Final answer:
The correct option is C. Bimetallic strip. A mechanical humidistat uses a bimetallic strip as its sensing element, which reacts to temperature changes by bending or coiling to adjust humidity levels.
Step-by-step explanation:
The sensing element used in a mechanical humidistat is a bimetallic strip. A bimetallic strip consists of two different metals, such as brass and steel, that are bonded together. As the temperature changes, the metals expand or contract at different rates, which causes the strip to bend or coil. This mechanical movement can then be used to control the electrical contacts in the humidistat, making adjustments to the humidity level as needed. This is different from a thermometer, which may use a thermistor to measure temperature by changes in electrical resistance or a mercury-in-glass thermometer, where the liquid expands and contracts within a glass tube.
A bimetallic strip consists of two different metals that expand or contract at different rates when heated or cooled. When the humidity level changes, the bimetallic strip bends due to the expansion or contraction of the metals, causing a mechanical switch to open or close, depending on the direction of the bending. This switch is connected to a control circuit that regulates the humidifier or dehumidifier accordingly.
Therefore, the correct option is C. Bimetallic strip is used as the sensing element in a mechanical humidistat.