Friday, March 20, 2015

Superconducting Electrons as a Frictionless Superfluid

While doing research for an article I'm writing about Janet Tate and her Gravity Probe B experiment[4], I found a few cool things regarding superconductors, frictionless bearings, and the Egg of Columbus experiment this morning.

The Egg of Columbus demonstration was first performed by Nicola Tesla in 1893 at the World's Columbian Exposition[1].  Here's a brief video from MIT showing a modern day version of the demonstration[2]:

The MIT site[2] describes the apparatus as follows:

"A toroid with three different wire windings is connected to 220 VAC 3-phase voltage. The voltage phase of each of the three windings lags 120 degrees behind the next, creating a changing induced magnetic field. The changing field causes metal objects to rotate when placed inside.
Motors using this principle are very common. In fact, power lines are often seen in sets of three because they are carrying three phases. For more information on 3-phase voltage,"
Alfred Leitner made use of a similar apparatus to demonstrate one of the properties of liquid helium.  Interstingly he points out that the rotating cylinder in the liquid helium Dewar is made of copper,(a non-superocnducting matieral).

Today's interesting find has to do with what happens when you replace the copper with a superconductor, a lead sphere in this case.  I. Simon reported[3] that while their Egg of Columbus style apparatus worked just fine, spinning a  lead sphere at room temperature, when the lead was cooled to its superconducting state, the sphere would no longer spin!  As long as care was taken not to trap residual magnetic fields in the superconductor as it cooled, the transverse magnetic field of the stator coils was unable to effect the cylinder in the least!

1. Tesla's Egg of Columbus on Wikipedia
2.  MIT demonstration of the Egg of Columbus
3.  Simon, I, "Forces Acting on Superconductors in Magnetic Fields", Journal of Applied Physics, 24, (1953), p. 19
4.  Tate, J, "Precise Determination of the Cooper-Pair Mass", Physical Review Letters, 62, (1989), 845

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