Biography, France history

Léon Foucault and the Pendulum


Léon Foucault and the Pendulum

Niall C.E.J. O’Brien

     On the left bank of the Seine, high over the Latin Quarter and the city of Paris stands the Pantheon, the temple of the French nation. Within its walls are images of the great people of France while in its underground crypt the tombs of many famous people lie in peace and honour. The building is designed like a Greek cross with four equal sides capped by a large colonnaded dome. Beneath this dome, in the central crossing of the building, hangs a twenty-eight kilogramme pendulum on a sixty-seven meter cable. This is Foucault’s pendulum where the rotation of the Earth on its axis is measured.


Jean Bernard Léon Foucault

Early life

     The inventor of this pendulum that bears his name was Jean Bernard Léon Foucault. He was born in Paris on September 18, 1819.[1]Foucault was the son of a male prostitute in the city. 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 L. J. M. Daguerre’s photographic processes. For three years he was experimental assistant to Alfred Donné (1801–1878) in his course of lectures on microscopic anatomy.[2]

Experiments with Light

     From this work Foucault developed an interest in the properties of light and the movements of the Earth. In September 1855 he discovered the existence of eddy currents. These are produced in a conductor moving in a magnetic field.[3] Foucault observed that the force required for the rotation of a copper disc becomes greater when it is made to rotate with its rim between the poles of a magnet, the disc at the same time becoming heated by the eddy current induced in the metal.[4]

     Foucault continued his scientific studies. In the seventeenth century, a Dutch physicist, Christian Huygens had studied the polarization of light and proposed a wave theory for the movement of light. During the early 1800s, two physicists, Thomas Young of England and Augustine J. Fresnel of France did much to prove Huygens’s theory. By the mid-1800s Foucault was working with another French physicist, Armand Hippolyte Louis Fizeau on measuring the speed of light.[5]  Their collaboration was encouraged by another French physician, François Arago. In 1838 Arago had proposed an experimental method of calculating the speed of light but different circumstances prevented him from conducting practical experiments. By 1850 Arago was ready to implement his theories but his poor eye sight failed him. It was left to Foucault and Fizeau to make the dream a reality.[6] Foucault and Fizeau were both Parisians of the same age (both born in September 1819) and had a shared interest in the properties of light.[7] Also like Foucault, Hippolyte Fizeau’s name appears on the Eiffel Tower honour list of scientists.[8] Using a revolving mirror they made accurate measurements of its speed. In 1862, Foucault determined the speed of light to be 298,000 km/s. This was 10,000 km/s less than that obtained by previous experimenters and only 0.6% off the currently accepted value.[9]

     In 1850 Foucault proved that light travels more slowly in water than in air. He further showed that the speed varies inversely with the index of reflection.[10] This work, using what became known as the Foucault-Fizeau apparatus, dismissed Isaac Newton’s corpuscle theory of light; a French victory over the English.[11]

     This experience with mirrors allowed Foucault to make improvements in the mirrors used in reflecting telescopes.[12] In 1857, he invented the polarizer which bears his name, and in the following year he devised a method of testing the mirror of a reflecting telescope to determine its shape. The so-called “Foucault knife-edge test” allows the worker to tell if the mirror is perfectly spherical or has non-spherical deviation in its figure. Prior to Foucault’s publication of his findings, the testing of reflecting telescope mirrors was a “hit or miss” proposition.[13]

Earth’s rotation

     In 1851 Foucault wanted to prove the Earth’s rotation. For this he needed a large pendulum for as he said “since the arc of a pendulum’s swing is fixed in relation to the atmosphere, the rotation of the earth will be rendered visible by a rotation of the arc of the swing.”[14] Foucault’s friend, François Arago had in 1819 used the pendulum to take measurements of the Earth and this was a source for exploration.[15]

     It was many centuries before Foucault’s time that the pendulum as a time measuring instrument was discovered. Galileo Galilei from Italy is credited with that discovery. One day as a young man he was attending service in the Cathedral of Pisa and noticed a lamp hanging from the roof was swinging slowly to and fro on its chain. Using his own pulse he observed that no matter how much or how little it swung, the action was perfectly regular. From that time the pendulum was developed to measure time in clocks.[16]  

    But where to place what would have to be a large pendulum. An outside location would not do as the wind would move the pendulum and so give a false reading. Foucault needed a tall building and the Pantheon was perfect for the job. After some debate he got permission to conduct his experiment there. In the centre of the building, under the great dome, where the reliquary of Saint Genevieve was once supported by four caryatids, Foucault installed his pendulum. A weight of twenty-eight kilogrammes, called a bob, was suspended from beneath the dome using an iron cable sixty-seven meters long.[17]



    The pendulum in the Pantheon 


     Foucault understood that a pendulum does not just swing back and forth at a constant rate but it does so through a small arc. Using this he concluded that as the Earth turns on its axis, the plane of swing of a pendulum rotates because of the heavy bob’s inertia resistance to change of its absolute direction, until it has returned to its original orientation. Foucault also found that this rotation of the pendulum is not uniform. Rather its rotation depends on its latitude. For example, in London it rotates in thirty hours.[18]

    The experiment was a success and there after a pendulum used to determine the rotation of the Earth was called a Foucault pendulum. Numerous visitors were attracted to the Pantheon to see this wonder of physics. In 1855, he received the Copley Medal of the Royal Society for his ‘very remarkable experimental researches’. Earlier in the same year he was made physicist at the imperial observatory at Paris.[19]But if Foucault had successfully timed the rotation of the Earth, his political timing was way off the mark.

Changing political climate

     Many years before in 1816 Louis XVIII signed a decree conferring the Pantheon in its totality back to the Catholic Church. In the period 1806 to 1815 the building had a dual function with the upper part as a religious church and the crypt as a ‘hall of fame’ to the great figures of France. Thus the French Revolution’s home of heroes became a religious church again as it had been up to 1791. The relics of Saint Genevieve were restored and buildings decorations were changed to reflect its new function.[20]

     Thus the Church of Saint Genevieve remained until July 1830. The three day revolution of that month ended the reign of the Bourbons and Charles X was replaced by Louis-Philippe d’Orleans. The new regime initially supported the revolutionary values of the people and on August 26, 1830 a decree once again turned the Church of Saint Genevieve into the Pantheon. But that was as far as Louis-Philippe was prepared to go. Throughout his reign the building remained unused, the crypt was closed to the public and no pantheonizations took place.[21]

     In February 1848 Louis-Philippe abdicated the throne and the Second Republic was declared. The provisional government designated the Pantheon as the “Temple of Humanity” and planned a major redecoration project. Towards the end of 1848 the government became more conservative in its outlook and the big project was slowly abandoned.



The Pantheon, ex Church of Saint Genevieve

     When Léon Foucault arrived in 1851 to use the Pantheon for his experiment the building was in need of something new to revive its duping spirit. The numerous visitors showed that the building could not just honour the great people of France who were dead but show case the great people still living. Yet it was not just Foucault’s pendulum that was swing in the year of 1851. The political pendulum was also moving.

     The president of the Republic, Louis-Napoleon Bonaparte, was elected in December 1848 with 5,434,226 votes out of 7,327,345 votes casted to serve a four year term. Yet from the outset the president faced a delicate relationship with politicians in the National Assembly and even hostility from within his own cabinet.[22] Thus as Louis-Napoleon faced elections in 1852 he was very conscious that his famous surname was not enough to secure victory and he needed the support of powerful lobby groups.

     One such group that the president sought was the Catholic party, an important political party of the time who could rally a large vote come the election day. The Catholic party had long opposed Foucault’s experiments as coming too near to realising a world where God was only a small part and science was the new creator.[23] Events now moved at a faster pace than Foucault’s pendulum. On December 2, 1851, Louis-Napoleon staged a coup d’état. The heads of the Republican and Monarchical parties were arrested and the National Assembly was dissolved. Troops stationed across Paris sternly repressed any attempts of a popular uprising. Later in that December a plebiscite was organised to elect a president for ten years.[24] In return for the political support of the Catholic Party, Louis-Napoleon had ordered the discontinuation of the Foucault pendulum at the Pantheon on December 1st. On December 6, a decree was issued returning the Pantheon to the church of Saint Genevieve and elevating it to a “National Basilica.”[25] On December 21, Louis-Napoleon was re-elected president of France for ten years.[26]

The pendulum without a home

      Léon Foucault had now no home for his pendulum experiment and placed it in storage. His mind on the other hand was far from storage. Necessity is the mother of invention and so in 1852 Foucault invented a gyroscope to continue to demonstrate the Earth’s rotation. He used the word gyroscope for this new invention because it comes from two Greeks words: gyros, meaning revolution and skopein which means to view. Thus a gyroscope means to view the revolutions of the Earth.[27]

     Foucault continued with many other experiments. In 1865 his papers on a modification of Watt’s governor appeared, upon which he had for some time been experimenting with a view to making its period of revolution constant, and on a new apparatus for regulating the electric light. In the year Foucault showed how, by the deposition of a transparently thin film of silver on the outer side of the object glass of a telescope, the sun could be viewed without injuring the eye.[28] In this latter work he collaborated with his old friend Armand Fizeau. Together they took the first clear photograph of the sun.[29]

     For these and other works his contemporaries recognised his ability. In 1862, he was made a member of the Bureau des Longitudes and an officer of the Légion d’Honneur. In 1864 he was made a member of the Royal Society of London, and the next year a member of the mechanical section of the Institute.[30]

The new home of the pendulum

     In later times Foucault’s pendulum came out of storage and found a home in the Musée National des Arts et Métiers.[31] Today it is suspended from the chancel ceiling of the church of Saint Martin des Champs which is attached to the Musée. This chancel was built between 1130 and 1140 and shows the transition from Romanesque to Gothic. The church was part of the larger abbey of Saint Martin des Champs, established in 1061 and reconstructed in the 13th century.[32]

France after 1870

     Meanwhile the political pendulum was changing again. After the battle of Sedan, on September 2, 1870, the Second Empire collapsed and the Third Republic was declared. But the war with Prussia continued. During the siege of Paris, the dome and other parts of the Church of Saint Genevieve were damaged. Following a harsh armistice the people of Paris revolted against their government and formed the Paris Commune. The Church of Saint Genevieve was one of their chief strong holds and one of the last places to surrender. Further damage was done to its dome, arches and exterior walls. The Paris Commune was ruthlessly suppressed in May 1871 and the new National Assembly was governed by a conservative Catholic royalist party.

     For a time it appeared that the republican ideas of the various revolutions would just fade away in the new order which was referred to as the Ordre moral. But the royalist majority in the National Assembly could not agree on who should be the next king of France. The Republicans slowly gained numbers in the Chamber of Deputies. On January 30, 1879 President Patrick McMahon resigned and a Republican majority government was formed. But the new government procrastinated as much as the previous government. It took the death of the ardent republican, Victor Hugo in May 1885 to move the pendulum to full change. The Church of Saint Genevieve again became the Pantheon.[33]

The death of Léon Foucault

     Sometime afterwards a replica Foucault pendulum was installed under the dome of the Pantheon where it fascinates visitors to this day. Foucault did not live to see that day of honour. Léon Foucault died on February 11, 1868 from what was probably a rapidly developing case of multiple sclerosis. He was buried in the Cimetière de Montmartre.[34]

Later honours for Léon Foucault

     Far above the Pantheon and beyond planet Earth Léon Foucault is remembered by having a moon crater named in his honour. TheFoucault is a small lunar impact crater that lies along the southern edge of Mare Frigoris, to the southeast of the crater Harpalus. The outer perimeter of Foucault forms a somewhat irregular circle, with slight outward bulges to the south and northeast. The inner wall of the rim is not notably terraced, and slopes down directly to the uneven floor.[35]

     More closer to home Foucault is named among the scientists and engineers upon the Eiffel Tower. His named is on the south-east side between Poinsot and Delaunay under the first floor balcony.[36]

Further reading

The following books cover the life and work of Léon Foucault. Amir D. Aczel, Pendulum: Léon Foucault and the Triumph of Science (Washington Square Press, 2003); Umberto Eco, Foucault’s Pendulum (Secker & Warburg, 1989);William Tobin, Perfecting the Modern Reflector (Sky & Telescope, October 1987);William Tobin, Léon Foucault (Scientific American, July 1998); and William Tobin, The Life and Science of Léon Foucault: The Man who Proved the Earth Rotates (Cambridge University Press, 2003).


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[1][1] R.T. Ellickson, “Jean Bernard Leon Foucault”, in The World Book (Chicago, 1981), vol. 7, p. 368

[2] = access on 30 June 2011

[3] R.T. Ellickson, “Jean Bernard Leon Foucault”, in The World Book (Chicago, 1981), vol. 7, p. 368

[4] = access on 30 June 2011

[5] Brian J. Thompson, “Optics”, in The World Book, vol. 14, pp. 613-4

[6] = access on 3 July 2011

[7] Anon, “Armand Hippolyte Louis Fizeau”, in Joy of Knowledge (1981), fact index, p. 266

[8] = access on 3 July 2011

[9] = access on 30 June 2011

[10] R.T. Ellickson, “Jean Bernard Leon Foucault”, in The World Book, vol. 7, p. 368

[11] = access on 30 June 2011

[12] R.T. Ellickson, “Jean Bernard Leon Foucault”, in The World Book, vol. 7, p. 368

[13] = accessed on 30 June 2011

[14] Alexia Lebeurre, The Pantheon: Temple of the Nation (Paris, 2000), p. 32

[15] = accessed on 3 July 2011

[16] Anon, “The early astronomers”, in The Pictorial Knowledge, edited by H.A. Pollock  (London, 1940), vol. 7, pp. 304-305

[17] Alexia Lebeurre, The Pantheon: Temple of the Nation (Paris, 2000), p. 32

[18] Anon, “Foucault pendulum”, in Joy of Knowledge (1981), fact index, p. 274

[19] = accessed on 30 June 2011

[20] Alexia Lebeurre, The Pantheon: Temple of the Nation (Paris, 2000), pp. 25, 28

[21] Alexia Lebeurre, The Pantheon: Temple of the Nation (Paris, 2000), p. 30

[22] Ernest D’Hauterive (ed.), The Second Empire and its Downfall: correspondence of Emperor Napoleon III and his cousin Prince Napoleon (London, 1930), p. 52 

[23] Alexia Lebeurre, The Pantheon: Temple of the Nation (Paris, 2000), p. 32

[24] Ernest D’Hauterive (ed.), The Second Empire and its Downfall, p. 56

[25] Alexia Lebeurre, The Pantheon: Temple of the Nation (Paris, 2000), pp. 32-3

[26] Danielle Chadych & Dominique Leborgne, Paris: the story of a great city (London, 2010), p. 80

[27] C.A. Frische, “Gyroscope”, in The World Book, vol. 8, p. 438

[28] = accessed on 30 June 2011

[29] Anon, “Armand Hippolyte Louis Fizeau”, in Joy of Knowledge (1981), fact index, p. 266

[30] = accessed on 30 June 2011

[31] Alexia Lebeurre, The Pantheon: Temple of the Nation (Paris, 2000), p. 32

[32] Giovanna Magi, Paris: a complete guide for visiting the city (Firenze, 1994), pp. 128-9

[33] Alexia Lebeurre, The Pantheon: Temple of the Nation (Paris, 2000), pp. 34, 37

[34] = accessed on 30 June 2011

[35] = accessed on 30 June 2011

[36] = accessed 30 June 2011


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