428 THE POPULAR SCIENCE MONTHLY 



fore this star reached the lower plate the electrical field was thrown on 

 and it straightway began to rise again toward the plate M. This was 

 because in the atomizing process the droplet in general received a fric- 

 tional charge; for, as is well known, strong frictional processes always 

 produce electrification. If this charge was of the wrong sign to cause 

 the drop to rise, rather than descend, when the 10,000-volt battery was 

 thrown on, the signs of the charges on M and N were reversed. When 

 the drop had been pulled up close to M the plates were discharged and 

 the drop allowed to fall under gravity again until it was close to N. In 

 this way, by alternately throwing on and off the electrical field, the oil 

 drop detective was kept pacing its beat up and down between the 

 plates in the hope that it would catch and hold some unwary ion which 

 came within its reach. The first time the experiment was tried an ion 

 was caught within a few minutes and the fact of the capture had been 

 signalled to the observer by the change in the speed with which the drop 

 moved up when the electrical field was on; for since the ion carried an 

 electrical charge, its advent upon the drop changed the charge on the 

 latter and therefore changed the speed with which it was pulled up 

 toward M. If the sign of M was positive, then the drop itself, in order 

 to be pulled up by the field, must have had a negative charge and in 

 that case the capture of a positive ion reduced this negative charge and 

 therefore reduced the speed in the field, while the capture of a nega- 

 tive ion increased the negative charge and hence increased the speed in 

 the field. From the sign, then, and the magnitude of this change in 

 speed, taken in connection with the constant speed under gravity, the 

 sign and the exact value of the charge carried by the captured ion could 

 be easily determined. 



A drop would often be kept traveling back and forth in the manner 

 described for four or five hours at a time, in the course of which it 

 would change its charge twenty or thirty times because of the capture 

 of ions and the value of each of these different charges would be com- 

 puted. The beauty and precision of the measurements and the cer- 

 tainty with which the atomic theory of electricity follows from the 

 results obtained can best be appreciated by inserting in full the record 

 of an experiment made upon a particular drop. The column headed G 

 gives the successive times which the droplet required to fall between two 

 fixed cross-hairs in the observing telescope whose distance apart corre- 

 sponded in this case to an actual distance of fall of .5222 centimeter. 

 It will be seen that these numbers are all the same within the limits of 

 error of a stop watch measurement. The column marked F gives the 

 successive times which the droplet required to rise under the influence 

 of the electrical field produced by applying in this case 5,051 volts of 

 potential difference to the plates M and N. It will be seen that after the 

 second trip up, the time changed from 12.4 to 21.8, indicating, since in 



