E guarda questo invece:
`
\chapter{The critical magnetic field}
Critical current density $f_c=$ limit for resistanceless current in superconductor.
Meissner effect: Resistenceless surface current cancel flux from inside material ($\Phi=Li$). This current we call shielding current $f_L$.
If strength of the external magnetic field increases, the shielding current $f_L$ increases.
Eventually shielding current $f_L$ reaches value equal to critical current density, $f_L$ becomes $=f_C$. Then material loses its superconductivity and flux is no longer cancelled.
Exist a limit $B_C$, critical flux density when supersonductivity is lost. This is all way written in a form of critical magnetic field strength, $H_C=B_C/\mu_0$. People talk about critical magnetic field, and use symbol $H_C$.
In addition to shielding current $f_L$, in superconductor can exist current from external current source. We call this \lq\lq{transport current}\rq\rq $f_T$.
Total current $\overline{f_S}=\overline{f_T}+\overline{f_L}$ must remain below critical current $f_C$ to have material in superconducting state.
`
E ora come la mettiamo?