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The present investigation is concerned with the conditions associated with the development of an anode spot for metal-vapor (vacuum) arcs. Chief among the aims of the investigation is the determination of the threshold current density for anode-spot formation for a variety of electrode materials spanning a wide range of thermal and electrical properties. Electrodes of Sn, Al, Ag, Cu, Mo, and W were chosen for study in a plane-parallel electrode geometry. Arcing was over one-half cycle of a 60-Hz current wave. The onset of anode-spot formation was determined from high-speed streak photographs of the discharge. An oscillographic record of the arc voltage was obtained simultaneously with the streak picture. From the data obtained particular interest attaches to the threshold current for anode-spot formation, the threshold current density derived from it, and the arc voltage-current characteristic. The threshold current densities (peak values) range from a low of 0.9×102 A/cm2 for tin to a high of 5.4×102 A/cm2 for tungsten, copper having the value 4.0×102 A/cm2. In general, high-current metal-vapor arcs have positive volt-ampere characteristics and exhibit an hysteresis effect. Rapid changes in arc voltage, noise voltage, and in the magnitude of the hysteresis effect are associated with the formation of an anode spot. The noise voltage and arc drop decrease as the spot develops. In addition a marked increase in luminosity appears in the plasma at the site of the spot. There is a strong correlation between the threshold current for anode-spot formation and the thermal characteristic Tm(kρc)1/2 of the electrode material.

Critical values for post-zero current and power loss were measured in an orifice-type air-blast interrupter of reduced size for dI/dt values of 10-35 A mu s-1. The measured values were one order smaller than predicted by turbulent theory. For 10 A mu s-1 the power loss was only 1 kW.