Pre-breakdown electrical conductivity of nearly saturated mercury vapour at medium pressures

Abstract

Comprehensive measurements of the electrical conductivity of nearly saturated mercury vapour at medium pressures prior to the transition to glow discharge are shown to fit an analytical formula. The electrical conductivity exhibits an exponentially growing curve with values of electrical conductivity many orders of magnitude higher than the theoretically observed values. The electrical conductivity depends pre dominantly on the discharge pressure near saturation, but changes over to dependence mainly on discharge temperature as the degree of saturation is decreased. This behaviour is radically different from that which could be predicted from any known analysis, for example; Saha's thermal equilibrium analysis, and is thought to be a new phenomenon. The literature on arc discharges in general and vapour arc cathode mechanisms in particular is extensively reviewed. Possible approaches to a model to account for the present measurements are discussed and proposals for future work, with the hindsight derived from the experience of the present investigations, are suggested. Similar behaviour has been observed in caesium vapour under similar conditions, and if the same effect is observed in other metallic vapours, such as copper and sodium, it will lead to an improved understanding of the phenomena taking place in industrial cold cathode discharges and arcs, which have been observed for many years and through many investigations without satisfactory explanation.