There is a conservation law for superconductors " the flux through a superconducting ring cannot change". This can be shown using Faraday's law: - E.M.F=-d/dt (BA) where B is the measured magnetic field passing through the ring and A is the area enclosed by the ring therefore for a ring -d/dt(BA)= iR+Ldi/dt where L is the inductance of the ring and i is the current flowing through the ring since R=0 in a superconductor, integrating we have BA+Li = a constant. So the flux through a superconducting ring can't change. In the above figure we have some superconducting material in its superconducting state. Magnetic flux from a permanent magnet or a coil is directed through the ring R. As the section P is rotated the flux through R is increased because the integration path wraps around R. What I don't understand is how the conservation law for magnetic flux applies to this diagram.