758 Prof. R. A. Millikan and Mr. H. Fletcher on the 



amount of the charge added by the ion to the drop can be 

 computed from the observed change in the speed of the 

 drop. 



For the sake of convenience in the measurement of suc- 

 cessive speeds, a scale containing 70 equal divisions was placed 

 in the eyepiece of the observing cathetometer telescope, which 

 in these experiments produced a magnification of about 15 

 diameters. The method of procedure was in general to first 

 get the drop nearly balanced by shaking off its initial charge 

 by holding a little radium near the observing chamber, then, 

 with a switch, to throw on the X rays until a sudden start 

 in the drop revealed the fact that an ion had been caught, 

 then to throw off the rays and take the time required for it 

 to move over 10 divisions, then to throw on the rays until 

 another sudden quickening in speed indicated the capture of 

 another ion, then to measure this speed, and to proceed in 

 this way without throwing off the field at all until the drop 

 got too close to the upper plate, when the rays were thrown 

 off and the drop allowed to fall under gravity to the desired 

 distance from the upper plate. In order to remove the 

 excess of positive ch.tr ge which the drop now had because 

 of its recent captures, some radium was brought near the 

 chamber and the field thrown off for a small fraction of a 

 second. As explained in the preceding papers, ions are 

 caught by the drop many times more rapidly when the field 

 is off than when it is on; also, as heretofore explained, nega- 

 tives are caught more easily than positives. Hence it was 

 in general an easy matter to bring the positively charged 

 drop back to its balanced condition, or indeed to any one of 

 the small number of working speeds which it was capable 

 of having, and then to repeat the series of catches described 

 above. In this way we kept the same drop under observation 

 for hours at a time and in one instance we recorded 100 

 successive captures of ions by a given drop, and determined 

 in each case whether the ion captured carried a single or a 

 double charge. 



The process of making this determination is exceedingly 

 simple and very reliable. For, since electricity is atomic in 

 structure, there are only, for example, three possible speeds 

 which a drop can have when it carries 1, 2, or 3 elementary 

 charges, and it is a perfectly simple matter to adjust con- 

 ditions so that, these speeds are of such different values that 

 each one can be unfailingly recognized even without a stop- 

 watch measurement. Indeed, the fact that electricity is 

 atomic is in no way more beautifully shown than by the way 

 in which in the following tables these relatively few possible 



