What is mean by Superconductor ?

In 1911, a scientist named Kamerling Onnes (a Dutch physicist) was conducting an experiment on mercury. He was investigating the effect of low temperature on mercury's properties. He found one surprising thing that as the temperature of the mercury went down ,the resistance offered by the mercury to the current flow was decreasing and after the particular temperature (Critical Temperature), the resistance offered by the Mercury became zero (0 ohm) and the current flew without any voltage drop.

He did this experiment again and again and found and results. Nobody knew this before him about this kind of behaviour of the Mercury.

The metal or any other material which is cooled below a particular temperature does not offer any kind of electrical resistance (offers zero electrical resistance), that metal or material is called as "Superconductor" and the property of of having zero electrical resistance to the current by any metal or material is called as "Superconductivity".

Not only Mercury but also Kathil, Lead, Indium, Thallium etc also has superconductivity property but their critical temperature is much lower than Mercury. From 265.8°C to 270.6°C is the range of the critical temperature for most of the metals. it is very difficult and expensive to cool down any metal to that temperature. Now scientists are focusing on the materials which will show the superconductive property of higher temperature (higher than above mentioned temperature range).

Now the question is 'How these metals are being super-cooled to such a low temperature...?'. the answer is by using liquid gases like nitrogen (N) and helium (He). Liquid Nitrogen can be used to cool the metal up to 66K and liquid helium can be used to cool the metals up to 5K. 

So, where does this "Superconductors" are being used, what are their applications...?

• One of the best application for superconductivity is in producing the large-volume, stable, and high-intensity magnetic fields required for MRI and NMR. 

MRI Machine

• Particle accelerators such as the Large Hydrogen Collidar (LHC) can include many high field electromagnets requiring large quantities of superconductors. 

LHC Cut view

• Germany has the world's longest superconducting power cable in production at 1 kilometer. It is a 10 kV liquid nitrogen cooled cable. The cable is smaller than an equivalent 110 kV regular cable and the lower voltage has the additional benefit of smaller transformers. 

Superconductor power cables

• Superconducting magnets are used to produce a levitating train. The concept behind it is to put a powerful light superconducting magnets below the train floor and then use copper coils in the track which use repulsion to elevate the train up to make it levitate. So, theoretically there will be no direct contact between track and train so the train will run on track without any friction. These trains are know as "Maglev Trains".

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  Maglev Train