Thermal Expansion, the Bimetallic Strip

Heat makes the atoms in a solid vibrate more and sit slightly further apart, so it expands, by an amount ΔL = α·L₀·ΔT, where α is the coefficient of linear expansion. Bond two metals with different α and heat them: the higher-α metal grows more, so the strip must curl, bending toward the lower-α side. That controllable, repeatable movement is used everywhere, a thermostat opens a heater contact when the room is warm enough, and a thermal circuit breaker trips when overcurrent heats the strip. Cool it and it straightens back.

• Try this: Heat the strip and watch it curl and lift off the contact, the circuit opens and the lamp goes out.
• Try this: Compare metal pairs, a bigger difference in α (Δα) curls the strip more for the same temperature.
• Try this: Lower the trip point and the thermostat opens at a cooler temperature.
• Try this: Cool it back down and the strip straightens and the contact re-closes.