  The electromagnetic field solution for a spherical invisibility cloak with an
active device inside is established. Extraordinary electric and magnetic
surface voltages are induced at the inner boundary of a spherical cloak, which
prevent electromagnetic waves from going out. The phase and handness of
polarized waves obliquely incident on such boundaries is kept in the reflected
waves. The surface voltages due to an electric dipole inside the concealed
region are found equal to the auxiliary scalar potentials at the inner
boundary, which consequently gain physical counterparts in this case.
