Effects of strong Magnetic Field upon Discharges in Vacuo* 179 



14 The Effects of a strong Magnetic Field upon Electric Dis- 

 charges in Vacuo." By A. A. C. SwiNTON. Communicated 

 by LORD KELVIN, F.R.S. Received June 10, Read June 

 18, 1896. 



As is well known, when the lines of force of a magnetic field cut 

 the path of the cathode rays in a vacuum tube, the rays are deflected 

 in one direction or another, according to the polarity of the lines of 

 force. If, on the other hand, the relative positions of the vacuum 

 tube and the magnet are such that the lines of force and the cathode 

 rays are parallel, the rays are not sensibly deflected. 



Under certain circumstances, however, I have found that with the 

 rays and lines of force. parallel, other phenomena occur both in regard 

 to the appearance of the discharge and in connexion with the internal 

 resistance of the tube. 



The apparatus employed consisted of a Crookes tube of the form 

 illustrated, supported vertically over one pole of a straight electro- 

 magnet. The tube, which was excited by means of a 10-inch Ruhm- 

 korff coil, working much below full power, was about 11 inches in 

 length. The cathode terminal consisted of an aluminium plate at 

 one end of the tube, and the anode a similar plate at one side. The 

 tube was exhausted to a degree that gave considerable green fluor- 

 escence of the glass, with a very slight trace of blue luminescence of 

 the residual gas in the neighbourhood of the cathode and anode. 

 The magnet ^mployed had a soft iron core 12 inches in length and 

 1|- inches diameter. It was wound with 2376 turns of No. 18 S.W.Gr. 

 copper wire, which, when supplied with continuous electric current 

 at 100 volts pressure, allowed from 13 to 14 amperes to pass, and 

 magnetised the iron core practically to saturation. 



When the Ruhmkorff discharge passed through the tube, the 

 magnet not being excited, the general appearance was as shown in 

 ftff. 1, the walls of the tube showing everywhere green fluorescence, 

 which was especially strong all over the rounded end of the tube 

 opposite the cathode. A very small amount of blue luminescence 

 could also be faintly seen just below the cathode, and also in the 

 vicinity of the anode. 



With the tube and magnet placed as in fig. 2, as soon as the 

 magnet was excited, the whole appearance of the discharge in the 

 tube was found to alter immediately to what is shown in the illus- 

 tration. Excepting for a very little at the top of the tube near the 

 cathode, and a very bright spot at the bottom immediately over the 

 magnet pole, all the green fluorescence of the glass disappeared, 

 while extending from near the cathode to the bright spot at the 



