The charge on each ion was assumed to be due to the excess or defect of one electron, and hence the experiment gave an estimate of the charge, e, associated with each corpuscle. The value of e at first obtained by Thom son in this way ranged from 5.5 X 10—'° to 8.4 X electrostatic units, and he adopted 6.5 X as the concluded value. In 1903 he published a later determination of e, obtained by following the same general plan as before but with certa_n improvements in technique, and gave the value e =3.4 X 10-1°.
Beautiful and ingenious as this determina tion of the charge on the individual corpuscle was, the method was open to certain criticisms, inasmuch as it involved certain assumptions which had not been shown to be valid, and which, in fact, were only approximately true. They were near enough to the truth for the method to yield a rough estimate of the value of e, but they were too imperfect to provide us with an accurate and dependable determination. It was not 'mown, for example, that Stokes' formula for the rate of fall of spheres in a viscous fluid would apply with sufficient ac curacy in the case of droplets of the txceed ingly small size here under consideration. Nor was it known that every ion actually did sur round itself by a liquid droplet, nor that there were no droplets containing more than one ion. Nor did the method make allowance for the effect of differences in the sizes of the droplets, nor for possible evaporation from their surfaces after they were formed. It is not possible, in the present place, to discuss these various points, but it must suffice to say that they have all received the most careful consideration in later researches, and Prof. R. A. Millikan, of the University of Chi cago, has recently been able to publish a definitive and probably very accurate value of e, obtained by a method which apparently leaves little to be desired on the score of soundness or of experimental excellence. It does not detract in any way from the admira tion that we must feel for Thomson's original work, to say that Millilcan's research was still more ingemous and beautiful. He succeeded in trapping single corpuscles, and in measuring the value of the ((electron') directly; and the account of his work that he gives in his book, (The Electron,' is extremely fascinating.
Mill:kan's fundamental idea was exceedingly simple, but in its practical auplication it called for an immense amount of ingenuity, experi mental skill and patient labor. A tiny spherical droplet of oil was electrified and caused to take tip a position, suspended in the air, between two horizontal metallic plates that could be electrified or grounded, at will. The drop was strongly illuthinated from two opposite sides, and was observed by means of a telescope di rected at right angles to the light-rays. It appeared, in the field of the telescope, *like a bright star against a black background?) The drop was first allowed to fall freely through a known distance (approximately equal to half a centimeter or one-fifth of an inch)! the limits of which were marked by a pair of cross-hairs in the telescope. The time of fall through this distance, in one set of experiments, was about 13 seconds. Before the drop reached the lower metallic plate, both plates were elec trified by connecting them to the terminals of a battery having a total electromotive force of from 5,000 to 10,000 volts, the charge of the loyver plate having the same sign as the elec tnfication on the oil drop. When the experi ment was rightly conducted, the drop (already carrying an electric charge) would begin to rise, under the influence of the electric field to which it was exposed, and the time required for it to make its upward journey from the lower cross-hair of the telescope to the upper one was noted. Before it reached the upper
plate the electric field would be destroyed by grounding the metal plates. The. drop would then fall again, and the time of its descent from the upper cross-hair to the lower one was once more observed, and so the experiment proceeded—keeping the droplet always in the air, and continually recording the times of its ascent and descent. (A single drop could thus be kept under constant observation for hours.) The size of the drop was determined from the measured time of its fall by means of a modi fied form of Stokes' formula for the descent of small spheres in viscous media — the original formula having been studied with great care (especially by Dr. H. D. Arnold) with refer ence to its accuracy in connection with droplets of the size used in these experiments. The diameter of the droplet being lcnown, its weight was readily ascertained, because the density of the oil of which it was composed was known. Then from a knowledge of the weight of the drop, and of the titne of its downward passage under the influence of gravity and of its upward passage under the influence of the known electric field, it was easy to cal culate the electric charge on the drop. An ingenious means was provided for chang ing the electrification of the drop at will, and in either direction, by ionizing the air between the plates by means of an X-ray discharge, and then throwing ions against the drop by electric repulsion. The original posi tive electrification of the drop was reduced every time a negative ion was taken in, and increased every time a positive ion was re ceived. After a positive ion had been taken in, the upward journey would be performed more quickly than before, and the inclusion of a negative ion would cause a correspond ing slowing of the upward motion. It was found to be quite possible to determine, from the circumstances of the motion, the number (as well as the sign) of the ions thus entering the drop; and by calculating the electric charges for all the different upward journeys that were observed, it became evident that these various charges either showed no change, or differed from one another either by a certain constant quantity, or by a low multiple of that quantity. It was even found that the original charge of the droplet was also an apparently exact multiple of this same quantity. The doctrine that electrification is a discontinuous process, and that it consists in adding to a body (or subtracting from it) a certain number of small-sized yet finite and equal charges, or gelectrons,p thereby received an exceedingly striking and definite confirmatlon; and the data available made it quite easy to calculate the magnitude of this elementary unit charge. After several years of study and observation, cul minating in two years of work with a special apparatus constructed with exceeding care, the final conclusion was, that the charge on the electron is invariably e=4.774 X 10-2° abso lute electrostatic units; and Millikan believes (apparently with good grounds) that the un certainty in this result is not greater than the thousandth part of its own magnitude. (The corresponding value of the charge, in absolute electromagnetic units is 1.592 X 10—n.