The Shot Effect and Electrical Breakdown in Insulators

Abstract

Measurements are made of the fluctuations in currents passing through insulating materials subjected to high, constant electric fields. Applying the theory of the shot effect to these fluctuations we are able to calculate the average number of elementary charges (electrons or ions) which discharge onto the electrodes as a unit. In glass, this number increases exponentially with the field from about one at low fields to about 10⁶ at fields just below breakdown. Considered with respect to various theories offered in explanation of electrical breakdown, our results indicate that the shot effect is caused by ``avalanches'' of electrons discharging through paths in the material over which breakdown occurs at higher fields, the avalanches increasing in size with the field until the material is ruptured. By making use of the Johnson effect (thermal agitation of electricity) a new and simple method has been worked out for calibrating the amplifier for shot effect measurements.