180 Prof. Thomsou on the Theory of Magnetic Induction 



Laws of Magnetic Induction according to Poisson's Theory. 



I. TMien a given body, susceptible of inductive magnetization 

 (whether it be ferromagnetic or diamagnetic), is placed in the 

 neighbourhood of a magnet, it becomes magnetized in a manner 

 dependent solely on the field of force which it is made to occupy. 



II. Superposition of Magnetic Inductions. — Different magnets 

 placed simultaneously in the neighbourhood of an inductively 

 magnetizable (ferromagnetic or diamagnetic) body induce in it a 

 distribution of magnetism which is the resultant of the different 

 distributions that woidd be induced by the separate influences 

 of the different magnets, each in its own position, with the others 

 removed. 



The fu'st of these two propositions merely imphes that any 

 magnet, whether an electro-magnet, or a magnet consisting of 

 magnetized substance, which produces at each point of a certain 

 space the same " force " as another magnet of any kind, would 

 produce the same inductive effect on a magnetizable substance 

 occupying that space. Eveiything that is known of inductive 

 action is consistent with it ; and it is, I belie\e, universally ad- 

 mitted as an axiomatic principle. 



The second proposition, which asserts the mutual indepen- 

 dence of superimposed magnetic inductions, is equivalent to an 

 assertion that, if the force at every point of a magnetic field be 

 altered in a certain ratio, the magnetization of a substance placed 

 in it will be altered proportionately. This is undoubtedly not a 

 principle of universal application. It is not applicable to steel, 

 nor to the substances of which natural magnets are composed ; 

 nor, in general, to substances possessing in any degree that pro- 

 perty of resisting magnetization or demagnetization, called by 

 Poisson " coercive force," in vii-tue of which they can penna- 

 nently retain magnetism. Neither is it, as Joule^s experiments, 

 and the more recent experiments of Gartenhauser and Midler 

 demonstrate, applicable to soft iron, except as an approximate 

 law of the magnetization when the magnetizing force does not 

 exceed certain limits of intensity. But, that it is vei-y approxi- 

 mately, if not rigorously, fulfilled in the magnetization of aU 

 homogeneous substances of veiy feeble inductive capacity, and 

 destitute of " coercive force" (as all known diamagnetics and all 

 ferromagnetics which contain no iron or nickel, or only veiy small 

 proportions in chemical combination, appear to be), is, I think, 

 extremely probable. The foundation of a complete theoiy of 

 magnetic induction requires an expei'imental investigation of the 

 laws according to which the " coercive force " acts in various 

 substances, and of the variation of inductive capacity produced 

 in soft iron, and it may be in other substances, by actual mag- 



