Arcing at Electrical Contacts on Closure. Part II. The Initiation of an Arc

Abstract

The capacity of the plates of an oscilloscope charged to 35 or 40 volts is discharged repeatedly by approaching electrodes of carbon, active silver, and inactive silver. Facts about the discharges, which are arcs of very short duration, are inferred from resulting open circuit potentials and calculated electrode separations. The separation at the first arc varies in different experiments but corresponds on the average to a nominal electric field of 0.6×10⁶ volts/cm for carbon or active silver and to 2×10⁶ volts/cm for inactive silver. Each arc is initiated by a very small number of field emission electrons. The hypothesis that a single electron may perhaps be sufficient is consistent with observations at later stages of each closure when the electrodes are closer and the field much higher. The earlier observation, that the potential across a short arc is constant and independent of current, is not true if the arc time is sufficiently short. For active silver a time comparable with 2×10⁻⁸ sec is required to establish the steady arc voltage characteristic of later stages of arcs which last longer than this. The initial time during which the potential is decreasing toward its final steady value is 100 times the transit time of a silver ion across the gap.