Translated for the Journal of the Franklin Institute

Physical Demonstration of the Rotation of the Earth by Means of the Pendulum

By M.L. Foucault

At the crown of the vault of a cellar, a strong piece of cast iron was secured to give support to the suspension wire, which hung freely from a small piece of tempered steel, whose free surface was perfectly horizontal. This wire of steel, strongly compressed by the action of the drawing-plate; its diameter varied from 6/10 to 11/10 of a millimetre; it was two metres long (78.75 inches,) and carried at its lower end a ground and polished sphere of brass, whose centre of gravity had been made to coincide with its centre of figure. This sphere weighed five kilogrammes (10 lbs.) and carried a sharp projection, which appeared to be a continuation of the suspension wire. When it was desired to try an experiment, we began by destroying the torsion of the wire, and stopping the turning of the sphere. Then to draw it aside from its position of equilibrium, it was included in a loop of organic thread, whose free extremity was attached to a fixed point in the all at a small height above the ground. The length of oscillation of the pendulum was determined by the length given to the thread; generally in my experiments these oscillations were at first from 15 degrees to 20 degrees.
 Foucault's Paper
Before proceding it is necessary to destroy, by an obstacle gradually withdrawn, the oscillating motion which the pendulum has in its restraint by the two threads. Then when we have succeeded in bringing it to rest, the organic thread is burned at some point of its length; and the pendulum obeying gravity alone, begins to move, and gives a long series of oscillations, whose plane quickly experiences a sensible displacement. At the end of a half hour, this displacement is such that it is evident to the eye; but it is more interesting to follow the phenomenon more closely, so as to be satisfied of the continuousness of the effect. For this purpose a vertical point or style is made use of, mounted on a support and placed on the ground, so that in its swinging motion the projection from the pendulum, at the limit of its excursion, will just touch the fixed point. In less than a minute, the exact coincidence of the two points ceases to be reproduced, the oscillating point being displaced constantly towards the left of the observer; which indicates that the direction of the plane of oscillation takes place in the same direction as the horizontal component of the apparent motion of the celestrial sphere. The mean value of this motion for any time, shews comformably to the indications of the theory, that in our latitude the horizontal trace of the plane of oscillation does not make a complete revolution in twenty-four hours.