The Diffusion of Electrons Against an Electric Field in the Non-Oscillatory Abnormal Low Voltage Arc

Abstract

Oscillatory abnormal low voltage arc in pure helium.---Experiments with a tube provided with an anode and a hot filament cathode, built and used with elaborate precautions for excluding mercury vapor, disproved the suggestion that traces of mercury vapor as an impurity play an essential role in permitting arcs in helium to operate at abnormally low voltages.Non-oscillatory low voltage arc.---It has been found that, under certain conditions, arcs may be maintained at abnormally low voltages (below the lowest critical potential) and without oscillations, in helium, mercury vapor and argon, this type of arc being particularly striking and easy to obtain in argon. An arc tube, provided with a movable exploring electrode which was used according to Langmuir's method, enabled measurements of potential, of ion concentration, and of average energy to be made in all parts of the arc. With arcs operating on about 6 volts, the cathode drop was invariably very near to 11.5 volts (the minimum radiating potential of argon), so that there was a reverse field of about 5 volts existing throughout the greater part of the arc. The fact that the arc current of almost an ampere flows against this field is due to the effect of diffusion arising from the large concentration gradient. The electron concentration varied from the order of ${10}^{12}$ per cc just outside the cathode to about ${10}^{10}$ near the anode. The reverse field is due to the difference between the mobilities of electrons and of positive ions and is therefore most pronounced in the case of argon, in which electron free paths are abnormally long. The most interesting single feature of this research is the proof of the importance of ion diffusion in low voltage arcs.