laid on and baked in succession.
+
+The characteristics sought in good enamel insulation for magnet wire
+may be thus briefly set forth: It is desirable for the insulation to
+possess the highest insulating qualities; to have a glossy, flawless
+surface; to be hard without being brittle; to adhere tenaciously and
+stand all reasonable handling without cracking or flaking; to have a
+coefficient of elasticity greater than the wire itself; to withstand
+high temperatures; to be moisture-proof and inert to corrosive
+agencies; and not to "dry out" or become brittle over a long period of
+time.
+
+_Space Utilization_. The utilization of the winding space in an
+electromagnet is an important factor in design, since obviously the
+copper or other conductor is the only part of the winding that is
+effective in setting up magnetizing force. The space occupied by the
+insulation is, in this sense, waste space. An ideally perfect winding
+may be conceived as one in which the space is all occupied by wire;
+and this would necessarily involve the conception of wire of square
+cross-section and insulation of infinite thinness. In such a winding
+there would be no waste of space and a maximum amount of metal
+employed as a conductor. Of course, such a condition is not possible
+to attain and in practice some insulating material must be introduced
+between the layers of wire and between the adjacent convolutions of
+wire. The ratio of the space occupied by the conductor to the total
+space occupied by the winding, that is, by the conductor and the
+insulation, is called the _coefficient of space utilization of the
+coil_. For the ideal coil just conceived the coefficient of space
+utilization would be 1. Ordinarily the coefficient of space
+utilization is greater for coarse wire than for fine wire, since
+obviously the ratio of the diameter of the wire to the thickness of
+the insulation increases as the size of the wire grows larger.
+
+The chief advantage of enamel insulation for magnet wire is its
+thinness, and the high coefficient of space utilization which may be
+secured by its use. In good enamel wire the insulation will average
+about one-quarter the thickness of the standard single silk
+insulation, and the dielectric strength is equal or greater. Where
+economy of winding space is desirable the advantages of this may
+readily be seen. For instance, in a given coil wound with No. 36
+single silk wire about one-half of the winding space is taken up with
+the insulation, whereas when the same coil is wound with No. 36
+enameled wire only about one-fifth of the winding space is taken up by
+the insulation. Thus the coefficient of space utilization is increased
+from .50 to .80. The practical result of this is that, in the case of
+any given winding space where No. 36 wire is used, about 60 per cent
+more turns can be put on with enameled wire than with single silk
+insulation, and of course this ratio greatly increases when the
+comparison is made with double silk insulation or with cotton
+insulation. Again, where it is desired to reduce the winding space and
+keep the same number of turns, an equal number of turns may be had
+with a corresponding reduction of winding space where enameled wire is
+used in place of silk or cotton.
+
+In the matter of heat-resisting properties the enameled wire possesses
+a great advantage over silk and cotton. Cotton or silk insulation will
+char at about 260 deg. Fahrenheit, while good enameled wire will stand
+400 deg. to 500 deg. Fahrenheit without deterioration of the insulation. It is
+in the matter of liability to injury in rough or careless handling, or
+in winding coils having irregular shapes, that enamel wire is
+decidedly inferior to silk or cotton-covered wire. It is likely to be
+damaged if it is allowed to strike against the sharp corners of the
+magnet spool during winding, or run over the edge of a hard surface
+while it is being fed on to the spool. Coils having other than round
+cores, or having sharp corners on their spool heads, should not
+ordinarily be wound with enamel wire.
+
+The dielectric strength of enamel insulation is much greater than that
+of either silk or cotton insulation of equal thickness. This is a
+distinct advantage and frequently a combination of the two kinds of
+insulation results in a superior wire. If wire insulated with enamel
+is given a single wrapping of silk or of cotton, the insulating and
+dielectric properties of the enamel is secured, while the presence of
+the silk and cotton affords not only an additional safeguard against
+bare spots in the enamel but also a certain degree of mechanical
+protection to the enamel.
+
+Winding Methods. In winding a coil, the spool, after being properly
+prepared, is placed upon a spindle which may be made to revolve rapidly.
+Ordinarily the wire is guided on by hand; sometimes, however, machinery
+is used, the wire being run over a tool which moves to and fro along the
+length of the spool, just fast enough to lay the wire on at the proper
+rate. The movement of this tool is much the same as that of the tool in
+a screw cutting lathe.
+
+Unless high voltages are to be encountered, it is ordinarily not
+necessary to separate the layers of wire with paper, in the case of
+silk-or cotton-insulated magnet wire; although where especially high
+insulation resistance is needed this is often done. It is necessary to
+separate the successive layers of a magnet that is wound with enamel
+wire, by sheets of paper or thin oiled cloth.
+
+[Illustration: Fig. 99. Electromagnet with Bare Wire]
+
+In Fig. 99 is shown a method, that has been used with some success, of
+winding magnets with bare wire. In this the various adjacent turns are
+separated from each other by a fine thread of silk or cotton wound on
+beside the wire. Each layer of wire and thread as it is placed on the
+core is completely insulated from the subsequent layer by a layer of
+paper. This is essentially a machine-wound coil, and machines for
+winding it have been so perfected that several coils are wound
+simultaneously, the paper being fed in automatically at the end of
+each layer.
+
+Another method of winding the bare wire omits the silk thread and
+depends on the permanent positioning of the wire as it is placed on
+the coil, due to the slight sinking into the layer of paper on which
+it is wound. In this case the feed of the wire at each turn of the
+spool is slightly greater than the diameter of the wire, so that a
+small distance will be left between each pair of adjacent turns.
+
+Upon the completion of the winding of a coil, regardless of what
+method is used, it is customary to place a layer of bookbinders' cloth
+over the coil so as to afford a certain mechanical protection for the
+insulated wire.
+
+_Winding Terminals_. The matter of bringing out the terminal ends of
+the winding is one that has received a great deal of attention in the
+construction of electromagnets and coils for various purposes. Where
+the winding is of fine wire, it is always well to reinforce its ends
+by a short piece of larger wire. Where this is done the larger wire is
+given several turns around the body of the coil, so that the finer
+wire with which it connects may be relieved of all strain which may be
+exerted upon it from the protruding ends of the wire. Great care is
+necessary in the bringing out of the inner terminal--_i.e._, the
+terminal which connects with the inner layer--that the terminal wire
+shall not come in contact with any of the subsequent layers that are
+wound on.
+
+[Illustration Fig. 100. Electromagnet with Terminals]
+
+Where economy of space is necessary, a convenient method of
+terminating the winding of the coil consists in fastening rigid
+terminals to the spool head. This, in the case of a fiber spool head,
+may be done by driving heavy metal terminals into the fiber. The
+connections of the two wires leading from the winding are then made
+with these heavy rigid terminals by means of solder. A coil having
+such terminals is shown in its finished condition in Fig. 100.
+
+_Winding Data_. The two things principally affecting the manufacture
+of electromagnets for telephone purposes are _the number of turns in a
+winding_ and _the resistance of the wound wire_. The latter governs
+the amount of current which may flow through the coil with a given
+difference of potential at its end, while the former control the
+amount of magnetism produced in the core by the current flowing. While
+a coil is being wound, it is a simple matter to count the turns by any
+simple form of revolution counter. When the coil has been completed it
+is a simple matter to measure its resistance. But it is not so simple
+to determine in advance how many turns of a given size wire may be
+placed on a given spool, and still less simple to know what the
+resistance of the wire on that spool will be when the desired turns
+shall have been wound.
+
+TABLE IV
+
+Winding Data for Insulated Wires--Silk and Cotton Covering
+
+A.W.G. B & S  |   20       21       22       23        24       25
+---------------------------------------------------------------------
+DIAMETER      |
+Mils          |   31.961   28.462   25.347   22.571   20.100   17.900
+---------------------------------------------------------------------
+AREA          |
+Circular Mils | 1021.20   810.10   642.70   509.45   404.01   320.40
+---------------------------------------------------------------------
+DIAMETER OVER |
+INSULATION    |
+  SINGLE      |
+  COTTON      |   37.861   34.362   31.247   28.471   26.000   23.800
+              |
+  DOUBLE      |
+  COTTON      |   42.161   38.662   35.547   32.771   30.300   28.100
+              |
+  SINGLE SILK |   34.261   30.762   27.647   24,871   22.401   20.200
+              |
+  DOUBLE SILK |   36.161   32.662   29.547   26.771   24.300   22.100
+---------------------------------------------------------------------
+TURNS PER     |
+LINEAR INCH   |
+  SINGLE      |
+  COTTON      |   25.7     28.3     31.0     34.4     36.9     38.0
+              |
+  DOUBLE      |   22.5     24.5     26.7     28.97    31.35    33.92
+  COTTON      |
+              |
+  SINGLE SILK |   27.70    30.97    34.39    38.19    42.37    47.02
+              |
+  DOUBLE SILK |   26.22    29.07    32.11    35.53    39.14    42.94
+---------------------------------------------------------------------
+TURNS PER     |
+SQUARE INCH   |
+  SINGLE      |
+  COTTON      |  660.5    800.9    961.0   1183.0   1321.6   1444.0
+              |
+  DOUBLE      |
+  COTTON      |  506.3    600.2    712.9    839.2     982.8  1150.8
+              |
+  SINGLE SILK |  767.3    959.1   1182.7   1458.5    1795.2  2210.9
+              |
+  DOUBLE SILK |  687.5    845.0   1031.0   1262.4    1532.0  1843.8
+---------------------------------------------------------------------
+OHMS PER      |
+CUBIC INCH    |
+  SINGLE      |
+  COTTON      |     .646     .981    1.502    2.359     3.528   5.831
+              |
+  DOUBLE      |
+  COTTON      |     .533     .795    1.188    1.772     2.595   3.802
+              |
+  SINGLE SILK |     .801    1.261    1.956    3.049     4.739   7.489
+---------------------------------------------------------------------
+
+
+A.W.G. B & S  |   26       27       28       29       30       31
+---------------------------------------------------------------------
+DIAMETER      |
+Mils          |   15.940   14.195   12.641   11.257   10.025    8.928
+---------------------------------------------------------------------
+AREA          |
+Circular Mils |  254.01   201.50   159.79   126.72   100.50    79.71
+---------------------------------------------------------------------
+DIAMETER OVER |
+INSULATION    |
+  SINGLE      |
+  COTTON      |   21.840   20.095   18.541   17.157   15.925   14.828
+              |
+  DOUBLE      |
+  COTTON      |   26.140   24.395   22.841   21.457   20.225   19.128
+              |
+  SINGLE SILK |   18.240   16.495   14.941   13.557   12.325   11.228
+              |
+  DOUBLE SILK |   20.140   18.395   16.841   15.457   14.225   13.128
+---------------------------------------------------------------------
+TURNS PER     |
+LINEAR INCH   |
+  SINGLE      |
+  COTTON      |   42.0     48.0     53.0     56.5     59.66    64.125
+              |
+  DOUBLE      |
+  COTTON      |   36.29    38.95    41.61    44.27    46.93    49.78
+              |
+  SINGLE SILK |   52.06    57.67    63.36    70.11    77.14    84.64
+              |
+  DOUBLE SILK |   46.81    51.59    56.43    61.56    66.79    72.39
+---------------------------------------------------------------------
+TURNS PER     |
+SQUARE INCH   |
+  SINGLE      |
+  COTTON      | 1764.0   2304.0   2809.9   3192.3   3359.2   4112.2
+              |
+  DOUBLE      |
+  COTTON      | 1317.0   1517.2   1731.0   1959.9   2202.5   2478.0
+              |
+  SINGLE SILK | 2710.3   3326.0   4014.5   4915.5   5950.2   7164.0
+              |
+  DOUBLE SILK | 2191.2   2661.6   3184.5   3789.8   4461.0   5240.0
+---------------------------------------------------------------------
+OHMS PER      |
+CUBIC INCH    |
+  SINGLE      |
+  COTTON      |    6.941   10.814   17.617   25.500   34.800   48.5
+              |
+  DOUBLE      |
+  COTTON      |    5.552    8.078   11.54    16.47    23.43    32.83
+              |
+  SINGLE SILK |    9.031   13.92    26.86    41.29    62.98    95.70
+---------------------------------------------------------------------
+
+
+A.W.G. B & S  |    32       33       34       35       36       37
+----------------------------------------------------------------------
+DIAMETER      |
+Mils          |     7.950    7.080    6.304    5.614    5.000    4.453
+----------------------------------------------------------------------
+AREA          |
+Circular Mils |    63.20    50.13    39.74    31.52    25.00    19.83
+----------------------------------------------------------------------
+DIAMETER OVER |
+INSULATION    |
+  SINGLE      |
+  COTTON      |    13.850   12.980   12.204   11.514   10.900   10.353
+              |
+  DOUBLE      |
+  COTTON      |    18.150   17.280   16.504   15.814   15.200   14.653
+              |
+  SINGLE SILK |    10.250    9.380    8.504    7.914    7.300    6.753
+              |
+  DOUBLE SILK |    12.150   11.280   10.504    9.814    9.200    8.653
+----------------------------------------------------------------------
+TURNS PER     |
+LINEAR INCH   |
+  SINGLE      |
+  COTTON      |    68.600   73.050   77.900   82.600   87.100   91.870
+              |
+  DOUBLE      |
+  COTTON      |    52.34    55.10    57.57    60.04    62.51    64.70
+              |
+  SINGLE SILK |    92.72   101.65   112.11   119.7    130.15   140.6
+              |
+  DOUBLE SILK |    78.19    84.17    90.44    96.90   103.55   110.20
+----------------------------------------------------------------------
+TURNS PER     |
+SQUARE INCH   |
+  SINGLE      |  4692.5   5333.5   6068.5   6773.3   7586.5   8440.0
+  COTTON      |
+              |
+  DOUBLE      |
+  COTTON      |  2739.5   3036.1   3314.2   3605.0   3907.5   4186.1
+              |
+  SINGLE SILK |  8597.5  10332.0  12570.0  14327.0  16940.0  19770.0
+              |
+  DOUBLE SILK |  6114.0   7085.0   8179.5   9389.5  10772.0  12145.0
+---------------------------------------------------------------------
+OHMS PER      |
+CUBIC INCH    |
+  SINGLE      |
+  COTTON      |    73.8    104.5    151.4    202.0    298.8    418.0
+              |
+  DOUBLE      |
+  COTTON      |    46.19    64.30    70.58   125.9    166.3    225.6
+              |
+  SINGLE SILK |   144.70   217.8    342.1    489.0    721.1   1062.0
+---------------------------------------------------------------------
+
+
+A.W.G. B & S  |    38        39       40
+--------------------------------------------
+DIAMETER      |
+Mils          |     3.965     3.531    3.144
+--------------------------------------------
+AREA          |
+Circular Mils |    15.72     12.47     9.89
+--------------------------------------------
+DIAMETER OVER |
+INSULATION    |
+  SINGLE      |
+  COTTON      |     9.865     9.431    9.044
+              |
+  DOUBLE      |
+  COTTON      |    14.165    13.731   13.344
+              |
+  SINGLE SILK |     6.265     5.831    5.344
+              |
+  DOUBLE SILK |     8.165     7.731    7.344
+--------------------------------------------
+TURNS PER     |
+LINEAR INCH   |
+  SINGLE      |
+  COTTON      |    95.000   100.700  106.000
+              |
+  DOUBLE      |
+  COTTON      |    66.80     68.80    71.20
+              |
+  SINGLE SILK |   151.05    163.04   177.65
+              |
+  DOUBLE SILK |   116.85    122.55   129.20
+--------------------------------------------
+TURNS PER     |
+SQUARE INCH   |
+  SINGLE      |
+  COTTON      |  9025.0   10140.5  11236.0
+              |
+  DOUBLE      |  4462.2    4733.6   5069.8
+  COTTON      |
+              |
+  SINGLE SILK | 22820.0   26700.0  31559.0
+              |
+  DOUBLE SILK | 13655.0   15018.0  16692.0
+--------------------------------------------
+OHMS PER      |
+CUBIC INCH    |
+  SINGLE      |
+  COTTON      |  567.0    811.0    1113.0
+              |
+  DOUBLE      |  305.5    409.8     545.5
+  COTTON      |
+              |
+  SINGLE SILK | 1557.0   2266.0    3400.0
+-------------------------------------------
+
+If the length and the depth of the winding space of the coil as well
+as the diameter of the core are known, it is not difficult to
+determine how much bare copper wire of a given size may be wound on
+it, but it is more difficult to know these facts concerning copper
+wire which has been covered with cotton or silk. Yet something may be
+done, and tables have been prepared for standard wire sizes with
+definite thicknesses of silk and cotton insulation. As a result of
+facts collected from a large number of actually wound coils, the
+number of turns per linear inch and per square inch of B.&S. gauge
+wires from No. 20 to No. 40 have been tabulated, and these,
+supplemented by a tabulation of the number of ohms per cubic inch of
+winding space for wires of three different kinds of insulation, are
+given in Table IV.
+
+Bearing in mind that the calculations of Table IV are all based upon
+the "diameter over insulation," which it states at the outset for each
+of four different kinds of covering, it is evident what is meant by
+"turns per linear inch." The columns referring to "turns per square
+inch" mean the number of turns, the ends of which would be exposed in
+one square inch if the wound coil were cut in a plane passing through
+the axis of the core. Knowing the distance between the head, and the
+depth to which the coil is to be wound, it is easy to select a size of
+wire which will give the required number of turns in the provided
+space. It is to be noted that the depth of winding space is one-half
+of the difference between the core diameter and the complete diameter
+of the wound coil. The resistance of the entire volume of wound wire
+may be determined in advance by knowing the total cubic contents of
+the winding space and multiplying this by the ohms per cubic inch of
+the selected wire; that is, one must multiply in inches the distance
+between the heads of the spool by the difference between the squares
+of the diameters of the core and the winding space, and this in turn
+by .7854. This result, times the ohms per cubic inch, as given in the
+table, gives the resistance of the winding.
+
+There is a considerable variation in the method of applying silk
+insulation to the finer wires, and it is in the finer sizes that the
+errors, if any, pile up most rapidly. Yet the table throughout is
+based on data taken from many samples of actual coil winding by the
+present process of winding small coils. It should be said further that
+the table does not take into account the placing of any layers of
+paper between the successive layers of the wires. This table has been
+compared with many examples and has been used in calculating windings
+in advance, and is found to be as close an approximation as is
+afforded by any of the formulas on the subject, and with the further
+advantage that it is not so cumbersome to apply.
+
+_Winding Calculations._ In experimental work, involving the winding of
+coils, it is frequently necessary to try one winding to determine its
+effect in a given circuit arrangement, and from the knowledge so
+gained to substitute another just fitted to the conditions. It is in
+such a substitution that the table is of most value. Assume a case in
+which are required a spool and core of a given size with a winding of,
+say No. 25 single silk-covered wire, of a resistance of 50 ohms.
+Assume also that the circuit regulations required that this spool
+should be rewound so as to have a resistance of, say 1,000 ohms. What
+size single silk-covered wire shall be used? Manifestly, the winding
+space remains the same, or nearly so. The resistance is to be
+increased from 50 to 1,000 ohms, or twenty times its first value.
+Therefore, the wire to be used must show in the table twenty times as
+many ohms per cubic inch as are shown in No. 25, the known first size.
+This amount would be twenty times 7.489, which is 149.8, but there is
+no size giving this exact resistance. No. 32, however, is very nearly
+of that resistance and if wound to exactly the same depth would give
+about 970 ohms. A few turns more would provide the additional thirty
+ohms.
+
+Similarly, in a coil known to possess a certain number of turns, the
+table will give the size to be selected for rewinding to a greater or
+smaller number of turns. In this case, as in the case of substituting
+a winding of different resistance, it is unnecessary to measure and
+calculate upon the dimensions of the spool and core. Assume a spool
+wound with No. 30 double silk-covered wire, which requires to be
+wound with a size to double the number of turns. The exact size to do
+this would have 8922. turns per square inch and would be between No.
+34 and No. 35. A choice of these two wires may be made, using an
+increased winding depth with the smaller wire and a shallower winding
+depth for the larger wire.
+
+Impedance Coils. In telephony electromagnets frequently serve, as
+already stated, to perform other functions than the producing of
+motion by attracting or releasing their armatures. They are required
+to act as impedance coils to present a barrier to the passage of
+alternating or other rapidly fluctuating currents, and at the same
+time to allow the comparatively free passage of steady currents. Where
+it is desired that an electromagnet coil shall possess high impedance,
+it is usual to employ a laminated instead of a solid core. This is
+done by building up a core of suitable size by laying together thin
+sheets of soft iron, or by forming a bundle of soft iron wires. The
+use of laminated cores is for the purpose of preventing eddy currents,
+which, if allowed to flow, would not only be wasteful of energy but
+would also tend to defeat the desired high impedance. Sometimes in
+iron-clad impedance coils, the iron shell is slotted longitudinally to
+break up the flow of eddy currents in the shell.
+
+Frequently electromagnetic coils have only the function of offering
+impedance, where no requirements exist for converting any part of the
+electric energy into mechanical work. Where this is the case, such
+coils are termed _impedance_, or _retardation_, or _choke coils_,
+since they are employed to impede or to retard or to choke back the
+flow of rapidly varying current. The distinction, therefore, between
+an impedance coil and the coil of an ordinary electromagnet is one of
+function, since structurally they may be the same, and the same
+principles of design and construction apply largely to each.
+
+_Number of Turns_. It should be remembered that an impedance coil
+obstructs the passage of fluctuating current, not so much by ohmic
+resistance as by offering an opposing or counter-electromotive force.
+Other things being equal, the counter-electromotive force of
+self-induction increases directly as the number of turns on a coil and
+directly as the number of lines of force threading the coil, and this
+latter factor depends also on the reluctance of the magnetic circuit.
+Therefore, to secure high impedance we need many turns or low
+reluctance, or both. Often, owing to requirements for direct-current
+carrying capacity and limitations of space, a very large number of
+turns is not permissible, in which case sufficiently high impedance to
+such rapid fluctuations as those of voice currents may be had by
+employing a magnetic circuit of very low reluctance, usually a
+completely closed circuit.
+
+_Kind of Iron. _An important factor in the design of impedance coils
+is the grade of iron used in the magnetic circuit. Obviously, it
+should be of the highest permeability and, furthermore, there should
+be ample cross-section of core to prevent even an approach to
+saturation. The iron should, if possible, be worked at that density of
+magnetization at which it has the highest permeability in order to
+obtain the maximum impedance effects.
+
+_Types._ Open-Circuit:--Where very feeble currents are being dealt
+with, and particularly where there is no flow of direct current, an
+open magnetic circuit is much used. An impedance coil having an open
+magnetic circuit is shown in section in Fig. 101, Fig. 102 showing its
+external appearance and illustrating particularly the method of
+bringing out the terminals of the winding.
+
+[Illustration: Fig. 101. Section of Open-Circuit Impedance Coil]
+
+[Illustration: Fig. 102. Open-Circuit Impedance Coil]
+
+[Illustration: Fig. 103. Closed-Circuit Impedance Coil]
+
+Closed-Circuit:--A type of retardation coil which is largely used in
+systems of simultaneous telegraphy and telephony, known as _composite
+systems_, is shown in Fig. 103. In the construction of this coil the
+core is made of a bundle of fine iron wires first bent into U-shape,
+and then after the coils are in place, the free ends of the core are
+brought together to form a closed magnetic circuit. The coils have a
+large number of turns of rather coarse wire. The conditions
+surrounding the use of this coil are those which require very high
+impedance and rather large current-carrying capacity, and fortunately
+the added requirement, that it shall be placed in a very small space,
+does not exist.
+
+Toroidal:--Another type of retardation coil, called the toroidal type
+due to the fact that its core is a torus formed by winding a
+continuous length of fine iron wire, is shown in diagram in Fig. 104.
+The two windings of this coil may be connected in series to form in
+effect a single winding, or it may be used as a "split-winding" coil,
+the two windings being in series but having some other element, such
+as a battery, connected between them in the circuit. Evidently such a
+coil, however connected, is well adapted for high impedance, on
+account of the low reluctance of its core.
+
+[Illustration: Fig. 104. Symbol of Toroidal Impedance Coil]
+
+This coil is usually mounted on a base-board, the coil being enclosed
+in a protecting iron case, as shown in Fig. 105. The terminal wires of
+both windings of each coil are brought out to terminal punchings on
+one end of the base-board to facilitate the making of the necessary
+circuit connections.
+
+[Illustration: Fig. 105. Toroidal Impedance Coil]
+
+The usual diagrammatic symbol for an impedance coil is shown in Fig.
+106. This is the same as for an ordinary bar magnet, except that the
+parallel lines through the core may be taken as indicating that the
+core is laminated, thus conveying the idea of high impedance. The
+symbol of Fig. 104 is a good one for the toroidal type of impedance
+coil.
+
+[Illustration: Fig. 106. Symbol of Impedance Coil]
+
+Induction Coil. An induction coil consists of two or more windings
+of wire interlinked by a common magnetic circuit. In an induction coil
+having two windings, any change in the strength of the current flowing
+in one of the windings, called the _primary_, will cause corresponding
+changes in the magnetic flux threading the magnetic circuit, and,
+therefore, changes in flux through the other winding, called the
+_secondary_. This, by the laws of electromagnetic induction, will
+produce corresponding electromotive forces in the secondary winding
+and, therefore, corresponding currents in that winding if its circuit
+be closed.
+
+_Current and Voltage Ratios._ In a well-designed induction coil the
+energy in the secondary, _i.e._, the induced current, is for all
+practical purposes equal to that of the primary current, yet the
+values of the voltage and the amperage of the induced current may vary
+widely from the values of the voltage and the amperage of the primary
+current. With simple periodic currents, such as the commercial
+alternating lighting currents, the ratio between the voltage in the
+primary and that in the secondary will be equal to the ratio of the
+number of turns in the primary to the number of turns in the
+secondary. Since the energy in the two circuits will be practically
+the same, it follows _that the ratio between the current in the
+primary and that in the secondary will be equal to the ratio of the
+number of turns in the secondary to the number of turns in the
+primary_. In telephony, where the currents are not simple periodic
+currents, and where the variations in current strength take place at
+different rates, such a law as that just stated does not hold for all
+cases; but it may be stated in general that _the induced currents will
+be of higher voltage and smaller current strength than those of the
+primary in all coils where the secondary winding has a greater number
+of turns than the primary_, and _vice versa_.
+
+_Functions._ The function of the induction coil in telephony is,
+therefore, mainly one of transformation, that is, either of stepping
+up the voltage of a current, or in other cases stepping it down. The
+induction coil, however, does serve another purpose in cases where no
+change in voltage and current strength is desired, that is, it serves
+as a means for electrically separating two circuits so far as any
+conductive relation exists, and yet of allowing the free transmission
+by induction from one of these circuits to the other. This is a
+function that in telephony is scarcely of less importance than the
+purely transforming function.
+
+_Design._ Induction coils, as employed in telephony, may be divided
+into two general types: first, those having an open magnetic circuit;
+and, second, those having a closed magnetic circuit. In the design of
+either type it is important that the core should be thoroughly
+laminated, and this is done usually by forming it of a bundle of soft
+Swedish or Norway iron wire about .02 of an inch in diameter. The
+diameter and the length of the coil, and the relation between the
+number of turns in the primary and in the secondary, and the
+mechanical construction of the coil, are all matters which are subject
+to very wide variation in practice. While the proper relationship of
+these factors is of great importance, yet they may not be readily
+determined except by actual experiment with various coils, owing to
+the extreme complexity of the action which takes place in them and to
+the difficulty of obtaining fundamental data as to the existing facts.
+It may be stated, therefore, that the design of induction coils is
+nearly always carried out by "cut-and-try" methods, bringing to bear,
+of course, such scientific and practical knowledge as the experimenter
+may possess.
+
+[Illustration: Fig. 107. Induction Coil]
+
+[Illustration: Fig. 108. Section of Induction Coil]
+
+_Use and Advantage._ The use and advantages of the induction coil in
+so-called local-battery telephone sets have already been explained in
+previous chapters. Such induction coils are nearly always of the open
+magnetic circuit type, consisting of a long, straight core comprised
+of a bundle of small annealed iron wires, on which is wound a primary
+of comparatively coarse wire and having a small number of turns, and
+over which is wound a secondary of comparatively fine wire and having
+a very much larger number of turns. A view of such a coil mounted on a
+base is shown in Fig. 107, and a sectional view of a similar coil is
+shown in Fig. 108. The method of bringing out the winding terminals is
+clearly indicated in this figure, the terminal wires _2_ and _4_ being
+those of the primary winding and _1_ and _3_ those of the secondary
+winding. It is customary to bring out these wires and attach them by
+solder to suitable terminal clips. In the case of the coil shown in
+Fig. 108 these clips are mounted on the wooden heads of the coil,
+while in the design shown in Fig. 107 they are mounted on the base, as
+is clearly indicated.
+
+Repeating Coil. The so-called repeating coil used in telephony is
+really nothing but an induction coil. It is used in a variety of ways
+and usually has for its purpose the inductive association of two
+circuits that are conductively separated. Usually the repeating coil
+has a one to one ratio of turns, that is, there are the same number of
+turns in the primary as in the secondary. However, this is not always
+the case, since sometimes they are made to have an unequal number of
+turns, in which case they are called _step-up _or _step-down_
+repeating coils, according to whether the primary has a smaller or a
+greater number of turns than the secondary. Repeating coils are almost
+universally of the closed magnetic circuit type.
+
+_Ringing and Talking Considerations._ Since repeating coils often
+serve to connect two telephones, it follows that it is sometimes
+necessary to ring through them as well as talk through them. By this
+is meant that it is necessary that the coil shall be so designed as to
+be capable of transforming the heavy ringing currents as well as the
+very much smaller telephone or voice currents. Ringing currents
+ordinarily have a frequency ranging from about 16 to 75 cycles per
+second, while voice currents have frequencies ranging from a few
+hundred up to perhaps ten thousand per second. Ordinarily, therefore,
+the best form of repeating coil for transforming voice currents is not
+the best for transforming the heavy ringing currents and _vice versa_.
+If the comparatively heavy ringing currents alone were to be
+considered, the repeating coil might well be of heavy construction
+with a large amount of iron in its magnetic circuit. On the other
+hand, for carrying voice currents alone it is usually made with a
+small amount of iron and with small windings, in order to prevent
+waste of energy in the core, and to give a high degree of
+responsiveness with the least amount of distortion of wave form, so
+that the voice currents will retain as far as possible their original
+characteristics. When, therefore, a coil is required to carry both
+ringing and talking currents, a compromise must be effected.
+
+_Types._ The form of repeating coil largely used for both ringing and
+talking through is shown in Fig. 109. This coil comprises a soft iron
+core made up of a bundle of wires about .02 inch in diameter, the ends
+of which are left of sufficient length to be bent back around the
+windings after they are in place and thus form a completely closed
+magnetic path for the core. The windings of this particular coil are
+four in number, and contain about 2,400 turns each, and have a
+resistance of about 60 ohms. In this coil, when connected for local
+battery work, the windings are connected in pairs in series, thus
+forming effectively two windings having about 120 ohms resistance
+each. The whole coil is enclosed in a protecting case of iron. The
+terminals are brought out to suitable clips on the wooden base, as
+shown. An external perspective view of this coil is shown in Fig. 110.
+By bringing out each terminal of each winding, eight in all, as shown
+in this figure, great latitude of connection is provided for, since
+the windings may be connected in circuit in any desirable way, either
+by connecting them together in pairs to form virtually a primary and a
+secondary, or, as is frequently the case, to split the primary and the
+secondary, connecting a battery between each pair of windings.
+
+[Illustration: Fig. 109. Repeating Coil]
+
+[Illustration: Fig. 110. Repeating Coil]
+
+Fig. 111 illustrates in section a commercial type of coil designed
+for talking through only. This coil is provided with four windings of
+1,357 turns each, and when used for local battery work the coils are
+connected in pairs in series, thus giving a resistance of about 190
+ohms in each half of the repeating coil. The core of this coil
+consists of a bundle of soft iron wires, and the shell which forms the
+return path for the magnetic lines is of very soft sheet iron. This
+shell is drawn into cup shape and its open end is closed, after the
+coil is inserted, by the insertion of a soft iron head, as indicated.
+As in the case of the coil shown in Figs. 109 and 110, eight terminals
+are brought out on this coil, thus providing the necessary flexibility
+of connection.
+
+[Illustration: Fig. 111. Repeating Coil]
+
+[Illustration: Fig. 112. Diagram of Toroidal Repeating Coil]
+
+[Illustration: Fig. 113. Toroidal Repeating Coil]
+
+Still another type of repeating coil is illustrated in diagram in Fig.
+112, and in view in Fig. 113. This coil, like the impedance coil shown
+in Fig. 104, comprises a core made up of a bundle of soft iron wires
+wound into the form of a ring. It is usually provided with two primary
+windings placed opposite each other upon the core, and with two
+secondary windings, one over each primary. In practice these two
+primary windings are connected in one circuit and the two secondaries
+in another. This is the standard repeating coil now used by the Bell
+companies in their common-battery cord circuits.
+
+[Illustration: THE OPERATING ROOM OF THE EXCHANGE AT WEBB CITY,
+MISSOURI]
+
+[Illustration: Fig. 114. Symbol of Induction Coil]
+
+Conventional Symbols. The ordinary symbol for the induction coil
+used in local battery work is shown in Fig. 114. This consists merely
+of a pair of parallel zig-zag lines. The primary winding is usually
+indicated by a heavy line having a fewer number of zig-zags, and the
+secondary by a finer line having a greater number of zig-zags. In this
+way the fact that the primary is of large wire and of comparatively
+few turns is indicated. This diagrammatic symbol may be modified to
+suit almost any conditions, and where a tertiary as well as a
+secondary winding is provided it may be shown by merely adding another
+zig-zag line.
+
+[Illustration: Fig. 115. Repeating-Coil Symbols]
+
+The repeating coil is indicated symbolically in the two diagrams of
+Fig. 115. Where there is no necessity for indicating the internal
+connections of the coil, the symbol shown in the left of this figure
+is usually employed. Where, however, the coil consists of four
+windings rather than two and the method of connecting them is to be
+indicated, the symbol at the right hand is employed. In Fig. 116
+another way of indicating a four-winding repeating coil or induction
+coil is shown. Sometimes such windings may be combined by connection
+to form merely a primary and a secondary winding, and in other cases
+the four windings all act separately, in which case one may be
+considered the primary and the others, respectively, the secondary,
+tertiary, and quaternary.
+
+[Illustration: Fig. 116. Symbol of Four-Winding Repeating Coil]
+
+Where the toroidal type of repeating coil is employed, the diagram of
+Fig. 112, already referred to, is a good symbolic representation.
+
+
+
+
+CHAPTER XI
+
+NON-INDUCTIVE RESISTANCE DEVICES
+
+
+It is often desired to introduce simple ohmic resistance into
+telephone circuits, in order to limit the current flow, or to create
+specific differences of potential at given points in the circuit.
+
+Temperature Coefficient. The design or selection of resistance
+devices for various purposes frequently involves the consideration of
+the effect of temperature on the resistance of the conductor employed.
+The resistance of conductors is subject to change by changes in
+temperature. While nearly all metals show an increase, carbon shows a
+decrease in its resistance when heated.
+
+The temperature coefficient of a conductor is a factor by which the
+resistance of the conductor at a given temperature must be multiplied
+in order to determine the change in resistance of that conductor
+brought about by a rise in temperature of one degree.
+
+TABLE V
+
+Temperature Coefficients
+
++---------------------------+-----------------------------+
+|       PURE METALS         |  TEMPERATURE  COEFFICIENTS  |
++---------------------------+--------------+--------------+
+|                           |  CENTIGRADE  |  FAHRENHEIT  |
++---------------------------+--------------+--------------+
+| Silver (annealed)         |   0.00400    |   0.00222    |
+| Copper (annealed)         |   0.00428    |   0.00242    |
+| Gold (99.9%)              |   0.00377    |   0.00210    |
+| Aluminum (99%)            |   0.00423    |   0.00235    |
+| Zinc                      |   0.00406    |   0.00226    |
+| Platinum (annealed)       |   0.00247    |   0.00137    |
+| Iron                      |   0.00625    |   0.00347    |
+| Nickel                    |   0.0062     |   0.00345    |
+| Tin                       |   0.00440    |   0.00245    |
+| Lead                      |   0.00411    |   0.00228    |
+| Antimony                  |   0.00389    |   0.00216    |
+| Mercury                   |   0.00072    |   0.00044    |
+| Bismuth                   |   0.00354    |   0.00197    |
++---------------------------+--------------+--------------+
+
+_Positive and Negative Coefficients._ Those conductors, in which a
+rise in temperature produces an increase in resistance, are said to
+have positive temperature coefficients, while those in which a rise in
+temperature produces a lowering of resistance are said to have
+negative temperature coefficients.
+
+The temperature coefficients of pure metals are always positive and
+for some of the more familiar metals, have values, according to
+Foster, as in Table V.
+
+Iron, it will be noticed, has the highest temperature coefficient of
+all. Carbon, on the other hand, has a large negative coefficient, as
+proved by the fact that the filament of an ordinary incandescent lamp
+has nearly twice the resistance when cold as when heated to full
+candle-power.
+
+Certain alloys have been produced which have very low temperature
+coefficients, and these are of value in producing resistance units
+which have practically the same resistance for all ordinary
+temperatures. Some of these alloys also have very high resistance as
+compared with copper and are of value in enabling one to obtain a high
+resistance in small space.
+
+One of the most valuable resistance wires is of an alloy known as
+_German silver_. The so-called eighteen per cent alloy has
+approximately 18.3 times the resistance of copper and a temperature
+coefficient of .00016 per degree Fahrenheit. The thirty per cent alloy
+has approximately 28 times the resistance of copper and a temperature
+coefficient of .00024 per degree Fahrenheit.
+
+For facilitating the design of resistance coils of German silver wire,
+Tables VI and VII are given, containing information as to length,
+resistance, and weight of the eighteen per cent and the thirty per
+cent alloys, respectively, for all sizes of wire smaller than No. 20
+B. & S. gauge.
+
+Special resistance alloys may be obtained having temperature
+coefficients as low as .000003 per degree Fahrenheit. Other alloys of
+nickel and steel are adapted for use where the wire must carry heavy
+currents and be raised to comparatively high temperatures thereby; for
+such use non-corrosive properties are specially to be desired. Such
+wire may be obtained having a resistance of about fifty times that of
+copper.
+
+TABLE VI
+
+18 Per Cent German Silver Wire
+
++---------+----------+-----------------+----------------+---------------+
+|   No.   |          |                 |                |               |
+| B. & S. | DIAMETER |     WEIGHT      |     LENGTH     |  RESISTANCE   |
+| GAUGE   |  INCHES  | POUNDS PER FOOT | FEET PER POUND | OHMS PER FOOT |
++---------+----------+-----------------+----------------+---------------+
+|   21    |  .02846  |    .002389      |     418.6      |     .2333     |
+|   22    |  .02535  |    .001894      |     527.9      |     .2941     |
+|   23    |  .02257  |    .001502      |     665.8      |     .3710     |
+|   24    |  .02010  |    .001191      |     839.5      |     .4678     |
+|   25    |  .01790  |    .0009449     |    1058.       |     .5899     |
+|   26    |  .01594  |    .0007493     |    1335.       |     .7438     |
+|   27    |  .01419  |    .0005943     |    1683.       |     .9386     |
+|   28    |  .01264  |    .0004711     |    2123.       |    1.183      |
+|   29    |  .01126  |    .0003735     |    2677.       |    1.491      |
+|   30    |  .01003  |    .0002962     |    3376.       |    1.879      |
+|   31    |  .008928 |    .0002350     |    4255.       |    2.371      |
+|   32    |  .007950 |    .0001864     |    5366.       |    2.990      |
+|   33    |  .007080 |    .0001478     |    6766.       |    3.771      |
+|   34    |  .006304 |    .0001172     |    8532.       |    4.756      |
+|   35    |  .005614 |    .00009295    |   10758.       |    5.997      |
+|   36    |  .005000 |    .00007369    |   13569.       |    7.560      |
+|   37    |  .004453 |    .00005845    |   17108.       |    9.532      |
+|   38    |  .003965 |    .00004636    |   21569.       |   12.02       |
+|   39    |  .003531 |    .00003675    |   27209.       |   15.16       |
+|   40    |  .003145 |    .00002917    |   34282.       |   19.11       |
++---------+----------+-----------------+----------------+---------------+
+
+Inductive Neutrality. Where the resistance unit is required to be
+strictly non-inductive, and is to be in the form of a coil, special
+designs must be employed to give the desired inductive neutrality.
+
+Provisions Against Heating. In cases where a considerable amount of
+heat is to be generated in the resistance, due to the necessity of
+carrying large currents, special precautions must be taken as to the
+heat-resisting properties of the structure, and also as to the
+provision of sufficient radiating surface or its equivalent to provide
+for the dissipation of the heat generated.
+
+Types. _Mica Card Unit._ One of the most common resistance coils
+used in practice is shown in Fig. 117. This comprises a coil of fine,
+bare German silver wire wound on a card of mica, the windings being so
+spaced that the loops are not in contact with each other. The winding
+is protected by two cards of mica and the whole is bound in place by
+metal strips, to which the ends of the winding are attached. Binding
+posts are provided on the extended portions of the terminals to assist
+in mounting the resistance on a supporting frame, and the posts
+terminate in soldering terminals by which the resistance is connected
+into the circuit.
+
+TABLE VII
+
+30 Per Cent German Silver Wire
+
++---------+----------+-----------------+----------------+---------------+
+|   No.   |          |                 |                |               |
+| B. & S. | DIAMETER |     WEIGHT      |     LENGTH     |  RESISTANCE   |
+| GAUGE   |  INCHES  | POUNDS PER FOOT | FEET PER POUND | OHMS PER FOOT |
++---------+----------+-----------------+----------------+---------------+
+|   21    |  .02846  |    .002405      |     415.8      |     .3581     |
+|   22    |  .02535  |    .001907      |     524.4      |     .4513     |
+|   23    |  .02257  |    .001512      |     661.3      |     .5693     |
+|   24    |  .02010  |    .001199      |     833.9      |     .7178     |
+|   25    |  .01790  |    .0009513     |    1051.       |     .9051     |
+|   26    |  .01594  |    .0007544     |    1326.       |    1.141      |
+|   27    |  .01419  |    .0005983     |    1671.       |    1.440      |
+|   28    |  .01264  |    .0004743     |    2108.       |    1.815      |
+|   29    |  .01126  |    .0003761     |    2659.       |    2.287      |
+|   30    |  .01003  |    .0002982     |    3353.       |    2.883      |
+|   31    |  .008928 |    .0002366     |    4227.       |    3.638      |
+|   32    |  .007950 |    .0001876     |    5330.       |    4.588      |
+|   33    |  .007080 |    .0001488     |    6721.       |    5.786      |
+|   34    |  .006304 |    .0001180     |    8475.       |    7.297      |
+|   35    |  .005614 |    .00009358    |   10686.       |    9.201      |
+|   36    |  .005000 |    .00007419    |   13478.       |   11.60       |
+|   37    |  .004453 |    .00005885    |   16994.       |   14.63       |
+|   38    |  .003965 |    .00004668    |   21424.       |   18.45       |
+|   39    |  .003531 |    .00003700    |   27026.       |   23.26       |
+|   40    |  .003145 |    .00002937    |   34053.       |   29.32       |
++---------+----------+-----------------+----------------+---------------+
+
+
+_Differentially-Wound Unit._ Another type of resistance coil is that
+in which the winding is placed upon an insulating core of
+heat-resisting material and wound so as to overcome inductive effects.
+In order to accomplish this, the wire to be bound on the core is
+doubled back on itself at its middle portion to form two strands, and
+these are wound simultaneously on the core, thus forming two spirals
+of equal number of turns. The current in traversing the entire coil
+must flow through one spiral in one direction with relation to the
+core, and in the opposite direction in the other spiral, thereby
+nullifying the inductive effects of one spiral by those of the other.
+This is called a _non-inductive winding_ and is in reality an example
+of differential winding.
+
+_Lamp Filament._ An excellent type of non-inductive resistance is the
+ordinary carbon-filament incandescent lamp. This is used largely in
+the circuits of batteries, generators, and other sources of supply to
+prevent overload in case of short circuits on the line. These are
+cheap, durable, have large current-carrying capacities, and are not
+likely to set things afire when overheated. An additional advantage
+incident to their use for this purpose is that an overload on a
+circuit in which they are placed is visibly indicated by the glowing
+of the lamp.
+
+[Illustration: Fig. 117. Mica Card Resistance]
+
+[Illustration: Fig. 118. Iron-Wire Ballast]
+
+Obviously, the carbon-filament incandescent lamp, when used as a
+resistance, has, on account of the negative temperature coefficient of
+carbon, the property of presenting the highest resistance to the
+circuit when carrying no current, and of presenting a lower and lower
+resistance as the current and consequent heating increases. For some
+conditions of practice this is not to be desired, and the opposite
+characteristic of presenting low resistance to small currents and
+comparatively high resistance to large currents would best meet the
+conditions of practice.
+
+_Iron-Wire Ballast._ Claude D. Enochs took advantage of the very high
+positive temperature coefficient of iron to produce a resistance
+device having these characteristics. His arrangement possesses the
+compactness of the carbon-filament lamp and is shown in Fig. 118. The
+resistance element proper is an iron wire, wound on a central stem of
+glass, and this is included in an exhausted bulb so as to avoid
+oxidation. Such a resistance is comparatively low when cold, but when
+traversed by currents sufficient to heat it considerably will offer a
+very large increase of resistance to oppose the further increase of
+current. In a sense, it is a self-adjusting resistance, tending
+towards the equalization of the flow of current in the circuit in
+which it is placed.
+
+
+
+
+CHAPTER XII
+
+CONDENSERS
+
+
+Charge. A conducting body insulated from all other bodies will
+receive and hold a certain amount of electricity (a charge), if
+subjected to an electrical potential. Thus, referring to Fig. 119, if
+a metal plate, insulated from other bodies, be connected with, say,
+the positive pole of a battery, the negative pole of which is
+grounded, a current will flow into the plate until the plate is raised
+to the same potential as that of the battery pole to which it is
+connected. The amount of electricity that will flow into the plate
+will depend, other things being equal, on the potential of the source
+from which it is charged; in fact, it is proportional to the potential
+of the source from which it is charged. This amount of electricity is
+a measure of the capacity of the plate, just as the amount of water
+that a bath-tub will hold is a measure of the capacity of the
+bath-tub.
+
+Capacity. Instead of measuring the amount of electricity by the
+quart or pound, as in the case of material things, the unit of
+electrical quantity is the _coulomb_. The unit of capacity of an
+insulated conductor is the _farad_, and a given insulated conductor is
+said to have unit capacity, that is, the capacity of one farad, when
+it will receive a charge of one coulomb of electricity at a potential
+of one volt.
+
+Referring to Fig. 119, the potential of the negative terminal of the
+battery may be said to be zero, since it is connected to the earth. If
+the battery shown be supposed to have exactly one volt potential, then
+the plate would be said to have the capacity of one farad if one
+coulomb of electricity flowed from the battery to the plate before the
+plate was raised to the same potential as that of the positive pole,
+that is, to a potential of one volt above the potential of the earth;
+it being assumed that the plate was also at zero potential before the
+connection was made. Another conception of this quantity may be had by
+remembering that a coulomb is such a quantity of current as will
+result from one ampere flowing one second.
+
+The capacity of a conductor depends, among other things, on its area.
+If the plate of Fig. 119 should be made twice as large in area, other
+things remaining the same, it would have twice the capacity. But there
+are other factors governing the capacity of a conductor. Consider the
+diagram of Fig. 120, which is supposed to represent two such plates as
+are shown in Fig. 119, placed opposite each other and connected
+respectively with the positive and the negative poles of the battery.
+When the connection between the plates and the battery is made, the
+two plates become charged to a difference of potential equal to the
+electromotive force of the battery. In order to obtain these charges,
+assume that the plates were each at zero potential before the
+connection was made; then current flows from the battery into the
+plates until they each assume the potential of the corresponding
+battery terminal. If the two plates be brought closer together, it
+will be found that more current will now flow into each of them,
+although the difference of potential between the two plates must
+obviously remain the same, since each of them is still connected to
+the battery.
+
+[Illustration: Fig. 119. Condenser Plate]
+
+Theory. Due to the proximity of the plates, the positive electricity
+on plate _A_ is drawn by the negative charge on plate _B_ towards
+plate _B_, and likewise the negative electricity on plate _B_ is drawn
+to the side towards plate _A_ by the positive charge on that plate.
+These two charges so drawn towards each other will, so to speak, bind
+each other, and they are referred to as _bound charges_. The charge on
+the right-hand side of plate _A_ and on the left-hand side of plate
+_B_ will, however, be free charges, since there is nothing to attract
+them, and these are, therefore, neutralized by a further flow of
+electricity from the battery to the plate.
+
+[Illustration: Fig. 120. Theory of Condenser]
+
+Obviously, the closer together the plates are the stronger will be the
+attractive influence of the two charges on each other. From this it
+follows that in the case of plate _A_, when the two plates are being
+moved closer together, more positive electricity will flow into plate
+_A_ to neutralize the increasing free negative charges on the
+right-hand side of the plate. As the plates are moved closer together
+still, a new distribution of charges will take place, resulting in
+more positive electricity flowing into plate _A_ and more negative
+electricity flowing into plate _B_. The closer proximity of the
+plates, therefore, increases the capacity of the plates for holding
+charges, due to the increased inductive action across the dielectric
+separating the plates.
+
+Condenser Defined. A condenser is a device consisting of two
+adjacent plates of conducting material, separated by an insulating
+material, called a _dielectric_. The purpose is to increase by the
+proximity of the plates, each to the other, the amount of electricity
+which each plate will receive and hold when subjected to a given
+potential.
+
+Dielectric. We have already seen that the capacity of a condenser
+depends upon the area of its plates, and also upon their distance
+apart. There is still another factor on which the capacity of a
+condenser depends, _i.e._, on the character of the insulating medium
+separating its plates. The inductive action which takes place between
+a charged conductor and other conductors nearby it, as between plate
+_A_ and plate _B_ of Fig. 120, is called _electrostatic induction_,
+and it plays an important part in telephony. It is found that the
+ability of a given charged conductor to induce charges on other
+neighboring conductors varies largely with the insulating medium or
+dielectric that separates them. This quality of a dielectric, by which
+it enables inductive action to take place between two separated
+conductors, is called _inductive capacity_. Usually this quality of
+dielectrics is measured in terms of the same quality in dry air, this
+being taken as unity. When so expressed, it is termed _specific
+inductive capacity_. To be more accurate the specific inductive
+capacity of a dielectric is the ratio between the capacity of a
+condenser having that substance as a dielectric, to the capacity of
+the same condenser using dry air at zero degrees Centigrade and at a
+pressure of 14.7 pounds per square inch as the dielectric. To
+illustrate, if two condensers having plates of equal size and equal
+distance apart are constructed, one using air as the dielectric and
+the other using hard crown glass as the dielectric, the one using
+glass will have a capacity of 6.96 times that of the one using air.
+From this we say that crown glass has a specific inductive capacity of
+6.96.
+
+Various authorities differ rather widely as to the specific inductive
+capacity of many common substances. The values given in Table VIII
+have been chosen from the Smithsonian Physical Tables.
+
+TABLE VIII
+
+Specific Inductive Capacities
+
++-----------------------+------------------------+
+|DIELECTRIC             |  REFERRED TO AIR AS 1  |
++-----------------------+------------------------+
+|Vacuum                 |   .99941               |
+|Hydrogen               |   .99967               |
+|Carbonic Acid          |  1.00036               |
+|Dry Paper              |  1.25 to 1.75          |
+|Paraffin               |  1.95 to 2.32          |
+|Ebonite                |  1.9  to 3.48          |
+|Sulphur                |  2.24 to 3.90          |
+|Shellac                |  2.95 to 3.73          |
+|Gutta-percha           |  3.3  to 4.9           |
+|Plate Glass            |  3.31 to 7.5           |
+|Porcelain              |  4.38                  |
+|Mica                   |  4.6  to 8.0           |
+|Glass--Light Flint     |  6.61                  |
+|Glass--Hard Crown      |  6.96                  |
+|Selenium               | 10.2                   |
++-----------------------+------------------------+
+
+This data is interesting as showing the wide divergence in specific
+inductive capacities of various materials, and also showing the wide
+divergence in different observations of the same material.
+Undoubtedly, this latter is due mainly to the fact that various
+materials differ largely in themselves, as in the case of paraffin,
+for instance, which exhibits widely different specific inductive
+capacities according to the difference in rapidity with which it is
+cooled in changing from a liquid to a solid state.
+
+We see then that the capacity of a condenser varies as the area of its
+plates, as the specific inductive capacity of the dielectric employed,
+and also inversely as the distance between the plates.
+
+Obviously, therefore, in making a condenser of large capacity, it is
+important to have as large an area of the plate as possible; to have
+them as close together as possible; to have the dielectric a good
+insulating medium so that there will be practically no leakage between
+the plates; and to have the dielectric of as high a specific inductive
+capacity as economy and suitability of material in other respects will
+permit.
+
+Dielectric Materials. _Mica_. Of all dielectrics mica is the most
+suitable for condensers, since it has very high insulation resistance
+and also high specific inductive capacity, and furthermore may be
+obtained in very thin sheets. High-grade condensers, such as are used
+for measurements and standardization purposes, usually have mica for
+the dielectric.
+
+[Illustration: Fig. 121. Rolled Condenser]
+
+_Dry Paper. _The demands of telephonic practice are, however, such as
+to require condensers of very cheap construction with large capacity
+in a small space. For this purpose thin bond paper, saturated with
+paraffin, has been found to be the best dielectric. The conductors in
+condensers are almost always of tinfoil, this being an ideal material
+on account of its cheapness and its thinness. Before telephony made
+such urgent demands for a cheap compact condenser, the customary way
+of making them was to lay up alternate sheets of dielectric material,
+either of oiled paper or mica and tinfoil, the sheets of tinfoil being
+cut somewhat smaller than the sheets of dielectric material in order
+that the proper insulation might be secured at the edges. After a
+sufficient number of such plates were built up the alternate sheets of
+tinfoil were connected together to form one composite plate of the
+condenser, while the other sheets were similarly connected together to
+form the other plate. Obviously, in this way a very large area of
+plates could be secured with a minimum degree of separation.
+
+[Illustration: Fig. 122. Rolled Condenser]
+
+There has been developed for use in telephony, however, and its use
+has since extended into other arts requiring condensers, what is
+called the _rolled condenser_. This is formed by rolling together in a
+flat roll four sheets of thin bond paper, _1_, _2_, _3_, and _4_, and
+two somewhat narrower strips of tinfoil, _5_ and _6_, Fig. 121. The
+strips of tinfoil and paper are fed on to the roll in continuous
+lengths and in such manner that two sheets of paper will lie between
+the two strips of tinfoil in all cases. Thin sheet metal terminals _7_
+and _8_ are rolled into the condenser as it is being wound, and as
+these project beyond the edges of the paper they form convenient
+terminals for the condenser after it is finished. After it is rolled,
+the roll is boiled in hot paraffin so as to thoroughly impregnate it
+and expel all moisture. It is then squeezed in a press and allowed to
+cool while under pressure. In this way the surplus paraffin is
+expelled and the plates are brought very close together. It then
+appears as in Fig. 122. The condenser is now sealed in a metallic
+case, usually rectangular in form, and presents the appearance shown
+in Fig. 123.
+
+[Illustration: Fig. 123. Rolled Condenser]
+
+A later method of condenser making which has not yet been thoroughly
+proven in practice, but which bids fair to produce good results,
+varies from the method just described in that a paper is used which in
+itself is coated with a very thin conducting material. This conducting
+material is of metallic nature and in reality forms a part of the
+paper. To form a condenser of this the sheets are merely rolled
+together and then boiled in paraffin and compressed as before.
+
+Sizes. The condensers ordinarily used in telephone practice range in
+capacity from about 1/4 microfarad to 2 microfarads. When larger
+capacities than 2 microfarads are desired, they may be obtained by
+connecting several of the smaller size condensers in multiple. Table
+IX gives the capacity, shape, and dimensions of a variety of
+condensers selected from those regularly on the market.
+
+TABLE IX
+
+Condenser Data
+
++------------+---------------+---------------------------------+
+|            |               |      DIMENSIONS IN INCHES       |
+|  CAPACITY  |     SHAPE     |----------+----------+-----------+
+|            |               |  Height  |   Width  | Thickness |
++------------+---------------+----------+----------+-----------+
+|    2 m. f. |  Rectangular  |  9-1/6   |  4-3/4   |   11/16   |
+|    1 m. f. |       "       |  9-1/6   |  4-3/4   |   11/16   |
+|    1 m. f. |       "       |  4-3/4   |  2-3/32  |   13/16   |
+|  1/2 m. f. |       "       |  2-3/4   |  1-1/4   |    3/4    |
+|    1 m. f. |       "       |  4-13/16 |  2-1/32  |   25/32   |
+|  1/2 m. f. |       "       |  4-3/4   |  2-3/32  |   13/16   |
+| 3/10 m. f. |       "       |  4-3/4   |  2-3/32  |   13/16   |
+|    1 m. f. |       "       |  2-3/4   |  3       |     l     |
++------------+---------------+----------+----------+-----------+
+
+
+Conventional Symbols. The conventional symbols usually employed to
+represent condensers in telephone diagrams are shown in Fig. 124.
+These all convey the idea of the adjacent conducting plates separated
+by insulating material.
+
+[Illustration: Fig. 124. Condenser Symbols]
+
+Functions. Obviously, when placed in a circuit a condenser offers a
+complete barrier to the flow of direct current, since no conducting
+path exists between its terminals, the dielectric offering a very high
+insulation resistance. If, however, the condenser is connected across
+the terminals of a source of alternating current, this current flows
+first in one direction and then in the other, the electromotive force
+in the circuit increasing from zero to a maximum in one direction, and
+then decreasing back to zero and to a maximum in the other direction,
+and so on. With a condenser connected so as to be subjected to such
+alternating electromotive forces, as the electromotive force begins to
+rise the electromotive force at the condenser terminals will also rise
+and a current will, therefore, flow into the condenser. When the
+electromotive force reaches its maximum, the condenser will have
+received its full charge for that potential, and the current flow into
+it will cease. When the electromotive force begins to fall, the
+condenser can no longer retain its charge and a current will,
+therefore, flow out of it. Apparently, therefore, there is a flow of
+current through the condenser the same as if it were a conductor.
+
+Means for Assorting Currents. In conclusion, it is obvious that the
+telephone engineer has within his reach in the various coils--whether
+non-inductive or inductive, or whether having one or several
+windings--and in the condenser, a variety of tools by which he may
+achieve a great many useful ends in his circuit work. Obviously, the
+condenser affords a means for transmitting voice currents or
+fluctuating currents, and for excluding steady currents. Likewise the
+impedance coil affords a means for readily transmitting steady
+currents but practically excluding voice currents or fluctuating
+currents. By the use of these very simple devices it is possible to
+sift out the voice currents from a circuit containing both steady and
+fluctuating currents, or it is possible in the same manner to sift out
+the steady currents and to leave the voice currents alone to traverse
+the circuit.
+
+Great use is made in the design of telephone circuits of the fact that
+the electromagnets, which accomplish the useful mechanical results in
+causing the movement of parts, possess the quality of impedance. Thus,
+the magnets which operate various signaling relays at the central
+office are often used also as impedance coils in portions of the
+circuit through which it is desired to have only steady currents pass.
+If, on the other hand, it is necessary to place a relay magnet, having
+considerable impedance, directly in a talking circuit, the bad effects
+of this on the voice currents may be eliminated by shunting this coil
+with a condenser, or with a comparatively high non-inductive
+resistance. The voice currents will flow around the high impedance of
+the relay coil through the condenser or resistance, while the steady
+currents, which are the ones which must be depended upon to operate
+the relay, are still forced in whole or in part to pass through the
+relay coil where they belong.
+
+In a similar way the induction coil affords a means for keeping two
+circuits completely isolated so far as the direct flow of current
+between them is concerned, and yet of readily transmitting, by
+electromagnetic induction, currents from one of these circuits to the
+other. Here is a means of isolation so far as direct current is
+concerned, with complete communication for alternating current.
+
+
+
+
+CHAPTER XIII
+
+CURRENT SUPPLY TO TRANSMITTERS
+
+
+The methods by which current is supplied to the transmitter of a
+telephone for energizing it, may be classified under two divisions:
+first, those where the battery or other source of current is located
+at the station with the transmitter which it supplies; and second,
+those where the battery or other source of current is located at a
+distant point from the transmitter, the battery in such cases serving
+as a common source of current for the supply of transmitters at a
+number of stations.
+
+The advantages of putting the transmitter and the battery which
+supplies it with current in a local circuit with the primary of an
+induction coil, and placing the secondary of the induction coil in the
+line, have already been pointed out but may be briefly summarized as
+follows: When the transmitter is placed directly in the _line circuit_
+and the line is of considerable length, the current which passes
+through the transmitter is necessarily rather small unless a battery
+of high potential is used; and, furthermore, the total change in
+resistance which the transmitter is capable of producing is but a
+small proportion of the total resistance of the line, and, therefore,
+the current changes produced by the transmitter are relatively small.
+On the other hand, when the transmitter is placed in a _local circuit_
+with the battery, this circuit may be of small resistance and the
+current relatively large, even though supplied by a low-voltage
+battery; so that the transmitter is capable of producing relatively
+large changes in a relatively large current.
+
+To draw a comparison between these two general classes of transmitter
+current supply, a number of cases will be considered in connection
+with the following figures, in each of which two stations connected by
+a telephone line are shown. Brief reference to the local battery
+method of supplying current will be made in order to make this chapter
+contain, as far as possible, all of the commonly used methods of
+current supply to transmitters.
+
+[Illustration: A TYPICAL MEDIUM-SIZED MULTIPLE SWITCHBOARD EQUIPMENT]
+
+Local Battery. In Fig. 125 two stations are shown connected by a
+grounded line wire. The transmitter of each station is included in a
+low-resistance primary circuit including a battery and the primary
+winding of an induction coil, the relation between the primary
+circuits and the line circuits being established by the inductive
+action between the primary and the secondary windings of induction
+coils, the secondary in each case being in the line circuits with the
+receivers.
+
+[Illustration: Fig. 125. Local-Battery Stations with Grounded Circuit]
+
+Fig. 126 shows exactly the same arrangement but with a metallic
+circuit rather than a grounded circuit. The student should become
+accustomed to the replacing of one of the line wires of a metallic
+circuit by the earth, and to the method, employed in Figs. 125 and
+126, of indicating a grounded circuit as distinguished from a metallic
+circuit.
+
+[Illustration: Fig. 126. Local-Battery Stations with Metallic Circuit]
+
+In Fig. 127 is shown a slight modification of the circuit shown in
+Fig. 126, which consists of connecting one end of the primary winding
+to one end of the secondary winding of the induction coil, thus
+linking together the primary circuit and the line circuit, a portion
+of each of these circuits being common to a short piece of the local
+wiring. There is no difference whatever in the action of the circuits
+shown in Figs. 126 and 127, the latter being shown merely for the
+purpose of bringing out this fact. It is very common, particularly in
+local-battery circuits, to connect one end of the primary and the
+secondary windings, as by doing so it is often possible to save a
+contact point in the hook switch and also to simplify the wiring.
+
+[Illustration: Fig. 127. Local-Battery Stations with Metallic Circuit]
+
+The advantages to be gained by employing a local battery at each
+subscriber's station associated with the transmitter in the primary
+circuit of an induction coil are attended by certain disadvantages
+from a commercial standpoint. The primary battery is not an economical
+way to generate electric energy. In all its commercial forms it
+involves the consumption of zinc and zinc is an expensive fuel. The
+actual amount of current in watts required by a telephone is small,
+however, and this disadvantage due to the inexpensive method of
+generating current would not in itself be of great importance. A more
+serious objection to the use of local batteries at subscribers'
+stations appears when the subject is considered from the standpoint of
+maintenance. Batteries, whether of the so-called "dry" or "wet" type,
+gradually deteriorate, even when not used, and in cases where the
+telephone is used many times a day the deterioration is comparatively
+rapid. This makes necessary the occasional renewals of the batteries
+with the attendant expense for new batteries or new material, and of
+labor and transportation in visiting the station. The labor item
+becomes more serious when the stations are scattered in a sparsely
+settled community, in which case the visiting of the stations, even for
+the performance of a task that would require but a few minutes' time,
+may consume some hours on the part of the employes in getting there and
+back.
+
+Common Battery. _Advantages._ It would be more economical if all of
+the current for the subscribers' transmitters could be supplied from a
+single comparatively efficient generating source instead of from a
+multitude of inefficient small sources scattered throughout the
+community served by the exchange. The advantage of such centralization
+lies not only in more economic generating means, but also in having
+the common source of current located at one place, where it may be
+cared for with a minimum amount of expense. Such considerations have
+resulted in the so-called "common-battery system," wherein the current
+for all the subscribers' transmitters is furnished from a source
+located at the central office.
+
+Where such a method of supplying current is practiced, the result has
+also been, in nearly all cases, the doing away with the subscriber's
+magneto generators, relying on the central-office source of current to
+furnish the energy for enabling the subscriber to signal the operator.
+Such systems, therefore, concentrate all of the sources of energy at
+the central office and for that reason they are frequently referred to
+as central-energy systems.
+
+     NOTE. In this chapter the central-energy or common-battery system
+     will be considered only in so far as the supply of current for
+     energizing the subscribers' transmitters is concerned, the
+     discussion of the action of signaling being reserved for
+     subsequent chapters.
+
+_Series Battery._ If but a single pair of lines had to be considered,
+the arrangement shown in Fig. 128 might be employed. In this the
+battery is located at the central office and placed in series with the
+two grounded lines leading from the central office to the two
+subscribers' stations. The voltage of this battery is made sufficient
+to furnish the required current over the resistance of the entire line
+circuit with its included instruments. Obviously, changes in
+resistance in the transmitter at Station A will affect the flow of
+current in the entire line and the fluctuations resulting from the
+vibration of the transmitter diaphragm will, therefore, reproduce
+these sounds in the receiver at Station B, as well as in that at
+Station A.
+
+[Illustration: Fig. 128. Battery in Series with Two Lines]
+
+An exactly similar arrangement applied to a metallic circuit is shown
+in Fig. 129. In thus placing the battery in series in the circuit
+between the two stations, as shown in Figs. 128 and 129, it is
+obvious that the transmitter at each station is compelled to vary the
+resistance of the entire circuit comprising the two lines in series,
+in order to affect the receiver at distant stations. This is in effect
+making the transmitter circuit twice as long as is necessary, as will
+be shown in the subsequent systems considered. Furthermore, the
+placing of the battery in series in the circuit of the two combined
+lines does not lend itself readily to the supply of current from a
+common source to more than a single pair of lines.
+
+[Illustration: Fig. 129. Battery in Series with Two Lines]
+
+_Series Substation Circuit._ The arrangement at the
+substations--consisting in placing the transmitter and the receiver in
+series in the line circuit, as shown in Figs. 128 and 129--is the
+simplest possible one, and has been used to a considerable extent, but
+it has been subject to the serious objection, where receivers having
+permanent magnets were used, of making it necessary to so connect the
+receiver in the line circuit that the steady current from the battery
+would not set up a magnetization in the cores of the receiver in such
+a direction as to neutralize or oppose the magnetization of the
+permanent magnets. As long as the current flowed through the receiver
+coils in such a direction as to supplement the magnetization of the
+permanent magnets, no harm was usually done, but when the current
+flowed through the receiver coils in such a way as to neutralize or
+oppose the magnetizing force of the permanent magnets, the action of
+the receiver was greatly interfered with. As a result, it was
+necessary to always connect the receivers in the line circuit in a
+certain way, and this operation was called _poling_.
+
+In order to obviate the necessity for poling and also to bring about
+other desirable features, it has been, until recently, almost
+universal practice to so arrange the receiver that it would be in the
+circuit of the voice currents passing over the line, but would not be
+traversed by direct currents, this condition being brought about by
+various arrangements of condensers, impedance coils, or induction
+coils, as will be shown later. During the year 1909, however, the
+adoption by several concerns of the so-called "direct-current"
+receiver has made it necessary for the direct current to flow through
+the receiver coils in order to give the proper magnetization to the
+receiver cores, and this has brought about a return to the very simple
+form of substation circuit, which includes the receiver and the
+transmitter directly in the circuit of the line. This illustrates well
+an occurrence that is frequently observed by those who have
+opportunity to watch closely the development of an art. At one time
+the conditions will be such as to call for complicated arrangements,
+and for years the aim of inventors will be to perfect these
+arrangements; then, after they are perfected, adopted, and
+standardized, a new idea, or a slight alteration in the practice in
+some other respect, will demand a return to the first principles and
+wipe out the necessity for the things that have been so arduously
+striven for.
+
+[Illustration: Fig. 130. Bridging Battery with Repeating Coil]
+
+_Bridging Battery with Repeating Coil._ As pointed out, the placing of
+the battery in series in the line circuit in the central office is not
+desirable, and, so far as we are aware, has never been extensively
+used. The universal practice, therefore, is to place it in a bridge
+path across the line circuit, and a number of arrangements employing
+this basic idea are in wide use. In Fig. 130 is shown the standard
+arrangement of the Western Electric Company, employed by practically
+all the Bell operating companies. In this the battery at the central
+office is connected in the middle of the two sides of a repeating coil
+so that the current from the battery is fed out to the two connected
+lines in multiple.
+
+Referring to the middle portion of this figure showing the
+central-office apparatus, _1_ and _2_ may be considered as the two
+halves of one side of a repeating coil divided so that the battery may
+be cut into their circuit. Likewise, _3_ and _4_ may be considered as
+the two halves of the other side of the repeating coil similarly
+divided for the same purpose. The windings of this repeating coil are
+ordinarily alike; that is, _1_ and _2_ combined have the same
+resistance, number of turns, and impedance as _3_ and _4_ combined.
+The two sides of this coil are alternately used as primary and
+secondary, _1_ and _2_ forming the primary when Station A is talking,
+and _3_ and _4_, the secondary; and _vice versa_ when Station B is
+talking.
+
+As will be seen, the current flowing from the positive pole of the
+battery will divide and flow through the windings _2_ and _4_; thence
+over the upper limb of each line, through the transmitter at each
+station, and back over the lower limbs of the line, through the
+windings _1_ and _3_, where the two paths reunite and pass to the
+negative pole of the battery. It is evident that when neither
+transmitter is being used the current flowing through both lines will
+be a steady current and that, therefore, neither line will have an
+inductive effect on the other. When, however, the transmitter at
+Station A is used the variations in the resistance caused by it will
+cause undulations in the current. These undulations, passing through
+the windings _1_ and _2_ of the repeating coil, will cause, by
+electromagnetic induction, alternating currents to flow in the
+windings _3_ and _4_, and these alternating currents will be
+superimposed on the steady currents flowing in that line and will
+affect the receiver at Station B, as will be pointed out. The reverse
+conditions exist when Station B is talking.
+
+_Bell Substation Arrangement._ The substation circuits at the stations
+in Fig. 130 are illustrative of one of the commonly employed methods
+of preventing the steady current from the battery from flowing through
+the receiver coil. This particular arrangement is that employed by the
+common-battery instruments of the various Bell companies. Considering
+the action at Station B, it is evident that the steady current will
+pass through the transmitter and through the secondary winding of the
+induction coil, and that as long as this current is steady no current
+will flow through the telephone receiver. The receiver, transmitter,
+and primary winding of the induction coil are, however, included in a
+local circuit with the condenser. The presence of the condenser
+precludes the possibility of direct current flowing in this path.
+Considering Station A as a receiving station, it is evident that the
+voice currents coming to the station over the line will pass through
+the secondary winding and will induce alternating currents in the
+primary winding which will circulate through the local circuit
+containing the receiver and the condenser, and thus actuate the
+receiver. The considerations are not so simple when the station is
+being treated as a transmitting station. Under this condition the
+steady current passes through the transmitter in an obvious manner. It
+is clear that if the local circuit containing the receiver did not
+exist, the circuit would be operative as a transmitting circuit
+because the transmitter would produce fluctuations in the steady
+current flowing in the line and thus be able to affect the distant
+station. The transmitter, therefore, has a direct action on the
+currents flowing in the line by the variation in resistance which it
+produces in the line circuit. There is, however, a subsidiary action
+in this circuit. Obviously, there is a drop of potential across the
+transmitter terminals due to the flow of steady current. This means
+that the upper terminal of the condenser will be charged to the same
+potential as the upper terminal of the transmitter, while the lower
+terminal of the condenser will be of the same potential as the lower
+terminal of the transmitter. When, now, the transmitter varies its
+resistance, a variation in the potential across its terminals will
+occur; and as a result, a variation in potential across the terminals
+of the condenser will occur, and this means that alternating currents
+will flow through the primary winding of the induction coil. The
+transmitter, therefore, by its action, causes alternating currents to
+flow through the primary of this induction coil and it causes, by
+direct action on the circuit of the line, fluctuations in the steady
+current flowing in the line. The alternating currents flowing in the
+primary of the coil induce currents in the secondary of the coil which
+supplement and augment the fluctuations produced by the direct action
+of the transmitter. This circuit may be looked at, therefore, in the
+light of combining the direct action which the transmitter produces in
+the current in the line with the action which the transmitter produces
+in the local circuit containing the primary of the induction coil,
+this action being repeated in the line circuit through the secondary
+of the induction coil.
+
+The receiver in this circuit is placed in the local circuit, and is
+thus not traversed by the steady currents flowing in the line. There
+is thus no necessity for poling it. This circuit is very efficient,
+but is subject to the objection of producing a heavy side tone in the
+receiver of the transmitting station. By "side tone" is meant the
+noises which are produced in the receiver at a station by virtue of
+the action of the transmitter at that station. Side tone is
+objectionable for several reasons: first, it is sometimes annoying to
+the subscriber; second, and of more importance, the subscriber who is
+talking, hearing a very loud noise in his own receiver, unconsciously
+assumes that he is talking too loud and, therefore, lowers his voice,
+sometimes to such an extent that it will not properly reach the
+distant station.
+
+[Illustration: Fig. 131. Bridging Battery with Impedance Coils]
+
+_Bridging Battery with Impedance Coils._ The method of feeding current
+to the line from the common battery, shown in Fig. 130, is called the
+"split repeating-coil" method. As distinguished from this is the
+impedance-coil method which is shown in Fig. 131. In this the battery
+is bridged across the circuit of the combined lines in series with two
+impedance coils, _1_ and _2_, one on each side of the battery. The
+steady currents from the battery find ready path through these
+impedance coils which are of comparatively low ohmic resistance, and
+the current divides and passes in multiple over the circuits of the
+two lines. Voice currents, however, originating at either one of the
+stations, will not pass through the shunt across the line at the
+central office on account of the high impedance offered by these
+coils, and as a result they are compelled to pass on to the distant
+station and affect the receiver there, as desired.
+
+This impedance-coil method seems to present the advantage of greater
+simplicity over the repeating-coil method shown in Fig. 130, and so
+far as talking efficiency is concerned, there is little to choose
+between the two. The repeating-coil method, however, has the advantage
+over this impedance-coil method, because by it the two lines are
+practically divided except by the inductive connection between the two
+windings, and as a result an unbalanced condition of one of the
+connected lines is not as likely to produce an unbalanced condition in
+the other as where the two lines are connected straight through, as
+with the impedance-coil method. The substation arrangement of Fig. 131
+is the same as that of Fig. 130.
+
+[Illustration: Fig. 132. Double-Battery Kellogg System]
+
+_Double Battery with Impedance Coils._ A modification of the
+impedance-coil method is used in all of the central-office work of the
+Kellogg Switchboard and Supply Company. This employs a combination of
+impedance coils and condensers, and in effect isolates the lines
+conductively from each other as completely as the repeating-coil
+method. It is characteristic of all the Kellogg common-battery systems
+that they employ two batteries instead of one, one of these being
+connected in all cases with the calling line of a pair of connected
+lines and the other in all cases with the called line. As shown in
+Fig. 132, the left-hand battery is connected with the line leading to
+Station A through the impedance coils _1_ and _2_. Likewise, the
+right-hand battery is connected to the line of Station B through the
+impedance coils _3_ and _4_. These four impedance coils are wound on
+separate cores and do not have any inductive relation whatsoever with
+each other. Condensers _5_ and _6_ are employed to completely isolate
+the lines conductively. Current from the left-hand battery, therefore,
+passes only to Station A, and current from the right-hand battery to
+Station B. Whenever the transmitter at Station A is actuated the
+undulations of current which it produces in the line cause a varying
+difference of potential across the outside terminals of the two
+impedance coils _1_ and _2_. This means that the two left-hand
+terminals of condensers _5_ and _6_ are subjected to a varying
+difference of potential and these, of course, by electrostatic
+induction, cause the right-hand terminals of these condensers to be
+subject to a correspondingly varying difference of potential. From
+this it follows that alternating currents will be impressed upon the
+right-hand line and these will affect the receiver at Station B.
+
+A rough way of expressing the action of this circuit is to consider it
+in the same light as that of the impedance-coil circuit shown in Fig.
+131, and to consider that the voice currents originating in one line
+are prevented from passing through the bridge paths at the central
+office on account of the impedance, and are, therefore, forced to
+continue on the line, being allowed to pass readily by the condensers
+in series between the two lines.
+
+_Kellogg Substation Arrangement._ An interesting form of substation
+circuit which is employed by the Kellogg Company in all of its
+common-battery telephones is shown in Fig. 132. In passing, it may be
+well to state that almost any of the substation circuits shown in this
+chapter are capable of working with any of the central-office
+circuits. The different ones are shown for the purpose of giving a
+knowledge of the various substation circuits that are employed, and,
+as far as possible, to associate them with the particular
+central-office arrangements with which they are commonly used.
+
+In this Kellogg substation arrangement the line circuit passes first
+through the transmitter and then divides, one branch passing through
+an impedance coil _7_ and the other through the receiver and the
+condenser _8_, in series. The steady current from the central-office
+battery finds ready path through the transmitter and the impedance
+coil, but is prevented from passing through the receiver by the
+barrier set up by the condenser _8_. Voice currents, however, coming
+over the line to the station, find ready path through the receiver and
+the condenser but are barred from passing through the impedance coil
+by virtue of its high impedance.
+
+In considering the action of the station as a transmitting station,
+the variations set up by the transmitter pass through the condenser
+and the receiver at the same station, while the steady current which
+supplies the transmitter passes through the impedance coil. Impedance
+coils used for this purpose are made of low ohmic resistance but of a
+comparatively great number of turns, and, therefore, present a good
+path for steady currents and a difficult path for voice currents. This
+divided circuit arrangement employed by the Kellogg Company is one of
+the very simple ways of eliminating direct currents from the receiver
+path, at the same time allowing the free passage of voice currents.
+
+[Illustration: Fig. 133. Dean System]
+
+_Dean Substation Arrangement._ In marked contrast to the scheme for
+keeping steady current out of the receiver circuit employed by the
+Kellogg Company, is that shown in Fig. 133, which has been largely
+used by the Dean Electric Company, of Elyria, Ohio. The central-office
+arrangement in this case is that using the split repeating coil, which
+needs no further description. The substation arrangement, however, is
+unique and is a beautiful example of what can be done in the way of
+preventing a flow of current through a path without in any way
+insulating that path or placing any barrier in the way of the current.
+It is an example of the prevention of the direct flow of current
+through the receiver by so arranging the circuits that there will
+always be an equal potential on each side of it, and, therefore, no
+tendency for current to flow through it.
+
+In this substation arrangement four coils of wire--_1_, _2_, _3_, and
+_4_--are so arranged as to be connected in the circuit of the line,
+two in series and two in multiple. The current flowing from the
+battery at the central office, after passing through the transmitter,
+divides between the two paths containing, respectively, the coils _1_
+and _3_ and the coils _2_ and _4_. The receiver is connected between
+the junction of the coils _2_ and _4_ and that of _1_ and _3_. The
+resistances of the coils are so chosen that the drop of potential
+through the coil _2_ will be equal to that through the coil _1_, and
+likewise that through the coil _4_ will be equal to that through the
+coil _3_. As a result, the receiver will be connected between two
+points of equal potential, and no direct current will flow through it.
+How, then, do voice currents find their way through the receiver, as
+they evidently must, if the circuit is to fulfill any useful function?
+The coils _2_ and _3_ are made to have high impedance, while _1_ and
+_4_ are so wound as to be non-inductive and, therefore, offer no
+impedance save that of their ohmic resistance. What is true,
+therefore, of direct currents does not hold for voice currents, and as
+a result, the voice currents, instead of taking the divided path which
+the direct currents pursued, are debarred from the coils _2_ and _3_
+by their high impedance and thus pass through the non-inductive coil
+_1_, the receiver, and the non-inductive coil _4_.
+
+This circuit employs a Wheatstone-bridge arrangement, adjusted to a
+state of balance with respect to direct currents, such currents being
+excluded from the receiver, not because the receiver circuit is in any
+sense opaque to such direct currents, but because there is no
+difference of potential between the terminals of the receiver circuit,
+and, therefore, no tendency for current to flow through the receiver.
+In order that fluctuating currents may not, for the same reason, be
+caused to pass by, rather than through, the receiver circuit, the
+diametrically-opposed arms of the Wheatstone bridge are made to
+possess, in large degree, self-induction, thereby giving these two
+arms a high impedance to fluctuating currents. The conditions which
+exist for direct currents do not, therefore, exist for fluctuating
+currents, and it is this distinction which allows alternating currents
+to pass through the receiver and at the same time excludes direct
+currents therefrom.
+
+In practice, the coils _1_, _2_, _3_, and _4_ of the Dean substation
+circuit are wound on the same core, but coils _1_ and _4_--the
+non-inductive ones--are wound by doubling the wire back on itself so
+as to neutralize their self-induction.
+
+_Stromberg-Carlson._ Another modification of the central-office
+arrangement and also of the subscribers' station circuits, is shown in
+Fig. 134, this being a simplified representation of the circuits
+commonly employed by the Stromberg-Carlson Telephone Manufacturing
+Company. The battery feed at the central office differs only from that
+shown in Fig. 132, in that a single battery rather than two batteries
+is used, the current being supplied to one of the lines through the
+impedance coils _1_ and _2_, and to the other line through the
+impedance coils _3_ and _4_; condensers _5_ and _6_ serve conductively
+to isolate the two lines. At the subscriber's station the line circuit
+passes through the secondary of an induction coil and the transmitter.
+The receiver is kept entirely in a local circuit so that there is no
+tendency for direct current to flow through it, but it is receptive to
+voice currents through the electromagnetic induction between the
+primary and the secondary of the induction coil.
+
+[Illustration: Fig. 134. Stromberg-Carlson System]
+
+[Illustration: Fig. 135. North Electric Company System]
+
+_North._ Another arrangement of central-office battery feed is
+employed by the North Electric Company, and is shown in Fig. 135. In
+this two batteries are used which supply current respectively to the
+two connected lines, condensers being employed to conductively isolate
+the lines. This differs from the Kellogg arrangement shown in Fig. 132
+in that the two coils _1_ and _2_ are wound on the same core, while
+the coils _3_ and _4_ are wound together upon another core. In this
+case, in order that the inductive action of one of the coils may not
+neutralize that of the other coil on the same core, the two coils are
+wound in such relative direction that their magnetizing influence
+will always be cumulative rather than differential.
+
+The central-office arrangements discussed in Figs. 130 to 135,
+inclusive, are those which are in principal use in commercial practice
+in common-battery exchanges.
+
+_Current Supply over Limbs of Line in Parallel._ As indicating further
+interesting possibilities in the method of supplying current from a
+common source to a number of substations, several other systems will
+be briefly referred to as being of interest, although these have not
+gone into wide commercial use. The system shown in Fig. 136 is one
+proposed by Dean in the early days of common-battery working, and this
+arrangement was put into actual service and gave satisfactory results,
+but was afterwards supplanted by the Bell equipment operating under
+the system shown in Fig. 130, which became standardized by that
+company. In this the current from the common battery at the central
+office is not fed over the two line wires in series, but in multiple,
+using a ground return from the subscriber's station to the central
+office. Across the metallic circuit formed by two connected lines
+there is bridged, at the central office, an impedance coil _1_, and
+between the center point of this impedance coil and the ground is
+connected the common battery. At the subscriber's station is placed an
+impedance coil _2_, also bridged across the two limbs of the line, and
+between the center point of this impedance coil and the ground is
+connected the transmitter, which is shunted by the primary winding of
+an induction coil. Connected between the two limbs of the line at the
+substation there is also the receiver and the secondary of an
+induction coil in series.
+
+[Illustration: Fig. 136. Current Supply over Parallel Limbs of Line]
+
+The action of this circuit at first seems a little complex, but if
+taken step by step may readily be understood. The transmitter supply
+circuit may be traced from the central-office battery through the two
+halves of the impedance coil _1_ in multiple; thence over the two
+limbs of the line in multiple to Station A, for instance; thence in
+multiple through the two halves of impedance coil _2_, to the center
+point of that coil; thence through the two paths offered respectively
+by the primary of the induction coil and by the transmitter; then to
+ground and back to the other pole of the central-office battery. By
+this circuit the transmitter at the substation is supplied with
+current.
+
+Variations in the resistance of the transmitter when in action, cause
+complementary variations in the supply current flowing through the
+primary of the induction coil. These variations induce similar
+alternating currents in the secondary of this coil, which is in series
+in the line circuit. The currents, so induced in this secondary, flow
+in series through one side of the line to the distant station; thence
+through the secondary and the receiver at that station to the other
+side of the line and back through that side of the line to the
+receiver. These currents are not permitted to pass through the bridged
+paths across the metallic circuit that are offered by the impedance
+coils _1_ and _2_, because they are voice currents and are, therefore,
+debarred from these paths by virtue of the impedance.
+
+[Illustration: Fig. 137. Current Supply over Parallel Limbs of Line]
+
+An objection to this form of current supply and to other similar
+forms, wherein the transmitter current is fed over the two sides of
+the line in multiple with a ground return, is that the ground-return
+circuit formed by the two sides of the line in multiple is subject to
+inductive disturbances from other lines in the same way as an ordinary
+grounded line is subject to inductive disturbance. The current-supply
+circuit is thus subject to external disturbances and such disturbances
+find their way into the metallic circuit and, therefore, through the
+instruments by means of the electromagnetic induction between the
+primary and the secondary coils at the substations.
+
+Another interesting method of current supply from a central-office
+battery is shown in Fig. 137. This, like the circuit just considered,
+feeds the energy to the subscriber's station over the two sides of the
+line in multiple with a ground return. In this case, however, a local
+circuit is provided at the substation, in which is placed a storage
+battery _1_ and the primary _2_ of an induction coil, together with
+the transmitter. The idea in this is that the current supply from the
+central office will pass through the storage battery and charge it.
+Upon the use of the transmitter, this storage battery acts to supply
+current to the local circuit containing the transmitter and the
+primary coil _2_ in exactly the same manner as in a local battery
+system. The fluctuating current so produced by the action of the
+transmitter in this local circuit acts on the secondary winding _3_ of
+the induction coil, and produces therein alternating currents which
+pass to the central office and are in turn repeated to the distant
+station.
+
+_Supply Many Lines from Common Source._ We come now to the
+consideration of the arrangement by which a single battery may be made
+to supply current at the central office to a large number of pairs of
+connected lines simultaneously. Up to this point in this discussion it
+has been shown only how each battery served a single pair of connected
+lines and no others.
+
+Repeating Coil:--In Fig. 138 is shown how a single battery supplies
+current simultaneously to four different pairs of lines, the lines of
+each pair being connected for conversation. It is seen that the pairs
+of lines shown in this figure are arranged in each case in accordance
+with the system shown in Fig. 130. Let us inquire why it is that,
+although all of these four pairs of lines are connected with a common
+source of energy and are, therefore, all conductively joined, the
+stations will be able to communicate in pairs without interference
+between the pairs. In other words, why is it that voice currents
+originating at Station A will pass only to the receiver at Station B
+and not to the receivers at Station C or Station H, for instance? The
+reason is that separate supply conductors lead from the points such as
+_1_ and _2_ at the junctions of the repeating-coil windings on each
+pair of circuits to the battery terminals, and the resistance and
+impedance of the battery itself and of the common leads to it are so
+small that although the feeble voice currents originating in the pair
+of lines connecting Station A and Station B pass through the battery,
+they are not able to alter the potential of the battery in any
+appreciable degree. As a result, therefore, the supply wires leading
+from the common-battery terminals to the points _7_ and _8_, for
+instance, cannot be subjected to any variations in potential by virtue
+of currents flowing through the battery from the points _1_ and _2_ of
+the lines joining Station A and Station B.
+
+[Illustration: MAIN OFFICE, KEYSTONE TELEPHONE COMPANY, PHILADELPHIA,
+PA.]
+
+[Illustration: Fig. 138. Common Source for Many Lines]
+
+[Illustration: Fig. 139. Common Source for Many Lines]
+
+Retardation Coil--Single Battery:--In Fig. 139 is shown in similar
+manner the current supply from a single battery to four different
+pairs of lines, the battery being associated with the lines by the
+combined impedance coil and condenser method, which was specifically
+dealt with in connection with Fig. 133. The reasons why there will be
+no interference between the conversations carried on in the various
+pairs of connected lines in this case are the same as those just
+considered in connection with the system shown in Fig. 138. The
+impedance coils in this case serve to keep the telephone currents
+confined to their respective pairs of lines in which they originate,
+and this same consideration applies to the system of Fig. 138, for
+each of the separate repeating-coil windings of Fig. 138 is in itself
+an impedance coil with respect to such currents as might leak away
+from one pair of lines on to another.
+
+Retardation Coil--Double Battery:--The arrangement of feeding a number
+of pairs of lines according to the Kellogg two-battery system is
+indicated in Fig. 140, which needs no further explanation in view of
+the description of the preceding figures. It is interesting to note in
+this case that the left-hand battery serves only the left-hand lines
+and the right-hand battery only the right-hand lines. As this is
+worked out in practice, the left-hand battery is always connected to
+those lines which originate a call and the right-hand battery always
+to those lines that are called for. The energy supplied to a calling
+line is always, therefore, from a different source than that which
+supplies a called line.
+
+[Illustration: Fig. 140. Two Sources for Many Lines]
+
+[Illustration: Fig. 141. Current Supply from Distant Point]
+
+_Current Supply from Distant Point._ Sometimes it is convenient to
+supply current to a group of lines centering at a certain point from a
+source of current located at a distant point. This is often the case
+in the so-called private branch exchange, where a given business
+house or other institution is provided with its own switchboard for
+interconnecting the lines leading to the various telephones of that
+concern or institution among themselves, and also for connecting them
+with lines leading to the city exchange. It is not always easy or
+convenient to maintain at such private switchboards a separate battery
+for supplying the current needed by the local exchange.
+
+In such cases the arrangement shown in Fig. 141 is sometimes employed.
+This shows two pairs of lines connected by the impedance-coil system
+with common terminals _1_ and _2_, between which ordinarily the common
+battery would be connected. Instead of putting a battery between these
+terminals, however, at the local exchange, a condenser of large
+capacity is connected between them and from these terminals circuit
+wires _3_ and _4_ are led to a battery of suitable voltage at a
+distant central office. The condenser in this case is used to afford a
+short-circuit path for the voice currents that leak from one side of
+one pair of lines to the other, through the impedance coils bridged
+across the line. In this way the effect of the necessarily high
+resistance in the common leads _3_ and _4_, leading to the storage
+battery, is overcome and the tendency to cross-talk between the
+various pairs of connected lines is eliminated. Frequently, instead of
+employing this arrangement, a storage battery of small capacity will
+be connected between the terminals _1_ and _2_, instead of the
+condenser, and these will be charged over the wires _3_ and _4_ from a
+source of current at a distant point.
+
+A consideration of the various methods of supplying current from a
+common source to a number of lines will show that it is essential that
+the resistance of the battery itself be very low. It is also necessary
+that the resistance and the impedance of the common leads from the
+battery to the point of distribution to the various pairs of lines be
+very low, in order that the voice currents which flow through them, by
+virtue of the conversations going on in the different pairs of lines,
+shall not produce any appreciable alteration in the difference of
+potential between the battery terminals.
+
+
+
+
+CHAPTER XIV
+
+THE TELEPHONE SET
+
+
+We have considered what may be called the elemental parts of a
+complete telephone; that is, the receiver, transmitter, hook switch,
+battery, generator, call bell, condenser, and the various kinds of
+coils which go to make up the apparatus by which one is enabled to
+transmit and receive speech and signals. We will now consider the
+grouping of these various elements into a complete working
+organization known as a telephone.
+
+Before considering the various types it is well to state that the term
+telephone is often rather loosely used. We sometimes hear the receiver
+proper called a telephone or a hand telephone. Since this was the
+original speaking telephone, there is some reason for so calling the
+receiver. The modern custom more often applies the term telephone to
+the complete organization of talking and signaling apparatus, together
+with the associated wiring and cabinet or standard on which it is
+mounted. The name telephone set is perhaps to be preferred to the word
+telephone, since it tends to avoid misunderstanding as to exactly what
+is meant. Frequently, also, the telephone or telephone set is referred
+to as a subscriber's station equipment, indicating the equipment that
+is to be found at a subscriber's station. This, as applying to a
+telephone alone, is not proper, since the subscriber's station
+equipment includes more than a telephone. It includes the local wiring
+within the premises of the subscriber and also the lightning arrester
+and other protective devices, if such exist.
+
+To avoid confusion, therefore, the collection of talking and signaling
+apparatus with its wiring and containing cabinet or standard will be
+referred to in this work as a telephone or telephone set. The receiver
+will, as a rule, be designated as such, rather than as a telephone.
+The term subscriber's station equipment will refer to the complete
+equipment at a subscriber's station, and will include the telephone
+set, the interior wiring, and the protective devices, together with
+any other apparatus that may be associated with the telephone line and
+be located within the subscriber's premises.
+
+Classification of Sets. Telephones may be classified under two
+general headings, magneto telephones and common-battery telephones,
+according to the character of the systems in which they are adapted to
+work.
+
+_Magneto Telephone._ The term magneto telephone, as it was originally
+employed in telephony, referred to the type of instrument now known as
+a receiver, particularly when this was used also as a transmitter. As
+the use of this instrument as a transmitter has practically ceased,
+the term magneto telephone has lost its significance as applying to
+the receiver, and, since many telephones are equipped with magneto
+generators for calling purposes, the term magneto telephone has, by
+common consent, come to be used to designate any telephone including,
+as a part of its equipment, a magneto generator. Magneto telephones
+usually, also, include local batteries for furnishing the transmitter
+with current, and this has led to these telephones being frequently
+called local battery telephones. However, a local battery telephone is
+not necessarily a magneto telephone and _vice versa_, since sometimes
+magneto telephones have no local batteries and sometimes local battery
+telephones have no magnetos. Nearly all of the telephones which are
+equipped with magneto generators are, however, also equipped with
+local batteries for talking purposes, and, therefore, the terms
+magneto telephone and local battery telephone usually refer to the
+same thing.
+
+_Common-Battery Telephone._ Common-battery telephones, on the other
+hand, are those which have no local battery and no magneto generator,
+all the current for both talking and signaling being furnished from a
+common source of current at the central office.
+
+_Wall and Desk Telephones._ Again we may classify telephones or
+telephone sets in accordance with the manner in which their various
+parts are associated with each other for use, regardless of what parts
+are contained in the set. We may refer to all sets adapted to be
+mounted on a wall or partition as _wall telephones_, and to all in
+which the receiver, transmitter, and hook are provided with a standard
+of their own to enable them to rest on any flat surface, such as a
+desk or table, as _desk telephones_. These latter are also referred to
+as portable telephones and as portable desk telephones.
+
+In general, magneto or local battery telephones differ from
+common-battery telephones in their component parts, the difference
+residing principally in the fact that the magneto telephone always has
+a magneto generator and usually a local battery, while the
+common-battery telephone has no local source of current whatever. On
+the other hand, the differences between wall telephones and desk
+telephones are principally structural, and obviously either of these
+types of telephones may be for common-battery or magneto work. The
+same component parts go to make up a desk telephone as a wall
+telephone, provided the two instruments are adapted for the same class
+of service, but the difference between the two lies in the structural
+features by which these same parts are associated with each other and
+protected from exposure.
+
+[Illustration: Fig. 142. Magneto Wall Set]
+
+[Illustration: Fig. 143. Magneto Wall Set]
+
+Magneto-Telephone Sets. _Wall._ In Fig. 142 is shown a familiar type
+of wall set. The containing box includes within it all of the working
+parts of the apparatus except that which is necessarily left outside
+in order to be within the reach of the user. Fig. 143 shows the same
+set with the door open. This gives a good idea of the ordinary
+arrangement of the apparatus within. It is seen that the polarized
+bell or ringer has its working parts mounted on the inside of the door
+or cover of the box, the tapper projecting through so as to play
+between the gongs on the outside. Likewise the transmitter arm, which
+supports the transmitter and allows its adjustment up and down to
+accommodate itself to the height of the user, is mounted on the front
+of the door, and the conductors leading to it may be seen fastened to
+the rear of the door in Fig. 143.
+
+In some wall sets the wires leading to the bell and transmitter are
+connected to the wiring of the rest of the set through the hinges of
+the door, thus allowing the door to be opened and closed repeatedly
+without breaking off the wires. In order to always insure positive
+electrical contact between the stationary and movable parts of the
+hinge a small wire is wound around the hinge pin, one end being
+soldered to the stationary part and the other end to the movable part
+of the hinge. In other forms of wall set the wires to the bell and the
+transmitter lead directly from the stationary portion of the cabinet
+to the back of the door, the wires being left long enough to have
+sufficient flexibility to allow the door to be opened and closed
+without injuring the wires.
+
+At the upper portion of the box there is mounted the hook switch, this
+being, in this case, of the short lever type. The lever of the hook
+projects through the side of the box so as to make the hook available
+as a support for the receiver. Immediately at the right of the hook
+switch is mounted the induction coil, and immediately below this the
+generator, its crank handle projecting through the right-hand side of
+the box so as to be available for use there. The generator is usually
+mounted on a transverse shelf across the middle of the cabinet, this
+shelf serving to form a compartment below it in which the dry battery
+of two or three cells is placed.
+
+The wall telephone-set cabinets have assumed a multitude of forms.
+When wet cells rather than dry cells were ordinarily employed, as was
+the case up to about the year 1895, the magneto generator, polarized
+bell, and hook switch were usually mounted in a rectangular box placed
+at the top of a long backboard. Immediately below this on the
+backboard was mounted the transmitter arm, and sometimes the base of
+this included the induction coil. Below this was the battery box, this
+being a large affair usually adapted to accommodate two and sometimes
+three ordinary LeClanche cells side by side.
+
+The dry cell has almost completely replaced the wet cell in this
+country, and as a result, the general type of wall set as shown in
+Figs. 142 and 143, has gradually replaced the old wet-cell type, which
+was more cumbrous and unsightly. It is usual on wall sets to provide
+some sort of a shelf, as indicated in Fig. 142, for the convenience of
+the user in making notes and memoranda.
+
+_Desk._ In the magneto desk-telephone sets, the so-called desk stand,
+containing the transmitter, the receiver, and the hook switch, with
+the standard upon which they are mounted, is shown in Fig. 144. This
+desk stand evidently does not comprise the complete equipment for a
+magneto desk-telephone set, since the generator, polarized bell, and
+battery are lacking. The generator and bell are usually mounted
+together in a box, either on the under side of the desk of the user or
+on the wall within easy reach of his chair. Connections are made
+between the apparatus in the desk stand proper and the battery,
+generator, and bell by means of flexible conducting cords, these
+carrying a plurality of conductors, as required by the particular
+circuit of the telephone in question. Such a complete magneto
+desk-telephone set is shown in Fig. 145, this being one of the types
+manufactured by the Stromberg-Carlson Manufacturing Company.
+
+[Illustration: Fig. 144. Desk Stand]
+
+A great variety of arrangements of the various parts of magneto
+desk-telephone apparatus is employed in practice. Sometimes, as shown
+in Fig. 145, the magneto bell box is equipped with binding posts for
+terminating all of the conductors in the cord, the line wires also
+running to some of these binding posts.
+
+In the magneto-telephone set illustrated the box is made large enough
+to accommodate only the generator and call bell, and the batteries are
+mounted elsewhere, as in a drawer of the desk, while in other cases
+there is no other equipment but that shown in the cut, the batteries
+being mounted within the magneto bell box itself. In still other
+cases, the polarized bell is contained in one box, the generator in
+another, the batteries in the drawer of the desk, the induction coil
+being mounted either in the base of the desk stand, in the bell box,
+or in the generator box. In such cases all of the circuits of the
+various scattered parts are wired to a terminal strip, located at some
+convenient point, this strip containing terminals for all the wires
+leading from the various parts and for the line wires themselves. By
+combining the various wires on the terminals of this terminal strip,
+the complete circuits of the telephone are built up. In still other
+cases the induction coil is mounted on the terminal strip and separate
+wires or sets of wires are run to the polarized bell and generator, to
+the desk stand itself, and to the batteries. These various
+arrangements are subject largely to the desire or personal ideas of
+the manufacturer or user. All of them work on the same principle so
+far as the operation of the talking and signaling circuits is
+concerned.
+
+[Illustration: Fig. 145. Magneto Desk Set]
+
+Circuits of Magneto-Telephone Sets. Magneto telephones, whether of
+the wall or desk type, may be divided into two general classes, series
+and bridging, according to whether the magnet of the bell is included
+in series or bridge relation with the telephone line when the hook is
+down.
+
+_Series._ In the so-called series telephone line, where several
+telephones are placed in series in a single line circuit, the
+employment of the series type of telephone results in all of the
+telephone bells being in series in the line circuit. This means that
+the voice currents originating in the telephones that are in use at a
+given time must pass in series through the magnets of the bells of the
+stations that are not in use. In order that these magnets, through
+which the voice currents must pass, may interfere to as small a degree
+as possible with the voice currents, it is common to employ
+low-resistance magnets in series telephones, these magnets being wound
+with comparatively few turns and on rather short cores so that the
+impedance will be as small as possible. Likewise, since the generators
+are required to ring all of the bells in series, they need not have a
+large current output, but must have sufficient voltage to ring through
+all of the bells in series and through the resistance of the line. For
+this reason the generators are usually of the three-bar type and
+sometimes have only two bars.
+
+In Fig. 146 are shown, in simplified form, the circuits of an ordinary
+series telephone. The receiver in this is shown as being removed from
+the hook and thus the talking apparatus is brought into play. The line
+wires _1_ and _2_ connect respectively to the binding posts _3_ and
+_4_ which form the terminals of the instrument. When the hook is up,
+the circuit between the binding posts _3_ and _4_ includes the
+receiver and the secondary winding of the induction coil, together
+with one of the upper contacts _5_ of the switch hook and the hook
+lever itself. This completes the circuit for receiving speech. The
+hook switch is provided with another upper contact _6_, between which
+and the contact _5_ is connected the local circuit containing the
+transmitter, the battery, and the primary of the induction coil in
+series. The primary and the secondary windings are connected together
+at one end and connected with the switch contact _5_, as shown. It is
+thus seen that when the hook is up the circuit through the receiver is
+automatically closed and also the local circuit containing the
+primary, the battery, and the transmitter. Thus, all the conditions
+for transmitting and receiving speech are fulfilled.
+
+[Fig. 146. Circuit of Series Magneto Set]
+
+When the hook is down, however, the receiving and transmitting
+circuits are broken, but another circuit is completed by the
+engagement of the hook-switch lever with the lower hook contact _7_.
+Between this contact and one side of the line is connected the
+polarized ringer and the generator. With the hook down, therefore, the
+circuit may be traced from the line wire _1_ to binding post _3_,
+thence through the generator shunt to the call bell, and thence
+through the lower switching contact _7_ to the binding post _4_ and
+line wire _2_. The generator shunt, as already described in Chapter
+VIII, normally keeps the generator shunted out of circuit. When,
+however, the generator is operated the shunt is broken, which allows
+the armature of the generator to come into the circuit in series with
+the winding of the polarized bell. The normal shunting of the
+generator armature from the circuit of the line is advantageous in
+several ways. In the first place, the impedance of the generator
+winding is normally cut out of the circuit so that in the case of a
+line with several stations the talking or voice currents do not have
+to flow through the generator armatures at the stations which are not
+in use. Again, the normal shunting of the generator tends to save the
+generator armature from injury by lightning.
+
+[Illustration: Fig. 147. Circuit of Series Magneto Set.]
+
+The more complete circuits of a series magneto telephone are shown in
+Fig. 147. In this the line binding posts are shown as _1_ and _2_. At
+the bottom of the telephone cabinet are four other binding posts
+marked _3_, _4_, _5_, and _6_. Of these _3_ and _4_ serve for the
+receiver terminals and _5_ and _6_ for the transmitter and battery
+terminals. The circuits of this diagram will be found to be
+essentially the same as those of Fig. 146, except that they are shown
+in greater detail. This particular type of circuit is one commonly
+employed where the generator, ringer, hook switch, and induction coil
+are all mounted in a so-called magneto bell box at the top of the
+instrument, and where the transmitter is mounted on an arm just below
+this box, and the battery in a separate compartment below the
+transmitter. The only wiring that has to be done between the bell box
+and the other parts of the instrument in assembling the complete
+telephone is to connect the receiver to the binding posts _3_ and _4_
+and to connect the battery and transmitter circuit to the binding
+posts _5_ and _6_.
+
+_Bridging._ In other cases, where several telephones are placed on a
+single-line circuit, the bells are arranged in multiple across the
+line. For this reason their magnets are wound with a very great number
+of turns and consequently to a high resistance. In order to further
+increase the impedance, the cores are made long and heavy. Since the
+generators on these lines must be capable of giving out a sufficient
+volume of current to divide up between all of the bells in multiple,
+it follows that these generators must have a large current output, and
+at the same time a sufficient voltage to ring the bells at the
+farthest end of the line. Such instruments are commonly called
+bridging instruments, on account of the method of connecting their
+bells across the circuit of the line.
+
+[Illustration: Fig. 148. Circuit of Bridging Magneto Set]
+
+The fundamental characteristic of the bridging telephone is that it
+contains three possible bridge paths across the line wires. The first
+of these bridge paths is through the talking apparatus, the second
+through the generator, and the third through the ringer. This is shown
+in simplified form in Fig. 148. The talking apparatus is associated
+with the two upper contacts of the hook switch in the usual manner and
+needs no further description. The generator is the second separate
+bridge path, normally open, but adapted to be closed when the
+generator is operated, this automatic closure being performed by the
+movement of the crank shaft. The third bridge contains the polarized
+bell, and this, as a rule, is permanently closed. Sometimes, however,
+the arrangement is such that the bell path is normally closed through
+the switch which is operated by the generator crank shaft, and this
+path is automatically broken when the generator is operated, at which
+time, also, the generator path is automatically closed. This
+arrangement brings about the result that the generator never can ring
+its own bell, because its switch always operates to cut out the bell
+at its own station just before the generator itself is cut into the
+circuit.
+
+In Fig. 149 is shown the complete circuit of a bridging telephone.
+The circuit given in this figure is for a local-battery wall set
+similar in type to that shown in Figs. 142 and 143. A simplified
+diagrammatic arrangement is shown in the lower left-hand corner of
+this figure, and from a consideration of this it will be seen that the
+bell circuit across the line is normally completed through the two
+right-hand normally closed contacts of the switch on the generator.
+When, however, the generator is operated these two contacts are made
+to disengage each other while the long spring of the generator switch
+engages the left-hand spring and thus brings the generator itself into
+the circuit.
+
+[Illustration: Fig. 149. Circuit of Bridging Magneto Set]
+
+Of the three binding posts, _1_, _2_, and _3_, at the top of Fig. 149,
+_1_ and _2_ are for connecting with the line wires, while _8_ is for a
+ground connection, acting in conjunction with the lightning arrester
+mounted at the top of the telephone and indicated at _4_ in Fig. 149.
+This has no function in talking or ringing, and will be referred to
+more fully in Chapter XIX. Suffice it to say at this point that these
+arresters usually consist of two conducting bodies, one connected
+permanently to each of the line binding posts, and a third conducting
+body connected to the ground binding post. These three conducting
+bodies are in close proximity but carefully insulated from each other;
+the idea being that when the line wires are struck by lightning or
+subjected otherwise to a dangerous potential, the charge on the line
+will jump across the space between the conducting bodies and pass
+harmlessly to ground.
+
+     NOTE. The student should practice making simplified diagrams from
+     actual wiring diagrams. The difference between the two is that
+     one is laid out for ease in understanding it, while the other is
+     laid out to show the actual course of the wires as installed.
+
+If the large detailed circuit of Fig. 149 be compared with the small
+theoretical circuit in the same figure, the various conducting
+paths will be found to be the same. Such a simplified circuit does
+more to enable one to grasp the fundamental scheme of a complex
+circuit than much description, since it shows at a glance the general
+arrangement. The more detailed circuits are, however, necessary to
+show the actual paths followed by the wiring.
+
+The circuits of desk stands do not differ from those of wall sets in
+any material degree, except as may be necessitated by the fact that
+the various parts of the telephone set are not all mounted in the same
+cabinet or on the same standard. To provide for the necessary relative
+movement between the desk stand and the other portions of the set,
+flexible conductors are run from the desk stand itself to the
+stationary portions of the equipment, such as the battery and the
+parts contained in the generator and bell box.
+
+[Illustration: Fig. 150. Circuit of Bridging Magneto Desk Set]
+
+In Fig. 150 is shown the circuit of the Stromberg-Carlson magneto
+desk-telephone set, illustrated in Fig. 145. This diagram needs no
+explanation in view of what has already been said. The conductors,
+leading from the desk-stand group of apparatus to the bell-box group of
+apparatus, are grouped together in a flexible cord, as shown in Fig.
+145, and are connected respectively to the various binding posts or
+contact points within the desk stand at one end and at the base of the
+bell box at the other end. These flexible conductors are insulated
+individually and covered by a common braided covering. They usually are
+individualized by having a colored thread woven into their insulating
+braid, so that it is an easy matter to identify the two ends of the
+same conductor at either end of the flexible cord or cable.
+
+[Illustration: Fig. 151. Common-Battery Wall Set]
+
+[Illustration: Fig. 152. Common-Battery Wall Set]
+
+Common-Battery Telephone Sets. Owing to the fact that common-battery
+telephones contain no sources of current, they are usually somewhat
+simpler than the magneto type. The component parts of a
+common-battery telephone, whether of the wall or desk type, are the
+transmitter, receiver, hook switch, polarized bell, condenser, and
+sometimes an induction coil. The purpose of the condenser is to
+prevent direct or steady currents from passing through the windings of
+the ringer while the ringer is connected across the circuit of the
+line during the time when the telephone is not in use. The
+requirements of common-battery signaling demand that the ringer shall
+be connected with the line so as to be receptive of a call at any time
+while the telephone is not in use. The requirements also demand that
+no conducting path shall normally exist between the two sides of the
+line. These two apparently contradictory requirements are met by
+placing a condenser in series with the ringer so that the ringer will
+be in a path that will readily transmit the alternating ringing
+currents sent out from the central-office generator, while at the same
+time the condenser will afford a complete bar to the passage of steady
+currents. Sometimes the condenser is also used as a portion of the
+talking apparatus, as will be pointed out.
+
+[Illustration: MAIN OFFICE, KANSAS CITY HOME TELEPHONE CO., KANSAS
+CITY, MO.]
+
+_Wall._ In Figs. 151 and 152 are given two views of a characteristic
+form of common-battery wall-telephone set, made by the
+Stromberg-Carlson Manufacturing Company. The common-battery wall set
+has usually taken this general form. In it the transmitter is mounted
+on an adjustable arm at the top of the backboard, while the box
+containing the bell and all working parts of the instrument is placed
+below the transmitter, the top of the box affording a shelf for
+writing purposes. In Fig. 151 are shown the hook switch and the
+receiver; just below these may be seen the magnets of the polarized
+bell, back of which is shown a rectangular box containing the
+condenser. Immediately in front of the ringer magnets is the induction
+coil.
+
+[Illustration: Fig. 153. Stromberg-Carlson Common-Battery Wall Set]
+
+In Fig. 153 are shown the details of the circuit of this instrument.
+This figure also includes a simplified circuit arrangement from which
+the principles involved may be more readily understood. It is seen
+that the primary of the induction coil and the transmitter are
+included in series across the line. The secondary of the induction
+coil, in series with the receiver, is connected also across the line
+in series with a condenser and the transmitter.
+
+_Hotel._ Sometimes, in order to economize space, the shelf of
+common-battery wall sets is omitted and the entire apparatus mounted
+in a small rectangular box, the front of which carries the transmitter
+mounted on the short arm or on no arm at all. Such instruments are
+commonly termed hotel sets, because of the fact that their use was
+first confined largely to the rooms in hotels. Later, however, these
+instruments have become very popular in general use, particularly in
+residences. Sometimes the boxes or cabinets of these sets are made of
+wood, but of recent years the tendency has been growing to make them
+of pressed steel. The steel box is usually finished in black enamel,
+baked on, the color being sometimes varied to match the color of the
+surrounding woodwork. In Figs. 154 and 155 are shown two views of a
+common-battery hotel set manufactured by the Dean Electric Company.
+
+Such sets are extremely neat in appearance and have the advantage of
+taking up little room on the wall and the commercial advantage of
+being light and compact for shipping purposes. A possible disadvantage
+of this type of instrument is the somewhat crowded condition which
+necessarily follows from the placing of all the parts in so confined a
+space. This interferes somewhat with the accessibility of the various
+parts, but great ingenuity has been manifested in making the parts
+readily get-at-able in case of necessity for repairs or alterations.
+
+[Illustration: Fig. 154. Steel Box Hotel]
+
+[Illustration: Fig. 155. Steel Box Hotel Set]
+
+_Desk_. The common-battery desk telephone presents a somewhat simpler
+problem than the magneto desk telephone for the reason that the
+generator and local battery, the two most bulky parts of a magneto
+telephone, do not have to be provided for. Some companies, in
+manufacturing desk stands for common-battery purposes, mount the
+condenser and the induction coil or impedance coil, or whatever device
+is used in connection with the talking circuit, in the base of the
+desk stand itself, and mount the polarized ringer and the condenser
+used for ringing purposes in a separate bell box adapted to be
+mounted on the wall or some portion of the desk. Other companies mount
+only the transmitter, receiver, and hook switch on the desk stand
+proper and put the condenser or induction coil, or other device
+associated with the talking circuit, in the bell box. There is little
+to choose between the two general practices. The number of conducting
+strands in the flexible cord is somewhat dependent on the arrangement
+of the circuit employed.
+
+[Illustration: Fig. 156. Common-Battery Desk Set]
+
+[Illustration: Fig. 157. Bell for Common-Battery Desk Set.]
+
+The Kellogg Switchboard and Supply Company is one which places all the
+parts, except the polarized ringer and the associated condenser, in
+the desk stand itself. In Fig. 156 is shown a bottom view of the desk
+stand with the bottom plate removed. In the upper portion of the
+circle of the base is shown a small condenser which is placed in the
+talking circuit in series with the receiver. In the right-hand portion
+of the circle of the base is shown a small impedance coil, which is
+placed in series with the transmitter but in shunt relation with the
+condenser and the receiver.
+
+[Illustration: Fig. 158. Bell for Common-Battery Desk Set]
+
+In Figs. 157 and 158 are shown two views of the type of bell box
+employed by the Kellogg Company in connection with the common-battery
+desk sets, this box being of pressed-steel construction and having a
+removable lid, as shown in Fig. 158, by which the working parts of the
+ringer are made readily accessible, as are also the terminals for the
+cord leading from the desk stand and for the wires of the line
+circuit. The condenser that is placed in series with the ringer is
+also mounted in this same box. By employing two condensers, one in the
+bell box large enough to transmit ringing currents and the other in
+the base of the desk stand large enough only to transmit voice
+currents, a duplication of condensers is involved, but it has the
+corresponding advantages of requiring only two strands to the flexible
+cord leading from the bell box to the desk stand proper.
+
+[Illustration: Fig. 159. Microtelephone Set]
+
+A form of desk-telephone set that is used largely abroad, but that has
+found very little use in this country, is shown in Fig. 159. In this
+the transmitter and the receiver are permanently attached together,
+the receiver being of the watch-case variety and so positioned
+relatively to the transmitter that when the receiver is held at the
+ear, the mouthpiece of the transmitter will be just in front of the
+lips of the user. In order to maintain the transmitter in a vertical
+position during use, this necessitates the use of a curved mouthpiece
+as shown. This transmitter and receiver so combined is commonly
+called, in this country, the _microtelephone set_, although there
+seems to be no logical reason for this name. The combined transmitter
+and receiver, instead of being supported on an ordinary form of hook
+switch, are supported on a forked bracket as shown, this bracket
+serving to operate the switch springs which are held in one position
+when the bracket is subjected to the weight of the microtelephone, and
+in the alternate position when relieved therefrom. This particular
+microtelephone set is the product of the L.M. Ericsson Telephone
+Manufacturing Company, of Buffalo, New York. The circuits of such sets
+do not differ materially from those of the ordinary desk telephone
+set.
+
+[Illustration: Fig. 160. Kellogg Common-Battery Desk Set]
+
+[Illustration: Fig. 161. Dean Common-Battery Set]
+
+Circuits of Common-Battery Telephone Sets. The complete circuits of
+the Kellogg desk-stand arrangement are shown in Fig. 160, the
+desk-stand parts being shown at the left and the bell-box parts at
+the right. As is seen, but two conductors extend from the former to
+the latter. A simplified theoretical sketch is also shown in the upper
+right-hand corner of this figure.
+
+The details of the common-battery telephone circuits of the Dean
+Electric Company are shown in Fig. 161. This involves the use of the
+balanced Wheatstone bridge. The only other thing about this circuit
+that needs description, in view of what has previously been said about
+it, is that the polarized bell is placed in series with a condenser so
+that the two sides of the circuit may be insulated from each other
+while the telephone is not in use, and yet permit the passage of
+ringing current through the bell.
+
+[Illustration: Fig. 162. Monarch Common-Battery Wall Set]
+
+The use of the so-called direct-current receiver has brought about a
+great simplification in the common-battery telephone circuits of
+several of the manufacturing companies. By this use the transmitter
+and the receiver are placed in series across the line, this path being
+normally opened by the hook-switch contacts. The polarized bell and
+condenser are placed in another bridge path across the line, this path
+not being affected by the hook-switch contacts. All that there is to
+such a complete common-battery telephone set, therefore, is a
+receiver, transmitter, hook switch, bell, condenser, and cabinet, or
+other support.
+
+The extreme simplicity of the circuits of such a set is illustrated in
+Fig. 162, which shows how the Monarch Telephone Manufacturing Company
+connect up the various parts of their telephone set, using the
+direct-current receiver already described in connection with Fig. 54.
+
+[Illustration: VENTILATING PLANT FOR LARGE TELEPHONE OFFICE BUILDING]
+
+
+
+
+CHAPTER XV
+
+NON-SELECTIVE PARTY-LINE SYSTEMS
+
+
+A party line is a line that is for the joint use of several stations.
+It is, therefore, a line that connects a central office with two or
+more subscribers' stations, or where no central office is involved, a
+line that connects three or more isolated stations with each other.
+The distinguishing feature of a party line, therefore, is that it
+serves more than two stations, counting the central office, if there
+is one, as a station.
+
+Strictly speaking, the term _party_ line should be used in
+contradistinction to the term _private_ line. Companies operating
+telephone exchanges, however, frequently lease their wires to
+individuals for private use, with no central-office switchboard
+connections, and such lines are, by common usage, referred to as
+"private lines." Such lines may be used to connect two or more
+isolated stations. A _private_ line, in the parlance of telephone
+exchange working, may, therefore, be a _party_ line, as inconsistent
+as this may seem.
+
+A telephone line that is connected with an exchange is an exchange
+line, and it is a party line if it has more than one station on it. It
+is an individual line or a single party line if it has but a single
+station on it. A line which has no central-office connection is called
+an "isolated line," and it is a party line if it has more than two
+stations on it.
+
+The problem of mere speech transmission on party lines is comparatively
+easy, being scarcely more complex than that involved in private or
+single party lines. This is not true, however, of the problem of
+signaling the various stations. This is because the line is for the
+common use of all its patrons or subscribers, as they are termed, and
+the necessity therefore exists that the person sending a signal, whether
+operator or subscriber, shall be able in some way to inform a person at
+the desired station that the call is intended for that station. There
+are two general ways of accomplishing this purpose.
+
+(_1_) The first and simplest of these ways is to make no provision for
+ringing any one bell on the line to the exclusion of the others, and
+thus allow all bells to ring at once whenever any station on the line
+is wanted. Where this is done, in order to prevent all stations from
+answering, it is necessary, in some way, to convey to the desired
+station the information that the call is intended for that station,
+and to all of the other stations the information that the call is not
+intended for them. This is done on such lines by what is called "code
+ringing," the code consisting of various combinations of long and
+short rings.
+
+(_2_) The other and more complex way is to arrange for selective
+ringing, so that the person sending the call may ring the bell at the
+station desired, allowing the bells at all the other stations to
+remain quiet.
+
+[Illustration: Fig. 163. Grounded-Circuit Series Line]
+
+These two general classes of party-line systems may, therefore, be
+termed "non-selective" and "selective" systems. Non-selective party
+lines are largely used both on lines having connection with a central
+office, and through the central office the privilege of connection
+with other lines, and on isolated lines having no central-office
+connection. The greatest field of usefulness of non-selective lines is
+in rural districts and in connection with exchanges in serving rather
+sparsely settled districts where the cost of individual lines or even
+lines serving but a few subscribers, is prohibitive.
+
+Non-selective telephone party lines most often employ magneto
+telephones. The early forms of party lines employed the ordinary
+series magneto telephone, the bells being of low resistance and
+comparatively low impedance, while the generators were provided with
+automatic shunting devices, so that their resistance would normally be
+removed from the circuit of the line.
+
+Series Systems. The general arrangement of a series party line
+employing a ground return is shown in Fig. 163. In this three ordinary
+series instruments are connected together in series, the end stations
+being grounded, in order to afford a return path for the ringing and
+voice currents.
+
+[Illustration: Fig. 164. Metallic-Circuit Series Line]
+
+In Fig. 164 there is shown a metallic-circuit series line on which
+five ordinary series telephones are placed in series. In this no
+ground is employed, the return being through a line wire, thus making
+the circuit entirely metallic.
+
+[Illustration: Fig. 165. Series Party Line]
+
+The limitations of the ordinary series party line may be best
+understood by reference to Fig. 165, in which the circuits of three
+series telephones are shown connected with a single line. The receiver
+of Station A is represented as being on its hook, while the receivers
+of Stations B and C are removed from their hooks, as when the
+subscribers at those two stations are carrying on a conversation. The
+hook switches of Stations B and C being in raised positions, the
+generators and ringers of those stations are cut out of the circuit,
+and only the telephone apparatus proper is included, but the hook
+switch of Station A being depressed by the weight of its receiver,
+includes the ringer of that station in circuit, and through this
+ringer, therefore, the voice currents of Stations B and C must pass.
+
+The generator of Station A is not in the circuit of voice currents,
+however, because of the automatic shunt with which the generator is
+provided, as described in Chapter VIII.
+
+A slight consideration of the series system as shown in this figure,
+indicates that the voice currents of any two stations that are in use,
+must pass (as indicated by the heavy lines) through the ringers of all
+the stations that are not in use; and when a great number of stations
+are placed upon a single line, as has been frequently the case, the
+impedance offered by these ringers becomes a serious barrier to the
+passage of the voice currents. This defect in the series party line is
+fundamental, as it is obvious that the ringers must be left in the
+circuit of the stations which are not in use, in order that those
+stations may always be in such condition as to be able to receive a
+call.
+
+This defect may in some measure be reduced by making the ringers of
+low impedance. This is the general practice with series telephones,
+the ringers ordinarily having short cores and a comparatively small
+number of turns, the resistance being as a rule about 80 ohms.
+
+Bridging Systems. Very much better than the series plan of
+party-line connections, is the arrangement by which the instruments
+are placed in bridges across the line, such lines being commonly known
+as bridged or bridging lines. This was first strongly advocated and
+put into wide practical use by J.J. Carty, now the Chief Engineer of
+the American Telephone and Telegraph Company.
+
+A simple illustration of a bridging telephone line is shown in Fig.
+166, where the three telephones shown are each connected in a bridge
+path from the line wire to ground, a type known as a "grounded
+bridging line." Its use is very common in rural districts.
+
+A better arrangement is shown in Fig. 167, which represents a
+metallic-circuit bridging line, three telephone instruments being
+shown in parallel or bridge paths across the two line wires.
+
+The actual circuit arrangements of a bridging party line are better
+shown in Fig. 168. There are three stations and it will be seen that
+at each station there are three possible bridges, or bridge paths,
+across the two limbs of the line. The first of these bridges is
+controlled by the hook switch and is normally open. When the hook is
+raised, however, this path is closed through the receiver and
+secondary of the induction coil, the primary circuit being also closed
+so as to include the battery and transmitter. This constitutes an
+ordinary local-battery talking set.
+
+[Illustration: Fig. 166. Grounded Bridging Line]
+
+[Illustration: Fig. 167. Metallic Bridging Line]
+
+[Illustration: Fig. 168. Metallic Bridging Line]
+
+A second bridge at each station is led through the ringer or
+call-bell, and this, in most bridging telephones, is permanently
+closed, the continuity of this path between the two limbs of the line
+not being affected either by the hook switch or by the automatic
+switch in connection with the generator.
+
+A third bridge path at each station is led through the generator.
+This, as indicated, is normally open, but the automatic cut-in switch
+of the generator serves, when the generator is operated, to close its
+path across the line, so that it may send its currents to the line and
+ring the bells of all the stations.
+
+When any generator is operated, its current divides and passes over
+the line wires and through all of the ringers in multiple. It is seen,
+therefore, that the requirements for a bridging generator are that it
+shall be capable of generating a large current, sufficient when
+divided up amongst all the bells to ring each of them; and that it
+shall be capable of producing a sufficient voltage to send the
+required current not only to the near-by stations, but to the stations
+at the distant end of the line.
+
+It might seem at first that the bridging system avoided one difficulty
+only to encounter another. It clearly avoids the difficulty of the
+series system in that the voice currents, in order to reach distant
+stations, do not have to pass through all of the bells of the idle
+stations in series. There is, however, presented at each station a
+leakage path through the bell bridged across the line, through which
+it would appear the voice currents might leak uselessly from one side
+of the line to the other and not pass on in sufficient volume to the
+distant station. This difficulty is, however, more apparent than real.
+It is found that, by making the ringers of high impedance, the leakage
+of voice currents through them from one side of the line to the other
+is practically negligible.
+
+It is obvious that in a heavily loaded bridged line, the bell at the
+home station, that is at the station from which the call is being sent,
+will take slightly more than its share of the current, and it is also
+obvious that the ringing of the home bell performs no useful function.
+The plan is frequently adopted, therefore, of having the operation of
+the generator serve to cut its own bell out of the circuit. The
+arrangement by which this is done is clearly shown in Fig. 169. The
+circuit of the bell is normally complete across the line, while the
+circuit of the generator is normally open. When, however, the generator
+crank is turned these conditions are reversed, the bell circuit being
+broken and the generator circuit closed, so as to allow its current all
+to pass the line. This feature of having the local bell remain silent
+upon the operation of its own generator is also of advantage because
+other parties at the same station are not disturbed by the ringing of
+the bell when a call is being made by that station.
+
+A difficulty encountered on non-selective bridging party lines, which
+at first seems amusing rather than serious, but which nevertheless is
+often a vexatious trouble, is that due to the propensity of some
+people to "listen in" on the line on hearing calls intended for other
+than their own stations. People whose ethical standards would not
+permit them to listen at, or peep through, a keyhole, often engage in
+this telephonic eavesdropping.
+
+Frequently, not only one but many subscribers will respond to a call
+intended for others and will listen to the ensuing conversation. This
+is disadvantageous in several respects: It destroys the privacy of
+conversation between any two parties; it subjects the local batteries
+to an unnecessary and useless drain; and it greatly impairs the
+ringing efficiency of the line. The reason for this interference with
+ringing is that the presence of the low-resistance receivers across
+the line allows the current sent out by any of the generators to pass
+in large measure through the receivers, thus depriving the ringers,
+which are of comparatively high resistance and impedance, of the
+energy necessary to operate them. As a result of this it is frequently
+impossible for one party to repeat the call for another because,
+during the interval between the first and second call, a number of
+parties remove their receivers from their hooks in order to listen.
+Ring-off or clearing-out signals are likewise interfered with.
+
+[Illustration: Fig. 169. Circuits of Bridging Station]
+
+A partial remedy for this interference with ringing, due to
+eavesdropping, is to introduce a low-capacity condenser into the
+receiver circuit at each station, as shown in Fig. 169. This does not
+seriously interfere with the speech transmission since the condensers
+will readily transmit the high-frequency voice currents. Such
+condensers, however, have not sufficient capacity to enable them
+readily to transmit the low-frequency ringing currents and hence
+these are forced, in large measure, to pass through the bells for
+which they are intended rather than leaking through the low-resistance
+receiver paths.
+
+The best condenser for this use is of about 1/2-microfarad capacity,
+which is ample for voice-transmitting purposes, while it serves to
+effectively bar the major portion of the generator currents. A higher
+capacity condenser would carry the generator currents much more
+readily and thus defeat the purpose for which it was intended.
+
+In order that the requisite impedance may be given to the ringers
+employed for bridging party lines, it is customary to make the cores
+rather long and of somewhat larger diameter than in series ringers and
+at the same time to wind the coils with rather fine wire so as to
+secure the requisite number of turns. Bridging bells are ordinarily
+wound to a resistance of 1,000 or 1,600 ohms, these two figures having
+become standard practice. It is not, however, the high resistance so
+much as the high impedance that is striven for in bridging bells; it
+is the number of turns that is of principal importance.
+
+As has already been stated, the generators used for bridging lines are
+made capable of giving a greater current output than is necessary in
+series instruments, and for this purpose they are usually provided with
+at least four, and usually five, bar magnets. The armature is made
+correspondingly long and is wound, as a rule, with about No. 33 wire.
+
+Sometimes where a bridged party line terminates in a central-office
+switchboard it is desired to so operate the line that the subscribers
+shall not be able to call up each other, but shall, instead, be able
+to signal only the central-office operator, who, in turn, will be
+enabled to call the party desired, designating his station by a
+suitable code ring. One common way to do this is to use biased bells
+instead of the ordinary polarized bells. In order that the bells may
+not be rung by the subscribers' generators, these generators are made
+of the direct-current type and these are so associated with the line
+that the currents which they send out will be in the wrong direction
+to actuate the bells. On the other hand, the central-office generator
+is of direct-current type and is associated with the line in the right
+direction to energize the bells. Thus any subscriber on the line may
+call the central office by merely turning his generator crank, which
+action will not ring the bells of the subscribers on the line. The
+operator will then be able to receive the call and in turn send out
+currents of the proper direction to ring all the bells and, by code,
+call the desired party to the telephone.
+
+[Illustration: ONE WING OF OPERATING ROOM, BERLIN, GERMANY Ultimate
+Capacity 24,000 Subscribers' Lines and 2,100 Trunk Lines.
+Siemens-Halske Equipment. Note Horizontal Disposal of Multiple]
+
+Signal Code. The code by which stations are designated on
+non-selective party lines usually consists in combinations of long and
+short rings similar to the dots and dashes in the Morse code. Thus,
+one short ring may indicate Station No. 1; two short rings Station No.
+2; and so on up to, say, five short rings, indicating Station No. 5.
+It is not good practice to employ more than five successive short
+rings because of the confusion which often arises in people's minds as
+to the number of rings that they hear. When, therefore, the number of
+stations to be rung by code exceeds five, it is better to employ
+combinations of long and short rings, and a good way is to adopt a
+partial decimal system, omitting the numbers higher than five in each
+ten, and employing long rings to indicate the tens digits and short
+rings to indicate the units digit, Table X.
+
+TABLE X
+
+Signal Code
++--------------+---------------+--------------+---------------+
+|STATION NUMBER|RING           |STATION NUMBER|RING           |
+|1             |1 short        |12            |1 long, 2 short|
+|2             |2 short        |13            |1 long, 3 short|
+|3             |3 short        |14            |1 long, 4 short|
+|4             |4 short        |15            |1 long, 5 short|
+|5             |5 short        |21            |2 long, 1 short|
+|11            |1 long, 1 short|22            |2 long, 2 short|
++--------------+---------------+--------------+---------------+
+
+Other arrangements are often employed and by almost any of them a
+great variety of readily distinguishable signals may be secured. The
+patrons of such lines learn to distinguish, with comparatively few
+errors, between the calls intended for them and those intended for
+others, but frequently they do not observe the distinction, as has
+already been pointed out.
+
+Limitations. With good telephones the limit as to the number of
+stations that it is possible to operate upon a single line is usually
+due more to limitations in ringing than in talking. As the number of
+stations is increased indefinitely a condition will be reached at
+which the generators will not be able to generate sufficient current
+to ring all of the bells, and this condition is likely to occur before
+the talking efficiency is seriously impaired by the number of bridges
+across the line.
+
+Neither of these considerations, however, should determine the maximum
+number of stations to be placed on a line. The proper limit as to the
+number of stations is not the number that can be rung by a single
+generator, or the number with which it is possible to transmit speech
+properly, but rather the number of stations that may be employed
+without causing undue interference between the various parties who may
+desire to use the line. Overloaded party lines cause much annoyance,
+not only for the reason that the subscribers are often not able to use
+the line when they want it, but also, in non-selective lines, because
+of the incessant ringing of the bells, and the liability of confusion
+in the interpretation of the signaling code, which of course becomes
+more complex as the number of stations increases.
+
+The amount of business that is done over a telephone line is usually
+referred to as the "traffic." It will be understood, however, in
+considering party-line working that the number of calls per day or per
+hour, or per shorter unit, is not the true measure of the traffic and,
+therefore, not the true measure of the amount of possible interference
+between the various subscribers on the line.
+
+An almost equally great factor is the average length of the
+conversation. In city lines, that is, in lines in city exchanges, the
+conversation is usually short and averages perhaps two minutes in
+duration. In country lines, however, serving people in rural
+districts, who have poor facilities for seeing each other,
+particularly during the winter time, the conversations will average
+very much longer. In rural communities the people often do much of
+their visiting by telephone, and conversations of half an hour in
+length are not unusual. It is obvious that under such conditions a
+party line having a great many stations will be subject to very grave
+interference between the parties, people desiring to use the line for
+business purposes often being compelled to wait an undue time before
+they may secure the use of the line.
+
+It is obvious, therefore, that the amount of traffic on the line,
+whether due to many short conversations or to a comparatively few
+long ones, is the main factor that should determine the number of
+stations that, economically, may be placed on a line. The facilities
+also for building lines enter as a factor in this respect, since it is
+obvious that in comparatively poor communities the money may not be
+forthcoming to build as many lines as are needed to properly take care
+of the traffic. A compromise is, therefore, often necessary, and the
+only rule that may be safely laid down is to place as few parties on a
+given line as conditions will admit.
+
+No definite limit may be set to apply to all conditions but it may be
+safely stated that under ordinary circumstances no more than ten
+stations should be placed on a non-selective line. Twenty stations
+are, however, common, and sometimes forty and even fifty have been
+connected to a single line. In such cases the confusion which results,
+even if the talking and the ringing efficiency are tolerable, makes
+the service over such overloaded lines unsatisfactory to all
+concerned.
+
+
+
+
+CHAPTER XVI
+
+SELECTIVE PARTY-LINE SYSTEMS
+
+
+The problem which confronts one in the production of a system of
+selective ringing on party lines is that of causing the bell of any
+chosen one of the several parties on a circuit to respond to a signal
+sent out from the central office without sounding any of the other
+bells. This, of course, must be accomplished without interfering with
+the regular functions of the telephone line and apparatus. By this is
+meant that the subscribers must be able to call the central office and
+to signal for disconnection when desired, and also that the
+association of the selective-signaling devices with the line shall not
+interfere with the transmission of speech over the line. A great many
+ways of accomplishing selective ringing on party lines have been
+proposed, and a large number of them have been used. All of these ways
+may be classified under four different classes according to the
+underlying principle involved.
+
+Classification. (_1_) _Polarity_ systems are so called because they
+depend for their operation on the use of bells or other responsive
+devices so polarized that they will respond to one direction of
+current only. These bells or other devices are so arranged in
+connection with the line that the one to be rung will be traversed by
+current in the proper direction to actuate it, while all of the others
+will either not be traversed by any current at all, or by current in
+the wrong direction to cause their operation.
+
+(_2_) The _harmonic_ systems have for their underlying principle the
+fact that a pendulum or elastic reed, so supported as to be capable of
+vibrating freely, will have one particular rate of vibration which it
+may easily be made to assume. This pendulum or reed is placed under
+the influence of an electromagnet associated with the line, and owing
+to the fact that it will vibrate easily at one particular rate of
+vibration and with extreme difficulty at any other rate, it is clear
+that for current impulses of a frequency corresponding to its natural
+rate the reed will take up the vibration, while for other frequencies
+it will fail to respond.
+
+Selection on party lines by means of this system is provided for by
+tuning all of the reeds on the line at different rates of vibration
+and is accomplished by sending out on the line ringing currents of
+proper frequency to ring the desired bell. The current-generating
+devices for ringing these bells are capable of sending out different
+frequencies corresponding respectively to the rates of vibration of
+each of the vibrating reed tongues. To select any one station,
+therefore, the current frequency corresponding to the rate of
+vibration of the reed tongue at that station is sent and this, being
+out of tune with the reed tongues at all of the other stations,
+operates the tongue of the desired station, but fails to operate those
+at all of the other stations.
+
+(_3_) In the _step-by-step_ system the bells on the line are normally
+not in operative relation with the line and the bell of the desired
+party on the line is made responsive by sending over the line a
+certain number of impulses preliminary to ringing it. These impulses
+move step-by-step mechanisms at each of the stations in unison, the
+arrangement being such that the bells at the several stations are each
+made operative after the sending of a certain number of preliminary
+impulses, this number being different for all the stations.
+
+(_4_) The _broken-line_ systems are new in telephony and for certain
+fields of work look promising. In these the line circuit is normally
+broken up into sections, the first section terminating at the first
+station out from the central office, the second section at the second
+station, and so on. When the line is in its normal or inactive
+condition only the bell at the first station is so connected with the
+line circuit as to enable it to be rung, the line being open beyond.
+Sending a single preliminary impulse will, however, operate a
+switching device so as to disconnect the bell at the first station and
+to connect the line through to the second station. This may be carried
+out, by sending the proper number of preliminary impulses, so as to
+build up the line circuit to the desired station, after which the
+sending of the ringing current will cause the bell to ring at that
+station only.
+
+Polarity Method. The polarity method of selective signaling on party
+lines is probably the most extensively used. The standard selective
+system of the American Telephone and Telegraph Company operates on
+this principle.
+
+_Two-Party Line._ It is obvious that selection may be had between two
+parties on a single metallic-circuit line without the use of biased
+bells or current of different polarities. Thus, one limb of a metallic
+circuit may be used as one grounded line to ring the bell at one of
+the stations, and the other limb of the metallic circuit may be used
+as another grounded line to ring the bell of the other station; and
+the two limbs may be used together as a metallic circuit for talking
+purposes as usual.
+
+This is shown in Fig. 170, where the ringing keys at the central office
+are diagrammatically shown in the left-hand portion of the figure as
+_K_^{1} and _K_^{2}. The operation of these keys will be more fully
+pointed out in a subsequent chapter, but a correct understanding will
+be had if it be remembered that the circuits are normally maintained by
+these keys in the position shown. When, however, either one of the keys
+is operated, the two long springs may be considered as pressed apart so
+as to disengage the normal contacts between the springs and to engage
+the two outer contacts, with which they are shown in the cut to be
+disengaged. The two outer contacts are connected respectively to an
+ordinary alternating-current ringing generator and to ground, but the
+connection is reversed on the two keys.
+
+[Illustration: Fig. 170. Simple Two-Party Line Selection]
+
+At Station A the ordinary talking set is shown in simplified form,
+consisting merely of a receiver, transmitter, and hook switch in a
+single bridge circuit across the line. An ordinary polarized bell is
+shown connected in series with a condenser between the lower limb of
+the line and ground. At Station B the same talking circuit is shown,
+but the polarized bell and condenser are bridged between the upper
+limb of the line and ground.
+
+If the operator desires to call Station A, she will press key _K_^{1}
+which will ground the upper side of the line and connect the lower
+side of the line with the generator _G_^{1}, and this, obviously, will
+cause the bell at Station A to ring. The bell at Station B will not
+ring because it is not in the circuit. If, on the other hand, the
+operator desires to ring the bell at Station B, she will depress key
+_K_^{2}, which will allow the current from generator _G_^{2} to pass
+over the upper side of the line through the bell and condenser at
+Station B and return by the path through the ground. The object of
+grounding the opposite sides of the keys at the central office is to
+prevent cross-ringing, that is, ringing the wrong bell. Were the keys
+not grounded this might occur when a ringing current was being sent
+out while the receiver at one of the stations was off its hook; the
+ringing current from, say, generator _G_^{1} then passing not only
+through the bell at Station A as intended, but also through the bell
+at Station B by way of the bridge path through the receiver that
+happened to be connected across the line. With the ringing keys
+grounded as shown, it is obvious that this will not occur, since the
+path for the ringing current through the wrong bell will always be
+shunted by a direct path to ground on the same side of the line.
+
+In such a two-party-line selective system the two generators _G_^{1}
+and _G_^{2} may be the same generator and may be of the ordinary
+alternating-current type. The bells likewise may be of the ordinary
+alternating-current type.
+
+The two-party selective line just described virtually employs two
+separate circuits for ringing. Now each of these circuits alone may be
+employed to accomplish selective ringing between two stations by using
+two biased bells oppositely polarized, and employing pulsating ringing
+currents of one direction or the other according to which bell it is
+desired to ring. One side of a circuit so equipped is shown in Fig.
+171. In this the two biased bells are at Station A and Station B,
+these being bridged to ground in each case and adapted to respond only
+to positive and negative impulses respectively. At the central office
+the two keys _K_^{1} and _K_^{2} are shown. A single
+alternating-current generator _G_ is shown, having its brush _1_
+grounded and brush _2_ connected to a commutator disk _3_ mounted on
+the generator shaft so as to revolve therewith. One-half of the
+periphery of this disk is of insulating material so that the brushes
+_4_ and _5_, which bear against the disk, will be alternately
+connected with the disk and, therefore, with the brush _2_ of the
+generator. Now the brush _2_, being one terminal of an
+alternating-current machine, is alternately positive and negative, and
+the arrangement of the commutator is such that the disk, which is
+always at the potential of the brush _2_, will be connected to the
+brush _5_ only while it is positively charged and with the brush _4_
+only while it is negatively charged. As a result, brush _5_ has a
+succession of positive impulses and brush _4_ a succession of negative
+ones. Obviously, therefore, when key _K_^{1} is depressed only the
+bell at Station A will be rung, and likewise the depression of key
+_K_^{2} will result only in the ringing of the bell at Station B.
+
+[Illustration: Fig. 171. Principle of Selection by Polarity]
+
+_Four-Party Line._ From the two foregoing two-party line systems it is
+evident that a four-party line system may be readily obtained, that
+is, by employing two oppositely polarized biased bells on each side of
+the metallic circuit. The selection of any of the four bells may be
+obtained, choosing between the pairs connected, respectively, with the
+two limbs of the line, by choosing the limb on which the current is to
+be sent, and choosing between the two bells of the pair on that side
+of the line by choosing which polarity of current to send.
+
+Such a four-party line system is shown in Fig. 172. In this the
+generators are not shown, but the wires leading from the four keys are
+shown marked plus or minus, according to the terminal of the generator
+to which they are supposed to be connected. Likewise the two bells
+connected with the lower side of the line are marked positive and
+negative, as are the two bells connected with the upper side of the
+line. From the foregoing description of Figs. 170 and 171, it is clear
+that if key _K_^{1} is pressed the bell at Station A will be rung, and
+that bell only, since the bells at Station C and Station _D_ are not
+in the circuit and the positive current sent over the lower side of
+the line is not of the proper polarity to ring the bell at Station B.
+
+The system shown in Fig. 172 is subject to one rather grave defect. In
+subsequent chapters it will be pointed out that in common-battery
+systems the display of the line signal at the central office is
+affected by any one of the subscribers merely taking his receiver off
+its hook and thus establishing a connection between the two limbs of
+the metallic circuit. Such common-battery systems should have the two
+limbs of the line, normally, entirely insulated from each other. It is
+seen that this is not the case in the system just described, since
+there is a conducting path from one limb of the line through the two
+bells on that side to ground, and thence through the other pair of
+bells to the other limb of the line. This means that unless the
+resistance of the bell windings is made very high, the path of the
+signaling circuit will be of sufficiently low resistance to actuate
+the line signal at the central office.
+
+[Illustration: Fig. 172. Four-Party Polarity Selection]
+
+It is not feasible to overcome this objection by the use of condensers
+in series with the bells, as was done in the system shown in Fig. 170,
+since the bells are necessarily biased and such bells, as may readily
+be seen, will not work properly through condensers, since the placing
+of a condenser in their circuit means that the current which passes
+through the bell is alternating rather than pulsating, although the
+original source may have been of pulsating nature only.
+
+[Illustration: Fig 173. Standard Polarity System]
+
+The remedy for this difficulty, therefore, has been to place in series
+with each bell a very high non-inductive resistance of about 15,000 or
+20,000 ohms, and also to make the windings of the bells of
+comparatively high resistance, usually about 2,500 ohms. Even with
+this precaution there is a considerable leakage of the central-office
+battery current from one side of the line to the other through the two
+paths to ground in series. This method of selective signaling has,
+therefore, been more frequently used with magneto systems. An endeavor
+to apply this principle to common-battery systems without the
+objections noted above has led to the adoption of a modification,
+wherein a relay at each station normally holds the ground connection
+open. This is shown in Fig. 173 and is the standard four-party line
+ringing circuit employed by the American Telephone and Telegraph
+Company and their licensees.
+
+In this system the biased bells are normally disconnected from the
+line, and, therefore, the leakage path through them from one side of
+the line to the other does not exist. At each station there is a relay
+winding adapted to be operated by the ringing current bridged across
+the line in series with a condenser. As a result, when ringing current
+is sent out on the line all of the relays, _i.e._, one at each
+station, are energized and attract their armatures. This establishes
+the connection of all the bells to line and really brings about
+temporarily a condition equivalent to that of Fig. 172. As a result,
+the sending of a positive current on the lower line with a ground
+return will cause the operation of the bell at Station A. It will not
+ring the bell at Station B because of the wrong polarity. It will not
+ring the bells of Station C and Station D because they are in the
+circuit between the other side of the line and ground. As soon as the
+ringing current ceases all of the relays release their armatures and
+disconnect all the bells from the line.
+
+By this very simple device the trouble, due to marginal working of
+the line signal, is done away with, since normally there is no leakage
+from one side of the line to the other on account of the presence of
+the condensers in the bridge at each station.
+
+[Illustration: Fig. 174. Ringing-Key Arrangement]
+
+In Fig. 174, the more complete connections of the central-office
+ringing keys are shown, by means of which the proper positive or
+negative ringing currents are sent to line in the proper way to cause
+the ringing of any one of the four bells on a party line of either of
+the types shown in Figs. 172 and 173.
+
+In this the generator _G_ and its commutator disk _3_, with the
+various brushes, _1_, _2_, _4_, and _5_, are arranged in the same
+manner as is shown in Fig. 171. It is evident from what has been said
+that wire _6_ leading from generator brush _2_ and commutator disk _3_
+will carry alternating potential; that wire _7_ will carry positive
+pulsations of potential; and that wire _8_ will carry negative
+pulsations of potential. There are five keys in the set illustrated in
+Fig. 174, of which four, viz, _K_^{1}, _K_^{2}, _K_^{3}, and _K_^{4},
+are connected in the same manner as diagrammatically indicated in
+Figs. 172 and 173, and will, obviously, serve to send the proper
+current over the proper limb of the line to ring one of the bells. Key
+_K_^{5}, the fifth one in the set, is added so as to enable the
+operator to ring an ordinary unbiased bell on a single party line when
+connection is made with such line. As the two outside contacts of this
+key are connected respectively to the two brushes of the
+alternating-current dynamo _G_, it is clear that it will impress an
+alternating current on the line when its contacts are closed.
+
+_Circuits of Two-Party Line Telephones._ In Fig. 175 is shown in
+detail the wiring of the telephone set usually employed in connection
+with the party-line selective-ringing system illustrated in Fig. 170.
+In the wiring of this set and the two following, it must be borne in
+mind that the portion of the circuit used during conversation might
+be wired in a number of ways without affecting the principle of
+selective ringing employed; however, the circuits shown are those most
+commonly employed with the respective selective ringing systems which
+they are intended to illustrate. In connecting the circuits of this
+telephone instrument to the line, the two line conductors are
+connected to binding posts _1_ and _2_ and a ground connection is made
+to binding post _3_. In practice, in order to avoid the necessity of
+changing the permanent wiring of the telephone set in connecting it as
+an A or B Station (Fig. 170), the line conductors are connected to the
+binding posts in reverse order at the two stations; that is, for
+Station A the upper conductor, Fig. 170, is connected to binding post
+_1_ and the lower conductor to binding post _2_, while at Station B
+the upper conductor is connected to binding post _2_ and the lower
+conductor to binding post _1_. The permanent wiring of this telephone
+set is the same as that frequently used for a set connected to a line
+having only one station, the proper ringing circuit being made by the
+method of connecting up the binding posts. For example, if this
+telephone set were to be used on a single station line, the binding
+posts _1_ and _2_ would be connected to the two conductors of the line
+as before, while binding post _3_ would be connected to post _1_
+instead of being grounded.
+
+[Illustration: Fig. 175. Circuit of Two-Party Station]
+
+_Circuits of Four-Party-Line Telephones._ The wiring of the telephone
+set used with the system illustrated in Fig. 172 is shown in detail in
+Fig. 176. The wiring of this set is arranged for local battery or
+magneto working, as this method of selective ringing is more frequently
+employed with magneto systems, on account of the objectionable features
+which arise when applied to common-battery systems. In this figure the
+line conductors are connected to binding posts _1_ and _2_, and a
+ground connection is made to binding post _3_. In order that all sets
+may be wired alike and yet permit the instrument to be connected for
+any one of the various stations, the bell is not permanently wired to
+any portion of the circuit but has flexible connections which will
+allow of the set being properly connected for any desired station. The
+terminals of the bell are connected to binding posts _9_ and _10_, to
+which are connected flexible conductors terminating in terminals _7_
+and _8_. These terminals may be connected to the binding posts _4_,
+_5_, and _6_ in the proper manner to connect the set as an A, B, C, or
+D station, as required. For example, in connecting the set for Station
+A, Fig. 172, terminal _7_ is connected to binding post _6_ and _8_ to
+_5_. For connecting the set for Station B terminal _7_ is connected to
+binding post _5_ and _8_ to _6_. For connecting the set for Station C
+terminal _7_ is connected to binding post _6_ and _8_ to _4_. For
+connecting the set for Station D terminal _7_ is connected to binding
+post _4_ and _8_ to _6_.
+
+[Illustration: Fig. 176. Circuit of Four-Party Station without Relay]
+
+[Illustration: Fig. 177. Circuit of Four-Party Station with Relay]
+
+The detailed wiring of the telephone set employed in connection with
+the system illustrated in Fig. 173 is shown in Fig. 177. The wiring of
+this set is arranged for a common-battery system, inasmuch as this
+arrangement of signaling circuit is more especially adapted for
+common-battery working. However, this arrangement is frequently
+adapted to magneto systems as even with magneto systems a permanent
+ground connection at a subscriber's station is objectionable inasmuch
+as it increases the difficulty of determining the existence or
+location of an accidental ground on one of the line conductors. The
+wiring of this set is also arranged so that one standard type of
+wiring may be employed and yet allow any telephone set to be connected
+as an A, B, C, or D station.
+
+Harmonic Method. _Principles._ To best understand the principle of
+operation of the harmonic party-line signaling systems, it is to be
+remembered that a flexible reed, mounted rigidly at one end and having
+its other end free to vibrate, will, like a violin string, have a
+certain natural period of vibration; that is, if it be started in
+vibration, as by snapping it with the fingers, it will take up a
+certain rate of vibration which will continue at a uniform rate until
+the vibration ceases altogether. Such a reed will be most easily
+thrown into vibration by a series of impulses having a frequency
+corresponding exactly to the natural rate of vibration of the reed
+itself; it may be thrown into vibration by very slight impulses if
+they occur at exactly the proper times.
+
+It is familiar to all that a person pushing another in a swing may
+cause a considerable amplitude of vibration with the exertion of but a
+small amount of force, if he will so time his pushes as to conform
+exactly to the natural rate of vibration of the swing. It is of course
+possible, however, to make the swing take up other rates of vibrations
+by the application of sufficient force. As another example, consider a
+clock pendulum beating seconds. By gentle blows furnished by the
+escapement at exactly the proper times, the heavy pendulum is kept in
+motion. However, if a person grasps the pendulum weight and shakes it,
+it may be made to vibrate at almost any desired rate, dependent on the
+strength and agility of the individual.
+
+The conclusion is, therefore, that a reed or pendulum may be made to
+start and vibrate easily by the application of impulses at proper
+intervals, and only with great difficulty by the application of
+impulses at other than the proper intervals; and these facts form the
+basis on which harmonic-ringing systems rest.
+
+The father of harmonic ringing in telephony was Jacob B. Currier, an
+undertaker of Lowell, Mass. His harmonic bells were placed in series
+in the telephone line, and were considerably used in New England in
+commercial practice in the early eighties. Somewhat later James A.
+Lighthipe of San Francisco independently invented a harmonic-ringing
+system, which was put in successful commercial use at Sacramento and a
+few other smaller California towns. Lighthipe polarized his bells and
+bridged them across the line in series with condensers, as in modern
+practice, and save for some crudities in design, his apparatus closely
+resembled, both in principle and construction, some of that in
+successful use today.
+
+Lighthipe's system went out of use and was almost forgotten, when
+about 1903, Wm. W. Dean again independently redeveloped the harmonic
+system, and produced a bell astonishingly like that of Lighthipe, but
+of more refined design, thus starting the development which has
+resulted in the present wide use of this system.
+
+The signal-receiving device in harmonic-ringing systems takes the form
+of a ringer, having its armature and striker mounted on a rather stiff
+spring rather than on trunnions. By this means the moving parts of the
+bell constitute in effect a reed tongue, which has a natural rate of
+vibration at which it may easily be made to vibrate with sufficient
+amplitude to strike the gongs. The harmonic ringer differs from the
+ordinary polarized bell or ringer, therefore, in that its armature
+will vibrate most easily at one particular rate, while the armature of
+the ordinary ringer is almost indifferent, between rather wide limits,
+as to the rate at which it vibrates.
+
+As a rule harmonic party-line systems are limited to four stations on
+a line. The frequencies employed are usually 16-2/3, 33-1/3, 50, and
+66-2/3 cycles per second, this corresponding to 1,000, 2,000, 3,000,
+and 4,000 cycles per minute. The reason why this particular set of
+frequencies was chosen is that they represent approximately the range
+of desirable frequencies, and that the first ringing-current machines
+in such systems were made by mounting the armatures of four different
+generators on a single shaft, these having, respectively, two poles,
+four poles, six poles, and eight poles each. The two-pole generator
+gave one cycle per revolution, the four-pole two, the six-pole three,
+and the eight-pole four, so that by running the shaft of the machine
+at exactly 1,000 revolutions per minute the frequencies before
+mentioned were attained. This range of frequencies having proved
+about right for general practice and the early ringers all having been
+attuned so as to operate on this basis, the practice of adhering to
+these numbers of vibrations has been kept up with one exception by all
+the manufacturers who make this type of ringer.
+
+_Tuning._ The process of adjusting the armature of a ringer to a
+certain rate of vibration is called tuning, and it is customary to
+refer to a ringer as being tuned to a certain rate of vibration, just
+as it is customary to refer to a violin string as being tuned to a
+certain pitch or rate of vibration.
+
+The physical difference between the ringers of the various frequencies
+consists mainly in the size of the weights at the end of the vibrating
+reed, that is, of the weights which form the tapper for the bell. The
+low-frequency ringers have the largest weights and the high-frequency
+the smallest, of course. The ringers are roughly tuned to the desired
+frequencies by merely placing on the tapper rod the desired weight and
+then a more refined tuning is given them by slightly altering the
+positions of the weights on the tapper rod. To make the reed have a
+slightly lower natural rate of vibration, the weight is moved further
+from the stationary end of the reed, while to give it a slightly
+higher natural rate of vibration the weight is moved toward the
+stationary. In this way very nice adjustments may be made, and the aim
+of the various factories manufacturing these bells is to make the
+adjustment permanent so that it will never have to be altered by the
+operating companies. Several years of experience with these bells has
+shown that when once properly assembled they maintain the same rate of
+vibration with great constancy.
+
+There are two general methods of operating harmonic bells. One of
+these may be called the in-tune system and the other the under-tune
+system. The under-tune system was the first employed.
+
+[Illustration: OPERATING ROOM AT TOKYO, JAPAN]
+
+_Under-Tune System._ The early workers in the field of
+harmonic-selective signaling discovered that when the tapper of the
+reed struck against gongs the natural rate of vibration of the reed
+was changed, or more properly, the reed was made to have a different
+rate of vibration from its natural rate. This was caused by the fact
+that the elasticity of the gongs proved another factor in the set
+of conditions causing the reeds to take up a certain rate of
+vibration, and the effect of this added factor was always to
+accelerate the rate of vibration which the reed had when it was not
+striking the gongs. The rebound of the hammer from the gongs tended,
+in other words, to accelerate the rate of vibration, which, as might
+be expected, caused a serious difficulty in the practical operation of
+the bells. To illustrate: If a reed were to have a natural rate of
+vibration, when not striking the gongs, of 50 per second and a current
+of 50 cycles per second were impressed on the line, the reed would
+take up this rate of vibration easily, but when a sufficient amplitude
+of vibration was attained to cause the tapper to strike the gongs, the
+reed would be thrown out of tune, on account of the tendency of the
+gongs to make the reed vibrate at a higher rate. This caused irregular
+ringing and was frequently sufficient to make the bells cease ringing
+altogether or to ring in an entirely unsatisfactory manner.
+
+In order to provide for this difficulty the early bells of Currier and
+Lighthipe were made on what has since been called the "under-tuned"
+principle. The first bells of the Kellogg Switchboard and Supply
+Company, developed by Dean, were based on this idea as their cardinal
+principle. The reeds were all given a natural rate of vibration, when
+not striking the gongs, somewhat below that of the current frequencies
+to be employed; and yet not sufficiently below the corresponding
+current frequency to make the bell so far out of tune that the current
+frequency would not be able to start it. This was done so that when
+the tapper began to strike the gongs the tapper would be accelerated
+and brought practically into tune with the current frequency, and the
+ringing would continue regularly as long as the current flowed. It
+will be seen that the under-tuned system was, therefore, one involving
+some difficulty in starting in order to provide for proper regularity
+while actually ringing.
+
+Ringers of this kind were always made with but a single gong, it being
+found difficult to secure uniformity of ringing and uniformity of
+adjustment when two gongs were employed. Although no ringers of this
+type are being made at present, yet a large number of them are in use
+and they will consequently be described. Their action is interesting
+in throwing better light on the more improved types, if for no other
+reason.
+
+Figs. 178 and 179 show, respectively, side and front views of the
+original Kellogg bell. The entire mechanism is self-contained, all
+parts being mounted on the base plate _1_. The electromagnet is of the
+two-coil type, and is supported on the brackets _2_ and _3_. The
+bracket _2_ is of iron so as to afford a magnetic yoke for the field of
+the electromagnet, while the bracket _3_ is of brass so as not to
+short-circuit the magnetic lines across the air-gap. The reed
+tongue--consisting of the steel spring _5_, the soft-iron armature
+pieces _6_, the auxiliary spring _7_, and the tapper ball _8_, all of
+which are riveted together, as shown in Fig. 178--constitutes the only
+moving part of the bell. The steel spring _5_ is rigidly mounted in the
+clamping piece _9_ at the upper part of the bracket _3_, and the reed
+tongue is permitted to vibrate only by the flexibility of this spring.
+The auxiliary spring _7_ is much lighter than the spring _5_ and has
+for its purpose the provision of a certain small amount of flexibility
+between the tapper ball and the more rigid portion of the armature
+formed by the iron strips _6-6_. The front ends of the magnet pole
+pieces extend through the bracket _3_ and are there provided with
+square soft-iron pole pieces _10_ set at right angles to the magnet
+cores so as to form a rather narrow air-gap in which the armature may
+vibrate.
+
+[Illustration: Fig. 178. Under-Tuned Ringer]
+
+The cores of the magnet and also the reed tongue are polarized by
+means of the =L=-shaped bar magnet _4_, mounted on the iron yoke _2_
+at one end in such manner that its other end will lie quite close to
+the end of the spring _5_, which, being of steel, will afford a path
+for the lines of force to the armature proper. We see, therefore, that
+the two magnet cores are, by this permanent magnet, given one
+polarity, while the reed tongue itself is given the other polarity,
+this being exactly the condition that has already been described in
+connection with the regular polarized bell or ringer.
+
+The electromagnetic action by which this reed tongue is made to
+vibrate is, therefore, exactly the same as that of an ordinary
+polarized ringer, but the difference between the two is that, in this
+harmonic ringer, the reed tongue will respond only to one particular
+rate of vibrations, while the regular polarized ringer will respond to
+almost any.
+
+As shown in Fig. 178, the tapper ball strikes on the inside surface of
+the single gong. The function of the auxiliary spring _7_ between the
+ball and the main portion of the armature is to allow some resilience
+between the ball and the balance of the armature so as to counteract
+in some measure the accelerating influence of the gong on the
+armature. In these bells, as already stated, the natural rate of
+vibration of the reed tongue was made somewhat lower than the rate at
+which the bell was to be operated, so that the reed tongue had to be
+started by a current slightly out of tune with it, and then, as the
+tapper struck the gong, the acceleration due to the gong would bring
+the vibration of the reed tongue, as modified by the gong, into tune
+with the current that was operating it. In ether words, in this system
+the ringing currents that were applied to the line had frequencies
+corresponding to what may be called the _operative rates of vibration_
+of the reed tongues, which operative rates of vibration were in each
+case the resultant of the natural pitch of the reed as modified by the
+action of the bell gong when struck.
+
+[Illustration: Fig. 179. Under-Tuned Ringer]
+
+_In-Tune System._ The more modern method of tuning is to make the
+natural rate of vibration of the reed tongue, that is, the rate at
+which it naturally vibrates when not striking the gongs, such as to
+accurately correspond to the rate of vibration at which the bells are
+to be operated--that is, the natural rate of vibration of the reed
+tongues is made the same as the operative rate. Thus the bells are
+attuned for easy starting, a great advantage over the under-tuned
+system. In the under-tuned system, the reeds being out of tune in
+starting require heavier starting current, and this is obviously
+conducive to cross-ringing, that is, to the response of bells to other
+than the intended frequency.
+
+Again, easy starting is desirable because when the armature is at
+rest, or in very slight vibration, it is at a maximum distance from
+the poles of the electromagnet, and, therefore, subject to the weakest
+influence of the poles. A current, therefore, which is strong enough
+to start the vibration, will be strong enough to keep the bell ringing
+properly.
+
+[Illustration: Fig. 180. Dean In-Tune Ringer]
+
+When with this "in-tune" mode of operation, the armature is thrown
+into sufficiently wide vibration to cause the tapper to strike the
+gong, the gong may tend to accelerate the vibration of the reed
+tongue, but the current impulses through the electromagnet coils
+continue at precisely the same rates as before. Under this condition
+of vibration, when the reed tongue has an amplitude of vibration wide
+enough to cause the tapper to strike the gongs, the ends of the
+armature come closest to the pole pieces, so that the pole pieces have
+their maximum magnetic effect on the armature, with the result that
+even if the accelerating tendency of the gongs were considerable, the
+comparatively large magnetic attractive impulses occurring at the same
+rate as the natural rate of vibration of the reed tongue, serve wholly
+to prevent any actual acceleration of the reed tongue. The magnetic
+attractions upon the ends of the armature, continuing at the initial
+rate, serve, therefore, as a check to offset any accelerating
+tendency which the striking of the gong may have upon the vibrating
+reed tongue.
+
+It is obvious, therefore, that in the "in-tune" system the
+electromagnetic effect on the armature should, when the armature is
+closest to the pole pieces, be of such an overpowering nature as to
+prevent whatever accelerating tendency the gongs may have from
+throwing the armature out of its "stride" in step with the current.
+For this reason it is usual in this type to so adjust the armature
+that its ends will actually strike against the pole pieces of the
+electromagnet when thrown into vibration. Sufficient flexibility is
+given to the tapper rod to allow it to continue slightly beyond the
+point at which it would be brought to rest by the striking of the
+armature ends against the pole pieces and thus exert a whipping action
+so as to allow the ball to continue in its movement far enough to
+strike against the gongs. The rebound of the gong is then taken up by
+the elasticity of the tapper rod, which returns to an unflexed
+position, and at about this time the pole piece releases the armature
+so that it may swing over in the other direction to cause the tapper
+to strike the other gong.
+
+[Illustration: Fig. 181. Tappers for Dean Ringers]
+
+The construction of the "in-tune" harmonic ringer employed by the Dean
+Electric Company, of Elyria, Ohio, is illustrated in Figs. 180, 181,
+and 182. It will be seen from Fig. 180 that the general arrangement of
+the magnet and armature is the same as that of the ordinary polarized
+ringer; the essential difference is that the armature is
+spring-mounted instead of pivoted. The armature and the tapper rod
+normally stand in the normal central position with reference to the
+pole pieces of the magnet and the gongs. Fig. 181 shows the complete
+vibrating parts of four ringers, adapted, respectively, to the four
+different frequencies of the system. The assembled armature, tapper
+rod, and tapper are all riveted together and are non-adjustable. All
+of the adjustment that is done upon them is done in the factory and
+is accomplished, first, by choosing the proper size of weight, and
+second, by forcing this weight into the proper position on the tapper
+rod to give exactly the rate of vibration that is desired.
+
+[Illustration: Fig. 182. Dean In-Tune Ringer]
+
+An interesting feature of this Dean harmonic ringer is the gong
+adjustment. As will be seen, the gongs are mounted on posts which are
+carried on levers pivoted to the ringer frame. These levers have at
+their outer end a curved rack provided with gear teeth adapted to
+engage a worm or screw thread mounted on the ringer frame. Obviously,
+by turning this worm screw in one direction or the other, the gongs
+are moved slightly toward or from the armature or tapper. This affords
+a very delicate means of adjusting the gongs, and at the same time one
+which has no tendency to work loose or to get out of adjustment.
+
+[Illustration: Fig. 183. Kellogg In-Tune Ringer]
+
+In Fig. 183 is shown a drawing of the "in-tune" harmonic ringer
+manufactured by the Kellogg Switchboard and Supply Company. This
+differs in no essential respect from that of the Dean Company, except
+in the gong adjustment, this latter being affected by a screw passing
+through a nut in the gong post, as clearly indicated.
+
+In both the Kellogg and the Dean in-tune ringers, on account of the
+comparative stiffness of the armature springs and on account of the
+normal position of the armature with maximum air gaps and consequent
+minimum magnetic pull, the armature will practically not be affected
+unless the energizing current is accurately attuned to its own natural
+rate. When the proper current is thrown on to the line, the ball will
+be thrown into violent vibration, and the ends of the armature brought
+into actual contact with the pole pieces, which are of bare iron and
+shielded in no way. The armature in this position is very strongly
+attracted and comes to a sudden stop on the pole pieces. The gongs are
+so adjusted that the tapper ball will have to spring about one
+thirty-second of an inch in order to hit them. The armature is held
+against the pole piece while the tapper ball is engaged in striking
+the gong and in partially returning therefrom, and so strong is the
+pull of the pole piece on the armature in this position that the
+accelerating influence of the gong has no effect in accelerating the
+rate of vibration of the reed.
+
+[Illustration: Fig. 184. Circuits of Dean Harmonic System]
+
+_Circuits_. In Fig. 184 are shown in simplified form the circuits of a
+four-station harmonic party line. It is seen that at the central
+office there are four ringing keys, adapted, respectively, to impress
+on the line ringing currents of four different frequencies. At the
+four stations on the line, lettered A, B, C, and D, there are four
+harmonic bells tuned accordingly. At Station A there is shown the
+talking apparatus employing the Wheatstone bridge arrangement. The
+talking apparatus at all of the other stations is exactly the same,
+but is omitted for the sake of simplicity. A condenser is placed in
+series with each of the bells in order that there may be no
+direct-current path from one side of the line to the other when all of
+the receivers are on their hooks at the several stations.
+
+In Fig. 185 is shown exactly the same arrangement, with the exception
+that the talking apparatus illustrated in detail at Station A is that
+of the Kellogg Switchboard and Supply Company. Otherwise the circuits
+of the Dean and the Kellogg Company, and in fact of all the other
+companies manufacturing harmonic ringing systems, are the same.
+
+_Advantages_. A great advantage of the harmonic party-line system is
+the simplicity of the apparatus at the subscriber's station. The
+harmonic bell is scarcely more complex than the ordinary polarized
+ringer, and the only difference between the harmonic-ringing telephone
+and the ordinary telephone is in the ringer itself. The absence of all
+relays and other mechanism and also the absence of the necessity for
+ground connections at the telephone are all points in favor of the
+harmonic system.
+
+[Illustration: Fig. 185. Circuits of Kellogg Harmonic System]
+
+_Limitations_. As already stated, the harmonic systems of the various
+companies, with one exception, are limited to four frequencies. The
+exception is in the case of the North Electric Company, which sometimes
+employs four and sometimes five frequencies and thus gets a selection
+between five stations. In the four-party North system, the frequencies,
+unlike those in the Dean and Kellogg systems, wherein the higher
+frequencies are multiples of the lower, are arranged so as to be
+proportional to the whole numbers 5, 7, 9, and 11, which, of course,
+have no common denominator. The frequencies thus employed in the North
+system are, in cycles per second, 30.3, 42.4, 54.5, and 66.7. In the
+five-party system, the frequency of 16.7 is arbitrarily added.
+
+While all of the commercial harmonic systems on the market are
+limited to four or five frequencies, it does not follow that a greater
+number than four or five stations may not be selectively rung. Double
+these numbers may be placed on a party line and selectively actuated,
+if the first set of four or five is bridged across the line and the
+second set of four or five is connected between one limb of the line
+and ground. The first set of these is selectively rung, as already
+described, by sending the ringing currents over the metallic circuit,
+while the second set may be likewise selectively rung by sending the
+ringing currents over one limb of the line with a ground return. This
+method is frequently employed with success on country lines, where it
+is desired to place a greater number of instruments on a line than
+four or five.
+
+Step-by-Step Method. A very large number of step-by-step systems
+have been proposed and reduced to practice, but as yet they have not
+met with great success in commercial telephone work, and are nowhere
+near as commonly used as are the polarity and harmonic systems.
+
+_Principles_. An idea of the general features of the step-by-step
+systems may be had by conceiving at each station on the line a ratchet
+wheel, having a pawl adapted to drive it one step at a time, this pawl
+being associated with the armature of an electromagnet which receives
+current impulses from the line circuit. There is thus one of these
+driving magnets at each station, each bridged across the line so that
+when a single impulse of current is sent out from the central office
+all of the ratchet wheels will be moved one step. Another impulse will
+move all of the ratchet wheels another step, and so on throughout any
+desired number of impulses. The ratchet wheels, therefore, are all
+stepped in unison.
+
+Let us further conceive that all of these ratchet wheels are provided
+with a notch or a hole or a projection, alike in all respects at all
+stations save in the position which this notch or hole or projection
+occupies on the wheel. The thing to get clear in this part of the
+conception is that all of these notches, holes, or projections are
+alike on all of the wheels, but they occupy a different position on
+the wheel for each one of the stations.
+
+Consider further that the bell circuit at each of the stations is
+normally open, but that in each case it is adapted to be closed when
+the notch, hole, or projection is brought to a certain point by the
+revolution of the wheel.
+
+Let us conceive further that this distinguishing notch, hole, or
+projection is so arranged on the wheel of the first station as to
+close the bell circuit when one impulse has been sent, that that on
+the second station will close the bell circuit after the second
+impulse has been sent, and so on throughout the entire number of
+stations. It will, therefore, be apparent that the bell circuits at
+the various stations will, as the wheels are rotated in unison, be
+closed one after the other. In order to call a given station,
+therefore, it is only necessary to rotate all of the wheels in unison,
+by sending out the proper stepping impulses until they all occupy such
+a position that the one at the desired station is in such position as
+to close the bell circuit at that station. Since all of the notches,
+holes, or projections are arranged to close the bell circuits at their
+respective stations at different times, it follows that when the bell
+circuit at the desired station is closed those at all of the other
+stations will be open. If, therefore, after the proper number of
+stepping impulses has been sent to the line to close the bell circuit
+of the desired station, ringing current be applied to the line, it is
+obvious that the bell of that one station will be rung to the
+exclusion of all others. It is, of course, necessary that provision be
+made whereby the magnets which furnish the energy for stepping the
+wheels will not be energized by the ringing current. This is
+accomplished in one of several ways, the most common of which is to
+have the stepping magnets polarized or biased in one direction and the
+bells at the various stations oppositely biased, so that the ringing
+current will not affect the stepping magnet and the stepping current
+will not affect the ringer magnets.
+
+After a conversation is finished, the line may be restored to its
+normal position in one of several ways. Usually so-called release
+magnets are employed, for operating on the releasing device at each
+station. These, when energized, will withdraw the holding pawls from
+the ratchets and allow them all to return to their normal positions.
+Sometimes these release magnets are operated by a long impulse of
+current, being made too sluggish in their action to respond to the
+quick-stepping impulses; sometimes the release magnets are tapped from
+one limb of the line to ground, so as not to be affected by the
+stepping or ringing currents sent over the metallic circuit; and
+sometimes other expedients are used for obtaining the release of the
+ratchets at the proper time, a large amount of ingenuity having been
+spent to this end.
+
+As practically all step-by-step party-line systems in commercial use
+have also certain other features intended to assure privacy of
+conversation to the users, and, therefore, come under the general
+heading of lock-out party-line systems, the discussion of commercial
+examples of these systems will be left for the next chapter, which is
+devoted to such lock-out systems.
+
+Broken-Line Method. The broken-line system, like the step-by-step
+system, is also essentially a lock-out system and for that reason only
+its general features, by which the selective ringing is accomplished,
+will be dealt with here.
+
+_Principles_. In this system there are no tuned bells, no positively
+and negatively polarized bells bridged to ground on each side of the
+line, and no step-by-step devices in the ordinary sense, by which
+selective signaling has ordinarily been accomplished on party lines.
+Instead of this, each instrument on the line is exclusively brought
+into operative relation with the line, and then removed from such
+operative relation until the subscriber wanted is connected, at which
+time all of the other instruments are locked out and the line is not
+encumbered by any bridge circuits at any of the instruments that are
+not engaged in the conversation. Furthermore, in the selecting of a
+subscriber or the ringing of his bell there is no splitting up of
+current among the magnets at the various stations as in ordinary
+practice, but the operating current goes straight to the station
+desired and to that station alone where its entire strength is
+available for performing its proper work.
+
+In order to make the system clear it may be stated at the outset that
+one side of the metallic circuit line is continued as in ordinary
+practice, passing through all of the stations as a continuous
+conductor. The other side of the line, however, is divided into
+sections, its continuity being broken at each of the subscriber's
+stations. Fig. 186 is intended to show in the simplest possible way
+how the circuit of the line may be extended from station to station in
+such manner that only the ringer of one station is in circuit at a
+time. The two sides of the line are shown in this figure, and it will
+be seen that limb _L_ extends from the central office on the left to
+the last station on the right without a break. The limb _R_, however,
+extends to the first station, at which point it is cut off from the
+extension _R_{x}_ by the open contacts of a switch. For the purpose of
+simplicity this switch is shown as an ordinary hand switch, but as a
+matter of fact it is a part of a relay, the operating coil of which is
+shown at _6_, just above it, in series with the ringer.
+
+[Illustration: Fig. 186. Principle of Broken-Line System]
+
+Obviously, if a proper ringing current is sent over the metallic
+circuit from the central office, only the bell at Station A will
+operate, since the bells at the other stations are not in the circuit.
+If by any means the switch lever _2_ at Station A were moved out of
+engagement with contact _1_ and into engagement with contact _3_, it
+is obvious that the bell of Station A would no longer be in circuit,
+but the limb _R_ of the line would be continued to the extension
+_R_{x}_ and the bell of Station B would be in circuit. Any current
+then sent over the circuit of the line from the central office would
+ring the bell of this station. In Fig. 187 the switches of both
+Station A and Station B have been thus operated, and Station C is thus
+placed in circuit. Inspection of this figure will show that the bells
+of Station A, Station B, and Station D are all cut out of circuit, and
+that, therefore, no current from the central office can affect them.
+This general scheme of selection is a new-comer in the field, and for
+certain classes of work it is of undoubted promise.
+
+[Illustration: Fig. 187. Principle of Broken-Line System]
+
+
+
+
+CHAPTER XVII
+
+LOCK-OUT PARTY-LINE SYSTEMS
+
+
+The party-line problem in rural districts is somewhat different from
+that within urban limits. In the latter cases, owing to the closer
+grouping of the subscribers, it is not now generally considered
+desirable, even from the standpoint of economy, to place more than
+four subscribers on a single line. For such a line selective ringing
+is simple, both from the standpoint of apparatus and operation; and
+moreover owing to the small number of stations on a line, and the
+small amount of traffic to and from such subscribers as usually take
+party-line service, the interference between parties on the same line
+is not a very serious matter.
+
+For rural districts, particularly those tributary to small towns,
+these conditions do not exist. Owing to the remoteness of the stations
+from each other it is not feasible from the standpoint of line cost to
+limit the number of stations to four. A much greater number of
+stations is employed and the confusion resulting is distressing not
+only to the subscribers themselves but also to the management of the
+company. There exists then the need of a party-line system which will
+give the limited user in rural districts a service, at least
+approaching that which he would get if served by an individual line.
+
+The principal investment necessary to provide facilities for telephone
+service is that required to produce the telephone line. In many cases
+the cost of instruments and apparatus is small in comparison with the
+cost of the line. By far the greater number of subscribers in rural
+districts are those who use their instruments a comparatively small
+number of times a day, and to maintain an expensive telephone line for
+the exclusive use of one such subscriber who will use it but a few
+minutes each day is on its face an economic waste. As a result, where
+individual line service is practiced exclusively one of two things
+must be true: either the average subscriber pays more for his service
+than he should, or else the operating company sells the service for
+less than it costs, or at best for an insufficient profit. Both of
+these conditions are unnatural and cannot be permanent.
+
+The party-line method of giving service, by which a single line is
+made to serve a number of subscribers, offers a solution to this
+difficulty, but the ordinary non-selective or even selective party
+line has many undesirable features if the attempt is made to place on
+it such a large number of stations as is considered economically
+necessary in rural work. These undesirable features work to the
+detriment of both the user of the telephone and the operating company.
+
+Many attempts have been made to overcome these disadvantages of the
+party line in sparsely settled communities, by producing what are
+commonly called lock-out systems. These, as their name implies, employ
+such an arrangement of parts that when the line is in use by any two
+parties, all other parties are locked out from the circuit and cannot
+gain access to it until the parties who are using it are through.
+System after system for accomplishing this purpose has been announced
+but for the most part these have involved such a degree of complexity
+and have introduced so many undesirable features as to seriously
+affect the smooth operation of the system and the reliability of the
+service.
+
+We believe, however, in spite of numerous failures, that the lock-out
+selective-signaling party line has a real field of usefulness and that
+operating companies as well as manufacturing companies are beginning
+to appreciate this need, and as a result that the relief of the rural
+subscriber from the almost intolerable service he has often had to
+endure is at hand. A few of the most promising lock-out party-line
+systems now before the public will, therefore, be described in some
+detail.
+
+Poole System. The Poole system is a lock-out system pure and simple,
+its devices being in the nature of a lock-out attachment for
+selective-signaling lines, either of the polarity or of the harmonic
+type wherein common-battery transmission is employed. It will be here
+described as employed in connection with an ordinary harmonic-ringing
+system.
+
+In Fig. 188 there is shown a four-station party line equipped with
+Poole lock-out devices, it being assumed that the ringers at each
+station are harmonic and that the keys at the central office are the
+ordinary keys adapted to impress the proper frequency on the line for
+ringing any one of the stations. In addition to the ordinary talking
+and ringing apparatus at each subscriber's station, there is a relay
+of special form and also a push-button key.
+
+[Illustration: Fig. 188. Poole Lock-Out System]
+
+Each of the relays has two windings, one of high resistance and the
+other of low resistance. Remembering that the system to which this
+device is applied is always a common-battery system, and that,
+therefore, the normal condition of the line will be one in which there
+is a difference of potential between the two limbs, it will be evident
+that whenever any subscriber on a line that is not in use raises his
+receiver from its hook, a circuit will be established from the upper
+contact of the hook through the lever of the hook to the
+high-resistance winding _1_ of the relay and thence to the other side
+of the line by way of wire _6_. This will result in current passing
+through the high-resistance winding of the relay and the relay will
+pull up its armature. As soon as it does so it establishes two other
+circuits by the closure of the relay armature against the contacts _4_
+and _5_.
+
+The closing of the contact _4_ establishes a circuit from the upper
+side of the line through the upper contact of the switch hook, thence
+through the contacts of the push button _3_, thence through the
+low-resistance winding _2_ of the relay to the terminal _4_, thence
+through the relay armature and the transmitter to the lower side of
+the line. This low-resistance path across the line serves to hold the
+relay armature attracted and also to furnish current to the
+transmitter for talking. The establishment of this low-resistance path
+across the line does another important thing, however; it practically
+short-circuits the line with respect to all the high-resistance relay
+windings, and thus prevents any of the other high-resistance relay
+windings from receiving enough current to actuate them, should the
+subscriber at any other station remove his receiver from the hook in
+an attempt to listen in or to make a call while the line is in use. As
+a subscriber can only establish the proper conditions for talking and
+listening by the attraction of this relay armature at his station, it
+is obvious that unless he can cause the pulling up of his relay
+armature he can not place himself in communication with the line.
+
+The second thing that is accomplished by the pulling up of the relay
+armature is the closure of the contacts _5_, and that completes the
+talking circuit through the condenser and receiver across the line in
+an obvious fashion. The result of this arrangement is that it is the
+first party who raises his receiver from its hook who is enabled to
+successfully establish a connection with the line, all subsequent
+efforts, by other subscribers, failing to do so because of the fact
+that the line is short-circuited by the path through the
+low-resistance winding and the transmitter of the station that is
+already connected with the line.
+
+A little target is moved by the action of the relay so that a visual
+indication is given to the subscriber in making a call to show whether
+or not he is successful in getting the use of the line. If the relay
+operates and he secures control of the line, the target indicates the
+fact by its movement, while if someone else is using the line and the
+relay does not operate, the target, by its failure to move, indicates
+that fact.
+
+When one party desires to converse with another on the same line, he
+depresses the button _3_ at his station until after the called party
+has been rung and has responded. This holds the circuit of his
+low-resistance winding open, and thus prevents the lock-out from
+becoming effective until the called party is connected with the line.
+The relay armature of the calling party does not fall back with the
+establishment of the low-resistance path at the called station,
+because, even though shunted, it still receives sufficient current to
+hold its armature in its attracted position. After the called party
+has responded, the button at the calling station is released and both
+low-resistance holding coils act in multiple.
+
+[Illustration: ONE WING OF OPERATING ROOM, BERLIN, GERMANY Ultimate
+Capacity 24,000 Subscribers' Lines and 2,100 Trunk Lines.
+Siemens-Halske Equipment. Note Horizontal Disposal of Multiple Jack
+Field.]
+
+No induction coil is used in this system and the impedance of the
+holding coil is such that incoming voice currents flow through the
+condenser and the receiver, which, by reference to the figure, will be
+seen to be in shunt with the holding coil. The holding coil is in
+series with the local transmitter, thus making a circuit similar to
+that of the Kellogg common-battery talking circuit already discussed.
+
+A possible defect in the use of this system is one that has been common
+to a great many other lock-out systems, depending for their operation
+on the same general plan of action. This appears when the instruments
+are used on a comparatively long line. Since the locking-out of all the
+instruments that are not in use by the one that is in use depends on
+the low-resistance shunt that is placed across the line by the
+instrument that is in use, it is obvious that, in the case of a long
+line, the resistance of the line wire will enter into the problem in
+such a way as to tend to defeat the locking-out function in some cases.
+Thus, where the first instrument to use the line is at the remote end
+of the line, the shunting effect that this instrument can exert with
+respect to another instrument near the central office is that due to
+the resistance of the line plus the resistance of the holding coil at
+the end instrument. The resistance of the line wire may be so high as
+to still allow a sufficient current to flow through the high-resistance
+coil at the nearer station to allow its operation, even though the more
+remote instrument is already in use.
+
+Coming now to a consideration of the complete selective-signaling
+lock-out systems, wherein the selection of the party and the locking
+out of the others are both inherent features, a single example of the
+step-by-step, and of the broken-line selective lock-out systems will
+be discussed.
+
+Step-by-Step System. The so-called K.B. system, manufactured by the
+Dayton Telephone Lock-out Manufacturing Company of Dayton, Ohio,
+operates on the step-by-step principle. The essential feature of the
+subscriber's telephone equipment in this system is the step-by-step
+actuating mechanism which performs also the functions of a relay. This
+device consists of an electromagnet having two cores, with a permanent
+polarizing magnet therebetween, the arrangement in this respect being
+the same as in an ordinary polarized bell. The armature of this magnet
+works a rocker arm, which, besides stepping the selector segment
+around, also, under certain conditions, closes the bell circuit and
+the talking circuit, as will be described.
+
+[Illustration: Fig. 189. K.B. Lock-Out System]
+
+Referring first to Fig. 189, which shows in simplified form a
+four-station K.B. lock-out line, the electromagnet is shown at _1_ and
+the rocker arm at _2_. The ratchet _3_ in this case is not a complete
+wheel but rather a segment thereof, and it is provided with a series
+of notches of different depths. It is obvious that the depth of the
+notches will determine the degree of movement which the upper end of
+the rocker arm may have toward the left, this being dependent on the
+extent to which the pawl _6_ is permitted to enter into the segment.
+The first or normal notch, _i.e._, the top notch, is always of such a
+depth that it will allow the rocker-arm lever _2_ to engage the
+contact lever _4_, but will not permit the rocker arm to swing far
+enough to the left to cause that contact to engage the bell contact
+_5_. As will be shown later, the condition for the talking circuit to
+be closed is that the rocker arm _2_ shall rest against the contact
+_4_; and from this we see that the normal notch of each of the
+segments _3_ is of such a depth as to allow the talking circuit at
+each station to be closed. The next notch, _i.e._, the second one in
+each disk, is always shallow, as are all of the other notches except
+one. A deep notch is placed on each disk anywhere from the third to
+the next to the last on the segment. This deep notch is called the
+_selective notch_, and it is the one that allows of contact being made
+with the ringer circuit of that station when the pawl _6_ drops into
+it. The position of this notch differs on all of the segments on a
+line, and obviously, therefore, the ringer circuit at any station may
+be closed to the exclusion of all the others by stepping all of the
+segments in unison until the deep notch on the segment of the desired
+station lies opposite to the pawl _6_, which will permit the rocker
+arm _2_ to swing so far to the left as to close not only the circuit
+between _2_ and _4_, but also between _2_, _4_, and _5_. In this
+position the talking and the ringing circuits are both closed.
+
+The position of the deepest notch, _i.e._, the selective notch, on the
+circumference of the segment at any station depends upon the number of
+that station; thus, the segment of Station 4 will have a deep notch in
+the sixth position; the segment for Station 9 will have a deep notch
+in the eleventh position; the segment for any station will have a deep
+notch in the position corresponding to the number of that station plus
+two.
+
+From what has been said, therefore, it is evident that the first, or
+normal, notch on each segment is of such a depth as to allow the
+moving pawl _6_ to fall to such a depth in the segment as to permit
+the rocker arm _2_ to close the talking circuit only. All of the other
+notches, except one, are comparatively shallow, and while they permit
+the moving pawl _6_ under the influence of the rocker arm _2_ to move
+the segment _3_, yet they do not permit the rocker arm _2_ to move so
+far to the left as to close even the talking circuit. The exception is
+the deep notch, or selective notch, which is of such depth as to
+permit the pawl _6_ to fall so far into the segment as to allow the
+rocker arm _2_ to close both the talking and the ringing circuits.
+Besides the moving pawl _6_ there is a detent pawl _7_. This always
+holds the segment _3_ in the position to which it has been last moved
+by the moving pawl _6_.
+
+The actuating magnet _1_, as has been stated, is polarized and when
+energized by currents in one direction, the rocker arm moves the pawl
+_6_ so as to step the segment one notch. When this relay is energized
+by current in the opposite direction, the operation is such that both
+the moving pawl _6_ and the detent pawl _7_ will be pulled away from
+the segment, thus allowing the segment to return to its normal position
+by gravity. This is accomplished by the following mechanism: An
+armature stop is pivoted upon the face of the rocker arm so as to swing
+in a plane parallel to the pole faces of the relay, and is adapted,
+when the relay is actuated by selective impulses of one polarity, to be
+pulled towards one of the pole faces where it acts, through impact with
+a plate attached to the pole face of the relay, as a limiting means
+for the motion of the rocker arm when the rocker arm is actuated by the
+magnet. When, however, the relay is energized by current in the
+opposite direction, as on a releasing impulse, the armature stop swings
+upon its pivot towards the opposite pole face, in which position the
+lug on the end of the armature stop registers with a hole in the plate
+on the relay, thus allowing the full motion of the rocker arm when it
+is attracted by the magnet. This motion of the rocker arm withdraws the
+detent pawl from engagement with the segment as well as the moving
+pawl, and thereby permits the segment to return to its normal position.
+As will be seen from Fig. 189, each of the relay magnets _1_ is
+permanently bridged across the two limbs of the line.
+
+Each station is provided with a push button, not shown, by means of
+which the subscriber who makes a call may prevent the rocker arm of
+his instrument from being actuated while selective impulses are being
+sent over the line. The purpose of this is to enable one party to make
+a call for another on the same line, depressing his push button while
+the operator is selecting and ringing the called party. The segment at
+his own station, therefore, remains in its normal position, in which
+position, as we have already seen, his talking circuit is closed; all
+of the other segments are, however, stepped up until the ringing and
+talking circuits of the desired station are in proper position, at
+which time ringing current is sent over the line. The segments in Fig.
+189, except at Station C, are shown as having been stepped up to the
+sixth position, which corresponds to the ringing position of the
+fourth station, or Station D. The condition shown in this figure
+corresponds to that in which the subscriber at Station C originated
+the call and pressed his button, thus retaining his own segment in its
+normal position so that the talking circuits would be established with
+Station D.
+
+When the line is in normal position any subscriber may call central by
+his magneto generator, not shown in Fig. 189, which will operate the
+drop at central, but will not operate any of the subscribers' bells,
+because all bell circuits are normally open. When a subscriber desires
+connection with another line, the operator sends an impulse back on
+the line which steps up and locks out all instruments except that of
+the calling subscriber.
+
+[Illustration: Fig. 190. K.B. Lock-Out Station]
+
+A complete K.B. lock-out telephone is shown in Fig. 190. This is the
+type of instrument that is usually furnished when new equipment is
+ordered. If, however, it is desired to use the K.B. system in
+connection with telephones of the ordinary bridging type that are
+already in service, the lock-out and selective mechanism, which is
+shown on the upper inner face of the door in Fig. 190, is furnished
+separately in a box that may be mounted close to the regular telephone
+and connected thereto by suitable wires, as shown in Fig. 191. It is
+seen that this instrument employs a local battery for talking and also
+a magneto generator for calling the central office.
+
+The central-office equipment consists of a dial connected with an
+impulse wheel, together with suitable keys by which the various
+circuits may be manipulated. This dial and its associated mechanism
+may be mounted in the regular switchboard cabinet, or it may be
+furnished in a separate box and mounted alongside of the cabinet in
+either of the positions shown at _1_ or _2_ of Fig. 192.
+
+In order to send the proper number of impulses to the line to call a
+given party, the operator places her finger in the hole in the dial
+that bears the number corresponding to the station wanted and rotates
+the dial until the finger is brought into engagement with the fixed
+stop shown at the bottom of the dial in Fig. 192. The dial is then
+allowed to return by the action of a spring to its normal position,
+and in doing so it operates a switch within the box to make and break
+the battery circuit the proper number of times.
+
+_Operation._ A complete description of the operation may now be had in
+connection with Fig. 193, which is similar to Fig. 189, but contains
+the details of the calling arrangement at the central office and also
+of the talking circuits at the various subscribers' stations.
+
+[Illustration: Fig. 191. K.B. Lock-Out Station]
+
+Referring to the central-office apparatus the usual ringing key is
+shown, the inside contacts of which lead to the listening key and to
+the operator's telephone set as in ordinary switchboard practice.
+Between the outside contact of this ringing key and the ringing
+generator there is interposed a pair of contact springs _8-8_ and
+another pair _9-9_. The contact springs _8_ are adapted to be moved
+backward and forward by the impulse wheel which is directly controlled
+by the dial under the manipulation of the operator. When these springs
+_8_ are in their normal position, the ringing circuit is continued
+through the release-key springs _9_ to the ringing generator. These
+springs _8_ occupy their normal position only when the dial is in its
+normal position, this being due to the notch _10_ in the contact wheel.
+At all other times, _i.e._, while the impulse wheel is out of its
+normal position, the springs _8-8_ are either depressed so as to engage
+the lower battery contacts, or else held in an intermediate position so
+as to engage neither the battery contacts nor the generator contacts.
+
+[Illustration: Fig. 192. Calling Apparatus K.B. System]
+
+When it is desired to call a given station, the operator pulls the
+subscriber's number on the dial and holds the ringing key closed,
+allowing the dial to return to normal. This connects the impulse
+battery to the subscriber's line as many times as is required to move
+the subscriber's sectors to the proper position, and in such direction
+as to cause the stepping movement of the various relays. As the
+impulse wheel comes to its normal position, the springs _8_,
+associated with it, again engage their upper contacts, by virtue of
+the notch _10_ in the impulse wheel, and this establishes the
+connection between the ringing generator and the subscriber's line,
+the ringing key being still held closed. The pulling of the
+transmitter dial and holding the ringing key closed, therefore, not
+only sends the stepping impulses to line, but also follows it by the
+ringing current. The sending of five impulses to line moves all of the
+sectors to the sixth notch, and this corresponds to the position
+necessary to make the fourth station operative. Such a condition is
+shown in Fig. 193, it being assumed that the subscriber at Station C
+originated the call and pressed his own button so as to prevent his
+sector from being moved out of its normal position. As a result of
+this, the talking circuit at Station C is left closed, and the talking
+and the ringing circuit of Station D, the called station, are closed,
+while both the talking and the ringing circuits of all the other
+stations are left open. Station D may, therefore, be rung and may
+communicate with Station C, while all of the other stations on the
+line are locked out, because of the fact that both their talking and
+ringing circuits are left open.
+
+[Illustration: Fig. 193. Circuit K.B. System]
+
+When conversation is ended, the operator is notified by the usual
+clearing-out signal, and she then depresses the release button, which
+brings the springs _9_ out of engagement with the generator contact
+but into engagement with the battery contact in such relation as to
+send a battery current on the line in the reverse direction from that
+sent out by the impulse wheel. This sends current through all of the
+relays in such direction as to withdraw both the moving and the
+holding pawls from the segments and thus allow all of the segments to
+return to their normal positions. Of course, in thus establishing the
+release current, it is necessary for the operator to depress the
+ringing key as well as the release key.
+
+A one-half microfarad condenser is placed in the receiver circuit at
+each station so that the line will not be tied up should some
+subscriber inadvertently leave his receiver off its hook. This permits
+the passage of voice currents, but not of the direct currents used in
+stepping the relays or in releasing them.
+
+The circuit of Fig. 193 is somewhat simplified from that in actual
+practice, and it should be remembered that the hook switch, which is
+not shown in this figure, controls in the usual way the continuity of
+the receiver and the transmitter circuits as well as of the generator
+circuits, the generator being attached to the line as in an ordinary
+telephone.
+
+Broken-Line System. The broken-line method of accomplishing
+selective signaling and locking-out on telephone party lines is due to
+Homer Roberts and his associates.
+
+[Illustration: Fig. 194. Roberts Latching Relay]
+
+To understand just how the principles illustrated in Figs. 186 and 187
+are put into effect, it will be necessary to understand the latching
+relay shown diagrammatically in its two possible positions in Fig. 194,
+and in perspective in Fig. 195. Referring to Fig. 194, the left-hand
+cut of which shows the line relay in its normal position, it is seen
+that the framework of the device resembles that of an ordinary
+polarized ringer. Under the influence of current in one direction
+flowing through the left-hand coil, the armature of this device
+depresses the hard rubber stud _4_, and the springs _1_, _2_, and _3_
+are forced downwardly until the spring _2_ has passed under the latch
+carried on the spring _5_. When the operating current through the coil
+_6_ ceases, the pressure of the armature on the spring _1_ is relieved,
+allowing this spring to resume its normal position and spring _3_ to
+engage with spring _2_. The spring _2_ cannot rise, since it is held by
+the latch _5_, and the condition shown in the right-hand cut of Fig.
+194 exists. It will be seen that the spring _2_ has in this operation
+carried out just the same function as the switch lever performed as
+described in connection with Figs. 186 and 187. An analysis of this
+action will show that the normal contact between the springs _1_ and
+_2_, which contact controls the circuit through the relay coil and the
+bell, is not broken until the coil _6_ is de-energized, which means
+that the magnet is effective until it has accomplished its work. It is
+impossible, therefore, for this relay to cut itself out of circuit
+before it has caused the spring _2_ to engage under the latch _5_. If
+current of the proper direction were sent through the coil _7_ of the
+relay, the opposite end of the armature would be pulled down and the
+hard rubber stud at the left-hand end of the armature would bear
+against the bent portion of the spring _5_ in such manner as to cause
+the latch of this spring to release the spring _2_ and thus allow the
+relay to assume its normal, or unlatched, position.
+
+A good idea of the mechanical construction of this relay may be
+obtained from Fig. 195. The entire selecting function of the Roberts
+system is performed by this simple piece of apparatus at each station.
+
+[Illustration: Fig. 195. Roberts Latching Relay]
+
+The diagram of Fig. 196 shows, in simplified form, a four-station
+line, the circuits being given more in detail than in the diagrams of
+Chapter XVI.
+
+It will be noticed that the ringer and the relay coil _6_ at the
+first station are bridged across the sides of the line leading to the
+central office. In like manner the bell and the relay magnets are
+bridged across the two limbs of the line leading into each succeeding
+station, but this bridge at each of the stations beyond Station A is
+ineffective because the line extension _R__{x} is open at the next
+station nearest the central office.
+
+[Illustration: Fig. 196. Simplified Circuits of Roberts System]
+
+In order to ring Station A it is only necessary to send out ringing
+current from the central office. This current is in such direction as
+not to cause the operation of the relay, although it passes through
+the coil _6_. If, on the other hand, it is desired to ring Station B,
+a preliminary impulse would be sent over the metallic circuit from the
+central office, which impulse would be of such direction as to operate
+the relay at Station A, but not to operate the bell at that station.
+The operation of the relay at Station A causes the spring _2_ of this
+relay to engage the spring _3_, thus extending the line on to the
+second station. After the spring _2_ at Station A has been forced into
+contact with the spring _3_, it is caught by the latch of the spring
+_5_ and held mechanically. When the impulse from the central office
+ceases, the spring _1_ resumes its normal position, thus breaking the
+bridge circuit through the bell at that station. It is apparent now
+that the action of coil _6_ at Station A has made the relay powerless
+to perform any further action, and at the same time the line has been
+extended on to the second station. A second similar impulse from the
+central office will cause the relay at Station B to extend the line on
+to Station C, and at the same time break the circuit through the
+operating coil and the bell at Station B. In this way any station may
+be picked out by sending the proper number of impulses to operate the
+line relays of all the stations between the station desired and the
+central office, and having picked out a station it is only necessary
+to send out ringing current, which current is in such direction as to
+ring the bell but not to operate the relay magnet at that station.
+
+In Fig. 197, a four-station line, such as is shown in Fig. 196, is
+illustrated, but the condition shown in this is that existing when two
+preliminary impulses have been sent over the line, which caused the
+line relays at Station A and Station B to be operated. The bell at
+Station C is, therefore, the only one susceptible to ringing current
+from the central office.
+
+[Illustration: Fig. 197. Simplified Circuits of Roberts System]
+
+Since only one bell and one relay are in circuit at any one time, it
+is obvious that all of the current that passes over the line is
+effective in operating a single bell or relay only. There is no
+splitting up of the current among a large number of bells as in the
+bridging system of operating step-by-step devices, which method
+sometimes so greatly reduces the effective current for each bell that
+it is with great difficulty made to respond. All the energy available
+is applied directly to the piece of apparatus at the time it is being
+operated. This has a tendency toward greater surety of action, and the
+adjustment of the various pieces of apparatus may be made with less
+delicacy than is required where many pieces of apparatus, each having
+considerable work to do, must necessarily be operated in multiple.
+
+The method of unlatching the relays has been briefly referred to.
+After a connection has been established with a station in the manner
+already described, the operator may clear the line when it is proper
+to do so by sending impulses of such a nature as to cause the line
+relays of the stations beyond the one chosen to operate, thus
+continuing the circuit to the end of the line. The operation of the
+line relay at the last station brings into circuit the coil _8_, Figs.
+196 and 197, of a grounding device. This is similar to the line relay,
+but it holds its operating spring in a normally latched position so as
+to maintain the two limbs of the line disconnected from the ground.
+The next impulse following over the metallic circuit passes through
+the coil _8_ and causes the operation of this grounding device which,
+by becoming unlatched, grounds the limb _L_ of the line through the
+coil _8_. This temporary ground at the end of the line makes it
+possible to send an unlocking or restoring current from the central
+office over the limb _L_, which current passes through all of the
+unlocking coils _7_, shown in Figs. 194, 196, and 197, thus causing
+the simultaneous unlocking of all of the line relays and the
+restoration of the line to its normal condition, as shown in Fig. 196.
+
+[Illustration: Fig. 198. Details of Latching Relay Connections]
+
+As has been stated, the windings _7_ on the line relays are the
+unlatching windings. In Figs. 196 and 197, for the purpose of
+simplicity, these windings are not shown connected, but as a matter of
+fact each of them is included in series in the continuous limb _L_ of
+the line. This would introduce a highly objectionable feature from the
+standpoint of talking over the line were it not for the balancing
+coils _7_^{1}, each wound on the same core as the corresponding
+winding _7_, and each included in series in the limb _R_ of the line,
+and in such direction as to be differential thereto with respect to
+currents passing in series over the two limbs of the line.
+
+The windings _7_ are the true unlocking windings, while the windings
+_7_^{1} have no other function than to neutralize the inductive
+effects of these unlocking windings necessarily placed in series in
+the talking circuit. All of these windings are of low ohmic
+resistance, a construction which, as has previously been noted, brings
+about the desired effect without introducing any self-induction in
+the line, and without producing any appreciable effect upon the
+transmission. A study of Fig. 198 will make clear the connections of
+these unlocking and balancing windings at each station.
+
+The statement of operation so far given discloses the general method
+of building up the line in sections in order to choose any party and
+of again breaking it up into sections when the conversation is
+finished. It has been stated that the same operation which selects the
+party wanted also serves to give that party the use of the line and to
+lock the others off. That this is true will be understood when it is
+stated that the ringer is of such construction that when operated to
+ring the subscriber wanted, it also operates to unlatch a set of
+springs similar to those shown in Fig. 194, this unlatching causing
+the proper connection of the subscriber's talking circuit across the
+limbs of the line, and also closing the local circuit through his
+transmitter. The very first motion of the bell armature performs this
+unlatching operation after which the bell behaves exactly as an
+ordinary polarized biased ringer.
+
+[Illustration: Fig. 199. Broken-Back Ringer]
+
+The construction of this ringer is interesting and is shown in its two
+possible positions in Fig. 199. The group of springs carried on its
+frame is entirely independent of the movement of the armature during
+the ringing operation. With reversed currents, however, the armature
+is moved in the opposite direction from that necessary to ring the
+bells, and this causes the latching of the springs into their normal
+position. In order that this device may perform the double function
+of ringer and relay the tapper rod of the bell is hinged on the
+armature so as to partake of the movements of the armature in one
+direction only. This has been called by the inventor and engineers of
+the Roberts system a _broken-back ringer_, a name suggestive of the
+movable relation between the armature and the tapper rod. The
+construction of the ringer is of the same nature as that of the
+standard polarized ringer universally employed, but a hinge action
+between the armature and the tapper rod, of such nature as to make the
+tapper partake positively of the movements of the armature in one
+direction, but to remain perfectly quiescent when the armature moves
+in the other direction, is provided.
+
+[Illustration: Fig. 200. Details of Ringer Connection]
+
+How this broken-back ringer controls the talking and the locking-out
+conditions may best be understood in connection with Fig. 200. The
+ringer springs are normally latched at all stations. Under these
+conditions the receiver is short-circuited by the engagement of
+springs _10_ and _11_, the receiver circuit is open between springs
+_10_ and _12_, and the local-battery circuit is open between springs
+_9_ and _12_. The subscribers whose ringers are latched are,
+therefore, locked out in more ways than one.
+
+When the bell is rung, the first stroke it makes unlatches the springs,
+which assume the position shown in the right-hand cut of Fig. 199, and
+this, it will be seen from Fig. 200, establishes proper conditions for
+enabling the subscriber to transmit and to receive speech.
+
+The hook switch breaks both transmitter and receiver circuits when down
+and in raising it establishes a momentary circuit between the ground
+and the limb _L_ of the line, both upper and lower hook contacts
+engaging the hook lever simultaneously during the rising of the hook.
+
+The mechanism at the central office by which selection of the proper
+station is made in a rapid manner is shown in Fig. 201. It has already
+been stated that the selection of the proper subscriber is brought
+about by the sending of a predetermined number of impulses from the
+central office, these impulses passing in one direction only and over
+the metallic circuit. After the proper party has been reached, the
+ringing current is put on in the reverse direction.
+
+[Illustration: Fig. 201. Central-Office Impulse Transmitter]
+
+The operator establishes the number of impulses to be sent by placing
+the pointer opposite the number on the dial corresponding to the
+station wanted. The ratchet wheel is stepped around automatically by
+each impulse of current from an ordinary pole changer such as is
+employed in ringing biased bells. When the required number of impulses
+has been sent, a projection, carried on a group of springs, drops into
+a notch on the drum of the selector shaft, which operation instantly
+stops the selecting current impulses and at the same time throws on
+the ringing current which consists of impulses in the reverse
+direction. So rapidly does this device operate that it will readily
+follow the impulses of an ordinary pole changer, even when this is
+adjusted to its maximum rate of vibration.
+
+[Illustration: VIEW OF A LARGE FOREIGN MULTIPLE SWITCHBOARD]
+
+_Operation._ Space will not permit a full discussion of the details of
+the central-office selective apparatus, but a general resume of the
+operation of the system may now be given, with the aid of Fig. 202,
+which shows a four-station line with the circuits of three of the
+stations somewhat simplified. In this figure Station A, Station B,
+and Station D are shown in their locked-out positions, A and B having
+been passed by the selection and ringing of Station C, while Station D
+is inoperative because it was not reached in the selection and the
+line is still broken at Station C. Station C, therefore, has
+possession of the line.
+
+When the subscriber at Station C raised his receiver in order to call
+central, a "flash" contact was made as the hook moved up, which
+momentarily grounded the limb _L_ of the line. (See Fig. 200.) This
+"flash" contact is produced by the arrangement of the hook which
+assures that the lower contact shall, by virtue of its flexibility,
+follow up the hook lever until the hook lever engages the upper
+contact, after which the lower contact breaks. This results in the
+momentary connection of both the upper and the lower contacts of the
+hook with the lever, and, therefore, the momentary grounding of the
+limb _L_ of the line. This limb always being continuous serves, when
+this "flash" contact is made, to actuate the line signal at the
+central office.
+
+[Illustration: Fig. 202. Circuits of Roberts Line]
+
+Since, however, all parties on the line are normally locked out of
+talking circuits, some means must be provided whereby the operator may
+place the signaling party in talking connection and leave all the
+other instruments on the line in their normally locked-out position.
+In fact, the operator must be able automatically to pick out the
+station that signaled in, and operate the ringer to unlatch the
+springs controlling the talking circuit of that station. Accordingly
+the operator sends impulses on the line, from a grounded battery,
+which are in the direction to operate the line relays and to continue
+the line circuit to the station calling. When, after a sufficient
+number of impulses, this current reaches that station it finds a path
+to ground from the limb _L_. This path is made possible by the fact
+that the subscriber's receiver is off its hook at that station. In
+order to understand just how this ground connection is made, it must
+be remembered that each of the ringer magnets is energized with each
+selecting impulse, but in such a direction as not to ring the bells,
+it being understood that all of the ringer mechanisms are normally
+latched. When the selecting impulse for Station C arrives, it passes
+through the ringer and the selecting relay coils at that station and
+starts to operate the remainder of the ringers sufficiently to cause
+the spring _12_ to engage the spring _13_. This establishes the ground
+connection from the limb _L_ of the line, the circuit being traced
+through limb _L_ through the upper contact of the switch, thence
+through springs _12_ and _13_ to ground, and this, before the line
+relay has time to latch, operates the quick-acting relay at the
+central office, which acts to cut off further impulses, and thus
+automatically stops at the calling station. Ringing current in the
+opposite direction is then sent to line; this unlatches the ringer
+springs and places the calling subscriber in talking circuit. When the
+operator has communicated with the calling subscriber, and found, for
+example, that another party on another similar line is desired, she
+turns the dial pointer on the selector to the number corresponding to
+the called-for party's number on that line, and presses the signal
+key. Pressing this key causes impulses to "run down the line,"
+selecting the proper party and ringing his bell in the manner already
+described. The connection between the two parties is then established,
+and no one else can in any possible way, except by permission of the
+operator, obtain access to the line.
+
+It is obvious that some means must be provided for restoring the
+selecting relays to normal after a conversation is finished. By
+referring to Fig. 194 it will be seen that the upper end of the latch
+spring _5_ is bent over in such a manner that when the armature is
+attracted by current flowing through the coil _7_, the knob on the
+left-hand end of the armature on rising engages with the bent cam
+surface and forces back the latch, permitting spring _2_ to return to
+its normal position.
+
+To restore the line the operator sends out sufficient additional
+selective impulses to extend the circuit to the end of the line, and
+thus brings the grounder into circuit. The winding of the grounder is
+connected in such a manner that the next passing impulse throws off
+its latch, permitting the long spring to contact with the ground
+spring. The operator now sends a grounded impulse over the continuous
+limb _L_ of the line which passes through the restoring coils _7_ at
+all the stations and through the right-hand coil of the grounding
+device to ground. The selecting relays are, therefore, simultaneously
+restored to normal. The grounder is also energized and restored to its
+normal position by the same current.
+
+If a party in calling finds that his own line is busy and he cannot
+get central, he may leave his receiver off its hook. When the party
+who is using the line hangs up his receiver the fact that another
+party desires a connection is automatically indicated to the operator,
+who then locks out the instrument of the party who has just finished
+conversation and passes his station by. When the operator again throws
+the key, the waiting subscriber is automatically selected in the same
+manner as was the first party. If there are no subscribers waiting for
+service, the stop relay at central will not operate until the grounder
+end of the line is unlatched, the selecting relays being then restored
+automatically to normal.
+
+The circuits are so organized that at all times whether the line is
+busy or not, the movement up and down of the switch hook, at any
+sub-station, operates a signal before the operator. Such a movement,
+when made slowly and repeatedly, indicates to the operator that the
+subscriber has an emergency call and she may use her judgment as to
+taking the line away from the parties who are using it, and finding
+out what the emergency call is for. If the operator finds that the
+subscriber has misused this privilege of making the emergency call,
+she may restore the connection to the parties previously engaged in
+conversation.
+
+One of the salient points of this Roberts system is that the operator
+always has control of the line. A subscriber is not able even to use
+his own battery till permitted to do so. A subscriber who leaves his
+receiver off its hook in order that he may be signaled by the operator
+when the line is free, causes no deterioration of the local battery
+because the battery circuit is held open by the switch contacts
+carried on the ringer. It cannot be denied, however, that this system
+is complicated, and that it has other faults. For instance, as
+described herein, both sides of the line must be looped into each
+subscriber's station, thus requiring four drop, or service, wires
+instead of two. It is possible to overcome this objection by placing
+the line relays on the pole in a suitably protected casing, in which
+case it is sufficient to run but two drop wires from the nearer line
+to station. There are undoubtedly other objections to this system, and
+yet with all its faults it is of great interest, and although radical
+in many respects, it teaches lessons of undoubted value.
+
+
+
+
+CHAPTER XVIII
+
+ELECTRICAL HAZARDS
+
+
+All telephone systems are exposed to certain electrical hazards. When
+these hazards become actively operative as causes, harmful results
+ensue. The harmful results are of two kinds: those causing damage to
+property and those causing damage to persons. The damage to persons
+may be so serious as to result in death. Damage to property may
+destroy the usefulness of a piece of apparatus or of some portion of
+the wire plant. Or the property damage may initiate itself as a harm
+to apparatus or wiring and may result in greater and extending damage
+by starting a fire.
+
+Electrical currents which endanger life and property may be furnished
+by natural or artificial causes. Natural electricity which does such
+damage usually displays itself as lightning. In rare cases, currents
+tending to flow over grounded lines because of extraordinary
+differences of potential between sections of the earth's surface have
+damaged apparatus in such lines, or only have been prevented from
+causing such damage by the operation of protective devices.
+
+Telegraph and telephone systems have been threatened by natural
+electrical hazards since the beginning of the arts and by artificial
+electrical hazards since the development of electric light and power
+systems. At the present time, contrary to the general supposition, it
+is in the artificial, and not in the natural electrical hazards that
+the greater variety and degree of danger lies.
+
+Of the ways in which artificial electricity may injure a telephone
+system, the entrance of current from an external electrical power
+system is a greater menace than an abnormal flow of current from a
+source belonging to the telephone system itself. Yet modern practice
+provides opportunities for a telephone system to inflict damage upon
+itself in that way. Telephone engineering designs need to provide
+means for protecting _all_ parts of a system against damage, from
+external ("foreign") as well as internal ("domestic") hazards, and to
+cause this protection to be inclusive enough to protect persons
+against injury and property from damage by any form of overheating or
+electrolytic action.
+
+A part of a telephone system for which there is even a remote
+possibility of contact with an external source of electrical power,
+whether natural or artificial, is said to be _exposed_ to electrical
+hazard. The degree or character of possible contact or other
+interference often is referred to in relative terms of _exposure_. The
+same terms are used concerning inductive relations between circuits.
+The whole tendency of design, particularly of wire plants, is to
+arrange the circuits in such a way as to limit the exposure as greatly
+as possible, the intent being to produce a condition in which all
+parts of the system will be _unexposed_ to hazards.
+
+Methods of design are not yet sufficiently advanced for any plant to
+be formed of circuits wholly unexposed, so that protective means are
+required to safeguard apparatus and circuits in case the hazard,
+however remote, becomes operative.
+
+Lightning discharges between the clouds and earth frequently charge
+open wires to potentials sufficiently high to damage apparatus; and
+less frequently, to destroy the wires of the lines themselves.
+Lightning discharges between clouds frequently induce charges in lines
+sufficient to damage apparatus connected with the lines. Heavy rushes
+of current in lines, from lightning causes, occasionally induce
+damaging currents in adjacent lines not sufficiently exposed to the
+original cause to have been injured without this induction. The
+lightning hazard is least where the most lines are exposed. In a small
+city with all of the lines formed of exposed wires and all of them
+used as grounded circuits, a single lightning discharge may damage
+many switchboard signals and telephone ringers if there be but 100 or
+200 lines, while the damage might have been nothing had there been 800
+to 1,000 lines in the same area.
+
+Means of protecting lines and apparatus against damage by lightning
+are little more elaborate than in the earliest days of telegraph
+working. They are adequate for the almost entire protection of life
+and of apparatus.
+
+Power circuits are classified by the rules of various governing bodies
+as high-potential and low-potential circuits. The classification of
+the National Board of Fire Underwriters in the United States defines
+low-potential circuits as having pressures below 550 volts;
+high-potential circuits as having pressures from 550 to 3,500 volts,
+and extra high-potential circuits as having pressures above 3,500
+volts. Pressures of 100,000 volts are becoming more common. Where
+power is valuable and the distance over which it is to be transmitted
+is great, such high voltages are justified by the economics of the
+power problem. They are a great hazard to telephone systems, however.
+An unprotected telephone system meeting such a hazard by contact will
+endanger life and property with great certainty. A very common form of
+distribution for lighting and power purposes is the three-wire system
+having a grounded neutral wire, the maximum potential above the earth
+being about 115 volts.
+
+Telephone lines and apparatus are subject to damage by any power
+circuit whether of high or low potential. The cause of property damage
+in all cases is the flow of current. Personal damage, if it be death
+from shock, ordinarily is the result of a high potential between two
+parts of the body. The best knowledge indicates that death uniformly
+results from shock to the heart. It is believed that death has
+occurred from shock due to pressure as low as 100 volts. The critical
+minimum voltage which can not cause death is not known. A good rule is
+never willingly to subject another person to personal contact with any
+electrical pressure whatever.
+
+Electricity can produce actions of four principal kinds:
+physiological, thermal, chemical, and magnetic. Viewing electricity as
+establishing hazards, the physiological action may injure or kill
+living things; the thermal action may produce heat enough to melt
+metals, to char things which can be burned, or to cause them actually
+to burn, perhaps with a fire which can spread; the chemical action may
+destroy property values by changing the state of metals, as by
+dissolving them from a solid state where they are needed into a state
+of solution where they are not needed; the magnetic action introduces
+no direct hazard. The greatest hazard to which property values are
+exposed is the electro-thermal action; that is, the same useful
+properties by which electric lighting and electric heating thrive may
+produce heat where it is not wanted and in an amount greater than can
+safely be borne.
+
+The tendency of design is to make all apparatus capable of carrying
+without overheating any current to which voltage within the telephone
+system may subject it, and to provide the system so designed with
+specific devices adapted to isolate it from currents originating
+without. Apparatus which is designed in this way, adapted not only to
+carry its own normal working currents but to carry the current which
+would result if a given piece of apparatus were connected directly
+across the maximum pressure within the telephone system itself, is
+said to be self-protecting. Apparatus amply able to carry its maximum
+working current but likely to be overheated, to be injured, or perhaps
+to destroy itself and set fire to other things if subjected to the
+maximum pressure within the system, is not self-protecting apparatus.
+
+To make all electrical devices self-protecting by surrounding them
+with special arrangements for warding off abnormal currents from
+external sources, is not as simple as might appear. A lamp, for
+example, which can bear the entire pressure of a central-office
+battery, is not suitable for direct use in a line several miles long
+because it would not give a practical signal in series with that line
+and with the telephone set, as it is required to do. A lamp suitable
+for use in series with such a line and a telephone set would burn out
+by current from its own normal source if the line should become
+short-circuited in or near the central office. The ballast referred to
+in the chapter on "Signals" was designed for the very purpose of
+providing rapidly-rising resistance to offset the tendency toward
+rapidly-rising current which could burn out the lamp.
+
+As another example, a very small direct-current electric motor can be
+turned on at a snap switch and will gain speed quickly enough so that
+its armature winding will not be overheated. A larger motor of that
+kind can not be started safely without introducing resistance into the
+armature circuit on starting, and cutting it out gradually as the
+armature gains speed. Such a motor could be made self-protecting by
+having the armature winding of much larger wire than really is
+required for mere running, choosing its size great enough to carry the
+large starting current without overheating itself and its insulation.
+It is better, and for long has been standard practice, to use starting
+boxes, frankly admitting that such motors are not self-protecting
+until started, though they are self-protecting while running at normal
+speeds. Such a motor, once started, may be overloaded so as to be
+slowed down. So much more current now can pass through the armature
+that its winding is again in danger. Overload circuit-breakers are
+provided for the very purpose of taking motors out of circuit in cases
+where, once up to speed, they are mechanically brought down again and
+into danger. Such a circuit-breaker is a device for protecting against
+an _internal_ hazard; that is, internal to the power system of which
+the motor is a part.
+
+Another example: In certain situations, apparatus intended to operate
+under impulses of large current may be capable of carrying its normal
+impulses successfully but incapable of carrying currents from the same
+pressure continuously. Protective means may be provided for detaching
+such apparatus from the circuit whenever the period in which the
+current acts is not short enough to insure safety. This is cited as a
+case wherein a current, normal in amount but abnormal in duration,
+becomes a hazard.
+
+The last mentioned example of damage from internal hazards brings us
+to the law of the electrical generation of heat. _The greater the
+current or the greater the resistance of the conductor heated or the
+longer the time, the greater will he the heat generated in that
+conductor._ But this generated heat varies directly as the resistance
+and as the time and as the square of the current, that is, the law is
+
+Heat generated = _C^{2}Rt_
+
+in which _C_ = the current; _R_=the resistance of the conductor; and
+_t_ = the time.
+
+It is obvious that a protective device, such as an overload
+circuit-breaker for a motor, or a protector for telephone apparatus,
+needs to operate more quickly for a large current than for a small
+one, and this is just what all well-designed protective devices are
+intended to do. The general problem which these heating hazards
+present with relation to telephone apparatus and circuits is: _To
+cause all parts of the telephone system to be made so as to carry
+successfully all currents which may flow in them because of any
+internal or external pressure, or to supplement them by devices which
+will stop or divert currents which could overheat them._
+
+Electrolytic hazards depend not on the heating effects of currents but
+on their chemical effects. The same natural law which enables primary
+and secondary batteries to be useful provides a hazard which menaces
+telephone-cable sheaths and other conductors. When a current leaves a
+metal in contact with an electrolyte, the metal tends to dissolve into
+the electrolyte. In the processes of electroplating and electrotyping,
+current enters the bath at the anode, passes from the anode through
+the solution to the cathode, removing metal from the former and
+depositing it upon the latter. In a primary battery using zinc as the
+positive element and the negative terminal, current is caused to pass,
+within the cell, from the zinc to the negative element and zinc is
+dissolved. Following the same law, any pipe buried in the earth may
+serve to carry current from one region to another. As single-trolley
+traction systems with positive trolley wires constantly are sending
+large currents through the earth toward their power stations, such a
+pipe may be of positive potential with relation to moist earth at some
+point in its length. Current leaving it at such a point may cause its
+metal to dissolve enough to destroy the usefulness of the pipe for its
+intended purpose.
+
+Lead-sheathed telephone cables in the earth are particularly exposed
+to such damage by electrolysis. The reasons are that such cables often
+are long, have a good conductor as the sheath-metal, and that metal
+dissolves readily in the presence of most aqueous solutions when
+electrolytic differences of potential exist. The length of the cables
+enables them to connect between points of considerable difference of
+potential. It is lack of this length which prevents electrolytic
+damage to masses of structural metal in the earth.
+
+Electrical power is supplied to single-trolley railroads principally
+in the form of direct current. Usually all the trolley wires of a city
+are so connected to the generating units as to be positive to the
+rails. This causes current to flow from the cars toward the power
+stations, the return path being made up jointly of the rails, the
+earth itself, actual return wires which may supplement the rails, and
+also all other conducting things in the earth, these being principally
+lead-covered cables and other pipes. These conditions establish
+definite areas in which the currents tend to leave the cables and
+pipes, _i.e._, in which the latter are positive to other things. These
+positive areas usually are much smaller than the negative areas, that
+is, the regions in which currents tend _to enter_ the cables form a
+larger total than the regions in which the currents tend _to leave_
+the cables. These facts simplify the ways in which the cables may be
+protected against damage by direct currents leaving them and also they
+reduce the amount, complication, and cost of applying the corrective
+and preventive measures.
+
+All electric roads do not use direct current. Certain simplifications
+in the use of single-phase alternating currents in traction motors
+have increased the number of roads using a system of
+alternating-current power supply. Where alternating current is used,
+the electrolytic conditions are different and a new problem is set,
+for, as the current flows in recurrently different directions, an area
+which at one instant is positive to others, is changed the next
+instant into a negative area. The protective means, therefore, must be
+adapted to the changed requirements.
+
+
+
+
+CHAPTER XIX
+
+PROTECTIVE MEANS
+
+
+Any of the heating hazards described in the foregoing chapter may
+cause currents which will damage apparatus. All devices for the
+protection of apparatus from such damage, operate either to stop the
+flow of the dangerous current, or to send that flow over some other
+path.
+
+Protection Against High Potentials. Lightning is the most nearly
+universal hazard. All open wires are exposed to it in some degree.
+Damaging currents from lightning are caused by extraordinarily high
+potentials. Furthermore, a lightning discharge is oscillatory; that
+is, alternating, and of very high frequency. Drops, ringers,
+receivers, and other devices subject to lightning damage suffer by
+having their windings burned by the discharge. The impedance these
+windings offer to the high frequency of lightning oscillations is
+great. The impedance of a few turns of heavy wire may be negligible to
+alternating currents of ordinary frequencies because the resistance of
+the wire is low, its inductance small, and the frequency finite. On
+the other hand, the impedance of such a coil to a lightning discharge
+is much higher, due to the very high frequency of the discharge.
+
+Were it not for the extremely high pressure of lightning discharges,
+their high frequency of oscillation would enable ordinary coils to be
+self-protecting against them. But a discharge of electricity can take
+place through the air or other insulating medium if its pressure be
+high enough. A pressure of 70,000 volts can strike across a gap in air
+of one inch, and lower pressures can strike across smaller distances.
+When lightning encounters an impedance, the discharge seldom takes
+place through the entire winding, as an ordinary current would flow,
+usually striking across whatever short paths may exist. Very often
+these paths are across the insulation between the outer turns of a
+coil. It is not unusual for a lightning discharge to plow its way
+across the outer layer of a wound spool, melting the copper of the
+turns as it goes. Often the discharge will take place from inner turns
+directly to the core of the magnet. This is more likely when the core
+is grounded.
+
+_Air-Gap Arrester_. The tendency of a winding to oppose lightning
+discharges and the ease with which such discharge may strike across
+insulating gaps, points the way to protection against them. Such
+devices consist of two conductors separated by an air space or other
+insulator and are variously known as lightning arresters, spark gaps,
+open-space cutouts, or air-gap arresters. The conductors between which
+the gap exists may be both of metal, may be one of metal and one of
+carbon, or both of carbon. One combination consists of carbon and
+mercury, a liquid metal. The space between the conductors may be
+filled with either air or solid matter, or it may be a vacuum.
+Speaking generally, the conductors are separated by some insulator.
+Two conductors separated by an insulator form a condenser. The
+insulator of an open-space arrester often is called the dielectric.
+
+[Illustration Fig. 203. Saw Tooth Arrester]
+
+Discharge Across Gaps:--Electrical discharges across a given distance
+occur at lower potentials if the discharge be between points than if
+between smooth surfaces. Arresters, therefore, are provided with
+points. Fig. 203 shows a device known as a "saw-tooth" arrester
+because of its metal plates being provided with teeth. Such an
+arrester brings a ground connection close to plates connected with the
+line and is adapted to protect apparatus either connected across a
+metallic circuit or in series with a single wire circuit.
+
+Fig. 201 shows another form of metal plate air-gap arrester having the
+further possibility of a discharge taking place from one line wire to
+the other. Inserting a plug in the hole between the two line plates
+connects the line wires directly together at the arrester. This
+practice was designed for use with series lines, the plug
+short-circuiting the telephone set when in place.
+
+A defect of most ordinary types of metal air-gap lightning arresters
+is that heavy discharges tend to melt the teeth or edges of the
+plates and often to weld them together, requiring special attention
+to re-establish the necessary gap.
+
+Advantages of Carbon:--Solid carbon is found to be a much better
+material than metal for the reasons that a discharge will not melt it
+and that its surface is composed of multitudes of points from which
+discharges take place more readily than from metals.
+
+[Illustration Fig. 204. Saw-Tooth Arrester]
+
+[Illustration Fig. 205. Carbon Block Arrester]
+
+Carbon arresters now are widely used in the general form shown in Fig.
+205. A carbon block connected with a wire of the line is separated
+from a carbon block connected to ground by some form of insulating
+separator. Mica is widely used as such a separator, and holes of some
+form in a mica slip enable the discharge to strike freely from block
+to block, while preventing the blocks from touching each other.
+Celluloid with many holes is used as a separator between carbon
+blocks. Silk and various special compositions also have their uses.
+
+[Illustration Fig. 206. Arrester Separators]
+
+Dust Between Carbons:--Discharges between the carbon blocks tend to
+throw off particles of carbon from them. The separation between the
+blocks being small--from .005 to .015 inch--the carbon particles may
+lodge in the air-gap, on the edges of the separator, or otherwise, so
+as to leave a conducting path between the two blocks. Slight moisture
+on the separator may help to collect this dust, thus placing a ground
+on that wire of the line. This ground may be of very high resistance,
+but is probably one of many such--one at each arrester connected to
+the line. In special forms of carbon arresters an attempt has been
+made to limit this danger of grounding by the deposit of carbon dust.
+The object of the U-shaped separator of Fig. 206 is to enable the
+arrester to be mounted so that this opening in the separator is
+downward, in the hope that loosened carbon particles may fall out of
+the space between the blocks. The deposit of carbon on the inside
+edges of the U-shaped separator often is so fine and clings so tightly
+as not to fall out. The separator projects beyond the blocks so as to
+avoid the collection of carbon on the outer edges.
+
+Commercial Types:--Fig. 207 is a commercial form of the arrangement
+shown in Fig. 205 and is one of the many forms made by the American
+Electric Fuse Company. Line wires are attached to outside binding
+posts shown in the figure and the ground wire to the metal binding
+post at the front. The carbon blocks with their separator slide
+between clips and a ground plate. The air-gap is determined by the
+thickness of the separator between the carbon blocks.
+
+[Illustration: Fig. 207. Carbon Block Arrester]
+
+[Illustration: Fig. 208 Roberts "Self-Cleaning" Arrester]
+
+The Roberts carbon arrester is designed with particular reference to
+the disposal of carbon dust and is termed self-cleaning for that
+reason. The arrangement of carbons and dielectric in this device is
+shown in Fig. 208; mica is cemented to the line carbon and is large
+enough to provide a projecting margin all around. The spark gap is not
+uniform over the entire surface of the block but is made wedge-shaped
+by grinding away the line carbon as shown. It is claimed that a
+continuous arcing fills the wedge-shaped chamber with heated air or
+gas, converting the whole of the space into a field of low resistance
+to ground, and that this gas in expanding drives out every particle of
+carbon that may be thrown off. It seems obvious that the wedge-shaped
+space offers greater freedom for carbon dust to fall out than in the
+case of the parallel arrangement of the block faces.
+
+An outdoor arrester for metallic circuits, designed by F.B. Cook, is
+shown in Fig. 209. The device is adapted to mount on a pole or
+elsewhere and to be covered by a protecting cap. The carbons are large
+and are separated by a special compound intended to assist the
+self-cleaning feature. The three carbons being grouped together as a
+unit, the device has the ability to care for discharges from one
+terminal to either of the others direct, without having to pass
+through two gaps. In this particular, the arrangement is the same as
+that of Fig. 204.
+
+[Illustration: Fig. 209. Cook Air-Gap Arrester]
+
+A form of Western Electric arrester particularly adapted for outside
+use on railway lines is shown with its cover in Fig. 210.
+
+[Illustration: Fig. 210. Western Electric Air-Gap Arrester]
+
+The Kellogg Company regularly equips its magneto telephones with
+air-gap arresters of the type shown in Fig. 211. The two line plates
+are semicircular and of metal. The ground plate is of carbon,
+circular in form, covering both line plates with a mica separator.
+This is mounted on the back board of the telephone and permanently
+wired to the line and ground binding posts.
+
+[Illustration: OLD SWITCHBOARD OF BELL EXCHANGE SERVING CHINATOWN,
+SAN FRANCISCO, CALIFORNIA]
+
+[Illustration: Fig. 211. Kellogg Air-Gap Arrester]
+
+Vacuum Arresters:--All of the carbon arresters so far mentioned depend
+on the discharge taking place through air. A given pressure will
+discharge further in a fairly good vacuum than in air. The National
+Electric Specialty Company mounts three conductors in a vacuum of the
+incandescent lamp type, Fig. 212. A greater separation and less
+likelihood of short-circuiting can be provided in this way. Either
+carbon or metal plates are adapted for use in such vacuum devices. The
+plates may be further apart for a given discharge pressure if the
+surfaces are of carbon.
+
+[Illustration: Fig. 212. Vacuum Arrester]
+
+Introduction of Impedance:--It has been noted that the existence of
+impedance tends to choke back the passage of lightning discharge
+through a coil. Fig. 213 suggests the relation between such an
+impedance and air-gap arrester. If the coil shown therein be considered
+an arrangement of conductors having inductance, it will be seen that a
+favorable place for an air-gap arrester is between that impedance and
+the line. This fact is made known in practice by frequent damage to
+aerial cables by electricity brought into them over long open wires,
+the discharge taking place at the first turn or bend in the aerial
+cable; this discharge often damages both core and sheath. It is well to
+have such bends as near the end of the cable as possible, and turns or
+goosenecks at entrances to terminals have that advantage.
+
+[Illustration: Fig. 213. Impedance and Air-Gap]
+
+This same principle is utilized in some forms of arresters, such as
+the one shown in Fig. 214, which provides an impedance of its own
+directly in the arrester element. In this device an insulating base
+carries a grounded carbon rod and two impedance coils. The impedance
+coils are wound on insulating rods, which hold them near, but not
+touching, the ground carbon. The coils are arranged so that they may
+be turned when discharges roughen the surfaces of the wires.
+
+[Illustration: Fig. 214. Holtzer-Cabot Arrester]
+
+Metallic Electrodes:--Copper or other metal blocks with roughened
+surfaces separated by an insulating slip may be substituted for the
+carbon blocks of most of the arresters previously described. Metal
+blocks lack the advantage of carbon in that the latter allows
+discharges at lower potentials for a given separation, but they have
+the advantage that a conducting dust is not thrown off from them.
+
+[Illustration: Fig. 215. Carbon Air-Gap Arrester]
+
+Provision Against Continuous Arc:--For the purpose of short-circuiting
+an arc, a globule of low-melting alloy may be placed in one carbon
+block of an arrester. This feature is not essential in an arrester
+intended solely to divert lightning discharges. Its purpose is to
+provide an immediate path to ground if an arc arising from artificial
+electricity has been maintained between the blocks long enough to melt
+the globule. Fig. 215 is a plan and section of the Western Electric
+Company's arrester used as the high potential element in conjunction
+with others for abnormal currents and sneak currents; the latter are
+currents too small to operate air-gap arresters or substantial fuses.
+
+Protection Against Strong Currents. _Fuses._ A fuse is a metal
+conductor of lower carrying capacity than the circuit with which it is
+in series at the time it is required to operate. Fuses in use in
+electrical circuits generally are composed of some alloy of lead,
+which melts at a reasonably low temperature. Alloys of lead have lower
+conductivity than copper. A small copper wire, however, may fuse at
+the same volume of current as a larger lead alloy wire.
+
+Proper Functions:--A fuse is not a good lightning arrester. As
+lightning damage is caused by current and as it is current which
+destroys a fuse, a lightning discharge _can_ open a circuit over which
+it passes by melting the fuse metal. But lightning may destroy a fuse
+and at the same discharge destroy apparatus in series with the fuse.
+There are two reasons for this: One is that lightning discharges act
+very quickly and may have destroyed apparatus before heating the fuse
+enough to melt it; the other reason is that when a fuse is operated
+with enough current even to vaporize it, the vapor serves as a
+conducting path for an instant after being formed. This conducting
+path may be of high resistance and still allow currents to flow
+through it, because of the extremely high pressure of the lightning
+discharge. A comprehensive protective system may include fuses, but it
+is not to be expected that they always will arrest lightning or even
+assist other things in arresting lightning. They should be considered
+as of no value for that purpose. Furthermore, fuses are best adapted
+to be a part of a general protective system when they do all that they
+must do in stopping abnormal currents and yet withstand lightning
+discharges which may pass through them. Other things being equal, that
+system of protection is best in which all lightning discharges are
+arrested by gap arresters and in which no fuses ever are operated by
+lightning discharges.
+
+Mica Fuse:--A convenient and widely used form of fuse is that shown in
+Fig. 216. A mica slip has metal terminals at its ends and a fuse wire
+joins these terminals. The fuse is inserted in the circuit by clamping
+the terminals under screws or sliding them between clips as in Figs.
+217 and 218. Advantages of this method of fuse mounting for protecting
+circuits needing small currents are that the fuse wire can be seen, the
+fuses are readily replaced when blown, and their mountings may be made
+compact. As elements of a comprehensive protective system, however, the
+ordinary types of mica-slip fuses are objectionable because too short,
+and because they have no means of their own for extinguishing an arc
+which may follow the blowing of the fuses. As protectors for use in
+distributing low potential currents from central-office power plants
+they are admirable. By simple means, they may be made to announce
+audibly or visibly that they have operated.
+
+[Illustration: Fig. 216. Mica Slip Fuse]
+
+[Illustration: Fig. 217. Postal Type Mica Fuse]
+
+[Illustration: Fig. 218. Western Union Type Mica Fuse]
+
+Enclosed Fuses:--If a fuse wire within an insulating tube be made to
+connect metal caps on that tube and the space around the tube be
+filled with a non-conducting powder, the gases of the vaporized fuse
+metal will be absorbed more quickly than when formed without such
+imbedding in a powder. The filling of such a tubular fuse also muffles
+the explosion which occurs when the fuse is vaporized.
+
+[Illustration: Fig. 219. Pair of Enclosed Fuses]
+
+Fuses of the enclosed type, with or without filling, are widely used
+in power circuits generally and are recommended by fire insurance
+bodies. Fig. 219 illustrates an arrester having a fuse of the enclosed
+type, this example being that of the H. W. Johns-Manville Company.
+
+[Illustration Fig. 220. Bank of Enclosed Fuses]
+
+In telephony it is frequently necessary to mount a large number of
+fuses or other protective devices together in a restricted space. In
+Fig. 220 a group of Western Electric tubular fuses, so mounted, is
+shown. These fuses have ordinarily a carrying capacity of 6 or 7
+amperes. It is not expected that this arrester will blow because 6 or
+7 amperes of abnormal currents are flowing through it and the
+apparatus to be protected. What is intended is that the fuse shall
+withstand lightning discharges and when a foreign current passes
+through it, other apparatus will increase that current enough to blow
+the fuse. It will be noticed that the fuses of Fig. 220 are open at
+the upper end, which is the end connected to the exposed wire of the
+line The fuses are closed at the lower end, which is the end connected
+to the apparatus. When the fuse blows, its discharge is somewhat
+muffled by the lining of the tube, but enough explosion remains so
+that the heated gases, in driving outward, tend to break the arc which
+is established through the vaporized metal.
+
+A pair of Cook tubular fuses in an individual mounting is shown in
+Fig. 221. Fuses of this type are not open at one end like a gun, but
+opportunity for the heated gases to escape exists at the caps. The
+tubes are made of wood, of lava, or of porcelain.
+
+Fig. 222 is another tubular fuse, the section showing the arrangement
+of asbestos lining which serves the two purposes of muffling the sound
+of the discharge and absorbing and cooling the resulting gases.
+
+[Illustration: Fig. 221. Pair of Wooden Tube Fuses]
+
+_Air-Gap vs. Fuse Arresters._ It is hoped that the student grasps
+clearly the distinction between the purposes of air-gap and fuse
+arresters. The air-gap arrester acts in response to high voltages,
+either of lightning or of high-tension power circuits. The fuse acts in
+response to a certain current value flowing through it and this minimum
+current in well-designed protectors for telephone lines is not very
+small. Usually it is several times larger than the maximum current
+apparatus in the line can safely carry. Fuses _can_ be made so delicate
+as to operate on the very smallest current which could injure apparatus
+and the earlier protective systems depended on such an arrangement. The
+difficulty with such delicate fuses is that they are not robust enough
+to be reliable, and, worse still, they change their carrying capacity
+with age and are not uniform in operation in different surroundings and
+at different temperatures. They are also sensitive to lightning
+discharges, which they have no power to stop or to divert.
+
+Protection Against Sneak Currents. For these reasons, a system
+containing fuses and air-gap arresters only, does not protect against
+abnormal currents which are continuous and small, though large enough
+to injure apparatus _because_ continuous. These currents have come to
+be known as sneak currents, a term more descriptive than elegant.
+Sneak currents though small, may, when allowed to flow for a long time
+through the winding of an electromagnet for instance, develop enough
+heat to char or injure the insulation. They are the more dangerous
+because insidious.
+
+[Illustration: Fig. 222. Tubular Fuse with Asbestos Filling]
+
+_Sneak-Current Arresters._ As typical of sneak-current arresters,
+Fig. 223 shows the principle, though not the exact form, of an
+arrester once widely used in telephone and signal lines. The normal
+path from the line to the apparatus is through a small coil of fine
+wire imbedded in sealing wax. A spring forms a branch path from the
+line and has a tension which would cause it to bear against the ground
+contact if it were allowed to do so. It is prevented from touching
+that contact normally by a string between itself and a rigid support.
+The string is cut at its middle and the knotted ends as thus cut are
+imbedded in the sealing wax which contains the coil.
+
+[Illustration: Fig. 223. Principle of Sneak-Current Arrester]
+
+A small current through the little coil will warm the wax enough to
+allow the string to part. The spring then will ground the line. Even
+so simple an apparatus as this operates with considerable accuracy.
+All currents below a certain critical amount may flow through the
+heating coil indefinitely, the heat being radiated rapidly enough to
+keep the wax from softening and the string from parting. All currents
+above this critical amount will operate the arrester; the larger the
+current, the shorter the time of operating. It will be remembered that
+the law of these heating effects is that the heat generated =
+_C^{2}Rt_, so that if a certain current operates the arrester in, say
+40 seconds, twice as great a current should operate the arrester in 10
+seconds. In other words, the time of operation varies inversely as the
+square of the current and inversely as the resistance. To make the
+arrester more sensitive for a given current--_i.e._, to operate in a
+shorter time--one would increase the resistance of the coil in the wax
+either by using more turns or finer wire, or by making the wire of a
+metal having higher specific resistance.
+
+The present standard sneak-current arrester embodies the two elements
+of the devices of Fig. 223: a _resistance_ material to transform the
+dangerous sneak current into localized heat; and a _fusible_ material
+softened by this heat to release some switching mechanism.
+
+The resistance material is either a resistance wire or a bit of
+carbon, the latter being the better material, although both are good.
+The fusible material is some alloy melting at a low temperature. Lead,
+tin, bismuth, and cadmium can be combined in such proportions as will
+enable the alloy to melt at temperatures from 140 deg. to 180 deg. F. Such an
+alloy is a solder which, at ordinary temperatures, is firm enough to
+resist the force of powerful springs; yet it will melt so as to be
+entirely fluid at a temperature much less than that of boiling water.
+
+[Illustration: Fig. 224. Heat Coil]
+
+_Heat Coil._ Fig. 224 shows a practical way of bringing the heating and
+to-be-heated elements together. A copper spool is wound with resistance
+wire. A metal pin is soldered in the bore of the spool by an easily
+melting alloy. When current heats the spool enough, the pin may slide
+or turn in the spool. It may slide or turn in many ways and this
+happily enables many types of arresters to result. For example, the pin
+may pull out, or push in, or push through, or rotate like a shaft in a
+bearing, or the spool may turn on it like a hub on an axle. Messrs.
+Hayes, Rolfe, Cook, McBerty, Kaisling, and many other inventors have
+utilized these combinations and motions in the production of
+sneak-current arresters. All of them depend on one action: the
+softening of a low-melting alloy by heat generated in a resistance.
+
+When a heat coil is associated with the proper switching springs, it
+becomes a sneak-current arrester. The switching springs always are
+arranged to ground the line wire. In some arresters, the line wire is
+cut off from the wire leading toward the apparatus by the same
+movement which grounds it. In others, the line is not broken at all,
+but merely grounded. Each method has its advantages.
+
+Complete Line Protection. Fig. 225 shows the entire scheme of protectors
+in an exposed line and their relation to apparatus in the central-office
+equipment and at the subscriber's telephone. The central-office
+equipment contains heat coils, springs, and carbon arresters. At some
+point between the central office and the subscriber's premises, each
+wire contains a fuse. At the subscriber's premises each wire contains
+other fuses and these are associated with carbon arresters. The figure
+shows a central battery equipment, in which the ringer of the telephone
+is in series with a condenser. A sneak-current arrester is not required
+at the subscriber's station with such equipment.
+
+Assume the line to meet an electrical hazard at the point _X_. If this
+be lightning, it will discharge to ground at the central office or at
+the subscriber's instrument or at both through the carbon arresters
+connected to that side of the line. If it be a high potential from a
+power circuit and of more than 350 volts, it will strike an arc at the
+carbon arrester connected to that wire of the line in the central
+office or at the subscriber's telephone or at both, if the separation
+of the carbons in those arresters is .005 inch or less. If the carbon
+arresters are separated by celluloid, it will burn away and allow the
+carbons to come together, extinguishing the arc. If they are separated
+by mica and one of the carbons is equipped with a globule of
+low-melting alloy, the heat of the arc will melt this,
+short-circuiting the gap and extinguishing the arc. The passage of
+current to ground at the arrester, however, will be over a path
+containing nothing but wire and the arrester. The resulting current,
+therefore, may be very large. The voltage at the arrester having been
+350 volts or more, in order to establish the arc, short-circuiting the
+gap will make the current 7 amperes or more, unless the applied
+voltage miraculously falls to 50 volts or less. The current through
+the fuse being more than 7 amperes, it will blow promptly, opening the
+line and isolating the apparatus. It will be noted that this
+explanation applies to equipment at either end of the line, as the
+fuse lies between the point of contact and the carbon arrester.
+
+[Illustration: Fig. 225. Complete Line Protection]
+
+Assume, on the other hand, that the contact is made at the point _Y_.
+The central-office carbon arrester will operate, grounding the line and
+increasing the amount of current flowing. There being no fuse to blow,
+a worse thing will befall, in the overheating of the line wire and the
+probable starting of a fire in the central office. It is obvious,
+therefore, that a fuse must be located between the carbon arrester and
+any part of the line which is subject to contact with a potential which
+can give an abnormal current when the carbon arrester acts.
+
+Assume, as a third case, that the contact at the point _X_ either is
+with a low foreign potential or is so poor a contact that the
+difference of potential across the gap of the carbon arrester is lower
+than its arcing point. Current will tend to flow by the carbon
+arrester without operating it, but such a current must pass through
+the winding of the heat coil if it is to enter the apparatus. The
+sneak current may be large enough to overheat the apparatus if allowed
+to flow long enough, but before it has flowed long enough it will have
+warmed the heat-coil winding enough to soften its fusible alloy and to
+release springs which ground the line, just as did the carbon arrester
+in the case last assumed. Again the current will become large and will
+blow the fuse which lies between the sneak-current arrester and the
+point of contact with the source of foreign current. In this case,
+also, contact at the point _Y_ would have operated mechanism to ground
+the line at the central office, and, no fuse interposing, the wiring
+would have been overheated.
+
+_Exposed and Unexposed Wiring._ Underground cables, cables formed of
+rubber insulated wires, and interior wiring which is properly done,
+all may be considered to be wiring which is unexposed, that is, not
+exposed to foreign high potentials, discharges, sneak, or abnormal
+currents. _All other wiring_, such as bare wires, aerial cables, etc.,
+should be considered as _exposed_ to such hazards and a fuse should
+exist in each wire between its exposed portion and the central office
+or subscriber's instrument. The rule of action, therefore, becomes:
+
+_The proper position of the fuse is between exposed and unexposed
+wiring._
+
+It may appear to the student that wires in an aerial cable with a lead
+sheath--that sheath being either grounded or ungrounded--are not
+exposed to electrical hazards; in the case of the grounded sheath,
+this would presume that a contact between the cable and a high
+potential wire would result merely in the foreign currents going to
+ground through the cable sheath, the arc burning off the
+high-potential wire and allowing the contact to clear itself by the
+falling of the wire. If the assumption be that the sheath is not
+grounded, then the student may say that no current at all would flow
+from the high-potential wire.
+
+Both assumptions are wrong. In the case of the grounded sheath, the
+current flows to it at the contact with the high-potential wire; the
+lead sheath is melted, arcs strike to the wires within, and currents
+are led directly to the central office and to subscribers' premises.
+In the case of the ungrounded sheath, the latter charges at once
+through all its length to the voltage of the high-potential wire; at
+some point, a wire within the cable is close enough to the sheath for
+an arc to strike across, and the trouble begins. All the wires in the
+cable are endangered if the cross be with a wire of the primary
+circuit of a high-tension transmission line. Any series arc-light
+circuit is a high-potential menace. Even a 450-volt trolley wire or
+feeder can burn a lead-covered cable entirely in two in a few seconds.
+The authors have seen this done by the wayward trolley pole of a
+street car, one side of the pole touching the trolley wire and the
+extreme end just touching the telephone cable.
+
+The answer lies in the foregoing rule. Place the fuse between the wires
+which _can_ and the wires which _can not_ get into contact with high
+potentials. In application, the rule has some flexibility. In the case
+of a cable which is aerial as soon as it leaves the central office,
+place the fuses in the central office; in a cable wholly underground,
+from central office to subscriber--as, for example, the feed for an
+office building--use no fuses at all; in a cable which leaves the
+central office underground and becomes aerial, fuse the wires just
+where they change from underground to aerial. The several branches of
+an underground cable into aerial ones should be fused as they branch.
+
+Wires properly installed in subscribers' premises are considered
+unexposed. The position of the fuse thus is at or near the point of
+entrance of the wires into that building if the wires of the
+subscriber's line outside the premises are exposed, as determined by
+the definitions given. If the line is unexposed, by those definitions,
+no protector is required. If one is indicated, it should be used, as
+compliance with the best-known practice is a clear duty. Less than
+what is known to be best is not honest practice in a matter which
+involves life, limb, and indefinite degrees of property values.
+
+Protectors in central-battery subscribers' equipments need no
+sneak-current arresters, as the condenser reduces that hazard to a
+negligible amount. Magneto subscribers' equipments usually lack
+condensers in ringer circuits, though they may have them in talking
+circuits on party lines. The ringer circuit is the only path through
+the telephone set for about 98 per cent of the time. Sneak-current
+arresters, therefore, should be a part of subscribers' station
+protectors in magneto equipment, except in such rural districts as may
+have no lighting or power wires. When sneak-current arresters are so
+used the arrangement of the parts then is the same as in the
+central-office portion of Fig. 225.
+
+Types of Central-Office Protectors. A form of combined heat coil and
+air-gap arrester, widely used by Bell companies for central-office
+protection, is shown in Fig. 226. The two inner springs form the
+terminals for the two limbs of the metallic-circuit line, while the
+two outside springs are terminals for the continuation of the line
+leading to the switchboard. The heat coils, one on each side, are
+supported between the inner and outer springs. High-tension currents
+jump to ground through the air-gap arrester, while sneak currents
+permit the pin of the heat coil to slide within the sleeve, thus
+grounding the outside line and the line to the switchboard.
+
+[Illustration: Fig. 226. Sneak-Current and Air-Gap Arrester]
+
+_Self-Soldering Heat Coils._ Another form designed by Kaisling and
+manufactured by the American Electric Fuse Company is shown in Fig.
+227. In this the pin in the heat coil projects unequally from the ends
+of the coil, and under the action of a sneak current the melting of
+the solder which holds it allows the outer spring to push the pin
+through the coil until it presses the line spring against the ground
+plate and at the same time opens the path to the switchboard. When the
+heat-coil pin assumes this new position it cools off, due to the
+cessation of the current, and _resolders_ itself, and need only be
+turned end for end by the attendant to be reset. Many are the
+variations that have been made on this self-soldering idea, and there
+has been much controversy as to its desirability. It is certainly a
+feature of convenience.
+
+[Illustration: Fig. 227. Self-Soldering Heat-Coil Arrester]
+
+Instead of using a wire-wound resistance element in heat-coil
+construction some manufacturers employ a mass of high-resistance
+material, interposed in the path of the current. The Kellogg Company
+has long employed for its sneak-current arrester a short graphite rod,
+which forms the resistance element. The ends of this rod are
+electroplated with copper to which the brass terminal heads are
+soldered. These heads afford means for making the connection with the
+proper retaining springs.
+
+[Illustration: Fig. 228. Cook Arrester]
+
+Another central-office protector, which uses a mass of special metal
+composition for its heat producing element is that designed by Frank B.
+Cook and shown in Fig. 228. In this the carbon blocks are cylindrical
+in form and specially treated to make them "self-cleaning." Instead of
+employing a self-soldering feature in the sneak-current arrester of
+this device, Cook provides for electrically resoldering them after
+operation, a clip being designed for holding the elements in proper
+position and passing a battery current through them to remelt the
+solder.
+
+In small magneto exchanges it is not uncommon to employ combined fuse
+and air-gap arresters for central-office line protection, the fuses
+being of the mica-mounted type already referred to. A group of such
+arresters, as manufactured by the Dean Electric Company, is shown in
+Fig. 229.
+
+[Illustration: Fig. 229. Mica Fuse and Air-Gap Arresters]
+
+Types of Subscribers' Station Protectors. Figs. 230 and 231 show types
+of subscribers' station protectors adapted to the requirements of
+central-battery and magneto systems. These, as has been said, should be
+mounted at or near the point of entrance of the subscriber's line into
+the premises, if the line is exposed outside of the premises. It is
+possible to arrange the fuses so that they will be safe and suitable
+for their purposes if they are mounted out-of-doors near the point of
+entrance to the premises. The sneak-current arrester, if one exists,
+and the carbon arrester also, must be mounted inside of the premises or
+in a protecting case, if outside, on account of the necessity of
+shielding both of these devices from the weather. Speaking generally,
+the wider practice is to put all the elements of the subscriber's
+station protector inside of the house. It is nearer to the ideal
+arrangement of conditions if the protector be placed immediately at the
+point of entrance of the outside wires into the building.
+
+[Illustration: Fig. 230. Western Electric Station Arrester]
+
+[Illustration: Fig. 231. Cook Arrester for Magneto Stations]
+
+_Ribbon Fuses_. A point of interest with relation to tubular fuses is
+that in some of the best types of such fuses, the resistance material
+is not in the form of a round wire but in the form of a flat ribbon.
+This arrangement disposes the necessary amount of fusible metal in a
+form to give the greatest amount of surface, while a round wire offers
+the least surface for a given weight of metal--a circle encloses its
+area with less periphery than any other figure. The reason for giving
+the fuse the largest possible surface area is to decrease the
+likelihood of the fuse being ruptured by lightning. The fact that such
+fuses do withstand lightning discharges much more thoroughly than
+round fuses of the same rating is an interesting proof of the
+oscillating nature of lightning discharges, for the density of the
+current of those discharges is greater on and near the surface of the
+conductor than within the metal and, therefore, flattening the fuse
+increases its carrying capacity for high-frequency currents, without
+appreciably changing its carrying capacity for direct currents. The
+reason its capacity for direct currents is increased at all by
+flattening it, is that the surface for the radiation of heat is
+increased. However, when enclosed in a tube, radiation of heat is
+limited, so that for direct currents the carrying capacity of fuses
+varies closely with the area of cross-section.
+
+City-Exchange Requirements. The foregoing has set down the
+requirements of good practice in an average city-exchange system.
+Nothing short of the general arrangement shown in Fig. 225 meets the
+usual assortment of hazards of such an exchange. It is good modern
+practice to distribute lines by means of cables, supplemented in part
+by short insulated drop wires twisted in pairs. Absence of bare wires
+reduces electrical hazards enormously. Nevertheless, hazards remain.
+
+Though no less than the spirit of this plan of protection should be
+followed, additional hazards may exist, which may require additional
+elements of protection. At the end of a cable, either aerial or
+underground, long open wires may extend into the open country as rural
+or long-distance circuits. If these be longer than a mile or two, in
+most regions they will be subjected to lightning discharges. These may
+be subjected to high-potential contacts as well.
+
+If a specific case of such exposure indicates that the cables may be
+in danger, the long open lines then are equipped with additional
+air-gap arresters at the point of junction of those open lines with
+the cable. Practice varies as to the type. Maintenance charges are
+increased if carbon arresters separated .005 inch are used, because of
+the cost of sending to the end of the long cable to clear the blocks
+from carbon dust after each slight discharge. Roughened metal blocks
+do not become grounded as readily as do carbon blocks. The occasions
+of visit to the arresters, therefore, usually follow actual heavy
+discharges through them.
+
+The recommendations and the practice of the American Telephone and
+Telegraph Company differ on this point, while the practice of other
+companies varies with the temperaments of the engineers. The American
+Company specifies copper-block arresters where long country lines
+enter cables, if those lines are exposed to lightning discharges only.
+The exposed line is called _long_ if more than one-half mile in
+length. If it is exposed to high-potential hazards, carbon blocks are
+specified instead of copper. Other specifications of that company have
+called for the use of copper-block arresters on lines exposed to
+hazards above 2,500 volts.
+
+[Illustration: ONE OF THE FOUR WINGS OF THE OLD KELLOGG DIVIDED
+MULTIPLE BOARD OF THE CUYAHOGA TELEPHONE COMPANY, CLEVELAND, OHIO
+Ultimate Capacity, 24,000 Lines. One of the Two Examples in the United
+States of a Multiple Switchboard Having an Ultimate Capacity over
+18,000 Lines. Replaced Recently by a Kellogg Straight Multiple Board
+Having an Ultimate Capacity of 18,000 Lines and a Present Capacity of
+10,000 Lines.]
+
+The freedom of metal-block arresters from dust troubles gives them a
+large economical advantage over carbon. For similar separations, the
+ratio of striking voltages between carbon blocks and metal blocks
+respectively is as 7 to 16. In certain regions of the Pacific Coast
+where the lightning hazard is negligible and the high tension hazard
+is great, metal-block arresters at the outer ends of cables give
+acceptable protection.
+
+High winds which drive snow or dust against bare wires of a long line,
+create upon or place upon those wires a charge of static electricity
+which makes its way from the line in such ways as it can. Usually it
+discharges across arresters and when this discharge takes place, the
+line is disturbed in its balance and loud noises are heard in the
+telephones upon it.
+
+[Fig. 232. Drainage Coils]
+
+A telephone line which for a long distance is near a high-tension
+transmission line may have electrostatic or electromagnetic
+potentials, or both, induced upon it. If the line be balanced in its
+properties, including balance by transposition of its wires, the
+electrostatic induction may neutralize itself. The electromagnetic
+induction still may disturb it.
+
+_Drainage Coils_. The device shown in Fig. 232, which amounts merely
+to an inductive leak to earth, is intended to cure both the snowstorm
+and electromagnetic induction difficulties. It is required that its
+impedance be high enough to keep voice-current losses low, while being
+low enough to drain the line effectively of the disturbing charges.
+Such devices are termed "drainage coils."
+
+Electrolysis. The means of protection against the danger due to
+chemical action, set forth in the preceding chapter, form such a
+distinct phase of the subject of guarding property against electrical
+hazards as to warrant treatment in a separate chapter devoted to the
+subject of electrolysis.
+
+[Illustration: MAIN EXCHANGE, CLEVELAND, OHIO.
+Largest Four-Party Selective Ringing Switchboard in the World. Kellogg
+Switchboard and Supply Co.]
+
+
+
+
+CHAPTER XX
+
+GENERAL FEATURES OF THE TELEPHONE EXCHANGE
+
+
+Up to this point only those classes of telephone service which could be
+given between two or more stations on a single line have been
+considered. Very soon after the practical conception of the telephone,
+came the conception of the telephone exchange; that is, the conception
+of centering a number of lines at a common point and there terminating
+them in apparatus to facilitate their interconnection, so that any
+subscriber on any line could talk with any subscriber on any other
+line.
+
+The complete equipment of lines, telephone instruments, and switching
+facilities by which the telephone stations of the community are given
+telephone service is called a telephone exchange.
+
+The building where a group of telephone lines center for
+interconnection is called a central office, and its telephonic
+equipment the central-office equipment. The terms telephone office and
+telephone exchange are frequently confused. Although a telephone office
+building may be properly referred to as a telephone exchange building,
+it is hardly proper to refer to the telephone office as a telephone
+exchange, as is frequently done. In modern parlance the telephone
+exchange refers not only to the central office and its equipment but to
+the lines and instruments connected therewith as well; furthermore, a
+telephone exchange may embrace a number of telephone offices that are
+interconnected by means of so-called trunk lines for permitting the
+communication of subscribers whose lines terminate in one office with
+those subscribers whose lines terminate in any other office.
+
+Since a given telephone exchange may contain one or more central
+offices, it is proper to distinguish between them by referring to an
+exchange which contains but a single central office as a single office
+exchange, and to an exchange which contains a plurality of central
+offices as a multi-office exchange.
+
+In telephone exchange working, three classes of lines are dealt
+with--subscribers' lines, trunk lines, and toll lines.
+
+Subscribers' Lines. The term subscriber is commonly applied to the
+patron of the telephone service. His station is, therefore, referred
+to as a subscriber's station, and the telephone equipment at any
+subscriber's station is referred to as a subscriber's station
+equipment. Likewise, a line leading from a central office to one or
+more subscribers' stations is called a subscriber's line. A
+subscriber's line may, as has been shown in a previous chapter, be an
+individual line if it serves but one station, or a party line if it
+serves to connect more than one station with the central office.
+
+Trunk Lines. A trunk line is a line which is not devoted to the
+service of any particular subscriber, but which may form a connecting
+link between any one of a group of subscribers' lines which terminate
+in one place and any one of a group of subscribers' lines which
+terminate in another place. If the two groups of subscribers' lines
+terminate in the same building or in the same switchboard, so that the
+trunk line forming the connecting link between them is entirely within
+the central-office building, it is called a local trunk line, or a
+local trunk. If, on the other hand, the trunk line is for connecting
+groups of subscribers' lines which terminate in different central
+offices, it is called an inter-office trunk.
+
+Toll Lines. A toll line is a telephone line for the use of which a
+special fee or toll is charged; that is, a fee that is not included in
+the charges made to the subscriber for his regular local exchange
+service. Toll lines extend from one exchange district to another, more
+or less remote, and they are commonly termed _local_ toll and
+_long-distance_ toll lines according to the degree of remoteness. A
+toll line, whether local or long-distance, may be looked upon in the
+nature of an inter-exchange trunk.
+
+Districts. The district in a given community which is served by a
+single central office is called an office district. Likewise, the
+district which is served by a complete exchange is called an exchange
+district. An exchange district may, therefore, consist of a number of
+central-office districts, just as an exchange may comprise a number of
+central offices. To illustrate, the entire area served by the exchange
+of the Chicago Telephone Company in Chicago, embracing the entire city
+and some of its suburbs, is the Chicago exchange district. The area
+served by one of the central offices, such as the Hyde Park office,
+the Oakland office, the Harrison office, or any of the others, is an
+office district.
+
+Switchboards. The apparatus at the central office by which the
+telephone lines are connected for conversation and afterwards
+disconnected, and by which the various other functions necessary to
+the giving of complete telephone service are performed, is called a
+switchboard. This may be simple in the case of small exchanges, or of
+vast complexity in the case of the larger exchanges.
+
+Sometimes the switchboards are of such nature as to require the
+presence of operators, usually girls, to connect and disconnect the
+line and perform the other necessary functions, and such switchboards,
+whether large or small, are termed _manual_.
+
+Sometimes the switchboards are of such a nature as not to require the
+presence of operators, the various functions of connection,
+disconnection, and signaling being performed by the aid of special
+forms of apparatus which are under the control of the subscriber who
+makes the call. Such switchboards are termed _automatic_.
+
+Of recent years there has appeared another class of switchboards,
+employing in some measure the features of the automatic and in some
+measure those of the manual switchboard. These boards are commonly
+referred to as _semi-automatic_ switchboards, presumably because they
+are supposed to be half automatic and half manual.
+
+_Manual_. Manual switchboards may be subdivided into two classes
+according to the method of distributing energy for talking purposes.
+Thus we may have _magneto_ switchboards, which are those capable of
+serving lines equipped with magneto telephones, local batteries being
+used for talking purposes. On the other hand, we may have
+_common-battery_ switchboards, adapted to connect lines employing
+common-battery telephones in which all the current for both talking
+and signaling is furnished from the central office. In still another
+way we may classify manual switchboards if the method of distributing
+the energy for talking and signaling purposes is ignored. Thus,
+entirely irrespective of whether the switchboards are adapted to serve
+common-battery or local-battery lines, we may have non-multiple
+switchboards and multiple switchboards.
+
+The term _multiple_ switchboard is applied to that class of
+switchboards in which the connection terminals or jacks for all the
+lines are repeated at intervals along the face of the switchboard, so
+that each operator may have within her reach a terminal for each line
+and may thus be able to complete by herself any connection between two
+lines terminating in the switchboard.
+
+The term _non-multiple_ switchboard is applied to that class of boards
+where the provision for repeating the line terminals at intervals along
+the face of the board is not employed, but where, as a consequence,
+each line has but a single terminal on the face of the board.
+Non-multiple switchboards have their main use in small exchanges where
+not more than a few hundred lines terminate. Where such is the case, it
+is an easy matter to handle all the traffic by one, two, or three
+operators, and as all of these operators may reach all over the face of
+the switchboard, there is no need for giving any line any more than one
+connection terminal. Such boards may be called _simple_ switchboards.
+
+There is another type of non-multiple switchboard adaptable for use in
+larger exchanges than the simple switchboard. A correct idea of the
+fundamental principle involved in these may be had by imagining a row
+of simple switchboards each containing terminals or jacks for its own
+group of lines. In order to provide for the connection of a line in
+one of these simple switchboards with a line in another one, out of
+reach of the operator at the first, short connecting lines extending
+between the two switchboards are provided, these being called
+_transfer_ or _trunk_ lines. In order that connections may be made
+between any two of the simple boards, a group of transfer lines is run
+from each board to every other one.
+
+In such switchboards an operator at one of the boards or positions may
+complete the connection herself between any two lines terminating at
+her own board. If, however, the line called for terminates at another
+one of the boards, the operator makes use of the transfer or trunk line
+extending to that board, and the operator at this latter board
+completes the connection, so that the two subscribers' lines are
+connected through the trunk or transfer line. A distinguishing feature,
+therefore, in the operation of so-called transfer switchboards, is that
+an operator can not always complete a connection herself, the
+connection frequently requiring the attention of two operators.
+
+Transfer systems are not now largely used, the multiple switchboard
+having almost entirely supplanted them in manual exchanges of such size
+as to be beyond the limitation of the simple switchboard. At
+multi-office manual exchanges, however, where there are a number of
+multiple switchboards employed at various central offices, the same
+sort of a requirement exists as that which was met by the provision of
+trunk lines between the various simple switchboards in a transfer
+system. Obviously, the lines in one central office must be connected to
+those of another in order to give universal service in the community in
+which the exchange operates. For this purpose inter-office trunk lines
+are used, the arrangement being such that when an operator at one
+office receives a call for a subscriber in another office, she will
+proceed to connect the calling subscriber's line, not directly with the
+line of the called subscriber because that particular line is not
+within her reach, but rather with a trunk line leading to the office in
+which the called-for subscriber's line terminates; having done this she
+will then inform an operator at that second office of the connection
+desired, usually by means of a so-called order-wire circuit. The
+connection between the trunk line so used and the line of the
+called-for subscriber will then be completed by the connecting link or
+trunk line extending between the two offices.
+
+In such cases the multiple switchboard at each office is divided into
+two portions, termed respectively the _A_ board and the _B_ board.
+Each of these boards, with the exception that will be pointed out in a
+subsequent chapter, is provided with a full complement of multiple
+jacks for all of the lines entering that office. At the _A_ board are
+located operators, called _A_ operators, who answer all the calls from
+the subscribers whose lines terminate in that office. In the case of
+calls for lines in that same office, they complete the connection
+themselves without the assistance of the other operators. On the other
+hand, the calls for lines in another office are handled through trunk
+lines leading to that other office, as before described, and these
+trunk lines always terminate in the _B_ board at that office. The _B_
+operators are, therefore, those operators who receive the calls over
+trunk lines and complete the connection with the line of the
+subscriber desired.
+
+To define these terms more specifically, an _A_ board is a multiple
+switchboard in which the subscriber's lines of a given office district
+terminate. For this reason the _A_ board is frequently referred to as
+a subscribers' board, and the operators who work at these boards and
+who answer the calls of the subscribers are called _A_ operators or
+subscribers' operators. _B_ boards are switchboards in which terminate
+the incoming ends of the trunk lines leading from other offices in the
+same exchange. These boards are frequently called incoming trunk
+boards, or merely trunk boards, and the operators who work at them and
+who receive the directions from the _A_ operators at the other boards
+are called _B_ operators, or incoming trunk operators.
+
+The circuits which are confined wholly to the use of operators and
+over which the instructions from one operator to another are sent, as
+in the case of the _A_ operator giving an order for a connection to a
+_B_ operator at another switchboard, are designated _call circuits_ or
+_order wire circuits_.
+
+Sometimes trunk lines are so arranged that connections may be
+originated at either of their ends. In other cases they are so arranged
+that one group of trunk lines connecting two offices is for the traffic
+in one direction only, while another group leading between the same two
+offices is for handling only the traffic in the other direction. Trunk
+lines are called _one-way_ or _two-way_ trunks, according to whether
+they handle the traffic in one direction or in two. A trunking system,
+where the same trunks handle traffic both ways, is called a
+_single-track system_; and, on the other hand, a system in which there
+are two groups of trunks, one handling traffic in one direction and the
+other in the other, is called a _double-track system_. This
+nomenclature is obviously borrowed from railroad practice.
+
+There is still another class of manual switchboards called the _toll
+board_ of which it will be necessary to treat. Telephone calls made by
+one person for another within the limits of the same exchange district
+are usually charged for either by a flat rate per month, or by a
+certain charge for each call. This is usually regardless of the
+duration of the conversation following the call. On the other hand,
+where a call is made by one party for another outside of the limits of
+the exchange district and, therefore, in some other exchange district,
+a charge is usually made, based on the time that the connecting
+long-distance line is employed. Such calls and their ensuing
+conversations are charged for at a very much higher rate than the
+purely local calls, this rate depending on the distance between the
+stations involved. The making up of connections between a
+long-distance and a local line is usually done by means of operators
+other than those employed in handling the local calls, who work either
+by means of special equipment located on the local board, or by means
+of a separate board. Such equipments for handling long-distance or
+toll traffic are commonly termed toll switchboards.
+
+They differ from local boards (a) in that they are arranged for a very
+much smaller number of lines; (b) in that they have facilities by
+which the toll operator may make up the connections with a minimum
+amount of labor on the part of the assisting local operators; and (c)
+in that they have facilities for recording the identification of the
+parties and timing the conversations taking place over the toll lines,
+so that the proper charge may be made to the proper subscriber.
+
+
+
+
+CHAPTER XXI
+
+THE SIMPLE MAGNETO SWITCHBOARD
+
+
+Definitions. As already stated those switchboards which are adapted
+to work in conjunction with magneto telephones are called magneto
+switchboards. The signals on such switchboards are electromagnetic
+devices capable of responding to the currents of the magneto
+generators at the subscribers' stations. Since, as a rule, magneto
+telephones are equipped with local batteries, it follows that the
+magneto switchboard does not need to be arranged for supplying the
+subscribers' stations with talking current. This fact is accountable
+for magneto switchboards often being referred to as local-battery
+switchboards, in contradistinction to common-battery switchboards
+which are equipped so as to supply the connected subscribers' stations
+with talking current.
+
+The term _simple_ as applied in the headings of this and the next
+chapter, is employed to designate switchboards adapted for so small a
+number of lines that they may be served by a single or a very small
+group of operators; each line is provided with but a single connection
+terminal and all of them, without special provision, are placed
+directly within the reach of the operator, or operators if there are
+more than one. This distinction will be more apparent under the
+discussion of transfer and multiple switchboards.
+
+Mode of Operation. The cycle of operation of any simple manual
+switchboard may be briefly outlined as follows: The subscriber desiring
+a connection transmits a signal to the central office, the operator
+seeing the signal makes connection with the calling line and places
+herself in telephonic communication with the calling subscriber to
+receive his orders; the operator then completes the connection with the
+line of the called subscriber and sends ringing current out on that
+line so as to ring the bell of that subscriber; the two subscribers
+then converse over the connected lines and when the conversation is
+finished either one or both of them may send a signal to the central
+office for disconnection, this signal being called a clearing-out
+signal; upon receipt of the clearing-out signal, the operator
+disconnects the two lines and restores all of the central-office
+apparatus involved in the connection to its normal position.
+
+Component Parts. Before considering further the operation of manual
+switchboards it will be well to refer briefly to the component pieces
+of apparatus which go to make up a switchboard.
+
+_Line Signal._ The line signal in magneto switchboards is practically
+always in the form of an electromagnetic annunciator or drop. It
+consists in an electromagnet adapted to be included in the line
+circuit, its armature controlling a latch, which serves to hold the
+drop or shutter or target in its raised position when the magnet is not
+energized, and to release the drop or shutter or target so as to permit
+the display of the signal when the magnet is energized. The symbolic
+representation of such an electromagnetic drop is shown in Fig. 233.
+
+[Illustration: Fig. 233. Drop Symbol]
+
+_Jacks and Plugs._ Each line is also provided with a connection
+terminal in the form of a switch socket. This assumes many forms, but
+always consists in a cylindrical opening behind which are arranged one
+or more spring contacts. The opening forms a receptacle for plugs
+which have one or more metallic terminals for the conductors in the
+flexible cord in which the plug terminates. The arrangement is such
+that when a plug is inserted into a jack the contacts on the plug will
+register with certain of the contacts in the jack and thus continue
+the line conductors, which terminate in the jack contacts, to the cord
+conductors, which terminate in the plug contacts. Usually also when a
+plug is inserted certain of the spring contacts in the jack are made
+to engage with or disengage other contacts in the jack so as to make
+or break auxiliary circuits.
+
+[Illustration: Fig. 234. Spring Jack]
+
+A simple form of spring jack is shown in section in Fig. 234. In Fig.
+235 is shown a sectional view of a plug adapted to co-operate with
+the jack of Fig. 234. In Fig. 236 the plug is shown inserted into the
+jack. The cylindrical portion of the jack is commonly called the
+_sleeve_ or _thimble_ and it usually forms one of the main terminals
+of the jack; the spring, forming the other principal terminal, is
+called the _tip spring_, since it engages the tip of the plug. The tip
+spring usually rests on another contact which may be termed the
+_anvil_. When the plug is inserted into the jack as shown in Fig. 236,
+the tip spring is raised from contact with this anvil and thus breaks
+the circuit leading through it. It will be understood that spring
+jacks are not limited to three contacts such as shown in these figures
+nor are plugs limited to two contacts. Sometimes the plugs have three,
+and even more, contacts, and frequently the jacks corresponding to
+such plugs have not only a contact spring adapted to register with
+each of the contacts of the plug, but several other auxiliary contacts
+also, which will be made or broken according to whether the plug is
+inserted or withdrawn from the jack. Symbolic representations of plugs
+and jacks are shown in Fig. 237. These are employed in diagrammatic
+representations of circuits and are supposed to represent the
+essential elements of the plugs and jacks in such a way as to be
+suggestive of their operation. It will be understood that such symbols
+may be greatly modified to express the various peculiarities of the
+plugs and jacks which they represent.
+
+[Illustration: Fig. 235. Plug]
+
+[Illustration: Fig. 236. Plug and Jack]
+
+[Illustration: Fig. 237. Jack and Plug Symbols]
+
+_Keys_. Other important elements of manual switchboards are ringing
+and listening keys. These are the devices by means of which the
+operator may switch the central-office generator or her telephone set
+into or out of the circuit of the connected lines. The details of a
+simple ringing and listening key are shown in Fig. 238. This consists
+of two groups of springs, one of four and one of six, the springs in
+each group being insulated from each other at their points of
+mounting. Two of these springs _1_ and _2_ in one group--the ringing
+group--are longer than the others, and act as movable levers engaging
+the inner pair of springs _3_ and _4_ when in their normal positions,
+and the outer pair _5_ and _6_ when forced into their alternate
+positions. Movement is imparted to these springs by the action of a
+cam which is mounted on a lever, manipulated by the operator. When
+this lever is moved in one direction the cam presses the two springs
+_1_ and _2_ apart, thus causing them to disengage the springs _3_ and
+_4_ and to engage the springs _5_ and _6_.
+
+[Illustration: Fig. 238. Ringing and Listening Key]
+
+The springs of the other group constitute the switching element of the
+listening key and are very similar in their action to those of the
+ringing key, differing in the fact that they have no inner pair of
+springs such as _3_ and _4_. The two long springs _7_ and _8_,
+therefore, normally do not rest against anything, but when the key
+lever is pressed, so as to force the cam between them, they are made
+to engage the two outer springs _9_ and _10_.
+
+[Illustration: Fig. 239. Ringing-and Listening-Key Symbols]
+
+The design and construction of ringing and listening keys assume many
+different forms. In general, however, they are adapted to do exactly
+the same sort of switching operations as that of which the device of
+Fig. 238 is capable. Easily understood symbols of ringing and
+listening keys are shown in Fig. 239; the cam member which operates on
+the two long springs is usually omitted for ease of illustration. It
+will be understood in considering these symbols, therefore, that the
+two long curved springs usually rest against a pair of inner contacts
+in case of the ringing key or against nothing at all in case of the
+listening key, and that when the key is operated the two springs are
+assumed to be spread apart so as to engage the outer pair of contacts
+with which they are respectively normally disconnected.
+
+_Line and Cord Equipments._ The parts of the switchboard that are
+individual to the subscriber's line are termed the _line equipment;_
+this, in the case of a magneto switchboard, consists of the line drop
+and the jack together with the associated wiring necessary to connect
+them properly in the line circuit. The parts of the switchboard that
+are associated with a connecting link--consisting of a pair of plugs
+and associated cords with their ringing and listening keys and
+clearing-out drop--are referred to as a _cord equipment_. The circuit
+of a complete pair of cords and plugs with their associated apparatus
+is called a _cord circuit_. In order that there may be a number of
+simultaneous connections between different pairs of lines terminating
+in a switchboard, a number of cord circuits are provided, this number
+depending on the amount of traffic at the busiest time of the day.
+
+_Operator's Equipment._ A part of the equipment that is not individual
+to the lines or to the cord circuits, but which may, as occasion
+requires, be associated with any of them is called the _operator's
+equipment_. This consists of the operator's transmitter and receiver,
+induction coil, and battery connections together with the wiring and
+other associated parts necessary to co-ordinate them with the rest of
+the apparatus. Still another part of the equipment that is not
+individual to the lines nor to the cord circuits is the
+calling-current generator. This may be common to the entire office or
+a separate one may be provided for each operator's position.
+
+Operation in Detail. With these general statements in mind we may
+take up in some detail the various operations of a telephone system
+wherein the lines center in a magneto switchboard. This may best be
+done by considering the circuits involved, without special regard to
+the details of the apparatus.
+
+The series of figures showing the cycle of operations of the magneto
+switchboard about to be discussed are typical of this type of
+switchboard almost regardless of make. The apparatus is in each case
+represented symbolically, the representations indicating type rather
+than any particular kind of apparatus within the general class to
+which it belongs.
+
+_Normal Condition of Line._ In Fig. 240 is shown the circuit of an
+ordinary magneto line. The subscriber's sub-station apparatus, shown
+at the left, consists of the ordinary bridging telephone but might
+with equal propriety be indicated as a series telephone. The
+subscriber's station is shown connected with the central office by the
+two limbs of a metallic-circuit line. One limb of the line terminates
+in the spring _1_ of the jack, and the other limb in the sleeve or
+thimble _2_ of the jack. The spring _1_ normally rests on the third
+contact or anvil _3_ in the jack, its construction being such that
+when a plug is inserted this spring will be raised by the plug so as
+to break contact with the anvil _3_. It is understood, of course, that
+the plug associated with this jack has two contacts, referred to
+respectively as the tip and the sleeve; the tip makes contact with the
+tip spring _1_ and the sleeve with the sleeve or thimble _2_.
+
+[Illustration: Fig. 240. Normal Condition of Line]
+
+The drop or line signal is permanently connected between the jack
+sleeve and the anvil _3_. As a result, the drop is normally bridged
+across the circuit of the line so as to be in a receptive condition to
+signaling current sent out by the subscriber. It is evident, however,
+that when the plug is inserted into the jack this connection between
+the line and the drop will be broken.
+
+In this normal condition of the line, therefore, the drop stands
+ready at the central office to receive the signal from the subscriber
+and the generator at the sub-station stands ready to be bridged across
+the circuit of the line as soon as the subscriber turns its handle.
+Similarly the ringer--the call-receiving device at the sub-station--is
+permanently bridged across the line so as to be responsive to any
+signal that may be sent out from the central office in order to call
+the subscriber. The subscriber's talking apparatus is, in this normal
+condition of the line, cut out of the circuit by the switch hook.
+
+_Subscriber Calling._ Fig. 241 shows the condition of the line when
+the subscriber at the sub-station is making a call. In turning his
+generator the two springs which control the connection of the
+generator with the line are brought into engagement with each other so
+that the generator currents may pass out over the line. The condition
+at the central office is the same as that of Fig. 240 except that the
+drop is shown with its shutter fallen so as to indicate a call.
+
+[Illustration: Fig. 241. Subscriber Calling]
+
+[Illustration: A SPECIALLY FORMED CABLE FOR KEY SHELF OF MONARCH
+SWITCHBOARD]
+
+_Operator Answering._ The next step is for the operator to answer the
+call and this is shown in Fig. 242. The subscriber has released the
+handle of his generator and the generator has, therefore, been
+automatically cut out of the circuit. He also has removed his receiver
+from its hook, thus bringing his talking apparatus into the line
+circuit. The operator on the other hand has inserted one of the plugs
+_P__{a} into the jack. This action has resulted in the breaking of the
+circuit through the drop by the raising of the spring _1_ from the
+anvil _3_, and also in the continuance of the line circuit through the
+conductors of the cord circuits. Thus, the upper limb of the line is
+continued by means of the engagement of the tip spring _1_ with the
+tip _4_ of the plug to the conducting strand _6_ of the cord circuit;
+likewise the lower limb of the line is continued by the engagement
+of the thimble _2_ of the jack with the sleeve contact _5_ of the plug
+_P__{a} to the strand _7_ of the cord circuit. The operator has also
+closed her listening key _L.K._ In doing so she has brought the
+springs _8_ and _9_ into engagement with the anvils _10_ and _11_ and
+has thus bridged her head telephone receiver with the secondary of her
+induction coil across the two strands _6_ and _7_ of the cord.
+Associated with the secondary winding of her receiver is a primary
+circuit containing a transmitter, battery, and the primary of the
+induction coil. It will be seen that the conditions are now such as to
+permit the subscriber at the calling station to converse with the
+operator and this conversation consists in the familiar "Number
+Please" on the part of the operator and the response of the subscriber
+giving the number of the line that is desired. Neither the plug
+_P__{c}, nor the ringing key _R.K._, shown in Fig. 242, is used in
+this operation. The clearing-out drop _C.O._ is bridged permanently
+across the strands _6-7_ of the cord, but is without function at this
+time; the fact that it is wound to a high resistance and impedance
+prevents its having a harmful effect on the transmission.
+
+[Illustration: Fig. 242. Operator Answering]
+
+It may be stated at this point that the two plugs of an associated
+pair are commonly referred to as the answering and calling plugs. The
+answering plug is the one which the operator always uses in answering
+a call as just described in connection with Fig. 242. The calling plug
+is the one which she next uses in connecting with the line of the
+called subscriber. It lies idle during the answering of a call and is
+only brought into play after the order of the calling subscriber has
+been given, in which case it is used in establishing connection with
+the called subscriber.
+
+[Illustration: Fig. 243. Operator Calling]
+
+_Operator Calling._ We may now consider how the operator calls the
+called subscriber. The condition existing for this operation is shown
+in Fig. 243. The operator after receiving the order from the calling
+subscriber inserts the calling plug _P__{c} into the jack of the line
+of the called station. This act at once connects the limbs of the line
+with the strands _6_ and _7_ of the cord circuit, and also cuts out the
+line drop of the called station, as already explained. The operator is
+shown in this figure as having opened her listening key _L.K._ and
+closed her ringing key _R.K._ As a result, ringing current from the
+central-office generator will flow out over the two ringing key springs
+_12_ and _13_ to the tip and sleeve contacts of the calling plug
+_P__{c}, then to the tip spring _1_ and the sleeve or thimble _2_ of
+the jack, and then to the two sides of the metallic-circuit line to the
+sub-station and through the bell there. This causes the ringing of the
+called subscriber's bell, after which the operator releases the ringing
+key and thereby allows the two springs _12_ and _13_ of that key to
+again engage their normal contacts _14_ and _15_, thus making the two
+strands _6_ and _7_ of the cord circuit continuous from the contacts of
+the answering plug _P__{a} to the contacts of the calling plug
+_P__{c}. This establishes the condition at the central office for
+conversation between the two subscribers.
+
+[Illustration: Fig. 244. Subscribers Connected for Conversation.]
+
+_Subscribers Conversing._ The only other thing necessary to establish a
+complete set of talking conditions between the two subscribers is for
+the called subscriber to remove his receiver from its hook, which he
+does as soon as he responds to the call. The conditions for
+conversation between the two subscribers are shown in Fig. 244. It is
+seen that the two limbs of the calling line are connected respectively
+to the two limbs of the called line by the two strands of the cord
+circuit, both the operator's receiver and the central-office generator
+being cut out by the listening and ringing keys, respectively. Likewise
+the two line drops are cut out of circuit and the only thing left
+associated with the circuit at the central office is the clearing-out
+drop _C. O._, which remains bridged across the cord circuit. This, like
+the two ringers at the respective connected stations, which also remain
+bridged across the circuit when bridging instruments are used, is of
+such high resistance and impedance that it offers practically no path
+to the rapidly fluctuating voice currents to leak from one side of the
+line circuit to the other. Fluctuating currents generated by the
+transmitter at the calling station, for instance, are converted by
+means of the induction coil into alternating currents flowing in the
+secondary of the induction coil at that station. Considering a
+momentary current as passing up through the secondary winding of the
+induction coil at the calling station, it passes through the receiver
+of that station through the upper limb of the line to the spring _1_ of
+the line jack belonging to that line at the central office; thence
+through the tip _4_ of the answering plug to the conductor _6_ of the
+cord; thence through the pair of contacts _14_ and _12_ forming one
+side of the ringing key to the tip _4_ of the calling plug; thence to
+the tip spring _1_ of the jack of the called subscriber's line; thence
+over the upper limb of his line through his receiver and through the
+secondary of the induction to one of the upper switch-hook contacts;
+thence through the hook lever to the lower side of the line, back to
+the central office and through the sleeve contact _2_ of the jack and
+the sleeve contact _5_ of the plug; thence through the other ringing
+key contacts _13_ and _15_; thence through the strand _7_ of the cord
+to the sleeve contact _5_ and the sleeve contact _2_ of the answering
+plug and jack, respectively; thence through the lower limb of the
+calling subscriber's line to the hook lever at his station; thence
+through one of the upper contacts of this hook to the secondary of the
+induction coil, from which point the current started.
+
+[Illustration: Fig. 245. Clearing-Out Signal]
+
+Obviously, when the called subscriber is talking to the calling
+subscriber the same path is followed. It will be seen that at any time
+the operator may press her listening key _L.K._, bridge her telephone
+set across the circuit of the two connected lines, and listen to the
+conversation or converse with either of the subscribers in case of
+necessity.
+
+_Clearing Out_. At the close of the conversation, either one or both
+of the subscribers may send a clearing-out signal by turning their
+generators after hanging up their receivers. This condition is shown
+in Fig. 245. The apparatus at the central office remains in exactly
+the same position during conversation as that of Fig. 244, except that
+the clearing-out drop shutter is shown as having fallen. The two
+subscribers are shown as having hung up their receivers, thus cutting
+out their talking apparatus, and as operating their generators for the
+purpose of sending the clearing-out signals. In response to this act
+the operator pulls down both the calling and the answering plug, thus
+restoring them to their normal seats, and bringing both lines to the
+normal condition as shown in Fig. 240. The line drops are again
+brought into operative relation with their respective lines so as to
+be receptive to subsequent calls and the calling generators at the
+sub-stations are removed from the bridge circuits across the line by
+the opening of the automatic switch contacts associated with those
+generators.
+
+_Essentials of Operation_. The foregoing sequence of operations while
+described particularly with respect to magneto switchboards is, with
+certain modifications, typical of the operation of nearly all manual
+switchboards. In the more advanced types of manual switchboards,
+certain of the functions described are sometimes done automatically,
+and certain other functions, not necessary in connection with the
+simple switchboard, are added. The essential mode of operation,
+however, remains the same in practically all manual switchboards, and
+for this reason the student should thoroughly familiarize himself with
+the operation and circuits of the simple switchboard as a foundation
+for the more complex and consequently more-difficult-to-understand
+switchboards that will be described later on.
+
+Commercial Types of Drops and Jacks. _Early Drops_. Coming now to
+the commercial types of switchboard apparatus, the first subject that
+presents itself is that of magneto line signals or drops. The very
+early forms of switchboard drops had, in most cases, two-coil magnets,
+the cores of which were connected at their forward ends by an iron
+yoke and the armature of which was pivoted opposite the rear end of
+the two cores. To the armature was attached a latch rod which
+projected forwardly to the front of the device and was there adapted
+to engage the upper edge of the hinged shutter, so as to hold it in
+its raised or undisplayed position when the armature was unattracted.
+Such a drop, of Western Electric manufacture, is shown in Fig. 246.
+
+[Illustration: Fig. 246 Old-Style Drop]
+
+Liability to Cross-Talk:--This type of drop is suitable for use only
+on small switchboards where space is not an important consideration,
+and even then only when the drop is entirely cut out of the circuit
+during conversation. The reason for this latter requirement will be
+obvious when it is considered that there is no magnetic shield around
+the winding of the magnet and no means for preventing the stray field
+set up by the talking currents in one of the magnets from affecting by
+induction the windings of adjacent magnets contained in other talking
+circuits. Unless the drops are entirely cut out of the talking
+circuit, therefore, they are very likely to produce cross-talk between
+adjacent circuits. Furthermore, such form of drop is obviously not
+economical of space, two coils placed side by side consuming
+practically twice as much room as in the case of later drops wherein
+single magnet coils have been made to answer the purpose.
+
+_Tubular Drops._ In the case of line drops, which usually can readily
+be cut out of the circuit during conversation, this cross-talk feature
+is not serious, but sometimes the line drops, and always the
+clearing-out drops must be left in connection with the talking circuit.
+On account of economy in space and also on account of this cross-talk
+feature, there has come into existence the so-called tubular or
+iron-clad drop, one of which is shown in section in Fig. 247. This was
+developed a good many years ago by Mr. E.P. Warner of the Western
+Electric Company, and has since, with modifications, become standard
+with practically all the manufacturing companies. In this there is but
+a single bobbin, and this is enclosed in a shell of soft Norway iron,
+which is closed at its front end and joined to the end of the core as
+indicated, so as to form a complete return magnetic path for the lines
+of force generated in the coil. The rear end of the shell and core are
+both cut off in the same plane and the armature is made in such form as
+to practically close this end of the shell. The armature carries a
+latch rod extending the entire length of the shell to the front portion
+of the structure, where it engages the upper edge of the pivoted
+shutter; this, when released by the latch upon the attraction of the
+armature, falls so as to display a target behind it.
+
+[Illustration: Fig. 247. Tubular Drop]
+
+[Illustration: Fig. 248. Strip of Tubular Drops]
+
+These drops may be mounted individually on the face of the
+switchboard, but it is more usual to mount them in strips of five or
+ten. A strip of five drops, as manufactured by the Kellogg Switchboard
+and Supply Company, is shown in Fig. 248. The front strip on which
+these drops are mounted is usually of brass or steel, copper plated,
+and is sufficiently heavy to provide a rigid support for the entire
+group of drops that are mounted on it. This construction greatly
+facilitates the assembling of the switchboard and also serves to
+economize space--obviously, the thing to economize on the face of a
+switchboard is space as defined by vertical and horizontal dimensions.
+These tubular drops, having but one coil, are readily mounted on
+1-inch centers, both vertically and horizontally. Sometimes even
+smaller dimensions than this are secured. The greatest advantage of
+this form of construction, however, is in the absolute freedom from
+cross-talk between two adjacent drops. So completely is the magnetic
+field of force kept within the material of the shell, that there is
+practically no stray field and two such drops may be included in two
+different talking circuits and the drops mounted immediately adjacent
+to each other without producing any cross-talk whatever.
+
+_Night Alarm._ Switchboard drops in falling make but little noise, and
+during the day time, while the operator is supposed to be needed
+continually at the board, the visual signal which they display is
+sufficient to attract her attention. In small exchanges, however, it
+is frequently not practicable to keep an operator at the switchboard
+at night or during other comparatively idle periods, and yet calls
+that do arrive during such periods must be attended to. For this
+reason some other than a visual signal is necessary, and this need is
+met by the so-called night-alarm attachment. This is merely an
+arrangement by which the shutter in falling closes a pair of contacts
+and thus completes the circuit of an ordinary vibrating bell or buzzer
+which will sound until the shutter is restored to its normal position.
+Such contacts are shown in Fig. 249 at _1_ and _2_. Night-alarm
+contacts have assumed a variety of forms, some of which will be
+referred to in the discussion of other types of drops and jacks.
+
+[Illustration: Fig. 249. Drop with Night-Alarm Contacts]
+
+_Jack Mounting._ Jacks, like drops, though frequently individually
+mounted are more often mounted in strips. An individually mounted jack
+is shown in Fig. 250, and a strip of ten jacks in Fig. 251. In such a
+strip of jacks, the strips supporting the metallic parts of the
+various jacks are usually of hard rubber reinforced by brass so as to
+give sufficient strength. Various forms of supports for these strips
+are used by different manufacturers, the means for fastening them in
+the switchboard frame usually consisting of brass lugs on the end of
+the jack strip adapted to be engaged by screws entering the stationary
+portion of the iron framework; or sometimes pins are fixed in the
+framework, and the jack is held in place by nuts engaging
+screw-threaded ends on such pins.
+
+[Illustration: Fig. 250. Individual Jack]
+
+[Illustration: Fig. 251. Strip of Jacks]
+
+_Methods of Associating Jacks and Drops._ There are two general
+methods of arranging the drops and jacks in a switchboard. One of
+these is to place all of the jacks in a group together at the lower
+portion of the panel in front of the operator and all of the drops
+together in another group above the group of jacks. The other way is
+to locate each jack in immediate proximity to the drop belonging to
+the same line so that the operator's attention will always be called
+immediately to the jack into which she must insert her plug in
+response to the display of a drop. This latter practice has several
+advantages over the former. Where the drops are all mounted in one
+group and the jacks in another, an operator seeing a drop fall must
+make mental note of it and pick out the corresponding jack in the
+group of jacks. On the other hand, where the jacks and drops are
+mounted immediately adjacent to each other, the falling of a drop
+attracts the attention of the operator to the corresponding jack
+without further mental effort on her part.
+
+The immediate association of the drops and jacks has another
+advantage--it makes possible such a mechanical relation between the
+drop and its associated jack that the act of inserting the plug into
+the jack in making the connection will automatically and mechanically
+restore the drop to its raised position. Such drops are termed
+_self-restoring drops_, and, since a drop and jack are often made
+structurally a unitary piece of apparatus, they are frequently called
+_combined_ drops and jacks.
+
+_Manual vs. Automatic Restoration._. There has been much difference of
+opinion on the question of manual versus automatic restoration of
+drops. Some have contended that there is no advantage in having the
+drops restored automatically, claiming that the operator has plenty of
+time to restore the drops by hand while receiving the order from the
+calling subscriber or performing some of her other work. Those who
+think this way have claimed that the only place where an automatically
+restored drop is really desirable is where, on account of the lack of
+space on the front of the switchboard, the drops are placed on such a
+portion of the board as to be not readily reached by the operator.
+This resulted in the electrically restored drop, mention of which will
+be made later.
+
+Others have contended that even though the drop is mounted within easy
+reach of the operator, it is advantageous that the operator should be
+relieved of the burden of restoring it, claiming that even though
+there are times in the regular performance of the operator's duties
+when she may without interfering with other work restore the drops
+manually, such requirement results in a double use of her attention
+and in a useless strain on her which might better be devoted to the
+actual making of connections.
+
+Until recently the various Bell operating companies have adhered, in
+their small exchange work, to the manual restoring method, while most
+of the so-called independent operating companies have adhered to the
+automatic self-restoring drops.
+
+Methods of Automatic Restoration. Two general methods present
+themselves for bringing about the automatic restoration of the drop.
+First, the mechanical method, which is accomplished by having some
+moving part of the jack or of the plug as it enters the jack force the
+drop mechanically into its restored position. This usually means the
+mounting of the drop and the corresponding jack in juxtaposition, and
+this, in turn, has usually resulted in the unitary structure
+containing both the drop and the jack. Second, the electrical method
+wherein the plug in entering the jack controls a restoring circuit,
+which includes a battery or other source of energy and a restoring
+coil on the drop, the result being that the insertion of the plug into
+the jack closes this auxiliary circuit and thus energizes the
+restoring magnet, the armature of which pulls the shutter back into
+its restored position. This practice has been followed by Bell
+operating companies whenever conditions require the drop to be mounted
+out of easy reach of the operator; not otherwise.
+
+_Mechanical--Direct Contact with Plug._ One widely used method of
+mechanical restoration of drops, once employed by the Western
+Telephone Construction Company with considerable success, was to hang
+the shutter in such position that it would fall immediately in front
+of the jack so that the operator in order to reach the jack with the
+plug would have to push the plug directly against the shutter and thus
+restore it to its normal or raised position. In this construction the
+coil of the drop magnet was mounted directly behind the jack, the
+latch rod controlled by the armature reaching forward, parallel with
+the jack, to the shutter, which, as stated, was hung in front of the
+jack. This resulted in a most compact arrangement so far as the space
+utilization on the front of the board was concerned and such combined
+drops and jacks were mounted on about 1-inch centers, so that a bank
+of one hundred combined drops and jacks occupied a space only a little
+over 10 inches square.
+
+A modification of this scheme, as used by the American Electric
+Telephone Company, was to mount the drop immediately over the jack so
+that its shutter, when down, occupied a position almost in front of,
+but above, the jack opening. The plug was provided with a collar,
+which, as it entered the jack, engaged a cam on the base of the
+shutter and forced the latter mechanically into its raised position.
+
+Neither of these methods of restoring--_i.e._, by direct contact
+between the shutter or part of it and the plug or part of it--is now
+as widely used as formerly. It has been found that there is no real
+need in magneto switchboards for the very great compactness which the
+hanging of the shutter directly in front of the drop resulted in, and
+the tendency in later years has been to make the combined drops and
+jacks more substantial in construction at the expense of some space on
+the face of the switchboard.
+
+[Illustration: Fig. 252. Kellogg Drop and Jack]
+
+Kellogg Type:--A very widely used scheme of mechanical restoration is
+that employed in the Miller drop and jack manufactured by the Kellogg
+Switchboard and Supply Company, the principles of which may be
+understood in connection with Fig. 252. In this figure views of one of
+these combined drops and jacks in three different positions are shown.
+The jack is composed of the framework _B_ and the hollow screw _A_,
+the latter forming the sleeve or thimble of the jack and being
+externally screw-threaded so as to engage and bind in place the front
+end of the framework _B_. The jack is mounted on the lower part of the
+brass mounting strip _C_ but insulated therefrom. The tip spring of
+the jack is bent down as usual to engage the tip of the plug, as
+better shown in the lower cut of Fig. 252, and then continues in an
+extension _D_, which passes through a hole in the mounting plate _C_.
+This tip spring in its normal position rests against another spring as
+shown, which latter spring forms one terminal of the drop winding.
+
+The drop or annunciator is of tubular form, and the shutter is so
+arranged on the front of the mounting strip _C_ as to fall directly
+above the extension _D_ of the tip spring. As a result, when the plug
+is inserted into the jack, the upward motion of the tip spring forces
+the drop into its restored position, as indicated in the lower cut of
+the figure. These drops and jacks are usually mounted in banks of
+five, as shown in Fig. 253.
+
+[Illustration: Fig. 253. Strip of Kellogg Drops and Jacks]
+
+Western Electric Type:--The combined drop and jack of the Western
+Electric Company recently put on the market to meet the demands of the
+independent trade, differs from others principally in that it employs
+a spherical drop or target instead of the ordinary flat shutter. This
+piece of apparatus is shown in its three possible positions in Fig.
+254. The shutter or target normally displays a black surface through a
+hole in the mounting plate. The sphere forming the target is out of
+balance, and when the latch is withdrawn from it by the action of the
+electromagnet it falls into the position shown in the middle cut of
+Fig. 254, thus displaying a red instead of a black surface to the view
+of the operator. When the operator plugs in, the plug engages the
+lower part of an =S=-shaped lever which acts on the pivoted sphere to
+restore it to its normal position. A perspective view of one of these
+combined line signals and jacks is shown in Fig. 255.
+
+A feature that is made much of in recently designed drops and jacks
+for magneto service is that which provides for the ready removal of the
+drop coil, from the rest of the structure, for repair. The drop and
+jack of the Western Electric Company, just described, embodies this
+feature, a single screw being so arranged that its removal will permit
+the withdrawal of the coil without disturbing any of the other parts or
+connections. The coil windings terminate in two projections on the
+front head of the spool, and these register with spring clips on the
+inside of the shell so that the proper connections for the coil are
+automatically made by the mere insertion of the coil into the shell.
+
+[Illustration: Fig. 254. Western Electric Drop and Jack]
+
+[Illustration: Fig. 255. Western Electric Drop and Jack]
+
+Dean Type:--The combined drop and jack of the Dean Electric Company is
+illustrated in Figs. 256 and 257. The two perspective views show the
+general features of the drop and jack and the method by which the
+magnet coil may be withdrawn from the shell. As will be seen the
+magnet is wound on a hollow core which slides over the iron core, the
+latter remaining permanently fixed in the shell, even though the coil
+be withdrawn.
+
+Fig. 258 shows the structural details of the jack employed in this
+combination and it will be seen that the restoring spring for the drop
+is not the tip spring itself, but another spring located above and
+insulated from it and mechanically connected therewith.
+
+[Illustration: Fig. 256. Dean Drop and Jack]
+
+[Illustration: Fig. 257. Dean Drop and Jack]
+
+[Illustration: Fig. 258. Details of Dean Jack]
+
+Monarch Type:--Still another combined drop and jack is that of the
+Monarch Telephone Manufacturing Company of Chicago, shown in sectional
+view in Fig. 259. This differs from the usual type in that the
+armature is mounted on the front end of the electromagnet, its latch
+arm retaining the shutter in its normal position when raised, and
+releasing it when depressed by the attraction of the armature. As is
+shown, there is within the core of the magnet an adjustable spiral
+spring which presses forward against the armature and which spring is
+compressed by the attraction of the armature of the magnet. The
+night-alarm contact is clearly shown immediately below the strip which
+supports the drop, this consisting of a spring adapted to be engaged
+by a lug on the shutter and pressed upwardly against a stationary
+contact when the shutter falls. The method of restoration of the
+shutter in this case is by means of an auxiliary spring bent up so as
+to engage the shutter and restore it when the spring is raised by the
+insertion of a plug into the jack.
+
+[Illustration: Fig. 259. Monarch Drop and Jack]
+
+_Code Signaling._ On bridging party lines, where the subscribers
+sometimes call other subscribers on the same line and sometimes call
+the switchboard so as to obtain a connection with another line, it is
+not always easy for the operator at the switchboard to distinguish
+whether the call is for her or for some other party on the line. On
+such lines, of course, code ringing is used and in most cases the
+operator's only way of distinguishing between calls for her and those
+for some sub-station parties on the line is by listening to the
+rattling noise which the drop armature makes. In the case of the
+Monarch drop the adjustable spring tension on the armature is intended
+to provide for such an adjustment as will permit the armature to give
+a satisfactory buzz in response to the alternating ringing currents,
+whether the line be long or short.
+
+[Illustration: Fig. 260. Code Signal Attachment]
+
+The Monarch Company provides in another way for code signaling at the
+switchboard. In some cases there is a special attachment, shown in
+Fig. 260, by means of which the code signals are repeated on the
+night-alarm bell. This is in the nature of a special attachment
+placed on the drop, which consists of a light, flat spring attached to
+the armature and forming one side of a local circuit. The other side
+of the circuit terminates in a fixture which is mounted on the drop
+frame and is provided with a screw, having a platinum point forming
+the other contact point; this allows of considerable adjustment. At
+the point where the screw comes in contact with the spring there is a
+platinum rivet. When an operator is not always in attendance, this
+code-signaling attachment has some advantages over the drop as a
+signal interpreter, in that it permits the code signals to be heard
+from a distance. Of course, the addition of spring contacts to the
+drop armature tends to complicate the structure and perhaps to cut
+down the sensitiveness of the drop, which are offsetting
+disadvantages.
+
+[Illustration: Fig. 261. Combined Drop and Ringer]
+
+For really long lines, this code signaling by means of the drop is
+best provided for by employing a combined drop and ringer, although in
+this case whatever advantages are secured by the mechanical
+restoration of the shutter upon plugging in are lost. Such a device as
+manufactured by the Dean Electric Company is shown in Fig. 261. In
+this the ordinary polarized ringer is used, but in addition the tapper
+rod carries a latch which, when vibrated by the ringing of the bell,
+releases a shutter and causes it to fall, thus giving a visual as well
+as an audible signal.
+
+_Electrical_. Coming now to the electrical restoration of drop
+shutters, reference is made to Fig. 262, which shows in side section
+the electrical restoring drop employed by the Bell companies and
+manufactured by the Western Electric Company. In this the coil _1_ is
+a line coil, and it operates on the armature _2_ to raise the latch
+lever _3_ in just the same manner as in the ordinary tubular drop.
+The latch lever _3_ acts, however, to release another armature _4_
+instead of a shutter. This armature _4_ is pivoted at its lower end at
+the opposite end of the device from the armature _2_ and, by falling
+outwardly when released, it serves to raise the light shutter _5_. The
+restoring coil of this device is shown at _6_, and when energized it
+attracts the armature _4_ so as to pull it back under the catch of the
+latch lever _3_ and also so as to allow the shutter _5_ to fall into
+its normal position. The method of closing the restoring circuit is by
+placing coil _6_ in circuit with a local battery and with a pair of
+contacts in the jack, which latter contacts are normally open but are
+bridged across by the plug when it enters the jack, thus energizing
+the restoring coil and restoring the shutter.
+
+[Illustration: Fig. 262. Electrically Restored Drop]
+
+A perspective view of this Western Electric electrical restoring drop
+is shown in Fig. 263, a more complete mention being made of this
+feature under the discussion of magneto multiple switchboards, wherein
+it found its chief use. It is mentioned here to round out the methods
+that have been employed for accomplishing the automatic restoration of
+shutters by the insertion of the plug.
+
+[Illustration: Fig. 263. Electrically Restored Drop]
+
+Switchboard Plugs. A switchboard plug such as is commonly used in
+simple magneto switchboards is shown in Fig. 264 and also in Fig. 235.
+The tip contact is usually of brass and is connected to a slender
+steel rod which runs through the center of the plug and terminates
+near the rear end of the plug in a connector for the tip conductor of
+the cord. This central core of steel is carefully insulated from the
+outer shell of the plug by means of hard rubber bushings, the parts
+being forced tightly together. The outer shell, of course, forms the
+other conductor of the plug, called the sleeve contact. A handle of
+tough fiber tubing is fitted over the rear end of the plug and this
+also serves to close the opening formed by cutting away a portion of
+the plug shell, thus exposing the connector for the tip conductor.
+
+[Illustration: Fig. 264. Switchboard Plug]
+
+_Cord Attachment._ The rear end of the plug shell is usually bored out
+just about the size of the outer covering of the switchboard cord, and
+it is provided with a coarse internal screw thread, as shown. The cord
+is attached by screwing it tightly into this screw-threaded chamber,
+the screw threads in the brass being sufficiently coarse and of
+sufficiently small internal diameter to afford a very secure
+mechanical connection between the outer braiding of the cord and the
+plug. The connection between the tip conductor of the cord and the tip
+of the plug is made by a small machine screw connection as shown,
+while the connection between the sleeve conductor of the plug and the
+sleeve conductor of the cord is made by bending back the latter over
+the outer braiding of the cord before it is screwed into the shank of
+the plug. This results in the close electrical contact between the
+sleeve conductor of the cord and the inner metal surface of the shank
+of the plug.
+
+Switchboard Cords. A great deal of ingenuity has been exerted toward
+the end of producing a reliable and durable switchboard cord. While
+great improvement has resulted, the fact remains that the cords of
+manual switchboards are today probably the most troublesome element,
+and they need constant attention and repairs. While no two
+manufacturers build their cords exactly alike, descriptions of a few
+commonly used and successful cords may be here given.
+
+_Concentric Conductors._ In one the core is made from a double strand
+of strong lock stitch twine, over which is placed a linen braid. Then
+the tip conductor, which is of stranded copper tinsel, is braided on.
+This is then covered with two layers of tussah silk, laid in reverse
+wrappings, then there is a heavy cotton braid, and over the latter a
+linen braid. The sleeve conductor, which is also of copper tinsel, is
+then braided over the structure so formed, after which two reverse
+wrappings of tussah silk are served on, and this is covered by a cotton
+braid and this in turn by a heavy linen or polished cotton braid. The
+plug end of the cord is reinforced for a length of from 12 to 18 inches
+by another braiding of linen or polished cotton, and the whole cord is
+treated with melted beeswax to make it moisture-proof and durable.
+
+[Illustration: Fig. 265. Switchboard Cord]
+
+_Steel Spiral Conductors._ In another cord that has found much favor
+the two conductors are formed mainly by two concentric spiral
+wrappings of steel wire, the conductivity being reinforced by adjacent
+braidings of tinsel. The structure of such a cord is well shown in
+Fig. 265. Beginning at the right, the different elements shown are, in
+the order named, a strand of lock stitch twine, a linen braiding, into
+the strands of which are intermingled tinsel strands, the inner spiral
+steel wrapping, a braiding of tussah silk, a linen braiding, a loose
+tinsel braiding, the outer conductor of round spiral steel, a cotton
+braid, and an outside linen or polished cotton braid. The inner tinsel
+braiding and the inner spiral together form the tip conductor while
+the outer braiding and spiral together form the sleeve conductor. The
+cord is reinforced at the plug end for a length of about 14 inches by
+another braiding of linen. The tinsel used is, in each case, for the
+purpose of cutting down the resistance of the main steel conductor.
+These wrappings of steel wire forming the tip and sleeve conductors
+respectively, have the advantage of affording great flexibility, and
+also of making it certain that whatever strain the cord is subjected
+to will fall on the insulated braiding rather than on the spiral
+steel which has in itself no power to resist tensile strains.
+
+_Parallel Tinsel Conductors._ Another standard two-conductor
+switchboard cord is manufactured as follows: One conductor is of very
+heavy copper tinsel insulated with one wrapping of sea island cotton,
+which prevents broken ends of the tinsel or knots from piercing
+through and short-circuiting with the other conductor. Over this is
+placed one braid of tussah silk and an outer braid of cotton. This
+combines high insulation with considerable strength. The other
+conductor is of copper tinsel, not insulated, and this is laid
+parallel to the thrice insulated conductor already described. Around
+these two conductors is placed an armor of spring brass wire in spiral
+form, and over this a close, stout braid of glazed cotton. This like
+the others is reinforced by an extra braid at the plug end.
+
+Ringing and Listening Keys. The general principles of the ringing
+key have already been referred to. Ringing keys are of two general
+types, one having horizontal springs and the other vertical.
+
+[Illustration: Fig. 266. Horizontal-Spring Listening and Ringing Key]
+
+_Horizontal Spring Type._ Various Bell operating companies have
+generally adhered to the horizontal spring type except in individual
+and four-party-line keys. The construction of a Western Electric
+Company horizontal spring key is shown in Fig. 266. In this particular
+key, as illustrated, there are two cam levers operating upon three
+sets of springs. The cam lever at the left operates the ordinary
+ringing and listening set of springs according to whether it is pushed
+one way or the other. In ringing on single-party lines the cam lever
+at the left is the one to be used; while on two-party lines the lever
+at the left serves to ring the first party and the ringing key at the
+right the second party.
+
+In order that the operator may have an indication as to which station
+on a two-party line she has called, a small target _1_ carried on a
+lever _2_ is provided. This target may display a black or a white
+field, according to which of its positions it occupies. The lever _2_
+is connected by the links _3_ and _4_ with the two key levers and the
+target is thus moved into one position or the other, according to
+which lever was last thrown into ringing position.
+
+It will be noticed that the springs are mounted horizontally and on
+edge. This on-edge feature has the advantage of permitting ready
+inspection of the contacts and of avoiding the liability of dust
+gathering between the contacts. As will be seen, at the lower end of
+each switch lever there is a roller of insulating material which
+serves as a wedge, when forced between the two long springs of any
+set, to force them apart and into engagement with their respective
+outer springs.
+
+[Illustration: Fig. 267. Vertical-Spring Listening and Ringing Key]
+
+_Vertical Spring Type._ The other type of ringing and listening key
+employing vertical springs is almost universally used by the various
+independent manufacturing companies. A good example of this is shown
+in Fig. 267, which shows partly in elevation and partly in section a
+double key of the Monarch Company. The operation of this is obvious
+from its mode of construction. The right-hand set of springs of the
+right-hand key in this cut are the springs of the listening key, while
+the left-hand set of the right-hand key are those of the calling-plug
+ringing key. The left-hand set of the left-hand key may be those of a
+ring-back key on the answering plug, while the right-hand set of the
+left-hand key may be for any special purpose. It is obvious that these
+groups of springs may be grouped in different combinations or omitted
+in part, as required. This same general form of key is also
+manufactured by the Kellogg Company and the Dean Company, that of the
+Kellogg Company being illustrated in perspective, Fig. 268. The keys
+of this general type have the same advantages as those of the
+horizontal on-edge arrangement with respect to the gathering of dust,
+and while perhaps the contacts are not so readily get-at-able for
+inspection, yet they have the advantage of being somewhat more simple,
+and of taking up less horizontal space on the key shelf.
+
+[Illustration: Fig. 268. Vertical Listening and Ringing Key]
+
+[Illustration: Fig. 269. Four-Party Listening and Ringing Key]
+
+_Party-Line Ringing Keys._ For party-line ringing the key matter
+becomes somewhat more complicated. Usually the arrangement is such
+that in connection with each calling plug there are a number of keys,
+each arranged with respect to the circuits of the plug so as to send
+out the proper combination and direction of current, if the polarity
+system is used; or the proper frequency of current if the harmonic
+system is used; or the proper number of impulses if the step-by-step
+or broken-line system is used. The number of different kinds of
+arrangements and combinations is legion, and we will here illustrate
+only an example of a four-party line ringing key adapted for harmonic
+ringing. A Kellogg party-line listening and ringing key is shown in
+Fig. 269. In this, besides the regular listening key, are shown four
+push-button keys, each adapted, when depressed, to break the
+connection back of the key, and at the same time connect the proper
+calling generator with the calling plug.
+
+_Self-Indicating Keys._ A complication that has given a good deal of
+trouble in the matter of party-line ringing is due to the fact that it
+is sometimes necessary to ring a second or a third time on a
+party-line connection, because the party called may not respond the
+first time. The operator is not always able to remember which one of
+the four keys associated with the plug connected with the desired
+party she has pressed on the first occasion and, therefore, when it
+becomes necessary to ring again, she may ring the wrong party. This is
+provided for in a very ingenious way in the key shown in Fig. 269, by
+making the arrangement such that after a given key has been depressed
+to its full extent in ringing, and then released, it does not come
+quite back to its normal position but remains slightly depressed. This
+always serves as an indication to the operator, therefore, as to which
+key she depressed last, and in the case of a re-ring, she merely
+presses the key that is already down a little way. On the next call if
+she is required to press another one of the four keys, the one which
+remained down a slight distance on the last call will be released and
+the one that is fully depressed will be the one that remains down as
+an indication.
+
+Such keys, where the key that was last used leaves an indication to
+that effect, are called _indicating_ ringing keys. In other forms the
+indication is given by causing the key lever to move a little target
+which remains exposed until some other key in the same set is moved.
+The key shown in Fig. 266 is an example of this type.
+
+     NOTE. The matter of automatic ringing and other special forms of
+     ringing will be referred to and discussed at their proper places
+     in this work, but at this point they are not pertinent as they
+     are not employed in simple switchboards.
+
+Operator's Telephone Equipment. Little need be said concerning the
+matter of the operator's talking apparatus, _i.e._, the operator's
+transmitter and receiver, since as transmitters and receivers they are
+practically the same as those in ordinary use for other purposes. The
+watch-case receiver is nearly always employed for operators' purposes
+on account of its lightness and compactness. It is used in connection
+with a head band so as to be held continually at the operator's ear,
+allowing both of her hands to be free.
+
+The transmitter used by operators does not in itself differ from the
+transmitters employed by subscribers, but the methods by which it is
+supported differ, two general practices being followed. One of these
+is to suspend the transmitter by flexible conducting cords so as to be
+adjustable in a vertical direction. A good illustration of this is
+given in Fig. 270. The other method, and one that is coming into more
+and more favor, is to mount the transmitter on a light bracket
+suspended by a flexible band from the neck of the operator, a breast
+plate being furnished so that the transmitter will rest on her breast
+and be at all times within proper position to receive her speech. To
+facilitate this, a long curved mouthpiece is commonly employed, as
+shown clearly in Fig. 47.
+
+[Illustration: Fig. 270. Operator's Transmitter Suspension]
+
+_Cut-in Jack._ It is common to terminate that portion of the apparatus
+which is worn on the operator's person--that is, the receiver only if
+the suspended type of transmitter is employed, and the receiver and
+transmitter if the breast plate type of transmitter is employed--in a
+plug, and a flexible cord connecting the plug terminates with the
+apparatus. The portions of the operator's talking circuit that are
+located permanently in the switchboard cabinet are in such cases
+terminated in a jack, called an operator's _cut-in jack_. This is
+usually mounted on the front rail of the switchboard cabinet just
+below the key shelf. Such a cut-in jack is shown in Fig. 271 and it is
+merely a specialized form of spring jack adapted to receive the short,
+stout plug in which the operator's transmitter, or transmitter and
+receiver, terminate. By this arrangement the operator is enabled
+readily to connect or disconnect her talking apparatus, which is worn
+on her person, whenever she comes to the board for work or leaves it
+at the end of her work. A complete operator's telephone set, or that
+portion that is carried on the person of the operator, together with
+the cut-in plug, is shown in Fig. 272.
+
+[Illustration: Fig. 271. Operator's Cut-in Jack]
+
+[Illustration: Fig. 272. Operator's Talking Set]
+
+Circuits of Complete Switchboard. We may now discuss the circuits of
+a complete simple magneto switchboard. The one shown in Fig. 273 is
+typical. Before going into the details of this, it is well to inform
+the student that this general form of circuit representation is one
+that is commonly employed in showing the complete circuits of any
+switchboard. Ordinarily two subscribers' lines are shown, these
+connecting their respective subscribers' stations with two different
+line equipments at the central office. The jacks and signals of these
+line equipments are turned around so as to face each other, in order
+to clearly represent how the connection between them may be made by
+means of the cord circuit. The elements of the cord circuit are also
+spread out, so that the various parts occupy relative positions which
+they do not assume at all in practice. In other words it must be
+remembered that, in circuit diagrams, the relative positions of the
+parts are sacrificed in order to make clear the circuit connections.
+However, this does not mean that it is often not possible to so locate
+the pieces of apparatus that they will in a certain way indicate
+relative positions, as may be seen in the case of the drop and jack in
+Fig. 273, the drop being shown immediately above the jack, which is
+the position in which these parts are located in practice.
+
+[Illustration: Fig. 273. Circuit of Simple Magneto Switchboard]
+
+Little need be said concerning this circuit in view of what has
+already been said in connection with Figs. 240 to 245. It will be seen
+in the particular sub-station circuit here represented, that the
+talking apparatus is arranged in the usual manner and that the ringer
+and generator are so arranged that when the generator is operated the
+ringer will be cut out of circuit, while the generator will be placed
+across the circuit; while, when the generator is idle, the ringer is
+bridged across the circuit and the generator is cut out.
+
+The line terminates in each case in the tip and sleeve contacts of the
+jack, and in the normal condition of the jack the line drop is bridged
+across the line. The arrangement by which the drop is restored and at
+the same time cut out of circuit when the operator plugs in the jack,
+is obvious from the diagrammatic illustration. The cord circuit is the
+same as that already discussed, with the exception that two ringing
+keys are provided, one in connection with the calling plug, as is
+universal practice, and the other in connection with the answering plug
+as is sometimes practiced in order that the operator may, when occasion
+requires, ring back the calling subscriber without the necessity of
+changing the plug in the jack. The outer contacts of these two ringing
+keys are connected to the terminals of the ringing generator and, when
+either key is operated, the connection between the plug, on which the
+ringing is to be done, and the rest of the cord circuit will be broken,
+while the generator will be connected with the terminals of the plug.
+The listening key and talking apparatus need no further explanation, it
+being obvious that when the key is operated the subscriber's telephone
+set will be bridged across the cord circuit and, therefore, connected
+with either or both of the talking subscribers.
+
+[Illustration: Fig. 274. Night-Alarm Circuit]
+
+Night-Alarm Circuits. The circuit of Fig. 273, while referred to as
+a complete circuit, is not quite that. The night-alarm circuit is not
+shown. In order to clearly indicate how a single battery and bell, or
+buzzer, may serve in connecting a number of line drops, reference is
+made to Fig. 274 which shows the connection between three different
+line drops and the night-alarm circuit. The night-alarm apparatus
+consists in the battery _1_ and the buzzer, or bell, _2_. A switch _3_
+adapted to be manually operated is connected in the circuit with the
+battery and the buzzer so as to open this circuit when the night alarm
+is not needed, thus making it inoperative. During the portions of the
+day when the operator is needed constantly at the board it is
+customary to leave this switch _3_ open, but during the night period
+when she is not required constantly at the board this switch is closed
+so that an audible signal will be given whenever a drop falls. The
+night-alarm contact _4_ on each of the drops will be closed whenever a
+shutter falls, and as the two members of this contact, in the case of
+each drop, are connected respectively with the two sides of the
+night-alarm circuit, any one shutter falling will complete the
+necessary conditions for causing the buzzer to sound, assuming of
+course that the switch _3_ is closed.
+
+_Night Alarm with Relay._ A good deal of trouble has been caused in
+the past by uncertainty in the closure of the night-alarm circuit at
+the drop contact. Some of the companies have employed the form of
+circuit shown in Fig. 275 to overcome this. Instead of the night-alarm
+buzzer being placed directly in the circuit that is closed by the
+drop, a relay _5_ and a high-voltage battery _6_ are placed in this
+circuit. The buzzer and the battery for operating it are placed in a
+local circuit controlled by this relay. It will be seen by reference
+to Fig. 275 that when the shutter falls, it will, by closing the
+contact _4_, complete the circuit from the battery _6_ through the
+relay _5_--assuming switch _3_ to be closed--and thus cause the
+operation of the relay. The relay, in turn, by pulling up its
+armature, will close the circuit of the buzzer _2_ through the battery
+_7_ and cause the buzzer to sound.
+
+[Illustration: Fig. 275. Night-Alarm Circuit with Relay]
+
+The advantage of this method over the direct method of operating the
+buzzer is that any imperfection in the night-alarm contact at the drop
+is much less likely to prevent the flow of current of the high-voltage
+battery _6_ than of the low-voltage battery _1_, shown in connection
+with Fig. 274. This is because the higher voltage is much more likely
+to break down any very thin bit of insulation, such as might be caused
+by a minute particle of dust or oxide between contacts that are
+supposed to be closed by the falling of the shutter. It has been
+common to employ for battery _6_ a dry-cell battery giving about 20
+or 24 volts, and for the operation of the buzzer itself, a similar
+battery of about two cells giving approximately 3 volts.
+
+_Night-Alarm Contacts._ The night-alarm contact _4_ of the drop shown
+diagrammatically in Figs. 274 and 275 would, if taken literally,
+indicate that the shutter itself actually forms one terminal of the
+circuit and the contact against which it falls, the other. This has
+not been found to be a reliable way of closing the night-alarm
+contacts and this method is indicated in these figures and in other
+figures in this work merely as a convenient way of representing the
+matter diagrammatically. As a matter of fact the night-alarm contacts
+are ordinarily closed by having the shutter fall against one spring,
+which is thereby pressed into engagement with another spring or
+contact, as shown in Fig. 249. This method employs the shutter only as
+a means for mechanically causing the one spring to press against the
+other, the shutter itself forming no part of the circuit. The reason
+why it is not a good plan to have the shutter itself act as one
+terminal of the circuit is that this necessitates the circuit
+connections being led to the shutter through the trunnions on which
+the shutter is pivoted. This is bad because, obviously, the shutter
+must be loosely supported on its trunnions in order to give it
+sufficiently free movement, and, as is well known, loose connections
+are not conducive to good electrical contacts.
+
+Grounded-and Metallic-Circuit Lines. When grounded circuits were the
+rule rather than the exception, many of the switchboards were
+particularly adapted for their use and could not be used with
+metallic-circuit lines. These grounded-circuit switchboards provided
+but a single contact in the jack and a single contact on the plug, the
+cords having but a single strand reaching from one plug to the other.
+The ringing keys and listening keys were likewise single-contact keys
+rather than double. The clearing-out drop and the operator's talking
+circuit and the ringing generator were connected between the single
+strand of the cord and the ground as was required.
+
+The grounded-circuit switchboard has practically passed out of
+existence, and while a few of them may be in use, they are not
+manufactured at present. The reason for this is that while many
+grounded circuits are still in use, there are very few places where
+there are not some metallic-circuit lines, and while the
+grounded-circuit switchboard will not serve for metallic-circuit
+lines, the metallic-circuit switchboard will serve equally well for
+either metallic-circuit or grounded lines, and will interconnect them
+with equal facility. This fact will be made clear by a consideration
+of Figs. 276, 277, and 278.
+
+[Illustration: Fig. 276. Connection Between Metallic Lines]
+
+[Illustration: Fig. 277. Connection Between Grounded Lines]
+
+_Connection between Two Similar Lines._ In Fig. 276 a common magneto
+cord circuit is shown connecting two metallic-circuit lines; in Fig.
+277 the same cord circuit is shown connecting two grounded lines. In
+this case the line wire _1_ of the left-hand line is, when the plugs
+are inserted, continued to the tip of the answering plug, thence
+through the tip strand of the cord circuit to the tip of the calling
+plug, then to the tip spring of the right-hand jack and out to the
+single conductor of that line. The entire sleeve portion of the cord
+circuit becomes grounded as soon as the plugs are inserted in the
+jacks of such a line. Hence, we see that the sleeve contacts of the
+plug and the sleeve conductor of the cord are connected to ground
+through the permanent ground connection of the sleeve conductors of
+the jack as soon as the plug is inserted into the jack. Thus, when the
+cord circuit of a metallic-circuit switchboard is used to connect two
+grounded circuits together, the tip strand of the cord is the
+connecting link between the two conductors, while the sleeve strand of
+the cord merely serves to ground one side of the clearing-out drop
+and one side each of the operator's telephone set and the ringing
+generator when their respective keys are operated.
+
+_Connection between Dissimilar Lines._ Fig. 278 shows how the same
+cord circuit and the same arrangement of line equipment may be used
+for connecting a grounded line to a metallic-circuit line. The
+metallic circuit line is shown on the left and the grounded line on
+the right. When the two plugs are inserted into the respective jacks
+of this figure, the right-hand conductor of the metallic circuit shown
+on the left will be continued through the tip strand of the cord
+circuit to the line conductor of the grounded line shown on the right.
+The left-hand conductor of the metallic-circuit line will be connected
+to ground because it will be continued through the sleeve strand of
+the cord circuit to the sleeve contact of the calling plug and thence
+to the sleeve contact of the jack of the grounded line, which sleeve
+contact is shown to be grounded. The talking circuit between the two
+connected lines in this case may be traced as follows: From the
+subscriber's station at the left through the right-hand limb of the
+metallic-circuit line, through the tip contact and tip conductor of
+the cord circuit, to the single limb of the grounded-circuit line,
+thence to the sub-station of that line and through the talking
+apparatus there to ground. The return path from the right-hand station
+is by way of ground to the ground connection at the central office,
+thence to the sleeve contact of the grounded line jack, through the
+sleeve conductor of the cord circuit, to the sleeve contact of the
+metallic-circuit line jack, and thence by the left-hand limb of the
+metallic-circuit line to the subscriber's station.
+
+[Illustration: Fig. 278. Connection Between Dissimilar Lines]
+
+A better way of connecting a metallic-circuit line to a grounded line
+is by the use of a special cord circuit involving a repeating coil,
+such a connection being shown in Fig. 279. The cord circuit in this
+case differs in no respect from those already shown except that a
+repeating coil is associated with it in such a way as to conductively
+divide the answering side from the calling side. Obviously, whatever
+currents come over the line connected with the answering plug will
+pass through the windings _1_ and _2_ of this coil and will induce
+corresponding currents in the windings _3_ and _4_, which latter
+currents will pass out over the circuit of the line connected with the
+calling plug. When a grounded circuit is connected to a metallic
+circuit in this manner, no ground is thrown onto the metallic circuit.
+The balance of the metallic circuit is, therefore, maintained.
+
+To ground one side of a metallic circuit frequently so unbalances it
+as to cause it to become noisy, that is, to have currents flowing in
+it, by induction or from other causes, other than the currents which
+are supposed to be there for the purpose of conveying speech.
+
+[Illustration: Fig. 279. Connection of Dissimilar Lines through
+Repeating Coil]
+
+_Convertible Cord Circuits._ The consideration of Fig. 279 brings us
+to the subject of so-called convertible cord circuits. Some
+switchboards, serving a mixture of metallic and grounded lines, are
+provided with cord circuits which may be converted at will by the
+operator from the ordinary type shown in Fig. 276 to the type shown in
+Fig. 279. The advantage of this will be obvious from the following
+consideration. When a call originates on any line, either grounded or
+metallic, the operator does not know which kind of a line is to be
+called for. She, therefore, plugs into this line with any one of her
+answering plugs and completes the connection in the usual way. If the
+call is for the same kind of a circuit as that over which the call
+originated, she places the converting key in such a position as will
+connect the conductors of the cord circuit straight through; while if
+the connection is for a different kind of a line than that on which
+the call originated she throws the converting key into such a
+position as to include the repeating coil. A study of Fig. 280 will
+show that when the converting key, which is commonly referred to as
+the repeating-coil key, is in one position, the cord conductors will
+be cut straight through, the repeating coil being left open in both
+its windings; and when it is thrown to its other position, the
+connection between the answering and calling sides of the cord circuit
+will be severed and the repeating coil inserted so as to bring about
+the same effects and circuit arrangements as are shown in Fig. 279.
+
+[Illustration: Fig. 280. Convertible Cord Circuit]
+
+Cord-Circuit Considerations. _Simple Bridging Drop Type._ The matter
+of cord circuits in magneto switchboards is deserving of much
+attention. So far as talking requirements are concerned, the ordinary
+form of cord circuit with a clearing-out drop bridged across the two
+strands is adequate for nearly all conditions except those where a
+grounded-and a metallic-circuit line are connected together, in which
+case the inclusion of a repeating coil has some advantages.
+
+[Illustration: Fig. 281. Bridging Drop-Cord Circuit]
+
+From the standpoint of signaling, however, this type of cord circuit
+has some disadvantages under certain conditions. In order to simplify
+the discussion of this and other cord-circuit matters, reference will
+be made to some diagrams from which the ringing and listening keys and
+talking apparatus have been entirely omitted. In Fig. 281 the regular
+bridging type of clearing-out drop-cord circuit is shown, this being
+the type already discussed as standard. For ordinary practice it is
+all right. Certain difficulties are experienced with it, however,
+where lines of various lengths and various types of sub-station
+apparatus are connected. For instance, if a long bridging line be
+connected with one end of this cord circuit and a short line having a
+low-resistance series ringer be connected with the other end, then a
+station on the long line may have some difficulty in throwing the
+clearing-out drop, because of the low-resistance shunt that is placed
+around it through the short line and the low-resistance ringer. In
+other words, the clearing-out drop is shunted by a comparatively
+low-resistance line and ringer and the feeble currents arriving from a
+distant station over the long line are not sufficient to operate the
+drop thus handicapped. The advent of the various forms of party-line
+selective signaling and the use of such systems in connection with
+magneto switchboards has brought in another difficulty that sometimes
+manifests itself with this type of cord circuit. If two ordinary
+magneto telephones are connected to the two ends of this cord circuit,
+it is obvious that when one of the subscribers has hung up his
+receiver and the other subscriber rings off, the bell of the other
+subscriber will very likely be rung even though the clearing-out drop
+operates properly; it would be better in any event not to have this
+other subscriber's bell rung, for he may understand it to be a recall
+to his telephone. When, however, a party line is connected through
+such a cord circuit to an ordinary line having bridging instruments,
+for instance, the difficulty due to ringing off becomes even greater.
+When the subscriber on the magneto line operates his generator to give
+the clearing-out signal, he is very likely to ring some of the bells
+on the other line and this, of course, is an undesirable thing. This
+may happen even in the case of harmonic bells on the party line, since
+it is possible that the subscriber on the magneto line in turning his
+generator will, at some phase of the operation, strike just the proper
+frequency to ring some one of the bells on the harmonic party line. It
+is obvious, therefore, that there is a real need for a cord circuit
+that will prevent _through ringing._
+
+One way of eliminating the through-ringing difficulty in the type of
+cord circuit shown in Fig. 281 would be to use such a very low-wound
+clearing-out drop that it would practically short-circuit the line
+with respect to ringing currents and prevent them from passing on to
+the other line. This, however, is not a good thing to do, since a
+winding sufficiently low to shunt the effective ringing current would
+also be too low for good telephone transmission.
+
+[Illustration: Fig. 282. Series Drop-Cord Circuit]
+
+_Series Drop Type._ Another type of cord circuit that was largely used
+by the Stromberg-Carlson Telephone Manufacturing Company at one time is
+shown in Fig. 282. In this the clearing-out drop was not bridged but
+was placed in series in the tip side of the line and was shunted by a
+condenser. The resistance of the clearing-out drop was 1,000 ohms and
+the capacity of the condenser was 2 microfarads. It is obvious that
+this way of connecting the clearing-out drop was subject to the
+_ringing-through_ difficulty, since the circuit through which the
+clearing-out current necessarily passed included the telephone
+instrument of the line that was not sending the clearing-out signal.
+This form was also objectionable because it was necessary for the
+subscriber to ring through the combined resistance of two lines, and in
+case the other line happened to be open, no clearing-out signal would
+be received. While this circuit, therefore, was perhaps not quite so
+likely as the other to tie up the subscriber, that is, to leave him
+connected without the ability to send a clearing-out signal, yet it was
+sure to ring through, for the clearing-out drop could not be thrown
+without the current passing through the other subscriber's station.
+
+[Illustration: Fig. 283. Dean Non-Ring-Through Cord Circuit]
+
+_Non-Ring-Through Type._ An early attempt at a non-ring-through cord
+is shown in Fig. 283, this having once been standard with the Dean
+Electric Company. It made use of two condensers of 1 microfarad each,
+one in each side of the cord circuit. The clearing-out drop was of 500
+ohms resistance and was connected from the answering side of the tip
+conductor to the calling side of the sleeve conductor. In this way
+whatever clearing-out current reached the central office passed
+through at least one of the condensers and the clearing-out drop. In
+order for the clearing-out current to pass on beyond the central
+office it was necessary for it to pass through the two condensers in
+series. This arrangement had the advantage of giving a positive
+ring-off, regardless of the condition of the connected line.
+Obviously, even if the line was short-circuited, the ringing currents
+from the other line would still be forced through the clearing-out
+drop on account of the high effective resistance of the 1-microfarad
+condenser connected in series with the short-circuited line. Also the
+clearing-out signal would be properly received if the connected line
+were open, since the clearing-out drop would still be directly across
+the cord circuit. This arrangement also largely prevented through
+ringing, since the currents would pass through the 1-microfarad
+condenser and the 500-ohm drop more readily than through the two
+condensers connected in series.
+
+[Illustration: Fig. 284. Monarch Non-Ring-Through Cord Circuit]
+
+In Fig. 284 is shown the non-ring-through arrangement of cord circuit
+adopted by the Monarch Company. In this system the clearing-out drop
+has two windings, either of which will operate the armature. The two
+windings are bridged across the cord circuit, with a 1/2-microfarad
+condenser in series in the tip strand between the two winding
+connections. While the low-capacity condenser will allow the
+high-frequency talking current to pass readily without affecting it to
+any appreciable extent, it offers a high resistance to a low-frequency
+ringing current, thus preventing it from passing out on a connected
+line and forcing it through one of the windings of the coil. There is
+a tendency to transformer action in this arrangement, one of the
+windings serving as a primary and the other as a secondary, but this
+has not prevented the device from being highly successful.
+
+A modification of this arrangement is shown in Fig. 285, wherein a
+double-wound clearing-out drop is used, and a 1/2-microfarad condenser
+is placed in series in each side of the cord circuit between the
+winding connections of the clearing-out drop. This circuit should give
+a positive ring-off under all conditions and should prevent through
+ringing except as it may be provided by the transformer action between
+the two windings on the same core.
+
+[Illustration: Fig. 285. Non-Ring-Through Cord Circuit]
+
+Another rather ingenious method of securing a positive ring-off and
+yet of preventing in a certain degree the undesirable ringing-through
+feature is shown in the cord circuit, Fig. 286. In this two
+non-inductive coils _1_ and _2_ are shown connected in series in the
+tip and sleeve strands of the coils, respectively. Between the neutral
+point of these two non-inductive windings is connected the
+clearing-out drop circuit. Voice currents find ready path through
+these non-inductive windings because of the fact that, being
+non-inductive, they present only their straight ohmic resistance. The
+impedance of the clearing-out drop prevents the windings being shunted
+across the two sides of the cord circuit. With this circuit a positive
+ring-off is assured even though the line connected with the one
+sending the clearing-out signal is short-circuited or open. If it is
+short-circuited, the shunt around the clearing-out drop will still
+have the resistance of two of the non-inductive windings included in
+it, and thus the drop will never be short-circuited by a very
+low-resistance path. Obviously, an open circuit in the line will not
+prevent the clearing-out signal being received. While this is an
+ingenious scheme, it is not one to be highly recommended since the
+non-inductive windings, in order to be effective so far as signaling
+is concerned, must be of considerable resistance and this resistance
+is in series in the talking circuit. Even non-inductive resistance is
+to be avoided in the talking circuit when it is of considerable
+magnitude and where there are other ways of solving the problem.
+
+[Illustration: Fig. 286. Cord Circuit with Differential Windings]
+
+_Double Clearing-out Type. _Some people prefer two clearing-out drops
+in each cord circuit, so arranged that the one will be responsive to
+currents sent from the line with which the answering plug is connected
+and the other responsive only to currents sent from the line with
+which the calling plug is connected. Such a scheme, shown in Fig. 287,
+is sometimes employed by the Dean, the Monarch, and the Kellogg
+companies. Two 500-ohm clearing-out drops of ordinary construction are
+bridged across the cord circuit and in each side of the cord circuit
+there is included between the drop connections a 1-microfarad
+condenser. Ringing currents originating on the line with which the
+answering plug is connected will pass through the clearing-out drop,
+which is across that side of the cord circuit, without having to pass
+through any condensers. In order to reach the other clearing-out drop
+the ringing current must pass through the two 1-microfarad condensers
+in series, this making in effect only 1/2-microfarad. As is well
+known, a 1/2-microfarad condenser not only transmits voice currents
+with ease but also offers a very high apparent resistance to ringing
+currents. With the double clearing-out drop system the operator is
+enabled to tell which subscriber is ringing off. If both shutters fall
+she knows that both subscribers have sent clearing-out signals and
+she, therefore, pulls down the connection without the usual precaution
+of listening to see whether one of the subscribers may be waiting for
+another connection. This double clearing-out system is analogous to
+the complete double-lamp supervision that will be referred to more
+fully in connection with common-battery circuits. There is not the
+need for double supervision in magneto work, however, that there is in
+common-battery work because of the fact that in magneto work the
+subscribers frequently fail to remember to ring off, this act being
+entirely voluntary on their part, while in common-battery work, the
+clearing-out signal is given automatically by the subscriber when he
+hangs up his receiver, thus accomplishing the desired end without the
+necessity of thoughtfulness on his part.
+
+[Illustration: Fig. 287. Double Clearing-Out Drops]
+
+Another form of double clearing-out cord circuit is shown in Fig. 288.
+In this the calling and the answering plugs are separated by repeating
+coils, a condenser of 1-microfarad capacity being inserted between each
+pair of windings on the two ends of the circuit. The clearing-out
+drops are placed across the calling and answering cords in the usual
+manner. The condenser in this case prevents the drop being
+short-circuited with respect to ringing currents and yet permits the
+voice currents to flow readily through it. The high impedance of the
+drop forces the voice currents to take the path through the repeating
+coil rather than through the drop. This circuit has the advantage of a
+repeating-coil cord circuit in permitting the connection of metallic
+and grounded lines without causing the unbalancing of the metallic
+circuits by the connection to them of the grounded circuits.
+
+[Illustration: Fig. 288. Double Clearing-Out Drops]
+
+Recently there has been a growing tendency on the part of some
+manufacturers to control their clearing-out signals by means of relays
+associated with cord circuits, these signals sometimes being ordinary
+clearing-out drops and sometimes incandescent lamps.
+
+[Illustration: Fig. 289. Relay-Controlled Clearing-Out Drop]
+
+In Fig. 289 is shown the cord circuit sometimes used by the L.M.
+Ericsson Telephone Manufacturing Company. A high-wound relay is
+normally placed across the cord and this, besides having a
+high-resistance and impedance winding has a low-resistance locking
+winding so arranged that when the relay pulls up its armature it will
+close a local circuit including this locking winding and local battery.
+When once pulled up the relay will, therefore, stay up due to the
+energizing of this locking coil. Another contact operated by the relay
+closes the circuit of a low-wound clearing-out drop placed across the
+line, thus bridging it across the line. The condition of high impedance
+is maintained across the cord circuit normally while the subscribers
+are talking; but when either of them rings off, the high-wound relay
+pulls up and locks, thus completing the circuit of the clearing-out
+drop across the cords. The subsequent impulses sent from the
+subscribers' generators operate this drop. The relay is restored or
+unlocked and the clearing-out drop disconnected from the cord circuit
+by means of a key which opens the locking circuit of the relay. This
+key is really a part of the listening key and serves to open this
+locking circuit whenever the listening key is operated. The
+clearing-out drop is also automatically restored by the action of the
+listening key, this connection being mechanical rather than electrical.
+
+Recall Lamp:--The Monarch Company sometimes furnishes what it terms a
+recall lamp in connection with the clearing-out drops on its magneto
+switchboards. The circuit arrangement is shown in Fig. 290, wherein
+the drop is the regular double-wound clearing-out drop like that of
+Fig. 284. The armature carries a contact spring adapted to close the
+local circuit of a lamp whenever it is attracted. The object of this
+is to give the subscriber, whose line still remains connected by a
+cord circuit, opportunity to recall the central office if the operator
+has not restored the clearing-out drop.
+
+[Illustration: Fig. 290. Cord Circuit with Recall Lamp]
+
+_Lamp-Signal Type._ There has been a tendency on the part of some
+manufacturing companies to advocate, instead of drop signals,
+incandescent lamp signals for the cord circuits, and sometimes for the
+line circuits on magneto boards. In most cases this may be looked upon
+as a "frill." Where line lamps instead of drops have been used on
+magneto switchboards, it has been the practice to employ, instead of a
+drop, a locking relay associated with each lamp, which was so arranged
+that when the relay was energized by the magneto current from the
+subscriber's station, it would pull up and lock, thus closing the lamp
+circuit.
+
+The local circuit, or locking circuit, which included the lamp was
+carried through a pair of contacts in the corresponding jacks so
+arranged that when the plug was inserted in answer to the call, this
+locking lamp circuit would be open, thereby extinguishing the lamp and
+also unlocking the relay. There seems to be absolutely no good reason
+why lamp signals should be substituted for mechanical drops in magneto
+switchboards. There is no need for the economy in space which the lamp
+signal affords, and the complications brought in by the locking
+relays, and the requirements for maintaining a local battery suitable
+for energizing the lamps are not warranted for ordinary cases.
+
+[Illustration: Fig. 291. Cord Circuit with Double Lamp Signals]
+
+In Fig. 291 is shown a cord circuit, adaptable to magneto
+switchboards, provided with double lamp signals instead of
+clearing-out drops. Two high-wound locking relays are bridged across
+the line, the cord strands being divided by 1-microfarad condensers.
+When the high-wound coil of either relay is energized by the magneto
+current from the subscriber's station, the relay pulls up and closes a
+locking circuit including a battery and a coil _2_, the contact _3_ of
+the locking relay, and also the contact _4_ of a restoring key. This
+circuit may be traced from the ground through battery, coil _2_,
+contact _3_ controlled by the relay, and contact _4_ controlled by the
+restoring key, and back to ground. In multiple with the locking coil
+_2_ is the lamp, which is illuminated, therefore, whenever the locking
+circuit is closed. Pressure on the restoring key breaks the locking
+circuit of either of the lamps, thereby putting out the lamp and at
+the same time restoring the locking relay to its normal position.
+
+_Lamps vs. Drops in Cord Circuits._ So much has been said and written
+about the advantages of incandescent lamps as signals in switchboards
+and about the merits of the common-battery method of supplying current
+to the subscribers, that there has been a tendency for people in
+charge of the operation of small exchanges to substitute the lamp for
+the drop in a magneto switchboard in order to give the general
+appearance of common-battery operations. There has also been a
+tendency to employ the common-battery system of operation in many
+places where magneto service should have been used, a mistake which
+has now been realized and corrected. In places where the simple
+magneto switchboard is the thing to use, the simpler it is the better,
+and the employment of locking relays and lamp signals and the
+complications which they carry with them, is not warranted.
+
+Switchboard Assembly. The assembly of all the parts of a simple
+magneto switchboard into a complete whole deserves final
+consideration. The structure in which the various parts are mounted,
+referred to as the cabinet, is usually of wood.
+
+_Functions of Cabinet._ The purpose of the cabinet is not only to form
+a support for the various pieces of apparatus but also to protect them
+from dust and mechanical injury, and to hold those parts that must be
+manipulated by the operator in such relation that they may be most
+convenient for use, and thus best adapted for carrying out their
+various functions. Other points to be provided for in the design of
+the cabinet and the arrangement of the various parts within are: that
+all the apparatus that is in any way liable to get out of order may be
+readily accessible for inspection and repairs; and that provision
+shall be made whereby the wiring of these various pieces of apparatus
+may be done in a systematic and simple way so as to minimize the
+danger of crossed, grounded, or open circuits, and so as to provide
+for ready repair in case any of these injuries do occur.
+
+
+_Wall-Type Switchboards._ The simplest form of switchboard is that for
+serving small communities in rural districts. Ordinarily the telephone
+industry in such a community begins by a group of farmers along a
+certain road building a line connecting the houses of several of them
+and installing their own instruments. This line is liable to be
+extended to some store at the village or settlement, thus affording
+communication between these farmers and the center of their community.
+Later on those residing on other roads do the same thing and connect
+their lines to the same store or central point. Then it is that some
+form of switchboard is established, and perhaps the storekeeper's
+daughter or wife is paid a small fee for attendance.
+
+[Illustration: Fig. 292. Wall Switchboard with Telephone]
+
+A switchboard well-adapted for this class of service where the number
+of lines is small, is shown in Fig. 292. In this the operator's
+talking apparatus and her calling apparatus are embodied in an
+ordinary magneto wall telephone. The switchboard proper is mounted
+alongside of this, and the two line binding posts of the telephone are
+connected by a pair of wires to terminals of the operator's plug,
+which plug is shown hanging from the left-hand portion of the
+switchboard. The various lines centering at this point terminate in
+the combined drops and jacks on the switchboard, of which there are 20
+shown in this illustration. Beside the operator's plug there are a
+number of pairs of plugs shown hanging from the switchboard cabinet.
+These are connected straight through in pairs, there being no
+clearing-out drops or keys associated with them in the arrangement.
+Each line shown is provided with an extra jack, the purpose of which
+will be presently understood.
+
+The method of operation is as follows: When a subscriber on a certain
+line desires to get connection through the switchboard he turns his
+generator and throws the drop. The operator in order to communicate
+with him inserts the plug in which her telephone terminates into the
+jack, and removes her receiver from its hook. Having learned that it
+is for a certain subscriber on another line, she withdraws her plug
+from the jack of the calling line and inserts it into the jack of the
+called line, then, hanging up her receiver, she turns the generator
+crank in accordance with the proper code to call that subscriber. When
+that subscriber responds she connects the two lines by inserting the
+two plugs of a pair into their respective jacks, and the subscribers
+are thus placed in communication. The extra jack associated with each
+line is merely an open jack having its terminals connected
+respectively with the two sides of the line. Whenever an operator
+desires to listen in on two connected lines she does so by inserting
+the operator's plug into one of these extra jacks of the connected
+lines, and she may thus find out whether the subscribers are through
+talking or whether either one of them desires another connection. The
+drops in such switchboards are commonly high wound and left
+permanently bridged across the line so as to serve as clearing-out
+drops. The usual night-alarm attachment is provided, the buzzer being
+shown at the upper right-hand portion of the cabinet.
+
+[Illustration: Fig. 293. Combined Telephone and Switchboard]
+
+Another type of switchboard commonly employed for this kind of
+service is shown in Fig. 293, in which the telephone and the
+switchboard cabinet are combined. The operation of this board is
+practically the same as that of Fig. 292, although it has
+manually-restored drops instead of self-restoring drops; the
+difference between these two types, however, is not material for this
+class of service. For such work the operator has ample time to attend
+to the restoring of the drop and the only possible advantage in the
+combined drop-and-jack for this class of work is that it prevents the
+operator from forgetting to restore the drops. However, she is not
+likely to do this with the night-alarm circuit in operation, since the
+buzzer or bell would continue to ring as long as the drop was down.
+
+[Illustration: Fig. 294. Upright Magneto Switchboard]
+
+[Illustration: Fig. 295. Upright Magneto Switchboard--Rear View]
+
+_Upright Type Switchboard._ By far the most common type of magneto
+switchboard is the so-called upright type, wherein the drops and
+jacks are mounted on the face of upright panels rising from a
+horizontal shelf, which shelf contains the plugs, the keys, and any
+other apparatus which the operator must manipulate. Front and rear
+views of such a switchboard, as manufactured by the Kellogg Company,
+are shown in Figs. 294 and 295. This particular board is provided with
+fifty combined drops and jacks and, therefore, equipped for fifty
+subscribers' lines. The drops and jacks are mounted in strips of five,
+and arranged in two panels. The clearing-out drops, of which there are
+ten, are arranged at the bottom of the two panels in a single row and
+may be seen immediately above the switchboard plugs. There are ten
+pairs of cords and plugs with their associated ringing and listening
+keys, the plugs being mounted on the rear portion of the shelf,
+while the ringing and listening keys are mounted on the hinged portion
+of the shelf in front of the plugs.
+
+[Illustration: Fig. 296. Details of Drop, Jack, Plug, and Key
+Arrangement]
+
+[Illustration: Fig. 297. Cross-Section of Upright Switchboard]
+
+A better idea of the arrangement of drops, jacks, plugs, and keys may
+be had from an illustration of a Dean magneto switchboard shown in
+Fig. 296. The clearing-out drops and the arrangement of the plugs and
+keys are clearly shown. The portion of the switchboard on which the
+plugs are mounted is always immovable, the plugs being provided with
+seats through which holes are bored of sufficient size to permit the
+switchboard cord to pass beneath the shelf. When one of these plugs is
+raised, the cord is pulled up through this hole thus allowing the plug
+to be placed in any of the jacks.
+
+The key arrangement shown in this particular cut is instructive. It
+will be noticed that the right-hand five pairs of plugs are provided
+with ordinary ringing and listening keys, while the left-hand five are
+provided with party-line ringing keys and listening keys. The
+listening key in each case is the one in the rear and is alike for all
+of the cord pairs. The right-hand five ringing keys are so arranged
+that pressing the lever to the rear will ring on the answering cord,
+while pressing it toward the front will cause ringing current to flow
+on the calling plug. In the left-hand five pairs of cords shown in
+this cut, the pressure of any one of the keys causes a ringing current
+of a certain frequency to flow on the calling cord, this frequency
+depending upon which one of the keys is pressed.
+
+[Illustration: Fig. 298. Cord Weight]
+
+An excellent idea of the grouping of the various pieces of apparatus
+in a complete simple magneto switchboard may be had from Fig. 297.
+While the arrangement here shown is applicable particularly to the
+apparatus of the Dean Electric Company, the structure indicated is
+none-the-less generally instructive, since it represents good practice
+in this respect. In this drawing the stationary plug shelf with the
+plug seat is clearly shown and also the hinged key shelf. The hinge of
+the key shelf is an important feature and is universally found in all
+switchboards of this general type. The key shelf may be raised and
+thus expose all of the wiring leading to the keys, as well as the
+various contacts of the keys themselves, to inspection.
+
+[Illustration: Fig. 299. Magneto Switchboard, Target Signals]
+
+As will be seen, the switchboard cords leading from the plugs extend
+down to a point near the bottom of the cabinet where they pass through
+pulley weights and then up to a stationary cord rack. On this cord
+rack are provided terminals for the various conductors in the cord,
+and it is at this point that the cord conductors join the other wires
+leading to the other portions of the apparatus as required. A good
+form of cord weight is shown in Fig. 298; and obviously the function
+of these weights is to keep the cords taut at all times and to prevent
+their tangling.
+
+[Illustration: Fig. 300. Rear View of Target Signal, Magneto
+Switchboard]
+
+The drawing, Fig. 297, also gives a good idea of the method of
+mounting the hand generator that is ordinarily employed with such
+magneto switchboards. The shaft of the generator is merely continued
+out to the front of the key shelf where the usual crank is provided,
+by means of which the operator is able to generate the necessary
+ringing current. Beside the hand generator at each operator's
+position, it is quite common in magneto boards, of other than the
+smallest sizes, to employ some form of ringing generator, either a
+power-driven generator or a pole changer driven by battery current for
+furnishing ringing current without effort on the part of the operator.
+
+[Illustration: Fig. 301. Dean Two-Position Switchboard]
+
+Switchboards as shown in Figs. 294 and 295, are called single-position
+switchboards because they afford room for a single operator.
+Ordinarily for this class of work a single operator may handle from
+one to two hundred lines, although of course this depends on the
+amount of traffic on the line, and this, in turn, depends on the
+character of the subscribers served, and also on the average number of
+stations on a line. Another single-position switchboard is shown in
+Figs. 299 and 300, being a front and rear view of the simple magneto
+switchboard of the Western Electric Company, which is provided with
+the target signals of that company rather than the usual form of drop.
+
+Where a switchboard must accommodate more lines than can be handled by
+a single operator, the cabinet is made wider so as to afford room for
+more than one operator to be seated before it. Sometimes this is
+accomplished by building the cabinet wider, or by putting two such
+switchboard sections as are shown in Figs. 294 or 299 side by side. A
+two-position switchboard section is shown in front and rear views in
+Figs. 301 and 302.
+
+[Illustration: Fig. 302. Rear View of Dean Two-Position Switchboard]
+
+_Sectional Switchboards._ The problem of providing for growth in a
+switchboard is very much the same as that which confronts one in
+buying a bookcase for his library. The Western Electric Company has
+met this problem, for very small rural exchanges, in much the same way
+that the sectional bookcase manufacturers have provided for the
+possible increase in bookcase capacity. Like the sectional bookcase,
+this sectional switchboard may start with the smallest of equipment--a
+single sectional unit--and may be added to vertically as the
+requirements increase, the original equipment being usable in its more
+extended surroundings.
+
+[Illustration: Fig. 303. Sectional Switchboard--Wall Type]
+
+This line of switchboards is illustrated in Figs. 303 to 306. The
+beginning may be made with either a wall type or an upright type of
+switchboard, the former being mounted on brackets secured to the wall,
+and the latter on a table. A good idea of the wall type is shown in
+Fig. 303. Three different kinds of sectional units are involved in
+this: first, the unit which includes the cords, plugs, clearing-out
+drops, listening jacks, operator's telephone set and generator;
+second, the unit containing the line equipment, including a strip of
+ten magneto line signals and their corresponding jacks; third, the
+finishing top, which includes no equipment except the support for the
+operator's talking apparatus.
+
+[Illustration: Fig. 301. Sectional Switchboard--Wall Type]
+
+The first of the units in Fig. 303 forms the foundation on which the
+others are built. Two of the line-equipment units are shown; these
+provide for a total of twenty lines. The top rests on the upper
+line-equipment unit, and when it becomes necessary to add one or more
+line-equipment units as the switchboard grows, this top is merely
+taken off, the other line-equipment units put in place on top of those
+already existing, and the top replaced. The wall type of sectional
+switchboard is so arranged that the entire structure may be swung out
+from the wall, as indicated in Fig. 304, exposing all of the apparatus
+and wiring for inspection. Each of the sectional units is provided
+with a separate door, as indicated, so that the rear door equipment is
+added to automatically as the sections are added. In the embodiment of
+the sectional switchboard idea shown in these two figures just
+referred to, no ringing and listening keys are provided, but the
+operator's telephone and generator terminate in a special plug--the
+left-hand one shown in Fig. 303--and when the operator desires to
+converse with the connected subscribers, she does so by inserting the
+operator's plug into one of the jacks immediately below the
+clearing-out drop corresponding to the pair of plugs used in making
+the connection. The arrangement in this case is exactly the same in
+principle as that described in Fig. 292. The operator's generator is
+so arranged in connection with this left-hand operator's plug that the
+turning of the generator crank automatically switches the operator's
+telephone set off and switches the generator on, just the same as a
+switch hook may do in a subscriber's series telephone.
+
+[Illustration: Fig. 305. Sectional Switchboard--Table Type]
+
+[Illustration: Fig. 306. Sectional Switchboard--Table Type]
+
+The upright type of sectional switchboard is shown in Figs. 305 and
+306, which need no explanation in view of the foregoing, except to say
+that, in the particular instrument illustrated, ringing and listening
+keys are provided instead of the jack-and-plug arrangement of the wall
+type. In this case also, the top section carries an arm for supporting
+a swinging transmitter instead of the hook support for the combined
+transmitter and receiver.
+
+
+
+
+REVIEW QUESTIONS
+
+[Blank Page]
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 11--62
+
+       *       *       *       *       *
+
+1. When was the telephone invented and by whom?
+
+2. State the velocity of sound in air. Is it higher in air than in a
+denser medium?
+
+3. State and define the characteristics of sound.
+
+4. Make sketch of Bell's original magneto telephone without permanent
+magnets.
+
+5. Describe and sketch Hughes' microphone.
+
+6. Which is, at present, the best material for varying the resistance
+in transmitters?
+
+7. Give the fundamental differences between the magneto transmitter
+and the carbon transmitter.
+
+8. What is the function of the induction coil in the telephone
+circuit?
+
+9. Describe and sketch the different kinds of visible signals.
+
+10. What should be the diameter of hard drawn copper wire in order to
+allow economical spacing of poles?
+
+11. State the four principal properties of a telephone line.
+
+12. If in testing a line the capacity is changed what are the results
+found on the receiver and transmitter end?
+
+13. Why is paper used as an insulator of telephone cables?
+
+14. How does a conductor behave in connection with direct current and
+how with alternating current?
+
+15. What influence has inductance on the telephone?
+
+16. Define impedance and give the formula for it.
+
+17. What is the usual specification for insulation of resistance in
+telephone cables?
+
+18. If 750 feet of cable have an insulation resistance of 9,135
+megohms, how great is the insulation resistance for 7 miles and 1,744
+feet of cable?
+
+19. What is the practical limiting conversation distance for No. 10 B.
+and S. wire?
+
+20. Describe Professor Pupin's method of inserting inductance into the
+telephone line.
+
+21. What does _mho_ denote?
+
+22. Why are Pupin's coils not so successful on open wires?
+
+23. What is a repeater?
+
+24. Define _reactive interference_.
+
+25. State the frequencies of the pitches of the human voice.
+
+26. What is the office of a diaphragm in a telephone apparatus?
+
+27. What transmitter material has greatly increased the ranges of
+speech?
+
+28. Describe the different methods of measurements of telephone
+circuits.
+
+29. What are the two kinds of _electric calls_?
+
+30. How many conductors has a telephone line?
+
+31. Give formula for capacity reactance and the meaning of the
+symbols.
+
+32. Which American cities are joined by underground lines at present?
+
+33. State the two practical ways of improving telephone transmission.
+
+
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 63--141
+
+       *       *       *       *       *
+
+1. On what general principle are most of the telephone transmitters of
+today constructed?
+
+2. Make sketch of the new Western Electric transmitter and describe
+its working.
+
+3. Make sketch and describe the Kellogg transmitter.
+
+4. What troubles were encountered in the earlier forms of granular
+carbon transmitters and how were they overcome?
+
+5. What limits the current-carrying capacity of the transmitter? How
+may this capacity be increased?
+
+6. State in what kind of transmitters a maximum degree of
+sensitiveness is desirable.
+
+7. Show the conventional symbols for transmitters.
+
+8. Describe a telephone receiver.
+
+9. Sketch a Western Electric receiver and point out its deficiencies.
+
+10. Make a diagram of the Kellogg receiver.
+
+11. Describe the direct-current receiver of the Automatic Electric
+Company.
+
+12. Describe and sketch the Dean receiver.
+
+13. Show the conventional symbols of a receiver.
+
+14. Describe exactly how, in a cell composed of a tin and a silver
+plate with dilute sulphuric acid as electrolyte, the current inside
+and outside of the cell will flow.
+
+15. Describe the phenomenon of polarization.
+
+16. What is _local action_ of a cell? How may it be prevented?
+
+17. Into how many classes may cells be divided? Which class is most
+used in telephony?
+
+18. Describe the LeClanche cell.
+
+19. Sketch and describe an excellent form of dry cell.
+
+20. Show the conventional symbols for batteries.
+
+21. Sketch and describe the generator shunt switch and the generator
+cut-in switch.
+
+22. How may a pulsating current be derived from a magneto generator?
+
+23. Show conventional symbols for magneto generators.
+
+24. Sketch and describe the Western Electric polarized bell.
+
+25. Give conventional ringer symbols.
+
+26. What is the purpose of the hook switch?
+
+27. Make sketch and give description of Kellogg's long lever hook
+switch.
+
+28. Describe and sketch the Western Electric short lever hook switch.
+
+29. Point out the principal difference between the desk stand hook
+switches of the Western Electric Company and of the Kellogg
+Switchboard and Supply Company.
+
+30. Give conventional symbols of hook switches.
+
+
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 143--225
+
+       *       *       *       *       *
+
+1. Describe an electromagnet and its function in telephony.
+
+2. Sketch an iron-clad electromagnet.
+
+3. What is a differential electromagnet? Sketch and describe one type.
+
+4. State the desirable characteristics of good enamel insulation for
+magnet wire.
+
+5. If you have a coil of No. 23 double cotton B. and S. wire of 115
+ohms resistance and you have to rewind it for 1,070 ohms resistance
+with double cotton wire, what number of wire would you take? Show
+calculation.
+
+     NOTE. No. 23 d. c wire has res. 1.772 ohms per cubic inch; for
+     the core, 115 ohms. There are required in the coil 1,070 ohms,
+     that is, 9.3 times as much. 1.772 x 9.3 = 16.47 ohms, which must
+     be the resistance per cu. in. This resistance gives, according to
+     Table IV, No. 29 wire.
+
+6. What is an impedance coil? State how it differs from an
+electromagnet coil.
+
+7. Describe the different kinds of impedance coils.
+
+8. Give symbol of impedance coil.
+
+9. What are the principal parts of an induction coil?
+
+10. What is the function of an induction coil in telephony?
+
+11. What is a repeating coil and how does it differ from an induction
+coil?
+
+12. Give conventional symbols of induction coils and repeating coils.
+
+13. Enumerate the different types of non-inductive resistance devices
+and give a short description of each.
+
+14. Define condenser.
+
+15. What is the meaning of the word _dielectrics_?
+
+16. State what you understand by the specific inductive capacity of a
+dielectric.
+
+17. Upon what factors does the capacity of a condenser depend?
+
+18. What is the usual capacity of condensers in telephone practice?
+
+19. Give conventional condenser symbols.
+
+20. By what two methods may the current be supplied to a telephone
+transmitter?
+
+21. Make sketch of local-battery stations with metallic circuit.
+
+22. Sketch common-battery circuit in series with two lines.
+
+23. State the objections against the preceding arrangement.
+
+24. Make sketch of the standard arrangement of the Western Electric
+Company in bridging the common battery with repeating coils.
+
+25. Sketch the arrangement of bridging the battery with impedance
+coils and state the purpose of the coils.
+
+26. Make diagram of a common-source current supply for many lines with
+repeating coils and point out the travel of the voice currents.
+
+27. Name the different parts which comprise a telephone set.
+
+28. What is a magneto telephone?
+
+29. Make diagram of the circuit of a series magneto set with receiver
+on the hook and explain how the different currents are flowing.
+
+30. Show diagram of the Stromberg-Carlson magneto desk telephone
+circuit and describe its working.
+
+31. Give sketch of the Stromberg-Carlson common-battery wall set
+circuit.
+
+32. Describe briefly the microtelephone set.
+
+33. Make sketch of the Monarch common-battery wall set.
+
+
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 227--286
+
+       *       *       *       *       *
+
+1. What is a party line?
+
+2. What is usually understood by private lines?
+
+3. What problem is there to overcome in connection with party lines?
+
+4. State the two general classes of party-line systems.
+
+5. Point out the defects of the series system.
+
+6. Make sketch of a metallic bridging line and show the circuit for
+the voice currents.
+
+7. What is a signal code?
+
+8. Give classification of selective party-line systems with short
+definitions.
+
+9. Describe the principle of selection by polarity and make sketch
+illustrating this principle.
+
+10. Make diagram of the circuit of a four-party station with relay.
+
+11. Describe the process of tuning in the harmonic system.
+
+12. What is the difference between the under-tune and in-tune systems?
+
+13. Sketch circuit of Kellogg's harmonic system.
+
+14. Illustrate the principle of a broken-line system by a sketch.
+
+15. In what particulars does the party-line system in rural districts
+differ from that within urban limits?
+
+16. Describe and sketch Pool's lock-out system.
+
+17. Make diagram of the K.B. lock-out system.
+
+18. What is the object of the ratchet in this system?
+
+19. Make diagram of simplified circuits of Roberts system.
+
+20. Sketch and describe Roberts latching key and connections.
+
+21. Sketch circuits of bridging station for non-selective party line.
+
+22. How would you arrange the signal code for six stations on a
+non-selective party line?
+
+23. What is the limit of number of stations on a non-selective party
+line under ordinary circumstances?
+
+24. State the objections against the party polarity system as shown in
+Fig. 172.
+
+25. What are the advantages of the harmonic party-line system?
+
+26. To how many frequencies is the harmonic system usually limited?
+
+27. What can you say about the commercial success of the step-by-step
+method?
+
+28. State the principles of a lock-out party line.
+
+29. For what purpose is a condenser placed in the receiver circuit of
+each station in the K.B. lock-out system?
+
+30. How are the selecting relays in Roberts line restored to their
+normal position after a conversation is finished?
+
+31. What are the objections against the Roberts system?
+
+
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 287--315
+
+       *       *       *       *       *
+
+1. What are electrical hazards?
+
+2. When is the lightning hazard least?
+
+3. What actions can electricity produce? Which involves the greater
+hazard to the value of property?
+
+4. When is a piece of apparatus called "self-protecting"?
+
+5. Why must a protector for telephone apparatus work more quickly for
+a large current than for a small one?
+
+6. State the general problem which heating hazards present with
+relation to telephone apparatus.
+
+7. What is the most nearly universal electrical hazard?
+
+8. Sketch and describe the saw-tooth lightning arrester.
+
+9. Make diagram of the carbon-block arrester and state its advantages.
+
+10. Describe a vacuum arrester.
+
+11. Explain the reason for placing an impedance in connection with the
+lightning arrester.
+
+12. What is the purpose of the globule of low-melting alloy in the
+Western Electric Company's arrester?
+
+13. Why are not fuses good lightning arresters?
+
+14. What is the proper function of a fuse?
+
+15. Make sketch of a mica slip fuse.
+
+16. Define _sneak currents_.
+
+17. Make a diagram of a sneak-current arrester and describe its
+principles and working.
+
+18. Describe a heat coil.
+
+19. Sketch a complete line protection.
+
+20. Where is the proper position of the fuse?
+
+21. Which wires are considered exposed and which unexposed?
+
+22. Why is it not necessary to install sneak-current arresters in
+central-battery subscribers' stations?
+
+23. Sketch and describe the action of a combined sneak-current and
+air-gap arrester, as widely used by Bell companies.
+
+24. Describe the self-soldering heat-coil arrester.
+
+25. What is the purpose of ribbon fuses?
+
+26. What is a drainage coil?
+
+
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 317--386
+
+       *       *       *       *       *
+
+1. What is a central office?
+
+2. What are (_a_) subscriber's lines? (_b_) Trunk lines? (_c_) Toll
+lines?
+
+3. For what purpose is the switchboard?
+
+4. Give short descriptions of the different classes of switchboards.
+
+5. How are manual switchboards subdivided? Describe briefly the
+different types.
+
+6. Define A and B boards.
+
+7. What is a call circuit?
+
+8. What kind of calls are handled on a toll switchboard?
+
+9. Give drop symbol and describe its principles.
+
+10. What is a jack?
+
+11. Make a sketch of a plug inserted into a jack.
+
+12. Give jack and plug symbols.
+
+13. What are ringing and listening keys?
+
+14. Show symbols for ringing and listening keys.
+
+15. State the parts of which a cord equipment consists.
+
+16. Show step by step the various operations of a telephone system
+wherein the lines center in a magneto switchboard. Make all the
+necessary diagrams and give brief descriptions to show that you
+understand each operation.
+
+17. On what principle does a drop with night-alarm contact operate?
+
+18. What is the advantage of associating jacks and drops?
+
+19. Describe the mechanical restoration as employed in the Miller
+drop and jack.
+
+20. Describe the electrical restoration of drop shutters as
+manufactured by the Western Electric Company.
+
+21. What complications arise in ringing of party lines and how are
+they overcome?
+
+22. Give diagram of the complete circuit of a simple magneto
+switchboard.
+
+23. Sketch night-alarm circuit with relay.
+
+24. What is a convertible cord circuit?
+
+25. State what disadvantages may be encountered under certain
+conditions with a bridging drop-cord circuit.
+
+26. Are lamps in cord circuits to be advocated on magneto
+switchboards?
+
+27. What is the function of the cabinet?
+
+28. Give cross-section of upright switchboard as used in the magneto
+system.
+
+29. What is the purpose of a sectional switchboard?
+
+30. Give a short description of the essential parts of a sectional
+switchboard.
+
+
+
+
+INDEX
+
+
+
+
+INDEX
+
+_The page numbers of this volume will be found at the bottom of the
+pages; the numbers at the top refer only to the section._
+
+
+A
+
+Acousticon transmitter
+Acoustics
+    characteristics of sound
+        loudness
+        pitch
+        timbre
+    human ear
+    human voice
+    propagation of sound
+Air-gap vs. fuse arresters
+Amalgamated zincs
+Arrester separators
+Audible signals
+    magneto bell
+    telegraph sounder
+    telephone receiver
+    vibrating bell
+Automatic Electric Company
+    direct-current receiver
+    transmitter
+Automatic shunt
+
+
+B
+
+Bar electromagnet
+Battery bell
+Battery symbols
+Blake single electrode
+Brazed bell
+Broken-back ringer
+Broken-line method of selective signaling
+
+
+C
+
+Capacity reactance
+Carbon
+    adaptability
+    limitations
+    preparation of
+    superiority
+Carbon air-gap arrester
+Carbon-block arrester
+Carrying capacity of transmitter
+Central-office protectors
+Characteristics of sound
+    loudness
+    pitch
+    timbre
+Chloride of silver cell
+Closed-circuit cells
+Closed-circuit impedance coil
+Common-battery telephone sets
+Condensers
+    capacity
+    charge
+    conventional symbols
+    definition of
+    dielectric
+    dielectric materials
+    functions
+    means for assorting current
+    sizes
+    theory
+Conductivity of conductors
+Conductors, conductivity of
+Conventional symbols
+Cook
+    air-gap arrester
+    arrester
+    arrester for magneto stations
+Crowfoot cell
+Current supply to transmitters
+    common battery
+        advantages
+        bell substation arrangement
+        bridging battery with impedance coils
+        bridging battery with repeating coil
+        current supply from distant point
+        current supply over limbs of line in parallel
+        Dean substation arrangement
+        double battery with impedance coil
+        Kellogg substation arrangement
+        North Electric Company system
+        series battery
+        series substation arrangement
+        Stromberg-Carlson system
+        supply many lines from common source
+            repeating coil
+            retardation coil
+  local battery
+
+
+D
+
+Dean
+  drop and jack
+  receiver
+  wall telephone hook
+Desk stand hooks
+  Kellogg
+  Western Electric
+Dielectric
+Dielectric materials
+  dry paper
+  mica
+Differential electromagnet
+Direct-current receiver
+Drainage coils
+
+
+E
+
+Electric lamp signal
+Electrical hazards
+Electrical reproduction of speech
+  carbon
+  conversion from sound waves to vibration of diaphragm
+  conversion from vibration to voice currents
+  conversion from voice currents to vibration
+  cycle of conversion
+  detrimental effects of capacity
+  early conceptions
+  electrostatic telephone
+  induction coil
+  limitations of magneto transmitter
+  loose contact principle
+  magneto telephone
+  measurements of telephone currents
+  variation of electrical pressure
+  variation of resistance
+Electrical signals
+  audible
+    magneto-bell
+    telegraph sounder
+    telephone receiver
+    vibrating bell
+  visible
+    electric lamp signal
+    electromagnetic signal
+Electrodes
+  arrangement of
+  carbon preparation
+  multiple
+  single
+Electrolysis
+Electromagnetic method of measuring telephone currents
+Electromagnetic signal
+Electromagnets and inductive coils
+  conventional symbols
+  differential electromagnet
+  direction of armature motion
+  direction of lines of force
+  electromagnets
+  low-resistance circuits
+    horseshoe form
+    iron-clad form
+    special horseshoe form
+  impedance coils
+    kind of iron
+    number of turns
+    types
+      closed-circuit
+      open-circuit
+      toroidal
+  induction coil
+    current and voltage ratios
+    design
+    functions
+    use and advantage
+  magnet wire
+    enamel
+    silk and cotton insulation
+    space utilization
+    wire gauges
+  magnetic flux
+  magnetization curves
+  magnetizing force
+  mechanical details
+  permeability
+  reluctance
+  repeating coil
+  winding methods
+    winding calculations
+    winding data
+    winding terminals
+Electrostatic capacity
+  unit of
+Electrostatic telephone
+Enamel
+
+
+F
+
+Five-bar generator
+Fuller cell
+
+
+G
+
+Galvani
+Generator armature
+Generator cut-in switch
+Generator shunt switch
+Generator symbols
+Granular carbon
+Gravity cell
+
+
+H
+
+Hand receivers
+Harmonic method of selective signaling
+  advantages
+  circuits
+  in-tune system
+  limitations
+  principles
+  tuning
+  under-tune system
+Head receivers
+Heat coil
+Holtzer-Cabot arrester
+Hook switch
+  automatic operation
+  contact material
+  design
+  desk stand hooks
+    Kellogg
+    Western Electric
+  purpose
+  symbols
+  wall telephone hooks
+    Dean
+    Kellogg
+    Western Electric
+Horseshoe electromagnet
+Human ear
+Human voice
+
+
+I
+
+Impedance coils
+  kind of iron
+  number of turns
+  symbols of
+  types
+    closed-circuit
+    open-circuit
+    toroidal
+Inductance vs. capacity
+Induction coil
+  current and voltage ratios
+  design
+  functions
+  use and advantage
+Inductive neutrality
+Inductive reactance
+Insulation of conductors
+Introduction to telephony
+Iron-clad electromagnet
+Iron wire ballast
+
+
+K
+
+Kellogg
+  air-gap arrester
+  desk stand hook
+  drop and jack
+  receiver
+  ringer
+  transmitter
+  wall telephone hook
+
+L
+
+Lalande cell
+Lamp filament
+Le Clanche cell
+Lenz law
+Line signals
+Lines of force, direction of
+Loading coils
+Lock-out party-line systems
+  broken-line method
+  operation
+  Poole system
+  step-by-step system
+Loudness of sound
+Low-reluctance circuits
+  horseshoe form
+  iron-clad form
+
+
+M
+
+Magnetic flux
+Magnetization curves
+Magnetizing force
+Magneto bell
+Magneto operator
+Magneto signaling apparatus
+  armature
+  automatic shunt
+  battery bell
+  generator symbols
+  magneto bell
+  magneto generator
+  method of signaling
+  polarized ringer
+  pulsating current
+  ringer symbols
+  theory
+Magneto switchboard
+  automatic restoration
+    mechanical
+      Dean type
+      Kellogg type
+      Monarch type
+      Western Electric type
+  circuits of complete switchboard
+  code signaling
+  commercial types of drops and jacks
+    early drops
+    jack mounting
+    manual vs. automatic restoration
+    methods of associating
+    night alarm
+    tubular drops
+  component parts
+    jacks and plugs
+    keys
+    line and cord equipments
+    line signal
+    operators' equipment
+  cord-circuit considerations
+    double clearing-out type
+    lamp-signal type
+    non-ring through type
+    series drop type
+    simple bridging drop type
+  definitions
+  electrical restoration
+  grounded and metallic-circuit lines
+  mode of operation
+  night-alarm circuits
+  operation in detail
+    clearing out
+    essentials of operation
+    normal condition of line
+    operator answering
+    operator calling
+    subscriber calling
+    subscribers conversing
+    operator's telephone equipment
+    cut-in jack
+  ringing and listening keys
+    horizontal spring type
+    party-line ringing keys
+    self-indicating keys
+    vertical spring type
+  switchboard assembly
+    functions of cabinet
+    sectional switchboards
+    upright type of switchboard
+    wall type switchboard
+  switchboard cords
+    concentric conductors
+    parallel tinsel conductors
+    steel spiral conductors
+  switchboard plugs
+Magneto telephone
+Magneto telephone sets
+Mica card resistance
+Mica slip fuse
+Microtelephone set
+Monarch drop and jack
+Monarch receiver
+Monarch transmitter
+Multiple electrode
+Mutual induction
+
+
+N
+
+Non-inductive resistance devices
+  inductive neutrality
+  provisions against heating
+  temperature coefficient
+  types
+    differentially-wound unit
+    iron wire ballast
+    lamp filament
+    mica card unit
+Non-selective party-line systems
+  bridging
+  limitations
+  series
+  signal code
+
+
+O
+
+Open-circuit cells
+Open-circuit impedance coil
+Operator's receiver
+
+
+P
+
+Packing of transmitters
+Permeability
+Pitch
+  Doppler's principle
+  vibration of diaphragms
+Polarity method of selective signaling
+Polarization of cells
+Polarized ringer
+  brazed bell
+  Kellogg
+  Western Electric
+Poole lock-out system
+Primary cells
+  conventional symbol
+  series and multiple connections
+  simple voltaic
+  types of
+    closed-circuit
+      Fuller
+      gravity
+      Lalande
+      prevention of creeping
+      setting up
+    open-circuit
+      Le Clanche
+    standard
+      chloride of silver
+Propagation of sound
+Protective means
+  against high potentials
+    air-gap arrester
+      advantages of carbon
+      commercial types
+      continuous arcs
+      discharge across gaps
+      dust between carbons
+      introduction of impedance
+      metallic electrodes
+      vacuum arresters
+    against sneak currents
+      heat coil
+      sneak-current arresters
+    against strong currents
+      fuses
+        enclosed
+        mica
+        proper functions
+    central-office protectors
+        self-soldering heat coils
+        sneak-current and air-gap arrester
+    city exchange requirements
+    complete line protection
+    electrolysis
+    subscribers' station protectors
+      ribbon fuses
+Pulsating-current commutator
+
+
+R
+
+Receivers
+  Dean
+  direct-current
+  early
+  Kellogg
+  modern
+  Monarch
+  operator's
+  single-pole
+  symbols
+  Western Electric
+Reluctance
+Repeating coil
+Ribbon fuses
+Ringer symbols
+Ringing and listening key
+Robert's latching relay
+Robert's self-cleansing arrester
+Rolled condenser
+
+
+S
+
+Saw-tooth arrester
+Selective party-line systems
+  broken-line method
+  classification
+    broken-line systems
+    harmonic systems
+    polarity systems
+    step-by-step systems
+  harmonic method
+  polarity method
+  step-by-step method
+Self-induction
+Signal code
+Signaling, method of
+Silk and cotton insulation
+Single electrode
+Single-pole receiver
+Sneak-current arresters
+Solid-back transmitter
+Sound
+  characteristics of
+    loudness
+    pitch
+    timbre
+Standard cell
+Step-by-step lock-out system
+Step-by-step method of selective signaling
+Subscribers' station protectors
+Switchboard cords
+Switchboard plugs
+Switchboard transmitter
+Symbols
+  battery
+  condenser
+  generator
+  hook switch
+  impedance coil
+  induction coil
+  receiver
+  repeating coil
+  ringer
+  ringing and listening key
+  transmitter
+
+
+T
+
+Table
+  condenser data
+  copper wire
+  German silver wire--18 per cent
+  German silver wire--30 per cent
+  metals, behavior of, in different electrolysis
+  signal code
+  specific inductive capacities
+  temperature coefficients
+  transmission distances, limiting
+  winding data for insulating wires
+Tandem differential electromagnet
+Telegraph sounder
+Telephone currents, measurements of
+  electromagnetic method
+  thermal method
+Telephone exchange, features of
+  districts
+  subscribers' lines
+  switchboards
+  toll lines
+  trunk lines
+Telephone lines
+  conductivity of conductors
+  electrostatic capacity
+  inductance of circuit
+  inductance vs. capacity
+  insulation of conductors
+  transmission
+Telephone sets
+  classification of
+    common-battery telephone
+    magneto telephone
+    wall and desk telephones
+  common-battery
+    desk
+    hotel
+    wall
+  magneto
+    circuits of
+      bridging
+      series
+    desk
+    wall
+Temperature coefficients
+Thermal method of measuring telephone currents
+Timbre
+Toroidal impedance coil
+Toroidal repeating coil
+Transmission, ways of improving
+Transmitters
+  acousticon
+  Automatic Electric Company
+  carrying capacity
+  conventional diagram
+  electrode
+    arrangement of
+    multiple
+    single
+  granular carbon
+  Kellogg
+  materials
+  Monarch
+  packing
+  sensitiveness
+  switchboard
+  symbols
+  variable resistance
+  Western Electric solid-back
+
+
+U
+
+Under-tuned ringer
+
+
+V
+
+Vacuum arrester
+Variable resistance
+Vibrating bell
+Visible signals
+  electric lamp
+  electromagnetic
+Volta
+Voltaic cell
+  amalgamated zincs
+  difference of potential
+  local action
+  polarization
+  theory
+
+
+W
+
+Wall telephone hooks
+  Dean
+  Kellogg
+  Western Electric
+Western Electric
+  air-gap arrester
+  desk stand hook
+  drop and jack
+  receiver
+  ringer
+  solid-back transmitter
+  station arrester
+  wall telephone hook
+White transmitter
+Wire gauges
+
+
+
+
+
+End of the Project Gutenberg EBook of Cyclopedia of Telephony & Telegraphy
+Vol. 1, by Kempster Miller, George Patterson, Charles Thom, Robert Millikan,
+Samuel McMeen
+
+*** END OF THIS PROJECT GUTENBERG EBOOK CYCLOPEDIA OF TELEPHONY 1 ***
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