358 Mr. Faraday on Magneto-electric Induction ; 



phaenomena of Arago's revolving disc ; but as I have shown 

 that the theory is in general based upon two conclusions con- 

 trary to truth, it is unnecessary to make a minute examina- 

 tion of it. It is impossible for it to exhibit the phaenomena 

 with exactitude. Those who are anxious for full satisfaction 

 on the subject, may decide, by means of a few experiments, 

 whether the opinions which I put forth in the paper which 

 first announced the discovery of these currents be true, or 

 whether the Italian philosophers were justified in declaring 

 that I was in error, and that they had published more just ideas 

 on the subject. 



Everybody knows that when M. Arago published his re- 

 markable discovery, he said the action of the disc upon the 

 magnet was resolvable into three forces : the FIRST, perpen- 

 dicular to the disc, which he found to be repulsive : the SE- 

 COND, horizontal and perpendicular to the vertical plane con- 

 taining the radius beneath the magnetic pole; this is a tan- 

 gential force, and occasions the rotation of the pole with the 

 metal : the THIRD, horizontal and parallel to the same radius; 

 it becomes null at a certain point towards the circumference ; 

 but when nearer the centre, it has a tendency to impel the 

 pole towards the centre; and when nearer the circumference, 

 to impel it in the contrary direction. 



At page 289, Messrs. Nobili and Antenoii give an ex- 

 planation of the first of these forces. As has been already 

 said, these gentlemen consider that the parts adjacent to the 

 magnet have currents contrary to those which are found near 

 the pole to which they approach, and consequently they are 

 repulsive ; and they consider that the parts that recede have 

 currents identical in direction with those which are near the 

 magnet from which they recede, and consequently these parts 

 are attractive. The sum of each of these various forces is 

 equal one to the other, but in what relates to the needle 

 or magnet this distribution differs ; for " the repulsive forces 

 being the nearest, invade the disc as far as the parts under 

 the needle, and thus obtain a preponderance over the action 

 of the contrary forces, which are exerted more obliquely, and 

 at a greater distance. In short, it is only a part of the re- 

 pulsive forces which is balanced by the attractive forces; the 

 remainder meets with no opposition, and it is this remainder 

 that produces the effect." 



But I have shown in this letter, that the currents in the 

 parts adjacent or distant are exactly contrary to what is sup- 

 posed by Messrs. Nobili and Antenori; and that conse- 

 quently where they expect attraction they would find repul- 

 sion 5 and attraction where they expected repulsion ; so that, 



