On the velocity of rotation of the electric discharge in gases at low pressures in a radial magnetic field

Abstract

The following paper contains an account of a series of experiments on the motion of an electric discharge in a magnetic field perpendicular to the direction of the discharge current. The fact that the discharge moves in a magnetic field like a flexible conductor carrying a current was discovered long ago, and De La Rive showed that it could be made to rotate continuously round one pole of a magnet placed inside the vacuum tube. The apparatus used in the present experiments was similar in principle to De La Rive’s, but was arranged so that fairly exact measurements of the various quantities concerned could be obtained. Fig. 1 shows a vertical section of the vacuum tube and magnet used. The tube consisted of two concentric glass tubes cemented with sealing wax into aluminium discs. The discs had grooves turned in them to fit the glass tubes, and the part of the discs between the tubes projected a few millimetres, so that there was no danger of the discharge passing through the sealing wax. Polished platinum rings were fixed on to the aluminium discs between the glass tubes, and these formed the electrodes between which the discharge was passed. The ends of the tubes were carefully ground truly perpendicular to their axes, and the two platinum rings were accurately parallel. To keep the electrodes cool, a ring of narrow brass tubing was soldered on to the back of each disc and a stream of water was kept flowing through these whenever a discharge was passed. This arrangement enabled comparatively large currents to be used without softening the sealing wax. A narrow copper tube was soldered into one of the discs and communicated with the interior of the vacuum tube through a fine hole. The tube was connected by a mercury sealed joint with a glass tube leading to a bulb containing pure phosphorus pentoxide, a Töpler pump, and a McLeod gauge.