Letters to the editor

One who has occasion to read widely in electromagnetic theory as concerns its application in electrical-machine design cannot help but note how little, comparatively speaking, has been done in connection with field-mapping problems in regions containing two or more materials of different permeabilities. Among the major contributors to this field have been G. F. C. Searle, W. Rogowski, T. Lehmann, E. Roth, and B. Hague. The subject is summarized in the latter´s unique and important “Electromagnetic Problems in Electrical Engineering” which is, practically speaking, the only reference book in this branch of electrical engineering. For this very reason errors in this text are of special concern. It is the purpose of this letter to point out one such, rendered the more important by the fact that this incorrect formula has been used by others.