Magnetic Shielding with Multiple Cylindrical Shells

The increased effectiveness of multiple over single shields is shown and formulas (including a recursion formula suitable for automatic computation) are given for the computation of the shielding ratio (field outside/field inside) for any number of concentric shells of contemporary high-permeability materials in any combination. Sample designs are given for shields 2¼, 3⅛, and 5¼ in. i.d. Scaling rules and charts are given for finding other designs without calculation. Graphs are given for the penetration of the field into the open end of the shield. The alternating-field case is treated and formulas are given for finding the shielding ratio for alternating fields.