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A mercury switch is an
electrical Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...
that opens and closes a circuit when a small amount of the liquid metal mercury 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 the ''mercury 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. The ''mercury 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 inside
hermetically sealed A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers but, as technology advanced, it applied to a larger ca ...
containers.


History

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),U.S. Patent 1598874.
"Electrical apparatus", filed January 19, 1922.
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 of
Honeywell Honeywell International Inc. is an American publicly traded, multinational conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four areas of business: aerospace, building automation, industrial automa ...
).U.S. Patent 2232626.
"Electric switch ", filed October 7, 1937.


Description

Mercury switches have one or more sets of
electrical contact An electrical contact is an Electronic component, electrical circuit component found in electrical switches, relays, Electrical connector, connectors and circuit breakers. Each contact is a piece of electrically conductive material, typically meta ...
s in a sealed
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
envelope that contains a small quantity of mercury. The envelope may also contain hydrogen at pressure, an
inert gas An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. Though inert gases have a variety of applications, they are generally used to prevent u ...
, 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. The switch may contain multiple sets of contacts, closing different sets at different angles, allowing, for example, single-pole, double-throw ('' SPDT'') operation.


Advantages

Mercury switches offer several advantages over other switch types: * The contacts are enclosed, so oxidation of the contact points is impossible. * In hazardous locations, interrupting the circuit does not emit a spark that could ignite flammable gases. * Contacts stay clean, and even if an internal arc occurs, the contact surfaces renew on every operation, so they don't wear out. * Even a small drop of mercury has low resistance, so switches can carry useful amounts of current in a small size. * Sensitivity of the drop to gravity provides a unique sensing function, and lends itself to simple, low-force mechanisms for manual or automatic operation. * The switches are quiet, as no contacts abruptly snap together. * The mass of the moving mercury drop provides an over center effect to avoid
chattering Chatter may refer to: * A person who chats (talks): ** OnlyFans chatter#Chatters and chatbots, OnlyFans chatter, a person employed to chat with clients by an OnlyFans model ** Project CHATTER (1947–53), a U.S. Navy truth serum project * The soun ...
as the switch tilts. * The envelope can include contacts for two or more circuits.


Disadvantages

Mercury switches have several disadvantages: * There is a tendency for the intermittently exposed electrode to become damaged by the intense heat and destructive force of the electrical arc that forms whenever the circuit opens or closes, particularly if the circuit is being opened under a large inductive load. Certain refractory materials have been used to encase this electrode to mitigate this effect. * Their relatively slow operating rate (due to the inertia of the mercury drop) makes them unsuitable for applications that require many operating cycles per second. * Glass envelopes and wire electrodes may be fragile and require flexible leads to prevent damage to the envelope. * The mercury drop forms a common electrode, so circuits are not isolated from each other in a multi-pole switch. * Their sensitivity to gravity may make them unsuitable in portable or mobile devices that can change orientation or vibrate. * Mercury compounds are highly toxic and accumulate in any
food chain A food chain is a linear network of links in a food web, often starting with an autotroph (such as grass or algae), also called a producer, and typically ending at an apex predator (such as grizzly bears or killer whales), detritivore (such as ...
, so safety codes exclude mercury in many new designs.


Uses


Roll sensing

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.


Automotive uses

Automobile manufacturers once used mercury switches for lighting controls (for example, trunk lid lights), ride control, and
anti-lock braking system An anti-lock braking system (ABS) is a Automotive safety, safety anti-Skid (automobile), skid Brake, braking system used on aircraft and on land motor vehicle, vehicles, such as cars, motorcycles, trucks, and buses. ABS operates by preventing t ...
s. 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.


Electrification of church bells

In the 1950s, Austrian bellfounder from
Vienna Vienna ( ; ; ) is the capital city, capital, List of largest cities in Austria, most populous city, and one of Federal states of Austria, nine federal states of Austria. It is Austria's primate city, with just over two million inhabitants. ...
, 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 in
Herford Herford (; ) is a town in North Rhine-Westphalia, Germany, located in the lowlands between the hill chains of the Wiehen Hills and the Teutoburg Forest. It is situated in the cultural region of Ostwestfalen-Lippe (OWL) and the Detmold (administrat ...
, 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 and ''mercury 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. 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 in
Croatia Croatia, officially the Republic of Croatia, is a country in Central Europe, Central and Southeast Europe, on the coast of the Adriatic Sea. It borders Slovenia to the northwest, Hungary to the northeast, Serbia to the east, Bosnia and Herze ...
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 , image_flag = Flag of Serbia.svg , national_motto = , image_coat = Coat of arms of Serbia.svg , national_anthem = () , image_map = , map_caption = Location of Serbia (gree ...
, 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 from
Bečej Bečej (, ; , ) is a town and municipality located in the South Bačka District of the autonomous province of Vojvodina, Serbia. The town has a population of 19,492, while the municipality has 30,681 inhabitants. History Bečej was mentioned f ...
still maintains his work, mercury switches, and motors with brakes by himself.


Fall alarms

Work performed in confined space (such as a welder inside a tank) raises special safety concerns. Tilt switches sound an alarm if a worker falls over.


Aircraft attitude indicators/artificial horizons

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.)


Thermostats

Mercury switches were once common in bimetal
thermostat A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint. Thermostats are used in any device or system tha ...
s. The weight of the movable mercury drop provided some
hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
by a degree of over-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.


Doorbells

Some old doorbells, for example, the Soviet ZM-1U4, use mercury switches as current interrupters.


Pressure switches

Some
pressure switch A pressure switch is a form of switch that operates an electrical contacts, electrical contact when a certain set fluid pressure has been reached on its input. The switch may be designed to make contact either on pressure rise or on pressure fall ...
es use a
Bourdon tube Bourdon derives from the French for bumblebee, and may refer to: * Bourdon (bell), the lowest bell in a set * Bourdon (organ pipe), a stopped organ pipe of a construction favored for low pitches * Bourdon (surname) * Drone (music): The lowest cou ...
and a mercury switch. The small force generated by the tube reliably operates the switch.


Vending and game machines

Mercury switches are still used in electro-mechanical systems where physical orientation of actuators or rotors is a factor. They are also commonly used in
vending machine A vending machine is an automated machine that dispenses items such as snacks, beverages, cigarettes, and lottery tickets to consumers after cash, a credit card, or other forms of payment are inserted into the machine or payment is otherwise m ...
s, and used to be common in
slot machine A slot machine, fruit machine (British English), poker machine or pokie (Australian English and New Zealand English) is a gambling machine that creates a game of chance for its customers. A slot machine's standard layout features a screen disp ...
s and pinball machines, functioning as a ''tilt 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.


Bombs

A tilt switch can trigger a bomb. Mercury tilt switches can be found in some bomb and landmine
fuze In military munitions, a fuze (sometimes fuse) is the part of the device that initiates its function. In some applications, such as torpedoes, a fuze may be identified by function as the exploder. The relative complexity of even the earliest fu ...
s, typically in the form of
anti-handling device An anti-handling device is an attachment to or an integral part of a landmine or other munition such as some fuze types found in general-purpose air-dropped bombs, cluster bombs and sea mines. It is designed to prevent tampering or disabling, ...
s, for example, a variant of the VS-50 mine.


Toxicity

Since mercury is a
toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
heavy metal, devices containing mercury switches must be treated as
hazardous waste Hazardous waste is waste that must be handled properly to avoid damaging human health or the environment. Waste can be hazardous because it is Toxicity, toxic, Chemical reaction, reacts violently with other chemicals, or is Corrosion, corrosive, ...
for disposal. Because it is now
RoHS The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Uni ...
restricted, most modern applications have eliminated it. A metal ball and contact wires can directly replace it, but may require additional circuitry to eliminate switch bounce. Low-precision thermostats use a bimetal strip and a switch contact. Precision thermostats use a
thermistor A thermistor is a semiconductor type of resistor in which the resistance is strongly dependent on temperature. The word ''thermistor'' is a portmanteau of ''thermal'' and ''resistor''. The varying resistance with temperature allows these devices ...
or silicon temperature sensor. Low-cost
accelerometer An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change (mathematics), rate of change of velocity) of the object relative to an observer who is in free fall (tha ...
s replace the mercury tilt switch in precision applications. In the United States, the Environmental Protection Agency (EPA) regulates the disposition and release of mercury. Individual states and localities may enact further regulations on the use or disposition of mercury.


See also

*
Mercury-arc valve A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high- current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-f ...
, a rectifier device intended for high electrical voltages/currents *
Mercury battery A mercury battery (also called mercuric oxide battery, mercury cell, button cell, or Ruben-Mallory) is a non-rechargeable electrochemical battery, a primary cell. Mercury batteries use a reaction between mercuric oxide and zinc electrodes in a ...
, an electrochemical battery * Mercury coulometer, an electro analytical chemistry device that determines the amount of matter transformed during a mercury reaction *
Mercury probe {{distinguish, Exploration of Mercury The mercury probe is an electrical probing device to make rapid, non-destructive contact to a sample for electrical characterization. Its primary application is semiconductor measurements where otherwise time- ...
, 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 A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switc ...
*
Mercury relay A mercury relay (mercury displacement relay, mercury contactor) is a relay that uses mercury as the switching element. They are used as high-current switches or contactors, where contact erosion from constant cycling would be a problem for conv ...


References


External links


How to Adapt a Mercury Switch for Robot Control
{{DEFAULTSORT:Mercury Switch Heating, ventilation, and air conditioning Switches Mercury (element) fr:Interrupteur#Mercure