SECT. XXXIII. DIRECTION OF THE CURRENTS. 331 



made to revolve rapidly the galvanometer needle was deflected 

 sometimes as much as 90, and by a uniform rotation the deflec- 

 tion was constantly maintained at 45. When the motion of the 

 copper plate was reversed, the needle was deflected in the con- 

 trary direction, 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 con- 

 ceived by considering that, merely by moving a single wire, like 

 the spoke of a wheel, before a magnetic pole, a current of elec- 

 tricity 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 pro- 

 duced in it as it passes the pole. Now, as the circular plate is 

 nothing more than an infinite number of radii or spokes in con- 

 tact, 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 elec- 

 tricity 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 conveys 

 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 elec- 

 tricity conveyed from the centre. 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 

 are only induced by motion across the magnetic lines of force. 

 When the plate is placed edgewise so as to be parallel to these 

 lines of force, no revolution of it with the most powerful magnet 

 produces the slightest signs of a current at the galvanometer. 

 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 



