Page:Proceedings of the Royal Society of London Vol 60.djvu/196

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A.s is well known, when the lines of force of a m agnetic 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 th e lines of force. If, on the other hand, the relative positions of the vacuum tube and the m agnet are such th a t th e lines of force and the cathode rays are parallel, the rays are not sensibly deflected. Under certain circumstances, however, I have found th a t with the rays and lines of force parallel, other phenom ena occur both in regard to the appearance of the discharge and in connexion w ith 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 electromagnet. The tube, w hich was excited by means of a 10-inch Ruhm - korff coil, w orking m uch below full power, was about 11 inches in length. The cathode term inal consisted of an alum inium plate at on6 end of the tube, and the anode a sim ilar plate a t one side. The tube was exhausted to a degree th at gave considerable green fluorescence of the glass, w ith a very slight trace of blue luminescence of the residual gas in the neighbourhood of the cathode and anode. The magnet employed had a soft iron core 12 inches in length and 1|- inches diam eter. I t was wound w ith 2376 tu rn s of No. 18 S.W.Gr. copper wire, which, when supplied w ith continuous electric current at 100 volts pressure, allowed from 13 to 14 am peres 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 H". 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 am ount of blue luminescence could also be faintly seen ju st 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 w hat is shown in the illustration. 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