A thermostat is a component of a control system which senses the temperature of a system so that the system's temperature is maintained near a desired setpoint. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature. The name is derived from the Greek words thermos "hot" and statos "a standing".
A thermostat may be a control unit for a heating or cooling system or a component part of a heater or air conditioner. Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus. A Thermostat may switch on and off at temperatures either side of the setpoint the extent of the difference is known as hysteresis and prevents too frequent switching of the controlled equipment.
The first electric room thermostat was invented in 1883 by Warren S. Johnson. Early technologies included mercury thermometers with electrodes inserted directly through the glass, so that when a certain (fixed) temperature was reached the contacts would be closed by the mercury. These were accurate to within a degree of temperature.
Common sensor technologies in use today include:
- Bimetallic mechanical or electrical sensors
- Expanding wax pellets
- Electronic thermistors and semiconductor devices
- Electrical thermocouples
These may then control the heating or cooling apparatus using:
- Direct mechanical control
- Electrical signals
- Pneumatic signals
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