10 Experimental Researches in Electricity. 



idea of the electro-tonic condition as he calls it, we conceive 

 it unnecessary to take up our time and space in dwelling upon 

 it, and therefore proceed at once to the fourth section, which 

 is occupied by explanations of certain extraordinary magnetic 

 phenomena, discovered by M. Arago, and which had long 

 defied the efforts of the most able philosophers to account 

 for them. 



In the year 1824, M. Arago shewed, that if a plate of 

 copper, or indeed of any other substance, be placed immedi- 

 ately under a magnetic needle, it diminishes sensibly the 

 extent of its oscillations^ without however affecting their 

 duration; and the needle is brought to rest in a shorter time 

 than if no such substance were placed near it. The converse 

 of this experiment however exhibited much more remark- 

 able results ; for if a plate of copper be revolved close to a 

 magnetic needle, so suspended that the latter may rotate in 

 a plane parallel to that of the former, the magnet tends to 

 follow the motion of the plate; or if the magnet be revolved, 

 the plate tends to follow its motion even although both 

 may be of several pounds weight. M. Arago asserts that 

 the phenomena alluded to may be produced with all sub- 

 stances, although this assertion has not been found capable 

 of verification by other experimenters. The subject was 

 investigated in England by Mr. Babbage and Sir John 

 Herschell ; but their efforts proved completely unsuccessful, 

 and to Faraday belongs the undivided merit of having re- 

 moved all that obscurity which, up to the date of his re- 

 searches, had enveloped it. 



Employing the large compound magnet previously alluded 

 to, he found that by rotating between its poles a copper 

 disc, a permanent deflection of the galvanometer needle, to 

 the extent of 45° could be maintained, thus demonstrating 

 the production of a permanent, whereas before he could 

 only obtain a momentary, current of electricity by common 

 magnetism. Motion of the plate was found to be essential 



