EXERCISES 129 



United States and Canada the compass points west of 

 north; in the western part, east of north. 



Columbus knew of the declination of the needle, and 

 kept a record of its amount for many places in the Atlantic ; 

 but to his sailors it was a source of great uneasiness, for 

 they thought that the very laws of nature were changed 

 in the new regions they passed through on 

 their long voyage. 



Dipping Needle. If a balanced needle is supported 

 so that it will swing in a vertical plane (Fig. 116), and is 

 then magnetized, it is called a dipping needle. Its north- 

 seeking end will then dip noticeably in the northern hemi- 

 sphere. Over the north magnetic pole it will stand verti- FIG. lie. 

 cal, the north-seeking end being downward. At the south "SSSBe? 8 

 magnetic pole the north-seeking end of the compass points 

 upward. It is plain that the compass cannot tell north and south 

 directions at the earth's magnetic poles, because there the magnetic 

 force is all vertical. 



140. Exercises. 



1. If you had a compass and a bar magnet, how could you tell, 

 without suspending the bar magnet, which of its ends was north- 

 seeking? 



2. How would a dipping needle behave at the " magnetic equator"; 

 that is, on a line half-way between the earth's magnetic poles? 



3. How would a dipping needle behave if brought near a large 

 deposit of iron? How would a compass behave? 



4. We believe that in a magnet the iron molecules are themselves 

 magnets, set end to end. Draw a sketch of such an arrangement, 

 marking the poles of each small magnet N and S respectively. 



5. When a steel bar is held in the direction taken by a dipping 

 needle, that is, pointing to the earth's magnetic pole, sharp blows on 

 one end of the bar cause it to become a magnet. Suggest the reason. 



