258 Mr, 'Christie on the Magnetism developed in [Oct. 



is sufficiently near to indicate that the copper acts as 1 have 

 supposed. A thick copper ring would be best adapted for 

 obtaining results for comparison ; and when I have leisure I 

 propose making ilse' of one. 



Lor the purpose of determining the law according to which 

 magnets act upon a copper disc at different distances, I sus- 

 pended, successively, two copper discs over the bar magnets 

 placed horizontally by the side of each other, with their poles of 

 the same name adjacent. The magnets were made to revolve 

 imtil the torsion of the wire caused the disc to return in the 

 contrary direction, when I considered that the force of torsion 

 wonld be double the force with which the magnets urged the 

 disc. The time in which this took place was noted, and also 

 the degree of torsion. After this the magnets were made to 

 revolve again with the same velocity, and the torsion noted 

 where the disc remained stationary by the action of the opposite 

 forces of torsion and of the magnets. This was done at several 

 distances ; and those distances, between the magnets and the 

 disc ascertained very accurately. In the observations with the 

 disc which I have named A, the magnets w^ere made to revolve 

 with two different velocities; one of nearly 12 revolutions per 

 second, the other of nearly 24 revolutions per second; but with 

 the disc C the magnets always revolved with the velocity 24 

 revolutions per second, as I found that 1 could keep more 

 steadily to this velocity than to the other. The length of the 

 suspending wire (No. 22) was the same in both cases 34*25 

 inches. The thickness of the magnets is one-fifth of an inch, 

 so that I have added -^ to the measured distances between the 

 upper surface of the magnets and the copper, to reduce them to 

 the distances between the plane of the copper and a horizontal 

 plane passing through the axes of the magnets. 



It is evident from these results, that the force with which the 

 magnets urge the disc, as the distance increases, decreases much 

 less rapidly than in the case of the copper-plate revolving. If 



we suppose it to vary as ^-- , then calling c and c' two distances 



and T and T^ the corresponding torsions, which are equal to the 



forces of the maarnets, n = -^ — ~ "^' — . 

 ° ' log. c' — log. c 



Comparing the preceding results, the several values of n 

 will be. 



