Q.The phenomenon of perfect diamagnetism in superconductor is called ____. (Fill up the gap)

Superconductivity and the Meissner Effect

Most metals lower their electrical resistance as they cool, but the resistance never quite vanishes. In a superconductor something dramatic happens: below a sharp critical temperature $T_c$, the resistance drops abruptly to exactly zero. A current started in a superconducting loop can circulate for years with no measurable decay, because there is no resistance to dissipate it.

Zero resistance

Below $T_c$ the material carries current with no voltage drop and no heating ($P = I^2R = 0$). Mercury, the first superconductor found (by Kamerlingh Onnes in 1911), becomes superconducting near $4.2$ K. Different materials have different $T_c$; each also has a critical magnetic field and critical current above which superconductivity is destroyed.

The Meissner effect

Zero resistance alone does not define a superconductor. The deeper signature is the Meissner effect: when a material is cooled below $T_c$, it expels magnetic flux from its interior — the field inside becomes zero. The superconductor behaves as a perfect diamagnet, actively pushing out the field rather than merely trapping whatever was there before. This expulsion is what lets a magnet levitate above a superconductor.

Why it matters …