Called "The Round®" by industry leader Honeywell, this single pole - double throw low 24 volt heating & cooling thermostat is a beige round device showing current temperature and desired temperature from 40 - 90 degrees. It works by rotating the clear plastic bezel to which a mercury switch is attached. The mercury switch is simply a glass tube with electrodes at one end. When the tube is tilted so the mercury contacts the electrodes, the circuit is completed. When it tilts away, the circuit is broken. A bimetallic coil acts as a thermometer by combining two different metals into a strip. Since they expand and contract at different rates, the coil expands or contracts in response to temperature. Depending upon the relationship of the mercury switch to the coil, the device connected to the thermostat goes on or off.