SECT. XXXIV. ELECTRICITY FROM ROTATION. 325 



Tested, a brilliant spark takes place every time the cop- 

 per point leaves the surface of the mercury. Platina 

 wire is ignited, shocks smarts enough to be disagreeable 

 are given, and water is decomposed with astonishing 

 rapidity by the same means; which proves beyond a 

 doubt the identity of the magnetic and electric agencies, 

 and places Dr. Faraday, whose experiments established 

 the principle, in the first rank of experimental philoso- 

 phers. 



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 mag- 

 netism in rotatory motion. If a circular plate of copper 

 be made to revolve immediately above or below a mag- 

 netic 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 circum- 

 volution 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 independent of 

 the motion of the air, since it is the same when a pane 

 of glass is interposed between the magnet and the cop- 

 per. 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 

 phenomenon, 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 

 horseshoe magnet, 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 

 EE 



