All of the material (that is below the critical temperature, critical current, and has no magnetic field penetrating it). Which is to say not all of the materi

All of the material (that is below the critical temperature, critical current, and has no magnetic field penetrating it). Which is to say not all of the material. It is common to have pockets of non-superconducting material in superconductors. If they get too big in the wrong places then you quench. Applied magnetic fields are common because, unsurprisingly, superconductors often find themselves used to make large magnetic fields without having to pay the electric company or cooling ohmic losses. Some applications can get by with just one magnet. Many others cannot.