A thermostat is an electro-mechanical on/off switch that is activated by temperature changes. It is typically used to control a heating or cooling system. The sensing element is usually a spiral bimetallic strip that coils and uncoils in response to temperature changes because of differential expansion of the two bonded metals. The switch element is either a set of electrical contacts or a glass-encapsulated mercury switch that controls a low-voltage relay. The relay can actuate a motor starter and igniter for an oil burner, a heavy-duty switch for electrical units, or a solenoid-operated valve on a gas furnace. The thermostat may also control a house-type air conditioner or heat pump. To reduce temperature swings, a small electrical heater unit is energized during the warming period, causing the switch to break prematurely in anticipation of room-heater override.
