in a Letter to M. Gay-Lussac. 361 



on each side in the parts which, being more distant from the 

 pole, are subject to a feebler inductive force : the current will 

 thus be completed or discharged (see fig. 3.). Let A B C D 

 represent a piece of copper moving in the direction of the ar- 

 row E, and N the north end of the magnet placed above ; 

 currents of electricity will be produced in the piece of metal ; 

 and though they undoubtedly extend from the part below the 

 pole to a great distance around (F. 92.), and at the same time 

 diminish in intensity and alter in direction as they recede 

 thence, yet the two circles may serve to represent the resultant 

 of these currents; and it will be evident that the point of 

 most intense action will be where they touch, and immediately 

 under the magnetic pole, or, on account of the time required, 

 a little in advance of it. Hence that portion of the forces 

 which acts parallel to the plane of the metal will carry the 

 pole forward in the direction of the arrow E, because the forces 

 are equally powerful on the side of the pole A B, as on the 

 side C D; and this portion, which on account of the time ne- 

 cessary for the production of the currents excited is perpen- 

 dicular to the direction of the metal, will be, as we have said, 

 repulsive, and have a tendency to impel the pole upwards and 

 away. 



But suppose that instead of the metal moving in a recti- 

 linear direction, a circular disc revolving upon its axis be sub- 

 stituted ; and then let us consider, in the first place, the case 

 of the magnetic pole placed upon its centre (fig. 4.) ; there is 

 then no production of electric currents, not because there is 

 no tendency to their formation, for I have stated in this letter, 

 and shown in my memoirs (F. 149. 166. 217.), that from the 

 time the disc begins to move, currents are also ready to move ; 

 but because they have a tendency to be formed in the direc- 

 tion of radii from the circumference to the centre; and as all 

 the parts are equally influenced, none of them having an ex- 

 cess of power over the others, and all equally distant from the 

 centre, no discharge can take place, and consequently no cur- 

 rent can be developed. As no current can exist, no effect de- 

 pendent on the action of a current upon the pole can be pro- 

 duced, and consequently there is then neither revolution nor 

 repulsion of the magnet. Hence the cause of the verticality 

 without repulsion which occurs at this place. 



Let us now consider the case in which the pole of the mag- 

 net, instead of being placed over the centre of the metal, is at 

 one of its sides, as in N, figure 5. The tendency to form elec- 

 tric currents is due to the movement of the parts of the disc 

 through the magnetic curves (F. 116. 217.)> and when these 

 curves are of equal intensity, the electric currents increase in 



