The spring contact to be inserted in the door jamb or window frame is so constructed that while under pressure the contacts are kept apart and the circuit is open. But when the door or window is opened, the pressure is released and a spring forces the contacts together.
[Fig. 44] is an open-circuit window spring fitted in the window frame so that when the window is closed the spring lug S is pressed inwards, breaking contact with the base B.
If the window is raised, the lug flies to the position shown by the dotted lines, and making contact with B, completes the circuit through bell and battery. These springs are fitted in the side of the window frame in a vertical position and are entirely concealed when the window is shut.
Fig. 44
In the closed-circuit system the reverse happens. The pressure of the closed door or window keeps the contacts together and its opening enables them to spring apart.
Fig. 45
In [Fig. 45] is a diagram of a closed-circuit burglar alarm, C a cell of gravity battery, R a relay, F the fixed contact and M the movable contact of the spring, S a stud projecting through the base of the spring and pushed in by the closed door.
When the door is closed, S being pushed in, the circuit of C, R, F and M is closed. The magnets of the relay hold the armature arm A forward against a hard rubber contact. But when S is released, the relay circuit is opened, R loses its power and A flies back, making contact, and throwing in circuit bell B and battery L.