Now, almost as soon as he discovered the phenomenon of superconductivity, cameling on us, was proposing that superconductors could be used to generate high magnetic fields. But his attempts to do this met with a total failure simply because the current densities were far too low. And then the course of doing this, he discovered that in addition to superconductivity only occurring below a certain critical temperature, once you were below that critical temperature, the superconductor could be driven normal if you exceeded a certain critical current. The current density, whether a superconductor or for a normal material, J is equal to current carriers, which are usually taken to be electrons, times the individual charge on the current carrier, times their velocity. The velocity depends upon the electric field. The acceleration is equal to EQ over M, the charge. So if there was an electric field on a superconductor, you can see that the charge particles, the charge carriers would continue to accelerate. If they did, then the current would continue to increase. But we know this doesn't happen. We have a steady current. In a superconductor, the electric field, E, is identically equal to zero. Nevertheless, the same component on this recognizes that besides this beautiful graph, there is another unpleasant dependence of the critical temperature as a function of magnetic field. Transition from a normal metal to the superconductor occurs at the transition temperature, which is Tc. That appears when you do the measurements with a very, very weak magnetic field. It appears to be quite sharp as camoing on us and holds found right and back in 1911, as to lower the temperature, in this case with mercury, it has a resistance above 4.1 degrees. And the moment you get below that critical temperature, the resistance disappears entirely. What they hadn't expected to find later, that if you have a magnetic field present, the transition occurs at lower temperature, and also it occurs abruptly with a latent heat. Let me try and explain thermodynamics of this. The entropy of a metal is proportional to the absolute temperature. The superconductor, this is normal. The superconductor has a transition at Tc, and at that transition nothing happens to the entropy, but the entropy starts to fall. There's no change in entropy and no latent heat. The transition teaches a measure of the change of entropy at transition. But now what they found was, when you apply a magnetic field, the transition takes place at a lower temperature, the magnetic field present. You're running up a superconducting state like this, but at this point, suddenly, besides, it can't start it, it can't manage any more. So it goes up there and there's a discontinuity in entropy. And so, the latent heat of absorption, the specific heat of the metal, is the derivative, effectively, derivative of entropy at temperature. So this is what is called a second-order transition, and that is what's called a first-order transition. Now, the second-order transition marks not a change into something quite different. It marks the beginning of a change. You can say, does not matter, okay, let's operate without magnetic field. Impossible, because we need superconductivity to transfer currents and current produced magnetic field. So, namely, the phenomenon which we need for, which for, we need superconductivity kills superconductivity. Honest himself failed to make a connection between the two. It was left to, Cillsby, to point out that the critical current was simply that current, which generated the critical field at the surface of the conductor. In order to understand what controlled current density, we really have to start with the basic phenomenology of the superconductor. Very important phenomenon was discovered by Meister and Oxenfeld in 1933. It consists in complete expulsion of magnetic field from the ball superconductor. As we see that the existence of the Meisner effect has led concept of the penetration depth.