MAGNETS 247 



Since the magnetic north pole is not at the actual north 

 pole, a compass does not point directly north in the 

 United States except on one line. This line comes south 

 from the magnetic pole and passes through central Ohio 

 and thence turns slightly toward the southeast. A 

 compass anywhere on this line points due north, but a 

 compass east of the line points west of true north, and 

 one west of the line points east of true north. At New 

 York the compass points about 9 degrees west of true 

 north, at Pittsburgh about 3 degrees west of true north, 

 and at San Francisco about i6f degrees east of true 

 north. This variation of the compass is called decli- 

 nation. The declination of the compass gave Columbus 

 trouble when he crossed the Atlantic the first time. 

 Mariners now have charts showing the declination for 

 all parts of the earth. The declination of the compass 

 at any one place changes slightly from one season to 

 another, but it makes greater changes during a period of 

 years. From 1580 to 1816 the declination at London 

 changed from 11 if east to 24 30' west, making a 

 total change- of 35 47' during a period of 236 years. 



QUESTIONS AND EXERCISES 



1. What is the history of lodestone? 



2. How can you magnetize your knife blade? 



3. Why does a suspended magnet point north? 



4. Does a magnet point due north where you are? Why? 



5. Test the law of magnets by having one magnet suspended 

 and bring near its poles the poles of another magnet. 



6. How does a magnet pick up a tack? 



7. With a small compass trace some lines of force about a 

 bar magnet. 



8. How do you care for your magnets? Why? 



9. If the earth were not a magnet, would a compass be of 

 any value in determining directions? Why? 



