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When a particular object, for example a sphere, shall be protected not against em or magnetic fields in general, only against a specific static, quasistatic or low frequency magnetic field coming from outside the object, can this magnetic field be hindered from penetrating into the sphere through a conductive shield; through wrapping the whole sphere with a current-carrying copper cable (fed by a 9 or 4,5 v-battery)? Are then shielding eddy currents created?

That the shall be shielding currents create magnetic fields themselves and can penetrate into the sphere, shall play no role, because my shielding problem is not about em interference.

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  • $\begingroup$ I'm having trouble understanding the question, especially the start of the second paragraph. Could this be reworded a bit? $\endgroup$
    – Nat
    Commented May 17, 2018 at 22:21

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In order to weaken an outside magnetic field, the field produced by the current in the cable (or protecting coil) has to have similar magnitude and opposite direction everywhere in the space of interest. By optimizing the shape of the protecting coil and adjusting its current you may be able to cancel some of the outside magnetic field in some areas.

This method would not work for a variable magnetic field, since, to cancel such field, the current in the protecting coil would have to change in sync with the changes of the magnetic field, which would not be possible if your current is produced by a battery (since it would be a constant current, which would generate a constant magnetic field).

An alternative shielding for constant or slow changing magnetic fields could be achieved by encasing your protected space in a shell made out of a material with high magnetic permeability. Instead of generating a cancelling magnetic field, such shell just redirects magnetic lines of the outside field away from the protected space.

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