Superconducting Magnets

The fact that iron saturates, the fact that we can do no better than aligning all the iron atoms current loops, places a fundamental limit on the usefulness of electromagnets for producing strong magnetic fields. Instead it is necessary to return to air core solenoids or other arrangements of coils of wire, and simply use huge currents.
The problem with using copper wire for coils that produce magnetic fields stronger than 1 tesla is that such strong currents are required that even the small resistance in copper produces enough heat to melt the wire. The only solutions for copper are to use an elaborate cooling system to keep the copper from heating, or do the experiment so fast that either the copper does not have time to heat, or you do not mind if it melts.

The introduction in the early 1970s of superconducting wire that could carry huge currents yet had zero electrical resistance revolutionized the design of strong field magnets. Magnets made from superconducting wire, called superconducting magnets are routinely designed to create magnetic fields of strengths up to 5 tesla. Such magnets will be used in the superconducting supercollider discussed earlier, and are now found in the magnetic resonance imaging devices in most large hospitals. The major problem with the superconducting magnets is that the superconducting wire has to be cooled by liquid helium to keep the wire in its superconducting state. And helium is a rare substance (at least on earth) that is difficult to liquefy and hard to maintain as a liquid.
In the late 1980s substances were discovered that are superconducting when immersed in liquid nitrogen, an inexpensive substance to create and maintain. So far we have not been able to make wires out of these “high temperature” superconductors that can carry the huge currents needed for big superconducting magnets. But this seems to be an engineering problem that when solved, may have a revolutionary effect not just on the design and use of superconducting magnets, but on technology in general.