Arc Cathodes of Low Current Density at High Amperage

Abstract

From photographs, and from the marking on the electrodes, Slepian and Haverstick found that short duration arcs of 25 amperes in gases at a pressure of a few centimeters had apparent current densities at the cathode of less than 100 amperes per cm2. This is in sharp contradiction to the density of several thousand amperes per cm2 which seems to be required by generally accepted theories of the cold cathode arc. Similar results have been found up to currents of 5000 amperes, burning for 1/120 sec. with electrodes of various metals in air at less than 10-cm pressure. Apparent cathode current densities calculated from the observed marking on the electrodes were less than 1000 amperes per cm2. The markings on the electrodes were apparently produced by oxidation only, the energy density developed at the electrodes being insufficient to bring the electrode surface to the melting point in the 1/120 second duration of the arc. The circuit interrupting capacity of these short low current density arcs are found to be not less than that of short arcs at atmospheric pressure.