Current Density of the Arc Cathode Spot

Abstract

New measurements of the area of the arc cathode spot from measurements of width of tracks left on metals by a cathode spot moving in a magnetic field indicate that the current density is greater than values given by previous workers. At 2.6 amperes, we find the following values in air at atmospheric pressure: Cu 124,000 amp./${\mathrm{cm}}^{2}$, Al 29,500 amp./${\mathrm{cm}}^{2}$, W 74,000 amp./${\mathrm{cm}}^{2}$, Hg on Cu 20,700 amp./${\mathrm{cm}}^{2}$.The Cu, Al, and W electrodes were first oxidized in a bunsen flame. For an arc burning in mercury vapor with a mercury cathode at a few microns pressure we find at 2.6 amperes a current density of 50,000 to 220,000 amperes per ${\mathrm{cm}}^{2}$. Values at currents above 5 amperes could not be obtained because of the formation of multiple spots.