130 ELECTRIC THEORY OF INERTIA [CH. xm. 



had reference to the proportion of radiation to thermal 

 energy developed when corpuscles encounter a target 

 which suddenly stops them. In so far as they consist 

 of non- electric matter they would produce only heat 

 by their dead collision, without any direct generation 

 of ethereal waves ; in so far as they consist of elec- 

 tric charges they would disperse a certain amount of 

 radiation energy ; and so the proportion of radiation 

 to heat might afford a criterion.* Hitherto, however, 

 no adequate measurements have been made in this 

 direction. 



But there is another more likely avenue to a con- 

 clusive result. We have seen that when an electric 

 charge moves with a speed approaching that of light, 

 its inertia is theoretically no longer constant, but 

 rapidly increases and becomes infinite when the 

 light- velocity itself is reached ; and rather complicated 

 and different expressions for this high-speed inertia 

 have been calculated by several mathematical physi- 

 cists, on different views of the constitution of the 

 electron. See Appendix K for a discussion of this 

 difficult subject. It is possible that this fact will 

 give us the necessary clue. 



For in certain cases of the production of cathode 

 rays, or at any rate of beta rays, a speed not far 

 short of that of light is reached, and in such cases 

 the effects of the increased inertia can be observed. 

 Such an experimental determination has been quite 

 recently undertaken and executed with great skill 

 by Dr. Kaufmann,t who employed the method indi- 

 cated above (Chap. V.) of comparing simultaneously 

 the electric and the magnetic deflexion of the same 

 set of rays from a speck of radium submitted 



*See J. J. Thomson, Phil. Mag., April, 1899, p. 416. 

 tSee Comptes Rendus for October 13, 1902. 



