412 Sig. Nobili an<f Antinori on the Electro-motive Force 



all other kinds, and which rendered it not easily assailable 

 before the discovery of Mr. Faraday. This character does 

 not consist only in momentary duration, which it has in common 

 with soft iron, but also in being a double magnetism, inverse 

 and direct ; inverse, at the moment of its production, opposite 

 to the producing cause ; direct, at the moment after, when 

 this cause disappears. 



Mr. Faraday considers Arago's magnetism of rotation as 

 entirely connected with a phasnomenon which he discovered 

 about ten years ago {Exp.IicsA2\.). '■'■He then ascertained" so 

 says the notice, ^Hhat by the rotation of a metallic disc under 

 the injluence of a magnet, there may he formed, in the direction 

 of the radii of that disc, electric currents in srifficient number to 

 render the disc a nes electric machine." We are quite igno- 

 rant how Mr. Faraday has ascertained this fact; and we do 

 not know how a result of such a nature could remain so long 

 a time generally unknown, and as it were lost in the hands of 

 the author of the discovery *. Besides, there is something 

 here very problematical to us ; and before we leave the subject 

 we will describe the experiment we have made relative to it. 



A disc of copper was revolved, and two long copper wires 

 prepared, attached at one set of ends to the galvanometer, 

 and at the other held by the hand against the disc, the one at 

 the centre, and the other at the circumference, in the direc- 

 tion of ihe radii. In the rotation of the disc, the points of 

 copper pressed against it will be heated, but unequally ; that 

 pressed against the circumference will be most heated, and 

 that at the centre the least. This difference is quite sufficient 

 to determine an electric current capable of moving the nee- 

 dle of the galvanometer, and retaining it after a few vibrations 

 at a certain degree of the division f . When the needle is 



[• Sig. Nobili and Antinori here seriously mistake the sense of my let- 

 ter to M. Hachette. I did not write " I then ascertained." The French 

 translation of my letter in Le Lycee, No. 35, sent to nie by M. Hachette, 

 does not say so. " M. Faraday considere le phenomene qui se manifeste 

 dans cette experience, comme intimement lie a celui de la rotation mag- 

 netique qu'il a eu le bonheur de trouver il y a dix ans. II a reconnn que 

 par la rotation du disc metallique, &c. &c." I am not Italian scholar 

 enough to say how Sig. Nobili and Antinori themselves at first expressed it; 

 but the phrase used in the present part of their paper is, " Egli riconohhe fin 

 d'allorache.Sfc.-" whilst that which they used at the head of the paper, to 

 express the same words of my letter is, " Egli ha riconosciido che, ^r. S^-c." 

 It was in consequence of tlie recent researches detailed in my paper that 

 I ascertained the state of the revolving plate, and could then refer the effect 

 in its kind to that which I had so long before discovered. The succeeding 

 remarks of Sig. Nobili and Antinori have no reference therefore except 

 to their mistake of my meaning. — M. F.] 



[+ All these causes of error were fully guarded against in every part of 

 iry researches {Exj). Res. 91. 1 13. 186.).-M. F.] 



thus 



