• A simple form of temperature sensor that can be used to provide an on/off signal when a particular temperatureis reached is the bimetal element.• This consists of two strips of different metals, such as brass and iron, bonded together.• The two metals have different coefficients of expansion. Thus, when the temperature of the bimetal stripincreases, the strip curves in order that one of the metals can expand more than the other.• The higher expansion metal is on the outside of the curve. As the strip cools, the bending effect is reversed.• This movement of the strip can be used to make or break electrical contacts and hence, at some particulartemperature, give an on/off current in an electrical circuit.• The device is not very accurate but is commonly used in domestic central heating thermostats because it is avery simple, robust device.