Léon Foucault quit medicine over blood and proved the Earth turns
Léon Foucault gave up medical school because he could not stand the sight of blood. In 1851 he hung a long, heavy pendulum from the dome of the Panthéon in Paris and let visitors watch its swing slowly turn, visible proof that the floor beneath them was spinning with the planet.
Foucault was born in Paris on 18 September 1819, the son of a publisher, and was mostly educated at home. After abandoning medicine he started out improving Louis Daguerre's photographic methods and spent three years assisting Alfred Donné with lectures on microscopic anatomy. Working alongside Hippolyte Fizeau, he compared the brightness of sunlight with arc lamps and limelight and studied interference and polarisation of light.
Light remained his obsession. In 1849 he showed that the dark absorption lines and bright emission lines at a given wavelength come from the same substance, the difference depending on the temperature of the source. In 1850 a rotating-mirror experiment revealed that light moves more slowly through water than through air, a blow widely seen as finishing off Newton's idea that light is made of particles. Twelve years later, with Charles Wheatstone's revolving mirror, he put its speed at 298,000 kilometres per second, within 0.6 percent of today's accepted value.
The pendulum made him famous. The idea had been tried earlier by Vincenzo Viviani, but Foucault's public version caused a sensation, and copies soon drew crowds in cities across Europe and America. A year later he used a spinning wheel to make the same point more simply and gave the device its name, the gyroscope. In 1855 he noticed that a copper disc spun between magnet poles resists turning and heats up, the effect of what are still called eddy currents or Foucault currents.
He also transformed telescope making. His knife-edge test of 1858 reveals whether a mirror is truly spherical by measuring focal lengths across its zones, replacing guesswork, and amateur mirror grinders still use it because the kit is cheap and simple. He joined the Royal Society in 1864. Foucault died in Paris on 11 February 1868, probably of fast-progressing multiple sclerosis, and his name is among the 72 engraved on the Eiffel Tower.
Source: Léon Foucault
