CH. XIIL] VARIATION OF INERTIA 135 



used another formula for calculating it (given in 

 Appendix K), which assumes that the charged body 

 behaves like a conducting sphere. But when the 

 correct deductions from the Heaviside expressions 

 above referred to were applied, with the collaboration 

 of M. Abraham, results practically equivalent to the 

 above were obtained. The above agreement is attained 

 by Professor J. J. Thomson, who applied his own 

 theory to the results of Kaufmann, working it out on 

 the assumption that the charge behaves like an actual 

 point. 



If it is to be urged in future that an electron 

 contains a material nucleus in addition to its electric 

 charge, the burden of proof rests with those who 

 maintain that thesis. The hypothesis which now 

 holds the field is the purely electrical one. 



But it must be remembered that this is not the 

 same thing as establishing an electrical theory for 

 all matter. The inertia of an electron is purely 

 electrical, but what about the inertia of an atom ? 

 Who knows that the atom is wholly composed of 

 electrons ? We do not know that as yet. 



Nevertheless we are now in a very central chapter 

 of modern physics, and it is desirable to enter into 

 the matter somewhat more in detail than in the 

 above preliminary sketch. 



