Superconductor expels magnetic lines of force

Below $T_c$, a superconductor expels all magnetic flux from its interior. This is the Meissner effect, and it makes the material a perfect diamagnet ($B = 0$ inside, $\chi_m = -1$).

How the flux is expelled

1. When the material is cooled below $T_c$, electrons form Cooper pairs (BCS theory). The pairs move together as one coherent quantum state, without scattering.
2. When a magnetic field is present, the pairs set up persistent surface (screening) currents in a thin layer of thickness about the London penetration depth $\lambda_L$ (10 to 100 nm).
3. These loss-free currents produce a magnetic field equal and opposite to the applied field inside the body, so the net field inside is zero:

$$B = \mu_0(H + M) = 0 \;\Rightarrow\; M = -H$$

4. The field does not stop sharply at the surface. By London's equation it decays exponentially inside:

$$B(x) = B_0\,e^{-x/\lambda_L}$$

So beyond a few $\lambda_L$ from the surface, $B \approx 0$.

  B
 B0|\
   | \
   |  `.
   |    `-._
   |        `--.____
   +--|-------------- x (into sample)
   0  λL

5. Because the screening currents never decay (zero resistance), the expulsion is permanent as long as $T < T_c$ and $H < H_c$.

The flux lines therefore bend around the sample instead of passing through it. This expulsion happens whether the field was applied before or after cooling, which shows that superconductivity is a true thermodynamic state and not just perfect conduction.