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XLIV. On the Charge of the Electron. By Jean Perrin v 

 Professeur de Chimie Physique a V University cle Paris*. 



PROFESSOR MILLIKAN has recently published in this 

 volume (pp. 210-228) a careful determination of the- 

 charge on an electron. He has employed the well-known 

 method of condensation by sudden expansion invented by 

 J. J. Thomson and afterwards improved upon by H. A.. 

 Wilson and himself, and he has obtained the value 4*65 x 10 -10 ,. 

 exactly equal to that which Rutherford and Geiger obtained 

 in 1908 from the study of the u rays. The new method: 

 which I based on the observation of the Brownian movement 

 (May 1908) gave the value 4'lx 10" 10 (Oct. 1908). 



The difference does not reach 12 per cent., and there is 

 thus a striking agreement between these very different 

 methods. There remains to be examined the degree of pre- 

 cision which each of them admits of. In regard to this I ani; 

 not of the same opinion as Prof. Millikan, and the objections 

 which he takes to my method lead me to think that he has- 

 not yet seen two at least of the Notes which I have published;' 

 in the Comptes Renclus or the detailed discussion which I 

 have given in the Annates de Ch. et de Phys. (Sept. 1908,, 

 pp. 1-114). I may be permitted to refer to these communi- 

 cations which are not open to the objections cited by Prof.. 

 Millikan. In effect: 



(1) I have been able to determine the radius of the- 

 grains used by me without having recourse to Stokes'' 

 law ; at the same time, far from assuming this law 

 a priori, I have been able to establish experimentally that it 

 applies exactly to the grains showing the Brow-nian move- 

 ment ((7. R. vol. cxlvii. p. 475 ; and Ann. pp. 45-52). I may 

 even say that I have there brought forward an indirect 

 contribution to the method of condensation by sudden ex- 

 pansion, where this law is applied, since in every case, on- 

 account of the feeble viscosity of the air, the droplets formed 

 have a notable Brownian movement which a strong magni- 

 fication would reveal, 



(2) I have not had to assume, and I have proved experi- 

 mentally, that the particles follow Maxwell's law (Ann. 

 pp. 81-86). 



(3) I have not assumed, in the final measurements, that 

 the density of the particles was that of gum gamboge in 

 mass, and I have determined this density by two different 

 methods (C. R. vol. cxlvii. p. 531, and Ann. pp. 38-40). 



* Communicated by the Author. 



