390 GENERAL SCIENCE 



by the ancients. The electro-magnet is made by winding a piece 

 of soft iron with wire, and allowing a current of electricity to run 

 through the wire. Such magnets lose their magnetism almost as 

 soon as the current of electricity is broken. Magnets of this type 



FIG. 268. Iron filings on a paper placed over a bar magnet. 



are used in electric bells, telegraph instruments, induction coils, 

 motors and dynamos. 



Experiments with Magnets. Place a horseshoe magnet or bar 

 magnet under a sheet of paper or glass. Shake iron filings evenly 



FIG. 269. Put a bar magnet on top of some small tacks spread out on a table, 

 and then lift it up. The tacks will be found to cling to the magnet, but in 

 unequal numbers at different parts. The magnet has polarity; that is, the 

 places near the ends where most tacks collect are called poles. 



over the paper with a salt shaker. Observe the force of magnetism, 

 and the direction of the magnetic lines. 



Experiment with Electro-Magnet. Wind a piece of soft iron 

 with insulated copper wire. Attach the end of the wire to an 

 electric battery. Bring the end of the magnet near tacks or other 

 pieces of iron. Break the current of electricity. Observe what 

 happens. 



