Conditions favouring the start of an arc discharge between cold activated electrodes at 50 cycles per second

Abstract

It has been shown by experimental investigation that the start of an alternating current (50 cycles per second) arc-discharge between two activated electrodes in a rare-gas filling takes place in two stages, the first being a glow discharge with high voltage and low current, the second a change-over to the arc-discharge proper with low voltage and high current. The change-over is favoured by electrodes comprising a pellet of special physical structure and chemical composition producing small amounts of active barium. This has been proved by experiments with discharges under conditions where no positive column is present. In long tubes where the main part of the voltage is taken up by a positive column, the start of the glow discharge is hampered by wall charges. These can be removed by auxiliary devices applied on the outside wall of the tube and preferably extending over its whole length. A design dispensing with such auxiliary devices has been described, by which the wall discharges are removed through almost continuous glow discharge between two auxiliary electrode wires extending through the greater part of the length, one or both being connected through small condensers to the main electrodes.