432 



WELLS'S NATURAL PHILOSOPHY. 



In what man- 

 ner can 



be made to ex- 

 cite magn«;t- 

 ismf 



What is a 

 Electro-mag- 



uctf 



WTiat is 

 HeUi? 



Fig. 364. 



805. If a piece of soft iron, entirely wanting 

 eufctrircurren" in magnetism, be placed within a coil of wire 

 tlirous2:h which an electric current is circulat- 

 ing, it will be rendered intensely magnetic, so 

 long as the current continues ; but the moment the cur- 

 rent ceases, the iron loses its magnetism. 



Magnets formed in this way, through the 

 agency of electricity, are called Electro-mag- 

 nets, and are more powerful than any others. 

 The coil, or spiral line of wire used for excit- 

 ing magnetism in the iron by conducting a 

 current of electricity about it, is called a Helix. 



It is usually made of copper wire, coated with 

 some non-conducting substance, such as silk wuuud 

 round it. The coils of the wire are generally re- 

 peated one over the other, until the size of the Lelix 

 is sufficient, since the magnetic action of an electric 

 current upon a bar of iron increases to a certain ex- 

 tent with the number of revolutions it performs aoout 

 it Fig. 3C4 represents the appearance of a hells. 



It is necessary for the induction of magnetism in ii*oa 

 bars by electricity, that the current should flow at right 

 angles to the axis of the bars. 



What deter- ^^ ^^^^ ^ar be steel, the magnetism thus in- 

 "'"an^'^iectro! cluccd will bc permanent ; and the direction in 

 magnet? wliich tlic currcut moves round the helix de- 



termines which of its extremities shall constitute its north, 

 and which its south pole. 



When the current circulates in the direction of the hands 

 of a watch, the north polo of the bar wiE be at the farthest 

 end of the hcUx. 



If a bar of soft iron, bent in the form of a horse shoe mag- 

 net, be wound with insulated wire, as is represented in Fig. 

 3G5, and a current of electricity transmitted through it, it 

 becomes a most powerful magnet. 



Electro-magnets of this character have been formed capa- 

 ble of supporting more than a ton weight. The magnetic 

 power thus developed is wholly dependent upon the ex- 

 istence of the current, and th3 moment it ceases the weights 

 ^ away by the action of gravity. 



Fig. 365. 



