There are some metals and chemical compounds whose resistivity becomes zero when their temperature is brought near 0 Kelvin (-273oC). At this stage such metals or compounds are said to have attained superconductivity. For example, mercury becomes superconducting at approximately 4.5 K (-268.5oC). The transition from normal conductivity to superconductivity takes place abruptly, it occurs over a very narrow range of temperature about 0.05 K. The temperature at which the transition takes place from the state of normal conductivity to that of superconductivity is called the transition temperature. There are two types of superconductors commonly known as Type I and Type II superconductors. Type I superconductors are soft superconductors. They are usually pure specimens of some applications. Type II superconductors are hard superconductors. They are usually alloys of metals with high value of resistivity in normal state. These are very useful as compared to Type I materials.