100. Automatic switches may be placed on windows and doors, in connection with alarm bells, to indicate when entrance into a building is being forced. There are two methods of installing these alarms: the open-circuit and the closed-circuit system. The open-circuit system is usually employed, in which the connections are entirely similar to those of an ordinary electric-bell circuit, the automatic circuit-closing device being substituted for the push button. A window spring employed in this system is shown in Fig. 80. This is let into the window frame, the cam c alone projecting; when the window is raised, the cam is pressed in, revolving about the pin p, and makes contact with the spring s, which is insulated from the plate by a washer at the lower end, and is normally prevented from touching the cam by an insulating wheel w. The wires from the bell and battery are connected to the plate and spring respectively.

101. In the closed-circuit system, automatic switches of various styles are employed, but the contacts are held together when the alarm is set, and a movement of the window or door breaks this contact. Two batteries are required for this system, one being a closed-circuit gravity battery, indicated at b in Fig. 81, connected in series with a resistance r of about 200 ohms, the magnet coils of the bell a, and the alarm switch s. The armature of the bell is thus held away from the back contact c. The open-circuit Leclanche battery b' is connected to this contact and to the far terminal of the magnet coils; the circuit is, therefore, normally open, but, as soon as the main circuit is opened, by moving the alarm switch, the spring on the armature of the bell presses it against the back contact, thereby closing the local circuit through the bell.

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Fig. 80.

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Fig. 81.