A thermostat is a device for controlling heating and cooling systems. It consists of a circuit controlled by a temperature sensitive device and connected to the environmental system. The most common thermostat, such as the one seen in homes and offices, is based on a bimetallic strip. As its name suggests, a bimetallic strip consists of thin strips of two different metals bonded together. One metal of the strip expands significantly with changes in temperature, the other metal changes very little with temperature increase or decrease. When the temperature increases, for example, one side of the strip will expand more than the other side, causing the strip to bend to one side. When it bends far enough, it closes the circuit that directs the air conditioner to turn on. The thermostat adjustment knob varies the distance that the bimetallic strip must bend to close the circuit, allowing selection of temperature level. As the air in the room gets cooler, the metal that expanded with heat will now contract, causing the bimetallic strip to straighten out until it no longer completes the circuit that allows the air conditioner to operate. The air conditioner will turn off until the air becomes warm enough to cause the strip to deform and close the circuit once again.
A number of thermostats that are variations on this theme have been developed. Some are based on a brass bellows filled with a thermally sensitive vapor. When the vapor is heated, it expands, pushing out the bellows until it closes the circuit and triggers the heater/air conditioner. Another thermostat design is the bulb type, which includes a bulb and capillary, similar to a thermometer, and a diaphragm. When the bulb is heated, the material expands and travels down the capillary to the diaphragm. The diaphragm in turn moves, moving a lever or spring post and eventually controlling the heating and cooling system.
Electronic thermostats have become increasingly popular in the past few years, offering the dual benefits of low cost and minimal moving parts. The active element is a thermistor, a semiconductor device whose resistance to electrical current changes with temperature. Temperature change is signified by a change in measured voltage, which can be used to pass information to the control systems of the heating/cooling units.