Shapiro’s Bathtub Experiment


Over forty years ago, in the 1960s, the world briefly became captivated with how a bathtub drains. Did something called the Coriolis effect influence the twirling water?

The Earth’s rotation influences how fluids swirl on the planet’s surface. It’s why low-pressure systems in the northern hemisphere twist counterclockwise. This phenomenon, known as the Coriolis effect, is the appearance of an object to deflect to one side in a rotating reference frame. Since it is such a tiny effect on small scales, no one had yet proven that this inertial force actually affects how water leaves a bathtub, despite many previous efforts.

In 1962, the same year that Watson and Crick received their Nobel Prize for the discovery of the double helix, MIT professor Ascher Shapiro, an expert in fluid mechanics, set up an elaborate test to try to change that. Shapiro’s elementary experiment, which started with a bathtub, quickly turned into a complicated and ambitious undertaking that involved a tank six feet wide and six inches deep.

The Coriolis effect at MIT’s latitude, 42°, was just “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 that covered the results of Shapiro’s experiment. After much tinkering to cancel out these interferences, and presumably a hefty water bill, Shapiro found the answer: the Coriolis effect does indeed cause a bathtub vortex in the northern hemisphere to swirl counterclockwise. Full Article »