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 [Pg 162] 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 were made on different particles. So far as concerns , these measurements, as everyone agrees, were essentially cloud measurements like Wilson’s. [Pg 163]