The problems considered are whether the breakdown that occurs between two parallel‐plane electrodes in a vacuum is an anode‐induced or cathode‐induced breakdown and what conditions determine which type of breakdown is dominant. The critical anode power density for an anode‐induced breakdown and the critical cathode current density for a cathode‐induced breakdown were measured and the experimental results were compared with predictions from theory of heat conduction. An analysis of these experimental results led to a criterion for determining the type of breakdown that predominates as a function of separation between electrodes and of the thermal and electrical conductivities of the material of the electrodes. It was shown that there were four distinct regions of separation: two anode‐induced regions, one cathode‐induced region, and one transition region of both types.