Meissner Effect
The Meissner effect is the expulsion of magnetic field lines from the interior of a superconducting material as it is cooled below its critical temperature — a defining property of superconductivity distinct from zero electrical resistance, and the physical basis for magnetic levitation of superconductors above a magnet.
Curious
You might expect that a superconductor, having zero electrical resistance, would simply behave like an ideal ordinary conductor — and an ideal conductor does resist changes to any magnetic field already threading through it (via induced currents opposing the change, following Lenz's law). But Walther Meissner and Robert Ochsenfeld discovered in 1933 something genuinely different and unexpected: a superconductor actively expels magnetic field from its interior as it's cooled through the critical temperature, regardless of whether the field was already present before cooling began.
This distinction matters enormously for correctly understanding superconductivity: it shows that superconductivity is a genuinely distinct thermodynamic phase of matter, not merely "a conductor with zero resistance" — the final magnetic state of a superconductor depends only on its current temperature and applied field, not on the history of how it got there, which is not true for an ordinary conductor with merely very low (but nonzero) resistance.
Exploring
| Symbol | Name | SI Unit |
|---|---|---|
| B(x) | Magnetic field at depth x below the surface | tesla (T) |
| λ | London penetration depth | metre (m), typically tens to hundreds of nanometres |
Deep Dive
The Meissner-Ochsenfeld discovery, made 22 years after Kamerlingh Onnes's original 1911 discovery of superconductivity itself, was crucial in establishing that superconductivity must be understood thermodynamically as a genuine phase transition to a new state of matter, rather than merely as an extreme, idealised limit of ordinary conduction — a conceptual distinction with significant theoretical consequences that shaped subsequent theoretical work, eventually including BCS theory (1957).
The London equations: Brothers Fritz and Heinz London developed a phenomenological electromagnetic theory in 1935 (predating the full microscopic BCS explanation by over two decades) that successfully described the Meissner effect mathematically, introducing the London penetration depth λ as the characteristic length scale over which magnetic field decays exponentially inside a superconductor. The London equations remain a useful macroscopic, phenomenological description of superconducting electrodynamics, providing a simpler practical framework than the full microscopic BCS theory for many applications, even though they don't themselves explain the underlying microscopic origin of superconductivity.
The distinction between Type I and Type II superconductors (see the Codex's forthcoming Type I and Type II Superconductors entry) is fundamentally about how each type handles the Meissner effect under increasingly strong applied magnetic fields — Type I superconductors abruptly lose all superconductivity (and hence the Meissner effect) above a single critical field, while Type II superconductors instead allow partial, quantised magnetic field penetration (in the form of vortices) above a lower critical field, while retaining superconductivity and a partial Meissner effect up to a much higher critical field — a distinction with major practical consequences for which materials are useful in high-field superconducting magnet applications.
Concept Relationship Map
Requires: superconductivity
Enables: Type I and Type II superconductors [PENDING FULL ENTRY]
Commonly confused with: assuming a superconductor is simply an ideal conductor with zero resistance (the Meissner effect shows it's a genuinely distinct thermodynamic phase)
Sources
- Meissner, W., Ochsenfeld, R. (1933). Naturwissenschaften, 21(44), 787–788.
- London, F., London, H. (1935). "The Electromagnetic Equations of the Supraconductor." Proceedings of the Royal Society A, 149(866), 71–88.