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Léon Foucault's Pendulum

Léon Foucault was a prolific scientist and inventor and is most famously known for pendulums and inventing the gyroscope. In 1851 he caused what can be described as pendulum Mania. After doing a small scale experiment in his basement and a successful larger test and demonstration in an observatory, he published a paper explaining how a pendulum shows the rotation of the earth. This caused a great deal of interest and a more impressive demonstration for the public was created with a larger pendulum in the Pantheon in Paris. It had a 28kg 17cm diameter bob on a 67 meter wire. Although the swing amplitude started at 2.5 meters and shortened over time, it could comfortability show the earth rotating for over 24 hours. The pendulum period was slightly over 16 seconds. The pendulum caught the peoples' imagination with its slow swing, simplicity, long running time, impressive size, telling the time and showing the earth's rotation. It enthralled the public so much, that similar pendulums appeared in most of the major cities around the world over the next few years. The pendulum in the Pantheon was removed by the 1851 French coup d'état but since 1995 has been a permanent exhibit. Despite its simplicity there was one aspect that perplexed many if not most people. Depending on the pendulum's latitude it would take a different amount of time to rotate a full 360 degrees. On the North Pole it would be 360 degrees every 24 hours and on the equator it would not turn at all. There is an equation to work out the time in hours for it to rotate 360 degrees.

Léon Foucault's Gyroscope

In the winter of 1851/1852 Léon Foucault begin the work on the gyroscope[1]. It was made by the Parisian instrument maker Paul-Gustave Froment[2] under Léon's instruction. In September 1852 he published two papers[3] regarding the results. The gyroscope was sensitive enough to not only detect the earth's rotation but also become the world's first working gyro-compass by locking on to true north. It was a sensation for the scientific community at the time but had huge impact on society and commercialisation roughly half a century later. The precision of the gyroscope was very advanced for its time. In order to detect the earth's rotation the gimbals needed to be as friction free as possible. Modern ball race bearing had not been invented[4] but even if modern high grade ball bearings were used it would still create too much friction. The outer gimbal was suspended from a thin wire or string (possibly silk thread) allowing the whole structure to rotate with extremely low friction. I strongly suspect Foucault's previous experience with the pendulums was a deciding factor in suspending it in this manner. I wonder if the Foucault's gyroscope experiment would have been as successful if he had not made the pendulums the year before. The spinning disk also needed to be of sufficient size and speed to operate for a good length of time. A special device was used to spin the gyroscope. The gyroscope was placed in a tower like structure that had a series of gears that ultimately connected to a spur gear on the shaft of the gyroscope. A hand crank was used and the gearing gave a large ratio so the gyroscope could be spun between 9000 to 12,000rpm. I therefore assume gearing ratio must have been about 100 to 1.

The original gyroscope was given to the Collége de France and lost. However at least 5 copies made by Dumoulin-Froment exist.
  • Museum of Arts and Crafts France [5]
  • London science museum acquired a copy in 1883 [6]
  • Columbia University in New York City acquired circa 1870 [7]
  • The University of Rennes: Physics collection c1875 [8]
  • Washington: Smithsonian institute (one box missing)[8]

How the experiment was performed.

  1. The base of the gyroscope has a tripod arrangement to ensure the base does not wobble on the table. A thumbscrew at each end of the tripod sets the height for that leg. Since the gyroscope is hanging on a thread it acts as a levelling bob.
  2. There is a thumbscrew at the very top of the apparatus. Allowing lifting or lowering of the gyroscope thread. Initially the thumbscrew is lifted a few turns to establish if the apparatus is flat. E.g. is the base of the gimbals in the centre.
  3. The thumbscrews on the tripod are adjusted until the apparatus is level.
  4. Once level there are side screws that lock the feet in place. E.g. the thumbscrews can no longer move.
  5. The top thumbscrew is lowered until the bottom tip of the gimbals is just touching the base. Too high and the gyroscope will float around with air currents and minor vibrations. Too low and far too much friction will be applied to the base and the experiment will not work. The experiment relies on the fact the gyroscope gimbals are suspended by a thin thread and hence extremely low friction.
  6. The outer gimbal is locked (there is a single thumbscrew to lock/unlock)
  7. The inner gimbal is locked. There is a thin steel sheets that can be lifted into place to lock the inner gimbals.
  8. The gyroscope is now placed on the tower like structure to spin it up. The hand crank is wound and the gyroscope is spun between 9000 to 12,000rpm.
  9. The gyroscope is now placed back into the gimbals.
  10. The outer gimbals are unlocked.
  11. The outer gimbals are gently rotated, with the inner gimbals still locked. The gyroscope oscillates and after a period of time the settles on true north.
  12. At this point the inner gimbal is unlocked.
  13. The rotation of the gyroscope can be observed.

Unanswered questions

  1. Why are there two disks and inner gimbals. The only different between the two appears to be the pivot/bearing. One has knife edges. The other has plain shafts. Was this to prove with doubt the knife edges did not impact the result?
  2. Locking the inner gimbals appears to be an afterthought as the steel shims are not in-keeping with the rest of the design. Do they really work by unscrews adjusting position and then tightening the screw
  3. What was the string material? Cotton? silk? String? Wire?
  4. What was the gear ratio of the gears when starting the gyroscope. Hand crank could be operated at maybe 120rpm. Max gyroscope speed 12,000rpm hence 100 to 1 ratio?

The missing original Foucault Gyroscope

William Tobin writter of "The life and science of Léon Foucault" states the original gyroscope was gifted to the Collége de France (was this in his will?) and has been lost. I've gone through all my books on gyroscopes and found no pictures of the original which makes me believe that it was lost after his death 1868 and before abundant photography in the early 20th century. Mysteriously as of May 2024 there is a distinct lack of information about its disappearance both in books and online. Even in older books from around 1900. Maybe the lack of photos suggests it was lost before photography was commonplace. Or it simply could have been in storage and never on display before then being lost somehow.

  1. Tobin, William. The Life and Science of Leon Foucault. Cambridge : Cambridge Univeristy Press, 2003. ISBN: 0-521-80855-3.
  2. Paul-Gustave Froment. Wikipedia. [Online] [Cited: 29 5 2024.] https://en.wikipedia.org/wiki/Paul-Gustave_Froment
  3. Foucault, Léon (1852a). "Astronomy: On a new experimental demonstration of the movement of the Earth, based on the fixity of the plane of rotation" . Weekly reports of the sessions of the Academy of Sciences (in French). 35 (Week of Monday September 27, 1852). Gauthier-Villars: 421-424.
  4. Payne, Dean. The Complete History of Bearings You Need to Know. BDS bearings. [Online] [Cited: 29 5 2024.] https://www.bdsbearing.com/blog/bearing-history
  5. musée des Arts et Métiers. Foucault gyroscope. musée des Arts et Métiers. [Online] [Cited: 29 5 2024.] https://www.arts-et-metiers.net/musee/gyroscope-de-foucault
  6. Science Museum Group. Gyroscopes for demonstrating the Earth's rotation, 1883. Science Museum Group. [Online] [Cited: 29 5 2024.] https://collection.sciencemuseumgroup.org.uk/objects/co56298
  7. National Museum of American History. [Online] [Cited: 23 5 2024.] https://americanhistory.si.edu/collections/nmah_1185008
  8. Bernard, Dominique and Tobin, William. Foucault gyroscope and induction-current apparatus in the University of Rennes physics collection by William TOBIN and Dominique BERNARD. Slideplayer. [Online] [Cited: 11 6 2024.] https://slideplayer.com/slide/8743810/

About the Author

Glenn Turner has been designing and manufacturing gyroscopes since the late 1990s. He founded Gyroscope.com and has sold gyroscopes to 138 countries. He has developed a number of custom gyroscopes including for gyros for specialist medical applications and has done consultancy work for a F1 racing team. He developed the first afford control moment gyroscope for teaching space gyroscopes. His other passions include Stirling engines and Tesla Turbines.