Capacity and Eddy Current Effects in Inductometers

Abstract

A theoretical investigation has been made upon the effects of self and intercapacities and of eddy-current losses in mutual inductances for which the coils have a common point. The following are the main conclusions arrived at. Small capacities introduce variations in the effective self and mutual inductances which change as the square of the frequency. Corresponding variations are introduced in the effective resistances of the coils. The mutual inductance is no longer pure, that is, the secondary E.M.F. is no longer in exact quadrature with the primary current. The primary and secondary circuits behave as if they have a resistance in common which varies as the square of the frequency. Eddy currents produce impurity and changes of effective resistance which are initially of the same nature as those due to capacity. Eddy current and capacity effects may be so combined that the variations of mutual inductance and of impurity cancel simultaneously, so that it is theoretically possible to obtain a pure mutual inductance having no variation with frequency over the whole range of audio frequencies. The conditions of balance of three alternating-current bridges, viz.: - The Heaviside inductance bridge. The Carey-Foster bridge. A modified Campbell frequency bridge have been worked out on the basis that the mutual inductance is not pure and the mutual and self-inductances are not invariable with frequency. It is shown how combined observations on these bridges may be used to determine the frequency corrections of a given inductometer. An experimental study has been made upon an inductometer of the Campbell type. The results show that if such an inductometer is used at the higher audio-frequencies for the measurements of the effective resistance of coils or of condensers, the inductometer corrections may in certain cases be larger than the quantities measured. A method for automatically correcting an inductometer is suggested in order that it may be used for the measurement of power factors of condensers without having recourse to tables of corrections.