DETERMINATION OF THE COEFFICIENT OF MAGNETISATION. 485 



point. This being the only effective force, the value of the intensity 

 of magnetisation at the point in question is 



The induction is equal to ^-rrkfa so that the total flow of induction 

 across the section S of the soft iron, comprising still that which arises 

 from the currents, is 



Q = 



/d S 

 *' 



If the soft iron does not occupy the whole of the space bounded 

 by the currents, but merely a portion S' of the section S, the total 

 flow of induction, across the surface of the currents, is 



T f (VS , (VS'1 

 = 4 7r 1 I J + 4**J 



Suppose, for instance, that the section of the iron is a circle of 

 radius a! concentric with the circular section of a torus ; the total 

 flow of induction in the torus will be 



Q' = 4*^1 [~R ~ 



-* + 



If the section of the coil were a rectangle of height b parallel to 

 the axis of revolution, and of the thickness 20, the mean radius 

 being R, we should have 



x R-a 



IR-a 



The iron ring, in like manner, might have a rectangular section 

 of height b\ of thickness 20', and of mean radius R'; the total flow 

 of induction would then be 



We shall see further on (559) the use which can be made of 

 these various formulae. 



