History of the Foucault Pendulum

This pendulum is named after the French physicist Léon Foucault, who was the first to use it as a demonstration of the Earth’s daily rotation. It represented a significant shift in scientific methodology, as it utilized laboratory apparatus instead of relying solely on astronomical observations, igniting considerable excitement in the scientific community.

Initially, Foucault employed a two-meter-long pendulum in his Paris basement. Later, he expanded this experiment at the Paris Observatory with a pendulum measuring about 11 meters. The first public demonstration occurred in 1851 beneath the dome of the Paris Panthéon, where he used a 28-kilogram ball suspended from a 67-meter wire. This substantial length and mass enabled the pendulum to complete one oscillation every 16.4 seconds, taking several hours to come to a complete stop.

Foucault's setup included a circle of sand measuring 6 meters in diameter, allowing the pendulum's tip to trace lines in the sand as it moved. The circle was secured and burned to prevent any lateral movement when the pendulum was set in motion. Over an hour of oscillation, the pendulum rotated more than 11 degrees clockwise, taking a total of 32 hours for a complete rotation.

Principle of Operation

To visualize the Foucault pendulum, imagine it hanging at the North Pole from a point directly above the pole. If you displace the pendulum slightly from its vertical position and release it, it will swing back and forth in the same plane. Meanwhile, the Earth rotates beneath it once every 24 hours. Observing the pendulum over time, you'd witness its oscillation plane shifting sideways (clockwise). However, this shift is due to the Earth moving relative to the pendulum's oscillation plane.

At the North Pole, the oscillation plane completes a full rotation in 24 hours. Moving south, for instance to New York, complicates the scenario, as the city—and the pendulum—rotate around the Earth's axis. Consequently, the pendulum's oscillation plane rotates more slowly, taking longer to complete a turn. The farther south you go, the slower this apparent rotation becomes; at the equator, this effect is negligible. South of the equator, the pendulum's apparent rotation resumes, but in the reverse direction.

Building a Foucault Pendulum

Creating your own Foucault pendulum and conducting this experiment is quite achievable. A longer suspension and a heavier weight are essential. A 5-meter rope with a weight of about 20 kilograms should suffice for visible movement, but using a heavier weight and longer rope will enhance the observable displacement.

The first video titled "Direction of Earth's Spin demo" provides a practical demonstration of the pendulum's function and the principles of Earth's rotation.

The second video, "PHYSICS MADE EASY - the Earth's axis of rotation - basics," simplifies the concepts surrounding Earth's rotation and the mechanics of the pendulum.

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