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Superconductivity
When metals cool their resistance falls steadily as the motion of the atoms of the metal and the
free electrons gets less and so the number of electron-atom collisions is reduced.
However it was found that at as the metal was cooled further a temperature was reached where
the resistance suddenly fell to zero – when this happens the metal is said to be superconducting
and the phenomenon is called superconductivity.
The temperature at which this happens for a given metal is called the transition temperature or
critical temperature for the metal. For pure metals the transition temperature is very low but
scientists have made compounds that have relatively high transition temperature. However as far
as I know of no materials have a transition temperature as high as room temperature.
The transition temperatures of a few common metals are given below:
Mercury 4.15 K, copper at 1.19 K, aluminium 1.19 K, zinc 0.85 K.
Recently a number of high temperature superconductors with transition temperatures as high as
200 K (-73oC) have been discovered.
In these conditions a current will flow for long periods of time without any external electromotive
force being applied. In 1911 Kammerlingh Onnes showed that this period could be up to many
hours!
The importance of superconductivity and the transition temperature is that if a material is
superconducting it has no resistance, this means that an electric current can flow through it without
energy loss in the form of resistive heating. This has tremendous benefits in electric motors and
electrical circuits. They have already been used in superconducting electromagnets in the
levitation of experimental trains.
Unfortunately there is a critical magnetic field above which the superconductivity breaks down and
normal resistance is restored. This means that the actual strength of a superconducting magnet is
limited.