14 Mr Christie on the Magnetism of Iron in Rotation. 



experiments, and I stated to him that, independently of the 

 deviation of the needle caused by the mass of iron, the devia- 

 tions due to the rotation of the plate were very nearly the 

 same in amount, as would arise from a polarizing of the iron 

 in a direction perpendicular to the line of the dip. 



Although, before the end of the year 1822, I had written 

 all but the theoretical part, at the conclusion of my paper, on 

 this subject, which was read before the Royal Society last 

 May, and printed in the Transactions, being engaged about 

 that time with other experiments, and otherwise much occu- 

 pied, I was obliged to defer finishing the paper for a consider- 

 able time. In a note to a paper on the effects of temperature, 

 on the intensity of magnetic forces, &c, read before the Royal 

 Society in June 1824, and printed, I, however, stated, that 

 I had discovered that a peculiar polarity was imparted to iron 

 by simple rotation, and mentioned some of the effects produced 

 by the rotation of an iron plate. It is, therefore, evident, that 

 the publication of the discovery which I had made of the 

 magnetical effects produced on iron by its rotation, took place 

 at least nine months before we had any account of M. Arago's 

 experiments, and six months before Mr Barlow undertook his, 

 on the effects produced by the rapid rotation of iron. No 

 one, you may be assured, is more disposed than myself to give 

 to Mr Barlow all the credit which is due for his observations ; 

 but as he had, for nearly three years previous to making his 

 experiments, been in possession of the facts which I had al- 

 ready observed, and found that the effects he observed were 

 explicable on the same principle of polarization, which I had 

 then pointed out, I think that his experiments must be allow- 

 ed to be simply a variation of my original ones, whatever im- 

 portance may be attached to such a variation of the experi- 

 ment, and that I may justly lay claim to the discovery, that 

 rotation has a considerable influence on magnetism, at least 



en, where the dip is more than 88°, he found that, in one position of the 

 plate, its rotation in opposite directions caused a difference of no less than 

 108° in the directions of the needle in the two cases, the same point, after 

 rotation, heing brought to coincide with a fixed mark in both. This was 

 an extreme case, but several of the deviations due to the rotation of the 

 correcting plate amounted to 30° or 40°. 



