Relative cathode spot and cell areas and currents in a copper cathode vacuum arc

High‐speed photographs of a decreasing‐current vacuum arc are used to examine the luminous zone area of cathode spots. The luminous zone was observed because it provides an upper limit to the area of the spot and because it provides for relative comparisons to other published data. The photographs are used to obtain experimental values for relative total emitting area, spot area, and spot current as a function of time for decreasing current. These data were taken both for periods when a spot extinguished and when it did not. The data were obtained from a series of high‐speed photographs with exposure times as short as 2.5 μs. The electrode orientation was such that a direct line of sight was used.