THE ISOLATION OF AN ION, A PRECISION MEASURE- 

 MENT OF ITS CHARGE, AND THE CORRECTION OF 

 STOKES'S LAW. 1 



By R. A. Millikan. 



INTRODUCTION. 



In a preceding paper 2 a method of measuring the elementary elec- 

 trical charge was presented which differed essentially from methods 

 which had been used by earlier observers only in that all of the meas- 

 urements from which the charge was deduced were made upon one 

 individual charged carrier. This modification eliminated the chief 

 sources of uncertainty which inhered in preceding determinations by 

 similar methods such as those made by Sir Joseph Thomson, 3 H. A. 

 Wilson, 4 Ehrenhaft, 5 and Broglie, 6 all of whom had deduced the ele- 

 mentary charge from the average behavior in electrical and gravita- 

 tional fields of swarms of charged particles. 



The method used in the former work consisted essentially in catch- 

 ing ions by C. T. R. Wilson's method on droplets of water or alcohol, 

 in then isolating by a suitable arrangement a single one of these 

 droplets, and measuring its speed first in a vertical electrical and 

 gravitational field combined, then in a gravitational field alone. 7 



The sources of error or uncertainty which still inhered in the 

 method arose from: (1) The lack of complete stagnancy in the air 

 through which the drop moved; (2) the lack of perfect uniformity 

 in the electrical field used; (3) the gradual evaporation of the 

 drops, rendering it impossible to hold a given drop under observa- 

 tion for more than a minute, or to time the drop as it fell under 

 gravity alone through a period of more than five or six seconds; 

 (4) the assumption of the exact validity of Stokes's law for the drops 

 used. The present modification of the method is not only entirely 



1 Reprinted with abridgment, by permission of the author and the American Physical 

 Society, from The Physical Review, Ithaca, N. Y., vol. 32, No. 4, April, 1911. A prelim- 

 inary account of this work was read on Apr. 23 before the American Physical Society and 

 was published in Science, vol. 32, p. 436, September, 1910. 



2 Millikan, Phys. Rev., December, 1909, and Phil. Mag., 19, p. 209. 



3 Thomson, Phil. Mag., 46, p. 528, 1898 ; 48, p. 547, 1899 ; 5, p. 346, 1903. 

 4 H. A. Wilson, Phil. Mag., 5, p. 429, 1903. 



B Ehrenhaft, Phys. Zeit., Mai, 1909. 



6 Broglie, Le Radium, Juillet, 1909. 



7 In work reported since this paper was first presented, Ehrenhaft (Phys. Zeit., July, 

 1910) has .adopted this vertical field arrangement so that he also now finds it possible to 

 make all his measurements upon individual charged particles. 



231 



