What Is the Meissner Effect, and Why Do Superconductors Levitate Magnets?

A small magnet hovering over a dish of cold ceramic, steaming with liquid nitrogen, is one of the best-known images in physics. The material underneath is a superconductor, and the magnet floats because the superconductor refuses to let magnetic field inside. That refusal is the Meissner effect, and it is the job of our Expulsieur sticker, the character who shows field lines the door. It is one of the strangest things that happens as matter is cooled toward the bottom of the temperature scale, the subject of our explainer What Happens at Absolute Zero?.

What is the Meissner effect?

Superconductivity was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes. When a superconducting material is cooled below its critical temperature, the Nobel Committee explains, “an electric current can then flow with no resistance whatsoever”. Simultaneously, “there occurs what is termed the Meissner effect. This means that a magnetic field cannot, or can only partly, penetrate the material” (NobelPrize.org, 1987 press release^[1]).

The effect was reported in 1933 by Walther Meissner and Robert Ochsenfeld in a short paper in Die Naturwissenschaften (Meissner & Ochsenfeld 1933^[2]). They measured the magnetic field near and inside cylinders of single-crystal tin and polycrystalline lead. A historical review by Vladimir Kozhevnikov summarises their main conclusion: the magnetic induction B inside the superconducting phase “is always zero” (Kozhevnikov 2021^[3]).

Isn’t that just what a perfect conductor would do?

No, and this is the subtle part. Faraday’s law says a changing magnetic field drives currents in a conductor. In a perfect conductor with zero resistance, those currents never die away, so the field inside can never change. Whatever field was inside when the resistance vanished stays frozen there.

• Perfect conductor, cooled in a field: the field is trapped inside. Remove the outside magnet and the trapped field stays.
• Superconductor, cooled in a field: the field is actively pushed out as it becomes superconducting.

The end state of a superconductor therefore does not depend on its history. Kozhevnikov notes that “the hallmark of perfect conductors is the irreversibility of their magnetic properties”, and that the Meissner effect is specifically about field-cooled samples. The 1933 experiments established a reversible state, the Meissner state, “characterized by zero induction simultaneously with zero resistivity” (Kozhevnikov 2021^[3]). That made superconductivity a true thermodynamic phase of matter, not just a material with very good wiring. The idea that magnetic fields drive currents is the same physics behind our post Why a Magnet Falls Slowly Through a Copper Pipe.

Why does a magnet float above a superconductor?

To keep field out, the superconductor runs screening currents in a thin layer at its surface. Those currents create a field that cancels the magnet’s field inside the material, and outside they repel the magnet, like two like poles facing each other. OpenStax describes it this way: a superconductor “excludes magnetic flux and, thus, repels other magnets”, and a small magnet levitated above a high-temperature superconductor cooled by liquid nitrogen “gives evidence that the material is superconducting” (OpenStax College Physics, §34.6^[4]). When the material warms above its critical temperature, the magnet drops.

How far does the field get in? The London penetration depth

The field is not cut off perfectly at the surface. In 1935 Fritz and Heinz London wrote down electromagnetic equations for a superconductor (London & London 1935^[5]). Their equations predict that a field entering the surface dies away exponentially with depth x:

B(x) = B₀ e^(−x/λ_L)

where B₀ is the field at the surface (tesla, T) and λ_L is the London penetration depth. In SI units,

λ_L = √( m / (μ₀ n_s e²) )

with m the electron mass, 9.109 383 7 × 10⁻³¹ kg (NIST^[6]); μ₀ the vacuum magnetic permeability, 1.256 637 06 × 10⁻⁶ N A⁻² (NIST^[7]); e the elementary charge, 1.602 176 634 × 10⁻¹⁹ C (NIST^[8]); and n_s the number density of superconducting electrons, in m⁻³.

A worked number

Take an illustrative n_s = 10²⁸ m⁻³ (a round value chosen for this example, not a measured property of a particular metal):

λ_L = √( 9.109 × 10⁻³¹ / (1.2566 × 10⁻⁶ × 10²⁸ × (1.602 × 10⁻¹⁹)²) ) ≈ 5.3 × 10⁻⁸ m ≈ 53 nm

So the field is shut out within a few tens of nanometres of the surface. Kozhevnikov describes λ_L as the “effective width of a surface layer” containing the screening current (Kozhevnikov 2021^[3]).

Type I and type II: when the field gets in anyway

Push the outside field too high and superconductivity gives up. The Nobel Committee’s 2003 background explains that type-I superconductors are metals “characterised by the Meissner effect”, and if the surrounding field becomes too strong, they lose their superconductivity. This does not happen with type-II superconductors, “which accommodate strong magnetic fields by letting the magnetic field in” (NobelPrize.org, 2003^[9]).

In type II, the field threads through the material in thin channels called vortices, while the rest stays superconducting. Alexei Abrikosov worked out the theory, which predicts a regular lattice of these vortices. The same source notes that “all high-temperature superconductors are type-II”, and that the properties of type-II superconductors “cannot be described by the BCS theory” on its own, the 1957 microscopic theory of Bardeen, Cooper and Schrieffer (Bardeen, Cooper & Schrieffer 1957^[10]).

The ceramic discs used in the classroom demo are type-II, so some of the magnet’s field does get in as vortices, and defects in the material hold those vortices in place. This flux pinning is a big part of why the magnet hovers stably at a fixed height instead of sliding off the edge. Roughly speaking, the screening currents supply the push and pinning supplies the stability.

Why does liquid nitrogen matter?

For most of the 20th century superconductors needed liquid helium, which boils at −269 °C (NobelPrize.org^[1]). Then Georg Bednorz and Alex Müller turned to oxide materials containing copper, work that won the 1987 Nobel Prize and opened a new family of “high-temperature superconductors” that can be cooled with liquid nitrogen (NobelPrize.org^[1]). In 1987 M. K. Wu, C. W. Chu and colleagues reported superconductivity at 93 K in a Y-Ba-Cu-O compound (Wu et al. 1987^[11]), above the 77 K boiling point of liquid nitrogen. That is why the levitating-magnet demo can now be done in a classroom. For the even colder world of superfluids, see The Cool Science of Superfluidity.

Is the Meissner effect fully understood?

The standard textbook account explains it with the London equations and BCS theory. Kozhevnikov’s 2021 review takes a dissenting view, arguing that “the physics of this well known phenomenon … still remains an unsolved problem” and proposing his own model (Kozhevnikov 2021^[3]). This is one author’s position, not the consensus, and we flag it as debated.

Find Expulsieur and the rest of the field crew in our Electromagnetism collection.

FAQ

Who discovered superconductivity?

Heike Kamerlingh Onnes, in 1911, who received the Nobel Prize in 1913. The associated Meissner effect, in the Nobel Committee’s words, “was observed for the first time in 1933”.

Does the magnet float forever?

The screening currents flow without resistance, so as long as the superconductor stays below its critical temperature, the magnet stays up.

Is the Meissner effect the same as zero resistance?

No. They appear together in a superconductor, but a material with zero resistance alone would trap field rather than expel it. The expulsion is the extra, defining property.

References

1. NobelPrize.org, Press release: The Nobel Prize in Physics 1987 (Bednorz, Müller). https://www.nobelprize.org/prizes/physics/1987/press-release/
2. W. Meissner & R. Ochsenfeld (1933), “Ein neuer Effekt bei Eintritt der Supraleitfähigkeit,” Die Naturwissenschaften 21, 787–788 (DOI 10.1007/BF01504252; metadata verified via Crossref). https://doi.org/10.1007/BF01504252
3. V. Kozhevnikov (2021), “Meissner Effect: History of Development and Novel Aspects,” J. Supercond. Nov. Magn. 34, 1979–2009 (arXiv:2105.11065). https://arxiv.org/pdf/2105.11065
4. P. P. Urone & R. Hinrichs, College Physics 2e (OpenStax, Rice University), §34.6 “High-temperature Superconductors”. https://openstax.org/books/college-physics-2e/pages/34-6-high-temperature-superconductors
5. F. London & H. London (1935), “The electromagnetic equations of the supraconductor,” Proc. R. Soc. Lond. A 149, 71–88 (DOI 10.1098/rspa.1935.0048; metadata verified via Crossref). https://doi.org/10.1098/rspa.1935.0048
6. NIST CODATA 2022 recommended values: electron mass. https://physics.nist.gov/cgi-bin/cuu/Value?me
7. NIST CODATA 2022 recommended values: vacuum magnetic permeability. https://physics.nist.gov/cgi-bin/cuu/Value?mu0
8. NIST CODATA 2022 recommended values: elementary charge (exact). https://physics.nist.gov/cgi-bin/cuu/Value?e
9. NobelPrize.org, popular science background, The Nobel Prize in Physics 2003 (Abrikosov, Ginzburg, Leggett). https://www.nobelprize.org/prizes/physics/2003/popular-information/
10. J. Bardeen, L. N. Cooper & J. R. Schrieffer (1957), “Theory of Superconductivity,” Phys. Rev. 108, 1175–1204. https://journals.aps.org/pr/abstract/10.1103/PhysRev.108.1175
11. M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang & C. W. Chu (1987), “Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure,” Phys. Rev. Lett. 58, 908–910. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.58.908

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.