Fig. 50.
§ 56. The next form of contact to which our attention must be directed, is that known as the burglar alarm, with its variant of door-contacts, sash-contacts, till-contacts, etc.
The "burglar's pest" (as the contrivance we illustrate is called) is one of the most useful applications of
electricity for the protection of property against thieves. It consists usually, first, of a brass plate ([Fig. 50]), upon which a platinum contact piece is fixed, and second, of a spring made of hardened brass or steel insulated from the plate; or of a cylindrical box with a spiral spring inside (see [Fig. 51]). It is so arranged that as long as the stud is kept pressed in, the platinum points of contact are kept apart; this is the position when fixed in the rebate of a closed door or window; but as soon as opened, the stud passes outward through the hole, and the points of contact come together and complete the circuit of the wires in connection with the bell. The bell is best to be a continuous ringing one. It may be
fixed in the master's bedroom, or outside the premises in the street.
Fig. 51.