What Is Superconductivity?

Answer 1: 

The electrical resistivity of some metals and alloys drops suddenly to zero when their specimens are cooled to a low temperature in liquid helium range. This was first observed by K. Onnes in 1911 in mercury.

The resistivity of mercury vanished completely below 4.2 degree Kelvin. The transition from normal conductivity occurs over a narrow range of temperature of order 0.05 degree Kelvin. This phenomenon is referred to as ``superconductivity.'' The specimens are called superconductors.

Thus, superconductors are materials which possess much more than infinite conductivity . They are perfect electrical conductors under special conditions. Superconductivity occurs in many metallic elements of the periodic table.

Experimental facts regarding superconductors revealed that if a superconductor has the form of a ring, a current can be induced in it by electromagnetic induction, then this current continues to persist with undiminished strength for days. This is ``persistant current.

Answer 2:

At low temperatures, metals have high conductivity of electricity, showing low resistance to the passage of current. The electrical resistance of metals decreases with decrease in temperature.

Superconductivity reveals that it is not confined to a few metals or alloys, but may be present in all metals and alloys provided they can be cooled to temperatures nearer absolute zero. K. Onnes noticed that at 4.2 kevlin the electrical resistance of pure mercury (Hg) became nil and the metal acquired the property of superconductivity.

The temperature at which the metal acquires high conductivity or superconductivity is known as transition temperature (Te). It has been illustrated that a current of about 1000 amperes passing through a tin wire at about 3K shows no heating at all.

Published in The Hindu on Nov 8, 2001.