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A plasma is a gas which has been heated to a condition where it is at least partially ionized and is, therefore, capable of conducting electric current. When an arc is established through the gaseous column separating two electrodes, some of the gas molecules in the column become ionized. The ionized zone, or plasma stream, consists of positively charged atoms of the arc gas, an essentially equal number of electrons, and neutral atoms or molecules. The electrical conductivity of a plasma stream varies with the degree of ionization but for argon plasma it is about 0.05% that of copper. More comprehensive discussions of plasma physics may be found in the literature. A plasma exists during any arc occurrence. In nature, the air that is ionized by a lightning bolt constitutes a plasma. The same ionization phenomenon occurs in welding arcs, carbon arc lights and arc furnaces. In recent years, however, the expression "plasma arc" has become associated with those processes employing a constricted arc. Arc constriction is brought about by forcing the arc to pass through a small nozzle or opening as it passes from the electrode to the workpiece.