What principle do metal thermo-controls usually operate on?

Metal thermo-controls primarily operate on the principle of unequal expansion of different metals when heated. This principle utilizes a bimetallic strip made of two different metals that expand at different rates when temperature increases. As the strip heats up, the metal that expands more causes the strip to bend or curve, which can activate a switch or adjust a control mechanism.

This ability to convert thermal energy into mechanical movement makes bimetallic elements essential in thermostats and other temperature-sensitive controls. When a temperature change occurs, the differential expansion of the metals responds predictably, thereby enabling the regulation of the system's temperature.

Other principles, such as capacitance change or resistance change, while relevant to other types of sensors, do not effectively explain the mechanism of operation for metal thermo-controls. Additionally, electromagnetic induction pertains to the generation of electric current in a conductor exposed to a changing magnetic field and is not applicable in the functioning of thermo-controls based on material expansion.