The Meissner Effect
- (The University of Chicago - Alvin Wei-Cheng Wong)
- Overview
The Meissner effect, the expulsion of the magnetic field from within a material that is becoming a superconductor, loses its resistance to the flow of electric current when cooled below a certain temperature (often called the transition temperature). close to absolute zero. The Meissner effect is a property of all superconductors and was discovered in 1933 by German physicists W. Meissner and R. Ochsenfeld.
When a superconductor in a magnetic field is cooled to a temperature where it suddenly loses electrical resistance, all or part of the magnetic field within the material is expelled. Except for a surface layer a millionth of an inch thick, the interior of all superconductors completely repels relatively weak magnetic fields. However, the external magnetic field may be so strong that it prevents the transition to the superconducting state and the Meissner effect does not occur.
In general, the intermediate range of magnetic field strengths present during cooling produces a partial Meissner effect, as the original magnetic field is weakened within the material but not completely exhausted. Some superconductors, known as type I superconductors, such as tin and mercury, can exhibit the full Meissner effect by eliminating various chemical impurities and physical defects and choosing appropriate geometries and dimensions. Other superconductors known as Type II superconductors, such as vanadium and niobium, exhibit only partial Meissner effects at moderate magnetic field strengths, regardless of their geometry or size. Type II superconductors show a decrease in magnetic field repulsion as their strength increases, until they suddenly cease to be superconductors in relatively strong magnetic fields.
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