There are a number of spectra which are seen under appropriate conditions of excitation in vacuum tubes containing helium with a small trace of some other gas, but which are difficult to isolate under other conditions. In particular the arc spectrum of carbon and the band spectrum known as the comet tail spectrum can be observed very favourably under these conditions. This latter spectrum, which is characteristic of the tails of comets, was first observed in the laboratory by Fowler in carbon monoxide at very low pressures, and recent analyses of the spectrum leave no doubt that it is to be attributed to the carbon monoxide molecule. In the course of the investigation of this spectrum in vacuum tubes containing helium some very peculiar types of striated discharge were observed, and further investigation leads to the conclusion that these striae are of a very special type and are quite different to those which are usually observed in vacuum tubes. They exhibit certain peculiarities which may perhaps justify the suggestion that this type of discharge is akin to the rare phenomenon known as ball lightning. The striae usually observed in vacuum tubes may be classified as stationary and moving striæ. The former have been observed since the earliest investigations of the electric discharge in gases at low pressures, and their appearance and behaviour is so well known as to need no description. The moving striations, first observed by Wullner, have recently been the subject of more critical investigations by Aston and Kikuchi and Whiddington, who by the application of stroboscopic methods have observed in pure gases striations moving with velocities of from 10
3
to 10
6
cm. per second. More recently Langmuir has described a peculiar type of streamer discharge which he has observed in argon containing a small amount of tungsten vapour which was introduced by the sputtering of a tungsten filament. Whilst the phenomena described by Langmuir bear a certain resemblance to those described in the present communication, inasmuch as in both cases the formation of a particulate cloud has been observed, the conditions of experiment are so different as to make it almost certain that the phenomena are different in origin. Langmuir’s observations were made with a direct current discharge, using an electrically heated tungsten filament as the cathode, and the streamer discharge observed by him was very sensitive to weak magnetic fields, whilst the disc discharge which has been the subject of the present investigation can only be maintained with an alternating current discharge, and is entirely unaffected by weak magnetic fields.
Magnetic white dwarfs: Observations, theory and future prospects