Pendulum: Léon Foucault and the Triumph of Science by Amir Aczel. Atria, $25, 275 pages.


The Chapel Hill News


November 9, 2003

Watching the World Turn


By Phillip Manning


Nicolaus Copernicus stated the hypothesis over 450 years ago in his classic “On the Revolutions of Heavenly Spheres”: the Earth is a ball rotating on its own axis. Almost nobody believed him. And why should one believe him? When you ride a horse, the motion creates a steady breeze in your face, but no constant gale blows when you stand still on the supposedly moving Earth. How can one be certain that the Earth is rotating?

Léon Foucault, the man who proved Copernicus’s big idea, developed no high-powered mathematical theories like Newton or Einstein, but even as a boy, he was a genius with his hands, designing and building precision toys and gadgets. He entered medical school, but dropped out because he couldn’t stand the sight of blood. Despite his abbreviated education, science intrigued him, and he attacked the problem of proving that the Earth rotated. In the basement of his mother’s Paris house, using wires and weights and a clever universal joint he devised, Foucault assembled a pendulum that could swing freely in any direction. He tested it, got the result he expected, and began to prepare for one of the great public experiments in the history of science.

Amir Aczel tells Foucault’s story in his new book “Pendulum” (Atria, $25). On February 2, 1851, Foucault sent the following invitation to every scientist in Paris: “You are invited to see the Earth turn tomorrow, from three to five, at Meridian Hall of the Paris Observatory.” The scientists who assembled the next day saw a wire over 30 feet long suspended from the ceiling. At the end of the wire was a symmetrical brass bob designed to swing in a perfectly straight line. The bob was attached to a wall by a short strand of woolen thread, which would enable Foucault to start the pendulum with no sideward motion. Marked on the floor beneath the pendulum was a long straight line. Then, Foucault burned the string holding the bob, and the released pendulum began its stately arc over the floor.

As the scientists watched, the swing of the pendulum began to change in relation to the line on the floor. But no force other than Earth’s gravity was acting on it, and the crowd knew what they were seeing. The pendulum’s arc had not changed; the Earth was rotating beneath it.

The key to the experiment is the universal joint, which allows the pendulum to swing in an unchanging plane as the Earth rotates. If the pendulum were hung from a hook, say, the hook would rotate with the Earth, twisting the wire and causing the pendulum’s plane of rotation to change.

To visualize how a Foucault pendulum works, consider one located at the North Pole. Although the plane of the pendulum never changes, it appears to move in a full circle every 24 hours because the Earth is rotating beneath it. As the pendulum is moved farther south, a complete rotation takes increasingly longer, and at the equator, the plane of oscillation doesn’t change at all. Foucault realized that the pendulum’s rate of rotation would change with its location, and he derived an equation that predicted rotational time at any latitude.

A few months later, Foucault suspended his pendulum in the Panthéon, one of the grandest buildings in Paris. Beneath its dome, Foucault placed a large wooden circle marked off in degrees, so the public could watch the world turn below the pendulum’s arc. Emperor Napoleon III came, and thousands of ordinary Parisians poured in to watch the pendulum and hear its inventor explain the demonstration. The plane of the pendulum was “fixed in absolute space,” he said, “while we and the planet rotated right under it.” Foucault’s experiment was repeated all over the world, from London to Rio de Janeiro; each time it performed exactly as Foucault predicted.

Foucault was famous, and his fame grew after he invented the gyroscope, a spinning disc that maintains a constant orientation in space. Unfortunately, public celebrity did not translate into scientific respect. Despite Foucault’s numerous applications and pleading letters, the Academy of Sciences, France’s most prestigious scientific body, repeatedly turned him down for membership. The Academy was dominated by theoreticians, and Foucault was an undereducated hands-on experimenter, a mere engineer.

Foucault didn’t give up; he continued to apply for every opening in the Academy. Finally, 14 years after his grand experiment, Foucault was admitted. It was more than a personal triumph, Aczel writes, “it announced that not only theoreticians could practice it [science] effectively,” but that experimentalists could contribute as well.

Léon Foucault died three years after his admission to the Academy, but the honors continued. In a final tribute, his name was etched into the structure of a tower constructed by another engineer, Gustave Eiffel.
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