THE MAGNETISM OF IRON. 65 



magnet poles, r is their distance apart in centimeters, and F is 

 the force in dynes with which they attract or repel each other. 



Algebraic sign of magnet pole. The poles m' and m" are 

 alike in sign when both are north poles or when both are south 

 poles. On the other hand, m' and m" are unlike in sign 

 when one is a north pole and the other is a south pole. It is 

 customary to consider a north pole as positive and the south 

 pole as negative. The force in equation (15) is considered as 

 positive when it is a repulsion. 



Two poles of a magnet always equal in strength and opposite in 

 sign. The behavior of a magnet in what is called a uniform 

 magnetic field, as described in Art. 41, shows that the poles 

 of a magnet are always equal in strength and opposite in sign. 

 A bar of steel may be irregularly magnetized so as to have one 

 or more north poles and one or more south poles, but the sum 

 total of the north polarity is equal to the sum total of the south 

 polarity. When a magnet is broken in two, each piece is found 

 to be a complete magnet with a north pole and a south pole. 

 It is often convenient, nevertheless, to speak of an isolated magnet 

 pole, meaning one'pole of a very long magnet, the other pole being 

 so far away as to be negligible in its effects. 



34. Magnetic figures. The magnetic field. When iron filings 

 are dusted upon a pane of glass which is placed over a magnet, 

 the filings tend to arrange themselves in regular filaments. Slight 

 tapping of the glass facilitates the arrangement of the filings. 

 Figure 30 is a photographic reproduction of a magnetic figure 

 obtained in this way. This magnetic figure conveys the idea that 

 something emanates from one end of the magnet, traverses the 

 surrounding region in beautifully curved lines, -and enters the 

 other end of the magnet. In fact, the entire region surrounding 

 a magnet is in a peculiar physical condition as is shown by the 

 behavior of a compass needle when the compass is brought into 

 the neighborhood of a large magnet. 



Wherever a compass may be placed in the neighborhood of a 

 magnet, the compass needle points in a definite direction, the 

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