Final answer:
The bimetallic strip is the most common temperature sensing element in mechanical thermostats, utilizing the differential thermal expansion of two bonded metals to mechanically control a switch in response to temperature changes.
Step-by-step explanation:
The most common temperature sensing element used for mechanical thermostats is the bimetallic strip. A bimetallic strip consists of two different metals bonded together that expand at different rates when heated. This difference in thermal expansion causes the strip to bend or curve. In thermostats, this curving action can be used to control the mechanical movement of a switch, opening or closing an electrical circuit in response to temperature changes. The bimetallic strip acts as a simple and reliable mechanism for temperature regulation without needing to be converted into electrical signals. While thermistors are commonly used in various thermometers for automated measurements, in the context of mechanical thermostats, it's the bimetallic strip that's predominantly employed for its simplicity and durability.