When magnetic field is applied to a super conductor at T < 𝑇𝑐
then magnetic lines of force does not passes through it and behaves as a perfect diamagnetic material. This effect is first observed by Meissner and familiarized as Meissner effect.
We know B = µo (H + M)
In superconducting state, the magnetic lines are not passing through it. So B = 0
0 = µo (H + M)
H = - M
𝑀 = 𝜒 = −1
The above expression indicates that, only diamagnetic materials can show negative
susceptibility. It concludes that, when weak magnetic field (H < Hc) is applied to a
superconductor then the magnetic lines of force does not penetrate through it and shows
perfect diamagnetic property. This property is called as Meissner effect.