The Abnormal Low Voltage Arc

Abstract

Oscillating and stable stages of low-voltage arcs in pure helium.---Measurements with a peak voltmeter, confirmed by photographs with a Braun oscillograph, showed that with resistance in the circuit the arc went through five stages as the current was increased from 0 to 1 amp. for the tube studied; (a) voltage stable above the ionizing potential 25.4; (b) voltage oscillating from above 25.4 to below the lowest radiating potential 19.8; (c) voltage stable at practically 19.8; (d) voltage oscillating from between 20 and 24 to below 19; (e) voltage stable above 19.8. The oscillating stages were eliminated by reducing the external resistance to a low value. These phenomena are given a semi-quantitative theoretical explanation on the basis of a negative space charge around the filament for stages a,b,c and a positive space charge for stages d,e, and of the probabilities of excitation and ionization by electron impacts at different voltages. Spectroscopic observations. Lines 7065, 6678, 5876 and 5016 were relatively weak during stage c and increased in intensity with current in stages d,e.Explanation of abnormal low-voltage arcs.---Arcs can be maintained, using a hot cathode, with an applied voltage less than the minimum critical potential in He and Hg, but not in ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$. Such abnormal arcs are found to be oscillating, the maximum voltage in a cycle always exceeding a critical potential. The arc is maintained momentarily at a lower voltage by ionization of atoms previously excited at a higher voltage, but not in the case of gases such as ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$, which do not exist in a metastable excited state and are not ionized by cumulative action. More recent work shows that with very intense ionization, abnormal arcs may be maintained without oscillations, because of the negative potential gradient from the anode to the boundary of the cathode drop.