1822.] Historical Sketch of Electro-magnetism. Ill 



one above being in one direction, the one below in the opposite 

 direction. This opposition in the direction of the magnetic 

 current in each of these axes is necessary, because of the abso- 

 lutely inverted manner in which the phenomena are presented 

 when the needle is above and below the wire. 



It is difficult to understand how the above theory is to explain 

 the phenomena described by Oersted, but it would not be fair 

 here to give an opinion on its merits, as the account is not taken 

 from the original paper, but from an abstract drawn up in another 

 language. 



The Marquis Ridolfi appears to have formed the idea that 

 electricity may be a compound of magnetism and heat, and many 

 experiments are described in the Bibhotheque Universelle, Feb. 

 1821, p. 114, &c. made with a view of separating electricity into 

 these elements, or of composing it from them. No experimental 

 proofs of the correctness of the opinion were obtained. 



Of all the theoretical views that have been given of electro- 

 magnetical phenomena, those by M. Ampere are the most exten- 

 sive and precise, and have been tested by the application of facts 

 and calculation very far beyond any of the rest. Indeed it is 

 these alone among all those that have been given to the pubHc, 

 which deserve, if any do, the title of A Theory. If I had pro- 

 fessed to send you any thing more than a sketch of electro-mag- 

 netism, I should have been afraid to touch this theory, but as it 

 is, I trust that M. Ampere will excuse the imperfections he may 

 see in the following account, if for nothing else, yet for the 

 humble professions of this letter. 



M. Ampere commences by assuming the existence of two 

 electric fluids, according to the theory which is now general, 1 

 believe, in France. There appears to be no doubt about his 

 meaning on this point, for though he uses the term electricity 

 very frequently, and in a way which might be understood, per- 

 haps, as applying equally either to a particular state of a body, 

 or to a particular fluid existing among its particles, yet by the 

 use of the term electric fluids in one place, and by the mention of 

 electric currents as currents of matter^ it is nearly certain that 

 M. Ampere means to speak of electricity as consisting of two 

 distinct fluids, which, though the one is called positive, and the 

 other negative electricity, are to be considered as equally posi- 

 tive in their existence, and possessed of equal powers. 



The voltaic battery is considered as an instrument possessing 

 the power of conveying one of these electricities to the one end, 

 the other to the other end. That which goes to the zinc end df 

 the battery is called positive electricity ; that which goes to the 

 copper end negative: these names being retained, it may be 

 presumed merely in deference to custom, and not because they 

 have any reference to particular qualities of either the one or the 

 other fluid. 



When a metaUic wire is made to touch the two poles of the 



