258 Mr. J, Parker on the Theory of Magnetism and 



ml cos 7 along Oz, are — -^-cos 7 parallel to Oz and 



5 — ' - along Or. 



TYl i 



These forces combined give —3 parallel to BA and 

 ?>ml (x y 



(X V z \ 



- cos a + - cos 6 + - cos 7 1 , 

 r r r ' y 



or —j- cos 0, along OP. We may therefore say that the 



action of a small magnet AB is equal to the sum of the actions 

 of its components. 



A molecule may be supposed made up of several elementary 

 magnets such as AB. As each of these constituent elemen- 

 tary magnets is equivalent to three component magnets 

 parallel respectively to the three rectangular axes, the whole 

 molecule is equivalent to three elementary magnets parallel 

 respectively to the axes, and therefore equivalent to a single 

 resultant elementary magnet. The magnetic moment and 

 direction of this resultant magnet may be called the magnetic 

 moment and axis of the molecule. 



As neighbouring molecules may be magnetized differently, 

 we shall avoid the irregularities by considering a volume 

 dv which, though very small, is still large enough to con- 

 tain many molecules. Since each molecule in the volume 

 is equivalent to three small component magnets parallel to 

 the axes, the whole volume dv is equivalent to three small 

 component magnets, and therefore to a single small magnet. 

 If we denote the moment of this single magnet by Idv, 

 I is defined to be the intensity of magnetization of the ele- 

 ment, or at a point in the element, and the direction of I is 

 defined to be the direction of magnetization. 



If A, B, C be the components of I parallel to the axes, it 

 is evident that the external action of the element dv is equal 

 to the sum of the actions of three equal volumes placed suc- 

 cessively in the same position, whose magnetizations are 

 respectively parallel to the axes and equal to A, B, and C. 



If we draw a curve such that the tangent at any point is 

 the direction of magnetization at that point, the curve may be 

 called a line of magnetization. It is generally continuous so 

 long as we keep to the same body. If at any point two con- 

 secutive tangents cut at a finite angle, we shall consider, that, 



