Electric Currents. 69 



science mathematically but to set out the 

 fundamental physical facts and applications of 

 electricity. 



Under the conditions above named magnet- 

 ism is developed in the soft iron bar. If we 

 open the key the current will cease and the 

 magnetism will vanish that is to say, the 

 molecules will turn back to their neutral 

 position by their own attractions, as has been 

 described in a previous chapter. Magnetism 

 developed in this way is called electromag- 

 netism. (See Chap. IV.) If we use a piece 

 of hardened steel instead of the soft iron it 

 will become magnetic and remain so when the 

 circuit is opened, because the natural tendency 

 of the molecules to turn back to the neutral 

 position is not great enough to overcome the 

 coercive force, or molecular friction, of hard- 

 em d steel, as has been also described in a 

 previous chapter. To make the best electro- 

 magnet we need qualities of iron just the op- 

 posite from those of the permanent magnet. 

 For the former we need the purest of soft iron, 

 well annealed (heated to redness and slowly 

 1, making it less brittle), so that its mole- 

 ml.-s are free to turn; while for the latter we 

 need hardened steel, so that when the mole- 

 c\\\i-< are once wrenched into the magnetic 

 rendition tlicy rannot, of themselves, turn 

 back to tin- neutral state. The great value of 

 <-tromagnct lies in its ability to readily 



