Physics
Answer
A superconductor is a substance of zero resistance (or infinite conductance) at a very low temperature.
Example — Mercury below 4.2 K, lead below 7.25 K and niobium below 9.2 K
Resistances of these substances decrease tremendously with the decrease in temperature and become almost zero in the low temperature range near absolute zero. Zero resistance of a superconductor means it's infinite conductivity (i.e., once a current starts flowing in a superconductor, it persists even when there is no potential across it).
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