178 ON FARADAY'S LINES OP FORCE. 



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

 quantities of electricity 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 attraction near the surface itself is per- 

 pendicular to it 



TJieory 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 application 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 qxiantity 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*. 



Ttieory of Paramagnetic and Diamagnetic Induction. 



Faraday t has shewn that the effects of paramagnetic and diamagnetic bodies 

 in the magnetic field may be explained by supposing paramagnetic bodies to 



* See Professor Thomson On the Mathematical Theory of Magnetism, Chapters in. and v. Phil. 

 Trant. 1851. 



t Experimental ReteanJut (3292). 



