Superconductor Story: Finding the Source of Renewable Electric Flow

We all know about the resistance of conductors for the sake of Ohm's formula we read in school life. If the temperature continues to drop, where will the resistance go? 

In search of the answer to this well-known question, history is made - the beginning of a new chapter. Superconductivity - One of the most important and exciting chapters in physics is narrated by Shamashis Sengupta.

A magnet floats on a superconductor's disc
A magnet floats on a superconductor's disc

Electricity is used everywhere in our daily lives. In a dark room, when we turn on the light by pressing the switch, countless electrons run through the electric wire. Where there is an application of electricity, there is a game of running electrons. What is the movement of electrons in a metal? One of the most exciting discoveries in physics revolves around this question.
Today we will discuss one such discovery, which is called superconductivity.

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Conductive and superconducting

Metal is the conductor of electricity. In some cases, it is not only conductive but surprisingly it can also be called superconducting. To maintain the flow of electricity in metal, one has to go through a constant supply of energy. 

When a voltage is applied between the two ends of electrically conductive wire (which is mostly done by a battery) current is generated from the movement of moving electrons. 

To maintain a certain amount of electricity, how much energy must be supplied from the outside of the electrons depends on the resistance of the metal. Making it with low resistance metal requires less energy and saves cost.

One of the main reasons for the existence of resistance is that the running electrons constantly collide with the molecular center of gravity lined up inside the solid. No matter how little, a little resistance will always be present. 

This is because the electrons collide with the surrounding molecules, even within themselves. It is justifiable to form such an idea from the earliest lessons of physics.

But laboratory experiments often create new surprises. The same thing happened in this case. A groundbreaking discovery was made in 1911. 

In the city of Leiden in the Netherlands, Kamarling Ones was researching the electrical conductivity of certain metals. He observed that in very cold conditions an electric current flows through solid-sized mercury with zero resistance as if there is no friction or reaction at all with the electrons surrounding the matter! 

Solid mercury then transcended the conductor into the superconducting of electricity. This new phase of metal is called a superconductor. The birth is of a subject called superconductivity.
Which theory of physics can explain it? The answer took more than four decades. Later new questions arose, the answers to which are still unknown. If it can be solved, it will be one of the best achievements of the basic physics of today. 

Why was the importance of Ones' research so immense? How did it give birth to new technology? We will discuss this in this article.

This topic is divided into three parts. Today in the composition of this first part we will look at the historical context of Ones' discovery. In the second part, we will see what the answer was after the problem of superconductivity was solved and how superconductor has shown the way to modern technology. And in the third part, we will discuss how superconductivity works at normal temperatures and so on...

Explanation of the resistance of the substance

The movement of electrons in metallic substances causes electric current or electrical current. Suppose there is a wire. If you connect the two sides of it with a battery, electricity will flow through it. The battery has a certain voltage, which will create an electric field inside the wire.

This electric field will give rise to electric current by moving the free electrons. The current is measured by the total number of charges per second passing through the cross-section of the wire. The higher the voltage of the battery, the higher the amount of running electricity.

This formula was discovered by Georg Ohm. He observed that voltage and current are proportional to metals. The proportional constant is called resistance.

What is the reason for having immunity? What happens inside a substance that blocks the passage of electrons and prevents the transmission of electricity? It was not possible to find an answer to this question at the time of Ohm. That was in the 1820s. 

At the time, scientists could measure voltage and current in a laboratory, but there were tiny particles of matter called atoms or electrons - all that was not known.

Many years later, by 1900, when the elementary particle called the electron was discovered, Paul Drude explained the resistance. Any substance is the sum of innumerable molecules. 

At the center of the molecule is the nucleus, which has a positive charge, and is surrounded by electrons carrying multiple negative charges. The characteristic of metallic matter is that not all the electrons inside it are bound to the nucleus. 

Many of them can move from one molecule to another. These are called free electrons. Their freedom of movement makes it possible to conduct electricity through metal. When an electron moves freely from a molecule, the charge on the molecule's total becomes positive, which is then called a positive ion.

"Electrons collide with massive ions along the way, and that's what makes them resistant," said Drude. The electrons move in a zigzag path, constantly colliding with rows of leaf ions.
If the temperature continues to drop, where will the resistance go? Scientist Ones set out to find the answer to this controversial question. 

Discovery of superconductivity

Let's see what Kamarling Ones was looking for in 1911. In that era, laboratory tests showed that as the temperature decreased, the resistance of the metal decreased. The reason for this can be inferred from Drude's theory. The ions of solid matter continue to move around their place. 

As the temperature decreases, the level of their instability gradually decreases. Then it is easier for the electrons to avoid the shock. If the temperature continues to drop, where will the resistance go? This question was very much on the minds of physicists. 

Some people thought that if the temperature was reduced, the resistance would continue to decrease, and even at zero temperatures, it could stop at zero. Some people said that electrons would also stagnate at zero temperature, so there would come a time when the resistance would increase drastically and become infinite, meaning that the metal would no longer be able to conduct electricity.

Ones and his colleagues worked tirelessly to find the answer. First, they found a way to liquefy helium from gas. This enabled them to reduce the temperature to 1 Kelvin (-272 degrees Celsius). At that time no other laboratory in the world knew how to reach such a low temperature. One was the guide. He used this cooling method to cool the solid mercury.

When the temperature reached 4 Kelvin, an unexpected event happened - the resistance of mercury suddenly became almost zero! The minimum resistance that can be measured accurately in the laboratory is even lower than that level in the case of mercury. 

That is so small that Ones says that in reality it can be taken to be zero. The image below is the result of that famous test. Two things here were very surprising. First, the resistance is reduced to zero before reaching absolute zero temperature. Second, this change was abrupt, as if something terrible were happening in the electron world.

It was not long before Ones realized the importance of this discovery. Zero resistance means that the speed of the electrons will never decrease. There is no loss of this current. This state of the metal is called a superconductor. 

Within a few years, superconductivity was observed in other metals as well. Tin and lead showed zero resistance at temperatures of 4 and 6 Kelvin, respectively. Superconductivity is a kind of phase change - which is completely different from the normal phase of free electrons2. Ones' discovery sparked a stir in the world of physics. 

He was awarded the Nobel Prize in 1913 for his discovery of the helium liquefaction method and his research on the properties of metals at low temperatures. 
A magnet floats on a superconductor's disc
Image 1: The result of that historic test of Kamarling Ones - the resistance of mercury (on the left axis) is almost zero when the temperature (on the lower axis) drops below 4.2 Kelvin.
Image Source: 
Centenary of the discovery of superconductivity, CERN

At 4 Kelvin, the resistance to hard mercury is lost - the beginning of a new chapter.


Different types of superconductors in the magnetic field

There is a close connection between the electric and magnetic fields. So the superconductivity that can be seen when the metal is cooled, scientists also worked to understand their behavior in the field of magnetism. 

Research on superconductors was only possible for those who knew enough helium liquefaction technology. The number of scientists capable of this work was innumerable.

Walther Mysner was one of the leading scientists in the 1920's. He made a very important discovery in 1933, with Robert Oxenfeld, that the magnetic field inside a superconductor disappears. When the superconductor is placed close to a magnet, it creates a field exactly opposite, so that the total magnetic field inside it is zero. 

A repulsive force acts between the superconductor and the magnet. This phenomenon is called the Mysner-Oxenfeld effect. A fancy test shows that if a light magnet is placed on a superconductor, it floats in space.

Read the second part

Author: Shamashis Sengupta, University of Paris — XI alumni.

Acknowledgments: I am grateful to Anil Ananthaswamy for his help in making this article.

(Cover image: Mysner-Oxenfeld Effect: A magnet floats on a superconductor's disc (courtesy: Wikipedia / Mai-Linh Doan); a video address of the event -

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