Fig. 38
Thermostats act on the principle that heat causes expansion whether of substances, liquids, or gases.
The degree in which different substances expand varies for the same increase in temperature. This fact is used in a common form of thermostat shown in [Fig. 38]. A strip of wood or hard rubber R has a strip of thin sheet metal S riveted to it. This compound strip is held at one end by a lug L screwed fast to a baseboard. Upon an increase of temperature the hard rubber expands more than the metal strip and the compound strip bends towards the adjustable contact screw A. Upon touching the latter, the circuit through the bell B, battery C and the metal strip S is completed, and the bell rings. A contact screw can be arranged at the other side of S R, which will give warning of a decrease in temperature, as the rubber contracts more than the metal strip.
Fig. 39
In some thermostats of this character two metals having different coefficients of expansion, such as steel and brass, are used instead of metal and hard rubber.
Thermostats of this nature are much used in incubators, and they can readily be combined with electric apparatus to open or close hot-air valves, dampers, etc., and thus regulate the supply of hot air, hot water, or gas.
A thermostat much used in fire alarm work has a thin metal chamber which is air tight. An increase of temperature causes the air to expand, which swells out the walls of the chamber and closes an electric circuit.
