Can You Detect The Coriolis Effect in your Sink?

18 December 2005

Interview with

Dr Karl Kruszelnicki, University of Sydney, Australia

Chris - What we've been doing this evening is asking people all around the East of England to fill sinks and baths with water, pull out the plug and see which way the water swirls down the plug hole. We've got a mixture of results this evening, but is this experiment actually possible? Can we detect the spinning of the Earth using this approach?

Karl - You can get it to work. However, you're looking at a thing called the Coriolis force, which is actually angular momentum under a different name. The Coriolis force on the small bodies of water you're working on is roughly 10 million times smaller than the gravity force, so you really need to do the experiments delicately. Let's just back up a bit here. The thing about angular momentum is the same as when ice skaters go faster when they pull their hands into their body. They speed up because they bring more of their mass to the spin axis of the body. If you think about the Earth spinning, at the equator it's a long way from the spin axis, and at the poles it's right on the spin axis. If you get a storm brewing just above the equator, they spin and move away from the equator towards the poles. As they do so, they head towards the spin axis of the Earth. There's a bit of angular momentum that needs to be accounted for. If you do the equations, this leads to clockwise rotation of a hurricane in the southern hemisphere and anticlockwise rotation in the northern hemisphere. But here you're looking at something tens or hundreds of kilometres across. How can you hope to see that same effect in a tiny tub? The answer is that if you do the experiment very carefully, you should be able to see it. This has been done once or twice.

Chris - So it is possible?

Karl - Yes. In a fine journal called Nature in 1962, there was a paper by Shapiro who did the experiment at MIT. A few years later at the University of Sydney, also published in Nature, a paper by Trefethen in 1964 about the bath tub vortex in the Southern hemisphere. What you do is get a special bath tub, which is two yards across, six inches deep and has a tiny tiny central hole. You put a cork there so you can see which way the water's going. You let the water settle for a day or two so you lose all the residual spin from putting it in there and then you open the drain plug. The water begins to flow out very slowly and nothing very significant happens for about twelve to fifteen minutes. At around that stage, you can begin to see the cork take on a clockwise or anticlockwise rotation depending on your hemisphere. It happens slowly at first and then increases to one rotation every four seconds by the end. Shapiro wrote that when all the precautions prescribed were taken, the vortex was invariably in the anti clockwise direction. Soif you're a fair way away from the equator and you it carefully, you can see it. However if you just rush off the plane at Singapore which is one degree from the equator, and the surface of the Earth is about parallel to the spin axis, put some water in an oval bowl and pull the plug straight out, you're only going to see local effects.

Chris - So in other words, Michael Palin was fooled into thinking this was true at the equator.

Karl - Mate, there's an old Polish saying. If you've got a dog, don't bark. It's fairly obscure, but what it means is stick to your speciality. The number of areas of ignorance we have are huge. In this particular case in the TV series Pole to Pole, Michael Plain meets a man called Michael McCleary, who says that this line here on the ground is the equator. He has a little square tub which he's holding in his hands with floating matchsticks. He then walks off in one direction and spins as he turns around to face the tourists. That gives a spin to the water. He takes his finger off the bottom and you can see the matchsticks going round clockwise or anti clockwise as he's being told. Poor Mr Palin is being conned!

Chris - It just goes to show that even the great Michael Palin can be conned sometimes. Thanks very much for joining us Dr Karl and helping us to avoid throwing out the baby with the bath water and debunking the myth of the Coriolis effect and how it effects spin when water goes down the plug hole.


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