Some folks at MIT Center for Bits and Atoms have made an object float. Ok, I have a little trouble with them talking about this in terms of "no gravity," but still it is quite cool. Some time ago I thought of doing exactly this experiment. The idea is that an electromagnet it controlled by a computer that has sensors (video cameras in this case) to sense the location of a metal ball. If the ball is too low, crank up the electromagnet above it. If it is too high, repel a little. With the appropriate algorithm (probably a PID algorithm) you should be able to get the ball to float just as the MIT folks in the video did.
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