The Meissner effect (also known as the Meissner-Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor. Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the flux distribution outside of tin and lead specimens as they were cooled below their transition temperature in the presence of a magnetic field. They found that below the superconducting transition temperature the specimens became perfectly diamagnetic, cancelling all flux inside. The experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconducting state.

Supercondutors don't allow magnetic fields to penetrate, so a magnet placed on top of superconductor is pushed upward into the air.

The Meissner Field (or Effect) is produced when a superconductor has an external magnetic field applied to it -- with the strength of the Meissner field/effect being dependent upon the initially applied external magnetic field. Once the Meissner effect has been initiated, it acts as a magnetic barrier and resists any further entry of an applied magnetic field into the superconductor. The effect is dramatic. Once established the Meissner field forces all other magnetic fields to effectively go around the superconductor, leaving the superconducting sample unaffected in terms of its superconductive characteristics.

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