Flux trapping and magnetisation of hollow superconducting cylinders

The magnetisation of hollow cylinders of high T_c materials and the field trapped inside them have been measured by integrating the signal from coils outside and inside the cylinders. The two coils allow the field trapped in the grains themselves to be separated from the field due to the circulating currents in the bulk material. The trapped field indicates the maximum field that can be expected in a magnet. For practical purposes the most important parameter is the field at which J_c drops to half its zero-field value. Cylinders have been made from yttrium-barium-copper oxide and from bismuth-strontium-calcium-copper oxide. Various preparation methods have been used