XXXI. ELECTRO-MAGNETIC INDUCTION 317 



the wire is to urge the north pole of the magnet in one 

 direction, and the south pole in the other. The force 

 thus exerted is multiplied in degree and increased in ex- 

 tent by each repetition of the turns of the wire, and in 

 consequence of these opposing forces the bar remains 

 suspended. This helix has all the properties of a mag- 

 net while the electrical current is flowing through it, 

 and may be substituted for one in almost every experi- 

 ment. It acts as if it had a north pole at one extremity 

 and a south pole at the other, and is attracted and re- 

 pelled by the poles of a magnet exactly as if it were one 

 itself. All these results depend upon the course of the 

 electricity ; that is, on the direction of the turns of the 

 screw, according as it is from right to left, or from left 

 to right, being contrary in the two cases. 



The action of Voltaic electricity on a magnet is not 

 only precisely the same with the action of two magnets 

 on one another, but its influence in producing temporary 

 magnetism in iron and steel is also the same with mag- 

 netic induction. The term induction, when appb'ed to 

 electric currents, expresses the power which these 

 currents possess of inducing any particular state upon 

 matter in their immediate neighborhood, otherwise neu- 

 tral or indifferent. For example, the connecting wire 

 of a galvanic battery holds iron filings suspended like an 

 artificial magnet, as long as the current continues to 

 flow through it ; and the most powerful temporary mag- 

 nets that have ever been made are obtained by bending 

 a thick cylinder of soft iron into the form of a horse- 

 shoe, and surrounding it with a coil of thick copper wire 

 covered with silk, to prevent communication between 

 its parts. When this wire forms part of a galvanic cir- 

 cuit, the iron becomes so highly magnetic, that a tem- 

 porary magnet of this kind, made by Professor Henry, 

 of the Albany Academy, in the United States, sustained 

 nearly a ton weight. The iron loses its magnetic power 

 the instant the electricity ceases to circulate, and ac- 

 quires it again as instantaneously when the circuit is re- 

 newed. Temporary magnets have been made by Pro- 

 fessor Moll of Utrecht, upon the same principle, capable 

 of supporting 200 pounds' weight, by means of a battery 

 of one plate less than half an inch square, consisting of 



DD2 



