Bimetal

Bimetal:
A bimetal (bi means 2 or double) consists of two metal strips firmly bonded together. These two strips have different coefficients of expansion. Upon heating, the strips bend relative to each other. The degree of bending depends on the temperature. The warmer the metal gets, the more it bends.
A construction can be made where the bimetal breaks the contact when it bends too much from heating. Therefore, a bimetal is often used as a temperature switch or safety switch.

Embedded bimetal strips in cars are used in pistons, automatic chokes, certain dashboard instruments, and thermal switches (e.g., the thermal switch mounted on the radiator, which activates the electric fan at higher coolant temperatures) and in the viscous coupling of the engine cooling fan.