444 



SCIENCE 



[N. S. Vol. XXXII. No. 822 



which varied in the extreme cases 360 fold. 

 These velocities lay between the limits .0013 

 cm. and .47 cm. per second. Complete records 

 of a few of these observations are given in 

 tables Vni., IX., X. and XI. 



On account of the obvious importance of 

 obtaining accurate readings on the larger 

 drops, for which Stokes's law should most 

 nearly hold, the times of fall of such drops 

 under gravity were taken with a chronograph 

 with as great care as possible. Also wherever 

 it was possible, the same drop was timed by 

 both Mr. Fletcher and myself in order to elim- 

 inate the personal equation. The degree of 

 precision which we attained can be judged 

 from the readings recorded in the columns 

 headed G in tables VIII., IX., X. and XI. 

 It will be seen that we very seldom made a 

 reading of the time interval involved in the 

 passage of our star between the cross hairs 

 which differed from the mean time inter- 

 val by more than one twenty-fifth of a second. 

 Furthermore, Mr. Fletcher's and my own 

 mean times on a given drop generally differ 

 from each other by less than one one-hun- 

 dredth of a second. 



All of the times recorded under F in these 

 tables were taken with a stop watch for the 

 reason that in view of the way in which v^ 

 and V, enter into formula (4) and also in view 

 of the fact that F was in all these observa- 

 tions very much larger than no increase in 

 the accuracy of e^ could be obtained by the use 

 of a chronograph in the observations on v,. 



The volts were read just before and just 

 after the observations on a given drop by 

 dividing the bank of storage cells into eleven 

 parts and reading the PD of each part by 

 means of a 900 volts Kelvin and White electro- 

 static voltmeter which we calibrated with an 

 accuracy of one tenth of one per cent, by com- 

 paring it with a Weston voltmeter which had 

 been standardized at the Bureau of Standards. 



The letter F before a reading means that it 

 was taken by Fletcher, the letter M that it was 

 taken by Millikan. 



It will be seen from the tables that even in 

 the case of the largest drops, which were 

 charged with as many as 130 elementary units, 



the values of n are in every case unmistakably 

 determined by the differences summarized at 



TABLE VIII 



Negative drop No. 20 

 Distance between cross hairs = 1.314 cm. 

 Temperature = 23.4° C. 



.0SS43 



Mean ej = 5.102 



TABLE rS 



Negative drop No. 27 

 Distance between cross hairs ;= 1.317 em. 

 Temperature = 25.2° C. 



.16436 



Mean e^ = 5.050 



F's mean G = 8.023. M's mean 6 = 8.007. 



Differences 



e\ 



141.78 — 131.58 = 10.20 -4- 2 = 5.10 

 136.34 — 131.58= 4.76-4-1 = 4.76 

 151.69 — 136.34 = 15.35 -H 3 = 5.12 

 161.41 — 141.20 = 20.20 = 4 = 5.05 

 141.20—136.17= 5.03 -Hi =5.03 



Prob. error 



1 per cent. 



2 per cent. 

 2 per cent. 



1 per cent. 



2 per cent. 



Weighted mean difference = 5.03. 



