SECT, xxxiv.] DIRECTION OF THE CURRENTS. 373 



its own 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 experi- 

 ment's 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 ex- 

 traneous 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 be- 

 tween 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 pos- 

 sess ordinary magnetic properties ; those affected when in 

 motion, in which electric currents are evolved by the in- 

 ductive force of the magnet, such as copper; and, lastly, 

 those which are perfectly indifferent to the magnet, whether 

 at rest or in motion. 



