Arc-electrode phenomena

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Abstract
The paper gives a survey of the most important aspect of arc-electrode processes (excluding externally heated cathodes) which concern engineers in a variety of applications. Although it is not possible to avoid discussion of the basic mechanisms and of the number of conflicting and still unresolved theories for them, the emphasis is, as far as possible, on those characteristics which affect the design and operation of devices or processes in which they occur. There is a brief general description of arc discharges and of important differences between them and glow discharges. After distinguishing between thermionic and nonthermionic cathodes (and commenting on the transitions of emission type which occur with some metals), the main body of the paper deals with nonthermionic cathodes. This begins with some of the chief observational data, such as current density, root splitting, electrode erosion, magnetic deflection, plasma jets and force on the cathode, and goes on to consider some of the important factors which affect cathode processes, including electrode material and surface state, current, gas and its pressure or vacuum. After commenting on zones within the cathode fall, there is a discussion of the many theories which have been put forward to explain the emission mechanism of cold-cathode arcs. It is suggested that no single process is responsible for all conditions, and that transitions in mode can occur, either abruptly or gradually, and the most significant principles and facts which must be taken into account are outlined. There is a short Section describing the main features of arc anodes, and the paper concludes with a discussion of electrode effects in mercury-arc convertors, gas-blast and oil circuit breakers, vacuum switches, arc heaters and plasma torches, arc welding, commutation and arc furnaces.

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