The Followers of Maxwell. 343 



a perfectly conducting sphere whose centre is at the point, the 

 only chaftge being that E and H would now vanish inside the 

 sphere. This inference was subsequently found* to be incorrect : 

 a distribution of electric charge on a moving sphere could in 

 fact not be in equilibrium if the electric force were radial, since 

 there would then be nothing to balance the mechanical force 

 exerted on the moving charge (which is equivalent to a current) 

 by the magnetic field. The moving system which gives rise to 

 the same field as a moving point-charge is not a sphere, but an 

 oblate spheroid whose polar axis (which is in the direction of 

 motion) bears to its equatorial axis the ratio (1 - tf/c*)^ : !.) 



The energy of the field surrounding a charged sphere is 

 greater when the sphere is in motion than when it is at rest. 

 To determine the additional energy quantitatively (retaining 

 only the lowest significant powers of v/c), we have only to 

 integrate, throughout the space outside the sphere, the expression 

 H 2 /87r, which represents the electrokinetic energy per unit 

 volume : the result is e z v~/3a, where e denotes the charge, v the 

 velocity, and a the radius of the sphere. 



It is evident from this result that the work required to be 

 done in order to communicate a given velocity to the sphere 

 is greater when the sphere is charged than when it is uncharged ; 

 that is to say, the virtual mass of the sphere is increased by an 

 amount 2e 2 /3a, owing to the presence of the charge. This may 

 be regarded as arising from the self-induction of the convection- 

 current which is formed when the charge is set in motion. It 

 was suggested by J. LarmorJ and by W. Wien that the inertia 

 of ordinary ponderable matter may ultimately prove to be of 

 this nature, the atoms being constituted of systems of electrons. || 



By G. F. C. Searle. 



t Cf. Searle, Phil. Trans, clxxxvii (1896), p. 675, and Phil. Ma ? . xliv (1897), 

 p. 329. On the theory of the moving electrified sphere, cf. also J. J. Thomson, 

 Recent Researches in Elect, and Mag., p. 16; 0. Heaviside, Electrical Papers, ii, 

 p. 514; Electromag. Theory, i, p. 269; W. B. Morton, Phil. Mag, xli (1896), 

 p. 488 ; A. Schuster, Phil. Mag. xliii (1897), p. 1. 



+ Phil. Trans, clxxxvi (1895), p. 697. Arch. Neerl (3) v (1900), p. 96. 



|| Experimental evidence that the inertia of electrons is purely electromagnetic 

 was afterwards furnished hy W. Kaufmann, Gott. Nach., 1901, p. 143 ; 1902, p 291. 



