When the oscillating electric spark is examined in a rapidly rotating mirror, the successive oscillations render themselves evident in the image as a series of lumnious curved streamers which emanate from the poles and extend towards the centre of the spark gap. These streamers were first observed by Feddersen in 1862, but the work of Schuster and Hemsalech in 1900 may be said to have opened up a new era in the subject. These workers threw the image of the spark on the slit of a spectroscope, and photographed the resulting spectrum on a film which was maintained in rapid rotation in a direction at right angles to that of the incident light. In their photographs they found that the air lines extended straight across from pole to pole, but that the metal lines were represented by curved bands drawn out in the centre of the spark gap. There is a close relation between these bands and the streamers seen in the unanalysed inductive spark. Schuster and Hemsalech carried out their experiments with the smallest possible inductance in series with the spark, and thus made the period of the oscillations so small that the drawing out on the film was insufficient to separate the individual oscillations from each other. Thus their curved lines represent a composite structure, consisting of all the streamers due to the successive oscillations superposed on each other. It follows from their results that the light of the streamers in the spark is entirely produced by the glowing of the metallic vapour of the electrodes, and that, while the luminosity of the air is practically instantaneous in its occurrence, that due to the metal vapour occurs in the centre of the spark gap an appreciable time later than near the poles. The actual process which goes on in the spark and gives rise to this delay in the arrival of the metallic vapour at the centre of the gap is not yet thoroughly understood. Schuster and Hemsalech make the natural supposition that it is due to the fact that the metal of the electrode is vaporised and rendered incandescent by the heat of the spark, and that the vapour takes an appreciable time to diffuse from the electrodes to the centre of the gap. The exception which has been taken to this view has arisen in part from the difficulty of observing the Doppler effect on the metallic lines which should be a concomitant of the diffusion of the vapour from the poles, and in part from the extraordinary results which the authors themselves obtained in some metals for the velocity of the diffusion corresponding to the different lines. In the case of bismuth and, in a less degree, of cadmium the different metallic lines could be divided into groups of different curvatures which indicated different velocities of diffusion towards the centre of the gap. As regards the former matter, there does not seem to be involved any real difficulty to the explanation, as Dr. Schuster has himself recently shown. The curious effect of the different curvatures of the lines of the same element has, however, always remained more or less of a difficulty in the way of a complete acceptance of their view. Schuster and Hemsalech themselves refer to the possibility in the case of bismuth that the metal may be a compound, and that the two kinds of molecules give rise to the differently curved lines. Other explanations have been made by different writers, but it cannot be said that any explanation adequately supported by experiment has been forthcoming. In view of this incompleteness in our knowledge of the constitution of the streamers it seemed to me that further observations with a rotating mirror would possibly be of value, and the investigations recorded below succeed, I think, in throwing a clearer light on the nature of the streamers, and on certain other phenomena which are characteristic of the spark.
Unidirectional emission of phase-controlled second harmonic generation from a plasmonic nanoantenna
