Arc, Electric

(also called voltaic arc), one kind of self-maintaining arc discharge in a gas, in which the discharge phenomena are concentrated in a narrow, brightly glowing column of plasma. If the electrodes are horizontal, the column becomes arc-shaped under the influence of the upward currents of hot gas heated by the discharge.

An electric arc between two carbon electrodes in air was first observed (1802) and described (1803) by the Russian scientist V. V. Petrov and the English scientist H. Davy (1808–09), who called it a voltaic arc. Papers by the Russian scientists N. N. Benardos (welding with carbon electrodes, 1882, and AC welding) and N. G. Slavianov (welding with metal electrodes, 1888–91) were devoted to the development of the theory of the electric arc and the study of problems of its use in industry.

An electric arc can exist in any gas at near-atmospheric and higher pressures. At atmospheric pressure and a current strength of several amperes (amps), the plasma temperature in the column is about 5000°K; at high pressures and current strengths it is up to 12,000°K; and if the column is blasted with a powerful gas flow, the temperature reaches 50,000°K. The temperature distribution for various parts of an electric arc between carbon electrodes with a current of 200 amps is shown in Figure 1.

The magnetic field formed by the current of the electric arc interacts with the current, causing compression (constriction) of the column. With an increase in pressure in the surrounding medium, the current strength in the electric arc increases and the cross section of its column decreases. Near

Figure 1. Temperature distribution in various parts of the column of an electric arc

the electrodes the arc column is constricted even more, forming bright cathode and anode spots on their surfaces. The current density at the arc cathode depends on the cathode material and kind of gas and is usually 10⁴-10⁵ amps/cm², but under specific conditions it can rise to 10⁷ amps/cm².

The current-voltage characteristic of an electric arc is a descending curve: a rise in current is accompanied by a drop in voltage between the electrodes.

Electric arcs are used in electrometallurgy (to make pure and refractory metals) and in lighting technology, and particularly extensively in electric welding. In some technical fields (for example, high-voltage technology), it is necessary to combat electric arc phenomena. Switches with various arc quenchers, including air, oil, and elegaz (sulfur hexafluoride) switches, and quenching by magnetic field are used to extinguish arcs that arise when high-voltage circuits are broken.