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Figure 4250 shows an external magnetic field penetrates the ordinary conductor. The electrons of the conductor behave in a disorderly way. In contrast, the Meissner effect, as an "invincible shield", is the expulsion of a magnetic field from a superconductor. Walther Meissner and Robert Ochsenfeld discovered this effect in 1933 by measuring the magnetic field distribution outside superconducting tin and lead materials. Their sample, in the presence of an applied magnetic field, was cooled below superconducting transition temperature. Below this temperature the sample almost canceled all magnetic fields inside. The electrons of the superconductor function in a conherent collective manner-invincibility.

Figure 4250. The external magnetic field penetrates the ordinary conductor and
it is excluded from the interior of a superconductor.

The Meissner Effect takes place at the quantum level of superconductivity in strongly diamagnetic materials. A superconductor can “expel” nearly all magnetic flux by setting up electric currents near its surface. The magnetic field of these surface currents cancels the applied magnetic field within the bulk. In the superconductor, the coherent functioning of the electrons spontaneously excludes an external, disruptive magnetic field.