Spectroscopic Estimation of dc Arc Electrode Vapor-Jet Velocities*

Abstract

When a dc iron arc is run in the silent mode in air, bands of the second positive system of nitrogen are emitted from two narrow zones near the anode, in addition to the iron spectrum and bands of the first positive system of nitrogen. It is suggested that following initial thermal excitation to the A3Σμ and B3Πσ levels, the C3Πμ levels are reached by inelastic collisions with neutral iron atoms contained in vapor jets ejected with supersonic velocity by the anode. Spectroscopic data and the assumption that the jet velocities diminish exponentially then enable calculation of the initial ejection velocity, which is of the same order of magnitude (20×105 cm sec−1) as those obtained by other methods. The velocity is independent of the arc gap but directly proportional to the current in accordance with Maecker’s formula. A proportionality constant, which is independent of the arc gap and current, for a given band head, is interpreted as an excitation coefficient. The initial velocity of ejection and excitation coefficient are less in nitrogen than in air.Oscillograms show that small amplitude oscillations superposed on the arc current of both hissing and silent modes cover most of the audio frequency range and correlate with the hissing sound.It is proposed that the current oscillations and hissing sound are caused by the vapor jets.