. American telephone practice . FIG. 253—COMMON-BATTERY SWITCH-BOARD FOR SMALL EXCHANGES. sufficiently to accommodate the different class of apparatus. A com-plete switch-board adaptable to exchanges, having not over onehundred lines, is shown in Fig. 253. CHAPTER XIX. COMMON BATTERY SUB-STATION EQUIPMENT. The essential features of the sub-station equipment for commonbattery work are the speech-receiving and transmitting apparatus,or receiver and transmitter; the call receiving apparatus, or ringer,the switch-hook for alternately bringing the talking apparatus andthe call receiving apparatus i
. American telephone practice . FIG. 253—COMMON-BATTERY SWITCH-BOARD FOR SMALL EXCHANGES. sufficiently to accommodate the different class of apparatus. A com-plete switch-board adaptable to exchanges, having not over onehundred lines, is shown in Fig. 253. CHAPTER XIX. COMMON BATTERY SUB-STATION EQUIPMENT. The essential features of the sub-station equipment for commonbattery work are the speech-receiving and transmitting apparatus,or receiver and transmitter; the call receiving apparatus, or ringer,the switch-hook for alternately bringing the talking apparatus andthe call receiving apparatus in proper relation with the line; and adevice, usually a condenser, for preventing direct currents from flow-ing over the metallic circuit of the line when the telephone is not inuse, but adapted to allow alternating currents to pass for the purposeof ringing the bell. It has been shown in the three preceding chapters that the condi-tions required of the sub-station in order to bring about the oper-. FIG. 254.—SIMPLE SUB-STATION CIRCUIT. ation of the various signals at the central office, are that the circuitof the line, when the telephone is not in use, shall be open to directcurrents, at the same time allowing alternating currents to pass forthe purpose of ringing the subscribers bell. When the telephone isnot in use the circuit between the two sides of the line must be com-plete with respect to both direct and alternating currents. The simplest form of circuit for the subscribers station adapted tomeet these requirements, is shown diagrammatically in Fig. 254. Inthis the talking apparatus consists of the transmitter and receiver,placed directly across the two limbs of the line when the hook israised. The call bell is permanently bridged across the line, no con-denser being used in its circuit, and, in order to prevent the opera-tion of the signals at the central office by the flow of direct currentthrough the call bell, the magnets of the latter are wound to a veryhigh re
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