ELECTRONIC MAGNITUDES 311 



amount of electricity or of some whole number of times 

 this amount. His observations are, therefore, to be 

 taken as a final demonstration of the existence of elec- 

 tricity only in definite amounts. This definite amount 

 of electricity he found to be independent of the manner 

 in which it was produced, as for example by friction, 

 or by the ionization of gases. It is independent of 

 the substance from which it is obtained or with which 

 it is later associated, since he used for his drops con- 

 ductors like mercury, poor conductors like glycerin, 

 and non-conducting oil. 



If for (n r ri) there is substituted unity, and for 

 (%' #2) the smallest change in velocity due to a change 

 in charge, the value of e may be computed from equa- 

 tion (4) above, providing m is known. Stokes's Law 

 for a falling drop, as given in equation (2), page 275, may 

 be solved to give the radius of the drop in terms of the 

 velocity and viscosity, as follows : 



r = 



2(D-d)g 



(5) 



Now the numerator of the expression given in Stokes's 

 equation is mg, since 4r 3 (D -d)g/3 is the weight of 

 the sphere in a vacuum less the weight of the liquid 

 displaced. Hence, making this substitution in equa- 

 tion (4) and also substituting r from equation (5), gives 



_4/9c\f/ TT 



sir) (rtp^ 



Millikan, however, found that the values he obtained 

 for e by using this equation depended upon the size 

 of the drop, being larger as the drop experimented with 



