As shown in the diagram, adjusting the temperature setting lever varies the glass vessel’s initial orientation, and changes the temperatures at which the liquid mercury completes or breaks the circuit.
Work Step by Step
The bimetallic strip coils more tightly as the temperature is lowered. This process tilts the glass vessel and the liquid mercury joins the two wire ends, completing the circuit and turning on the furnace. As the temperature rises, the strip uncoils, tilting the vessel the other way, so the mercury slides away from the wires. The furnace shuts off.