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LW1DSE > TECH 06.12.20 14:03l 64 Lines 3386 Bytes #999 (0) @ WW
BID : 1112-LW1DSE
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Subj: Arcs
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From: LW1DSE@LU7DQP.#LAN.BA.ARG.SOAM
To : TECH@WW
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* Electric Arc *
*****************
An electric arc is an electrical breakdown of a gas which produces an
ongoing plasma discharge, resulting from a current flowing through normally
nonconductive media such as air. A synonym is arc discharge. The phenomenon
was first described by Vasily V. Petrov, a Russian scientist who discovered
it in 1802. An archaic term is voltaic arc as used in the phrase "voltaic arc
lamp".
The various shapes of electric arc are emergent properties of nonlinear
patterns of current and electric field. The arc occurs in the gas-filled space
between two conductive electrodes (often made of carbon) and it results in a
very high temperature, capable of melting or vaporizing most materials. An
electric arc is a continuous discharge, while a similar electric spark
discharge is momentary. An electric arc may occur either in direct-current
circuits or in alternating current circuits. In the latter case, the arc may
re-strike on each half cycle of the current. An electric arc differs from a
glow discharge in that the current density is quite high, and the voltage
drop within the arc is low; at the cathode the current density may be as high
as one million amps per square centimeter.
On a commercial basis, electric arcs are used for welding, plasma
cutting, for electrical discharge machining, as an arc lamp in movie theater
projectors and Followspots in stage lighting. Electric arc furnaces are used
to produce steel and other substances. Calcium carbide is made in this way as
it requires a large amount of energy to promote an endothermic reaction (at
temperatures of 2500 ψC).
Low-pressure electric arcs are used for lighting, e.g., fluorescent
tubes, mercury and sodium street lamps, and camera flash lamps.
Undesired or unintended electric arcing can have detrimental effects on
electric power transmission and distribution systems and electronic equipment.
An electric arc has a non-linear relationship between current and
voltage. Once the arc is established (either by progression from a glow
discharge or by momentarily touching the electrodes then separating them),
increased current results in a lower voltage between the arc terminals. This
negative impedance effect requires that some positive form of impedance to be
placed in the circuit, if it is desired to maintain a stable arc. This
property is the reason uncontrolled electrical arcs in apparatus become so
destructive, since once initiated an arc will draw more and more current from
a fixed-voltage supply until the apparatus is destroyed.
Extracted from Wikipedia.com.
Compiled and translated to ASCCI by LW1DSE Osvaldo
Almirante Brown
Buenos Aires
Argentina
02/11/2007
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