of Determining the Elementary Electrical Charge. 211 



attempted two years ago to perfect Wilson's method by 

 using radium instead of X-rays for the ionizing agent, by 

 emploving stronger electrical fields for the sake of increasing 

 the difference between v 2 and v x in equation (4), and by 

 observing the fall of the cloud through smaller distances and 

 shorter times in order to reduce the error due to the 

 evaporation of the cloud during the time of observation. 



We obtained at once much more consistent values than 

 those reported by Wilson, and made a preliminary report" 

 in which we gave as the mean of ten observations which 

 varied from 3'66 to 4'37 the value <?=4*06 x 10" 10 . We 

 stated at the time that although we had not eliminated 

 altogether the error due to evaporation, we thought that we 

 had rendered it relatively harmless, and that our final value, 

 although considerably larger than either Wilson's or 

 Thomson's (3'1 and 3*4 respectively) must be considered an 

 approach at least toward the correct value. All of the 

 determinations which have since been made have placed the 

 value of e as high as 4 x 10 -10 and some considerably 

 higher. 



In this former work Mr. Begeman and I used Wilson'.- 

 constant 3*1 x 10~ 9 for the value of the quantity within the 

 brackets in equation 4 ; for we assumed that any errors 

 involved in this constant must be small in comparison with 

 the other errors of the experiment, and confined our attention 

 v\ holiy to improving the consistency and accuracy of the 

 determination of the other factors in the equation. As will 

 presently appear this assumption was not jus.ified. 



The outstanding causes of uncertainty in Wilson's method, 

 as used by ourselves, were as follows : — 



1. There is an experimental difficulty involved in obtaining 

 clouds which fall without any distortion of the upper surface 

 because of air currents. 



2. The npper surface of a cloud falling in an electrical 

 field is exceedingly difficult to follow on account of the 

 scattering of the cloud which is usually produced by throwing 

 on the field. 



3 The method necessitates the assumption that it is 

 possible to obtain in successive expansions exactly identical 

 drops, so that r 2 and t^ can be used in equation (5) as though 

 they applied to the same drop. 



4. The assumption is made that the cloud falls uniformly, 

 and that there is no appreciable evaporation during the time 

 of observation. 



5. The assumption is made that the temperature of the air 

 * Millikan and Begemaa, Phvs. Rev. vol. xxvi. p. 198 (1908. 



*P2 



