A mercury switch (also known as a mercury tilt switch) is a switch whose purpose is to allow or interrupt the flow of electric current in an electrical circuit in a manner that is dependent on the switch's physical position or alignment relative to the direction of the "pull" of earth's gravity.
Mercury switches consist of one or more sets of electrical contacts in a sealed glass envelope which contains a bead of mercury. The envelope may also contain air, an inert gas, or a vacuum. Gravity is constantly pulling the drop of mercury to the lowest point in the envelope. When the switch is tilted in the appropriate direction, the mercury touches a set of contacts, thus completing the electrical circuit through those contacts. Tilting the switch the opposite direction causes the mercury to move away from that set of contacts, thus breaking that circuit. The switch may contain multiple sets of contacts, closing different sets at different angles allowing, for example, Single-Pole, Double-Throw (SPDT'') operation.
Tilt switches may be used for a rollover or tip over warning for construction equipment and lift vehicles operating in rugged off-highway terrain. There are several non-mercury types but few are implemented due to sensitivity to shock and vibration - causing false tripping. However devices resistant to this do exist.
Mercury switches were commonly used in bimetal thermostats
. The weight of the movable mercury drop provided some hysteresis
by moving the bimetal spring slightly beyond the point it would normally assume, thereby holding the thermostat off slightly longer before flipping to the on state and then holding the thermostat on slightly longer before flipping back to the off state. The mercury also provided a very positive on/off switching action and could withstand millions of cycles without degradation of the contacts.
Mercury switches are still used in mechanical systems that are controlled electrically where the physical orientation of actuators or rotors is a factor. They are also commonly used in vending machines
that have 'tilt alarms'. When the machine is rocked or tilted in an attempt to gain a product, the mercury switch activates, sounding an alarm.
used a mercury tilt switch and also microswitches
to trigger the car bomb
which killed Airey Neave
during the Troubles
. Other paramilitary
groups also used the device, including the Red Hand Defenders
who used a mercury switch to detonate
the bomb that killed Rosemary Nelson
Additionally, mercury tilt switches can be found in some bomb and landmine fuzes
, typically in the form of anti-handling devices
e.g. a notable variant of the VS-50 mine
Since mercury has toxic
properties, defective devices containing mercury switches must be treated as chemical waste
. Because of this, mercury switches have been eliminated in most applications; for example, modern thermostats tend to use thermistors
or other electronic devices to measure the temperature and a relay
provides the switching action formerly performed by the mercury switch.