What is the Foucault pendulum in the new Parliament building?

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The Foucault pendulum, named for a French physicist, is a deceptively simple device used to illustrate the earth’s rotation.

 

The new Parliament building in New Delhi inaugurated by Prime Minister Narendra Modi on May 28, 2023. | Photo Credit: PTI

 

One of the features of the new Parliament building in New Delhi, inaugurated on May 28, is a Foucault pendulum suspended from its ‘Constitutional Gallery’ area. It has been designed and installed by the National Council of Science Museums (NCSM), Kolkata.

 

The Foucault pendulum is named for Léon Foucault (1819-1868), the French physicist who first devised the apparatus in the mid-19th century. It is a deceptively simple device used to illustrate the earth’s rotation. At the time Foucault set up the first public display of the pendulum, the earth’s rotation was a well established fact. His achievement, instead, was to provide a proof that didn’t involve intricate astronomical observations and calculations.

The pendulum consists of a heavy bob suspended at the end of a long, strong wire from a fixed point in the ceiling. As the pendulum swings, the imaginary surface across which the wire and the bob swipe is called the plane of the swing.

 

If the pendulum is installed at the North Pole, the pendulum will basically be swinging as the earth rotates ‘below’. But someone standing on the earth’s surface doesn’t notice the planet’s rotation (without e.g. looking up at the sky from time to time); instead, to them, the plane of the swing will seem to rotate by a full circle as the earth completes one rotation.

 

If the pendulum is installed over the equator, the plane won’t appear to shift at all because it will be rotating along with the earth. On any other latitude, the plane will shift through 360º in “one sidereal day divided by the sine of the latitude of its location”, per a Brown University note.

 

A Foucault pendulum is not a simple matter of setting up a pendulum with large parts. It must be designed, installed, and set swinging in such a way that the bob’s motion is influenced to the extent possible only by gravity.

 

In 1991, the then-new Inter-University Centre for Astronomy and Astrophysics, Pune, commissioned the country’s first Foucault pendulum for public display from NCSM. After several studies and failed tests, NCSM installed the setup in 1993.

 

NCSM subsequently installed another Foucault pendulum in the Queensland Science Museum, Brisbane. E. Islam, a member of the team that built these setups and later director of the Birla Industrial and Technological Museum, Kolkata, wrote in 2010:

 

“As was expected from theory, the pendulum at Pune apparently turned 4.86º per hour clockwise, while in Brisbane it turned 6.92º per hour anti-clockwise. The success of our new design was thus unmistakably established by experimental results from both the hemispheres.”

 

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