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What is a Mercury Switch?

A mercury switch can be used along with a car battery to activate hood and trunk lights.
A mercury switch contains a small amount of liquid mercury.
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  • Written By: Michael Pollick
  • Edited By: Lindsay D.
  • Last Modified Date: 12 December 2014
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A mercury switch consists of a sealed glass tube containing two unconnected electrodes and a small amount of liquid mercury. As long as the liquid metal remains on the opposite end of the tube, the electrodes remain disconnected and no current will flow. Once the tube is moved past a certain angle, however, the mercury will pool between the two electrodes and a connection is made. The result is electricity flowing through a completed circuit. Once the liquid metal has returned to its original position, the electrical current stops immediately.

The use of a mercury switch has become controversial in recent years. Mercury is considered to be a very hazardous substance, especially if allowed to vaporize. Mercury in its liquid form can be handled safely under controlled conditions, but accidental ingestion or skin contact can cause serious nerve damage or even death. For this reason, many manufacturers have begun to replace mercury switches with other mechanical methods of disrupting electrical current. School science classes are also phasing out experiments which may expose students to mercury.

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One common use of a mercury switch can be found in older room thermostats. A heat-sensitive metal coil expands and contracts according to the ambient room temperature. A mercury switch is balanced near the end of this coil, connected to the central heating and cooling system. If the room temperature rises beyond the preset range, the expanding coil will cause the liquid mercury to move to the other side of the switch. This causes two electrodes to form a complete circuit, which in turn causes the cooling system to send out colder air. As the coil in the thermostat contracts, the liquid metal will eventually return to the opposite end of the switch and the connection is broken.

Because a mercury switch is motion-based, some car manufacturers use them to activate hood and trunk lights. As the lid is raised, the mercury switch becomes tilted and an electrical connection is made between the car's battery and the light bulb. Once the lid is closed, the liquid mercury returns to its original location and the light is extinguished. Controversy over the safety of mercury, however, has prompted many owners to change out their old mercury switches in favor of mechanical ones. Some refrigerator units, such as chest freezers, may also use mercury switches in their lids.

Mercury switches are also popular as detonators in motion-activated bombs. Any attempt to shake or move the explosive device will cause the mercury to complete the circuit between a detonator and the explosive charge. This is why law enforcement officers caution by-standers not to touch or move any suspicious packages. An explosive device armed with a mercury switch must be defused in place or moved without changing its original orientation.

A broken mercury switch is considered toxic waste, so untrained and unprotected individuals should never attempt to remove them. If you find an older appliance or thermostat with a broken switch, contact the local Hazardous Materials unit for advice. Car owners may also want to learn more about removing older mercury switches safely.

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