The bimetallic strip on an iron serves as a thermostatic switch to stop electricity from flowing to the heating element once the appliance reaches the desired temperature. The differences in the coefficients of expansion for the two metals that form the strip cause it to curve as it heats, eventually to the point it pulls away from the electrical contact and breaks the circuit.
Breaking the circuit causes the iron and the bimetallic strip to begin to cool. As the strip cools, it loses the curvature, eventually touching the contact again, allowing electricity to flow and the iron to heat. One finds bimetal strips in other appliances, such as heaters, air conditioners and ovens. In some appliances, the strip serves to prevent overheating and fire danger. Other applications include the automobile. Automobile fan clutches operate using the bimetal strip to engage the fan by allowing the expanding metal to create friction for engaging the fan. Mercury thermostats attach a vial of mercury to the bimetal spring. Heating and cooling of the strip cause the mercury to move back and forth to complete the circuit without the mechanical wear of contacts. However, mercury is toxic. Heating and cooling experts recommend replacing mercury containing thermostats with digital devices.