president's address— section ▲. 41 



or, I is the induction that would be necessary to cause a force 

 H (i.e., a variation in the magneto-motive force of H per 



unit length) in a medium of permeabihty - . ^ . The 



magnetic susceptibility K is simply ^ . ^ . Both these 



quantities I and K depend, therefore, on the permeability of 

 the surrounding medium as well as on that of the iron, and 

 cannot properly be said to belong to the iron. 



I should like here to suggest that in future B and H be 

 never treated as of like dimensions ; that the equation 

 B = H + 4 TT I, if it is ever necessary that it should be 

 written at all, should be written B = a*' H + 4 tt I, /u' being 

 the permeability of the air, or, in other words, we should 

 make ju the absolute, not the specific, permeability of a sub- 

 stance. As for the equation itself, it is most interesting 

 historically, but it is not necessary to the development of the 

 theory. What would be thought of us if we wrote : — 

 Weight of a substance = its volume -{- 4 tt x something to 

 make this equation a correct one ? And the statement is not 

 much bettered by substituting for " its volume " the words 

 "weight of an equal volume of water." 



The analogy, being correct, gives a true value of the energy 

 of a magnetised medium. The energy per unit volume in 

 the analogy is i E rf*, which (translating) 



1 B^ 1 „2 1 TJU 



O TT JU OTT OTT 



It is easy to picture by this analogy the events that take 

 place when a circuit carrying a current, or a permanent 

 magnetic shell, attracts a piece of iron. Suppose a plane 

 circular shell, uniformly and permanently magnetised, to be 

 immersed in a medium of permeability ju', and a piece of iron 

 of permeability fi to be in the neighbourhood. We know 

 then that the magnet will attract the iron, the energy of the 

 field diminishing as the two approach, for the permanent 

 magnet gives rise to so much induction, as the iron approaches 

 the magnet it takes up a larger share of the total induction. 

 But the energy of so much induction in iron is much less than 



1 B* 



in air, for energy per unit volume is q- • — . On the other 



