The effective inductance and resistance of screened coils

In this paper an effort has been made to determine simple and workable formulae for the changes produced in the characteristics of an air-core coil by placing it in a screening container. The formulae, which have been derived empirically, are based upon the simplified case of a solenoidal coil of infinite length, placed coaxially in an infinitely long cylindrical screen. Another simplification, namely that of regarding the coil-and-screen system as a fiat band of wires placed parallel to a conducting plane, is also mentioned, and from these two simple cases it has been possible to proceed, by application of the results of measurement, to formulae for inductance and resistance change so simple that they almost amount to workshop rules. These formulae are shown to cover all practical cases of finite coils and coaxial screens to an accuracy within 2%. The effects of proximity of ends, small deformations and displacements, and temperature changes, are also examined and estimated. The shape of the curve of eddy-current distribution in the screen is examined, and it is found that the shape of curve derived from search-coil measurements is very similar to that derived from calculation on the basis of regarding the coil as a flat system.