Not Very Educational, Just Pretty
I'm a little crunched for time since today's a travel day, so to get things started back off, here's a question. What do the two pictures below have to do with each other?
Maglev trains like the one announced this week by Japan, (shown on the left), use pairs of superconductors and magnets to levitate. Testing has to be done to ensure that the levitation won't become unstable because of mechanical vibrations or stray magnetic fields. The picture on the right is a capture from the lab outtake video shown below. The video is from a playday, but the apparatus was used in an experiment that characterized the levitation of a superconductor by an alternating current magnetic field.
As quantitative data, the video is fairly useless because I'm changing way to many variables at once. It's fun for me at least to see all the things going on. Oh yeah, please excuse my super-classy super-scientific use of the phrase 'crappy signal'.
What you're looking at is a small pair of magnets levitated above a superconductor contained in the Styrofoam cup of liquid nitrogen. The cup is sitting atop an electromagnet that's being driven with 25 kHz current. As the current is increased, you'll see the magnet begin to lose stability as it's effected by the increasing field from the solenoid. A while later, you'll see the liquid nitrogen begin to heat up and boil off because of the heat generated by the solenoid's iron core. The boiling off utlmiately makes a pretty little dithered fog over the cup. Finally, there's a surprise at the end. I'll leave it at this, the superconductor itself isn't very sensitive to the 25 kHz magnetic field. However, when I ramp down the current I'm superimposing an apparent current that vibrates at only a few Hz. The superconductor sees that and reacts.
1. Paper on levitation in an AC magnetic field