Meissner Effect

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff

Categories: Magnetism, Liquid Nitrogen

Alternative titles: Magnetic Field Expulsion, Floating Magnet

Summary

When cooled below its critical temperature using liquid nitrogen, a superconductor expels magnetic fields and becomes a perfect diamagnet. This phenomenon, known as the Meissner effect, allows a magnet to levitate above the superconductor until it warms above its transition temperature.

Procedure

Place a sample of yttrium-barium-copper-oxide (YBCO) superconductor in a container.
Pour liquid nitrogen over the sample until it is fully cooled below its critical temperature.
Hold a small permanent magnet above the superconductor.
Observe the magnet levitate as the superconductor expels the magnetic field.
Allow the liquid nitrogen to evaporate; when the superconductor warms, the magnet will gradually lose levitation and fall.

Links

Superconductor Meissner effect - pl4nb33:

Meissner Effect - Physlab.org:

📄 Meissner Effect - PhysLab: https://physlab.org/class-demo/meissner-effect/

Variations

Use different shapes or sizes of magnets to see how the levitation changes.
Demonstrate flux pinning by slightly displacing the magnet and observing how it locks into position.
Compare the effect using different superconducting materials.

Safety Precautions

Wear cryogenic gloves and face protection when handling liquid nitrogen.
Handle superconductors and magnets with care; avoid direct skin contact with extremely cold surfaces.
Ensure good ventilation to prevent oxygen displacement from evaporating nitrogen.
Do not confine liquid nitrogen in a sealed container (risk of explosion).

Questions to Consider

Why does a superconductor expel magnetic fields when cooled below its critical temperature? (It becomes a perfect diamagnet due to the Meissner effect.)
What role does temperature play in this demonstration? (Below the critical temperature, the material becomes superconducting; above it, superconductivity is lost.)
How is flux pinning different from simple levitation? (Flux pinning allows the magnet to remain suspended in fixed positions due to trapped magnetic field lines in the superconductor.)
What practical applications arise from the Meissner effect? (Maglev trains, frictionless bearings, superconducting power lines.)