London field

Inside a superconductor, the valence electrons are free to move -- so free, in fact, that in the aggregate they behave like a superfluid, not subject to friction. As a consequence, during rotation of the superconductor around an axis of symmetry, the electrons stay still while the positively charged atoms move. By Ampère's law (moving charges create a magnetic field) a magnetic field results. This magnetic field is called a London field.