File:The practical telephone handbook and guide to the telephonic exchange (1906) (14569522649).jpg

Identifier: practicaltelepho00pool (find matches)
Title: The practical telephone handbook and guide to the telephonic exchange
Year: 1906 (1900s)
Authors: Poole, Joseph
Subjects: Telephone
Publisher: New York, Macmillan Co.
Contributing Library: Northeastern University, Snell Library
Digitizing Sponsor: Boston Library Consortium Member Libraries

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withan insulating plug when its companion line-jack is in use. Onepair of jacks is fitted on one of the ordinary sections, and theother on one of the concentration sections. At the provincial exchange end the line is also connectedto a 7-point jack, the inner contact springs of such jack beingconnected through the two windings of a differentially-woundrelay (r8) to a 24-volt battery (20 v. and 4 v.). In place of acalling lamp a polarised indicating relay (P.I.R.) is used, theindicating needle (n) of which is, under normal conditions,kept deflected to the right by the 4-volt battery (4 v.).It will be noticed that at both ends of the trunk line, connec-tion is made to a 24-volt battery, in such a manner that eachbattery opposes the other. No current, therefore, passes untila connecting plug is inserted in a jack at one end, when thebattery at that end is cut off, and the battery at the oppositeend becomes effective if a circuit is formed. This is the &2 PRACTICAL TELEPHONE HANDBOOK
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TRUNK-LINE EXCHANGES 263 equipotential method of working, which was described inthe preceding chapter, as used on both-way junction hues. Method of Working.—When London wishes to call the pro-vincial exchange a plug is inserted in the jack. This cuts offthe permanent battery. A ringing key (s in Fig. 270) is thenpressed, which connects another 24-volt battery in the reversedirection, so as to assist the battery at the far-end of the line.This operates the relay k8, which is so biassed as not to operatewith a 24-volt battery. The r8 relay causes the operation ofthe e9 retaining relay by closing the circuit of a 20-volt batterythrough it, and the latter relay cuts off the 4-volt batteryfrom the indicator, P.I.E.. The needle of the latter is causedto deflect to the left by current from the 20-volt battery, thusattracting the attention of the operator. When the latterinserts an answering plug, the circuit through P.I.R. is broken,and the needle returns to its centre position, where it r

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