254 Mr, Christie on the Magnetism developed in [Oct. 



horizontally parallel to each other, and having their poles of the 

 same name contiguous. These produced quick rotation in a 

 heavy disc of copper six inches in diameter, and suspended by 

 a wire, No. 20. 



A bar-magnet, four inches long, and having both its ends 

 south poles, was made to revolve rapidly under a copper disc. 

 The disc revolved in the same direction as the magnets. 



The two bar-magnets before-mentioned were acfjusted to the 

 axis of rotation, so that their upper ends were at the distance of 

 five inches from each other, and their lower ends 1*8 inch 

 apart. They were first made to revolve rapidly under the copper 

 disc with poles of the same name nearest to the disc, and then 

 with poles of a contrary name : the times in which the several 

 rotations of the disc took place were as nearly as possible the 

 same in the two cases. 



In the first three, I could only remark the time to the nearest 

 second, having no assistance. Should the times agree precisely, 

 which I have very little doubt they would be found to do, the 

 result would, I think, be singular. It would show that the 

 magnetism in the disc is instantaneously developed by one pole 

 of the magnet, and as instantaneously destroyed, and a contrary 

 magnetism developed by the contrary pole ; or rather it would 

 indicate, that the time during which the disk retained the 

 induced magnetism was less than the time of half a revolution 

 "of the magnet. 



The same two bar-magnets were laid horizontally by the side 

 of each other, four-tenths of an inch a part. They were first 

 made to revolve rapidly under the disc with their poles of the 

 same name adjacent, and then with those of a contrary name 

 adjacent. 



From these it appears that the effect was but little diminished 

 by placing poles of a contrary name so close to each other. 



The adjacent poles being of the same name, they were con- 

 nected by a piece of soft iron, one-eighth of an inch thick, and 

 half an inch wide. After 4^ revolutions of the disk (screw), the 

 torsion of the wire was equal to the force of the magnets, and 

 the same was the case at 4-|- revolutions (unscrew). So that 

 although the effects were greatly diminished by connecting the 

 poles, they were by no means destroyed. 



The magnets were now placed over each other, first with 

 poles of a contrary name, and then with those of the same name 

 Contiguous ; so that although the upper magnet was nearer to 

 the disc by its own thickness than in the fourth experiment, the 

 feffect when poles of Contrary name were contiguous was not half 

 what it was when they were connected by the iron. 



A thick copper plate, eight inches in diameter, and one inch 

 thick, was placed on the axis of rapid rotation, its plane hori- 

 zontal. A thin copper disc, four inches diameter, and weighing 



