326 DIRECTION OF THE CURRENTS SECT. XXXIV. 



was made to revolve rapidly, the galvanometer needle 

 was deflected sometimes as much as 90, and, by a uni- 

 form rotation, the deflection was constantly maintained 

 at 45. When the motion of the copper plate was je- 

 versed, the needle was deflected in the contrary direc- 

 tion, and thus a permanent current of electricity was 

 evolved by an ordinary magnet. The intensity of the 

 electricity collected by the wires, and conveyed by them 

 to the galvanometer, varied with the position of the 

 plate relatively to the poles of the magnet. 



The motion of the electricity in the copper plate may 

 be conceived by considering, that merely by moving a 

 single wire like the spoke of a wheel before a magnetic 

 pole, a current of electricity tends to flow through it 

 from one end to the other. Hence, if a wheel be con- 

 structed of a great many such spokes, and revolved 

 near the pole of a magnet in the manner of the copper 

 disc, each radius or spoke will tend to have a current 

 produced in it as it passes the pole. Now, as the 

 circular plate is nothing more than an infinite number 

 of radii or spokes in contact, the currents will flow in 

 the direction of the radii if a channel be open for their 

 return, and in a continuous plate that channel is afforded 

 by the lateral portions on each side of the particular 

 radius close to the magnetic pole. This hypothesis is 

 confirmed by observation, for the currents of positive 

 electricity set from the center to the circumference, and 

 the negative from the circumference to the center, and 

 vice versa, according to the position of the magnetic 

 poles and the direction of rotation. So that a collecting 

 wire at the center of the copper plate conveys positive 

 electricity to the galvanometer in one case, and negative 

 in another ; that collected by a conducting wire in con- 

 tact with the circumference of the plate is always the 

 opposite of the electricity conveyed from the center. 

 It is evident that when the plate and magnet are both 

 at rest, no effect takes place, since the electric currents 

 which cause the deflection of the galvanometer cease 

 altogether. The same phenomena may be produced by 

 electro-magnets. The effects are similar when the 

 magnet rotates and the plate remains at rest. When 

 the magnet revolves uniformly, about its own axis, elec- 



