SECT. XXXIV. MAGNETIC POLARITY. 339 



is a perfect magnet. Fig. 2, plate 7, shows tlie lines of force in 

 a fractured magnet when the ends are not yet separated ; fig. 3 

 shows them when they are. 



Currents of electricity are produced in conducting bodies moved 

 across these lines of magnetic force. If a copper wire at a little 

 distance above the north pole of a bar magnet be moved from 

 left to right, at any angle across the lines of magnetic force, they 

 will induce a current of electricity in the wire flowing from 

 right to left ; if the wire be moved with the same velocity in the 

 contrary direction, the induced current will be of equal intensity, 

 but it will flow from left to right. Similar results are obtained 

 from the south pole, and the phenomena are the same when the 

 magnet is moved and the wire is at rest; in both cases the 

 intensity is greater the swifter the motion,. It appears that the 

 quantity of electricity induced is directly as the amount of the 

 magnetic curves intersected, and when a wire is moving uni- 

 formly in a field of equal magnetic force, the current of electricity 

 generated is proportional to the time, and also to the velocity of 

 motion ; for when a metallic disc is made to revolve through the 

 lines of force, the current induced is strongest near the edge where 

 the velocity is greatest ; and in different substances moving across 

 the lines of force the intensity of the induced current is directly 

 as the conducting power of the substance. Thus bodies moved 

 near a magnet have an electrical current developed in them, and 

 conversely bodies affected by an electric current are definitely 

 moved by a magnet near them. 



By the preceding experiments it appears that magnetic polarity 

 is manifested in two ways ; in the magnetised needle, by attrac- 

 tion and repulsion, and in a wire moving a'cross lines of magnetic 

 force it is shown by the opposite directions in which the induced 

 current flows according as the body is moved from the right to 

 the left, or left to right. Hence polarity consists in the opposite 

 and antithetical actions manifested at the opposite ends or oppo- 

 site sides of a limited or unlimited line of force. Antithesis is 

 the true and most general character of magnetism, whatever may 

 be its mode of action. 



It was by the induction of electric currents in copper wires 

 moving across the lines of magnetic force that Dr. Faraday 

 proved that the lines of force issuing from a magnet are closed 

 curves which return again and pass through the interior of the 



. Q2 



