The emission of light from spark discharges

Abstract

The investigation is concerned with the transitory effects occurring in spark channels, after the discharge gap has been' bridged completely by a streamer. Observations have been made for various gases, in particular for hydrogen and argon, at pressures of the order of 1 atm. The main features studied are the light emitted from the spark channel during the period of current flow, and also the after-glow which persists after the current has fallen to a negligible value. The results indicate that the after-glow in hydrogen is probably a thermal effect only, whilst in argon it is due also, indeed largely, to other causes of which the persistence of atoms in metastable states is the most likely. Two main experimental methods were employed: ( a ) a revolving mirror camera, similar in principle to those used previously by several investigators, and ( b ) a photoelectric electron multiplier tube coupled directly to a cathode ray oscillograph. The latter method, which is new in investigations of this character, enables quantitative light emission results to be obtained, and methods of calibrating the apparatus are described in detail. The overall time constant of the circuit was sufficiently small to enable after-glows of duration as low as 1 μsec. to be clearly distinguished. The measurements show that for currents of about 100 amp., lasting for 2-4/isec., the after-glow in argon at a pressure of about 1 atm. could be detected after some 30 μsec., whereas in hydrogen, for similar conditions, the after-glow lasted for only some 3 μsec. Other gases showed after-glows of durations varying between the limits set by hydrogen and argon. In order to correlate the light output with energy dissipation in the spark channel, calorimetric measurements were made from which the mean voltage drop during the passage of current was estimated. The channel radii were measured with photographic plates sensitive to the visible and near ultraviolet light. Observations were also made of the spectra of the light emitted from the channel. The argon spectrum showed a strong continuum, and, for hydrogen, only the Balmer lines, much broadened, were seen. The density of ionization in the spark channels is deduced approximately in several ways, from the Stark effect, in hydrogen, and from a consideration of the energy balance in the channel, in both hydrogen and argon. The various calculations are in fairly close agreement and give N i ~10 17 ions per c.c. Channel temperatures, as determined on the basis of Saha’s equation, the limited applicability of which is discussed, are shown to be about 10,000- 15,000° K. The mechanism of light emission from the channel is discussed in some detail, and it is shown that either normal excitation or electron-ion recombination could be entirely responsible for the observed effects.