Wednesday, 26 March 2008

WRONG: Baths drain in different directions either side of the equator

Gaspard-Gustave de Coriolis was the French mathematician who gave his name to the effect that the earth’s rotation has on large-scale fluid dynamics. And if you think that means he didn’t know how to party, he also wrote a mathematical treatise on billiards[1].

The Coriolis effect is the reason weather systems spin anti-clockwise in the northern hemisphere and clockwise in the south. As air circulates north and south between areas of high and low pressure in a more-or-less straight line, the earth moves underneath it. From the moving perspective of someone on the earth, large-scale weather systems in the air appear to rotate, though this is really a result of the shape of the earth and the movement of the observer.

But none of this has anything to do with your plughole.

Water does not spin one way down the plug hole in the northern hemisphere and another in the south. This is a myth, reinforced by cheeky types who, for a small fee, will “demonstrate” the effect at the equator using sleight of hand. You may remember a guy getting away with this on Michael Palin's Pole To Pole. If you’ve noticed that your bath has a tendency to always drain itself one particular way, it’s due to the shape, angle and design of your plug hole and its position relative to your taps. Baths, sinks and toilets drain far too quickly, and are much too small – thousands of miles too small - for the relatively minuscule influence of the earth’s movement to have any visible effect. That doesn’t mean you can’t be impressed by the collapse of a chaotic flow of water into a relatively stable vortex, however, whichever way it spins.

[1] Théorie mathématique des effets du jeu de billiard, Gaspard-Gustave de Coriolis, 1835

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