INDUCED MAGNETISATION 



it is placed from the distribution of poles throughout the system, 

 these poles acting according to the inverse law. 



Now suppose that instead of making the tunnel we make a cut 

 through the iron perpendicular to the lines of force and remove a 

 very thin wafer-shaped portion of the iron. The lines of induction 

 go straight across the crevasse thus formed and the tubes of induc- 

 tion have the same cross-section in the crevasse as in the iron on 

 either side. But in the iron -free space in the crevasse the induction 

 is equal to the intensity. The intensity will be that previously 

 existing + that due to the new surface layers formed on the two 

 faces of the crevasse, say I, and these will produce intensity 4)71-1. 

 Hence within the cavity the intensity is 



B = H + 47rl 



where H is the intensity before the crevasse was made, or putting 

 I = AcH, the intensity in the crevasse which is equal to the induc- 

 tion in the iron before the crevasse was made is 



B = H + 



Putting B = ^H 



we get /UL = 1 4* 4?r/c. 



The quantities B and H are to be regarded as the two funda- 

 mental quantities. The quantity I, which is equal to(B H)/47r, 

 is a convenient quantity to introduce in that it enables us to express 

 simply the polarity which we may suppose to be distributed over 

 the system where /m changes in order to calculate the intensity of 

 the field, using the inverse square law. This supposed polarity is 

 somewhat artificial if we accept the principle of continuity of flux 

 of induction. In electricity the tubes actually end on conductors 

 and the charges at their ends may be regarded as having a real 

 existence. But in magnetism the tubes of induction never end. 

 Either they come from an infinite distance and go off to another 

 infinite distance or they are re-entrant and form closed rings. The 

 polarity or which we suppose to exist where a tube passes from one 

 medium to another with different /u, is merely a device introduced 

 to enable us to calculate the effect of change of medium. 



Permeability and the molecular theory. We can give 

 some account of the greater permeability of iron as compared with 

 air on the molecular theory in the following way, in which we only 

 attempt to give a general idea. 



Let NS, Fig. 186, represent a magnet and consider one of its lines 

 of force as drawn through P. Now introduce just outside this line 

 a small magnet NS, Fig. 187, turned so that its lines run in the 

 opposite direction through P, and let us suppose for simplicity that 

 it is of such strength and in such a position that it just neutralises 



Q 



