Anonymous Portrait of Jean Bernard Leon Foucault
Medium Salt print from wet plate negative
Photo Date 1850s Print Date 1850s
Dimensions 8-3/4 x 5-1/2 in. (222 x 140 mm)
Photo Country France
Photographer Country France
Contact Alex Novak and Marthe Smith
About This Image
Jean Bernard Leon Foucault was born in 1819, the son of a French publisher. He showed early skill in making mechanical toys, studied medicine, but shifted to physical sciences at the Paris Observatory. With Armand Fizeau (who was to make considerable advancements to photography), 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 velocity 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 m (220 feet) long, he proved that the 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 utilizing 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. It is interesting to surmise if his friend and collaborator, Fizeau might have take this early image of Foucault.
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