Fig. 60.
Fig. 61.
At [Fig. 61] is illustrated a novel form, in which the expansion of air causes contact to be made. It consists in an air chamber hermetically closed by a corrugated metal plate I, similar to that used in the aneroid barometers. When the temperature rises to a certain point, the expansion of the air in the chamber brings the
metallic plate into contact with the screw, as shown below. This closes the circuit and rings the bell in the usual manner. In all these fire or thermometer alarms, the exact degree of heat at which the bell shall ring, can be pretty accurately adjusted by means of the contact screws.
§ 59. Closely allied to these forms of contacts are the devices whereby an ordinary clock or watch can be made to arouse the over-drowsy sleeper by the ringing of an electric bell, which in this case should be of the continuous type. All these depend in their action upon some arrangement whereby when the hour hand of the clock or watch arrives at a certain given point in its travel, it makes contact between the battery and bell. In general the contact piece is attached bodily to the clock, but in the very ingenious arrangement illustrated at [Fig. 62] (devised by Messrs. Binswanger) the contacts are attached to an outer case, and as the case of the watch itself forms one point of contact, any watch that will slip in the case, may be set to ring the bell.
