MAGNETISM 



the field of the other at P. Then the lines will destroy each other 

 at P and the line from N will run through the small magnet some- 

 what as drawn in Fig. 188, or the line has been pulled out of the 

 air into the iron of NS. 



If a piece of previously unmagnetised iron is put into the 

 position of NS its molecular magnets first tend to set along the lines 



FIG. 186. 



of force of NS, and then each molecular magnet so setting has an 

 action like that of NS as above considered. It draws out of the 



Flo. 187. 



field round it lines of force due to NS, concentrating them on itself. 

 Regarding induction rather than force, we may say that each 

 molecular magnet setting along the lines in the field draws into 



FIG. 188. 



itself some of the induction due to NS and makes it continuous 

 with its own induction. 



AVe shall now consider some cases of induction. 



Induction in a sphere of uniform permeability /m 

 placed in a field in air uniform before its introduction. 

 Let the uniform field into which the sphere is introduced be H. Let 

 us assume that the sphere is uniformly magnetised by induction 



