14.1 Introduction

Superconductivity was a scientific curiosity for a long time, but by now there are some actual applications like producing high magnetic fields for Magnetic Resonance Imaging, and there are lots of potential applications. The big prize would of course be superconductive lines for power transmission which would eliminate losses. But quite apart from applications, I feel that some acquaintance with superconductivity should be part of modern engineering education. Superconductivity is, after all, such an extraordinary phenomenon, so much in contrast with everything we are used to. It is literarily out of this world. Our world is classical, but superconductivity is a quantum phenomenon a quantum phenomenon on macroscopic scale. The wavefunctions, for example, that lead an artificial existence in quantum mechanics proper appear in superconductivity as measurable quantities.

The discovery of superconductivity was not very dramatic. When Kamerlingh Onnes succeeded in liquefying helium in 1908, he looked round for something worth measuring at that temperature range. His choice fell upon the resistivity of metals. He tried platinum first and found that its resistivity continued to decline at lower temperatures, tending to some small but finite value as the temperature approached the absolute zero. He could have tried a large number of other metals with similar prosaic results. But he was in luck. His second metal, mercury, showed quite unorthodox behaviour. Its resistivity (as shown in Fig. 14.1) suddenly decreased to such a small value that he was unable to measure it and...