Mercury switch

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A Single-Pole, Single-Throw (SPST) mercury switch on millimetre graph paper Mercury Switch without housing.jpg
A Single-Pole, Single-Throw (SPST) mercury switch on millimetre graph paper
Another mercury switch design Mercury switch.jpg
Another mercury switch design

A mercury switch is an electrical switch 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.

In electrical engineering, a switch is an electrical component that can "make" or "break" an electrical circuit, interrupting the current or diverting it from one conductor to another. The mechanism of a switch removes or restores the conducting path in a circuit when it is operated. It may be operated manually, for example, a light switch or a keyboard button, may be operated by a moving object such as a door, or may be operated by some sensing element for pressure, temperature or flow. A switch will have one or more sets of contacts, which may operate simultaneously, sequentially, or alternately. Switches in high-powered circuits must operate rapidly to prevent destructive arcing, and may include special features to assist in rapidly interrupting a heavy current. Multiple forms of actuators are used for operation by hand or to sense position, level, temperature or flow. Special types are used, for example, for control of machinery, to reverse electric motors, or to sense liquid level. Many specialized forms exist. A common use is control of lighting, where multiple switches may be wired into one circuit to allow convenient control of light fixtures.

Mercury (element) Chemical element with atomic number 80

Mercury is a chemical element with the symbol Hg and atomic number 80. It is commonly known as quicksilver and was formerly named hydrargyrum. A heavy, silvery d-block element, mercury is the only metallic element that is liquid at standard conditions for temperature and pressure; the only other element that is liquid under these conditions is the halogen bromine, though metals such as caesium, gallium, and rubidium melt just above room temperature.

Contents

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 containers.

Description

Mercury switches have one or more sets of electrical contacts in a sealed glass envelope that contains a small quantity of mercury. The envelope may also contain air, an inert 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. [1] The switch may contain multiple sets of contacts, closing different sets at different angles, allowing, for example, single-pole, double-throw ( SPDT ) operation.

Glass Amorphous solid that exhibits a glass transition

Glass is a non-crystalline, amorphous solid that is most often transparent and has widespread practical, technological, and decorative uses in, for example, window panes, tableware, optics and optoelectronics. The most familiar, and historically the oldest, types of manufactured glass are "silicate glasses" based on the chemical compound silica (silicon dioxide, or quartz), the primary constituent of sand. The term glass, in popular usage, is often used to refer only to this type of material, which is familiar from use as window glass and in glass bottles. Of the many silica-based glasses that exist, ordinary glazing and container glass is formed from a specific type called soda-lime glass, composed of approximately 75% silicon dioxide (SiO2), sodium oxide (Na2O) from sodium carbonate (Na2CO3), calcium oxide (CaO), also called lime, and several minor additives.

An inert gas is a gas that does not undergo chemical reactions under a set of given conditions. The noble gases often do not react with many substances and were historically referred to as the inert gases. Inert gases are used generally to avoid unwanted chemical reactions degrading a sample. These undesirable chemical reactions are often oxidation and hydrolysis reactions with the oxygen and moisture in air. The term inert gas is context-dependent because several of the noble gases can be made to react under certain conditions.

Advantages

Mercury switches offer several advantages over other switch types:

Electrical equipment in hazardous areas Electrical equipment in places where fire or explosion hazards may exist

In electrical engineering, hazardous locations are defined as places where fire or explosion hazards may exist due to flammable gases, flammable liquid–produced vapors, combustible liquid–produced vapors, combustible dusts, or ignitable fibers/flyings present in the air in quantities sufficient to produce explosive or ignitable mixtures. Electrical equipment that must be installed in such classified locations should be specially designed and tested to ensure it does not initiate an explosion, due to arcing contacts or high surface temperature of equipment.

Bistability

In a dynamical system, bistability means the system has two stable equilibrium states. Something that is bistable can be resting in either of two states. These rest states need not be symmetric with respect to stored energy. An example of a mechanical device which is bistable is a light switch. The switch lever is designed to rest in the "on" or "off" position, but not between the two. Bistable behavior can occur in mechanical linkages, electronic circuits, nonlinear optical systems, chemical reactions, and physiological and biological systems.

Disadvantages

Mercury switches have several disadvantages:

A food chain is a linear network of links in a food web starting from producer organisms and ending at apex predator species, detritivores, or decomposer species. A food chain also shows how the organisms are related with each other by the food they eat. Each level of a food chain represents a different trophic level. A food chain differs from a food web, because the complex network of different animals' feeding relations are aggregated and the chain only follows a direct, linear pathway of one animal at a time. Natural interconnections between food chains make it a food web. A common metric used to the quantify food web trophic structure is food chain length. In its simplest form, the length of a chain is the number of links between a trophic consumer and the base of the web and the mean chain length of an entire web is the arithmetic average of the lengths of all chains in a food web.

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 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. [4] [5]

Anti-lock braking system safety anti-skid braking system

An anti-lock braking system (ABS) is a safety anti-skid braking system used on aircraft and on land vehicles, such as cars, motorcycles, trucks, and buses. ABS operates by preventing the wheels from locking up during braking, thereby maintaining tractive contact with the road surface.

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 thermostats. The weight of the movable mercury drop provided some hysteresis 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 switches use a Bourdon tube and a mercury switch. The small force generated by the tube reliably operates the switch.

Vending

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 machines for tilt alarms that detect when someone tries to rock or tilt the machine to make it vend a product.

Bombs

A tilt switch can trigger a bomb. [6] [7] Mercury tilt switches can be found in some bomb and landmine fuzes, typically in the form of anti-handling devices, for example, a variant of the VS-50 mine.

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, 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 or silicon temperature sensor. Low-cost accelerometers replace the mercury tilt switch in precision applications.

In the United States, the Environmental Protection Agency (EPA) regulates the disposition and release of mercury. [8] Individual states and localities may enact further regulations on the use or disposition of mercury.

See also

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Rectifier AC-DC conversion device; electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction

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Thermostat component which maintains a setpoint temperature

A thermostat is a component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.

Reed switch

The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by Walter B. Ellwood. In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal reeds contacts in a hermetically sealed glass envelope. The contacts are usually normally open, closing when a magnetic field is present, or they may be normally closed and open when a magnetic field is applied. The switch may be actuated by an electromagnetic coil, making a reed relay, or by bringing a permanent magnet near the switch. When the magnetic field is removed, the reeds in the reed switch return to their original position.

Gas-filled tube arrangement of electrodes in a gas within an insulating, temperature-resistant envelope

A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.

Bimetallic strip strip used to convert a temperature change into mechanical displacement

A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases steel and brass. The strips are joined together throughout their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.

Ignitron type of gas-filled tube used as a controlled rectifier

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Mercury-arc valve electrical equipment for converting high-voltage or -current alternating current into direct current

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-filled tube, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring. As a result, mercury-arc valves were much more rugged and long-lasting, and could carry much higher currents than most other types of gas discharge tube.

Xenon arc lamp Gas discharge lamp

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Contactor Electronic circuit element serving as a switch

A contactor is an electrically-controlled switch used for switching an electrical power circuit. A contactor is typically controlled by a circuit which has a much lower power level than the switched circuit, such as a 24-volt coil electromagnet controlling a 230-volt motor switch.

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Fan heater heater that works by using a fan to pass air over a heat source

A fan heater, also called a blow heater, is a heater that works by using a fan to pass air over a heat source. This heats up the air, which then leaves the heater, warming up the surrounding room. They can heat an enclosed space such as a room faster than a heater without fan, but, like any fan, create audible noise.

Pressure switch

A pressure switch is a form of switch that closes an 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. Pressure switches are widely used in industry to automatically supervise and control systems that use pressurized fluids.

Solid-state relay

A solid-state relay (SSR) is an electronic switching device that switches on or off when a small external voltage is applied across its control terminals. SSRs consist of a sensor which responds to an appropriate input, a solid-state electronic switching device which switches power to the load circuitry, and a coupling mechanism to enable the control signal to activate this switch without mechanical parts. The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.

An infinite switch, simmerstat, energy regulator or infinite controller is a type of switch that allows variable power output of a heating element of an electric stove. It is called "infinite" because its average output is infinitely variable rather than being limited to a few switched levels. It uses a bi-metallic strip conductive connection across terminals that disconnects with increased temperature. As current passes through the bimetal connection, it will heat and deform, breaking the connection and turning off the power. After a short time, the bimetal will cool and reconnect. Therefore, infinite switches vary the average power delivered to a device by oscillating quickly between on and off states. They may be used for situations that are not sensitive to such changes, such as the resistive heating elements in electric stoves and kilns.

Mercury relay

A mercury relay is a relay that uses mercury as the switching element.

References

  1. Fraden, Jacob (2004). Handbook of Modern Sensors - Physics, Designs and Applications (3rd Edition). Springer - Verlag. pp. 256–257.
  2. Martin J. Heathcote, The J & P transformer book: a practical technology of the power transformer Newnes, 2007 ISBN   0-7506-8164-0 p.285
  3. David W. Pessen, Industrial automation: circuit design and components,Wiley-IEEE, 1989 ISBN   0-471-60071-7, page 44
  4. United States, Mercury study report to Congress, DIANE Publishing, 1997 ISBN   1-4289-0372-0, page 2-11
  5. Organisation for Economic Co-operation and Development, Instrument mixes for environmental policy OECD Publishing, 2007, ISBN   92-64-01780-1, pg.145
  6. Vallely, Paul (22 February 2002). "The Airey Neave Files". The Independent. London. Retrieved 12 May 2010.
  7. "1979: Car bomb kills Airey Neave". BBC News. 30 March 1979. Retrieved 26 March 2010.
  8. "Mercury: Laws and regulations". United States Environmental Protection Agency. 2008-04-16. Retrieved 2008-05-30.