Electrical resistance of certain materials, known as superconductors, changes abruptly from a nonzero value to zero as their temperature is lowered below a critical temperature $$T_c(0)$$. An interesting property of superconductors is that their critical temperature becomes smaller than $$T_c(0)$$ if they are placed in a magnetic field, i.e., the critical temperature $$T_c(B)$$ is a function of the magnetic field strength B. The dependence of $$T_c(B)$$ on B is shown in the figure.