Thermostats control ambient temperature by sensing the temperature in a room and responding to that by activating heating or cooling equipment or remaining dormant. Most modern home thermostats rely on a bimetallic strip to determine the current temperature at or near the thermostat's housing.
Bimetallic strips feature two metals that expand at different rates, allowing the device to gauge the ambient temperature in the area. The center of the coiled strip has an adjustment lever attached that activates heating or cooling systems based on the setting of the thermostat. Digital systems may use the reading from the strips to activate these systems directly, while older models often rely on physical contact between the strip and mechanical components to switch on or off various systems.
Wax thermostats sense temperature in cars and other heavy-duty systems, including hot-water heaters in the home. They sense the ambient temperature in an area using the physical changes to the wax as the area heats. Melted wax expands and moves components that shut off systems in case of overheating, allowing the ambient temperature to return to normal. Systems that require fast responses to potentially subtle changes in temperature often rely on thermostats that use metal discs and bellows in a setup similar to bimetallic coils.