in a Letter to M. Gay-Lussac. 359 



following their opinion, corrected by experiment, the result 

 should be attraction instead of repulsion. But Arago was 

 right in saying that it is repulsion ; and consequently the 

 theory of the effect given cannot be the true one. 



My views upon the subject in question may be found in my 

 first paper. I examined whether it were possible or probable 

 (F. 125.) that time could be a necessary element for the de- 

 velopment of the maximum current in the metal. In this case 

 the resultant of all the forces would be in advance of the mag- 

 net, when the plate was rotated, or in the rear of it, if it (the 

 magnet) were rotated; and a line joining this resultant with 

 the pole would be oblique to the plane of motion ; then the 

 force in the direction of this line might be resolved into two 

 others, one parallel, the other perpendicular to the plane of 

 movement or rotation ; the latter would be a repulsive force, 

 producing an effect analogous to that remarked by M. Arago. 



The second force is that which occasions the magnet and 

 the disc mutually to follow each other. Referring to page 

 290, fig. 1. or 2. (my figure 2. may also be made use of,) we 

 read, " Forces of attraction exist in s, s, s, towards which it 

 (the magnet) is attracted, and repulsive forces in w, w, w, which 

 impel it in the same direction;" consequently the magnet moves 

 either after, or with the metal ; but the currents, and conse- 

 quently the forces, are exactly contrary to what has been sup- 

 posed, as I have just shown ; the magnet and the disc should 

 therefore move in opposite directions, if the forces act in the 

 manner that has been supposed. But as they do not move, in 

 fact, in opposite directions, it is evident that the theory which 

 explains their movement by reversing the facts must be itself 

 erroneous. 



The third force is that which has a tendency to remove the 

 magnetic pole either towards the centre or the circumference, 

 on each side of a neutral point situated upon the radius above 

 which the magnet is placed ; this effect is described at page 

 281, and in fig. 4, which accompanies the memoir, and which 

 I believe to be perfectly correct. The memoir proceeds to 

 explain the effect by referring to the repulsive force admitted 

 (p. 289.) to account for the first effect observed by Arago, viz. 

 the vertical repulsion of the disc; and supposing that this re- 

 pulsive force be distributed over a certain extent of the disc, 

 beneath the magnet, it is established (p. 292. fig. 5.) that if the 

 pole be situated very near the circumference, the portion of 

 the body whence this force emanates will be lessened, being 

 cut by the circumference itself; consequently the parts that 

 are nearest to the centre are more powerful, and impel the pole 

 in an outward direction ; while if the pole be placed very near 



