142 VARIABLE MASS [CH. xiv. 



clear a trace as possible, for purposes of measurement, 

 and then by trial and error choosing a constant k l9 

 such as to make the squares of discrepancies of the 

 ratio here called k 2 , from its mean value, as small as 

 possible. 



If it is possible to find a value for the constant k t 

 which shall bring out the calculated value k 2 constant 

 within the limits of experimental uncertainty, then 

 the form of the theoretical function ^ is to that 

 extent verified ; and inasmuch as that function was 

 calculated on the hypothesis of purely electrical 

 mass, the hypothesis of the purely electrical nature 

 of the inertia of ft rays is thereby similarly verified. 



Kaufmann in one of his papers says the experimental 

 errors in his concluding series only amounted to 1*4 

 per cent. ; which, considering the difficulties to be 

 overcome, is remarkably good. 



It is also of interest to record that the numerical 

 value obtained for the normal or low speed value 



S) 



of for the ft rays from radium is 1'84 x 10 7 c.g.s. ; 



m Q 



while Dr. Simon's independent determination, by 

 other means, of the same quantity for cathode rays 

 was l*865x!0 7 ; which is likewise a satisfactory 

 agreement. It is needless to emphasise the agreement 

 with J. J. Thomson's much earlier measurement of the 

 same quantity for rays from other sources. 



The formula employed by Dr. Kaufmann, as repre- 

 senting the inertia, was erroneously deduced from 

 results in a paper by Mr. Searle of Cambridge ; and on 

 the strength of that he concluded at first that only a 

 fraction of the mass was electric. But it was pointed 

 out by Dr. Abraham of Gottingen that the inertia thus 

 calculated was only appropriate to direct acceleration, 

 or acceleration in the line of motion ; whereas what 



