358 Permanent Magnetism [OH. xi 



The quantity I is called the " intensity of magnetisation " of the magnet. 

 This magnetisation has direction as well as magnitude. In the present 

 instance the direction is that of the axis of x. 



406. In general, we define the intensity and direction of magnetisation 

 as follows : 



The intensity of magnetisation at any point of a magnetised body is defined 

 to be the ratio of the magnetic moment of any small particle at this point to 

 the volume of the particle. 



The direction of magnetisation at any point of a magnetised body is defined 

 to be the direction of the magnetic axis of a small particle of magnetic matter 

 at the point. 



Instead of specifying the magnetisation of a body in terms of its poles, it 

 is both more convenient from the mathematical point of view, and more in 

 accordance with truth from the physical point of view, to specify the intensity 

 at every point in magnitude and direction. Thus the bar-magnet which has 

 been under consideration would be specified by the statement that its in- 

 tensity of magnetisation at every point is / parallel to the axis of x. A body 

 such that the intensity is the same at every point, both in magnitude and 

 direction, is said to be uniformly magnetised. 



THE MAGNETIC FIELD OF FORCE. 



407. The field of force produced by a collection of magnets is in many 

 respects similar to an electrostatic field of force, so that the various concep- 

 tions which were found of use in electrostatic theory will again be employed. 



The first of these conceptions was that of electric intensity at a point. In 

 electrostatic theory, the intensity at any point was defined to be the force 

 per unit charge which would act on a small charged particle placed at the 

 point. It was necessary to suppose the charge to be of infinitesimal amount, 

 in order that the charges on the conductors in the field might not be dis- 

 turbed by induction. 



There is, as we shall see later, a phenomenon of magnetic induction, 

 which is in many respects similar to that of electrostatic induction, so that in 

 defining magnetic intensity we have again to introduce a condition to ex- 

 clude effects of induction. 



Also, to avoid confusion between the magnetic intensity and the intensity 

 of magnetisation defined in 406, it will be convenient to speak of magnetic 

 force at a point, rather than of magnetic intensity. We accordingly have the 

 following definition, analogous to that given in 30. 



