= Solution
A closed conducting enclosure redistributes surface charge so that an empty interior is protected from an external static <electric field>. Grounding fixes a reference and drains deposited charge, but an ideal isolated closed conductor also provides inward <electrostatic shielding>. Real seams, apertures and cables need careful bonding, shielding and feedthrough filtering. For time-varying fields the relevant <wavelength>, conductivity, geometry and penetration depth matter; a static argument alone is insufficient. Differential wiring and small pickup-loop areas can complement shielding, while careless multiple ground connections create unwanted current loops.
Ordinary copper or aluminium does not generally exclude a static <magnetic field>. High-permeability shells instead guide magnetic flux around the protected volume. Their effectiveness depends on shape, continuity, openings and saturation; magnetic history and mechanical stress can worsen performance. Nested shields and degaussing can reduce residual fields. Superconducting shielding can exploit the <Meissner effect>, but trapped flux, openings and critical-field limits require attention.
At nonzero <frequency>, <eddy currents> in conducting shields oppose changing magnetic flux. The <skin depth> is
$$
\boxed{\delta=\sqrt{\frac{2}{\mu\sigma\omega}}}
$$
for a good conductor in the usual diffusion approximation. Thickness of several <skin depths> suppresses penetration, but the very-low-frequency limit is weak for a normal metal. High-permeability and conducting layers can therefore complement each other. Active compensation coils are an additional method, not passive shielding. Test performance by applying known fields along relevant axes and measuring the residual field or induced signal over the apparatus <bandwidth>; room-temperature shielding materials and electronics can themselves add noise.
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