492 Sir Oliver Lodge on the 



the massiveness o£ the hypothetical magnetic circulation, 

 without any assumption of that kind. 

 Its energy will be 



JO J a 



;9..2«r8in0.rri0.<Zr. 



Substituting for w its value just found in §§ 8, 9,' 



a 2 

 w = — . us'm 0, 

 T i 



the term to be integrated becomes 



o 



irpa^u 2 . -y sin 3 0^0; 



and so the energy comes out 



4:7rpa 4 u 2 



'da 



Expressing this as the kinetic energy of a certain mass m 

 moving with speed u, this gives 



or twice the mass of the content of the sphere, considered 

 as composed of the surrounding magnetically circulating 

 medium of density p. In so far as w and u are not equal, 

 this expression for m becomes multiplied by the factor 



I — ) , which justifies the remark of § 9. 



12. All the estimates of inertia agree, therefore, as regards 

 order of magnitude : — 



(1) The simple sphere composed of the coagulated liquid; 



(2) The sphere moving through effectively displaced simple 

 incompressible fluid ; 



(3) The more elaborate and truer consideration of the 

 magnetic circulation outside the sphere. 



For they are to one another merely in the ratio 



1 s li : 2 ; 



and they all give the effective mass of the electron as due to 

 a disturbance, moving and identified with itself, affecting a 

 region comparable with its own bulk. That region therefore 

 must have a certain density. 



What is that density ? 



We all admit now that the mass of an electron is comparable 



