Superconductors:

• A superconductor is an element or metallic alloy which, when cooled to near absolute zero, dramatically loses all electrical resistance.
• In principle, superconductors can allow electric current to flow without any energy loss (although, in practice, an ideal superconductor is very hard to produce). This type of current is called a super current.
• In addition, superconductors exhibit the Meissner effect in which they cancel all magnetic flux inside, becoming perfectly diamagnetic.
• In this case, the magnetic field lines actually travel around the cooled superconductor. It is this property of superconductors which is frequently used in magnetic levitation experiments.

Critical Temperature:

• The critical temperature of a substance is the temperature at and above which vapour of the substance cannot be liquefied, no matter how much pressure is applied.
• Every substance has a critical temperature, of which some examples are shown below

Critical magnetic field:

• If an increasing magnetic field is applied to a superconductor at a constant temperature below the critical temperature, then the material will make a transition from the superconducting state to the normal state when the field reaches a well-defined strength. This field at which the superconductivity is destroyed is known as the critical magnetic field strength Bc
• If the field is reduced, with the temperature held constant, the material returns to the superconducting state at the same critical field strength Bc
• Critical magnetic field strength depends on temperature, and its temperature dependence is shown