"Heat developed" and "powder" Lichtenberg figures and the ionization of dielectric surfaces produced by electrical impulses

A brief outline of the characteristics of Lichtenberg figures is given. Some experiments on "heat developed" and "powder" figures are reported. With regard to the former it is shown that the effect is restricted to certain kinds of solid dielectrics and associated with the state of the surface; an explanation of the mode of their formation is suggested. The effect of repetition of impulses of alternating polarity is investigated by means of "powder" figures. It is shown that the effect of a discharge of given polarity is not neutralized or cancelled by a succeeding discharge of the opposite polarity. This leads to the conclusion that local electrical or mechanical failure in the surface may occur owing to the chance that positively and negatively charged particles will in the course of time be brought very close together. A difference between the Lichtenberg figures produced by negative impulses of short and long wave front and tail is demonstrated; an explanation based on the theory of spark discharge is given. The electrostatic charges produced on the dielectric surface by single and multiple impulses are measured. The results indicate that in order to produce the charge, electron emission from the dielectric surface, when this is the cathode, is necessary; this is evidence that formation of a cathode spot on the dielectric surface is possible, and may be capable of triggering the process which results in failure of the dielectric.