The properties of mutual inductance standards at telephonic frequencies

It is well known that when a mutual inductance carries alternating current, the value of the effective mutual inductance varies with the frequency, and further that the secondary P.D. is not exactly in quadrature with the primary current. Thus certain frequency corrections are introduced into all alternating-current bridges in which a mutual inductance standard is used. An experimental investigation has been made of the actual magnitude of such corrections for mutual inductance standards of three types: - A stranded wire fixed standard, with spaced winding to minimise capacity effects. The Campbell variable mutual inductance standard. The Tinsley variable mutual inductance standard made in accordance with Butterworth's recommendations. Values of the corrections are given for all these instruments under various conditions. It is shown that whereas the variation of mutual inductance with frequency is mainly due to capacity effects, dielectric losses in the insulation, or alternating current conductance, play an important part in the determination of the phase defect, the impurity being found to be proportional to a power of the frequency higher than the second, over the telephonic range of frequency.