A method is given for obtaining the equivalent circuit of a set of concentric circular cylindrical conducting tubes within an alternating axial magnetic field. First, a general form of field solution is obtained using transfer matrices. A variable transformation is then made which makes it possible to derive an equivalent circuit for each annular region in the device. By joining the equivalent circuits in cascade, an equivalent circuit for the complete device is obtained. The applications of the theory include: electromagnetic shielding, flaw detectors and eddy-current heating. By using an equivalent circuit instead of the analytical form of field solution, much effort can be saved in both analysis and calculation. Experimental results are shown for shielding and impedance measurement.