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NCERT's Class 12 chapter Magnetism and Matter says that superconductors show perfect diamagnetism, that "the field lines are completely expelled", and that this is "called the Meissner effect, after the name of its discoverer". There were in fact two discoverers, and the effect is not just a fact to memorise. It answered a real question about what superconductivity is. This post tells the story and works through the physics.
Who was Walther Meissner?
The Neue Deutsche Biographie (1990) records that Meissner was born in Berlin on 16 December 1882 and died in Munich on 15 November 1974. He started in engineering at the Technische Hochschule Charlottenburg, switched to physics and took his doctorate under Max Planck in 1906. In 1908 he joined the Physikalisch-Technische Reichsanstalt (PTR), Germany's national physics laboratory, and from 1912 he built up its low-temperature research.
After the First World War he built a helium liquefier with the Linde company. That made Berlin the third place, after Leiden and Toronto, where liquid helium was available. His group then searched for new superconductors. In 1928 they found that tantalum superconducts, the sixth superconducting element known. Thorium, titanium and vanadium followed, then copper sulfide, the first superconducting compound, and carbides such as niobium carbide that superconduct at about 10 K.
For how superconductivity was first seen in mercury, read our post on Heike Kamerlingh Onnes.
What did Meissner and Ochsenfeld find in 1933?
Meissner wanted to know how current flows inside a superconductor. The NDB explains that Max von Laue suggested measuring the magnetic field between two closely spaced superconductors carrying current. During these measurements in spring 1933, Meissner and his colleague Robert Ochsenfeld saw something new: magnetic flux was pushed out of the inside of a superconductor as soon as it was cooled below its transition temperature.
They reported it in a short note, Ein neuer Effekt bei Eintritt der Supraleitfähigkeit ("A new effect at the onset of superconductivity"), in Die Naturwissenschaften in November 1933. In his 1972 Nobel lecture, J. Robert Schrieffer summed it up: a superconductor "is a perfect diamagnet as well as a perfect conductor", and the field vanishes inside a bulk specimen "even when cooled down below the transition temperature in the presence of a magnetic field".
Why is a perfect conductor not enough?
This is the heart of the discovery, and you can follow it with Faraday's law. In a material with zero resistance, any change in magnetic flux would drive an unlimited current, so the flux inside can never change. Compare two orders of doing things:
Cool first, then switch on the field. A perfect conductor keeps its original flux, which was zero, so the field stays out. A superconductor also keeps the field out. No difference.
Switch on the field, then cool. A perfect conductor keeps the flux that was already inside, so the field stays in. A superconductor pushes the field out.
Case 2 is what Meissner and Ochsenfeld saw. The NDB notes that it proved the change from the normal to the superconducting state is fully reversible, so the final state does not depend on how you got there. That made it a true thermodynamic state, and it opened the way to thermodynamic theories and to the London brothers' theory of currents and fields in superconductors.
Worked example: how strongly does a superconductor magnetise?
Inside a perfect diamagnet B = 0. Using B = μ0(H + M) with χ = M/H, this needs M = -H, so χ = -1 and μr = 1 + χ = 0, exactly as NCERT states.
Take a sample applied field B0 = 0.010 T, below the sample's critical field. Then H = B0/μ0 = 0.010 / (4π × 10^-7) ≈ 7.96 × 10^3 A/m, so M ≈ -7.96 × 10^3 A/m.
Compare that with an ordinary diamagnet. For a material with a sample susceptibility of -1 × 10^-5, M = χH ≈ -0.08 A/m. The superconductor's magnetisation is 10^5 times larger, which is why a magnet can float above one. Our post on magnetic materials and hysteresis covers ordinary diamagnets and paramagnets.
Mistakes students make about the Meissner effect
Equating it with zero resistance. Zero resistance alone would trap a field already inside. Expulsion is a separate property.
Writing μr = 1 for a superconductor. For perfect diamagnetism μr = 0 and χ = -1.
Assuming it works in any field. A strong enough field destroys superconductivity, as Kamerlingh Onnes found.
Saying the field stops exactly at the surface. Schrieffer notes the shielding currents flow in a thin penetration layer near the surface.
When the signs of χ and M blur together, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise magnetisation on its own before returning to superconductors.
What came after Berlin?
The NDB records that conflicts with Johannes Stark, the new president of the PTR, helped push Meissner to leave in 1934 for the Technische Hochschule Munich. The Walther-Meissner-Institut adds that he was, in its words, politically clean, and became President of the Bavarian Academy of Sciences in 1946. His low-temperature commission grew into the institute, renamed after him in 1982 on the centenary of his birth.
What can a student learn from how Meissner worked?
The discovery came from a careful measurement made to answer a narrower question, how current spreads inside a superconductor. Meissner did not ignore the result that did not fit. When an experiment surprises you, ask first what a simpler model would predict, as the perfect-conductor comparison does.
Try it now: for B0 = 0.020 T, find H and M inside a superconductor, and say what would happen to M above the critical field.
