2/5§ 



Mr, Christie on the Magnetism developed in [Oct. 



.fFrom tlxe latter observations, it is evident that the eflfec^t 

 produced de onds upon the intensity of the magnetism in the 

 needle employed ; and this, I think, proves clearly that the 

 effect arises from the magnetism induced in the copper from 

 the needle itself. ^^^. 



If we suppose the tang, of the deviation to vary as -p — , then 

 6 and 6^ being two deviations at the distances d and d% we shall 



have n = l °g- tan, e; -Jog.jaruj^ 

 log. d — log. d. 



Computing n from this, by a comparison of every two obser- 

 vations we have the following values of n : 



5-04 • 



4-60 



4-62 



4-29 



4-20 



4-45 



4-10 



4-65 



4-07 



3-59 



Mean 4-361. 





a> 



B -^^ 

 .2 S 



Is 



4-37 

 3-93 

 3-88 

 3-51 

 3-60 

 3-64 

 3-31 

 3-80 

 3-23 

 2-78 



Mean?605J 



a> 



(D Oh 



S> <W 0) 



la 



0) p 



If we suppose that the poles of the needle are urged by forces 

 in the direction of the motion of the copper, which being con- 

 stant in the copper, would affect the needle reciprocally as the 

 square of the distance ; then these forces in the copper being 

 derived from the needle itself, we must suppose that their inten- 

 sity will vary also reciprocally as the square of the distance ; so 

 that the force on the needle arising from this mutual action, 

 would vary reciprocally as the fourth power of the distance. 

 Taking the mean between the mean values of w above, when the 

 distance is measured from the centre of the copper and from its 

 surface, would give the value of n for an intermediate point 

 3-983, which is as near to 4, supposing that such ought to be the 

 value, as we could expect the observations to give. 



The next experiments which I made were with the view of 

 determining the law of force as regards the distance, when 

 magnets act upon a copper disc. For this purpose I made use 

 of the suspending wire as a balance of torsion. The results 

 which I have obtained in this manner give a much less rapid 

 diminution of the force, as the distance increases, than appears 

 to take place when a thick copper-plate acts upon a small 

 magnet, as in the former experiments, which agrees with what 

 you have mentioned as following from your results. The results 

 obtaiixed in the former case appear to indicate, that every 



