A bimetallic strip thermostat, when included in, say, an electrical air conditioning unit, makes and breaks the circuit in response to variations in the room temperature. Two dissimilar pieces of metal, whose coefficients of thermal expansion differ appreciably, are welded together to form the strip. When the temperature rises, the unequal expansions of the two metals cause the strip to bend; this completes the circuit and switches on the cooling system. As the temperature falls, the metals contract unequally and the strip straightens and breaks the circuit.

A) At the required temperature, the circuit is broken.
B) When the temperature rises, the strip bends and completes the circuit.
C) Complete thermostat with adjusting screw. By moving the fixed contact, the required temperature is adjusted.
1) bimetallic strip
2) fixed contact
3) insulator
4) temperature-adjusting screw.

A thermostat is a device for maintaining a material or enclosure at a constant temperature by automatically regulating its heat supply. This is cut off if the temperature rises and reconnected if it falls below that required. A thermostat comprises a sensor whose dimensions of physical properties change with temperature and a relay device which controls a switch or valve accordingly. Bimetallic strips are widely used in thermostats; they consist of two metals with widely different linear thermal coefficients fused together. As the temperature rises, the strip bends away from the side with the larger coefficient. This motion may be sufficient to control a heater directly.