PHYSICS OF NINETEENTH CENT. ELECTRICITY. 581 



obtaining a spark by the electricity developed according to this prin- 

 ciple from a natural magnet. These, together with certain other dis- 

 coveries, were communicated by him to the Royal Society, in a paper 

 read on the 24th of November, 1831. 



It will be obvious that, as the proximity of the pole of a permanent 

 magnet converts a piece of soft iron into a magnet, we may obtain a 

 succession of currents by taking a soft iron cylinder wound round with 

 insulated copper wire, and causing the pole of a magnet to approach 



FIG. 294. A MAGNETO-ELECTRIC MACHINE. 



and recede. But as the currents thus formed are in opposite direc- 

 tions alternately, they would, in general, neutralize each other's effect. 

 But there are various expedients by which the direction of currents 

 can be instantly reversed, and by employing some of them in connec- 

 tion with the movements of the magnet, it is easy to send currents in 

 one direction only through a conductor external to the coil. H. Pixii, 

 soon after Faraday's discovery had been announced, constructed an 

 apparatus in which a large horse-shoe magnet was made to revolve 

 beneath soft iron armatures covered with silk-covered copper wire. He 

 was thus able to produce chemical decompositions, give shocks, exhibit 

 sparks, and cause divergence of the gold leaves of an electroscope. An 



