The critical temperature is the transition temperature at which the normal state of a conductor changes to superconducting state.

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 $T \lt T_c$. The effect is reversible.

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<$T_c$ 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.