of Magnetic Polarity in Metallic Bodies. 271 



polarity in thin sheet-iron, and in other metallic bodies, when 

 in the shape of discs. 



The distribution and retention of magnetic polarity in thin 

 discs of soft iron by rotation, when placed in various positions, 

 with reference to the magnetic meridian, &c., were first no- 

 ticed and investigated by Mr. Christie, in an extensive series 

 of highly interesting experiments which were published in the 

 Phil. Trans, for 1825*.' Mr. Christie very politely showed 

 me several of his experiments at the time he was carrying 

 them on ; and when published, presented me with a copy of 

 the paper in which they were described. In a very short time 

 afterwards I repeated some of those experiments with a tem- 

 porary apparatus which I fitted up for the purpose, and ob- 

 tainecl the most satisfactory and corroborative results. Besides 

 repeating the original experiments, however, I was led by cu- 

 riosity to institute others, which, as some of the results are 

 rather curious, I now propose to detail. 



Experiment: 1. — Let abed (Plate III. fig. 1.) represent a 

 disc of sheet-iron, about fifteen inches in diameter, placed 

 south of, and in the same horizontal plane as, the needle ns, 

 with its centre in the magnetic meridian ; and a neutral point 

 a, in its northern edge, about one inch distant from the 

 pole n. 



If the north f pole of a powerful bar-magnet, having its 

 axis vertical, be now passed over a diameter of the disc from c 

 to a, as indicated by the arrow, and about half an inch above 

 it|, and taken away suddenly from the point a, the plate will 

 exhibit south polarity at that point; and the pole n of the 

 needle will be attracted with a considerable force. If now a 

 small delicate dipping-needle be carried gently over tiie sur- 

 face of the place, feeble poles of both characters will sometimes 

 be found in various parts of the iron, varying in power and 

 situation almost every time the magnet repeats the excitation ; 

 but in no case will a north pole be found of equal vigour with 

 the south pole determined at the point a. In several instances 

 I have found no regular pole only at a; all the rest of the 

 plate exhibiting a diffused polarity without any apparent cen- 

 tral pole. It sometimes happens, however, that all the south 

 polarity of tiie plate is separated from its north polarity by 

 an irregular curve line, as represented by the dotted line e q 

 (fig. 1.), which line may be regarded as a magnetic equator; 



* Mr. Christie's paper was read before tiie Royal Society, May 12, 1825. 

 [An alistiac-t of it will be Ibtiiul in Fhil. IVlai,'. vol. ixvi. [). !)o. — Kurr.] 



t That pole of the needle which is solicited by the southern parts of 

 tlie earth is liere called noitli. 



X A round [liece of board of the same diameter as the iron |)late, and 

 of a proper thickness, I have found very convenient to prevent the magnet 

 from toufliing the [liate. 



every 



