Cathode drop in arcs and glow discharges

THE ARC is defined as an electrical discharge in a gas or a vapor in which the cathode drop is of the order of 10 or 20 volts and the current density to the cathode spot is of the order of hundreds or thousands of amperes per square centimeter. For an arc to exist it is necessary that there be some mechanism for producing electrons at or near the cathode. Unfortunately, there is no general agreement among physicists as to the mechanism which produces this low cathode drop with the correspondingly high current density which is characteristic of an electric arc. In this paper the author reviews briefly the different prevalent theories regarding the low cathode drop in an arc, offering discussions and interpretations based upon extensive research work carried out at the California Institute of Technology.