MAGNETISM 151 



174. Permanent Magnets. We have learned that 

 when a piece of steel is magnetized it remains a magnet 

 after it is removed from the magnetic field. For this 

 reason a magnet made of steel is called permanent. Of 

 course there are many kinds of steel, and 



these vary greatly in their value as perma- 

 nent magnets. 



Experiment 103. Make an electro-magnet as 

 in Experiment 102. Across one end of it draw a 

 small piece of steel (a needle, knife blade, or steel 

 pen) several times, always in the same direction. Try 

 to pick up small tacks with this. 



Two forms of permanent magnets are 

 common, the horseshoe (Fig. 109) and the Fm - 109 

 bar magnet (a straight bar of steel). They are not 

 made nearly as powerful as some electro-magnets are. 

 Their use in small dynamos and in telephones is most 

 important. 



175. Magnetic Poles. In using magnets we have 

 perhaps noticed that the force seems to be greatest at 

 the ends, while at the center none at all is felt. For 

 this reason many magnets, both permanent and electro- 

 magnets, are made in a horseshoe form, so as to bring 



the ends near together 

 and exert greatest force 

 FIG. no at that point. 



Experiment 104. Lay a bar magnet down upon a mass of 

 iron filings ; lift it carefully by the center. Notice the arrange- 

 ment of the filings that cling to i (Fig. 110). Where are they 

 most numerous? Where are they least in number? Does the 

 number change gradually or sharply? 



