In 1911, Kammerlingh onnes while studying the electrical behaviour of metals at low temperatures, he observed that the resistivity of impure metals decreased with temperature and had a finite valve at 0K. But for mercury the resistivity decreased gradually as the temperature is reduced, but abruptly dropped to zero at 4.2K (Figure). Below this temperature, the resistivity was zero. This state with zero resistance is known as the superconducting state.

Critical temperature (or) Transition Temperature (T₂)

The temperature at which the transition from normal state to superconducting state takes place on cooling in the absence of magnetic field is called the Critical Temperature (T_c).

If the transistion temperature is low, then the super conductors ae known as Low Temperature Super Conductors.

If the transition temperature is high (even 30 Kelvin) then it is known as High Temperature Super Conductors.

The transition temperature is different for different superconducting materials. Table lists the superconducting transition temperature for some of the materials.

It is seen that metals which are normally good conductors at room temperature are poor superconductors. But ceramic materials which are poor conductors at room temperature are very good superconductors. Presently scientists are trying to synthesize new materials with higher transition temperatures so that practical application of superconducting materials become more and more feasible. Such materials are know as “High T_c” Superconductors.

Table : Transition temperature of various elements and compounds