CH. XIIL] VARIATION OF INERTIA 131 



simultaneously to an electric and a magnetic field 

 coincident in direction. As a matter of fact the 

 speck gives off rays of various speeds, which are 

 differently deflected into a thin streak like a 

 comet's tail (see fig. 18) : and it is the faint 

 impression they make on a photographic plate in 

 high vacuum that is measured and gives the data. 

 Thus the velocity and the e/m ratio are both 

 known, and to summarise briefly the result 

 Kaufman n concluded that when the speeds ap- 

 proached perceptibly near the velocity of light, the 

 electrochemical equivalent m/e increased by just 

 the amount required in accordance with pure electric 

 theory the theory which attributes the whole of 

 inertia to electric influence. There appeared to be 

 no quantitative room for any extra inertia, such 

 as that of an inert particle of non-electric matter 

 travelling with each projectile, retaining its inertia 

 constant at all speeds, and so contributing nothing to 

 the rise of inertia perceived when the speed approaches 

 within hail of that of light. 



We will now enter more into detail concerning this 

 important matter. 



Proof of the purely electrical nature of the inertia 

 of the ft particles shot out by Radium. 



There is every reason to believe that the /3 rays 

 emitted by radium are identical with the cathode 

 rays observable in a vacuum tube ; for both consist 

 of a multitude of electrons or corpuscles travelling at 

 excessively high speed ; and if a determination be 

 made of this speed and of the electro-chemical equiva- 

 lent for the case of P rays for instance, by the 



