In a Geissler discharge containing ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, CO, mixtures of hydrogen and the rare gases or mixtures of mercury vapor in the rare gases at pressures of 0.5 to 12 mm and electrode separation of 4 to 30 cm, the anode glow breaks up into a number of bright hemispherical spots, more or less symmetrically arranged when the polarity of a direct current discharge is reversed. Anode spots do not form in ${\mathrm{O}}_2$, A or Ne but faint spots form in He which contain a trace of ${\mathrm{H}}_2$. It is shown that one necessary condition for the formation of spots is the emission of gas from the surface of the anode. As many as 75 spots arranged in 5 concentric rings have been observed on an anode 34 mm in diameter. Conditions governing the number, size and duration of the spots have been determined. Spots formed on all the metals tried; Fe, Ni, Al, Cu, Hg and brass. The formation of anode spots has been used to prove that in a Geissler discharge between cold electrodes, the cathode absorbs gas and the anode emits gas at rates which are functions of the current density, gas pressure, kind of gas and electrode temperature. It is shown that the increase of the potential drop of a discharge with time, is due almost wholly to an increase in the gas content of the cathode, and that the potential drop immediately after starting a discharge is lowered by previously degassing the cathode by means of an induction furnace or by using this electrode as anode.

• Received 23 September 1929

DOI:https://doi.org/10.1103/PhysRev.35.72