a bimetallic strip

Here mismatch is put to work. Bond a strip of one metal to a strip of another that expands more, and heating makes the pair curl toward the low-expansion side — because the top layer has to grow longer than the bottom and the only way to do that while staying glued is to bow. This simple curl is the heart of old dial thermostats, oven thermometers, and thermal switches.

The two metals — say brass (alpha about 19 x 10^-6 per K) bonded to Invar (alpha about 1.2 x 10^-6 per K) — have very different expansion coefficients. On heating by delta_T the brass wants to lengthen far more than the Invar, but forced to share a single length, the strip bends into an arc, with curvature proportional to (alpha_1 - alpha_2) x delta_T and inversely proportional to thickness. A thin, long strip of two well-mismatched metals gives a big, sensitive deflection per degree. Cool it and it straightens back; the motion is repeatable and reversible.

The curling strip mechanically opens or closes an electrical contact: classic dial thermostats, older iron and toaster controls, thermal circuit breakers (which trip when overheating current bends the strip enough), and cheap dial thermometers. It is a neat lesson that thermal-expansion mismatch — usually a villain that cracks joints — becomes a useful, robust actuator when you deliberately choose a big difference in alpha.