A New Phenomenon in the High-Pressure Mercury Vapour Discharge

Abstract

THE mechanism of the positive column of the high-pressure mercury vapour discharge appears to be now well understood¹. It has been a basic assumption in all the theoretical work—and this has been borne out by general experience of the behaviour of the discharge—that the positive column is substantially uniform in all properties along the whole of the distance between the electrodes, at least to within less than a millimetre from the discharge terminations. It is true that there are some exceptions : the column often tends to contract near the electrodes, and in long vertical burning tubes this effect is more marked at the lower end. This contraction is not difficult to understand, and its accentuation at the lower electrode is due to the higher density there compared with that at the upper end of the tube. It is true also that very close to the cathode small intense bright spots may often be seen², but it has always seemed likely that this was associated with a purely surface effect on the electrode; and there has been no evidence to suggest that in the high-pressure condition any appreciable degree of axial non-uniformity could be produced.