Electric Arc

Gases consist of neutral molecules, and are, therefore, good insulators. Yet under certain conditions, a breakdown of the insulating property occurs, and current can pass through the gas. Several phenomena are associated with the electric discharge in gases; among them are spark, dark (Townsend) discharge, glow, corona, and arc.

In order to conduct electricity, two conditions are required. First, the normally neutral gas must create charges or accept them from external sources, or both. Second, an electric field should exist to produce the directional motion of the charges. A charged atom or molecule, or ion, can be positive or negative; electrons are negative charges. In electrical devices, an electric field is produced between two electrodes, called anode and cathode, made of conducting materials. The process of changing a neutral atom or molecule into an ion is called ionization. Ionized gas is called plasma. Conduction in gases is distinguished from conduction in solids and liquids in that the gases play an active role in the process. The gas not only permits free charges to pass though, but itself may produce charges. Cumulative ionization occurs when the original electron and its offspring gain enough energy, so each can produce another electron. When the process is repeated over and over, the resulting process is called an avalanche.

For any gas at a given pressure and temperature there is a certain voltage value, called breakdown potential, that will produce ionization. Application of a voltage above the critical value would initially cause the current to increase due to cumulative ionization, and the voltage is then decreased. If the pressure is not too low, conduction is concentrated into a narrow, illuminated, "spark" channel. By receiving energy from the current, the channel becomes hot and may produce shock-waves. Natural phenomena are the lightning and the associated thunder, that consist of high voltages and currents that cannot be artificially achieved.

An arc can be produced in high pressure following a spark. This occurs when steady conditions are achieved, and the voltage is low but sufficient to maintain the required current. In low pressures, the transient stage of the spark leads to the glow discharge, and an arc can later be formed when the current is further increased. In arcs, the thermionic effect is responsible for the production of free electrons that are emitted from the hot cathode. A strong electric field at the metallic surface lowers the barrier for electron emission, and provides a field emission. Because of the high temperature and the high current involved, however, some of the mechanisms of arcs cannot be easily studied.