Most materials will only superconduct, if at all, at very low temperatures, near absolute zero. Above the critical temperature, the material may have conventional metallic conductivity or may even be an insulator. As the temperature drops below the critical point, resistivity rapidly drops to zero and current can flow freely without any resistance whatsoever. It can even jump small insulating gaps (the Josephson effect).
The earliest superconductors to be discovered would only exhibit the effect at temperatures approaching that of liquid helium. More recently, mixed oxide compounds have been discovered which superconduct below the temperature of liquid nitrogen, which is much less expensive than liquid helium, and are therefore of more practical use.
Superconductors are still not entirely understood, but the race is on to develop a material that will superconduct at room temperature.
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