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Measurements have been made of the current-voltage characteristics of abnormal glow discharges in mercury vapour and xenon, both with a mercury cathode, over a pressure range from 0·5 to 15 mm Hg. Measurements have also been made of the cathode fall distance as a function of the voltage, but at low voltages only.A theoretical description is presented and is in good agreement with experiment. The main features of the theory are: at high current densities, the gas density increases towards the cathode as a result of momentum-exchanging collisions between the positive ions and the gas molecules; the dominant secondary emission process is photoelectric emission by photons from the negative glow, except near the glow-arc transition where emission by excited atom bombardment makes a small but significant contribution; and only a small fraction of the photons produced in the negative glow reach the cathode, because of frequent reabsorption and subsequent ionization.

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