SECT, xxxiv.] ELECTRICITY FROM ROTATION. 371 



SECTION XXXIV. 



Electricity produced by Rotation Direction of the Currents Electricity from 

 the Rotation of a Magnet M. Arago's Experiment explained Rotation of 

 a Plate of Iron between the Poles of a Magnet Relation of Substances to 

 Magnets of three Kinds Thermo-Electricity. 



M. ARAGO discovered an entirely new source of magnetism 

 in rotatory motion. If a circular plate of copper be made 

 to revolve immediately above or below a magnetic needle or 

 magnet, suspended in such a manner that the magnet may 

 rotate in a plane parallel to that of the copper plate, the 

 magnet tends to follow the circumvolution of the plate ; or, if 

 the magnet revolves, the plate tends to follow its motion ; 

 so powerful is the effect, that magnets and plates of many 

 pounds' weight have been carried round. This is quite in- 

 dependent of the motion of the air, since it is the same 

 when a pane of glass is interposed between the magnet and 

 the copper. When the magnet and the plate are at rest, 

 not the smallest effect, attractive, repulsive, or of any kind, 

 can be perceived between them. In describing this phe- 

 nomenon, M. Arago states that it takes place not only with 

 metals, but with all substances, solids, liquids, and even 

 gases, although , the intensity depends upon the kind of 

 substance in motion. Experiments made by Dr. Faraday 

 explain this singular action. A plate of copper, twelve 

 inches in diameter and one fifth of an inch thick, was 

 placed between the poles of a powerful horse-shoe mag- 

 net, and connected at certain points with a galvanometer 

 by copper wires. When the plate was at rest, no effect was 

 produced ; but as soon as the plate was made to revolve 

 rapidly the galvanometer needle was deflected sometimes as 

 much as 90, and, by a uniform rotation, the deflection was 



B B 2 



