Page:Popular Science Monthly Volume 18.djvu/71

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jar. Four of these elements put up in a wooden case constitute the battery usually furnished.

These appliances provided, the most common way of using the system is to make it complete in each building, the alarm apparatus being placed in a sleeping-apartment in a private house, and in the

Fig. 3.
PSM V18 D071 Alarm system battery.jpg

watchman's room in a place of business. So arranged, the condition of the circuit is this: In the daytime, when the doors and windows are open, the circuit is continuous at all points except at the alarm apparatus. At night this is reversed, the circuit being closed at the instrument, and broken at all the points protected, A movement at any of these points which closes the circuit gives the alarm and turns the proper needle on the annunciator. The connection with the alarm is made at night by an attendant, and broken at any desired time in the morning. In private houses fitted with electric bells, a clock is often provided that disconnects the alarm in the morning and turns the current on to a bell placed in the servants' room. The movement by which this is done is something similar to that of the ordinary alarm-clock.

The protection afforded with such an apparatus in good working order is probably as perfect as it can be made. It is generally impossible to cut the wires from the outside of the building, and unless this is done intrusion will start the alarm. Even if the wires be cut, but-