Magnetomotive Force. 213 



(1) Maxwell, vol. ii. p. 45 : — " If a magnet could be con- 

 structed so that the distribution of its magnetization is not 

 altered by any magnetic force brought to act on it, it might 

 be called a rigidly magnetized body/' 



(2) Gordon, ' Electricity and Magnetism/ i. p. 148 : — 

 u Into however many pieces we cut a magnet, each will 

 have two opposite poles, whose strength is equal to that of the 

 poles of the original magnet/' P. 151 — " The moment of 

 a magnet is not altered by cutting it in pieces." P. 155 — " If 

 from any magnetized substance we cut any piece whatever, 

 its magnetic moment is simply proportional to its volume." 



Now these two hypotheses may apply to a theoretical mag- 

 netism which can be imagined; but they are both far from 

 representing the actual behaviour of permanent magnets. 



With respect to (1), I shall develop a hypothesis which 

 leads to an account of the properties of permanent magnets 

 at all events nearer the truth than (1). (2) is very far from 

 being true. 



(1) After this passage Maxwell proceeds: — " The only 

 known body which fulfils this condition " (rigid magnetiza- 

 tion) " is a conducting circuit round which a constant cur- 

 rent is made to flow. Such a circuit exhibits magnetic pro- 

 perties, and may therefore be called an electro-magnet; but 

 these magnetic properties are not affected by the other mag- 

 netic forces in the field."" 



Now from the point of view of the preceding investigation, we 

 should not call the current-circuit a rigid electromagnet. We 

 should speak of it as possessing a definite magnetomotive force, 

 and say that it magnetizes the space or other objects in its neigh- 

 bourhood. And this magnetism is by no means rigid, but 

 depends on the resistance (or on the permeability) of the 

 magnetic circuits through which it flows; i. e. it is modified 

 by the introduction of iron into the field, which rigid mag- 

 netism would not be. 



If, then, the hypothesis of frictionless Amperian currents 

 in magnets be at all correct, even as an analogy, the first 

 supposition as to the nature of permanent magnets will be 

 that they possess a constant magnetomotive force in their 

 substance in virtue of the Amperian currents; or this may be 

 simply assumed without further hypothesis. And this mag- 

 netomotive force maintains a magnetic induction in the 

 magnet which depends on the total resistance of the magnetic 

 circuit. 



Now, if this be true, suppose a long steel ; bar-magnet to be 

 cut up into short pieces. The resistance of the whole con- 

 sists of two parts — that of the steel, and the air-resistances at 



