A Short Biography of the Foucault Pendulum.

November 30, 2011

When you walk into the Wiess Energy Hall, the very first thing you see is our Foucault pendulum.

It is a metal ball suspended by a cable that swings back and forth encircled by pegs. Children and adults will run through the rest of the museum, reach the pendulum, and wait with baited breath to watch a peg topple. When one of the pegs finally falls, you can hear a cheer erupt from the area. It is one of the most memorable parts of the museum. As the pendulum swings, it moves clockwise knocking down pegs as the Earth turns. It swings back and forth, back and forth (you are getting sleepy).

Foucault Pendulum
Foucault Pendulum at the Houston Museum of Natural Science

It is interesting to sit around the pendulum and listen to people try to explain it.

Some will talk about how it is a clock.  Others will put the time between pegs being knocked down between 10 minutes and 1 hour.  Our pendulum knocks down a peg on an average of every fifteen minutes. While the pendulum looks like it rotates around the circle, it is the Earth that is rotating and the pendulum that just swings. The pendulum is a visualization of a rotating Earth. To describe it in a different way, T = 24/sin q where T equals the amount of time to make one complete revolution and q is the latitude of the pendulum. At least that’s what Foucault said.

Star TrailsCreative Commons License photo credit: monkeymanforever

Leon Foucault was born in Paris (France, not Texas) on September 18, 1819.

As a young boy he did not show an inclination towards science or study.  In fact his teacher considered him lazy because he did not turn in his work. He did, however, enjoy building mechanical devices, such as a small steam engine and a telegraph, and tinkering.  He entered medical school to become a surgeon, but found that he fainted at the sight of blood.  Instead of becoming a blindfolded surgeon, he switched to physics. 

At the age of 25, not having learnt anything at school nor from book, enthusiastic about science but not about study, Léon Foucault took on the task of making the work of scientists understandable to the public and of passing judgment on the value to the work of leading men of science – J Bertrand, Éloge historique de Léon Foucault.

Foucault proved his worth in being able to take mathematical proofs and construct a mechanical proof, his pendulum being one of those.

He also constructed a device to prove that light moves slower through water than air. The mathematics describing the proof had been around for over a decade, but Foucault was the first to prove that it worked. His first pendulum on public display opened on February 3, 1851 in the Paris Observatory (again France, not Texas). Instead of knocking down pins as the pendulum moved, the first Foucault pendulum drew in sand.  He also invented the gyroscope, which stays in place as the Erath moves around it. This invention has proved essential for planes, space craft, and even the Hubble Telescope.  

Hubble Space Telescope
Creative Commons License photo credit: NASA Goddard Photo and Video

After he came to power, Napoleon III, an amateur scientist, created a job for Foucault at the newly named Imperial Observatory. There, Foucault developed his knife edge test to measure the conic shape of mirrors. This led to a more constant quality of lenses for use in telescopes.  He died on February 11, 1868 from multiple sclerosis.  His legacy lives on today.  He has an asteroid named in his honor. But he is honored around the world by his plethora of pedula that swing to and fro, showing people that the Earth keeps on spinning.

Authored By Daniel Burch

An inveterate punster, amateur chef, and fencer, Daniel B has a double degree in History and Museum Science from Baylor. He currently serves as the Assistant Program Coordinator for the Wiess Energy Hall and Adult Education at HMNS.

5 responses to “A Short Biography of the Foucault Pendulum.”

  1. Sharon says:

    Daniel…thanks, you make it sound so easy (except for the T equals stuff!) I’m going to try to explain it better to the students!

  2. Daniel says:

    It’s a great mechanical proof of something that we know, but usually don’t think about.

  3. Jim says:

    Daniel: love the story and agree that the pendulum is a great exhibit. However, I think the time between peg falls is really quite close to 10 minutes. There is a 12 pointed star on the floor and there are about 12 pegs between each point, giving a total of 144 pegs. Since the Museum is located at (approximately) 30 degrees N latitude, the pendulum makes a full circle in approximately T = 24/Sin(30) = 24/0,5 = 48 hours. Since it knocks pegs down on each end of its swing, all of the pegs will be toppled in approximately 24 hours. All of that should result in an average time between pegs of (24 * 60)minutes/ 144 pegs or 10 minutes/peg. Keep up the great work!

  4. Jack H says:

    As a volunteer at the museum, it is always fun to watch the visitors wait for the pendulum to knock over a peg. When a peg is knocked over, it always fascinates me! Thanks for sharing the history of the pendulum, and how it works! In response to what Jim mentioned, this particular pendulums average time to knock a peg down is about every 8.25 minutes. However, I have seen it occur in less than 8.25 minutes as in the morning there are about 2-4 pegs that are placed closer together than the rest. If you look closely at the picture at the very top that is in your article, to the left of the pendulum there are two pegs very close together.

    Thanks for the information!

  5. R Wenner says:

    It would be interesting to see if the period between pegs knocked down varies during the year: particularly whether it is faster or slower at the solstices (~June 21 and ~ Dec 21) than at the equinoxes (~March 21 & ~Sept 21).

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