Jean-Bernard-Léon Foucault Biography (1819-1868)


Jean-Bernard-Léon Foucault, born in Paris on September 19, 1819, was an extremely successful experimental physicist. He, like Galileo, believed experimentation and innovation were the best ways to accurately assess the properties of the natural world.

Originally trained as a physician, Foucault made his living as a highly successful science and mathematics writer, and until he was appointed to a post atthe Paris Observatory later in his career, he conducted his experimental work at home. Some of Foucault's most fruitful work was done in collaboration with other prominent scientists of the day, such as French physicist Armand H.Fizeau (1819-1896).

Using the daguerreotype developed by Louis Daguerre, Foucault and Fizeau produced the first photographs of the sun's surface in 1845. Since Daguerre's photos required long exposure times to produce a clear picture, Foucault had todesign a device for their solar camera capable of following the sun on its daily path across the sky. (This apparent motion is a result of the earth rotating beneath the sun and stars.)

Toward this end, Foucault resurrected the concept of the siderostat, or clock-drive, which had originally been conceived by Christiaan Huygens. Foucault's device used a pendulum to regulate a clock which rotated the solar camera.As he worked with the clock-drive, Foucault noticed that regardless of how the device was turned, the pendulum tended to keep swinging in the direction inwhich it was started. It occurred to Foucault that this tendency could be used to demonstrate that the earth rotates. Foucault surmised that the direction in which a pendulum swung would change with respect to the earth as the earth rotated about its axis, provided the pendulum was allowed to swing freely.

After he had carried out his demonstration successfully in private, Foucaultconstructed a massive pendulum, consisting of a large iron ball suspended from a wire more than 200 feet (60 m) in length. Staging a dramatic exhibition with the device in 1852, Foucault set the pendulum swinging before a large crowd in the Panthéon in Paris. A spike attached to the bottom of the ball scratched a line in a plot of sand scattered on the floor. As time passed,the line shifted, proving the earth was rotating under the pendulum. Three hundred years after Copernicus proposed that the earth rotates about its axis,Foucault had provided the observational evidence.

Foucault applied the same priniciple to a heavy wheel by mounting it onto a shaft and spinning it rapidly. When he turned the shaft with his hands, the wheel resisted his attempts to shift the plane in which it was spinning. This wheel, called a gyroscope, had obvious uses. Navigators quickly found that a spinning gyroscope could accurately detect deviations from a straight course.In addition, the gyroscope could be used to regulate motors and stabilize mechanical devices. Based on his gyroscope, Foucault developed a series of devices in the 1860s that were much more effective at regulating engine velocity than James Watt 's governor. The regulators were used in telescope clock-drives and large steam engine s.

Foucault also provided an improved measurement of the velocity of light. Armand Fizeau (1819-1896) had been the first person to measure light using a terrestrial method in 1849, but his result was 5 percent too high. Foucault, working independently in 1862, built an ingenious device that worked as follows:a small mirror was spun at a rate of 800 times per second. A beam of light was bounced from the spinning mirror onto a stationary mirror 65 feet distant,returning to the spinning mirror. Since the mirror had moved since the beam of light had left it, the beam was deflected slightly. Foucault made calculations based on the amount of deflection, arriving at a rate of 185,058 miles (298,000 km) per second--within 1 percent of the actual speed.

Foucault also introduced the use of silvered glass in the construction of reflecting telescopes in 1857. Originally invented by Isaac Newton (1642-1727),reflecting telescopes use mirrors to gather light. In Newton's time, however,it was necessary to build the mirror from polished speculum metal which tarnished very easily. Foucault's silvered glass was lighter, cleaner and easy tore-coat when necessary. Foucault devised means of testing mirrors and lensesto be used in optical devices.

A scientist with obvious mechanical skills, Foucault inevitably made significant contributions to practical technology. He improved the arc lamp by developing a regulator that allowed the gas fuel to be replaced with electricity, bringing the arc lamp into theaters. He invented a photometer (a device to measure the brightness of light), and improved the performance of induction coils by creating a mercury interrupter.

During his short but productive lifetime, Foucault was also involved with a number of theoretical matters such as the conductivity in liquids, stellar spectra, and the conversion of mechanical work into heat. He died at the age of48 on February 11, 1868.

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