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 (Василий В. Петров), a Russian scientist who discovered it in 1802. An archaic term is voltaic arc as used in the phrase "voltaic arc lamp".

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Overview

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. ^[1]

Uses

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 °C).

Low-pressure electric arcs are used for lighting, e.g., fluorescent tubes, mercury and sodium street lamps, and camera flash lamps.

Electric arcs have been studied for electric propulsion of spacecraft.

Undesired arcing

Undesired or unintended electric arcing can have detrimental effects on electric power transmission and distribution systems and electronic equipment. Undesired arcing in electrical contactors can be suppressed by various devices, including:

• immersion in oil, inert gas or vacuum
• arc chutes
• magnetic blowouts

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.

See also

• Arc transmitter
• Arc welding
• Arc lamp
• Spark gap
• Vacuum arc

References

1. ^ A. H. Howatson, An Introduction to Gas Discharges, Pergamon Press, Oxford pgs. 80-95