The story that the direction of the swirl you see in the water when bathtubs or toilets are drained is determined by the hemisphere you are on will probably sound familiar. I think most of you will also know that this is a bit of an urban legend, as the force of the Coriolis effect which is supposed to determine this direction is rather small compared to other influences like the shape of the basin and turbulence still present in the water before draining. In practice, you’ll find that toilets flush either way on the northern and southern hemisphere, and many times a particular basin can be found to show different swirls on separate tries.
However, on several spots where you can cross the equator by road, you’ll find performances for tourists which apparently show the effect of the Coriolis effect. It is not very difficult to do this trick. Look for instance at this guy working at Uganda Equator, who has been doing this for quite some years. You can find numerous videos of his performance on YouTube.
Probably he manages to force the direction of the swirl by lifting up his ‘stabilizing’ divider in a subtle way. Or the bowls might be formed to drain in a particular direction. Note that for a more convincing experiment he should use the same bowl in all three positions.
But there’s another thing about his performance which made me laugh when I noticed. He actually messed up the theoretical directions the bowls should drain because of the Coriolis effect: on the northern hemisphere it should be counter-clockwise and on the southern hemisphere clockwise! Or does he realise that he is showing the wrong directions and this is just an extra layer in his performance to please the bystanders who know that this is a trick and also know how the Coriolis effect works? 😉
A nice explanation of the Coriolis effect can be seen in this YouTube by Veritasium:
Update 28 April 2018
I’ve made contact with the guy in Uganda via Facebook. He denies he is using tricks (not that I was expecting him to acknowledge that), but also completely ignores my comments on him having mixed up the directions. I’m definitely not the first person to notice his mistake (see for instance Travel Myths: Water Drains the Opposite Direction in the Southern Hemisphere), but I’m quite curious whether other people have pointed this out to him (or his Kenyan colleague) during a live performance on the spot and how he would respond 😉
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Thanks for your post. That person made me confused and wasted my time.
Little late to the game here, but Is this a demo that he charges for?
I guess he just expects some tips.
Check the air bubbles in the water.
In addition to incorrect direction of swirl, at 0.25x speed you can see when he ‘stops’ L and R bowls the air bubbles turning same direction as spin painted onto bowls.
The 2 or 3 second halt is nothing more than a brief pause, in which air bubbles slow down, but do not halt.
Lifting blade results in air bubbles resuming same direction of travel they were initially going.
Middle bowl has 0 air bubbles visible, indicating calm water with 0 inclination to spin either direction due to 0 rotation of water.
Where do the rotation vectors come from? Check the video. As he pours the water, the angle of pour starts water slightly in direction of arrows, as pour is slightly off-center.
Pour angle determines rotation direction. Rotation direction visible due to air bubbles on top rotating same direction.
Definitely a scam.
I think you’re confused as to the “correct” direction of the effect. In the northern hemisphere, things moving south will not move as quickly eastward, so they will appear to get swept to the west; similarly, fluids moving north will have more eastward velocity so they will move “to the right”. Overall, therefore, the effect of fluids in the northern hemisphere should be to rotate clockwise. (In the southern hemisphere, you will of course see the opposite.)
You start off well, but your conclusion that “the effect of fluids in the northern hemisphere should be to rotate clockwise” is not correct. The effect is well explained in the Veritasium video, maybe you should watch it again.
Sorry. The water moving north to the center of the bowl will deflect to the right forcing a counterclockwise spin in the northern hemisphere. All water in the bowl will deflect to the right to make the spin happen. So, the bowls were mislabeled. However, this is ignoring the fact that within a few hundred kilometers of the equator, the Coriollis effect does not happen. Look at a complete hurricane map. NO hurricanes develop on or cross the equator. Something so small as a bowl of water certainly will show NO effect.
How sad. Like telling kids St Nicholas does not exist. These guys give tourists an experience, and most will be wiser afterwards. Who would have otherwise ever have heard of the coriolis effect?
Children have to find out one day that St Nicholas doesn’t exist. I do not have a big problem with these guys making a little money by fooling tourists. I just think it is funny that in some cases they mess up the alleged explanation.