Abridgment of the electric arc

DEFINITION OF ARC. An arc is a discharge of electricity, between electrodes in a gas or vapor, which has a voltage drop at the cathode of the order of the minimum ionizing or minimum exciting potential of the gas or vapor. ARC CHARACTERISTICS. The relation of arcs to glow discharges and coronas is illustrated by discussion of “generalized” curve of the gas discharge characteristic. Empirical equations for arc characteristics are interpreted, and a dependence on the boiling temperature of the anode is shown. Seeliger's experiments on the transition from glow to arc, accompanied by the development of a cathode spot, show that the mechanism of the current at the cathode is fundamentally different in the two types of discharge. CATHODE SPOT. An analysis based on heat conduction in the cathode shows that the cathode spot has no sharp thermal definition, but does have a sharp boundary if defined by visual brightness or by thermionic emission. The phenomenon of moving cathode spots presents the problem of accounting for the observed temperatures. THEORIES OF CATHODE FALL. Comptons theory is based on space charge considerations and the assumption that the thickness of the fall space is equal to the electronic mean free path. Langmuir's theory differs from Compton's in assuming this thickness to be considerably less than a free path. Considerations of energy balance at the cathode definitely support Langmuir's rather than Compton's theory. ENERGY BALANCE AT CATHODE. Calorimetric measurements permit an estimate of the fraction of the current at the cathode which is carried by electrons. Though uncertain, the data are accurate enough definitely to support Langmuir's theory and to indicate that, in many cases, thermionic emission of electrons from the cathode is supplemented by a “pulling out” of electrons by the electric field which is concentrated at the cathode surface. Factors which determine the anode drop and the potential fall and ionization in the negative glow and the positive column are briefly discussed.