30 Mr, Christie on the Magnetism of Iron [July, 



diap, would be easterly, and with another point coinciding, 

 westerly ; whereas had the iron possessed no partial magnetism, 

 which was the case I wished to investigate, there would have 

 been no deviation when its centre was on the meridian. My 

 first object was to find what points on the plate must coincide 

 with the limb, in order that the plate, when its centre was on the 

 meridian, should cause no deviation in the needle ; and it was 

 in my attempts to effect this, which at first sight appears suffi- 

 ciently easy, that I discovered the leading feature in all the 

 phainbraena which 1 am about to describe. 



" General Description of the PhtBtiomena arising from the Rotar 

 tion of' an Iron Plate, 



" In order to find the points which I have mentioned, I adjusted 

 the instrument so that the plane of the fixed limb was exactly 

 in the magnetic meridian, and then brought the other limb into 

 the same plane : the centre of the plate was then on the magnetic 

 meridian, and its plane perpendicular to that plane, as represented 

 in fig. 1. I now made the plate revolve in its own plane about 

 the axis B h, and noted very carefully its effect on the needle: 

 In doing this I found that if I placed the plate on the arm, so 

 that a certain point, c for instance, coincided with the plane of 

 the limb, the deviation was different when the same point, by 

 the revolution of the plate, coincided with the limb again. As 

 it appeared by this that the revolution of the plate had an effect 

 upon the needle, independent of the partial magnetism of parti- 

 cular points, I considered that if the plate were made to revolve 

 the contrary way, the deviation ought to be on the opposite side, 

 and this I found to be the case. I will illustrate this by the 

 observations made when I first noticed the effect. The plate 

 was divided at every 30° of its circumference (fig. 2) by lines 

 drawn through the centre, and being placed on the arm, so that 

 0° coincided with the upper part of the limb, the north end of 

 the needle pointed 10' east ; but when this point again coincided 

 with the limb, by the upper edge of the plate revolving from 

 west to east, the needle pointed 30' east : making the plate 

 revolve the contrary way, that is, its upper edge from east to 

 west, when 0° coincided with the limb, the north end of the 

 needle pointed 28' west: so that there was a difference of 58', 

 when eveiy point of the plate had the same position with respect 

 to the needle, according as the plate was brought into that po- 

 sition by revolving from west to east, or from east to west. As 

 this appeared e^ftraordinary, I made repeated observations at the 

 time to ascertain that the effect was independent of any acci- 

 dental circumstances, and found that the results always accorded 

 with the first, the difference caused by the rotation of the plate 

 being however greater or less according to the position of the 

 plate. 



