Mercury switch
A mercury switch is a switch whose contact is made by mercury.
Design and function
Hermetically, usually in a glass tube, closed under a protective gas such switches contain metallic current supply and a small amount (about 0.1 to 3 ml) of mercury. Electrically conductive and liquid at room temperature metal mercury includes, depending on its position the circuit.
Mercury tilt switches react to changes in position and were formerly used as a tilt sensor, in older time relay, in case Ironing controllers in automatic stairway lighting switch and pressure switches ( level measurement at a water pump control with boiler). More mercury relay, see also at relay.
The interior lighting of cabinets was switched with a mercury tilt switch in the lid.
Mercury switches and relays are extremely reliable and can switch large loads (usually up to 16 A at 230 V AC supply voltage).
The durability of the mercury switches, even when frequent switching of large loads due to the fact that the contact wear is omitted, in particular when the pair of contacts is made by means of suitable internals in the ceramic vessel on both sides of mercury. Both metallic current leads are constantly surrounded by mercury and do not themselves work as a switching contact.
Contact thermometers are in principle also mercury-switches, are not referred to as such. You can only switch low loads.
Use and problem
In the first affordable coffee machines for homes were up to the mid-1960s still in use bimetallic influenced the tendency of a mercury switch. Later "real" - and considerably cheaper - Bimetallic came off this technique. Mercury switches are still occasionally in anti-theft devices ( tilt sensor) and alarm systems for use, but also it replaced on semiconductor basis already by electronic sensors and switches. There was also turntables, where the drive motor was turned off by a moving by the tone arm mercury switches upon reaching the end position.
In some bombs, a mercury switch is in the electrical firing circuit to close the ignition circuit with change in position or acceleration and ultimately trigger the explosion of the bomb. In some landmines is a mercury switch part of the cancellation protection.
Furthermore, there are switches with mercury-wetted switch contacts. However, they are not usually referred to as a mercury switch.
In the 1980s, mercury switches were installed in the first motion-sensitive controllers for computers. Example, there was some joystick models that were sensitive tilt and got along because of the mercury switches with no micro switches or potentiometers. They are considered the principal precursor of motion and acceleration -sensitive controllers, as they come, for example, to Nintendo's Wii console to use.
Due to the toxicity of mercury and the associated problems with the disposal of old products mercury switches are used only rarely. To reduce the risk of breakage of the glass tube and hence the leakage of mercury, mercury switches have recently been often are additionally surrounded by a plastic capsule. A further disadvantage is the dependence of the position and vibration function. Mercury switches must be used due to the RoHS Regulation only in exceptional cases since March 2005. Mercury switches used as a tilt switch can often be replaced by mercury-free tilt switches in which a metal ball as a function of their position closes a pair of contacts. The only disadvantage of these systems over mercury switches, the occurring contact wear, which is why this tilt switches are possible, do not subjected to high switching voltages or currents.