A Unified Explanation of Phenomena in Short Metal-Vapour Arcs in Air

Two types of short metal-vapour arcs between closely spaced noble-metal electrodes are distinguished: the anode and the cold cathode arc. Examination of electrode damage due to low-current (2-10A) field-emission-breakdown arcs and inductive break arcs in atmospheric air has led to the observation of two phenomena apparently contradicting to each other, firstly the existence of Germer's critical electrode distance and secondly, the occurrence of a steady transition from the anode to the cathode type with increasing arc duration. The physical interpretation of this are behaviour communicated in this report is based on the appearance of internal ionization instabilities of the self-sustained plasma-cathode system of the fast moving cold cathode spot for electrode separations near the axial extent of the spot ionization region. The concept includes the question of arc striking mechanism and a model of arc evolution with arc time.