Peering up, you will see a long wire extending towards the ceiling. This is the Focaultsches Pendel. It is not just a pendulum. It is a demonstration of Earth’s rotation. The Focaultsches Pendel is a testament to human curiosity.
Let’s delve into the story of the Focaultsches Pendel. French physicist Léon Foucault conceived it in 1851. He wanted a simple experiment to show Earth’s rotation. He succeeded. The Focaultsches Pendel offered the first direct evidence of this phenomenon.
Imagine a long, heavy pendulum swinging above a circular area. Over time, its plane of oscillation changes. This change isn’t the pendulum itself moving. It is the Earth rotating beneath it. Foucault was inspired by a vibrating rod on a lathe. It vibrated in the same plane despite the lathe’s rotation.
The first public display was in February 1851 at the Paris Observatory. Weeks later, he created a more famous version. This Focaultsches Pendel featured a 28-kilogram brass-coated lead bob. It hung from a 67-metre wire in the Panthéon, Paris.
At the Panthéon’s latitude, the pendulum’s swing rotated about 11.3 degrees per hour. It completed a full circle in approximately 32 hours. Foucault reported a 2.3 mm deflection every oscillation. This required a swing angle of just over two degrees.
Foucault explained his findings in an 1851 paper. He described how, at the North Pole, the pendulum’s plane would remain constant. The Earth’s rotation would be visible against this fixed plane. If the oscillations lasted 24 hours, the plane would complete a revolution.
The original bob was moved in 1855 to the Conservatoire des Arts et Métiers. It was temporarily reinstalled in the Panthéon for the 50th anniversary in 1902. During museum renovations in the 1990s, it was again displayed at the Panthéon in 1995. It returned to the Musée des Arts et Métiers before reopening in 2000.
In 2010, disaster struck. The cable snapped causing irreparable damage to the bob and the marble floor. The damaged original is now displayed separately. An exact copy operates under the Panthéon’s dome since 1995.
At the North or South Pole, the pendulum’s oscillation plane stays fixed relative to the universe. Earth rotates underneath it in one sidereal day. At the equator, the plane remains fixed relative to Earth. Elsewhere, it precesses but slower than at the poles.
The angular speed depends on latitude. A pendulum at 30 degrees south rotates 360 degrees in two days when viewed by an earthbound observer. Setting up a Focaultsches Pendel requires precision. Imperfections can cause veering, masking the Earth’s effect.
Heike Kamerlingh Onnes studied this extensively. He observed the pendulum shifting from linear to elliptic oscillation. Imperfections or wire elasticity can cause a beat between horizontal modes. Launching the pendulum is also critical. Burning a thread avoids unwanted sideways motion.
Air resistance slows the pendulum. Many museum versions use electromagnetic drives to keep it swinging. Ensuring no preferred swing direction is a key engineering challenge. The Focaultsches Pendel here provides a compelling visual. It embodies scientific discovery. It bridges theory and observable reality.