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 frictional 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 negative 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 computed. The beauty and precision of the measurements and the certainty 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 corresponded 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
