the Lines of Magnetic Force. 415 



sphondyloid external to the magnet, a cm-rent of electricity is 

 generated, and that current is definite and the same for any or 

 every intersection of the given sphondyloid. At the same time, 

 whether the wire be quiescent or in motion, it does not cause 

 derangement, or expansion, or contraction of the lines of force ; 

 the state of the power in the neighbouring or other parts of the 

 sphondyloid remaining sensibly the same (3176.). 



3272. The old experiment of a wire when carrying an electric 

 current* moving round a magnetic pole, or of a current being 

 produced in the same wire when it is carried per force round the 

 same pole (114.), shows the electrical dependence of the magnet 

 and the wire, both when the current is employed from the first, 

 and when it is generated by the motion. It coincides in prin- 

 ciple with the results already quoted, and it includes, experiment- 

 ally, all currents of electricity, whatever the medium in which 

 they occur, even up to the discharge of the Leyden jar and that 

 between the electrodes of the voltaic battery. I think it also 

 indicates the state of magnetic or electric tension in the sur- 

 rounding space, not only when that space is occupied by metal 

 or a wire, but also by air and other bodies ; for whatever be the 

 state in one case, it is probably general and therefore common 

 to all (3173.). 



3273. I will now venture for a time to assume the physical 

 existence of the external lines of magnetic force, for the purpose 

 of considering how the idea will accord with the general pheno- 

 mena of magnetism. The magnet is evidently the sustaining 

 power, and in respect of its internal condition or that of its par- 

 ticles, tbere is no idea put forth to represent it which at all ap- 

 proaches in probability and beauty to that of Ampere (1659.). 

 Its analogy with the helix is wonderful ; nevertheless there is, 

 as yet, a striking experimental distinction between them; for 

 whereas an unchangeable magnet can never raise up a piece 

 of soft iron to a state more than equal to its own, as measured 

 by the moving wire (3219.), a helix carrying a current can de- 

 velope in an iron core magnetic lines of force, of a hundred or 

 more times as much power as that possessed by itself, when 

 measured by the same means. In every point of view, therefore, 

 the magnet deserves the utmost exertions of the philosopher for 



body may be either that contained by the surface of revolution of E, or 

 that contained between the two xirfaccs of E and F, and which, for the 

 sake ofbrevitVj I have (by the advice of a kind friend) called supply the 

 Sphondyloid. The parts of the solid described, which are within and with- 

 out the magnet, arc in power equivalent to each other. When it is needful 

 ■ "■ !. of them separate!), they arc easily distinguished as the inner and 



outer sphoiidyloids ; the surface of the magnet being then part of the 

 bounding surface. 



* Experimental Researches, 8vo edition, vol. ii. p. 127. 



