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 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.
Although the elimination of the assumption of equality of the number of droplets and the number of ions makes Wilson’s determination of
more reliable as to method than its predecessors, the accuracy actually attained was not great, as can best be seen from his own final summary of results. He made eleven different determinations which varied from
