Determination of e, N, and Related Constants. 9 



the mean values of the positive and negative units are the 

 same within 3 parts in 6700, but that the time t g under gravity 

 is quite the same when the drop contains 22 electrons as when 

 it contains but 6 (see column n) ; further, that the value of e x 

 computed by (1) from the speeds when the drop carries 

 from 11 to 22 electrons is the same as that found when 

 it carries 6 or 7 electrons. These numbers show, first, that 

 the speed produced by a given field is an exact measure of 

 the charge ; second, that the speed under gravity and hence 

 the apparent resistance of the medium to the motion of the 

 drop through it is independent of the charge; and, third, 

 that the un-ionized atom is strictly neutral, or that the 

 positive and negative electrons are alike in charge. 



6. The Coefficient of Viscosity of Air. 



The only remaining element of uncertainty is in the coeffi- 

 cient of viscosity of air. In 1913, in view of measurements 

 made in this laboratory upon this constant, in addition to 

 measurements made elsewhere, I published * as the most 

 probable value at 23° C. t? = -0001824, and estimated that 

 this could not contain an uncertainty of more than *l per 

 cent. The correctness of this estimate was questioned by 

 Vogel f and Gille X, who, while reducing the value obtained 

 by the Halle observers by one per cent., still retained a 

 value which was half a per cent, higher than that which I 

 had adopted. It is to be pointed out that if this result were 

 correct, my value of e instead of coming down would go up 

 by three-fourths per cent,, but in any case it was obviously 

 necessary to institute further tests as to the correct value. 

 These tests were carried out most thoroughly b}^ Dr. E. L. 

 Harrington, who, using the constant deflexion apparatus 

 designed by Dr. Gilchrist and the author, succeeded, by the 

 introduction of improvements in condition and perfections 

 in detail, in making a determination of tj which is, I think, 

 altogether unique in its reliability and precision. I give to 

 it alone greater weight than to all the other work of the past 

 fifty years in this field taken together. For the individual 

 determinations, though made with different suspensions and 

 in such a way as to eliminate all constant sources of error 

 save the dimensions of the cylinders, never differ among 

 themselves by as much as *1 per cent, and the error in the 

 final mean can scarcely be more than one part in 2000. 

 Indeed, the work has since been repeated by another 



* Annalen der Physik, xli. p. 759 (1913) . 



t Ibid, xliii. p. 1235 (1914). t Ibid, xlviii. p. 799 (1915). 



