10 Prof. R. A. Millikan on a new 



observer, Mr. Stacy, and the result found dependable to- 

 within that limit of uncertainty. Dr. Harrington's * value 

 is ?7 2 3= , 00018227. This value is within less than a tenth 

 per cent, of my 1913 value. The constants of the sus- 

 pensions were determined by taking the periods in vacuo, 

 and it is interesting that they differed from the period 

 in air by as much as '2 per cent., because of the moment 

 of inertia of the air which is dragged along With the rotating 

 cylinder. 



7. The Observations. 



The results of the final series of observations on 25 con- 

 secutive drops are given in Table II. and fig. 2. The numbers 

 at the top of the sheet in the figure represent the approximate 

 times of fall under gravity of the drops opposite which they 

 stand. They are inserted to show the reader at a glance that 

 the value of the slope and of the intercept on the e$ axis,, 

 that is, the value of the electron, is not in any way a function 

 of the radius of the drop. One can get this slope by com- 

 paring only relatively large drops at different pressures (for 

 example, drops falling in from 14 to 20 seconds), or quite 

 small ones (such as those falling in from 44 to 57 seconds), or 

 by comparing drops of different sizes at the same pressure. 

 The starred drops were those taken when the conditions of 

 observation were considered as perfect as possible. No 

 attempt was made to take observations on drops which 

 fell through the fixed cross-hair distance of 1*0220 cm. 

 in less than 14 seconds, since it was desired to keep the 

 timing errors negligible. The value of e* — namely, the 

 intercept on the e^ axis — wag taken from the graph, as 

 was also the slope divided by the intercept, which is the 

 value of b in equation (4). The values thus found were 



et= 61-13 xlO- 8 , 

 b = -000618, 



p being measured in centimetres of mercury at 23° C. and, 

 a in centimetres. The value of A (see foregoing footnote) 

 corresponding to this value of b is '864, instead of '874 as 

 found in 1913. The difference is due, I think, to small 

 errors which were then made in the calibration of both the 

 Hipp chronoscope and the voltmeter, which, however, com- 

 pensated each other in their effect on e, though not in that 

 on A. The numbers given in the last column of Table II.. 

 are the values of et obtained algebraically from (4), and the 



* Phys. Rev. Dec. 1916. 



