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A mercury switch is a glass tube containing liquid mercury and two electrodes that connect when tilted past a certain angle, allowing electricity to flow. However, mercury is dangerous, so many manufacturers are replacing them. They are used in thermostats, car lights, and even motion-activated bombs. Broken switches are toxic waste and should only be handled by professionals.
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 at the opposite end of the tube, the electrodes remain disconnected and no current flows. Once the tube is moved past a certain angle, however, mercury builds up between the two electrodes and a connection is made. The result is electricity flowing through a complete circuit. Once the liquid metal has returned to its original position, the electric current is immediately cut off.
The use of a mercury switch has become controversial in recent years. Mercury is considered a very dangerous 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 severe nerve damage or even death. For this reason, many manufacturers have begun to replace mercury switches with other mechanical methods of interrupting the electric current. School science classes are also phasing out experiments that could expose students to mercury.
A common use of a mercury switch can be found in older room thermostats. A heat sensitive metal coil expands and contracts based on ambient temperature. A mercury switch is balanced near the end of this coil, connected to the central heating and cooling system. If the room temperature exceeds the preset range, the expanding coil will cause 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 blow cooler air. When the thermostat coil contracts, the liquid metal will eventually flow back to the opposite end of the switch and the connection will be severed.
Because a mercury switch is motion-based, some automakers use them to activate their hood and trunk lights. When the lid is lifted, the mercury switch tilts and an electrical connection is made between the car battery and the bulb. Once the lid is closed, the liquid mercury returns to its original position and the light goes out. The mercury safety controversy, however, has prompted many owners to swap out their aging mercury switches in favor of mechanical ones. Some refrigeration 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 agencies warn passers-by 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 it. If you find an older appliance or thermostat with a broken switch, contact your local hazardous materials unit for advice. Car owners may also want to learn more about safely removing old mercury switches.
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