In the present edition of this book I have endeavored to present a simple treatment of all the developments in physics to date which have caused a modification or extension of any of the viewpoints expressed just seven years ago. In its preparation I have been very much impressed to find how uniformly the changes represent additions rather than subtractions—a striking illustration of the great truth that science, like a plant, grows in the main by the process of accretion. If I have succeeded in interesting some old friends and making a few new ones for one of the most fascinating of subjects, I shall be content.

ROBERT ANDREWS MILLIKAN

NORMAN BRIDGE LABORATORY OF PHYSICS
CALIFORNIA INSTITUTE OF TECHNOLOGY
PASADENA, CALIFORNIA
MAY 18, 1924

CONTENTS

PAGE
INTRODUCTION [1]
CHAPTER
I.EARLY VIEWS OF ELECTRICITY[6]
II.THE EXTENSION OF THE ELECTROLYTIC LAWS TO CONDUCTION IN GASES[25]
III.EARLY ATTEMPTS AT THE DIRECT DETERMINATION OF [45]
IV.GENERAL PROOF OF THE ATOMIC NATURE OF ELECTRICITY[66]
V.THE EXACT EVALUATION OF [90]
VI.THE MECHANISM OF IONIZATION OF GASES BY X-RAYS AND RADIUM RAY[125]
VII.BROWNIAN MOVEMENTS IN GASES[145]
VIII.IS THE ELECTRON ITSELF DIVISIBLE?[158]
IX.THE STRUCTURE OF THE ATOM[182]
X.THE NATURE OF RADIANT ENERGY[232]
APPENDIX A. FROM MOBILITIES AND DIFFUSION COEFFICIENTS[262]
APPENDIX B.TOWNSEND’S FIRST ATTEMPT AT A DETERMINATION OF [265]
APPENDIX C.THE BROWNIAN-MOVEMENT EQUATION[268]
APPENDIX D.THE INERTIA OR MASS OF AN ELECTRICAL CHARGE ON A SPHERE OF RADIUS [272]
APPENDIX E.MOLECULAR CROSS-SECTION AND MEAN FREE PATH[275]
APPENDIX F.NUMBER OF FREE POSITIVE ELECTRONS IN THE NUCLEUS OR AN ATOM BY RUTHERFORD’S METHOD[277]
APPENDIX G.BOHR’S THEORETICAL DERIVATION OR THE VALUE OR THE RYDBERG CONSTANT[282]
APPENDIX H.A. H. COMPTON’S THEORETICAL DERIVATION OF THE CHANGE IN THE WAVE-LENGTH OR ETHER-WAVES BECAUSE OR SCATTERING BY FREE ELECTRON[284]
APPENDIX I.THE ELEMENTS, THEIR ATOMIC NUMBERS, ATOMIC WEIGHTS, AND CHEMICAL POSITION[286]
INDEXES. [287]

INTRODUCTION

Perhaps it is merely a coincidence that the man who first noticed that the rubbing of amber would induce in it a new and remarkable state now known as the state of electrification was also the man who first gave expression to the conviction that there must be some great unifying principle which links together all phenomena and is capable of making them rationally intelligible; that behind all the apparent variety and change of things there is some primordial element, out of which all things are made and the search for which must be the ultimate aim of all natural science. Yet if this be merely a coincidence, at any rate to Thales of Miletus must belong a double honor. For he first correctly conceived and correctly stated, as far back as 600 B.C., the spirit which has actually guided the development of physics in all ages, and he also first described, though in a crude and imperfect way, the very phenomenon the study of which has already linked together several of the erstwhile isolated departments of physics, such as radiant heat, light, magnetism, and electricity, and has very recently brought us nearer to the primordial element than we have ever been before.

Whether this perpetual effort to reduce the complexities of the world to simpler terms, and to build up the infinite variety of objects which present themselves to our senses out of different arrangements or motions of the least possible number of elementary substances, is a modern heritage from Greek thought, or whether it is a native instinct of the human mind may be left for the philosopher and the historian to determine. Certain it is, however, that the greatest of the Greeks aimed at nothing less than the complete banishment of caprice from nature and the ultimate reduction of all her processes to a rationally intelligible and unified system. And certain it is also that the periods of greatest progress in the history of physics have been the periods in which this effort has been most active and most successful.

Thus the first half of the nineteenth century is unquestionably a period of extraordinary fruitfulness. It is at the same time a period in which for the first time men, under Dalton’s lead, began to get direct, experimental, quantitative proof that the atomic world which the Greeks had bequeathed to us, the world of Leucippus and Democritus and Lucretius, consisting as it did of an infinite number and variety of atoms, was far more complex than it needed to be, and that by introducing the idea of molecules built up out of different combinations and groupings of atoms the number of necessary elements could be reduced to but about seventy. The importance of this step is borne witness to by the fact that out of it sprang in a very few years the whole science of modern chemistry.

And now this twentieth century, though but twenty-four years old, has already attempted to take a still bigger and more significant step. By superposing upon the molecular and the atomic worlds of the nineteenth century a third electronic world, it has sought to reduce the number of primordial elements to not more than two, namely, positive and negative electrical charges. Along with this effort has come the present period of most extraordinary development and fertility—a period in which new viewpoints and indeed wholly new phenomena follow one another so rapidly across the stage of physics that the actors themselves scarcely know what is happening—a period too in which the commercial and industrial world is adopting and adapting to its own uses with a rapidity hitherto altogether unparalleled the latest products of the laboratory of the physicist and the chemist. As a consequence, the results of yesterday’s researches, designed for no other purpose than to add a little more to our knowledge of the ultimate structure of matter, are today seized upon by the practical business world and made to multiply tenfold the effectiveness of the telephone or to extract six times as much light as was formerly obtained from a given amount of electric power.