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Water Going Down the Plughole
Clockwise or the other way round in the Southern Hemisphere or the Northern Hemisphere?

Does water always go down the plughole in a clockwise direction in the Southern hemisphere? And does it go down anticlockwise/counterclockwise in the Northern hemisphere? It is an interesting question, and there is an extra level of intrigue which suggests something about society and the way people think.

Legend has it that if you have a bath/tub/basin/sink full of water and you pull the plug and let the water go down the plughole into the drain, then if you are at any place South of the equator, the water will rotate in a vortex as it goes down the plughole in a clockwise direction, whereas if you are located anywhere North of the equator, then the water will go around in the other direction, counterclockwise or anticlockwise. Those directions again: In the Southern hemisphere, water is said to go down the plughole clockwise; and in the Northern hemisphere, anticlockwise/counterclockwise. These are the directions which most large storm systems go round.

The idea that rotating fluids adopt a direction clockwise/anticlockwise according to North/South position is seen in the rotation direction of hurricanes, cyclones, typhoons, and other rotating storm systems. We hear it is caused by The Coriolis Effect, which is a principle in physics to do with things going round. As the earth goes round once every 24 hours, things on the equator are going round as if on the edge of the roundabout and so are going faster, relative to their adjacent neighbours, so there is a bias created which gives storms and other revolving systems a nudge to start their vortices going round in the favoured direction.

Tornadoes are devastating but are much smaller than hurricanes, but still you see they often go round in the expected direction. But not always. Plus, dust devils are tiny rotating air systems, and as far as I know they can be of either polarity wherever they are on the earth, or on other planets.

The idea goes that this fabled Coriolis Effect which causes the great storm systems on the earth to go round clockwise/anticlockwise influences all things and will mean that water in a handbasin will go down the drain in the preferred direction. Some people add "always" to their statement of the direction.

So, is it true or not? Well let's just leave that issue aside a moment, because there is a much more interesting issue here! Why don't you find out for yourself by experiment?! You've got a sink, and you know where you are on the earth, so how about letting some water go down the plughole and see which way round it goes down. In fact, you wouldn't even need to fill the basin specially or waste any water; You could just observe and make a note each time you use it. After ten or twenty tests it would soon become apparent whether the legend is true or not.

An experiment into plughole water descending directionality was performed between July 10th 2011 and July 17th 2011 on a Royal Doulton basin located about 120 miles north of London (which is of course in the Northern Hemisphere). The results were as follows:

Anticlockwise: 15

Clockwise: 6

Straight down: 4

Drained*: 10

* The experiment was optimised to have undisturbed water in the basin. So, the basin was emptied and the direction of the water noted, then half-filled ready for the next visit to the bathroom. *Drained refers to the rubber plug not quite fitting and the basin being found to be empty prior to the intended emptying.

Conclusions: Although there is a suggestion of a preference of direction for anticlockwise, to put this in perspective: a fifty-fifty plughole would produce a 6 v 15 out of 21 results approximately 2.5% of the time. On traditional "95% statistically significant" standards, at 97.5% this passes. However, there's more to things than that. You can decide for yourself how conclusive you think the experimental results are. Something, though: the experiment proves conclusively that "It always goes down anticlockwise in the Northern Hemisphere" is false.

As you may have guessed, I'm not a fan of belief based systems where you're just supposed to believe in some religious fabled notions because someone else tells you that you should, or because "it is written", or because someone a long time ago said it and everyone just nods in submission to their authority. I am more in favour of an enlightened age where things make sense and can be tested scientifically. So on the small matter of the directionality of water in plugholes, I consider it a sad reflection on society that so many people are more willing to believe what they are taught rather than testing it themselves and finding out the scientific truth. Also, I think the idea of doing an experiment to be encouraged, as it would help to get rid of a lot of nonsense which is believed by far too many people.

But, back to the plugholes, although big storms tend to go round the expected direction, these are large scale systems which are affected by the big effects of the earth, and it's not necessarily true when taken to a tiny scale like in a bathtub. To take another example of the appropriateness of round earth considerations: Maps of countries have to be carefully distorted to make them fit, as the maps are flat, but the countries have to fit on the surface of the earth, which is obviously not flat. So, if you measure distances around a map or across it, there are minor discrepancies. But this does not mean that when you buy a piece of land and you get a map on the title deeds you should ask the surveyors whether it's flat or curved around the surface of the earth or not, because even if your land was hundreds of acres, the difference will be only a few inches, as the scale is so much smaller than the size and curvature of the earth.

If you're going to do an experiment, you need to repeat it many times, because it may just be that the direction of water in your plughole is random, like tossing a coin, in which case you could only start to make conclusions about it being biased one way or the other after seeing if it came down heads or tails ten out of ten times, etc.

Also, you don't need to be near the equator and try this on one side and then the other. Indeed, if you are near the equator, the science suggests the difference will be much less. So, only one location required, preferably in the temperate latitudes either North or South, several basins of water, and there you have it; a set of experimental results. (If your water is so clean so you can't see the surface easily, spread a sprinkling of talc or pepper on it).

If you live in New York, which is plenty far enough from the equator, and you let ten sinks of water go down the plughole, you would expect, if the legend were true, that at least nine out of ten would go round counterclockwise, but I think you may find it's more like four out of ten either one way or the other. London, similarly, except it's termed "anticlockwise". And in Sydney Australia, expected direction clockwise, but results may be quite different in practice. On the equator itself, there is general agreement that water going down a plughole will go round in a vortex of random orientation. And at the poles, the water probably freezes before it gets a chance to go down the plughole ;-)

Some people have speculated that the direction which water chooses to go down a plughole is mostly a function of the plumbing system. This might also be worth experimenting into. As most buildings have more than one sink/basin/bath etc it may be possible to spot differences in the clockwise/anticlockwise nature of different plugholes.

Other useful references: Science , The Earth , Water , Baths, Basins, and Sinks , and Why do clocks go round clockwise?

There's another page of Bathroom fittings here.

Another interesting thought: The Dyson vacuum cleaner is a revolutionary invention which involves a revolving cyclone of air. The Dyson company manufactures these machines for worldwide distribution and has no plans to make left-handed and right-handed cyclonic vacuums for different hemispheres!