[thelazyone]

I think it's a fair question. I guess the answer is "because we have superconductors, and no material with 'very small resistance' instead".

Also note that while there is zero resistance you still have parasitic currents and general interaction with the rest of the environment. So no perpetual motion for us today!

[jacquesm]

Even a 'very small resistance' would likely be prohibitive. Pure silver for instance is an excellent conductor, but when you're talking about the last little bits it's orders of magnitude difference between that and a superconductor, and those orders of magnitude difference in resistance translate into orders of magnitude more current. So even a small resistance would cause your MRI machine to have a resolution so low as to be unusable.

[obrix]

MR imaging is possible with permanent magnets and resistive (copper wire wound) magnets at low field strengths, below 0.35T or so. Above that the heating of the magnet windings becomes excessive and it would be very difficult to maintain a stable enough field strength.

Superconducting magnets are very nice as long as there is no quenching. The material used for conductors must be mechanically stable and perform consistently from one production batch to another. One reason why current high tc superconductors are not popular...

[jychang]

Actually, there are infinite orders of magnitude between the resistance of silver and that of a superconductor.

[thelazyone]

I think that the initial question was "why a 0 resistance compared to a ridicously low resistance". And my point is that it's easier to get a superconductor than some material with "ridicously low" resistance. As you said, silver is unusable for potent magnets, and such is any other non-superconducting magnets.

Probably if we had materials with a billionth of the resistance of silver they would work, but we haven't. And we have superconductors, luckly. :)