The French physicist Jean Bernard Léon Foucault (1819-1868) is remembered for the Foucault pendulum, by which he demonstrated the diurnal rotation of the earth, and for the first accurate determination of the velocity of light.

Léon Foucault, son of a Paris bookseller, was born on Sept. 18, 1819. He began to study medicine but turned to physics, probably as a result of becoming assistant to Alfred Donné, who was developing a photoengraving process by etching daguerreotypes in connection with his anatomy lectures. This brought Foucault contact with the physicist Hippolyte Fizeau, who was at that time attempting to improve the daguerreotype process, and they collaborated for several years on optical topics. From 1845 Foucault was editor of the scientific section of the Journal de débats. In 1855 he was appointed physicist at the Paris Observatory; in 1864 he was elected a foreign member of the Royal Society of London; and in 1865 he became a member of the Académie des Sciences.

Rotating Frames of Reference

Foucault's first important experimental demonstration was of the earth's rotation, for which he used a pendulum. The plane of motion of a freely suspended simple pendulum appears to rotate; in fact, it is spatially fixed while the earth rotates. Foucault published his account of this in 1851, together with an equation connecting the apparent angular rotation of the pendulum's plane with the angular velocity of the earth and the latitude of the place of the experiment. It created great interest, and the experiment, readily repeatable with simple apparatus, was, and still is, frequently performed in public. In 1852 Foucault gave a further demonstration of the earth's rotation with a freely mounted gyroscope and derived some laws describing its behavior. These experiments, in combination with earlier theoretical work by Gustave Coriolis, led to a clearer understanding of rotating frames of reference. For his work Foucault was awarded the Copley Medal of the Royal Society in 1855.

Determining the Velocity of Light

In 1850 Foucault joined the debate over the then-competing particle and wave theories of light. D. F. J. Arago had demonstrated in 1838 that a crucial test could be made by comparing the velocities of light in air and in a dense medium, and he was experimenting to determine the velocity of light with a rotating-mirror method devised by Charles Wheatstone in 1834. Lack of success and ill health led Arago to pass the task on to Foucault in 1850. Success came in the same year, when Foucault observed a retardation of the velocity of light in water, giving support to the wave theory. He then saw how the rotating-mirror method could be refined to measure the absolute velocity of light in a restricted space. Foucault overcame the technical problems and in 1862 obtained a value of 2.98 x 10¹⁰ centimeters per second, the first accurate measure of this fundamental physical constant.

From 1855, as physicist at the Imperial Observatory, Foucault worked to improve the design of telescopes. As a member of the Bureau of Longitudes from 1862 he improved certain surveying instruments, particularly the centrifugal governor, which aided timekeeping in the use of field-transit instruments. The 1860s saw Foucault turning toward precision engineering and electricity, but he was incapacitated by a stroke in July 1867 and died in Paris on Feb. 11, 1868.

Foucault's ability to recognize fruitful lines of research, so sadly lacking in many of his contemporary countrymen, was combined with an experimental ability of the first order. His early death was a great loss to French science.

Further Reading

• One of Foucault's experimental findings is reprinted in Harlow Shapley and Helen E. Howarth, A Source Book in Astronomy (1929). For general background see Henry Smith Williams, The Great Astronomers (1930).