We know that electric current is a directed movement of charged particles. Under normal conditions the materials that conduct electricity must this faculty to the existence of electrons with freedom of movement; it is about those who are at the so-called Fermi level. In effect, as said electrons, like the others, they have a negative charge; A collective movement in a certain direction is the current. On the way they suffer multiple dispersions during which they lose energy and change the orientation of their movement. So, for the current does not disappear, it is necessary to replenish the lost energy and return them to move in the original direction; This is achieved by applying a voltage. In the superconducting state, part of the electrons that were at the Fermi level are grouped in pairs forming what is known as Cooper pairs. These entities have a net charge: they have freedom of movement and the interesting thing is that they do not experience dispersions during their movement. Therefore, a current can last indefinitely without the need to provide power to the load carriers. Let's now a little history. In the year 1911, the Dutch physicist Heike Kamerlingh Onnes conducted experiments to measure the resistance of metals at low temperatures. The choice of the first substance to be analyzed fell on mercury. Why this preference? Everything is very simple; at that time it was the only metal that-by means of distillation-could be cleaned quite well from impurities. The influence of secondary factors had to be eliminated when carrying out the experiments. We can then characterize the materials in superconducting state by having an equal steel resistance, although there is another feature that distinguishes them. In 1933 Meissner and Ochsenfeld determined -using samples of tin and lead- that inside the superconductors the magnetic field is null, although on the outside it adopts values other than zero. A substance that has this property is called perfect diamagnetic. The discovery of this phenomenon modified the conceptions on the subject that were held at that time. It was understood then that a superconductor is something more than a perfect conductor, since the latter can not be perfect diamagnetic at temperatures other than absolute zero