Foucault's Gyroscope

The Foucault gyroscope was a device created by French physicist Léon Foucault in 1852, conceived as a follow-up experiment to his Foucault pendulum in order to further demonstrate the Earth's rotation. It was the prototype of the modern gyroscope.

Foucault felt that the results of the pendulum experiment had been misunderstood. He therefore endeavored to create an apparatus with a "body freely suspended by its center of gravity and rotating around one of its principal axes", allowing the study of a plane with "absolute directional stability".: "C'est faute d'avoir compris dans son acception véritable la fixité du plan d'oscillation, que beaucoup de personnes se sont fait, de la déviation, une idée inexacte, et ont méconnu sa valeur et son uniformité. Mais, si au plan d'oscillation du pendule on substitue le plan de rotation d'un corps librement suspendu par son centre de gravité et tournant autour d'un de ses axes principaux, on a à considérer un plan physiquement défini et qui possède réellement une fixité de direction absolue. C'est pour réaliser cette conception et en obtenir de nouveaux signes de la rotation de la Terre, que j'ai composé et fait exécuter un nouvel appareil que je puis mettre dès à présent sous les yeux de l'Académie."

Design

Together with Paul-Gustave Froment, Foucault built an apparatus in which the inner gimbal was balanced on knife edge bearings on the outer gimbal and the outer gimbal was suspended by a fine, torsion-free thread in such a manner that the lower pivot point carried almost no weight.

The gyro was spun to 9,000–12,000 revolutions per minute with an arrangement of gears before being placed into position, which was sufficient time to balance the gyroscope and carry out 10 minutes of experimentation. The instrument could be observed either with a microscope viewing a tenth of a degree scale or by a long pointer.

Publications

Foucault published two papers in 1852, one focused on astronomy with the weight free to move on all three axes (''On a new experimental demonstration of the motion of the Earth, based on the fixity of the plane of rotation'') and the other on mechanics with the weight free to move on only two axes (''On the orientation phenomena of rotating bodies driven by a fixed axis on the Earth's surface. New sensitive signs of daily movement'').

In the paper on mechanics, Foucault explained that if one axis of rotation is fixed in line with the surface of the Earth, the other two axes of rotation tend to the same direction, similar to "a magnetic needle", making it possible to use the instrument to highlight a directing force.

Naming

Foucault coined the name gyroscope in the 1852 publication of his experiment:: "L'appareil spécialement destiné à mettre en évidence et à mesurer approximativement la déviation d'un corps tournant en toute liberté, peut servir également à produire et à observer les phénomènes d'orientation que je viens d'énoncer et de décrire. Comme tous ces phénomènes dépendent du mouvement de la Terre et en sont des manifestations variées, je propose de nommer gyroscope l'instrument unique qui m'a servi à les constater."

This apparatus specially designed to highlight and approximate the deviation of a freely rotating body can also be used to produce and observe the phenomena of orientation that I have just stated and described. As all these phenomena depend on the movement of the Earth and are its varied manifestations, I propose to name the sole instrument which has served me to observe them ''gyroscope''.

Copies

At least three more copies of a Foucault gyro were made in convenient travelling and demonstration boxes and copies survive in the UK, France, and the US.

The Foucault gyroscope became a challenge and source of inspiration for skilled science hobbyists such as D. B. Adamson.

Gallery

Foucault's gyroscope, Astronomie populaire.jpg, Sketch of the gyroscope
Launching device for Foucault's gyroscope, Astronomie populaire.jpg, Launching device for the gyroscope

Citations

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References

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Gyroscopes
Physics experiments
French inventions