The Meissner Effect, or How to Levitate a Magnet
Hello everyone, and welcome to another installment of "Cool Things About Superconductors." Oh, wait. That's not a thing I'm doing. Despite that, there are so many cool things about superconductors, and I'm going to cover one of them today (well, technically, one and a bit): the Meissner Effect. What is the Meissner Effect? Well, it's something that happens when a normal conductor hits its critical temperature. That temperature is the temperature that, below it, the ordinary run-of-the-mill conductor begins superconducting. This means that it goes from having a normal, everyday amount of resistance to electric currents, to having exactly zero resistance to electric currents. And while there are so many other cool things that happen as a consequence of this, we're going to talk about what happens right at that moment.
The Meissner effect was discovered by Walther Meissner and Robert Ochsenfeld in 1933, and describes an interesting phenomenon that occurs when a superconductor transitions to its superconducting state. At the moment it passes below its critical temperature and begins superconducting, it pushes out all internal magnetic fields, so that, regardless of the presence of a magnetic field outside the superconductor, there is nothing inside of it. The physicists Fritz and Heinz London showed why this is with the London Equation, but I'm not going to really get into that here because it's by no means a simple explanation. Regardless of explanation, the phenomenon remains: when a superconductor begins superconducting, it excludes all internal magnetic fields.
In order to do this, the superconductor must produce an opposed magnetic field to even field like that would be internal. This process comes with a neat side effect that you may have seen before: you can levitate a magnet above a superconductor that is superconducting. This is precisely because the superconductor is excluding all internal fields. Normally, the magnetic field of the magnet would penetrate the superconductor, and the magnet would have no opposition to it (depending on the material, an induced field may occur which attracts the magnet to it). However, when the superconductor is in its superconducting state, every field line fromt he magnet that would normally penetrate it induces a current in the superconductor that creates an opposed magnetic field (as I touched upon in my post on light, a moving eletric field creates a moving magnetic field), which pushes back on the magnet. No matter how the magnet is positioned, all the field lines emanating from it will be opposed by the superconductor, and there will be an equal force distributes across the magnet, which will keep it levitating. You can move it around, spin it, whatever, but the induced fields from the superconductor will always follow it.
Technically, the levitating magnet does not demonstrate the Meissner Effect, because the Meissner Effect is the phenomenon of going from having internal fields lines to having no internal electric field. The demonstration of this would be to place the magnet on top of the superconductor, and then cool it down to the superconducting state and watch the magnet rise up and begin levitating above the superconductor, because it totally does that, and it's so cool. There are so many more cool things about superconductors, but I am far too busy this week to be able to fully cover them (I'm writing this post on Monday). So stay tuned for next week's installment of "More Cool Things About Superconductors."