Mr. W. Sturgeon on the Theory of Magnetic Electricity. 447 



advancing with its external surface against the exciting polar 

 magnetic lines. Hence the direction of the current in the ring 

 will appear to be reversed ; though, with regard to the exciting 

 lines which called it forth and gave it motion, the direction re- 

 mains constantly the same. 



The compound vortices in fig. 16, 17, 21, and 22. (Plate I. 

 vol. i.) are easily explained in the same manner, by considering 

 each vortex as a simple ring. In fig. 16 and 21. the interior 

 surface of the supposed ring strikes the magnetic lines in the 

 vortex on the right-hand side of each figure. But the exterior 

 surface of the ring receives the exciting impressions in the 

 vortex represented on the left side of each figure. The com- 

 pound vortices represented in fig. 1 7 and 22. are explained in 

 the same way, by considering them to be receiving the exci- 

 ting impressions in the contrary order. 



By taking advantage of this beautiful law, I have been en- 

 abled to exalt the force on the edge of a revolving disc to a 

 considerable extent, as will be shown by the following experi- 

 ment. 



Experiment 22. — Let fig. 10. (Plate I.) represent a disc of 

 copper, revolving in a vertical plane between the poles of two 

 horse-shoe magnets, situated as in the figure, having the north 

 pole of one magnet and the south pole of the other on the same 

 side of the disc. 



With this arrangement the electric forces will be distri- 

 buted as indicated by the small arrows in the interior of the 

 circular plate; when it is rotated in the direction of the large 

 exterior arrow. By this distribution the resulting forces in 

 the upper and lower edges of the plate have the same general 

 direction. In the lower edge the aggregate force or current 

 is in the same direction as that in which the plate revolves ; 

 but in the upper edge the aggregate current is in the opposite 

 direction to that of the revolving plate. By reversing the ro- 

 tatory motion, the whole systems of currents become reversed 

 also. 



There is a very great advantage by this disposition of the 

 magnets and the copper disc ; for not only is the force in the 

 upper and lower edges very much exalted, but by the arrange- 

 ment of the magnetic poles they very nearly neutralize each 

 other's effects on the needle. To accomplish this point the 

 most decidedly, which is an important consideration in the ex- 

 periment, the exciting magnets ought, as nearly as they can 

 possibly be procured, to be of the same power. 



If, instead of a single disc, the compound disc, described in 

 Experiment 21. in my last communication, be employed, the 

 excited forces are still more powerful. A large straight needle 



