Impedance of Magnetic Coaxial Pipe

It was found that a coaxial pipe consisting of two ferromagnetic pipes exhibits interesting properties, as the skin effect is considerable even at low frequencies when current is passed in the pipes. Among other results, it was found that the impedances of the inner and outer pipes Zin and Zout were considerably different, and that there exists a current frequency at which Zout is minimum. A qualitative vector diagram and quantitative formulae are presented relating the impedances to the current densities and magnetic fluxes in the pipes.