DEVEL0PMEl!5rTS IN ELECTROMAGNETISM BLOCH. 225 



to the orientation of the accelleration with regard to the velocity. 

 The transA^erse mass, detectable only in the experiments with the 

 deviations of the cathode rays, is given according to Max Abraham 

 by the relation 



m^ 4^2i 2^ 1-^ 



This formula seemed completely verified by the observations of 

 Katifmann ^ (1900 and 1903). He measured the variation of the 

 ratio e/m with the velocity for the ^ rays from radium, utilizing 

 the electric and magnetic deviations of the electrons having veloci- 

 ties reaching ninety-five one-hundredths of the velocity of light. 



Since then other formula? have been proposed in the place of this. 

 Langevin and Bucherer,- basing their formula upon the hypothesis 

 of a deformable electron of constant volume, obtained 



1 



- =(1-/?^)' 



o 



Further, as a consequence of tlie development of the theory of rela- 

 tivity (see Sec. II of this article), H. A. Lorentz, postulating an 

 electron of constant equatorial diameter, deduced a third formula : 



_1 

 m ^ 



These new formulae also appear to fit the experiments of Kaufmann. 

 It became necessary, therefore, to make new experiments more pre- 

 cise than those of Kaufmann in order to choose between the various 

 formulae. Several attempts to do this have been made. 



Bucherer" placed a grain of radium fluoride at the center of a 

 condenser formed of two flat disks 8 cm. in diameter and separated 

 by 0.25 mm. This condenser was inclosed in an air-tight cylindrical 

 box, the walls of which carried a photographic film. This was all 

 placed in a uniform magnetic field parallel to the plates and a ver}- 

 perfect vacuum produced. When the condenser is charged, the ^ 

 rays trace upon the film a line the analysis of which permits the 

 calculation of the variation of e/m with the velocity. In this case 

 the formula of Lorentz is found to fit best, confirming nicely the 

 principle of relativity. 



1 See Ions, electrons and eoi-puscles, vol. 1. 

 - Langevin, Revue g^nerale des Sciences, p. 267, 1905. 



" Bucherer, Physik. Zeitschrift, vol, 9, p. 755, 1908 ; Annalen der Physik, vol. 28, p. 513, 

 1909. 



44863°— SM 1913 15 



