330 ELECTRICITY BY ROTATION. SECT. XXXIII. 



SECTION XXXIII. 



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 a 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 it may rotate in a plane parallel to that of 

 the copper plate, the magnet tends to follow the circumvolu- 

 tion 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 

 copper. When the magnet and tlie plate are at rest, not the 

 smallest effect, attractive, repulsive, or of any kind, can be per- 

 ceived between them. In describing this phenomenon, M. Arago 

 states that it takes place not only with metals, but with all sub- 

 stances, 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. He found 

 that, if a piece of metal or a metallic wire forming a circuit of any 

 form be moved from right to left across the lines of force pro- 

 ceeding from the pole of a bar magnet, these lines of force induce 

 a current of electricity flowing in one direction ; and when the 

 motion of the metal or wire is reversed, the direction of the 

 current is reversed also : the rotation of the magnet about its axis 

 has no effect on these results, and no current is induced when the 

 metal or wire is at rest. 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, consequently crossing the 

 magnetic lines of force at right angles, and connected at certain 

 jtoints with a galvanometer by copper wires. When the plate 

 was at rest no effect was produced ; but as soon as the plate was 



