IONIZATION IN CATHODE-FALL REGION OF MOVING ARCS

Measurements have been made of the sum of the cathode- and anode-fall voltages of arcs moving in a transverse magnetic field between copper and brass electrodes in air, argon and helium. The experiments have covered a range of arc current of 40-700 A and a velocity range up to 470 m s-1. The gas pressure was mainly about 1 atm but a check at 11 atm showed no major difference. Comparison between these voltages, the excitation and ionization potentials of the gases and recent data on cathode-fall voltages of static arcs show that even with arcs moving at speeds up to nearly 500 m s-1 on cold-cathode emitting materials, ionization of electrode vapour rather than gas appears to have occurred in the cathode-fall region. The ionized vapour is generally that of the cathode, but it is possible for anode material carried by vapour jets to be ionized in the cathode-fall region when the arc is moving sufficiently slowly.