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Equipment sometimes requires isolation from external magnetic fields. For static or slowly varying magnetic fields (below about 100 kHz) Faraday shielding is ineffective. In these cases shields made of high magnetic permeability metal alloys can be used, such as sheets of Permalloy and Mu-Metal or with highly conductive metals. These materials don’t block the magnetic field, as with electric shielding, but rather draw the field into themselves, providing a path for the magnetic field lines around the shielded volume. The best shape for magnetic shields is thus a closed container surrounding the shielded volume. The effectiveness of this type of shielding depends on a material’s permeability (in the case of steel), which generally drops off at both very low magnetic field strengths and at high field strengths where the material becomes saturated and a materials conductivity (in the case of aluminum and copper). So to achieve low residual fields, magnetic shields often consist of several enclosures one inside the other, each of which successively reduces the field inside it.
Because of the above limitations of passive shielding, an alternative used with static or low-frequency fields is active shielding; using a field created by electromagnets to cancel the ambient field within a volume. Solenoids and Helmholtz coils are types of coils that can be used for this purpose. (Source: WikiPedia – edited)