132 ELECTRIC THEORY OF INERTIA [CH. xm. 



method of subjecting them both to magnetic and to 

 electrostatic deflexions, which is the easiest way the 

 numbers come out quite similar to the number 

 obtained for cathode rays, viz., for m/e the value 

 10 ~ 7 in E.M. units, and for u something of the order 

 10 9 centimetres per second. 



But radium under favourable conditions is found 

 to shoot out its particles with a speed exceeding even 

 this, and in some cases to approach within hail of 

 the limiting speed, the velocity of light. This is 

 the very important result obtained by the German 

 physicist W. Kaufmann, who has made an admirable 

 series of determinations of speed and of electro- 

 chemical equivalent for this case. The importance 

 of obtaining these excessively high speeds should be 

 obvious, for thereby we are enabled to test the elec- 

 trical theory of inertia. Theoretically the inertia at 

 high speeds is not constant, but increases according 

 to a complicated but calculated law ; we cannot 

 suppose that the electric charge varies in any 

 way with motion ; hence the electrochemical equiva- 

 lent m/e is proportional simply to the mass, and 

 ought to be a function of the velocity u, nearly 

 constant for ordinary values, but increasing rapidly 

 as it approaches within hail of the velocity of 

 light. 



To obtain numerical values we may apply the 

 theory developed by Mr. Heaviside and by Prof. J. 

 J. Thomson, with regard to the increase in momentum 

 of a flying electric charge, over and above the natural 

 mu value, with m considered constant, which is the 

 value at all ordinary speeds. 



The formula which the latter used for the purpose 

 of numerical calculation is one of those given in his 

 Recent Researches ; it is quoted in his American 



