Measurement of arc spot formation delay times at cold cathodes in air

The column of a high current arc is magnetically blown against a cathode positioned perpendicularly to the arc axis. A linearly increasing voltage is applied to this electrode. The delay time of the arc spot formation which initiates the arc commutation was measured for smooth electrodes made of Cu, steel, graphite, Mg, and Ce. The cumulative frequency distribution of this delay time was found to be characteristic function of the electrode material and to be related to its work function. Experiments with copper electrodes covered with oxide layers of different thicknesses show that the thickness of the layer has only a weak influence on the delay time