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XXVIII. On Magnetomotive Force. 

 By R. H. M. Bosanquet, St. John's College, Oxford. 



To the Editors of the Philosophical Magazine and Journal. 



GrENTLEMEN, 



THE following paper is an attempt to develop the analogy 

 between magnetism and the voltaic circuit, which was 

 enunciated by Faraday. The assumptions of this theory are 

 generally admitted to be true; but they have not, so far as I 

 know, been consistently pushed to their consequences. 



It appears to me, further, that this point of view is the only 

 one for which there exists any experimental evidence. The 

 fundamental assumptions of Poisson's theory are admittedly 

 false ; and it is only by the introduction of fictitious quantities 

 that the existing mathematical theory has been rendered in any 

 degree capable of representing the facts. 



Faraday compared a magnet to a voltaic battery immersed 

 in water*; and he established by experiment the principal 

 analogies on which this comparison is founded. 



The first principle of the voltaic circuit is, that the current 

 produced by a given electromotive force in a circuit depends 

 on the resistance of the circuit as a whole. 



I shall use the expression "magnetomotive force" to indi- 

 cate the analogue of electromotive force. It is a difference 

 of magnetic potential, just as electromotive force is a differ- 

 ence of electric potential. 



Now the fundamental hypothesis at the base of the ordinary 

 mathematical theory of magnetism is, that there are magneti- 

 zing forces ^ which are of the dimensions of the magnetic 

 induction 33 which they produce, and that the magnetizing 

 force permeates every medium, and produces in magnetic 

 media magnetic induction proportional to the force and to a 

 coefficient of permeability /j,, quite independently of the exist- 

 ence of any magnetic circuit. 



This is the simplest way of putting it. I will state the case 

 presently in terms of the quantity known as " magnetization," 

 which is the quotient of moment by volume. 



There are two objections to this. First, in relation to com- 

 plete circuits. 



Consider a sphere of iron, or a disk magnetized normally to 

 its plane. Then (Maxwell, ii. p. 66) the magnetic induction 

 through it is small. Now let it form part of a bar, and let the 

 bar be bent round into a ring, so as to establish a magnetic 

 circuit. We know that the magnetic induction through the 

 same piece under the same force will become enormous. Can 

 * Experimental Researches, iii. p. 424, par. 3276. 



