of Magnetic Polariiy in Metallic Bodies. 277 



Messrs. Babbage and Herschel, by Mr. Christie, Mr. Barlow, 

 Mr. Marsh, and by myself; and variously modified according 

 to the views of the respective experimenters. The experiments 

 and observations of Messrs. Babbage and Herschel, and those 

 of Mr. Christie, were published in the Philosophical Transac- 

 tions for 1825, and those of INIr. Barlow and Mr. Marsh in 

 the Edinburgh Journal of Science *. 



I repeated M. Arago's experiments about the same time that 

 they were repeated by those gentlemen, and the observations 

 which I had made were intended for early publication ; but 

 whilst drawing up an account of the experiments, and diversi- 

 fying them in variousways, I found reason not to publish 

 them quite so hastily. Some of my experiments, however, 

 were well known to several scientific gentlemen in London and 

 its vicinity in 1825, and, without my knowledge, were published 

 in the Edinburgh Journal of Science. Since that time I have 

 frequently exhibited such ot' them as are calculated for the 

 lecture table ; and on that account they may not, perhaps, be 

 quite so interesting to some readers as they otherwise would 

 have been. 



The novel phasnomenon of a magnetic needle rotating on its 

 pivot by simplj' placing it above a revolving plate of copper, 

 had something in it so exceedingly fascinating, and presented 

 so striking a similitude to the electro-magnetic rotations with 

 which the minds of philosophers were at that time so familia- 

 rized, — that for a while it seemed doubtful whether or not the 

 revolving plate possessed electric properties. If not, another 

 question presented itself: Does rotation produce magnetism? 

 Another mode of solving the problem was, — that the copper 

 plate, like all ferruginous bodies, actuated the needle by what 

 is frecjuently called inchiced magnetism by the influence of the 

 earth ; and that the needle was put into motion by a rapid 

 succession of transient magnetic poles induced in the plate. 

 When, however, it was found that the revolutions of the 

 needle became more decisive, and were performed with greater 

 promptitude in proportion to its polar energies, and that rapid 

 revolutions were accomplished in light copper discs delicately 

 suspended over a revolving horse-shoe or other powerfid mag- 

 net, — it readily occurred that all the phaenoniena emanated 

 from the action of the magnet employed exciting polarity in the 

 metals under examination ; and that those metals possessing 

 some degree of retention of magnetism, the poles excited in 

 them, although transient, would necessarily lag behind when- 

 ever the celerity of motion exceeded the decay of polarity. The 

 poles thus excited being of an opposite character to those of the 



* Abstracts of all tlie papers in which these experiments arc ilescribecl, 

 will he found in Phil. Mag. vol. ixvi. p. [)T—VJ.—V,mi\ 



exciting 



