Household thermostat, Photo 1 of 3. A household thermostat is used to regulate the inside temperature. Shown here is an older thermostat with its cover removed to show the internals. The lever at the bottom is used to set the temperature. It is connected to the bimetallic spiral (center) which serves as a temperature sensor. The mercury switch (top) is attached to the spiral and is essentially a sealed glass tube with two contacts and a drop of mercury. In this photo the switch is tilted so that mercury does not touch the contacts and the circuit is opened. The complete series includes photo numbers BR9710, BR9711 and BR9712.