Millikan obtained his oil drop by perforating the upper of the two metallic plates by means of a minute pinhole, and then sending a fine spray of the oil into the space above the plate, by blowing a puff of air through an atomizer. In the course of time one of the droplets of the spray would fall through the pinhole into the region between the plates, and the experi ment could be started. The friction to which the oil was subjected in the atomizer electrified the droplets of spray positively, and, as has been stated above, the charge communicated to the droplet in this way was always found to be an exact multiple of the value given above. This fact is highly interesting, because here we have, for the first time, direct evidence that an electric charge communicated to a body by friction consists in an excess or deficit of a definite, finite number of electrons. In one ex periment, for example, the positive charge com municated to the droplet by the initial friction of the atomizer was found to correspond to a loss (or deficiency) of nine negative electrons.
Millikan varied his drop-experiments in many ways, using numerous substances (in cluding mercury) for the drops, and experi with drops of widely different sizes, and with various gases between his electrified plates; and he concludes that "the apparent value of the electron is not in general a func tion of the gas in which the particle falls, of the materials used, or of the radius of the drop on which it is caught.° In other words, he strikingly confirmed the theory that the negative corpuscle has an actual, physical existence,, apart from the existence of the kinds of matter heretofore contemplated by the chemist.
The determination of the mass m of a free, slowly-moving negative corpuscle is an easy matter after and e have been separately determined; for we have the simple relation t tn, With Millikan's value of e and no Bucherer's value (both expressed in terms of absolute electrostatic units) we have ps==(4.774 X + (5.299 X 11r)1901 X grammes.
(It may be shown, from this, that it would require 1,845 slowly-moving negative corpuscles, to have a combined mass equal to the mass of one hydrogen atom.) We do not yet know the shape of the negative corpuscle, nor do we positively know that the word has any definite mean ing when applied to it. Larmor, for purposes of discussion, assumed the corpuscle to be a mathematical point endowed with a finite charge of electricity, which creates a certain type of strain in the surrounding ether; but the pre vailing conception (in which Larmor would doubtless concur) is that the actual, physical corpuscle has some kind of spatial extension, though it may not have definite boundaries.
Nicholson, in a paper read before the Physical Society of London in October 1917, suggested that the corpuscle is a region of strain in the ether, the strain being intense in the immediate vicinity of a certain central point, and diminish ing with extreme rapidity as we pass away from that point. According to this view the corpuscle would have no definite boundaries, and therefore (in a strict sense) no definite shape, though on account of the intense localiza tion of the region in which the strain is really significant, we might treat the corpuscle for most purposes almost as though it were a mathematical point. If we desited to assign a ((radius)) to such a corpuscle, we should have to define the radius arbitrarily, either as ex tending to a region where the strain is some definite fraction of the maximum central strain, or in some other way.
In the absence of data concerning the shape of the negative corpuscle, it is natural to try, first, the simplest assumption we can make with regard to it and to see how well this fits such facts as we have. The simplest shape, from a mathematical standpoint, is a sphere ; and we find that the three best-known theories as to the shape of the negative corpuscle assume it to be spherical, at all events when it is at rest.
(1) Abraham considers the corpuscle to be rigid and spherical at all times, whether it is moving rapidly or at rest.
(2) Lorentz considers it to be spherical when at rest, but assumes that when it moves it be comes transformed into an ellipsoid of revolu tion With its equatorial radius unchanged, but with its polar radius (which is parallel to the direction of the motion) shortened to r 1—x° where r is the original radius and x is the ratio that the speed of the corpuscle bears to the speed of light.
(3) Bucherer and Langevin also consider the corpuscle to be spherical when at rest and assume that when it is in motion it takes the form of an ellipsoid of revolution with its polar radius shortened and directed parallel to the motion; but they assume that the polar radius becomes and that the equatorial radii are increased in consequence of the mo tion, so that each becomes equal to r(1—?)--i. where r and x have the same significance as before. (It is to be observed that these rela tions of Bucherer. and Langevin leave the vol ume of the corpuscle unchanged, whatever the speed may be).