July 13, 1900.] 



SCIENCE. 



47 



as a conducting screen, which protects the 

 rays from electrostatic influences. This ex- 

 planation of the failure to obtain electro- 

 static deflection was suggested by Schuster * 

 as early as 1890 ; but the importance of 

 this source of error was not generally ap- 

 preciated until much later. The fact that 

 a conductor placed in the path of the 

 kathode rays usually takes a positive charge 

 instead of a negative one is doubtless due 

 to the same cause. Being surrounded by 

 a conducting medium, the conductor will 

 receive its charge partly from the kathode 

 rays and partly by induction. The induc- 

 tive charge will usually be positive, and 

 may be sufBciently strong to determine the 

 sign of the resultant. Doubtless the almost 

 universal employment of the induction coil 

 by the earlier observers was also in part to 

 blame for the contradictory results. The 

 use of a fluctuating current is now seen to 

 introduce manj' annoying complications. 

 In quantitative work especiallj^, some source 

 of steady current, such as a large Holtz 

 machine or a storage battery, is much to 

 be preferred. 



The discovery that the kathode rays 

 carry a negative charge and are subject to 

 electrostatic deflection afibrded so strong 

 an argument in favor of the Crookes theory, 

 that attempts were at once made to subject 

 the theory to -quantitative tests. The ques- 

 tion of the size of the kathode ray parti- 

 cles and the charge carried by them was 

 attacked independently and almost simul- 

 taneously by Wiechertf ^nd J. J. Thom- 

 son. J It is interesting to observe that al- 

 though the conclusions reached were prac- 

 tically the same, the methods employed 

 were radically different. Wiechert's first 



*Froc. Boy. Soc, 47, p. 526, 1890. 



t Physikal.-okonom. Gesellsohaft in Konigaberg. 

 Jan. 7, 1897. Wiedemann's Beibldtier, 21, p. 443. 



t Royal Institution Lecture. April 30, 1897. 

 Land. Elect., 39, p. 104, 1897. Phil. Mag. 44, p. 

 293, 1897. 



determinations were based upon the con- 

 sideration that since the motion of the kath- 

 ode ray particle is due to the electrical 

 forces, the kinetic energy acquired by each 

 particle must be equal to the potential en- 

 ergy which it possessed at the surface of 

 the kathode. A relation is thus obtained 

 connecting the charge, mass, and velocity 

 of the particles with the potential of the 

 kathode. A second relation between these 

 same quantities is obtained by measuring 

 the deflection of the rays in a magnetic 

 field of known strength. By elimination 

 it is then possible to determine both the 

 velocity of the rays and the ratio of the 

 charge carried by each particle to its mass. 

 The results indicated a velocity not far 

 from 10'° cm. per second, or nearly one- 

 third that of light. That a material par- 

 ticle should move at such an enormous ve- 

 locity seems almost incredible. It is not 

 surprising that Wiechert felt the need of 

 checking this result by some independent 

 method. He did so by employing a method 

 that had been suggested by Des Coudres* in 

 1895, and which is independent of any as- 

 sumption regarding the nature of the kath- 

 ode rays ; the results obtained were of the 

 same order of magnitude as before. That 

 the kathode rays often have a velocity 

 closely approaching that of light has since 

 been abundantly confirmed. 



"Wiechert's values for the ratio e/m — i. e., 

 the ratio of the charge carried by a kathode 

 rays particle to the mass, — lay between 

 20x10" and 40x10'^ (c. g. s., electro-mag- 

 netic units). This is about three thou- 

 sand times greater than the corresponding 

 ratio for the hydrogen ion in ordinary 

 electrolysis. We must therefore conclude 

 either that the particles carry a much 

 larger charge than is carried by an ion in 

 electrolysis, or else that they are smaller 

 than the hydrogen atom. The latter alter- 

 native, which harmonizes so well with the 



* Wiedematm' s Beilldtter, 21, p. 648. 



