The electrical resistivity of pure metals was known to drop rapidly with temperature. Several attempts were going on to find the residual resistivity of the metals which would replicate the inherent residual impurities. On Eighth April 1911, Dutch physicist Kamerlingh Onnes found that the electrical resistivity of mercury tended to become zero suddenly at about 4.25 K (about 269 degrees below the temperature of ICE. On earth the minimum temperature that exists in the pole is ~ 240 K). Later on, this reversible and reproducible phenomena was seen in Lead at 7.2 K, Tin at 3.7 K and few other metals.
During those days, due to the limitation in instrumentation, the question of resistance dropping to immeasurable level at temperature below the critical temperature arose. In order to validate this, Onnes made a ring of Tin and cooled it with a background field. As the ring went pass its critical temperature, the field was removed which induced a circular current in the ring. It was pointed that even if small resistance is present in Tin, then the induced current would decay in finite time.
But Onnes could not predict any measurable change in the current after several days (In present days, with high purity of metals and increased measurement accuracy, the time has been extended to years). It was concluded that the resistance is far below the practicable limits and hence it can be considered as `zero'.
The state of metals showing this phenomena below a critical temperature was termed as "Superconducting state."
Now the question for you: