154 



ELECTRICITY 



FIG. 112 



magnetic pole of the earth, and its negative end of 

 course toward the positive magnetic pole. Such a mag- 

 netized bit of steel may be used as a compass needle. 

 This needle is finely balanced, 

 and usually swings over a card 

 on which the "points of the com- 

 pass" are marked (Fig. 112). 



The compass has long been a 

 valuable aid to sailors and trav- 

 elers, who often have little else to 

 guide them. But owing to the 

 position of the magnetic poles 

 and variations in the earth's 

 magnetism at different points, 

 the needle points to the true 

 north at only a few places on earth. To find the true 

 north, the user of the compass has to know how far wrong 

 the compass direction is at that point, and allow for it. 



QUESTIONS 



1. What is magnetic force ? Define magnetism. 



2. What is a magnet ? Name two kinds of magnets. 



3. How is an electro-magnet made? How is it magnetized? 

 Why should the wire be insulated ? 



4. Name various uses of electro-magnets, showing why this 

 form of magnet is particularly useful in each case. How may the 

 power of an electro-magnet be increased? 



5. Of what are permanent magnets made? How are they 

 magnetized ? Show any points of difference between permanent 

 magnets and electro-magnets. Which form is most used in elec- 

 trical devices, and why ? 



6. What are the poles of a magnet ? How are they named ? 

 Why are magnets often made in horseshoe form ? 



