THEORY OF MAGNETISM 41 



A fixed line on the binnacle shows the direction of the 

 keel of the ship. The card being attached to the needle 

 always has its "north" pointing toward the north. To 

 determine the direction of the ship it is only necessary to 

 notice on the card in what direction the keel line is point- 

 ing. The mariner of course must know the declination 

 at the place where he is and make the proper corrections. 

 The different governments furnish tables and charts show* 

 ing these corrections. 



21. Theory of Magnetism. Experiment 16. Heat a No. 20 



knitting needle red hot and plunge it quickly into cold water. This 

 tempers the needle so that it will break readily. Magnetize the needle 

 as was done in Experiment 11. When it has become well magnetized, 

 break it in the middle. Test each half with a suspended magnet, as 

 was done in Experiment 13. Is each half a full magnet or only half 

 a magnet? Break these halves again and test. What effect does 

 breaking a magnet have upon the magnet? 



In Experiment 16 it was found that if a magnet is 

 broken in two, each half is a perfect magnet. If these 

 halves are broken, each piece is a perfect magnet, and 

 so on as long as the division is kept up. It is also found 

 that if a magnet is heated or suddenly jarred or pounded 

 it loses its magnetism. If a magnet is filed into filings and 

 these filings are put into a glass tube the tube will have 

 no magnetic properties but will act to a magnet like an 

 ordinary iron bar. 



If now the tube is held vertically and tapped several 

 times on a strong magnet, the tube will be found to 

 have acquired the properties of a magnet. The tapping 

 joggled the particles so that they could arrange themselves 

 under the influence of the magnetic pole and when they 

 became so arranged a magnet was the result. If the fil- 

 ings are now poured out of the tube and then put back 

 again, there will be no magnetization. 



