. American telephone practice . ircuit, Fig. 314, it will be seen thatthe trunk circuit terminates at the private branch exchange end ina plug and cord, and in fact resembles somewhat in its equip-ment the incoming end of the trunk circuits used in regular trunk-in-g between large exchanges. The equipment of this trunk circuitmay be best described in connection with the description of itsoperation. 420 AMERICAN TELEPHONE PRACTICE. Assuming that a private branch subscriber calls for a main ex-change subscriber, the operator at the private exchange will, afteranswering the call with an answering
. American telephone practice . ircuit, Fig. 314, it will be seen thatthe trunk circuit terminates at the private branch exchange end ina plug and cord, and in fact resembles somewhat in its equip-ment the incoming end of the trunk circuits used in regular trunk-in-g between large exchanges. The equipment of this trunk circuitmay be best described in connection with the description of itsoperation. 420 AMERICAN TELEPHONE PRACTICE. Assuming that a private branch subscriber calls for a main ex-change subscriber, the operator at the private exchange will, afteranswering the call with an answering plug of a regular cord cir-cuit, withdraw this plug and insert in its place the plug of a trunkline. The relay, A, is polarized and is kept normally in the positionshown by current flowing from the main exchange battery. As soonas the trunk plug is inserted into the jack of the private branchline, the line relay at the main exchange, which had previously re-mained de-energized, due to the high resistance of relay A, will oper-. FIG. 316.—PRIVATE-BRANCH EXCHANGE SWITCH-BOARD. ate, and thus cause the line lamp at the main exchange to be dis-played. The main exchange operator will answer in the ordinaryway, and after finding out the number of the line with which con-nection is to be established, establish this connection in the ordinarymanner. When the private branch subscriber hangs up, the answeringsupervisory signal at the main office will be lighted and the ringinglamp at the private branch exchange will also be lighted, due to thefalling back of the armature of the relay C. To this the privatebranch attendant pays no attention. When, however, the main PRIVATE BRANCH EXCHANGE SERVICE. 421 office operator, in response to the signals, withdraws the answeringplug from the private branch trunk line jack, the relay A, at theprivate branch end of the trunk, will be restored to its normal posi-tion and allow its armature to fall back, thus lighting the disconnectlamp, which is now
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