Jean Bernard Léon Foucault | Biography

Léon Foucault was born in Paris on September 19, 1819. Due to his poor health, he was privately educated at home. He chose medicine as his vocation and practiced as a physician, but he soon left the profession because he could not stand the sight of blood. He became a science theorist, producing textbooks of arithmetic, geometry and chemistry and writing a column covering scientific principles for a newspaper. Foucault's interest in science and technology was unquenchable, and he collaborated with French physicist Armand Fizeau in a number of ventures and discoveries.

Foucault and Fizeau were the first to use the daguerreotype to photograph the surface of the Sun in 1845. Such photographs required long exposures from Foucault's camera, so he invented a pendulum-driven device with the ability to track the sun (or stars, for that matter) as their positions in the sky changed through the day. He noticed that the pendulum, if it was turned, tried to keep swinging in the same direction in which it was started.

It occurred to him that since the pendulum had this tendency to stay swinging in one plane, it could, in effect, detect the Earth's rotation. Though the theory of a rotating earth was already accepted as correct at this time, no one had ever been able to provide experimental proof that the Earth, and not the Sun and stars, were moving. Foucault made numerous experiments at home in 1851, which led him to believe he needed a larger pendulum. He set up a pendulum 36 feet (11 m) long in the Paris Observatory, and by the following year felt ready to "go public." He gave a spectacular demonstration before a large crowd at the Panthéon in Paris in which he suspended a large iron ball from a wire that was more than 200 ft. (60 m) long. When he set the pendulum swinging, a spike on the bottom of the ball scratched a line in sand on the floor. As time passed, the line shifted. Since the pendulum could not arbitrarily change the direction it was moving, the shift had to be the result of the Earth rotating. The system of planetary motion devised by Nicholas Copernicus three hundred years earlier was at last verified with direct proof.

Taking the results of the pendulum experiment one step farther, Foucault realized that other swiftly rotating objects would behave the same way. When he spun a wheel on a shaft and found that it resisted his attempts to change the direction in which it was pointed, the gyroscope was born. It eventually became an integral part of compasses.

Meanwhile, Fizeau had become the first person to measure the speed of light using an outdoor earth-based method. (His result was close, but was 5 percent too high.) Obtaining this fundamental yardstick of astronomy had proved to be a major problem since the time of Johannes Kepler and Galileo. Working independently of Fizeau, and entirely within a laboratory, Foucault came to within 1 percent of the actual speed. In 1850 he measured the speed of light as it passed through water and found that it decreased slightly. That had important implications; at the time there were two theories regarding the nature of light. According to one theory, light moved as a wave; according to another, it moved as a stream of particles. Foucault's results tipped the balance toward the wave theory, and he received a Ph.D. for his work.

In 1857 Foucault developed a method of silvering concave glass to make mirrors for reflecting telescopes. Isaac Newton had invented a design that used a polished speculum (composed of several metals) to reflect light through an eyepiece. The speculum was prone to tarnishing and that limited the reflecting telescope's ability. The refracting telescope was superior, but it was limited by the size of its lenses. Foucault's mirrors were lighter, less prone to tarnishing, and easy to re-silver if necessary. They gave reflecting telescopes a clear advantage over refractors.

Foucault's personal life was fairly uneventful. He was slavishly devoted to his work, which eroded his health. On February 11, 1868, he died at the age of 48 from a brain disease.