Magnetic Properties of a Hollow Superconducting Lead Sphere

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Abstract
The magnetic hysteresis of a hollow superconducting lead sphere at 4.2°K has been studied by measuring the magnetic field distributions along the equatorial plane with bismuth probes. After cooling in the absence of a magnetic field, the hollow sphere was found to be a perfect magnetic shield in the superconducting state. The magnetic field penetrated into the interior of the sphere when the magnetic field at the surface of the sphere exceeded the threshhold value. Frozen-in fields were observed upon demagnetizing the sphere from the normal state. Upon reversing the applied field, the frozen-in field could be made to vanish, so that the sphere again became a perfect magnetic shield. The magnitude of the applied field necessary to extinguish the frozen-in field was found to be the same independent of the path of approach. The internal frozen-in fields did not change over specified ranges of the applied field, so that the existence of other equilibrium superconducting states are indicated for the hollow sphere. In the intermediate and frozen-in states, 10 to 30 minutes were required for the magnetic field to reach an equilibrium value.

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