A study of mechanical forces on Type I superconductors in a magnetic field
Smith, David Robert, 1939-
Houston, William V.
Master of Arts
An investigation of the mechanical forces on superconducting strips of indium about 10-1 cm in thickness and strips of vacuum deposited tin about 8 x 10°5 cm thick due to their passage through magnetic fields has been made with a torsion pendulum. At 4.2°K the thin films showed no observable eddy current damping, while the indium strips showed damping which increased with the magnetic field intensity. At temperatures corresponding to critical fields of about 50 gauss both materials showed damping which increased with the magnetic field intensity. This damping is clearly not due to eddy currents in the normal regions in the case of the tin films, since no eddy current damping was observed at 4.2°K. It is believed to be associated with the motion of normal superconducting boundaries and to the motion of flux lines against a resistive pinning force. An analysis of the data, assuming the dissipative force to be made up of a constant force plus a force proportional to the first power of the velocity with which the samples pass through the field regions, indicates that the constant force is about the same for the films and the strips for corresponding degrees of flux penetration. There was also evidence of a conservative force which tended to expel the samples from the field regions and produced a flux penetration barrier opposing the entrance of the samples into the field regions.