THE CONQUEST OF NATURE 



through which a current of electricity is passing is 

 placed across a magnetic field, the wire is impelled to 

 move in a plane at right angles to the direction of the 

 lines of force. It is forcibly thrust aside. This side- 

 thrust acting on coils of wire is what produces the 

 revolution of the armature of the electric motor. 



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THE ORIGIN OF THE DYNAMO 



The very first studies that had to do with the mutual 

 relations of electricity and magnetism were made by 

 Hans Christian Oersted, the Dane, as early as 1815. 

 He discovered that a magnetic needle is influenced by 

 the passage near it of a current of electricity, demon- 

 strating, therefore, that the electric current in some 

 way invades the medium surrounding any conductor 

 along which it is passing. Oersted's experiments were 

 repeated, and some new phenomena observed by the 

 Frenchman Andre' Marie Ampere and Dominique Fran- 

 cois Arago. Arago constructed an interesting device, in 

 which a metal disk was made to revolve in the presence of 

 a current of electricity; but neither he nor anyone else at 

 the time was able to explain the phenomenon. 



In 1824 an advance was made through the construc- 

 tion of the first electric magnet by Sturgeon. Hitherto 

 it had not been known that a magnet could be made 

 artificially, except by contact with a previously existing 

 magnet. Sturgeon showed that any core of iron may 

 be rendered magnetic if wound with a conducting wire, 

 through which a current of electricity is passed. The 

 experiments thus inaugurated were followed up in 



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