Monday, December 5, 2011

Generation of Magnetic Field using Superconductors : An Initial Set-back

The disappearance of electrical conductivity in many metals at certain critical temperature gave a boost to the hope of people thinking about powerful electromagnets and other electrical technologies. They thought, if a specimen can take current without any Ohmic (resistive) losses then the dream of constructing high-field electromagnets can be fulfilled. In 1913, Onnes fabricated a small coil of Lead keeping this in mind and cooled it down to its superconducting state, i.e. below 4.2 K. What was driven him to do so was -- he wanted to flow high current through it without significant power loss, and generate high magnetic field. However, he along with the entire science community were greatly disappointed when it was found that the magnet was unable to produce more than few hundred Gauss even when it is operated below its superconducting temperature. Above a certain field, it was behaving as if a normal conductor is being charged with current. This superconducting to normal transition was termed as quench. With the help of further investigations, it was found that the superconducting behavior of the metals was also disappearing at a certain field generated either internally or externally (few hundred Gausses). Thus, to the disappointment of all, it was found that the superconductors can be operated within the critical temperature and critical field (which was too low to make an electromagnet).
For straight superconductors carrying high current, the quenching occurs when the field generated at the surface of the wire crosses the critical field. 

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