438 



SCIENCE 



[N. S. Vol.. XXXII. No. 822 



gives the charge e„ in the form 



It is from this equation that the values of e„, 

 in tables I.-XI. are obtained. 



§ 4. Preliminary Ohservations upon the 

 Catching of Ions hy Oil-drops. — Table I. pre- 

 sents the record of the observations taken upon 

 a drop which was watched through a period of 

 four and one half hours as it was alternately 

 moved up and down between the cross-hairs 

 of the observing telescope under the influence 

 of the field F and gravity G. How completely 

 the error arising from evaporation, convection 

 currents, or any sort of disturbances in the 

 air, are eliminated, is shovm by the constancy 

 during all this time in the value of the velocity 

 under gravity. This constancy was not at- 

 tained without a considerable amount of ex- 

 perimenting which will be described in full 

 elsewhere. It is sufficient here to state that 

 the heating effects of the illuminating arc were 

 eliminated, first by filtering the light through 

 about two feet of water, and second, by shut- 

 ting off the light from the arc altogether ex- 

 cept at occasional instants, when the shutter 

 was opened to see that the star was in place, 

 or to make an observation of the instant of its 

 transit across a cross-hair. Further evidence 

 of the complete stagnancy of the air is fur- 

 nished by the fact that for an hour or more 

 at a time the drop would not drift more than 

 two or three millimeters to one side or the 

 other of the point at which it entered the field. 



The observations in Table I. are far less 

 accurate than many of those which follow, the 

 timing being done in the case of Table I. with 

 a stop-watch, while many of the later timings 

 were taken with a chronograph. Nevertheless, 

 this series is presented because of the unusual 

 length of time over which the drop was ob- 

 served, and because of the rather unusual 

 variety of phenomena which it presents. 



The column headed shows the successive 

 times, in seconds, taken by the droplet to fall 



under gravity the distance between the cross- 

 hairs. It will be seen that in the course of 

 the four and one half hours the value of the 

 time increases very slightly, thereby showing 

 that the drop is very slowly evaporating. 

 Furthermore, there are rather marked fluctua- 

 tions recorded in the first ten observations 

 which are probably due to the fact that, in 

 this part of the observation, the shutter was 

 open so much as to produce very slight con- 

 vection currents. 



The column headed F is the time of ascent 

 of the drop between the cross hairs under the 

 action of the field. The column headed e„ 

 is the value of the charge carried by the drop 

 as computed from equation (4). The column 

 headed n gives the number by which the values 

 of the preceding column must be divided to 

 obtain the numbers in the last column. The 

 numbers in the e,, colu mn are in general aver- 

 ages of all the observations of the table which 

 are designated by the same numeral in the n 

 column. If a given observation is not in- 

 cluded in the average in the e„ column, a 

 blank appears opposite that observation in the 

 last two columns. On account of the slow 

 change in the value of G, the observations are 

 arranged in groups and the average value of 

 G for each group is placed opposite that group 

 in the first column. The reading of the PD 

 between the plates, taken at the mean time 

 corresponding to each group, is labeled V and 

 placed just below or just above the mean G 

 corresponding to that group. The PD was 

 applied by means of a storage battery. 



§ 5. Discussion of Taile I. — Since the orig- 

 inal drop was in this ease negative, it is evi- 

 dent that a sudden increase in the speed due 

 to the field, that is, a decrease in the time 

 given in column F, .means that the drop has 

 caught a negative ion from the air, while a 

 decrease in the speed means that it has caught 

 a positive ion. If attention be directed, first, 

 to the latter part of the table, where the ob- 

 servations are most accurate, it will be seen 

 that beginning with the group for which G = 



