Studies on the Low-Voltage Arc in Mercury Vapor and its Relation to Fluorescence

Abstract

Low-voltage arc in mercury vapor.—(1) Variation of the striking and breaking voltages with conditions. Using a simple tube, with incandescent tungsten filament as cathode and nearby mercury surface as anode, the arc was found to strike at voltages as low as 4.4 and to persist down to 1.8 volts, if the pressure was from 2-5 mm. and the cathode was sufficiently hot. (2) Explanation. If, however, correction is made for the initial velocity distribution of the electrons, the minimum electron energy necessary to maintain the arc comes out 5.5 volts which is the difference between the ionization potential, 10.4 volts, and the resonance potential 4.9 volts. Ionization is then effected by two successive mpacts, at 4.9 and 5.5 volts, in agreement with the theory of K. T. Compton. (3) Effect of the absence of freshly distilled mercury vapor. When a nickel anode was used and the mercury surface was removed to a considerable distance, 70 to 180 cm., the arc could not be maintained at a net voltage of 5.5, but 6.7 volts was required. This fact was demonstrated with three pieces of apparatus. This indicates that while freshly distilled mercury vapor, as is well known, is particularly active in fluorescent absorption of $\lambda 2536$ radiation (4.9 volts), old vapor is more responsive to $\lambda 1849$ radiation (6.7 volts).