[Electric engineering.] . e by a washer at the lower end and is nor-mally prevented from touching the cam by aninsulating wheel w. The wires from the belland battery are connected to the plate andspring, respectively. The annunciator used ismuch the same as that employed for bellwork,but additional, convenient attachments are usu-ally placed on it, such as a device to keep thebell ringing until the annunciator is reset, aclock to connect and disconnect the system atcertain hours, etc. The annunciator is usuallyequipped with a small button over each drop,which when pushed will complete the circ
[Electric engineering.] . e by a washer at the lower end and is nor-mally prevented from touching the cam by aninsulating wheel w. The wires from the belland battery are connected to the plate andspring, respectively. The annunciator used ismuch the same as that employed for bellwork,but additional, convenient attachments are usu-ally placed on it, such as a device to keep thebell ringing until the annunciator is reset, aclock to connect and disconnect the system atcertain hours, etc. The annunciator is usuallyequipped with a small button over each drop,which when pushed will complete the circuitand cause the drop to fall if there happens tobe any door or window open. These are veryuseful for testing out to see if everything isclosed. All these appliances belong to the an-nunciator itself and do not affect the general plan of wiring,which is carried out in the same way as for bell wiring. 58. In the closed-circuit system, automatic switchesof various styles are used, but the contacts are held together Fig. Fig. 28. when the alarm is set, and a movement of the window ordoor breaks this contact. Two batteries are required for 42 INTERIOR WIRING. 28 this system, one being a closed-circuit gravity battery indi-cated at b in Fig. 28, connected in series with a resistance rof about 200 ohms, the magnet coils of the bell a, and thealarm switch s. The armature of the bell is thus held awayfrom the back contact c. The open-circuit Leclanche bat-tery b is connected to this contact and to the far terminalof the magnet coils; this circuit is therefore normally open,but as soon as the main circuit is opened by moving thealarm switch, the spring on the armature of the bell pressesit against the back contact, thereby closing the local circuitthrough the bell. 59. Fig. 29 shows a crowfoot or gi*avity cell, the typegenerally used when a small current has to be furnished con-tinually. A sheet-copper electrode is placed at the bottom of the jar along withcrystals of coppersulphat
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