A shade spring ([Fig. 49]), is made for either open or closed circuits. In operation, the shade is pulled down and its string or ring hooked on to H. This draws H up a trifle against a spiral spring and its lower end makes contact with an insulated spring S closing the circuit. If the shade is disturbed, the spiral spring on the lower part of H is released and it causes a break of contact with S in the direction of the arrow.
Fig. 49
When made for open circuit, S is bent so that while under tension no contact is made, but release of tension causes the contact.
[Fig. 50] gives the wiring of two windows and a door on the closed-circuit system. It will be seen that the contact springs are all in series, opening a window or the door will thus break the circuit.
When setting the alarm at night by connecting up the batteries, relay and bell, should any one of these springs be open the relay armature will not hold, and the bell rings.
Fig. 50
In this figure the relay is replaced by an electromagnet holding up a drop shutter by magnetic attraction. Upon the circuit opening, this shutter falls, exposing a number painted on it. At the same time it hits a spring contact placed below it and closes the bell and local battery circuit.