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<p>A very thorough examination is made of the various formulae which have been proposed from time to time for calculating the mutual inductance of two circular coils of rectangular section. Numerous numerical calculations are made by the various formulae. It is pointed out which are the most accurate and where some of them fail. New formulae are deduced which are more convenient in special cases. Curves are drawn showing the relative accuracy of various formulae for particular coils at varying distances. Weinstein's, Nagaoka's (using Jacobi's /b q/-series) and Coffin's formulae for circles are also given and numerical examples worked out. Rowland's and Lyle's formulae agree almost exactly, both being based on the same%
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<br>Subjects: electromagnetic induction; Electrical instruments and techniques</div>
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