## Tuesday, December 6, 2011

### Magnetic Susceptibility (Another Surprise!!)

Another consequence with the superconducting magnets is that they offer very low magnetic susceptibility. Whenever a metal is exposed to external magnetic field, Eddy current is set up in the material, which then decays depending on the resistance offered by the material. However, in case of superconductors, Eddy current does not decay and hence expels the magnetic field from its vicinity.
Mathematically, from the first Maxwell equation

\nabla V = -{\dot B}/c

But the voltage inside a superconductor is zero in steady D.C. state and hence, B = B$_0$. Thus, the flux in a material in its superconducting state becomes constant at a level which was inherent to it when the resistance of the material was lost. This phenomena gave a back up support to the previously experienced problem of critical field in superconductors.

With all such obstacles, the technological implementation of the superconductor could not be done for more than two decades. It is not that the researches in this area were stopped. In fact, a number of other researchers and young scientists were motivated towards making a no-loss magnet. However, practical uses remain a dream for people for quite a long time.

IN ADDITION TO THIS, THE REASON BEHIND MATERIAL LOSING ITS RESISTANCE COULD NOT BE FULLY EXPLAINED BY THE SCIENTISTS OF THAT ERA.