Thermostat with mercury switch for controlling home heating system. The desired temperature is set using the dial at the lower left. The mercury switch (sealed glass tube with metallic liquid mercury) activates the heating system when tilted. The tilting of the switch is accomplished by the bimetallic coiled strip (above temperature dial). Bimetallic strips are composed of two different metals laminated together. These metals have different rates of expansion which caused the coil to wind or unwind with the changing ambient temperature. When the coil has expanded or contracted enough the mercury switch is activated causing the home heating system to either turn on or turn off.