THE INTERFERENCE OF ELECTRONS TLdited by P. DEBYE Professor of Experimental Physics and Director of the Physical Institute at the University of Leipzig Aut iwrixed Translation by WINIFRED M. DEANS, M. A., B. Sc. BLACKIE SON LIMITED LONDON AND GLASGOW Printed in Great Britain by Blackie Sf Son, Ltd., Glasgow PREFACIj TO GERMAN EDITION In the summer of 1930 it was for the third time found possible to hold a conference in Leipzig, thanks to the official suppont which was again forthcoming. The subject of this years conference was the interference of electrons. The publishing firm of S. Hirzel was again kind enough to undertake the publication of th6 lectured in book form. It was thought desirable that the lectures should not merely describe experiments concerned solely with the diffraction and in terference of electrons, but should consider in detail how far wave mechanics can contribute to the explanation of the electronic con ductivity of metals. The first four lectures were accordingly given by Rupp, Wierl, Mark, and Mott, the series being concluded with lectures by Grlineisen, Bloch, and Peierls. Rupp deals not 50 much with the diffraction of electrons by crystals as with the special phenomena which are observed with slow electrons. He was thus led to give an account of the discontinuity of potential at the sur face of the diffracting substance this gave rise to an interesting discussion on the structure of the surface field, in which von Laue, Bethe, and Joffe took part. Here, as always, the lecturers subse quently incorporated the results of discussion in their papers. Wierl gives an account of his experiments with free molecules, which, apart from their importance in connexion with the measurement of intramolecular distances, make it necessary just as in the case of X-rays to consider the behaviour of the atomic scattering factor for electrons. What theory has to say on this last problem is dis cussed by Mott in the fourth paper he shows that no contradiction arises and that the properties of the atom form factor are actually reproduced by the theory, for both X-rays and electron beams. Previous to this, however, comes Marks paper, in which he de scribes the refinements which, as is well known, have to be made in the simple classical theory of crystal diffraction if the interference of secondary waves is to be taken into account. He considers ki vi PREFACE TO GERMAN EDITION detail what results capable of experimental verification it is possible to deduce from the Darwin-Ewald dispersion theory of X-rays and Bethes dispersion theory of electron beams. The first of the three reports on the behaviour of electrons in metals is a paper by Grlineisen, which is chiefly concerned with a comparison of the experimental results on the electrical and thermal conductivities with the results of the modern interference theory. There is no doubt that Sommerfelds striking advance in this direc tion, together with the work which has resulted from it, has very notably promoted our understanding of the phenomena exhibited by metals. It is equally certain, however, that there still remains a whole series of problems raised by experiment, towards the solution of which theory has as yet contributed the merest hints or nothing at all. Accordingly, Blochs paper deals with the mutual action of the electrons, which is usually neglected entirely, although this practice is not justifiable without further examination. Peierls gives an account of the changes in conductivity brought about by a mag netic field, a subject in which our knowledge has been so greatly advanced by the experiments of Kapitza and of Meissner and SchefTers. Here again it is made clear that we are only at the be ginnings of a theory, which, however, is extremely valuable even in its present form. In these papers the reader will find many suggestions for ex perimental or theoretical work. For this reason I consider their publication in book form amply justified...