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More cool science, from Nature:

Earth's magnetic field reversals mimicked in the lab

Every few hundred thousand years or so, the Earth's north and south magnetic poles switch places. No one knows what triggers these geomagnetic field reversals, but a team in France has now reproduced them in the lab.

When the disks were spun at equal but opposite rates, this field stayed constant. But if the rotation rates were different, the behaviour was more complex, says Stéphan Fauve, group leader at the École Normale Supérieure. "This means that there is overall rotation of the entire contents," he says, "which mimics the spinning of the Earth".

Under these conditions the magnetic field switches polarity apparently at random, typically every minute or so. It happens just as it happens to the Earth, with the field declining slowly to zero and then reappearing quickly in the opposite orientation.