THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 

 FEBRUAR Y 1910. 



XXII. A New Modification of the Cloud Method of Deter- 

 mining the Elementary Electrical Charge and the most 

 Probable Value of that Charge. By Prof. R. A. Millikan, 



University of Chicago *. 



§1. Introduction. 



AMONG all physical constants there are two which will 

 be universally admitted to be of predominant im- 

 portance ; the one is the velocity of light, which now appears 

 in many of the fundamental equations of theoretical physics, 

 and the other is the ultimate, or elementary, electrical 

 ■charge, a knowledge of which m dies possible a determination 

 of the absolute values of all atomic and molecular weights, 

 the absolute number of molecules in a gi\>en weight of any 

 substance, the kinetic energy of agitation of any molecule at 

 a given temperature, and a considerable number of other 

 important physical quantities. 



While the velocity of light is now known with a precision 

 of one part in twenty thousand, the value of the elementary 

 electrical charge has until very recently been exceedingly 

 uncertain. The results herewith presented s> em to show 

 that the method here used for its determination — a modi- 

 fication of the Thomson- Wi son cloud method — furnishes the 

 value of e with a directness, certainty, and precision, easily 

 comparable with that obtained by any of the methods which 



* Communicated by the Author. 

 Phil. Mag. S. 6. Vol. 19. No 110. Feb. 1910. P 



