A bimetallic strip consists of two metal strips riveted or welded together in the longitudinal direction. The two blades are made of two different metals having different coefficients of thermal expansion. A variation in temperature causes a deformation of the bimetal. The deformation is all the more important as the length of the bimetallic strip is great. This is what can be observed on bimetals coiled in a spiral :
Since these are metals, it is sufficient that one of the two is a good enough conductor to be able to exploit this property in an electrical circuit. The bimetallic strip then becomes a switch that reacts depending on the temperature. There are bimetallic strips in circuit breakers, fire alarms, blinking bulbs and thermostats.
See also the eduMedia animation "bimetallic strip".
Place the heating source close to the bimetallic strip.