Foucault pendulum | Earth’s rotation on display

Invented by the 19th century French physicist Jean Bernard Léon Foucault, the pendulum provides simple physical proof that the earth rotates on its axis

Foucault pendulum, the 19th-century experiment that exemplified the earth’s rotation without complex calculations, has found a new home in the recently inaugurated Parliament building of India. The pendulum features in Parliament’s Constitutional Gallery area. It was designed and installed by the National Council of Science Museums (NCSM), Kolkata.

Invented by the 19th century French physicist Jean Bernard Léon Foucault, the pendulum provided simple physical proof that the earth rotated on its axis.

However, as E. Islam, a member of the NCSM team that has built Foucault pendulum models in India and abroad, pointed out in a research paper, the invention was actually an accident. According to Mr. Islam, Foucault was setting up a long and thin metal rod in a lathe when he accidentally plucked it, causing the end of the metal rod to vibrate in the same plane. Its other end rotated while being fixed on the headstock of the lathe.

The incident paved the way for the current version of Foucault pendulum. To test the theory, Foucault suspended a short pendulum from the chuck of a vertical drill press and set it to oscillation. He then started the drill press, and noticed that the pendulum swung in its original plane, irrespective of the fact that its mounted end was rotating. Foucault knew he was onto something. He set up an 11-metre-long wire in the Paris Observatory for analysis and found that it too rotated clockwise.

The physicist first set up the public display of the pendulum at the Pantheon in Paris in 1851. It consisted of a hollow brass sphere filled with lead to reach 28 kg in mass. It measured 17cm in diameter and was suspended from a 67-metre-long pendulum.

According to his findings, it is much easier to understand the phenomenon of the earth’s rotation using the pendulum at the Poles than it is at lower latitudes. At the Poles, the pendulum’s plane rotates once every 24 hours (which is the approximate period for one rotation of the earth), while at the Equator, it does not rotate at all. This is because the earth rotates faster at the Equator than it does at the Poles because it is wider in the centre and hence needs to cover more area in the same time period as compared to the North or South Pole.

The pendulum swings across a plane, which is the surface swiped by the motion of the sphere, also called the pendulum’s bob. As the earth rotates, the plane of the pendulum’s swing appears to rotate slowly. However, it’s not the pendulum, but the earth itself, that is rotating.

Coriolis effect

This relative motion explains the Coriolis effect. Coriolis force is a phenomenon that appears to act on objects in motion in a rotating reference frame, like the earth. In the Northern Hemisphere, Coriolis force causes moving objects to be deflected to the right, while its effect is the opposite in the Southern Hemisphere. This deflection is called the Coriolis effect. The direction in which Foucault pendulum swings is in line with the Coriolis effect. With each swing, the bob of Foucault pendulum moves a little to the right in the Northern Hemisphere and vice versa in the South. This is why the plane of the swing is observed to have rotated in the clockwise direction in the Northern Hemisphere over a period of time.

The Royal Astronomical Society, London, states that certain conditions must be satisfied for accuracy of Foucault pendulum. The pendulum must be allowed to swing freely, independent of any torque, in any plane. The bob must be heavy, and the string must be long to reduce air resistance effect. The pendulum must be released from rest smoothly to avoid any knee-jerk motions and to ensure that it swings in a plane.

For people standing on the surface of the earth, rotation is not a noticeable part of the daily life. This is why if the pendulum is installed at the North Pole, it will swing as the earth rotates underneath it, and the plane of the swing will appear to rotate one full circle in 24 hours, like the earth’s rotation. However, a pendulum at the Equator appears to remain in the same plane because it rotates along with the earth.

India’s first Foucault pendulum display was commissioned in 1991 by the Inter-University Centre for Astronomy and Astrophysics, Pune. It was installed by the NCSM in 1993.

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