256 Mr. J. Parker on the Theory of Magnetism and 



magnetism, we have only to mention that, according to the 

 physical theories of magnetism, all that is meant is a finite 

 pressure or tension per unit area on the surface. 



The reason why, in an ordinary bar-magnet, there is little 

 manifestation of the magnetic properties except near the ends, 

 is supposed to be that the positive end of one atom or mole- 

 cule and the negative end of the next partially neutralize 

 one another^ 



QOOOOGOO 



as indicated by the figure. 



To calculate to what extent the magnetisms of successive 

 molecules neutralize one another, we require some preliminary 

 propositions. 



Suppose that a very short, thin, straight magnet of length I 

 is placed with its centre at and let the quantities of mag- 

 netism at its two ends A, B, which may be treated as mere 



Fig. 6. 



points, be — m and + m. Let a unit positive pole be situated 

 at a point P whose distance from is r, and let 6 be the 

 angle between OP and the line AB, or the axis of the magnet. 



7YI 



Then the magnet AB exerts on P a repulsive force -pub 

 along BP and an attraction p 2 along PA. Now -prT 2 



along BP is equivalent to p^ pp parallel to BA and ™ 



PO m PA 2 

 X p-r\ parallel to OP ; and tjj^ along PA is equivalent to 



x =p-r parallel to BA and-j-r- 2 ^p-r parallel to PO. Hence the 



action of the elementary magnet AB on P is equivalent to a 



ml / 1 1 \ 



force -^-1 pp- 3 +pX3 ) acting at P parallel to BA and a force 



'" po (i4 3 ~i4') atraloiisOP - 



