In a perfectly conducting body the lines of magnetic force move with the material; the total number crossing the surface in either direction is therefore constant, and cannot be changed by motions of the material forming the body. Both in terrestrial magnetism and in sunspots, large changes in the external field are observed which do involve a change in the total number of lines and cannot be explained by a mere crowding together of lines of force that already cross the surface. In these phenomena loops of field must emerge through the surface, and the theory must involve the finite conductivity of the material.