XII - SUPERCONDUCTORS

Summary

What is a superconductor?

I have noticed that physicists are rarely given to excesses of exuberance in talking about their subject, even among themselves. So when they use a prefix like super for a material, it must be something quite extraordinary. I shall first tell you what the phenomenon is, and then explain why it is extraordinary. Suppose I take a wire of the metal lead which has a resistance of one ohm at room temperature, about 290 K, and measure its electrical resistance at different temperatures as I cool it down. I show the result in fig. XII-1. The resistance drops smoothly as the temperature drops. Near 7 K, the resistance drops abruptly by a factor of more than 10²⁰ below its value at room temperature, and remains so at all lower temperatures. Put differently, the wire now conducts electric currents at least 10²⁰ times better than it did before. In fact, we can only set an upper limit to this number because of the limits on the sensitivity of the measuring instruments. For all we know, the wire in this condition may be a perfect conductor, which means that a current can pass through it with zero voltage applied to it. Well, super is a bit less than perfect, and so we are content to say that lead has become superconducting, and to call the phenomenon itself superconductivity.