Two Distinct Types of Short Arcs

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Short field emission arcs are of two types, those which vaporize metal predominantly from the anode by electron bombardment, and those in which the metal of the arc is supplied from the cathode largely by the melting of points by the field emission currents flowing through them. These are appropriately called ``anode arcs'' and ``cathode arcs.'' A single anode arc erodes a pit in the anode and leaves a corresponding roughened area on the cathode. A single cathode arc, on the other hand, leaves on the cathode a widely dispersed array of individual pits which tend strongly to lie along scratch lines; in many cases no mark at all can be found on the anode after an arc of this type. Anode and cathode arcs differ in arc voltage, which is higher for cathode arcs than for anode arcs, and in other ways. Both types of arcs have been observed for many metals, but the data reported here are for palladium only. For clean surfaces of palladium all arcs are of the cathode type at a striking potential of 400 volts, but at 300 volts and at lower voltages anode arcs are observed also, becoming more frequent as the voltage is lowered. For surfaces activated by carbonaceous material only arcs of the cathode type occur at all striking potentials. In an anode arc, metal is transferred in both directions, as measured by radioactive tracers, but the net transfer is from anode to cathode. A cathode arc transfers metal from cathode to anode only, and the magnitude per unit of arc energy is less than the transfer in an anode arc.