1826.] On the Magnetism arising from Rotation. 183 



Article II. 



Abstracts of Papers in the Philosophical Transactions for 1825, 

 on the peculiar Magnetic Effect induced in Iron, and on the 

 Magnetism manifested in other Metals, S^c. during the Act of 

 Rotation. By Messrs. Barlow, Christie, Babbage, and 

 Herschel. 



{Continued from p. 116.) 



Account of the Repetition of M. Arago's Experiments on the 

 Magnetism manifested by various Substances during the Act of 

 Rotation. By C. Babbage, Esq. FRS. and J. F. W. Herschel, 

 Esq. Sec. RS. 



The curious experiments of M. Arago described by M. Gay- 

 Lussac during his visit to London in the spring of the year 

 1825, in which plates of copper and other substances set in rapid 

 rotation beneath a magnetized needle, caused it to deviate from 

 its direction, and finally dragged it round vnth them, naturally 

 excited much attention, and the investigation of their various 

 circumstances, and of their connexion with the effects observed 

 by Mr. Barlow in December, to be produced by the rotation of 

 masses of iron, and described by him in a paper read to the 

 Royal Society,* became an object of considerable interest. 

 Accordingly, having erected at Mr. Babbage's house, in Devon- 

 shire-street, an apparatus for setting a copper-plate in rotation 

 about a vertical axis by the aid of a turning lathe, the authors of 

 this paper proceeded to try its effect on a magnetized needle 

 suspended over it. The first attempt failed from the use of too 

 small a needle ; but this being replaced by a magnetic bar of 

 considerable weight delicately suspended by a silk thread, they 

 had the satisfaction of seeing it deviate several degrees from its 

 point of rest in a direction corresponding with that of the rota- 

 tion of the copper-plate ; and on employing instead of this bar 

 a very delicate azimuth compass, belonging to and the invention 

 of Capt. Kater, the influence of zinc, brass, and lead, was simi- 

 larly rendered sensible. 



" In this first trial," they observe, *' having neither the com- 

 mand of a very rapid rotation, nor of massive metallic discs, the 

 deviation of the compass observed did not exceed 10 or 11 

 degrees. In order therefore to enlarge the visible effect, and at 

 the same time disencumber ourselves of the limit set to it by the 

 polarity of the needle, it occurred to us to reverse the experi- 

 ment, and ascertain whether discs of copper or other non- 

 magnetic substances (in the usual acceptation of the word) 



* See the last volume of the Annals, p. 444. 



