ELEMENTARY MAGNETISM. 419 



As an expansion of the note at p. 409, the following extract may 

 find a place here : 



' It is well known that a voltaic current exerts an attractive 

 force upon a second current, flowing in the same direction ; and 

 that when the directions are opposed to each other the force exerted 

 is a repulsive one. By coiling wires into spirals, Ampere was 

 enabled to make them produce all the phenomena of attraction 

 and repulsion exhibited by magnets, and from this it was but a 

 step to his celebrated theory of molecular currents. He supposed 

 the molecules of a magnetic body to be surrounded by such currents, 

 which, however, in the natural state of the body mutually neutral- 

 ised each other, on account of their confused grouping. The act of 

 magnetisation he supposed to consist in setting these molecular 

 currents parallel to each other ; and, starting from this principle, 

 he reduced all the phenomena of magnetism to the mutual action 

 of electric currents. 



' If we reflect upon the experiments recorded in the foregoing 

 pages from first to last, we can hardly fail to be convinced that 

 diamagnetic bodies operated on by magnetic forces possess a polarity 

 " the same in kind as, but the reverse in direction of, that acquired 

 by magnetic bodies." But if this be the case, how are we to 

 conceive the physical mechanism of this polarity ? According to 

 Coulomb's and Poisson's theory, the act of magnetisation consists 

 in the decomposition of a neutral magnetic fluid ; the north pole of 

 a magnet, for example, possesses an attraction for the south fluid 

 of a piece of soft iron submitted to its influence, draws the said 

 fluid towards it, and with it the material particles with which the 

 fluid is associated. To account for diamagnetic phenomena this 

 theory seems to fail altogether ; according to it, indeed, the oft- 

 used phrase, " a north pole exciting a north pole, and a south pole 

 a south pole," involves a contradiction. For if the north fluid be 

 supposed to be attracted towards the influencing north pole, it is 

 absurd to suppose that its presence there could produce repulsion. 

 The theory of Ampere is equally at a loss to explain diamagnetic 

 action ; for if we suppose the particles of bismuth surrounded by 

 molecular currents, then, according to all that is known of electro- 

 dynamic laws, these currents would set themselves parallel to, and 

 in the same direction as, those of the magnet, and hence attraction, 

 and not repulsion, would be the result. The fact, however, of this 

 not being the case, proves that these molecular currents are not 

 the mechanism by which diamagnetic induction is effected. The 

 consciousness of this, I doubt not, drove M. Weber to the assumption 

 that the phenomena of diamagnetism are produced by molecular 

 currents, not directed, but actually excited in the bismuth by the 



B B 2 



