A mercury switch, sometimes called a tilt switch, consists of a sealed glass tube containing two electrodes at one end and a small quantity of mercury that is free to move along the tube.
The switch is connected to an electrical circuit that controls a device. such as a light or motor, and accurately positioned. Consequently, the mercury pools in the appropriate end of the tube, connects the electrodes and completes the electrical circuit only when the switch is tilted beyond a certain angle.
A chest freezer may use a mercury switch to activate its interior light when the door is raised, while a thermostat may use one connected to a bimetallic coil that bends according to temperature. Other devices that contain a mercury switch include washing machines, space heaters and clothes irons.