Wilson’s modification of Thomson’s work consisted in placing inside the chamber A two horizontal brass plates 3½ cm. in diameter and from 4 to 10 mm. apart and connecting to these plates the terminals of a 2,000-volt battery. He then formed a negative cloud by a sudden expansion of amount between 1.25 and 1.3, and observed first the rate of fall of the top surface of this cloud between the plates when no electrical field was on; then he repeated the expansion and observed the rate of fall of the cloud when the electrical field as well as gravity was driving the droplets downward. If represents the force of gravity acting on the droplets in the top surface of the cloud and the force of gravity plus the electrical force arising from the action of the field on the charge , and if is the velocity of fall under the action of gravity alone, and the velocity when both gravity and the electrical field are acting, then, if the ratio between the force acting and the [Pg 55] velocity produced is the same when the particle is charged as when it is uncharged, we have Combining this with the Stokes’s Law equation which runs in which is the radius, the density, the velocity of the drop under gravity , and is the viscosity of the air, and then eliminating in by means of Wilson obtained after substituting for and the appropriate values (not accurately known, it is true, for saturated air at the temperature existing immediately after the expansion), Wilson’s method constitutes a real advance in that it eliminates the necessity of making the very awkward assumption that the number of droplets is equal to the number of negative ions, for since he observes only the rate of fall of the top of the cloud, and since the more heavily charged droplets will be driven down more rapidly by the field than the less heavily charged ones, his actual measurements would always be made upon the least heavily charged droplets. All of the other difficulties and assumptions contained in either Townsend’s or Thomson’s experiments inhere also in Wilson’s, and in addition one fresh and rather serious assumption is introduced, namely, that the [Pg 56] clouds formed in successive expansions are identical as to size of droplets. For we wrote down the first equation of Wilson’s method as though the and were measurements made upon the same droplet, when as a matter of fact the measurements are actually made on wholly different droplets. I have myself found the duplication of cloud conditions in successive expansions a very uncertain matter. Furthermore, Wilson’s method assumes uniformity in the field between the plates, an assumption which might be quite wide of the truth.