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)
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Electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. Shielding is typically applied to enclosures to isolate electrical devices from the 'outside world', and to cables to isolate wires from the environment through which the cable runs. Electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as RF shielding.
The shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. A conductive enclosure used to block electrostatic fields is also known as a Faraday cage. The amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic field.