INDUCED MAGNETISATION 



233 



flux across the gap must be equal and opposite to the flux over the 

 curved hemispherical surface. 



All this may be regarded as experimental. We now make the 

 fundamental supposition that the lines and tubes of induction are 

 continued through the iron, that the tubes crossing the air gap are 

 the continuations of those in the air outside, and that along each 

 tube, induction x cross-section is constant, and has the same value 



FIG. 179. 



as the same product for the same tube outside the iron. We shall 

 see later that on a certain supposition the tubes will go straight 

 through the iron, or that the induction within it is uniform. 



We may now close up the gap, having only used it to emphasise 

 the idea that the induction runs through the iron, and we have the 

 lines of induction as in Fig. 179, and along any tube induction x cross- 

 section = constant whether it is in iron or air. Let us now, Fig. 180, 



FIG. 180. 



