Study of the Visible Grayish-Blue Radiation of the Focal Spot of a Lilienfeld Tube

J. E. Lilienfeld and Franz Rohler. Untersuchungen über die sichtbare blaugraue Brennfleckstrahlung an der Lilienfeldröhre. Phys. Zeitschr. 21, p. 249 (1920).

The paper being reviewed is a continuation of an investigation published by one of the authors about a year ago. The authors study chiefly the polarization and the spectral composition of the radiation. As before, the object of study was a Lilienfeld tube powered by a transformer.

Subjective observation of the polarization of the light under investigation, with the aid of a Nicol prism, leads to the conclusion that its rays are rectilinearly polarized. On the basis of these observations, the electric vector oscillates parallel to the surface of the focal spot, and the maximum of the energy lies in the section by the plane of symmetry of the tube of the anticathode mirror. An objective check of these data by means of photography (a Voigt spectrometer, a Savart plate, and a Nicol) fully confirmed this conclusion. The extremely weak light, nevertheless transmitted by the Nicol prisms set at minimum brightness, may be ascribed to light polarized differently, or else attributed to the radiation of dust particles that always cover the anticathode mirror, or to defects in the polishing of the mirror. In any case, the photographs revealed an increase in the brightness of this light when the defects of the anticathode mirror were increased.

For the spectral investigations, the Voigt polarization apparatus was replaced by a Steinheil spectrograph with a Rutherford prism. The photographs were taken chiefly with the tube in the profile position for oblique rays. The spectrum obtained was continuous, similar to the spectrum of a temperature source. For comparison, on the same plate, spectra were obtained of the metallic filament of an incandescent lamp, the temperature of which had been measured as 1950° (with a Wanner pyrometer). Two photographs of this filament were obtained in such a way that the first produced the same blackening of the plate in the orange-red part of the spectrum as the light of the focal spot, while the second was obtained with an exposure 2.5 times greater. Comparison of the spectra establishes that the spectrum of the focal spot in the region of shorter waves extends farther than the spectra of both photographs of the filament; this indicates an extraordinarily high temperature, if it were the cause of the radiation under investigation. Further investigations to determine this temperature are being undertaken by one of the authors.

The visible light of the focal spot of the tube may be caused by oscillations of electrons covering the anticathode in the form of a cloud. Under the action of the cathode rays they are set in motion, and this energy is distributed among them in the same way as thermal energy is. There are small velocities of motion, but there are also velocities equal to those of the cathode rays. This state of motion, of course, is not limited to the surface layer of electrons. Electrons inside the metal also come into motion, but these latter can give rise in space only to X-ray radiation, since visible

rays will not be able to emerge from the metal. In the fields of the tube the electrons covering the anticathode are set in motion that is not distorted by collisions with the atoms of the substance; their motion is, to a high degree, ordered; the light of the radiation is rectilinearly polarized. To the X-ray radiation arising in the depth of the anticathode one can no longer ascribe such order; this light is either not polarized, or is polarized only in part. It is possible to imagine that the fields existing in the tube are indeed capable of giving rise to the structure of double electric layers, within which a motion of electrons such as that described takes place. The joint appearance of visible radiation and X-rays enables the author to suggest that, at least, some part of the continuous X-ray spectrum is a continuation of the visible radiation.

Vl. Karyakin.