The Machine of Bohnenberger
   The specimen of Bohnenberger’s machine recently rediscovered. 0.686640471513

Date manufactured: Around 1813

Period/Dates when in use: The instrument served as a device to illustrate the inertia of the axis of free rotating bodies and to illustrate the precession motion of the earth. It was used during lectures of physics and astronomy by J.G.F. Bohnenberger at the University of Tübingen, Germany, during the first decades of the 19th century. Furthermore, its principle spread quickly through France and became the basis for Foucault’s epochal work on gyros.

Description: The instrument is a relatively inconspicuous device. Its overall height is only 16.2 cm. The rotor symbolizing the earth is made of ivory and has in the middle a line indicating the equator. It is not exactly spherical but shows a slight flattening at the poles. The two gimbals and the fastening ring (outer diameter 8.5 cm) are made of brass coated by a Zapon varnish. The little drums on the rotating axis have the function to wind up a silk thread, which serves to start the rotor moving by strongly pulling this string. The bearings are point suspensions and can be adjusted by screws being orientated radial to the gimbals and the outer ring. Although all pivots and bearing bushes are made of steel, they are still rust-free. The whole mechanism operates even now very smoothly and can be handled as intended.

Item History: Working on a simple experimental proof for the rotational motion of the earth, the French physicist Léon Foucault introduced in 1852 the term Gyroscope for an instrument being able to observe such movements. Besides his well-known pendulum, these investigations concentrated on gyros with cardanic suspension. Foucault recognized especially that a well-directed restraint of the motion of the gimbals (like blocking one degree of freedom of the suspension) leads to specific indicators detecting different rotation components. With that, he paved the way for such important navigation instruments like the artificial horizon, the gyrocompass, the directional gyro, and inertial navigation systems. Nevertheless, L. Foucault is not the originator of that mechanical principle. He was familiar with it because such gyros were especially popular in France during the 19th century: they were already employed in numerous French schools to explain the precession of the earth rotation axis. This matter was born of the initiative of the French mathematician Pierre-Simon Laplace, who recommended introducing that kind of gyros as teaching aid. It is likely that P.-S. Laplace referred initially to a specimen that was in possession of the École Polytechnique in Paris. This apparatus is mentioned in 1813 by one of his former students, the French mathematician S. Denis Poisson: In a paper on analysing the dynamics of rotating bodies (Poisson 1813), he uses the gyro with cardanic suspension as an example, and he names the inventor of the mechanism. It was his contemporary German colleague J.G. Friedrich Bohnenberger being Professor for mathematics, astronomy, and physics at the University of Tübingen, Germany. This scientist explained for the first time the design and the use of such an apparatus (Bohnenberger 1817). As Bohnenberger could inevitably not yet know the term gyroscope, he called the device simply Machine. Therefore, the Machine of Bohnenberger (often less correctly named the Apparatus or Gyroscope of Bohnenberger) was the basis for Foucault’s epochal work on gyros.

In his original publication of 1817, Bohnenberger indicates at the beginning that several copies of the first Machine had been manufactured in Tübingen, and at the end of the article he names the manufacturer and the price of the Machine: All devices were assembled by the “Universitätsmechanicus” Johann W.G. Buzengeiger and could be bought for 18 Gulden (roughly one monthly salary for Bohnenberger). Despite this hint, all of the initial specimens of Bohnenberger’s Machine seemed to be lost since a long time. Fortunately, in December 2004 one of them was rediscovered during a stocktaking at the Kepler-Gymnasium (high school) in Tübingen.

For additional information see: Wagner, J.; Sorg, H.; Renz, A.: The Machine of Bohnenberger. European Journal of Navigation 3 (2005), No. 4, pp. 69-77

Additional Photos:
The specimen of Bohnenberger’s machine recently rediscovered.
Rotor bearing and attachment point of the precession momentum weight.
Drawing of Bohnenberger’s original publication (Bohnenberger 1817)
Johann Gottlieb Friedrich Bohnenberger, June 5, 1765 – April 19, 1831

Supporting documentation:
Bohnenberger, J.G.F. (1817): Beschreibung einer Maschine zur Erläuterung der Geseze der Umdrehung der Erde um ihre Axe, und der Veränderung der Lage der letzteren. Tübinger Blätter für Naturwissenschaften und Arzneikunde 3, pp. 72-83 (433064 bytes)

Poisson, D. (1813): Mémoire Sur un Cas particulier du Mouvement de rotation des Corps pesans. Journal de l'École Polytechnique 9, No. 16, pp. 247-262 (1322028 bytes)

City of Tübingen
City Museum (Stadtmuseum) of Tübingen

Submission authored by:
Jörg F. Wagner
Universität Stuttgart, Institute for Statics and Dynamics of Aerospace Structures