News in Science

Water Down a Plughole

According to popular myth, in the Southern Hemisphere, all cyclones spin clockwise - and so does the water in toilet bowls, bath tubs and hand basins. The story is correct about the cyclones. But the water going down a drain - well, half the time it's right, and half the time it's wrong.

The supposed force behind this phenomenon is the Coriolis Force, named after Gustave-Gaspard Coriolis. He first described it way back in 1835. Actually, there's no such thing as "Coriolis Force" - it's just a fancy name for "Conservation Of Angular Momentum". You can see angular momentum being conserved or saved with an ice skater spinning on one foot. When the skater pulls in their arms close to the centre, or spin axis of their body, they speed up - and when they stretch out their arms, they slow down again.

But the weather people call this phenomenon the Coriolis Force, just to keep the name short. A big storm is called a cyclone in Australia, a typhoon in Asia, or a hurricane in the USA. Anyhow, one of these big storms will start up near the equator, and then move in the direction of either the North Pole, or the South Pole. As it moves closer to one of the Poles, it also gets closer to the Spin Axis of the Earth, which runs through the North and South Poles. This is similar to the ice skater pulling their arms in closer to the spin axis of their body. The storm is moving in two directions at the same time - towards the Pole, and closer to the spin axis of the Earth. I won't go into it now, but to balance the momentum equations, the storm spins clockwise in the Southern Hemisphere, and anti-clockwise in the Northern Hemisphere.

As I said, the weather scientists call the force that makes cyclones spin the Coriolis Force. The Coriolis Force is zero at the Equator, and strongest at the North and South Poles. The infantry see the Coriolis Force in action in long-range guns. The operators in charge of fire-control of long-range guns have to apply different corrections to the left or right, depending on how far from the equator they are, and in which hemisphere they are. These situations involve large distances - a cyclone can span hundreds of kilometres, while a big gun could easily lob a shell 50 kilometres.

But a toilet or hand basin is much smaller - only about 30 cm across. And if you do the numbers, the Coriolis Force on such a small body of water is about 10 million times smaller than the gravity force of the Earth pulling on the water in the toilet or hand basin. So could you possibly see such a small Coriolis Effect, when the "gravity effect" is so much bigger?

The answer is "yes" - but only if you are extremely careful. The rotation effect is so small, that it is vastly overshadowed by the direction in which the water entered the toilet bowl or basin, or wind effects over the surface of the water. The experiment has been done in both hemispheres. It was done in 1962 by Ascher H. Shapiro at MIT in Massachusetts, and repeated in 1965 by Lloyd M. Trefethen at the University of Sydney.

Shapiro had a very shallow dish about 2 metres across and 150 mm deep. The outlet hole was about 9 mm across. He added the water through a hose, deliberately swirling it clockwise (the opposite of the expected draining direction). He then covered it with a plastic sheet, and let the water stand for 24 hours, to reduce the initial rotation of the water. He placed a small floating cork on the water, and then released the stopper plug from below. The water took 20 minutes to drain out, with no visible rotation for the first 12-15 minutes. Then he could see the float begin to spin anti-clockwise - slowly at first, then gradually increasing to one rotation every four seconds by the end. The University of Sydney team used very similar apparatus, and got consistent clockwise rotations.

Shapiro wrote in Nature, "When all the precautions described were taken, the vortex was invariably in the counter-clockwise direction". But don't expect to see the water spinning the right way down the plug hole, if you lurch off the plane at Singapore (about 1o from the Equator), rush to the toilet, fill the hand-basin with water and pull the plug.

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