Dynamics of the Electron, 85 



(N.B. — We have for small velocities ds = cdt approximately, 



2^ d?x__2 £ ffo 

 3 a els' 1 



&c. 



We therefore observe that the quantity ~ — % plays here the 



2 e 2 

 same part as the mass m . We can therefore call o — 2 



the rest-mass of the electron, and put it equivalent to m .) 



Now a few remarks on the equations (2). These were 

 first introduced into Mathematical Physics by Maxwell 

 about 1865. Ever since their introduction, various efforts 

 have been made by different investigators for getting some- 

 thing out of them, and in certain cases they have yielded 

 very valuable information, and led to many important 

 results. We may cite for example, Maxwell's prediction 

 of the existence of Radiation Pressure. The close analogy 

 of the equations (2) with the equations of elasticity led 

 Maxwell to propose his famous theory of " Stresses," i. e. to 

 imagine that the electric forces are due to a distribution of 

 the stresses (X x} Xy . . . ) in aether, which behaves in this case 

 like an elastic solid. But this theory is fraught with many 

 difficulties, which have been pointed out from time to time 

 by several investigators *. In a paper f communicated to 

 the Phil. Mag., the author observed that though the forces 

 can be well accounted for, the Energy of Electrification 

 cannot be accounted for on Maxwell's hypothesis. 



Another direction in which the equations (2) have been 

 exploited is the subject of Electromagnetic mass of an 

 electron. When an electron moves with a certain velocity, 

 it creates round it an electric as well as a magnetic field. 

 We can say with Maxwell that the energy is stored in the 

 aether, and the electron by its motions exerts a force on 

 every particle of aether. 



If we now integrate this force over the whole space f 



* Maxwell, 'Electricity and Magnetism,' third edition, vol. i. chap, v., 

 footnote p. 165. 



t Phil. Mag;. March 1917. 



j N.B. This space is the absolute space of the Pre-Relativity 

 Period. 



