. American telephone practice . /, thelocal circuit through the relay, L, will be broken between contactsa and b of the jack, thus restoring this relay to its normal position PRIVATE BRANCH EXCHANGE SERVICE. 411 and putting out the line lamp, as well as breaking the ground con-nection at the contact, d. This relay cannot, therefore, be againoperated except by ringing current from the central office. In establishing a connection in a reverse direction—that is, inresponse to the private branch subscriber making a call, the attend-ant at the private branch exchange may call central by pluggingint
. American telephone practice . /, thelocal circuit through the relay, L, will be broken between contactsa and b of the jack, thus restoring this relay to its normal position PRIVATE BRANCH EXCHANGE SERVICE. 411 and putting out the line lamp, as well as breaking the ground con-nection at the contact, d. This relay cannot, therefore, be againoperated except by ringing current from the central office. In establishing a connection in a reverse direction—that is, inresponse to the private branch subscriber making a call, the attend-ant at the private branch exchange may call central by plugginginto the jack and throwing her listening key, this act allowing theflow of current over the trunk line and operating the line relay be-longing to the trunk line. The call will then be answered by themain office operator in exactly the same manner as if it were thatof an ordinary line subscriber. It has been previously stated that the battery at the private branchoffice is frequently charged from the main office battery over one. IMr FIG. 310.—PRIVATE-BRANCH EXCHANGE TRUNK CIRCUIT OF CHICAGOTELEPHONE COMPANY. or more of the trunk lines when idle. This is accomplished in thetrunk line circuit shown in Fig. 310, it being obvious that when thetrunk line is not in use a flow of current will exist from the live sideof the battery, A, at the central office over the tip side of the trunk-line to the tip contact of the jack, /, at the private branch exchangeto the ungrounded pole of the battery, B, at the private branch ex-change, and thence to ground. In order that there may be suffi-cient flow of current over this circuit, the battery at the privatebranch exchange is usually made to consist of a fewer number ofcells than that af. the main office. Eight cells are frequently usedat the branch exchange, while 11 are commonly used at the mainoffice. The proper pole of the battery at the private branch exchangeis grounded, it being necessary to ground the same pole as is 412 AMERICAN TELE
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