## Types 1 and II Superconductors

Materials that exhibit superconductivity do so below a certain temperature, characteristic of the material. When the material is cooled below this temperature in the region of a magnetic field, the magnetic field inside the material is zero - the Meissner effect. A surface current is induced on the surface of the superconductor generates external magnetic field so that the field inside the conductor is zero.
The temperature below which superconductivity occurs decreases with increasing applied magnetic field. For magnetic fields above a critical value
$B_c$
superconductivity is not exhibited at any temperature.

Only some superconductors, called Type I cancel the magnetic field completely inside the superconductor. The strength of
$B_c$
for Type I superconductors is too weak for many uses. Type II superconductors may exhibit superconductivity for applied magnetic fields several hundred times those for Type I superconductors, but fail to observe the Meissner effect fully. These materials are typically metals or alloys, with larger resistivities.