. American telephone practice . justment between the circuit arrangements of these resist-ances with respect to the lamps and the voltage of the battery whenboth the condition of the lamps and the voltage of the battery aresubject to variation. The difficulty in this respect is enhanced bythe fact that after an exchange has been in operation some time newlamps have to be added to replace those burned out which givesa condition wherein lamps of various ages must be used under thesame conditions. Such considerations as this have not, however,prevented such systems from being successful in practi
. American telephone practice . justment between the circuit arrangements of these resist-ances with respect to the lamps and the voltage of the battery whenboth the condition of the lamps and the voltage of the battery aresubject to variation. The difficulty in this respect is enhanced bythe fact that after an exchange has been in operation some time newlamps have to be added to replace those burned out which givesa condition wherein lamps of various ages must be used under thesame conditions. Such considerations as this have not, however,prevented such systems from being successful in practice. The line and cord circuits of the North Electric Company, ofCleveland, Ohio, is shown in Fig. 277. The arrangement of theline relay in this circuit is not unlike that of the Sterling circuitjust described, this relay having a split winding, including between 340 AMERICAN TELEPHONE PRACTICE. them the common battery, the two windings and the battery in seriesbeing connected permanently across the metallic circuit of the FIG. 277.—NORTH ELECTRIC COMPANYS MULTIPLE SWITCH-BOARD CIRCUIT. The third contact on the jack is wired through the coil of a cut-offrelay to ground, the arrangement in this case beig similar to that THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 341 employed by the Western Electric circuit. The cut-off relay, how-ever, in the North circuit, instead of serving to separate the linerelay and signaling apparatus entirely from the line, simply servesto open the circuit of the line lamp, this circuit being controlledat two places, one by a normally open pair of contacts in the linerelay, and the other by a normally closed pair of contacts in thecut-off relay. The cord circuit has three-conductor cords and three-contactplugs, two batteries being used as in the Kellogg system. Currentis supplied from battery, A, to the two talking strands of the an-swering cord through the two windings, respectively, of the relay,R, current being similarly supplied to the talking
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