25S Exp, of the Voltaic Pile on the Magnetic Needle. 



north face ; so that I have caused demi-revohitions to the hori- - 

 zontal needle, by bringing the south pole of the magnetised bar 

 from the south face of the suspended needle, or the north pole ; 

 from the north side. I could even produce an entire revolution; 

 for the attracted * face follows the movement of the face which 

 attracts it. 



A tendency might also be observed in the needle to direct one 

 of its faces towards the north, and the other towards the mag- 

 netic south ; for when diverted from it, it returned by a series 

 of oscillations, and if it had made an entire revolution, it main- 

 tained itself there; which shows that the torsion of the silk thread 

 was too feeble to remove the needle from its position of equili- 

 brium. 



These experiments would seem to prove that, independently of 

 magnetic poles in a longitudinal direction, the suspended needle 

 had also poles in a diametrical direction. It is easy however to 

 be convinced that no such poles as the latter exist. 



In fact, supposing that the suspended needle were a curved 

 plane, the magnetic action of the earth will always direct the 

 plane of that curve in the line of the meridian, and if turned away 

 from it, it will come back to that line. Further, the concavity 

 will be turned towards the pole of the earth which attracts the 

 inferior pole of the needle. If a magnetised bar is brought near 

 which has more action than the earth on the suspended needle, 

 the concavity or convexity of the arch will be directed according 

 as the bar exercises an attraction or a repulsion upon the inferior 

 pole of the needle. 



This curve in the needle suffices, then, to produce the effects 

 observed. That it is the true cause of this apparent polarization, 

 any one may convince himself. 



It will, in the first place, be readily admitted, that it is at least 

 very difficult to make a steel wire without a slight curve, what- 

 ever care may be taken to rectify it perfectly. It will be seen, 

 besides, by rendering this curve sensible to the eye, and present- 

 ing it successively in different directions, that it is always the 

 curve which determines the disposition of the faces of the needle. 



It may hence be concluded, that the case is the same with a 

 curve less sensible, or even altogether invisible. The same ex- 

 planation v/ill apply to the case where the curve is complicated, 

 and cannot be comprehended in a plane. A needle not mag- 

 netised presents analogous phaenomena. 



I have dwelt on these latter facts, because they include all 

 the circumstances necessary for the detection of error, and may 

 perhaps explain the magnetic phjenomena announced by some 

 philosophers. 



* This expression is not quite exact, but is more descriptive than any other. 



XL. Upon 



