Theory of a single-layer, bifilar, absolute standard of mutual inductance
C. Snow · Journal of research of the National Bureau of Standards · 1940
A formula is derived for the mutual inductance of two h elical wires wound bifilarly upon the same cylindrical form. This aims at a precision sufficient for use in absolute electrical measurements, so that terms are retained which, compared with the principal term, are r elatively small quantities of the order of the square of the ratio of pitch to cylindrical radius. Nominally the two helical windings consist of wires of equal diameters, equally spaced, with equal cylindrical radii, but allowance is made for the case where these three equalities are not quite attained. The axial lengths are assumed to be equal, so that, since their pitch must be the same, the total number of turns is the same. The latter is considered to be integral, which permits of the two return currents being carried each by a straight wire parallel to a generator of the cylindrical form and close to it. The effect of these lead wires is included in the formula. The azimuth angle between these generators is assumed to lie between -7r and 7r, so that the point of current entrance for the primary is on a turn adjacent to that of the secondary. Allowance is made for a general type of current distribution.