CONTRIBUTIONS TO SCIENCE. 539 



depend on the energy of these vortices, and this must 

 depend on the relations of the several portions of the wire 

 to one another and to the medium, as well as on the density 

 of the medium. The density of the medium Maxwell 

 identified with its magnetic permeability. This is greater 

 in (para-) magnetic substances than in air or vacuum ; 

 greatest of all in iron. In fact, it is so great in the case of 

 iron, that Maxwell supposed the particles of the iron itself 

 to take part in the vortex motion. Hence the energy of the 

 field, and therefore the self-induction of the wire, is greater 

 the greater the magnetic permeability of the surrounding 

 medium, and the presence of an iron core in a coil immensely 

 increases its self-induction and the energy corresponding to 

 a given current flowing in the coil. 



If, after a current has been established in a wire, the 

 circuit be broken or the electro-motive force removed, the 

 molecular vortices refuse to come to rest till they have 

 expended their energy. The only outlet for this energy is 

 a current in the wire, since there is no opportunity of doing 

 work in a non-conducting medium, where there can be no 

 slipping between the elements of the mechanism. The vor- 

 tices, therefore, keep the electricity moving in the wire after 

 the battery has been removed, until they have expended all 

 their energy in doing work against the resistance of the wire. 



But if there be another conductor in the field parallel or 

 slightly inclined to the first, there is another partial outlet 

 for the energy of the system, and a " secondary " current will 

 be set up in the second wire in the same direction as the 

 current in the primary, while that in the primary will be 

 less than it would have been if no secondary circuit had 

 existed. In this way the hypothesis of molecular vortices 

 affords an explanation both of the mutual induction of two 

 circuits and the self-induction of one. 



Suppose a wire to be placed in a magnetic field at right 

 angles to the lines of force, and then to be moved so as to 

 cut the lines at right angles, we should expect that in front 

 of the moving wire the lines of force or threads of vortices 

 would be squeezed together transversely, but extended in 



