= Exterior magnetic field of a wire with an eccentric cylindrical cavity
A uniform <electric current density> outside a parallel cylindrical cavity can be constructed by the <superposition principle>: add a full cylinder carrying that density and a displaced cylinder carrying its negative. At an exterior point, each contribution is the external <magnetic flux density> of its own net <electric current>. The combined <magnetic flux density> generally lacks <rotational symmetry> even though <Ampère's law> fixes its <line integral> around the whole wire.
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