THE UNIVERSITY OF CHICAGO SCIENCE SERIES Editorial Committee ELIAKIM HASTINGS MOORE, Chairman JOHN MERLE COULTER PRESTON KYES THE UNIVERSITY OF CHICAGO SCIENCE SERIES, established by the Trustees of the University, owes its origin to a belief that there should be a medium of publication occupying a position between the technical journals with their short articles and the elaborate treatises which attempt to cover several or all aspects of a wide field. The volumes of the series will differ from the discussions generally appearing in technical journals in that they will present the complete results of an experiment or series of investigations which previously have appeared only in scattered articles, if published at all. On the other hand, they will differ from detailed treatises by confining themselves to specific problems of current interest, and in presenting the subject in as summary a manner and with as little technical detail as is consistent with sound method. They will be written not only for the specialist but for the educated layman. THE ELECTRON THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS THE BAKER & TAYLOR COMPANY NEW YORK THE CAMBRIDGE UNIVERSITY PRESS LONDON THE MARUZEN-KABUSHIKI-KAISHA TOKYO, OSAKA, KYOTO, FUKUOKA, SENDAI THE MISSION BOOK COMPANY SHANGHAI THE ELECTRON ITS ISOLATION AND MEASUREMENT AND THE DETERMINATION OF SOME OF ITS PROPERTIES BY Robert Andrews Millikan Formerly Professor of Physics, the University of Chicago Director Norman Bridge Laboratory of Physics California Institute of Technology THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS COPYRIGHT 1917 BY THE UNIVERSITY OF CHICAGO All Rights Reserved Published August 1917 Second Impression February 1918 Third Impression November 1918 Fourth Impression December 1910 Fifth Impression February 1921 Second Edition October 1924 Composed and Printed By The University of Chicago Press Chicago, Illinois, U.S.A. TO ALBERT A. MICHELSON AND MARTIN A. RYERSON THIS SMALL OUTGROWTH OF THEIR INSPIRATION AND GENEROSITY IS RESPECTFULLY DEDICATED PREFACE It is hoped that this volume may be of some interest both to the physicist and to the reader of somewhat less technical training. It has been thought desirable for the sake of both classes of readers, not to break the thread of the discussion in the body of the book with the detailed analyses which the careful student demands. It is for this reason that all mathematical proofs have been thrown into appendixes. If, in spite of this, the general student finds certain chapters, such as VII and VIII, unintelligible, it is hoped that without them he may yet gain some idea of certain phases at least of the progress of modern physics. R. A. MILLIKAN May 18, 1917 PREFACE TO THE SECOND EDITION