340 Mr. E. H. M. Bosanquet on Electromagnets. 



but the magnetizing force existing within the metal of the 

 bar, for instance, was supposed not to be the same as the 

 external magnetizing force, but to be diminished by demag- 

 netizing forces, which resided on the ends of the bar. In 

 rings, however, where there were no ends, the theoretical 

 magnetizing force was the same as the external magnetizing 

 force, and the permeability of a magnetic metal could be 

 determined by measures of rings. 



In dealing practically with bars, what we want to know is 

 the connection between the magnetism developed and the 

 external magnetizing force. The ratio of these two quantities, 

 which I call conductivity, is a quantity analogous to the 

 permeability of the theory, but not identical with it ; the 

 conductivity does not take into account the supposed demag- 

 netizing forces at the ends of the bar. The system that I 

 have adopted prefers to attribute the diminished magnetism in 

 the case of bars with ends to the increased resistance experi- 

 enced by the magnetism in traversing space external to the 

 magnetic metal ; and I employ the word conductivity in 

 connection with an entire magnetic circuit, with its air 

 resistances ; leaving permeability with its original application 

 to circuits or parts of circuits lying wholly within the 

 magnetized substance. 



The permeability of rings and the conductivity of bars 

 have, then, precisely the same meaning. In both cases the 

 meaning is, 



magnetic induction 

 external magnetizing force' 



If we suppose the magnetizing force uniformly extended 

 along the bar, as by a uniformly wound coil, we have 



External magnetizing force x length = external potential. 



Conductivity _ magnetic induction 1 

 length magnetic potential ~~ p' 



where p is magnetic resistance, according to our definition of 

 magnetic resistance. 



Whence permeability of rings or conductivity of bars 

 _ length 



~ f 

 I invariably reserve \l * for the 'permeability ', since it is thus 



* In a paper presented to the Boyal Society, Dec. 20, 1880 (Proc. E. 

 S. vol. xxxw. p. 445), in which the system of broken magnetic circuits 

 with air resistances^ now so generally used by practical men, was first 

 developed, I distinguished insufficiently between permeability and conduc- 

 tivity, and used jx to represent both ideas. 



