On the condition ‘h normal to the wall’ in magnetic field problems

In finite element computations of magnetic fields, the boundary condition n × h = 0 is usually enforced on walls made of a material ofgreat permeability. We purport here to question the validity of this approximation. It is shown how this condition stems from a suitable passage to the limit (when the ratio of permeabilities μ 0 /μ tends to zero), provided a condition of topological nature (‘no loops’) is fulfilled. What to do when this condition does not hold is discussed.