Complete Diamagnetic - The Most Important Feature of Superconductors
In 1933, German physicists W.Meissner and R.Ochsebfekd measured the distribution of the magnetic field of a tin single crystal ball superconductor. It was found that when the metal was cooled into a superconducting state in a small magnetic field, the magnetic line of force in the body was discharged and the magnetic line of force could not pass through its body, that is to say, superconductor was superconducting. In the state, the magnetic field in the body is equal to zero. This effect is called the "Meisner effect".
Once the superconductor enters the superconducting state, the magnetic flux in the body will be completely removed from the body, and the magnetic induction intensity is always zero, and the magnetic field is added to the conductor first and the magnetic field is added, or the magnetic field is cooled first. As long as the superconductor is in the state, the superconductor will discharge all the magnetic flux. In addition, superconductors are completely anti - magnets, and the external magnetic field can not enter or exist in a large range of superconductors, which is another basic characteristic of superconductors.
Later, people have also done the experiment in a shallow tin plate, put in a very small magnetic very strong NdFeB permanent magnets, and then reduce the temperature, so that the tin superconductivity. At this time, it can be seen that the small magnet left the tin plate surface and floated up and kept a distance from the tin plate. This is because superconductors are "not allowed" to have any magnetic field inside. If there is a magnetic field to pass through the superconductor, the superconductor will inevitably produce an opposite magnetic field, which ensures that the internal magnetic field is zero. This makes a repulsion. When a magnet is placed underneath a superconductor and perpendicular to the superconductor, the superconductor will get vertical buoyancy. When the force is exactly equal to the gravity of the superconductor, the superconductor can be suspended in the air.
The Meisner effect indicates that the superconducting state is a dynamic equilibrium state and has nothing to do with how to enter the superconducting state. The zero resistance phenomenon of the superconducting state and the Meisner effect are two independent and interrelated basic properties of the superconducting state. The pure zero resistance does not guarantee the existence of Meisner effect, but the zero resistance effect is a necessary condition for the Meisner effect. Therefore, to measure whether a material is a superconductor, we must consider whether there is zero resistance and Meisner effect simultaneously.