Almost from the moment that Kamerlingh Onnes first realized that a superconductor has no electrical resistance, he appreciated the potential for producing commercial magnets. He hoped that it might be possible to generate powerful magnetic fields by passing high currents through superconducting wire coils (usually called solenoids). The zero resistance of a superconducting solenoid avoids the severe drawback of large Joule heating in coils made from resistive metals. Once the magnetic field has been set up in a coil of superconducting wire carrying a persistent resistanceless current, it needs no further electrical power to maintain it: there is no energy loss.