Orf^i DIRECTION OF THE CURRENTS. [SECT, xxxiv. 



constantly maintained at 45. When the motion of the 

 copper plate was reversed, the needle was deflected in the 

 contrary direction, and thus a permanent current of elec- 

 tricity 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 constructed 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 centre to the circumference, and the negative from the 

 circumference to the centre, and vice versa, according to the 

 position of the magnetic poles and the direction of rotation ; 

 so that a collecting wire at the centre of the copper plate con- 

 veys positive electricity to the galvanometer in one case, and 

 negative in another ; that collected by a conducting wire in 

 contact with the circumference of the plate is always the 

 opposite of the electricity conveyed from the centre. It ig 

 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 



