ELECTRONIC MAGNITUDES 309 



between the plates. On the other hand, if it carried 

 a negative charge, it could be made to move up by 

 making the upper plate positive with respect to the 

 lower (by the proper motion of the switch and the 

 commutator). Similarly, if the droplet were positive, 

 the upward motion would be accomplished by reversing 

 the battery connection. 



We must remember that the little drops with which 

 Millikan worked were so small that they would only 

 fall about a millimeter a second and yet they were a 

 thousand times or more the diameter of the molecules 

 of the ah* through which they fell. The change in 

 the inertia of the drop due to its picking up a molecule 

 of the air in a collision would then be negligible. Now 

 Millikan observed thousands of these drops at various 

 times and would sometimes observe the actions of 

 a single drop for hours. By allowing it to fall and 

 then by applying the potential and moving it up again 

 he could keep it in the field of his telescope. When he 

 wanted it to fall he short-circuited the plates, thereby 

 destroying the electrical field between them. He made 

 observations of the tune of fall and the time of rise 

 consecutively. 



The time of fall for a given droplet, when there was 

 no electrical field, was found to be always the same, 

 within the limits of the experimental error involved 

 in observing the times when it passed the cross hairs 

 of the telescope. The effect of the charges which the 

 drop carried hi increasing the resistance of the gaseous 

 medium was therefore inappreciable. There was no 

 appreciable acceleration under the action of gravity, 

 and the motion of the drop was that of a body hi a 



