Report of the National Research Council Committee on Electrodynamics of Moving Media* PART I By W. F. G. SWANN Professor of Physics, University of Minnesota PART II By JOHN T. TATE Professor of Physics, University of Minnesota PART III By H. BATEMAN Professor of Mathematics, California Institute of Technology PART IV By E. H. KENNARD Professor of Mathematics, Cornell University *This committee of the Division of Physical Sciences of the National Research Council consists of the following members: J. S. Ames, Professor of Physics, and Director, Physical Laboratory, Johns Hopkins University, Chairman; S. J. Barnett, H. Bateman, E. H. Kennard, C. M. Sparrow, W. F. G. Swann, J. T. Tate. 1 3+1 CONTENTS The laws of inverse squares and of conservation of charge. Case where electric field is the quantity fundamentally defined. Note 3. Concerning the significance of v in the circuital relations. Note 4. Concerning definitions in terms of the motion of an electron.. The ultimate form assumed by the circuital relations..... Relations between the above definition of H and that ordinarily Note 5. The role played by definition and by the force equation in relation to the invariance of the circuital relations under the transformation of the restricted theory of relativity.. 68 Part III-EQUATIONS FOR THE DESCRIPTION OF ELEC- Introductory remarks on electricity, space and time. PART I THE FUNDAMENTALS OF ELECTRODYNAMICS BY W. F. G. SWANN INTRODUCTION The present report purposes to make a critical survey of some of the more familiar aspects of the subject with a view of inquiring as to what parts may be considered as relying upon experiment, or upon deduction from fundamental principles, and what parts are mere definition or scaffolding, convenient for the correlation of the experimental laws but in no sense calling for experimental proof themselves. Any individual experiment usually covers a very limited region of the whole field of investigation of which it forms a part. It is a representative of a much larger class, from which it is chosen on the grounds of practicability. In citing an experimental foundation in any particular case we shall not therefore confine ourselves to individual experiments which have been performed, but shall speak rather of general classes of experiment of which any particular experiment will form a special case. One method of procedure would be to take the complete scheme of equations as formulated by Lorentz, and discuss everything in terms of these. To do this, however, would be to clothe the subject with considerable artificiality as regards certain important fields of its application. The attitude which one adopts toward the subject, his way of thinking about it, even the method of expression of its laws is to some extent bound up with the degree of generality which he has in mind for its application. Thus, for example, there is a perfectly consistent scheme of hypotheses applicable to the case where one confines himself to unvarying currents in closed wire circuits; and while this subject may be treated as part of the general case of electronic motion, it may as far as its own requirements are concerned, be discussed in different and somewhat simpler language. The field of application of this special case is moreover so great that it constitutes as it were a little subject in itself, and it is therefore convenient to discuss what would be its laws from the standpoint of one who never had occasion to deal with more general cases. Following out this plan, we shall consider the whole subject in four stages applicable respectively to the following cases. 1. Electrostatics. 2. Steady currents in closed circuits. 3. Varying currents in closed circuits. 4. The general case as symbolized by the Lorentzian equations for free aether and their adaptation to a material medium. Certain matters of detail will be relegated to notes in an appendix to this report. 5 |