CHAPTER 11. MAGNETISM 



4. The magnetic compass. — The principle of the present day mag- 

 netic compass is in no way different from that of the compass used 

 by the ancients. It consists of a inagnetized needle, or array of needles, 

 pivoted so that it will rotate in a horizontal plane. The superiority of 

 the present day compass results from a better knowledge of the laws 

 of magnetism which govern the behavior of the compass, and from 

 greater precision in construction. 



5. Magnetism. — Any piece of metal on becoming magnetized, that 

 is, acquiring the property of attractmg small particles of iron or steel, 

 will assume regions of concentrated magnetism, called poles. Any 

 such magnet will have at least two poles, of unlike polarity. Mag- 

 netic lines of force (flux) connect one pole of such a magnet with the 

 other pole as indicated in figure 2. The number of such lines per unit 

 area represent the intensity of the so-called magnetic field in that area. 



N \ Lines of Force 



/ ^' ^ \ 



/ / ,^ — ^ ^ N 



, // .---.V' (Flux) 



V^i.^'^ 



'-^ — \mo 



w^ yj:i 





SS]^ - ^ - - Bar Magnet 



// » \\ '^ 





i 



Figure 2. — Lines of magnetic force about a magnet. 



If two such magnetized bars or magnets are placed side by side, the 

 like poles will repel each other and the unlike poles will attract each 

 other. 



6. Magnetism is in general of two types, permanent and induced. 

 A bar having permanent magnetism will retain its magnetism when 



