By Hamish Johnston
The above video shows a very nice demonstration of flux pinning in a superconductor. This effect occurs in high-temperature superconductors, which when exposed to a magnetic field will allow some magnetic field lines to penetrate their bulk. This is unlike most conventional superconductors, such as lead, which expel all magnetic field lines.
The field lines inside the superconductor don’t like to move around, which pins the magnetic field in place. The result is that the magnet and the superconductor don’t want to move relative to each other, which is demonstrated in the video.
Exceptions occur when there is a degree of symmetry in the magnet field. This is illustrated nicely by showing that a superconductor will rotate on a magnetic disk but not on a rectangular-shaped magnet. Even better is when the superconductor is placed above – and then below – a magnetic track and given a shove.