of Determining the Elementary Electrical Charge. 223 



SUMMABT. 



Series. 



Charge. 



1~ 



Value of e. 



Weight 



assigned. 



No. 1 



4-59 



7 



No. 2 



i. 



4-56 





No. 3 



■2 c 



464 



6 



No. 4 



5r 



4-83 



4 



No. 5 



2e 



4-7 



1 



No. 6 



6e 



469 



3 



Simple mean c = 4 - 70x 10~~ lu . 

 Weighted „ e =4'35xlO~ LO 



It will be observed that we did not succeed in balancing 

 an)' singly charged drop. This is because the field was not 

 strong enough to hold singly charged drops unless they were 

 exceedingly small. Such drops, both because of the small- 

 ness of their size and the smallness of their charge, are notiu 

 equilibrium with multiply-charged drops (see above) and con- 

 sequently evaporate so rapidly that their life is relatively 

 short. The single observation mentioned above was probablv 

 upon such a drop, but it was evaporating so rapidly that I 

 obtained a poor value of e. Big drops and heavily charged 

 drops are those which evaporate most -lowly, and hence those 

 which are most easy to hold stationary. On the other hand 

 the error in the time determination becomes greater if the 

 rate of fall under gravity is large. All of these considera- 

 tions have been taken into account in assigning the weights 

 given in the summary. In finding the mean time in a given 

 series the observations through one space have been given one 

 half the weight of the observations through two spaces. 



The measurement of the potentials applied to the plates was 

 made with a Braun electrometer reading to 3000 volts. 1 

 calibrated this instrument with an accuracy of about 4 of 

 1 per cent., by comparing it with a Weston voltmeter winch 

 was sent to the Bureau of Standards for standardization. The 

 distance between the plates was measured witli a cathetometer 

 after all of the observations upon the rates of fall had been 

 made. The distance between the cross-hairs was also 

 measured with the aid of the vertical cathetometer scale. 



