442 Prof. Challis on the Hydrodynamical Theory of the . 



by the intervention of the aether, between two galvanic elements, 

 according to the law of the inverse square (see art. 69). 



75. The processes by which different physicists have calculated 

 the action of a galvanic coil on a small magnet rest entirely on 

 the results of experiments made by Ampere, the details of which 

 are given in his work entitled Theorie des Phenomenes Electro- 

 dynamiques uniquement deduite de l* experience. Ampere, with 

 the view of explaining magnetic phenomena by galvanic action, 

 conceived that each molecule of a magnet might be surrounded 

 by a complete galvanic circuit, and by following out the conse- 

 quences of this hypothesis was led to give to a galvanic conductor 

 the form of a coil, or solenoid, the action of which is found prac- 

 tically to be nearly the same as that of a magnet. Advantage 

 being taken of this experimental result, the calculation of the 

 action of a coil on an external small magnet is reduced to the 

 calculation of magnetic action. On these principles Professor 

 Clerk Maxwell, in his f Treatise on Electricity and Magnetism ' 

 (vol. ii. p. 131, art. 482), compares " the action of a finite [gal- 

 vanic] current with that of a magnetic shell of which the circuit 

 is the bounding edge;" and then adds, "If the circuit be sup- 

 posed to be filled up by a surface bounded by the circuit and 

 thus forming a diaphragm, and if a magnetic shell coinciding 

 with this surface be substituted for the electric [i. e. galvanic] 

 current, then the magnetic action of the shell on all distant 

 points will be identical with that of the current." This conclu- 

 sion entirely depends on the assumption that the magnetic shell 

 has a negative side as well as & positive side, which is equivalent 

 to assuming the simultaneous action of the red and blue magne- 

 tisms mentioned above. In consequence of this hypothesis the 

 law of the magnetic action of any element of the shell is changed 

 from the inverse square to the inverse cube, and is thus made to 

 accord with the law deduced from the a priori principles of the 

 hydrodynamical theory. 



76. In the f Treatise on Natural Philosophy/ by Professor Sir 

 William Thomson and Professor Tait, the following passage 

 occurs in vol. i. § 477 (e) : — 



ff For magnetic and electromagnetic applications a very useful 

 case [of the action according to the law of the inverse square] 

 is that of two equal disks, each perpendicular to the line joinr 

 ing their centres, on any point in that line — their masses being 

 of opposite sign — that is, one repelling and the other attracting. 

 Let a be the radius, p the mass of a superficial unit of either, c 

 their distance, x the distance of the attracted point from the 

 nearest disk. The whole action is evidently 



o f__f+^ g \ 



