332 ARAGO'S EXPERIMENT. SECT. XXXIII. 



axis, electricity of the same kind is collected at its poles, and the 

 opposite electricity at its equator. 



The phenomena which take place in M. Arago's experiments 

 may be explained on this principle. When "both the copper 

 plate and the magnet are revolving, the action of the induced 

 electric current tends continually to diminish their relative 

 motion, and to bring the moving bodies into a state of relative 

 rest ; so that, if one be made to revolve by an extraneous force, 

 the other will tend to revolve about it in the same direction, and 

 with the same velocity. 



When a plate of iron, or of any substance capable of being 

 made either a temporary or permanent magnet, revolves between 

 the poles of a magnet, it is found that dissimilar poles on oppo- 

 site sides of the plate neutralize each other's effects, so that no 

 electricity is evolved ; while similar poles on each side of the 

 revolving plate increase the quantity of electricity, and a single 

 pole end-on is sufficient. But when copper, and substances not 

 sensible to ordinary magnetic impressions, revolve, similar poles 

 on opposite sides of the plate neutralize each other ; dissimilar 

 poles on each side exalt the action ; and a single pole at the edge 

 of the revolving plate, or end-on, does nothing. This forms a 

 test for distinguishing the ordinary magnetic force from that pro- 

 duced by rotation. If unlike poles, that is, a north and south 

 pole, produce more effect than one pole, the force will be due to 

 electric currents ; if similar poles produce more effect than one, 

 then the power is not electric. These investigations show that 

 there are really very few bodies magnetic in the manner of iron. 

 Dr. Faraday therefore arranges substances in three classes, with 

 regard to their relation to magnets : those affected by the mag- 

 net when at rest, like iron, steel, and nickel, which possess ordi- 

 nary magnetic properties ; those affected when in motion, in 

 which electric currents are evolved by the inductive force of the 

 magnet, such as copper ; and, lastly, those which are perfectly 

 indifferent to the magnet, whether at rest or in motion. 



It has already been observed that three bodies are requisite to 

 form a galvanic circuit, one of which must be fluid. But, in 

 1822, Professor Seebeck, of Berlin, discovered that electric currents 

 may be produced by the partial application of heat to a circuit 

 formed of two solid conductors. For example, when a semicircle 

 of bismuth, joined to a semicircle of antimony, so as to form a 



