and on the Theory of Magnetism. 95 



wire, and the powers emanating from them, I have merely 

 spoken of them as two, to distinguish the one set of effects 

 from the other. The high authority of Dr. Wollaston is at- 

 tached to the opinion that a single electro-magnetic current 

 passing round the axis of the wire in a direction determined by 

 the position of the voltaic poles, is sufficient to explain all the 

 phenomena. 



M. Ampere, who has been engaged so actively in this branch of 

 natural philosophy, drew from his theory, the conclusion that a 

 circular wire forming part of the connexion between the poles of 

 the battery, should be directed by the earth's magnetism, and 

 stand in a plane perpendicular to the magnetic meridian and the 

 dipping needle. This result was said to be actually obtained, but 

 its accuracy has been questioned, both on theoretical and experi- 

 mental grounds. As the magnet directs the wire when in form 

 of a curve, and the curve a needle, I endeavoured to repeat the 

 experiment, and succeeded in the following manner : — A voltaic 

 combination of two plates was formed, which were connected by 

 a copper wire, bent into a circular form ; the plates were put into 

 a small glass jar with dilute acid, and the jar floated on the sur- 

 face of water: being then left to itself in a quiet atmosphere, 

 the instrument so arranged itself that the curve was in a plane 

 perpendicular to the magnetic meridian ; when moved from this 

 position, either one way or the other, it returned again ; and on 

 examining the side of the curve towards the north, it was found 

 to bi that, which, according to the law already stated, would 

 be attracted by a south pole. A voltaic circle made in a silver 

 capsule, and mounted with a curve, also produced the same 

 effect; as did likewise, very readily, M. de la Rive's small ring 

 apparatus*. ^Vhen placed on acidulated water, the gas liberated 

 from the plates prevented its taking up a steady position ; but 

 when put into a little floating cell, made out of the neck of a 

 Florence flask, the whole readily took the position mentioned 

 above, and even vibrated slowly about it. 



As tlie straight connecting wire is directed by a magnet, there 



* Sl'i; Miscellanea. 



