Thermostats and thermal switches are electro-mechanical on/off switches that are activated by temperature changes. They are used to control heating and cooling systems. The sensing element is usually a spiral bimetallic strip that coils and uncoils in response to temperature changes because of the 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. Thermostats and thermal switches may also control residential air conditioners or industrial heat pumps. To reduce temperature swings, a small electrical heater may be energized during the warming period, causing the switch to break prematurely in anticipation of room-heater override.