102 



SCIENCE. 



[N. S. Vol. XII. No. 290. 



neglecting the influence of retarding forces 

 when such are really present would be to 

 give values of e/m that are larger than the 

 true value. For this reason, Schuster 

 looked upon the method as giving merely a 

 superior limit for the ratio. The experi- 

 ments of Lenard make it unlikely that re- 

 tarding forces can be present after the rays 

 have emerged from the dark space. But it 

 appears to me that in the immediate neigh- 

 borhood of the kathode their equivalent 

 might well be present. Before the electri- 

 fied corpuscles can yield to the repulsion of 

 the kathode and fly ofif to form the kathode 

 rays, they must be torn loose from the 

 molecules of which they form a part. Is it 

 not possible that an appreciable fraction 

 of the whole potential energy is expended 

 in effecting this separation? Again, al- 

 though it is certain that the kathode rays 

 start from points very close to the kathode, 

 have we any reason to suppose that they 

 originate exactly at the surface? If the 

 rays start a little in front of the kathode, 

 the effect is the same, so far as the results 

 obtained by Schuster's method are con- 

 cerned, as if the corpuscles were subjected 

 to retarding forces. 



The most serious reason for doubting the 

 correctness of the values obtained for e/m 

 arises from the almost incredible velocity 

 of the kathode rays. What right have we 

 to suppose that ordinary electrical and me- 

 chanical laws are applicable to a particle 

 moving at one-third the velocity of light ? 

 It appears to me that we have before us the 

 most stupendous piece of extrapolation in 

 the whole history of physics. . Let us con- 

 sider briefly the assumptions that are made 

 and their experimental basis. The chief as- 

 sumptions are as follows : 



(1) The force exerted upon a corpuscle 

 when passing through a magnetic field is 

 proportional to the speed, being equal to 

 Hev, where J3" is the field strength, e the 

 charge, and •vthe speed. 



(2) The force exerted upon a corpuscle 

 when passing through an electric field is the 

 same as though the corpuscle were at rest. 



The experiments of Kowland and Him- 

 stedt afford indirect experimental evidence 

 that the law stated in (1) is true for veloci- 

 ties up to about 10,000 cm. per second. In 

 computing e/m the assumption is made that 

 the same law holds for velocities a million 

 times greater ! 



So far as I am aware, the question of the 

 force exerted upon a moving charge by a 

 stationary electrostatic field has never been 

 made the subject of direct experimental 

 inquiry. Lenard,* however, has made some 

 experiments upon the kathode rays them- 

 selves which are of the greatest importance 

 in connection with this question. Upon 

 passing the rays through an intense electro- 

 static field in a direction parallel to the 

 lines of force, he found that the rays were 

 either accelerated or retarded according to 

 the direction of the field. The change in ve- 

 locity was determined by measurements of 

 the magnetic deflection and was in some 

 cases as great as fifty per cent. The observed 

 change was the same in amount as would 

 be expected if the force upon the charged 

 corpuscles was the same as though they 

 were at rest. 



The dynamics and electrodynamics of a 

 charged body in rapid motion have been 

 attacked from a theoretical standpoint by 

 J. J. Thomson,-)- Heaviside,t and Schuster.g 

 Rowland 1 1 has recently called attention to 

 the fact that this is a case for the applica- 

 tion of an extremely fundamental scientific 

 law, namely, that of the ' conservation of 

 knowledge.' Our real knowledge of the 

 subject, based upon experiment, is practic- 



* Wied. Ann., 65, p. 504, 1898. 



t Recent Researches in Electricity and Magnetism. 



t Electrical Papers, Vol. 2. 



§ Phil. Mag., 43, p. 1, 1897. 



II Presidential Address before the American Phys- 

 ical Society, Bulletin of the American Physical Society, 

 Vol. I., No. 1. 



