Abstract :
Study of the mechanism of the electric spark has been mainly confined to the short spark in homogeneous fields, or to the long spark provided by Nature in the form of the lightning flash. Following some early work of the author´s on the characteristics of the impulse-voltage spark from a negatively or positively charged point electrode to earth, the author has further studied these discharges with the rotating camera to find the chronological sequence of discharge processes. Discharges preceding the main spark were observed irrespective of the polarity of the high-voltage electrode, the separation in time of the pre-discharge manifestation from the main spark being almost equal to the time-to-sparkover as recorded by the cathode-ray oscillograph. These discharges have been studied in air at atmospheric and lower pressures, and it has been shown that the pre-discharge is a ¿leader stroke¿ in the strict sense of the word as applied to the lightning discharge: a ¿leader¿ blazes an ionized path from the high-voltage electrode (of either polarity) to earth and is then followed by the return or main stroke of the spark from earth to the high-voltage electrode. Under certain conditions leader strokes have been shown to develop from both electrodes simultaneously. The leader stroke always exhibits branching in the direction of propagation; the main stroke follows some of the more important branches but does not develop fresh branches. The direction of branching of the spark discharge has been shown to furnish the criterion for the direction of propagation of the leader stroke.
