Faraday's Discoveries 



And now he turned the light of these discover- 

 ies upon the darkest physical phenomenon of 

 that day. Arago had discovered in 1824, that 

 a disk of non-magnetic metal had the power of 

 bringing a vibrating magnetic needle suspended 

 over it rapidly to rest; and that on causing the 

 disk to rotate the magnetic needle rotated along 

 with it. When both were quiescent, there was 

 not the slightest measurable attraction or re- 

 pulsion exerted between the needle and the disk ; 

 still when in motion the disk was competent 

 to drag after it, not only a light needle, but a 

 heavy magnet. The question had been probed 

 and investigated with admirable skill by both 

 Arago and Ampere, and Poisson had published a 

 theoretic memoir on the subject; but no cause 

 could be assigned for so extraordinary -an action. 

 It had also been examined in this country by 

 two celebrated men, Mr. Babbage and Sir John 

 Herschel; but it still remained a mystery. Fara- 

 day always recommended the suspension of 

 judgment in cases of doubt. "I have always 

 admired," he says, "the prudence and philo- 

 sophical reserve shown by M. Arago in resisting 

 the temptations to give a theory of the effect he 

 had discovered, so long as he could not devise one 

 which was perfect in its application, and in re- 

 fusing to assent to the imperfect theories of 

 others." Now, however, the time for theory 

 had come. Faraday saw mentally the rotating 

 disk, under the operation of the magnet, flooded 

 with his induced currents, and from the known 

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