A bimetallic strip acts as a mechanical temperature switch. It fuses two different metals, like brass and iron, side-by-side. When heated, brass expands faster than iron. Because they are bonded, this mismatch forces the strip to bend toward the iron side. When it cools, it straightens out. This bending action can open or close electrical switch contacts or physically move mechanical levers.
Mechanics see these strips inside older dial dashboard temperature gauges, classic HVAC thermostats, circuit breakers, and carburetor choke coils. As the engine warms up, the warm air or electrical current heats the bimetallic spring coil, which slowly rotates to open the carburetor choke plate.