"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."