Levitated Cheerio Experiment

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The Levitated Cheerio Experiment (LCX) is a small demonstration of the technique of magnetic levitation by feedback control. The small "cheerio" sized wood block is levitated magnetically by the electromagnet coil above. (The cheerio is "loaded" with a small neodymium magnet.)

If you've every played with trying to levitate a magnet with another one above it, you would have found that the floating magnet either wants to fall or jumps up very quickly and sticks to the upper magnet. In 1842, Earnshaw showed that this frustrating behavior will occur no matter how one arranges these two magnets. However, by allowing the strength of the upper magnet strength to change in time, we can get around this problem. (For a general explanation of other methods, see this FAQ on magnetic levitation.)

In the LCX, an analog electrical circuit senses the position of the cheerio as it breaks an infrared beam emitted on the right side of the picture (the light beam appears bluish/white to the camera, but is invisible to the eye.) The circuit then adjusts the current in the electromagnet, changing its strength. In this way, if the cheerio starts going up, the circuit will make the upper magnet weaker and the cheerio will start to fall. If the cheerio gets too low, the circuit will increase the strength of the electromagnet to raise it back up. A schematic of the circuit is available here.

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Return to the Levitated Dipole Experiment.

Last modified Fri, Feb 12, 1999 by Darren Garnier