Why and how did Kamerlingh Onnes discover superconductivity?

 

At the beginning of the 20^th century, K. Onnes wanted to measure and understand the properties of metals at very low temperatures. Until then, we only knew that the resistance in a metal progressively drops when the temperature is decreased. But what would happen if we cooled the metal to absolute zero? Would the resistance continue to slowly decrease, would it reach an absolute minimum or would it approach an infinite value?

The first measurements showed a saturation to a minimum of resistance, but this limit seemed to depend on the purity of the metal. This is the reason why Onnes decided to measure a metal he knew how to purify: mercury.

On April 8^th 1911 Onnes discovered to his own amazement that below 4.2 K the resistance of mercury suddenly dropped to zero!

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These words in Dutch Kwik nagenoeg nul werefound in
Onnes’s notebook. They were written on April 8^th 1911.

In English it means the resistance of mercury is zero.

 

This experiment can be repeated showing a very sudden drop which is completely unexpected. This happens as if the metal electrons suddenly started to move forward with nothing to slow them down.

K.Onnes called this phenomenon of perfect conductivity – superconductivity.

In the same experiment, he also observed the superfluid transition of helium at 2.2 K., without recognizing its significance. The precise date and the circumstances of the discovery were only reconstructed a century later, when Onnes’s notebook was found. In subsequent decades, superconductivity was observed in several other materials. In 1913 lead was found to superconduct at 7 K, in 1941 niobium nitride (нитрид ниобия) was found to superconduct at 16 K

Subsequent tests of tin and lead showed that superconductivity was a property of numerous metals if they were cooled sufficiently.

Although superconductivity remained an esoteric scientific research area during his lifetime, Onnes firmly believed that the resistance-free current would eventually allow for the creation of many practical devices.

 

24 Watch the video “What is a superconductor – Magic Marks ” and complete these dialogues.

 

Student A: What is meant by the critical current density when we speak about superconductors?

Student B: I suppose it is current density above which…

Student A: What does the critical current density depend on?

Student B; As far as I remember it depends on…

 

Student A; You know, some conditions can break the superconductivity of a material. What are they?

Student B: They mentioned three conditions: a)… b)… c)…

 

Student A: Could you explain the term the critical temperature for a superconductor?

StudentB I think it is the temperature at which...

25 Complete each figure with the correct term.

 

Cooper pairs The Meissner effect The Josephson effect

 

 

Fig. 1 __________ Fig 2 ____________ Fig.3 __________