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A Single-Pole, Single-Throw (SPST) mercury switch

A mercury 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.

Mercury switch as depicted in MacGyver season three, espisode nine: Hell Week. Mercury blob in a shallow dish; should it tilt and the blob touch any two of the edge wires, the device would trigger.

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.

The IRA frequently used mercury tilt switches to detonate car bombs during the Troubles. Other terrorist groups also used the device, including the Red Hand Defenders who used a mercury switch to detonate the bomb that killed Rosemary Nelson.

Mercury switches are often used as an exotic trigger in media depictions of bombs, as in Blown Away and MacGyver.

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.

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