THE KINETIC THEORY OF MATTER 429 



this case the drop was positive, that a negative ion had been caught 

 from the air. The next time recorded under F, namely, 34.8, indicates 

 that another negative ion has been caught. The next time, 84.5, indi- 

 cates the capture of still another negative ion. This charge was held for 

 two trips, when the speed changed back again to 34.6, showing that a 

 positive ion had now been caught which carried precisely the same 

 charge as the negative ion which before caused the inverse change in 

 time, i. e., that from 34.8 to 84.5. 



13.6 12.5 



13.8 12.4 



13.4 21.8 



13.4 34.8 

 13.6 84.5 



13.6 85.5 



13.7 34.6 



13.5 ' 34.8 

 13.5 16.0 



13.8 34.8 



13.7 34.6 



13.8 21.9 

 13.6 



13.5 

 13.4 

 13.8 

 33.4 



Mean 13T595 



Now all of the successive values of the charge carried by the drop 

 throughout the experiment can be easily computed from the constant 

 speed under gravity and the successive values of the speed in the 

 electric field. To find the absolute values of these charges it is indeed 

 necessary to know the weight of the drop, and the determination of this 

 weight may involve an error of a fraction of a per cent, at most, but 

 since this weight remains constant throughout the experiment the rela- 

 tive values of the successive charges can be found with absolute certainty 

 and with great precision without any knowledge of this weight. They 

 are in fact simply proportional to the successive values assumed by the 

 sum of the two speeds, viz., that under gravity and that in the field. ^ 



* For in the case of bodies moving slowly and uniformly through a resisting 

 medium any two forces produce velocities which are proportional to the forces. 

 The downward force due to gravity is here 7ng and the upward force due to the 

 field is Fe, in whicli F denotes the strength of the field and e the charge on the 

 drop. Hence, if i\ is the downward velocity due to gravity and v^ the upward 

 velocity due to the excess of the upward pull of the field over the downward pull 

 of gravity, we have 



V, mq mq , ^ 



?'2 Fe — ing Fi\ 



