Influence of humidity on the breakdown voltage of sphere-gaps and uniform-field gaps

Studies were made of the influence of humidity on the breakdown voltage of gaps in air. The voltages were measured for gaps between uniform-field electrodes and between spheres. The sphere-gaps comprised 2, 6.25, 12.5 and 25cm-diameter spheres which were enclosed in an air-tight chamber in which the humidity was varied between 0 and 17 mm Hg of water-vapour pressure. An increase in breakdown voltage of up to 5.5% was observed for some gaps when the humidity was increased. The change in voltage for a given change of humidity varied with gap length and with the shape of the electrodes. It was largest for uniform-field electrodes and decreased with decreasing sizes of sphere. With sphere-gaps the effect increased with gap length, reached a maximum at a spacing which depended on the sphere diameter and then decreased to about half the maximum as the gap was increased still further. The results were obtained with sphere-gaps which were varied up to a maximum length approximately equal to the sphere diameters.