Electromagnetic Forces on a Superconductor

Measurements of the eddy current damping of the rotational oscillations of a tin sphere suspended in a magnetic field from a torsion fiber give the expected results as a function of the magnetic field and the conductivity as long as the tin is a normal conductor. Below the transition temperature, however, the damping torque becomes less than ${10}^{-5\mathrm{}}$ times its value in the normal state and no changes in period are observed except those attributable to residual frozen-in moments.