Professor Joseph Henry's Invention 



repel, each other. On this fact Ampere founded 

 his celebrated theory, that magnetism consists 

 merely in the attraction of electrical currents 

 revolving at right angles to the line joining the 

 two poles of the magnet. The magnetization of 

 a bar of steel or iron, according to this theory 

 consists in establishing within the metal by in- 

 duction a series of electrical currents, all revolv- 

 ing in the same direction at right angles to the 

 axis or length of the bar. 



It was this theory which led Arago, as he 

 states, to adopt the method of magnetizing 

 sewing needles and pieces of steel wire, shown in 

 Fig. 3. This method consists in transmitting 



Fig. 3 



a current of electricity through a helix surround- 

 ing the needle or wire to be magnetized. For 

 the purpose of insulation the needle was enclosed 

 in a glass tube, and the several turns of the helix 

 were at a distance from each other to insure the 

 passage of electricity through the whole length 

 of the wire, or, in other words, to prevent it from 

 seeking a shorter passage by cutting across from 

 one spire to another. The helix employed by 

 Arago obviously approximates the arrangement 

 required by the theory of Ampere, in order to 

 develop by induction the magnetism of the iron. 

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