PHYSICAL FORCES 127 



This wonderful form of energy, as before noted, is not 

 confined to iron or steel, and no substance is completely 

 indifferent to it. Bodies which only manifest its in- 

 fluence by being attracted, have been named magnetics. 

 Those, the far larger class, which are only repelled 

 repelled by both poles are called diamagnetics. The 

 wonderful polarity of the magnet will even modify the 

 polarity of light ; but such matters are beyond the range 

 of those elementary notions of science with which alone 

 this work has to do. 



We have before spoken of the relations between 

 magnetism and electricity, aijd they are indeed very 

 close relations. An electric current passing near iron or 

 steel tends to make it magnetic, while the movements of 

 a magnet tend to produce electric currents. The rotation 

 of a magnet on its axis will produce electric currents 

 through it, if the universally necessary condition of a 

 current, a complete circuit, exists. On the other hand, 

 the passage of electric currents around an axis will make 

 the axis it surrounds magnetic.* Let a rod of iron have 

 an insulated copper wire wound a number of times about 

 it. Then so soon as a current of electricity is made to 

 pass through the wire, so soon will the rod of iron 

 become a magnet an electro-magnet but its magnetic 

 energy will cease the moment the current of electricity 

 is discontinued. The iron may be bent as a horse-shoe 

 and then if, through a copper wire copiously investing it, 

 a strong current of electricity be made to pass, the iron 

 horse-shoe will, as long as the current so passes, be a 

 very powerful electro-magnet, and sustain a great weight 

 of iron, which, however, will instantly fall away from it, 

 the moment the current ceases. 



* For a very important consequence of this, see pout, p. 171. 



