Impedance of a loop surrounding a conducting cylinder

The change in complex impedance between an ideal one-turn coil surrounding and coaxial with an infinitely long circular cylinder of conductivity δ and permeability μ and a similar coil without the core has been calculated. From the exact expression, a power series in the quantity δ/b (δ=skin depth; b=radius of core) has been developed. From this result, the change in impedance of a physically realistic multiturn coil can be estimated. The theory permits a rational approach to optimization of the design of eddy-current test coils and provides a possible means of detecting changes in the radius and conductivity of the cylinder.