TUANSACTIONS OF THE SKCTIO.NS. 11 



On the Heat produced ht/ the Influence of the Magnet upon Bodies m Motion. 

 Bi/ M. Leon Foucault, Paris. 



In 1821, Arago observed the remarkable fact of the attraction of the magnetic 

 needle by conducting bodies in motion. The phsenomenon appeared very singular, 

 and remained without explanation until Faraday announced the important discovery 

 of currents of induction. It was then evident, 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 direction. It may be said, in 

 general terms, that the magnet and the conducting body tend towards a state of rela- 

 tive repose by a mutual influence. 



If, notwithstanding this influence, it is desired to continue the motion, a certain 

 amount of force {travail) must be constantly furnished ; the moveable part seems to 

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

 duces a dynamic effect, which I have thought must be represented by heat. 



We arrive at the same inference by taking into consideration the currents of 

 induction which succeed one another in the interior of bodies in motion ; but an 

 idea of the quantity of heat produced would only be acquired with great difficulty 

 by this mode of regarding the affair, whilst by considering this heat as due to a trans- 

 formation of force, it a|)peared 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 b. gyroscope, and which was 

 previously employed in experiments of a very different nature. This solid is a torus 

 of bronze connected 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 magnet more effective, two pieces of 

 soft iron added to the helices prolonged the magnetic poles, and concentrated thera 

 in the vicinity of the revolving body. 



When the apparatus is going with the greatest rapidity, the current 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 developed by Faraday. But if the handle be then pushed, so as 

 to restore to the apparatus the movement which it has lost, the resistance experienced 

 requires the application of a certain amount of force, the equivalent of which reap- 

 pears and accumulates in heat in the interior of the revolving body. 



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

 tion of temperature. Having, for example, taken the apparatus at the surrounding 

 temperature of 60°'8 F., I saw the thermometer rise successively to 68°, 77°, 86°, and 

 93^'2 F. ; but the phfenomenon had previously become sufficiently developed to 

 render the employment of the thermometer unnecessary, as the heat produced had 

 become sensible by the hand. 



If the experiment appear worthy of interest, it would be easy to arrange an appa- 

 ratus to reproduce and augment this phsenomenon. There is no doubt, that by 

 means of a machine properly constructed, and composed only of permanent magnets, 

 high temperatures might be produced, so as to place before the eyes of the public 

 assembled in lecture rooms a curious example of the conversion of force into heat. 



On a Machine for Polishing Specula. By Dr. Green. 



On the Optical Properties of Cadmacetite. 

 By William Haidinger, Vieniia. 

 [Crystals of the salt were laid before the Section by Sir David Brewster.] 

 I have the honour to lay before the Association a short notice on the Absorption 

 of the Crystals of Acetate of Cadmium, or to denote them by a single word, of Cad. 

 macetite, together with some of the crystals, which form the subject of the commu- 

 nication. 



