False friends are common. Yes, but where True nature links a friendly pair, The blessing is as rich as rare.
The magnet of their course is gone, or only points in vain The shore to which their shiver'd sail shall never stretch again.
Superconductivity was discovered in 1911 by H. Kamerlingh Onnes soon after he succeeded in liquefying helium (Onnes, 1911). He observed that the resistivity of mercury dropped suddenly as its temperature was lowered below a certain critical value TC (for Hg, TC = 4.2 K). Over the years, it was found that many additional elements and compounds similarly transition to a superconducting state. In this state, materials exhibit properties that are strikingly different from the normal state. Below we discuss the most important features of superconductors.
Properties of superconductors
The first important property of a material that undergoes a superconducting transition is that its resistivity drops to zero belowa critical temperature (see Figure 12.1). In a superconducting ring, a persistent electric current flows without any observable attenuation for as long as one is willing to watch.