Mar 5, 2007
This isn’t exactly your average high school science fair project, in fact please don’t try this for yourself without proper supervision, as you’ll need a magnet, a chunk of high temperature superconductor and a flask of liquid nitrogen, oh and a pot that won’t crack when you pour in the liquid nitrogen. (Standard cyro lab safety equipment is recommended) When this video first hit the streets, lots of people said it was a fake, but what it is, is a classic demonstration from modern physics of non-classical behaviour in materials cooled below a certain critical temperature. The effect in question is well-known to scientists working with superconductors and is known as the Meissner-Ochsenfeld effect, or more commonly the Meissner effect. Poor old Ochsenfeld rarely gets a name check. It was discovered by the pair in 1933, so it’s nothing new to physics, relatively speaking.
So, what’s going on? How does the high temperature superconductor levitate the magnet and if it’s high temperature why does it have to be cooled with liquid nitrogen to near 77 Kelvin, that’s almost minus 200 Celsius). Well, we’ll answer the last question first, it’s the easiest. The High is relative! Low temperature superconductors only work at close to absolute zero, minus 273 K, so anything at the almost balmy temperature of liquid nitrogen is positively smoking!
Now to the hard bit. Superconductors are unusual ceramic materials. They have lots of weird properties not least the fact that they superconduct, which means they can carry an electric current with zero resistance. So, picture the magnetic field around the magnet, if you bring the magnet close to the ceramic when it’s at room temperature, the magnetic field lines pass straight through. But, when the ceramic is chilled below its critical temperature to make it a superconductor, those magnetic field lines can no longer penetrate the ceramic. But, that doesn’t really answer the question, it just begs another – why can the magnetic field not penetrate the superconductor?
The final answer lies in the fact that the magnet induces tiny electrical currents in the superconductor as it is lowered towards the superconductor (remember, a moving magnetic field induces a current in a conductor, it’s the basis of the dynamo and electrical generation). However, this is no ordinary conductor, it’s a superconductor and so those electrical currents keep flowing round and round in infinite circles within the superconductor. Now, conversely to a moving magnetic field producing a current in a conductor, an electrical field will induce a magnetic field and because of Fleming’s right-hand rule, that induced magnetic field matches the pole to which the superconductor is exposed by the placing of the magnet in the first place.
The resulting repulsion is counteracted by the downward force of gravity and the magnet hovers neatly above the superconductor, at least until it warms to above its critical temperature.
Just for completeness, I should also point out that the magnet is effectively pinned in position by an effect known as flux pinning, which is caused by magnetic field lines getting snarled up by impurities in the superconductor. But, if you set the magnet spinning it will spin without friction (well not in this demo because there is air friction). Incidentally, this levitation effect has a serious application in some types of Mag-lev train.