The influence of nearby earthed objects and of the polarity of the voltage on the direct-voltage breakdown of horizontal sphere-gaps

Experiments were made to determine, the influence of nearby earthed objects on the breakdown voltages of 6.25 and 12.5cm-diameter sphere-gaps. The effect was found to be a function of both gap length and sphere diameter. A method has been discussed for establishing clearances which are suitable for use when the sphere-gaps are employed for accurate voltage measurements. Recommended clearances have been quoted which are suitable for all sizes of spheres, and a distinction has been made between the clearance conditions required for experimental calibrations of the breakdown voltages, and those required when a calibrated gap is used for the measurement of voltage. The polarity of the direct voltage had only a small influence on the breakdown voltages of the gaps.