ELECTRONIC MAGNITUDES 



307 



of the cloud per unit volume was obtained by passing 

 it through drying tubes and observing their increase 

 in weight. From this and the size of the particles he 

 found the number, and upon the assumption that 

 each is abnormal by only one electron he obtained from 

 his electrometer reading the value of e. 



The most exact determination of the electronic 

 quantity is that of Millikan. 1 The method which he 

 devised involved the observation of the motion of a 

 small particle of oil (about 10" 4 cm. diam.) under the 

 action of gravity and of 

 an electrical field. The 

 particle usually carried 

 an electrical charge pro- 

 duced "by friction" hi 

 its own formation, for 

 the drops were obtained 

 by blowing carefully fil- 



jj "? 



Larth Earth 



tered air through an Earth 



ordinary atomizer con- 

 taining the oil. The electrical field was maintained 

 between two parallel plates, M and N of Fig. 38, by 

 means of the battery B. The switch S t if to the right, 

 permitted the plates to be raised to the difference of 

 potential of the battery or to discharge through the 

 connecting wires in case it was moved to the left. The 

 four circles at the right of the figure represent a com- 

 mutator, a device of four mercury cups, two of which 

 connect to the battery and the other two to the parallel 

 plate condenser (in one case through the earth) . These 

 mercury cups could be connected hi any desired way 

 Cf. Millikan, "The Electron," Univ. of Chicago Press, 1917. 



