thermostat, automatic device that regulates temperature in an enclosed area by controlling heating or refrigerating systems. It is commonly connected to one of these systems, turning it on or off in order to maintain a predetermined temperature. Its operating principle is based on the fact that one of its components expands or contracts significantly during a temperature change. This expansion or contraction actuates a control on a furnace, cooling system, or piece of machinery. The thermostat sometimes uses mercury, which expands when heated and rises in a glass tube until, at a predetermined point, it touches an electrical contact to complete a circuit and thereby actuate a control; conversely, during a lowering of temperature the mercury descends in the tube and breaks the circuit. The thermostat often uses a bimetallic strip, which is made of two thin metallic pieces of different composition that are bonded together. As the temperature of the strip changes, the two pieces change length at different rates, forcing the strip to bend. This bending causes the strip to make or break a circuit.