-particle has been directly measured and that it comes out, within the limits of error of the measurement, at exactly two electrons—as it should according to the evidence furnished by
measurements on the
-particles.
II. THE EVIDENCE FOR THE EXISTENCE OF A SUB-ELECTRON
Now, the foregoing contention has actually been made, and evidence has been presented which purports to show that electric charges exist which are much smaller than the electron. Since this raises what may properly be called the most fundamental question of modern physics, the evidence needs very careful consideration. This evidence can best be appreciated through a brief historical review of its origin.
The first measurements on the mobilities in electric fields of swarms of charged particles of microscopically visible sizes were made by H. A. Wilson[99] in 1903, as detailed in [chap. III]. These measurements were repeated with modifications by other observers, including ourselves, during the years immediately following. De Broglie’s modification, published in 1908,[100] consisted in sucking the metallic clouds discovered by Hemsalech and De Watteville,[101] produced by sparks or arcs between metal electrodes, into the focal plane of an ultra-microscope and observing the motions of the individual particles in this cloud in a horizontal electrical field produced by applying a potential difference to two vertical parallel plates in front of the objective of his microscope. In this paper De Broglie first commented upon the fact that some of these particles were charged positively, some negatively, and some not at all, and upon the further fact that holding radium near the chamber caused changes in the charges of the particles. He promised quantitative measurements of the charges themselves. One year later he fulfilled the promise,[102] and at practically the same time Dr. Ehrenhaft[103] published similar measurements made with precisely the arrangement described by De Broglie a year before. Both men, as Dr. Ehrenhaft clearly pointed out,[104] while observing individual particles, obtained only a mean charge, since the different measurements entering into the evaluation of
