Current Density Per Cathode Spot in Vacuum Arcs

Two methods of computing the current density in a cathode spot of a metal arc are compared. The first method computes the spot area in terms of a crater left on the metal. Detailed arguments are presented as to why this method is not correct. Evidence is presented supporting a second method, that of estimating the cathode spot from the luminous glow observed during the discharge. The current density is estimated to be less than 105 A/cm2 during the lifetime of the spot.