Mercury switch
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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, or other inertia.
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.
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[edit] Uses
[edit] Roll sensing
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.
[edit] Sleepy driver alarms
Tilt switches can be found in alarms that are triggered when a driver's head tips, alerting them if they are falling asleep at the wheel.
[edit] Fall alarms
Work performed in confined space (such as a welder inside a tank) has special labor safety requirements. Tilt switches are used to sound an alarm if a worker falls over.
If a person wearing a tilt switch falls over, an emergency phone number could be dialed. I've fallen and I can't get up! popularized this application.
[edit] Thermostats
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.
[edit] Vending
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. They were used in pinball machines, they were the origin of the "tilt" indicator.
[edit] Bombs
The INLA used a mercury tilt switch and also microswitches to trigger the car bomb which killed Airey Neave during the Troubles. [1][2] 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[1]. 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.
[edit] Toxicity
Since mercury is a poisonous heavy metal, devices containing mercury switches must be treated as hazardous waste for disposal. Because it is now RoHS restricted, it has been eliminated in most modern applications. A metal ball and contact wires can directly replace it. Low-precision thermostats simply use a bimetal strip and a switch contact; precision thermostats use a thermistor or silicon temperature sensor. Low-cost accelerometers replace the mercury tilt switch in precision applications.
[edit] See also
- Mercury arc valve, a rectifier device intended for high electrical voltages/currents
- Mercury battery, an electrochemical battery
- Mercury coulometer, an electro analytical chemistry device that determines the amount of matter transformed during a mercury reaction
- Mercury probe, an electrical probing device to sample for electrical characterization
- Mercury swivel commutator, an electrical circuit, current-reversing switch using the element mercury
- Mercury-wetted relay
