ISOLATION OF AN ION MILLIKAN. 



245 



of either sign or in any desired number, within limits, upon a given 

 drop. We did this as follows : When it was desired to load the drop 

 up negatively, for example, we held it with the aid of the field fairly 

 close to the positive plate, and placed the radium so that it would 

 produce uniform ionization throughout the chamber. Under these 

 conditions if the positive and negative ions were alike in both num- 

 ber and mobility the chance that the drop would catch a negative ion 

 would be as many times its chance of catching a positive ion as the 

 distance from the drop to the negative plate was times the distance 

 from the drop to the positive plate. Similarlj^, if we wished to 

 load the drop positively it was held by the field close to the nega- 

 tive plate. On account of the slightly greater mobility of the nega- 

 tive ion, and also on account of the somewhat greater numbers in 

 which they occur, we found, in general, a greater tendency of the 

 drops to take up negative than positive charges. In view, there- 

 fore, of the greater ease with which negative drops could be held for 

 long intervals without being lost to the plates most of the drops 

 studied have been of negative sign. 



THE FAILURE OF STOKES's LAW. 



"When the values of e^ Avere computed as above for different drops, 

 although each individual drop showed the same sort of consistency 

 which was exhibited by the drop of Table I, the value of e^ at first 

 came out differently, even for drops showing the same value of the 

 velocity under gravity. This last irregularity was practically elimi- 

 nated by blowing the drops into air which was strictly dust free, but 

 even then drops of different sizes, as determined by v^^ always gave 

 consistently different values of e^. This is illustrated by the observa- 

 tions shown in Tables III, IV, V, VI, VII, and VIII. 



Table III. — 'Negative drop No. 5. 

 [Distance between cross hairs= 1.303 cm. Temperature= 24.6° C. Density of oil at 25.0° C.= .9041.] 



