In February 1851, Léon Foucault published in the Comptes rendus his famous pendulum experiment performed at the “Observatoire de Paris”. This ended two centuries of quest for an experimental demonstration of Earth rotation. One month later, the experiment was reproduced at larger scale in the Panthéon and, as early as the summer of 1851, it was being repeated in many places across the world. The next year, Foucault invented the gyroscope to get a still more direct proof of Earth rotation. The theory relied on the masterpiece treatise of Laplace on celestial mechanics, published in 1805, which already contained the mathematical expression of the force that would be discovered by Gustave Coriolis 30 years later. The idea of a fictitious inertial force proposed by Coriolis prevailed by the end of 19th century, as it was conceptually simpler than Laplace's approach. The full theory of the Foucault pendulum, taking into account its unavoidable imperfections, was not obtained until three decades later by Kamerlingh Onnes, the future discoverer of liquid helium and superconductivity. Today, Foucault's exceptional creativity is still a source of inspiration for research and the promotion of science through experimental proofs widely available to the public.
Léon Foucault, also called Jean Foucault, in full Jean-Bernard-Léon Foucault, (born September 18, 1819, Paris, France—died February 11, 1868, Paris), French physicist whose “Foucault pendulum” provided experimental proof that Earth rotates on its axis. He also introduced and helped develop a technique of measuring the absolute speed of light with extreme accuracy.
Foucault was educated for the medical profession, but his interests turned to experimental physics. With Armand Fizeau, he began a series of investigations of light and heat. By 1850 he established that light travels slower in water than in air. In the same year he measured the speed of light, finding a value that is within 1 percent of the true figure.
In 1851, by interpreting the motion of a heavy iron ball swinging from a wire 67 metres (220 feet) long, he proved that Earth rotates about its axis. Such a “Foucault pendulum” always swings in the same vertical plane. But on a rotating Earth, this vertical plane slowly changes, at a rate and direction dependent on the geographic latitude of the pendulum. For this demonstration and a similar one using a gyroscope, Foucault received in 1855 the Copley Medal of the Royal Society of London and was made physical assistant at the Imperial Observatory, Paris.
He discovered the existence of eddy currents, or “Foucault currents,” in a copper disk moving in a strong magnetic field, constructed an improved mirror for the reflecting telescope, and in 1859 invented a simple but extremely accurate method of testing telescope mirrors for surface defects.