Prebreakdown electrical conductivity of nearly saturated caesium vapour

Comprehensive measurements of the electrical conductivity of mixtures of nearly saturated caesium vapour and xenon prior to the transition to the glow or arc are shown to fit an empirically derived analytical formula. As saturation is approached, the electrical conductivity exhibits a regular double-exponentially-growing periodic oscillation as a function of the degree of saturation. Experimental points cover the range from 12 periods to 3 periods from saturation. Extrapolation of the empirical curve to saturation predicts a change in electrical conductivity from a metallic level to an extremely low level between the last peak and the last trough of the oscillation. This behaviour is quite different from that predicted by Saha equilibrium analysis for example, and is thought to be a new effect. Possible approaches to a theory are discussed. If similar behaviour is confirmed in other metallic vapours, e.g. copper, mercury and sodium, it will shed considerable light on puzzling phenomena that have been observed in industrial arcs and discharge tubes for many years without satisfactory explanation.