1826.1 Magnetism of Metals, S^c, arising from their Rotation. 247: 



readily admit, at least the probability, of all bodies in nature 

 participating in it more or less. Yet if the electro-dynamical 

 theory of magnetism be well founded, it is difficult to conceive 

 how that internal circulation of electricity, which has been 

 regarded as necessary for the production of magnetism, can be 

 excited or maintained in non-conducting bodies. Without 

 pretending to draw a line, however, in what is perhaps at last 

 only a question of degree, one thing is certain, that all the 

 unequivocal cases of magnetic action observed by us, lie among 

 the best conductors of electricity. Another feature, no less 

 striking, is the extreme feebleness of this species of action 

 compared with that which takes place in cases of sensible 

 attraction and polarity. This will appear more evidently, if we 

 consider the mode of action which probably obtains in these 

 experiments, and the mechanism, if we may so express it, by 

 which the effects of such almost infinitesimal forces are rendered 

 perceptible in them. 



The rationale of these phsenomena, as well as of those 

 observed by Mr. Barlow in the rotation of iron, which form only 

 a particular case (though certainly the most prominent of any) 

 of the class in question, seems to depend on a principle which, 

 whether it has or has not been before entertained or distinctly 

 stated in words, it may be as well, once for all, to assume here 

 as a postulatum, viz. that in the induction of magnetism, time 

 enters as an essential element, and that no finite degree of magnetic 

 polarity can be communicated to, or taken from, any body what- 

 ever susceptible of magnetism, in an instant. 



This principle will, if we mistake not, be found to afford at 

 least a plausible explanation of most, if not all the phsenomena 

 above described, without the necessity of calling in any addi- 

 tional hypothesis, or new doctrine in magnetism. For the other 

 principle we shall have occasion to employ, that magnetic bodies 

 differ exceedingly, both in susceptibility of this quality and in 

 the degree of the pertinacity with which they retain it, (which 

 may be called their retentive power,) is not an hypothesis, but 

 an acknowledged fact. It is only in the mode of its extension 

 to new cases of magnetics that we can be led into any fallacies. 

 Whether these two qualities (susceptibility and retentive power) 

 be, or be not mutually dependent, this is not the place to 

 inquire. Probably they are not so, at least directly : and the 

 new facts almost convert this probability into certainty ; at all 

 events, at present, we shall, for greater generality, suppose thena 

 independent. 



Conceive now a plate of any thickness, and of indefinite 

 superficial extent, of a metal or other magnetic, whose retentive 

 power is very small. If either pole (suppose the north) of a 

 magnet be brought vertically over a point in its surface, it will 

 therq produce ^ pole of the cQiitrary name iu the plate, the 



