The electric spark in air

The inadequacy of the classical theory of spark discharge when applied to long gaps at atmospheric pressure is discussed. A brief description is then given of the streamer theory of the spark. This latter theory not only explains the observed sparking phenomena but also incorporates a criterion, based on atom-physical considerations, which facilitates the calculation of the breakdown potentials of different gaps in which the field distribution is known. For those gaps in which corona is observed to precede sparkover, the corona onset voltage is that determined. The results of such calculations for various types of gaps are described, with particular reference to the sphere-sphere gap. Various features of point-plane breakdown are also mentioned, and an explanation is given for the occurrence of the upward-growing positive streamer from the earthed plane when the high-voltage point is of negative polarity. It is further shown that in the case of lightning discharge from a negative cloud to earth in open country, upward-growing positive streamers from the earth are not likely to be more than a few metres long and thus will rarely be observed. The subject of irradiation and statistical time-lag is then dealt with in some detail. Various experiments are described and it is shown that quite different results can be obtained according to whether the gap is illuminated or not by the light from neighbouring spark-gaps. The effect of such irradiation is found to be important not only for short gaps but also for gaps several centimetres long, and in particular for the positive impulse breakdown of a sphere-gap when the spacing is greater than that corresponding to the Toepler discontinuity. In this regard certain criticisms are given of the published standard tables for voltage measurement by sphere-gaps. The lowering of sparkover voltage for uniform and nonuniform gaps by the intense light radiated from a nearby spark source was also investigated. An explanation of the vario- us results obtained is given in terms of the new theory of the spark.