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.