1821.] Historical Sketch of Electro-magnetism. 283 



deduction of fact from fact may be beautifully traced in it. 

 The following is little more than an enumeration of the facts 

 contained in it. 



Viz. that the wire in connexion affected the needle as M. 

 Oersted described ; the effect was immediately attributed to the 

 wire itself becoming a magnet; and this was instantly proved by 

 bringing it near iron filings, which were attracted, and remained 

 attached to it as long as the communication continued. This is 

 in fact the same experiment as that made by M. Arago (see page 

 276) but it was made by the two philosophers independent of each 

 other; and as no detail of M. Arago's experiment has as yet 

 been published, the accurate description of Sir H. Davy's will 

 be found highly interesting. This effect took place in any part 

 of the wire, and any where in the battery. Steel needles placed 

 on the connecting wire became magnetic; those parallel to the 

 wire acted like the wire itself, those across it had each two poles ; 

 such as were placed under the wire, the positive end of the bat- 

 tery, being east, had north poles on the south of the wire, and 

 south poles to the north ; those needles above were in the oppo- 

 site direction; and this was constantly the case, whatever the 

 inclination of the needle to the wire. This position, it will be 

 observed, is precisely that which has been referred to in the 

 account of M. Oersted's experiments. On breaking the connexion, 

 the steel needles across retained their magnetism, while those 

 parallel to the wire lost it at the moment. 



Wires of platina, silver, &c. in the same situation, were not 

 rendered magnetic, except when, by accident, they formed part 

 of the circuit. Whatever the position of the battery, or wire, the 

 effect was the same. Contact was found not necessary: instan- 

 taneous effect was produced by mere juxtaposition, though thick 

 glass intervened ; filings arranged themselves in right lines across 

 the wire on a glass plate held over it at a quarter of an inch 

 distance. The effect was proportional to the quantity of elec- 

 tricity passing through a given space, without any relation to 

 tire metal transmitting it. Increasing the size of the plates pro- 

 portionally increased the magnetic effects of the connecting 

 wires. The wire connecting a battery of 60 pairs of plates did 

 not take up half so much filings as when the battery was arranged 

 so as to form 30 pairs of plates twice the size. The magnetic 

 powers of the wire rose with its heat. 



Considering that a great quantity of electricity was necessary 

 to produce sensible magnetism, Sir H. Davy concluded that a 

 current from the common machine would have no effect, while a 

 discharge would; and this was found to be true ; the poles of 

 the needle magnetized being situated exactly as before. In these 

 experiments a battery of 17 square feet, highly charged, being dis- 

 charged through a silver wire, l-20th of an inch in diameter, 

 rendered bars of steel two inches long, and from l-10th to l-20th 

 thick, so magnetic, as to lift up pieces of steel wire and needles, 



