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The Meissner effect is the expulsion of magnetic flux from a material as it enters the superconducting state. Currents that form near the superconductor’s surface create a magnetic field opposing the applied field, so the field is strongly excluded from the material’s bulk. That opposing response can produce a repulsive force on a nearby magnet and, when the force balances gravity, make the magnet levitate.
Meissner expulsion is not merely another name for zero electrical resistance. It is a separate magnetic property of superconductivity, with limits set by temperature, applied field, material type and magnetic history.
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What the Meissner effect means
When an ordinary material is placed in a magnetic field, the field generally passes through it, with the amount of distortion depending on the material’s magnetic susceptibility. A superconductor behaves differently after it is cooled below its critical temperature and enters its superconducting state: magnetic flux is expelled from most of its interior.
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This condition is called the Meissner state. “Expelled” does not mean that the field is absent at every point. Magnetic field penetrates a very thin surface region, where the currents responsible for screening flow. Away from that surface layer, the field in the bulk is strongly suppressed.
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OpenStax describes this behavior as perfect diamagnetism. The superconductor’s magnetic response opposes the applied field, rather than simply allowing an already-present field to remain trapped. That change in magnetic state is the defining physical idea behind the Meissner effect.
How screening currents create magnetic repulsion
Surface currents oppose the applied field
Suppose a permanent magnet approaches a superconducting sample. The magnet’s field reaches the sample and changes the electronic motion near its surface. A current is established in that surface region, and the current generates its own magnetic field. By Lenz’s-law behavior, this induced field opposes the change and reduces the applied field inside the superconductor.
The two fields oppose one another in the region between the magnet and the sample. The resulting magnetic-energy gradient produces a force that pushes the magnet away. The same interaction can be described from the opposite viewpoint: the superconductor experiences a force away from the magnet.
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Levitation occurs when the upward magnetic force equals the downward gravitational force on the magnet. If the magnetic force is smaller, the magnet falls; if it is larger, the magnet moves upward until the forces or the geometry change. A classroom demonstration therefore makes the Meissner effect visible, but levitation is a consequence of the magnetic force, not a complete definition of the effect.
The exact appearance of a demonstration depends on the sample’s shape, the magnet’s field, temperature, material type and whether magnetic flux is pinned in the material. A floating magnet is evidence of strong magnetic interaction, but the details of the state must still be identified.
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Why zero resistance is not enough
Superconductivity is commonly introduced through resistance dropping to zero. That property is important, but it does not by itself imply Meissner expulsion.
Imagine a hypothetical perfect conductor whose resistance suddenly becomes zero while a magnetic field is already inside it. Faraday’s law says that changing magnetic flux would require an electric field. With no resistance, the existing flux could remain effectively frozen in place instead of being expelled. The final field would then depend on how the sample was cooled and on its previous magnetic history.
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| Property | Hypothetical perfect conductor | Superconductor in the Meissner state |
|---|---|---|
| Electrical resistance | Zero | Zero |
| Pre-existing magnetic flux | May remain, depending on cooling and field history | Expelled from the bulk as the superconducting state forms |
| Magnetic response | Not necessarily perfect diamagnetism | Strong diamagnetic screening, within its allowed field and temperature range |
This is why “a superconductor is just a perfect conductor” is an incomplete explanation. The Meissner effect is an additional thermodynamic property of the superconducting phase.
Temperature determines whether expulsion is possible
Every superconducting material has a critical temperature, usually written as Tc. Below Tc, it can support a superconducting state; above Tc, thermal effects drive it into its normal state. The Meissner effect requires the material to be in the superconducting regime, so warming a sample above its critical temperature removes the conditions needed for bulk flux exclusion.
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Cooling below Tc is necessary but not always sufficient for a particular field arrangement. The applied magnetic field must also remain below the relevant critical field, and the sample may have geometry-dependent edge effects or trapped flux. A demonstration that works in a cryogenic vessel can stop working as the sample warms, even if the magnet and its position have not changed.
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The statement that superconductors “repel magnetic fields” needs a field qualification. Increasing the applied field eventually changes the superconducting state. The transition depends on whether the material is type I or type II.
Type I superconductors
A type I superconductor has a single critical field in the simplified bulk description. Below that field, it can remain in the Meissner state and exclude magnetic flux from its interior. Above it, superconductivity is destroyed and the material becomes normal, allowing magnetic flux to enter.
Type II superconductors
A type II superconductor has two critical fields: a lower critical field, often denoted Hc1, and an upper critical field, Hc2. Its behavior is therefore divided into three regions:
| Applied-field range | Magnetic state | What happens to flux |
|---|---|---|
| Below Hc1 | Meissner state | Flux is strongly excluded from the bulk, apart from the surface penetration region. |
| Between Hc1 and Hc2 | Mixed or vortex state | Some flux penetrates through quantized vortices while the surrounding material remains superconducting. |
| Above Hc2 | Normal state | Bulk superconductivity is lost and the material no longer provides superconducting flux exclusion. |
The mixed state is not a complete failure of superconductivity. A type II material can retain zero resistance and superconducting regions while admitting a controlled amount of flux. This is why a type II sample may not look like a perfectly field-free shield even though it remains superconducting.
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Four conditions that determine what you observe
To interpret an experiment or video correctly, check four variables together:
- Material type: Type I and type II superconductors have different critical-field behavior.
- Temperature: The sample must be below its critical temperature, and cooling margin matters when the temperature approaches Tc.
- Applied field: The magnet’s field must be compared with the relevant critical field. A field strong enough to exceed that limit can drive a transition out of the Meissner state.
- Magnetic state and history: A field-cooled sample, a zero-field-cooled sample and a sample containing pinned vortices can show different levitation or trapping behavior.
These variables explain why two demonstrations using different materials can produce different heights, stability and apparent amounts of repulsion.
Meissner expulsion versus flux pinning
In an idealized Meissner-state explanation, screening currents keep flux out of the bulk. In many practical type II superconductors, however, magnetic flux can enter as vortices in the mixed state. Imperfections in the crystal or material can pin those vortices in place.
Flux pinning can make a magnet appear locked above, below or beside a superconducting track rather than simply pushed away. That striking stability is related to the interaction of the magnet’s field with pinned vortices; it should not be described as proof that every part of the sample is in a complete Meissner state. The distinction matters because flux penetration and superconductivity can coexist in the mixed state.
A practical way to analyze a levitation demonstration
- Identify the sample and its type. Determine whether it is described as type I or type II and whether the demonstration uses a bulk piece, thin film or another geometry.
- Check the cooling condition. Confirm that the sample is below its critical temperature and note whether it was cooled before or after the magnet was placed nearby.
- Estimate the field regime. Compare the magnet’s field with the stated critical field, remembering that the field varies strongly with distance and around edges.
- Look for evidence of the state. A freely repelled magnet suggests screening; a magnet that remains at a fixed position or follows a track may indicate flux pinning in a type II material.
- Separate force from definition. The visible lift demonstrates magnetic force balancing weight. The Meissner effect itself is the underlying expulsion or strong exclusion of flux as superconductivity develops.
This checklist prevents a common mistake: treating any floating magnet as proof of identical magnetic behavior in all superconductors.
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Illustrative material values are not universal limits
OpenStax’s 2016 table lists an illustrative critical temperature of 92 K and a critical field greater than 100 T for YBa2Cu3O7. Those figures describe that material entry in the textbook; they are not general values for all superconductors, nor a guarantee that a demonstration operates at those conditions. Critical temperature and critical fields vary with composition, sample quality, field orientation and measurement conditions.
Common misconceptions
“The superconductor pushes away every magnetic field.”
Only a qualified statement is accurate. A material can exclude fields in its Meissner range, admit vortices in the type II mixed state or become normal when a critical field or temperature is exceeded.
“Zero resistance automatically expels trapped flux.”
Zero resistance alone does not determine what field was present before cooling. Flux expulsion is an additional superconducting property.
“Levitation is the definition of the Meissner effect.”
Levitation is one visible result of magnetic repulsion. A superconductor can show flux exclusion without a magnet visibly floating, and a levitation setup can involve flux pinning and mixed-state physics.
“Flux penetration means superconductivity has ended.”
For a type II material between its lower and upper critical fields, flux enters through vortices while superconductivity and zero resistance remain.
Why the effect matters
The Meissner effect demonstrates that superconductivity changes a material’s magnetic thermodynamics, not just its electrical losses. It provides a direct way to distinguish a superconducting phase from an idealized zero-resistance conductor and gives engineers a basis for magnetic shielding, levitation experiments and systems that exploit controlled vortex behavior. In every application, the operating temperature, field strength and material’s critical-field regime determine whether the desired behavior is complete expulsion, partial flux penetration or a return to the normal state.
Quick Recap
Key points to remember
- The Meissner effect is magnetic-flux expulsion from the bulk when a material enters the superconducting state.
- Surface screening currents create a field opposing the applied field and can produce magnetic repulsion.
- Levitation occurs when that magnetic force balances gravity; it is an illustration, not the definition.
- Meissner expulsion is distinct from zero electrical resistance.
- Type I superconductors lose superconductivity above one critical field, while type II materials pass through a mixed vortex state between lower and upper critical fields.
- Temperature, applied field, material type and magnetic history determine what a real sample does.
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