248 



VOLTAIC ELECTRICITY 



armature" shown in Fig. 194. The armature is of soft iron 

 divided into two semicircles with brass handles. 

 When the helix is placed in a closed circuit, the 

 semicircles resist a considerable force tending to 

 draw them apart; when the circuit is broken they 

 fall asunder of their own weight. 



393. Electro-Magnets. The bar of 



Fig. 193, and the ring of Fig. 194, are electro- 

 magnets. The electro-magnet more often has 

 the horse-shoe form shown in Fig. 195. The 



middle of the bent bar is bare, the direction of the windings 



on the ends being such that, were the bar straightened, the 



current would move in the same 



direction round every part. 



Electro-magnets have been made 



capable of supporting several 



tons. 



(a.) When the circuit is broken and 

 the current thus interrupted, the iron 

 is generally not wholly demagnetized. 

 The small magnetism remaining is 

 called residual magnetism. The resid- 

 ual magnetism seems to vary with 

 the degree of impurity of the iron. 



394. Making Perma- 

 nent Magnets. A steel bar 

 may be permanently magnetized 

 ( 320) by drawing it, from its centre, in one direction over 

 one pole of a powerful electro-magnet, and then, from its 

 centre, in the opposite direction over the other pole, and 

 repeating the process a few times. A bar of steel placed 

 within a helix through which a strong current is passing, 

 will be permanently magnetized. The arrangement is sub- 

 stantially like that shown in Fig. 193. 



FIG. 195. 



