440 



SCIENCE 



[X. S. Vol. XXXII. No. 822 



caught in. the first change, i. e., from 71 to 

 3S0, carried exactly the same charge as the 

 negative ion caught in the change from 380 

 to 71 ; or again, that the negative ion caught 

 in the change from 71 to 39, had exactly the 

 same charge as the positive ion caught in 

 the change from 39 to 71. 



Furthermore, the exact value of the charge 

 caught in each of the above cases is obtained 

 in terms of mg from the differences in the 

 values of e^, given by equation (1), and if it 

 be assumed that the value of m is approxi- 

 mately known through Stokes's law, then the 

 approximately correct value of the charge on 

 the captured ion is given by the difference 

 between the values of e„ obtained through 

 equation (4). The mean value of this dif- 

 ference obtained from all the changes in the 

 latter haK of table I. (see Differences) is 

 4.93 X 10"". 



iN'ow it will be seen from the first observa- 

 tion given in the table that the charge which 

 was originally upon this drop and which was 

 obtained not from the ions in the air, but from 

 the frietional process involved in blowing the 

 spray, was 34.47 X 10"^°. This number comes 

 within one seventh of one per cent, of being 

 exactly seven times the charge on the positive 

 or on the negative ion caught in the ob- 

 servations under consideration. Mr. Harvey 

 Fletcher and myself, who have worked to- 

 gether on these experiments since December, 

 1909, studied in this way between December 

 and May from one to two hundred drops 

 which had initial charges varying between the 

 limits 1 and 150, and which were upon as 

 diverse substances as oil, mercury and gly- 

 cerine, and found in every case the original 

 charge on the drop an exact multiple of the 

 smallest charge which we found that the drop 

 caught from the air. The total number of 

 changes which we have observed would be be- 

 tween one and two thousand, and in not one 

 single instance has there ieen any change 

 which did not represent the advent upon the 

 drop of one definite invariable quantity of 

 , electricity, or a very small exact multiple of 



that quantity. These observations are the 

 justification for assertions 1 and 2 of the in- 

 troduction. 



Before discussing assertion 4 it is desirable 

 to direct attention to three additional con- 

 clusions which can be drawn from table I. : 



First, since the time of the drop in the field 

 varied in these observations from 380 seconds to 

 6.7 seconds, it will be seen that the resultant 

 moving force acting upon the drop was varied 

 in the ratio 1 to 55, without bringing to light 

 the slightest indication of a dependence of e^ 

 upon the velocity. Independently of theory, 

 therefore, we can assert that the velocity of this 

 drop was strictly proportional to the moving 

 force. The certainty with which this conclusion 

 can be dravm may be seen from a consideration 

 of the following numerical data. Although we 

 had upon our drop all possible multiples of 

 the unit 4.917 X 10"^° between 4 and 17, save 

 only 15, there is not a single value of e^ given 

 in the table which differs by as much as .5 

 per cent, from the final mean e^. It is true 

 that the observational error in a 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 .5 per cent. 

 In no case is there here found a divergence 

 from the final value of e^ ai mote than .4 per 

 cent. 



Second, since the charge on the drop was 

 multiplied more than four times without 

 changing at all the value of 0, or the value of 

 Bj, 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. 



Third, 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 col- 

 umn under F, while in all of the four and one 

 half hours during which these experiments 

 lasted, there is but one such. A moment's 

 consideration will show why this is. When 

 the field is on, the ions are driven with enor- 



