CHAPTER II 

 MAGNETISM AND ELECTROMAGNETICS 



30. Magnetization. When bodies are magnetized magnetic 

 forces act at every point throughout their volume and lines of 

 magnetic induction pass through them. There are two kinds of 

 magnetic poles just as there are two kinds of electric charges; 

 as a positive electric charge appears where a dielectric flux leaves 

 a surface, so a positive magnetic pole appears where a magnetic 

 flux leaves a surface. The positive magnetic pole is called a 

 north pole. Similarly a negative magnetic pole or south pole 

 appears where a magnetic flux enters a surface. 



Thus a body which is magnetized has a north pole at one part 

 of its surface and an equal south pole at another part unless the 

 magnetic path forms a closed circuit as in Fig. 36. 



Fig. 35 represents a horseshoe magnet. The lines of magnetic 

 induction pass through it in the direction shown, leaving the surface 

 at N and entering it again at S. Thus TV is a positive magnetic 

 pole or a north pole and S is a negative magnetic pole or a south 

 pole. 



Fig. 36 represents the same magnet with its armature on. 

 The lines of magnetic induction pass in the same direction as 

 before, but the circuit is closed and the poles do not appear until 

 a gap is made by removing the armature. 



31. Laws of Magnetism. First Law. Like magnetic poles 

 repel one another; unlike magnetic poles attract one another. 



Second Law. The force exerted between two magnetic poles 

 is proportional to the product of their strengths and is inversely 

 proportional to the square of the distance between them. This 

 law can be expressed by the formula 



(69) 



where m and m\ are the pole strengths, r is the distance between 

 them in centimeters, and / is the force exerted between them in 

 dynes. If m and mi are like poles the force is a repulsion 

 and / is positive. 



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