A HISTORY OF SCIENCE 



nomenon of Arago's rotating disk, the disk inducing the 

 current from the magnet, and, in reacting, deflecting 

 the needle. To prove this, he constructed a disk that 

 revolved between the poles of an electro-magnet, con- 

 necting the axis and the edge of the disk with a gal- 

 vanometer. " . . .A disk of copper, twelve inches in 

 diameter, fixed upon a brass axis," he says, "was 

 mounted in frames so as to be revolved either vertically 

 or horizontally, its edge being at the same time intro- 

 duced more or less between the magnetic poles. The 

 edge of the plate was well amalgamated for the purpose 

 of obtaining good but movable contact ; a part round 

 the axis was also prepared in a similar manner. 



"Conductors or collectors of copper and lead were 

 constructed so as to come in contact with the edge of the 

 copper disk, or with other forms of plates hereafter to 

 be described. These conductors were about four inches 

 long, one-third of an inch wide, and one -fifth of an inch 

 thick ; one end of each was slightly grooved, to allow 

 of more exact adaptation to the somewhat convex edge 

 of the plates, and then amalgamated. Copper wires, 

 one-sixteenth of an inch in thickness, attached in the 

 ordinary manner by convolutions to the other ends of 

 these conductors, passed away to the galvanometer. 



"All these arrangements being made, the copper 

 disk was adjusted, the small magnetic poles being 

 about one-half an inch apart, and the edge of the plate 

 inserted about half their width between them. One 

 of the galvanometer wires was passed twice or thrice 

 loosely round the brass axis of the plate, and the other 

 attached to a conductor, which itself was retained by 

 the hand in contact with the amalgamated edge of the 



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