The Variation of Sparking Potential with Intense Ultraviolet Illumination

The static sparking potential has been found to be lowered by illuminating the cathode of a spark gap with intense ultraviolet light, an auxiliary spark gap being used as the source of ultraviolet illumination. The photoelectric current released from the cathode of the illuminated gap was about ${10}^6$ times larger with this source than with a quartz mercury arc for illumination. The amount of lowering of the sparking potential was found to increase as the intensity of the spark illumination was increased and reductions up to about 10 percent were noted in air. The reduced sparking potentials in air were found to be a function of gas pressure times gap length and therefore obey Paschen's law. The approximate law for the amount of lowering of the sparking potential can be expressed by the condition that a certain number remains constant. This number is the number of positive ions generated by electron collision in the gas as the result of a given number of photoelectrons being liberated at the cathode by one flash of the illuminating spark. One may interpret this as meaning that the breakdown is a space charge phenomenon and that some fixed value of the field is necessary for breakdown to occur. Then if the distribution of the space charge does not change appreciably, the amount necessary to produce the given field will be constant.