An inexpensive temperature-sensing mechanism is the ”bimetallic strip:” a thin strip of two
metals, joined back-to-back, each metal having a different rate of thermal expansion. When
the strip heats or cools, differing rates of thermal expansion between the two metals causes
it to bend. The bending of the strip can then be used to actuate a switch contact mechanism.
Other temperature switches use a brass bulb filled with either a liquid or gas, with a tiny
tube connecting the bulb to a pressure-sensing switch. As the bulb is heated, the gas or liquid
expands, generating a pressure increase which then actuates the switch mechanism..