Directional Effects in the Electric Breakdown of Single Crystals of KCl and NaCl

In alkali halide crystals the discharge paths formed when electric breakdown occurs about a point electrode follow definite crystallographic directions, but many believe that the intrinsic electric strength of these materials does not depend on the direction of the applied field. This belief is difficult to reconcile with the above fact and examination of the literature shows that it is based on very scanty evidence. The experiments described in this paper were made to extend the available information, and measurements of intrinsic strength were made with both KCl and NaCl between - 200°C and + 230°C. The results of the measurements provide little support for the above belief. A fair measure of agreement existed between the discharge track directions and the directions of smallest electric strength. This result suggests that the mechanism of breakdown is dependent on the properties of high-energy conduction electrons.