From his journal, we know that he made the discovery at exactly two o'clock in the morning on January 6, 1851. He was down in the cellar of the house he shared with his mother, located at the corner of the rue de Vaugirard and rue d'Assas -- in the heart of the intellectual Left Bank of Paris and within the immediate area in which Gertrude Stein and Picasso would live during the next century. He had been working feverishly in the cellar for weeks, but no one walking on the fashionable street above could suspect that down below an experiment was being prepared -- one that would forever change the way we view the world.
Jean Bernard Léon Foucault (Léon Foucault to all who knew him) was thirty-two years old. He was not a trained scientist, but he already had a few scientific achievements to his credit, including a clever experiment to measure the speed of light. And he could claim credit for some inventions as well, including a design for light in microscopy and a way of regulating theatrical lighting. But during the last few months of 1850 and into 1851, Léon Foucault had been concentrating all his efforts on a different kind of problem. He was attempting to solve the most persistent scientific problem of all time: one that had plagued Copernicus, Kepler, Descartes, Galileo, and Newton in the sixteenth to the eighteenth centuries, and that -- surprisingly -- remained unresolved as late as Foucault's own time.
He had prepared his experiment carefully, perfecting it during long hours of concentrated work in his cellar over a period of months. Foucault's remaining problems with the experiment were technical ones, and he was an expert at doing precision work with his hands. He worked with wires, metal cutters, measuring devices, and weights. He finally secured one end of a 2-meter long steel wire to the ceiling of the cellar in a way that allowed it to rotate freely without resulting torque. At the other end of the wire, he attached a 5-kilogram bob made of brass. Foucault had thus created a free-swinging pendulum, suspended from the ceiling.
Once the pendulum was set in motion, the plane in which it oscillated back and forth could change in any direction. Designing a mechanism that would secure this property was the hardest part of his preparations. And the pendulum had to be perfectly symmetric: Any imperfection in its shape or distribution of weight could skew the results of the experiment, denying Foucault the proof he desired. Finally, the pendulum's swing had to be initiated in such a way that it would not favor any particular direction because a hand pushed it slightly in one direction or another. The initial conditions of the pendulum's motion had to be perfectly controlled.
Since such a pendulum had never been made before, the process of building it also required much trial and error, and Foucault had been experimenting with the mechanism for a month. Finally, he got it right. His pendulum could swing in any direction without hindrance.
On January 3, 1851, Foucault's apparatus was ready, and he set the device in motion. He held his breath as the pendulum began to swing. Suddenly the wire snapped, and the bob fell heavily to the ground. Three days later, he was ready to try again. He carefully set the pendulum in motion and waited. The bob swung slowly in front of his eyes, and Foucault attentively followed every oscillation.
Finally, he saw it. He detected the slight but clearly perceptible change he was looking for in the plane of the swing of the pendulum. The pendulum's plane of oscillation had moved away from its initial position, as if a magic hand had intervened and pushed it slowly but steadily away from him. Foucault knew he had just observed the impossible. The mathematicians -- and among them France's greatest names: Laplace, Cauchy, and Poisson -- had all said that such motion could not occur or, if it did, could never be detected. Yet he, not a mathematician and not a trained physicist, somehow always knew that the mysterious force would be there. And now, he finally found it. He saw a clear shift in the plane of the swing of the pendulum. Léon Foucault had just seen the Earth turn.
Copyright © 2003 by Amir D. Aczel
Leon Foucault and the Triumph of Science
Leon Foucault and the Triumph of Science
Pendulum is a fascinating journey through the mind and findings of one of the most important and lesser-known characters in the history of science. Through careful research and lively anecdotes, world-renowned author Amir D. Aczel reveals the astonishing range and breadth of Foucault's discoveries. For, in addition to offering the first unequivocal proof of Earth's rotation, Foucault gave us the modern electric compass and microscope, was a pioneer in photographic technology, and made remarkable deductions about color theory, heat waves, and the speed of light.
At its heart, Pendulum is a story about the illustrious period in France during the Second Empire; the crucial triumph of science over religion; and, most compelling, the life of a struggling, self-made man whose pursuit of knowledge continues to inform our notions about the universe today.