Surge Impulse Breakdown of Air

This paper describes a phenomenon which is used as a method of studying the development of high-voltage sparkover. It has been found that if a voltage which rises very rapidly to a value more than sufficient to cause breakdown if continuously applied is impressed upon a sphere-gap, and this is very quickly reduced to a low value, luminous streamers are produced instead of the usual intense spark. These streamers can be observed with the eye and can be recorded on a photographic plate by the use of a quartz lens. The phenomenon described and recorded is of intense interest because it presents the intermediate stages between the initiation of and the actual flashover between the spheres. The effect on time lag of flashover with special reference to the state of ionization of the gas previous to flashover is also discussed. Besides giving information on the mechanism of development of spark-over, this method of ``suppressed discharge'' has been used to determine the nature of the electrostatic field about insulators and between electrodes of various shapes.