134 ELECTRIC THEORY OF INERTIA [CH. xm. 



theoretical ratio </> (0) ; namely, the ratio which 

 expresses the number of times the mass of an electric 

 charge, at specified high speed, exceeds its mass at 

 low or zero speed. 



The successive values of <(#) come out, according 

 o J. J. Thomson, for the above set of velocities, 



1-5, 1'66, 2*0, 2-42, 3'1, 



and these are what must be compared with direct 

 observation or measurement of the apparent or 

 effective mass in each case. 



Now the corresponding values observed experi- 

 mentally by Kaufmann for these same quantities that 

 is to say the factor by which the moving mass 

 exceeded the same mass when stationary were 



1'65, 1*83, 2-04, 2'43, 3'09, 



showing a very remarkable degree of approximate 

 agreement between experiment and theory, especi- 

 ally at the higher speeds. 



Thus at the highest speed ever yet observed for 

 what may be called a particle of matter, at any rate 

 for an electron namely 2*85 x 10 10 cm. per sec. or 

 six hundred million miles per hour the mass of the 

 particle is three times as great as its usual value ; 

 and naturally its momentum and energy are increased 

 in the same proportion. 



Such a surprising agreement as the above, between 

 theory and observation, removes from my mind all 

 reasonable doubt as to the truth of the hypothesis 

 that the inertia of electrons is electrical inertia. 

 I regard this closeness of agreement as specially 

 surprising, for it was not the first deduction of the 

 experimenter, W. Kaufmann, himself: his deduction 

 rather was that the electrical mass constitutes about 

 one-third or one-fourth of the whole; but then he 



