Cathode Spot

Abstract

The properties and motions of the mercury arc cathode spot are calculated on the basic assumptions: (1) that the current density is ${10}^5$ amp/${\mathrm{cm}}^2$, with positive ion component $\frac{1}{20}$; (2) that the spot is made up of a number of small "emitting areas," of current density ${10}^7$ amp/${\mathrm{cm}}^2$ moving randomly within the spot. It is shown that these assumptions lead to explanations of the mechanism of emission ("field" emission); retrograde motion, both "turning points" and velocity; pressure on the spot; and evaporation from the spot; in good agreement with experimental values.