180 



MAGNETISM 



the NSP, bending round through the air, entering the SSP, travel- 

 ling through the steel body of the magnet, and joining on at the 

 point of issue. The condition symbolised by the lines of force is 



somehow maintained by what goes 

 on in the body of the magnet. 



We may roughly illustrate the 

 idea of continuity by imagining a 

 number of steel flexible rings of 

 different sizes a, &, c, d, e, f, bound 

 together along NS as in Fig. 126. 

 The part where they are bound 

 together represents the magnet, the 

 region where they bend round repre- 

 sents the field in air. 



Magnetisation by induc- 

 tion. Permeability. We may 

 use the filing method to show some- 

 thing of what is going on in magnetisation by induction. If a pure 

 of soft iron is placed in a field for instance, in the field between two 

 unlike poles as in Fig. 127 (a) and the field thus produced is com- 

 pared with that when the soft iron is removed, Fig. 127 (/;), it is 

 seen that the lines from N bend round and enter the iron a^ if tU-y 



FIG. 126. 





FIG. 127. 



found it easier to go through the iron than through the air. Where 

 they enter the soft iron the passage from air to iron is manifested 

 by a SSP ; where they leave the iron at the other end the passage 

 into air is manifested by a NSP. But the idea to be dwelt upon is 

 that the iron forms an easier passage for the magnetic action is, 

 as Faraday put it, a better conductor than the air, or, as we now 

 say, using terms introduced by Lord Kelvin, is more " permeable," 

 or has greater " permeability." 



