MOLECULAR MAGNETISM BY W. PEDDIE, D. Sc., F. R. S. E. . HARRIS PROFESSOR OF PHYSICS, UNIVERSITY COLLEGE, DUNDEE UNIVERSITY OF ST. ANDREWS I believe that we have there in Webers representation the explanation of magnetism in solids. It is interesting to see all these things come out as they do. KELVIN LONDON EDWARD ARNOLD CO. 1929 All Rights Reserved TO MY WIFE iELPER AND COMRADE NOW AS EVER And this word, Yet once more, signifyeth the removing of those things that are shaken, as of things that are made, that those things which cannot be shaken may icmain. If the vision tarry, wait for it, for it will come and it will not be late. Made and Printed in Great Britain by UNW1N BROTHERS LIMITED, LONDON AND WOKING PREFACE THE foundations of every one of the great branches of Natural Philosophy have been probed beyond the merely phenomenal or molar stage of their description. They are now treated, in large part at least, in the light of molecular or even sub-atomic ideas. Upon every one of them has been cast the illumination of the two great speculative modern expansions of the older physical theory. And, in turn, it lies with them, by appeal to the decision of experiment to discriminate amongst the various forms which these speculations may assume, and to lead to the recognition of their underlying compulsions, and so to the elimination of what is arbitrary in their formulations. Equally with all the other branches, the subject of Magnetism has shared in the general advance and the attention of investigators, theoretical and practical alike, is now, and is increasingly, turned upon it. Optics and Chemjptry in particular wait upon its developments. But, whereas all the other subjects have their representa tion amongst the honours courses in Universities, and their aids to study amongst the series of textbooks, this subject has lain hitherto outside these courses, possibly for the reason that no textbook has yet appeared. Indeed, until the recent publication of Stoners work, Magnetism and Atomic Structure, in Britain, and of Le Magnetisme in France, original papers were largely the only source of information available to the student. It is not the aim of this book to occupy also the ground taken by these works. It is rather to furnish a textbook which may, on the one hand, serve as an accompaniment to a short University course of lectures on the subject and, on the other, be of use as an introduction to these more detailed treatments. A very large bibliography, given by Stoner, will be advantageous to the student who desires to proceed further. And I trust that, with whatever aid or stimulus may be found vi MOLECULAR MAGNETISM herein, some students may be induced to adopt the subject as one in which they will become original investigators. My thanks are specially due to members of my staff for much voluntary aid to Dr. Forrest for reading all the chapters in manuscript, and for frequent helpful discussions to Dr. Winifred Smith for careful proof reading and drawing of all the diagrams and to Mr. Raymond Smart, M. A., for verification of formulae. I have also to thank Mr. G. N. Hunter, B. Sc., for carrying out troublesome calculations, and plotting several curves by their aid. W. PEDDIE. July 17, 1929- CONTENTS CHAPTER I PAGE THE ELEMENTARY PHENOMENA OF MAGNETISM AND i THE MAGNETIC FIELD i. Magnetism and Electricity. 2. Electric and Magnetic Quantity. 3. The Characteristics of Electric and Magnetic Forces and Fields of Force. 4. Electric and Magnetic Induction. 5. The Electric Origin of a Magnetic Field. 6. Equivalent Magnets. 7. Equivalent Electric Circuits. 8. The Equivalent Magnetic Moment. 9. The Magnetic Potential and the Force Components. 10. Diamagnetism and Paramagnetism. n. Ferromagnetism. 12. Susceptibility Per meability. CHAPTER II THE FOUNDATION OF THE MOLECULAR THEORY OF MAGNETISM AND DIAMAGNETISM . . . . 16 13. The Non-Molecular Theory. 14. Webers Foundation of the Molecular Theory. 15...
