224 Prof. Millikan : New Modification of the Cloud Method 



§ 7. Discussion of Results. 



It will be observed that the only possible elementary charge 

 of which the observed charges are multiples is 4*65 x 10~ 10 , 

 and further that the measured charges represent all the 

 possible multiples of this charge between 2 and 6. This 

 shows that the elementary charge cannot possibly be the 

 smallest charge which we observed: namely, 9'3xl0~ 10 , 

 since we obtained odd as well as even multiples of one half 

 of this quantity. Furthermore, the elementary charge can 

 scarcely be a sub-multiple (e. g. a half) of 4*65 x 10 ~ 10 , since 

 the observed charges would then represent only even 

 multiples of this elementary charge, and there is no reason 

 why odd as well as even multiples of the elementary charge 

 should not have been observed. 



As indicated above the only theoretical assumption involved 

 in this determination is the assumption of the validity of 

 Stokes' law for these drops. Since this law has been shown 

 by direct experiment to hold for large spheres falling slowly 

 in a viscous fluid *, and since the spheres under observation in 

 these experiments have diameters which vary from *00034 cm. 

 to '00047 cm. — numbers which are from 35 to 50 times the 

 mean free path of the air molecules — it is scarcely conceivable 

 that Stokes' law fails to hold for them. Furthermore if 

 Stokes' law did not hold for these drops, that is, if their 

 actunl velocity were the velocity given by Stokes' law 

 multiplied by some constant factor, then the above series of 

 values would simply be multiplied by this factor raised to the 

 3/2 power. So far as these observations, taken by themselves, 

 are concerned, there is no reason why this may not be the 

 'case. If, for example, the final multiplying factor were 2, 

 T-hen the elementary charge would be 9*3 x 10~ 10 , and our 

 series would contain all multiples of this charge up to 6. If 

 this were the case then the charge carried by the a particle, 

 which Rutherford has found to be 9*3 x 10" 10 , would be the 

 elementary charge instead of twice the elementary charge, 

 and the a particle could not then be helium. If it be con- 

 sidered as proven that the a particle is helium, and that the 

 charge which it carries is 9*3 x 10~ 10 , then the above obser- 

 vations may be taken as experimental verification of the 

 validity of Stokes' law for water drops of the size used in 

 this experiment. In other words thes« observations become 

 altogether irreconcilable with Rutherford's experiments if 

 Stokes' law does not hold. 



* H. S. Allen, Phil. Mag. ser. 5, vol. 1. p. 519 (1900). 



