Abstract
The factors are discussed which control the initial rate of increase of current in the arc-like discharge obtained when a condenser of several microfarads is discharged through a small tube containing rarefied gas. It is suggested that at higher pressures ionization probably proceeds thermally, but that at lower pressures ionization takes place by direct electron collision, the cathode spot having a negligible retarding effect except possibly at very low pressures. The current density of emission from the cathode spot is at all times enormous, appearing sometimes greater than 106 amperes per cm2 and very seldom less than 105 amperes per cm2 even for a discharge prolonged until it has the appearance of a normal arc. It is suggested, therefore, that the value (Druyvesteyn and Penning 1940) of the emission current density of about 104 amperes per cm2 usually accepted for normal vacuum arcs is in error (Froome 1946, 1947). The rapid movement of the cathode spot and rate of growth of current exclude the possibility of the cathode emission being derived thermionically, while the very high current densities observed favour field emission.
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