200 FoucauWs Pendulum Experiment 



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* Bibl. Universelle, March, 1851. 



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Art. XXII. — FoiicauWs Pendulum Experiment. 



The peculiar interest of Foiicaiilt's experiment with the Pen- 

 duUim, by which the rotation of the earth is proved even within 

 the walls of an ordinary lecture room, has obtained for it very 

 general attention, and it has already been repeated in numerous 

 places in this and other countries. The occurrence from which 

 M. FoucauU was led to his discovery is thus related by him.* 

 '* Having fixed on the arbor of a lathe and in the direction of the 

 axis, a round and flexible steel rod, it was put in vibration by de- 

 flecting it from its position of equilibrium and leaving it to itself. 

 A plane of oscillation is thus determined, which, from the per- 

 sistence of the visual impressions, is clearly delineated in space. 

 Now it was remarked that on turning around with the hand, the 

 arbor which formed the support of this vibrating rod; the plane ^ 



of oscillation was not carried with it, but always retained the . 



same direction in space." From this came the conclusion that a [ 



pendulum set in motion will continue in the same plane of vibra- 

 tion, however the point of suspension be rotated — a fact easily 

 proved by a simple trial with a weight at the end of a cord. The 

 rotation of the point of suspension may make the pendulum re- 

 volve on its axis; but the plane of vibration will remain the 

 same. The reason for this is obvious: the swinging pendulum, 

 when about to return (after an outward swing) from its point of 

 rest, is made to move from that point by gravity alone, and can 

 therefore fall but in one direction; and the momentum acquired 

 by falUng carries it beyond this centre in the same direction to 

 the point of rest on the other side ; here again it is in a like con- 

 dition, and must return under the force of gravity in one and the I 

 same line, gravity acting in the same direction whether the point | 

 of suspension be rotated or not. Thus the plane of vibration is 

 fixed from the very nature of the forces at work. 



It is evident, therefore, that if a pendulum were swinging at 

 the pole of the earth, the plane of vibration, as it would not 

 change with the revolution of the earth, should mark this revo- 

 kition by seeming to revolve in the contrary direction, and in 24 

 hours it would make apparently the whole circuit of 360 de- 

 grees. But at the equator, the plane of vibration is carried for- 

 ward by the revolution of the earth, and so undergoes no change 

 with reference to the meridians. Between the equator and poleSj 

 the time requi^ed for the pendulum to make 360^ varies according 

 to the latitude, being greater the farther from the pole. For these 

 intermediate latitudes the problem is beautifully solved on a globe. 



Having a globe for the trial, select, for instance, latitude 30 

 and apply the principle that the plane of vibration is constant 



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