Mr. H. A. Rowland on Magnetic Distribution. 355 



formula is applied to the special case considered by Green, it 

 agrees with it where the permeability of the material is great. 

 My formula, however, is far more general than Green's. 



It is to Green that we owe the important remark that the 

 distribution in a steel magnet may be nearly represented by the 

 same formula that applies to electromagnets. 



As Green uses what is known as the surface-density of mag- 

 netization, let us first see how this quantity compares with those 

 I have used. 



Suppose that a long thin steel wire is so magnetized in the 

 direction of its length that when broken up the pieces will have 

 the same magnetic moment. While the rod is together, if we 

 calculate its effect on exterior bodies, we shall see that the ends 

 are the only portions which seem to act. Hence we may mathe- 

 matically consider the whole action of the rod to be due to the 

 distribution of an imaginary magnetic fluid over the ends of the 

 rod. As any case of magnetism can be represented by a proper 

 combination of these rods, we see that all cases of this sort can 

 be calculated on the supposition of there being two magnetic 

 fluids distributed over the surfaces of the bodies, a unit quan- 

 tity of which will repel another unit of like nature at a unit's 

 distance w 7 ith a unit of force. The surface-density at any point 

 will then be the quantity of this fluid on a unit surface at the 

 given point ; and the linear density along a rod will be the quan- 

 tity along a unit of length, supposing the density the same as 

 at the given point. 



"Where we use induced currents to measure magnetism we 

 measure the number of lines of force, or rather induction, cut 

 by the wire, and the natural unit used is the number of lines of 

 a unit field which will pass through a unit surface placed per- 

 pendicular to the lines of force. The unit pole produces a unit 

 field at a unit's distance ; hence the number of lines of force 

 coming from the unit pole is 47r, and the linear density is 



^SSB" <"> 



and the surface-density 



S =^Al ( 12 ) 



These really apply only to steel magnets; but as in the case of 

 electromagnets the action of the helix is very small compared 

 with that of the iron, especially when it is very long and the iron 

 soft*, we can apply these to the cases w T e consider. 



* I take this occasion to correct an error in Jenkin's ' Textbook of 

 Electricity,' where it is stated that, by the introduction of the iron bar into 



2 A2 



