[Illustration:  Fig. 274.  Night-Alarm Circuit]

Night-Alarm Circuits.  The circuit of Fig. 273, while referred to as a complete circuit, is not quite that.  The night-alarm circuit is not shown.  In order to clearly indicate how a single battery and bell, or buzzer, may serve in connecting a number of line drops, reference is made to Fig. 274 which shows the connection between three different line drops and the night-alarm circuit.  The night-alarm apparatus consists in the battery 1 and the buzzer, or bell, 2.  A switch 3 adapted to be manually operated is connected in the circuit with the battery and the buzzer so as to open this circuit when the night alarm is not needed, thus making it inoperative.  During the portions of the day when the operator is needed constantly at the board it is customary to leave this switch 3 open, but during the night period when she is not required constantly at the board this switch is closed so that an audible signal will be given whenever a drop falls.  The night-alarm contact 4 on each of the drops will be closed whenever a shutter falls, and as the two members of this contact, in the case of each drop, are connected respectively with the two sides of the night-alarm circuit, any one shutter falling will complete the necessary conditions for causing the buzzer to sound, assuming of course that the switch 3 is closed.

Night Alarm with Relay. A good deal of trouble has been caused in the past by uncertainty in the closure of the night-alarm circuit at the drop contact.  Some of the companies have employed the form of circuit shown in Fig. 275 to overcome this.  Instead of the night-alarm buzzer being placed directly in the circuit that is closed by the drop, a relay 5 and a high-voltage battery 6 are placed in this circuit.  The buzzer and the battery for operating it are placed in a local circuit controlled by this relay.  It will be seen by reference to Fig. 275 that when the shutter falls, it will, by closing the contact 4, complete the circuit from the battery 6 through the relay 5—­assuming switch 3 to be closed—­and thus cause the operation of the relay.  The relay, in turn, by pulling up its armature, will close the circuit of the buzzer 2 through the battery 7 and cause the buzzer to sound.

[Illustration:  Fig. 275.  Night-Alarm Circuit with Relay]

The advantage of this method over the direct method of operating the buzzer is that any imperfection in the night-alarm contact at the drop is much less likely to prevent the flow of current of the high-voltage battery 6 than of the low-voltage battery 1, shown in connection with Fig. 274.  This is because the higher voltage is much more likely to break down any very thin bit of insulation, such as might be caused by a minute particle of dust or oxide between contacts that are supposed to be closed by the falling of the shutter.  It has been common to employ for battery 6 a dry-cell battery giving about 20 or 24 volts, and for the operation of the buzzer itself, a similar battery of about two cells giving approximately 3 volts.