A superconducting material kept in a magnetic field expels the magnetic flux out its body when cooled below the critical temperature and exhibits perfect diamagnetism. This is called MEISSNER EFFECT.

• It is found that as the temperature of the specimen is lowered to T_c, the magnetic flux is suddenly and completely expelled from it. The flux expulsion continues for T<T_c. The effect is reversible.</li>
• When the temperature is raised from below T_c. The flux density penetrates the specimen again at T=T_c and the material turns to the normal state.

• For the normal state the magnetic induction inside the specimen is given by: $B=μ_o (H+M)=μ_o (1+χ)H$………………………………………..(1)

Here H is the applied magnetic field , m is the magnetization produced within the specimen , is χ the susceptibility of the material and $μ_o$ is the permeability of free space.

• At $T \lt T_c$ as seen above

B = 0

Hence equation (1) reduced to,

M = -H

And thus χ = M/H = -1

• The specimen is therefore a perfect diamagnetic. The diamagnetism produces strong repulsion to the external magnets.
• This effect is used to identify a superconductor , in levitation effect and suspension effect.