Foucault’s Pendulum

Foucault’s Pendulum at the Panthéon, Paris, © Javi Masa

On January 3, 1851French physicist Leon Foucault started to experiment with his eponymous pendulum, by which he was able to proof the earth‘s rotation. Actually, how can you prove that the earth is a rotating orb in an easy-to-see experiment and – of course – without spaceflight? By today, Foucault’s simple device is part of numerous natural science museums around the world.

Ok, how does Foucault’s pendulum work? The apparatus consists of a tall pendulum free to swing in any vertical plane. The actual plane of swing appears to rotate relative to the Earth. There are only two places on earth, where the plane of oscillation of the pendulum remains fixed: at either the North Pole or South Pole. ‘Fixed’ means fixed relative to the distant masses of the universe while Earth rotates underneath it. Thus, it takes one sidereal day to complete a rotation. So, relative to Earth, the plane of oscillation of a pendulum at the North Pole undergoes a full clockwise rotation during one day; a pendulum at the South Pole rotates counterclockwise.

Leon Foucault was born on September 18, 1819 as the son of a publisher in Paris. After an education received chiefly at home, he studied medicine, which he abandoned in favour of physics due to a fear of blood. He first directed his attention to the improvement of the photographic processes of of Louis Daguerre. Together with Hippolyte Fizeau he carried out a series of investigations on different characteristics of light [1]. In 1850, he was able to measure the speed of light in the course of the so-called Foucault–Fizeau experiment, which is considered as final counterargument of Newton’s corpuscle theory of light when it showed that light travels more slowly through water than through air.

In optics, the Foucault cutting method he developed is still used today to inspect optical surfaces or entire optical systems. Foucault also investigated eddy currents in metals, for which he was awarded the Copley Medal in 1855. He developed a powerful mirror telescope and, based on Johann Gottlieb Friedrich von Bohnenberger‘s Gyroscope of 1810, the gyro compass in 1851-1852.

The same year he started experimenting with the pendulum and in 1851 the very first public exhibition of a Foucault pendulum took place in the Meridian of the Paris Observatory. A few weeks later Foucault was able to display his most famous pendulum at the Paros Pantheon, when he suspended a 28 kg brass-coated lead bob with a 67 meter long wire from its dome. The plane of the pendulum’s swing rotated clockwise 11° per hour, making a full circle in 32.7 hours. The original bob used in 1851 at the Panthéon was moved in 1855 to the Conservatoire des Arts et Métiers in Paris. The experiment caused a sensation in both the learned and popular worlds. In the following year he used (and named) the gyroscope as a conceptually simpler experimental proof.

With a pendulum at the North or South Pole, the plane of oscillation would show a full rotation per star Sunday (23 hours, 56 minutes, 4.091 seconds). This is due to the fact that at these points the earth simply rotates away under the pendulum while it maintains its plane of vibration unchanged (apart from the orbit around the sun). The observed rotation takes place contrary to the sense of the earth’s rotation, i. e. on the North Pole around “right” (i. e. clockwise), on the South Pole “left”. At the equator, however, the oscillation plane of the pendulum does not rotate at all in relation to the ground. The closer you get to the poles, the stronger the rotation. The physical interpretation is that in relation to an earth-solid coordinate system – i. e. seen from the natural position of the human being – a Coriolis force acts on the pendulum body at right angles to its direction of movement, which causes a deviation to the right in the northern hemisphere and a deviation to the left in the southern hemisphere; only at the equator the distracting force is absent, because here the Coriolis force has no component parallel to the earth’s surface.

Foucault’s pendulum also has prominent literary references, as e.g. in Umberto Eco‘s eponymous novel,  often also cited as ‘The thinking man’s DaVinci Code‘, in which the ‘mother of all conspiracy theories’ culminates in an arcane ritual carried out unter the pendulum.

At yovisto you can learn more about the life and works of Leon Foucault in the lecture of Dr. Matt Nehring from Adams State College, Colorado, entiteled ‘The Scientific Contributions of Leon Foucault‘.

References and further Reading:


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