The magnetic circuit model acts as a unifying principle in descriptive magnetostatics, and as an approximate computational aid in electrical machine design. It was the subject of repeated rediscoveries through the period 1855 to 1886, taking different forms and being provided with different justifications but all motivated by the mathematical difficulty of existing magnetic theory. The process culminated in several competitively-slanted announcements of the principle made during 1884 to 1886, arising in connection with the already comparatively efficient designs of contemporary (...) dynamos which made its application plausible. The preferred conceptual imagery, of induction flow against a ‘resistance’, originated in the magnetic theories of Faraday, though these underwent considerable changes before adoption. (shrink)