Mr. Christie's Paper. 95 



of these plisenomena holds good in the case of the earth, viz. 

 the non-coincidence of its polarized axis with that of its diur- 

 nal rotation. 



On the Alteration in the Magnetism of an Iron Plate, occasioned 

 by a Rotation on its Axis. By S. H. CHrcisxiE, Esq. 



The effects obsei'ved and described in this paper, although 

 minute in themselves, appeared, in the author's opinion, to 

 point out a species of magnetic action not hitherto described. 

 It has long been weW known that striking, twisting, or filing 

 iron, in different directions, with regard to the magnetic axis, 

 materially influences its polarity, but it does not appear to 

 have been remarked that the simple rotation of iron in diffe- 

 rent directions has any such influence. This, however, the 

 author has ascertained to be the case, and that the laws which 

 govern this peculiar action are so regular, that there can re- 

 main no doubt of a coi'responding regularity in their causes. 



The attention of the author was first drawn to these phaeno- 

 mena by some apparent anomalies in the magnetic action of 

 an iron plate on the compass, observed in the course of a dif- 

 ferent investigation. In order to avoid or allow for the dis- 

 turbing influence of partial magnetism in the iron, it became 

 necessary to attend minutely to the position of certain points 

 in its circumference, which corresponded to the maxima and 

 minima of this magnetism. It was then found that these 

 points were not constant, but shifted their position as the plate 

 was made to revolve in its own plane ; or, in other words, that 

 a plate which, in a given position, produced a certain devia- 

 tion in a compass, no longer produced the same deviation 

 after making an exact revolution in its own plane, although 

 brought to rest, and every part of the apparatus restored pre- 

 cisely to its former place. 



It appeared from this, that the revolution of the plate in its 

 own plane had an influence on its power of deviating the 

 needle independent of the partial magnetism of particular points 

 in it ; and the justice of this idea was proved by giving it a ro- 

 tation in an opposite direction, when the effect on its directive 

 power was also reversed. 



The change produced by rotation in the directive power of 

 the plate was found to be a maximum when its plane was pa- 

 rallel to the line of dip on the magnetic axis, and at the same 

 time as little inclined to the horizon as this condition would 

 allow ; but when the plane of the plate was parallel to the ho- 

 rizon, the effect was diminished in the ratio of 5 to 1, and 

 when perpendicular to the horizon, and coincident with the 

 magnetic meridian, was altogether destroyed. 



The 



