A bridgewire or bridge wire, also known as a hot bridge wire (HBW), is a relatively thin

resistance wire Resistance wire is wire intended for making electrical resistors (which are used to control the amount of current in a circuit). It is better if the alloy used has a high resistivity, since a shorter wire can then be used. In many situations, the ...

used to set off a

pyrotechnic composition A pyrotechnic composition is a substance or mixture of substances designed to produce an effect by heat, light, sound, gas/smoke or a combination of these, as a result of non-detonative self-sustaining exothermic chemical reactions. Pyrotechnic s ...

serving as

pyrotechnic initiator In pyrotechnics, a pyrotechnic initiator (also initiator or igniter) is a device containing a pyrotechnic composition used primarily to ignite other, more difficult-to-ignite materials, such as thermites, gas generators, and solid-fuel rockets. T ...

. By passing of electric current it is heated to a high temperature that starts the exothermic

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

of the attached composition. After successful firing, the bridgewire melts, resulting in an

open circuit Open circuit may refer to: *Open-circuit scuba, a type of SCUBA-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled air * Open-circuit test, a method used in electrical engineerin ...

. Usually a thin nichrome wire is used. Some applications also use

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

alloy; other bridgewire materials in use are

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

, etc. Care has to be taken when selecting the material as it is in direct contact with the pyrotechnic composition and should not undergo corrosion in such conditions. Another material, able to actively release chemical energy, is Pyrofuze,

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

wire clad with

palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...

; when being heated it undergoes strongly exothermic reaction as the molten metals form an alloy. A variant with the same function consists of laminated thin alternate layers of aluminium and

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

. Carbon bridge is a thin spot of colloidal

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...

used as the bridgewire. Some variants use a conductive pyrotechnic composition as the resistive material. In amateur rocketry, grossly overloaded low wattage metal film resistors and 0805 Surface-mount technology resistors are also used. A shallow notch cut into the center of the bridgewire promotes gross localized overheating instead of homogeneous heating of the entire bridgewire. This may improve the bridgewire performance in some applications. Bridgewires are used in diverse applications; to trigger detonators,

electric match An electric match is a device that uses an externally applied electric current to ignite a combustible compound. Electric matches use a bridgewire consisting of a heating element to ignite a pyrogen, which is a quantity of readily ignited pyrotec ...

es, squibs, electric

blasting cap A detonator, frequently a blasting cap, is a device used to trigger an explosive device. Detonators can be chemically, mechanically, or electrically initiated, the last two being the most common. The commercial use of explosives uses electri ...

pyrotechnic fastener A pyrotechnic fastener (also called an explosive bolt, or pyro, within context) is a fastener, usually a nut or bolt, that incorporates a pyrotechnic charge that can be initiated remotely. One or more explosive charges embedded within the bolt a ...

s, and more. Bridgewires dipped in a suitable pyrotechnic composition ( pyrogen) are known as

es. Pyrogens with content of

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

allow reaching very high combustion temperatures. Devices using bridgewires, whether for initiating an explosion ("electroexplosive") or for nonexplosive purposes, are called bridge wire actuated devices (BWAD). Bridgewires, especially connected to longer cables, may be susceptible to initiation by currents induced by external electromagnetic fields. By passing an extremely high amount of electric current through the bridgewire, it gets rapidly vaporized, causing a small explosion. This is exploited in

exploding-bridgewire detonator The exploding-bridgewire detonator (EBW, also known as exploding wire detonator) is a type of detonator used to initiate the detonation reaction in explosive materials, similar to a blasting cap because it is fired using an electric current. EBW ...

s (EBWs), used for very safe and highly precise initiation of explosives, e.g. in

nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...

Notes

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External links

Bridgewire heating
Pyrotechnic initiators Firelighting using electricity