ISOLATION OF AN ION MILLIKAN. 241 



few of the smaller times is as much as 1 or 2 per cent, but the 

 observational error in the last half of the table should nowhere 

 exceed 0.5 per cent. In no case is there here found a divergence 

 from the final value of e x of more than 0.4 per cent. 



2. Since the charge on the drop was multiplied more than four 

 times without changing at all the value of G, or the apparent value of 

 <?!, the observations prove conclusively that in the case of drops like 

 this, the drag which the air exerts upon the drop is independent of 

 whether the drop is charged or uncharged. In other words, the 

 apparent viscosity of the air is not affected by the charge in the 

 case of drops of the sort used in these experiments. 



3. It will be seen from the table that in general a drop catches an 

 ion only when the field is off. Were this not the case there would 

 be many erratic readings in the column under F, while in all the four 

 and one-half hours during which these experiments lasted there is 

 but one such, and the significance of this one will presently be dis- 

 cussed. A moment's consideration will show why this is. When the 

 field is on, the ions are driven with enormous speed to the plates as 

 soon as they are formed, their velocities in the fields here used being 

 not less than 10,000 centimeters per second. Hence an ion can 

 not be caught when the field is on unless the molecule which is 

 broken up into ions happens to be on the line of force running from 

 the plates through the drop. With minute drops and relatively 

 small ionization this condition is very unlikely to occur. When the 

 field is off, however, the ions are retained in the space between the 

 plates, and sooner or later one or more of them, by virtue of its 

 energy of agitation, makes impact upon the drop and sticks to it. 



These considerations lead up to assertion 4 in the introduction. 

 It will be seen from the readings in the first half of the table that 

 even when the drop had a negative charge of from 12 to 17 units 

 it was not only able to catch more negative ions, but it apparently 

 had an even larger tendency to catch the negatives than the posi- 

 tives. Whence, then, does a negative ion obtain an amount of energy 

 which enables it to push itself up against the existing electrostatic 

 repulsion and to attach itself to a drop already strongly negatively 

 charged? It can not obtain it from the field, since the phenomenon 

 occurs when the field is not on. It can not obtain it from any explo- 

 sive process which frees the ion from the molecule at the instant of 

 ionization, since again in this case, too, ions would be caught as well, 

 or nearly as well, when the field is on as when it is off. Here., then, 

 is an absolutely direct proof that the ion must be endowed with a 

 kinetic energy of agitation which is sufficient to push it up to the 

 surface of the drop against the electrostatic repulsion of the charge on 

 the drop. 



97578°— sm 1910 16 



