Electric Discharge on a Dielectric Surface (Lichtenberg Figures)

THE following method of developing a latent Lichtenberg figure on the surface of certain types of solid dielectrics is very successful. So far, only a `silicon' type of synthetic resin, `Novalak' (fusible phenol formaldehyde polymer), ester gum and cumar resin have been used; but it appears necessary merely that the dielectric should be hard and brittle at ordinary temperatures and should soften quickly at some higher temperature. For example, in one experiment a piece of the resin was fused and poured on a clean glass plate to form, when cool, a hard smooth and uniform coating. This was placed between a needle point and an earthed metal plate. A single positive (or negative) voltage surge of magnitude 10 kV. was applied. No change of the surface was visible. On gently heating the resin coating, however, until the softening temperature was just reached, thread-like grooves appeared which, in a few moments, assumed the characteristic form of a Lichtenberg figure. At this instant the heating was stopped, otherwise the figure disappeared rapidly as the resin liquefied.