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+The Project Gutenberg EBook of Cyclopedia of Telephony and Telegraphy,
+Vol. 2, by Kempster Miller and George Patterson and Charles Thom and Robert Millikan and 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 and Telegraphy, Vol. 2
+       A General Reference Work on Telephony, etc. etc.
+
+Author: Kempster Miller
+        George Patterson
+        Charles Thom
+        Robert Millikan
+        Samuel McMeen
+
+Release Date: August 15, 2010 [EBook #33437]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK TELEPHONY AND TELEGRAPHY, VOL 2 ***
+
+
+
+
+Produced by Ronald Holder, Stephen H. Sentoff and the
+Online Distributed Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+
+[Illustration: THOMAS A. EDISON Pioneer Electrical Investigator and
+Inventor of Numerous Telegraph, Telephone, Lighting, and Other
+Electrical Devices.]
+
+
+
+
+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
+
+
+COPYRIGHT, 1911, 1912,
+BY
+AMERICAN SCHOOL OF CORRESPONDENCE
+
+
+COPYRIGHT, 1911, 1912
+BY
+AMERICAN TECHNICAL SOCIETY
+
+
+Entered at Stationers' Hall, London
+All Rights Reserved
+
+
+
+
+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, _College 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: GROSSE POINT EXCHANGE RACK Detroit Home Telephone
+Company, Detroit, Mich. _The Dean Electric Co._]
+
+[Illustration: LINE SIDE OF LARGE MAIN DISTRIBUTING FRAME]
+
+
+
+
+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 II
+
+
+MANUAL SWITCHBOARDS _By K. B. Miller and S. G. McMeen_[A] Page[B] 11
+
+Common-Battery Switchboards--Line Signals--Cord
+Circuit--Lamps--Mechanical Signals--Relays--Jacks--Switchboard
+Assembly--Transfer Switchboard--Transfer Lines--Handling
+Transfers--Multiple Switchboard--Busy Test--Influence of
+Traffic--Magneto-Multiple Switchboard--Multiple Boards: Series,
+Branch-Terminal, Modern Magneto, Common-Battery--Western Electric No. 1
+Relay Board--Western Electric No. 10 Board--Types of Multiple
+Boards--Apparatus--Trunking--Western Electric and Kellogg Trunk Circuits
+
+AUTOMATIC SYSTEMS _By K. B. Miller and S. G. McMeen_ Page 135
+
+Automatic vs. Manual--Operation--Selecting Switch--Line Switch--Trunking
+Systems--Two- and Three-Wire Systems--Subscriber's Station
+Apparatus--First and Second Selector Operation--Connector--Release after
+Conversation--Multi-Office System--Automatic Sub-Offices--Rotary
+Connector--Party Lines--Two-Wire Automatic System--Lorimer
+System--Central-Office Apparatus--Operation--Automanual
+System--Operation--Subscriber's Apparatus--Operator's
+Equipment--Switching Equipment--Distribution of Calls--Connection--Speed
+
+POWER PLANTS AND BUILDINGS _By K. B. Miller and S. G. McMeen_ Page 227
+
+Currents Employed--Types--Operator's Transmitter Supply--Ringing-Current
+Supply--Auxiliary Signaling Current--Primary Sources--Duplicate
+Apparatus--Storage Batteries--Power
+Switchboards--Circuits--Central-Office Building--Arrangement of
+Apparatus--Manual Offices--Automatic Offices
+
+SPECIAL SERVICE FEATURES _By K. B. Miller and S. G. McMeen_ Page 271
+
+Private-Branch Exchanges--Switchboards--Supervision--With Automatic
+Offices--Battery Supply--Ringing Current--Inter-Communicating
+Systems--Magneto System--Common-Battery Systems--Types--Long-Distance
+Switching--Operator's Orders--Trunking--Way Stations--Traffic--Measured
+Service--Charging--Rates--Toll Service--Local Service
+
+TELEGRAPH AND RAILWAY WORK _By K. B. Miller and S. G. McMeen_ Page 321
+
+Phantom, Simplex, and Composite Circuits--Ringing--Railway
+Composite--Telephone Train Dispatching--Railroad
+Conditions--Transmitting Orders--Apparatus--Telephone Equipment--Types
+of Circuits--Test Boards--Blocking Sets--Dispatching on Electric
+Railways
+
+REVIEW QUESTIONS Page 359
+
+INDEX Page 373
+
+[Footnote A: For professional standing of authors, see list of Authors
+and Collaborators at front of volume.]
+
+[Footnote B: For page numbers, see foot of pages.]
+
+[Illustration: PORTION OF TERMINAL ROOM OF LARGE COMMON-BATTERY OFFICE
+Prospect Office, New York Telephone Co.]
+
+
+
+
+CHAPTER XXII
+
+THE SIMPLE COMMON-BATTERY SWITCHBOARD
+
+
+=Advantages of Common-Battery Operation.= The advantages of the
+common-battery system of operation, alluded to in Chapter XIII, may be
+briefly summarized here. The main gain in the common-battery system of
+supply is the simplification of the subscribers' instruments, doing away
+with the local batteries and the magneto generators, and the
+concentration of all these many sources of current into one single
+source at the central office. A considerable saving is thus effected
+from the standpoint of maintenance, since the simpler common-battery
+instrument is not so likely to get out of order and, therefore, does not
+have to be visited so often for repairs, and the absence of local
+batteries, of course, makes the renewal of the battery parts by members
+of the maintenance department, unnecessary. Another decided advantage in
+the common-battery system is the fact that the centralized battery
+stands ready always to send current over the line when the subscriber
+completes the circuit of the line at his station by removing his
+receiver from its hook. The common-battery system, therefore, lends
+itself naturally to the purposes of automatic signaling, since it is
+only necessary to place at the central office a device in the circuit of
+each line that will be responsive to the current which flows from the
+central battery when the subscriber removes his receiver from its hook.
+It is thus that the subscriber is enabled automatically to signal the
+central office when he desires a connection; and as will be shown, it is
+by the same sort of means, associated with the cord circuits used in
+connecting his line with some other line, that the operator is
+automatically notified when a disconnection is desired, the cessation of
+current through the subscriber's line when he hangs up his receiver
+being made to actuate certain responsive devices which are associated
+with the cord at that time connected with his line, and which convey the
+proper disconnect signal to the operator.
+
+Concentration of sources of energy into a single large unit, the
+simplification of the subscriber's station equipment, and the ready
+adaptability to automatic signaling from the subscriber to the central
+office are, therefore, the reasons for the existence of the
+common-battery system.
+
+=Common Battery vs. Magneto.= It must not be supposed, however, that the
+common-battery system always has advantages over the magneto system, and
+that it is superior to the magneto or local-battery system for all
+purposes. It is the outward attractiveness of the common-battery system
+and the arguments in its favor, so readily made by over-zealous
+salesmen, that has led, in many cases, to the adoption of this system
+when the magneto system would better have served the purpose of utility
+and economy.
+
+To say the least, the telephone transmission to be had from
+common-battery systems is no better than that to be had from
+local-battery systems, and as a rule, assuming equality in other
+respects, it is not as good. It is perhaps true, however, that under
+average conditions common-battery transmission is somewhat better,
+because whereas the local batteries at the subscribers' stations in the
+local-battery system are not likely to be in uniformly first-class
+condition, the battery in a common-battery system will be kept up to its
+full voltage except under the grossest neglect.
+
+The places in which the magneto, or local-battery, system is to be
+preferred to the common-battery system, in the opinion of the writers,
+are to be found in the small rural communities where the lines have a
+rather great average length; where a good many subscribers are likely to
+be found on some of the lines; where the sources of electrical power
+available for charging storage batteries are likely either not to exist,
+or to be of a very uncertain nature; and where it is not commercially
+feasible to employ a high-grade class of attendants, or, in fact, any
+attendant at all other than the operator at the central office.
+
+In large or medium-sized exchanges it is always possible to procure
+suitable current for charging the storage batteries required in
+common-battery systems, and it is frequently economical, on account of
+the considerable quantity of energy that is thus used, to establish a
+generating plant in connection with the central office for developing
+the necessary electrical energy. In very small rural places there are
+frequently no available sources of electrical energy, and the expense of
+establishing a power plant for the purpose cannot be justified. But even
+if there is an electric light or railway system in the small town, so
+that the problem of available current supply does not exist, the
+establishment of a common-battery system with its storage battery and
+the necessary charging machinery requires the daily attendance at the
+central office of some one to watch and care for this battery, and this,
+on account of the small gross revenue that may be derived from a small
+telephone system, often involves a serious financial burden.
+
+There is no royal road to a proper decision in the matter, and no sharp
+line of demarcation may be drawn between the places where common-battery
+systems are superior to magneto and _vice versa_. It may be said,
+however, that in the building of all new telephone plants having over
+about 500 local subscribers, the common-battery system is undoubtedly
+superior to the magneto. If the plant is an old one, however, and is to
+be re-equipped, the continuance of magneto apparatus might be justified
+for considerably larger exchanges than those having 500 subscribers.
+
+Telephone operating companies who have changed over the equipment of old
+plants from magneto to common battery have sometimes been led into
+rather serious difficulty, owing to the fact that their lines, while
+serving tolerably well for magneto work, were found inadequate to meet
+the more exacting demands of common-battery work. Again in an old plant
+the change from magneto to common-battery equipment involves not only
+the change of switchboards, but also the change of subscribers'
+instruments that are otherwise good, and this consideration alone often,
+in our opinion, justifies the replacing of an old magneto board with a
+new magneto board, even if the exchange is of such size as to demand a
+small multiple board.
+
+Where the plant to be established is of such size as to leave doubt as
+to whether a magneto or a common-battery switchboard should be employed,
+the questions of availability of the proper kind of power for charging
+the batteries, the proper kind of help for maintaining the batteries and
+the more elaborate central-office equipment, the demands and previous
+education of the public to be served, all are factors which must be
+considered in reaching the decision.
+
+It is not proper to say that anything like all exchanges having fewer
+than 500 local lines, should be equipped with magneto service. Where all
+the lines are short, where suitable power is available, and where a good
+grade of attendants is available--as, for instance, in the case of
+private telephone exchanges that serve some business establishment or
+other institution located in one building or a group of buildings--the
+common-battery system is to be recommended and is largely used, even
+though it may have but a dozen or so subscribers' lines. It is for such
+uses, and for use in those regular public-service exchange systems where
+the conditions are such as to warrant the common-battery system, and yet
+where the number of lines and the traffic are small enough to be handled
+by such a small group of operators that any one of them may reach over
+the entire face of the board, that the simple non-multiple
+common-battery system finds its proper field of usefulness.
+
+=Line Signals.= The principles and means by which the subscriber is
+enabled to call the central-office operator in a common-battery system
+have been referred to briefly in Chapter III. We will review these at
+this point and also consider briefly the way in which the line signals
+are associated with the connective devices in the subscribers' lines.
+
+_Direct-Line Lamp._ The simplest possible way is to put the line signal
+directly in the circuit of the line in series with the central-office
+battery, and so to arrange the jack of the corresponding line that the
+circuit through the line signal will be open when the operator inserts a
+plug into that jack. This arrangement is shown in Fig. 307 where the
+subscriber's station at the left is indicated in the simplest of its
+forms. It is well to repeat here that in all common-battery manual
+systems, the subscriber's station equipment, regardless of the
+arrangement or type of its talking and signaling apparatus, must have
+these features: First, that the line shall be normally open to direct
+currents at the subscriber's station; second, that the line shall be
+closed to direct currents when the subscriber removes his receiver from
+its hook in making or in answering a call; third, that the line
+normally, although open to direct currents, shall afford a proper path
+for alternating or varying currents through the signal receiving device
+at the sub-station. The subscriber's station arrangement shown in Fig.
+307, and those immediately following, is the simplest arrangement that
+possesses these three necessary features for common-battery service.
+
+[Illustration: Fig. 307. Direct-Line Lamp]
+
+Considering the arrangement at the central office, Fig. 307, the two
+limbs of the line are permanently connected to the tip and sleeve
+contacts of the jack. These two main contacts of the jack normally
+engage two anvils so connected that the tip of the jack is ordinarily
+connected through its anvil to ground, while the sleeve of the jack is
+normally connected through its anvil to a circuit leading through the
+line signal--in this case a lamp--and the common battery, and thence to
+ground. The operation is obvious. Normally no current may flow from the
+common battery through the signal because the line is open at the
+subscriber's station. The removal of the subscriber's receiver from its
+hook closes the circuit of the line and allows the current to flow
+through the lamp, causing it to glow. When the operator inserts the plug
+into the jack, in response to the call, the circuit through the lamp is
+cut off at the jack and the lamp goes out.
+
+This arrangement, termed the direct-line lamp arrangement, is largely
+used in small common-battery telephone systems where the lines are very
+short, such as those found in factories or other places where the
+confines of the exchange are those of a building or a group of
+neighboring buildings. Many of the so-called private-branch exchanges,
+which will be considered more in detail in a later chapter, employ this
+direct-line lamp arrangement.
+
+[Illustration: Fig. 308. Direct-Line Lamp with Ballast]
+
+_Direct-Line Lamp with Ballast._ Obviously, however, this direct-line
+lamp arrangement is not a good one where the lines vary widely in length
+and resistance. An incandescent lamp, as is well known, must not be
+subjected to too great a variation in current. If the current that is
+just right in amount to bring it to its intended degree of illumination
+is increased by a comparatively small amount, the life of the lamp will
+be greatly shortened, and too great an increase will result in the
+lamp's burning out immediately. On the other hand, a current that is too
+small will not result in the proper illumination of the lamp, and a
+current of one-half the proper normal value will just suffice to bring
+the lamp to a dull red glow. With lines that are not approximately
+uniform in length and resistance the shorter lines would afford too
+great a flow of current to the lamps and the longer lines too little,
+and there is always the danger present, unless means are taken to
+prevent it, that if a line becomes short-circuited or grounded near the
+central office, the lamp will be subjected to practically the full
+battery potential and, therefore, to such a current as will burn it out.
+One of the very ingenious and, we believe, promising methods that has
+been proposed to overcome this difficulty is that of the iron-wire
+ballast, alluded to in Chapter III. This, it will be remembered,
+consists of an iron-wire resistance enclosed in a vacuum chamber and so
+proportioned with respect to the flow of current that it will be
+subjected to a considerable heating effect by the amount of current that
+is proper to illuminate the lamp. As has already been pointed out,
+carbon has a negative temperature coefficient, that is, its resistance
+decreases when heated. Iron, on the other hand, has a positive
+temperature coefficient, its resistance increasing when heated. When
+such an iron-wire ballast is put in series with the incandescent lamp
+forming the line signal, as shown in Fig. 308, it is seen that the
+resistance of the carbon in the lamp filament and of the iron in the
+ballast will act in opposite ways when the current increases or
+decreases. An increase of current will tend to heat up the iron wire of
+the ballast and, therefore, increase its resistance, and the ballast is
+so proportioned that it will hold the current that may flow through the
+lamp within the proper maximum and minimum limits, regardless of the
+resistance of the line in which the lamp is used. This arrangement has
+not gone into wide use up to the present time.
+
+_Line Lamp with Relay._ By far the most common method of associating the
+line lamp with the line is to employ a relay, of which the actuating
+coil is in the line circuit, this relay serving to control a local
+circuit containing the battery and the lamp. This arrangement and the
+way in which these parts are associated with the jack are clearly
+indicated in Fig. 309. Here the relay may receive any amount of current,
+from the smallest which will cause it to pull up its armature, to the
+largest which will not injure its winding by overheat. Relays may be
+made which will attract their armatures at a certain minimum current and
+which will not burn out when energized by currents about ten times as
+large, and it is thus seen that a very large range of current through
+the relay winding is permissible, and that, therefore, a very great
+latitude as to line resistance is secured. On the other hand, it is
+obvious that the lamp circuit, being entirely local, is of uniform
+resistance, the lamp always being subjected, in the arrangement shown,
+to practically the full battery potential, the lamp being selected to
+operate on that potential.
+
+[Illustration: Fig. 309. Line Lamp with Relay]
+
+_Pilot Signals._ In the circuits of Figs. 307, 308, and 309, but a
+single line and its associated apparatus is shown, and it may not be
+altogether clear to the uninitiated how it is that the battery shown in
+those figures may serve, without interference of any function, a larger
+number of lines than one. It is to be remembered that this battery is
+the one which serves not only to operate the line signals, but also to
+supply talking current to the subscribers and to supply current for the
+operation of the cord-circuit signals after the cord circuits are
+connected with the lines.
+
+In Fig. 310 this matter is made clear with respect to the association of
+this common battery with the lines for operating the line signals, and
+also another important feature of common-battery work is brought out,
+viz, the pilot lamp and its association with a group of line lamps.
+Three subscribers' lines only are shown, but this serves clearly to
+illustrate the association of any larger number of lines with the common
+battery. Ignoring at first the pilot relay and the pilot lamp, it will
+be seen that each of the tip-spring anvils of the jacks is connected to
+a common wire _1_ which is grounded. Each of the sleeve-contact anvils
+is connected through the coil of the line relay to another common wire
+_2_, which connects with the live side of the common battery. Obviously,
+therefore, this arrangement corresponds with that of Fig. 309, since the
+battery may furnish current to energize any one of the line relays upon
+the closure of the circuit of the corresponding line. Each of the relay
+armatures in Fig. 310 is connected to ground.
+
+Here we wish to bring out an important thing about telephone circuit
+diagrams which is sometimes confusing to the beginner, but which really,
+when understood, tends to prevent confusion. The showing of a separate
+ground for each of the line-relay armatures does not mean that literally
+each one of these armatures is connected by a separate wire to earth,
+and it is to be understood that the three separate grounds shown in
+connection with these relay armatures is meant to indicate just such a
+set of affairs as is shown in connection with the tip-spring anvils of
+the jacks, all of which are connected to a common wire which, in turn,
+is grounded. Obviously, the result is the same, but in the case of this
+particular diagram it is seen that a great deal of crossing of lines is
+prevented by showing a separate ground at each one of the relay
+armatures. The same practice is followed in connection with the common
+battery. Sometimes it is very inconvenient in a complicated diagram to
+run all of the wires that are supposed to connect with one terminal of
+the battery across the diagram to represent this connection. It is
+permissible, therefore, and in fact desirable, that separate battery
+symbols be shown wherever by so doing the diagram will be simplified,
+the understanding being, in the absence of other information or of other
+indications, that the same battery is referred to, just as the same
+ground is referred to in connection with the relay armatures in the
+figure under discussion.
+
+Each line lamp in Fig. 310 is shown connected on one hand to its
+corresponding line relay contact and on the other hand to a common wire
+which leads through the winding of the pilot relay to the live side of
+the battery. It is obvious here that whenever any one of the line relays
+attracts its armature the local circuit containing the corresponding
+lamp and the common battery will be closed and the lamp illuminated.
+
+Whenever any line relay operates, the current, which is supplied to its
+lamp, must come through the pilot-relay winding, and if a number of line
+relays are energized, then the current flow of the corresponding lamps
+must flow through this relay winding. Therefore, this relay winding must
+be of low resistance, so that the drop through its winding may not be
+sufficient to interfere with the proper burning of the lamps, even
+though a large number of lamps be fed simultaneously through it. The
+pilot relay must be so sensitive that the current, even through one
+lamp, will cause it to attract its armature. When it does attract its
+armature it causes illumination of the pilot lamp in the same way that
+the line relays cause the illumination of the line lamps.
+
+The pilot lamp, which is commonly associated with a group of line lamps
+that are placed on any one operator's position of the switchboard, is
+located in a conspicuous place in the switchboard cabinet and is
+provided with a larger lens so as to make a more striking signal. As a
+result, whenever any line lamp on a given position lights, the pilot
+lamp does also and serves to attract the attention, even of those
+located in distant portions of the room, to the fact that a call exists
+on that position of the board, the line lamp itself, which is
+simultaneously lighted, pointing out the particular line on which the
+call exists.
+
+Pilot lamps, in effect, perform similar service to the night alarm in
+magneto boards, but, of course, they are silent and do not attract
+attention unless within the range of vision of the operator. They are
+used not only in connection with line lamps, but also in connection with
+the cord-circuit lamps or signals, as will be pointed out.
+
+[Illustration: Fig. 311. Battery Supply Through Impedance Coils]
+
+[Illustration: Fig. 312. Battery Supply through Repeating Coils]
+
+[Illustration: Fig. 313. Battery Supply with Impedance Coils and
+Condensers]
+
+=Cord Circuit.= _Battery Supply._ Were it not for the necessity of
+providing for cord-circuit signals in common-battery switchboards, the
+common-battery cord circuit would be scarcely more complex than that for
+magneto working. Stripped of all details, such as signals, ringing and
+listening keys, and operator's equipment, cord circuits of three
+different types are shown in Figs. 311, 312, and 313. These merely
+illustrate the way in which the battery is associated with the cord
+circuits and through them with the line circuits for supplying current
+for talking purposes to the subscribers. It is thought that this matter
+will be clear in view of the discussion of the methods by which current
+is supplied to the subscribers' transmitters in common-battery systems
+as discussed in Chapter XIII. While the arrangements in this respect of
+Figs. 311, 312, and 313 illustrate only three of the methods, these
+three are the ones that have been most widely and successfully used.
+
+_Supervisory Signals._ The signals that are associated with the cord
+circuits are termed supervisory signals because of the fact that by
+their means the operator is enabled to supervise the condition of the
+lines during times when they are connected for conversation. The
+operation of these supervisory signals may be best understood by
+considering the complete circuits of a simple switchboard and must be
+studied in conjunction with the circuits of the lines as well as those
+of the cords.
+
+[Illustration: Fig. 314. Simple Common-Battery Switchboard]
+
+_Complete Circuit._ Such complete circuits are shown in Fig. 314. The
+particular arrangement indicated is that employed by the Kellogg
+Company, and except for minor details may be considered as typical of
+other makes also. Two subscribers' lines are shown extending from
+Station A and Station B, respectively, to the central office. The line
+wires are shown terminating in jacks in the same manner as indicated in
+Figs. 307, 308, and 309, and their circuits are normally continued from
+these jacks to the ground on one side and to the line relay and battery
+on the other. The jack in this case has three contacts adapted to
+register with three corresponding contacts in each of the plugs. The
+thimble of the jack in this case forms no part of the talking circuit
+and is distinct from the two jack springs which form the line terminals.
+It and the auxiliary contact _1_ in each of the plugs with which it
+registers, are solely for the purpose of co-operating in the control of
+the supervisory signals.
+
+The tip and sleeve strands of the cord are continuous from one plug to
+the other except for the condensers. The two batteries indicated in
+connection with the cord circuit are separate batteries, a
+characteristic of the Kellogg system. One of these batteries serves to
+supply current to the tip and sleeve strand of the cord circuit through
+the two windings _3_ and _4_, respectively, of the supervisory relay
+connected with the answering side of the cord circuit, while the other
+battery similarly supplies current through the windings _5_ and _6_ of
+the supervisory relay associated with the calling side of the cord
+circuit. The windings of these relays, therefore, act as impedance coils
+and the arrangement by which battery current is supplied to the cord
+circuits and, therefore, to the lines of the connected subscribers, is
+seen to be the combined impedance coil and condenser arrangement
+discussed in Chapter XIII.
+
+As soon as a plug is inserted into the jack of a line, the line relay
+will be removed from the control of the line, and since the two strands
+of the cord circuit now form continuations of the two line conductors,
+the supervisory relay will be substituted for the line relay and will be
+under control of the line. Since all of the current which passes to the
+line after a plug is inserted must pass through the cord-circuit
+connection and through the relay windings, and since current can only
+flow through the line when the subscriber's receiver is off its hook, it
+follows that the supervisory relays will only be energized after the
+corresponding plug has been inserted into a jack of the line and after
+the subscriber has removed his receiver. Unlike the line relays, the
+supervisory relays open their contacts to break the local circuits of
+the supervisory lamps _7_ and _8_ when the relay coils are energized,
+and to close them when de-energized; but the armatures of the
+supervisory relays do alone control the circuits of the supervisory
+lamps. These circuits are normally held open in another place, that is,
+between the plug contacts _1_ and the jack thimbles. It is only,
+therefore, when a plug is inserted into a jack and when the supervisory
+relay is de-energized, that the supervisory lamp may be lighted. When a
+plug is inserted into a jack and when the corresponding supervisory
+relay is de-energized, the circuit may be traced from ground at the
+cord-circuit batteries through the left-hand battery, for instance,
+through lamp _7_, thence through the contacts of the supervisory relay
+to the contact _1_ of the plug, thence through the thimble of the jack
+to ground. When a plug is inserted into the jack, therefore, the
+necessary arrangements are completed for the supervisory lamp to be
+under the control of the subscriber. Under this condition, whenever the
+subscriber's receiver is on its hook, the circuit of the line will be
+broken, the supervisory relay will be de-energized, and the supervisory
+lamp will be lighted. When, on the other hand, the subscriber's receiver
+is off its hook, the circuit of the line will be complete, the
+supervisory relay will be energized, and the supervisory lamp will be
+extinguished.
+
+_Salient Features of Supervisory Operation._ It will facilitate the
+student's understanding of the requirements and mode of operation of
+common-battery supervisory signals in manual systems, whether simple or
+multiple, if he will firmly fix the following facts in his mind. In
+order that the supervisory signal may become operative at all, some act
+must be performed by the operator--this being usually the act of
+plugging into a jack--and then, until the connection is taken down, the
+supervisory signal is under the control of the subscriber, and it is
+displayed only when the subscriber's receiver is placed on its hook.
+
+_Cycle of Operations._ We may now trace through the complete cycle of
+operations of the simple common-battery switchboard, the circuits of
+which are shown in Fig. 314. Assume all apparatus in its normal
+condition, and then assume that the subscriber at Station A removes his
+receiver from its hook. This pulls up the line relay and lights the line
+lamp, the pilot relay also pulling up and lighting the common pilot lamp
+which is not shown. In response to this call, the operator inserts the
+answering plug and throws her listening key _L.K._ The operator's
+talking set is thus bridged across the cord circuit and she is enabled
+to converse with the calling subscriber. The answering supervisory lamp
+_7_ did not light when the operator inserted the answering plug into the
+jack, because, although the contacts in the lamp circuit were closed by
+the plug contact _1_ engaging the thimble of the jack, the lamp circuit
+was held open by the attraction of the supervisory relay armature, the
+subscriber's receiver being off its hook. Learning that the called-for
+subscriber is the one at Station B, the operator inserts the calling
+plug into the jack at that station and presses the ringing key _R.K._,
+in order to ring the bell. The act of plugging in, it will be
+remembered, cuts off the line-signaling apparatus from connection with
+that line. As the subscriber at Station B was not at his telephone when
+called and his receiver was, therefore, on its hook, the insertion of
+the calling plug did not energize the supervisory relay coils _5_ and
+_6_, and, therefore, that relay did not attract its armature. The
+supervisory lamp _8_ was thus lighted, the circuit being from ground
+through the right-hand cord-circuit battery, lamp _8_, back contacts of
+the supervisory relay, third strand of the cord to contact _1_ of the
+calling plug, and thence to ground through the thimble of the jack. The
+lighting of this lamp is continued until the party at Station B responds
+by removing his receiver from its hook, which completes the line
+circuit, energizes relay windings _5_ and _6_, causes that relay to
+attract its armature, and thus break the circuit of the lamp _8_. Both
+supervisory lamps remain out as long as the two subscribers are
+conversing, but when either one of them hangs up his receiver the
+corresponding supervisory relay becomes de-energized and the
+corresponding lamp lights. When both of the lamps become illuminated,
+the operator knows that both subscribers are through talking and she
+takes down the connection.
+
+Countless variations have been worked in the arrangement of the line and
+cord circuits, but the general mode of operation of this particular
+circuit chosen for illustration is standard and should be thoroughly
+mastered. The operation of other arrangements will be readily understood
+from an inspection of the circuits, once the fundamental mode of
+operation that is common to all of them is well in mind.
+
+=Lamps.= The incandescent lamps used in connection with line and
+supervisory signals are specially manufactured, but differ in no sense
+from the larger lamps employed for general lighting purposes, save in
+the details of size, form, and method of mounting. Usually these lamps
+are rated at about one-third candle-power, although they have a somewhat
+larger candle-power as a rule. They are manufactured to operate on
+various voltages, the most usual operating pressures being 12, 24, and
+48 volts. The 24-volt lamp consumes about one-tenth of an ampere when
+fully illuminated, the lamp thus consuming about 2.4 watts. The 12- and
+48-volt lamps consume about the same amount of energy and corresponding
+amounts of current.
+
+[Illustration: Fig. 315. Switchboard Lamp]
+
+_Lamp Mounting._ The usual form of screw-threaded mounting employed in
+lamps for commercial lighting was at first applied to the miniature
+lamps used for switchboard work, but this was found unsatisfactory and
+these lamps are now practically always provided with two contact strips,
+one on each side of the glass bulb, these strips forming respectively
+the terminals for the two ends of the filament within. Such a
+construction of a common form of lamp is shown in Fig. 315, where these
+terminals are indicated by the numerals _1_ and _2_, _3_ being a dry
+wooden block arranged between the terminals at one end for securing
+greater rigidity between them.
+
+[Illustration: Fig. 316. Line Lamp Mounting]
+
+The method of mounting these lamps is subject to a good deal of
+variation in detail, but the arrangement is always such that the lamp is
+slid in between two metallic contacts forming terminals of the circuit
+in which the lamp is to operate. Such an arrangement of springs and the
+co-operating mounting forming a sort of socket for the reception of
+switchboard lamps is referred to as a _lamp jack_. These are sometimes
+individually mounted and sometimes mounted in strips in much the same
+way that jacks are mounted in strips. A strip of lamp jacks as
+manufactured by the Kellogg Company is shown in Fig. 316. The
+opalescent lens is adapted to be fitted in front of the lamp after it
+has been inserted into the jack. Fig. 317 gives an excellent view of an
+individually-mounted lamp jack with its lamp and lens, this also being
+of Kellogg manufacture. This figure shows a section of the plug shelf
+which is bored to receive a lamp. In order to protect the lamps and
+lenses from breakage, due to the striking of the plugs against them, a
+metal shield is placed over the lens, as shown in this figure, this
+being so cut away as to allow sufficient openings for the light to shine
+through. Sometimes instead of employing lenses in front of the lamps, a
+flat piece of translucent material is used to cover the openings of the
+lamp, this being protected by suitable perforated strips of metal. A
+strip of lamp jacks employing this feature is shown in Fig. 318, this
+being of Dean manufacture. An advantage of this for certain types of
+work is that the flat translucent plate in front of the lamp may readily
+carry designating marks, such as the number of the line or something to
+indicate the character of the line, which marks may be readily changed
+as required.
+
+[Illustration: Fig. 317. Supervisory Lamp Mounting]
+
+[Illustration: Fig. 318. Line Lamp Mounting]
+
+[Illustration: Fig. 319. Individual Lamp Jacks]
+
+In the types made by some manufacturers the only difference between the
+pilot lamp and the line lamp is in the size of the lens in front of it,
+the jack and the lamp itself being the same for each, while others use a
+larger lamp for the pilot. In Fig. 319 are shown two individual lamp
+jacks, the one at the top being for supervisory lamps and the one at the
+bottom being provided with a large lens for serving as a pilot lamp.
+
+[Illustration: TERMINAL ROOM APPARATUS IN PROCESS OF INSTALLATION
+Installed by Dean Electric Company at Detroit, Mich.]
+
+=Mechanical Signals.= As has been stated the so-called mechanical
+signals are sometimes used in small common-battery switchboards instead
+of lamps. Where this is done the coil of the signal, if it is a line
+signal, is substituted in the line circuit in place of the relay coil.
+If the signals are used in connection with cord circuits for supervisory
+signals, their coils are put in the circuit in place of the supervisory
+relay coils. (These signals are referred to in Chapter III in connection
+with Fig. 23.) They are so arranged that the attraction of the armature
+lifts a target on the end of a lever, and this causes a display of color
+or form. The release of the armature allows this target to drop back,
+thus obliterating the display. Such signals, often called _visual
+signals_ and _electromagnet signals_, should be distinguished from the
+drops considered in connection with magneto switchboards in which the
+attraction of the armature causes the display of the signal by the
+falling of a drop, the signal remaining displayed until restored by some
+other means, the restoration depending in no wise on when the armature
+is released.
+
+_Western Electric._ The mechanical signal of the Western Electric
+Company, shown in Fig. 320, has a target similar to that shown in Fig.
+254 but without a latch. It is turned to show a different color by the
+attraction of the armature and allowed to resume its normal position
+when the armature is released.
+
+[Illustration: Fig. 320. Mechanical Signal]
+
+_Kellogg._ Fig. 321 gives a good idea of a strip of mechanical signals
+as manufactured by the Kellogg Company. This is known as the _gridiron_
+signal on account of the cross-bar striping of its target. The white
+bars on the target normally lie just behind the cross-bars on the shield
+in front, but a slight raising of the target--about one-eighth of an
+inch--exposes these white bars to view, opposite the rectangular
+openings in the front shield.
+
+[Illustration: Fig. 321. Strip of Gridiron Signals]
+
+_Monarch._ In Fig. 322 is shown the visual signal manufactured by the
+Monarch Telephone Company.
+
+[Illustration: Fig. 322. Mechanical Signal]
+
+=Relays.= The line relays for common-battery switchboards likewise
+assume a great variety of forms. The well-known type of relay employed
+in telegraphy would answer the purpose well but for the amount of room
+that it occupies, as it is sometimes necessary to group a large number
+of relays in a very small space. Nearly all present-day relays are of
+the single-coil type, and in nearly all cases the movement of the
+armature causes the movement of one or more switching springs, which are
+thus made to engage or disengage their associated spring or springs. One
+of the most widely used forms of relays has an L-shaped armature hung
+across the front of a forwardly projecting arm of iron, on the
+knife-edge corner of which it rocks as moved by the attraction of the
+magnet. The general form of this relay was illustrated in Fig. 95.
+Sometimes this relay is made up in single units and frequently a large
+number of such single units are mounted on a single mounting plate. This
+matter will be dealt with more in detail in the discussion of
+common-battery multiple switchboards. In other cases these relays are
+built _en bloc_, a rectangular strip of soft iron long enough to afford
+space for ten relays side by side being bored out with ten cylindrical
+holes to receive the electromagnets. The iron of the block affords a
+return path for the lines of force. The L-shaped armatures are hung over
+the front edge of this block, so that their free ends lie opposite the
+magnet cores within the block. This arrangement as employed by the
+Kellogg Company is shown in two views in Figs. 323 and 324.
+
+[Illustration: Fig. 323. Strip of Relays]
+
+[Illustration: Fig. 324. Strip of Relays]
+
+A bank of line relays especially adapted for small common-battery
+switchboards as made by the Dean Company, is shown in Fig. 325.
+
+[Illustration: Fig. 325. Bank of Relays]
+
+=Jacks.= The jacks in common-battery switchboards are almost always
+mounted in groups of ten or twenty, the arrangement being similar to
+that discussed in connection with lamp strips. Ordinarily in
+common-battery work the jack is provided with two inner contacts so as
+to cut off both sides of the signaling circuit when the operator plugs
+in. A strip of such jacks is shown in Fig. 326.
+
+[Illustration: Fig. 326. Strip of Cut-Off Jacks]
+
+Ringing and listening keys for simple common-battery switchboards differ
+in no essential respect from those employed in magneto boards.
+
+[Illustration: Fig. 327. Details of Lamp, Plug, and Key Mounting]
+
+=Switchboard Assembly.= The general assembly of the parts of a simple
+common-battery switchboard deserves some attention. The form of the
+switchboard need not differ essentially from that employed in magneto
+work, but ordinarily the cabinet is somewhat smaller on account of the
+smaller amount of room required by its lamps and jacks. An excellent
+idea of the line jacks and lamps, plugs, keys, and supervisory signals
+may be obtained from Fig. 327, which is a detail view taken from a
+Kellogg board. In the vertical panel of the board above the plug shelf
+are arranged the line jacks and the lamps in rows of twenty each, each
+lamp being immediately beneath its corresponding jack. Such jacks are
+ordinarily mounted on 1/2-inch centers both vertically and horizontally,
+so that a group of one hundred lamps and line jacks will occupy a space
+only slightly over 10 by 5 inches. Such economy of space is not required
+in the simple magneto board, because the space might easily be made
+larger without in any way taxing the reach of the operator. The reason
+for this comparatively close mounting is a result, not of the
+requirements of the simple non-multiple common-battery board itself, but
+of the fact that the jack strips and lamp strips, which are required in
+very large numbers in multiple boards, have to be mounted extremely
+close together, and as the same lamp strips and jack strips are often
+available for simple switchboards, an economy in manufacture is effected
+by adherence to the same general dimensions.
+
+[Illustration: Fig. 328. Simple Common-Battery Switchboard with
+Removable Relay Panel]
+
+A rear view of a common form of switchboard cabinet, known as the
+_upright type_ and manufactured by the Dean Company, is shown in Fig.
+328. In this all the relays are mounted on a hinged rack, which, when
+opened out as indicated, exposes the wiring to view for inspection or
+repairs. Access to both sides of the relays is thus given to the
+repairman who may do all his work from the rear of the board without
+disturbing the operator.
+
+Fig. 329 shows a three-position cabinet of Kellogg manufacture, this
+being about the limit in size of boards that could properly be called
+simple. Obviously, where a switchboard cabinet must be made of greater
+length than this, _i. e._, than is required to accommodate three
+operators, it becomes too long for the operators to reach all over it
+without undue effort or without moving from their seats. The so-called
+_transfer board_ and the _multiple board_ (to be considered in
+subsequent chapters), constitute methods of relief from such a condition
+in larger exchanges.
+
+[Illustration: Fig. 329. Three-Position Lamp Board]
+
+
+
+
+CHAPTER XXIII
+
+TRANSFER SWITCHBOARD
+
+
+When the traffic originating in a switchboard becomes so great as to
+require so many operators that the board must be made so long that any
+one of the operators cannot reach over its entire face, the simple
+switchboard does not suffice. Either some form of transfer switchboard
+or of multiple switchboard must be used. In this chapter the transfer
+switchboard will be briefly discussed.
+
+The transfer switchboard is so named because its arrangement is such
+that some of the connections through it are handled by means of two
+operators, the operator who answers the call transferring it to another
+operator who completes the connection desired.
+
+=Limitations of Simple Switchboard.= Conceive a number of simple magneto
+switchboards, or a number of common-battery switchboards, arranged side
+by side, their number being so great as to form, by their combination, a
+board too long for the ordinary cords and plugs to reach between its
+extremities. On each of these simple switchboards, which we will say are
+each of the one-position type, there terminates a group of subscribers'
+lines so great in number, considering the traffic on them, that the
+efforts of one operator will just about be taxed to properly attend to
+their calls during the busiest hours of the day. If, now, these
+subscribers would be sufficiently accommodating to call for no other
+subscribers than those whose lines terminate on the same switchboard
+section or on one of the immediately adjacent switchboard sections, all
+would be well, but subscribers will not be so restricted. They demand
+universal service; that is, they demand the privilege of having their
+own lines connected with the line of any other person in the exchange.
+Obviously, in the arrangement just conceived, any operator may answer
+any call originating at her own board and complete the connection with
+the desired subscriber if that subscriber's jack terminates on her own
+section or on one of the adjacent ones. Beyond that she is powerless
+unless other means are provided.
+
+=Transfer Lines.= In the transfer board these other means consist in the
+provision of groups of local trunk lines or transfer lines extending
+from each switchboard position to each other non-adjacent switchboard
+position. When an operator receives a call for some line on a
+non-adjacent position, having answered this call with her answering
+plug, she inserts the calling plug into the jack of one of these
+transfer lines that leads to the proper other section. The operator at
+that section is notified either verbally or by signal, and she completes
+the connection between the other end of the transfer line and the line
+of the called subscriber; the connection between the two subscribers
+thus being effected through the cords of the two operators in question
+linked together by the transfer line. Such a transfer line as just
+described, requiring the connection at each of its ends by one of the
+plugs of the operator's cord pair, is termed a _jack-ended trunk_ or a
+_jack-ended transfer line_ because each of its ends terminates in a
+jack.
+
+[Illustration: Fig. 330. Jack-Ended Transfer Circuit]
+
+There is another method of accomplishing the same general result by the
+employment of the so-called _plug-ended trunk_ or _plug-ended transfer
+line_. In this the trunk or transfer line terminates at one end, the
+answering end, in a jack as before, and the connection is made with it
+by the answering operator by means of the calling plug of the pair with
+which she answered the originating call. The other end of this trunk,
+instead of terminating in a jack, ends in a plug and the second operator
+involved in the connection, after being notified, picks up this plug and
+inserts it in the jack of the called subscriber, thus completing the
+connection without employing one of her regular cord pairs.
+
+_Jack-Ended Trunk._ In Fig. 330 are shown the circuits of a commonly
+employed jack-ended trunk for transfer boards. The talking circuit, as
+usual, is shown in heavy lines and terminates in the tip and sleeve of
+the transfer jacks at each end. The auxiliary contacts in these jacks
+and the circuits connecting them are absolutely independent of the
+talking circuit and are for the purpose of signaling only, the
+arrangement of the jacks being such that when a plug is inserted, the
+spring _1_ will break from spring _2_ and make with spring _3_.
+Obviously, the insertion of a plug in either of the jacks will establish
+such connections as to light both lamps, since the engagement of spring
+_1_ with spring _3_ in either of the jacks will connect both of the
+lamps in multiple across the battery, this connection including always
+the contacts _1_ and _2_ of the other jack. From this it follows that
+the insertion of a plug in the other end of the trunk will, by breaking
+contact between springs _1_ and _2_, put out both the lamps. One plug
+inserted will, therefore, light both lamps; two plugs inserted or two
+plugs withdrawn will extinguish both lamps.
+
+[Illustration: Fig. 331. Jack-Ended Transfer Circuit]
+
+If an operator located at one end of this trunk answers a call and finds
+that the called-for subscriber's line terminates within reach of the
+operator near the other end of this trunk, she will insert a calling
+plug, corresponding to the answering plug used in answering a call, into
+the jack of this trunk and thus light the lamp at both its ends. The
+operator at the other end upon seeing this transfer lamp illuminated
+inserts one of her answering plugs into the jack, and by means of her
+listening key ascertains the number of the subscriber desired, and
+immediately inserts her calling plug into the jack of the subscriber
+wanted and rings him in the usual manner. The act of this second
+operator in inserting her answering plug into the jack extinguishes the
+lamp at her own end and also at the end where the call originated, thus
+notifying the answering operator that the call has been attended to. As
+long as the lamps remain lighted, the operators know that there is an
+unattended connection on that transfer line. Such a transfer line is
+called a _two-way_ line or a _single-track_ line, because traffic over
+it may be in either direction. In Fig. 331 is shown a trunk that
+operates in a similar way except that the two lamps, instead of being
+arranged in multiple, are arranged in series.
+
+[Illustration: Fig. 332. Jack- and Plug-Ended Transfer Circuit]
+
+_Plug-Ended Trunk._ In Fig. 332 is shown a plug-ended trunk, this
+particular arrangement of circuits being employed by the Monarch Company
+in its transfer boards. This is essentially a one-way trunk, and traffic
+over it can pass only in the direction of the arrow. Traffic in the
+opposite direction between any two operators is handled by another trunk
+or group of trunks similar to this but "pointed" in the other direction.
+For this reason such a system is referred to as a _double-track_ system.
+The operation of signals is the same in this case as in Fig. 330, except
+that the switching device at the left-hand end of the trunk instead of
+being associated with the jack is associated with the plug seat, which
+is a switch closely associated with the seat of a plug so as to be
+operated whenever the plug is withdrawn from or replaced in its seat.
+The operation of this arrangement is as follows: Whenever an operator at
+the right-hand end of this trunk receives a call for a subscriber whose
+line terminates within the reach of the operator at the left-hand end of
+the trunk, she inserts the calling plug of the pair used in answering
+the calling subscriber into the jack of the trunk, and thus lights both
+of the trunk lamps. The operator at the other end of the trunk, seeing
+the trunk lamp lighted, raises the plug from its seat and, having
+learned the wishes of the calling subscriber, inserts this plug into the
+jack of the called subscriber without using one of her regular pairs.
+When she raised the trunk plug from its seat, she permitted the long
+spring _1_ of the plug seat switch to rise, thus extinguishing both
+lamps and giving the signal to the originating operator that the trunk
+connection has received attention. On taking down the connection, the
+withdrawal of the plug from the right hand of the trunk lights both
+lamps, and the restoring of the trunk plug to its normal seat again
+extinguishes both lamps.
+
+=Plug-Seat Switch.= The plug-seat switch is a device that has received a
+good deal of attention not only for use with transfer systems, but also
+for use in a great variety of ways with other kinds of manual switching
+systems. The placing of a plug in its seat or withdrawing it therefrom
+offers a ready means of accomplishing some switching or signaling
+operation automatically. The plug-seat switch has, however, in spite of
+its possibilities, never come into wide use, and so far as we are aware
+the Monarch Telephone Manufacturing Company is the only company of
+prominence which incorporates it in its regular output. The Monarch
+plug-switch mechanism is shown in Fig. 333, and its operation is
+obvious. It may be stated at this point that one of the reasons why the
+plug-seat switch has not been more widely adopted for use, is the
+difficulty that has been experienced due to lint from the switchboard
+cords collecting on or about the contact points. In the construction
+given in the detailed cut, upper part, Fig. 333, is shown the means
+adopted by the Monarch Company for obviating this difficulty. The
+contact points are carried in the upper portion of an inverted cup
+mounted on the under side of the switchboard shelf, and are thus
+protected, in large measure, from the damaging influence of dust and
+lint.
+
+[Illustration: Fig. 333. Plug-Seat Switch]
+
+[Illustration: Fig. 334. Order-Wire Arrangement]
+
+=Methods of Handling Transfers.= One way of giving the number of the
+called subscriber to the second operator in a transfer system is to
+have that operator listen in on the circuit after it is continued to her
+position and receive the number either from the first operator or from
+the subscriber. Receiving it from the first operator has the
+disadvantage of compelling the first operator to wait on the circuit
+until the second operator responds; receiving it from the subscriber has
+the disadvantage of sometimes being annoying to him. This, however, is
+to be preferred to the loss of time on the part of the originating
+operator that is entailed by the first method. A better way than either
+of these is to provide between the various operators working in a
+transfer system, a so-called _order-wire_ system. An order wire, as
+ordinarily arranged, is a circuit terminating at one end permanently in
+the head receiver of an operator, and terminating at the other end in a
+push button which, when depressed, will connect the telephone set of the
+operator at that end with the order wire. The operator at the
+push-button end of the order wire may, therefore, at will, communicate
+with the other operator in spite of anything that the other operator may
+do. An order-wire system suitable for transfer switchboards consists in
+an order wire leading from each operator's receiver to a push button at
+each of the other operator's positions, so that every operator has it
+within her power to depress a key or button and establish communication
+with a corresponding operator. When, therefore, an operator in a
+transfer system answers a call that must be completed through a transfer
+circuit, she establishes connection with that transfer circuit and then
+informs the operator at the other end of that circuit by order wire of
+the number of the trunk and the number of the subscriber with which that
+trunk is to be connected. Fig. 334 shows a system of order-wire buttons
+by means of which each operator may connect her telephone set with that
+of every other operator in the room, the number in this case being
+confined to three. Assuming that each pair of wires leading from the
+lower portion of this figure terminates respectively in the operator's
+talking apparatus of the three respective operators, then it is obvious
+that operator No. 1, by depressing button No. 2, will connect her
+telephone set with that of operator No. 2; likewise that any operator
+may communicate with any other operator by depressing the key bearing
+the corresponding number.
+
+=Limitations of Transfer System.= It may be stated that the transfer
+system at present has a limited place in the art of telephony. The
+multiple switchboard has outstripped it in the race for popular approval
+and has demonstrated its superiority in practically all large manual
+exchange work. This is not because of lack of effort on the part of
+telephone engineers to make the transfer system a success in a broad
+way. A great variety of different schemes, all embodying the fundamental
+idea of having one operator answer the call and another operator
+complete it through a trunk line, have been tried. In San Francisco, the
+Sabin-Hampton system was in fairly successful service and served many
+thousands of lines for a number of years. It was, however, afterwards
+replaced by modern multiple switchboards.
+
+_Examples of Obsolete Systems._ The Sabin-Hampton system was unique in
+many respects and involved three operators in each connection. It was
+one of the very first systems which employed automatic signaling
+throughout and did away with the subscribers' generators. It did not,
+however, dispense with the subscribers' local batteries.
+
+Another large transfer system, used for years in an exchange serving at
+a time as many as 5,000, was employed at Grand Rapids, Michigan. This
+was later replaced by an automatic switchboard.
+
+[Illustration: Fig. 335. Three-Position Transfer Switchboard]
+
+=Field of Usefulness.= The real field of utility for the transfer system
+today is to provide for the growth of simple switchboards that have
+extended beyond their originally intended limits. By the adding of
+additional sections to the simple switchboard and the establishment of a
+comparatively cheap transfer system, the simple boards may be made to do
+continued service without wasting the investment in them by discarding
+them and establishing a completely new system. However, switchboards are
+sometimes manufactured in which the transfer system is included as a
+part of the original equipment. In Fig. 335 is shown a three-position
+transfer switchboard, manufactured by the Monarch Telephone Company. At
+first glance the switchboard appears to be exactly like those described
+in Chapter XXI, but on close observation, the transfer jacks and signals
+may be seen in the first and third positions, just below the line jacks
+and signals. There is no transfer equipment in the second position of
+this switchboard because the operator at that position is able to reach
+the jacks of all the lines and, therefore, is able to complete all calls
+originating on her position without the use of any transfer equipment.
+Referring to Fig. 301, which illustrates a two-position simple
+switchboard, it may readily be seen that if the demands for telephone
+service in the locality in which this switchboard is installed should
+increase so as to require the addition of more switchboard positions,
+this switchboard could readily be converted to a transfer switchboard by
+placing the necessary transfer jacks and signals in the vacant space
+between the line jacks and clearing-out drops.
+
+[Illustration: CABLE TURNING SECTIONS, BETWEEN A AND B BOARDS Cortlandt
+Office, New York Telephone Co.]
+
+
+
+
+CHAPTER XXIV
+
+PRINCIPLES OF THE MULTIPLE SWITCHBOARD
+
+
+=Field of Utility.= The multiple switchboard, unlike the transfer board,
+provides means for each operator to complete, without assistance, a
+connection with any subscriber's line terminating in the switchboard no
+matter how great the number of lines may be. It is used only where the
+simple switchboard will not suffice; that is, where the number of lines
+and the consequent traffic is so great as to require so many operators
+and, therefore, so great a length of board as to make it impossible for
+any one operator to reach all over the face of the board without moving
+from her position.
+
+=The Multiple Feature.= The fundamental feature of the multiple
+switchboard is the placing of a jack for every line served by the
+switchboard within the reach of every operator. This idea underlying the
+multiple switchboard may be best grasped by merely considering the
+mechanical arrangement and grouping of parts without regard to their
+details of operation. The idea is sometimes elusive, but it is really
+very simple. If the student at the outset will not be frightened by the
+very large number of parts that are sometimes involved in multiple
+switchboards, and by the great complexity which is apparent in the
+wiring and in the action of these parts; and will remember that this
+apparent complexity results from the great number of repetitions of the
+same comparatively simple group of apparatus and circuits, much will be
+done toward a mastery of the subject.
+
+The multiple switchboard is divided into sections, each section being
+about the width and height that will permit an ordinary operator to
+reach conveniently all over its face. The usual width of a section
+brought about by this limitation is from five and one-half to six feet.
+Such a section affords room for three operators to sit side by side
+before it. Now each line, instead of having a single jack as in the
+simple switchboard, is provided with a number of jacks and one of these
+is placed on each of the sections, so that each one of the operators may
+have within her reach a jack for each line. It is from the fact that
+each line has a multiplicity of jacks, that the term multiple
+switchboard arises.
+
+_Number of Sections._ Since there is a jack for each line on each
+section of the switchboard, it follows that on each section there are as
+many jacks as there are lines; that is, if the board were serving 5,000
+lines there would be 5,000 jacks. Let us see now what it is that
+determines the number of sections in a multiple switchboard. In the
+final analysis, it is the amount of traffic that arises in the busiest
+period of the day. Assume that in a particular office serving 5,000
+lines, the subscribers call at such a very low rate that even at the
+busiest time of the day only enough calls are made to keep, say, three
+operators busy. In this case there would be no need for the multiple
+switchboard, for a single section would suffice. The three operators
+seated before that section would be able to answer and complete the
+connections for all of the calls that arose. But subscribers do not call
+at this exceedingly low rate. A great many more calls would arise on
+5,000 lines during the busiest hour than could be handled by three
+operators and, therefore, a great many more operators would be required.
+Space has to be provided for these operators to work in, and as each
+section accommodates three operators the total number of sections must
+be at least equal to the total number of required operators divided by
+three.
+
+Let us assume, for instance, that each operator can handle 200 calls
+during the busy hour. Assume further that during the busy hour the
+average number of calls made by each subscriber is two. One hundred
+subscribers would, therefore, originate 200 calls within this busy hour
+and this would be just sufficient to keep one operator busy. Since one
+operator can handle only the calls of one hundred subscribers during the
+busy hour, it follows that as many operators must be employed as there
+are hundreds of subscribers whose lines are served in a switchboard, and
+this means that in an exchange of 5,000 subscribers, 50 operators'
+positions would be required, or 16-2/3 sections. Each of these sections
+would be equipped with the full 5,000 jacks, so that each operator could
+have a connection terminal for each line.
+
+_The Multiple._ These groups of 5,000 jacks, repeated on each of the
+sections are termed multiple jacks, and the entire equipment of these
+multiple jacks and their wiring is referred to as the multiple. It will
+be shown presently that the multiple jacks are only used for enabling
+the operator to connect with the called subscriber. In other words these
+jacks are for the purpose of enabling each operator to have within her
+reach any line that may be called for regardless of what line originates
+the call. We will now consider what arrangements are provided for
+enabling the operator to receive the signal indicating a call and what
+provisions are made for her to answer the call in response to such a
+signal.
+
+=Line Signals.= Obviously it is not necessary to have the line signals
+repeated on each section of the board as are the multiple jacks. If a
+line has one definite place on the switchboard where its signal may be
+received and its call may be answered, that suffices. Each line,
+therefore, in addition to having its multiple jacks distributed one on
+each section of the switchboard, has a line signal and an individual
+jack immediately associated with it, located on one only of the
+sections. This signal usually is in the form of a lamp and is termed the
+line signal, and this jack is termed the answering jack since it is by
+means of it that the operator always answers a call in response to the
+line signal.
+
+_Distribution of Line Signals._ It is evident that it would not do to
+have all of these line signals and answering jacks located at one
+section of the board for then they would not be available to all of the
+operators. They are, therefore, distributed along the board in such a
+way that one group of them will be available to one operator, another
+group to another operator, and so on; the number of answering jacks and
+signals in any one group being so proportioned with respect to the
+number of calls that come in over them during the busy hour that it will
+afford just about enough calls to keep the operator at that position
+busy.
+
+We may summarize these conditions with respect to the jack and
+line-signal equipment of the multiple switchboard by saying that each
+line has a multiple jack on each section of the board and in addition to
+this has on one section of the board an answering jack and a line
+signal. These answering jacks and line signals are distributed in groups
+along the face of the board so that each operator will receive her
+proper quota of the originating calls which she will answer and, by
+virtue of the multiple jack, be able to complete the connections with
+the desired subscribers without moving from her position.
+
+=Cord Circuits.= Each operator is also provided with a number of pairs
+of cords and plugs with proper supervisory or clearing-out signals and
+ringing and listening keys, the arrangement in this respect being
+similar to that already described in connection with the simple
+switchboard.
+
+=Guarding against Double Connections.= From what has been said it is
+seen that a call originating on a given line may be answered at one
+place only, but an outgoing connection with that line may be made at any
+position. This fact that a line may be connected with when called for at
+any one of the sections of the switchboard makes necessary the provision
+that two or more connections will not be made with the same line at the
+same time. For instance, if a call came in over a line whose signal was
+located on the first position of the switchboard for a connection with
+line No. 1,000, the operator at the first position would connect this
+calling line with No. 1,000 through the multiple jack on the first
+section of the switchboard. Assume now that some line, whose signal was
+located on the 39th position of the switchboard, should call also for
+line No. 1,000 while that line was still connected with the first
+calling subscriber. Obviously confusion would result if the operator at
+the 39th position, not knowing that line No. 1,000 was already busy,
+should connect this second line with it, thereby leaving both of the
+calling subscribers connected with line No. 1,000, and as a result all
+of these three subscribers connected together.
+
+The provisions for suitable means for preventing the making of a
+connection with a line that is already switched at some other section of
+the switchboard, has offered one of the most fertile fields for
+invention in the whole telephone art. The ways that have been proposed
+for accomplishing this are legion. Fortunately common practice has
+settled on one general plan of action and that is to so arrange the
+circuits that whenever a line is switched at one section, such an
+electrical condition will be established on the forward contacts of all
+of its multiple jacks that any operator at any other section in
+attempting to make a connection with that line will be notified of the
+fact that it is already switched by an audible signal, which she will
+receive in her head receiver. On the other hand the arrangement is such
+that when a line is not busy, _i. e._, it is not switched at any of the
+positions of the switchboard, the operator on attempting to make a
+connection with such a line will receive no such guarding signal and
+will, therefore, proceed with the connection.
+
+We may liken a line in a multiple switchboard to a lane having a number
+of gates giving access to it. One of these gates--the answering jack--is
+for the exclusive use of the proprietor of that lane. All of the other
+gates to the lane--the multiple jacks--are for affording means for the
+public to enter. But whenever any person enters one of these gates, a
+signal is automatically put up at all of the other gates forbidding any
+other person to enter the lane as long as the first person is still
+within.
+
+[Illustration: Fig. 336. Principle of Multiple Switchboard]
+
+=Diagram Showing Multiple Board Principle.= For those to whom the
+foregoing description of the multiple board is not altogether clear, the
+diagram of Fig. 336 may offer some assistance. Five subscribers' lines
+are shown running through four sections of a switchboard. Each of these
+lines is provided with a multiple jack on each section of the board.
+Each line is also provided with an answering jack and a line signal on
+one of the sections of the board. Thus the answering jacks and the line
+signals of lines _1_ and _2_ are shown in Section I, that of line _4_ is
+shown in Section II, that of line _3_ in Section III, and that of line
+_5_ in Section IV. At Section I, line _1_ is shown in the condition of
+having made a call and having had this call answered by the operator
+inserting one of her plugs into its answering jack. In response to the
+instructions given by the subscriber, the operator has inserted the
+other plug of this same pair in the multiple jack of line _2_, thus
+connecting these two lines for conversation. At Section III, line _3_ is
+shown as having made a call, and the operator as having answered by
+inserting one of her plugs into the answering jack. It happens that the
+subscriber on line _3_ requests a connection with line _1_, and the
+condition at Section III is that where the operator is about to apply
+the tip of the calling plug to the jack of line _1_ to ascertain whether
+or not that line is busy. As before stated, when the contact is made
+between the tip of the calling plug and the forward contact of the
+multiple jack, the operator will receive a click in the ear (by means
+that will be more fully discussed in later chapters), this click
+indicating to her that line _1_ is not available for connection because
+it is already switched at some other section of the switchboard.
+
+=Busy Test.= The busy signal, by which an operator in attempting to make
+a connection is informed that the line is already busy, has assumed a
+great variety of forms and has brought forth many inventions. It has
+been proposed by some that the insertion of a plug into any one of the
+jacks of a line would automatically close a little door in front of each
+of the other jacks of the line, therefore making it impossible for any
+other operator to insert a plug as long as the line is in use. It has
+been proposed by others to ring bells or to operate buzzers whenever the
+attempt was made by an operator to plug into a line that was already in
+use. Still others have proposed to so arrange the circuits that the
+operator would get an electric shock whenever she attempted to plug into
+a busy line. The scheme that has met with universal adoption, however,
+is that the operator shall, when the tip of her calling plug touches the
+forward contact of the jack of a line that is already switched, receive
+a click in her telephone which will forbid her to insert the plug. The
+absence of this click, or silence in her telephone, informs her that she
+may safely make the connection.
+
+_Principle._ The means by which the operator receives or fails to
+receive this click, according to whether the line is busy or idle, vary
+widely, but so far as the writers are aware they all have one
+fundamental feature in common. The tip of the calling plug and the test
+contact of all of the multiple jacks of an idle line must be absolutely
+at the same potential before the test, so that no current will flow
+through the test circuit when the test is actually made. The test
+thimbles of all the jacks of a busy line must be at a different
+potential from the tip of the test plug so that a current will flow and
+a click result when the test is made.
+
+_Potential of Test Thimbles._ It has been found an easy matter to so
+arrange the contacts in the jacks of a multiple switchboard that
+whenever the line is idle the test thimbles of that line will be a
+certain potential, the same as that of all the unused calling plug tips.
+It has also been easy to so arrange these contacts that the insertion of
+a plug into any one of the jacks will, by virtue of the contacts
+established, change the potential of all the test thimbles of that line
+so that they will be at a different potential from that of the tips of
+the calling plugs. It has not been so easy, however, to provide that
+these conditions shall exist under all conditions of practice. A great
+many busy tests that looked well on paper have been found faulty in
+practice. As is always the case in such instances, this has been true
+because the people who considered the scheme on paper did not foresee
+all of the conditions that would arise in practice. Many busy-test
+systems will operate properly while everything connected with the
+switchboard and the lines served by it remains in proper order. But no
+such condition as this can be depended on in practice. Switchboards, no
+matter how perfectly made and no matter with how great care they may be
+installed and maintained, will get out of order. Telephone lines will
+become grounded or short-circuited or crossed or opened. Such
+conditions, in a faulty busy-test system, may result in a line that is
+really idle presenting a busy test, and thus barring the subscriber on
+that line from receiving calls from other lines just as completely as if
+his line were broken. On the other hand, faulty conditions either in the
+switchboard or in the line may make a line that is really busy, test
+idle, and thus result in the confusion of having two or more subscribers
+connected to the same line at the same time.
+
+_Busy-Test Faults._ To show how elusive some of the faults of a busy
+test may be, when considered on paper, it has come within the
+observation of the writers that a new busy-test system was thought well
+enough of by a group of experienced engineers to warrant its
+installation in a group of very large multiple switchboards, the cost of
+which amounted to hundreds of thousands of dollars, and yet when so
+installed it developed that a single short-circuited cord in a position
+would make the test inoperative on all the cords of that
+position--obviously an intolerable condition. Luckily the remedy was
+simple and easily applied.
+
+In a well-designed busy-test system there should be complete silence
+when the test is made of an idle line, and always a well-defined click
+when the test is made of a busy line. The test on busy lines should
+result in a uniform click regardless of length of lines or the condition
+of the apparatus. It does not suffice to have a little click for an idle
+line and a big click for a busy line, as practice has shown that this
+results in frequent errors on the part of the operators.
+
+Good operating requires that the tip of the calling plug be tapped
+against the test thimble several times in order to make sure of the
+state of the called line.
+
+In some multiple switchboards the arrangement has been such that the
+jacks of a line would test busy as soon as the subscriber on that line
+removed his receiver from its hook to make a call, as well as while any
+plug was in any jack of that line. The advocates of this added feature,
+in connection with the busy test, have claimed that the receiver, when
+removed from its hook in making a call, should make the line test busy
+and that a line should not be connected with when the subscriber's
+receiver was off its hook any more than it should be when it was already
+connected with at some other section of the switchboard. While it is
+true that a line may be properly termed busy when the subscriber has
+removed his receiver in order to make a call, it is not true that there
+is any real necessity for guarding against a connection with it while he
+is waiting for the operator to answer. Leaving the line unguarded for
+this brief period may result in the subscriber, who intended to make the
+call, having to defer his call until he has conversed with the party who
+is trying to reach him. This cannot be said to be a detriment to the
+service, however, since the second party gets the connection he desires
+much sooner than he otherwise would, and the first party may still make
+his first intended call as soon as he has disposed of the party who
+reached him while he was waiting for his own operator to answer. It may
+be said, therefore, in connection with this matter of making the line
+test busy as soon as a subscriber has removed his receiver from the
+hook, that it is not considered an essential, and in case of those
+switchboard systems which naturally work out that way it is not
+considered a disadvantage.
+
+=Field of Each Operator.= It was stated earlier in this chapter that as
+each section accommodated three operators, the total number of sections
+in a switchboard will be at least one-third the total number of required
+operators. This thought needs further development, for to stop at that
+statement is to arrive somewhat short of the truth. In order to do this
+it is necessary to consider the field in the multiple, reached by each
+operator. The section is of such size, or should be, that an operator
+seated in the center position of it may, without undue effort, reach all
+over the multiple. But the operator at the right-hand position cannot
+reach the extreme left portion of the multiple of that section, nor can
+the operator at the left reach the extreme right. How then may each
+operator reach a jack for every line? Remembering that the multiple
+jacks are arranged exactly the same in each section, each jack always
+occupying the same relative position, it is easy to see that while the
+operator at a right-hand position of a section cannot reach the
+left-hand third of the multiple in her own section, she may reach the
+left-hand third of the multiple in the section at her right, and this,
+together with the center and right-hand thirds of her own section,
+represents the entire number of lines. So it is with the left-hand
+operator at any section, she reaches two-thirds of all the lines in the
+multiple of her own section and one-third in that of the section at her
+left.
+
+_End Positions._ This makes it necessary to inquire about the operators
+at the end positions of the entire board. To provide for these the
+multiple is extended one-third of a section beyond them, so as to supply
+at the ends of the switchboard jacks for those lines which the end
+operators cannot reach on their own sections. Sometimes instead of
+adding these end sections to the multiple for the end operators, the
+same result is accomplished by using only the full and regular sections
+of the multiple, and leaving the end positions without operators'
+equipment, as well as without answering jacks, line signals, and cords
+and plugs, so that in reality the end operator is at the middle position
+of the end section. This, in our opinion, is the better practice, since
+it leaves the sections standard, and makes it easier to extend the
+switchboard in length, as it grows, by the mere addition of new sections
+without disturbing any of the old multiple.
+
+=Influence of Traffic.= We wish again to emphasize the fact that it is
+the traffic during the busiest time of day and not the number of lines
+that determine the size of a multiple switchboard so far as its length
+is concerned. The number of lines determines the size of the multiple in
+any one section, but it is the amount of traffic, the number of calls
+that are made in the busiest period, that determines the number of
+operators required, and thus the number of positions. Had this now very
+obvious fact been more fully realized in the past, some companies would
+be operating at less expense, and some manufacturers would have sold
+less expensive switchboards.
+
+The whole question as to the number of positions boils down to how many
+answering jacks and line signals may be placed at each operator's
+position without overburdening the operator with incoming traffic at the
+busy time of day. Obviously, some lines will call more frequently than
+others, and hence the proper number of answering jacks at the different
+positions will vary. Obviously, also, due to changes in the personnel of
+the subscribers, the rates of calling of different groups of lines will
+change from time to time, and this may necessitate a regrouping of the
+line signals and answering jacks on the positions; and changes in the
+personnel of the operators or in their skill also demand such
+regrouping.
+
+_Intermediate Frame._ The intermediate distributing frame is provided
+for this purpose, and will be more fully discussed in subsequent
+chapters. Suffice it to say here that the intermediate distributing
+frame permits the answering jacks and line signals to be shifted about
+among the operators' positions, so that each position will have just
+enough originating traffic to keep each of the operators economically
+busy during the busiest time of the day.
+
+
+
+
+CHAPTER XXV
+
+THE MAGNETO MULTIPLE SWITCHBOARD
+
+
+=Field of Utility.= The principles of the multiple switchboard set forth
+in the last chapter were all developed long before the common-battery
+system came into existence, and consequently all of the first multiple
+switchboards were of the magneto type. Although once very widely used,
+the magneto multiple switchboard has almost passed out of existence,
+since it has become almost universal practice to equip exchanges large
+enough to employ multiple boards with common-battery systems.
+Nevertheless there is a field for magneto multiple switchboards, and in
+this field it has recently been coming into increasing favor. In those
+towns equipped with magneto systems employing simple switchboards or
+transfer switchboards, and which require new switchboards by virtue of
+having outgrown or worn out their old ones, the magneto multiple
+switchboard is frequently found to best fit the requirements of economy
+and good practice. The reason for this is that by its use the magneto
+telephones already in service may be continued, no change being required
+outside of the central office. Furthermore, with the magneto multiple
+switchboard no provision need be made for a power plant, which, in towns
+of small size, is often an important consideration. Again, many
+companies operate over a considerable area, involving a collection of
+towns and hamlets. It may be that all of these towns except one are
+clearly of a size to demand magneto equipment and that magneto equipment
+is the standard throughout the entire territory of the company. If,
+however, one of the towns, by virtue of growth, demands a multiple
+switchboard, this condition affords an additional argument for the
+employment of the magneto multiple switchboard, since the same standards
+of equipment and construction may be maintained throughout the entire
+territory of the operating company, a manifest advantage. On the other
+hand, it may be said that the magneto multiple switchboard has no proper
+place in modern exchanges of considerable size--say, having upward of
+one thousand subscribers--at least under conditions found in the United
+States.
+
+Notwithstanding the obsolescence of the magneto multiple switchboard for
+large exchanges, a brief discussion of some of the early magneto
+multiple switchboards, and particularly of one of the large ones, is
+worth while, in that a consideration of the defects of those early
+efforts will give one a better understanding and appreciation of the
+modern multiple switchboard, and particularly of the modern multiple
+common-battery switchboard, the most highly organized of all the manual
+switching systems. Brief reference will, therefore, be made to the
+so-called series multiple switchboard, and then to the branch terminal
+multiple switchboard, which latter was the highest type of switchboard
+development at the time of the advent of common-battery working.
+
+[Illustration: Fig. 337. Series Magneto Multiple Switchboard]
+
+=Series-Multiple Board.= In Fig. 337 are shown the circuits of a series
+magneto multiple switchboard as developed by the engineers of the
+Western Electric Company during the eighties. As is usual, two
+subscribers' lines and a single cord circuit are shown. One side of each
+line passes directly from the subscriber's station to one side of the
+drop, and also branches off to the sleeve contact of each of the jacks.
+The other side of the line passes first to the tip spring of the first
+jack, thence to the anvil of that jack and to the tip spring of the next
+jack, and so on in series through all of the jacks belonging in that
+line to the other terminal of the drop coil. Normally, therefore, the
+drop is connected across the line ready to be responsive to the signal
+sent from the subscriber's generator. The cord circuit is of the
+two-conductor type, the plugs being provided with tip and sleeve
+contacts, the tips being connected by one of the flexible conductors
+through the proper ringing and listening key springs, and the sleeve
+being likewise connected through the other flexible conductor and the
+other springs of the ringing and listening keys. It is obvious that when
+any plug is inserted into a jack, the circuit of the line will be
+continued to the cord circuit and at the same time the line drop will be
+cut out of the circuit, because of the lifting of the tip spring of the
+jack from its anvil. Permanently connected between the sleeve side of
+the cord circuit and ground is a retardation coil _1_ and a battery.
+Another retardation coil _2_ is connected between the ground and a
+point on the operator's telephone circuit between the operator's head
+receiver and the secondary of her induction coil. These two retardation
+coils have to do with the busy test, the action of which is as follows:
+normally, or when a line is not switched at the central office, the test
+thimbles will all be at substantially ground potential, _i. e._, they
+are supposed to be. The point on the operator's receiver circuit which
+is grounded through the retardation coil _2_ will also be of ground
+potential because of that connection to ground. In order to test, the
+operator always has to throw her listening key _L.K._ into the listening
+position. She also has to touch the tip of the calling plug _P_c to a
+sleeve or jack of the line that is being tested. If, therefore, a test
+is made of an idle or non-busy line, the touching of the tip of the
+calling plug with the test thimble of that line will result in no flow
+of current through the operator's receiver, because there will be no
+difference of potential anywhere in the test circuit, which test circuit
+may be traced from the test thimble of the line under test to the tip of
+the calling plug, thence through the tip strand of the cord to the
+listening key, thence to the outer anvil of the listening key on that
+side, through the operator's receiver to ground through the impedance
+coil _2_. If, however, the line had already been switched at some other
+section by the insertion of either a calling or answering plug, all of
+the test thimbles of that line would have been raised to a potential
+above that of the ground, by virtue of the battery connected with the
+sleeve side of the cord circuit through the retardation coil _1_. If the
+operator had made a test of such a line, the tip of her testing plug
+would have found the thimble raised to the potential of the battery and,
+therefore, a flow of current would occur which would give her the busy
+click. The complete test circuit thus formed in testing a busy line
+would be from the ungrounded pole of the battery through the impedance
+coil _1_ associated with the cord that was already in connection with
+the line, thence to the sleeve strand of that cord to the sleeve of the
+jack at which the line was already switched, thence through that portion
+of the line circuit to which all of the sleeve contacts were connected,
+and therefore to the sleeve or test thimble of the jack at which the
+test is made, thence through the tip of the calling plug employed in
+making the test through the tip side of that cord circuit to the outer
+listening key contact of the operator making the test, and thence to
+ground through the operator's receiver and the impedance coil _2_. The
+resultant click would be an indication to the operator that the line was
+already in use and that, therefore, she must not make the connection.
+
+The condenser _3_ is associated with the operator's talking set and with
+the extra spring in the listening key _L.K._ in such a manner that when
+the listening key is thrown, the tip strand of the cord circuit is
+divided and the condenser included between them. This is for the purpose
+of preventing any potentials, which might exist on the line with which
+the answering plug _P_a was connected, from affecting the busy-test
+conditions.
+
+_Operation._ The operation of the system aside from the busy-test
+feature is just like that described in connection with the simple
+magneto switchboard. Assuming that the subscriber at Station _A_ makes
+the call, he turns his hand generator, which throws the drop on his line
+at the central office. The operator, seeing the signal, inserts the
+answering plug of one of her idle pairs of cords into the answering jack
+and throws her listening key _L.K._ This enables the operator to talk
+with the calling subscriber, and having found that he desires a
+connection with the line extending to Station _B_, she touches the tip
+of her calling plug to the multiple jack of that line that is within her
+reach, it being remembered that each one of the multiple jacks shown is
+on a different section. She leaves the listening key in the listening
+position when she does this. If the line is busy, the click will notify
+her that she must not make the connection, in which case she informs the
+calling subscriber that the line is busy and requests him to call again.
+If, however, she received no click, she would insert the calling plug
+into the jack, thus completing the connection between the two lines. She
+would then press the ringing key associated with the calling plug and
+that momentarily disconnects the calling plug from the answering plug
+and at the same time establishes connection between the ringing
+generator and the called line. The release of the ringing key again
+connects the calling and answering plugs and, therefore, connects the
+two subscribers' lines ready for conversation. All that is then
+necessary is that the called subscriber shall respond and remove his
+receiver from its hook, the calling subscriber already having done this.
+When the conversation is finished, both of the subscribers (if they
+remember it) will operate their ringing generators, which will throw the
+clearing-out drop as a signal to the operator for disconnection. If it
+should become necessary for the operator to ring back on the line of the
+calling subscriber, she may do so by pressing the ringing key associated
+with the calling plug.
+
+Frequently this multiple switchboard arrangement was used with grounded
+lines, in which case the single line wire extending from the
+subscriber's station to the switchboard was connected with the tip
+spring of the first jack, the circuit being continued in series through
+the jack to the drop and thence to ground through a high non-inductive
+resistance.
+
+_Defects._ This series multiple magneto system was used with a great
+many variations, and it had a good many defects. One of these defects
+was due to the necessary extending of one limb of the line through a
+large number of series contacts in the jacks. This is not to be desired
+in any case, but it was particularly objectionable in the early days
+before jacks had been developed to their present high state of
+perfection. A particle of dust or other insulating matter, lodging
+between the tip spring and its anvil in any one of the jacks, would
+leave the line open, thus disabling the line to incoming signals, and
+also for conversation in case the break happened to occur between the
+subscriber and the jack that was used in connecting with the line.
+Another defect due to the same cause was that the line through the
+switchboard was always unbalanced by the insertion of a plug, one limb
+of the line always extending clear through the switchboard to the drop
+and the other, when the plug was inserted, extending only part way
+through the switchboard and being cut off at the jack where the
+connection was made. The objection will be apparent when it is
+remembered that the wires in the line circuit connecting the multiple
+jacks are necessarily very closely bunched together and, therefore,
+there is very likely to be cross-talk between two adjacent lines unless
+the two limbs of each line are exactly balanced throughout their entire
+length.
+
+Again the busy-test conditions of this circuit were not ideal. The fact
+that the test rings of the line were connected permanently with the
+outside line circuit subjected these test rings to whatever potentials
+might exist on the outside lines, due to any causes whatever, such as a
+cross with some other wire; thus the test rings of an idle line might by
+some exterior cause be raised to such a potential that the line would
+test busy. It may be laid down as a fundamental principle in good
+multiple switchboard practice that the busy-test condition should be
+made independent of any conditions on the line circuit outside of the
+central office, and such is not the case in this circuit just described.
+
+[Illustration: CABLE RUN FROM INTERMEDIATE FRAME TO MULTIPLE Cortlandt
+Office, New York Telephone Co.]
+
+=Branch-Terminal Multiple Board.= The next important step in the
+development of the magneto multiple switchboard was that which produced
+the so-called branch-terminal board. This came into wide use in the
+various Bell operating companies before the advent of the common-battery
+systems. Its circuits and the principles of operation may be understood
+in connection with Fig. 338. In the branch-terminal system there are no
+series contacts in the jacks and no unbalancing of the line due to a
+cutting off of a portion of the line circuit when a connection was made
+with it. Furthermore, the test circuits were entirely local to the
+central office and were not likely to be affected by outside conditions
+on the line. This switchboard also added the feature of the automatic
+restoration of the drops, thus relieving the operator of the burden of
+doing that manually, and also permitting the drops to be mounted on a
+portion of the switchboard that was not available for the mounting of
+jacks, and thus permitting a greater capacity in jack equipment.
+
+[Illustration: Fig. 338. Branch-Terminal Magneto Multiple Switchboard]
+
+Each jack has five contacts, and the answering and multiple jacks are
+alike, both in respect to their construction and their connection with
+the line. The drops are the electrically self-restoring type shown in
+Fig. 263. The line circuits extended permanently from the subscriber's
+station to the line winding of the drop and the two limbs of the line
+branched off to the tip and sleeve contacts _1_ and _2_ respectively of
+each jack. Another pair of wires extended through the multiple parallel
+to the line wires and these branched off respectively to the contact
+springs _3_ and _4_ of each of the jacks. This pair of wires formed
+portions of the drop-restoring circuit, including the restoring coil _6_
+and the battery _7_, as indicated. The test thimble _5_ of each of the
+jacks is connected permanently with the spring _3_ of the corresponding
+jack and, therefore, with the wire which connects through the restoring
+coil _6_ of the corresponding drop to ground through the battery _7_.
+
+The plugs were each provided with three contacts. Two of these were the
+usual tip and sleeve contacts connected with the two strands of the cord
+circuit. The third contact _8_ was not connected with any portion of the
+cord circuit, being merely an insulated contact on the plug adapted,
+when the plug was fully inserted, to connect together the springs _3_
+and _4_. The cord circuit itself is readily understood from the drawing,
+having two features, however, which merit attention. One is the
+establishing of a grounded battery connection to the center portion of
+the winding of the receiver for the purposes of the busy test, and the
+other is the provision of a restoring coil and restoring circuit for the
+clearing-out drop, this circuit being closed by an additional contact on
+the listening key so as to restore the clearing-out drop whenever the
+listening key was operated.
+
+_Operation._ An understanding of the operation of this system is easy.
+The turning of the subscriber's generator, when the line was in its
+normal condition, caused the display of the line signal. The insertion
+of the answering plug, in response to this call, did three things: (1)
+It extended the line circuit to the tip and sleeve strand of the cord
+circuit. (2) It energized the restoring coil _6_ of the drop by
+establishing the circuit from the contact spring _3_ through the plug
+contact _8_ to the other contact spring _4_, thus completing the circuit
+between the two normally open auxiliary wires. (3) The connecting of the
+springs _3_ and _4_ established a connection from ground to the test
+thimbles of all the jacks on a line, the spring _4_ being always
+grounded and the spring _3_ being always connected to the test thimble
+_5_.
+
+It is to be noted that on idle lines the test rings are always at the
+same potential as the ungrounded pole of the battery _7_, being
+connected thereto through the winding _6_ of the restoring coil. On all
+busy lines, however, the test rings are dead grounded through the
+contact _8_ of the plug that is connected with the line.
+
+The tip of the testing plug at the time of making a test will also be at
+the same potential as that of the ungrounded pole of the battery _7_,
+since this pole of the battery _7_ is always connected to the center
+portion of the operator's receiver winding, and when the listening key
+is thrown the tip of the calling plug is connected therewith and is at
+the same potential. When, therefore, the operator touches the tip of
+the calling plug to the test thimble of an idle line, she will get no
+click, since the tip of the plug and the test thimble will be at the
+same potential. If, however, the line has already been switched at
+another section of the board, there will be a difference of potential,
+because the test thimble will be grounded, and the circuit, through
+which the current which causes the click flows, may be traced from the
+ungrounded pole of the battery _7_ to the center portion of the
+operator's receiver, thence through one-half of the winding to the tip
+of the calling plug, thence to the test thimble of the jack under test,
+thence to the spring _3_ of the jack on another section at which the
+connection exists, through the contact _8_ on the plug of that jack to
+the spring _4_, and thence to ground and back to the other terminal of
+the battery _7_.
+
+_Magnet Windings._ Coils of the line and clearing-out drops by which
+these drops are thrown, are wound to such high resistance and impedance
+as to make it proper to leave them permanently bridged across the
+talking circuit. The necessity for cutting them out is, therefore, done
+away with, with a consequent avoidance, in the case of the line drops,
+of the provision of series contacts in the jacks.
+
+_Arrangement of Apparatus._ In boards of this type the line and
+clearing-out drops were mounted in the extreme upper portion of the
+switchboard face so as to be within the range of vision of the operator,
+but yet out of her reach. Therefore, the whole face of the board that
+was within the limit of the operator's reach was available for the
+answering and multiple jacks. A front view of a little over one of the
+sections of the switchboard, involving three complete operator's
+positions, is shown in Fig. 339, which is a portion of the switchboard
+installed by the Western Electric Company in one of the large exchanges
+in Paris, France. (This has recently been replaced by a common-battery
+multiple board.) In this the line drops may be seen at the extreme top
+of the face of the switchboard, and immediately beneath these the
+clearing-out drops. Beneath these are the multiple jacks arranged in
+banks of one hundred, each hundred consisting of five strips of twenty.
+At the extreme lower portion of the jack space are shown the answering
+jacks and beneath these on the horizontal shelf, the plugs and keys.
+These jacks were mounted on 1/2-inch centers, both vertically and
+horizontally and each section had in multiple 90 banks of 100 each,
+making 9,000 in all. Subsequent practice has shown that this involves
+too large a reach for the operators and that, therefore, 9,000 is too
+large a number of jacks to place on one section if the jacks are not
+spaced closer than on 1/2-inch centers. With the jack involving as many
+parts as that required by this branch terminal system, it was hardly
+feasible to make them smaller than this without sacrificing their
+durability, and one of the important features of the common-battery
+multiple system which has supplanted this branch-terminal magneto system
+is that the jacks are of such a simple nature as to lend themselves to
+mounting on 3/8-inch centers, and in some cases on 3/10-inch centers.
+
+[Illustration: Fig. 339. Face of Magneto Multiple Switchboard]
+
+=Modern Magneto Multiple Board.= Coming now to a consideration of modern
+magneto multiple switchboards, and bearing in mind that such boards are
+to be found in modern practice only in comparatively small installations
+and then only under rather peculiar conditions, as already set forth, we
+will consider the switchboard of the Monarch Telephone Manufacturing
+Company as typical of good practice in this respect.
+
+[Illustration: Fig. 340. Monarch Magneto Multiple Switchboard Circuits]
+
+_Line Circuit._ The line and cord circuits of the Monarch system are
+shown in Fig. 340. It will be seen that each jack has in all five
+contacts, numbered from _1_ to _5_ respectively, of which _1_ and _4_
+are the springs which register with the tip and ring contacts of the
+plug and through which the talking circuit is continued, while _2_ and
+_3_ are series contacts for cutting off the line drop when a plug is
+inserted, and _5_ is the test contact or thimble adapted to register
+with the sleeve contact on the plug when the plug is fully inserted. The
+line circuit through the drop may be traced normally from one side of
+the line through the drop coil, thence through all of the pairs of
+springs _2_ and _3_ in the jacks of that line, and thence to spring _1_
+of the last jack, this spring always being strapped to the spring _2_ in
+the last jack, and thence to the other side of the line. All the ring
+springs _1_ are permanently tapped on to one side of the line, and all
+of the tip springs _4_ are permanently tapped to the other side of the
+line. This system may, therefore, properly be called a branch-terminal
+system. It is seen that as soon as a plug is inserted into any of the
+jacks, the circuit through the drop will be broken by the opening of the
+springs _2_ and _3_ in that jack. The drop shown immediately above the
+answering jack is so associated mechanically with that jack as to be
+mechanically self-restored when the answering plug is inserted into the
+answering jack in response to a call. The arrangement in this respect is
+the same as that shown in Fig. 259, illustrating the Monarch combined
+drop and jack.
+
+_Cord Circuit._ The cord circuit needs little explanation. The tip and
+ring strands are the ones which carry the talking current and across
+these is bridged the double-wound clearing-out drop, a condenser being
+included in series in the tip strand between the two drop windings in
+the manner already explained in connection with Fig. 284. The third or
+sleeve strand of the cord is continuous from plug to plug, and between
+it and the ground there is permanently connected a retardation coil.
+
+_Test._ The test is dependent on the presence or absence of a path to
+ground from the test thimbles through some retardation coil associated
+with a cord circuit. Obviously, in the case of an idle line there will
+be no path to ground from the test thimbles, since normally they are
+merely connected to each other and are insulated from everything else.
+When, however, a plug is inserted into a multiple or answering jack, the
+test thimbles of that line are connected to ground through the
+retardation coil associated with the third strand of the plug used in
+making the connection. When the operator applies the tip of the calling
+plug to a test contact of a multiple jack there will be no path to
+ground afforded if the line is idle, while if it is busy the potential
+of the tip of the test plug will cause a current to flow to ground
+through the impedance coil associated with the plug used in making the
+connection. This will be made clearer by tracing the test circuit. With
+the listening key thrown this may be traced from the live side of the
+battery through the retardation coil _6_, which is common to an
+operator's position, thence through the tip side of the listening key to
+the tip conductor of the calling cord, and thence to the tip of the
+calling plug and the thimble of the jack under test. If the line is idle
+there will be no path to ground from this point and no click will
+result, but if the line is busy, current will flow from the tip of the
+test plug to the thimble of the jack tested, thence by the test wire in
+the multiple to the thimble of the jack at which a connection already
+exists, and thence to ground through the third strand of the cord used
+in making that connection and the impedance coil associated therewith.
+The current which flows in this test circuit changes momentarily the
+potential of the tip side of the operator's telephone circuit, thus
+unbalancing her talking circuit and causing a click.
+
+[Illustration: Fig. 341. Magneto Multiple Switchboard]
+
+If this test system were used in a very large board where the multiple
+would extend through a great many sections, there would be some
+liability of a false test due to the static capacity of the test
+contacts and the test wire running through the multiple. For small
+boards, however, where the multiple is short, this system has proven
+reliable. A multiple magneto switchboard employing the form of circuits
+just described is shown in Fig. 341. This switchboard consists of three
+sections of two positions each. The combined answering jacks and drops
+may be seen at the lower part of the face of the switchboard and
+occupying somewhat over one-half of the jack and drop space. The
+multiple jacks are above the answering jacks and drops and it may be
+noted that the same arrangement and number of these jacks is repeated in
+each section. This switchboard may be extended by adding more sections
+and increasing the multiple in those already installed to serve 1,600
+lines.
+
+_Assembly._ In connection with the assembly of these magneto multiple
+switchboards, as installed by the Monarch Company, Fig. 342 shows the
+details of the cord rack at the back of the board. It shows how the ends
+of the switchboard cords opposite to the ends that are fastened to the
+plugs are connected permanently to terminals on the cord rack, at which
+point the flexible conductors are brought out to terminal clips or
+binding posts, to which the wires leading from the other portions of the
+cord circuit are led. In order to relieve the conductors in the cords
+from strain, the outer braiding of the cord at the rack end is usually
+extended to form what is called a _strain cord_, and this attached to an
+eyelet under the cord rack, so that the weight of the cord and the cord
+weights will be borne by the braiding rather than by the conductors.
+This leaves the insulated conductors extending from the ends of the
+cords free to hang loose without strain and be connected to the
+terminals as shown. This method of connecting cords, with variations in
+form and detail, is practically universal in all types of switchboards.
+
+[Illustration: Fig. 342. Cord-Rack Connectors]
+
+A detail of the assembly of the drops and jacks in such a switchboard
+is shown in Fig. 343. The single pair of clearing-out drops is mounted
+in the lower part of the vertical face of the switchboard just above the
+space occupied by the plug shelf. Vertical stile strips extend above the
+clearing-out drop space for supporting the drops and jacks. A single row
+of 10 answering jacks and the corresponding line drops are shown in
+place. Above these there would be placed, in the completely assembled
+board, the other answering jacks and line signals that were to occupy
+this panel, and above these the strips of multiple jacks. The rearwardly
+projecting pins from the stile strips are for the support of the
+multiple jack strips, these pins supporting the strips horizontally by
+suitable multiple clips at the ends of the jack strips; the jack strips
+being fastened from the rear by means of nuts engaging the screw
+threads on these pins. This method of supporting drops and jacks is one
+that is equally adaptable for use in other forms of boards, such as the
+simple magneto switchboard.
+
+[Illustration: Fig. 343. Drop and Jack Mounting]
+
+[Illustration: Fig. 344. Keyboard Wiring]
+
+In Fig. 344 is shown a detail photograph of the key shelf wiring in one
+of these Monarch magneto switchboards. In this the under side of the
+keys is shown, the key shelf being raised on its hinge for that purpose.
+The cable, containing all of the insulated wires leading to these keys,
+enters the space under the key shelf at the extreme left and from the
+rear. It then passes to the right of this space where a "knee" is
+formed, after which the cable is securely strapped to the under side of
+the key shelf. By this construction sufficient flexibility is provided
+for in the cable to permit the raising and lowering of the key shelf,
+the long reach of the cable between the "knee" and the point of entry at
+the left serving as a torsion member, so that the raising of the shelf
+will give the cable a slight twist rather than bend it at a sharp angle.
+
+
+
+
+CHAPTER XXVI
+
+THE COMMON-BATTERY MULTIPLE SWITCHBOARD
+
+
+=Western Electric No. 1 Relay Board.= The common-battery multiple
+switchboard differs from the simple or non-multiple common-battery
+switchboard mainly in the provision of multiple jacks and in the added
+features which are involved in the provision for a busy test. The
+principles of signaling and of supplying current to the subscribers for
+talking are the same as in the non-multiple common-battery board. For
+purposes of illustrating the practical workings of the common-battery
+multiple switchboard, we will take the standard form of the Western
+Electric Company, choosing this only because it is the standard with
+nearly all the Bell operating companies throughout the United States.
+
+[Illustration: Fig. 345. Line Circuit Western Electric No. 1. Board]
+
+_Line Circuit._ We will first consider the line circuit in simplified
+form, as shown in Fig. 345. At the left in this figure the
+common-battery circuit is shown at the subscriber's station, and at the
+right the central-office apparatus is indicated so far as equipment of a
+single line is concerned. In this simplified diagram no attempt has been
+made to show the relative positions of the various parts, these having
+been grouped in this figure in such a way as to give as clear and simple
+an idea as possible of the circuit arrangements. It is seen at a glance
+that this is a branch terminal board, the three contacts of each jack
+being connected by separate taps or legs to three wires running
+throughout the length of the board, these three wires being individual
+to the jacks of one line. On this account this line circuit is commonly
+referred to as a three-wire circuit. By the same considerations it will
+be seen that the switchboard line circuit of the branch-terminal
+multiple magneto system, shown in Fig. 338, would be called a four-wire
+circuit. It will be shown later that other multiple switchboards in wide
+use have a still further reduction in the number of wires running
+through the jacks, or through the multiple as it is called, such being
+referred to as two-wire switchboards.
+
+The two limbs of the line which extend from the subscriber's circuit,
+beside being connected by taps to the tip and sleeve contacts of the
+jack respectively, connect with the two back contacts of a cut-off
+relay, and when this relay is in its normal or unenergized condition,
+these two limbs of the line are continued through the windings of the
+line relay and thence one to the ungrounded or negative side of the
+common-battery and the other to the grounded side. The subscriber's
+station circuit being normally open, no current flows through the line,
+but when the subscriber removes his receiver for the purpose of making a
+call the line circuit is completed and current flows through the coil of
+the line relay, thus energizing that relay and causing it to complete
+the circuit of the line lamp. The cut-off relay plays no part in the
+operation of the subscriber's calling, but merely leaves the circuit of
+the line connected through to the calling relay and battery. The coil of
+the cut-off relay is connected to ground on one side and on the other
+side to the third wire running through the switchboard multiple and
+which is tapped off to each of the test rings on the jacks. As will be
+shown later, when the operator plugs into the jack of a line, such a
+connection is established that the test ring of that jack will be
+connected to the live or negative pole of the common battery, which will
+cause current to flow through the coil of the cut-off relay, which will
+then operate to _cut off_ both of the limbs of the line from their
+normal connection with ground and the battery and the line relay. Hence
+the name _cut-off relay_.
+
+The use of the cut-off relay to sever the calling apparatus from the
+line at all times when the line is switched serves to make possible a
+very much simpler jack than would otherwise be required, as will be
+obvious to anyone who tries to design a common-battery multiple system
+without a cut-off relay. The additional complication introduced by the
+cut-off relay is more than offset by the saving in complexity of the
+jacks. It is desirable, on account of the great number of jacks
+necessarily employed in a multiple switchboard, that the jacks be of the
+simplest possible construction, thus reducing to a minimum their first
+cost and making them much less likely to get out of order.
+
+_Cord Circuit._ The cord circuit of the Western Electric standard
+multiple common-battery switchboard is shown in Fig. 346. This cord
+circuit involves the use of three strands in the flexible cords of both
+the calling and the answering plugs. Two of these are the ordinary tip
+and ring conductors over which speech is transmitted to the connected
+subscriber's wire. The third, the sleeve strand, carries the supervisory
+lamps and has associated with it other apparatus for the control of
+these lamps and of the test circuit.
+
+[Illustration: Fig. 346. Cord Circuit Western Electric No. 1 Board]
+
+The system of battery feed is the well-known split repeating-coil
+arrangement already discussed. The tip strand runs straight through to
+the repeating coil, while the ring strand contains, in each case, the
+winding of the supervisory relay corresponding to either the calling or
+the answering plug. In order that the presence in the talking circuit of
+a magnet winding possessing considerable impedance may not interfere
+with the talking efficiency, each of these supervisory relay windings is
+shunted by a non-inductive resistance. In practice the supervisory relay
+windings have each a resistance of about 20 ohms and the shunt around
+them each a resistance of about 31 ohms. In the third strand of each
+cord is placed a 12-volt supervisory lamp, and in series with it a
+resistance of about 80 ohms. Each supervisory relay is adapted, when
+energized, to close a 40-ohm shunt about its supervisory lamp. The
+arrangement and proportion of these resistances is such that when a plug
+is inserted into the jack of a line the lamp will receive current from a
+circuit traced from the negative pole of the battery in the center of
+the cord circuit through the lamp and the 80-ohm series resistance,
+through the third strand of the cord to the test thimble of the jack,
+and thence to the positive or grounded pole of the battery through the
+third conductor in the multiple and the winding of the cut-off relay.
+This current always flows as long as the plug is inserted, and it is
+just sufficient to illuminate the lamp when the supervisory relay
+armature is not attracted. When, however, the supervisory relay armature
+is attracted, the shunting of the lamp by the 40-ohm resistance cuts
+down the current to such a degree as to prevent the illumination of the
+lamp, although some current still flows through it.
+
+The usual ringing and listening key is associated with the calling plug,
+and in some cases a ring-back key is associated with the answering plug,
+but this is not standard practice.
+
+_Operation._ The operation of this cord circuit in conjunction with the
+line circuit of Fig. 345 may best be understood by reference to Fig.
+347. This figure employs a little different arrangement of the line
+circuit in order more clearly to indicate how the two lines may be
+connected by a cord; a study of the two line circuits, however, will
+show that they are identical in actual connections. It is to be
+remembered that all of the battery symbols shown in this figure
+represent in reality the same battery, separate symbols being shown for
+greater simplicity in circuit connections.
+
+We will assume the subscriber at Station _A_ calls for the subscriber at
+Station _B_. The operation of the line relay and the consequent lighting
+of the line lamp, and also the operation of the pilot relay will be
+obvious from what has been stated. The response of the operator by
+inserting the answering plug into the answering jack, and the throwing
+of her listening key so as to bridge her talking circuit across the cord
+in order to place herself in communication with the subscriber, is also
+obvious. The insertion of the answering plug into the answering jack
+completed the circuit through the third strand of the cord and the
+winding of the cut-off relay of the calling line, and this accomplishes
+three desirable results. The circuit so completed may be traced from the
+negative or ungrounded side of the battery to the center portion of the
+cord circuit, thence through the supervisory lamp _1_, resistance _2_,
+to the third conductor on the plug, test thimble on the jack, thence
+through the winding of the cut-off relay to ground, which forms the
+other terminal of the battery. The results accomplished by the closing
+of this circuit are: first, the energizing of the cut-off relay to cut
+off the signaling portion of the line; second, the flowing of current
+through the lamp that is almost sufficient to illuminate it, but not
+quite so because of the closure of the shunt about it, for the reason
+that will be described; third, the raising of the potential of all the
+contact thimbles on the jacks from zero to a potential different from
+that of the ground and equal in amount to the fall of potential through
+the winding of the cut-off relay. A condition is thus established at the
+test rings such that some other operator at some other section in
+testing the line will find it busy and will not connect with it.
+
+[Illustration: Fig. 347. Western Electric No. 1 Board]
+
+The reason why the lamp _1_, connected with the answering plug, was not
+lighted was that the supervisory relay _3_, associated with the
+answering plug, became energized when the operator plugged in, due to
+the flow of current from the battery through the calling subscriber's
+talking apparatus, this flow of current being permitted by the removal
+of the calling subscriber's receiver from its hook. The energizing of
+this relay magnet by causing the attraction of its armature, closed the
+shunt about the lamp _1_, which shunt contains the 40-ohm resistance
+_4_, and thus prevents the lamp from receiving enough current to
+illuminate it. Obviously, as soon as the calling subscriber replaces his
+receiver on its hook, the supervisory relay _3_ will be de-energized,
+the shunt around the lamp will be broken, and the lamp will be
+illuminated to indicate to the operator the fact that the subscriber
+with whose line her calling plug is connected has replaced his receiver
+on its hook.
+
+_Testing--Called Line Idle._ Having now shown how the operator connects
+with the calling subscriber's line and how that line automatically
+becomes guarded as soon as it is connected with, so that no other
+operator will connect with it, we will discuss how the operator tests
+the called line and subsequently connects with that line, if it is found
+proper to do so. If, on making the test with one of the multiple jacks
+of the line leading to Station _B_, that line is idle and free to be
+connected with, its test rings will all be at zero potential because of
+the fact that they are connected with ground through the cut-off relay
+winding with no source of current connected with them. The tip of the
+calling plug will also be at zero potential in making this test, because
+it is connected to ground through the tip side of the calling-plug
+circuit and one winding of the cord-circuit repeating coil. As a result
+no flow of current will occur, the operator will receive no click, and
+she will know that she is free to connect with the line. As soon as she
+does so, by inserting the plug, the third strand of the cord will be
+connected with the test thimble of the calling line and the resulting
+flow of current will bring about three results, two of which are the
+same, and one of which is slightly different from those described as
+resulting from the insertion of the answering plug into the jack of the
+calling line. First, the cut-off relay will be operated and cut off the
+line signaling apparatus from the called line; second, a flow of current
+will result through the calling supervisory lamp _5_, which in this case
+will be sufficient to illuminate that lamp for the reason that the
+called subscriber has not yet responded, the calling supervisory relay
+_6_ has, therefore, not yet been energized, and the lamp has not,
+therefore, been shunted by its associated resistance _7_; third, the
+test thimbles of the called line will be raised to a potential above
+that of the earth, and thus the line will be guarded against connection
+at another section of the switchboard. As soon as the called subscriber
+responds to the ringing current sent out by the operator, current will
+flow over the cord circuit and over his line through his transmitter.
+This will cause the calling supervisory relay to be energized and the
+calling lamp to be extinguished. Both lamps _1_ and _5_ remain
+extinguished as long as the connected subscribers are in conversation,
+but as soon as either one of them hangs up his receiver the
+corresponding lamp will be lighted, due to the de-energization of the
+supervisory relay and the breaking of the shunt around the lamp. The
+lighting of both lamps associated with a cord circuit is a signal to the
+operator for disconnection.
+
+[Illustration: TERMINAL ROOM IN MEDIUM-SIZED MANUAL OFFICE Relay Rack at
+Right. This Employs the Kellogg Parallel Arrangement of Frames.]
+
+_Testing--Called Line Busy._ If we now assume that the called line was
+already busy, by virtue of being connected with at another section, the
+test rings of that line would accordingly all be raised to a potential
+above that of the earth. As a result, when the operator applied the tip
+of her calling plug to a test thimble on that line, current would flow
+from this test thimble through the tip of the calling plug and tip
+strand of the cord and through one winding of the cord-circuit repeating
+coil to ground. This would cause a slight raising of potential of the
+entire tip side of the cord circuit and a consequent momentary flow of
+current through the secondary of the operator's circuit bridged across
+the cord circuit at that time.
+
+_Operator's Circuit Details._ The details of the operator's talking
+circuit shown in Fig. 347 deserve some attention. The battery supply to
+the operator's transmitter is through an impedance coil _9_. The
+condenser _12_ is bridged around the transmitter and the two primary
+windings _10_ and _11_, which windings are in parallel so as to afford a
+local circuit for the passage of fluctuating currents set up by the
+transmitter. The two primary windings _10_ and _11_ are on separate
+induction coils, the secondary windings _13_ and _14_ being, therefore,
+on separate cores. The winding _15_, in circuit with the secondary
+winding _14_ and the receiver, is a non-inductive winding and is
+supposed to have a resistance about equal to the effective resistance to
+fluctuating currents of a subscriber's line of average length. Owing to
+the respective directions of the primary and secondary windings _10_ and
+_11_, _13_ and _14_, the result is that the outgoing currents set up by
+the operator's transmitter are largely neutralized in the operator's
+receiver. Incoming currents from either of the connected subscribers,
+however, pass, in the main, through the secondary coil _13_ and the
+operator's receiver, rather than through the shunt path formed by the
+secondary _14_, and the non-inductive resistance _15_. This is known as
+an "anti-side tone" arrangement, and its object is to prevent the
+operator from receiving her own voice transmission so loudly as to make
+her ear insensitive to the feebler voice currents coming in from the
+subscribers.
+
+_Order-Wire Circuits._ The two keys _16_ and _17_, shown in connection
+with the operator's talking circuit in Fig. 347, play no part in the
+regular operation of connecting two local lines, as described above.
+They are order-wire keys, and the circuits with which they connect lead
+to the telephone sets of other operators at distant central offices, and
+by pressing either one of these keys the operator is enabled to place
+herself in communication over these so-called order-wire circuits with
+such other operators. The function and mode of operation of these
+order-wire circuits will be described in the next chapter, wherein
+inter-office connections will be discussed.
+
+_Wiring of Line Circuit._ The line circuits shown in Figs. 345 and 347
+are, as stated, simplified to facilitate understanding, although the
+connections shown are those which actually exist. The more complete
+wiring of a single line circuit is shown in Fig. 348. The line wires are
+shown entering at the left. They pass immediately, upon entering the
+central office, through the main distributing frame, the functions and
+construction of which will be considered in detail in a subsequent
+chapter. The dotted portions of the circuit shown in connection with
+this main distributing frame indicate the path from the terminals on one
+side of the frame to those on the other through so-called jumper wires.
+The two limbs of the line then pass to terminals _1_ and _2_ on one side
+of the so-called intermediate distributing frame. Here the circuit of
+each limb of the line divides, passing, on the one hand, to the tip and
+sleeve springs of all the multiple jacks belonging to that line; and, on
+the other hand, through the jumper wires indicated by dotted lines on
+the intermediate distributing frame, and thence to the tip and ring
+contacts of the answering jack. A consideration of this connection will
+show that the actual electrical connections so far as already described
+are exactly those of Figs. 345 and 347, although those figures omitted
+the main and intermediate distributing frames. Only two limbs of the
+line are involved in the main frame. In the intermediate frame the test
+wire running through the multiple is also involved. This test wire, it
+will be seen, leads from the test thimbles of all the multiple jacks to
+the terminal _3_ on the intermediate frame, thence through the jumper
+wire to the terminal _6_ of this frame, and to the test thimble of the
+answering jack. Here again the electrical connections are exactly those
+represented in Figs. 345 and 347, although those figures do not show the
+intermediate frame.
+
+The two terminals _4_ and _5_ of the intermediate frame, besides being
+connected to the tip and sleeve springs of the answering jack, are
+connected to the contacts of the cut-off relay, and thence through the
+coils of the line relay to ground on one side and to battery on the
+other. Thus the line relay and battery are normally included in the
+circuit of the line. The contact _6_ on the intermediate distributing
+frame, besides being connected to the test thimble of all the jacks, is
+connected through the coil of the cut-off relay to ground, thus
+establishing a path by which current is supplied to the cut-off relay
+when connection is made to the line at any jack. There is another
+contact _7_ on the intermediate distributing frame which merely forms a
+terminal for joining one side of the line lamp to the back contact of
+the line relay.
+
+_Functions of Distributing Frames._ Since the line circuit thus far
+described in connection with Fig. 348 is exactly the same as that of
+Fig. 345 in its electrical connections, it becomes obvious that the main
+and intermediate distributing frames play no part in the operation of
+the circuit any more than a binding post of a telephone plays a part in
+its operation. These frames carry terminals for facilitating the
+connection of the various wires in the line circuit and, as will be
+shown later, for facilitating certain changes in the line connection.
+
+[Illustration: Fig. 348. Line Circuit No. 1 Board]
+
+Remembering that the dotted lines in Fig. 348 indicate jumper wires of
+the main and intermediate distributing frames, and that these are in the
+nature of temporary or readily changeable connections, and that the full
+lines, whether heavy or light, are permanent connections not readily
+changeable, it will be seen that the wires leading through the multiple
+jacks of a certain line are permanently associated with each other, and
+with certain terminals on the main distributing frame and certain other
+terminals on the intermediate distributing frame. It will also be seen
+that the line lamp and the answering jack, together with the cut-off
+relay and line relay, are permanently associated with each other and
+with another group of terminals _4_, _5_, _6_, and _7_ on the
+intermediate distributing frame. It will also be apparent that by
+changing the jumper wires on the main frame, any outside line may be
+connected with any different set of line switchboard equipment, and also
+that by making changes in the jumper wires on the intermediate frame,
+any given answering jack and line lamp with its associated line cut-off
+relay may be associated with any set of multiple jacks.
+
+_Pilot Signals._ In a portion of the circuit leading from the battery
+that is common to a group of line lamps is the winding of the pilot
+relay, which is common to this group of line lamps. This controls, as
+already described, the circuit of the pilot lamp common to the same
+group of line lamps. In addition, a night-bell circuit is sometimes
+provided, this usually being in the form of an ordinary polarized
+ringer, the circuit of which is controlled by a night-bell relay common
+to the entire office. Normally, this relay is shunted out of the circuit
+of the common portion of the lead to the pilot relay contacts by the key
+_8_, but when the key _8_ is opened all current that is fed to the pilot
+lamps passes through the night-bell relay, and thus, whenever any pilot
+lamp is lighted, the night-bell relay will attract its armature and thus
+close the circuit of the calling generator through the night bell.
+
+A study of this figure will make clear to the student how the portions
+of the circuit that are individual to the line are associated with such
+things as the battery, that are common to the entire office, and such as
+the pilot relay and lamp, that are common to a group of lines
+terminating in one position.
+
+_Modified Relay Windings._ In some cases, the line relay instead of
+being double wound, as shown, is made with a single winding, this
+winding being normally included between the ring side of the cut-off
+relay and the battery, the tip side of the cut-off relay being run
+direct to ground. The present practice of the Western Electric Company
+is towards the double-wound relay, however, and that is considered
+standard in all of their large No. 1 multiple boards, except where the
+customer, owing to special reasons, demands a single wound relay on the
+ring side of the line. The prime reason for the two-winding line relay
+is the lessened click in the calling-subscriber's receiver which occurs
+when the operator answers. All line relays prior to 1902 were
+single-wound, but after that they were made double and used some turns
+of resistance wire to limit the normal calling current.
+
+_Relay Mounting._ In the standard No. 1 relay board of the Western
+Electric Company and, in fact, in nearly all common-battery multiple
+boards that are manufactured by other companies, the line and cut-off
+relays are mounted on separate racks outside the switchboard room and
+adjacent to the main and intermediate distributing frames, the wiring
+being extended from the relays to the jacks and lamps on the switchboard
+proper by means of suitable cables. The Western Electric Company has
+recently instituted a departure from this practice in the case of some
+of their smaller No. 1 switchboard installations. Where it is thought
+that the ultimate capacity required by the board will not be above 3,000
+lines, the relay rack is dispensed with and all of the line and cut-off
+relays, as well as the supervisory relays, are mounted in the rear of
+the switchboard frame. For this purpose the line and cut-off relays are
+specially made with the view to securing the utmost compactness. In
+still other cases, in switchboards of relatively small ultimate
+capacity, they use this small line and cut-off relay mounted on a
+separate relay rack, in which case the board is the standard No. 1 board
+except for the type of relays. In all of these modifications of the No.
+1 board adapted for the use of the smaller and cheaper relays, the line
+relay has but a single winding, the small size of the relay winding not
+lending itself readily to double winding with the added necessary coil
+terminals.
+
+_Capacity Range._ The No. 1 Western Electric board is made in standard
+sizes up to an ultimate capacity of 9,600 lines. For all capacities
+above 4,900 lines, a 3/8-inch jack, vertical and horizontal face
+dimensions, is employed. For this capacity the smaller types of cut-off
+and line relays are not employed. Up to ultimate capacities of 4,900
+lines, 1/2-inch jacks are employed, and either the small or the large
+relays mounted on a separate rack are available. Up to 3,000 lines
+ultimate capacity, the 1/2-inch jack is employed, and either the small
+or the large cut-off and line relays are available, but in case the
+small type is used the purchaser has the option of mounting them on a
+separate relay rack, as in ordinary practice, or mounting them in the
+switchboard cabinet and dispensing with the relay rack.
+
+=Western Electric No. 10 Board.= The No. 1 common-battery multiple
+switchboard, regardless of its size and type of arrangement of line and
+cut-off relays, involves two relays for each line, the line relay
+energized by the taking of the receiver off its hook, and the cut-off
+relay energized by the act of the operator on plugging in and serving to
+remove the line relay from the circuit whenever and as long as a plug is
+inserted into any jack of the line. This seems to involve a considerable
+expense in relays, but this, as has been stated, is warranted by the
+greater simplicity in jacks which the use of the cut-off relay makes
+possible. In addition to this expense of investment in the line and
+cut-off relays, the amount of current required to hold up the cut-off
+relays during conversations foots up to a considerable item of expense,
+particularly as the system of supervisory signals is one in which the
+supervisory lamp takes current not only while burning, but its circuit
+takes even more current when the lamp is extinguished during the time of
+a connection. For all of these reasons, and some other minor ones, it
+was deemed expedient by the engineers of the Western Electric Company to
+design a common-battery multiple switchboard for small and medium-sized
+exchanges in which certain sacrifices might be made to the end of
+accomplishing certain savings. The result has been a type of
+switchboard, designated the No. 10, which may be found in a number of
+Bell exchanges, it being considered particularly adaptable to
+installations of from 500 to 3,000 lines. Although this board has been
+subject to a good deal of adverse criticism, and although it seems
+probable that even for the cheaper boards the No. 1 type with some of
+the modifications just described will eventually supersede this No. 10
+board, yet the present extent of use of the No. 10 board and the
+instructive features which its type displays warrant its discussion
+here.
+
+_Circuits._ The circuits of this switchboard are shown in Fig. 349, this
+indicating two-line circuits and a connecting cord circuit, together
+with the auxiliary apparatus employed in connection with the operator's
+telephone circuit, the pilot and night alarm circuits. The most
+noticeable feature is that cut-off jacks are employed, the circuit of
+the line normally extending through the sets of jack springs in the
+multiple, and answering jacks to the line relay and battery on one side
+of the line, and to ground on the other side. Obviously, the additional
+complexity of the jack saves the use of a cut-off relay and the relay
+equipment of each line consists, therefore, of but a single line relay,
+which controls the lamp in an obvious manner.
+
+[Illustration: Fig. 349. Western Electric No. 10 Board]
+
+The cord circuit is of the three-conductor type, the two talking strands
+extending to the usual split repeating-coil arrangement, and battery
+current for talking purposes being fed through these windings as in the
+standard No. 1 board. The supervisory relay is included in the ring
+strand of the cord circuit and is shunted by a non-inductive resistance,
+so that its impedance will not interfere with the talking currents. The
+armature of the supervisory relay closes the lamp contact on its back
+stroke, so that the lamp is always held extinguished when the relay is
+energized. The supervisory lamp is included in a connection between the
+back contact of the supervisory relay and ground, this connection
+including the central-office battery. As a result, the illumination of
+the supervisory lamp is impossible until a plug has been inserted into a
+jack, in which case, assuming the supervisory relay to be de-energized,
+the lamp circuit is completed through the wire connecting all of the
+test thimbles and the resistance permanently bridged to ground from that
+wire.
+
+_Test._ For purposes of the test it is evident that the test rings of an
+idle line are always at ground potential, due to their connection to
+ground through the resistance coil. It is also evident that the tip of
+an unused calling plug will always be at ground potential and,
+therefore, that the testing of an idle line will result in no click in
+the operator's receiver. When a line is switched, however, the potential
+of all the test rings will be raised due to their being connected with
+the live pole of the battery through the third strand of the cord. When
+the operator in testing touches the test contact of the jack of a busy
+line, a current will, therefore, flow from this test contact to the tip
+strand of the cord and thence to ground through one of the repeating
+coil windings. The potential of the tip side of the cord will,
+therefore, be momentarily altered, and this will result in a click in
+the operator's receiver bridged across the cord circuit at the time. The
+details of the operator's cord circuit and of the pilot lamp and night
+alarm circuits will be clear from the diagram.
+
+_Operation._ A brief summary of the operation of this system is as
+follows:
+
+The subscriber removes his receiver from its hook, thus drawing up the
+armature of the line relay and lighting his line lamp. The operator
+answers. The line lamp is extinguished by the falling back of the
+line-relay armature, due to the breaking of the relay circuit at the
+jack contacts. The subscriber then receives current for his transmitter
+through the cord-circuit battery connections. The supervisory relay
+connected with the answering cord is not lighted, because, although the
+lamp-circuit connection is completed at the jack, the supervisory relay
+is operated to hold the lamp circuit open. Conversation ensues between
+the operator and the subscriber, after which the operator tests the line
+called for with the tip of the calling plug of the pair used in
+answering. If the called line is not busy, no click will ensue, because
+both the tested ring and the calling plug are at the same potential.
+Finding no click, the operator will insert the plug and ring by means of
+the ringing key. When the operator plugs in, the supervisory lamp,
+associated with the calling plug, becomes lighted because the circuit is
+completed at the jack and the supervisory relay remains de-energized,
+since the line circuit is open at the subscriber's station. When the
+called subscriber responds, the calling supervisory lamp goes out
+because of the energization of the supervisory relay. Both lamps remain
+out during the conversation, but when either subscriber hangs up, the
+corresponding supervisory lamp will be lighted because of the falling
+back of the supervisory relay armature.
+
+If the called line is busy, a click will be heard, for the reason
+described, and the operator will so inform the calling subscriber. It
+goes without saying, that in any multiple-switchboard system a plug may
+be found in the actual multiple jack that is reached for, in which case,
+although no test will be made, the busy condition will be reported back
+to the calling subscriber.
+
+_Economy._ It has been the belief of the Western Electric engineers that
+a real economy is accomplished in this type of board by the saving in
+relay equipment. It is, of course, apparent at a glance that with a
+switchboard long enough and of sections enough, the cost of extra jack
+springs and their platinum contacts must become great enough to offset
+the saving accomplished by omitting the cut-off relay. This makes it
+apparent that if there is any economy in this type of multiple
+switchboard, it must be found in the very small boards where there are
+but few jacks per line and where the extra cost of the cut-off jack is
+not enough to offset the extra cost of an added relay. It is the growing
+belief, however, among engineers, that the multiple switchboard must be
+very small indeed in order that the added complexity of the cut-off
+jacks and wiring may be able to save anything over the two-relay type of
+line; and it is believed that where economy is necessary in small
+boards, it may be best effected by employing cheaper and more compact
+forms of relays and mounting them, if necessary, directly in the
+switchboard cabinet.
+
+     NOTE. These two standard types of common-battery multiple
+     switchboards of the Western Electric Company represent the
+     development through long years of careful work on the part of
+     the Western Electric and Bell engineers, credit being
+     particularly due to Scribner, McBerty, and McQuarrie of the
+     Western Electric Company, and Hayes of the American Telephone
+     and Telegraph Company.
+
+=Kellogg Two-Wire Multiple Board.= The simplicity in the jacks permitted
+by the use of the cut-off relay in the Western Electric common-battery
+multiple switchboard for larger exchanges was carried a step further by
+Dunbar and Miller in the development of the so-called two-wire
+common-battery multiple switchboard, which for many years has been the
+standard of the Kellogg Switchboard and Supply Company. The particular
+condition which led to the development of the two-wire system was the
+demand at that time on the Kellogg Company for certain very large
+multiple switchboards, involving as many as 18,000 lines in the
+multiple. Obviously, this necessitated a small jack, and obviously a
+jack having only two contacts, a tip spring and a sleeve, could be made
+more easily and with greater durability of this very small size than a
+jack requiring three or more contacts. Other reasons that were
+considered were, of course, cheapness in cost of construction and
+extreme simplicity, which, other things being equal, lends itself to low
+cost of maintenance.
+
+_Line Circuit._ Like the standard Western Electric board for large
+offices, the Kellogg two-wire board employs two relays for each line,
+the line relay under the control of the subscriber and in turn
+controlling the lamp, and a cut-off relay under the control of the
+operator and in turn controlling the connection of the line relay with
+the line. The line circuit as originally developed and as widely used by
+the Kellogg Company is shown in Fig. 350. The extreme simplicity of the
+jacks is apparent, as is also the fact that but two wires lead through
+the multiple. Another distinguishing feature is, that all of the
+multiple and answering jacks are normally cut off from the line at the
+cut-off relay, but when the cut-off relay operates it serves, in
+addition to cutting off the line relay, to attach the two limbs of the
+line to the two wires leading through the multiple and answering jacks.
+The control of the line relay by the subscriber's switch hook is clear
+from the figure. The control of the cut-off relay is secured by
+attaching one terminal of the cut-off relay winding permanently to that
+wire leading through the multiple which connects with the sleeve
+contacts of the jack, the other terminal of the cut-off relay being
+grounded. The way in which this relay is operated will be understood
+when it is stated that the sleeve contacts of both the answering and
+calling plugs always carry full battery potential and, therefore,
+whenever any plug is inserted into any jack, current flows from the
+sleeve of the jack through the sleeve contact of the jack to ground,
+through the winding of the cut-off relay, which relay becomes energized
+and performs the functions just stated. It is seen that the wire
+running through the multiple to which the sleeve jack contacts are
+attached, is thus made to serve the double purpose of answering as one
+side of the talking circuit, and also of performing the functions
+carried out by the separate or third wire in the three-wire system. It
+will be shown also that, in addition, this wire is made to lend itself
+to the purposes of the busy test without any of these functions
+interfering with each other in any way.
+
+[Illustration: Fig. 350. Two-Wire Line Circuit]
+
+_Cord Circuit._ The cord circuit in somewhat simplified form is shown in
+Fig. 351. Here again there are but two conductors to the plugs and two
+strands to the cords. This greater simplicity is in some measure offset
+by the fact that four relays are required, two for each plug. This
+so-called four-relay cord circuit may be most readily understood by
+considering half of it at a time, since the two relays associated with
+the answering plug act in exactly the same way as those connected with
+the calling plug.
+
+[Illustration: Fig. 351. Two-Wire Cord Circuit]
+
+Associated with each plug of a pair are two relays _1_ and _2_, in the
+case of the answering cord, and _3_ and _4_ in the case of the calling
+cord. The coils of the relays _1_ and _2_ are connected in series and
+bridged across the answering cord, a battery being included between the
+coils in this circuit. The coils of the relays _3_ and _4_ are similarly
+connected across the calling cord. A peculiar feature of the Kellogg
+system is that two batteries are used in connection with the cord
+circuit, one of them being common to all answering cords and the other
+to all calling cords. The operation of the system would, however, be
+exactly the same if a single battery were substituted for the two.
+
+_Supervisory Signals._ Considering the relays associated with the
+answering cord, it is obvious that these two relays _1_ and _2_ together
+control the circuit of the supervisory lamp _5_, the circuit of this
+lamp being closed only when the relay _1_ is de-energized and the relay
+_2_ is energized. We will find in discussing the operation of these that
+the relay _2_ is wholly under the control of the operator, and that the
+relay _1_, after its plug has been connected with a line, is wholly
+under the control of the subscriber on that line. It is through the
+windings of these two relays that current is fed to the line of the
+subscriber connected with the corresponding cord.
+
+When a plug--the answering plug, for instance--is inserted into a jack,
+current at once flows from the positive pole of the left-hand battery
+through the winding of the relay _2_ to the sleeve of the plug, thence
+to the sleeve of the jack and through the cut-off relay to ground. This
+at once energizes the supervisory relay _2_ and the cut-off relay
+associated with the line. The cut-off relay acts, as stated, to continue
+the tip and sleeve wires associated with the jacks to the line leading
+to the subscriber, and also to cut off the line relay. The supervisory
+relay _2_ acts at the same time to attract its armature and thus
+complete its part in closing the circuit of the supervisory lamp.
+Whether or not the lamp will be lighted at this time depends on whether
+the relay _1_ is energized or not, and this, it will be seen, depends on
+whether the subscriber's receiver is off or on its hook. If off its
+hook, current will flow through the metallic circuit of the line for
+energizing the subscriber's transmitter, and as whatever current goes to
+the subscriber's line must flow through the relay _1_, that relay will
+be energized and prevent the lighting of the supervisory lamp _5_. If,
+on the other hand, the subscriber's receiver is on its hook, no current
+will flow through the line, the supervisory relay will not be energized,
+and the lamp _5_ will be lighted.
+
+In a nutshell, the sleeve supervisory relay normally prevents the
+lighting of the corresponding supervisory lamp, but as soon as the
+operator inserts a plug into the jack of the line, the relay _2_
+establishes such a condition as to make possible the lighting of the
+supervisory lamp, and the lighting of this lamp is then controlled
+entirely by the relay _1_, which is, in turn, controlled by the position
+of the subscriber's switch hook.
+
+_Battery Feed._ A 2-microfarad condenser is included in each strand of
+the cord, and battery is fed through the relay windings to the calling
+and called subscribers on opposite sides of these condensers, in
+accordance with the combined impedance coil and condenser method
+described in Chapter XIII. Here the relay windings do double duty,
+serving as magnets for operating the relays and as retardation coils in
+the system of battery supply.
+
+_Complete Cord and Line Circuits._ The complete cord and line circuits
+of the Kellogg two-wire system are shown in Fig. 352. In the more recent
+installations of the Kellogg Company the cord and line circuits have
+been slightly changed from those shown in Figs. 350 and 351, and these
+changes have been incorporated in Fig. 352. The principles of operation
+described in connection with the simplified figures remain, however,
+exactly the same. One of the changes is, that the tip side of the lines
+is permanently connected to the tips of the jacks instead of being
+normally cut off by the cut-off relay, as was done in the system as
+originally developed. Another change is, that the line relay is
+associated with the tip side of the line, rather than with the sleeve
+side, as was formerly done. The cord circuit shown in Fig. 352 shows
+exactly the same arrangement of supervisory relays and exactly the same
+method of battery feed as in the simplified cord circuit of Fig. 351,
+but in addition to this the detailed connections of the operator's
+talking set and of her order-wire keys are indicated, and also the
+ringing equipment is indicated as being adapted for four-party harmonic
+work.
+
+[Illustration: Fig. 352. Kellogg Two-Wire Board]
+
+In connection with this ringing key it may be stated that the springs
+_7_, _8_, _9_, and _10_ are individually operated by the pressure of one
+of the ringing key buttons, while the spring _17_, connected with the
+sleeve side of the calling plug, is always operated simultaneously with
+the operation of any one of the other springs. As a result the proper
+ringing circuit is established, it being understood that the upper
+contacts of the springs _7_, _8_, _9_, and _10_ lead to the terminals of
+their respective ringing generators, the other terminals of which are
+grounded. The circuit is, therefore, from the generator, through the
+ringing key, out through the tip side of the line, back over the sleeve
+side of the line, and to ground through the spring _17_, resistance
+_11_, and the battery, which is one of the cord-circuit batteries. The
+object of this coil _11_ and the battery connection through it to the
+ringing-key spring is to prevent the falling back of the cut-off relay
+when the ringing key is operated. This will be clear when it is
+remembered that the cut-off relay is energized by battery current fed
+over the sleeve strand of the cord, and obviously, since it is necessary
+when the ringing key is operated to cut off the supply wire back of the
+key, this would de-energize the cut-off relay when the ringing key was
+depressed, and the falling back of the cut-off relay contacts would make
+it impossible to ring because the sleeve side of the line would be cut
+off. The battery supply through the resistance _11_ is, therefore,
+substituted on the sleeve strand of the cord for the battery supply
+through the normal connection.
+
+_Busy Test._ The busy test depends on all of the test rings being at
+zero potential on an idle line and at a higher potential on a busy line.
+Obviously, when the line is not switched, the test rings are at zero
+potential on account of a ground through the cut-off relay. When,
+however, a plug is inserted in either the answering or multiple jacks,
+the test rings will all be raised in potential due to being connected
+with the live side of the battery through the sleeve strand of the cord.
+Conditions on the line external to the central office cannot make an
+idle line test busy because, owing to the presence of the cut-off relay,
+the sleeve contacts of all the jacks are disconnected from the line when
+it is idle. The test circuit from the tip of the calling plug to ground
+at the operator's set passes through the tip strand of the cord, thence
+through a pair of normally closed extra contacts on the supervisory
+relay _4_, thence in series through all the ringing key springs _10_,
+_9_, _8_, and _7_, thence through an extra pair of springs _12_ and _13_
+on the listening key--closed only when the listening key is
+operated--and thence to ground through a retardation coil _14_. No
+battery or other source of potential exists in this circuit between
+ground and the tip of the calling plug and, therefore, the tip is
+normally at ground potential. The sleeve ring of the jack being at
+ground potential if the line is idle, no current will flow and no click
+will be produced in testing such a line. If, however, the line is busy,
+the test ring will be at a higher potential and, therefore, current will
+flow from the tip of the calling plug to ground over the path just
+traced, and this will cause a rise in potential at the terminal of the
+condenser _15_ and a momentary flow of current through the tertiary
+winding _16_ of the operator's induction coil; hence the click.
+
+[Illustration: SWITCH ROOM OF CITIZENS' TELEPHONE COMPANY, GRAND RAPIDS,
+MICH. One of the Earliest Large Automatic Offices.]
+
+Obviously the testing circuit from the tip of the calling plug to ground
+at the operator's set is only useful during the time when the calling
+plug is not in a jack, and as the tip strand of the calling plug has to
+do double duty in testing and in serving as a part of the talking
+circuit, the arrangement is made that the testing circuit will be
+automatically broken and the talking circuit through the tip strand
+automatically completed when the plug is inserted into a jack in
+establishing a connection. This is accomplished by means of the extra
+contact on the relay _4_, which relay, it will be remembered, is held
+energized when its corresponding plug is inserted in a jack. During the
+time when the plug is not inserted, this relay is not energized and the
+test circuit is completed through the back contact of its right-hand
+armature. When connection is made at the jack, this relay becomes
+energized and the tip strand of the cord circuit is made complete by the
+right-hand lever being pulled against the front contact of this relay.
+The keys shown to the right of the operator's set are order-wire keys.
+
+_Summary of Operation._ We may give a brief summary of the operation of
+this system as shown in Fig. 352. The left-hand station calls and the
+line relay pulls up, lighting the lamp. The operator inserts an
+answering plug in the answering jack, thus energizing the cut-off relay
+which operates to cut off the line relay and to complete the connection
+between the jacks and the external line. The act of plugging in by the
+operator also raises the potential of all the test rings so as to guard
+the line against intrusion by other callers. The supervisory lamp _5_
+remains unlighted because, although the relay _2_ is operated, the relay
+_1_ is also operated, due to the calling subscriber's receiver being off
+its hook. The operator throws her listening key, communicates with the
+subscriber, and, learning that the right-hand station is wanted,
+proceeds to test that line. If the line is idle, she will get no click,
+because the tip of her calling plug and the tested ring will be at the
+same ground potential. She then plugs in and presses the proper
+ringing-key button to send out the proper frequency to ring the
+particular subscriber on the line--if there be more than one--the
+current from the battery through the coil _11_ and spring _17_ serving
+during this operation to hold up the cut-off relay.
+
+As soon as the operator plugs in with the calling plug, the supervisory
+lamp _6_ lights, assuming that the called subscriber had not already
+removed his receiver from its hook, due to the fact that the relay _4_
+is energized and the relay _3_ is not. As soon as the called subscriber
+responds, the relay _3_ becomes energized and the supervisory lamp goes
+out. If the line called for had been busy by virtue of being plugged at
+another section, the tip of the operator's plug in testing would have
+found the test ring raised to a potential above the ground, and, as a
+consequence, current would have flowed from the tip of this plug through
+the back contact of the right-hand lever of relay _4_, thence through
+the ringing key springs and the auxiliary listening-key springs to
+ground through the retardation coil _14_. This would have produced a
+click by causing a momentary flow of current through the tertiary
+winding _16_ of the operator's set.
+
+_Wiring of Line Circuit._ The more complete wiring diagram of a single
+subscriber's line, Fig. 353, shows the placing in the circuits of the
+terminals and jumper wires of the main distributing frame and of the
+intermediate distributing frame, and also shows how the pilot lamps and
+night-alarm circuits are associated with a group of lines. The main
+distributing frame occupies the same relative position in this line
+circuit as in the Western Electric, being located in the main line
+circuit outside of all the switchboard apparatus. The intermediate
+distributing frame occupies a different relative position from that in
+the Western Electric line. It will be recalled by reference to Fig. 348
+that the line lamp and the answering jack were permanently associated
+with the line and cut-off relays, such mutations of arrangement as were
+possible at the intermediate distributing frame serving only to vary the
+connection between the multiple of a line and one of the various groups
+of apparatus consisting of an answering jack and line lamp and
+associated relays. In the Kellogg arrangement, Fig. 353, the line and
+cut-off relays, instead of being permanently associated with the
+answering jack and line lamp, are permanently associated with the
+multiple jacks, no changes, of which the intermediate or main frames are
+capable, being able to alter the relation between a group of multiple
+jacks and its associated line and cut-off relays. In this Kellogg
+arrangement the intermediate distributing frame may only alter the
+connection of an answering jack and line lamp with the multiple and its
+permanently associated relays. The pilot and night alarm arrangements of
+Fig. 353 should be obvious from the description already given of other
+similar systems.
+
+[Illustration: Fig. 353. Kellogg Two-Wire Line Circuit]
+
+=Dean Multiple Board.= In Fig. 354 are shown the circuits of the
+multiple switchboard of the Dean Electric Company. The subscriber's
+station equipment shown at Station _A_ and Station _B_ will be
+recognized as the Wheatstone-bridge circuit of the Dean Company.
+
+_Line Circuit._ The line circuit is easily understood in view of what
+has been said concerning the Western Electric line circuit, the line
+relay _1_ being single wound and between the live side of the battery
+and the ring side of the line. The cut-off relay _2_ is operated
+whenever a plug is inserted in a jack and serves to sever the connection
+of the line with the normal line signaling apparatus.
+
+_Cord Circuit._ The cord circuit is of the four-relay type, but employs
+three conductors instead of two, as in the two-wire system. The relay
+_3_, being in series between the battery and the sleeve contact on the
+plug, is energized whenever a plug is inserted in the jack, its winding
+being placed in series with the cut-off relay of the line with which the
+plug is connected. This completes the circuit through the associated
+supervisory lamp unless the relay _4_ is energized, the local lamp
+circuit being controlled by the back contact of relay _4_ and the front
+contact of relay _3_. It is through the two windings of the relay _4_
+that current is fed to the subscriber's station, and, therefore, the
+armature of this relay is responsive to the movements of the
+subscriber's hook. As the relay _3_ holds the supervisory lamp circuit
+closed as long as a plug is inserted in a jack of the line, it follows
+that during a connection the relay _4_ will have entire control of the
+supervisory lamp.
+
+_Listening Key._ The listening key, as usual, serves to connect the
+operator's set across the talking strands of the cord circuit, and the
+action of this in connection with the operator's set needs no further
+explanation.
+
+_Ringing Keys._ The ringing-key arrangement illustrated is adapted for
+use with harmonic ringing, the single springs _5_, _6_, _7_, and _8_
+each being controlled by a separate button and serving to select the
+particular frequency that is to be sent to line. The two springs _9_ and
+_10_ always act to open the cord circuit back of the ringing keys,
+whenever any one of the selective buttons is depressed, in order to
+prevent interference by ringing current with the other operations of the
+circuit.
+
+Two views of these ringing keys are shown in Figs. 355 and 356. Fig. 356
+is an end view of the entire set. In Fig. 355 the listening key is shown
+at the extreme right and the four selective buttons at the left. When a
+button is released it rises far enough to cause the disengagement of the
+contacts, but remains partially depressed to serve as an indication that
+it was last used. The group of springs at the extreme left of Fig. 355
+are the ones represented at _9_ and _10_ in Fig. 354 and by the anvils
+with which those springs co-operate.
+
+[Illustration: Fig. 354. Dean Multiple Board Circuits]
+
+_Test._ The test in this Dean system is simple, and, like the Western
+Electric and Kellogg systems, it depends on the raising of the
+potential of the test thimbles of all the line jacks of a line when a
+connection is made with that line by a plug at any position. When an
+operator makes a test by applying the tip of the calling plug to the
+test thimble of a busy line, current passes from the test thimble
+through the tip strand of the cord to ground through the left-hand
+winding of the calling supervisory relay _4_. The drop of potential
+through this winding causes the tip strand of the cord to be raised to a
+higher potential than it was before, and as a result the upper plate of
+the condenser _11_ is thus altered in potential and this change in
+potential across the condenser results in a click in the operator's ear.
+
+[Illustration: Fig. 355. Dean Party Line Ringing Key]
+
+[Illustration: Fig. 356. Dean Party Line Ringing Key]
+
+=Stromberg-Carlson Multiple Board.= _Line Circuit._ In Fig. 357 is shown
+the multiple common-battery switchboard circuits employed by the
+Stromberg-Carlson Telephone Manufacturing Company. The subscriber's line
+circuits shown in this drawing are of the three-wire type and, with the
+exception of the subscriber's station, are the same as already described
+for the Western Electric Company's system.
+
+_Cord Circuit._ The cord circuit employed is of the two-conductor type,
+the plugs being so constructed as to connect the ring and thimble
+contacts of the jack when inserted. This cord circuit is somewhat
+similar to that employed by the Kellogg Switchboard and Supply Company,
+shown in Fig. 352, except that only one battery is employed, and that
+certain functions of this circuit are performed mechanically by the
+inter-action of the armatures of the relays.
+
+_Supervisory Signals._ When the answering plug is inserted in a jack, in
+response to a call, the current passing to the subscriber's station and
+also through the cut-off relay must flow through the relay _1_, thus
+energizing it. As the calling subscriber's receiver is at this time
+removed from the hook switch, the path for current will be completed
+through the tip of the jack, thence through the tip of the plug, through
+relay _2_ to ground, causing relay _2_ to be operated and to break the
+circuit of the answering supervisory lamp. The two relays _1_ and _2_
+are so associated mechanically that the armature of _1_ controls the
+armature of _2_ in such a manner as to normally hold the circuit of the
+answering supervisory lamp open. But, however, when the plug is inserted
+in a jack, relay _1_ is operated and allows the operation of relay _2_
+to be controlled by the hook switch at the subscriber's station. The
+supervisory relay _3_ associated with the calling cord is operated when
+the calling plug is placed in a jack, and this relay normally holds the
+armature of relay _4_ in an operated position in a similar manner as the
+armature of relay _1_ controlled that of relay _2_. Supervisory relay
+_4_ is under the control of the hook switch at the called subscriber's
+station.
+
+_Test._ In this circuit, as in several previously described, when a plug
+is inserted in a jack of a line, the thimble contacts of the jacks
+associated with that line are raised to a higher potential than that
+which they normally have. The operator in testing a busy line, of course
+having previously moved the listening key to the listening position,
+closes a path from the test thimble of the jack, through the tip of the
+calling plug, through the contacts of the relay _4_, the inside springs
+of the listening key, thence through a winding of the induction coil
+associated with her set to ground. The circuit thus established allows
+current to flow from the test thimble of the jack through the winding of
+her induction coil to ground, causing a click in her telephone receiver.
+The arrangement of the ringing circuit does not differ materially from
+that already described for other systems and, therefore, needs no
+further explanation.
+
+[Illustration: Fig. 357. Stromberg-Carlson Multiple Board Circuits]
+
+=Multiple Switchboard Apparatus.= Coming now to a discussion of the
+details of apparatus employed in multiple switchboards, it may be
+stated that much of the apparatus used in the simpler types is capable
+of doing duty in multiple switchboards, although, of course,
+modification in detail is often necessary to make the apparatus fit the
+particular demands of the system in which it is to be used.
+
+_Jacks._ Probably the most important piece of apparatus in the multiple
+switchboard is the jack, its importance being increased by the fact that
+such very large numbers of them are sometimes necessary. Switchboards
+having hundreds of thousands of jacks are not uncommon. The multiple
+jacks are nearly always mounted in strips of twenty and the answering
+jacks usually in strips of ten, the length of the jack strip being the
+same in each case in the same board and, therefore, giving twice as wide
+a spacing in the answering as in the multiple jacks. The distance
+between centers in the multiple jacks varies from a quarter of an
+inch--which is perhaps the extreme minimum--to half an inch, beyond
+which larger limit there seems to be no need of going in any case. It is
+customary that the jack strip shall be made of the same total thickness
+as the distance between the centers of two of its jacks, and from this
+it follows that the strips when piled one upon the other give the same
+vertical distance between jack centers as the horizontal distance.
+
+In Fig. 358 is shown a strip of multiple and a strip of answering jacks
+of Western Electric make, this being the type employed in the No. 1
+standard switchboards for large exchanges. In Fig. 359 are shown the
+multiple and answering jacks employed in the No. 10 Western Electric
+switchboard. The multiple jacks in the No. 1 switchboard are mounted on
+3/8-inch centers, the jacks having three branch terminal contacts. The
+multiple jacks of the No. 10 switchboard indicated in Fig. 359 are
+mounted on 1/2-inch centers, each jack having five contacts as indicated
+by the requirement of the circuits in Fig. 349.
+
+In Fig. 360 are shown the answering and multiple jacks of the Kellogg
+Switchboard and Supply Company's two-wire system. The extreme simplicity
+of these is particularly well shown in the cut of the answering jack,
+and these figures also show clearly the customary method of numbering
+jacks. In very large multiple boards it has been the practice of the
+Kellogg Company to space the multiple jacks on 3/10-inch centers, and in
+their smaller multiple work, they employ the 1/2-inch spacing. With the
+3/10-inch spacing that company has been able to build boards having a
+capacity of 18,000 lines, that many jacks being placed within the reach
+of each operator.
+
+In all modern multiple switchboards the test thimble or sleeve contacts
+are drawn up from sheet brass or German silver into tubular form and
+inserted in properly spaced borings in strips of hard rubber forming the
+faces of the jacks. These strips sometimes are reinforced by brass
+strips on their under sides. The springs forming the other terminals of
+the jack are mounted in milled slots in another strip of hard rubber
+mounted in the rear of and parallel to the front strip and rigidly
+attached thereto by a suitable metal framework. In this way desired
+rigidity and high insulation between the various parts is secured.
+
+[Illustration: Fig. 358. Answering and Multiple Jacks for No. 1 Board]
+
+_Lamp Jacks._ The lamp jacks employed in multiple work need no further
+description in view of what has been said in connection with lamp jacks
+for simple common-battery boards. The lamp jack spacing is always the
+same as the answering jack spacing, so that the lamps will come in the
+same vertical alignment as their corresponding answering jacks when the
+lamp strips and answering jack strips are mounted in alternate layers.
+
+[Illustration: Fig. 359. Answering and Multiple Jacks for No. 10 Board]
+
+[Illustration: Fig. 360. Answering and Multiple Jacks for Kellogg
+Two-Wire Board]
+
+_Relays._ Next in order of importance in the matter of individual parts
+for multiple switchboards is the relay. The necessity for reliability of
+action in these is apparent, and this means that they must not only be
+well constructed, but that they must be protected from dust and moisture
+and must have contact points of such a nature as not to corrode even in
+the presence of considerable sparking and of the most adverse
+atmospheric conditions. Economy of space is also a factor and has led to
+the almost universal adoption of the single-magnet type of relay for
+line and cut-off as well as supervisory purposes.
+
+[Illustration: Fig. 361. Type of Line Relay]
+
+[Illustration: Fig. 362. Type of Cut-Off Relay]
+
+The Western Electric Company employs different types of relays for line,
+cut-off, and supervisory purposes. This is contrary to the practice of
+most of the other companies who make the same general type of relay
+serve for all of these purposes. A good idea of the type of Western
+Electric line relay, as employed in its No. 1 board, may be had from
+Fig. 361. As is seen this is of the tilting armature type, the armature
+rocking back and forth on a knife-edge contact at its base, the part on
+which it rests being of iron and of such form as to practically
+complete, with the armature and core, the magnetic circuit. The cut-off
+relay, Fig. 362, is of an entirely different type. The armature in this
+is loosely suspended by means of a flexible spring underneath two
+L-shaped polar extensions, one extending up from the rear end of the
+core and the other from the front end. When energized this armature is
+pulled away from the core by these L-shaped pieces and imparts its
+motion through a hard-rubber pin to the upper pair of springs so as to
+effect the necessary changes in the circuit.
+
+[Illustration: Fig. 363. Western Electric Combined Line and Cut-off
+Relay]
+
+[Illustration: Fig. 364. Western Electric Supervisory Relay]
+
+[Illustration: Fig. 365. Line Relay No. 10 Board]
+
+Much economy in space and in wiring is secured in the type of
+switchboards employing cut-off as well as line relays by mounting the
+two relays together and in making of them, in fact, a unitary piece of
+apparatus. Since the line relay is always associated with the cut-off
+relay of the same line and with no other, it is obvious that this
+unitary arrangement effects a great saving in wiring and also secures a
+great advantage in the matter of convenience of inspection. Such a
+combined cut-off and line relay, employed in the Western Electric No. 1
+relay board, is shown in Fig. 363. These are mounted in banks of ten
+pairs, a common dust cap of sheet iron covering the entire group.
+
+The Western Electric supervisory relay, Fig. 364, is of the tilting
+armature type and is copper clad. The dust cap in this case fits on with
+a bayonet joint as clearly indicated. In Fig. 365 is shown the line
+relay employed in the Western Electric No. 10 board.
+
+[Illustration: Fig. 366. Kellogg Line and Cut-off Relays]
+
+[Illustration: Fig. 367. Strip of Kellogg Line and Cut-Off Relays]
+
+The Kellogg Company employs the type of relay of which the magnetic
+circuit was illustrated in Fig. 95. In its multiple boards it commonly
+mounts the line and cut-off relays together, as shown in Fig. 366. A
+single, soft iron shell is used to cover both of these, thus serving as
+a dust shield and also as a magnetic shield to prevent cross-talk
+between adjacent relays--an important feature, since it will be
+remembered the cut-off relays are left permanently connected with the
+talking circuit. Fig. 367, which shows a strip of twenty such pairs of
+relays, from five of which the covers have been removed, is an excellent
+detail view of the general practice in this respect; obviously, a very
+large number of such relays may be mounted in a comparatively small
+space. The mounting strip shown in this cut is of heavy rolled iron and
+is provided with openings through which the connection terminals--shown
+more clearly in Fig. 366--project. On the back of this mounting strip
+all the wiring is done and much of this wiring--that connecting adjacent
+terminals on the back of the relay strip--is made by means of thin
+copper wires without insulation, the wires being so short as to support
+themselves without danger of crossing with other wires. When these wires
+are adjacent to ground or battery wires they may be protected by
+sleeving, so as to prevent crosses.
+
+[Illustration: Fig. 368. Monarch Relay]
+
+An interesting feature in relay construction is found in the relay of
+the Monarch Telephone Manufacturing Company shown in Figs. 368 and 369.
+The assembled relay and its mounting strip and cap are shown in Fig.
+368. This relay is so constructed that by the lifting of a single latch
+not only the armature but the coil may be bodily removed, as shown in
+Fig. 369, in which the latch is shown in its raised position. As seen,
+the armature has an L-shaped projection which serves to operate the
+contact springs lying on the iron plate above the coil. The simplicity
+of this device is attractive, and it is of convenience not only from the
+standpoint of easy repairs but also from the standpoint of factory
+assembly, since by manufacturing standard coils with different
+characters of windings and standard groups of springs, it is possible to
+produce without special manufacture almost any combination of relay.
+
+[Illustration: Fig. 369. Monarch Relay]
+
+=Assembly.= The arrangement of the key and jack equipment in complete
+multiple switchboard sections is clearly shown in Fig. 370, which shows
+a single three-position section of one of the small multiple
+switchboards of the Kellogg Switchboard and Supply Company. The
+arrangement of keys and plugs on the key shelf is substantially the same
+as in simple common-battery boards. As in the simple switchboards the
+supervisory lamps are usually mounted on the hinged key shelf
+immediately in the rear of the listening and ringing keys and with such
+spacing as to lie immediately in front of the plugs to which they
+correspond. The reason for mounting the supervisory lamps on the key
+shelf is to make them easy of access in case of the necessity of lamp
+renewals or repairs on the wiring. The space at the bottom of the
+vertical panels, containing the jacks, is left blank, as this space is
+obstructed by the standing plugs in front of it. Above the plugs,
+however, are seen the alternate strips of line lamps and answering
+jacks, the lamps in each case being directly below the corresponding
+answering jacks. Above the line lamps and answering jacks in the two
+positions at the right there are blank strips into which additional line
+lamps and jacks may be placed in case the future needs of the system
+demand it. The space above these is the multiple jack space, and it is
+evident from the small number of multiple jacks in this little
+switchboard that the present equipment of the board is small. It is also
+evident from the amount of blank space left for future installations of
+multiple jacks that a considerable growth is expected. Thus, while there
+are but four banks of 100 multiple jacks, or 400 in all, there is room
+in the multiple for 300 banks of 100 multiple jacks, or 3,000 in all.
+The method of grouping the jacks in banks of 100 and of providing for
+their future growth is clearly indicated in this figure. The next
+section at the right of the one shown would contain a duplicate set of
+multiple jacks and also an additional equipment of answering jacks and
+lamps.
+
+[Illustration: A MULTIPLE MANUAL SWITCHING BOARD FOR TOLL CONNECTIONS IN
+AN AUTOMATIC SYSTEM Multiple Jacks are Provided for Each Line through
+Which Toll Connections are Handled Directly.]
+
+[Illustration: Fig. 370. Small Multiple Board Section]
+
+For ordinary local service no operator would sit at the left-hand
+position of the section shown, that being the end position, since the
+operator there would not be able easily to reach the extreme right-hand
+portion of the third position and would have nothing to reach at her
+left. This end position in this particular board illustrated is provided
+with toll-line equipment, a practice not uncommon in small multiple
+boards. To prevent confusion let us assume that the multiple jack space
+contains its full equipment of 3,000 jacks on each section. The
+operator in the center position of the section shown could easily reach
+any one of the jacks on that section. The operator at the third position
+could reach any jack on the second and third position of her section,
+but could not well reach multiple jacks in the first position. She
+would, however, have a duplicate of the multiple jacks in this first
+position in the section at her right, _i. e._, in the fourth position,
+and it makes no difference on what portion of the switchboard she plugs
+into the multiple so long as she plugs into a jack of the right line.
+
+
+
+
+CHAPTER XXVII
+
+TRUNKING IN MULTI-OFFICE SYSTEMS
+
+
+It has been stated that a single exchange may involve a number of
+offices, in which case it is termed a multi-office exchange. In a
+multi-office exchange, switchboards are necessary at each office in
+which the subscribers' lines of the corresponding office district
+terminate. Means for intercommunication between the subscribers in one
+office and those in any other office are afforded by inter-office trunks
+extended between each office and each of the other offices.
+
+If the character of the community is such that each of the offices has
+so few lines as to make the simple switchboard suffice for its local
+connections, then the trunking between the offices may be carried out in
+exactly the same way as explained between the various simple
+switchboards in a transfer system, the only difference being that the
+trunks are long enough to reach from one office to another instead of
+being short and entirely local to a single office. Such a condition of
+affairs would only be found in cases where several small communities
+were grouped closely enough together to make them operate as a single
+exchange district, and that is rather unusual.
+
+The subject of inter-office trunking so far as manual switchboards are
+concerned is, therefore, confined mainly to trunking between a number of
+offices each equipped with a manual multiple switchboard.
+
+=Necessity for Multi-Office Exchanges.= Before taking up the details of
+the methods and circuits employed in trunking in multi-office systems,
+it may be well to discuss briefly why the multi-office exchange is a
+necessity, and why it would not be just as well to serve all of the
+subscribers in a large city from a single huge switchboard in which all
+of the subscribers' lines would terminate. It cannot be denied, when
+other things are equal, that it is better to have only one operator
+involved in any connection which means less labor and less liability of
+error.
+
+The reasons, however, why this is not feasible in really large
+exchanges are several. The main one is that of the larger investment
+required. Considering the investment first from the standpoint of the
+subscriber's line, it is quite clear that the average length of
+subscriber's line will be very much greater in a given community if all
+of the lines are run to a single office, than will be the case if the
+exchange district is divided into smaller office districts and the lines
+run merely from the subscribers to the nearest office. There is a direct
+and very large gain in this respect, in the multi-office system over the
+single office system in large cities, but this is not a net gain, since
+there is an offsetting investment necessary in the trunk lines between
+the offices, which of course are separate from the subscribers' lines.
+
+Approaching the matter from the standpoint of switchboard construction
+and operation, another strong reason becomes apparent for the employment
+of more than one office in large exchange districts. Both the
+difficulties of operation and the expense of construction and
+maintenance increase very rapidly when switchboards grow beyond a
+certain rather well-defined limit. Obviously, the limitation of the
+multiple switchboard as to size involves the number of multiple jacks
+that it is feasible to place on a section. Multiple switchboards have
+been constructed in this country in which the sections had a capacity of
+18,000 jacks. Schemes have been proposed and put into effect with
+varying success, for doubling and quadrupling the capacity of multiple
+switchboards, one of these being the so-called divided multiple board
+devised by the late Milo G. Kellogg, and once used in Cleveland, Ohio,
+and St. Louis, Missouri. Each of these boards had an ultimate capacity
+of 24,000 lines, and each has been replaced by a "straight" multiple
+board of smaller capacity. In general, the present practice in America
+does not sanction the building of multiple boards of more than about
+10,000 lines capacity, and as an example of this it may be cited that
+the largest standard section manufactured for the Bell companies has an
+ultimate capacity of 9,600 lines.
+
+European engineers have shown a tendency towards the opposite practice,
+and an example of the extreme in this case is the multiple switchboard
+manufactured by the Ericsson Company, and installed in Stockholm, in
+which the jacks have been reduced to such small dimensions as to permit
+an ultimate capacity of 60,000 lines.
+
+The reasons governing the decision of American engineers in
+establishing the practice of employing no multiple switchboards of
+greater capacity than about 10,000 lines, briefly outlined, are as
+follows: The building of switchboards with larger capacity, while
+perfectly possible, makes necessary either a very small jack or some
+added complexity, such as that of the divided multiple switchboard,
+either of which is considered objectionable. Extremely small jacks and
+large multiples introduce difficulties as to the durability of the jacks
+and the plugs, and also they tend to slow down the work of operators and
+to introduce errors. They also introduce the necessity of a smaller
+gauge of wire through the multiple than it has been found desirable to
+employ. Considered from the standpoint of expense, it is evident that as
+a multiple switchboard increases in number of lines, its size increases
+in two dimensions, _i. e._, in length of board and height of section,
+and this element of expense, therefore, is a function of the square of
+the number of lines.
+
+The matter of insurance, both with respect to the risk as to property
+loss and the risk as to breakdown of the service, also points distinctly
+in the direction of a plurality of offices rather than one. Both from
+the standpoint of risk against fire and other hazards, which might
+damage the physical property, and of risk against interruption to
+service due to a breakdown of the switchboard itself, or a failure of
+its sources of current, or an accident to the cable approaches, the
+single office practice is like putting all one's eggs in one basket.
+
+Another factor that has contributed to the adoption of smaller
+switchboard capacities is the fact that in the very large cities even a
+40,000 line multiple switchboard would still not remove the necessity of
+multi-office exchanges with the consequent certainty that a large
+proportion of the calls would have to be trunked anyway.
+
+Undoubtedly, one of the reasons for the difference between American and
+European practice is the better results that American operating
+companies have been able to secure in the handling of calls at the
+incoming end of trunks. This is due, no doubt, in part to the
+differences in social and economic conditions under which exchanges are
+operated in this country and abroad, and also in part to the
+characteristics of the English tongue when compared to some of the other
+tongues in the matter of ease with which numbers may be spoken. In
+America it has been found possible to so perfect the operation of
+trunking under proper operating conditions and with good equipment as to
+relieve multi-office practice of many of the disadvantages which have
+been urged against it.
+
+=Classification.= Broadly speaking there are two general methods that
+may be employed in trunking between exchanges. The first and simplest of
+these methods is to employ so-called _two-way trunks_. These, as their
+name indicates, may be used for completing connections between offices
+in either direction, that is, whether the call originates at one end or
+the other. The other way is by the use of _one-way trunks_, wherein each
+trunk carries traffic in one direction only. Where such is the case, one
+end of the trunk is always used for connecting with the calling
+subscriber's line and is termed the _outgoing_ end, and the other end is
+always used in completing the connection with the called subscriber's
+line, and is referred to as the _incoming_ end. Traffic in the other
+direction is handled by another set of trunks differing from the first
+set only in that their outgoing and incoming ends are reversed.
+
+As has already been pointed out, a system of trunks employing two-way
+trunks is called a _single-track system_, and a system involving two
+sets of one-way trunks is called a _double-track system_. It is to be
+noted that the terms outgoing and incoming, as applied to the ends of
+trunks and also as applied to traffic, always refer to the direction in
+which the trunk handles traffic or the direction in which the traffic is
+flowing with respect to the particular office under consideration at the
+time. Thus an _incoming trunk_ at one office is an _outgoing trunk_ at
+the other.
+
+_Two-Way Trunks._ Two-way trunks are nearly always employed where the
+traffic is very small and they are nearly always operated by having the
+_A_-operator plug directly into the jack at her end of the trunk and
+displaying a signal at the other end by ringing over the trunk as she
+would over an ordinary subscriber's line. The operator at the distant
+exchange answers as she would on an ordinary line, by plugging into the
+jack of that trunk, and receives her orders over the trunk either from
+the originating operator or from the subscriber, and then completes the
+connection with the called subscriber. Such trunks are often referred to
+as "ring-down" trunks, and their equipment consists in a drop and jack
+at each end. In case there is a multiple board at either or both of the
+offices, then the equipment at each end of the trunk would consist of a
+drop and answering jack, together with the full quota of multiple jacks.
+It is readily seen that this mode of operation is slow, as the work that
+each operator has to do is the same as that in connecting two local
+subscribers, plus the time that it takes for the operators to
+communicate with each other over the trunk.
+
+_One-Way Trunks._ Where one-way trunks are employed in the double-track
+system, the trunks, assuming that they connect multiple boards, are
+provided with multiple jacks only at their outgoing ends, so that any
+operator may reach them for an outgoing connection, and at their
+incoming ends they terminate each in a single plug and in suitable
+signals and ringing keys, the purpose of which will be explained later.
+Over such trunks there is no verbal communication between the operators,
+the instructions passing between the operators over separate order-wire
+circuits. This is done in order that the trunk may be available as much
+as possible for actual conversation between the subscribers. It may be
+stated at this point that the duration of the period from the time when
+a trunk is appropriated by the operators for the making of a certain
+connection until the time when the trunk is finally released and made
+available for another connection is called the _holding time_, and this
+holding time includes not only the period while the subscribers are in
+actual conversation over it, but also the periods while the operators
+are making the connection and afterwards while they are taking it down.
+It may be said, therefore, that the purpose of employing separate order
+wires for communication between the operators is to make the holding
+time on the trunks as small as possible and, therefore, for the purpose
+of enabling a given trunk to take part in as many connections in a given
+time as possible.
+
+In outline the operation of a one-way trunk between common-battery,
+manual, multiple switchboards is, with modifications that will be
+pointed out afterwards, as follows: When a subscriber's line signal is
+displayed at one office, the operator in attendance at that position
+answers and finding that the call is for a subscriber in another office,
+she presses an order-wire key and thereby connects her telephone set
+directly with that of a _B_-operator at the proper other office. Unless
+she finds that other operators are talking over the order wire, she
+merely states the number of the called subscriber, and the _B_-operator
+whose telephone set is permanently connected with that order wire merely
+repeats the number of the called subscriber and follows this by
+designating the number of the trunk which the _A_-operator is to employ
+in making the connection. The _A_-operator, thereupon, immediately and
+without testing, inserts the calling plug of the pair used in answering
+the call into the trunk jack designated by the _B_-operator; the
+_B_-operator simultaneously tests the multiple jack of the called
+subscriber and, if she finds it not busy, inserts the plug of the
+designated trunk into the multiple jack of the called subscriber and
+rings his bell by pressing the ringing key associated with the trunk
+cord used. The work on the part of the _A_-operator in connecting with
+the outgoing end of the trunk and on the part of the _B_-operator in
+connecting the incoming end of the trunk with the line goes on
+simultaneously, and it makes no difference which of these operators
+completes the connection first.
+
+It is the common practice of the Bell operating companies in this
+country to employ what is called automatic or machine ringing in
+connection with the _B_-operator's work. When the _B_-operator presses
+the ringing key associated with the incoming trunk cord, she pays no
+further attention to it, and she has no supervisory lamp to inform her
+as to whether or not the subscriber has answered. The ringing key is
+held down, after its depression by the operator, either by an
+electromagnet or by a magnet-controlled latch, and the ringing of the
+subscriber's bell continues at periodic intervals as controlled by the
+ringing commutator associated with the ringing machine. When the
+subscriber answers, however, the closure of his line circuit results in
+such an operation of the magnet associated with the ringing key as to
+release the ringing key and thus to automatically discontinue the
+ringing current.
+
+When a connection is established between two subscribers through such a
+trunk the supervision of the connection falls entirely upon the
+_A_-operator who established it. This means that the calling supervisory
+lamp at the _A_-operator's position is controlled over the trunk from
+the station of the called subscriber, the answering supervisory lamp
+being, of course, under the control of the calling subscriber as in the
+case of a local connection. It is, therefore, the _A_-operator who
+always initiates the taking down of a trunk connection, and when, in
+response to the lighting of the two lamps, she withdraws her calling
+plug from the trunk jack, the supervisory lamp associated with the
+incoming end of the trunk at the other office is lighted, and the
+_B_-operator obeys it by pulling down the plug.
+
+If, upon testing the multiple jack of the called subscriber's line, the
+_B_-operator finds the line to be busy, she at once inserts the trunk
+plug into a so-called "busy-back" jack, which is merely a jack whose
+terminals are permanently connected to a circuit that is intermittently
+opened and closed, and which also has impressed upon it an alternating
+current of such a nature as to produce the familiar "buzz-buzz" in a
+telephone receiver. The opening and closing of this circuit causes the
+calling supervisory lamp of the _A_-operator to flash at periodic
+intervals just as if the called subscriber had raised and lowered his
+receiver, but more regularly. This is the indication to the _A_-operator
+that the line called for is busy. The buzzing sound is repeated back
+through the cord circuit of the _A_-operator to the calling subscriber
+and is a notification to him that the line is busy.
+
+Sometimes, as is practiced in New York City, for instance, the buzzing
+feature is omitted, and the only indication that the calling subscriber
+receives that the called-for line is busy is being told so by the
+_A_-operator. This may be considered a special feature and it is
+employed in New York because there the custom exists of telling a
+calling subscriber, when the line he has called for has been found busy,
+that the party will be secured for him and that he, the calling
+subscriber, will be called, if he desires.
+
+A modification of this busy-back feature that has been employed in
+Boston, and perhaps in other places, is to associate with the busy-back
+jack at the _B_-operator's position a phonograph which, like a parrot,
+keeps repeating "Line busy--please call again." Where this is done the
+calling subscriber, _if he understands what the phonograph says_, is
+supposed to hang up his receiver, at which time the _A_-operator takes
+down the connection and the _B_-operator follows in response to the
+notification of her supervisory lamp. The phonograph busy-back scheme,
+while ingenious, has not been a success and has generally been
+abandoned.
+
+As a rule the independent operating companies in this country have not
+employed automatic ringing, and in this case the _B_-operators have
+been required to operate their ringing keys and to watch for the
+response of the called subscriber. In order to arrange for this, another
+supervisory lamp, termed the _ringing lamp_, is associated with each
+incoming trunk plug, the going out of this lamp being a notification to
+the _B_-operator to discontinue ringing.
+
+=Western Electric Trunk Circuits.= The principles involved in
+inter-office trunking with automatic ringing, are well illustrated in
+the trunk circuit employed by the Western Electric Company in connection
+with its No. 1 relay boards. The dotted dividing line through the center
+of Fig. 371 represents the separating space between two offices. The
+calling subscriber's line in the first office is shown at the extreme
+left and the called subscriber's line in the second office is shown at
+the extreme right. Both of these lines are standard multiple switchboard
+lines of the form already discussed. The equipment illustrated in the
+first office is that of an _A_-board, the cord circuit shown being that
+of the regular _A_-operator. The outgoing trunk jacks connecting with
+the trunk leading to the other office are, it will be understood,
+multipled through the _A_-sections of the board and contain no relay
+equipment, but the test rings are connected to ground through a
+resistance coil _1_, which takes the place of the cut-off relay winding
+of a regular line so far as test conditions and supervisory relay
+operation are concerned. The equipment illustrated in the second office
+is that of a _B_-board, it being understood that the called subscriber's
+line is multipled through both the _A_- and _B_-boards at that office.
+The part of the equipment that is at this point unfamiliar to the reader
+is, therefore, the cord circuit at the _B_-operator's board. This
+includes, broadly speaking, the means: (1) for furnishing battery
+current to the called subscriber; (2) for accomplishing the ringing of
+the called subscriber and for automatically stopping the ringing when he
+shall respond; (3) for performing the ordinary switching functions in
+connection with the relays of the called subscriber's line in just the
+same way that an _A_-operator's cord carries out these functions; and
+(4) for causing the operation of the calling supervisory relay of the
+_A_-operator's cord circuit in just the same manner, under control of
+the connected called subscriber, as if that subscriber's line had been
+connected directly to the _A_-operator's cord circuit.
+
+[Illustration: Fig. 371. Inter-Office Connection--Western Electric
+System]
+
+The operation of these devices in the _B_-operator's cord circuit may
+be best understood by following the establishment of the connection.
+Assuming that the calling subscriber in the first office desires a
+connection with the subscriber's line shown in the second office, and
+that the _A_-operator at the first office has answered the call, she
+will then communicate by order wire with the _B_-operator at the second
+office, stating the number of the called subscriber and receiving from
+that operator in return the number of the trunk to be employed. The two
+operators will then proceed simultaneously to establish the connection,
+the _A_-operator inserting the calling plug into the outgoing trunk
+jack, and the _B_-operator inserting the trunk plug into the multiple
+jack of the called subscriber's line after testing. We will assume at
+first that the called subscriber's line is found idle and that both of
+the operators complete their respective portions of the work at the same
+time and we will consider first the condition of the calling supervisory
+relay at the _A_-operator's position.
+
+The circuit of the calling supervisory lamp will have been closed
+through the resistance coil _1_ connected with the outgoing trunk jacks
+and the lamp will be lighted because, as will be shown, it is not yet
+shunted out by the operation of its associated supervisory relay.
+Tracing the circuit of the calling supervisory relay of the
+_A_-operator's circuit, it will be found to pass from the live side of
+the battery to the ring side of the trunk circuit through one winding of
+the repeating coil of the _B_-operator's cord; beyond this the circuit
+is open, since no path exists through the condenser _2_ bridged across
+the trunk circuit or through the normally open contacts of the relay _3_
+connected in the talking circuit of the trunk. The association of this
+relay _3_ with the repeating coil and the battery of the trunk is seen
+to be just the same as that of a supervisory relay in the _A_-operator's
+cord, and it is clear, therefore, that this relay _3_ will not be
+energized until the called subscriber has responded. When it is
+energized it will complete the path to ground through the _A_-operator's
+calling supervisory relay and operate to shunt out the _A_-operator's
+calling supervisory lamp in just the same manner as if the
+_A_-operator's calling plug had been connected directly with the line of
+the calling subscriber. In other words, the called subscriber in the
+second office controls the relay _3_, which, in turn, controls the
+calling supervisory relay of the _A_-operator, which, in turn, shunts
+out its lamp.
+
+The connection being completed between the two subscribers, the
+_B_-operator depresses one or the other of the ringing keys _5_ or _6_,
+according to which party on the line is called, assuming that it is a
+two-party line. It will be noticed that the springs of these ringing
+keys are not serially arranged in the talking circuit, but the cutting
+off of the trunk circuit back of the ringing keys is accomplished by the
+set of springs shown just at the left of the ringing keys, which set of
+springs _7_ is operated whenever either one of the ringing keys is
+depressed. An auxiliary pair of contacts, shown just below the group of
+springs _7_, is also operated mechanically whenever either one of the
+ringing keys is depressed, and this serves to close one of two normally
+open points in the circuit of the ringing-key holding magnet _8_. This
+holding magnet _8_ is so arranged with respect to the contacts of the
+ringing key that whenever any one of them is depressed by the operator,
+it will be held depressed as long as the magnet is energized just the
+same as if the operator kept her finger on the key. The other normally
+open point in the circuit of the holding magnet _8_ is at the lower pair
+of contacts of the test and holding relay _9_. This relay is operated
+whenever the trunk plug is inserted in the jack of a called line,
+regardless of the position of the subscriber's equipment on that line.
+The circuit may be traced from the live side of the battery through the
+trunk disconnect lamp _4_, coil _9_, sleeve strand of cord, and to
+ground through the cut-off relay of the line. The insertion of the trunk
+plug into the jack thus leaves the completion of the holding-magnet
+circuit dependent only upon the auxiliary contact on the ringing key,
+and, therefore, as soon as the operator presses either one of these
+keys, the clutch magnet is energized and the key is held down, so that
+ringing current continues to flow at regular intervals to the called
+subscriber's station.
+
+The ringing current issues from the generator _10_, but the supply
+circuit from it is periodically interrupted by the commutator _11_
+geared to the ringing-machine shaft. This periodically interrupted
+ringing current passes to the ringing-key contacts through the coil of
+the ringing cut-off relay _12_, and thence to the subscriber's line. The
+ringing current is, however, insufficient to cause the operation of this
+relay _12_ as long as the high resistance and impedance of the
+subscriber's bell and condenser are in the circuit. It is, however,
+sufficiently sensitive to be operated by this ringing current when the
+subscriber responds and thus substitutes the comparatively low
+resistance and impedance path of his talking apparatus for the previous
+path through his bell. The pulling up of the ringing cut-off relay _12_
+breaks a third normally closed contact in the circuit of the holding
+coil _8_, de-energizing that coil and releasing the ringing key, thus
+cutting off ringing current. There is a third brush on the commutator
+_11_ connected with the live side of the central battery, and this is
+merely for the purpose of assuring the energizing of the ringing cut-off
+relay _12_, should the subscriber respond during the interval while the
+commutator _11_ held the ringing current cut off. The relay _12_ may
+thus be energized either from the battery, if the subscriber responds
+during a period of silence of his ringer, or from the generator _10_, if
+the subscriber responds during a period while his bell is sounding; in
+either case the ringing current will be promptly cut off by the release
+of the ringing key.
+
+The trunk operator's "disconnect lamp" is shown at _4_, and it is to be
+remembered that this lamp is lighted only when the _A_-operator takes
+down the connection at her end, and also that this lamp is entirely out
+of the control of the subscribers, the conditions which determine its
+illumination being dependent on the positions of the operators' plugs at
+the two ends of the trunk. With both plugs up, the lamp _4_ will receive
+current, but will be shunted to prevent its illumination. The path over
+which it receives this current may be traced from battery through the
+lamp _4_, thence through the coil of the relay _9_ and the cut-off relay
+of the called subscriber's line. This current would be sufficient to
+illuminate the lamp, but the lamp is shunted by a circuit which may be
+traced from the live side of battery through the contact of the relay
+_13_, closed at the time, and through the coil of the trunk cut-off
+relay coil _14_. The resistance of this coil is so proportioned to the
+other parts of the circuit as to prevent the illumination of the lamp
+just exactly as in the case of the shunting resistances of the lamps in
+the _A_-operator's cord. It will be seen, therefore, that the supply of
+current to the trunk disconnect lamp is dependent on the trunk plug
+being inserted into the jack of the subscriber's line and that the
+shunting out of this lamp is dependent on the energization of the relay
+_13_. This relay _13_ is energized as long as the _A_-operator's plug is
+inserted into the outgoing trunk jack, the path of the energizing
+circuit being traced from the live side of the battery at the second
+office through the right-hand winding of this relay, thence over the tip
+side of the trunk to ground at the first office. From this it follows
+that as long as both plugs are up, the disconnect lamp will receive
+current but will be shunted out, and as soon as the _A_-operator pulls
+down the connection, the relay _13_ will be de-energized and will thus
+remove the shunt from about the lamp, allowing its illumination. The
+left-hand winding of the relay _13_ performs no operating function, but
+is merely to maintain the balance of the talking circuit, it being
+bridged during the connection from the ring side of the trunk to ground
+in order to balance the bridge connection of the right-hand coil from
+the live side of battery to the tip side of the trunk circuit.
+
+The relay _14_, already referred to as forming a shunt for the trunk
+disconnect lamp, has for its function the keeping of the talking circuit
+through the trunk open until such time as the relay _13_ operates, this
+being purely an insurance against unnecessary ringing of a subscriber in
+case the _A_-operator should by mistake plug into the wrong trunk. It is
+not, therefore, until the _A_-operator has plugged into the trunk and
+the relay _13_ has been operated to cause the energization of the relay
+_14_ that the ringing of the called subscriber can occur, regardless of
+what the _B_-operator may have done.
+
+The relay _9_ has an additional function to that of helping to control
+the circuit of the ringing-key holding magnet. This is the holding of
+the test circuit complete until the operator has tested and made a
+connection and then automatically opening it. The test circuit of the
+_B_-operator's trunk may be traced, at the time of testing, from the
+thimble of the multiple jack under test, through the tip of the cord,
+thence through the uppermost pair of contacts of the relay _9_ to ground
+through a winding of the _B_-operator's induction coil. After the test
+has been made and the plug inserted, the relay _9_, which is operated by
+the insertion of the plug, acts to open this test circuit and at the
+same time complete the tip side of the cord circuit.
+
+In the upper portion of Fig. 371 the order-wire connections, by which
+the _A_-operator and the _B_-operator communicate, are indicated. It
+must be remembered in connection with these that the _A_-operator only
+has control of this connection, the _B_-operator being compelled
+necessarily to hear whatever the _A_-operators have to say when the
+_A_-operators come in on the circuit.
+
+[Illustration: Fig. 372. Incoming Trunk Circuit]
+
+The incoming trunk circuit employed by the Western Electric Company for
+four-party line ringing is shown in Fig. 372, it being necessarily
+somewhat modified from that shown in Fig. 371, which is adapted for
+two-party line ringing only. In addition to the provision of the
+four-party line ringing keys, by which positive or negative pulsating
+current is received over either limb of the line, and to the provision
+of the regular alternating current ringing key for ringing on single
+party lines, it is necessary in the ringing cut-off relay to provide for
+keeping the alternating and the pulsating ringing currents entirely
+separate. For this reason, the ringing cut-off relay _12_ is provided
+with two windings, that at the right being in the path of the
+alternating ringing currents that are supplied to the alternating
+current key, and that at the left being in the ground return path for
+all of the pulsating ringing currents supplied to the pulsating keys.
+With this explanation it is believed that this circuit will be
+understood from what has been said in connection with Fig. 371. The
+operation of the holding coil _8_ is the same in each case, the holding
+magnet in Fig. 372 serving to hold depressed any one of the five ringing
+keys that may have been used in calling the subscriber.
+
+[Illustration: AUTOMATIC EQUIPMENT, MAIN OFFICE, BERKELEY, CALIFORNIA A
+Feature of Interest Here is That the Cement Floor is Treated with a
+Filler and Painted, with No Other Covering.]
+
+[Illustration: Fig. 373. Western Electric Trunk Ringing Key]
+
+The standard four-party line, trunk ringing key of the Western Electric
+Company is shown in Fig. 373. In this the various keys operate not by
+pressure but rather by being pulled by the finger of the operator in
+such a way as to subject the key shaft to a twisting movement. The
+holding magnet lies on the side opposite to that shown in the figure and
+extends along the full length of the set of keys, each key shaft being
+provided with an armature which is held by this magnet until the magnet
+is de-energized by the action of the ringing cut-off relay.
+
+[Illustration: Fig. 374. Trunk Relay]
+
+[Illustration: Fig. 375. Trunk Relay]
+
+The standard trunk relays employed by the Western Electric Company in
+connection with the circuits just described are shown in Figs. 374 and
+375. In each case the dust-cap or shield is also shown. The relay of
+Fig. 374 is similar to the regular cut-off relay and is the one used for
+relays _9_ and _14_ of Figs. 371 and 372. The relay of Fig. 375 is
+somewhat similar to the subscriber's line relay in that it has a tilting
+armature, and is the one used at _13_ in Figs. 371 and 372. The trunk
+relay _3_ in Figs. 371 and 372 is the same as the _A_-operator's
+supervisory relays already discussed.
+
+It has been stated that under certain circumstances _B_-operator's trunk
+circuits devoid of ringing keys, and consequently of all keys, may be
+employed. This, so far as the practice of the Bell companies is
+concerned, is true only in offices where there are no party lines, or
+where, as in many of the Chicago offices, the party lines are worked on
+the "jack per station" basis. In "jack per station" working, the
+selection of the station on a party line is determined by the jack on
+which the plug is put, rather than by a ringing key, and hence the
+keyless trunk may be employed.
+
+[Illustration: Fig. 376. Keyless Trunk]
+
+A keyless trunk as used in New York is shown in Fig. 376. This has no
+manually operated keys whatever, and the relay _17_, when it is
+operated, establishes connection between the ringing generator and the
+conductors of the trunk plug. The relays _3_, _13_, and _12_ operate in
+a manner identical with those bearing corresponding numbers in Fig.
+371. As soon as the trunk operator plugs into the multiple jack of the
+called subscriber, the relay _16_ will operate for the same reason that
+the relay _9_ operated in connection with Fig. 371. The trunk disconnect
+lamp will receive current, but if the operator has already established
+connection with the other end of the trunk, this lamp will not be
+lighted because shunted by the relay _17_, due to the pulling up of the
+armature of the relay _13_. The relay _15_ plays no part in the
+operation so far described, because of the fact that its winding is
+short-circuited by its own contacts and those of relay _12_, when the
+latter is not energized. As a result of the operation of the relay _17_,
+ringing current is sent to line, the supply circuit including the coil
+of the relay _12_. As soon as the subscriber responds to this ringing
+current, the armature of the relay _12_ is pulled up, thus breaking the
+shunt about the relay _15_, which, therefore, starts to operate in
+series with the relay _17_, but as its armatures assume their attracted
+position, the relay _17_ is cut out of the circuit, the coil of the
+relay _15_ being substituted for that of the relay _17_ in the shunt
+path around the lamp _4_. The relay _17_ falls back and cuts off the
+ringing current. The relay _15_ now occupies the place with respect to
+the shunt around the lamp _4_ that the relay _17_ formerly did, the
+continuity of this shunt being determined by the energization of the
+relay _13_. When the _A_-operator at the distant exchange withdraws the
+calling plug from the trunk jack, this relay _13_ becomes de-energized,
+breaking the shunt about the lamp _4_ and permitting the display of that
+lamp as a signal to the operator to take down the connection. It may be
+asked why the falling back of relay _15_ will not again energize relay
+_17_ and thus cause a false ring on the called subscriber. This will not
+occur because both the relays _15_ and _17_ depend for their
+energization on the closure of the contacts of the relay _13_, and when
+this falls back the relay _17_ cannot again be energized even though the
+relay _15_ assumes its normal position.
+
+=Kellogg Trunk Circuits.= The provision for proper working of trunk
+circuits in connection with the two-wire multiple switchboards is not an
+altogether easy matter, owing particularly to the smaller number of
+wires available in the plug circuits. It has been worked out in a highly
+ingenious way, however, by the Kellogg Company, and a diagram of their
+incoming trunk circuit, together with the associated circuits involved
+in an inter-office connection, is shown in Fig. 377.
+
+[Illustration: Fig. 377. Inter-Office Connection--Kellogg System]
+
+This figure illustrates a connection from a regular two-wire multiple
+subscriber's line in one office, through an _A_-operator's cord circuit
+there, to the outgoing trunk jacks at that office, thence through the
+incoming trunk circuit at the other office to the regular two-wire
+multiple subscriber's line at that second office. The portion of this
+diagram to be particularly considered is that of the _B_-operator's cord
+circuit. The trunk circuit terminates in the multipled outgoing trunk
+jacks at the first office, the trunk extending between offices
+consisting, of course, of but two wires. We will first consider the
+control of the calling supervisory lamp in the _A_-operator's cord
+circuit, it being remembered that this control must be from the called
+subscriber's station. It will be noticed that the left-hand armature of
+the relay _1_ serves normally to bridge the winding of relay _2_ across
+the cord circuit around the condenser _3_. When, however, the relay _1_
+pulls up, the coil of relay _4_ is substituted in this bridge connection
+across the trunk. The relay _2_ has a very high resistance
+winding--about 15,000 ohms--and this resistance is so great that the tip
+supervisory relay of the _A_-operator's cord will not pull up through
+it. As a result, when this relay is bridged across the trunk circuit,
+the tip relay on the calling side of the _A_-operator's cord circuit is
+de-energized, just as if the trunk circuit were open, and this results
+in the lighting of the _A_-operator's calling supervisory lamp. The
+winding of the relay _4_, however, is of low resistance--about 50
+ohms--and when this is substituted for the high-resistance winding of
+the relay _2_, the tip relay on the calling side of the _A_-operator's
+cord is energized, resulting in the extinguishing of the calling
+supervisory lamp. The illumination of the _A_-operator's calling
+supervisory lamp depends, therefore, on whether the high-resistance
+relay _2_, or the low-resistance relay _4_, is bridged across the trunk,
+and this depends on whether the relay _1_ is energized or not. The relay
+_1_, being bridged from the tip side of the trunk circuit to ground and
+serving as the means of supply of battery current to the called
+subscriber, is operated whenever the called subscriber's receiver is
+removed from its hook. Therefore, the called subscriber's hook controls
+the operation of this relay _1_, which, in turn, controls the conditions
+which cause the illumination or darkness of the calling supervisory lamp
+at the distant office.
+
+Assuming that the _A_-operator has received and answered a call, and has
+communicated with the _B_-operator, telling her the number of the
+called subscriber, and has received, in turn, the number of the trunk to
+be used, and that both operators have put up the connection, then it
+will be clear from what has been said that the calling supervisory lamp
+of the _A_-operator will be lighted until the called subscriber removes
+his receiver from its hook, because the tip relay in the _A_-operator's
+cord circuit will not pull up through the 15,000-ohm resistance winding
+of the relay _2_. As soon as the subscriber responds, however, the relay
+_1_ will be operated by the current which supplies his transmitter. This
+will substitute the low-resistance winding of the relay _4_ for the
+high-resistance winding of the relay _2_, and this will permit the
+energizing of the tip supervisory relay of the _A_-operator and put out
+the calling supervisory lamp at her position. As in the Western Electric
+circuit, therefore, the control of the _A_-operator's calling
+supervisory lamp is from the called subscriber's station and is relayed
+back over the trunk to the originating office.
+
+In this circuit, manual instead of automatic ringing is employed,
+therefore, unlike the Western Electric circuit, means must be provided
+for notifying the B-operator when the calling subscriber has answered.
+This is done by placing at the _B_-operator's position a ringing lamp
+associated with each trunk cord, which is illuminated when the
+_B_-operator places the plug of the incoming trunk into the multiple
+jack of the subscriber's line, and remains illuminated until the
+subscriber has answered. This is accomplished in the following manner:
+when the operator plugs into the jack of the line called, relay _5_ is
+energized but is immediately de-energized by the disconnecting of the
+circuit of this relay from the sleeve conductor of the cord when the
+ringing key is depressed, the selection of the ringing key being
+determined by the particular party on the line desired. These ringing
+keys have associated with them a set of springs _9_, which springs are
+operated when any one of the ringing keys is depressed. Thus, with a
+ringing key depressed and the relay _5_ de-energized, the ringing lamp
+will be illuminated by means of a circuit as follows: from the live side
+of the battery, through the ringing lamp _12_, through the back contact
+and armature of the relay _6_, through the armature and contact of relay
+_4_, then through the armature and front contact of relay _2_--which at
+this time is the relay bridged across the trunk and, therefore,
+energized--and thence through the back contact and armature of relay
+_5_ to ground. When the subscriber removes his receiver from the hook,
+the relay _1_ will become energized as previously described, and will,
+therefore, operate relay _6_ to break the circuit of the ringing lamp.
+The circuit thus established by the operation of relay _1_ is as
+follows: from the live side of battery, through the winding of relay
+_6_, through the armature and contact of relay _1_, through the armature
+and contact of relay _4_, through the armature and front contact of
+relay _2_, thence through the armature and back contact of relay _5_ to
+ground. As soon as the _B_-operator notes that the ringing lamp has gone
+out, she knows that no further ringing is required on that line, thus
+allowing the operation of relay _5_ and accomplishing the locking out of
+the ringing lamp during the remainder of that connection. The relay _6_,
+after having once pulled up, remains locked up through the rear contact
+of the left-hand armature of relay _5_ and ground, until the plug is
+removed from the jack.
+
+At the end of the conversation, when the _A_-operator has disconnected
+her cord circuit on the illumination of the supervisory signals, both
+relays _2_ and _4_ will be in an unoperated condition and will provide a
+circuit for illuminating the disconnect lamp associated with the
+_B_-operator's cord. This circuit may be traced as follows: from battery
+through the disconnect lamp, through the armatures and contacts of
+relays _2_ and _4_, thence through the front contact and armature of
+relay _5_ to ground, thus illuminating the disconnect lamp. The ringing
+lamp will not be re-illuminated at this time, due to the fact that it
+has been previously locked out by relay _6_. The operator then removes
+the plug from the jack of the line called, and the apparatus in the
+trunk circuit is restored to normal condition.
+
+In the circuit shown only keys are provided for ringing two parties.
+This circuit, however, is not confined to the use of two-party lines,
+but may be extended to four parties by simply duplicating the ringing
+keys and by connecting them with the proper current for selectively
+ringing the other stations.
+
+The method of determining as to whether the called line is free or busy
+is similar to that previously described for the _A_-operator's cord
+circuit when making a local connection, and differs only in the fact
+that in the case of the trunk cord the test circuit is controlled
+through the contacts of a relay, whereas in the case of the
+_A_-operator's cord, the test circuit was controlled through the
+contacts of the listening key. The function of the resistance _10_ and
+the battery connected thereto is the same as has been previously
+described.
+
+The general make-up of trunking switchboard sections is not greatly
+different from that of the ordinary switchboard sections where no
+trunking is involved. In small exchanges where ring-down trunks are
+employed, the trunk line equipment is merely added to the regular jack
+and drop equipment of the switchboard. In common-battery multiple
+switchboards the _A_-boards differ in no respect from the standard
+single office multiple boards, except that immediately above the
+answering jacks and below the multiple there are arranged in suitable
+numbers the jacks of the outgoing trunks.
+
+Where the offices are comparatively small, the incoming trunk portions
+of the _B_-boards are usually merely a continuance of the _A_-boards,
+the subscriber's multiple being continuous with and differing in no
+respect from that on the _A_-sections. Instead of the usual pairs of
+_A_-operators' plugs, cords, and supervisory equipment, there are on the
+key and plug shelves of these _B_-sections the incoming trunk plugs and
+their associated equipment.
+
+In large offices it is customary to make the _B_-board entirely separate
+from the _A_-board, although the general characteristics of construction
+remain the same. The reason for separate _A_- and _B_-switchboards in
+large exchanges is to provide for independent growth of each without the
+growth of either interfering with the other.
+
+A portion of an incoming trunk, or _B_-board, is shown in Fig. 378. The
+multiple is as usual, and, of course, there are no outgoing trunk jacks
+nor regular cord pairs. Instead the key and plug shelves are provided
+with the incoming-trunk plug equipments, thirty of these being about the
+usual quota assigned to each operator's position.
+
+In multi-office exchanges, employing many central offices, such, for
+instance, as those in New York or Chicago, it is frequently found that
+nearly all of the calls that originate in one office are for subscribers
+whose lines terminate in some other office. In other words, the number
+of calls that have to be trunked to other offices is greatly in excess
+of the number of calls that may be handled through the multiple of the
+_A_-board in which they originate. It is not infrequent to have the
+percentage of trunked calls run as high as 75 per cent of the total
+number of calls originating in any one office, and in some of the
+offices in the larger cities this percentage runs higher than 90 per
+cent.
+
+[Illustration: Fig. 378. Section of Trunk Switchboard]
+
+[Illustration: Fig. 379. Section of Partial Multiple Switchboard]
+
+This fact has brought up for consideration the problem as to whether,
+when the nature of the traffic is such that only a very small portion of
+the calls can be handled in the office where they originate, it is worth
+while to employ the multiple terminals for the subscribers' lines on the
+_A_-boards. In other words, if so great a proportion as 90 per cent of
+the calls have to be trunked any way, is it worth while to provide the
+great expense of a full multiple on all the sections of the _A_-board in
+order to make it possible to handle the remaining 10 per cent of the
+calls directly by the _A_-operators?
+
+As a result of this consideration it has been generally conceded that
+where such a very great percentage of trunking was necessary, the full
+multiple of the subscribers' lines on each section was not warranted,
+and what is known as the partial multiple board has come into existence
+in large manual exchanges. In these the regular subscribers' multiple is
+entirely omitted from the _A_-board, all subscribers' calls being
+handled through outgoing trunk jacks connected by trunks to _B_-boards
+in the same as well as other offices. In these partial multiple
+_A_-boards, the answering jacks are multipled a few times, usually
+twice, so that calls on each line may be answered from more than one
+position. This multipling of answering jacks does not in any way take
+the place of the regular multipling in full multiple boards, since in no
+case are the calls completed through the multiple jacks. It is done
+merely for the purpose of contributing to team work between the
+operators.
+
+A portion of such a partial multiple _A_-board is shown in Fig. 379.
+This view shows slightly more than one section, and the regular
+answering jacks and lamps may be seen at the bottom of the jack space
+just above the plugs. Above these are placed the outgoing trunk jacks,
+those that are in use being indicated with white designation strips.
+Above the outgoing trunk jacks are placed the multiples of the answering
+jacks, these not being provided with lamps.
+
+The partial multiple _A_-section of Fig. 379 is a portion of the
+switchboard equipment of the same office to which the trunking section
+shown in Fig. 378 belongs. That this is a large multiple board may be
+gathered from the number of multiple jacks in the trunking section,
+8,400 being installed with room for 10,500. That the board is a portion
+of an equipment belonging to an exchange of enormous proportions may be
+gathered from the number of outgoing trunk jacks shown in the _A_-board,
+and in the great number of order-wire keys shown between each of the
+sets of regular cord-circuit keys. The switchboards illustrated in these
+two figures are those of one of the large offices of the New York
+Telephone Company on Manhattan Island, and the photographs were taken
+especially for this work by the Western Electric Company.
+
+     =Cable Color Code.= A great part of the wiring of switchboards
+     requires to be done with insulated wires grouped into cables.
+     In the wiring of manual switchboards as described in the seven
+     preceding chapters, and of automatic and automanual systems and
+     of private branch-exchange and intercommunicating systems
+     described in succeeding chapters, cables formed as follows are
+     widely used:
+
+     Tinned soft copper wires, usually of No. 22 or No. 24 B. & S.
+     gauge, are insulated, first with two coverings of silk, then
+     with one covering of cotton. The outer (cotton) insulation of
+     each wire is made of white or of dyed threads. If dyed, the
+     color either is solid red, black, blue, orange, green, brown,
+     or slate, or it is striped, by combining one of those colors
+     with white or a remaining color. The object of coloring the
+     wires is to enable them to be identified by sight instead of by
+     electrical testing.
+
+     Wires so insulated are twisted into pairs, choosing the colors
+     of the "line" and "mate" according to a predetermined plan. An
+     assortment of these pairs then is laid up spirally to form the
+     cable core, over which are placed certain wrappings and an
+     outer braid. A widely used form of switchboard cable has paper
+     and lead foil wrappings over the core, and the outer cotton
+     braid finally is treated with a fire-resisting paint.
+
+     STANDARD COLOR CODE FOR CABLES
+
+     +---------------+-------------------------------------------------+
+     |               |                     MATE                        |
+     |   LINE WIRE   +-------+-------+-------+-----------+-------------+
+     |               | White |  Red  | Black | Red-White | Black-White |
+     +---------------+-------+-------+-------+-----------+-------------+
+     | Blue          |   1   |  21   |  41   |     61    |      81     |
+     | Orange        |   2   |  22   |  42   |     62    |      82     |
+     | Green         |   3   |  23   |  43   |     63    |      83     |
+     | Brown         |   4   |  24   |  44   |     64    |      84     |
+     | Slate         |   5   |  25   |  45   |     65    |      85     |
+     | Blue-White    |   6   |  26   |  46   |     66    |      86     |
+     | Blue-Orange   |   7   |  27   |  47   |     67    |      87     |
+     | Blue-Green    |   8   |  28   |  48   |     68    |      88     |
+     | Blue-Brown    |   9   |  29   |  49   |     69    |      89     |
+     | Blue-Slate    |  10   |  30   |  50   |     70    |      90     |
+     | Orange-White  |  11   |  31   |  51   |     71    |      91     |
+     | Orange-Green  |  12   |  32   |  52   |     72    |      92     |
+     | Orange-Brown  |  13   |  33   |  53   |     73    |      93     |
+     | Orange-Slate  |  14   |  34   |  54   |     74    |      94     |
+     | Green-White   |  15   |  35   |  55   |     75    |      95     |
+     | Green-Brown   |  16   |  36   |  56   |     76    |      96     |
+     | Green-Slate   |  17   |  37   |  57   |     77    |      97     |
+     | Brown-White   |  18   |  38   |  58   |     78    |      98     |
+     | Brown-Slate   |  19   |  39   |  59   |     79    |      99     |
+     | Slate-White   |  20   |  40   |  60   |     80    |     100     |
+     +---------------+-------+-------+-------+-----------+-------------+
+
+     The numerals represent the pair numbers in the cable.
+
+     The wires of spare pairs usually are designated by solid red
+     with white mate for first spare pair, and solid black with
+     white mate for second spare pair. Individual spare wires
+     usually are colored red-white for first individual spare, and
+     black-white for second individual spare.
+
+
+
+
+CHAPTER XXVIII
+
+FUNDAMENTAL CONSIDERATIONS OF AUTOMATIC SYSTEMS
+
+
+=Definition.= The term automatic, as applied to telephone systems, has
+come to refer to those systems in which machines at the central office,
+under the guidance of the subscribers, do the work that is done by
+operators in manual systems. In all automatic telephone systems, the
+work of connecting and disconnecting the lines, of ringing the called
+subscriber, even though he must be selected from among those on a party
+line, of refusing to connect with a line that is already in use, and
+informing the calling subscriber that such line is busy, of making
+connections to trunk lines and through them to lines in other offices
+and doing the same sort of things there, of counting and recording the
+successful calls made by a subscriber, rejecting the unsuccessful, and
+nearly all the thousand and one other acts necessary in telephone
+service, are performed without the presence of any guiding intelligence
+at the central office.
+
+The fundamental object of the automatic system is to do away with the
+central-office operator. In order that each subscriber may control the
+making of his own connections there is added to his station equipment a
+call transmitting device by the manipulation of which he causes the
+central-office mechanisms to establish the connections he desires.
+
+We think that the automatic system is one of the most astonishing
+developments of human ingenuity. The workers in this development are
+worthy of particular notice. From occupying a position in popular regard
+in common with long-haired men and short-haired women they have recently
+appeared as sane, reasonable men with the courage of their convictions
+and, better yet, with the ability to make their convictions come true.
+The scoffers have remained to pray.
+
+=Arguments Against Automatic Idea.= Naturally there has been a bitter
+fight against the automatic. Those who have opposed it have contended:
+
+First: that it is too complicated and, therefore, could be neither
+reliable or economical.
+
+Second: that it is too expensive, and that the necessary first cost
+could not be justified.
+
+Third: that it is too inflexible and could not adapt itself to special
+kinds of service.
+
+Fourth: that it is all wrong from the subscribers' point of view as the
+public will not tolerate "doing its own operating."
+
+_Complexity._ This first objection as to complexity, and consequent
+alleged unreliability and lack of economy should be carefully analyzed.
+It too often happens that a new invention is cast into outer darkness by
+those whose opinions carry weight by such words as "it cannot work; it
+is too complicated." Fortunately for the world, the patience and
+fortitude which men must possess before they can produce meritorious,
+though intricate inventions, are usually sufficient to prevent their
+being crushed by any such offhand condemnation, and the test of time and
+service is allowed to become the real criterion.
+
+It would be difficult to find an art that has gone forward as rapidly as
+telephony. Within its short life of a little over thirty years it has
+grown from the phase of trifling with a mere toy to an affair of
+momentous importance to civilization. There has been a tendency,
+particularly marked during recent years, toward greater complexity; and
+probably every complicated new system or piece of apparatus has been
+roundly condemned, by those versed in the art as it was, as being unable
+to survive on account of its complication.
+
+To illustrate: A prominent telephone man, in arguing against the
+nickel-in-the-slot method of charging for telephone service once said,
+partly in jest, "The Lord never intended telephone service to be given
+in that way." This, while a little off the point, is akin to the
+sweeping aside of new telephone systems on the sole ground that they are
+complicated. These are not real reasons, but rather convenient ways of
+disposing of vexing problems with a minimum amount of labor. Important
+questions lying at the very root of the development of a great industry
+may not be put aside permanently in this offhand way. The Lord has
+never, so far as we know, indicated just what his intentions were in the
+matter of nickel service; and no one has ever shown yet just what
+degree of complexity will prevent a telephone system from working.
+
+It is safe to say that, if other things are equal, the simpler a machine
+is, the better; but simplicity, though desirable, is not all-important.
+Complexity is warranted if it can show enough advantages.
+
+If one takes a narrow view of the development of things mechanical and
+electrical, he will say that the trend is toward simplicity. The
+mechanic in designing a machine to perform certain functions tries to
+make it as simple as possible. He designs and re-designs, making one
+part do the work of two and contriving schemes for reducing the
+complexity of action and form of each remaining part. His whole trend is
+away from complication, and this is as it should be. Other things being
+equal, the simpler the better. A broad view, however, will show that the
+arts are becoming more and more complicated. Take the implements of the
+art of writing: The typewriter is vastly more complicated than the pen,
+whether of steel or quill, yet most of the writing of today is done on
+the typewriter, and is done better and more economically. The art of
+printing affords even more striking examples.
+
+In telephony, while every effort has been made to simplify the component
+parts of the system, the system itself has ever developed from the
+simple toward the complex. The adoption of the multiple switchboard, of
+automatic ringing, of selective ringing on party lines, of
+measured-service appliances, and of automatic systems have all
+constituted steps in this direction. The adoption of more complicated
+devices and systems in telephony has nearly always followed a demand for
+the performance by the machinery of the system of additional or
+different functions. As in animal and plant life, so in mechanics--the
+higher the organism functionally the more complex it becomes physically.
+
+Greater intricacy in apparatus and in methods is warranted when it is
+found desirable to make the machine perform added functions. Once the
+functions are determined upon, then the whole trend of the development
+of the machine for carrying them out should be toward simplicity. When
+the machine has reached its highest stage of development some one
+proposes that it be required to do something that has hitherto been done
+manually, or by a separate machine, or not at all. With this added
+function a vast added complication may come, after which, if it develops
+that the new function may with economy be performed by the machine, the
+process of simplification again begins, the whole design finally taking
+on an indefinable elegance which appears only when each part is so made
+as to be best adapted in composition, form, and strength to the work it
+is to perform.
+
+Achievements in the past teach us that a machine may be made to do
+almost anything automatically if only the time, patience, skill, and
+money be brought to bear. This is also true of a telephone system. The
+primal question to decide is, what functions the system is to perform
+within itself, automatically, and what is to be done manually or with
+manual aid. Sometimes great complications are brought into the system in
+an attempt to do something which may very easily and cheaply be done by
+hand. Cases might be pointed out in which fortunes and life-works have
+been wasted in perfecting machines for which there was no real economic
+need. It is needless to cite cases where the reverse is true. The matter
+of wisely choosing the functions of the system is of fundamental
+importance. In choosing these the question of complication is only one
+of many factors to be considered.
+
+One of the strongest arguments against intricacy in telephone apparatus
+is its greater initial cost, its greater cost of maintenance, and its
+liability to get out of order. Greater complexity of apparatus usually
+means greater first cost, but it does not necessarily mean greater cost
+of up-keep or lessened reliability. A dollar watch is more simple than
+an expensive one. The one, however, does its work passably and is thrown
+away in a year or so; the other does its work marvelously well and may
+last generations, being handed down from father to son. Merely reducing
+the number of parts in a machine does not necessarily mean greater
+reliability. Frequently the attempt to make one part do several diverse
+things results in such a sacrifice in the simplicity of action of that
+part as to cause undue strain, or wear, or unreliable action. Better
+results may be attained by adding parts, so that each may have a
+comparatively simple thing to do.
+
+[Illustration: WESTERN ELECTRIC COMPANY TYPICAL CHARGING OUTFIT AT
+DAWSON, GEORGIA]
+
+The stage of development of an art is a factor in determining the degree
+of complexity that may be allowed in the machinery of that art. A
+linotype machine, if constructed by miracle several hundred years ago,
+would have been of no value to the printer's art then. The skill was not
+available to operate and maintain it, nor was the need of the public
+sufficiently developed to make it of use. Similarly the automatic
+telephone exchange would have been of little value thirty years ago. The
+knowledge of telephone men was not sufficiently developed to maintain
+it, telephone users were not sufficiently numerous to warrant it, and
+the public was not sufficiently trained to use it. Industries, like
+human beings, must learn to creep before they can walk.
+
+Another factor which must be considered in determining the allowable
+degree of complexity in a telephone system is the character of the labor
+available to care for and manage it. Usually the conditions which make
+for unskilled labor also lend themselves to the use of comparatively
+simple systems. Thus, in a small village remote from large cities the
+complexity inherent in a common-battery multiple switchboard would be
+objectionable. The village would probably not afford a man adequately
+skilled to care for it, and the size of the exchange would not warrant
+the expense of keeping such a man. Fortunately no such switchboard is
+needed. A far simpler device, the plain magneto switchboard--so simple
+that the girl who manipulates it may also often care for its
+troubles--is admirably adapted to the purpose. So it is with the
+automatic telephone system; even its most enthusiastic advocate would be
+foolish indeed to contend that for all places and purposes it was
+superior to the manual.
+
+These remarks are far from being intended as a plea for complex
+telephone apparatus and systems; every device, every machine, and every
+system should be of the simplest possible nature consistent with the
+functions it has to perform. They are rather a protest against the
+broadcast condemnation of complex apparatus and systems just because
+they are complicated, and without regard to other factors. Such
+condemnation is detrimental to the progress of telephony. Where would
+the printing art be today if the linotype, the cylinder press, and other
+modern printing machinery of marvelous intricacy had been put aside on
+account of the fact that they were more complicated than the printing
+machinery of our forefathers?
+
+That the automatic telephone system is complex, exceedingly complex,
+cannot be denied, but experience has amply proven that its complexity
+does not prevent it from giving reliable service, nor from being
+maintained at a reasonable cost.
+
+_Expense._ The second argument against the automatic--that it is too
+expensive--is one that must be analyzed before it means anything. It is
+true that for small and medium-sized exchanges the total first cost of
+the central office and subscribers' station equipment, is greater than
+that for manual exchanges of corresponding sizes. The prices at which
+various sizes of automatic exchange equipments may be purchased vary,
+however, almost in direct proportion to the number of lines, whereas in
+manual equipment the price per line increases very rapidly as the number
+of lines increases. From this it follows that for very large exchanges
+the cost of automatic apparatus becomes as low, and may be even lower
+than for manual. Roughly speaking the cost of telephones and
+central-office equipment for small exchanges is about twice as great for
+automatic as for manual, and for very large exchanges, of about 10,000
+lines, the cost of the two for switchboards and telephones is about
+equal.
+
+For all except the largest exchanges, therefore, the greater first cost
+of automatic apparatus must be put down as one of the factors to be
+weighed in making the choice between automatic and manual, this factor
+being less and less objectionable as the size of the equipment increases
+and finally disappearing altogether for very large equipments. Greater
+first cost is, of course, warranted if the fixed charges on the greater
+investment are more than offset by the economy resulting. The automatic
+screw machine, for instance, costs many times more than the hand screw
+machine, but it has largely displaced the hand machine nevertheless.
+
+_Flexibility._ The third argument against the automatic telephone
+system--its flexibility--is one that only time and experience has been
+able to answer. Enough time has elapsed and enough experience has been
+gained, however, to disprove the validity of this argument. In fact, the
+great flexibility of the automatic system has been one of its surprising
+developments. No sooner has the statement been made that the automatic
+system could not do a certain thing than forthwith it has done it. It
+was once quite clear that the automatic system was not practicable for
+party-line selective ringing; yet today many automatic systems are
+working successfully with this feature; the selection between the
+parties on a line being accomplished with just as great certainty as in
+manual systems. Again it has seemed quite obvious that the automatic
+system could not hope to cope with the reverting call problem, _i. e._,
+enabling a subscriber on a party line to call back to reach another
+subscriber on the same line; yet today the automatic system may do this
+in a way that is perhaps even more satisfactory than the way in which it
+is done in multiple manual switchboards. It is true that the automatic
+system has not done away with the toll operator and it probably never
+will be advantageous to require it to do so for the simple reason that
+the work of the toll operator in recording the connections and in
+bringing together the subscribers is a matter that requires not only
+accuracy but judgment, and the latter, of course, no machine can supply.
+It is probable also that the private branch-exchange operator will
+survive in automatic systems. This is not because the automatic system
+cannot readily perform the switching duties, but the private
+branch-exchange operator has other duties than the mere building up and
+taking down of connections. She is, as it were, a door-keeper guarding
+the telephone door of a business establishment; like the toll operator
+she must be possessed of judgment and of courtesy in large degree,
+neither of which can be supplied by machinery.
+
+In respect to toll service and private branch-exchange service where, as
+just stated, operators are required on account of the nature of the
+service, the automatic system has again shown its adaptability and
+flexibility. It has shown its capability of working in harmony with
+manual switchboards, of whatever nature, and there is a growing tendency
+to apply automatic devices and automatic principles of operation to
+manual switchboards, whether toll or private branch or other kinds, even
+though the services of an operator are required, the idea being to do by
+machinery that portion of the work which a machine is able to do better
+or more economically than a human being.
+
+_Attitude of Public._ The attitude of the public toward the automatic is
+one that is still open to discussion; at least there is still much
+discussion on it. A few years ago it did seem reasonable to suppose that
+the general telephone user would prefer to get his connection by merely
+asking for it rather than to make it himself by "spelling" it out on the
+dial of his telephone instrument. We have studied this point carefully
+in a good many different communities and it is our opinion that the
+public finds no fault with being required to make its own connections.
+To our minds it is proven beyond question that either the method
+employed in the automatic or that in the manual system is satisfactory
+to the public as long as good service results, and it is beyond question
+that the public may get this with either.
+
+_Subscriber's Station Equipment._ The added complexity of the mechanism
+at the subscriber's station is in our opinion the most valid objection
+that can be urged against the automatic system as it exists today. This
+objection has, however, been much reduced by the greater simplicity and
+greater excellence of material and workmanship that is employed in the
+controlling devices in modern automatic systems. However, the automatic
+system must always suffer in comparison with the manual in respect of
+simplicity of the subscriber's equipment. The simplest conceivable thing
+to meet all of the requirements of telephone service at a subscriber's
+station is the modern common-battery manual telephone. The automatic
+telephone differs from this only in the addition of the mechanism for
+enabling the subscriber to control the central-office apparatus in the
+making of calls. From the standpoint of maintenance, simplicity at the
+subscriber's station is, of course, to be striven for since the proper
+care of complex devices scattered all over a community is a much more
+serious matter than where the devices are centered at one point, as in
+the central office. Nevertheless, as pointed out, complexity is not
+fatal, and it is possible, as has been proven, to so design and
+construct the required apparatus in connection with the subscribers'
+telephones as to make them subject to an amount of trouble that is not
+serious.
+
+=Comparative Costs.= A comparison of the total costs of owning,
+operating, and maintaining manual and automatic systems usually results
+in favor of the automatic, except in small exchanges. This seems to be
+the consensus of opinion among those who have studied the matter deeply.
+Although the automatic usually requires a larger investment, and
+consequently a larger annual charge for interest and depreciation,
+assuming the same rates for each case, and although the automatic
+requires a somewhat higher degree of skill to maintain it and to keep it
+working properly than the manual, the elimination of operators or the
+reduction in their number and the consequent saving of salaries and
+contributory expenses together with other items of saving that will be
+mentioned serves to throw the balance in favor of the automatic.
+
+The ease with which the automatic system lends itself to inter-office
+trunking makes feasible a greater subdivision of exchange districts into
+office districts and particularly makes it economical, where such would
+not be warranted in manual working. All this tends toward a reduction in
+average length of subscribers' lines and it seems probable that this
+possibility will be worked upon in the future, more than it has been in
+the past, to effect a considerable saving in the cost of the wire plant,
+which is the part of a telephone plant that shows least and costs most.
+
+=Automatic vs. Manual.= Taking it all in all the question of automatic
+versus manual may not and can not be disposed of by a consideration of
+any single one of the alleged features of superiority or inferiority of
+either. Each must be looked at as a practical way of giving telephone
+service, and a decision can be reached only by a careful weighing of all
+the factors which contribute to economy, reliability, and general
+desirability from the standpoint of the public. Public sentiment must
+neither be overlooked nor taken lightly, since, in the final analysis,
+it is the public that must be satisfied.
+
+=Methods of Operation.= In all of the automatic telephone systems that
+have achieved any success whatever, the selection of the desired
+subscriber's line by the calling subscriber is accomplished by means of
+step-by-step mechanism at the central office, controlled by impulses
+sent or caused to be sent by the acts of the subscriber.
+
+_Strowger System._ In the so-called Strowger system, manufactured by the
+Automatic Electric Company of Chicago, the subscriber, in calling,
+manipulates a dial by which the central-office switching mechanism is
+made to build up the connection he wants. The dial is moved as many
+times as there are digits in the called subscriber's number and each
+movement sends a series of impulses to the central office corresponding
+in number respectively to the digits in the called subscriber's number.
+During each pause, except the last one, between these series of
+impulses, the central-office mechanism operates to shift the control of
+the calling subscriber's line from one set of switching apparatus at the
+central office to another.
+
+In case a four-digit number is being selected first, the movement of
+the dial by the calling subscriber will correspond to the thousands
+digit of the number being called, and the resulting movement of the
+central-office apparatus will continue the calling subscriber's line
+through a trunk to a piece of apparatus capable of further extending his
+line toward the line terminals of the thousand subscribers whose numbers
+begin with the digit chosen. The next movement of the dial corresponding
+to the hundreds digit of the called number will operate this piece of
+apparatus to again extend the calling subscriber's line through another
+trunk to apparatus representing the particular hundred in which the
+called subscriber's number is. The third movement of the dial
+corresponding to the tens digit will pick out the group of ten
+containing the called subscriber's line, and the fourth movement
+corresponding to the units digit will pick out and connect with the
+particular line called.
+
+_Lorimer System._ In the Lorimer automatic system invented by the
+Lorimer Brothers, and now being manufactured by the Canadian Machine
+Telephone Company of Toronto, Canada, the subscriber sets up the number
+he desires complete by moving four levers on his telephone so that the
+desired number appears visibly before him. He then turns a handle and
+the central-office apparatus, under the control of the electrical
+conditions thus set up by the subscriber, establishes the connection. In
+this system, unlike the Strowger system, the controlling impulses are
+not caused by the movement of the subscriber's apparatus in returning to
+its normal position after being set by the subscriber. Instead, the
+conditions established at the subscriber's station by the subscriber in
+setting up the desired number, merely determine the point in the series
+of impulses corresponding to each digit at which the stepping impulses
+local to the central office shall cease, and in this way the proper
+number of impulses in the series corresponding to each digit is
+determined.
+
+_Magnet- vs. Power-Driven Switches._ These two systems differ radically
+in another respect. In the Strowger system it is the electrical impulses
+initiated at the subscriber's apparatus that actually cause the movement
+of the switching parts at the central office, these impulses energizing
+electromagnets which move the central-office switching devices a step at
+a time the desired number of steps. In the Lorimer system the switches
+are all power-driven and the impulses under the control of the
+subscriber's instrument merely serve to control the application of this
+power to the various switching mechanisms. These details will be more
+fully dealt with in subsequent chapters.
+
+_Multiple vs. Trunking._ It has been shown in the preceding portion of
+this work that the tendency in manual switchboard practice has been away
+from trunking between the various sections or positions of a board, and
+toward the multiple idea of operating, wherein each operator is able to
+complete the connection with any line in the same office without
+resorting to trunks or to the aid of other operators. Strangely enough
+the reverse has been true in the development of the automatic system. As
+long as the inventors tried to follow the most successful practice in
+manual working, failure resulted. The automatic systems of today are
+essentially trunking systems and while they all involve multiple
+connections in greater or less degree, all of them depend fundamentally
+upon the extending of the calling line by separate lengths until it
+finally reaches and connects with the called line.
+
+_Grouping of Subscribers._ In this connection we wish to point out here
+two very essential features without which, so far as we are aware, no
+automatic telephone system has been able to operate successfully. The
+first of these is the division of the total number of lines in any
+office of the exchange into comparatively small groups and the
+employment of correspondingly small switch units for each group. Many of
+the early automatic systems that were proposed involved the idea of
+having each switch capable in itself of making connection with any line
+in the entire office. As long as the number of lines was small--one
+hundred or thereabouts--this might be all right, but where the lines
+number in the thousands, it is readily seen that the switches would be
+of prohibitive size and cost.
+
+_Trunking between Groups._ This feature made necessary the employment of
+trunk connections between groups. By means of these the lines are
+extended a step at a time, first entering a large group of groups,
+containing the desired subscriber; then entering the smaller group
+containing that subscriber; and lastly entering into connection with the
+line itself. The carrying out of this idea was greatly complicated by
+the necessity of providing for many simultaneous connections through the
+switchboard. It was comparatively easy to accomplish the extension of
+one line through a series of links or trunks to another line, but it
+was not so easy to do this and still leave it possible for any other
+line to pick out and connect with any other idle line without
+interference with the first connection. A number of parallel paths must
+be provided for each possible connection. Groups of trunks are,
+therefore, provided instead of single trunks between common points to be
+connected. The subscriber who operates his instrument in making a call
+knows nothing of this and it is, of course, impossible for him to give
+any thought to the matter as to which one of the possible paths he shall
+choose. It was by a realization of these facts that the failures of the
+past were turned into the successes of the present. The subscriber by
+setting his signal transmitter was made to govern the action of the
+central-office apparatus in the selection of the proper _group_ of
+trunks. The group being selected, the central-office apparatus was made
+to act at once _automatically_ to pick out and connect with _the first
+idle trunk of such group_. Thus, we may say _that the subscriber by the
+act performed on his signal transmitter, voluntarily chooses the group
+of trunks, and immediately thereafter the central-office apparatus
+without the volition of the subscriber picks out the first idle one of
+this group of trunks so chosen_. This fundamental idea, so far as we are
+aware, underlies all of the successful automatic telephone-exchange
+systems. It provides for the possibility of many simultaneous
+connections through the switchboard, and it provides against the
+simultaneous appropriation of the same path by two or more calling
+subscribers and thus assures against interference in the choice of the
+paths.
+
+_Outline of Action._ In order to illustrate this point we may briefly
+outline the action of the Strowger automatic system in the making of a
+connection. Assume that the calling subscriber desires a connection with
+a subscriber whose line bears the number 9,567. The subscriber in making
+the call will, by the first movement of his dial, transmit nine impulses
+over his line. This will cause the selective apparatus at the central
+office, that is at the time associated with the calling subscriber's
+line, to move its selecting fingers opposite a group of terminals
+representing the ends of a group of trunk lines leading to apparatus
+employed in connecting with the ninth thousand of the subscribers'
+lines.
+
+While the calling subscriber is getting ready to transmit the next
+digit, the automatic apparatus, without his volition, starts to pick out
+the first idle one of the group of trunks so chosen. Having found this
+it connects with it and the calling subscriber's line is thus extended
+to another selective apparatus capable of performing the same sort of
+function in choosing the proper hundreds group.
+
+In the next movement of his dial the calling subscriber will send five
+impulses. This will cause the last chosen selective switch to move its
+selective fingers opposite a group of terminals representing the ends of
+a group of trunks each leading to a switch that is capable of making
+connection with any one of the lines in the fifth hundred of the ninth
+thousand. Again during the pause by the subscriber, the switch that
+chose this group of trunks will start automatically to pick out and
+connect with the first idle one of them, and will thus extend the line
+to a selective switch that is capable of reaching the desired line,
+since it has access to all of the lines in the chosen hundred. The third
+movement of the dial sends six impulses and this causes this last chosen
+switch to move opposite the sixth group of ten terminals, so that there
+has now been chosen the nine hundred and fifty-sixth group of ten lines.
+The final movement of the dial sends seven impulses and the last
+mentioned switch connects with the seventh line terminal in the group of
+ten previously chosen and the connection is complete, assuming that the
+called line was not already engaged. If it had been found busy, the
+final switch would have been prevented from connecting with it by the
+electrical condition of certain of its contacts and the busy signal
+would have been transmitted back to the calling subscriber.
+
+_Fundamental Idea._ This idea of subdividing the subscribers' lines in
+an automatic exchange, of providing different groups of trunks so
+arranged as to afford by combination a number of possible parallel paths
+between any two lines, of having the calling subscriber select, by the
+manipulation of his instrument, the proper group of trunks any one of
+which might be used to establish the connection he desires, and of
+having the central-office apparatus act automatically to choose and
+connect with an idle one in this chosen group, should be firmly grasped.
+It appears, as we have said, in every successful automatic system
+capable of serving more than one small group of lines, and until it was
+evolved automatic telephony was not a success.
+
+_Testing._ As each trunk is chosen and connected with, conditions are
+established, by means not unlike the busy test in multiple manual
+switchboards, which will guard that trunk and its associated apparatus
+against appropriation by any other line or apparatus as long as it is
+held in use. Likewise, as soon as any subscriber's line is put into use,
+either by virtue of a call being originated on it, or by virtue of its
+being connected with as a called line, conditions are automatically
+established which guard it against being connected with any other line
+as long as it is busy. These guarding conditions of both trunks and
+lines, as in the manual board, are established by making certain
+contacts, associated with the trunks or lines, assume a certain
+electrical condition when busy that is different from their electrical
+condition when idle; but unlike the manual switchboard this different
+electrical condition does not act to cause a click in any one's ear, but
+rather to energize or de-energize certain electromagnets which will
+establish or fail to establish the connection according to whether it is
+proper or improper to do so.
+
+_Local and Inter-Office Trunks._ The groups of trunks that are used in
+building up connections between subscribers' lines may be local to the
+central office, or they may extend between different offices. The action
+of the two kinds of trunks, local or inter-office, is broadly the same.
+
+
+
+
+CHAPTER XXIX
+
+THE AUTOMATIC ELECTRIC COMPANY'S SYSTEM
+
+
+Almost wherever automatic telephony is to be found--and its use is
+extensive and rapidly growing--the so-called Strowger system is
+employed. It is so named because it is the outgrowth of the work of
+Almon B. Strowger, an early inventor in the automatic telephone art.
+That the system should bear the name of Strowger, however, gives too
+great prominence to his work and too little to that of the engineers of
+the Automatic Electric Company under the leadership of Alexander E.
+Keith.
+
+=Principles of Selecting Switch.= The underlying features of this
+automatic system have already been referred to in the abstract. A better
+grasp of its principles may, however, be had by considering a concrete
+example of its most important piece of apparatus--the selecting switch.
+The bare skeleton of such a switch, sufficient only to illustrate the
+salient point in its mode of operation, is shown in Fig. 380. The
+essential elements of this are a vertical shaft capable of both
+longitudinal and rotary motion; a pawl and ratchet mechanism actuated by
+a magnet for moving the shaft vertically a step at a time; another pawl
+and ratchet mechanism actuated by another magnet for rotating the shaft
+a step at a time; an arm carrying wiper contacts on its outer end,
+mounted on and moving with the shaft; and a bank of contacts arranged on
+the inner surface of a section of a cylinder adapted to be engaged by
+the wiper contacts on this movable arm.
+
+These various elements are indicated in the merest outline and with much
+distortion in Fig. 380, which is intended to illustrate the principles
+of operation rather than the details as they actually are in the system.
+In the upper left-hand corner of this figure, the magnet shown will, if
+energized by impulses of current, attract and release its armature and,
+in doing so, cause the pawl controlled by this magnet to move the
+vertical shaft of the switch up a step at a time, as many steps as
+there are impulses of current. The vertical movement of this shaft will
+carry the wiper arm, attached to the lower end of the shaft, up the same
+number of steps and, in doing so, will bring the contacts of this wiper
+arm opposite, but not engaging, the corresponding row of stationary
+contacts in the semi-cylindrical bank. Likewise, through the ratchet
+cylinder on the intermediate portion of the shaft, the magnet shown at
+the right-hand portion of this figure will, when energized by a
+succession of electrical impulses, rotate the shaft a step at a time, as
+many steps as there are impulses. This will thus cause the contacts of
+the wiper arm to move over the successive contacts in the row opposite
+to which the wiper had been carried in its vertical movement.
+
+[Illustration: Fig. 380. Principles of Automatic Switch]
+
+At the lower left-hand corner of this figure, there is shown a pair of
+keys either one of which, when operated, will complete the circuit of
+the magnet to which it is connected, this circuit including a common
+battery. In a certain rough way this pair of key switches in the lower
+left-hand corner of the drawing may be taken as representing the
+call-transmitting apparatus at the subscriber's station, and the two
+wires extending therefrom may be taken as representing the line wires
+connecting that subscriber's station to the central office; but the
+student must avoid interpreting them as actual representations of the
+subscriber's station calling apparatus or the subscriber's line since
+their counterparts are not to be found in the system as it really
+exists. Here again accuracy has been sacrificed for ease in setting
+forth a feature of operation.
+
+Still referring to Fig. 380, it will be seen that the bank contacts
+consist of ten rows, each having ten pairs of contacts. Assume again,
+for the sake of simplicity, that the exchange under consideration has
+one hundred subscribers and that each pair of bank contacts represents
+the terminals of one subscriber's line. Assume further that the key
+switches in the lower left-hand corner of the figure are being
+manipulated by a subscriber at that station and that he wishes to obtain
+a connection with line No. 67. By pressing and releasing the left-hand
+key six times, he will cause six separate impulses of current to flow
+through the upper left-hand magnet and this will cause the switch shaft
+to move up six steps and bring the wiper arm opposite the sixth row of
+bank contacts. If he now presses and releases his right-hand key seven
+times, he will, through the action of the right-hand magnet, rotate the
+shaft seven steps, thus bringing the wipers into contact with the
+seventh contact of the sixth row and thus into contact with the desired
+line. As the wiper contacts on the switch arm form the terminals of the
+calling subscriber's line, it will be apparent that the calling
+subscriber is now connected through his switch with the line of
+subscriber No. 67.
+
+As stated, each of the pairs of bank contacts are connected with the
+line of a subscriber; the line, Fig. 380, is shown so connected to the
+forty-first pair of contacts, that is to the first contact in the fourth
+row. The selecting switch shown in Fig. 380 would be for the sole use of
+the subscriber on the line No. 41. Each of the other subscribers would
+have a similar switch for his own exclusive use. Since any of the
+switches must be capable of reaching line No. 67, for instance, when
+moved _up_ six rows and _around_ seven, it follows that the
+sixty-seventh pair of contacts in each bank of the entire one hundred
+switches must also be connected together and to line No. 67. The same
+is, of course, true of all the contacts corresponding to any other
+number. Multiple connections are thus involved between the corresponding
+contacts of the banks, in much the same way as in the corresponding
+jacks in the multiple of a manual switchboard. As a result of this
+multiple connection of the bank contacts, any subscriber may move the
+wiper arm of his selecting switch into connection with the line of any
+other subscriber.
+
+_The "Up-and-Around" Movement._ The elemental idea to be grasped by the
+discussion so far, is the so-called "up-and-around" method of action of
+the selecting switches employed in this system. This preliminary
+discussion may be carried a step further by saying that the arrangement
+is such that when a subscriber presses both his keys and grounds both of
+the limbs of his line, such a condition is brought about as will cause
+all holding pawls to be withdrawn from the shaft, and thus allow it to
+return to its normal position with respect to both its vertical and
+rotary movements. No attempt has been made in Fig. 380 to show how this
+is accomplished.
+
+=Function of Line Switch.= Such a system as has been briefly outlined in
+the foregoing would require a separate selecting switch for each
+subscriber's line and would be limited to use in exchanges having not
+more than one hundred lines. In the modern system of the Automatic
+Electric Company, the requirement that each subscriber shall have a
+selective switch, individual to his own line, has been eliminated by
+introducing what is called an _individual line switch_ by means of which
+any one of a group of subscribers' lines, making a call, automatically
+appropriates one of a smaller group of selecting switches and makes it
+its own only while the connection exists.
+
+=Subdivision of Subscribers' Lines.= The limitation as to the size of
+the exchange has been overcome, without increasing the number of bank
+contacts in any selecting switch, by dividing the subscribers' lines
+into groups of one hundred and causing selecting switches automatically
+to extend the calling subscriber's line first into a group of groups
+corresponding, for instance, to the thousand containing the called
+subscriber's line, and then into the particular group containing the
+line, and lastly, to connect with the individual line in that group.
+
+=Underlying Feature of Trunking System.= It will be remembered that in
+the chapter on fundamental principles of automatic systems, it was
+stated that the subscriber, by means of the signal transmitter at his
+station, was made to govern the action of the central-office apparatus
+in the selection of a proper group of trunks; and the group being
+selected, the central-office apparatus was made to act automatically to
+pick out and connect with the first idle trunk of such group. This
+selection by the subscriber of a group followed by the automatic
+selection from among that group forms the basis of the trunking system.
+It is impossible, by means of any simple diagram, to show a complete
+scheme of trunking employed, but Fig. 381 will give a fundamental
+conception of it. This figure shows how a single calling line, indicated
+at the bottom, may find access into any particular line in an office
+having a capacity for ten thousand.
+
+=Names of Selecting Switches.= Selecting switches of the "up-and-around"
+type are the means by which the calling line selects and connects with
+the trunk lines required in building up the connection, and finally
+selects and connects with the line of the called subscriber. Where such
+a switch is employed for the purpose of selecting a _trunk_, it is
+called a selector switch. It is a _first selector_ when it serves to
+pick out a major group of lines, _i. e._, a group containing a
+particular thousand lines or, in a multi-office system, a group
+represented by a complete central office. It is a _second selector_ when
+it serves to make the next subdivision of groups; a _third selector_ if
+further subdivision of groups is necessary; and finally it is _a
+connector_ when it is employed to pick out and connect with the
+_particular line in the final group of one hundred lines_ to which the
+connection has been brought by the selectors. In a single office of
+10,000-line capacity, therefore, we would have first and second
+selectors and connectors, the first selectors picking out the thousands,
+the second selectors the hundreds, and the connectors the individual
+line. In a multi-office system we may have first, second, and third
+selectors and connectors, the first selector picking out the office, the
+second selector the thousands in that office, the third selector the
+hundreds, and the connector the individual lines.
+
+=The Line Switch.= In addition to the selectors and connectors there are
+line switches, which are comparatively simple, one individual to each
+line. Each of these has the function, purely automatic, of always
+connecting a line, as soon as a call is originated on it, to some one of
+a smaller group of first selectors available to that line. This idea may
+be better grasped when it is understood that, in the earlier systems of
+the Automatic Electric Company, there was a first selector permanently
+associated with each line. By the addition of the comparatively simple
+line switch, a saving of about ninety per cent of the first selectors
+was effected, since the number of first selectors was thereby reduced
+from a number equal to the number of lines in a group to a number equal
+to the number of simultaneous connections resulting from calls
+originating in that group. In other words, by the line switch, the
+number of first selectors is determined by the traffic rather than by
+the number of lines.
+
+=Scheme of Trunking.= With this understanding as to the names and
+broader functions of the things involved, Fig. 381 may now be
+understood. The line switch of the single line, as indicated here, has
+only the power of selection among three trunks, but it is to be
+understood that in actual practice, it would have access to a greater
+number, usually ten. So, also, throughout this diagram we have shown the
+apparatus and trunks arranged in groups of three instead of in groups of
+ten, only the first three thousands groups being indicated and the first
+three hundreds groups in each thousand. Again only three levels instead
+of ten are indicated for each selecting switch, it being understood that
+in the diagram the various levels are represented by concentric arcs of
+circles, and the trunk contacts by dots on these arcs.
+
+_Line-Switch Action._ When the subscriber, whose line is shown at the
+bottom of the figure, begins to make a call, the line switch acts to
+connect his line with one of the first selector trunks available to it.
+This selection is entirely preliminary and, except to start it, is in no
+way under the control of the calling subscriber. The calling line now
+has under its control a first selector which, for the time being,
+becomes individual to it. Let it be assumed that the line switch found
+the first of the first selector trunks already appropriated by some
+other switch, but that the second one of these trunks was found idle.
+This trunk being appropriated by the line switch places the center one
+of the first selectors shown under the control of the subscriber's line.
+This first selector then acts in response to the first set of selective
+impulses sent out by his signal transmitter.
+
+[Illustration: DEAN HARMONIC CONVERTER Dry Cell Type for Magneto
+Exchange. _The Dean Electric Co._]
+
+[Illustration: Fig. 381. Scheme of Trunking]
+
+_First Selector Action._ We will assume that the calling subscriber
+desires to connect with No. 3213. The first movement of the subscriber's
+signal transmitter will send, therefore, three impulses over the line.
+These impulses will act on the vertical magnet of the first selector
+switch to move it up three steps. On this "level" of the contact bank of
+this switch all of the contacts will represent second selector trunks
+leading to the _third_ thousand group. The other ends of these trunks
+will terminate in the wipers and also in the controlling magnets of
+second selectors serving this thousand. This function on the part of the
+first selector controlled by the act of the subscriber will have thus
+selected a _group_ of trunks leading to the _third_ thousand, but the
+subscriber has nothing to do with which one of the trunks of this group
+will actually be used. Immediately following the vertical movement of
+the first selector switch the rotary movement of this switch will start
+and will continue until the wipers of that switch have found contacts of
+an idle trunk leading to a second selector. Assuming that the first
+trunk was the one found idle, the first selector wipers would pause on
+the first pair of contacts in the third level of its bank, and the trunk
+chosen may be seen leading from that contact off to the group of second
+selectors belonging to the third thousand. For clearness, the chosen
+trunks in this assumed connection are shown heavier than the others.
+
+_Second Selector Action._ The next movement of the dial by the
+subscriber in establishing his desired connection will send two
+impulses, it being desired to choose the _second_ hundred in the _third_
+thousand. The first selector will have become inoperative before this
+second series of impulses is sent and, therefore, only the second
+selector will respond. Its vertical magnet acting under the influence of
+these two impulses will step up its wiper contacts opposite the second
+row of bank contacts, and the subscriber will thus have chosen the
+_group_ of trunks leading to the _second_ hundred in the _third_
+thousand. Here, again, the automatic operation of picking out the first
+idle one of this chosen group of trunks will take place without the
+volition of the subscriber, and it will be assumed that the first two
+trunks on this level of the second selector were found already engaged
+and that the third was therefore chosen. The connection continues, as
+indicated by heavy lines in Fig. 381, to the third one of the connectors
+in the _second_ hundred of the _third_ thousand. Any one of these
+connectors would have accomplished the purpose but this is assumed to be
+the first one found idle by the second selector.
+
+_Connector Action._ The third movement of the subscriber's dial will
+send but one impulse, this corresponding to the _first_ group of ten in
+the _second_ hundred in the _third_ thousand. This impulse will move the
+connector shaft up to the first level of bank contacts; and from now on
+the action of the connector differs radically from that of the
+selectors. The connector is not searching for an idle trunk in the group
+but for a particular line and, therefore, having chosen the group of ten
+lines in the desired hundred, the connector switch waits for further
+guidance from the subscriber. This comes in the form of the final set of
+impulses sent by the subscriber's signal transmitter which, in this
+case, will be three in number, corresponding to the final digit in the
+number of the called subscriber. This series of impulses will control
+the rotary movement of the connector wipers which will move along the
+first level and stop on the third one. The process is seen to be one of
+successive selection, first of a large group, then of a smaller, again
+of a smaller, and finally of an individual.
+
+If the line is found not busy, the connection between the two
+subscribers is complete and the called subscriber's bell will be rung.
+If it is found busy, however, the connector will refuse to connect and
+will drop back to its normal position, sending a busy signal back to the
+calling subscriber. The details of ringing and the busy-back operation
+may only be understood by a discussion of drawings, subsequently to be
+referred to.
+
+=Two-Wire and Three-Wire Systems.= In most of the systems of the
+Automatic Electric Company in use today the impulses by which the
+subscriber controls the central-office apparatus flow over one side of
+the line or the other and return by ground. The metallic circuit is used
+for talking and for ringing the called subscriber's bell, while ground
+return circuits, on one side of the line or the other, are used for
+sending all the switch controlling impulses.
+
+Recently this company has perfected a system wherein no ground is
+required at the subscriber's station and no ground return path is used
+for any purpose between the subscriber and the central office.
+This later system is known as the "two-wire" system, and in
+contra-distinction to it, the earlier and most used system has been
+referred to as the "three-wire." It is not meant by this that the line
+circuits actually have three wires but that each line employs three
+conductors, the two wires of the line and the earth. The three-wire
+system will be referred to and described in detail, and from it the
+principles of the two-wire system will be readily understood.
+
+[Illustration: Fig. 382. Automatic Wall Set]
+
+[Illustration: Fig. 383. Automatic Desk Stand]
+
+=Subscriber's Station Apparatus.= The detailed operation of the
+three-wire system may be best understood by considering the subscriber's
+station apparatus first. The general appearance of the wall set is shown
+in Fig. 382, and of the desk set in Fig. 383. These instruments embody
+the usual talking and call-receiving apparatus of a common-battery
+telephone and in addition to this, the signal transmitter, which is the
+thing especially to be considered now. The diagrammatic illustration of
+the signal transmitter and of the relation that its parts bear to the
+other elements of the telephone set is shown in Fig. 384. It has already
+been stated that the subscriber manipulates the signal transmitter by
+rotating the dial on the face of the instrument. A clearer idea of this
+dial and of the finger stop for it may be obtained from Figs. 382 and
+383.
+
+[Illustration: Fig. 384. Circuits of Telephone Set]
+
+_Operation._ To make a call for a given number the subscriber removes
+his receiver from its hook, then places his forefinger in the hole
+opposite the number corresponding to the first digit of the desired
+number. By means of the grip thus secured, he rotates the dial until its
+movement is stopped by the impact of the finger against the stop. The
+dial is then released and in its return movement it sends the number of
+impulses corresponding to the first digit in the called number. A
+similar movement is made for each digit.
+
+In Fig. 384 is given a phantom view of the dial, in order to show more
+clearly the relation of the mechanical parts and contacts controlled by
+it. For a correct idea of its mechanical action it must be understood
+that the shaft _1_, the lever _2_, and the interrupter segment _3_ are
+all rigidly fastened to the dial and move with it. A coiled spring
+always tends to move the dial and these associated parts back to their
+normal positions when released by the subscriber, and a centrifugal
+governor, not shown, limits the speed of the return movement.
+
+The subscriber's hook switch is mechanically interlocked with the dial
+so as to prevent the dial being moved from its normal position until the
+hook is in its raised position. This interlocking function involves also
+the pivoted dog _4_. Normally the lower end of this dog lies in the path
+of the pin _5_ carried on the lever _2_, and thus the shaft, dial, and
+segment are prevented from any considerable movement when the receiver
+is on the hook. However, when the receiver is removed from its hook, the
+upwardly projecting arm from the hook engages a projection on the dog
+_4_ and moves the dog out of the path of the pin _5_. Thus the dial is
+free to be rotated by the subscriber. The pin _6_ is mounted in a
+stationary position and serves to limit the backward movement of the
+dial by the lever _2_ striking against it.
+
+Ground Springs:--Five groups of contact springs must be considered, some
+of which are controlled wholly by the position of the switch hook,
+others jointly by the position of the switch hook and the dial, others
+by the movement of the dial itself, and still others by the pressure of
+the subscriber's finger on a button. The first of these groups consists
+of the springs _7_ and _8_, the function of which is to control the
+continuity of the ground connection at the subscriber's station. The
+arrangement of these two springs is such that the ground connection will
+be broken until the subscriber's receiver is removed from its hook. As
+soon as the receiver is raised, these springs come together in an
+obvious manner, the dog _4_ being lifted out of the way by the action of
+the hook. The ledge on the lower portion of the spring _7_ serves as a
+rest for the insulated arm of the dog _4_ to prevent this dog, which is
+spring actuated, from returning and locking the dial until after the
+receiver has been hung up.
+
+Bell and Transmitter Springs:--The second group is that embracing the
+springs _9_, _10_, _11_, and _12_. The springs _10_ and _11_ are
+controlled by the lower projection from the switch hook, the spring
+_11_ engaging the spring _12_ only when the hook is down. The spring
+_10_ engages the spring _9_ only when the hook lever is up and not then
+unless the dial is in its normal position. While the hook is raised,
+therefore, the springs _9_ and _10_ break contact whenever the dial is
+moved and make contact again when it returns to its normal position. The
+springs _11_ and _12_ control the circuit through the subscriber's bell
+while the springs _9_ and _10_ control the continuity of the circuit
+from one side of the line to the other so as to isolate the limbs from
+each other while the signal transmitter is sending its impulses to the
+central office.
+
+Impulse Springs:--The third group embraces springs _13_, _14_, and _15_
+and these are the ones by which the central-office switches are
+controlled in building up a connection.
+
+Something of the prevailing nomenclature which has grown up about the
+automatic system may be introduced at this point. The movements of the
+selecting switches at the central office are referred to as _vertical_
+and _rotary_ for obvious reasons. On account of this the magnet which
+causes the vertical movement is referred to as the _vertical magnet_ and
+that which accomplishes the _rotary_ movement as the _rotary magnet_. It
+happens that in all cases the selecting impulses sent by the
+subscriber's station, corresponding respectively to the number of digits
+in the called subscriber's number, are sent over one side of the line
+and in nearly all cases these selecting impulses actuate the vertical
+movements of the selecting switches. For this reason the particular limb
+of the line over which the selecting impulses are sent is called the
+_vertical limb_. The other limb of the line is the one over which the
+single impulse is sent after each group of selecting impulses, and it is
+this impulse in every case which causes the selector switch to start
+rotating in its hunt for an idle trunk. This side of the line is,
+therefore, called _rotary_. For the same reasons the impulses over the
+vertical side of the line are called _vertical impulses_ and those over
+the rotary side, _rotary impulses_. The naming of the limbs of the line
+and of the current impulses _vertical_ and _rotary_ may appear odd but
+it is, to say the least, convenient and expressive.
+
+Coming back to the functions of the third group of springs, _13_, _14_,
+and _15_, _15_ may be called the _vertical spring_ since it sends
+vertical impulses; _13_, the _rotary spring_ since it sends rotary
+impulses; and _14_, the _ground spring_ since, when the hook is up, it
+is connected with the ground.
+
+On the segment _3_ there are ten projections or cams _16_ which, when
+the dial is moved, engage a projection of the spring _15_. When the dial
+is being pulled by the subscriber's finger, these cams engage the spring
+_15_ in such a way as to move it away from the ground spring and no
+electrical contact is made. On the return of the dial, however, these
+cams engage the projection on the spring _15_ in the opposite way and
+the passing of each cam forces this vertical spring into engagement with
+the ground spring. It will readily be seen, therefore, by a
+consideration of the spacing of these cams on the segment and the finger
+holes in the dial that the number of cams which pass the vertical spring
+_15_ will correspond to the number on the hole used by the subscriber in
+moving the dial.
+
+Near the upper right-hand corner of the segment _3_, as shown in Fig.
+384, there is another projection or cam _17_, the function of which is
+to engage the rotary spring _13_ and press it into contact with the
+ground spring. Thus, the first thing that happens in the movement of the
+dial is for the projection _17_ to ride over the hump on the rotary
+spring and press the contact once into engagement with the ground
+spring; and likewise, the last thing that happens on the return movement
+of the dial is for the rotary spring to be connected once to the ground
+spring after the last vertical impulse has been sent.
+
+If both the rotary and vertical sides of the line are connected with the
+live side of the central-office battery, it follows that every contact
+between the vertical and the ground spring or between the rotary and the
+ground spring will allow an impulse of current to flow over the vertical
+or the rotary side of the line.
+
+We may summarize the action of these impulse springs by saying that
+whenever the dial is moved from its normal position, there is, at the
+beginning of this movement, a single rotary impulse over the rotary side
+of the line; and that while the dial returns, there is a series of
+vertical impulses over the vertical side of the line; and just before
+the dial reaches its normal position, after the sending of the last
+vertical impulse, there is another impulse over the rotary side of the
+line.
+
+The mechanical arrangements of the interrupter segment _3_ and its
+associated parts have been greatly distorted in Fig. 384 in order to
+make clear their mode of operation. This drawing has been worked out
+with great care, with this in mind, at a sacrifice of accuracy in regard
+to the actual structural details.
+
+Ringing Springs:--The fourth group of springs in the subscriber's
+telephone is the ringing group and embraces the springs _18_, _19_, and
+_20_. The springs _19_ and _20_ are normally closed and maintain the
+continuity of the talking circuit. When, however, the button attached to
+the spring _19_--which button may be seen projecting from the instrument
+shown in Fig. 382, and from the base of the one shown in Fig. 383--is
+pressed, the continuity of the talking circuit is interrupted and the
+vertical side of the line is connected with the ground. It is by this
+operation, after the connection has been made with the desired
+subscriber's line, that the central-office apparatus acts to send
+ringing current out on that line.
+
+Release Springs:--The fifth set of springs is the one shown at the
+left-hand side of Fig. 384, embracing springs _21_, _22_, and _23_. The
+long curved spring _21_ is engaged by the projecting lug on the switch
+hook when it rises so as to press this spring away from the other two.
+On the return movement of the hook, however, this spring is pressed to
+the left so as to bring all three of them into contact, and this, it
+will be seen, grounds both limbs of the line at the subscriber's
+station. This combination cannot be effected by any of the other springs
+at any stage of their operation, and it is the one which results in the
+energization of such a combination of relays and magnets at the central
+office as will release all parts involved in the connection and allow
+them to return to their normal positions ready for another call.
+
+_Salient Points._ If the following things are borne in mind about the
+operation of the subscriber's station apparatus, an understanding of the
+central-office operations will be facilitated. First, the selective
+impulses always flow over the vertical side of the line; they are always
+preceded and always followed by a single impulse over the rotary side of
+the line. The ringing button grounds the vertical side of the line and
+the release springs ground both sides of the line simultaneously.
+
+=The Line Switch.= The first thing to be considered in connection with
+the central-office apparatus is the line switch. This, it will be
+remembered, is the device introduced into each subscriber's line at the
+central office for the purpose of effecting a reduction of the number of
+first selectors required at the central office, and also for bringing
+about certain important functional results in connection with trunking
+between central and sub-offices. The function of the line switch in
+connection with the subscriber's line, however, is purely that of
+reducing the number of first selectors.
+
+The line switches of one hundred lines are all associated to form a
+single unit of apparatus, which, besides the individual line switches,
+includes certain other apparatus common to those lines. Such a group of
+one hundred line switches and associated common apparatus is called a
+_line-switch unit_, or frequently, a _Keith unit_. Confusion is likely
+to arise in the mind of the reader between the individual line switch
+and the line-switch unit, and to avoid this we will refer to the piece
+of apparatus individual to the line as the line switch, and to the
+complete unit formed of one hundred of these devices as a line-switch
+unit.
+
+_Line and Trunk Contacts._ Each line switch has its own bank of contacts
+arranged in the arc of a circle, and in this same arc are also placed
+the contacts of each of the ten individual trunks which it is possible
+for that line to appropriate. The contacts individual to the
+subscriber's line in the line switch are all multipled together, the
+arrangement being such that if a wedge or plunger is inserted at any
+point, the line contacts will be squeezed out of their normal position
+so as to engage the contacts of the trunk corresponding to the
+particular position in the arc at which the wedge or plunger is
+inserted. A small plunger individual to each line is so arranged that it
+may be thrust in between the contact springs in the line-switch bank in
+such manner as to connect any one of the trunks with the line terminals
+represented in that row, the particular trunk so connected depending on
+the portion of the arc toward which the plunger is pointed at the time
+it is thrust in the contacts.
+
+These banks of lines and trunk contacts are horizontally arranged, and
+piled in vertical columns of twenty-five line switches each. The ten
+trunk contacts are multipled vertically through the line-switch banks,
+so that the same ten trunks are available to each of the twenty-five
+lines. We thus have, in effect, an old style, Western Union, cross-bar
+switchboard, the line contacts being represented in horizontal rows and
+the trunk contacts in vertical rows, the connection between any line and
+any trunk being completed by inserting a plunger at the point of
+intersection of the horizontal and the vertical rows corresponding to
+that line and trunk.
+
+_Trunk Selection._ The plungers by which the lines and trunks are
+connected are, as has been said, individual to the line, and all of the
+twenty-five plungers in a vertical row are mounted in such manner as to
+be normally held in the same vertical plane, and this vertical plane is
+made to oscillate back and forth by an oscillating shaft so as always
+_to point the plungers toward a vertical row of trunk contacts that
+represent a trunk that is not in use at the time_. The to-and-fro
+movement of this oscillating shaft, called the _master bar_, is
+controlled by a master switch and the function of this master switch is
+always to keep the plungers pointed toward the row of contacts of an
+idle trunk. The thrusting movement of the individual plungers into the
+contact bank is controlled by magnets individual to the line and under
+control of the subscriber in initiating a call. As soon as the plunger
+of a line has been thus thrust into the contact bank so as to connect
+the terminals of that line with a given trunk, the plunger is no longer
+controlled by the master bar and remains stationary. The master bar then
+at once moves all of the other plungers that are not in use so that they
+will point to the terminals of another trunk that is not in use. The
+plungers of all the line switches in a group of twenty-five are,
+therefore, subject to the oscillating movements of the master bar when
+the line is not connected to a first selector trunk. As soon as a call
+is originated on a line, the corresponding plunger is forced into the
+bank and is held stationary in maintaining the connection to a first
+selector trunk, and all of the other plungers not so engaged, move on so
+as to be ready to engage another idle trunk.
+
+_Trunk Ratio._ The assignment of ten trunks to twenty-five lines would
+be a greater ratio of trunks than ordinary traffic conditions require.
+This ratio of trunks to lines is, however, readily varied by multipling
+the trunk contacts of several twenty-five line groups together. Thus,
+ten trunks may be made available to one hundred subscribers' lines by
+multipling the trunks of four twenty-five line switch groups together.
+In this case the four master bars corresponding to the four groups of
+twenty-five line switches are all mechanically connected together so as
+to move in unison under the control of a single master switch. If more
+than ten and less than twenty-one trunks are assigned to one hundred
+lines, then each set of ten trunks is multipled to the trunk contacts of
+fifty line switches, the two master bars of these switches being
+connected together and controlled by a common master switch.
+
+_Structure of Line Switch._ The details of the parts of a line switch
+that are individual to the line are shown in Fig. 385, the line and
+trunk contact bank being shown in the lower portion of this figure and
+also in a separate view in the detached figure at the right. A detailed
+group of several such line switches with the oscillating master bar is
+shown in Fig. 386. This figure shows quite clearly the relative
+arrangement of the line and trunk contact banks, the plungers for each
+bank, and the master bar.
+
+[Illustration: Fig. 385. Line Switch]
+
+In practice, four groups of twenty-five line switches each are mounted
+on a single framework and the group of one hundred line switches,
+together with certain other portions of the apparatus that will be
+referred to later, form a line-switch unit. A front view of such a unit
+is shown in Fig. 387. In order to give access to all portions of the
+wiring and apparatus, the framework supporting each column of fifty line
+switches is hinged so as to open up the interior of the device as a
+whole. A line-switch unit thus opened out is shown in Fig. 388.
+
+[Illustration: Fig. 386. Portion of Line-Switch Unit]
+
+_Circuit Operation._ The mode of operation of the line switch may be
+best understood in connection with Fig. 389, which shows in a schematic
+way the parts of a line switch that are individual to a subscriber's
+line, and also those that are common to a group of fifty or one hundred
+lines. Those portions of Fig. 389 which are individual to the line are
+shown below the dotted line extending across the page. The task of
+understanding the line switch will be made somewhat easier if Figs. 385
+and 389 are considered together. The individual parts of the line switch
+are shown in the same relation to each other in these two figures with
+the exception that the bank of line and trunk springs in the lower
+right-hand corner of Fig. 389 have been turned around edgewise so as to
+make an understanding of their circuit connections possible.
+
+[Illustration: Fig. 387. Line-Switch Unit]
+
+[Illustration: Fig. 388. Line-Switch Unit]
+
+[Illustration: Fig. 389. Circuits of Line-Switch Unit]
+
+The vertical and rotary sides of the subscriber's line are shown
+entering at the lower left-hand corner of this figure, and they pass to
+the springs of the contact bank. Immediately adjacent to these springs
+are the trunk contacts from which the vertical and the rotary limbs of
+the first selector trunk proceed. The plunger is indicated at _1_, it
+being in the form of a wheel of insulating material. It is carried on
+the rod _2_ pivoted on a lever _3_, which, in turn, is pivoted at _4_ in
+a stationary portion of the framework. A spring _5_, secured to the
+underside of the lever _3_ and projecting to the left beyond the pivot
+_4_ of this lever, serves always to press the right-hand portion of the
+lever _3_ forward in such direction as to tend to thrust it into the
+contact bank. The plunger is normally held out of the contact bank by
+means of the latch _6_ carried on the armature _7_ of the trip magnet.
+When the trip magnet is energized it pulls the armature _7_ to the left
+and thus releases the plunger and allows it to enter the contact bank.
+
+[Illustration: POWER SWITCHBOARD FOR MEDIUM-SIZED OFFICE Mercury Arc
+Rectifier Panel and Transformer at Right.]
+
+The master bar is shown at _8_, and a feather on this bar engages a
+notch in the segment attached to the rear end of the plunger rod _2_.
+This master bar is common to all of the plunger rods and by its
+oscillatory movement, under the influence of the master switch, it
+always keeps all of the idle plunger bars pointed toward the contacts of
+an idle trunk. As soon, however, as the trip magnet is operated to cause
+the insertion of a plunger into the contact bank, the feather on the
+master bar is disengaged by the notch in the segment of the plunger rod,
+and the plunger rod is, therefore, no longer subject to the oscillating
+movement of the master bar.
+
+When the release magnet is energized, it attracts its armature _9_ and
+this lifts the armature _7_ of the trip magnet so that the latch _6_
+rides on top of the left-hand end of the lever _3_. Then, when the
+release magnet is de-energized, the spring _5_, which was put under
+tension by the latch, moves the entire structure of levers back to its
+normal position, withdrawing the plunger from the bank of contacts. The
+notch on the edge of the segment of the plunger rod, when thus released,
+will probably not strike the feather on the master bar, and the plunger
+rod will thus not come under the control of the master bar until the
+master bar has moved, in its oscillation, so that the feather registers
+with the notch, after which this bar will move with all the others.
+
+If, while the plunger is waiting to be picked up by the master bar, the
+same subscriber should call again, his line will be connected with the
+same trunk as before. There is no danger in this, however, that the
+trunk will be found busy, because the master bar will not have occupied
+a position which would make it possible for any of the lines to
+appropriate this trunk during the intervening time.
+
+_Master Switch._ Associated with each master bar there is a master
+switch which determines the position in which the master bar shall stop
+in order that the idle plungers may be pointed always to the contacts of
+an idle trunk. The arm _10_ of this switch is attached to the master bar
+and oscillates with it and serves to connect the segment _11_
+successively with the contacts _12_, which are connected respectively to
+the third, or release wire of each first selector trunk. In the figure
+the arm _10_ is shown resting on the sixth contact of the switch and
+this sixth contact is connected to a spring _13_ in the line-switch
+contact bank that has not yet been referred to. As soon as the plunger
+is inserted into the contact bank, the spring _14_ will be pressed into
+engagement with the spring _13_, and this spring _14_ is connected with
+the live side of the battery through the release magnet winding.
+
+The contact strip _11_ on the master switch is thus connected through
+the release magnet to the battery and from this current flows through
+the left-hand winding of the master-switch relay. This energizes this
+relay and causes the closure of the circuit of the locking magnet which
+magnet unlocks the master bar to permit its further rotation. The
+unlocking of the master bar brings the spring _15_ into engagement with
+_16_ and thus energizes the master magnet, the armature of which
+vibrates back and forth after the manner of an electric-bell armature,
+and steps the wheel _17_ around. The wheel _17_ is mechanically
+connected to the master bar so that each complete revolution of the
+wheel will cause one complete oscillation of the master bar. The master
+bar will thus be moved so as to cause all the idle plungers to sweep
+through an arc and this movement will stop as soon as the master-switch
+arm _10_ connects the arc _11_ with one of the contacts _12_ that is not
+connected to the live side of the battery through the springs _13_ and
+_14_ of some other line switch. It is by this means that the plungers of
+the line switches are always kept pointing at the contacts of an idle
+trunk. The way in which this feature has been worked out must demand
+admiration and accounts for the marvelous quickness of this line switch.
+The fact that the plungers are pointed in the right direction before the
+time comes for their use, leaves only the simple thrusting motion of the
+plunger to accomplish the desired connection immediately upon the
+initiation of a call by the subscriber.
+
+_Locking Segment._ It will be understood that the locking segment _18_
+and the master-switch contact finger _10_ are both rigidly connected
+with the master bar _8_ and move with it, the locking segment _18_
+serving always to determine accurately the angular position at which the
+master bar and the master-switch arm are brought to rest.
+
+_Bridge Cut-Off._ One important feature of automatic switching,
+particularly as exemplified in the system of the Automatic Electric
+Company, is the disconnection, after its use, of each operating magnet
+of each piece of apparatus involved in making a connection. Since these
+operating magnets are always bridged across the line at the time of
+their operation and then cut off after they have performed their
+function, this feature may be referred to as the _bridge cut-off_.
+
+_Guarding Functions._ Still another feature of importance is the means
+for guarding a line or a piece of apparatus that has already been
+appropriated or made busy, so that it will not be appropriated or
+connected with for use in some other connection. For this latter purpose
+contacts and wires are associated with each piece of apparatus, which
+are multipled to similar contacts on other pieces of apparatus in much
+the same way and for a similar purpose that the test thimbles in a
+multiple switchboard are multipled together. Such wires and contacts in
+the Automatic Electric Company's apparatus are called _private wires_
+and _contacts_.
+
+The bridge cut-off and guarding functions are provided for in the line
+switch by a bridge cut-off relay shown in Fig. 389 and also in Fig. 385,
+it being the upper one of the individual line relays in each of those
+figures. This bridge cut-off relay is operated as soon as the plunger of
+the line is thrust into the bank; the contacts _19_ and _20_, closed by
+the plunger, serving to complete the circuit of this relay. To make
+clear the bridge cut-off feature it will be noted that the trip magnet
+of a line switch is connected in a circuit traced from the rotary side
+of the line through the contacts _21_ and _22_ of the bridge cut-off
+relay, thence through the coil of the trip magnet to the common wire
+leading to the spring _23_ of the master-bar locking device and thence
+to the live side of the battery. Obviously, therefore, as soon as the
+bridge cut-off relay operates, the trip magnet becomes inoperative and
+can cause no further action of the line switch because its circuit is
+broken between the springs _21_ and _22_.
+
+The private or guarding feature is taken care of by the action of the
+plunger in closing contacts _19_ and _20_, since the private wire
+leading to the bridge cut-off relay is, as has already been stated,
+connected to ground when these contacts are closed. This private wire
+leads off and is multipled to the private contacts on all the connectors
+that have the ability to reach this line, and the fact that this wire
+is grounded by the line switch as soon as it becomes busy, establishes
+such conditions at all of the connectors that they will refuse to
+connect with this line as long as it is busy, in a way that will be
+pointed out later on.
+
+_Relation of Line Switch and Connectors._ The vertical and rotary wires
+of the subscriber's line are shown leading off to the connector banks at
+the left-hand side of Fig. 389, and one side of this connection passes
+through the contacts _24_ and _25_ of the bridge cut-off relay on the
+line switch. It is through this path that a connection from some other
+line through a connector to this line is established and it is seen that
+this path is held open until the bridge cut-off relay of the line switch
+is operated. For such a connection to this line the bridge cut-off relay
+of the line switch is operated over the private wire leading from the
+connector, and the operation of the bridge cut-off relay at this time
+serves to render inoperative the line switch, so that it will not
+perform its usual functions should the called subscriber start to make a
+call after his line had been seized.
+
+_Summary of Line-Switch Operation._ To summarize the operation of a line
+switch when a call is originated on its line, the first movement of the
+calling subscriber's dial will ground the rotary side of the line and
+operate the trip magnet. This will cause the plunger to be inserted into
+the bank, and thus extend the line to the first selector trunk through
+the closing of the right-hand set of springs shown in the lower
+right-hand corner of Fig. 389. The insertion of the plunger will also
+connect the battery through the left-hand winding of the master-switch
+relay and, by the sequence of operations which follows, cause the master
+bar to move all of the idle plungers so as to again point them to an
+idle trunk. The closure of contacts _19_ and _20_ by the plunger causes
+the operation of the bridge cut-off relay which opens the circuit of the
+trip magnet, rendering it inoperative; and also establishes ground
+potential on all the private wire contacts of that line in the banks of
+the connectors, so as to guard the line and its associated apparatus
+against intrusion by others. The line is cut through, therefore, to a
+first selector and all of the line-switch apparatus is completely cut
+off from the talking circuit.
+
+It must be remembered that all of the actions of the line switch, which
+it has taken so long to describe, occur practically instantaneously and
+as a result of the first part of the first movement of the subscriber's
+dial. The line switch has done its work and "gone out of business"
+before the selective impulses of the first digit begin to take place.
+
+=Selecting Switches.= The first selector is now in control of the
+calling subscriber. The circuits and elements of the first selector
+switch are shown in Fig. 390. The general mechanical structure of the
+first selectors, second selectors, and connectors, is the same and may
+be referred to briefly here. Fig. 391 shows a rear view of a first
+selector; Fig. 392, a side view of a second selector; and Fig. 393, a
+front view of a connector. The arrangement of the vertical and rotary
+magnets, of the selector shafts, and of the contact banks are identical
+in all three of these pieces of apparatus and all these switches work on
+the "up-and-around principle" referred to in connection with Fig. 380.
+It is thought that with the general structure shown in Figs. 391, 392,
+and 393 in mind, the actual operation may be understood much more
+readily from Fig. 390.
+
+Four magnets--the vertical, the rotary, the private, and the
+release--produce the switching movements of the machine. These magnets
+are controlled by various combinations brought upon the circuits by
+three relays--the vertical, the rotary, and the back release. The fourth
+relay shown, called the _off-normal_, is purely for signaling purposes,
+as will be described.
+
+_Side Switch._ Another important element of the selecting switches is
+the so-called side switch which might better be called a pilot
+switch--but we are not responsible for its name. This side switch has
+for its function the changing of the control of the subscriber's line to
+successive portions of the selector mechanism, rendering inoperative
+those portions that have already performed their functions and that,
+therefore, are no longer needed. This switch may be seen best in Fig.
+392 just above the upper bank of contacts. It is shown in Fig. 390
+greatly distorted mechanically so as to better illustrate its electrical
+functions.
+
+[Illustration: Fig. 390. Circuits of First Selector]
+
+The contact levers _1_, _2_, _3_, and _4_ of the side switch are carried
+upon the arm _5_ which is pivoted at _6_. All of these contact levers,
+therefore, move about _6_ as an axis. The side switch has three
+positions and it is shown, in Fig. 390, in the first one of these. When
+the private magnet armature is attracted and released once, the
+escapement carried by it permits the spring _7_ to move the arm _5_ so
+as to bring the wipers of the side switch into its second position; the
+second pulling up and release of the private magnet armature will cause
+the movement of the side switch wipers into the third position. It is to
+be noted that the escapement which releases the side switch arm may be
+moved either by the private or by the rotary magnet, since the armature
+of the latter has a finger which engages the private magnet armature.
+
+[Illustration: Fig. 391. Rear View of First Selector]
+
+_Functions of Side Switch._ The functions of the side switch may be
+briefly outlined in connection with the first selector, as an example.
+In the first position it extends the control of the subscriber's signal
+transmitter through the first selector trunk and line relays to the
+vertical and private magnets so that these magnets will be responsive to
+the selecting impulses corresponding to the first digit. In its second
+position it brings about such a condition of affairs that the rotary
+magnet will be brought into play and automatically move the wipers over
+the bank contacts in search of an idle trunk. In its third position,
+both the vertical and rotary relays are cut off and the line is cut
+straight through to the second selector trunk, and only those parts of
+the first selector apparatus are left in an operative state which have
+to do with the private or guarding circuits and with the release.
+Similar functions are performed by the side switch in connection with
+the other selecting switches.
+
+[Illustration: Fig. 392. Side View of Second Selector]
+
+_Release Mechanism._ Another one of the features of the switch that
+needs to be considered before a detailed understanding of its operation
+may be had, is the mechanical relation of the holding and the release
+dog. This dog is shown at _8_ and, in the language of the art, is called
+the _double dog_. As will be seen, it has two retaining fingers, one
+adapted to engage the vertical ratchet and the other, the rotary ratchet
+on the selector shaft. This double dog is pivoted at _9_ and is
+interlinked in a peculiar way with the armature of the vertical magnet,
+the armature of the release magnet, and the arm of the side switch. The
+function of this double dog is to hold the shaft in whatever vertical
+position it is moved by the vertical magnet and then, when the rotary
+magnet begins to operate, to hold the shaft in its proper angular
+position. It will be noted that the fixed dog _10_ is ineffective when
+the shaft is in its normal angular position. But as soon as the shaft is
+rotated, this fixed dog _10_ becomes the real holding pawl so far as the
+vertical movement is concerned. The double dog _8_ is normally held out
+of engagement with the vertical and the rotary ratchets by virtue of the
+link connection, shown at _11_, between the release magnet armature and
+the rear end of the double dog. On the previous release of the switch
+the attraction of the release magnet armature permitted the link _11_ to
+hook over the end of the dog _8_ and thus, on its return movement, to
+pull this dog out of engagement with its ratchets. This movement also
+resulted in pushing on the link _12_ which is pivoted to the side switch
+arm _5_, and thus the return movement of the release magnet is made to
+restore the side switch to its normal position. In order that the double
+dog may be made effective when it is required, and in order that the
+side switch may be free to move under the influence of the private
+magnet, the double dog is released from its connection with the release
+magnet armature by the first movement of the vertical magnet in a manner
+which is clear from the drawing.
+
+=First Selector Operation.= In discussing the details of operation of
+the various selectors it will be found convenient to divide the
+discussion according to the position of the side switch. This will bring
+about a logical arrangement because it is really the side switch which
+determines by its position the sequence of operation.
+
+[Illustration: Fig. 393. Front View of Connector]
+
+_First Position of Side Switch._ This is the position shown in Fig. 390,
+and is the normal position. The vertical and the rotary lines extending
+from the calling subscriber are continued by the levers _1_ and _2_ of
+the side switch through the vertical and the rotary relay coils,
+respectively, to the live side of battery. The lever _4_ of the side
+switch in this position connects to ground the circuit leading from the
+line switch through the release trunk, and the winding of the off-normal
+relay. This winding is thus put in series with the release magnet of
+the line switch, but on account of high resistance of the off-normal
+relay no operation of the release magnet is caused. This will, however,
+permit such current to flow through the release circuit as will energize
+the sensitive off-normal relay and cause it to attract its armature and
+light the off-normal lamp. If this lamp remains lighted more than a
+brief period of time, it will attract notice and will indicate that the
+corresponding selector has been appropriated by a line switch and that
+for some reason the selector has gone no further. This lamp, therefore,
+is an aid in preventing the continuance of this abnormal condition.
+
+The first thing that happens after the line switch has connected the
+calling subscriber with the first selector is a succession of impulses
+over the vertical side of the line, this being the set of impulses
+corresponding in number to the thousands digit or to the office, if
+there is more than one. It will be understood that here we are
+considering a single office of ten-thousand-line capacity or
+thereabouts, and that, therefore, this first set of impulses corresponds
+to the thousands digit in the called subscriber's line. Each one of
+these impulses will flow from the battery through the vertical relay and
+each movement of this relay armature will close the circuit of the
+vertical magnet and cause the shaft of the selector to be stepped up to
+the proper level. Immediately following the first series of selecting
+impulses from the subscriber's station, a single impulse follows over
+the rotary side of the line. This gives the rotary relay armature one
+impulse and this in turn closes the circuit of the private magnet once.
+The single movement of the private magnet armature allows the escapement
+finger on the arm _5_ to move one step and this brings the side switch
+contacts into the second position.
+
+_Second Position of Side Switch._ In this position lever _4_ of the side
+switch places a ground on the wire leading through the rotary magnet to
+a source of interrupted battery current. The impulses which thus flow
+through the rotary magnet occur at a frequency dependent upon the
+battery interrupter and this is at a rate of approximately fifteen
+impulses per second. The rotary magnet will step the selector shaft
+rapidly around until something occurs to stop these impulses. This
+something is the finding by the private wiper of an ungrounded private
+contact in the bank, since all of the contacts corresponding to busy
+trunks are grounded, as will be explained.
+
+The action of the private magnet enters into this operation in the
+following way: A circuit may be traced from the battery through the
+private magnet to the third side switch wiper when in its second
+position, thence through the back release relay to the private wiper. If
+the wiper is at the time on the private bank contact of a busy trunk, it
+will find that contact grounded and the private magnet will be
+energized. The energizing of this magnet will not, however, cause the
+release of the side switch. It must be energized and de-energized. The
+private magnet armature will, therefore, be operated by the finger of
+the rotary magnet armature on the first rotary step. The private magnet
+will be energized and hold its armature operated if the private wiper
+finds a ground on the first bank contact and will stay energized as long
+as the private wiper is passing over private contacts of busy trunks.
+Its armature will not be allowed to fall back during the passage of the
+wiper from one trunk to another, because during that interval the finger
+of the rotary magnet will hold it operated. As soon, however, as the
+private wiper reaches the private bank contact of an idle trunk, no
+ground will be found and the circuit of the private magnet will be left
+open. When the impulse through the rotary magnet ceases, the private
+magnet armature will fall back and the side switch will be released to
+its third position.
+
+_Third Position of Side Switch._ The first thing to be noted in this
+position is that the calling line is cut straight through to the second
+selector trunk, the connection being clean with no magnets bridged
+across or tapped off. The third wiper of the side switch, when in its
+third position, is grounded and this connects the release wire of the
+second selector trunk, on which the switch wipers rest, through the
+private wiper, the winding of the back release magnet, and the third
+wiper of the side switch to ground. This establishes a path for the
+subsequent release current through the back release magnet; and, of
+equal importance, it places a ground on the private bank contact of that
+trunk so that the private wiper of any other switch will be prevented
+from stopping on the contacts of this trunk in the same manner that the
+wiper of this switch was prevented from stopping on other trunks that
+were already in use.
+
+The fourth lever on the side switch, when in its third position, serves
+merely to close the circuit of the rotary off-normal lamp. This lamp is
+for the purpose of calling attention to any first selector switch that
+has been brought into connection with some second selector trunk and
+which, for some reason, has failed in its release. These off-normal
+lamps are so arranged that they may be switched off manually to avoid
+burning them during the hours of heaviest traffic. At night they afford
+a ready means of testing for switches that have been left off-normal,
+since the manual switches controlling these lamps may then be closed,
+and any lamps which burn will show that the switches corresponding to
+them are off-normal. Simple tests then suffice to show whether they are
+properly or improperly in their off-normal position.
+
+_Release of the First Selector._ As will be shown later, the normal way
+of releasing the switches is from the connector back over the release
+wire. It is sufficient to say at this point that when the proper time
+for release comes, an impulse of current will come back over the second
+selector trunk release wire through the private wiper, to the back
+release relay magnet, and thence to ground through the third wiper of
+the side switch which is in its third position. It may be asked why the
+back release magnet was not energized during the previous operations
+described, when current passed through it. The reason for this is that
+in those previous operations the private magnet was always included in
+series in the circuit and on account of the high resistance of the
+private magnet, sufficient current did not pass through the back release
+magnet to energize it.
+
+When the back release relay is energized, it closes the circuit of the
+release magnet and thus, through the link _11_, draws the double dog
+away from its engagement with the shaft ratchets and at the same time,
+through the link _12_, restores the side switch to its normal position.
+Whenever the release magnet is operated it acts as a relay to close a
+pair of contacts associated with it and thus to momentarily ground the
+release wire of the first selector trunk extending back to the line
+switch. Referring to Fig. 389, it will be seen that this path leads
+through the contacts _13_ and _14_ and the release magnet to the
+battery. It is by this means that the line switch is released, the
+release impulse being relayed back from the first selector.
+
+=Second Selector Operation.= For the purpose of considering the action
+of the second selector, we will go back to the point where the first
+selector had connected with a second selector trunk and where its side
+switch had moved into its third position. In this condition, it will be
+remembered, the trunk line was cut through to a second selector trunk
+and all first selector apparatus cleared from the talking circuit.
+
+The second selector chosen is one corresponding to the thousands group
+as determined by the first digit of the called subscriber's number. The
+circuits of a second selector are shown in Fig. 394 and it must be borne
+in mind that the mechanical arrangements for producing the vertical and
+the rotary movement of the shaft and for operating the side switch are
+practically the same as those of the first selector. As in the first
+selector, the sequence of operation is controlled by the successive
+positions of the side switch, the first position permitting the
+selection of the hundreds corresponding to the vertical impulses, the
+second position allowing the selector to search for an idle trunk in
+that hundred, and the third position cutting the trunk through and
+clearing the circuit of obstructing apparatus.
+
+_First Position of Side Switch._ The first thing that happens when the
+subscriber begins to move his dial in the transmission of the second
+series of selecting impulses is the sending of a preliminary impulse
+over the rotary side of the line. This, in the case of the second
+selector, energizes the rotary relay which, in turn, energizes the
+private magnet; but the private magnet in the case of the second
+selector can do nothing toward the release of the side switch because
+the projection _5'_, on the side switch arm _5_, meets a projection on
+the rear of the selector shaft which thus prevents the movement of the
+side switch arm _5_ until the selector shaft has been moved out of its
+normal position.
+
+Immediately after the establishment of the connection to the selector,
+the second set of selecting impulses comes in over the vertical wire
+from the subscriber's station. These impulses, corresponding in number
+to the hundreds digit, will energize the vertical relay and cause it, in
+turn, to energize the vertical magnet, stepping up the selector shaft to
+the level corresponding to the hundred sought. The single rotary
+impulse, which follows just before the subscriber's dial reaches its
+normal position, will energize the rotary relay of the second selector.
+This, in turn, energizes the private magnet which makes a single
+movement of its armature and allows the escapement finger on the side
+switch arm to move one step and bring the side switch contacts into the
+second position.
+
+[Illustration: Fig. 394. Circuits of Second Selector]
+
+_Second Position of Side Switch._ No detailed discussion of this is
+necessary, since, with the side switch in its second position, the
+actions which occur in causing the wipers of the second selector to seek
+and connect with an idle trunk line, are exactly the same as in the case
+of the first selector. When the second selector wipers finally reach a
+resting place on the bank contacts, the private magnet armature,
+operated during the hunting process, is released and the side switch is
+thus shifted into the third position.
+
+_Third Position of Side Switch._ The moving of the side switch into its
+final position brings about the same state of affairs with respect to
+the second selector that already exists with respect to the first
+selector. The trunk line is cut straight through and all bridge circuits
+or by-paths from it are cut off. The same guarding conditions are
+established to prevent other lines or other pieces of apparatus from
+making connections that will interfere with the one being established,
+and the same provisions are made for working the back release when the
+proper impulse comes from the connector, and for passing this back
+release impulse on to the first selector in the same way that the first
+selector passes it on to the line switch. The line of the calling
+subscriber has now been extended to a connector, and that connector is
+one of a group--usually ten--which alone has the ability to reach the
+particular hundred lines containing the line of the desired subscriber.
+The selection has, therefore, been narrowed down from one in ten
+thousand to one in one hundred.
+
+=The Connector=--_Its Functions._ It has already been stated that the
+connector is of the same general type of apparatus as the first and the
+second selectors. Unlike the first and the second selectors, however,
+the connector is required to make a double selection under the guidance
+of the subscriber. The first selector makes a single selection of a
+group under the guidance of the subscriber and then an automatic
+selection in that group not controlled by the subscriber. So it is with
+the second selector. The connector, however, makes a selection of a
+group of ten under the guidance of the subscriber and then, again under
+the guidance of the subscriber, it picks out a particular one of that
+group.
+
+The connector also has other functions in relation to the ringing of
+the called subscriber and the giving of a busy signal to the calling
+subscriber in case the line wanted is found busy. It has still other
+functions in that the talking current, which is finally supplied to
+connected subscribers, is supplied through paths furnished by it.
+
+_Location of the Connectors._ Connectors are the only ones of the
+selecting switches that are in any sense individual to the subscribers'
+lines. None of them is individual to a subscriber's line, but it may be
+said that a group of ten connectors is individual to a group of one
+hundred subscribers' lines. Since each group of one hundred lines has a
+group of connectors of its own and since each one hundred lines also has
+a line-switch unit of its own, and since the lines of this group must be
+multipled through the bank contacts of the connectors of this individual
+group and through the bank contacts of the line switches of this
+particular unit, it follows that on account of the wiring problems
+involved there is good reason for mounting the connectors in close
+proximity to the line switches representing the same group of lines.
+Some help in the grasping of this thought may result if it be remembered
+that the line switch is, so to speak, the point of entry of a call and
+that the connector is the point of exit, and, in order to reduce the
+amount of wiring and to economize space, the point of exit and the point
+of entry are made as close together as possible.
+
+The relative locations and grouping of the line switches and connectors
+are clearly shown in Fig. 395, which is a rear view of the same
+line-switch unit that was illustrated in Figs. 387 and 388.
+
+[Illustration: GAS ENGINE AND POWER BOARD Citizens' Telephone Co.,
+Racine, Wis. _The Dean Electric Co._]
+
+=Operation of the Connector.= The circuits of the connector are shown in
+Fig. 396. In addition to the features that have been pointed out in the
+first and the second selectors, all of which are to be found, with some
+modifications, perhaps, in the connector, there must be considered the
+features in the connector of busy-signal operation, of ringing the
+called subscriber, of battery supply to both subscribers, and of the
+trunk release operation. These may be best understood by tracing through
+the operations of the connector from the time it is picked up by a
+second selector until the connection is finally completed, or until the
+busy signal has been given in case completion was found impossible. As
+in the first and the second selectors, the sequence of operations is
+determined by the position of the side switch.
+
+[Illustration: Fig. 395. Connector Side of Line-Switch Unit]
+
+[Illustration: Fig. 396. Circuits of Connector]
+
+_First Position of Side Switch._ The connector in a ten-thousand-line
+system is the recipient of the impulses resulting from the third and
+fourth movements of the subscriber's dial. Considering the third
+movement of the subscriber's dial, the first impulse resulting from it
+comes over the rotary side of the line and results in the rotary relay
+attracting its armature once. This results in a single impulse through
+the private magnet which, however, does nothing because the projection
+_5'_ strikes against a projection on the selector shaft. These two
+projections interfere only when the selector shaft is in its normal
+position. Then follows the series of impulses from the subscriber's
+station corresponding to the tens digit in the called subscriber's
+number. These pass over the vertical side of the line and through the
+vertical relay, energizing that relay a corresponding number of times.
+
+The vertical magnet, as in the case of the first and the second
+selectors, is included in the circuit controlled by the vertical relay
+and this results in the connector shaft being stepped up to the level
+corresponding to the particular tens group containing the called
+subscriber's number. It will be noted that the impulses from the
+vertical side of the line, which cause this selection, pass through one
+winding _13_ of the calling battery supply relay. This relay is operated
+by these vertical selecting impulses, but in this position of the side
+switch the closure of its local circuits accomplishes nothing.
+
+Immediately after the tens group of selecting impulses over the vertical
+side of the line, there follows a single rotary impulse from the
+subscriber's station which, as in the case of the first and the second
+selectors, energizes the rotary relay and causes it to give one impulse
+to the private magnet. This impulse is now able, since the shaft has
+moved from its normal position, to release the side switch arm one
+notch, and the side switch, therefore, moves into its second position.
+
+_Second Position of Side Switch._ It is principally in this second
+position of the side switch that the connector selecting function
+differs from that of the first and the second selector. There is no
+trunk to be hunted, but rather the rotary movement of the connector
+wipers must be made in response to the impulses, from the subscriber's
+station, which correspond to the units digit in the selected number. The
+first impulse corresponding to the fourth movement of the subscriber's
+dial is a rotary one, and, as usual, it passes through the rotary relay
+winding and this, in turn, gives an impulse to the private magnet. The
+private magnet at this time has already released the side switch arm to
+its second position, but it is unable to release it further because of a
+feather on the wiper shaft--which projects just far enough to engage the
+lug _5'_, when the shaft is in its normal angular position--thus
+preventing the side switch arm from moving farther than its second
+position.
+
+Then follows over the vertical side of the line the last set of
+selecting impulses corresponding to the units digit. This, as before,
+energizes the vertical relay, but in the second position of the side
+switch, it is to be noted, that the vertical relay no longer controls
+the vertical magnet; the side switch has shifted the control of the
+vertical relay to the rotary magnet. The rotary magnet is, therefore,
+energized a number of times corresponding to the last digit in the
+called number and the wipers of the connectors are thus brought to the
+contacts of the line sought--their final goal. At this point many things
+may happen, and the things that do happen depend on whether the called
+subscriber's line is idle or busy.
+
+Called-Line Busy:--It will first be assumed that the called line is
+busy. The testing operation at the connectors occurs in the second
+position of the side switch. If the called line is busy, it will be
+either because it is connected to by some other connector or because it
+has itself made a call. In the former case the private contacts of that
+line in the banks of all the connectors serving that hundreds group of
+lines will be grounded through the private wiper of some other
+connector. That this is so, may be seen by tracing the circuit from the
+private wiper on the shaft to the third side switch wiper which is
+grounded in the third position; the other connector that has already
+engaged the line will, of course, have its side switch in its final, or
+third position. Again, if the line called is busy, because a call has
+already been made from this line to some other line, the private
+contacts on the connectors corresponding to the line will be grounded,
+as will be seen by tracing from the private bank contacts, which are
+shown in Fig. 396, through the private wire to the line switch, which is
+shown in Fig. 389, and from thence to ground through the springs _19_
+and _20_, which are brought together when the line switch is operated.
+
+In any event, therefore, the determining condition of a busy line is
+that its private bank contacts on all connectors of its group shall be
+grounded. Under the present assumed condition, therefore, the connector
+wipers, which have been brought to the bank contacts of the desired
+line, will find a ground at the private bank contact. The connector
+shaft stops for an instant on the contacts of this busy line and
+immediately there follows over the rotary side of the line the
+inevitable single rotary impulse. This energizes the rotary relay and
+this, as usual, energizes the private magnet. Remembering now that the
+connector side switch is in its second position and that the private
+wiper of the connector has found a ground, we may trace back from the
+private wiper through the third side switch wiper to its second contact;
+thence through the contact springs _14_ and _15_, closed by the private
+magnet; thence through the release magnet; thence through the contact
+springs _16_ and _17_ of the calling battery supply relay to the live
+side of the battery. This calling battery supply relay will, at this
+time, have its core energized because the coil _18_ is in series with
+the rotary relay coil which, as just stated, was energized by the last
+rotary impulse. This series of operations has led to the energizing of
+the release magnet, and, as a result, the double dog of the connector is
+pulled out of the connector shaft ratchets and the shaft and the side
+switch are restored to their normal position.
+
+Busy-Back Signal:--The connector has dropped back to normal in all
+respects. The calling subscriber, not knowing this, presses his ringing
+button. This grounds the vertical side of the line at his station and
+operates the vertical relay at the connector. This steps the shaft of
+the connector up one step and causes the closure of the contacts _19_
+and _20_ at the top of the connector shaft. This establishes a
+connection to a circuit carrying periodically interrupted battery
+current on which an inductive hum is placed. This circuit may be traced
+from this source through the springs _20_ and _19_ to the first wiper of
+the side switch, thence through the normally closed contacts of the
+ringing relay to the rotary side of the line, and the varying potential
+to which this path is subjected produces an inductive flow back to the
+calling subscriber's telephone, and gives him the necessary signal which
+consists of a hum or buzzing noise with which all users of automatic
+systems soon become familiar.
+
+Release on Busy Connection:--The connector, since its last release, has
+been stepped up one notch and must again be released. When the
+subscriber hangs up his receiver after receiving the busy signal, he
+grounds both sides of his line momentarily by the action of the springs
+_21_, _22_, and _23_ of Fig. 384. This operates the rotary and the
+vertical relays on the connector simultaneously and brings together for
+the first time the springs _21_ and _22_ of Fig. 396. This establishes a
+connection from the battery through the springs _16_ and _17_ on the
+calling battery supply relay, thence through the release magnet of the
+connector, thence through the springs _22_ and _21_ of the vertical and
+the rotary relay, thence through the release trunk back to the second
+selector. From here the circuit passes through the private wiper of that
+selector and the back release relay to ground through the third side
+switch wiper which is in the third position. Considering this circuit in
+respect to its action on the connector it is obvious that it energizes
+the release magnet on the connector which restores the connector to
+normal as before. At the second selector this circuit passed through the
+back release relay, which closed a circuit through the release magnet
+and through the back release relay contacts, thence back over the second
+selector release trunk to the back release relay of the first selector,
+and through the third wiper of the side switch on that selector to
+ground, since that side switch also is in its third position. The
+current through this circuit energizes the release magnet of the second
+selector and restores it to its normal position and also energizes the
+back release relay of the first selector. This, in turn, closes the
+circuit from the battery through the release magnet of the first
+selector and contacts of the back release relay to ground. This works
+the release magnet of the first selector and restores that selector to
+normal. The contacts on the first selector release magnet, shown in Fig.
+390, are closed by the action of the release magnet and this closes the
+path from ground back through the first selector release wire, and
+through the contacts _13_ and _14_ of the line switch, through the line
+switch release magnet to battery, and this restores the line switch to
+normal.
+
+The reason for the term _back release_ will now be apparent. The release
+operation at the connector is relayed back to the second selector; that
+of the second selector back to the first selector; and that of the first
+selector back to the line switch. Until this plan was adopted, the
+release magnet of each selector and connector involved in a connection
+was left bridged across the talking circuit so as to be available for
+release; and it sometimes occurred that a first selector would be
+released before a second selector or connector, which latter switches
+would thus be left off-normal until rescued by an attendant. The back
+release plan makes it impossible for the connection necessary for the
+release of a switch to be torn down until the release is actually
+accomplished.
+
+Called Line Found Idle:--It will be remembered that, before the
+digression necessary to trace through the operations occurring upon the
+finding of a busy line, the connector wipers had been brought, by the
+influence of the calling subscriber's impulses, into engagement with the
+contacts of the desired line; that the connector side switch was in its
+second position; and that the final rotary impulse following the last
+series of selecting impulses had not been sent. The condition now to be
+assumed is that the called subscriber's line is free and the private
+wiper, therefore, has found and rests on an ungrounded private bank
+contact. The final rotary impulse which immediately follows will operate
+the rotary relay and this, in turn, will operate the private magnet.
+This happened under the assumed condition that the line was busy, but in
+that case the release magnet was also operated at the same time and
+restored all conditions to normal. Under the present condition the
+operation of the private magnet will perform its usual function and move
+the side switch of the connector into its third position.
+
+_Third Position of Side Switch._ When the side switch of the connector
+moves to its third position, it, as usual, cuts the talking circuit
+straight through from the vertical and the rotary sides of the trunk
+leading from the previous selector to the outgoing terminal of the
+subscriber's line, which may be traced upon Fig. 396 back through the
+line switch, shown in Fig. 389. Several things are to be noted about the
+talking circuit so established: First, the inclusion of the condensers
+in the vertical and the rotary sides of the connector circuit. The
+purpose of this will be referred to later. Second, the inclusion in this
+circuit at the connector of a pair of normally closed contacts in the
+ringing relay. It may be said in passing that the ringing relay
+corresponds exactly in function to a ringing key in a manual
+switchboard. Third, the talking circuit leading from the connector to
+the called subscriber's line passes on one side through the springs _24_
+and _25_ of the bridge cut-off relay of the line switch, which is shown
+in Fig. 389. These springs are normally open and would prevent the
+completion of the talking circuit but for the fact that the bridge
+cut-off relay of the line switch is energized over the private wire
+leading to the connector bank and then through the connector wiper to
+the third side switch wiper which, at this time, is in its third
+position. The talking circuit is thus complete. The operation of this
+bridge cut-off relay on the line switch has not only completed the
+talking circuit but it has also opened the circuit of the trip magnet of
+the line switch so as to prevent the operation of the trip magnet by the
+subscriber on that line in case he should attempt to make a call during
+the interval between the time when his line was connected with by the
+connector and the time when he answers the call.
+
+The third wiper of the connector side switch when moved into its third
+position, puts the ground on all of the private bank contacts of the
+line chosen and thus guards that line against connection by others, as
+already described. It also operates the bridge cut-off relay of the line
+switch as just mentioned.
+
+The fourth wiper of the side switch, when moved into its third position,
+establishes such a connection as will place the ringing relay under the
+control of the vertical relay. This may be seen by tracing from ground
+to the vertical relay springs _23_ and _24_, thence through the normally
+closed upper pair of contacts on the private magnet, thence through the
+fourth wiper on the side switch to its third contact, thence through the
+ringing relay magnet, and through the springs _16_ and _17_ of the
+calling battery supply relay and to battery. The calling battery supply
+relay winding being in series with the vertical relay winding, the two
+operate together and close the two normally open points in the ringing
+relay circuit. This ringing relay acts as an ordinary ringing key and
+connects the generator to the called subscriber's line in an obvious
+manner, at the same time opening the talking circuit back of the ringing
+relay in order to prevent the ringing current chattering the relays in
+the circuit back of it. All that remains now is for the called
+subscriber to respond. When he does he closes the metallic circuit of
+the line through his talking apparatus.
+
+_Battery Supply to Connected Subscriber._ Throughout the whole process
+of building up a connection, it will be remembered that both sides of
+the calling line are connected through the respective vertical and
+rotary relays involved in building up the connection with the live side
+of the battery. At the time when the connection is finally established
+and the called subscriber rung, both sides of the calling line are
+connected through various relay windings to the live side of the
+battery. Such a condition leaves both sides of the line at the same
+potential and, therefore, there is no tendency for current to flow
+through the calling subscriber's talking apparatus, even though it is
+connected across the circuit of the line. It remains, therefore, to be
+seen how these conditions are so changed after the building up of a
+connection as to supply the calling subscriber with talking current.
+
+The calling subscriber can get no current until the called subscriber
+responds. When the connection is first made with the called subscriber's
+line, battery connection to his line is made from the live side of
+battery through the normally closed contacts of the calling battery
+supply relay, thence through the winding _25_ of the called battery
+supply relay to the vertical side of the called line. The grounded side
+of the battery is connected to the rotary side of his line through the
+third wiper of the connector and the coil _26_ of the called battery
+supply relay. As a result, this subscriber receives proper talking
+current through the coils _25_ and _26_, and this relay is operated by
+the flow of this current. The operation of this called battery supply
+relay merely shifts the connection of the rotary side of the calling
+subscriber's line from its normal battery connection, to ground, and
+thus the battery is placed straight across the calling subscriber's line
+so as to supply talking current. This supply circuit to the calling
+subscriber may be traced from the live side of the battery through the
+winding _13_ of the calling battery supply relay and the winding of the
+vertical relay to the vertical side of the line, and from the grounded
+side of battery through the third side switch wiper in its third
+position to the now closed pair of contacts in the called battery supply
+relay through the coil _18_ of the calling battery supply relay and the
+coil of the rotary relay to the rotary side of the line.
+
+It will be noted that the system of battery supply is that of the
+standard condenser and retardation coil scheme largely employed in
+manual practice; and that aside from the coils through which the battery
+current is supplied to the connected subscribers, there are no taps
+from, or bridges across, the two sides of the talking circuit.
+
+=Release after Conversation.= It remains now only to secure the
+disconnection of the subscribers after they are through talking. When
+the calling subscriber hangs up, the whole disconnection is brought
+about, all of the apparatus, including connector, selectors, and line
+switch, returning to normal. This is done by the back release system and
+is accomplished in almost the same way as has already been described in
+connection with the disconnect after an unsuccessful call. There is this
+difference, however: after an unsuccessful call when the line called for
+was found busy, the release was made while the connector side switch was
+in its normal position. In the present case, the release must be made
+with the connector side switch in its third position and with the
+talking battery bridged across the metallic circuit rather than
+connected between each limb of the line and ground. It must be
+remembered that the calling battery supply relay, while traversed by
+current during the conversation, is not magnetically energized because,
+with the current flowing through the metallic circuit of the line, the
+two windings exert a differential effect. As soon, however, as the
+calling subscriber hangs up his receiver, this differential action
+ceases, due to the grounding of both sides of the line at the
+subscriber's station. This relay, therefore, operates and cuts off
+battery from the called battery supply relay and this, in turn, releases
+its armature and thus changes the connection of the rotary side of the
+calling line from ground to live side of the battery. The normal
+condition of the battery connection now being restored, both the
+vertical and the rotary relays at the connector become operated, due to
+the ground on both sides of the line at the subscriber's station, and
+this, as we have seen, is the condition which brings about the operation
+of the connector release magnet, and the relaying back of the disconnect
+impulse successively through the selectors to the line switch.
+
+=Multi-Office System.= In exchanges involving more than one office, the
+same general principles and mode of operation already outlined apply. If
+the total number of subscribers in the multi-office exchange is to be
+less than ten thousand, then four digit numbers suffice, and the first
+movement of the dial may be made to select the office into which the
+connection is to go, the subscribers' lines being so numbered with
+respect to the offices that each office will contain only certain
+thousands. The choosing of the thousand by the calling subscriber,
+therefore, takes care in itself of the choice of offices. Where,
+however, a multi-office exchange is to provide for connections among a
+greater number of lines than ten thousand and less than one hundred
+thousand, then it will take five movements of the dial to make the
+selection--the five movements corresponding either to the five digits in
+a number or to the name of an office, as indicated on the dial, and the
+four digits of a smaller number. The lines may all carry five digit
+numbers or, what is considered better practice, may be designated by an
+office name followed by a four digit number. In this latter case the
+numbers of the subscribers' lines will in each case be contained in one
+or more of the tens of thousands groups, no number having more than four
+digits. And the first movement of the dial, whether the name or number
+plan be adopted, will select an office; or, looking at it another way,
+will select a group of ten thousand and this being done, the next four
+successive movements of the dial will select the numbers in that ten
+thousand in just the some way that has been already described.
+
+Certain difficulties arise, however, in multi-office working due to the
+fact that the three-wire trunks between offices would in most cases be
+objectionable. As long as the trunks extend between the various groups
+of apparatus in the same office, it is cheaper to provide three wires
+for each of them than it is to make any additional complication in the
+apparatus. Where the trunking is done between offices, however, the
+system may be so modified as to work over two wire inter-office trunks.
+
+_The Trunk Repeater._ The purpose of the trunk repeater is to enable the
+inter-office trunking to be done over two wires. It may be said that the
+trunk repeater is a device placed in the outgoing trunk circuit at the
+office in which a call originates, which will do over the two wires of
+the trunk leading from it to the distant office just the same thing that
+the subscriber's signal transmitter does over the two wires of the
+subscriber's lines. It has certain other functions in regard to feeding
+the battery for talking purposes back to the calling subscriber's line,
+taking the place in this respect of the calling battery feed relay in
+the connector in a single office exchange.
+
+[Illustration: Fig. 397. Circuits of Trunk Repeater]
+
+The circuits of a trunk repeater are shown in Fig. 397. In considering
+it, it must be understood that the three wires entering the figure at
+the left are the vertical, rotary, and release wires of a second
+selector trunk leading from the first selector banks in the same office.
+The two wires leading from the right of the figure are those extending
+to the distant office, and terminate there in second selectors. The
+vertical and the rotary sides of this trunk as shown at the left will
+receive the impulses from the subscriber's station coming through the
+line switch and the first selector, as usual. The vertical impulses will
+pass through the winding of the vertical relay and through the winding
+_1_ of the calling battery supply relay and thence to battery, the same
+as on a connector. These impulses will work the armatures of both of
+these relays in unison. The movements of the vertical relay armature in
+response to these impulses will cause corresponding impulses to flow
+over a circuit which may be traced from ground, through the springs _3_
+and _2_ of the vertical relay, the springs _4_ and _5_ of the bridged
+relay _6_ and thence to the vertical side of the trunk and to the
+distant office, where it passes into a second selector and through its
+vertical relay to battery. Thus the vertical impulses are passed on over
+the two-wire trunk to the second selector at the distant office. It
+becomes necessary, however, to prevent these impulses from passing back
+through the winding of the bridge relay _6_ and this is done by means of
+the sluggish relay _7_. This relay receives local battery impulses in
+unison with those sent over the trunk by the vertical relay, these being
+supplied from the battery at the local office through the contacts _8_
+and _9_ of the calling battery supply relay, which works in unison with
+the vertical relay. These rapidly recurring impulses are too fast for
+the sluggish relay _7_ to follow. And this relay merely pulls up its
+armature and cuts off both sides of the trunk leading back to the first
+selector. The rotary impulses are repeated to the rotary side of the
+two-wire trunk in a similar way.
+
+Considering now the operation of the trunk repeater in the reverse
+direction, the action of the bridging relay _6_ is of vital importance.
+Normally both sides of trunk line are connected to the live side of the
+battery and, therefore, there is no difference of potential between them
+and no tendency to operate the bridged relay. When the connection has
+been fully established to the subscriber at the distant office, and that
+subscriber has responded, the action of his battery supply relay will,
+as before stated, change the connection of the rotary side of the line
+from battery to ground, and thus bridge the battery at the distant
+exchange across the trunk. This action will pull up the bridged relay
+_6_ at the trunk repeater and will perform exactly the same function
+with respect to the connection of the battery with the calling
+subscriber's line. In other words, it will change the connection of the
+rotary side of the calling line from battery to ground, thus
+establishing the necessary difference in potential to give the calling
+subscriber the necessary current for transmission purposes. The
+disconnect feature is about the same as already described. When the
+calling subscriber hangs up his receiver both the vertical and rotary
+relays of the trunk repeater operate, which places the ground on both
+sides of the two-wire trunk to the distant office, which is the
+condition for releasing all of the apparatus there.
+
+For the purpose of convenience the simplified diagram of Fig. 398 has
+been prepared, which shows the complete connection from a calling
+subscriber to a called subscriber in a multi-office exchange, wherein
+the first movement of the dial is employed to establish the connection
+to the proper office and the four succeeding movements to make a
+selection among ten thousand lines in that office. This circuit,
+therefore, employs at the first office the line switch, the first
+selector, and the trunk repeater; and at the second office the second
+selector, third selector, connector, and line switch.
+
+The third selector is omitted from Fig. 398, but this will cause no
+confusion, since it is exactly like the second selector. The circuits
+shown are exactly like those previously described but in drawing them
+the main idea has been to simplify the connections to the greatest
+possible extent at a sacrifice in the clearness with which the
+mechanical inter-relation of parts is shown. No correct understanding of
+the circuits of an automatic system is possible without a clear idea of
+the mechanical functions performed by the different parts, and,
+therefore, we have described what are apparently the more complex
+circuit drawings first. It is believed that the student, in attempting
+to gain an understanding of this marvel of mechanical and electrical
+intricacy, will find his task less burdensome if he will refer freely to
+both the simplified circuit drawing of Fig. 398 and the more complex
+ones preceding it. By doing so he will often be enabled to clear up a
+doubtful circuit point from the simpler diagram and a doubtful
+mechanical point from those diagrams which represent more clearly the
+mechanical relation of parts.
+
+[Illustration: Fig. 398. Connection between a Calling and a Called
+Subscriber in an Automatic System]
+
+=Automatic Sub-Offices.= Obviously, the system of trunking employed in
+automatic exchanges lends itself with great facility to the subdivision
+of an exchange into a large number of comparatively small office
+districts and the establishment of branch offices or sub-offices at the
+centers of these districts.
+
+The trunking between large offices has already been described. An
+attractive feature of the automatic system is the establishment of
+so-called sub-stations or sub-offices. Where there is, in an outlying
+district, a distinct group of subscribers whose lines may readily be
+centered at a common point within that district and where the number of
+such subscribers and lines is insufficient to establish a fully equipped
+office, it is possible to establish a so-called sub-station or
+sub-office connected with the main office of that district by trunk
+lines. At this sub-office there are placed only line switches and
+connectors. When a call is originated on one of these sub-office lines,
+the line switch acts instantly to connect that line with one of the
+trunks leading to the main office of that district, at which this trunk
+terminates in a first selector. From there on, the connection is the
+same as that in a system in which no sub-offices are employed. Calls
+coming into this sub-office over trunk lines from the main office are
+received on the connectors at the sub-office and the connection is made
+with the sub-office line by the connector in the usual manner. This
+arrangement, it is seen, amounts merely to a stretching of the connector
+trunks for a given group of lines so that they will reach out from a
+main office to a sub-office, it being more economical to lengthen the
+smaller number of trunks and by so doing to decrease in length the
+larger number of subscribers' lines.
+
+=The Rotary Connector.= For certain purposes it becomes desirable in
+automatic work to employ a special form of connector which will have in
+itself a certain ability to make automatic selection of one of a group
+of previously chosen trunks in much the same manner as the first and
+second selectors automatically choose the first idle one of a group of
+trunks.
+
+Such a use is demanded in private branch-exchange working where a given
+business establishment, for instance, has a plurality of lines
+connecting its own private switchboard with the central office. The
+directory number of all these lines is, for convenience, made the same,
+and it is important, therefore, that when a person attempts to make a
+connection with this establishment, he will not fail to get his
+connection simply because the first one of these lines happens to be
+busy. For such use a given horizontal row of connector terminals or a
+part of such a row is assigned to the lines leading to the private
+branch exchange and the connector is so modified as to have a certain
+"discretionary" power of its own. As a result, when the common number of
+all these lines is called, the connector will choose the first one, if
+it is not already engaged by some other connector, but if it is, it will
+pass on to the next, and so on until an idle one is found. It is only
+when the connector has hunted through the entire group of lines and
+found them all busy that it will refuse to connect and will give the
+busy signal to the calling subscriber.
+
+=Party Lines.= The description of this system as given above has been
+confined entirely to direct line working; however, party lines may be
+and are frequently employed.
+
+The circuits and apparatus used with direct lines are, with slight
+modifications, applicable to use with party lines.
+
+The harmonic method of ringing is employed and the stations are so
+arranged with respect to the connectors that those requiring the same
+frequency for ringing the bells are in groups served by the same set of
+connectors.
+
+[Illustration: POWER MACHINERY Citizens' Telephone Company, Racine, Wis.
+_The Dean Electric Co._]
+
+The party lines are operated on the principle commonly known in manual
+practice as the jack per station arrangement. Each party line will,
+therefore, have sets of terminals appearing in separate hundreds; the
+connectors associated with each of these hundreds being so arranged as
+to impress the proper frequency of ringing current on the line.
+
+From the subscribers' standpoint the operation is the same as for direct
+lines, as the particular hundreds digit of a number serves to select one
+of a group of connectors capable of connecting the proper ringing
+current to the line.
+
+To avoid confusion, which would be caused by a subscriber on a party
+line attempting to make a call when the line is already in use by some
+other subscriber, the subscribers' stations are so arranged that when
+the line is in use all other stations on the line are locked out.
+
+[Illustration: Fig. 399. Wall Set for Two-Wire System]
+
+=The Two-Wire Automatic System.= The two-wire system that has recently
+been introduced by the Automatic Electric Company brings about the very
+important result of accomplishing all of the automatic switching over
+metallic circuit lines without the use of ground or common returns. The
+system is thus relieved of the disturbing influences to which the
+three-wire system is sometimes subjected, due to differences in earth
+potential between various portions of the system, which may add to or
+subtract from the battery potential and alter the net potential
+available between two distant points. The introduction of this system
+has also made possible certain other incidental features of advantage,
+one of which is a great simplification and reduction in size of the
+subscriber's station signal-transmitting apparatus.
+
+With the doing away of the ground as a return circuit, it becomes
+impossible to send vertical impulses over one side of the line and to
+follow them by single rotary impulses over the other side of the line.
+Yet it becomes necessary to distinguish between the pure selective
+impulses and those impulses which dictate a change of function at the
+central office. The plan has, therefore, been adopted of accomplishing
+the selection in each case by short and rapidly recurring impulses and
+of accomplishing those functions formerly brought about by the single
+impulse over the rotary side of the line by a pause between the
+respective series of selective impulses. This is accomplished at the
+central office by replacing the vertical and the rotary relays of the
+three-wire system by a quick-acting and a sluggish relay, respectively;
+the quick-acting relay performing the functions previously carried out
+by the vertical relay, and the sluggish relay acting only during the
+pauses between the successive series of quick impulses to do the things
+formerly done by the rotary relay. This has resulted in a delightful
+simplification of subscriber's apparatus, since it is now necessary only
+to provide a device which will connect the two sides of the line
+together the required number of times in quick succession and then allow
+a pause with the circuit closed while the subscriber is getting ready to
+transmit another set of impulses corresponding to another digit. The
+calling device has no mechanical function co-acting with any of the
+other parts of the telephone and may be considered as a separate
+mechanical device electrically connected with the line. The transmitting
+device is not much larger than a large watch and a good idea of it may
+be had from Fig. 399, which shows the latest wall set, and Fig. 400,
+which shows the latest desk set of the Automatic Electric Company. We
+regret the fact that this company has made the request that the complete
+details of their two-wire system be not published at this time.
+
+[Illustration: Fig. 400. Desk Stand for Two-Wire System]
+
+
+
+
+CHAPTER XXX
+
+THE LORIMER AUTOMATIC SYSTEM
+
+
+The Lorimer automatic telephone system has not been commercially used in
+this country but is in commercial operation in a few places in Canada.
+It is interesting from several points of view. It was invented, built,
+and installed by the Lorimer Brothers--Hoyt, George William, and
+Egbert--of Brantford, Ontario. These young men without previous
+telephonic training and, according to their statements, without ever
+having seen the inside of a telephone office, conceived and developed
+this system and put it in practical operation. With the struggles and
+efforts of these young men in accomplishing this feat we have some
+familiarity, and it impresses us as one of the most remarkable inventive
+achievements that has come to our attention, regardless of whatever the
+merits or demerits of the system may be.
+
+The Lorimer system is interesting also from the fact that, in most
+cases, it represents the mechanical rather than the electrical way of
+doing things. The switches are power driven and electrically controlled
+rather than electrically driven and electrically controlled, as in the
+system of the Automatic Electric Company.
+
+The subscriber's station apparatus consists of the usual receiver,
+speech transmitter, call bell, and hook switch, and in addition a signal
+transmitter arranged to be manipulated by the subscriber so as to
+control the operation of the central-office apparatus in connecting with
+any desired line in the system.
+
+The central-office apparatus is designed throughout upon the principle
+of switching by means of power-driven switches which are under the
+control of the signal transmitters of the calling subscriber's station.
+The switches employed in making a connection are all so arranged with
+respect to constantly rotating shafts that the movable member of such
+switches may be connected to the shafts by means of electromagnets
+controlled directly or indirectly by relays, which, in turn, are brought
+under the control of the signal transmitters.
+
+The circuits are so designed in many instances that the changes
+necessary for the different steps are brought about by the movement of
+the switches themselves, thus permitting the use of circuits which are
+rather simple. The switches employed are all of a rotary type; the
+co-ordinate selection, which is accomplished in the Automatic Electric
+Company's system by a vertical and rotary movement, being brought about
+in this system by the independent rotation of two switches.
+
+=Subscriber's Station Equipment.= A subscriber's desk-stand set, except
+the call bell, is shown in Fig. 401, and a wall set complete in Fig.
+402. In both of these illustrations may be seen the familiar
+transmitter, receiver, and hook switch, and in the wall set, the call
+bell. The portion of these telephone sets which is unfamiliar at present
+is the part which is enclosed in the enlarged base of the desk stand and
+the protruding device below the speech transmitter in the wall set--the
+signal transmitter referred to earlier in the chapter. The small push
+button and small plate through which the number may be seen directly
+below the transmitter in Fig. 402, are for the purpose of registering
+calls.
+
+[Illustration: Fig. 401. Lorimer Automatic Desk Stand]
+
+The signal transmitter is a device whose function is to record
+mechanically the number of the subscriber's station with which
+connection is desired, and to transmit that record to the central office
+by a system of electrical impulses over the line conductors. Instead of
+operating by its own initiative, the signal transmitter is adapted to
+respond to central-office control in transmitting electrically the
+number which has been recorded mechanically upon it.
+
+The signal transmitter shown removed from the base of the desk stand at
+the left in Fig. 403 comprises in part four sets of contact pins having
+ten pins in each set, one set for each of the digits of a four-digit
+number. There are also several additional contact pins for signaling and
+auxiliary controlling purposes. All of these contact pins are arranged
+upon the circumference of a circle and a movable brush mounted upon a
+shaft at the center of the circle is adapted to be rotated by a clock
+spring and to make contact with each of the pins successively. The call
+is started, after the number desired has been set on the dial, by giving
+the crank at the right of the signal transmitter a complete turn and
+thus winding the spring. The shaft carrying the signal transmitter brush
+carries also an escapement wheel, the pallet of which is directly
+controlled by an electromagnet.
+
+[Illustration: Fig. 402. Lorimer Automatic Wall Set]
+
+The four dials with the numerals printed on them are attached to four
+levers, respectively, and are moved by their levers opposite windows,
+near the top of the casing. Through each of these windows a single
+numeral may be seen on the corresponding one of the dials. The dials may
+be adjusted so that the four numerals seen will read from left to right
+to correspond to the number of the line with which connection is
+desired.
+
+The setting of the dials so that the number desired shows at the small
+circular opening results in connecting the earth or a common return
+conductor to one pin of each set of ten pins, the pin grounded in each
+set depending upon the numerical value of the digit for which the dial
+is set.
+
+The circle of contact pins is set in an insulating disk, the signal
+transmitting brush operates upon the pins on one side of the disk, and
+electrical fingers attached to the dials operate upon the pins on the
+other side of the disk. The escapement wheel is a single toothed disk
+attached directly to the shaft which carries the signal brush and its
+pallet is attached rigidly to the magnet armature.
+
+[Illustration: Fig. 403. Desk Stand with Signal Transmitter Removed]
+
+Once a call has been turned in, the entire subscriber's station
+equipment is locked beyond power of the subscriber to tamper with it in
+any way, rendering it impossible either to defeat the call which has
+been started or to prevent the subscriber's station as a whole from
+returning completely to normal position and thus restoring itself for
+regular service. The key shown just below the signal transmitter in the
+case of the desk stand, and at the right in the wall set, is for the
+purpose of operating a relay at the central office which, in turn,
+connects ringing current to the line of the subscriber with which
+connection has been made, and thus actuates the call bell.
+
+As the number set up at the signal transmitter remains in full view
+until reset for some other number, it is easily checked by inspection
+and also lessens the labor involved in making a second call for the same
+line, which is frequently necessary when the line is found busy the
+first time called.
+
+=Central-Office Apparatus.= The subscriber's lines are divided into
+groups of one hundred lines each at the central office, each group being
+served by a single unit of central-office apparatus. In a
+central-office unit there is "sectional apparatus" which appears but
+once for the unit of one hundred lines; "divisional apparatus" which
+appears a number of times for each unit, depending upon the traffic; and
+"line apparatus" which appears one hundred times for each unit or once
+for each line.
+
+The sectional apparatus comprises devices whose duties are, first, to
+detect a calling line, and second, to assign to the calling line a set
+of idle divisional apparatus which serves to perform the necessary
+switching functions and complete the connection.
+
+The sets of divisional apparatus, or, as called in this system,
+"divisions," are common to a section and are employed in a manner
+similar to the connecting cords of a manual switchboard. The number of
+these divisions provided for each section is, therefore, determined by
+the number of simultaneous connections resulting from calls originating
+in the section. It has been the custom in building this apparatus to
+provide each section with seven divisions or connective elements.
+
+The line apparatus comprises one relay, having a single winding, and two
+pairs of contacts operated by its armature. This device is substantially
+the well known cut-off relay almost universally employed in
+common-battery systems. The fixed multiple contacts of the lines in the
+switching banks of the connecting apparatus are considered as pertaining
+to the various pieces of apparatus on which they are found rather than
+to their respective lines. A good idea may be obtained of the
+arrangement of the sectional and divisional apparatus by referring to
+Fig. 404, which is one unit of a thousand-line equipment. The apparatus
+in the vertical row at the extreme left of the illustration is the
+sectional apparatus, while the remaining seven vertical rows of
+apparatus are the divisions.
+
+_The Section._ The sectional apparatus for each unit consists of three
+separate devices called for convenience a _decimal indicator_, a
+_division starter_, and a _decimal-register controller_. All of these
+devices are normally motionless when idle. The energization of the
+decimal indicator, in response to the inauguration of a call at a
+subscriber's station, results immediately in an action of the division
+starter which starts a division to connect with the line calling. It
+results also in the starting of the decimal-register controller, the
+remaining unit of sectional apparatus.
+
+It is thus seen that upon the starting of a call by a subscriber, all
+of the sectional apparatus belonging to his one hundred lines
+immediately becomes active, the division starter acting to start a
+division, the decimal indicator becoming energized to indicate the tens
+group in which the call has appeared, and the decimal-register
+controller becoming active to adjust the decimal register of the
+division assigned by the division starter. The division starter having
+assigned a division for the exclusive use of this particular call,
+passes to a position from which it may start a similar idle division
+when the next call is received. The decimal register controller makes
+its half revolution for the call and comes to rest, awaiting a
+subsequent call, and the decimal indicator continues energized but only
+momentarily, since it is released by the action of the cut-off relay
+when the call is taken in charge by the divisional connective devices.
+
+Calls may follow each other rapidly, the connective devices being
+entirely independent of each other after having been assigned to the
+respective calling lines. As has been described, the decimal indicator
+starts the division starter and the decimal-register controller in quick
+succession. The division starter, shown at the extreme bottom of the
+left-hand row of Fig. 404, is a cylinder switch of the same general type
+as used throughout this system. In it the terminals of a switch in each
+division appear as fixed contact points in a circle over which move the
+brushes of the division starter.
+
+The decimal-register controller has the duties of transmitting to the
+divisional apparatus a series of current impulses corresponding in
+number to the numerical value of the tens digit of the calling line.
+This is effected by providing before a movable brush ten contacts from
+which the brush may receive current. These contacts are normally not
+connected to battery, so that the brush in passing over them does not
+receive current from them; however, when the brush has reached the
+contact corresponding in number to the tens digit of the calling line, a
+relay associated with the decimal-register controller charges the
+contacts with the potential of the main battery, and each of the
+remaining contacts passed over by the brush sends a current impulse to a
+device designed to indicate on the division selected for the call the
+tens digit of the calling line.
+
+_The Connective Division._ The connective division, seven of which are
+shown in Fig. 404, is an assemblage of switches comprising, as a whole,
+a set suitable for a complete connection from calling to called
+subscriber. Each connective division in the unit illustrated is
+completely equipped to care for a called number of three digits, _i.
+e._, each division will connect its calling line with any one of one
+thousand lines which may be called. By a system of interconnecting
+between divisions, each division may be equipped with interconnecting
+apparatus so as to make it possible to complete a call with any one of
+ten thousand lines. Each connecting division of a ten-thousand-line
+exchange comprises six major switches. Of the six major switches, one is
+termed a _secondary connector_, another an _interconnector_, and the
+four remaining are termed the _primary portion_ of the division.
+
+[Illustration: Fig. 404. Unit of Switching Apparatus]
+
+Before taking up the operation of the switches, the mechanical nature of
+the switches themselves will be described. The switches are built with a
+contact bank cylindrical in form and with internal movable brushes
+traveling in a rotary manner in circular paths upon horizontal rows of
+contacts fixed in the cylindrical banks. For driving these brushes a
+constantly rotating main power-driven shaft is provided. Between each
+shaft and the rotating brushes of each major switch is an electric
+clutch, which, by the movement of an armature, causes the brushes of the
+switch to partake of the motion of the shaft and by the return of the
+armature to come again to rest. The motion of the brushes of the major
+switches, or cylinder switches, as they are frequently called because of
+their form, is constantly in the same direction. They have a normal
+position upon a set of the cylinder contacts. They leave their normal
+position and take any predetermined position as controlled by the
+magnets of the clutch, and, having served the transient purpose, they
+return to their normal position by traversing the remainder of their
+complete revolution and stopping in their position of rest or idleness.
+
+The mechanical construction of each of the cylinder switches is such
+that it may disengage its clutch and bring its brushes to rest only with
+the brushes in some one of a number of predetermined positions. The
+locations of the brushes in these positions of rest, or "stop"
+positions, as they are called, may differ with the different cylinder
+switches, according to the nature of the duty required of the switch,
+and the total number of stop positions also may vary. The primary and
+secondary connectors, the interconnector selectors, and the
+interconnectors each have eleven stop positions; the rotary switch has
+eight stop positions; the signal-transmitter controller has but two.
+
+In the six cylinder switches making up a connective division and
+required for any conversation, in a ten-thousand-line exchange some of
+the switches are set to positions which are determined by the control of
+the calling subscriber and represent by their selective positions the
+value of some digit of the calling or called subscriber's number. Others
+are switches controlling the call in its progress and controlling the
+switches responsive to the call. These latter switches take positions
+independent of the numbers.
+
+In addition to the major switches, there are upon each division four
+minor switches termed _registers_. Each consists of an arc of fixed
+contacts accompanied by a set of brushes which sweep over the contacts.
+Instead of being driven by an electromagnet, the register brushes are
+placed under tension of a spring which tends at all times to draw them
+forward. They are then restrained by an escapement device similar to a
+pallet escapement in a clock, the pallet being controlled by the
+register's magnets. When a series of impulses are received by the
+register magnets, the pallet is actuated a corresponding number of times
+and the register brushes are permitted to move forward under tension of
+their powerful propelling spring. Each register is associated with a
+major switch, and the register brushes are engaged by a cam upon the
+associated major switch, and are restored to normal position against the
+tension of their propelling spring, the force of restoration being
+obtained from the main shaft.
+
+The electrical clutches which connect and disconnect the movable brushes
+of the major switches from the main driving shaft are controlled in all
+instances by circuits local to the central office. In some instances
+these circuits include relay contacts and are controlled by a relay. In
+other instances they are formed solely through switch contacts. In all
+cases the control, when from a distance, is received upon relays
+suitable for being controlled by the small currents which are adapted to
+flow over long lines. In all instances the power for moving a brush is
+derived from the main shaft and only the control of the movement is
+derived from electromagnets, relays, or other electric sources. In many
+instances the clutch circuit is closed through contacts of its own
+switch and, therefore, may be closed only when its switch is in some
+predetermined position. All of the switches are mechanically powerful
+and designed particularly to sustain the wear of long-continued and
+oft-repeated usage. This is true also of the moving parts which carry
+the brushes and of the journals sustaining those parts.
+
+_The Switches of the Connective Division._ The six major switches of the
+connecting division are as follows:
+
+The Primary Connector:--The function of this switch is to connect the
+conductors of the calling line with the switching devices of the
+connective division. Associated with this switch is a register termed
+the _decimal register_. The one hundred lines of the section are
+terminated in fixed multiple contacts in the cylinder switch of the
+primary connector. The calling line is selected and connected with by
+adjusting the decimal register to a position corresponding to the
+calling line's tens digit and adjusting the brushes of the cylinder
+switch to a position corresponding to the calling line's unit digit.
+
+The Rotary Switch:--This is a master switch, or pilot switch,
+consisting of a cylinder switch without register. Its duty is the
+control of other switches and the completion of circuits formed in part
+through other switches. It is the pilot switch and the switch of
+initiative and control for the entire connective division.
+
+Signal-Transmitter Controller:--The primary function of this switch is
+the generation of signaling impulses of two classes. Impulses of the
+first class pass over central-office circuits only and are effective
+upon magnets of the divers major and minor switches; impulses of the
+second class pass over a line conductor of the calling line and are
+effective upon the signal transmitter at the subscriber's station. The
+impulses sent out over the line to the subscriber's station cause the
+brush to pass over the contacts and thereby indicate the numerical
+values of the various digits set by the dials. This switch also enters
+in an important manner into the circuits involved in the testing of the
+called line for the busy condition. It is controlled by the rotary
+switch.
+
+Interconnector Selector:--In an exchange using four digits in the
+numbers, the register of the interconnector selector is adjusted in each
+call to a position corresponding to the numerical value of the thousands
+digit of the called number. The cylinder switch then acts to select an
+idle trunk. The switch is controlled by the rotary switch in connection
+with the signal transmitter controller.
+
+Interconnector:--This switch is similar to the interconnector selector
+in design and in function. It is a cylinder switch with register. The
+register is adjusted in each call to a position corresponding to the
+numerical value of the hundreds digit of the number called and the
+cylinder switch then operates to select an idle trunk. The switch is
+controlled by the rotary switch in connection with the signal
+transmitter controller.
+
+Secondary Connector:--This switch contains in its cylinder bank of
+contacts the multiple points of one hundred subscribers' lines and its
+function is to connect the conductors of the called line to the
+conductors of the connective division. This is accomplished by adjusting
+the register to correspond to the value of the tens digit of the line
+desired and by adjusting the cylinder brushes to correspond to the value
+of the units digit of the line. The switch is controlled by the rotary
+switch in connection with the signal-transmitter controller.
+
+=Operation.= A brief description of the progress of a call from its
+institution to the complete connection and subsequent disconnection
+begins with the adjustment of the dial indicators of the telephone set
+and the turning of the crank of the signal transmitter one revolution.
+This act, performed by the calling subscriber, connects one of the line
+conductors to earth. Immediately the decimal indicator associated with
+the section in which the calling line terminates is energized and starts
+the division starter. The division starter instantly starts the rotary
+switch of an idle division. The rotary switch now starts the
+decimal-register controller and connects to it the decimal register of
+the primary connector of the division selected.
+
+All of the above acts in the central office occur practically
+simultaneously. The impulses generated by the controller are effective
+upon the decimal register of the started division and, therefore, adjust
+that register to a position corresponding to the tens value of the
+calling line.
+
+The rotary switch now disconnects the tens register and starts the
+cylinder brushes of the primary connector which automatically stop when
+they encounter the calling line. At this instant the cut-off relay of
+the line is energized and the decimal indicator is released. The call
+now is clear of all sectional apparatus and another call may come
+through immediately, being assigned in charge of another idle division.
+
+The total time in which any call is in charge of the sectional
+apparatus, _i. e._, the total time from the grounding of the line
+conductor at the sub-station until the line has been connected with by
+the primary connector of some division of that section and the sectional
+apparatus has been released by the operation of the cut-off relay,
+approximates two-fifths of a second.
+
+The next operation initiated by the rotary switch is the starting of the
+signal-transmitter controller of the connective division, which, in
+turn, adjusts the register of the interconnector selector to a position
+corresponding to the thousands digit of the number of the called line as
+indicated by the signal transmitter at the calling station. This selects
+an interconnector serving the lines of the selected thousand.
+
+This initial selection being completed the rotary switch readjusts the
+circuits of the connective division in such manner that in the further
+progress of the signal-transmitter controller, its impulses will be
+effective upon the register of the selected interconnector. In this
+manner, the register of the interconnector, which may be upon the same
+connective division as the rotary switch handling the call, or which may
+be the interconnector of some other division, as determined by the
+number of the called subscriber, is adjusted to a position corresponding
+to the second or hundreds digit of the number called. The cylinder
+switch of the interconnector then selects and appropriates an idle trunk
+extending to a secondary connector upon some connective division serving
+the hundred selected.
+
+The rotary switch again shifts the circuits of the connective division
+in such manner that the signal-transmitter controller is effective upon
+the secondary connector, both register and cylinder, and adjusts the
+register and cylinder, respectively, with their brushes in contact with
+the tens and units digits, respectively, of the number of the called
+line.
+
+The conductors of the called line now are connected through the
+secondary connector, the interconnector, and the interconnector selector
+to the rotary switch; the conductors of the calling line are connected
+through the primary connector to the rotary switch; thus completely
+connecting the lines except at the rotary switch. To effect the
+connecting together of the two lines, both rotary switch and
+signal-transmitter controller must pass forward into their next
+positions, the connection when thus effected being made through
+conductors containing a repeating coil and main battery connection for
+supplying talking current to the two lines and containing also ringing
+and supervisory relays.
+
+The called line is tested to determine if busy during the short interval
+in which the rotary switch takes a short step to connect the calling and
+the called lines. In this step of the rotary switch the busy-test relay
+is connected to the guard wire or busy-test wire of the called line, and
+if that line be busy, the relay interferes with the control exercised by
+the rotary switch upon the signal-transmitter controller, and the
+controller is prevented from taking the step required to connect the
+line. Thus, when a busy line is encountered, the final step of the
+rotary switch is taken to set up the conversation conditions, but the
+signal-transmitter controller does not take its final step; by this
+failure of the signal-transmitter controller due to the action of the
+busy-test relay, the calling line is not connected to the called line
+but is connected to a busy-back tone generator instead.
+
+Whether the line encountered be busy or idle, the connective division
+remains in its condition as then adjusted until the subscriber hangs his
+receiver upon the hook switch to obtain disconnection. The ringing of
+the bell of the called station is done directly by the calling
+subscriber in pressing the ringing key.
+
+The disconnection is effected, when the receiver of the calling line is
+hung up, by the supervisory relay in the central office, whose winding
+is included in the line circuit, and whose contacts act directly to
+start the rotary switch. In disconnecting, the rotary switch starts the
+primary and the secondary connectors and thus instantly releases both
+the calling and the called lines. Thereafter the rotary switch in
+passing from position to position restores switch after switch of the
+connective division to normal and finally itself returns to normal in
+preparation for its assignment to service in answering a subsequent
+call.
+
+
+
+
+CHAPTER XXXI
+
+THE AUTOMANUAL SYSTEM
+
+
+Two systems of telephony are now in common use in this country--the
+manual system and the automatic. With the growth of the automatic, and
+the gradually ripening conviction, which is now fully matured in the
+minds of most telephone engineers, that automatic switching is
+practical, there has been a growing tendency toward doing automatically
+many of the things that had previously been done manually. One of the
+results of this tendency has been the production of the _automanual_
+system, the invention of Edward E. Clement, an engineer and patent
+attorney, of Washington, D. C. In connection with Mr. Clement's name, as
+inventor, must be mentioned that of Charles H. North, whose excellent
+work as a designer and manufacturer has contributed much toward the
+present excellence of this highly interesting system.
+
+=Characteristics of System.= The name "automanual" is coined from the
+two words, automatic and manual, and is intended to suggest the idea
+that the system partakes in part of the features of the automatic system
+and in part of those of the manual system.
+
+We regret that neither space nor the professional relation which we have
+had with the development of this system will permit us to make public an
+extended and detailed description of its apparatus and circuits. Only
+the general features of the system may, therefore, be dealt with.
+
+[Illustration: POWER APPARATUS FOR COMMON-BATTERY MANUAL OFFICE OF
+MEDIUM SIZE]
+
+The underlying idea of the automanual system is to relieve the
+subscriber of all work in connection with the building up of his
+connection, except the asking for it; to complicate the subscriber's
+station equipment in no way, it being left the same as in the
+common-battery manual system; to do away with manual apparatus, such as
+jacks, cords and plugs, at the central office, and to substitute for it
+automatic switching apparatus which will be guided in its movements,
+not by the subscriber, but by a very much smaller number of operators
+than would be necessary to manipulate a manual switchboard.
+
+=General Features of Operation.= A broad view of the operation of the
+system is this. The subscriber desiring to make a call takes down his
+receiver, and this causes a lamp to light in front of an operator. The
+operator presses a button and is in telephonic communication with the
+subscriber. Receiving the number desired, the operator sets it up on a
+keyboard in just about the same way that a typist will set up the
+letters of a short word on a typewriting machine. The setting up of the
+number on the keyboard being accomplished, the proper condition of
+control of the associated automatic apparatus at the central office is
+established and the operator has no further connection with the call.
+The automatic switching apparatus guided by the conditions set up on the
+operator's keyboard proceeds to make the proper selection of trunks and
+to establish the proper connections through them to build up a talking
+circuit between the calling subscriber and the called and to ring the
+called subscriber's bell, or, if his line is found busy, the apparatus
+refuses to connect with it and sends a busy signal back to the calling
+subscriber. The operator performs no work in disconnecting the
+subscribers, that being automatically taken care of when they hang up
+their receivers at the close of the conversation.
+
+From the foregoing it will be seen that there is this fundamental
+difference between the automatic and the automanual--the automatic
+system dispenses entirely with the central-office operator for all
+ordinary switching functions; the automanual employs operators but
+attempts to so facilitate their work that they may handle very many more
+calls than would be possible in a manual system, and at the same time
+secures the advantages of secrecy which the automatic system secures to
+its subscribers.
+
+=Subscriber's Apparatus.= One of the main points in the controversy
+concerning automatic _versus_ manual systems is whether or not it is
+desirable to have the subscriber ask for his connection or to have him
+make certain simple movements with his fingers which will lead to his
+securing it. The developers of the automanual system have taken the
+position that the most desirable way, so far as the subscriber is
+concerned, is to let him ask for it. It is probable that this point
+will not be a deciding one in the choice of future systems, since it
+already seems to be proven that the subscribers in automatic systems are
+willing to go through the necessary movements to mechanically set up the
+call. The advantage which the automanual system shares with the manual,
+however, in the greater simplicity of its subscriber's station
+apparatus, cannot be gainsaid.
+
+[Illustration: Fig. 405. Operators' Key Tables]
+
+[Illustration: Fig. 406. Top View of Key Table]
+
+=Operator's Equipment.= The general form of the operator's equipment is
+shown in Fig. 405. A closer view of the top of one of the key tables is
+shown in Fig. 406. As will be seen, the equipment on each operator's
+position consists of three separate sets of push-button keys closely
+resembling in external appearance the keys of a typewriter or adding
+machine. Immediately above each set of keys are the signal lamps
+belonging to that set.
+
+The operator's keys are arranged in strips of ten, placed _across_
+rather than _lengthwise_ on the key shelf. One of these strips is shown
+in Fig. 407. There are as many strips of keys in each set as there are
+digits in the subscribers' numbers, _i. e._, three in a system having a
+capacity of less than one thousand; four in a system of less than ten
+thousand; and so on. In addition to the number keys of each set is a
+partial row of keys, including what is called a _starting key_ and also
+keys for making the party-line selection.
+
+[Illustration: Fig. 407. Strip of Selecting Keys]
+
+[Illustration: Fig. 408. Wiring of Key Shelf]
+
+The simplicity of the operator's key equipment is one of its attractive
+features. Fig. 408 shows one of the key shelves opened so as to expose
+to view all of the apparatus and wiring that is placed before the
+operator. The reason for providing more than one key set on each
+operator's position is, that after a call has been set up on one key
+set, a few seconds is required before the automatic apparatus controlled
+by the key set can do its work and release the key set ready for another
+call. The provision of more than one key set makes it possible for the
+operator to start setting up another call on another key set without
+waiting for the first to be released by the automatic apparatus.
+
+[Illustration: Fig. 409. Switch Room of Automanual Central Office]
+
+=Automatic Switching Equipment.= A general view of the arrangement of
+automatic switches in an exchange established by the North Electric
+Company at Ashtabula, Ohio, is shown in Fig. 409. The desk in the
+foreground is that of the wire chief. This automatic apparatus consists
+largely of relays and automatic selecting switches. The switches are of
+the step-by-step type, having vertical and rotary movements, and an idea
+of one of them, minus its contact banks, is given in Fig. 410. The
+control of the automatic switches by the operator's key sets is through
+the medium of a power-driven, impulse-sending machine. From this machine
+impulses are taken corresponding to the numbers of the keys depressed.
+
+[Illustration: Fig. 410. Selecting Switch]
+
+=Automatic Distribution of Calls.= A feature of great interest in this
+system is the manner in which the incoming calls are distributed among
+the operators. From each key set an operator's trunk is extended to what
+is called a secondary selector switch, through which it may be connected
+to a primary selector trunk and calling line. When a subscriber calls by
+taking down his receiver, his line relay pulls up and causes a primary
+selector switch to connect his line with an idle local trunk or link
+circuit, at the same time starting up a secondary selector switch which
+immediately connects the primary trunk and the calling line to an
+operator's idle key set. If an operator is at the time engaged in
+setting up a call on a key set, or if that key set is still acting to
+control the sending of impulses to the automatic switches, it may be
+said to be busy, and it is not selected by this preliminary selecting
+apparatus in response to an incoming call. As soon, however, as the
+necessary impulses have been taken from the key set by the automatic
+apparatus, that key set is released and is again ready to receive a
+call. In this way the calls come before each operator only as that
+operator is able and ready to receive them.
+
+=Setting up a Connection.= As soon as the key-set lamp lights, in
+response to such an incoming call, the operator presses a listening
+button, receives the number from the subscriber, and depresses the
+corresponding number buttons on that key set, thereby determining the
+numbers in each of the series of impulses to be sent to the selector and
+the connector switches to make the desired connection. The operator
+repeats this number to the calling subscriber as she sets it up, and
+then presses the starting button, whereupon her work is done so far as
+that call is concerned. If, upon repeating the call to the subscriber,
+the operator finds that she is in error, she may change the number set
+up at any time before she has pressed the starting button.
+
+=Building up a Connection.= The keys so set up determine the number of
+impulses that will be transmitted by the impulse-sending machine to the
+selector and the connector switches. These switches, impelled by these
+impulses, establish the connection if the line called for is not already
+connected to. If a party-line station is called for, the proper station
+on it will be selectively rung as determined by the party-line key
+depressed by the operator. If the line is found busy, the connector
+switch refuses to make the connection and places a busy-back signal on
+the calling line.
+
+=Speed in Handling Calls.= This necessarily brief outline gives an idea
+only of the more striking features of the automanual system. A study of
+the rapidity with which calls may be handled in actual practice shows
+remarkable results as compared with manual methods of operating. The
+operators set up the number keys corresponding to a called number with
+the same rapidity that the keys of a typewriter are pressed in spelling
+a word. In fact, even greater speed is possible, since it is noticed
+that the operators frequently will depress all of the keys of a number
+at once, as by a single striking movement of the fingers. The rapidity
+with which this is done defies accurate timing by a stop watch in the
+hands of an expert. It is practically true, therefore, that the time
+consumed by the operator in handling any one call is that which is taken
+in getting the number from the subscriber and in repeating it back to
+him.
+
+TABLE XI
+
+Total Time Consumed by Operator in Handling Calls on Automanual System
+
+  +-----------------------------------------------------------------+
+  |                        First 100 Calls                          |
+  +-----------------------------------------------------------------+
+  |Longest Individual Period                         12.40  seconds |
+  |Average five longest Individual Periods            7.44  seconds |
+  |Average ten longest Individual Periods             6.34  seconds |
+  |Shortest Individual Period                         1.60  seconds |
+  |Average five shortest Individual Periods           1.92  seconds |
+  |Average ten shortest Individual Periods            1.96  seconds |
+  |Average Entire 100 Calls                           3.396 seconds |
+  |Hourly Rate at which calls were being handled   1060             |
+  +-----------------------------------------------------------------+
+
+  +-----------------------------------------------------------------+
+  |                        Second 100 Calls                         |
+  +-----------------------------------------------------------------+
+  |Longest Individual Period                          7.60  seconds |
+  |Average five longest Individual Periods            5.52  seconds |
+  |Average ten longest Individual Periods             5.34  seconds |
+  |Shortest Individual Period                         2.00  seconds |
+  |Average five shortest Individual Periods           2.04  seconds |
+  |Average ten shortest Individual Periods            2.18  seconds |
+  |Average Entire 100 Calls                           3.374 seconds |
+  |Hourly Rate at which calls were being handled   1067             |
+  +-----------------------------------------------------------------+
+
+  +-----------------------------------------------------------------+
+  |                        Third 100 Calls                          |
+  +-----------------------------------------------------------------+
+  |Longest Individual Period                          5.40  seconds |
+  |Average five longest Individual Periods            5.32  seconds |
+  |Average ten longest Individual Periods             4.44  seconds |
+  |Shortest Individual Period                         1.60  seconds |
+  |Average five shortest Individual Periods           1.65  seconds |
+  |Average ten shortest Individual Periods            1.80  seconds |
+  |Average Entire 100 Calls                           3.160 seconds |
+  |Hourly Rate at which calls were being handled   1139             |
+  +-----------------------------------------------------------------+
+
+Owing to the difficulty of securing accurate traffic data by means of a
+stop watch, an automatic, electrical timing device, capable of
+registering seconds and hundredths of a second, has been used in
+studying the performance of this system in regular operation at
+Ashtabula Harbor. The operators were not informed that the records were
+being taken, and the data tabulated represents the work of two operators
+in handling regular subscribers' calls. The figures in Table XI are
+given by C. H. North as representing the total time consumed by the
+operator from the time her line lamp was lighted until her work in
+connection with the call was finished, and it included, therefore, the
+pressing of the listening button, the receiving of the number from the
+subscriber, repeating it back to him, setting up the connection on the
+keys, and pressing the starting key.
+
+It will be seen that the average time for each 100 calls is quite
+uniform and is slightly over three seconds. The considerable variation
+in the individual calls, ranging from a maximum of 12.40 seconds down to
+a minimum of 1.60 seconds, is due almost entirely to the difference
+between the subscribers in the speed with which they can give their
+numbers. These figures indicate that, in each of the tests, calls were
+being handled at the rate of more than one thousand per hour by each
+operator.
+
+The test of the subscriber's waiting time, _i. e._, the time that he
+waited for the operator to answer, for one hundred calls made without
+the knowledge of the operator, showed the results as given in Table XII,
+in which a split second stop watch was used in making the observations.
+
+TABLE XII
+
+Subscribers' Waiting Time
+
+  +----------------------------------------------------------+
+  |Number of Calls Tested                   100              |
+  |Longest Individual Period                  5.20 seconds   |
+  |Average 5 Longest Individual Periods       4.64 seconds   |
+  |Average 10 Longest Individual Periods      3.80 seconds   |
+  |Shortest Individual Period                 1.00 seconds   |
+  |Average 5 Shortest Individual Periods      1.28 seconds   |
+  |Average 10 Shortest Individual Periods     1.34 seconds   |
+  |Average Entire 100 Calls                   2.07 seconds   |
+  +----------------------------------------------------------+
+
+The length of time which the subscriber has to wait before receiving an
+answer from the operator is, of course, one of the factors that enters
+into the giving of good telephone service, and the times shown by this
+test are considerably shorter than ordinarily maintained in manual
+practice. The waiting time of the subscriber is not, of course, a part
+of the time that is consumed by the operator, and the real economy so
+far as the operator's time is concerned is shown in the tests recorded
+in Table XI.
+
+
+
+
+CHAPTER XXXII
+
+POWER PLANTS
+
+
+The power plant is an organization of devices to furnish to a telephone
+system the several kinds of current, at proper pressures, for the
+performance of the several general electrical tasks within the exchange.
+
+=Kinds of Currents Employed.= Sources of both direct and alternating
+current are required and a single exchange may employ these for one or
+more of the following purposes:
+
+_Direct Current._ Current which flows always in one direction whether
+steady or varying, is referred to as direct current, and may be required
+for transmitters, for relays, for line, supervisory, and auxiliary
+signals, for busy tests, for automatic switches, for call registers, for
+telegraphy, and in the form of pulsating current for the ringing of
+biased bells.
+
+_Alternating Current._ Sources of alternating current are required for
+the ringing of bells, for busy-back and other automatic signals to
+subscribers, for howler signals to attract the attention of subscribers
+who have left their receivers off their hooks, and for signaling over
+composite lines.
+
+=Types of Power Plants.= Clearly the requirements for current supply
+differ greatly for magneto and common-battery systems. There is,
+however, no great difference between the power plants required for the
+automatic and the manual common-battery systems.
+
+In the simplest form of telephone system--two magneto telephones on a
+private line--the power plant at each station consists of two elements:
+one, the magneto generator, which is a translating device for turning
+hand power into alternating current for ringing the bell of the distant
+station; and the other, a primary battery which furnishes current to
+energize the transmitter. In such a system, therefore, each telephone
+has its own power plant. The term power plant, however, as commonly
+employed in telephone work, refers more particularly to the organization
+of devices at the central office for furnishing the required kinds of
+current, and it is to power plants in this sense that this chapter is
+devoted.
+
+_Magneto Systems._ If magneto lines be connected to a switchboard, the
+current for throwing the drop at the switchboard is furnished by the
+subscriber's generator, and the current for energizing the subscriber's
+transmitter is furnished by the local battery at his station; but
+sources of current must be provided for enabling the central-office
+operator to signal or talk to the subscribers. These are about the only
+needs for which current must be furnished in an ordinary magneto central
+office. If a multiple board is employed, direct current is also needed
+for the purpose of the busy test and also for operating the drop
+restoring circuits, if the electrical method of restoring the drops is
+employed.
+
+_Common-Battery Systems._ In common-battery systems the requirements are
+very much more extensive. The subscribers' telephones have no power
+plants of their own, but are provided with a common source of direct
+current located at the central office for supplying the talking current,
+and for operating the central-office signals, and the operators are
+provided with one or more common sources of alternating or pulsating
+current for ringing the subscribers' bells. Common-battery equipment
+requires the use of currents of different kinds for a greater number of
+auxiliary purposes than does magneto equipment. These facts make the
+power plant in a common-battery office much more important than in a
+magneto office.
+
+=Operators' Transmitter Supply.= In a small magneto exchange, the
+transmitter current may be had from primary batteries, a separate
+battery being employed for each operator's set. When there are more than
+three or four operators, however, it is usual, even in magneto offices,
+to obtain the transmitter current from a common storage battery. A
+storage battery has the fortunate quality of very low internal
+resistance, therefore a number of operators' transmitters may be
+actuated by one source without introducing cross-talk. In other words, a
+storage battery is a current-furnishing device of good regulation, the
+variation of consumption in one circuit leading from it causing slight
+variation in the currents of other circuits leading from it. If this
+were not so, cross-talk would exist between the telephones of the
+operators' positions connected to the same battery. This regulating
+quality enables the multiple feeding of telephone circuits to be carried
+further than the mere supplying of operators' sets and is the quality
+which makes possible the successful use of a storage battery as the
+single source of transmitter current for common-battery central-office
+equipment.
+
+In furnishing a plurality of operators' transmitters from a common
+battery, the importance of low resistance and inductance in the portion
+of the path that is common to all of the circuits must not be
+overlooked. Not only is a battery of extremely low resistance required,
+but also conductors leading from it that are common to two or more of
+the circuits should be of very low resistance and consequently large in
+cross-section and as short as possible. In common-battery offices there
+is obviously no need of employing a separate battery for the operators'
+transmitters, since they may readily be supplied from the common storage
+battery which supplies direct current to the subscribers' lines.
+
+=Ringing-Current Supply.= _Magneto Generators._ As a central-office
+equipment is required to ring many subscribers' bells, only the small
+ones find it convenient to ring them by means of hand-operated magneto
+generators. Small magneto switchboards are usually equipped so that each
+operator is provided with a hand-generator, but even where such is the
+case some source of ringing current not manually operated is desirable.
+In larger switchboards the hand generators are entirely dispensed with.
+
+The magneto generator may be driven by a belt from any convenient
+constantly moving pulley, and the early telephone exchanges were often
+equipped with such generators having better bearings and more current
+capacity than those in magneto telephones. These were adapted to be run
+constantly from some source of power, delivering ringing current to the
+operators' keyboards at from 16 to 20 cycles per second.
+
+_Pole Changers._ Vibrating pole changers were also used in the early
+exchanges, but passed out of use, partly because of poor design, but
+more because of the absence of good forms of primary batteries for
+vibrating them and for furnishing the direct currents to be transformed
+into alternating line current for ringing the bells. The pole changer
+was redesigned after the beginning of the great spread of telephony in
+the United States in 1893. Today it is firmly established as an element
+of good telephone practice. Fig. 411 illustrates the principle upon
+which one of the well-known pole changers--the Warner--operates. In
+this _1_ is an electromagnet supplied by a constant-current battery _2_
+to keep the vibratory system continually in motion. This motor magnet
+and its battery work in a local circuit and cause vibration in exactly
+the same manner as the armature of an ordinary electric door bell is
+caused to vibrate. The battery from which the ringing current is derived
+is indicated at _3_, and the poles of this are connected, respectively,
+to the vibrating contacts _4_ and _5_. These contacts are merely the
+moving members of a pole changing switch, and a study of the action will
+readily show that when these moving parts engage the right-hand
+contacts, current will flow to the line supposed to be connected to the
+terminals _6_ and _7_ in one direction, while, when these parts engage
+the left-hand contacts, current will flow to the line in the reverse
+direction. The circuit of the condenser shown is controlled by the
+armature of the relay _8_.
+
+The winding of this relay is put directly in the circuit of the main
+battery _3_, so that whenever current is drawn from this battery to ring
+a distant bell, this relay will be operated and will bridge the
+condenser across the circuit of the line. The purpose of the condenser
+is to make the impulses flowing from the pole changer less abrupt, and
+the reason for having its bridged circuit normally broken is to prevent
+a waste of current from the battery _3_, due to the energy which would
+otherwise be consumed by the condenser if it were left permanently
+across the line.
+
+[Illustration: Fig. 411. Warner Pole Changer]
+
+[Illustration: Fig. 412. Pole Changers for Harmonic Ringing]
+
+Pole changers for ringing bells of harmonic party lines are required to
+produce alternating currents of practically constant frequencies. The
+ideal arrangement is to cause the direct currents from a storage battery
+to be alternated by means of the pole changers, and then transformed
+into higher voltages required for ringing purposes, the transformer
+also serving to smooth the current wave, making it more suitable for
+ringing purposes. In Fig. 412 such an arrangement, adapted to develop
+currents for harmonic ringing on party lines, is shown. The regular
+common battery of the central office is indicated at _1_, _2_ being an
+auxiliary battery of dry cells, the purpose of which will be presently
+referred to. At the right of the battery _1_ there is shown the calling
+plug with its associated party-line ringing keys adapted to impress the
+several frequencies on the subscribers' lines. The method by which the
+current from the main storage battery passes through the motor magnets
+of the several vibrators, and by which the primary currents through the
+transformers are made to alternate at the respective frequencies of
+these vibrators, will be obvious from the drawing. It is also clear that
+the secondary currents developed in these transformers are led to the
+several ringing keys so as to be available for connection with the
+subscribers' lines at the will of the operator. The condensers are
+bridged across the primary windings of the transformers for the purpose
+of aiding in smoothing out the current waves. The use of the auxiliary
+battery _2_ and the retardation coil _3_ in the main supply lead is for
+the purpose of preventing the pulsating currents drawn from the main
+battery _1_ from making the battery "noisy." These two batteries have
+like poles connected to the supply lead, and the auxiliary battery
+furnishes no current to the system except when the electromotive force
+of the impulse flowing from the main battery is choked down by the
+impedance coil and the deficiency is then momentarily supplied for each
+wave by the auxiliary battery. This is the method developed by the Dean
+Electric Company for preventing the pole-changer system from causing
+disturbances on lines supplied from the same main battery.
+
+[Illustration: Fig. 413. Multi-Cyclic Generator Set]
+
+_Ringing Dynamos._ Alternating and pulsating currents for ringing
+purposes are also largely furnished from alternating-current dynamos
+similar to those used in commercial power and lighting work, but
+specially designed to produce ringing currents of proper frequency and
+voltage. These are usually driven by electric motors deriving their
+current either from the commercial supply mains or from the
+central-office battery. In large exchanges harmonic ringers are usually
+operated by alternating-current generators driven by motors, a separate
+dynamo being provided to furnish the current of each frequency. Fig. 413
+shows a set of four such generators directly connected to a common
+motor. As no source of commercial power for driving such generators is
+absolutely uniform, and since the frequency of the ringing current must
+remain very close to a constant predetermined rate, some means must be
+employed for holding the generators at a constant speed of revolution,
+and this is done by means of a governor shown at the right-hand end of
+the shaft in Fig. 413. The principle of this governor is shown in Fig.
+414. A weighted spring acts, by centrifugal force, to make a contact
+against an adjustable screw, when the speed of the shaft rises a
+predetermined amount. This spring and its contact are connected to two
+collector rings _1_ and _2_ on the motor shaft, and connection is made
+with these by the brushes _3_ and _4_. The closing of the governor
+contact serves, therefore, merely to short-circuit the resistance _5_,
+which is normally included in the shunt field of the motor. This
+governor is based on the principle that weakening the field increases
+the speed. It acts to insert the resistance in series with the field
+winding when the speed falls, and this, in turn, results in restoring
+the speed to normal.
+
+[Illustration: Fig. 414. Governor for Harmonic Ringing Generators]
+
+=Auxiliary Signaling Currents.= Alternating currents, such as those
+employed for busy signals to subscribers in automatic systems, those for
+causing loud tones in receivers which have been left off the hook
+switch, and those for producing loud tones in calling receivers
+connected to composite lines, all need to be of much higher frequency
+than alternating current for ringing bells. The simplest way of
+producing such tones is by means of an interrupter like that of a
+vibrating bell; but this is not the most reliable way and it is usual to
+produce busy or "busy-back" currents by rotating commutators to
+interrupt a steady current at the required rate. As the usual busy-back
+signal is a series of recurrent tones about one-half second long,
+interspersed with periods of silence, the rapidly commuted direct
+current is required to be further commuted at a slow rate, and this is
+conveniently done by associating a high-speed commutator with a
+low-speed one. Such an arrangement may be seen at the left-hand end of
+the multicyclic alternating machine shown in Fig. 413. This commuting
+device is usually associated with the ringing machine because that is
+the one thing about a central office that is available for imparting
+continuous rotary motion.
+
+=Primary Sources.= Most telephone power plants consume commercial
+electric power and deliver special electric current. Usually some
+translating device, such as a motor-generator or a mercury-arc
+rectifier, is employed to transform the commercial current into the
+specialized current required for the immediate uses of the exchange.
+
+_Charging from Direct-Current Mains._ In some cases commercial direct
+current is used to charge the storage batteries without the intervention
+of the translating devices, resistances being used in series with the
+battery to regulate the amount of current. Commercial direct current
+usually is available at pressures from 110 volts and upward, while
+telephone power plants contain storage batteries rarely of pressures
+higher than 50 volts. To charge a 50-volt storage battery direct from
+110-volt mains results in the loss of about half the energy purchased,
+this lost energy being set free in the form of heat generated in the
+resistance devices. Notwithstanding this, it is sometimes economical to
+charge directly from the commercial direct-current power mains, but only
+in small offices where the total amount of current consumed is not large
+and where the greatest simplicity in equipment is desirable. It is
+better, however, in nearly all cases, to convert the purchased power
+from the received voltage to the required voltage by some form of
+translating device, such as a rotary converter or a mercury-arc
+rectifier.
+
+_Rotary Converters._ Broadly speaking, a rotary converter consists of a
+motor adapted to the voltage and kind of current received, mechanically
+coupled to a generator adapted to produce current of the required kind
+and voltage. The harmonic ringing machine shown in Fig. 413 is an
+example of this, this particular one being adapted to receive direct
+current at ordinary commercial pressure and to deliver four different
+alternating currents of suitable pressures and frequencies. It is to be
+understood, however, that the conversion may be from direct current to
+direct current, from alternating to direct, or from direct to
+alternating. Such a device where the motor is a separate and distinct
+machine from the generator or generators is called a _motor-generator_.
+It is usual to connect the motors and the generators together directly
+by a coupling having some flexibility, as shown in Fig. 413, so as to
+prevent undue friction in the bearings.
+
+[Illustration: THE POWER AND WIRE CHIEF'S ROOM OF THE EXCHANGE AT WEBB
+CITY, MISSOURI]
+
+As an alternative to the converting device made up of a motor coupled to
+a generator, both motor and generator windings may be combined on the
+same core and rotate within the same field. Such a rotary converter has
+been called a _dynamotor_. As a rule the dynamotor is only suitable for
+small power-plant work. It has the following objectionable features:
+(_a_) It is difficult to regulate its output, since the same field
+serves for both the motor and the dynamo windings. For this reason its
+main use is as a ringing machine where the regulation of the output is
+not an important factor. (_b_) Furthermore, the fact that the motor and
+dynamo armature windings are on the same core makes it difficult to
+guard against breakdowns of the insulation between the two windings,
+especially when the driving current is of high voltage.
+
+_Charging Dynamos._ The dynamo for charging the storage battery is, of
+course, a direct-current machine and may be a part of a motor generator
+or it may derive its power from some other than an electric motor, such
+as a gas or steam engine. It should be able to develop a voltage
+slightly above that of the voltage of the storage battery when at its
+maximum charge, so as always to be able to deliver current to the
+charging battery regardless of the state of charge. A 30-volt generator,
+for example, can charge eleven cells in series economically; a 60-volt
+generator can charge twenty-five cells in series economically.
+
+Battery-charging generators are controlled as to their output by varying
+a resistance in series with their fields. Such machines are usually
+shunt-wound. Sometimes they are compound-wound, but compounding is less
+important in telephone generators than in some other uses. A feature of
+great importance in the design of charging generators is smoothness of
+current. If it were possible to design generators to produce absolutely
+even or smooth current, the storage battery would not be such an
+essential feature to common-battery exchanges, because then the
+generator might deliver its current directly to the bus bars of the
+office without any storage-battery connection and without causing noise
+on the lines. Such generators have been built in small units. Even if
+these smooth current generators were commercially developed to a degree
+to produce absolutely no noise on the lines, the storage battery would
+still be used, since its action as a reservoir for electrical energy is
+important. It not only dispenses with the necessity of running the
+generators continuously, but it also affords a safeguard against
+breakdowns which is one of its important uses.
+
+The ability to carry the load of a central office directly on the
+charging generator without the use of a storage battery is of no
+importance except in an emergency which takes the storage battery wholly
+out of service. Since the beginning of common-battery working such
+emergencies have happened a negligible number of times. Far more
+communities have lacked telephone service because of accidents beyond
+human control than because of storage-battery failures.
+
+In power plants serving large offices, the demand upon the storage
+battery is great enough to require large plate areas in each cell. The
+internal resistance, therefore, is small and considerable fluctuations
+may exist in the charging current without their being heard in the
+talking circuits. The amount of noise to be heard depends also on the
+type of charging generator. Increasing the number of armature coils and
+commutator segments increases the smoothness of the charging current.
+The shape of the generator pole pieces is also a factor in securing such
+smoothness.
+
+If, with a given machine and storage battery, the talking circuits are
+disturbed by the charging current, relief may be obtained by inserting a
+large impedance in the charging circuit. This impedance requires to be
+of low resistance, because whatever heat is developed in it is lost
+energy. This means that the best conditions exist when the resistance is
+low and the inductance large. These conditions are satisfied by using in
+the impedance coil many turns of large wire and an ample iron core.
+
+Dynamotors are not generally suitable for charging purposes. Not only is
+the difficulty in regulating their output a disadvantage, but the fact
+that the primary and secondary windings are so closely associated on the
+armature core makes them carry into the charging current, not only the
+commutator noises of the generator end, but of the motor end as well.
+
+_Mercury-Arc Rectifiers._ In common-battery offices serving a few
+hundred lines, and where the commercial supply is alternating current,
+it is good practice to transform it into direct-battery charging current
+by means of a mercury-arc rectifier. It is a device broadly similar to
+the mercury-arc lamp produced by Peter Cooper Hewitt. It contains no
+moving parts and operates at high efficiency without introducing noises
+into the telephone lines. It requires little care and has good length of
+life.
+
+[Illustration: Fig. 415. Mercury-Arc Rectifier Circuits]
+
+The circuit of a mercury-arc rectifier charging outfit is shown in Fig.
+415. The mercury-arc rectifier proper consists of a glass bulb
+containing vacuum and a small amount of mercury. When its terminals are
+connected, as indicated--the two anodes across an alternating-current
+source and the cathode with a circuit that is to be supplied with direct
+current--this device has the peculiarity of action that current will
+flow alternately from the two anodes always to the cathode and never
+from it. The cathode, therefore, becomes a source of positive potential
+and, as such, is used in charging the storage battery through the series
+reactance coil and the compensating reactances, as indicated. The line
+transformer shown at the upper portion of Fig. 415, is the one for
+converting the high-potential alternating current to the comparatively
+low-potential current required for the action of the rectifier. The
+transformer below this has a one-to-one ratio, and is called the
+insulating transformer. Its purpose is to safeguard the telephone
+apparatus and circuits against abnormal potentials from the line, and
+also to prevent the ground, which is commonly placed on the neutral wire
+of transformers on commercial lighting circuits, from interfering with
+the ground that is commonly placed on the positive pole of the
+central-office battery.
+
+=Provision Against Breakdown.= In order to provide against breakdown of
+service, a well-designed telephone power plant should have available
+more than one primary source of power and more than one charging unit
+and ringing unit.
+
+_Duplicate Primary Sources._ In large cities where the commercial power
+service is highly developed and a breakdown of the generating station is
+practically impossible, it is customary to depend on that service alone.
+In order to insure against loss of power due to an accident to portions
+of the distributing system, it is the common custom to run two entirely
+separate power leads into the office, coming, if possible, from
+different parts of the system so that a breakdown on one section will
+not deprive the telephone exchange of primary power. In smaller places
+where the commercial service is not so reliable, it is usual to provide,
+in addition to the commercial electric-power service, an independent
+source of power in the form of a gas or steam engine. This may be run as
+a regular source, the commercial service being employed as an emergency
+or _vice versa_, as economy may dictate. In providing a gas engine for
+driving charging dynamos, it is important to obtain one having as good
+regulation as possible, in order to obtain a charging current of
+practically constant voltage.
+
+_Duplicate Charging Machines._ The storage batteries of telephone
+exchanges are usually provided of sufficient capacity to supply the
+direct-current needs of the office for twenty-four hours after a full
+charge has been given them. This in itself is a strong safeguard against
+breakdown. In addition to this the charging machines should be in
+duplicate, so that a burnt-out armature or other damage to one of the
+charging units will not disable the plant.
+
+_Duplicate Ringing Machines._ It is equally important that the ringing
+machines, whether of the rotary or vibrating type, be in duplicate. For
+large exchanges the ringing machines are usually dynamos, and it is not
+unusual to have one of these driven from the commercial power mains and
+the other from the storage battery. With this arrangement complete
+failure of all sources of primary power would still leave the exchange
+operative as long as sufficient charge remains in the storage battery.
+
+_Capacity of Power Units._ In designing telephone switchboards it is the
+common practice to so design the frameworks that the space for multiple
+jacks is in excess of that required for the original installation. In a
+like manner, the power plant is also designed with a view of being
+readily increased in capacity to an amount sufficient to provide current
+for the ultimate number of subscribers' lines for which the switchboard
+is designed. The motor generators, or whatever means are provided for
+charging the storage batteries, are usually installed of sufficient size
+to care for the ultimate requirements of the office. The ringing
+machines are also provided for the ultimate equipment. However, in the
+case of the storage battery, it is common practice to provide the
+battery tanks of sufficient size to care for the ultimate capacity,
+while the plates are installed for a capacity only slightly in excess of
+that required for the original installation. As the equipment of
+subscribers' lines is increased, additional plates may, therefore, be
+added to the cells without replacing the storage battery as a whole, and
+without making extraordinary provisions to prevent the interruption of
+service. It is also customary to provide charging and supply leads from
+the storage battery of carrying capacity sufficient for the ultimate
+requirements of the office.
+
+=Storage Battery.= The storage battery is the power plant element which
+has made common-battery systems possible. The common-battery system is
+the element which has made the present wide development of telephony
+possible.
+
+A storage-battery cell is an electro-chemical device in which a chemical
+state is changed by the passage of current through the cell, this state
+tending to revert when a current is allowed to flow in the opposite
+direction. A storage cell consists of two conductors in a solution, the
+nature and the relation of these three elements being such that when a
+direct current is made to pass from one conductor to the other through
+the solution, the compelled chemical change is proportional to the
+product of the current and its duration. When the two conductors are
+joined by a path over which current may flow, a current does flow in the
+opposite direction to that which charged the cell.
+
+All storage batteries so far in extensive use in telephone systems are
+composed of lead plates in a solution of sulphuric acid in water called
+the _electrolyte_. In charging, the current tends to oxidize the lead of
+one plate and de-oxidize the other. In discharging, the tendency is
+toward equilibrium.
+
+The containers, employed in telephone work, for the plates and
+electrolyte are either of glass or wood with a lead lining, the glass
+jars being used for the smaller sized plates of small capacity cells,
+while the lead-lined wooden tanks are employed with the larger capacity
+cells. The potential of a cell is slightly over two volts and is
+independent of the shape or size of the plates for a given type of
+battery. The storage capacity of a cell is determined by the size and
+the number of plates. Therefore, by increasing the number of plates and
+the areas of their surfaces, the ampere-hour capacity of the cell is
+correspondingly increased. The desired potential of the battery is
+obtained by connecting the proper number of cells in series.
+Storage-battery cells used in telephone work vary from 2 plates having
+an area of 12 square inches each, to cells having over 50 plates, each
+plate having an area of 240 square inches. The ampere-hour capacity of
+these batteries varies from 6 ampere hours to 4,000 ampere hours,
+respectively, when used at an average 8-hour discharge rate. In Fig. 416
+is illustrated a storage cell employing a glass container and having
+fifteen plates. Each plate is 11 inches high and 10-1/2 inches wide,
+with an area, therefore, of 115.5 square inches. Such a cell has a
+normal capacity of 560 ampere hours. The type illustrated is one made by
+the Electric Storage Battery Company of Philadelphia, Pa.[A]
+
+[Illustration: Fig. 416. Storage Cell]
+
+_Installation._ In installing the glass jars it is customary to place
+them in trays partially filled with sand. They are, however, at times
+installed on insulators so designed as to prevent moisture from causing
+leakage between the cells. The cells using wooden tanks are placed on
+glass or porcelain insulators, and the tanks are placed with enough
+clearance between them to prevent the lead lining of adjacent tanks from
+being in contact and thereby short-circuiting the cells. After the
+positive and the negative plates have been installed in the tanks, their
+respective terminals are connected to bus bars, these bus bars being,
+for the small types of battery, lead-covered clamping bolts, while in
+the larger types reinforced lead bus bars are employed, to which the
+plates are securely joined by a process called lead burning. This
+process consists in melting a portion of the bus bar and the terminal
+lug of the plate by a flame of very high temperature, thus fusing each
+individual plate to the proper bus bar. The plates of adjacent cells are
+connected to the same bus bar, thus eliminating the necessity of any
+other connection between the cells.
+
+_Initial Charge._ As soon as the plates have been installed in the tanks
+and welded to the bus bars, the cell should be filled with electrolyte
+having a specific gravity of 1.180 to 1.190 to one-half inch above the
+tops of the plates and then the charge should be immediately started at
+about the normal rate. In the case of a battery consisting of cells of
+large capacity, it is customary to place the electrolyte in the cells as
+nearly simultaneously as possible rather than to completely fill the
+cells in consecutive order. When the electrolyte is placed in the cells
+simultaneously, the charge is started at a very much reduced rate before
+the cells are completely filled, the rate being increased as the cells
+are filled, the normal rate of charge being reached when the cells are
+completely filled. Readings should be taken hourly of the specific
+gravity and temperature of the electrolyte, voltage of the cells, and
+amperage of charging current. A record or log should be kept of the
+specific gravity and voltage of each of the cells of the battery
+regularly during the life of the battery and it is well to commence this
+record with the initial charge.
+
+The initial charge should be maintained for at least ten hours after the
+time when the voltage and specific gravity have reached a maximum. If
+for any reason it is impractical to continue the initial charge
+uninterrupted, the first period of charging should be at least from
+twelve to fifteen hours. However, every effort should be made to have
+the initial charge continuous, as an interruption tends to increase the
+time necessary for the initial charge, and if the time be too long
+between the periods of the initial charge, the efficiency and capacity
+of the cells are liable to be affected. In case of a large battery,
+precaution should be taken to insure that the ventilation is
+exceptionally good, because if it is not good the temperature is liable
+to increase considerably and thereby cause an undue amount of
+evaporation from the cells.
+
+The object of the temperature readings taken during the charge is to
+enable corrections to be made to the specific gravity readings as
+obtained by the hydrometer, in order that the correct specific gravity
+may be ascertained. This correction is made by adding .001 specific
+gravity for each three degrees in temperature above 70 deg. Fahrenheit,
+or subtracting the same amount for each three degrees below 70 deg.
+Fahrenheit. At the time the cells begin to gas they should be gone over
+carefully to see that they gas evenly, and also to detect and remedy
+early in the charging period any defects which may exist. If there is
+any doubt in regard to the time at which the cells reach a maximum
+voltage and specific gravity, the charge should be continued
+sufficiently long before the last ten hours of the charge are commenced
+to eliminate any such doubt, as in many cases poor efficiency and low
+capacity of a cell later in its life may be traced to an insufficient
+initial charge.
+
+_Operation._ After the battery has been put in commission the periodic
+charges should be carefully watched, as excessive charging causes
+disintegration and decreases the life and capacity of the battery;
+while, on the other hand, undercharging will result in sulphating of the
+plates and decrease of capacity, and, if the undercharge be great, will
+result in a disintegration of the plates. It is, therefore, essential
+that the battery be charged regularly and at the rate specified for the
+particular battery in question. In order to minimize the chance of
+either continuously overcharging or undercharging the battery, the
+charges are divided into two classes, namely, regular charges and
+overcharges. The regular charges are the periodic charges for the
+purpose of restoring the capacity of the battery after discharge. The
+overcharges, which should occur once a week or once in every two weeks,
+according to the use of the battery, are for the purpose of insuring
+that all cells have received their proper charge, for reducing such
+sulphating as may have occurred on cells undercharged, and for keeping
+the plates, in general, in a healthy condition. The specific gravity of
+the electrolyte, the voltage of the battery, and the amount of gasing
+observed are all indications of the amount of charge which the battery
+has received and should all be considered when practicable. Either the
+specific gravity or voltage may be used as the routine method of
+determining the proper charge, but, however, if the proper charge is
+determined by the voltage readings, this should be frequently checked by
+the specific gravity, and _vice versa_.
+
+During the charging and discharging of a battery the level of the
+electrolyte in the cells will fall. As the portion of the electrolyte
+which is evaporated is mainly water, the electrolyte may be readily
+restored to its normal level by adding distilled water or carefully
+collected rain water.
+
+_Pilot Cell._ As the specific gravity of all the cells of a battery,
+after having once been properly adjusted, will vary the same in all the
+cells during use, it has been found satisfactory to use one cell,
+commonly termed the pilot cell, for taking the regular specific gravity
+readings and only reading the specific gravity of all the cells
+occasionally or on the overcharge. This cell must be representative of
+all the cells of the battery, and if the battery is so subdivided in use
+that several sets of cells are liable to receive different usage, a
+pilot cell should be selected for each group.
+
+_Overcharge._ If the battery is charged daily, it should receive an
+overcharge once a week, or if charged less frequently, an overcharge
+should be given at least once every two weeks. In making an overcharge
+this should be done at a constant rate and at a rate specified for the
+battery. During the overcharge the voltage of the battery and the
+specific gravity of the pilot cell should be taken every fifteen minutes
+from the time the gasing begins. The charge should be continued until
+five consecutive, specific-gravity readings are practically the same.
+The voltage of the battery should not increase during the last hour of
+the charge.
+
+As the principal object of the overcharge is to insure that all of the
+cells have received the proper charge, it must, therefore, be continued
+long enough to not only properly charge the most efficient cells, but
+also to properly charge those which are lower in efficiency. The longer
+the interval between overcharges, the greater will be the variation
+between the cells and, therefore, it is necessary to continue the
+overcharge longer when the interval between overcharges is as great as
+two weeks. Before the overcharge is made the cells should be carefully
+inspected for short circuits and other abnormal conditions. These
+inspections may best be made by submerging an electric lamp in the cell,
+if the cell be of wood, or of allowing it to shine through from the
+outside, if it be of glass. By this means any foreign material may be
+readily detected and removed before serious damage is caused. In making
+these inspections it must be borne in mind that whatever tools or
+implements are used must be non-metallic and of some insulating
+material.
+
+_Regular Charge._ Regular charges are the periodic charges for restoring
+the capacity of the battery, and should be made as frequently as the use
+of the battery demands. The voltage of the cells is a good guide for
+determining when the battery should be recharged. The voltage of a cell
+should never be allowed to drop below 1.8 volts, and it is usually
+considered better practice to recharge when the battery has reached 1.9
+volts. If a battery is to remain idle for even a short time, it should
+be left in a completely charged condition.
+
+The regular charges for cells completely equipped with plates should be
+continued until the specific gravity of the pilot cell has risen to five
+points below the maximum attained on the preceding overcharge, or, if
+only partially equipped with plates, until it has risen to three points
+below the previous maximum. The voltage per cell at this time should be
+from .05 volts to .1 volts below that obtained on the previous
+overcharge. At this time all the cells should be gasing, but not as
+freely as on an overcharge.
+
+_Low Cells._ An unhealthy condition in a cell usually manifests itself
+in one of the following ways: Falling off in specific gravity or voltage
+relative to the rest of the cells, lack of gasing when charged, and
+color of the plates, either noticeably lighter or darker than those of
+other cells of the battery. When any of the above conditions are found
+in a cell, the cell should receive immediate attention, as a delay may
+mean serious trouble. The cell should be thoroughly inspected to
+determine if a short-circuit exists, either caused by some foreign
+substance, by an excess of sediment in the bottom of the tank, or by
+portions of the plates themselves. If such a condition is found, the
+cause should be immediately removed and, if the defect has been of short
+duration, the next overcharge will probably restore it to normal
+condition. If the defect has existed for some time, it is often
+necessary to give the cell a separate charge. This may be done by
+connecting it directly to the charging generator with temporary leads
+and thus bring it back to its normal condition. It is sometimes found
+necessary to replace the cell in order to restore the battery to its
+normal condition.
+
+_Sediment._ The cells of the battery should be carefully watched to
+prevent the sediment which collects in the bottom of the jar or tank
+during use from reaching the bottom of the plates, thereby causing short
+circuits between them. When the sediment in the cell has reached within
+one-half inch of the bottom of the plates, it should be removed at once.
+With small cells using glass jars this can most easily be done directly
+after an overcharge by carefully drawing off the electrolyte without
+disturbing the sediment and then removing it from the jar. The plates
+and electrolyte should be replaced in the jar as soon as convenient to
+prevent the plates from becoming dry. If the plates are large and in
+wooden tanks, the sediment can most easily be removed by means of a
+scoop made especially for the purpose. The preferable time to clean the
+tanks is just before an overcharge.
+
+_Replacing Batteries._ There comes a time in the life of nearly every
+central-office equipment when the storage battery must be completely
+renewed. This is due to the fact that the life of even the best of
+storage batteries is not as great as the life of the average switchboard
+equipment. It may also be due to the necessity for greater capacity than
+can be secured with the existing battery tanks, usually caused by
+underestimating the traffic the office will be required to handle.
+Again, it is sometimes necessary to make extensive alterations in an
+existing battery, perhaps due to the necessity for changing its
+location. To change a battery one cell at a time, keeping the others in
+commission meanwhile, has often been done, but it is always expensive
+and unsatisfactory and is likely to shorten the life of the battery, due
+to improper and irregular forming of the plates during the initial
+charge. The advent of the electric automobile industry has brought with
+it a convenient means for overcoming this difficulty. Portable storage
+cells for automobile use are available in almost every locality and may
+often be rented at small cost. A sufficient number of such cells may be
+temporarily installed, enough of them being placed in multiple to give
+the necessary output. By floating a temporary battery so formed across
+the charging mains and running the generators continuously, a temporary
+source of current supply may be had at small expense for running the
+exchange during the period required for alterations. Usually a time of
+low traffic is chosen for making the changes, such as from Saturday
+evening to Monday morning. Very large central-office batteries, serving
+as many as 6,000 lines, have thus been taken out of service and replaced
+without interfering with the traffic and with the use of but a
+comparatively few portable cells. One precaution has to be observed in
+such work, and that is not to subject the portable cells to too great an
+overcharge, due to the great excess of generator over battery capacity.
+This is easily avoided by watching the ammeters to see that the input is
+not in too great excess of the output, and if necessary, by frequently
+stopping the machines to avoid this.
+
+=Power Switchboard.= The clearing-house of the telephone power plant is
+the power board. In most cases, it carries switches, meters, and
+protective devices.
+
+_Switches._ The switches most essential are those for opening and
+closing the motor and the generator circuits of the charging sets and
+with these usually are associated the starting rheostats of the motors
+and the field rheostats of the generators. The starting rheostats are
+adapted to allow resistance to be removed from the motor armature
+circuit, allowing the armature to gain speed and increase its
+counter-electromotive force without overheating. The accepted type has
+means for opening the driving circuit automatically in case its voltage
+should fall, thus preventing a temporary interruption of driving current
+from damaging the motor armature on its return to normal voltage.
+
+[Illustration: Fig. 417. Power-Plant Circuits]
+
+_Meters._ The meters usually are voltmeters and ammeters, the former
+being adapted to read the several voltages of direct currents in the
+power plant. An important one to be known is the voltage of the
+generator before beginning a battery charge, so that the generator may
+not be thrown on the storage battery while generating a voltage less
+than that of the battery. If this were done, the battery would discharge
+through the generator armature. The voltmeter enables the voltage of the
+charging generator to be kept above that of the battery, as the latter
+rises during charge. It enables the performance of several cells of the
+battery to be observed. A convenient way is to connect the terminals of
+the several cells to jacks on the power board and to terminate the
+voltmeter in a plug.
+
+The ammeter, with suitable connections, enables the battery-charge rate
+to be kept normal and the battery discharge to be observed. In order to
+economize power, it is best to charge the battery during the hours of
+heavy load. The generator output then divides, the switchboard taking
+what the load requires, the battery receiving the remainder.
+
+In systems requiring the terminal voltage of the equipment to be kept
+constant within close limits, either it is necessary to use two
+batteries--never drawing current from a battery during charge--or to
+provide means of compensating for the rise of voltage while the battery
+is under charge. The latter is the more modern method and is done either
+by using fewer cells when the voltage per cell is higher or by inserting
+counter-electromotive force cells in the discharge leads, opposing the
+discharge by more or fewer cells as the voltage of the battery is
+higher or lower. In either method, switches on the power board enable
+the insertion and removal of the necessary end cells or
+counter-electromotive force cells.
+
+_Protective Devices._ The protective devices required on a power board
+are principally _circuit-breakers_ and _fuses_. Circuit-breakers are
+adapted to open motor and generator circuits when their currents are too
+great, too small, or in the wrong direction. Fuses are adapted to open
+circuits when the currents in them are too great. The best type is that
+in which the operation of the fuses sounds or shows an alarm, or both.
+
+=Power-Plant Circuits.= The circuit arrangement of central-office power
+plants is subject to wide variation according to conditions. The type of
+telephone switchboard equipment, whether magneto or common-battery,
+automatic or manual, will, of course, largely affect the circuit
+arrangement of the power plant. Fig. 417 shows a typical example of good
+practice in this respect for use with a common-battery manual
+switchboard equipment. Besides showing the switches for handling the
+various machines and the charge-and-discharge leads from the storage
+battery, this diagram shows how current from the storage battery is
+delivered to various parts of the central-office equipment.
+
+[Footnote A: The instructions given later in this chapter are for
+batteries of this make, although they are applicable in many respects to
+all types commonly used in telephone work.]
+
+
+
+
+CHAPTER XXXIII
+
+HOUSING CENTRAL-OFFICE EQUIPMENT
+
+
+=The Central-Office Building.= Proper arrangement of the central-office
+equipment depends largely upon the design of the central-office
+building. The problem involved should not be solved by the architect
+alone. The most careful co-operation between the engineer and the
+architect is necessary in order that the various parts of the telephonic
+equipment may be properly related, and that the wires connecting them
+with each other and with the outside lines be disposed of with due
+regard to safety, economy, and convenience. So many factors enter into
+the design of a central-office building that it is impossible to lay
+down more than the most general rules. The attainment of an ideal is
+often impossible, because of the fact that the building is usually in
+congested districts, and its very shape and size must be governed by the
+lot on which it is built, and by the immediate surroundings. Frequently,
+also, the building must be used for other purposes than those of a
+telephone office, so that the several purposes must be considered in its
+design. Again, old buildings, designed for other purposes, must
+sometimes be altered to meet the requirements of a telephone office, and
+this is perhaps the most difficult problem of all.
+
+The exterior of the building is a matter that may be largely decided by
+the architect and owner after the general character of the building has
+been determined. One important feature, however, and one that has been
+overlooked in many cases that we know of, is to so arrange the building
+that switchboard sections and other bulky portions of the apparatus,
+which are necessarily assembled at the factory rather than on the site,
+may be brought into the building without tearing down the walls.
+
+_Fire Hazard._ The apparatus to be housed in a central-office building
+often represents a cost running into the hundreds of thousands of
+dollars; but whether of large or small first cost, it is evident that
+its destruction might incur a very much greater loss than that
+represented by its replacement value. In guarding the central-office
+equipment against destruction by fire or other causes, the telephone
+company is concerned to a very much greater extent than the mere cost of
+the physical property; since it is guarding the thing which makes it
+possible to do business. While the cost of the central office and its
+contents may be small in comparison with the total investment in outside
+plant and other portions of the equipment, it is yet true that these
+larger portions of the investment become useless with the loss of the
+central office.
+
+There is another consideration, and that is the moral obligation of the
+operating company to the public. A complete breakdown of telephone
+service for any considerable period of time in a large city is in the
+nature of a public calamity.
+
+For these reasons the safeguarding of the central office against damage
+by fire and water should be in all cases a feature of fundamental
+importance, and should influence not only the character of the building
+itself, but in many cases the choice of its location.
+
+_Size of Building._ It goes without saying that the building must be
+large enough to accommodate the switchboards and other apparatus that is
+required to be installed. The requirement does not end here, however.
+Telephone exchange systems have, with few exceptions, grown very much
+faster than was expected when they were originally installed. Many
+buildings have had to be abandoned because outgrown. In planning the
+building, therefore, the engineer should always have in mind its
+ultimate requirements. It is not always necessary that the building
+shall be made large enough at the outset to take care of the ultimate
+requirements, but where this is not done, the way should be left clear
+for adding to it when necessity demands.
+
+[Illustration: RINGING AND CHARGING MACHINES AND POWER BOARD Plaza
+Office, New York Telephone Co.]
+
+_Strength of Building._ The major portion of telephone central-office
+apparatus, whether automatic or manual, is not of such weight as to
+demand excessive strength in the floors and walls of buildings.
+Exceptions to this may be found in the storage battery, in the power
+machinery, especially where subject to vibration, and in certain cases
+in the cable runs. After the ultimate size of the equipment has been
+determined, the engineer and the architect should confer on this point,
+particularly with reference to the heavier portions of the apparatus,
+to make sure that adequate strength is provided. The approximate weights
+of all parts of central-office equipments may readily be ascertained
+from the manufacturers.
+
+_Provision for Employes._ In manual offices particularly it has been
+found to be not only humane, but economical to provide adequate quarters
+for the employes, both in the operating rooms and places where they
+actually perform their work, and in the places where they may assemble
+for recreation and rest. The work of the telephone operator,
+particularly in large cities, is of such a nature as often to demand
+frequent periods of rest. This is true not only on account of the
+nervous strain on the operator, but also on account of the necessity,
+brought about by the demands of economy, for varying the number of
+operators in accordance with the traffic load. These features accentuate
+the demand for proper rooms where recreation, rest, and nourishment may
+be had.
+
+_Provision for Cable Runways._ In very small offices no special
+structural provision need be made in the design of the building itself
+for the entrance of the outside cables, and for the disposal of the
+cables and wires leading between various portions of the apparatus. For
+large offices, however, this must necessarily enter as an important
+feature in the structure of the building itself. It is important that
+the cables be arranged systematically and in such a way that they will
+be protected against injury and at the same time be accessible either
+for repairs or replacement, or for the addition of new cables to provide
+for growth. Disorderly arrangement of the wires or cables results in
+disorder indeed, with increased maintenance cost, uneconomical use of
+space, inaccessibility, liability to injury, and general unsightliness.
+
+The carrying of cables from the basement to the upper floors or between
+floors elsewhere must be provided for in a way that will not be wasteful
+of space, and arrangements must be made for supporting the cables in
+their vertical runs. In the aggregate their weight may be great, and
+furthermore each individual cable must be so supported that its sheath
+will not be subject to undue strain. Another factor which must be
+considered in vertical cable runs is the guarding against such runs
+forming natural flues through which flames or heated gases would pass,
+in the event of even an unimportant fire at their lower ends.
+
+=Arrangement of Apparatus in Small Manual Offices.= Where a
+common-battery multiple switchboard equipment is used, at least three
+principal rooms should be provided--one for the multiple switchboard
+proper; one for the terminal and power apparatus, including the
+distributing frames, racks, and power machinery; and the third for the
+storage battery. These should adjoin each other for purposes of
+convenience and of economy in wiring.
+
+[Illustration: Fig. 418. Typical Small Office Floor Plan]
+
+_Floor Plans for Small Manual Offices._ As was pointed out, there are
+several plans of disposing of the main and intermediate distributing
+frames and the line and cut-off relay racks. The one most practiced is
+to mount the relay rack alongside the main and intermediate distributing
+frame in the terminal room. A typical floor plan of such an arrangement
+for a small office, employing as a maximum five sections of multiple
+switchboards, is shown in Fig. 418. This is an ideal arrangement well
+adapted for a rectangular floor space and on that account may often be
+put into effect. It should be noted that the switchboard grows from left
+to right, and that alternative arrangements are shown for disposing of
+those sections beyond the second. The cable turning section through
+which the multiple and answering jacks are led to the terminal frames is
+placed as close as possible to the terminal frames. This results in a
+considerable saving in cable. An interesting feature of this floor plan
+is the arrangement of unitary sections of main and intermediate frames
+and relay racks, representing recent practice of the Western Electric
+Company. The iron work of the three racks is built in sections and these
+are structurally connected across so that the first section of the main
+frame, the intermediate frame, and the relay rack form one unit, the
+structural iron work which ties them together forming the runway for the
+cables between them. But two of these units, including two sections of
+each frame, are shown installed, the provision for growth being
+indicated by dotted lines.
+
+The battery room in this case provides for the disposal of the battery
+cells in two tiers. This room is merely partitioned off from the
+distributing or terminal room. Where this is done the partition walls
+should be plastered on both sides so as to prevent, as far as possible,
+the entrance of any battery fumes into the apparatus rooms.
+
+The wire chief's desk, as will be noted, is located in such a position
+as to give easy access from it not only to the distributing frames and
+relay rack, but to the power apparatus as well.
+
+_Combined Main and Intermediate Frames._ For use in small exchanges, the
+Western Electric Company has recently put on the market a combined main
+and intermediate distributing frame. This is constructed about the same
+as an ordinary main frame, the protectors being on one side and the line
+and intermediate frame terminals on the other. The lower half of the
+terminals on each vertical bay is devoted to the outside line terminals
+and the upper half is devoted to intermediate frame terminals. This
+arrangement is indicated in the elevation in Fig. 419. With the use of
+this combined main and intermediate frame, the floor plan of Fig. 418
+may be modified, as shown in Fig. 420.
+
+[Illustration: Fig. 419. Combined Main and Intermediate Frames]
+
+[Illustration: Fig. 420. Small Office Floor Plan]
+
+[Illustration: Fig. 421. Terminal Apparatus--Small Office]
+
+In Fig. 421 is given an excellent idea of terminal-room apparatus
+carried out in accordance with the more usual plan of employing separate
+main and intermediate distributing frames. At the extreme right of this
+figure the protector side of the main frame is shown. It will be
+understood that the line cables terminate on the horizontal terminal
+strips on the other side of this frame and are connected through the
+horizontal and vertical runways of the frame to the protector terminals.
+The intermediate frame is shown in the central portion of the figure,
+the side toward the left containing the answering-jack terminals, and
+the side toward the right the multiple jack terminals, these latter
+being arranged horizontally. This horizontal and vertical arrangement of
+the terminals on the main and intermediate distributing frames has been
+the distinguishing feature between the Bell and Independent practice,
+the Bell Companies adhering to the horizontal and vertical arrangement,
+while the Independent Companies have employed the vertical arrangement
+on both sides. We are informed that in the future the new smaller
+installations of the Bell Companies will be made largely with the
+vertical arrangement on both sides. At the left of Fig. 421 is shown the
+relay rack in two sections of two bays each. This illustration also
+gives a good idea of the common practice in disposing of the cables
+between the frames in iron runways just below the ceiling of the
+terminal room.
+
+_Types of Line Circuits._ The design of the terminal-room floor plan
+will depend largely on the arrangement of apparatus in the subscribers'
+line circuits with respect to the distributing frames and relay racks.
+The Bell practice in this respect has already been referred to and is
+illustrated in Fig. 348. In this the line and cut-off relays are
+permanently associated with the answering jacks and lamps, resulting in
+the answering-jack equipment being subject to change with respect to the
+multiple and the line through the jumpers of the intermediate frame. The
+practice of the Kellogg Company, on the other hand, has been illustrated
+in Fig. 353, and in this the line and cut-off relays are permanently
+associated with the multiple and with the line, only the answering jacks
+and lamps being subject to change through the jumper wires on the
+intermediate frame. This latter arrangement has led to a very desirable
+parallel arrangement of the two distributing frames and the relay rack.
+These are made of equal length so as to correspond bay for bay, and are
+placed side by side with only enough space between them for the passage
+of workmen--the relay rack lying between the main and intermediate
+frames. In this scheme all the multiple and answering-jack cables run
+from the intermediate distributing frame, and the cabling between the
+intermediate frame and the relay rack and between the relay rack and the
+main frame is run straight across from one rack to the other. This
+results in a great saving of cable within the terminal room, over that
+arrangement wherein the cabling from one frame to another is necessarily
+led along the length of the frame to its end and then passes through a
+single runway to the end of the other frame.
+
+=Large Manual Offices.= For purposes of illustrating the practice in
+housing the apparatus in very large offices equipped with manual
+switchboards, we have chosen the Chelsea office of the New York
+Telephone Company as an excellent example of modern practice.
+
+[Illustration: Fig. 422. Floor Plan, Operating Room, Chelsea Office, New
+York City]
+
+The ground plan of the building is U-shaped, in order to provide the
+necessary light over the rather large floor areas. The plan of the
+operating floor--the sixth floor of the building--is shown in Fig. 422.
+As will be seen, this constitutes a single operating room, the _A_-board
+being located in the right wing and the _B_-board in the left. The point
+from which both boards grow is near the center of the front of the
+building, the boards coming together at this point in a common cable
+turning section. The disposal of the various desks for the manager,
+chief operator, and monitors is indicated. Those switchboard sections
+which are shown in full lines are the ones at present installed, the
+provision for growth being indicated in dotted lines.
+
+[Illustration: Fig. 423. Terminal Room and Operators' Quarters, Chelsea
+Office, New York City]
+
+The fifth floor is devoted to the terminal room and operators' quarters,
+the terminal room occupying the left-hand wing and the major portion of
+the front of the building, and the operators' quarters the right-hand
+wing. The line and the trunk cables come up from the basement of the
+building at the extreme left, being supported directly on the outside
+wall of the building. Arriving at the fifth floor, they turn
+horizontally and are led under a false flooring provided with trap
+doors, to the protector side of the main frame. The disposal of the
+cables between the various frames will be more readily understood by
+reference to the following photographs.
+
+A general view of a portion of the _A_-board of the Chelsea office is
+shown in Fig. 424, this view being taken from a point in the left-hand
+wing looking toward the front. In Fig. 425 is shown a closer view of a
+smaller portion of the board. Fig. 426 gives an excellent idea of the
+rear of this switchboard and of the disposal of the cables and wires.
+The main mass of cables at the top are those of the multiple.
+Immediately below these may be seen the outgoing trunk cables. The forms
+of the answering-jack cables lie below these and are not so readily
+seen, but the cables leading from these forms are led down to the runway
+at the bottom of the sections, and thence along the length of the board
+to the intermediate distributing frame on the floor below. The layer of
+cables, supported on the iron rack immediately above the answering-jack
+cable runway, shown at the extreme bottom of the view, are those
+containing the wires leading from the repeating coils to the cord
+circuits.
+
+An interesting feature of this board is the provisions for protection
+against injury by fire and water. On top of the boards throughout their
+entire length there is laid a heavy tarpaulin curtain with straps
+terminating in handles hanging down from its edges. These may be seen in
+Fig. 426 and also in Fig. 425. The idea of this is that if the board is
+exposed to a water hazard, as in the case of fire, the board may be
+completely covered, front and rear, with this tarpaulin curtain, by
+merely pulling the straps. The entire force--both operators and
+repairmen--is drilled to assure the carrying out of this plan.
+
+The rear of the boards is adapted to be enclosed by wooden curtains,
+similar to those employed in roll-top desks. These are all raised in the
+rear view of Fig. 426, the housing for the rolled-up curtain being shown
+at the extreme top of the sections. In order to guard the multiple
+cables and the multiple jacks against fire which might originate in the
+cord-circuit wiring, a heavy asbestos partition is placed immediately
+above the cord racks and is clearly shown in Fig. 426.
+
+[Illustration: Fig. 424. Subscribers' Board. Chelsea Office, New York
+City]
+
+[Illustration: Fig. 425. Subscribers' Board. Chelsea Office, New York
+City]
+
+[Illustration: Fig. 426. Rear View Chelsea Switchboard]
+
+[Illustration: Fig. 427. Terminal and Power Apparatus. Chelsea Office]
+
+A view of the terminal and power room is shown in Fig. 427. In the upper
+left-hand corner the cables may be seen in their passage downward from
+the cable turning section between the _A_- and _B_-boards. The large
+group of cables shown at the extreme left is the _A_-board multiple.
+This passes down and then along the horizontal shelves of the
+intermediate frame, which is the frame in the extreme left of this view.
+The _B_-board multiple comes down through another opening in the floor,
+and as is shown, after passing under the _A_-board multiple joins it in
+the same vertical run from which it passes to the intermediate frame.
+The cord-circuit cables lead down through the same opening as that
+occupied by the _A_-board multiple and pass off to the right-hand one of
+the racks shown, which contains the repeating coils. The cables leading
+from the opening in the ceiling to the right-hand side of the
+intermediate distributing frame are the answering-jack cables, and from
+the terminals on this side of this frame other cables pass in smaller
+groups to the relay terminals on the relay racks which lie between the
+intermediate frame and the coil rack.
+
+The power board is shown at the extreme right. The fuse panel at the
+left of the power board contains in its lower portion fuses for the
+battery supply leads to the operator's position and to private-branch
+exchanges, and in its upper portion lamps and fuses for the ringing
+generator circuits for the various operators' positions and also for
+private-branch exchanges.
+
+At the lower left-hand portion of this view is shown the battery
+cabinet. It is the practice of the New York Telephone Company not to
+employ separate battery rooms, but to locate its storage batteries
+directly in the terminal room and to enclose them, as shown, in a wooden
+cabinet with glass panels, which is ventilated by means of a lead pipe
+extending to a flue in the wall.
+
+One unit of charging machines, consisting of motor and generator, is
+shown in the immediate foreground. A duplicate of this unit is employed
+but is not shown in this view. The various ringing and message register
+machines are shown beyond the charging machines. Three of these smaller
+machines are for supplying ringing current and the remainder are for
+supplying 30-volt direct current for operating the message registers.
+One of the machines of each set is wound to run from the main storage
+battery in case of a failure of the general lighting service from which
+the current for operating is normally drawn.
+
+[Illustration: Fig. 428. Terminal Apparatus. Chelsea Office]
+
+[Illustration: Fig. 429. Floor Plan, Automatic Office, Lansing,
+Michigan]
+
+Another view of the terminal-room apparatus is given in Fig. 428. This
+is taken from the point marked _B_ on the floor plan of Fig. 423. At the
+right may be seen the message registers on which the calls of the
+subscribers in this office are counted as a basis for the bills for
+their service. At the extreme left is shown the private-line test board.
+Through this board run all of the lines leased for private use, and also
+all of the order wire or call lines passing through this office. The
+purpose of such an arrangement is to facilitate the testing of such line
+wires. At the right of this private-line test board is shown a
+four-position wire chief's desk, upon which are provided facilities for
+making all of the tests inside and outside.
+
+[Illustration: Fig. 430. Line-Switch Units]
+
+[Illustration: Fig. 431. Automatic Apparatus at Lansing Office]
+
+The main frame is shown at the right of Fig. 428, just to the right of a
+gallery from which a step-ladder leads. The left-hand side of this frame
+is the line or protector side, but the portion toward the observer in
+this picture is unequipped. These equipped protector strips carry 400
+pairs of terminals each, and the consequent length of these strips makes
+necessary the gallery shown, in order that all of them may be readily
+accessible.
+
+[Illustration: Fig. 432. Main Distributing Frame, Lansing Office]
+
+[Illustration: Fig. 433. Line Switches]
+
+[Illustration: POWER PLANT FOR AUTOMATIC SWITCHBOARD EQUIPMENT Bay
+Cities Home Telephone Company, Berkeley, Cal.]
+
+[Illustration: Fig. 434. Secondary Line Switches and First Selectors]
+
+=Automatic Offices.= There is no great difference in the amount of floor
+space required in central offices employing automatic and manual
+equipment. Whatever difference there is, is likely to be in favor of the
+automatic. The fact that no such rigid requirement exists in the
+arrangement of automatic apparatus, as that which makes it necessary to
+place the sections of a multiple board all in one row, makes it possible
+to utilize the available space more economically with automatic than
+with manual equipment.
+
+[Illustration: Fig. 435. Second Selectors]
+
+[Illustration: Fig. 436. Toll Distributing Frame and Harmonic
+Converters]
+
+In manual practice it is necessary to place the distributing frames and
+power apparatus in a separate room from that containing the switchboard,
+but in an automatic exchange no such necessity exists; in fact, so far
+as the distributing-frame equipment is concerned, it is considered
+desirable to have it located in the same room as the automatic switches.
+
+The battery room in an automatic exchange should be entirely separate
+from the operating room, since the fumes from the battery would be fatal
+to the proper working of the automatic switches.
+
+_Typical Automatic Office._ The floor-plan and views of a medium-sized
+automatic office at Lansing, Michigan, have been chosen as representing
+typical practice. The floor plan is shown in Fig. 429. The apparatus
+indicated in full lines represents the present equipment, and that in
+dotted lines the space that will be required by the expected future
+equipment.
+
+In Fig. 430 is shown a group of five line-switch units, representing a
+total of five hundred lines. The length of such a unit is practically
+fourteen feet and the breadth over all about twenty-two inches.
+
+Fig. 431 shows a general view of this Lansing office, taken from a point
+of view indicated at _A_ on the floor plan of Fig. 429. Fig. 432 shows
+the main distributing frame, which is of ordinary type; Fig. 433 shows a
+closer view of some of the primary line switches; Fig. 434 is a view of
+the secondary line switches and first selectors, the latter being on the
+right; Fig. 435 is a view of the frequency selectors and second
+selectors, the former being used in connection with party-line work; and
+Fig. 436 is a view of the toll distributing frame and harmonic
+converters for party-line ringing.
+
+A general view of the main switching room in the Grant Avenue office of
+the Home Telephone Company of San Francisco is given in Fig. 437, this
+being taken before the work of installation had been fully completed.
+The present capacity of the equipment is 6,000 and the ultimate 12,000
+lines. This office is one of a number of similar ones recently installed
+for the Home Telephone Company in San Francisco, the combination of
+which forms by far the largest automatic exchange yet installed. The
+scope of the plans is such as to enable 125,000 subscribers to be served
+without any change in the fundamental design, and by means merely of
+addition in equipment and lines as demanded by the future subscriptions
+for telephone service.
+
+[Illustration: Fig. 437. Grant Avenue Office--San Francisco]
+
+
+
+
+CHAPTER XXXIV
+
+PRIVATE BRANCH EXCHANGES
+
+
+=Definitions.= A telephone exchange devoted to the purely local uses of
+a private establishment such as a store, factory, or business office, is
+a private exchange. If, in addition to being used for such local
+communication, it serves also for communication with the subscribers of
+a city exchange, it becomes in effect a branch of the city exchange and,
+therefore, a private branch exchange. The term "P. B. X." has become a
+part of the telephone man's vocabulary as an abbreviation for private
+branch exchange.
+
+Private exchanges for purely local use require no separate treatment as
+any of the types of switching equipments for interconnecting the lines
+for communication, that have been or that will be described herein, may
+be used. The problem becomes a special one, however, when communication
+must also be had with the subscribers of a public exchange, since then
+trunking is involved in which the conditions differ materially from
+those encountered in trunking between the several offices in a
+multi-office exchange.
+
+For such communication one or more trunk lines are led from the private
+branch office usually to the nearest central office of the public
+exchange and such trunks are called private branch-exchange trunks. They
+are the paths for communication between the private exchange and the
+public exchange. For establishing the connections either between the
+local lines themselves or between the local lines and the trunks, and
+for performing other duties that will be referred to, one or more
+private branch-exchange operators are employed at the switchboard of the
+private establishment.
+
+The private branch exchange may operate in conjunction with a manual or
+an automatic public exchange, but whether manual or automatic, the
+private exchange is usually manually operated, although it is quite
+possible to make a private branch exchange that is wholly automatic and
+will, therefore, involve no operator at all.
+
+=Functions of the Private Branch-Exchange Operator.= It is possible, as
+just stated, entirely to dispense with the private branch-exchange
+operator so far as the mere connection and disconnection of the lines is
+concerned. But the real function of the private branch-exchange operator
+is a broader one than this and it is for this reason that even in
+connection with automatic public exchanges, operators are desirable at
+the private branches. The private branch-exchange operator is, as it
+were, the doorkeeper of the telephone entrance to the private
+establishment. She is the person first met by the public in entering
+this telephone door. There is the same reason, therefore, why she should
+be intelligent, courteous, and obliging as that the ordinary doorkeeper
+should possess these characteristics.
+
+As to incoming traffic to a private branch exchange, an intelligent
+operator may do much toward directing the calls to the proper department
+or person, even though the person calling may have little idea as to
+whom he desires to reach. This saves the time of the person who makes
+the call as well as that of the people at the private branch stations,
+since it prevents their being unnecessarily called.
+
+The functions of the private branch-exchange operator are no less
+important with respect to outgoing calls. It is the duty of the operator
+to obtain connections through the city exchange for the private branch
+subscriber, who merely asks for a certain connection and hangs up his
+receiver to await her call when she shall have obtained it. This saving
+of time of busy people by having the branch-exchange operator make their
+calls for them has one attending disadvantage, which is that the person
+in the city exchange who is called does not, when he answers his
+telephone, find the real party with whom he is to converse, but has to
+wait until that party responds to the private branch operator's call.
+This is akin to asking a person to call at one's office and then being
+out when he gets there. This drawback is greatly accentuated where both
+the parties that are to be involved in the connection are people high in
+authority in certain establishments at private branch exchanges. Some
+business houses have made the rule that the private branch operator
+shall not connect with their lines until she has actually heard the
+voice of the proper party at the other end. When two subscribers in two
+different private branch exchanges where this rule is enforced, attempt
+to get into communication with each other, the possibilities of trouble
+are obvious.
+
+All that may be said on this matter is that the person who calls another
+by telephone should extend that person the same courtesies that he would
+had he called him in person to his office; and that a person who is
+called by telephone by another should meet him with the same
+consideration as if he had received a personal call at his office or
+home. The arbitrary ruling made by some corporations and persons, which
+results always in the "other fellow's" doing the waiting, is not
+ethically correct nor is it good policy.
+
+=Private Branch Switchboards.= Private branch switchboards may be of
+common-battery or magneto types regardless of whether they work in
+conjunction with main office equipments having common-battery or magneto
+equipments. Usually a magneto private branch exchange works in
+conjunction with a magneto main office, but this is not always true.
+There are cases where the private branch equipment of modern
+common-battery type works in conjunction with main office equipment of
+the magneto type; and in some of these cases the private branch exchange
+has a much larger number of subscribers than the main office. This is
+likely to be true in large summer resort hotels located in small and
+otherwise unimportant rural districts. In one such case within our
+knowledge the private branch exchange has a larger number of stations
+than the total census population of the town, resulting in an apparent
+telephone development considerably greater than one hundred per cent.
+
+_Magneto Type._ Where both the private branch and the main office
+equipments are of the magneto type, the private branch requirements are
+met by a simple magneto switchboard of the requisite size, and the
+trunking conditions are met by ring-down trunks extending to the main
+office. In this case the supervision is that of the ordinary
+clearing-out drop type, the operators working together as best they may.
+
+_Common-Battery Type._ The cases where the private branch board is of
+common-battery type and the main office of magneto type are
+comparatively so few that they need not be treated here. Where they do
+occur they demand special treatment because the main portion of the
+traffic over the trunk lines to the city or town central office is
+likely to be toll traffic through that office over long-distance lines.
+The principal reason why the equipment of the town offices under such
+conditions is magneto rather than common battery is that the traffic
+conditions are those of short season and heavy toll, and common-battery
+switching equipment at the main office has no especial advantages for
+toll work.
+
+[Illustration: Fig. 438. Desk Type, Private Branch Board]
+
+For small private branch exchanges the desk type of switch board, shown
+in Fig. 438, is largely used. The operator frequently has other work to
+do and the desk is, therefore, a convenience. In larger private
+exchanges, such as those requiring more than one operator, some form of
+upright cabinet is employed, and if, as sometimes occurs, the branch
+exchange is of such size as to demand a multiple board, then the general
+form of the board does not differ materially from the standard types of
+multiple board employed in regular central office work. The most common
+private branch-exchange condition is that of a common-battery branch
+working into a common-battery main office. In such the main point to be
+considered is that of supervision of trunk-line connections.
+
+_Cord Type._ For the larger sizes of branch exchange switchboards, the
+switching apparatus is practically the same as that of ordinary manual
+switchboards wherein the connections are made between the various lines
+by means of pairs of cords and plugs. The private branch-exchange trunk
+lines usually terminate on the private branch board in jacks but in some
+cases plug-ended trunks are used.
+
+[Illustration: Fig. 439. Key Type, Private Branch Board]
+
+The line signals may consist in mechanical visual signals or in lamps,
+the choice between these depending largely on the source of battery
+supply at the branch exchange, a matter which will be considered later.
+The trunk-line signals at the private branch board are usually ordinary
+drops which are thrown when the main-exchange operator rings on the line
+as she would on an ordinary subscriber's line. Frequently, however, lamp
+signals are used for this purpose, being operated by locking relays
+energized when the main-office operator rings or, in some cases,
+operated at the time when the main-office operator plugs into the
+trunk-line jack.
+
+[Illustration: Fig. 440. Circuits, Key-Type Board]
+
+_Key Type._ For small private branch-exchange switchboards, a type
+employing no cords and plugs has come into great favor during recent
+years. Instead of connecting the lines by jacks and plugs, they are
+connected by means of keys closely resembling ordinary ringing and
+listening keys. Such a switchboard is shown in Fig. 439, this having a
+capacity of three trunks, seven local lines, and the equivalent of five
+cord circuits. The drops associated with the three trunks may be seen in
+the upper left-hand side of the face of the switchboard. Immediately
+below these in three vertical rows are the keys which are used in
+connecting the trunks with the "cord circuits" or connecting bus wires.
+At the right of the drop associated with the trunks are seven visual
+signals, these being the calling signals of the local lines. The seven
+vertical rows of keys, immediately to the right of the three trunk-line
+rows, are the line keys. The throwing of any one of these keys and of a
+trunk-line key in the same horizontal row in the same direction will
+connect a line with a trunk through the corresponding bus wires, leaving
+one of the supervisory visual signals, shown at the extreme top of the
+board, connected with the circuit. The keys in a single row at the right
+are those by means of which the operator may bridge her talking set
+across any of the "cord circuits." The circuits of this particular board
+are shown in Fig. 440. This is equipped for common-battery working, the
+battery feed wires being shown at the left.
+
+=Supervision of Private Branch Connections.= At the main office where
+common-battery equipment is used, the private branch trunks terminate
+before the _A_-operators exactly in the same way as ordinary
+subscribers' lines, _i. e._, each in an answering jack and lamp at one
+position and in a multiple jack on each section. It goes without saying,
+therefore, that the handling of a private branch call, either incoming
+or outgoing, should be done by the _A_-operator in the same manner as a
+call on an ordinary subscriber's line, and that the supervision of the
+connection should impose no special duties on the _A_-operator.
+
+There has been much discussion, and no final agreement, as to the proper
+method of controlling the supervisory lamp at the main office of a cord
+that is, at the time, connected to a private branch trunk. Three general
+methods have been practiced:
+
+The first method is to have the private branch subscriber directly
+control the supervisory lamp at the main office without producing any
+effect upon the private branch supervisory signal; this latter signal
+being displayed only after the connection has been taken down at the
+main office and in response to the withdrawal of the main office plug
+from the private branch jack. This is good practice so far as the
+main-office discipline is concerned but it results in a considerable
+disadvantage to both the city and private branch subscribers in that it
+is impossible for the private branch subscriber, when connected to the
+other, to re-signal the private branch operator without the connection
+being first taken down.
+
+The second method is to have the private branch subscriber control both
+the supervisory signal at the private branch board and at the main
+board. This has the disadvantage of bringing both operators in on the
+circuit when the private branch subscriber signals.
+
+The third method, and one that seems best, is to place the supervisory
+lamp of the private branch board alone under the control of the private
+branch subscriber, so that he may attract the attention of the private
+branch operator without disturbing the supervisory signal at the main
+office. The supervisory signal at the main office in this case is
+displayed only when the private branch operator takes down the
+connection. This practice results in a method of operation at the main
+office that involves no special action on the part of the _A_-operator.
+She takes down the connection only when the main-office subscriber has
+hung up his telephone and the private branch subscriber has disconnected
+from the trunk.
+
+Whatever method is employed, private branch disconnection is usually
+slow, and for this reason many operating companies instruct the
+_A_-operators to disconnect on the lighting of the supervisory lamp of
+the city subscriber.
+
+=With Automatic Offices.= Private branch exchanges most used in
+connection with automatic offices employ manual switchboards, with the
+cord circuits of which is associated a signal transmitting device by
+which the operator instead of the subscriber may manipulate the
+automatic apparatus of the public exchange by impulses sent over the
+private branch-exchange trunk lines. The subscriber's equipment at the
+private branch stations may be either automatic or manual. Frequently
+the same private branch exchange will contain both kinds. With the
+manual sub-station equipment the operation is exactly the same as in a
+private branch of a manual exchange, except that the private branch
+operator by means of her dial makes the central-office connection
+instead of telling the main-office operator to do so for her. With
+automatic sub-station equipment at the private branch the subscribers,
+by removing their receivers from their hooks, call the attention of the
+private branch operator, who may receive their orders and make the
+desired central-office connection for them, or who may plug their lines
+through to the central office and allow the subscribers to make the
+connection themselves with their own dials.
+
+In automatic equipment of the common-battery type, some change always
+takes place in the calling line at the time the called subscriber
+answers. In the three-wire system during the time of calling, both wires
+are of the same polarity with respect to earth. At the time of the
+answering of the called subscriber, the two wires assume different
+polarities, one being positive to the other. Such a change is sufficient
+for the actuation of devices local to the private exchange switchboard
+and may be interpreted through the calling supervisory signal in such a
+way as to allow it to glow during calling and not to glow after the
+called subscriber has answered. In the two-wire automatic system a
+similar change can be arranged for, with similar advantageous results.
+
+_Secrecy._ In private exchanges operating in connection with automatic
+central offices, the secret feature of individual lines may or may not
+be carried into the private exchange equipment. Some patrons of
+automatic exchanges set a high value on the absence of any operator in a
+connection and transact business over such lines which they would not
+transact at all over manual lines or would not transact in the same way
+over manual lines. To some such patrons, the presence of a private
+exchange operator, even though employed and supervised by themselves,
+seems to be a disadvantage. To meet such a feeling, it is not difficult
+to arrange the circuits of a private exchange switchboard so that the
+operator may listen in upon a cord circuit at any time and overhear what
+is being said upon it _so long as two subscribers are not in
+communication on that cord circuit_. That is, she may answer a call and
+may speak to the calling person at any time she wishes until the called
+person answers. When he does answer and conversation can take place,
+some device operates to disconnect her listening circuit from the cord
+circuit, not to be connected again until at least one of the subscribers
+has hung up his receiver. With private exchange apparatus so arranged,
+the secrecy of the system is complete.
+
+=Battery Supply.= There are three available methods of supplying direct
+current for talking and signaling purposes to private branch exchanges,
+each of which represents good practice under certain conditions. First,
+by means of pairs of wires extended from the central-office battery;
+second, by means of a local storage battery at the private branch
+exchange charged over wires from the central office; and third, by means
+of a local storage battery at the private exchange charged from a local
+source.
+
+The choice of these three methods depends always on the local conditions
+and it is a desirable feature, to be employed by large operating
+companies, to have all private branch-exchange switchboards provided
+with simple convertible features contained within the switchboard for
+adapting it to any one of these methods of supplying current.
+
+If a direct-current power circuit is available at the private branch
+exchange, it may be used for charging the local storage battery by
+inserting mere resistance devices in the charging leads. If the local
+power circuit carries alternating current, a converting device of some
+sort must be used and for this purpose, if the exchange is large enough
+to warrant it, a mercury rectifier is an economical and simple device.
+
+The supply of current to private branch exchanges over wires leading to
+the central-office battery has the disadvantage of requiring one or
+several pairs of wires in the cables carrying the trunk wires. No
+special wires are run, regular pairs in the paper insulated line or
+trunk cables being admirably suited for the purpose. Sufficient
+conductivity may be provided by placing several such pairs in multiple.
+
+If the amount of current required by the private exchange warrants it,
+pairs of charging wires from the central office may be fewer if a
+battery is charged over them than if they are used direct to the bus
+bars of the private exchange switchboard. If they are used in the latter
+way, and this is simpler for reasons of maintenance, some means must be
+provided to prevent the considerable resistance of the supply wires from
+introducing cross-talk into the circuit of the private exchange. This is
+accomplished by bridging a considerable capacity across the supply pairs
+at the private exchange--ten to twelve microfarads usually suffice. This
+point has already been referred to and illustrated in connection with
+Fig. 141.
+
+The number of pairs of wires, or, in other words, the amount of copper
+in the battery lead between the central office and the private
+branch-exchange switchboard needs to be properly determined not only to
+eliminate cross-talk when the proper condensers are used with them, but
+to furnish the proper difference of potential at the private exchange
+bus bars, so that the line and supervisory signals will receive the
+proper current. It is a convenience in installing and maintaining
+private exchange switchboards of this kind to prepare tables showing the
+number of pairs of No. 19 gauge and No. 22 gauge wires required for a
+private exchange at a given distance from its central office and of a
+probable amount of traffic. The traffic may be expressed in the maximum
+number of pairs of cords which will be in use at one time. With this
+fact and the distance, the number of pairs of wires required may be
+determined.
+
+=Ringing Current.= The ringing current may be provided in two ways: over
+pairs of wires from the city-office ringing machines or by means of a
+local hand generator, or both. A key should enable either of these
+sources of ringing current to be chosen at will.
+
+=Marking of Apparatus.= All apparatus should be marked with permanent
+and clear labels. That private exchange switchboard is best at which an
+almost uninformed operator could sit and operate it at once. It is not
+difficult to lay out a scheme of labels which will enable such a board
+to be operated without any detailed instructions being given.
+
+=Desirable Features.= The board should contain means of connecting
+certain of the local private exchange lines to the central-office trunks
+when the board is unattended. Also, it is desirable that it should
+contain means whereby any local private exchange line may be connected
+to the trunk so that its station will act as an ordinary subscriber's
+station. Whether the trunks of the private exchange lead to a manual or
+an automatic equipment, it often is desired to connect a local line
+through in that way, either so that the calling person may make his
+calls without the knowledge of the private exchange operator, because he
+wishes to make a large number of calls in succession, or because for
+some other reason he prefers to transact his business directly with or
+through the exchange than to entrust it to his operator.
+
+
+
+
+CHAPTER XXXV
+
+INTERCOMMUNICATING SYSTEMS
+
+
+=Definition.= The term "intercommunicating" has been given to a
+specialized type of telephone system wherein the line belonging to each
+station is extended to each of the other stations, resulting in all
+lines extending to all stations. Each station is provided with apparatus
+by means of which the telephone user there may connect his own telephone
+with the line of the station with which he wishes to communicate,
+enabling him to signal and talk with the person at that station.
+
+=Limitations.= The idea is simple. Each person does his own switching
+directly, and no operator is required. It is easy to see, however, that
+the system has limitations. The amount of line wire necessary in order
+to run each line to each station is relatively great, and becomes
+prohibitive except in exchanges involving a very small number of
+subscribers, none of which is remote from the others. Again, the amount
+of switching apparatus required becomes prohibitive for any but a small
+number of stations. As a result, twenty-five or thirty stations are
+considered the usual practical limit for intercommunicating systems.
+
+=Types.= An intercommunicating system may be either magneto or
+common-battery, according to whether it uses magneto or common-battery
+telephones. The former is the simpler; the latter is the more generally
+used.
+
+[Illustration: WESTERN ELECTRIC COMPANY BATTERY ROOM AT MONMOUTH,
+ILLINOIS]
+
+=Simple Magneto System.= The schematic circuit arrangement of an
+excellent form of magneto intercommunicating system is given in Fig.
+441. In this, five metallic circuit lines are led to as many stations,
+an ordinary two-contact open jack being tapped off of each line at each
+station. A magneto bell of the bridging type is permanently bridged
+across each line at the station to which that line belongs. The
+telephone at each station is an ordinary bridging magneto set except
+that its bell is, in each case, connected to the line as just stated.
+Each telephone is connected through a flexible cord to a two-contact
+plug adapted to fit into any of the jacks at the same station.
+
+The operation is almost obvious. If a person at Station _A_ desires to
+call Station _E_, he inserts his plug into the jack of line _E_ at his
+station and turns his generator crank. The bell of Station _E_ rings
+regardless of where the plug of that station may be. The person at
+Station _E_ responds by inserting his own plug in the jack of line _E_,
+after which the two parties are enabled to converse over a metallic
+circuit. It makes no difference whether the persons, after talking,
+leave these plugs in the jacks or take them out, since the position of
+the plug does not alter the relation of the bell with the line.
+
+[Illustration: Fig. 441. Magneto Intercommunicating System]
+
+This system has the advantage of great simplicity and of being about as
+"fool proof" as possible. It is, however, not quite as convenient to use
+as the later common-battery systems which require no turning of a
+generator crank.
+
+=Common-Battery Systems.= In the more popular common-battery systems two
+general plans of operation are in vogue, one employing a plug and jacks
+at each station for switching the "home" instrument into circuit with
+any line, and the other employing merely push buttons for doing the
+same thing. These may be referred to as the plug type and the
+push-button type, respectively.
+
+[Illustration: Fig. 442. Plug Type of Common-Battery Intercommunicating
+System]
+
+_Kellogg Plug Type._ The circuits of a plug type of intercommunicating
+system, as manufactured by the Kellogg Company, are shown in Fig. 442.
+While only three stations are shown, the method of connecting more will
+be obvious.
+
+This system requires as many pairs of wires running to all stations as
+there are stations, and in addition, two common wires for ringing
+purposes. The talking battery feed is through retardation coils to each
+line. When all the hooks are down, each call bell is connected between
+the lower common wire and the tip side of the talking circuit individual
+to the corresponding station. The ringing buttons at each station are
+connected between the tip of the plug at that station and the upper
+common wire. As a result, when a person at one station desires to call
+another, it is only necessary for him to insert his plug in the jack of
+the desired station and press his ringing button; the circuit being
+traced from one pole of the ringing battery through the upper common
+ringing wire, ringing key of the station making the call, tip of plug,
+tip conductor of called station's line, bell of called station, and back
+to the ringing battery through the lower common ringing wire.
+
+[Illustration: Fig. 443. Push-Button Wall Set]
+
+_Kellogg Push-Button Type._ Fig. 443 shows a Kellogg wall-type
+intercommunicating set employing the push-button method of selecting,
+and Fig. 444 shows the internal arrangement of this set.
+
+[Illustration: Fig. 444. Push-Button Wall Set]
+
+_Western Electric System._ The method of operation of the push-button
+key employed in the intercommunicating system of the Western Electric
+Company is well shown in Fig. 445. When the button is depressed all the
+way down, as shown in the center cut of Fig. 445, which represents the
+ringing position of the key, contact is made with the line wires of the
+station called, and ringing current is placed on the line. When the
+pressure is released, the button assumes an intermediate position, as
+shown in the right-hand cut, which represents the talking position of
+the key and in which the ringing contacts _1_ and _2_ are open, but
+contact with the line for talking purposes is maintained. The key is
+automatically held in this intermediate position by locking plate _3_
+until this plate is actuated by the operation of another button which
+releases the key so that it assumes its normal position as shown in the
+left-hand cut. When a button is depressed to call a station, it first
+connects the called station's line to the calling station through the
+two pairs of contacts _4_ and _5_ and then connects the ringing battery
+to that line by causing the spring _1_ to engage the contact _2_. The
+ringing current then passes through the bell at the called station,
+through the back contacts of the switch hook at that station, over one
+side of the line, and through the "way-down" contact _1_ of the button
+at the calling station, thence over the other side of the battery line
+back to the ringing battery, operating the bell at the called station.
+
+[Illustration: Fig. 445. Push-Button Action, Western Electric System]
+
+The circuits of the Western Electric system are similar to those of Fig.
+442, but adapted, of course, to the push-button arrangement of switches.
+Two batteries are employed, one for ringing and the other for talking,
+talking current being fed to the lines through retardation coils to
+prevent interference or cross-talk from other stations which might be
+connected together at the same time.
+
+_Monarch System._ As the making of connections in an intercommunicating
+system is entirely in the hands of the user, it is desirable that the
+operation be simple and that carelessness on the part of the user result
+in as few evil effects as possible. For instance, the leaving of the
+receiver off its hook will, in many systems, result in such a drain on
+the battery as to greatly shorten its life.
+
+The system of the Monarch Company has certain distinctive features in
+this respect. It is of the push-button type and as in the system just
+discussed, one pressure of the finger on one button clears the station
+of previous connections, rings the station called, and establishes a
+talking connection between the caller's telephone and the line desired.
+In addition to this, the system is designed to eliminate battery waste
+by so arranging the circuits that the battery current does not flow
+through either called or calling instrument until a complete connection
+is made--the calling button down at one station, the home button down at
+the called station, and both receivers off the hook. It does not hurt
+the batteries, therefore, if one neglects to hang up his receiver.
+
+[Illustration: Fig. 446. Push-Button Wall Set]
+
+[Illustration: Fig. 447. Push-Button Action, Monarch System]
+
+Three views of the wall set of this system are shown in Fig. 446, which
+illustrates how both the door and the containing box are separately
+hinged for easy access to the apparatus and connecting rack. As in the
+Western Electric and Kellogg push-button systems, each push-button key
+has three positions, as shown in Fig. 447. The first button shows all
+the springs open, the normal position of the key. The second button is
+in the half-way or talking position with all the springs, except the
+ringing spring, in contact. The third button shows the springs all in
+contact, the condition which exists when ringing a station.
+
+The mechanical construction of the key is shown in Fig. 448. Each button
+has a separate frame upon which the springs are mounted. Any one of the
+frames with its group of contact springs may be removed without
+interfering with either the electrical or the mechanical operation of
+the others. This is a convenient feature, making possible the
+installation of as few stations as are needed at first, and the
+subsequent addition of buttons as other stations are added.
+
+[Illustration: Fig. 448. Push-Button Keys]
+
+The restoring feature is a horizontal metal carriage, in construction
+very much like a ladder--one round pressing against each key frame, due
+to the tension on the carriage exerted by a single flat spring. The
+plunger of each button is equipped with a shoulder, which normally is
+above the round of the ladder. When the button is operated, this
+shoulder presses against a round of the carriage forcing it over far
+enough so that the shoulder can slip by. The upper surface of the
+shoulder is flat, and on passing below the pin, allows the carriage to
+slip back into its normal position and the pin rests on the top of the
+shoulder holding the plunger down. This position places the talking
+springs in contact. The ringing springs are open until the plunger is
+pressed all the way down, then the ringing contact is made. When the
+pressure is released, the plunger comes back to the half-way or talking
+position, leaving the ringing contacts open again.
+
+When another button is pressed, the same operation takes place and, by
+virtue of the carriage being temporarily displaced, the original key is
+left free to spring back to its normal position.
+
+Each station is provided with a button for each other station and a
+"home" button. The salient feature of the system is that before a
+connection may be established, the button at the calling station
+corresponding to the station called and also the home button of the
+station called must be depressed, if it is not already down. The home
+key at any station, when depressed, transposes the sides of the line
+with respect to the talking apparatus. The home key also has a spring
+which changes the normal connection of the line at that station from the
+negative to the positive side of the talking battery. Unless, therefore,
+a connection between two stations is made through the calling key at one
+station and the home key at the other, no current can flow even though
+both receivers are off their hooks, because in that case no connection
+will exist with the positive side of the battery. This relation is shown
+in Fig. 449, which gives a simplified circuit arrangement for two
+connected stations.
+
+[Illustration: Fig. 449. Monarch Intercommunicating System]
+
+Referring to Fig. 449, when the station called depresses the home button
+the talking circuit is then completed after the hook switch is raised.
+This is because the talking battery is controlled by the home key.
+Conductors from both the negative and the positive sides of the battery
+enter this key. In the normal position of the springs, the negative side
+of the battery is in contact with the master spring in the home key and
+through these springs the negative battery is applied to all the calling
+keys, and from there on to the hook switch. When, however, the home
+button is operated, the spring which carries the negative battery to
+the home key is opened, and the spring which carries the positive
+battery is closed. This puts the positive battery on at the hook switch
+instead of the negative battery, as in its normal condition.
+
+In this system it is seen that a separate pair of line wires is used for
+each station, and in addition to these, two common pairs are run to all
+stations, one for ringing and one for talking battery connections.
+
+=For Private Branch Exchanges.= So far the intercommunicating system has
+been discussed only with respect to its use in small isolated plants. It
+has a field of usefulness in connection with city exchange work, as it
+may be made to serve admirably as a private branch exchange. Where this
+is done, one or more trunk lines leading to an office of the city
+exchange are run through the intercommunicating system exactly as a
+local line in that system, being tapped to a jack or push button at
+every station. A person at any one of the stations may originate a call
+to the main office by inserting his plug in the trunk jack, or pushing
+his trunk push button. Also any station, within hearing or sight of the
+trunk-line signal from the main office, may answer a main-office call in
+the same way. In order that the convenience of a private branch exchange
+may be fully realized, however, it is customary to provide an
+attendant's station at which is placed the drop or bell on which the
+incoming trunk signal is received. The duty of this attendant during
+business hours is to answer trunk calls from the main office and finding
+out what party is desired, call up the proper station on the
+intercommunicating system. The party at that station may then connect
+himself with the trunk.
+
+The practice of the Dean Company, for instance, is as follows in regard
+to trunking between intercommunicating systems and main offices with
+common-battery equipment. The attendant's station telephone cabinet
+contains, besides the push-button keys for local and trunk connections,
+a drop signal and release key, together with relays in each trunk
+circuit. The latter are used to hold the trunks until the desired party
+responds.
+
+The main-exchange trunk lines, besides terminating at the attendant's
+station, are wired through the complete intercommunicating system so
+that any intercommunicating telephone can be connected direct to the
+central office by depressing the trunk key, which is provided with a
+button of distinctive color. The pressing of the trunk key allows the
+telephone to take its current from the main-office storage battery and
+to operate the main-office line and supervisory signals direct, without
+making it necessary to call on the attendant to set up the connection.
+
+[Illustration: Fig. 450. Junction Box]
+
+[Illustration: Fig. 451. Typical Arrangement of Intercommunicating
+System]
+
+Incoming calls from the common-battery main office to the
+intercommunicating system are all handled by the attendant. The
+main-office operator signals the intercommunicating system by ringing,
+the same as for a regular subscriber's line. This will operate a drop in
+the attendant's station cabinet, and through an armature contact, give a
+signal on a low-pitched buzzer. This alarm buzzer operates only when the
+main exchange is ringing and, therefore, does not require that the drop
+shutter be restored immediately. An extra key may be provided for an
+extension night-alarm bell, for use where the attendant also does work
+in a room separate from that containing the attendant's station
+telephone equipment.
+
+The attendant operator answers the main-line signal by pressing the
+proper trunk button, as designated by the operated drop on the
+attendant's cabinet. The answering of the trunk connects a locking relay
+across the circuit so that the attendant may call the desired party on
+the intercommunicating system without having to hold the trunk manually.
+The party desired is then notified which trunk to use and the attendant
+operator hangs up her receiver, no further attention being necessary on
+her part.
+
+The trunk-holding relay is automatically released when the desired party
+(with the telephone receiver off the hook) depresses the proper trunk
+button, thus clearing the trunk line of all bridged apparatus and making
+the talking circuit the same as in the regular type of private
+branch-exchange switchboard.
+
+The most convenient way of installing the wires of an intercommunicating
+system is to run a cable containing the proper number of pairs to
+provide for the ultimate number of stations to all the stations, tapping
+off from the conductors in the cable to the jacks or push buttons at
+each station. These tap connections are best made by means of junction
+boxes which contain terminals for all the conductors.
+
+Such a junction box, with the through cable and the tap cable in place,
+is illustrated in Fig. 450. A schematic lay-out of the various parts of
+a Dean intercommunicating system, provided with an attendant's station
+and with trunks to a city office, is given in Fig. 451.
+
+
+
+
+CHAPTER XXXVI
+
+LONG-DISTANCE SWITCHING
+
+
+=Definitions.= Telephone messages between communities are called
+long-distance messages. They are also called toll messages. Almost all
+long-distance traffic is handled by message-rate (measured-service)
+methods of charge. All measured-service messages are toll messages,
+whether they are completed within a given community or between
+communities. The term "long-distance," therefore, is more descriptive
+than the term "toll." The subject of local and long-distance measured
+service is treated exhaustively in a chapter of its own.
+
+Some telephone-exchange operating companies call their own inter-city
+business "toll," and use the term "long-distance" for business carried
+between exchanges for them by another company. The distinction seems to
+be unwarranted.
+
+=Use of Repeating Coil.= Most long-distance lines are magneto circuits.
+If they are switched to grounded circuits, repeating coils need to be
+inserted. Toll switching equipments contain means of inserting repeating
+coils in the connecting cords when required. Their use reduces the
+volume of transmitted speech, but often is essential even in connecting
+metallic circuit lines, as a quiet local metallic circuit may have a
+ground upon it which will cause excessive noises when a quiet
+long-distance line is connected to it.
+
+=Switching through Local Board.= In the simplest form of long-distance
+switching, the lines terminate in switchboards with local lines and may
+be connected with each other and with the local lines through the
+regular cord circuits, if the equipment be of the magneto type. The
+waystations on such a line are equipped with magneto generators. These
+waystations may signal each other by bell ringing; the central office
+may call any waystation by ringing the proper signal and may supervise
+in a way all traffic on such lines by noting the calls for other
+stations than the supervising exchange.
+
+=Operators' Orders.= _By Call Circuits._ Where the long-distance traffic
+between two communities is large, economy requires that the sending of
+signals by ringing over the line, waiting for an answer, and then
+reciting the details of the call, be improved upon. If the traffic is
+large and the distance between communities small, call circuits are
+established in the same way as between the switchboards in several
+manual central offices of an exchange. The long-distance operator
+handling the originating call passes the necessary details to the
+distant operator by telephone over the call circuit. Such circuits also
+are known as order circuits. They are accessible to originating
+operators at keys and are connected directly and permanently to the
+telephone sets of receiving operators. One call circuit can handle the
+orders for a large number of actual conversation circuits. The operator
+at the receiving end designates the conversation circuit which shall be
+used, the originating operator following that instruction.
+
+_By Telegraph._ Where traffic and distance are large, conversation lines
+cost more than in the case last assumed. It then is of greater
+importance to use all the possible talking circuits for actual
+conversations in order that the revenue may be as high as possible. A
+phantom circuit good enough for call circuit purposes would be good
+enough for actual commercial messages, therefore, it is customary to
+furnish such originating and receiving operators with Morse telegraph
+sets. The lines are obtained by applying composite apparatus to the
+conversation circuits. Two Morse circuits can be had from each
+long-distance line without impairing any quality of that line except the
+ability to ring over it. As one Morse circuit can carry information
+enough between two operators to enable them to keep many telephone
+circuits busy, they do not need to ring upon the composited lines, so
+that nothing is lost while revenue is gained.
+
+=Two-Number Calls.= In cases where the traffic between communities is
+large, where the rate is small, and where the conversations are short
+and more on the general order of local calls, it is usual to handle the
+switches exactly as local calls are trunked between central offices of
+the same exchange. That is, the subscriber's operator who answers the
+call trunks it, by the assistance of a call circuit and an incoming
+trunk operator. The subscriber's operator records only the numbers of
+the calling and called subscribers. No long-distance operators at all
+assist in these connections. They are known as "two-number calls." The
+calling subscriber remains at his telephone until the conversation is
+finished.
+
+=Particular-Party-Calls.= In cases where the traffic is smaller, and
+where the rate is large, it is customary to handle the calls through
+long-distance operators. The ticket records the particular party wished,
+and the calls are named "particular party" calls. In such connections
+the calling patron is allowed to hang up his receiver, after his call is
+recorded, and is called again when his correspondent is found and is
+ready to talk. This makes _all calls for conversations_ outgoing ones.
+Only recording operators receive calls _from_ patrons. Line operators
+make calls _to_ patrons.
+
+=Trunking.= Long-distance lines entering a city usually terminate in one
+office only, no matter how many offices the local exchange may have. It
+is possible to terminate these long-distance lines on a position of the
+multiple switchboard for local lines. For a variety of reasons this is
+not practiced except in special cases. The usual method is to terminate
+them in a special long-distance board and to provide trunk lines from
+this board to the one or more local switchboards of the exchange. In
+common-battery systems these toll trunks are so arranged that the called
+local subscriber receives transmitter current from the office nearest to
+him, yet is able to show the long-distance operator the position of his
+switch hook and is able to be called by the long-distance operator
+without the intervention of the switching operator in the local office,
+even though two repeating coils may be in the trunk circuit.
+
+_Through Ringing._ There is a distinct traffic advantage in having the
+ringing of the subscriber under the control of the long-distance
+operator. The latter may call for the subscriber by stating her wish
+over the call circuit associated with the long-distance trunk. The
+connection having been made by the switching operator, the long-distance
+operator may withhold ringing the subscriber's bell until all is in
+readiness for the conversation.
+
+_High-Voltage Toll Trunks._ In some systems, the long-distance trunks
+are further specialized by being enabled to furnish transmitter current
+to subscribers at a higher voltage than is used in local conversations.
+With a given construction of transmitters there is a critical maximum
+current which can be carried by the granular carbon of the instrument
+without excessive heating, consequent noises, and permanent damage. The
+shortest lines and the longest lines of an exchange district being
+served by a source of current common to all, the standard potential of
+this source must be such as to give the longest lines current enough
+without giving the shortest lines too much. The very longest local
+lines, however, do not receive current enough from the standard
+potential to give maximum efficiency when talking over long distances,
+though they get enough for local conversations. By providing a battery
+with a voltage twice that used for local conversations and connecting it
+into the current supply element of the toll trunk through non-inductive
+resistances, not too much current may be given to the shortest lines and
+considerably more than normal current to the longest lines.
+
+=Ticket Passing.= When only one operator is necessary in a town, her
+duty being to switch both local and long-distance lines, she may write
+her own tickets and execute them entire. In larger communities with
+larger long-distance traffic, the duties need to be specialized. The
+subscribers' wants as to long-distance connections are given by
+themselves to recording long-distance operators, who write them on
+tickets and pass these to operators who get the parties together. The
+problem of ticket-passing becomes important and many mechanical carriers
+have been tried, culminating in the system which utilizes vacuum tubes.
+This is in some ways similar to vacuum or compressed-air tube systems
+for carrying cash in retail stores. The ticket is carried, however,
+without any enclosing case and the tubes are flat instead of round, _i.
+e._, they are rectangular in section. By suitable means a vacuum is
+maintained in a large common tube having a tap to a box-like valve at
+each line operator's position. A ticket tube connects this valve with a
+distributing table at or near which the tickets are written. The tickets
+are of uniform size and are so made as to enable a flap to be bent up
+easily along one edge. The distributing operator has merely to insert
+the ticket, bent edge foremost, in the open end of the tube, whereupon
+the air pressure behind it will drive it through to its destination,
+near by or far away. The tickets travel thirty feet a second. The tube
+may be bent into almost any required form. The ticket, on arriving at a
+line operator's position, slides between two springs, breaking a shunt
+around a relay and allowing the latter to light the lamp.
+
+=Waystations.= Waystations on long-distance lines may be equipped in
+several ways. Most of them have magneto sets and can ring each other.
+Some are equipped with common-battery sets and get all current for
+signaling and transmission from a terminal central office. In the latter
+case, there is the advantage that the ringers are in series with
+condensers, assisting greatly in tests for fault locations. Such tests
+are hindered by the presence of ringer bridges across the line, as in
+magneto practice. Condensers can be inserted in series with ringers of
+magneto sets if the testing advantage is valued highly enough. A
+disadvantage of the use of common-battery sets in waystations on
+long-distance lines is the lessened transmission volume of the stations
+farthest from the current source.
+
+_Center Checking._ An operating advantage of common-battery sets on
+long-distance lines is that all calls are forced to be answered by the
+terminal station. Waystations can not call each other, as they have no
+calling means. With magneto sets, waystation agents sometimes call each
+other direct and neglect to record the call and to remit its price. When
+they can not call each other direct, the revenues of the company
+increase.
+
+A traffic method which requires all calls from waystations to be made to
+a central switching office is called a center-checking system. It is so
+called because all checking for stations so switched is done at the
+central point instead of each waystation keeping its own records of
+calls sent and received. In such practice it is usual to bill each
+station once a month for the messages it sent. Where center checking is
+not practiced, the agent makes a report and sends a remittance. Center
+checking comes about naturally for waystations having no ringing
+equipment.
+
+Center checking originated long before the invention of common-battery
+systems. It requires merely that no waystation shall have a generator
+which can ring a bell. The method most widely used is to equip the
+waystations with magneto generators which produce direct currents only;
+such a generator cannot operate a polarized ringer. It is not usual to
+produce the direct current by actually rectifying the alternating
+current, but merely by omitting half the impulses, sending to the line
+only alternate half-cycles of the current generated. Any drop or relay
+adapted to respond to regular ringing current will respond to this
+modified form of generator.
+
+
+
+
+CHAPTER XXXVII
+
+TELEPHONE TRAFFIC
+
+
+The term "traffic," with reference to telephone service, has come to
+mean the gross transaction of communication between telephone users.
+This traffic may be expressed in whatever terms are found convenient for
+the particular phase considered.
+
+=Unit of Traffic.= With reference to payment for local telephone
+service, the conversation is the unit of traffic. In the daily
+operations of telephone systems there are fewer conversations than there
+are connections and fewer connections than there are calls, because
+lines are found busy and all calls to subscribers are not answered.
+
+For these reasons, in traffic inquiries which have to do with the amount
+of business which subscribers attempt to transact, the total traffic in
+a given time usually is considered as so many calls originated by the
+subscribers in the community. From this condition arises the term
+"originating calls."
+
+For the reason that the purpose of the switching equipment in a central
+office is to make connections, the abilities of operators and of
+equipments frequently are measured in terms of connections per hour or
+per other unit of time.
+
+For the reason that in charging for service all unavailing calls are
+omitted, the conversation is the unit of traffic.
+
+=Traffic Variations.= Telephone-exchange traffic is subject to such
+general variations as are noted in the way a compass needle points
+north, the migrations of birds, the blowing of the trade winds, and
+other natural phenomena. There are variations in traffic which occur
+each day, others which change with the seasons, and still others which
+are related to holidays and other special commercial and social events.
+For instance, the day before Thanksgiving Day, in many regions, is the
+busiest telephone traffic day in the year.
+
+[Illustration: WESTERN ELECTRIC MOTOR-GENERATOR CHARGING SET]
+
+The daily variations in telephone traffic are closely related to
+commercial activities and certain general features of this daily
+variation are common to all telephone systems everywhere. Fig. 452 is a
+typical graphic record of the traffic of a telephone exchange and
+represents what happens in almost every town or city. The total calls in
+this figure are not given as absolute units but would vary to adapt the
+figure to a particular case. The figure shows principally that the
+traffic in the night is light; that it rises to its maximum height
+somewhere between 10 o'clock A.M. and noon; that though it is never as
+high again during that day, the afternoon peak is over 80 per cent as
+great; and that two minor peaks appear about the dinner hour and after
+evening entertainments.
+
+[Illustration: Fig. 452. Load Curve]
+
+_Busy-Hour Ratio._ If the story told by Fig. 452 were to be turned into
+a table of calls per hour, the busiest hour of the day would be found to
+correspond to the highest portion of the figure, and in that busiest
+hour of the day, if a number of selected days were to be compared, would
+be found a very constant traffic. The number of calls made, or the
+number of connections completed, in that particular hour, day by day,
+would be found to be much the same. The ratio of the number of units in
+that hour to the number of units in that entire day would be found to
+be practically the same ratio day by day. This ratio of busy hour to
+total day would be found to be much more nearly constant than the gross
+number of calls per hour or per day.
+
+In a large, busy city, about one-eighth of the total daily calls are in
+some one hour; in a smaller, less active city, probably one-tenth are so
+congested. This is reasonable when one remembers that in the larger city
+the active business of the day begins later and ends earlier.
+
+=Importance of Traffic Study.= A knowledge of the amount of traffic in
+an exchange, and its distribution as to time and as to the divisions of
+the exchange, is important for a number of reasons. Traffic knowledge is
+essential in order that the equipment may be designed and placed in the
+proper way and the total load distributed properly on that apparatus and
+its operators.
+
+For example, in an office equipped with a manual multiple switchboard,
+the length of the switchboard is governed entirely by the number of
+operators who must work before it. It is mechanically possible to make a
+switchboard for ten thousand lines only 15 feet long, seating seven
+operators. The entire multiple of ten thousand lines could appear three
+times in such a switchboard. The seven operators could not handle the
+traffic we know would be originated by ten thousand lines, with any
+present system of charging for service. Even a rough knowledge of the
+probable traffic would enable us to approximate the number of operators
+needed and to equip each position, not only with access to the ten
+thousand lines to be called, but also with just enough keyboard
+equipment, serving as tools, and just enough answering jacks, serving as
+means of bringing the traffic to her. It is foreknowledge of traffic
+which enables a switchboard to fit the task it is to perform.
+
+=Rates of Calling.= The rates of calling of different kinds of lines
+vary. The lines of business stations originate more calls than do the
+lines of residences. Some kinds of business originate more calls than
+others. Some kinds of business have a higher rate of calling in one
+season than in others. Flat-rate lines originate more calls than do
+message-rate lines. When a line changes from a flat rate to a message
+rate, the number of originating calls per day decreases. An operator's
+position, handling message-rate lines only, can serve more lines than if
+all of them were at flat rates. The number of message-rate or
+coin-prepayment lines which an operator's position can care for depends
+not only on the traffic but on the method of charging for service,
+whether by tickets or meters and upon the kind of meters; or it depends
+on the method of collecting the coins. In some regions, the rate of
+calling, on the introduction of a complete measured-service plan, has
+been reduced to one-fourth of what it was on the flat-rate plan.
+
+In manual switchboards of early types, wherein the position of the
+subscriber's answering jack was fixed by his telephone number, the
+inequality of traffic became a serious problem. Most of the subscribers
+who first installed telephones when the exchange was small, retained
+their telephones and numbers; as their use of the telephone grew with
+their business, it was customary to find the positions answering the
+lower numbers much more busy than the positions answering the higher
+numbers, the latter belonging to later and usually less active business
+places.
+
+_Functions of Intermediate Distributing Frame._ The intermediate
+distributing board was invented to meet these conditions of unequal
+traffic upon lines and of variations in traffic with changes of seasons
+and of charges. The intermediate distributing board enables a line to
+retain its number and its position in the multiple, but to keep its
+answering jack and lamp signal in any desired position. If a flat-rate
+subscriber changes to a message rate, his line may be moved to a
+message-rate position and be answered, in company with others like it,
+by an operator serving many more lines than she could serve if all of
+them were flat rate.
+
+=Methods of Traffic Study.= The best way to learn traffic facts for the
+purposes of designing and operating equipment is to conduct systematic
+series of observations in all exchanges; to record them in company with
+all related facts; and to compare them from time to time, recording the
+results of the comparisons. Then when it is required to solve a new
+problem, the traffic data will enable the probable future conditions to
+be known with as great exactness as is possible in studies with relation
+to transportation or any other human activity.
+
+TABLE XIII
+
+Calling Rates
+
+  +-------------------------+-------------------------------+
+  |                         |  CALLS PER DAY WITH DIFFERENT |
+  |  KIND OF SERVICE        |       METHODS OF CHARGE       |
+  |                         +-------------+-----------------+
+  |                         |  FLAT RATE  |   MESSAGE RATE  |
+  +-------------------------+-------------+-----------------+
+  |Residence                |         8   |             4   |
+  |Business                 |  12 to 20   |       8 to 14   |
+  |Private Exchange Trunk   |        40   |            25   |
+  |Hotel Exchange Trunk     |        50   |            30   |
+  |Apartment House Trunk    |        30   |            18   |
+  +-------------------------+-------------+-----------------+
+
+There are three general ways of observing traffic. A record of
+originating calls is known as a "peg count," because the counting
+formerly was done by moving a peg from place to place in a series of
+holes. The simplest exact way is to provide each operator with a small
+mechanical counter, the key of which she can depress once for each call
+to be counted. A second way is to determine a ratio which exists, for
+the particular time and place, between the number of calls in a given
+period and the average number of cord circuits in use. Knowing this
+ratio, the cord circuits can be counted, the ratio applied, and the
+probable total known. The third method, which is applicable to offices
+having service meters on all lines, is to associate one master meter per
+position or group of lines with all the meters of that position or
+group, so that each time any service meter of that position is operated,
+the master meter will count one unit. This method applies to either
+manual or automatic equipments.
+
+=Representative Traffic Data.= For purposes of comparison, the following
+are representative facts as to certain traffic conditions.
+
+_Calling Rates._ The number of calls originated per day by different
+kinds of lines with different methods of charge are shown in Table XIII.
+
+_Operators' Loads._ The abilities of subscribers' operators to switch
+these calls depend on the type of equipment used, on the kind of
+management exercised, and on the individual skill of operators. With
+manual multiple equipment of the common-battery type, and good
+management, the numbers of originating calls per busy hour given in
+Table XIV can be handled by an average operator. The number of calls per
+operator per busy hour depends upon the amount of trunking to other
+offices which that operator is required to do. In a small city, for
+example, where all the lines are handled by one switchboard, there is no
+local switching problem except to complete the connection in the
+multiple before each position. In a large city, where wire economy and
+mechanical considerations compel the lines to be handled by a number of
+offices with manual equipment, some portion of the total originating
+load of each office must be trunked to others. Table XIV shows that an
+increase of 90 per cent in the amount of out-trunking has decreased the
+operator's ability to less than 70 per cent of the possible maximum.
+
+TABLE XIV
+
+Effect of Out-Trunking on Operator's Capacity
+
+  +----------------------------+---------------------------------------+
+  |PER CENT ORIGINATING CALLS  |  CAPACITY OF SUBSCRIBERS' OPERATOR'S  |
+  |TRUNKED TO OTHER OFFICES    |    POSITION IN CALLS PER BUSY HOUR    |
+  +----------------------------+---------------------------------------+
+  |            0               |                  240                  |
+  |           10               |                  230                  |
+  |           30               |                  200                  |
+  |           50               |                  185                  |
+  |           75               |                  170                  |
+  |           90               |                  165                  |
+  +----------------------------+---------------------------------------+
+
+_Trunking Factor._ In providing the system of trunks interconnecting the
+offices, whether the equipment be manual or automatic, it is essential
+to know not only how much traffic originates in each office, but how
+much of it will be trunked to each other office and how many trunks will
+be required. An interesting phase of telephone traffic studies is that
+it is possible to determine in advance the amount of traffic which can
+be completed directly in the multiple of that office and how much must
+be trunked elsewhere. Theoretical considerations would indicate that if
+the local multiple contains one-eighth of the total lines of the city,
+one-eighth of the calls originating in that office could be completed
+locally and seven-eighths would be trunked out. In almost all cases,
+however, it is found that more than the theoretical percentage of
+originating calls are for the neighborhood of that office and can be
+completed in the multiple. This results in the determination of a factor
+by which the theoretical out-trunking can be multiplied to determine the
+probable real out-trunking. In most cases, the ratio of actual to
+theoretical out-trunking is 75 per cent, or approximately that. In
+special cases, it may be far from 75 per cent.
+
+_Trunk Efficiency._ The capacities of trunks vary with their methods of
+operation and with the number of trunks in a group. For example, in the
+manual system where trunk operators in distant offices are instructed
+over call circuits and make disconnections in response to lamp signals,
+such an incoming trunk operator can complete from 250 to 500 connections
+per busy hour. The actual ability depends upon the number of distant
+offices served by that operator and upon the amount of work she has to
+perform on each call.
+
+The number of messages which can be handled by one trunk in the busy
+hour will depend upon the number of trunks in the group and upon the
+system employed. It appears that the ability of trunks in this regard is
+higher in the automatic system than in the manual system. For the
+latter, Table XV gives representative facts.
+
+TABLE XV
+
+Messages per Trunk in Manual System
+
+  +----------------------------+------------------------+
+  | NUMBER OF TRUNKS IN GROUP, | MESSAGES PER TRUNK PER |
+  |      MANUAL SYSTEM         |        BUSY HOUR       |
+  +----------------------------+------------------------+
+  |             5              |             7          |
+  |            10              |             9          |
+  |            20              |            12          |
+  |            40              |            15          |
+  |            60              |            18          |
+  +----------------------------+------------------------+
+
+Some of the reasons for the higher efficiencies of trunks in the
+automatic system are not well defined, but unquestionably exist. They
+have to do partly with the prompter answering observable in automatic
+systems. The operation of calling being simple, a called subscriber
+seems to fear that unless he answers promptly the calling party will
+disconnect and perhaps may call a competitor. The introduction of
+machine-ringing on automatic lines, where existing in competition with
+manual ringing on manual lines, seems to encourage subscribers to answer
+even more promptly. The length of conversation in automatic systems
+seems to be shorter than in manual systems. Still more important,
+disconnection in automatic systems is instantaneous during all hours,
+whereas in manual systems it is less prompt in the busiest and least
+busy hours than in the hours of intermediate congestion. The practical
+results of trunk efficiencies in automatic systems are given in Table
+XVI.
+
+TABLE XVI
+
+Messages per Trunk in Automatic System
+
+  +----------------------------+------------------------+
+  | NUMBER OF TRUNKS IN GROUP, | MESSAGES PER TRUNK PER |
+  |    AUTOMATIC SYSTEM        |       BUSY HOUR        |
+  +----------------------------+------------------------+
+  |             5              |              15        |
+  |            10              |              22        |
+  |            20              |              28        |
+  |            40              |              32        |
+  |            60              |              34        |
+  +----------------------------+------------------------+
+
+_Toll Traffic._ Toll or long-distance traffic follows the general laws
+of local or exchange traffic. Conversations are of greater average
+length in long-distance traffic. The long-distance line is held longer
+for an average conversation than is a local-exchange line. The local
+trunks which connect long-distance lines with exchange lines for
+conversation are held longer than are the actual long-distance trunks
+between cities. Knowing the probable traffic to be brought to the
+long-distance switching center by the long-distance trunks from exchange
+centers, the number of trunks required may be determined by knowing the
+capacity of each trunk. These trunk capacities vary with the method of
+handling the traffic and they vary as do local trunks with the number of
+trunks in a group. Table XVII illustrates this variation of capacity
+with sizes of groups.
+
+TABLE XVII
+
+Messages per Trunk in Long-Distance Groups
+
+  +--------------------------+-------------------------+
+  | NUMBER OF LONG-DISTANCE  |  MESSAGES PER TRUNK PER |
+  |    TRUNKS IN GROUP       |       BUSY HOUR         |
+  +--------------------------+-------------------------+
+  |            5             |           2             |
+  |           10             |           3             |
+  |           20             |           3.2           |
+  |           40             |           3.5           |
+  |           60             |           4             |
+  |          100             |           4.6           |
+  +--------------------------+-------------------------+
+
+=Quality of Service.= The quality of telephone service rendered by a
+particular equipment managed in a particular way depends on a great
+variety of elements. The handling of the traffic presented by patrons is
+a true manufacturing problem. The quality of the service rendered
+requires continuous testing in order that the management may know
+whether the service is reaching the standard; whether the standard is
+high enough; whether the cost of producing it can be reduced without
+lowering the quality; and whether the patrons are getting from it as
+much value as they might.
+
+In manual systems, the quality of telephone service depends upon a
+number of elements. The following are some principal ones:
+
+     1. Prompt answering.
+
+     2. Prompt disconnection.
+
+     3. Freedom from errors in connecting with the called line.
+
+     4. Promptness in connecting with the called line.
+
+     5. Courtesy and the use of form.
+
+     6. Freedom from failure by busy lines and failure to answer.
+
+     7. Clear enunciation.
+
+     8. Team work.
+
+_Answering Time._ There is an interrelation between these elements. Team
+work assists both answering and prompt disconnection. The quality of
+telephone service can not be measured alone in terms of prompt
+answering. Formerly telephone service was boasted of as being
+"three-second service" if most of the originating calls were answered in
+three seconds. Often such prompt answering reacts to prevent prompt
+disconnecting. Patient, systematic work is required to learn the real
+quality of the service.
+
+As to answering, the clearest, truest statement concerning manual
+service is found by making test calls to each position, dividing them
+into groups of various numbers of whole seconds each, and comparing the
+percentage of these groups to the whole number of telephones to that
+position. For example, assume each of the calls to a given position to
+have been answered in ten seconds or less, in which
+
+     100 per cent are answered in ten seconds or less;
+
+     80 per cent in eight seconds or less;
+
+     60 per cent in six seconds or less.
+
+It is probable that a reasonably uniform manual service will show only a
+small percentage answered in three seconds or under. Such percentages
+may be drawn in the form of curves, so that at a glance one may learn
+efficiency in terms of prompt answering.
+
+_Disconnecting Time._ Prompt disconnection was improved enormously by
+the introduction of relay manual boards. Just before the installation of
+relay boards in New York City, the average disconnecting time was over
+seventeen seconds. On the completion of an entire relay equipment, the
+average disconnecting time was found to be under three seconds. The
+introduction of relay manual apparatus has led subscribers to a larger
+traffic and to the making of calls which succeed each other very
+closely. A most important rule is, _that disconnect signals shall be
+given prompt attention either by the operator who made the connection,
+by an operator adjacent, or by a monitor who may be assisting_; and
+another, still more important one is, _that a flashing keyboard lamp
+indicating a recall shall be given precedence over all originating and
+all other disconnect signals_.
+
+_Accuracy and Promptness._ Promptness and accuracy in connecting with
+the called line are vital, and yet a large percentage of errors in these
+elements might exist in an exchange having a very high average speed of
+answering the originating call. Indeed, it seems quite the rule that
+where the effort of the management is devoted toward securing and
+maintaining extreme speed of original answering, all the other elements
+suffer in due proportion.
+
+_Courtesy and Form._ It goes without saying that operators should be
+courteous; but it is necessary to say it, and keep saying it in the most
+effective form, in order to prevent human nature under the most
+exasperating circumstances from lapsing a little from the standard,
+however high. The use of form assists both the operators and the
+subscribers, because in all matters of strict routine it is much easier
+to secure high speed and great accuracy by making as many as possible of
+the operations automatic. The use of the word "number" and other
+well-accepted formalities has assisted greatly in securing speed, clear
+understanding, and accurate performance. The simple expedient of
+spelling numbers by repeating the figures in a detached form--as "1-2-5"
+for 125--has taught subscribers the same expedient, and the percentage
+of possible error is materially reduced by going one step further and
+having the operator, in repeating, use always the opposite form from
+that spoken by the calling subscriber.
+
+_Busy and Don't Answer Calls._ Notwithstanding the old impression of the
+public to the contrary, the operator has no control over the "busy line"
+and "don't answer" situation. It is, however, of high importance that
+the management should know, by the analysis of repeated and exhaustive
+tests of the service, to what extent these troubles are degrading it. In
+addition to improving the service by the elimination of busy reports,
+there is no means of increasing revenue which is so easy and so certain
+as that which comes from following up the tabulated results of busy
+calls.
+
+_Enunciation._ It must be remembered that clear enunciation for
+telephone purposes is a matter wholly relative, and the ability of an
+operator in this regard can be determined only by a close analysis of
+many observations from the standpoint of a subscriber. A trick of speech
+rather than a pleasant voice and an easy address has made the answering
+ability of many an operator captivating to a group of satisfied
+subscribers.
+
+_Team Work._ By team work is meant the ability of a group of operators,
+seated side by side, to work together as a unit in caring for the
+service brought to them by the answering jacks within their reach. In
+switchboards of the construction usual today, a call before any operator
+may be answered by her, or by the operator at either the right or the
+left of her position. In many exchanges this advantage is wholly
+overlooked. In the period of general re-design of central-office
+equipments about fourteen years ago, a switchboard was installed with
+mechanical visual signals and answering-jacks on a flat-top board, and
+an arrangement of operators such that the signal of any call was
+extremely prominent and in easy reach of each one of four or possibly
+five operators. Associated with the line signals within the reach of
+such a group was an auxiliary lamp signal which would light when a call
+was made by any of the lines so terminating. It was found that with this
+arrangement the calls were answered in a strictly even manner, special
+rushes being cared for by the joint efforts of the group rather than
+serving to swamp the operator who happened to be in charge of the
+particular section affected by the rush.
+
+This principle has been tried out in so many ways that it is astonishing
+that it is not recognized as being a vital one. The whole matter is
+accomplished by impressing upon each operator that her duty is, _not_ to
+answer the calls of a specific number of lines before her, but to
+answer, with such promptness as is possible, _any call which is within
+the reach of her answering equipment_.
+
+=Observation of Service.= All that is required to be known concerning
+the form of address and courtesy may be learned by a close observation
+of the operators' work by the chief operators and monitors, and by the
+use of listening circuits permanently connected to the operators' sets.
+It is naturally necessary that the use of these listening circuits by
+the chief operator or her assistants must not be known to the operators
+at the times of use, even though they may know of the existence of such
+facilities.
+
+With a well-designed and properly maintained automatic equipment, the
+eight elements of good manual service reduce themselves to only one or
+two. Freedom from failure by busy lines and failure to answer are
+service-qualities independent of the kind of switching apparatus. Too
+great a percentage of busy calls for a given line indicates that the
+telephone facilities for calls incoming to that subscriber are
+inadequate. The best condition would be for each subscriber to have
+lines enough so that none of them ever would be found busy. This is the
+condition the telephone company tries to establish between its various
+offices.
+
+In manual practice it is possible to keep such records as will enable
+the traffic department to know when the lines to a subscriber are
+insufficient for the traffic trying to reach him. As soon as such facts
+are known, they can be laid before the subscriber so that he may arrange
+for additional incoming lines. In automatic practice this is not so
+simple, as the source and destination of traffic in general is not so
+clearly known to the traffic department. Automatic recorders of busy
+calls are necessary to enable the facts to be tabulated.
+
+
+
+
+CHAPTER XXXVIII
+
+MEASURED SERVICE
+
+
+In the commercial relation between the public and a telephone system,
+the commodity which is produced by the latter and consumed by the former
+is telephone service. Users often consider that payment is made for
+rental of telephone apparatus and to some persons the payment per month
+seems large for the rental of a mere telephone which could be bought
+outright for a few dollars.
+
+The telephone instrument is but a small part of the physical property
+used by a patron of a telephone system. Even the _entire_ group of
+property elements used by a patron in receiving telephone service
+represents much less than what really is his proportion of the
+service-rendering effort. What the patron receives is service and its
+value during a time depends largely on how much of it he uses in that
+time, and less on the number of telephones he can call.
+
+_The cost of telephone service varies as the amount of use._ It is just,
+therefore, that the selling price should vary as the amount of use.
+
+=Rates.= There are two general methods of charging for telephone service
+and of naming rates for this charge. These are called flat rates and
+measured-service rates. The latter are also known as message rates,
+because the message or conversation is the unit. Flat rates are those
+which are also known as rentals. The service furnished under flat rates
+is also known as unlimited service, for the reason that under it a
+patron pays the same amount each month and is entitled to hold as many
+conversations--send as many messages and make as many calls--as he
+wishes, without any additional payment. In the measured-service plan,
+the amount of payment in a month varies in some way with the amount of
+use, depending on the plan adopted. The patron may pay a fixed base
+amount per month, entitling him to have equipment for telephone service
+and to receive messages, but being required to pay, in addition to this
+base amount, a sum which is determined by the number of messages which
+he sends. Or he may pay a base amount per month and be entitled to have
+the equipment, to receive calls, and to send a certain number of
+messages, paying specifically in addition only for messages exceeding
+that certain number.
+
+Whether flat rates or measured-service rates are practiced, the general
+tendency is to establish lower rates for service in homes than in
+business places. This is another recognition of the justice of
+graduating the rates in accordance with the amount of use.
+
+=Units of Charging.= While both the flat-rate and the measured-rate
+methods of charging for unlimited and measured service are practiced in
+local exchanges, long-distance service universally is sold at message
+rates. The unit of message rates in long-distance service is time. The
+charge for a message between two points joined by long-distance lines
+usually is a certain sum for a conversation three minutes long plus a
+certain sum for each additional minute or fraction of a minute. In local
+service, the message-rate time charge per message takes less account of
+the time unit. The conversation is almost universally the unit in
+exchanges. Some managements restrict messages of multi-party lines to
+five minutes per conversation, because of the desire to avoid
+withholding the line from other parties upon it for too long periods.
+Service sold at public stations similarly is restricted as to time, even
+though the message be local to the exchange. Three to five minutes local
+conversation is sold generally for five cents in the United States. The
+time of the average local message, counting actual conversation time
+only, is one hundred seconds.
+
+=Toll Service.= _Long Haul._ In long-distance service, there are two
+general methods of handling traffic, as to the relations between the
+calling and the called stations. For the greater distances, as between
+cities not closely related because not belonging to one general
+community, the calling patron calls a particular person and pays nothing
+unless he holds conversation with that person. In this method, the
+operator records the name of the person called for; the name, telephone
+number, or both, of the person calling; the names of the towns where the
+message originated and ended; the date, the time conversation began, and
+the length of time it lasted.
+
+_Short Haul._ Where towns are closely related in commercial and social
+ways and where the traffic is large and approaches local service in
+character, and yet where conversations between them are charged at
+different rates than are local calls within them, a more rapid system of
+toll charging than that just described is of advantage. In these
+conditions, patrons are not sold a service which allows a particular
+party to be named and found, nor is the identity of the calling person
+required. The operator needs to know merely of these calls that they
+originate at a certain telephone and are for a certain other. The facts
+she must record are fewer and her work is simpler. Therefore, the cost
+of such switching is less than for true long-distance calls and it can
+be learned by careful auditing just when traffic between points becomes
+great enough to warrant switching them in this way. Such switching, for
+example, exists between New York and Brooklyn, between Chicago and
+suburbs around it which have names of their own but really are part of
+the community of Chicago, and between San Francisco and other cities
+which cluster around San Francisco Bay.
+
+Calls of the "long-haul" class are known as "particular person" or
+"particular party" calls, while "short-haul" calls are known as
+"two-number" long-distance calls. It is customary to handle particular
+party calls on long-distance switchboards and to handle two-number calls
+in manual systems on subscribers' switchboards exactly like local calls,
+except that the two-number calls are ticketed. It is customary in
+automatic systems to handle two-number calls by means of the regular
+automatic equipment plus ticketing by a suburban or two-number operator.
+
+_Timing Toll Connections._ It formerly was customary to measure the time
+of long-distance conversations by noting on the ticket the time of its
+beginning and the time of its ending, the operator reading the time from
+a clock. For human and physical reasons, such timing seems not to be
+considered infallible by the patron who pays the charge, and in cases of
+dispute concerning overtime charges so timed, telephone companies find
+it wisest to make concessions. The physical cause of error in reading
+time from a clock is that of parallax; that is, the error which arises
+from the fact that the minute hand of a clock is some distance from the
+surface of the dial so that one can "look under it." On an ordinary
+clock having a large face and its minute hand pointing upward or
+downward, five people standing in a row could read five different times
+from it at the same instant. The middle person might see the minute
+hand pointing at 6, indicating the time to be half-past something;
+whereas, person No. 1 and person No. 5 in the row might read the time
+respectively 29 and 31 minutes past something. Operators far to the
+right or to the left of a clock will get different readings, and an
+operator below a clock will get different kinds of readings at different
+times and correct readings at few times.
+
+Timing Machines:--Machines which record time directly on long-distance
+tickets are of value and machines which automatically compute the time
+elapsing during a conversation are of much greater value. The
+calculagraph is a machine of the latter class. The use of some such
+machine uniformly reduces controversy as to time which really elapsed.
+Parallax errors are avoided. The record possesses a dignity which
+carries conviction.
+
+[Illustration: Fig. 453. Calculagraph Records]
+
+Calculagraph records are shown in Fig. 453. In the one shown in the
+upper portion of this figure, the conversation began at 10.44 P.M. This
+is shown by the right-hand dial of the three which constitute the
+record. The minutes past 10 o'clock are shown by the hand within the
+dial and the hour 10 is shown by the triangular mark just outside the
+dial between X and XI.
+
+The duration of the conversation is shown by the middle and the
+left-hand dials. The figures on both these dials indicate minutes. The
+middle dial indicates roughly that the conversation lasted for a time
+between 0 and 5 minutes. The left-hand dial indicates with greater
+exactness that the conversation lasted one and one-quarter minutes.
+
+The hand of the left-hand dial makes one revolution in five minutes; of
+the middle dial, one revolution in an hour. The middle dial tells how
+many full periods of five minutes have elapsed and the left-hand dial
+shows the excess over the five-minute interval.
+
+The lower portion of Fig. 453 is a similar record beginning at the same
+time of day, but lasting about five and one-half minutes. As before, the
+readings of the two dials are added to get the elapsed time.
+
+[Illustration: Fig. 454. Relative Position of Hands and Dials]
+
+The right-hand dial, showing merely time of day, stands still while its
+hands revolve. The dies which print the dials and hands of the middle
+and the left-hand records rotate together. Examining the machine, one
+finds that the hands of these dials always point to zero. The middle
+dial and hand make one complete revolution in an hour; the left-hand
+dial and hand, one in five minutes. In making the records, the dials are
+printed at the beginning and the hands at the end of the conversation.
+Therefore, the hands will have moved forward during the
+conversation--still pointing to zero in both cases--but when printed the
+hands will point to some other place than they were pointing when the
+dials were printed. In this way, their angular distances truly indicate
+the lapse of time. Fig. 454 shows the relative position of the hands and
+dials within the machine at all times. It will be noted that the arrow
+of the left-hand dial does not point exactly to zero. This is due to the
+fact that the dials and hands are printed by separate operations and
+cannot be printed simultaneously.
+
+[Illustration: WESTERN ELECTRIC RINGING MACHINE]
+
+Another method of timing toll connections has been developed by the
+Monarch Telephone Manufacturing Company. This employs a master clock of
+great accuracy, which may be mounted on the wall anywhere in the
+building or another building if desired. A circuit leads from this clock
+to a time-stamp device on the operator's key shelf, and the clock closes
+this circuit every quarter minute. The impulses thus sent over the
+circuit energize the magnet of the time stamp, which steps a train of
+printing wheels around so as always to keep them set in such position as
+to properly print the correct time on a ticket whenever the head of the
+stamp is moved by the operator into contact with the ticket. A large
+number of such stamps may be operated from the same master clock. By
+printing the starting time of a connection below the finishing time the
+computation of lapsed time becomes a matter of subtraction. A typical
+toll ticket with the beginning and ending time printed by the time stamp
+in the upper left-hand corner and the elapsed time recorded by hand in
+the upper right-hand corner is shown in Fig. 455. It is seen that this
+stamp records in the order mentioned the month, the day, the hour, the
+minute and quarter minute, the A.M. and P.M. division of the day, and
+the year.
+
+[Illustration: Fig. 455. Toll Ticket Used with Monarch System]
+
+An interesting feature of this system is that the same master clock may
+be made in a similar manner to actuate secondary clocks placed at
+subscribers' stations, the impulses being sent over wires in the same
+cables as those containing the subscribers' lines. This system,
+therefore, serves not only as a means for timing the toll tickets and
+operating time stamps wherever they are required in the business of the
+telephone company, but also to supply a general clock and time-stamp
+service to the patrons of the telephone company as a "by-product" of the
+general telephone business.
+
+Exchange service is measured in terms of conversations without much
+regard to their length. The payment for the service may be made at the
+time it is received, as in public stations and at telephones equipped
+with coin prepayment devices; or the calls from a telephone may be
+recorded and collection for them made at agreed intervals. In the
+prepayment method the price per call is uniform. In the deferred payment
+method the calls are recorded as they are made, their number summed up
+at intervals, and the amount due determined by the price per call. The
+price per call may vary with the number of calls sold. A large user may
+have a lower rate per call than a small user.
+
+=Local Service.= _Ticket Method._ Measured local service sometimes is
+recorded by means of tickets, similarly to the described method of
+charging long-distance calls, except that the time of day and the
+duration of conversation are not so important. Where local ticketing is
+practiced, it is usual to write on the ticket only the number of the
+calling telephone and the date, and to pass into the records only those
+tickets which represent actual conversations, keeping out tickets
+representing calls for busy lines and calls which were not answered.
+
+_Meter Method._ The requirements of speed in good local service are
+opposed to the ticketing method. Where measured service is supplied to a
+substantial proportion of the lines of a large exchange,
+electro-mechanical service meters are attached to the lines. These
+service meters register as a consequence of some act on the part of the
+switchboard operator, or may be caused to register by the answering of
+the called subscriber.
+
+[Illustration: Fig. 456. Connection Meter]
+
+In manual practice, meters of the type shown in Fig. 456 are associated
+with the lines as in Fig. 457. The meters are mounted separately from
+the switchboard, needing only to be connected to the test-strand of the
+line by cabled wires. If desired, the meter may be mounted on racks in
+quarters especially devoted to them, and the cases in which the racks
+are mounted may be kept locked. In such an arrangement the meters are
+read from time to time through the glass doors of the cases.
+
+The meters are caused to operate by pressure on the meter key _MK_,
+associated with the answering cord as in Fig. 458. This increases the
+normal potential to 30 volts. When the armature of the meter has made a
+part of its stroke, it closes a contact which places its 40-ohm winding
+in shunt with its 500-ohm winding, thus furnishing ample power for
+turning the meter wheels.
+
+[Illustration: Fig. 457. Western Electric Line Circuit and Service
+Meter]
+
+Such meters are in common use in large exchanges, notable examples being
+the cities of New York and London. In London, there is a zone within
+which the price per call is one penny and between which and other zones
+the price is twopence. Calls within the zone either are completed by the
+answering operator directly in the multiple before her or are trunked to
+other offices in that zone. Calls for points outside of that zone are
+trunked to other offices and in giving the order the operator finds that
+the call circuit key lights a special signal lamp before her. This
+reminds her that the call is at a twopence price, so in recording it she
+presses the meter key twice. This counts two units on the meter and the
+units are billed at a penny each.
+
+In automatic systems it is not possible to operate a meter system in
+which the operator will press a key for each call to be charged, because
+there is no operator. In such systems--a notable example being the
+measured-service automatic system in San Francisco--the meter registers
+only upon the answering of the called subscriber. Calls for lines found
+busy and calls which are not answered do not register. Calls for
+long-distance recording operators, two-number ticket operators,
+information, complaint, and other company departments are not
+registered. In the Chinatown quarter of San Francisco, where most calls
+begin and end in the neighborhood, service is sold at an unlimited flat
+rate for neighborhood calls and at a message rate for other calls. The
+meter system recognizes this condition and does not register calls
+_from_ Chinese subscribers _for_ Chinese subscribers, though it does
+register calls from Chinese subscribers to Caucasian subscribers. The
+nature of the system is such as to enable it to discriminate as to
+races, localities, or other peculiarities as may be desired.
+
+[Illustration: Fig. 458. Western Electric Cord Circuit and Service Meter
+Key]
+
+In the manual meter circuits of Figs. 457 and 458, the meter windings
+have no relation to the line conductors. In the automatic arrangement
+just described, there are meter windings in the line during times of
+calling, but none in the line during times of conversation. The balance
+of the line, therefore, is undisturbed at all times wherein balance is
+of any importance.
+
+In both systems just described, the meters of all lines are in their
+respective central offices. Meters for use at subscribers' stations have
+been devised and there is no fundamental reason why the record might not
+be made at the subscriber's station instead of, or in addition to, a
+central-office record. Experience has shown that confidence in a meter
+system can be secured if the meters be positive, accurate, and reliable.
+The labor of reading the meters is much less when they are kept in
+central offices. Subscribers may have access to them if they wish.
+
+_Prepayment Method._ Prepayment measured-service mechanisms permit a
+coin or token to be dropped into a machine at the subscriber's telephone
+at the time the conversation is held. A variety of forms of telephone
+coin collectors are in use, their operations being fundamentally either
+electrical or mechanical.
+
+Electrically operated coin collectors require either that the coin be
+dropped into the machine in order to enable the central office to be
+signaled in manual systems, or the switches to be operated in automatic
+systems, or they require that the coin be dropped into the machine after
+calling, but before the conversation is permitted.
+
+Western Electric Company coin collectors, shown in Fig. 459, may be
+operated in either way in connection with manual systems. The usual way
+is to require the coin to be dropped before the central-office line lamp
+can glow. The operator then rings the called subscriber and upon his
+answering places a sufficient potential upon the calling line to operate
+the polarized relay and to drop the coin into the cash box. If the
+called subscriber does not answer or his line is busy, potential is
+placed on the calling line, moving the polarized relay in the other
+direction and dropping the coin into a return chute so that the
+subscriber may take it. If it is preferred that the coin be paid only on
+the request of the operator, the return feature need not be provided.
+
+In both forms of operation, the Western Electric coin collector is
+adapted to bridge its polarized relay between one limb of the line and
+ground during the time a coin rests on the pins, as shown in Fig. 459.
+When no coin is on the pins--_i. e._, before calling and after the
+called station responds--the relay is not so bridged.
+
+[Illustration: Fig. 459. Principle of Western Electric Coin Collector]
+
+The armature of the relay responds only to a high potential and this is
+applied by the operator. If the coin is to be taken by the company, one
+polarity is sent; if it is to be returned to the patron, the other
+polarity is sent. These polarities are applied to a limb of the line
+proper. It will be recalled that pressures to actuate service meters are
+applied to the test-strand. If wished, keys may be arranged so as to
+apply 30 volts to the test-strand and the collecting potential to the
+line at the same operation. This enables the service meter to count the
+tokens placed in the cash box of the coin collector, and serves as a
+valuable check.
+
+In automatic systems, in one arrangement, coin collectors are arranged
+so that no impulses can be sent unless a coin has been deposited, the
+coin automatically passing to the cash box when the called subscriber
+answers, or to the patron if it is not answered. In another arrangement,
+calls are made exactly as in unlimited service, but a coin must be
+deposited before a conversation can be held. The calling person can hear
+the called party speak and may speak himself but can not be heard until
+the coin is deposited. No coin-return mechanism is required in this
+method.
+
+Coin collectors of these types usually are adapted to receive only one
+kind of coin, these, in the United States, being either nickels or
+dimes. For long-distance service, where the charges vary, it is
+necessary to signal to an operator just what coins are paid. It is
+uniformly customary to send these signals by sound, the collector being
+so arranged that the coins strike gongs. In coin collectors of the Gray
+Telephone Paystation Company, the coins strike these gongs by their own
+weight in falling through chutes. In coin collectors of the Baird
+Electric Company, the power for the signals is provided by hand power, a
+lever being pulled for each coin deposited. Both methods are in wide
+use.
+
+
+
+
+CHAPTER XXXIX
+
+PHANTOM, SIMPLEX, AND COMPOSITE CIRCUITS
+
+
+=Definitions.= Phantom circuits are arrangements of telephone wires
+whereby more working, non-interfering telephone lines exist than there
+are sets of actual wires. When four wires are arranged to provide three
+metallic circuits for telephone purposes, two of the lines are physical
+circuits and one is a phantom circuit.
+
+Simplex and composite circuits are arrangements of wires whereby
+telephony and telegraphy can take place at the same time over the same
+wires without interference.
+
+[Illustration: Fig. 460. Phantom Circuit]
+
+=Phantom.= In Fig. 460 four wires join two offices. _RR_ are repeating
+coils, designed for efficient transforming of both talking and ringing
+currents. The devices marked _A_ in this and the following figures are
+air-gap arresters. Currents from the telephones connected to either
+physical pair of wires pass, at any instant, in opposite directions in
+the two wires of the pair. The phantom circuit uses one of the physical
+pairs as a _wire_ of its line. It does this by tapping the middle point
+of the line side of each of the repeating coils. The impedance of the
+repeating-coil winding is lowered because, all the windings being on
+the same core, the phantom line currents pass from the middle to the
+outer connections so as to neutralize each other's influence. The
+currents of the phantom circuit, unlike those of the physical circuits,
+are _in the same direction_ in both wires of a pair at any instant.
+Their potentials, therefore, are equal and simultaneous.
+
+A phantom circuit is formed most simply when both physical lines end in
+the same two offices. If one physical line is longer than the other, a
+phantom circuit may be formed as in Fig. 461, wherein the repeating coil
+is inserted in the longer line where it passes through a terminal
+station of the shorter.
+
+[Illustration: Fig. 461. Phantom from Two Physical Circuits of Unequal
+Length]
+
+[Illustration: Fig. 463. Two Phantoms Joined by Physical Circuit]
+
+A circuit may be built up by adding a physical circuit to a phantom. A
+circuit may be made up of two or more phantom circuits, joined by
+physical ones. In Fig. 462 a phantom circuit is extended by the use of a
+physical circuit, while in Fig. 463, two phantom circuits are joined by
+placing between them a physical circuit.
+
+[Illustration: Fig. 462. Phantom Extended by Physical Circuit]
+
+_Transpositions._ In phantom circuits formed merely by inserting
+repeating coils in physical circuits and doing nothing else, an exact
+balance of the sides of the phantom circuit is lacking. The resistances,
+insulations, and capacities to earth of the sides may be equal, but the
+exposures to adjacent telephone and telegraph circuits and to power
+circuits will not be equal unless the phantom circuits are transposed.
+
+To transpose a set of lines of two physical wires each, is not
+complicated, though it must be done with care and in accordance with a
+definite, foreknown plan. Transposing phantom circuits is less simple,
+however, as four wires per circuit have to be transposed, instead of
+two.
+
+[Illustration: Fig. 464. Transposition of Phantom Circuits]
+
+In Fig. 464, the general spacing of transposition sections is the usual
+one, 1,300 feet, of the _ABCB_ system widely in use. The pole circuit,
+on pins _5_ and _6_ of the upper arm, is transposed once each two miles.
+The pole circuit of the second arm transposes either once or twice a
+mile. But neither pole circuit differs in transposition from any other
+regular scheme except in the frequency of transposition. All the other
+wires of each arm, however, are so arranged that each wire on either
+side of the pole circuit moves from pin to pin at section-ends, till it
+has completed a cycle of changes over all four of the pins on its side.
+In doing so, each phantom circuit is transposed with proper regard to
+each of the other three on that twenty-wire line.
+
+The "new transposition" lettering in Fig. 464 is for the purpose of
+identifying the exact scheme of wiring each transposition pole. The
+complication of wiring at each transposition pole is increased by the
+adoption of phantom circuits. Maintenance of all the circuits is made
+more costly and less easy unless the work at points of transposition is
+done with care and skill. Phantom circuits, to be always successful,
+require that the physical circuits be balanced and kept so.
+
+_Transmission over Phantom Circuits._ Under proper conditions phantom
+circuits are better than physical circuits, and in this respect it may
+be noted that some long-distance operating companies instruct their
+operators always to give preference to phantom circuits, because of the
+better transmission over them. The use of phantom circuits is confined
+almost wholly to open-wire circuits; and while the capacity of the
+phantom circuit is somewhat greater than that of the physical circuit,
+its resistance is considerably smaller. In the actual wire the phantom
+loop is only half the resistance of either of the physical lines from
+which it is made, for it contains twice as much copper. The resistance
+of the repeating coils, however, is to be added.
+
+=Simplex.= Simplex telegraph circuits are made from metallic circuit
+telephone lines, as shown in Fig. 465. The principle is identical with
+that of phantom telephone circuits. The potentials placed on the
+telephone line by the telegraph operations are equal and simultaneous.
+They cause no current to flow _around_ the telephone loop, only _along_
+it. If all qualities of the loop are balanced, the telephones will not
+overhear the telegraph impulses. In the figure, _AA_ are arresters, as
+before, _GG_ are Morse relays; a 2-microfarad condenser is shunted
+around the contact of each Morse key _F_ to quench the noises due to the
+sudden changes on opening the keys between dots and dashes.
+
+[Illustration: Fig. 465. Simplex Telegraph Circuit]
+
+A simplex arrangement even more simple substitutes impedance coils for
+the repeating coils of Fig. 465. The operation of the Morse circuit is
+the same. An advantage of such a circuit, as shown in Fig. 466, is that
+the telephone circuit does not suffer from the two repeating-coil losses
+in series. A disadvantage is, that in ringing on such a line with a
+grounded generator, the Morse relays are caused to chatter.
+
+[Illustration: Fig. 466. Simplex Telegraph Circuit]
+
+The circuit of Fig. 465 may be made to fit the condition of a through
+telephone line and a way telegraph station. The midway Morse apparatus
+of Fig. 467 is looped in by a combination of impedance coils and
+condensers. The plans of Figs. 465 and 466 here are combined, with the
+further idea of stopping direct and passing alternating currents, as is
+so well accomplished by the use of condensers.
+
+[Illustration: Fig. 467. Simplex Circuit with Waystation]
+
+[Illustration: Fig. 468. Composite Circuit]
+
+=Composite.= Composite circuits depend on another principle than that of
+producing equal and simultaneous potentials on the two wires of the
+telephone loop. The opposition of impedance coils to alternating
+currents and of condensers to direct currents are the fundamentals. The
+early work in this art was done by Van Rysselberghe, of Belgium. In Fig.
+468, one telephone circuit forms two Morse circuits, two wires carrying
+three services. Each Morse circuit will be seen to include, serially,
+two 50-ohm impedance coils, and to have shunts through condensers to
+ground. The 50-ohm coils are connected differentially, offering low
+consequent impedance to Morse impulses, whose frequency of interruption
+is not great. As the impedance coils are large, have cores of
+considerable length, and are wound with two separate though serially
+connected windings each, their impedance to voice currents is great.
+They act as though they were not connected differentially, so far as
+voice currents are concerned.
+
+Because of the condensers serially in the telephone line, voice currents
+can pass through it, but direct currents can not. Impulses due to
+discharges of cores, coils, and capacities in the Morse circuit _could_
+make sounds in the telephones, but these are choked out, or led to earth
+by the 30-ohm impedance coils and the heavy Morse condensers.
+
+=Ringing.= Ringing over simplex circuits is done in the way usual where
+no telegraph service is added. Both telegraphy and telephony over
+simplex circuits follow their usual practice in the way of calling and
+conversing. In composite working, however, ringing by usual methods
+either is impossible because of heavy grounds and shunts, or if it is
+possible to get ringing signals through at all, the relays of the Morse
+apparatus will chatter, interfering with the proper use of the telegraph
+portion of the service.
+
+It is customary, therefore, either to equip composite circuits with
+special signaling devices by which high-frequency currents pass over the
+telephone circuits, operating relays which in turn operate local ringing
+signals; or to refrain from ringing on composite circuits and to
+transmit orders for connections by telegraph. The latter is wholly
+satisfactory over composite lines between points having heavy telegraph
+traffic, and it is between such points as these that composite practice
+is most general.
+
+=Phantoms from Simplex and Composite Circuits.= Phantom and simplex
+principles are identical, and by adding the composite principle, two
+simplex circuits may have a phantom superadded, as in Fig. 469.
+Similarly, as in Fig. 470, two composite circuits can be phantomed. This
+case gives seven distinct services over four wires: three telephone
+loops--two physical and one phantom--and four Morse lines.
+
+[Illustration: Fig. 469. Phantom of Two Simplex Circuits]
+
+[Illustration: Fig. 470. Phantom of Two Composite Circuits]
+
+=Railway Composite.= The foregoing are problems of making telegraphy a
+by-product of telephony. With so many telegraph wires on poles over the
+country, it has seemed a pity not to turn the thing around and provide
+for telephony as a by-product of telegraphy. This has been accomplished,
+and the result is called a railway composite system. For the reason that
+the telegraph circuits are not in pairs, accurately matched one wire
+against another, and are not always uniform as to material, it has not
+been possible to secure as good telephone circuits from telegraph wires
+as telegraph circuits from telephone wires.
+
+Practical results are secured by adaptation of the original principle of
+different frequencies. A study of Fig. 468 shows that over such a
+composite circuit the usual method of ringing from station to station
+over the telephone circuit by an alternating current of a frequency of
+about sixteen per second is practically impossible. This is because of
+the heavy short-circuit provided by the two 30-ohm choke coils at each
+of the stations, the heavy shunt of the large condensers, and the
+grounding through the 50-ohm choke coils. If high-frequency speech
+currents can pass over these circuits with a very small loss, other
+high-frequency circuits should find a good path. There are many easy
+ways of making such currents, but formerly none very simple for
+receiving them. Fig. 471 shows one simple observer of such
+high-frequency currents, it being merely an adaptation of the familiar
+polarized ringer used in every subscriber's telephone. In either
+position of the armature it makes contact with one or the other of two
+studs connected to the battery, so that in all times of rest the relay
+_A_ is energized. When a high-frequency current passes through this
+polarized relay, however, there is enough time in which the armature is
+out of contact with either stud to reduce the total energy through the
+relay _A_ and allow its armature to fall away, ringing a vibrating bell
+or giving some other signal.
+
+[Illustration: Fig. 471. Ringing Device for Composite Circuits]
+
+Fig. 472 shows a form of apparatus for producing the high-frequency
+current necessary for signaling. It is evident that if a magneto
+generator, such as is used in ordinary magneto telephones, could be made
+to drive its armature fast enough, it also might furnish the
+high-frequency current necessary for signaling through condensers and
+past heavy impedances.
+
+[Illustration: Fig. 472. Ringing Current Device]
+
+Applying these principles of high-frequency signals sent and received to
+a single-wire telegraph circuit, the arrangement shown in Fig. 473
+results, this being a type of railway composite circuit. The principal
+points of interest herein are the insertion of impedances in series with
+the telegraph lines, the shunting of the telegraph relays by small
+condensers, the further shunting of the whole telegraph mechanism of a
+station by another condenser, and thus keeping out of the line circuit
+changes in current values which would be heard in the telephones if
+violent, and might be inaudible if otherwise.
+
+[Illustration: Fig. 473. Railway Composite Circuit]
+
+[Illustration: FRONT OF LONG-DISTANCE POWER BOARD U.S. Telephone
+Company, Cleveland, Ohio. _The Dean Electric Co._]
+
+A further interesting element is the very heavy shunting of the
+telephone receiver by means of an inductive coil. This shunt is applied
+for by-path purposes so that heavy disturbing currents may be kept out
+of the receiver while a sufficient amount of voice current is diverted
+through the receiver. It is well to have the inductance of this shunt
+made adjustable by providing a movable iron core for the shunt winding.
+When the core is drawn out of the coil, its impedance is diminished
+because the inductance is diminished. This reduces the amount of
+disturbing noise in the receiver. The core should be withdrawn as little
+as the amount of disturbance permits, as this also diminishes the
+loudness of the received speech.
+
+Because the signaling over lines equipped with this form of composite
+working results in the ringing of a bell by means of local current, it
+is of particular advantage in cases where the bell needs to ring loudly.
+Switch stations, crossings, and similar places where the attendant is
+not constantly near the telephone can be equipped with this type of
+composite apparatus and it so offers a valuable substitute for regular
+railway telegraph equipment, with which the attendant may not be
+familiar. The success of the local bell-ringing arrangement, however,
+depends on accurate relay adjustment and on the maintenance of a primary
+battery. The drain on the ringing battery is greater than on the talking
+battery.
+
+A good substitute for the bell signal on railway composite circuits is a
+telephone receiver responding directly to high-frequency currents over
+the line. The receiver is designed specially for the purpose and is
+known as a "howler." Its signal can be easily heard through a large
+room. The condenser in series with it is of small capacity, limiting the
+drain upon the line. Usually the howler is detached by the switch hook
+during conversation from a station.
+
+_Railway Composite Set._ The circuit of a set utilizing such an
+arrangement together with other details of a complete railway composite
+set is shown in Fig. 474. The drawing is arranged thus, in the hope of
+simplifying the understanding of its principles. It will be seen that
+the induction coil serves as an interrupter as well as for transmission.
+All of the contacts are shown in the position they have during
+conversation. The letters _Hc1_, _Hc2_, etc., and _Kc1_, _Kc2_, etc.,
+refer to hook contacts and key contacts, respectively, of the numbers
+given. The arrangements of the hook and key springs are shown at the
+right of the figure. _RR_ represent impedance coils connected serially
+in the line and placed at terminal stations. The composite telephone
+sets are bridged from the line to ground at any points between the
+terminal impedance coils.
+
+The direct currents of telegraphy are prevented from passing to ground
+through the telephone set during conversation by the 2-microfarad
+condenser which is in series with the receiver. They are prevented from
+passing to ground through the telephone set when the receiver is on the
+hook by a .05 microfarad condenser in series with the howler. The
+alternating currents of speech and interrupter signaling are kept from
+passing to ground at terminals by the impedance coils.
+
+Signals are sent from the set by pressing the key _K_. This operates the
+vibrator by closing contacts _Kc6_ and _Kc7_. The howler is cut off and
+the receiver is short-circuited by the same operation of the key. The
+impedance of the coil _I_ is changed by moving its adjustable core.
+
+[Illustration: Fig. 474. Railway Composite Set]
+
+=Applications.= A chief use of composite and simplex circuits is for
+ticket wire purposes. These are circuits over which long-distance
+operators instruct each other as to connecting and disconnecting lines,
+the routing of calls, and the making of appointments. One such wire will
+care for all the business of many long-distance trunks. The public also
+absorbs the telegraph product of telephone lines. Such telegraph service
+is leased to brokers, manufacturers, merchants, and newspapers. Railway
+companies use portable telephone adjuncts to telegraph circuits on
+trains for service from stations not able to support telegraph
+attendants, and in a limited degree for the dispatching of trains.
+Telephone train dispatching, however, merits better equipment than a
+railway composite system affords.
+
+
+
+
+CHAPTER XL
+
+TELEPHONE TRAIN DISPATCHING[A]
+
+
+It has been only within the past three few that the telephone has begun
+to replace the telegraph for handling train movements. The telegraph and
+the railroads have grown up together in this country since 1850, and in
+view of the excellent results that the telegraph has given in train
+dispatching and of the close alliance that has always naturally existed
+between the railway and the telegraph, it has been difficult for the
+telephone, which came much later, to enter the field.
+
+=Rapid Growth.= The telephone has been in general use among the
+railroads for many years, but only on a few short lines has it been used
+for dispatching trains. In these cases the ordinary magneto circuit and
+instruments have been employed, differing in no respect from those used
+in commercial service at the present time. Code ringing was used and the
+number of stations on a circuit was limited by the same causes that
+limit the telephones on commercial party lines at present.
+
+The present type of telephone dispatching systems, however, differs
+essentially from the systems used in commercial work, and is, in fact, a
+highly specialized party-line system, arranged for selective ringing and
+_many stations_. The first of the present type was installed by the New
+York Central and Hudson River Railroad in October, 1907, between Albany
+and Fonda, New York, a distance of 40 miles. This section of the road is
+on the main line and has four tracks controlled by block signals.
+
+The Chicago, Burlington, and Quincy Railroad was the second to install
+train-dispatching circuits. In December, 1907, a portion of the main
+line from Aurora to Mendota, Illinois, a distance of 46 miles, was
+equipped. This was followed in quick succession by various other
+circuits ranging, in general, in lengths over 100 miles. At the present
+time there are over 20 train-dispatching circuits on the Chicago,
+Burlington, and Quincy Railroad covering 125 miles of double track, 28
+miles of multi-track, and 1,381 miles of single track, and connecting
+with 286 stations.
+
+Other railroads entered this field in quick order after the initial
+installations, and at the present time nearly every large railroad
+system in the United States is equipped with several telephone
+train-dispatching circuits and all of these seem to be extending their
+systems.
+
+In 1910, several railroads, including the Delaware, Lackawanna, and
+Western, had their total mileage equipped with telephone dispatching
+circuits. The Atchison, Topeka, and Santa Fe Railroad is equipping its
+whole system as rapidly as possible and already is the largest user of
+this equipment in this country. From latest information, over 55
+railroads have entered this field, with the result that the telephone is
+now in use in railroad service on over 29,000 miles of line.
+
+=Causes of Its Introduction.= The reasons leading to the introduction of
+the telephone into the dispatching field were of this nature: First, and
+most important, was the enactment of State and Federal Laws limiting to
+nine hours the working day of railroad employes transmitting or
+receiving orders pertaining to the movement of trains. The second, which
+is directly dependent upon the first, was the inability of the railroads
+to obtain the additional number of telegraph operators which were
+required under the provisions of the new laws. It was estimated that
+15,000 additional operators would be required to maintain service in the
+same fashion after the new laws went into effect in 1907. The increased
+annual expense occasioned by the employment of these additional
+operators was roughly estimated at $10,000,000. A third reason is found
+in the decreased efficiency of the average railway and commercial
+telegraph operator. There is a very general complaint among the
+railroads today regarding this particular point, and many of them
+welcome the telephone, because, if for no other reason, it renders them
+independent of the telegrapher. What has occasioned this decrease in
+efficiency it is not easy to say, but there is a strong tendency to lay
+it, in part, to the attitude of the telegraphers' organization toward
+the student operator. It is a fact, too, that the limits which these
+organizations have placed on student operators were directly
+responsible for the lack of available men when they were needed.
+
+=Advantages.= In making this radical change, railroad officials were
+most cautious, and yet we know of no case where the introduction of the
+telephone has been followed by its abandonment, the tendency having been
+in all cases toward further installations and more equipment of the
+modern type. The reasons for this are clear, for where the telephone is
+used it does not require a highly specialized man as station operator
+and consequently a much broader field is open to the railroads from
+which to draw operators. This, we think, is the most far-reaching
+advantage.
+
+The telephone method also is faster. On an ordinary train-dispatching
+circuit it now requires from 0.1 of a second to 5 seconds to call any
+station. In case a plurality of calls is desired, the dispatcher calls
+one station after another, getting the answer from one while the next is
+being called, and so on. By speaking into a telephone many more words
+may be transmitted in a given time than by Morse telegraphy. It is
+possible to send fifty words a minute by Morse, but such speed is
+exceptional. Less than half that is the rule. The gain in high speed,
+therefore, which is obtained is obvious and it has been found that this
+is a most important feature on busy divisions. It is true that in the
+issuance of "orders," the speed, in telephonic train dispatching, is
+limited to that required to write the words in longhand. But all
+directions of a collateral character, the receipt of important
+information, and the instantaneous descriptions of emergency situations
+can be given and received at a speed limited only by that of human
+speech.
+
+The dispatcher is also brought into a closer personal relation with the
+station men and trainmen, and this feature of direct personal
+communication has been found to be of importance in bringing about a
+higher degree of co-operation and better discipline in the service.
+
+Telephone dispatching has features peculiar to itself which are
+important in improving the class of service. One of these is the
+"answer-back" automatically given to the dispatcher by the waystation
+bell. This informs the dispatcher whether or not the bell at the station
+rang, and excuses by the operators that it did not, are eliminated.
+
+Anyone can answer a telephone call in an emergency. The station
+operator is frequently agent also, and his duties often take him out of
+hearing of the telegraph sounder. The selector bell used with the
+telephone can be heard for a distance of several hundred feet. In
+addition, it is quite likely that anyone in the neighborhood would
+recognize that the station was wanted and either notify the operator or
+answer the call.
+
+In cases of emergency the train crews can get into direct communication
+with the dispatcher immediately, by means of portable telephone sets
+which are carried on the trains. It is a well-known fact that every
+minute a main line is blocked by a wreck can be reckoned as great loss
+to the railroad.
+
+It is also possible to install siding telephone sets located either in
+booths or on poles along the right-of-way. These are in general service
+today at sidings, crossings, drawbridges, water tanks, and such places,
+where it may be essential for a train crew to reach the nearest
+waystation to give or receive information.
+
+The advantage of these siding sets is coming more and more to be
+realized. With the telegraph method of dispatching, a train is ordered
+to pass another train at a certain siding, let us say. It reaches this
+point, and to use a railroad expression, "goes into the hole." Now, if
+anything happens to the second train whereby it is delayed, the first
+train remains tied up at that siding without the possibility of either
+reaching the dispatcher or being reached by him. With the telephone
+station at the siding, which requires no operator, this is avoided. If a
+train finds itself waiting too long, the conductor goes to the siding
+telephone and talks to the dispatcher, possibly getting orders which
+will advance him many miles that would otherwise have been lost.
+
+It is no longer necessary for a waystation operator to call the
+dispatcher. When one of these operators wishes to talk to the
+dispatcher, he merely takes his telephone receiver off the hook, presses
+a button, and speaks to the dispatcher.
+
+With the telephone it is a simple matter to arrange for provision so
+that the chief dispatcher, the superintendent, or any other official may
+listen in at will upon a train circuit to observe the character of the
+service. The fact that this can be done and that the operators know it
+can be done has a very strong tendency to improve the discipline.
+
+The dispatchers are so relieved, by the elimination of the strain of
+continuous telegraphing, and can handle their work so much more quickly
+with the telephone, that in many cases it has been found possible to
+increase the length of their divisions from 30 to 50 per cent.
+
+=Railroad Conditions.= One of the main reasons that delayed the
+telephone for so many years in its entrance to the dispatching field is
+that the conditions in this field are like nothing which has yet been
+met with in commercial telephony. There was no system developed for
+meeting them, although the elements were at hand. A railroad is divided
+up into a number of divisions or dispatchers' districts of varying
+lengths. These lengths are dependent on the density of the traffic over
+the division. In some cases a dispatcher will handle not more than 25
+miles of line. In other cases this district may be 300 miles long. Over
+the length of one of these divisions the telephone circuit extends, and
+this circuit may have upon it 5 or 50 stations, _all of which may be
+required to listen upon the line at the same time_.
+
+It will be seen from this that the telephone dispatching circuit
+partakes somewhat of the nature of a long-distance commercial circuit in
+its length, and it also resembles a rural line in that it has a large
+number of telephones upon it. Regarding three other characteristics,
+namely, that many of these stations may be required to be in on the
+circuit simultaneously, that they must all be signaled selectively, and
+that it must also be possible to talk and signal on the circuit
+simultaneously, a telephone train-dispatching circuit resembles nothing
+in the commercial field. These requirements are the ones which have
+necessitated the development of special equipment.
+
+=Transmitting Orders.= The method of giving orders is the same as that
+followed with the telegraph, with one important exception. When the
+dispatcher transmits a train order by telephone, he writes out the order
+as he speaks it into his transmitter. In this way the speed at which the
+order is given is regulated so that everyone receiving it can easily get
+it all down, and a copy of the transmitted order is retained by the
+dispatcher. All figures and proper names are spelled out. Then after an
+order has been given, it is repeated to the dispatcher by each man
+receiving it, and he underlines each word as it comes in. This is now
+done so rapidly that a man can repeat an order more quickly than the
+dispatcher can underline. The doubt as to the accuracy with which it is
+possible to transmit information by telephone has been dispelled by this
+method of procedure, and the safety of telephone dispatching has been
+fully established.
+
+=Apparatus.= The apparatus which is employed at waystations may be
+divided into two groups--the selector equipment and the telephone
+equipment. The selector is an electro-mechanical device for ringing a
+bell at a waystation when the dispatcher operates a key corresponding to
+that station. At first, as in telegraphy, the selector magnets were
+connected in series in the line, but today all systems bridge the
+selectors across the telephone circuit in the same way and for the same
+reasons that it is done in bridging party-line work. There are at the
+present time three types of selectors in general use, and the mileage
+operated by means of these is probably considerably over 95 per cent of
+the total mileage so operated in the country.
+
+[Illustration: Fig. 475. Western Electric Selector]
+
+[Illustration: Fig. 476. Western Electric Selector]
+
+_The Western Electric Selector._ This selector is the latest and perhaps
+the simplest. Fig. 475 shows it with its glass dust-proof cover on, and
+Fig. 476 shows it with the cover removed. This selector is adapted for
+operating at high speed, stations being called at the rate of ten per
+second.
+
+The operating mechanism, which is mounted on the front of the selector
+so as to be readily accessible, works on the central-energy
+principle--the battery for its operation, as well as for the operation
+of the bell used in connection with it, both being located at the
+dispatcher's office. The bell battery may, however, be placed at the
+waystation if this is desired.
+
+The selector consists of two electromagnets which are bridged in series
+across the telephone circuit and are of very high impedance. It is
+possible to place as many of these selectors as may be desired across a
+circuit without seriously affecting the telephonic transmission.
+Direct-current impulses sent out by the dispatcher operate these
+magnets, one of which is slow and the other quick-acting. The first
+impulse sent out is a long impulse and pulls up both armatures, thereby
+causing the pawls above and below the small ratchet wheel, shown in Fig.
+476, to engage with this wheel. The remaining impulses operate the
+quick-acting magnet and step the wheel around the proper number of
+teeth, but do not affect the slow-acting magnet which remains held up by
+them. The pawl connected to the slow-acting magnet merely serves to
+prevent the ratchet wheel from turning back. Attached to the ratchet
+wheel is a contact whose position can be varied in relation to the
+stationary contact on the left of the selector with which this engages.
+This contact is set so that when the wheel has been rotated the desired
+number of teeth, the two contacts will make and the bell be rung. Any
+selector may thus be adjusted for any station, and the selectors are
+thus interchangeable. When the current is removed from the line at the
+dispatcher's office, the armatures fall back and everything is restored
+to normal. An "answer-back" signal is provided with this selector
+dependent upon the operation of the bell. When the selector at a station
+operates, the bell normally rings for a few seconds. The dispatcher,
+however, can hold this ring for any length of time desired.
+
+The keys employed at the dispatcher's office for operating selectors are
+shown in Fig. 477. There is one key for each waystation on the line and
+the dispatcher calls any station by merely giving the corresponding key
+a quarter turn to the right. Fig. 478 shows the mechanism of one of
+these keys and the means employed for sending out current impulses over
+the circuit. The key is adjustable and may be arranged for any station
+desired by means of the movable cams shown on the rear in Fig. 478,
+these cams, when occupying different positions, serving to cover
+different numbers of the teeth of the impulse wheel which operate the
+impulse contacts.
+
+[Illustration: Fig. 477. Dispatcher's Keys]
+
+[Illustration: Fig. 478. Dispatcher's Key Mechanism]
+
+_The Gill Selector._ The second type of selector in extensive use
+throughout the country today is known as the Gill, after its inventor.
+It is manufactured for both local-battery and central-energy types, the
+latter being the latest development of this selector. With the
+local-battery type, the waystation bell rings until stopped by the
+dispatcher. With the central-energy type it rings a definite length of
+time and can be held for a longer period as is the case with the Western
+Electric selector. The selector is operated by combinations of
+direct-current impulses which are sent out over the line by keys in the
+dispatcher's office.
+
+[Illustration: Fig. 479. Gill Selector]
+
+The dispatcher has a key cabinet, and calls in the same way as already
+described, but these keys instead of sending a series of quick impulses,
+send a succession of impulses with intervals between corresponding to
+the particular arrangement of teeth in the corresponding waystation
+selector wheel. Each key, therefore, belongs definitely with a certain
+selector and can be used in connection with no other.
+
+A concrete example may make this clearer. The dispatcher may operate key
+No. 1421. This key starts a clockwork mechanism which impresses at
+regular intervals, on the telephone line, direct-current impulses, with
+intervals between as follows: 1-4-2-1. There is on the line one selector
+corresponding to this combination and it alone, of all the selectors on
+the circuit, will step its wheel clear around so that contact is made
+and the bell is rung. In all the others, the pawls will have slipped out
+at some point of the revolution and the wheels will have returned to
+their normal positions.
+
+The Gill selector is shown in Fig. 479. It contains a double-wound relay
+which is bridged across the telephone circuit and operates the selector.
+This relay has a resistance of 4,500 ohms and a high impedance, and
+operates the selector mechanism which is a special modification of the
+ratchet and pawl principle. The essential features of this selector are
+the "step-up" selector wheel and a time wheel, normally held at the
+bottom of an inclined track.
+
+The operation of the selector magnet pushes the time wheel up the track
+and allows it to roll down. If the magnet is operated rapidly, the wheel
+does not get clear down before being pushed back again. A small pin on
+the side of the pawl, engaging the selector wheel normally, opposes the
+selector wheel teeth near their outer points. When the time wheel rolls
+to the bottom of the track, however, the pawl is allowed to drop to the
+bottom of the tooth. Some of the teeth on the selector wheel are formed
+so that they will effectually engage with the pawl only when the latter
+is in normal position, while others will engage only while the pawl is
+at the bottom position; thus innumerable combinations can be made which
+will respond to certain combinations of rapid impulses with intervals
+between. The correct combination of impulses and intervals steps the
+selector wheel clear around so that a contact is made. The selector
+wheels at all other stations fail to reach their contact position
+because at some point or points in their revolution the pawls have
+slipped out, allowing the selector wheels to return "home."
+
+The "answer-back" is provided in this selector by means of a few
+inductive turns of the bell circuit which are wound on the selector
+relay. The operation of the bell through these turns induces an
+alternating current in the selector winding which flows out on the line
+and is heard as a distinctive buzzing noise by the dispatcher.
+
+[Illustration: Fig. 480. Cummings-Wray Dispatcher's Sender]
+
+_The Cummings-Wray Selector._ Both of the selectors already described
+are of a type known as the _individual-call_ selectors, meaning that
+only one station at a time can be called. If a plurality of calls is
+desired, the dispatcher calls one station after another. The third type
+of selector in use today is of a type known as the _multiple-call_, in
+which the dispatcher can call simultaneously as many stations as he
+desires.
+
+The Cummings-Wray selector and that of the Kellogg Switchboard and
+Supply Company are of this type and operate on the principle of
+synchronous clocks. When the dispatcher wishes to put through a call, he
+throws the keys of all the stations that he desires and then operates a
+starting key. The bells at all these stations are rung by one operation.
+
+The dispatcher's sending equipment of the Cummings-Wray system is shown
+in Fig. 480, and the waystation selector in Fig. 481. It is necessary
+with this system for the clocks at all stations to be wound every eight
+days.
+
+[Illustration: Fig. 481. Cummings-Wray Selector]
+
+In the dispatcher's master sender the clock-work mechanism operates a
+contact arm which shows on the face of the sender in Fig. 480. There is
+one contact for every station on the line. The clock at this office and
+the clocks at all the waystation offices start together, and it is by
+this means that the stations are signaled, as will be described later,
+when the detailed operation of the circuits is taken up.
+
+=Telephone Equipment.= Of no less importance than the selective devices
+is the telephone apparatus. That which is here illustrated is the
+product of the Western Electric Company, to whom we are indebted for all
+the illustrations in this chapter.
+
+_Dispatcher's Transmitter._ The dispatcher, in most cases, uses the
+chest transmitter similar to that employed by switchboard operators in
+every-day service. He is connected at all times to the telephone
+circuit, and for this reason equipment easy for him to wear is
+essential. In very noisy locations he is equipped with a double head
+receiver. On account of the dispatcher being connected across the line
+permanently and of his being required to talk a large part of the time,
+there is a severe drain on the transmitter battery. For this reason
+storage batteries are generally used.
+
+[Illustration: Fig. 482. Waystation Desk Telephone]
+
+_Waystation Telephones._ At the waystations various types of telephone
+equipment may be used. Perhaps the most common is the familiar desk
+stand shown in Fig. 482, which, for railroad service, is arranged with a
+special hook-switch lever for use with a head receiver.
+
+Often some of the familiar swinging-arm telephone supports are used, in
+connection with head receivers, but certain special types developed
+particularly for railway use are advantageous, because in many cases the
+operator who handles train orders is located in a tower where he must
+also attend to the interlocking signals, and for such service it is
+necessary for him to be able to get away from the telephone and back to
+it quickly. The Western Electric telephone arm developed for this use is
+shown in Fig. 483. In this the transmitter and the receiver are so
+disposed as to conform approximately to the shape of the operator's
+head. When the arm is thrown back out of the way it opens the
+transmitter circuit by means of a commutator in its base.
+
+[Illustration: Fig. 483. Telephone Arm]
+
+_Siding Telephones._ Two types of sets are employed for siding
+purposes. The first is an ordinary magneto wall instrument, which
+embodies the special apparatus and circuit features employed in the
+standard waystation sets. These are used only where it is possible to
+locate them indoors or in booths along the line. These sets are
+permanently connected to the train wire, and since the chances are small
+that more than one of them will be in use at a time, they are rung by
+the dispatcher, by means of a regular hand generator, when it is
+necessary for him to signal a switching.
+
+[Illustration: Fig. 484. Weather-Proof Telephone Set]
+
+In certain cases it is not feasible to locate these siding telephone
+sets indoors, and to meet these conditions an iron weather-proof set is
+employed, as shown in Figs. 484 and 485. The apparatus in this set is
+treated with a moisture-proofing compound, and the casing itself is
+impervious to weather conditions.
+
+[Illustration: Fig. 485. Weather-Proof Telephone Set]
+
+_Portable Train Sets._ Portable telephone sets are being carried
+regularly on wrecking trains and their use is coming into more and more
+general acceptance on freight and passenger trains. Fig. 486 shows one
+of these sets equipped with a five-bar generator for calling the
+dispatcher. Fig. 487 shows a small set without generator for conductors'
+and inspectors' use on lines where the dispatcher is at all times
+connected in the circuit.
+
+[Illustration: Fig. 486. Portable Telephone Set]
+
+[Illustration: Fig. 487. Portable Telephone Set]
+
+These sets are connected to the telephone circuit at any point on the
+line by means of a light portable pole arranged with terminals at its
+outer extremity for hooking over the line wires, and with flexible
+conducting cords leading to the portable set. The use of these sets
+among officials on their private cars, among construction and bridge
+gangs working on the line, and among telephone inspectors and repairmen
+for reporting trouble, is becoming more and more general.
+
+=Western Electric Circuits.= As already stated, a telephone
+train-dispatching circuit may be from 25 to 300 miles in length, and
+upon this may be as many stations as can be handled by one dispatcher.
+The largest known number of stations upon an existing circuit of this
+character is 65.
+
+[Illustration: Fig. 488. Dispatcher's Station--Western Electric System]
+
+_Dispatcher's Circuit Arrangement._ The circuits of the dispatcher's
+station in the Western Electric system are shown in Fig. 488, the
+operation of which is briefly as follows: When the dispatcher wishes to
+call any particular station, he gives the key corresponding to that
+station a quarter turn. This sends out a series of rapid direct-current
+impulses on the telephone line through the contact of a special
+telegraph relay which is operated by the key in a local circuit. The
+telegraph relay is equipped with spark-eliminating condensers around its
+contacts and is of heavy construction throughout in order to carry
+properly the sending current.
+
+_Voltage._ The voltage of the sending battery is dependent on the length
+of the line and the number of stations upon it. It ranges from 100 to
+300 volts in most cases. When higher voltages are required in order
+successfully to operate the circuit, it is generally customary to
+install a telegraph repeater circuit at the center of the line, in order
+to keep the voltage within safe limits. One reason for limiting the
+voltage employed is that the condensers used in the circuit will not
+stand much higher potentials without danger of burning out. It is also
+possible to halve the voltage by placing the dispatcher in the center of
+the line, from which position he may signal in two directions instead of
+from one end.
+
+_Simultaneous Talking and Signaling._ Retardation coils and condensers
+will be noticed in series with the circuit through which the signaling
+current must pass before going out on the line. These are for the
+purpose of absorbing the noise which is caused by high-voltage battery,
+thus enabling the dispatcher to talk and signal simultaneously. The
+250-ohm resistance connected across the circuit through one back contact
+of the telegraph relay absorbs the discharge of the 6-microfarad
+condenser.
+
+[Illustration: Fig. 489. Selector Set--Western Electric System]
+
+=Waystation Circuit.= The complete selector set for the waystations is
+shown in Fig. 489, and the wiring diagram of its apparatus in Fig. 490.
+The first impulse sent out by the key in the dispatcher's office is a
+long direct-current impulse, the first tooth being three or four times
+as wide as the other teeth. This impulse operates both magnets of the
+selector and attracts their armatures, which, in turn, cause two pawls
+to engage with the ratchet wheel, while the remaining quick impulses
+operate the "stepping-up" pawl and rotate the wheel the requisite number
+of teeth. Retardation coils are placed in series with the selector in
+order to choke back any lightning discharges which might come in over
+the line. The selector contact, when operated, closes a bell circuit,
+and it will be noted that both the selector and the bell are operated
+from battery current coming over the main line through variable
+resistances. There are, of course, a number of selectors bridged across
+the circuit, and the variable resistance at each station is so adjusted
+as to give each approximately 10 milliamperes, which allows a large
+factor of safety for line leakage in wet weather. The drop across the
+coils at 10 milliamperes is 38 volts. If these coils were not employed,
+it is clear that the selectors nearer the dispatcher would get most of
+the current and those further away very little.
+
+[Illustration: Fig. 490. Selector Set--Western Electric System]
+
+A time-signal contact is also indicated on the selector-circuit diagram
+of Fig. 490. This is common to all offices and may be operated by a
+special key in the dispatcher's office, thereby enabling him to send out
+time signals over the telephone circuit.
+
+[Illustration: Fig. 491. Gill Dispatcher's Station]
+
+=Gill Circuits.= The circuit arrangement for the dispatcher's outfit of
+the Gill system is shown in Fig. 491. This is similar to that of the
+Western Electric system just described. The method of operation also is
+similar, the mechanical means of accomplishing the selection being the
+main point of difference. In Fig. 492 the wiring of the Gill selector at
+a waystation for local-battery service is shown. The selector contact
+closes the bell circuit in the station and a few windings of this
+circuit are located on the selector magnets, as shown. These provide the
+"answer-back" by inductive means.
+
+[Illustration: Fig. 492. Gill Selector--Local Battery]
+
+Fig. 493 shows the wiring of the waystation, central-energy Gill
+selector. In this case, the local battery for the operation of the bell
+is omitted and the bell is rung, as is the case of the Western Electric
+selector, by the main sending battery in the dispatcher's office.
+
+[Illustration: Fig. 493. Gill Selector--Central Energy]
+
+The sending keys of these two types of circuits differ, in that with the
+local-battery selector the key contact is open after the selector has
+operated, and the ringing of the bell must be stopped by the dispatcher
+pressing a button or calling another station. Either of these operations
+sends out a new current impulse which releases the selector and opens
+its circuit.
+
+With the central-energy selector, however, the contacts of the sending
+key at the dispatcher's office remain closed after operation for a
+definite length of time. This is obviously necessary in order that
+battery may be kept on the line for the operation of the bell. In this
+case the contacts remain closed during a certain portion of the
+revolution of the key, and the bell stops ringing when that portion of
+the revolution is completed. If, however, the dispatcher desires to give
+any station a longer ring, he may do so by keeping the key contacts
+closed through an auxiliary strap key as soon as he hears the
+"answer-back" signal from the called station.
+
+=Cummings-Wray Circuits.= The Cummings-Wray system, as previously
+stated, is of the multiple-call type, operating with synchronous clocks.
+Instead of operating one key after another in order to call a number of
+stations, all the keys are operated at once and a starting key sets the
+mechanism in motion which calls all these stations with one operation.
+Fig. 494 shows the circuit arrangement of this system.
+
+[Illustration: Fig. 494. Cummings-Wray System]
+
+In order to ring one or more stations, the dispatcher presses the
+corresponding key or keys and then operates the starting key. This
+starting key maintains its contact for an appreciable length of time to
+allow the clock mechanism to get under way and get clear of the
+releasing magnet clutch. Closing the starting key operates the
+clock-releasing magnet and also operates the two telegraph-line relays.
+These send out an impulse of battery on the line operating the bridged
+2,500-ohm line relays and, in turn, the selector releasing magnets;
+thus, all the waystation clocks start in unison with the master clock.
+The second hand arbor of each clock carries an arm, which at each
+waystation is set at a different angle with the normal position than
+that at any other station. Each of these arms makes contact precisely at
+the moment the master-clock arm is passing over the contact
+corresponding to that station.
+
+If, now, a given station key is pressed in the master sender, the
+telegraph-line relays will again operate when the master-clock arm
+reaches that point, sending out another impulse of battery over the
+line. The selector contact at the waystation is closed at this moment;
+therefore, the closing of the relay contact operates the ringing relay
+through a local circuit, as shown. The ringing relay is immediately
+locked through its own contact, thus maintaining the bell circuit closed
+until it is opened by the key and the ringing is stopped.
+
+As the master-clock arm passes the last point on the contact dial, the
+current flows through the restoring relay operating the restoring magnet
+which releases all the keys. A push button is provided by means of which
+the keys may be manually released, if desired. This is used in case the
+dispatcher presses a key by mistake. Retardation coils and variable
+resistances are provided at the waystation just as with the other
+selector systems which have been described and for the same reasons.
+
+The circuits of the operator's telephone equipment shown in Fig. 495,
+are also bridged across the line. This apparatus is of high impedance
+and of a special design adapted to railroad service. There may be any
+number of telephones listening in upon a railroad train wire at the same
+time, and often a dispatcher calls in five or six at once to give
+orders. These conditions have necessitated the special circuit
+arrangement shown in Fig. 495.
+
+[Illustration: Fig. 495. Telephone Circuits]
+
+The receivers used at the waystations are of high impedance and are
+normally connected, through the hook switch, directly across the line in
+series with a condenser. When the operator, at a waystation wishes to
+talk, however, he presses the key shown. This puts the receiver across
+the line in series with the retardation coil and in parallel with the
+secondary of the induction coil. It closes the transmitter battery
+circuit at the same time through the primary of the induction coil.
+
+The retardation coil is for the purpose of preventing excessive side
+tone, and it also increases the impedance of the receiver circuit, which
+is a shunt on the induction coil. This latter coil, however, is of a
+special design which permits just enough current to flow through the
+receiver to allow the dispatcher to interrupt a waystation operator when
+he is talking.
+
+The key used to close the transmitter battery is operated by hand and is
+of a non-locking type. In some cases, where the operators are very busy,
+a foot switch is used in place of this key. The use of such a key or
+switch in practical operation has been found perfectly satisfactory, and
+it takes the operators but a short time to become used to it.
+
+The circuits of the dispatcher's office are similarly arranged, Fig.
+495, being designed especially to facilitate their operation. In other
+words, as the dispatcher is doing most of the work on the circuit, his
+receiver is of a low-impedance type, which gives him slightly better
+transmission than the waystations obtain. The key in his transmitter
+circuit is of the locking type, so that he does not have to hold it in
+while talking. This is for the reason that the dispatcher does most of
+the talking on this circuit. Foot switches are also employed in some
+cases by the dispatchers.
+
+=Test Boards.= It is becoming quite a general practice among the
+railroads to install more than one telephone circuit along their
+rights-of-way. In many cases in addition to the train wire, a message
+circuit is also equipped, and quite frequently a block wire also
+operated by telephone, parallels these two. It is desirable on these
+circuits to be able to make simple tests and also to be able to patch
+one circuit with another in cases of emergency.
+
+[Illustration: Fig. 496. Test Board]
+
+Test boards have been designed for facilitating this work. These consist
+of simple plug and jack boxes, the general appearance of which is shown
+in Fig. 496. The circuit arrangement of one of these is shown in Fig.
+497. Each wire comes into an individual jack as will be noted on one
+side of the board, and passes through the inside contact of this jack,
+out through a similar jack on the opposite side. The selector and
+telephone set at an office are taken off these inside contacts through a
+key, as shown. The outside contacts of this key are wired across two
+pairs of cords. Now, assume the train wire comes in on jacks _1_ and
+_3_, and the message wire on jacks _9_ and _11_. In case of an accident
+to the train wire between two stations, it is desirable to patch this
+connection with a message wire in order to keep the all-important train
+wire working. The dispatcher instructs the operator at the last station
+which he can obtain, to insert plugs _1_ and _2_ in jacks _1_ and _10_,
+and plugs _3_ and _4_ in jacks _3_ and _12_, at the same time throwing
+the left-hand key. Then, obtaining an operator beyond the break by any
+available means, he instructs him likewise to insert plugs _1_ and _2_
+in jacks _9_ and _2_, and plugs _3_ and _4_ in jacks _11_ and _4_,
+similarly throwing the left-hand key. By tracing this out, it will be
+observed that the train wire is patched over the disabled section by
+means of the message circuit, and that the selector and the telephone
+equipment are cut over on to the patched connections; in other words,
+bridged across the patching cords.
+
+[Illustration: Fig. 497. Circuits of Test Board]
+
+It will also be seen that with this board it is possible to open any
+circuit merely by plugging into a jack. Two wires can be short-circuited
+or a loop made by plugging two cords of corresponding colors into the
+two jacks. A ground jack is provided for grounding any wire. In this
+way, a very flexible arrangement of circuits is obtained, and it is
+possible to make any of the simple tests which are all that are usually
+required on this type of circuit.
+
+=Blocking Sets.= As was just mentioned, quite frequently in addition to
+train wires and message circuits, block wires are also operated by
+telephone. In some cases separate telephone instruments are used for the
+blocking service, but in others the same man handles all three circuits
+over the same telephone. The block wire is generally a converted
+telegraph wire between stations, usually of iron and usually grounded.
+It seldom ranges in length over six miles.
+
+[Illustration: Fig. 498. Blocking Set]
+
+Where the block wires are operated as individual units with their own
+instruments, it is unnecessary to have any auxiliary apparatus to be
+used in connection with them. Where, however, they are operated as part
+of a system and the same telephone is used on these that is used on the
+train wire and message wire, additional apparatus, called a blocking
+set, is required. This blocking set, shown in Figs. 498 and 499, was
+developed especially for this service by the Western Electric Company.
+As will be noted, a repeating coil at the top and a key on the front of
+the set are wired in connection with a pair of train wire cords. This
+repeating coil is for use in connecting a grounded circuit to a metallic
+circuit, as, for instance, connecting a block wire to the train wire,
+and is, of course, for the purpose of eliminating noise. Below the key
+are three combined jacks and signals. One block wire comes into each of
+these and a private line may be brought into the middle one. When the
+next block rings up, a visual signal is displayed which operates a bell
+in the office by means of a local circuit. The operator answers by
+plugging the telephone cord extending from the bottom of the set into
+the proper jack. This automatically restores the signal and stops the
+bell.
+
+[Illustration: Fig. 499. Blocking Set]
+
+Below these signals appear four jacks. One is wired across the train
+wire; one across the message wire; and the other two are bridged across
+the two pairs of patching cords on each side of the set. The operator
+answers a call on any circuit by plugging his telephone cord into the
+proper jack.
+
+If a waystation is not kept open in the evening, or the operator leaves
+it for any reason and locks up, he can connect two blocks together by
+means of the block-wire cords. These are arranged simply for connecting
+two grounded circuits together and serve to join two adjacent blocks,
+thereby eliminating one station. A jack is wired across these cords, so
+that the waystation operator can listen in on the connection if he so
+desires.
+
+In some cases not only are the telephone circuits brought into the test
+board, but also two telegraph wires are looped through this board before
+going to the peg switchboard. This is becoming quite a frequent practice
+and, in times of great emergency, enables patches to be made to the
+telegraph wires as well as to the telephone wires.
+
+=Dispatching on Electric Railways.= As interurban electric railways are
+becoming more extended, and as their traffic is becoming heavier, they
+approximate more closely to steam methods of operation. It is not
+unusual for an electric railway to dispatch its cars exactly as in the
+case of a steam road. There is a tendency, however, in this class of
+work, toward slightly different methods, and these will be briefly
+outlined.
+
+On those electric railways where the traffic is not especially heavy, an
+ordinary magneto telephone line is frequently employed with standard
+magneto instruments. In some cases the telephone sets are placed in
+waiting rooms or booths along the line of the road. In other cases it is
+not feasible to locate the telephone indoors and then iron weather-proof
+sets, such as are shown in Figs. 484 and 485, are mounted directly on
+the poles along the line of railway. With a line of this character there
+is usually some central point from which orders are issued and the
+trainmen call this number when arriving at sidings or wherever they may
+need to do so.
+
+Another method of installing a telephone system upon electric railways
+is as follows: Instead of instruments being mounted in booths or on
+poles along the line, portable telephone sets are carried on the cars
+and jacks are located at regular intervals along the right-of-way on the
+poles. The crew of the car wishing to get in touch with the central
+office or the dispatcher, plugs into one of these jacks and uses the
+portable telephone set. At indoor stations, in offices or buildings
+belonging to the railroad, the regular magneto sets may be employed, as
+in the first case outlined.
+
+On electric railway systems where the traffic is heavy, the train or car
+movements may be handled by a dispatcher just as on the steam railroad.
+There is usually one difference, however. On a steam road, the operators
+who give the train crews their orders and manipulate the semaphore
+signals are located at regular intervals in the different waystations.
+No such operators are usually found on electric railways, except,
+perhaps, at very important points, and, therefore, it is necessary for
+the dispatcher to be able to signal cars at any point and to get into
+communication with the crews of these cars. He does this by means of
+semaphores operated by telephone selectors over the telephone line. The
+telephone circuit may be equipped with any number of selectors desired,
+and the dispatcher can operate any particular one without operating any
+other one on the circuit. Each selector, when operated, closes a pair of
+contacts. This completes a local circuit which throws the semaphore arm
+to the "danger" position, at the same time giving the dispatcher a
+distinctive buzz in his ear, which informs him that the arm has actually
+moved to this position. He can get this signal only by the operation of
+the arm.
+
+Each semaphore is located adjacent to a telephone booth in which is also
+placed the restoring lever, by means of which the semaphore is set in
+the "clear" position by the crew of the car which has been signaled. The
+wall-type telephone set is usually employed for this class of service,
+but if desired, desk stands or any of the various transmitter arms may
+be used.
+
+It is necessary for the crew of the car which first approaches a
+semaphore set at "danger," to get out, communicate with the dispatcher,
+and restore the signal to the "clear" position. The dispatcher can not
+restore the signal. The signal is set only in order that the train crew
+may get into telephonic communication with the dispatcher, and in order
+to do this, it is necessary for them to go into the booth in any case.
+
+[Footnote A: We wish particularly to acknowledge the courtesy of the
+Western Electric Company in their generous assistance in the preparation
+of this chapter.]
+
+
+
+
+REVIEW QUESTIONS
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 11--68
+
+       *       *       *       *       *
+
+1. What are the advantages of a common-battery system?
+
+2. When is the local battery to be preferred to the common-battery?
+
+3. Enumerate the different kinds of line signals.
+
+4. Make a diagram of the arrangement of a direct line lamp signal.
+
+5. What is a direct line lamp with ballast? Give sketch.
+
+6. Describe a line lamp with relay.
+
+7. What is a pilot lamp and what are its functions?
+
+8. Sketch three different kinds of batteries applied to cord circuits.
+
+9. What is a supervisory signal?
+
+10. Make diagram of a complete simple common-battery switchboard
+circuit.
+
+11. When will the supervisory signal become operative?
+
+12. What is the candle-power of incandescent lamps used for line and
+supervisory signals?
+
+13. At what voltages do they operate?
+
+14. What are visual signals?
+
+15. Describe the mechanical signal of the Western Electric Company.
+
+16. Give a short description of the general assembly of the parts of a
+simple common-battery switchboard.
+
+17. What is a transfer switchboard?
+
+18. Outline the limitations of a simple switchboard.
+
+19. Describe and sketch a plug-ended transfer line.
+
+20. Why is the plug-seat switch not more widely adopted for use?
+
+21. Make diagram of an order-wire arrangement.
+
+22. What are the limitations of the transfer system?
+
+23. What are the fundamental features of the multiple switchboard?
+
+24. What is a multiple jack?
+
+25. What is an answering jack?
+
+26. Make a diagram showing the principle of multiple switchboards.
+
+27. What is the busy signal?
+
+28. What determines the size of a multiple switchboard?
+
+29. What is the use of the intermediate distributing frame?
+
+30. Make diagram of the series magneto multiple switchboard and describe
+its operation.
+
+31. What are the defects of this system?
+
+32. Give a diagram of the branch terminal magneto multiple switchboard.
+
+33. Give a diagram and a short description of the Monarch magneto
+multiple switchboard.
+
+
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 69--134
+
+       *       *       *       *       *
+
+1. Sketch and describe the line circuit of the common-battery multiple
+switchboard of the Bell companies.
+
+2. Make a diagram of the cord circuit of the Western Electric standard
+multiple common-battery switchboard.
+
+3. Describe the busy test in this system.
+
+4. What is the function of the order-wire circuits?
+
+5. What is jumper wire?
+
+6. Give a short description of the relay mounting in the standard No. 1
+relay board of the Western Electric Company.
+
+7. What is the ultimate capacity of the No. 1 Western Electric
+switchboard?
+
+8. What is the capacity of the No. 10 Western Electric switchboard?
+
+9. How does this switchboard No. 10 differ from No. 1?
+
+10. Give a diagram of the two-wire line circuit of the Kellogg Company.
+
+11. What is the capacity of the condenser of the cord circuit in the
+foregoing system?
+
+12. Give a complete diagram of the Kellogg two-wire board.
+
+13. Describe the busy test in this system.
+
+14. Give diagram of the Stromberg-Carlson multiple-board circuit.
+
+15. What is the most important piece of apparatus in a multiple
+switchboard?
+
+16. What is the spacing of the multiple jacks in the No. 1 Western
+Electric switchboard?
+
+17. How do the relays of the Western Electric Company differ from those
+of other companies?
+
+18. Describe the relay construction of the Monarch Telephone Company.
+
+19. What is meant by inter-office trunking?
+
+20. What is the present practice in America as to the capacity of
+multiple hoards?
+
+21. What is the tendency in Europe regarding the capacity of multiple
+boards?
+
+22. Discuss the preferences in American practice.
+
+23. State the different methods of trunking between exchanges.
+
+24. When are two-way trunks employed?
+
+25. Make diagram of the Western Electric inter-office connection system.
+
+26. Describe the standard four-party line trunk ringing key of the
+Western Electric Company.
+
+27. Sketch and describe a keyless trunk.
+
+28. Give diagram of the inter-office connection of the Kellogg system.
+
+29. How does this system differ from the Western Electric in regard to
+the ringing?
+
+30. Why are the A and B switchboards in large exchanges entirely
+separated?
+
+
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 135--226
+
+       *       *       *       *       *
+
+1. What is the general object of automatic telephone systems?
+
+2. What are the common arguments against these systems and how are they
+met?
+
+3. Give the operations that the calling subscriber has to go through in
+any one of the successful systems.
+
+4. During calling what is happening at the central office?
+
+5. Describe the action of the Strowger or Automatic Electric Company
+selecting switch.
+
+6. What is the function of a line switch?
+
+7. Describe the Strowger scheme of trunking and illustrate its action by
+diagram.
+
+8. Make a diagram of the sub-station apparatus and connections.
+
+9. Make a diagram of the line switch unit.
+
+10. Describe the action of the various guarding features necessary to
+protect a busy line.
+
+11. Make a simple diagram of the circuits of the first selector.
+
+12. Give the functions and operations of the connector.
+
+13. Give a diagram of connecting circuits.
+
+14. Tell all you can regarding the battery supply to the connected
+subscriber.
+
+15. How are subscribers disconnected after they are through talking?
+
+16. Describe a multi-office system.
+
+17. Give a diagram of circuits of the trunk repeater.
+
+18. Make a complete diagram of the connections between a calling and a
+called subscriber in an automatic system.
+
+19. What is the rotary connector?
+
+20. Describe the sub-station equipment of the Lorimer automatic system.
+
+21. Describe the Lorimer central-office apparatus.
+
+22. Give a description of the progress of a call from its institution to
+the final disconnection in the Lorimer system.
+
+23. What is the automanual system?
+
+24. Give general features of the operation in the automanual system.
+
+25. Describe the automanual system subscribers' apparatus.
+
+26. Give a description of the automanual central-office equipment.
+
+
+
+
+REVIEW QUESTIONS
+
+ON SUBJECT OF TELEPHONY
+
+PAGES 227--270
+
+       *       *       *       *       *
+
+1. What kinds of currents are employed?
+
+2. What types of power plants are used?
+
+3. Describe the sources of current supplied for the operator's
+transmitter current and ringing current.
+
+4. Make a diagram of the Warner pole changer.
+
+5. Make a diagram of pole changers for harmonic ringing.
+
+6. What is a multi-cyclic generator set?
+
+7. Make a diagram of governor for harmonic ringing generators.
+
+8. Describe the various primary sources of power.
+
+9. Make a diagram of the mercury-arc-rectifier circuits.
+
+10. What provision against breakdown is made?
+
+11. Tell all you can about the storage battery--its construction and its
+operation.
+
+12. What is a pilot cell?
+
+13. Describe the switches, meters, and protective devices used on the
+power switchboard.
+
+14. Give a diagram showing a typical example of a common-battery manual
+switchboard equipment and circuits.
+
+15. Give the main points concerning the construction of a central-office
+building.
+
+16. What provision should be made for cable runways?
+
+17. Make a sketch of a small central-office floor plan.
+
+18. Describe the Western Electric main and intermediate frames. Give
+diagrams.
+
+19. Give principal points regarding small office terminal apparatus.
+
+20. Give types of line circuits.
+
+21. Describe the typical equipment of a large manual office. Give floor
+plans.
+
+22. Give floor plan of an automatic office.
+
+
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 271--320
+
+       *       *       *       *       *
+
+1. What is a private-branch exchange?
+
+2. What does "P. B. X." mean?
+
+3. What is the function of the private-branch exchange operator?
+
+4. Describe the key type of a small private-branch exchange switchboard.
+
+5. Describe the different methods of supervision of private-branch
+connections.
+
+6. Describe the automatic equipment of the common-battery type in
+private-branch exchanges.
+
+7. How is secrecy of individual lines obtained in a private-exchange
+equipment?
+
+8. What is an intercommunicating system?
+
+9. Sketch a magneto intercommunicating system.
+
+10. Sketch and describe a plug type common-battery intercommunicating
+system.
+
+11. Sketch and describe the action of the push button in the Monarch
+system and in the Western Electric system.
+
+12. Sketch and describe the Monarch intercommunicating system.
+
+13. What is the office of the junction box in this system?
+
+14. What is a long-distance message?
+
+15. What is the function of the repeating coil in the long-distance
+line?
+
+16. Which is the simplest form of long-distance switch?
+
+17. What is a phantom circuit?
+
+18. Under what control is the ringing of the subscriber in
+long-distance calls?
+
+19. What is meant by ticket passing?
+
+20. What particular advantage has a common-battery set on long-distance
+lines?
+
+21. Give a typical load curve for telephone traffic.
+
+22. Why is traffic a study of importance?
+
+23. State the function of the intermediate distributing frame.
+
+24. State the different methods of traffic study.
+
+25. What is the trunking factor?
+
+26. Define _trunking efficiency_.
+
+27. Enumerate some of the elements upon which the quality of service in
+a manual system depends.
+
+28. What is team work?
+
+29. How does the cost of telephone service vary?
+
+30. What two general methods of charging for telephone service are in
+use?
+
+31. Describe a calculagraph and how is it used?
+
+32. How are toll connections timed by the Monarch Telephone Company?
+
+33. Sketch and describe the Western Electric Company line circuit and
+service meter.
+
+
+
+
+REVIEW QUESTIONS
+
+ON THE SUBJECT OF TELEPHONY
+
+PAGES 321--358
+
+       *       *       *       *       *
+
+1. Describe a phantom circuit with diagram.
+
+2. Explain how two phantoms may be joined by a physical circuit.
+
+3. Which are the better, phantom or physical circuits, and why?
+
+4. Explain how the simplex circuit differs from the phantom telephone
+circuit.
+
+5. Why are not telegraph wires as serviceable for telephone work as
+telephone wires are for telegraph work?
+
+6. Give the names of the different parts of a railway composite set and
+explain method of operating.
+
+7. State the causes of the introduction of the telephone into the train
+dispatching field and explain the advantages it has over the telegraph
+for this work.
+
+8. In transmitting orders for train dispatching, how are mistakes
+avoided?
+
+9. Describe the Western Electric selector and explain its use.
+
+10. In what way does the Gill selector differ from the Western Electric?
+
+11. What special feature does the multiple coil selector possess?
+
+12. What special arrangement is provided for the train dispatcher in
+noisy locations?
+
+13. How can a man on a wrecking train get connection with the train
+dispatcher?
+
+14. What is the usual limit in length of a telephone train dispatching
+circuit and what is the largest number of stations at present existing
+on such a circuit?
+
+15. What is the voltage of the sending battery for a train dispatcher's
+circuit and upon what is it dependent?
+
+16. For what purpose is a repeater circuit used?
+
+17. How is the noise caused by a high voltage battery absorbed so that
+the dispatcher may talk and signal simultaneously?
+
+18. Draw a diagram showing the circuit arrangement for the dispatcher's
+outfit of the Gill system.
+
+19. Explain fully the purpose of the retardation coil in connection with
+a waystation set.
+
+20. In case of accident to a train wire between two stations, how can
+the connection be patched if the road is also equipped with a message
+circuit in addition to the train wire?
+
+21. Why do some railroads have block wires in addition to train wires
+and message circuits?
+
+22. If a waystation on a block wire is to be cut out for any length of
+time, by what method can the two adjacent blocks be connected,
+eliminating the station between?
+
+23. What are some of the methods used for dispatching on electric
+railways where the traffic is not especially heavy?
+
+24. On an electric road in case a car approaches a semaphore set at
+"danger," what must the crew of the car do?
+
+
+
+
+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
+
+Automanual system 218
+  automatic distribution of calls 223
+  automatic switching equipment 222
+  building up a connection 224
+  characteristics of 218
+  operation 219
+  operator's equipment 220
+  setting up a connection 224
+  speed in handling calls 224
+  subscriber's apparatus 219
+
+Automatic desk stand 158
+
+Automatic Electric Company's telephone system 149
+  automatic sub-offices 201
+  connector 185
+    function of 185
+    location of 186
+    operation of 186
+  first selector operation 179
+  function of line switch 152
+  line switch 153, 163
+    bridge cut-off 173
+    circuit operations 167
+    guarding functions 173
+    line and trunk contacts 164
+    locking segment 172
+    master switch 171
+    relation of, to connectors 174
+    structure of 166
+    summary of operation 174
+    trunk ratio 165
+    trunk selection 165
+  multi-office system 196
+  party lines 202
+  release after conversation 196
+  rotary connector 202
+  second selector operation 182
+  selecting switches 153, 175
+    release mechanism 178
+    side switch 175
+  subdivision of subscribers' lines 152
+  subscribers' station apparatus 158
+    operation 160
+      bell and transmitter springs 160
+      ground springs 160
+      impulse springs 161
+      release springs 163
+      ringing springs 163
+    salient points 163
+  trunking 154
+    connector action 157
+    first selector action 156
+    line switch action 154
+    second selector action 156
+  two-wire automatic systems 203
+  two-wire and three-wire systems 157
+  underlying feature of trunking system 153
+
+Automatic telephone systems 135
+  arguments against 135
+    attitude of public 141
+    complexity 136
+    expense 140
+    flexibility 140
+    subscriber's station equipment 142
+  automatic vs. manual 143
+  comparative costs 142
+  definition 135
+  methods of operation 143
+    fundamental idea 147
+    grouping of subscribers 145
+    local and inter-office trunks 148
+    Lorimer system 144
+    magnet vs. power-driven switches 144
+
+Automatic telephone systems
+  methods of operation
+    multiple vs. trunking 145
+    outline of action 146
+    Strowger system 143
+    testing 148
+    trunking between groups 145
+
+Automatic wall set 158
+
+
+B
+
+Blocking sets 355
+
+Busy test 48
+  busy-test faults 50
+  potential of test thimbles 49
+  principle 49
+
+
+C
+
+Circuits 321
+  applications 322
+  composite 326
+  phantom 321
+    transmission over 324
+    transpositions 323
+  railway composite 327
+  ringing 327
+  simplex 324
+
+Common-battery multiple switchboard 69
+  assembly 106
+  Dean multiple board 93
+    cord circuit 94
+    line circuit 93
+    listening key 94
+    ringing keys 94
+    test 94
+  Kellogg two-wire multiple board 84
+    battery feed 88
+    busy test 90
+    complete cord and line circuit 88
+    cord circuit 86
+    line circuit 85
+    summary of operation 91
+    supervisory signals 87
+    wiring of line circuit 92
+  multiple switchboard apparatus 97
+    jacks 99
+    lamp jacks 100
+    relays 101
+  Stromberg-Carlson multiple board 96
+    cord circuit 96
+    supervisory signals 97
+    test 97
+  Western Electric No. 1 relay board 69
+    capacity range 80
+    cord circuit 71
+    functions of distributing frames 77
+    line circuit 69
+    modified relay windings 79
+    operation 72
+    operator's circuit detail 75
+    order-wire circuits 78
+    pilot signals 79
+    relay mounting 80
+    testing--called line busy 75
+    testing--called line idle 74
+    wiring of line circuit 76
+  Western Electric No. 10 board 80
+    circuits 81
+    economy 84
+    operation 83
+    test 83
+
+Common-battery switchboard 11
+  advantages of operation 11
+  common battery vs. magneto 12
+  cord circuit 20
+    battery supply 20
+    complete circuit 21
+    supervisory signals 21
+  cycle of operations 23
+  jacks 30
+  lamps 24
+    mounting 25
+  line signals 14
+    direct-line lamp 14
+    direct-line lamp with ballast 15
+    line lamp with relay 17
+    pilot signals 17
+  mechanical signals 27
+    Kellogg 28
+    Monarch 28
+    Western Electric 27
+  relays 28
+  switchboard assembly 31
+
+Composite circuits 326
+
+Connector 185
+
+Cord circuit 20
+
+Cord circuit
+  battery supply 20
+  complete circuit 21
+  supervisory signals 21
+
+Cord-rack connectors 66
+
+Cummings-Wray selector 342
+
+
+D
+
+Dean multiple board 93
+
+Dispatchers' keys 339
+
+Dispatching on electric railways 356
+
+
+G
+
+Gill selector 341
+
+
+H
+
+Housing central-office equipment 249
+  arrangement of apparatus in small manual offices 252
+    combined main and intermediate frames 253
+    floor plans for 252
+    types of line circuits 255
+  automatic offices 267
+    typical automatic office 270
+  central-office building 249
+    fire hazard 249
+    provision for cable runways 251
+    provision for employes 251
+    size of building 250
+    strength of building 250
+  large manual office 256
+
+
+I
+
+Intercommunicating systems 282
+  common-battery systems 283
+    Kellogg plug type 284
+    Kellogg push-button type 285
+    Monarch system 287
+    Western Electric system 285
+  definition 282
+  limitations 282
+  for private-branch exchanges 290
+  simple magneto system 282
+
+
+J
+
+Jacks 30
+
+
+K
+
+Kellogg mechanical signal 28
+
+Kellogg trunk circuits 125
+
+Kellogg two-wire multiple board 84
+
+Keyboard wiring 67
+
+
+L
+
+Lamp mounting 25
+
+Lamps 24
+
+Line signals 14
+  direct-line lamp 14
+  direct-line lamp with ballast 15
+  line lamp with relay 17
+  pilot signals 17
+
+Line switch 163
+
+Long-distance switching 293
+  definitions 293
+    center-checking 297
+  operators' orders 294
+    by call circuits 294
+    by telegraph 294
+  particular party calls 295
+  switching through local board 293
+  ticket passing 296
+  trunking 295
+    high-voltage toll trunks 295
+    through ringing 295
+  two-number calls 294
+  use of repeating coil 293
+  waystations 297
+
+Lorimer automatic system 144, 205
+  central-office apparatus 208
+    connective division 210
+    sectional apparatus 209
+    switches 213
+      interconnector 214
+      interconnector selector 214
+      primary connector 213
+      rotary switch 213
+      secondary connector 214
+      signal transmitter controller 214
+  operation 215
+  subscriber's station equipment 206
+
+
+M
+
+Magneto multiple switchboard 53
+  branch-terminal multiple board 58
+    arrangement of apparatus 61
+    magnet windings 61
+    operation 60
+  field of utility 53
+  modern magneto multiple board 63
+    assembly 66
+    cord circuit 64
+    test 62
+
+Magneto multiple switchboard
+  series-multiple board 54
+    defects 57
+    operation 56
+
+Measured service 310
+  local service 316
+    meter method 316
+    prepayment method 318
+    ticket method 316
+  rates 310
+  toll service 311
+    long haul 311
+    short haul 311
+    timing toll connections 312
+  units of charging 311
+
+Mechanical signals 27
+  Kellogg 28
+  Monarch 28
+  Western Electric 27
+
+Mercury-arc rectifier circuits 237
+
+Monarch visual signal 28
+
+Multi-office exchanges, necessity for 109
+
+Multiple switchboard 43
+  busy test 48
+  cord circuits 46
+  diagram showing principle of 47
+  double connections 46
+  field of each operator 51
+  field of utility 43
+  influence of traffic 52
+  line signals 45
+  multiple feature 43
+
+
+P
+
+Phantom circuit 321
+
+Pilot signals 17
+
+Plug-seat switch 38
+
+Pole changers for harmonic ringing 231
+
+Power plants 227
+  auxiliary signaling currents 233
+  currents employed 227
+    alternating current 227
+    direct current 227
+  operator's transmitter supply 228
+  power plant circuit 248
+  power switchboard 246
+    meters 246
+    protective devices 248
+    switches 246
+  primary sources 234
+    charging from direct-current mains 234
+    charging dynamos 235
+    mercury-arc rectifiers 236
+    rotary converters 234
+  provision against breakdown 237
+    capacity of power units 238
+    duplicate charging machines 238
+    duplicate primary sources 238
+    duplicate ringing machines 238
+  ringing-current supply 229
+    magneto generators 229
+    pole changers 229
+    ringing dynamos 232
+  storage battery 239
+    initial charge 241
+    installation 240
+    low cells 244
+    operation 242
+    overcharge 243
+    pilot cell 243
+    regular charge 244
+    replacing batteries 245
+    sediment 245
+  types 227
+    common-battery systems 228
+    magneto systems 228
+
+Power switchboard 246
+
+Private-branch exchanges 271
+  with automatic offices 278
+    secrecy 279
+  battery supply 279
+  circuits, key-type board 276
+  definitions 271
+  desirable features 281
+  functions of the private-branch exchange operator 272
+  marking of apparatus 281
+  private-branch switchboards 273
+    common-battery type 273
+    cord type 275
+    key type 275
+    magneto type 273
+  ringing current 280
+  supervision of private-branch connections 277
+
+
+R
+
+Relays 28
+
+Rotary connector 202
+
+
+S
+
+Selecting switches 175
+
+Selector 175
+
+Simplex circuits 324
+
+Storage battery 239
+
+Storage cell 240
+
+Stromberg-Carlson multiple board 96
+
+Strowger automatic system 143
+
+Subscribers' board 259-261
+
+Switchboard assembly 31
+
+
+T
+
+Table
+  automanual system time data 225
+  automatic systems, messages per trunk in 305
+  calling rates 302
+  long-distance groups, messages per trunk in 305
+  manual system, messages per trunk in 304
+  out-trunking, effect of, on operator's capacity 303
+  subscribers' waiting time 226
+
+Telephone traffic 298
+  importance of traffic study 300
+  methods of traffic study 301
+  observation of service 308
+  quality of service 305
+    accuracy and promptness 307
+    answering time 306
+    busy and don't answer calls 307
+    courtesy and form 307
+    disconnecting time 306
+    enunciation 308
+    team work 308
+  rates of calling 300
+  representative traffic data 302
+    calling rates 302
+    operators' loads 302
+    toll traffic 304
+    trunk efficiency 303
+    trunking factor 303
+  traffic variations 298
+    busy hour ratio 299
+  unit of traffic 298
+
+Telephone train dispatching 333
+  advantages 335
+  apparatus 338
+    Cummings-Wray selector 342
+    dispatcher's transmitter 343
+    Gill selector 341
+    portable train sets 345
+    siding telephones 345
+    waystation telephones 344
+    Western Electric selector 338
+  blocking sets 355
+  causes of its introduction 334
+  Cummings-Wray circuits 350
+  on electric railways 356
+  Gill circuits 349
+  railroad conditions 337
+  rapid growth 333
+  test boards 353
+  transmitting orders 337
+  waystation circuits 348
+  Western Electric circuits 347
+
+Telephone train-dispatching circuit
+    Cummings-Wray 350
+    Gill 349
+    waystation 348
+    Western Electric 347
+
+Test boards 353
+
+Transfer switchboard 34
+  field of usefulness 41
+  handling transfers 38
+  limitations 40
+  plug-seat switch 38
+  transfer lines 35
+    jack-ended trunk 35
+    plug-ended trunk 37
+
+Trunking in multi-office systems 109
+  classification 112
+    one-way trunks 103
+    two-way trunks 112
+  Kellogg trunk circuits 125
+  necessity for exchanges 109
+  Western Electric trunk circuits 116
+
+
+W
+
+Warner pole changer 230
+
+Waystation telephones 344
+
+Western Electric
+  mechanical signal 27
+  selector 338
+  trunk circuits 116
+
+
+
+
+Transcriber's Notes.
+
+Spelling variants where it wasn't possible to determine the author's
+intent were left as is. These include: "clockwork" and "clock-work;"
+"doorkeeper" and "door-keeper;" "interrelation" and "inter-relation;"
+"multicyclic" and "multi-cyclic;" "redesign" and "re-design," along with
+derivatives.
+
+Added closing double quote in Steinmetz entry in list of authorities:
+"Theoretical Elements of Electrical Engineering."
+
+Changed "switch-hook" to "switch hook" on page 88: "the subscriber's
+switch hook."
+
+Page 107 says there is room for 300 banks of 100 multiple jacks, but
+then says this allows for 3,000 multiple jacks in all, rather than
+30,000. Based on the figure, 300 banks should be 30 banks, which would
+correct the arithmetic. However, I did not change this.
+
+Changed "bi-paths" to "by-paths" on page 185: "circuits or by-paths."
+
+Changed "appararus" to "apparatus" on page 209: "The sectional
+apparatus."
+
+Changed "two number" to "two-number" on page 312: "the two-number calls
+are ticketed."
+
+On page 333, a paragraph begins with "It has been only within the past
+three few." Perhaps the author meant "It has been only within the past
+three years" or "It has been only within the past few years." But since
+I didn't know, I left is as is.
+
+Changed "them ain" to "the main" on page 333: "on the main line."
+
+Changed "weatherproof" to "weather-proof" on page 357: "iron
+weather-proof sets."
+
+Changed "interoffice" to "inter-office" three times on page 364, to
+match the spelling in the body of the document: "meant by inter-office
+trunking;" "inter-office connection system;" "of the inter-office
+connection."
+
+Changed "break-down" to "breakdown" on page 367: "provision against
+breakdown."
+
+Changed "way-station" to "waystation" twice on page 372: "with a
+waystation set;" and "a waystation on a block wire."
+
+Changed "way stations" to "waystations" on page 375, in the entry for
+Long-distance switching.
+
+Each page of the Index repeated this text: "Note.--For page numbers see
+foot of pages." They were removed.
+
+
+
+
+
+End of the Project Gutenberg EBook of Cyclopedia of Telephony and
+Telegraphy, Vol. 2, by Kempster Miller and George Patterson and Charles Thom and Robert Millikan and Samuel McMeen
+
+*** END OF THIS PROJECT GUTENBERG EBOOK TELEPHONY AND TELEGRAPHY, VOL 2 ***
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