44 Mr maxwell, ON FARADAY'S LINES OF FORCE. 



of one substance having a greater inductive capacity, or conducting the lines of inductive 

 action more freely than another. If we suppose that in our analogy of a fluid in a resisting 

 medium the resistance is different in different media, then by making the resistance less we 

 obtain the analogue to a dielectric which more easily conducts Faraday's lines. 



It is evident from (23) that in this case there will always be an apparent distribution of 

 electricity on the surface of the dielectric, there being negative electricity where the lines enter 

 and positive electricity where they emerge. In the case of the fluid there are no real sources 

 on the surface, but we use them merely for purposes of calculation. In the dielectric there 

 may be no real charge of electricity, but only an apparent electric action due to the surface. 



If the dielectric had been of less conductivity than the surrounding medium, we should 

 have had precisely opposite efl^ects, namely, positive electricity where lines enter, and negative 

 where they emerge. 



If the conduction of the dielectric is perfect or nearly so for the small quantities of elec- 

 tricity with which we have to do, then we have the case of (24). The dielectric is then 

 considered as a conductor, its surface is a surface of equal potential, and the resultant attrac- 

 tion near the surface itself is perpendicular to it. 



Theory of Permanent Magnets. 



A magnet is conceived to be made up of elementary magnetized particles, each of which has 

 its own north and south poles, the action of which upon other north and south poles is 

 governed by laws mathematically identical with those of electricity. Hence the same applica- 

 tion of the idea of lines of force can be made to this subject, and the same analogy of fluid 

 motion can be employed to illustrate it. 



But it may be useful to examine the way in which the polarity of the elements of a 

 magnet may be represented by the unit cells in fluid motion. In each unit cell unity of fluid 

 enters by one face and flows out by the opposite face, so that the first face becomes a unit 

 sink and the second a unit source with respect to the rest of the fluid. It may therefore be 

 compared to an elementary magnet, having an equal quantity of north and south magnetic 

 matter distributed over two of its faces. If we now consider the cell as forming part of a 

 system, the fluid flowing out of one cell will flow into the next, and so on, so that the source 

 will be transferred from the end of the cell to the end of the unit tube. If all the unit tubes 

 begin and end on the bounding surface, the sources and sinks will be distributed entirely 

 on that surface, and in the case of a magnet which has what has been called a solenoidal or 

 tubular distribution of magnetism, all the imaginary magnetic matter will be on the surface*. 



Theory of Paramagnetic and Diamagnetic Induction. 



V 



Faraday •)■ has shewn that the effects of paramagnetic and diamagnetic bodies in the magnetic 

 field may be explained by supposing paramagnetic bodies to conduct the lines of force better, 



• See Professor Thomson On the Mathematical Theory of Magnetism, Chapters III. & V. Phil. Trans. 1851. 

 ■^ Experimental Researches (3293). 



