S is the surface of a lump of magnetic material.

As we, all know that magnetic field lines for any magnetic substance, forms a continuous closed loops. Now as for the magnetic intensity, the magnetic intensity (H) for any magnet outside of its area is and for inside the magnet. The magnetic field is denoted by B and the permeability of the material is and relative permeability is denoted by.

Therefore, we can say that the magnetic field (B) is continuous across the lump of the magnetic material. On the other hand, the magnetic intensity (H) varies because of its dependence of the length thereby lines of H cannot be continuous across the lump S.