the sample. Put differently ijl0M is in the opposite direction to the applied field and
equal to it in magnitude. Thus, below Tc a superconductor is a perfectly diamagnetic
substance (xm = - 1). This should be contrasted with the behavior of a perfect conductor,
which only exhibits infinite conductivity, or p = 0, below Tc. If we place a perfect
conductor in a magnetic field and then cool it below Tc, the magnetic field is not rejected.
These two types of behavior are identified in Figure 8.45. If we switch off the
field, the field around the superconductor simply disappears. But switching off the field
means there is a decreasing applied field. This change in the field induces currents in the
perfect conductor by virtue of Faraday's law of induction. These currents generate a
magnetic field that opposes the change (Lenz's law); in other words, they generate a
field along the same direction as the applied field to reenforce the decreasing field. As
the current can be sustained (p = 0) without Joule dissipation, it keeps on flowing and
maintaining the magnetic field. The two final situations are shown in Figure 8.45 and
distinguish the Meissner effect, a distinct characteristic of a superconductor, from the
behavior of a perfect conductor (p = 0 only). The photograph showing the levitation of
a magnet above the surface of a superconductor (Figure 8.46) is the direct result of the
Meissner effect: the exclusion of the magnet's magnetic fields from the interior of the
superconductor.
The transition from the normal state to the superconducting state as the temperature
falls below the critical temperature has similarities with phase transitions such as solid
to liquid or liquid to vapor changes. At the critical temperature, there is a sharp change
in the heat capacity as one would observe for any phase change. In the superconducting