Define the critical temperature. Slow that in the superconducting state the material is perfectly diamagnetic.
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 Tc, the magnetic flux is suddenly and completely expelled from it. The flux expulsion continues for T < Tc. The
effect is reversible.
⦁ When the temperature is raised from below Tc . The flux density penetrates the specimen again at T = Tc and the material turns to the normal state.
⦁ For the normal state the magnetic induction inside the specimen is given by:
B = μo (H+M) = μO(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 μo is the permeability of free space.
At T<Tc 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.