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 LX^'^. Intelligence and Miscellaneous Articles. 



ON THE HEAT PRODFCED BY THE INFLUENCE OF THE MAGNET 

 CPON BODIES IN MOTION. BY M. LEON FOUCAULT. 



f N 1824, Arago observed the remarkable fact of the attraction of 

 -'- the magnetic needle by conducting bodies in motion. The phse- 

 nomenon appeared very singular, and remained without explanation 

 until Faraday announced the important discovery of currents of in- 

 duction. It was then proved, that in Arago's experiments the motion 

 gave rise to currents, which, by reacting upon the magnet, tended 

 to associate it with the moveable body and draw it in the same direc- 

 tion. It may be said, in general terms, that the magnet and the 

 conducting body tend towards a state of relative repose by a mutual 

 influence. 



If, notwithstanding this influence, the motion continue, a certain 

 amount of force must be constantly furnished, the moveable part 

 be, as it were, pressed by a break, and this force necessarily pro- 

 duces a dynamic eifect, which, according to the new doctrines, I 

 have thought must be found in heat. 



We arrive at the same consequence by taking into consideration 

 the currents of induction which succeed one another in the interior 

 of the body in motion ; but an idea of the quantity of heat produced 

 would only be acquired with great difiiculty by this mode of regard- 

 ing the affair, whilst by considering this heat as due to a transforma- 

 tion of force, it appeared certain to me that a sensible elevation of 

 temperature would be easily produced in a decisive experiment. 

 Having ready to my hand all the elements necessary for a prompt 

 verification, I proceeded to its execution in the following manner. 



Between the poles of a strong electro-magnet I partially introduced 

 the solid of revolution belonging to the apparatus which I have 

 called a gyroscope, and which was previously employed in experi- 

 ments of a very diff^erent nature. This solid is a ring of bronze con- 

 nected by a toothed pinion with an apparatus of wheels, by the action 

 of which, when turned by the hand, it may revolve with a rapidity 

 of 150 or 200 turns in a second. To render the action of the mag- 

 net more effective, two pieces of soft iron added to the helices pro- 

 longed the magnetic poles, and concentrated them in the vicinity 

 of the revolving body. 



When the apparatus is going with the greatest rapidity, the cur- 

 rent of six Bunsen's couples, passed into the electro-magnet, stops 

 the movement in a few seconds, as though an invisible break had 

 been applied to the moving body ; this is Arago's experiment, as de- 

 veloped by Faraday. But if the handle be then pushed on, so as to 

 restore to the apparatus the movement which it has lost, the resist- 

 ance experienced requires the application of a certain amount of force, 

 the equivalent of which reappears and accumulates in heat in the inte- 

 rior of the revolving body. 



By means of a thermometer inserted in the mass we may follow 

 the gradual elevation of temperature. Having, for example, taken 



Phil. Mag. S. 4. Vol. 10. No, 68. Dec. 1855. 2 H 



