Amercury switch is anelectricalswitch that opens and closes acircuit when a small amount of the liquid metalmercury connects metal electrodes to close the circuit. There are several different basic designs (tilt, displacement, radial, etc.) but they all share the common design strength of non-eroding switch contacts.
The most common is themercury tilt switch. It is in one state (open or closed) when tilted one direction with respect to horizontal, and the other state when tilted the other direction. This is what older style thermostats used to turn a heater or air conditioner on or off.
Themercury displacement switch uses a 'plunger' that dips into a pool of mercury, raising the level in the container to contact at least one electrode. This design is used in relays in industrial applications that need to switch high current loads frequently. These relays use electromagnetic coils to pull steel sleeves insidehermetically sealed containers.
From around 1905 to 1910 various mercury switches were invented, but the "mercury in glass envelope" switch got its start with patent 1598874 (filed on January 19, 1922 by Louis Phelan),[1] which evolved into a more modern mercury switch with a straight tubular glass envelope via patent 2232626 (filed on October 7, 1937 by Harold Olson ofHoneywell).[2]
Mercury switches have one or more sets ofelectrical contacts in a sealedglass envelope that contains a small quantity of mercury. The envelope may also contain hydrogen at pressure, aninert gas, or a vacuum. Gravity constantly pulls 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 an electrical circuit. Tilting the switch in the opposite direction moves the mercury away from that set of contacts, breaking that circuit.[3] The switch may contain multiple sets of contacts, closing different sets at different angles, allowing, for example, single-pole, double-throw (SPDT) operation.
Mercury switches offer several advantages over other switch types:
Mercury switches have several disadvantages:
Tilt switches provide a rollover or tip over warning for applications like construction equipment and lift vehicles that operate in rugged 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 shock and vibration do exist.
Automobile manufacturers once used mercury switches for lighting controls (for example, trunk lid lights), ride control, andanti-lock braking systems. Scrapped automobiles can leak mercury to the environment if these switches are not properly removed. Since 2003, new American-built cars no longer use mercury switches.[6][7]
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In the 1950s, Austrian bellfounder fromVienna, Josef Pfundner jr., decided to develop a new method of electrifying church bells. Austria's existing electric church bells were already motorized using motors from a German factory inHerford, combined with brakes and gearbox in order to change the direction of the motor, while maintaining an optimal swing angle and changing the swing direction of the bell. This method was expensive, thus, Pfundner decided to use a motor with a brake andmercury switch instead. This was cheaper, albeit significantly more sensitive. This type of "gearbox" on the yoke contained four mercury switches, specifically, two switches for each direction, which were wired to the motor in such a way as to change swinging direction while the bell was ringing. The bell's swing angle could be adjusted by carefully moving the switches, though, in an instance where a switch may be incorrectly installed on the yoke, or broken, the bell risked being fatally cracked/damaged.[citation needed]
Pfundner closed the bellfoundry in 1971. The bellfoundry Grassmayr from Innsbruck in Tyrol had bought a licence for using Pfundner's patented method, and they continued to use it for their own bell electrification until the early 2000s. Some church bell electrifiers inCroatia also adopted this same system (e.g. Alojz Domislovic his successor Luka Ivandija, and later Ivan Bosilj. Ivan Bosilj stopped with usage of mercury switches in 2000). In Vojvodina,Serbia, the same system was used by bell electrifier Mihaly Rozsa, but after his retirement, his systems weren't maintained until end of 2019. Now, one electrician fromBečej still maintains his work, mercury switches, and motors with brakes by himself.[citation needed]
Work performed inconfined space (such as a welder inside a tank) raises special safety concerns. Tilt switches sound an alarm if a worker falls over.
Electrically driven attitude indicators typically use mercury switches to keep the gyro axis vertical. When the gyro is off vertical, mercury switches trigger torque motors that move the gyro position back to the correct position. (Air driven attitude indicators use a different operating principle.)
Mercury switches were once common inbimetalthermostats. The weight of the movable mercury drop provided somehysteresis by a degree ofover-center action. The bimetal spring had to move further to overcome the weight of the mercury, tending to hold it in the open or closed position. The mercury also provided positive on-off switching, and could withstand millions of cycles without contact degradation.
Some old doorbells, for example, the Soviet ZM-1U4, use mercury switches as current interrupters.
Somepressure switches use aBourdon tube and a mercury switch. The small force generated by the tube reliably operates the switch.
Mercury switches are still used in electro-mechanical systems where physical orientation of actuators or rotors is a factor. They are also commonly used invending machines, and used to be common inslot machines andpinball machines, functioning as atilt alarms that detect when someone tries to rock or tilt the machine to make it vend a product or affect a play outcome. They are less common in modern pinball machines due to concerns over mercury toxicity and pinball machines' appeal to children.[8]
A tilt switch can trigger a bomb.[9][10] Mercury tilt switches can be found in some bomb andlandminefuzes, typically in the form ofanti-handling devices, for example, a variant of theVS-50 mine.
Since mercury is atoxicheavy metal, devices containing mercury switches must be treated ashazardous waste for disposal. Because it is nowRoHS restricted, most modern applications have eliminated it. A metal ball and contact wires can directly replace it, but may require additional circuitry to eliminateswitch bounce. Low-precision thermostats use a bimetal strip and a switch contact. Precision thermostats use athermistor or silicon temperature sensor. Low-costaccelerometers replace the mercury tilt switch in precision applications.
In the United States, theEnvironmental Protection Agency (EPA) regulates the disposition and release of mercury.[11] Individual states and localities may enact further regulations on the use or disposition of mercury.
