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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.

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