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Verifying a Vortex

A scientist’s quest to demonstrate the Coriolis effect in a bathtub
October 24, 2012

In the 1960s, thanks mainly to MIT mechanical-engineering professor Ascher Shapiro ’38, PhD ’46, the world became captivated by the question of how a bathtub drains.

Harold Edgerton captured this side view of a water vortex in 1939. More than two decades later, Ascher Shapiro would demonstrate the Coriolis effect at bathtub scale.

Scientists were aware that Earth’s rotation alters the trajectory of objects in motion. This phenomenon causes low-­pressure weather systems to twist counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The Coriolis effect, as it’s known, had long been well documented at such large scales.

But despite previous attempts, no one had shown that the effect—first described in 1835 by the French engineer and mathematician Gustave-Gaspard Coriolis—works on very small scales as well. Though in theory it should influence bathwater’s exit through a drain, the Coriolis effect was thought to be too small to see.

In 1962, the same year that Watson and Crick received their Nobel Prize for the discovery of the double helix, Shapiro set up an elaborate test to try to show once and for all that the Coriolis effect could, in fact, be seen at the scale of an ordinary household bathroom. 

For his experiment, Shapiro used a circular, flat-bottomed tub with a centered drain hole three-eighths of an inch in diameter to which he attached a 20-foot length of hose, plugged with a stopper at the end. He filled the tank six inches deep with clean, room-temperature water. 

Small variations—air movement, a temperature change, a surface disturbance—create buoyancy currents that overshadow the Coriolis effect. So Shapiro did much tinkering to cancel out these possible sources of interference—covering the tank with a sheet of plastic to keep out air currents, for example, and carefully controlling the room’s temperature. He also filled the tank by swirling water in clockwise, so that if the water drained counterclockwise, the direction would not have been influenced by how the tank was filled.

After 24 hours of letting the water settle, Shapiro carefully pulled the plug from the end of the hose, gently placing above the drain a small float made of two crossed slivers of wood an inch long. It took about 20 minutes for the tub to drain completely. For the first 12 to 15 minutes, the float remained motionless. Then it began to rotate almost imperceptibly, counterclockwise, reaching a peak speed of approximately one revolution every three to four seconds. 

Proving that the Coriolis effect can be detected in a bathtub-­size tank, albeit under carefully controlled conditions, was a remarkable achievement. At MIT’s latitude of 42°, the effect was “only thirty-millionths that of gravity, which is so small that it will be overcome by filling and even temperature differences and water impurities,” reported one of many newspapers and periodicals covering the experiment. 

Shapiro’s results were published in Nature and verified by colleagues who used his technique to demonstrate a clockwise flow in the Southern Hemisphere. The findings fascinated a curious public of all ages. Shapiro would also become known for explaining and improving the aerodynamics of golf ball dimples, as well as for helping to develop the intra-aortic balloon for heart patients and devices to treat blood clots, asthma, emphysema, and glaucoma. But for more than a decade after the bathtub test, he would receive letters and newspaper clippings from all over the world about what was dubbed the “bathtub vortex” controversy. 

Several students asked how to repeat the experiment for class projects, while a California schoolteacher who had visited 87 countries confessed that pulling bathtub plugs had “become a hobby” for her: “I’ve pulled them from Tierra del Fuego to Barrow, Alaska, and Narvik, Norway, to Capetown and Tasmania.” A baffling letter from a Pennsylvania publisher described attempts to use Shapiro’s work in experiments with plants: “I am trying to duplicate results now in my greenhouse, with geraniums.” A man from Lausanne, Switzerland, wrote several times, insisting that bathtub drainage rotation was linked to barometric pressure. His third letter to Shapiro, who’d graciously replied to the first two, ended “I hope you don’t mind this; I’m enjoying it.”

For a seemingly insignificant problem, the bathtub issue loomed large in his career. When he died in 2004, the first line of his Boston Globe obituary read: “Dr. Ascher Shapiro wanted to get a handle on how fluids move whether they were swirling down the bathtub drain, or flowing through the human body.” 

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