Final answer:
In a mechanical humidistat, a hygroscopic material (such as a metal coil or synthetic strip) that expands and contracts with humidity is used as the sensing element. This mechanism is similar to a sensor component that relays information to a control system, much like a thermistor detects temperature changes in thermometers.
Step-by-step explanation:
The sensing element used in a mechanical humidistat is often a hygroscopic material, such as a metal coil or a synthetic strip that expands and contracts with changes in humidity. These materials are sensitive to moisture in the air, and their mechanical changes are used to actuate a switch or to provide a measurement. This process can be akin to a sensor component of a homeostatic mechanism, which senses changes in a variable (in this case, humidity) and communicates this information to a control system for adjustment.
A commonly recognized sensor, although not used for measuring humidity, is a thermistor, which is utilized in thermometers for its temperature-dependent resistance. In response to temperature changes, a thermistor's resistance varies, and this can be automatically measured to determine the temperature. This principle is similar to how mechanical humidistats function by detecting humidity changes mechanically.