By the use of a cathode-ray tube it is shown that the a.c. breakdown current in a glass vessel with external electrodes, containing air at reduced pressures, consists of a number of separate pulses, the number in a half-cycle increasing with the excess voltage. The current flows in pulses because once a breakdown occurs in the gas, charges collect on the glass walls and produce reverse fields which stop the discharge. These wall-charge fields also determine the phase when the pulses occur. Ionization by collision of the molecules of the gas and secondary emission of electrons from the inner surfaces of the glass walls are the fundamental processes in the discharge. The breakdown conditions are shown to be similar to a d.c. discharge between metal electrodes. A theory is given explaining the shape of the pulses and their peak currents for different voltages and pressures.
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