36 Mr. "W. Sturgeon on the Distribictioji of 



success at which I arrived in the investigation I am now de^' 

 scribing*. 



If two or three discs of the same diameter be placed close 

 together on the same axis, so as to form a compound disc or 

 plate, the forces which operate on the needle are much more 

 powerful than when one disc only is employed. Much, how- 

 ever, depends upon the thickness of the metal, thick discs 

 having a great advantage over those which are very thin ; 

 notwithstanding which, a decided uniformity in the distribution 

 of polarity is displayed even in the thinnest copper or zinc foil. 



I made a compound disc by soldering the edges of two 

 single ones to a rim or hollow cylinder of copper, about half an 

 inch deep, so that when completed it formed a cylindrical box, 

 half an inch high, and about ten inches in diameter, having 

 a perforation through its centre for the introduction of the 

 spindle on which it was intended to rotate. When this cylinder 

 was mounted in the place of tl)e single disc in experiment 20. 

 and 21, the deflection of a four-inch needle (neutralized in the 

 usual way) would amount to about 40^ with a moderate velocity 

 of rotation. When the velocity was considerable, and the mo- 

 tion equable, the needle would be perfectly steady at that, or 

 at a greater angle of deflection. 



Straight needles, particularly when they are very long, are 

 by no means well adapted for obtaining the greatest effect from 

 the forces in the edge of the disc whilst rotating in a vertical 

 plane, because of the great distance at which the poles are 

 necessarily placed from those operating forces. It is much 

 better to employ needles which are bent into circular arcs, 

 having nearly the same curvature as the edge of the disc. 

 Two needles of this form may be advantageously employed at 

 the same lime; the one above, and the other below, and both 

 concentric with the edge, as in fig. 25. The needles are at- 

 tached to a straw, or thin slip of light wood, with their poles 

 placed in opposite directions. When thus arranged, their 

 directive force will, in a great measure, be neutralized, both 

 as regards the magnetism of the earth, and that of the exci- 



* At the time I was making these experiments, I found that the frame 

 of an electrical machine with a multiplying wheel and band was very con- 

 venient for giving the disc a considerable velocity in a vertical plane. A 

 spindle, supported in the pivot-holes of the frame, and furnished with a 

 pulley at one end, carried the revolving disc; and a pile of liooks formed 

 the stage for the support of the horse-shoe magnet. Some time last summer, 

 however, I constructed an aj)i)aratus for the purpose of rotating discs, cylin- 

 ders, &c., on a horizontal axis, which, as it very much resembles a y>/rt/<? 

 Mac/line, it is not necessary to describe in this place, — any further than 

 merely to mention, that it is furnished with neat ttages for the support of 

 the e-Nxiting magnet and the compass-needles. 



