216 Prof. Millikan : New Modification of the Cloud Method 



and for the viscosity of air saturated with the vapour of 

 alcohol 77 = -0001878. 



Under the conditions, then, which apply to the following 

 experiments the value of the constant within the brackets in 

 equation (4) is, for water, 3*422 x 10- 9 in place of 3 1 X 10~ 9 

 as in Wilson's formula, and for alcohol, 3*353 x 10~ 9 . 

 Furthermore if approximately the same temperature con- 

 ditions apply, as they probably do, to Wilson's method as 

 used in the preceding determination of e by Mr. Begeman 

 and myself, which was also made at 26° C, then the error in 

 our former result due to the use of the constant 3*1 X 10~~ 9 is 

 somewhat more than 10 per cent. When the value 

 4*06 X 10 " 10 is corrected for this error it becomes 4*5 X 10" 10 

 and when further corrected for a H per cent, error which 

 was found in the calibration of the voltmeter with which 

 the original potential-differences were measured, it becomes 

 4-57 x 10- 10 . 



§ 4. The Balancing of Individual Charged Drops hy an 

 Electrostatic Field. 



My original plan for eliminating the evaporation error was 

 to obtain, if possible, an electric field strong enough to exactly 

 balance the force of gravity upon the cloud and by means of 

 a sliding contact to vary the strength of this field so as to 

 hold the cloud balanced throughout its entire life. In this 

 way it was thought that the whole evaporation-history of the 

 cloud might be recorded, and suitable allowances then made 

 in the observations on the rate of fall to eliminate entirely 

 the error due to evaporation. It was not found possible to 

 balance the cloud as had been originally planned, but it was 

 found possible to do something very much better ; namely,, 

 to hold individual charged drops suspended by the field for 

 periods varying from 30 to 60 seconds. I have never actually 

 timed drops which lasted more than 45 seconds, although I 

 have several times observed drops which in my judgment 

 lasted considerably longer than this. The drops which it 

 was found possible to balance by an electrical field always 

 carried multiple charges, and the difficulty experienced in 

 balancing such drops was less than had been anticipated. 



The procedure is simply to form a cloud and throw on the 

 field immediately thereafter. The drops which have charges 

 of the same sign as that of the upper plate or too weak 

 charges of the opposite sign, rapidly fall, while those which 

 are charged with too many multiples of sign opposite to that 



