486 



POPULAR SCIENCE MONTHLY. 



teristic glow. For example, if a screen coated with, zinc sulphide is 

 placed within a discharge tube in the manner shown in Fig. 3, the 

 cathode rays which pass through the slit in the mica diaphragm just 

 opposite the cathode, light it up brilliantly in the parts along which 

 they graze, and thus trace a distinct outline of their path from one 

 end of the tube to the other. 



The nature of these rays was the subject of much dispute between 

 the years 1880, when they first began to be studied, and 1898. Some 

 thought them to be streams of minute negatively charged particles 

 shot off with enormous velocities from the cathode C, while others 



maintained that they did not con- 

 sist of particles at all, but were 

 waves in the ether, just like light 

 waves. The dispute was finally 

 ended by two very conclusive ex- 

 periments performed, the first by 

 Perrin, a Frenchman, and the other 

 by J. J. Thomson, professor of 

 physics in Cambridge University, 

 England. Perrin 's experiment 

 consisted in proving that under 

 all circumstances a body which 

 was placed along the path OP, 

 so that the cathode rays could 



Showing Fig. 3. Showing De- fall upon it, became charged with 

 Cathode flection of Cathode , . ^ < • •!_ • • 1 1 



Rays by a Magnet, negative electricity, just as would 

 be expected if the cathode rays 

 consisted of negatively charged particles. J. J. Thomson's experi- 

 ment consisted in showing that if a charge of positive electricity were 

 placed upon the plate E (see Fig. 1), and a charge of negative electricity 

 upon the plate D, the rays were deflected out of the line OP and into 

 the path OP'. This, too, was to have been expected if the rays con- 

 sist of negatively charged particles, for these particles would be repelled 

 by the negative electricity upon D and attracted by the positive elec- 

 tricity upon E. 



There is a further property of the rays, which, although it had 

 long been known, adds powerful support to the projected particle 

 theory. It is that when a magnet is brought near the cathode beam 

 in the manner shown in Fig. 3, the beam is deflected by it also, just 

 as would be expected if it consisted of a stream of negatively charged 

 particles. These three experiments settled the question in favor of 

 the projected particle theory, so that physicists are now all agreed in 

 regarding the cathode rays as streams of minute, negatively charged 

 corpuscles shot off in straight lines from the surface of the negative 

 electrode and in a direction at right angles to this surface. 



Fig. 2. 

 Beam of 

 Kays. 



