RELATION OF THE ELECTRIC AND MAGNETIC FORCES. 275 



direction, and tend by their joint force to deflect the needle of the instrument. The 

 whole arrangement was so effective and delicate, that moving the magnetic pole about 

 the eighth of an inch to and fro two or three times, in periods equal to those required 

 for the vibrations of the galvanometer needle, was sufficient to cause considerable 

 vibration in the latter ; thus showing readily the consequence of strengthening the 

 influence of the magnet on the one core and helix, and diminishing it on the other. 



1712. Then without disturbing the distances of the magnet and cores, plates of 

 substances were interposed. Thus calling the two cores A and B, a plate of shell-lac 

 was introduced between the magnetic pole and A for the time occupied by the needle 

 in swinging one way ; then it was withdrawn for the time occupied in the return 

 swing ; introduced again for another equal portion of time ; withdrawn for another 

 portion, and so on eight or nine times ; but not the least effisct was observed on the 

 needle. In other cases the plate was alternated, i, e. it was introduced between the 

 magnet and A for one period of time, withdrawn and introduced between the magnet 

 and B for the second period, withdrawn and restored to its first place for the third 

 period, and so on, but with no effect on the needle. 



1713. In these experiments shell-lac in plates 0'9 of an inch in thickness, sulphur 

 in a plate 0*9 of an inch in thickness, and copper in a plate 0*7 of an inch in thick- 

 ness were used without any effect. And I conclude that bodies, contrasted by the 

 extremes of conducting and insulating power, and opposed to each other as strongly 

 as metals, air, and sulphur, show no difference with respect to magnetic forces when 

 placed in their lines of action, at least under the circumstances described. 



1714. With a plate of iron, or even a small piece of that metal, as the head of a 

 nail, a very different effect was produced, for then the galvanometer immediately 

 showed its sensibility, and the perfection of the general arrangement. 



1715. I arranged matters so that a plate of copper 0*2 of an inch in thickness, and 

 ten inches in diameter, should have the part near the edge interposed between the 

 magnet and the core, in which situation it was first rotated rapidly, and then held 

 quiescent alternately, for periods according with that required for the swinging of 

 the needle ; but not the least effect upon the galvanometer was produced. 



1716. A plate of shell-lac 0*6 of an inch in thickness was applied in the same man- 

 ner, but whether rotating or not it produced no effect. 



1717- Occasionally the plane of rotation was directly across the magnetic curve : 

 at other times it was made as oblique as possible ; the direction of the rotation being 

 also changed in different experiments, but not the least effect was produced. 



171 8. I now removed the helices with their soft iron cores, and replaced them by 

 two flat helices wound upon card board, each containing forty-two feet of silked cop- 

 per wire, and having no associated iron. Otherwise the arrangement was as before, 

 and exceedingly sensible ; for a very slight motion of the magnet between the helices 

 produced an abundant vibration of the galvanometer needle. 



1719. The introduction of plates of shell-lac, sulphur, or copper into the intervals 



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