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

. By passing of

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movi ...

it is heated to a high temperature that starts the

exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity ...

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...

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 Nichrome (also known as NiCr, nickel-chromium or chromium-nickel) is a family of alloys of nickel, chromium, and often iron (and possibly other elements) commonly used as resistance wire, heating elements in devices like toasters, electrical kett ...

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 ...

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. It ha ...

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 ...

; 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 t ...

. Carbon bridge is a thin spot of

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...

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 la ...

used as the bridgewire. Some variants use a conductive pyrotechnic composition as the resistive material. In

amateur rocketry Amateur rocketry, sometimes known as experimental rocketry or amateur experimental rocketry, is a hobby in which participants experiment with fuels and make their own rocket motors, launching a wide variety of types and sizes of rockets. Amateur ro ...

, grossly overloaded low wattage metal film

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...

s and 0805

Surface-mount technology Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred ...

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

detonator 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 electr ...

s, electric matches, 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 electric matches. 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 ...

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 field An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field produced by (stationary or moving) electric charges. It is the field described by classical electrodynamics (a classical field theory) and is the classical ...

s. By passing an extremely high amount of electric current through the bridgewire, it gets rapidly vaporized, causing a small

explosion An explosion is a rapid expansion in volume associated with an extreme outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Supersonic explosions created by high explosives are known ...

. 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 bomb ...

Notes

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External links

Bridgewire heating
Pyrotechnic initiators Firelighting using electricity