408 



Professor Sir J. J. Thomson 



[March 26, 



with a hole bored through it to allow the cathode rays to pass 

 through and strike against the target B ; this is a copper plate which 

 is at the end of and in metallic communication with a cylinder of 

 wire gauze. The variation in the speed of the cathode particles is 

 produced by putting between this gauze and the anode a potential 

 difference tending to stop the rays. Thus, if \^ is the P.D. between 

 the anode and cathode, V2 that between the anode and the gauze, the 

 energy of the cathode rays when they strike the target is proportional 

 to Vj-Yo. To detect any radiation coming from the target a 

 camera was placed at the end of the side tube" T ; a slit was put in 

 front of the photographic plate, and half of it covered by thin slices 



Fig. ], 



of paraffin wax, collodion, mica, glass, or fluorite, so as to be able to 

 estimate the penetrating power of the radiation. A magnet was 

 placed between the target and photographic plate, so as to deflect 

 from the latter any corpuscular radiation from the target. The 

 vacuum was made as low as possible by charcoal and liquid air ; it 

 was so low that no luminosity could be detected between the anode 

 and the target. The plates used were Schumaun plates. The times 

 of exposure, which varied from one minute to two hours, were chosen 

 so as to make the energy in the cathode rays striking against the 

 target during the time of exposure constant. Thus, for cathode rays 

 which had fallen through 10 volts the time of exposure would be ten 

 times that for those which had fallen through 200 ; the latter gave 

 dense photographs with an exposure of two minutes. I have obtained 

 photographs with potential differences as low as 10 volts. These are 

 not due to ordinary light coming from the discharge tube, for (1) 

 they are not obtained when the beam of cathode rays is deflected 

 from the target by a magnet ; and (2) the rays which produce them 

 are quite unable to penetrate very thin films of glass. To test 

 whether or not they were due to corpuscular radiation from the 

 target two methods were employed. First, a magnet was placed 

 between the target and the photographic plate so as to deflect the 

 corpuscular radiation from the plate ; this did not affect the photo- 

 graphs. The second method was to place between the target and' 

 the photographic plate a pair of parallel plates and a^^ply to them a 



