318 AMPERE'S THEORY. SECT. XXXI. 



with the whole, yet the general effect is the same as if the mag- 

 netic properties were confined to the surface. Consequently, 

 Ampere concludes that the internal electro-currents must com- 

 pensate one another, and that the magnetism of a body must 

 therefore arise from a superficial current of electricity constantly 

 circulating in a direction perpendicular to the axis of the magnet ; 

 so that the reciprocal action of magnets and all the phenomena 

 of electro-magnetism are reduced to the action and reaction of 

 superficial currents of electricity, acting at right angles to their 

 direction. 



Notwithstanding the experiments made by Ampere to elucidate 

 the subject, there is still an uncertainty in the theory of the 

 induction of magnetism by an electric current in a body near it. 

 It does not appear whether electric currents which did not 

 previously exist are actually produced by induction, or if its 

 effect be only to give one uniform direction to the infinite num- 

 ber of electric currents previously existing in the particles of the 

 body, and thus rendering them capable of exhibiting magnetic 

 phenomena, in the same manner as polarization reduces the 

 undulations of light to one plane, which had previously been 

 performed in every plane. Possibly both may be combined in 

 producing the effect ; for the action of the electric current may 

 not only give a common direction to those already existing, but 

 may also increase their intensity. However that may be, by 

 assuming that the attractions and repulsions of the elementary 

 portions of electric currents vary inversely as the square of the 

 distance, the actions being at right angles to the direction of the 

 current, it is found that the attraction and repulsion of a current 

 of indefinite length on the elementary portion of a parallel current 

 at any distance from it are in the simple ratio of the shortest 

 distance between them : consequently, the reciprocal action of 

 electric currents is reduced to the composition and resolution of 

 forces, so that the phenomena of electro-magnetism are brought 

 under the laws of mechanics by the theory of Ampere. It 

 appears that Dr. Faraday's very remarkable experiment of elec- 

 trifying and magnetising a ray of polarized light may possibly 

 afford a demonstration of the reality of Ampere's explanation of 

 the ultimate nature of magnetism. 



In this experiment a copper wire 501 feet long was arranged 

 in four concentric spirals, the extremities of which were connected 



