- Meissner Effect Definition: The Meissner effect is defined as the expulsion of magnetic fields from a superconductor when it is cooled below its critical temperature.
- Discovery and Experiment: German physicists Walther Meissner and Robert Ochsenfeld discovered the Meissner effect in 1933 through experiments with tin and lead samples.
- Meissner State: The Meissner state occurs when a superconductor expels external magnetic fields, creating a state with zero magnetic field inside.
- Critical Magnetic Field: A superconductor returns to its normal state if the magnetic field exceeds the critical magnetic field, which varies with temperature.
- Application of Meissner Effect: The application of the Meissner effect in magnetic levitation is crucial for high-speed bullet trains, allowing them to float above tracks and reduce friction.
When Superconductors are cooled below their critical temperature, they expel magnetic field from the bulk. German physicists Walther Meissner and Robert Ochsenfeld reported that expulsion in 1933 and the response is now known as the Meissner effect. They cooled tin and lead in an applied field and measured a stronger field just outside the samples, which showed that flux had been pushed out. Type I materials stay fully expelled below one critical field; Type II stay fully expelled only below the lower critical field.
Meissner State
The fully expelled condition of a superconductor is the Meissner state. An example of the Meissner effect is shown in the figure below.
That state ends when the magnetic field, either applied from outside or produced by current in the wire, rises past a critical value. Type I then goes normal. Type II first admits flux vortices and only becomes fully normal above the upper critical field.
The field strength at which a superconductor leaves the Meissner state is the critical magnetic field. For Type I there is one such field. For Type II the lower critical field ends the Meissner state and the upper critical field ends superconductivity. Both rise as temperature falls below the critical temperature. The figure below shows how the critical magnetic field varies with temperature.
Application of Meissner Effect
A common classroom demo of the Meissner effect is magnetic levitation: a cooled superconductor can float a magnet, or the other way around, because the expelled magnetic field produces a repulsive force. Stable float often also needs flux pinning in Type II material. Superconducting magnets appear in some maglev research, including Japan’s SCMaglev, but those trains use electrodynamic suspension in a guideway rather than a pure Meissner float above steel rails.
