Leon Foucault – the man who showed that the earth rotates
"That's when I saw the pendulum. The bullet, with its tip pointing downward, hanging at the bottom of a long wire attached to the top of the cornucopia, described in isochronous majesty its sweeping motion"
UMBERTO ECO in Foucaults pendulum
|
On the morning of January 3, 1851, Foucault had completed his experimental setup in the basement where he lived on the Rue d'Ássas in Paris. He had suspended a five-kilo weight on a two-meter-long rope, which was suspended so that the weight could swing freely in all directions. He knew that the pendulum had to be suspended perfectly symmetrically for the experiment to succeed and he had put a lot of work into making it work. With his attempt, he hoped to be able to show what no one had previously been able to demonstrate with a clear experiment – namely that the earth spins on its axis. It was certainly known at this time that the earth spins on its axis, among other things through studies of fixed stars. But the final proof, where the common man could see it with his own eyes, was missing. |
|
Natural philosophers and other scholars had at this time made all kinds of experiments to try to show this with a demonstration. For example, bullets had been dropped from high altitudes, for example down mine shafts to try to measure deviations from the vertical upon impact. There were no clear results from these experiments. They had also tried shooting a cannonball straight up into the air to see if it returned to the launch point. The attempt failed (thankfully). But no one had yet managed to find a really tangible, simple proof that the earth really does spin on its axis. And now he, Leon Foucault, was close to doing this. He, who was only an autodidact, would be the first to demonstrate this, with an elegant experiment. But the rope to his pendulum broke and he had to start over with his experiment.
Foucault was born in 1819 and was the son of a publisher in Paris. One would think that a scientist who made such a beautiful experiment that no one had previously succeeded in would have an extensive scientific education. But Foucault had no such training. At the school College Stanislas in Paris, where he started in 1829, he did not particularly excel and his mother arranged private tuition at home so that he could pass his basic education and have the opportunity to study further.
Foucault's greatness lay in his ability to use his hands. During his high school years, he designed and built model boats and steam engines and even a fully functioning telegraph. He had a great talent for ingenious, original constructions and worked with extreme accuracy. His mother, seeing his skill in using his hands, saw her son's potential to become a surgeon. Foucault also started medical school in 1839 but quit after a while because he could not stand the sight of blood. But before he interrupted his medical studies, his teacher of microscopy and histology, Alfred Donne´, had seen the talent Foucault had and engaged him as his assistant and collaborator.
By this time, Louis Daguerre had created a first process for photography, called daguerreography. A problem when you wanted to image something permanent with this technique was that the exposure time was extremely long. The time for an exposure could be of the order of a number of minutes up to an hour. You could thereby depict, for example, landscapes, but pictures of moving subjects such as people were very difficult to take.
|
Leon Foucault had had a peer of the same age, Hippolyte Fizeau, who studied medicine medicine at the same time as Foucault and who was more interested in physics. Fizeau developed a method of development that significantly reduced exposure times. Foucault subsequently refined the method further, which increased the areas of use of daguerreography. |
|
|
In order to further improve daguerreography and reduce exposure times, strong illumination of what was to be imaged was required. In various ways, Foucault succeeded in developing new, better lighting techniques that could be used in microscopy and which made it possible to now also take pictures of, for example, biological preparations (1845). Together with Alfred Donne, Foucault made an atlas with 80 pictures of histological preparations (Cours de Microscopie). |
|
 Histological preparations in the atlas Cours de Microscopie. Imaged with daguerreography |
|
|
Francois Arago, permanent secretary at the French Academy of Sciences and a scientist himself, hired Foucault and Fizeau in 1844 to use this photographic technique to take sharp pictures of the sun. The goal was to increase knowledge about the structure of the sun. Foucault constructed a heliostat, a device that allows the sun to be reflected against a certain fixed point, and compensates for the sun's movement. They managed to take sharp pictures of the sun with this technique and thus contributed to the knowledge of the structure of the sun. |
 |
As part of his exploration of the nature of light, Francois Arago was determining the speed of light in water. But diabetes made his eyesight worse and worse, he could no longer experiment on his own. He then hired Foucault and Fizeau to continue his work and make this determination. Something seems to have happened in the relationship between the friends, possibly they disagreed on which methods to use, but at least the two friends started working separately. Fizeau continued to develop the method proposed by Arago while Foucault built his own experimental setup. Both methods were based on rapidly rotating mirrors, which rotated at a well-defined speed. In 1850, Foucault and Fizeau each succeeded in determining the speed of light in water. The result showed that the speed of light is lower in water than in air, something that became decisively important for the discussion about the nature of light that was conducted at that time. This was Foucault's greatest success to date. För detta belönades han med Copleymedaljen av brittiska Royal Society.
In January 1851, a week after the rope broke, he made another attempt with an improved suspension device. Foucault could see how the direction of the pendulum was constantly changing clockwise. He realized that this must be due to the rotation of the earth.
With each oscillation, the direction of the plane of oscillation changed ever so slightly clockwise, the earth rotated under the plane of oscillation, and as the changes for each oscillation added up, the effect eventually became visible even to the naked eye, without measuring instruments. The fact that the deviations were added meant that, in contrast to the previous experiments with released balls, the effect could be seen.
But how would he present his successful experiment to the world? How would he get recognition, especially from the French Academy of Sciences? Foucault was not at this time a recognized scientist. But there was one person who could help Foucault give the experiment the attention it deserved.
Foucault had previously been employed to carry out experiments by the permanent secretary of the French Academy of Sciences, the director of the Paris Observatory, Francois Arago, and they had a good relationship. He got Francois Arago's permission to do the demonstration in the prestigious Meridian Room of the Paris Observatory. It was very great to be able to do the experiment in this room. The Meridian Room was the largest, highest and most famous room at the Observatory. In the floor of the room the north-south meridian was marked. Here the pendulum could be started parallel to the north-south meridian and the clockwise deviation would soon be visible.
This time, Foucault suspended a five-kilogram ball from an eleven-meter-long line and carefully adjusted the suspension device. To ensure that the bullet was at rest when it was released, he tied the bullet to the wall with a cotton string, waited for the bullet not to move, and then burned the string. The pendulum was now set in motion parallel to the north-south meridian and the clockwise deviation soon became apparent. Now the experiment worked and everything was prepared for the presentation.
Foucault now sent out invitation cards to all famous scientists in Paris. In the invitation card he wrote "You are invited to come to see the earth rotate, tomorrow between three and five, in the Meridian Hall of the Paris Observatory".
On February 3, 1851, he made his big demonstration. At the same time, he provided a formula that could be used to calculate the time of rotation of the oscillating plane at all latitudes on the globe. För en pendel upphängd vid nord- eller sydpolen vrider sig svängningsplanet ett helt varv på 24 timmar (23 timmar 56 minuter 04 sekunder). Vid ekvatorn vrider sig svängningsplanet inte alls. Med formeln kunde man nu beräkna tiden på en godtycklig latitud. I Paris, (på latituden 48 grader 51 minuter) vred sig svängningsplanet ett varv på nästan 32 timmar.
Reaction to the experiment was mixed. The meaning of the beautiful experiment was clear to everyone and it was thought to be a very beautiful experiment. But how was it that no one had thought of this simple experiment before? And how did you prove Foucault's formula? Foucault was not a trained mathematician, nor had he left any explanation for the formula.
The mathematicians immediately set about trying to prove the formula theoretically. On February 17, a proof was presented before the French Academy of Sciences by the mathematician Jacques Binet. But the question remained and remains – how could Foucault “find” this formula?
But such a beautiful experiment naturally deserved to be announced to the whole world with pomp and circumstance. The President of the Republic at the time, Louis-Napoleon Bonaparte, the future Napoleon III, recognized the market value of such a demonstration. He himself was very interested in natural science and took the initiative for a public display under the Pantheon's dome.
|
|
|
|
The setting in this beautiful building and with a ceiling height that allowed a pendulum length of 67 meters made the demonstration extremely effective. A pendulum with this impressive length has a swing time of just over 16 seconds. This made the experiment extra effective.
Underneath the ball was a stick that drew tracks in a sand bank. Each time the bullet passed, it tore a piece of the sandbank. Even a normally impatient onlooker could soon see that the bullet changed direction clockwise.
The bullet's motion was of course eventually slowed down by air resistance, but it oscillated back and forth for about 5 to 6 hours. During this time, the direction of the swing plane changed by roughly 60°.
Advertisements in the Paris newspapers now promised "Come and see the proof that the earth spins on its axis". The public demonstration was carried out before large crowds of astonished Parisians and with president Louis-Napoleon Bonaparte at the head. Foucault stood for days on end in the Pantheon and explained the function to the spectators. The news quickly spread around the world, shuttles were built around the world, and a veritable shuttle mania broke out. The pendulum experiment was carried out, among other places, in Rio de Janeiro in the southern hemisphere. It twisted, just as expected, counterclockwise. This provided yet another confirmation of the validity of the sine formula.
This world-famous pendulum experiment by Foucault is still often ranked among the ten most beautiful of all physics experiments in the world.
Foucault's experiment was repeated, among other things, in 1852 in Kölner Dom with a pendulum that was at least 160 meters long. The pendulum time should have been about 25 seconds! Here, the validity of the sine formula was again confirmed. The experiment was repeated here many times with different starting directions to see if the result was the same. The starting direction did not appear to affect the result.
How then did Foucault get the idea for his experiment? He had noted that if you put a steel rod in a lathe and set it in swing, for example up-down, the swing plane is maintained regardless of the turning speed of the lathe. The steel bar thus continued to swing up-down all the time even though the lathe was started and was not affected by the rotation of the lathe. Foucault realized that this was comparable to the pendulum and the rotation of the earth.
After his great pendulum demonstration, Leon Foucault also constructed and named the gyroscope, which made the gyrocompass possible. The gyroscope also provided new proof that the earth spins on its axis.
In 1862, Foucault made another determination of the speed of light and arrived at the then best value of 298,000 km/second, only 0.6% of today's accepted value. He then used an experimental setup proposed by Thomas Wheatstone.
In 1865, on his sixth attempt, he was elected to the French Academy of Sciences. This he had longed for throughout his adult career.
|
|
Foucaults pendel in Pantheon
Leon Foucault died in 1868, aged 49, probably from multiple sclerosis.
/Bo Göransson
Appendix
Foucault's formula
At his demonstration in 1851, Foucault stated that the time for the plane of rotation to rotate one full revolution could be calculated by T = 24 / sin λ, where λ is the latitude of the locality.
The twisting effect would thus be greatest at the poles. At the poles, the latitude λ= +/- 90° applies. Since λ = 90° at the North Pole, the time T = 24/sin 90°, i.e. T = 24 h. The oscillating plane will therefore turn one full revolution in 24 h. At the equator, no turning at all would take place.
Foucault did not provide any proof of the sine formula, nor had he performed the experiment in any other place, and thus had no experimental data from places other than Paris. Nevertheless, he had managed to state a formula for the rate of rotation which, it turned out, applied generally to the entire globe.