362 Mr. Faraday on Magneto-electric Induction; 



force in proportion to the increase of rapidity in the motion 

 of the parts of the disc that intersect the magnetic curves 

 (F. 258.). Let us now trace a circle a b around a magnetic 

 pole as a centre, and it will represent the projection of mag- 

 netic curves of equal intensity upon the disc; a and b are 

 points situated at an equal distance from the pole, in the pass- 

 ing radius which is immediately under the pole; but as the 

 part or point a passes by the pole with much greater velocity 

 than the part b, the intensity of the electric current which is 

 excited in this part a is proportionably greater. This is also 

 true for the points in any other radius of the revolving plate 

 cutting the circle a b, and true likewise for any other circle 

 traced round N as a centre, and representing consequently 

 magnetic curves of equal intensity ; with the exception, that 

 when the circle extends beyond the centre C of the revolving 

 disc, as to c d, instead of the existence of a feebler current at 

 the point d than at the point c, there is then a tendency to 

 produce an opposite current. 



The natural consequence of these actions of the different 

 parts is, that as the sum of the forces tending to produce the 

 electric current in the direction from c to d is greater on the 

 side c of the magnetic pole than on the side d, the curvature 

 or return of these currents by the right and left also com- 

 mences on this side; and then the two circles, which as be- 

 fore may be considered as representing the resultants of these 

 currents, do not come into contact exactly under the pole, but 

 at a greater or less distance from it, towards the circumference, 

 as in figure 6. 



This circumstance of itself would not occasion any move- 

 ment in a pole restrained in its motion to the direction of the 

 radius only; but being combined with that which results from 

 the time necessary for the development of the current, and to 

 which reference has been already made, as explaining thejirst 

 of the three forces by which M. Arago exhibits the action of 

 the magnetic pole and disc, it will, I hope, fully elucidate all 

 the effects that we are investigating, and will also prove the 

 influence of time as an element. Let c (fig. 7.) be the centre 

 of a revolving disc, and r c a part of the radius under the mag- 

 netic pole p ; the contact of the two circles representing the 

 currents is, as we have just seen, on the side of the pole beyond 

 the centre c; but on account of the element of time and the 

 direction of the rotation R of the plate of metal, it is also a 

 little. to the left of the radius re ; so that the pole is brought 

 under the action of the two orders of currents, not symmetric- 

 ally but obliquely. The necessary consequence is, that if it 

 be free to move in the direction of the radius, and in that di- 



