The Meissner Effect

• As shown previously, a small magnet can levitate above a superconductor below its critical temperature ($T_c$)
• The reason for this is a phenomenon known as the Meissner effect:
• When a superconductor below $T_c$ is placed within an external magnetic field, it produces its own magnetic field to cancel out the external one.

• The phenomenon can be explained as follows¹:
  • If a magnet is brought near a superconductor, the change in flux induces a surface eddy current within the superconductor. (Faraday's Law)
  • The direction of this current is such that it produces a magnetic field that opposes the change in flux that caused it. (Lenz's Law)
  • Thus the induced magnetic field repels the external magnetic field.

• This repulsion balances out the weight of the magnet and causes the magnet to levitate.
• It does not occur in normal conductors as the resistance in the conductor quickly causes the current to disappear.
• Since there's not resistance in superconductors, the magnet will levitate unless disturbed.

Footnotes

1. This is the HSC explanation. The real version is much more complex