SECT. XXXIV. DIRECTION OF THE CURRENTS. 327 



tricity 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 then* relative motion, and to bring the mov- 

 ing 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 opposite 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 

 produced 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 magnet when at rest, like iron, steel, 

 and nickel, which possess ordinary magnetic properties ; 

 those affected when in motion, in which electric cur- 

 rents 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, 



