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The Meissner effect is the expulsion of magnetic flux from a material’s bulk when it becomes superconducting. Currents that form in a thin surface region create a magnetic field opposing the applied field, so the interior is strongly shielded. The resulting magnetic force can repel a nearby magnet and, when it balances the magnet’s weight, make it levitate.
This is more than ordinary zero electrical resistance. A hypothetical perfect conductor could preserve magnetic flux already inside it, depending on how it was cooled and exposed to a field. A superconductor actively changes its magnetic state as it crosses its transition, subject to its temperature, material type and critical magnetic fields.
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What the Meissner effect means
In the normal state, magnetic field lines can pass through a material. When a suitable material is cooled below its critical temperature and enters the superconducting state, magnetic flux is driven out of the bulk. The material behaves as a near-perfect diamagnet: its magnetic response opposes the applied field rather than reinforcing it.
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“Expelled” does not mean that the field is absent at every point. The magnetic field penetrates a very thin surface region, where screening currents flow. Beyond that surface layer, in the material’s bulk Meissner state, the field is strongly excluded. The exact penetration and the field limits depend on the superconductor.
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The effect is therefore a change in magnetic behavior associated with the superconducting transition. It is not simply a visual synonym for a magnet floating over a cold sample, nor is levitation required for the effect to exist.
How screening currents produce repulsion
Opposing the applied field
As the material becomes superconducting, electrical currents appear near its surface. These are called screening currents because their magnetic field screens the interior from the externally applied field. Their direction is such that the field they create opposes the field trying to enter the sample.
This opposition follows the same field-change logic expressed by Faraday’s law: the induced magnetic response resists a change in magnetic flux. In a superconductor, the response is persistent because the superconducting state supports current without the ordinary resistive dissipation that would quickly weaken it.
Turning field exclusion into a force
A permanent magnet brought close to the screening-current region experiences a magnetic force. The current-generated field and the magnet’s field oppose one another in the relevant region, producing repulsion. If the upward magnetic force equals the magnet’s downward weight, the magnet can remain suspended:
magnetic force = weight = mg
That balance explains the familiar levitation demonstration. Moving the magnet, changing its distance or changing the superconductor’s temperature alters the force, so a demonstration can lose its stable-looking position without the underlying physics being disproved.
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Why zero resistance is not enough
The perfect-conductor thought experiment
It is tempting to define a superconductor as a material with resistance equal to zero. That definition misses the Meissner effect. Suppose a material merely became a perfect conductor while a magnetic field was already inside it. With no resistance, Faraday’s law would prevent the existing flux from changing easily. The field could remain trapped in the sample rather than being expelled.
That hypothetical perfect conductor can therefore retain a magnetic state determined by its cooling and field history. A superconductor displays an additional thermodynamic property: on entering the superconducting state, it establishes screening currents that exclude flux from the bulk, within the material’s allowed field range.
Two tests of superconductivity
- Electrical test: resistance falls to an immeasurably small value, allowing persistent current.
- Magnetic test: the material enters a Meissner state in which bulk magnetic flux is expelled.
Both behaviors matter. A demonstration that shows only a long-lived current does not by itself establish complete Meissner expulsion, and a levitating object is evidence of a magnetic force but is not, by itself, a full definition of superconductivity.
Four conditions that determine whether flux is excluded
1. Temperature relative to the critical temperature
Every superconducting material has a critical temperature, commonly written as Tc. Above it, the material is in its normal state and does not maintain a Meissner state. Cooling below Tc permits superconductivity, provided the magnetic field and current demands remain within the material’s limits.
2. Applied field relative to a critical field
Superconductivity survives only below field thresholds that depend on the material and temperature. If the applied field exceeds the relevant threshold, the material can return to its normal state or move into a state with partial flux penetration. Saying that superconductors “repel magnetic fields” without stating this qualification is too broad.
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3. Material type
Type I and type II superconductors have different field responses. Type I materials have one principal critical-field boundary. Type II materials have a lower and an upper critical field, creating an intermediate mixed state.
4. Magnetic and cooling history
Whether a field was present before cooling, whether the sample was cooled in a field and how the applied field changes afterward can affect the observed magnetic configuration. This history is precisely why a perfect conductor and a true superconductor are not interchangeable concepts.
Type I versus type II superconductors
The following states describe the idealized bulk response; real samples can show geometry-dependent details at their surfaces and edges.
| Material and field range | Magnetic state | What happens to flux | Superconductivity |
|---|---|---|---|
| Type I, below its critical field | Meissner state | Bulk flux is expelled; only a thin surface region carries the screening response. | Present, with zero resistance. |
| Type I, above its critical field | Normal state | Superconductivity is lost and the material no longer maintains Meissner exclusion. | Absent. |
| Type II, below the lower critical field | Meissner state | Bulk flux is strongly excluded. | Present, with zero resistance. |
| Type II, between lower and upper critical fields | Mixed or vortex state | Some magnetic flux penetrates while superconducting regions and zero resistance remain. | Still present. |
| Type II, above the upper critical field | Normal state | The field penetrates without the superconducting shielding state. | Absent. |
The mixed state is an important exception to the simplistic phrase “a superconductor expels all magnetic fields.” A type II material can admit flux and remain superconducting. Partial penetration is not the same as complete loss of superconductivity.
What happens during a cooling experiment
- Start above the transition: keep the sample above its critical temperature. It is normal, so magnetic flux can occupy its interior.
- Set the magnetic field: place the magnet or apply the field in the arrangement you want to study. Record whether the field was present before cooling; that history matters.
- Cross below the critical temperature: cool the sample into the superconducting state while keeping the field below the appropriate critical limit.
- Watch the screening response: surface currents develop and oppose the applied field. Flux is expelled from the bulk in the Meissner state.
- Change one condition at a time: raising the temperature through the transition, increasing the field past a critical value or changing the sample’s material state can reduce or eliminate bulk exclusion.
This sequence distinguishes a genuine transition from simply placing a magnet near a conductive object. Ordinary conductors can develop transient induced currents, but resistance causes those currents to decay; the superconducting state sustains its screening response under the permitted conditions.
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How to interpret a levitation demonstration
What levitation shows
If a magnet rises or remains suspended above a cooled sample, the magnetic force is large enough to oppose gravity at that position. The observation is consistent with screening currents and magnetic-field exclusion. It makes an otherwise invisible field response visible.
What levitation does not show by itself
- It does not prove that every part of the sample is in a complete Meissner state.
- It does not establish that the material is below every relevant critical field.
- It does not show that all magnetic flux has vanished from the sample; a thin surface penetration region is expected, and type II materials may be in a mixed state.
- It does not distinguish zero resistance from flux expulsion unless the experiment also establishes the superconducting transition and field conditions.
Different demonstrations can therefore look different even when they involve superconductors. Temperature, sample type, applied field, geometry and magnetic history all affect the force and the amount of flux that penetrates.
A concrete material example: YBa₂Cu₃O₇
OpenStax’s University Physics Volume 3 (2016) lists illustrative values of approximately 92 K for the critical temperature of YBa₂Cu₃O₇ and a critical field greater than 100 T. Those are entries for that specific material in the textbook table, not universal values for superconductors and not a guarantee that a particular sample or experiment will reach those limits.
The example shows why a temperature and field must accompany any claim about magnetic expulsion. A material can be cold enough to be superconducting yet exposed to a field that pushes it out of the Meissner state; conversely, a field that is acceptable at one temperature may exceed the critical limit at another.
Common misconceptions and the precise correction
“Superconductors repel every magnetic field.”
They strongly exclude fields in the Meissner state, not under every possible condition. Critical fields, temperature and material type set the limits. Type II superconductors permit partial flux penetration in their mixed state.
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“The Meissner effect is just zero resistance.”
No. Zero resistance concerns electrical transport; the Meissner effect concerns the magnetic state and expulsion of bulk flux. A hypothetical perfect conductor could preserve pre-existing flux instead of expelling it.
“A floating magnet is the definition of the effect.”
Levitation is a consequence of magnetic repulsion when the force balances weight. The Meissner effect is the underlying flux expulsion, which can occur without a dramatic floating object.
“Any field inside a type II superconductor means superconductivity has failed.”
No. Between the lower and upper critical fields, a type II superconductor is in a mixed state: some flux enters, while superconductivity and zero resistance persist.
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The field penetrates a thin surface region where the screening currents flow. The useful statement is that flux is expelled from the bulk or strongly excluded in the Meissner state.
How to evaluate a claim about magnetic repulsion
When reading an experiment, video or specification, check these details before drawing a conclusion:
- What material is being used, and is it type I or type II?
- What critical temperature applies, and is the sample demonstrably below it?
- What magnetic field is applied, and how does it compare with the lower, upper or single critical field?
- Was the sample cooled in zero field or while a field was present?
- Does the evidence show magnetic flux expulsion, or only a persistent current, force or levitation effect?
- If the sample is type II, could the observation be in the mixed state rather than the complete Meissner state?
These checks turn the slogan “superconductors repel magnets” into the more accurate statement: a superconductor in its Meissner state generates screening currents that oppose an applied magnetic field and exclude it from the bulk, within the limits set by temperature, field and material.
Further reading
For a textbook treatment, OpenStax’s free University Physics Volume 3, section 9.8, “Superconductivity,” covers perfect diamagnetism, the distinction between zero resistance and flux expulsion, and type I and type II critical-field behavior. A Georgia Tech Physics Book explanation illustrates screening and levitation, while an NPTEL lecture on the two-fluid model and London equations discusses why a perfect conductor’s field history differs from a superconductor’s transition.
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