1826.] Mr, Christie on the Magnetism arising from Rotation, 253 



however, a metallic, i. e. a magnetic contact is established 

 between them, their mutual induction acts, and the result is a. 

 general developement of one polarity in the region adjacent to* 

 the magnet; and of the other, feebler and more diffused, in the 

 parts of the mass remote from it. This is probably the rationale 

 of the restoration of virtue which takes place when a cut disc is 

 soldered up. And it is not difficult to conceive that a weak 

 magnetism may be thus very faithfully transmitted through 

 substances, such as bismuth and lead, whose direct action is 

 very small, because, as we have seen, the intensity of their 

 direct action depends, for one of its causes, on the retentive 

 power of the substance, which is out of question in the indi- 

 rect mode of action here considered. In fact, if the retentive 

 power of the solder were reduced to nothing, i. e. if it gained 

 and lost magnetism instantaneously, it would still act as a 

 conductor, and probably the better for this quality ; so that 

 the communication between opposite sides of a slit, or contigu- 

 ous portions of two adjacent particles of a powder, would still 

 be kept up by it, provided it were susceptible of magnetism at 

 all. The observed and very striking fact then of the powerful 

 action of bismuth as a conductor, while its action as a magnet i» 

 so extremely feeble, is in itself a strong argument for the inde- 

 pendence of these two qualities, which we have designated by the " 

 expressions — susceptibility f^n6. retentive power y and may possibly 

 be made the foundation of a mode of distinguishing and measur- 

 ing their degrees in different substances. 



On the Magnetism developed in Copper and other Substances 

 during Rotation. In a Letter from Samuel Hunter Christie, 

 Esq. MA. &c. to J. F. W. Herschel, Esq. Sec. RS. 



^ After having made experiments with a thin copper disk 

 suspended over a horse-shoe magnet, similar to those which I 

 witnessed at Mr. Babbage's, I made the following. 



A disc of drawing paper was suspended by the finest brass 

 wire (No. 37) over the horse-shoe magnet, with a paper screen 

 between. A rapid rotation of the magnet (20 to 30 times per 

 second) caused no rotation in the paper, but it occasionally 

 dipped on the sides, as if attracted by the screen, which might 

 be the effect of electricity excited in the screen by the friction 

 of the air beneath it. 



A disc of glass was similarly suspended over the magnet : no 

 effect produced by the rotation. 



A disc of mica was similarly suspended : no effect. 



The horse-shoe magnet was replaced by two bar-magnets, 

 each 7-5 inches long, and weighing 3 oz, 16 dwt. each, placed 



