CATHODE RAYS.i 



By Prof. J. J. Thomson, F. R. S. 



The first observer to leave any record of what are now known as 

 the cathode rays seems to have been Pliicker, who in 1859 observed 

 the now well-known green phosphorescence on the glass in the neigh- 

 borhood of the negative electrode. Pliicker was the first physicist to 

 make experiments on the discharge through a tube in a state anything 

 approaching what we should now call a high vacuum. He owed the 

 opportunity to do this to his fellow-townsman Giessler, who first made 

 such vacua attainable. Pliicker, who had made a very minute study 

 of the effect of a magnetic field on the ordinary discharge which 

 stretches from one terminal to the other, distinguished the discharge 

 which produced the green phosphorescence from the ordinary dis- 

 charge by the difference in its behavior when in a magnetic field. 

 Pliicker ascribed these phosphorescent patches to currents of elec 

 tricity which went from the cathode to the walls of the tube and then 

 for some reason or other retraced their steps. 



The subject was next taken up by Pliicker's pupil, Hittorf, who 

 greatly extended our knowledge of the subject, and to whom we owe 

 the observation that a solid body placed between a pointed cathode 

 and the walls of the tube cast a well defined shadow. This observa- 

 tion was extended by Goldstein, who found that a well marked, though 

 not very sharply defined, shadow was cast by a small body placed 

 near a cathode of considerable area. This was a very imx)ortant obser- 

 vation, for it showed that the rays casting the shadow came in a defi- 

 nite direction from the cathode. If the eathode were replaced by a 

 luminous disk of the same size, this disk would not cast a shadow of 

 a small object placed near it, for though the object might intercept the 

 rays which came out normally from the disk, yet enough light would be 

 given out sideways from other parts of the disk to prevent the shadow 

 being at all well marked. Goldstein seems to have been the first to 

 advance the theory, which has attained a good deal of prevalence in 

 Germany, that these cathode rays are transversal vibrations in the 

 ether. 



^ Address before the Eoyal Institution of Great Britain, April 30, 1897. Printed iu 

 Proceedings of the Royal Institution, 1897. 



157 



