MAGNETIC METHODS 



67 



If a bar magnet is suspended at its mid-point and left free to turn, (as 

 is the case with a compass needle), one end of the magnet will always 

 point toward the magnetic north pole. The pole at the north-seeking end 

 of the magnet is arbitrarily called the north pole or plus pole, and is ordin- 

 arily marked with an N or -|-. The other end of the magnet is the south 

 pole or negative pole. 



Fig. 6. — Photograph 

 of iron filings illustra- 

 ting the lines of force 

 due to a bar magnet 

 N-S and the distortion 

 produced by a piece of 

 iron A. 



vmmm^ 



It is of interest that certain dike-type magnetic ore bodies exhibit 

 polarity. Under ordinary conditions, in the northern hemisphere, they will 

 have a south magnetic pole at their top, or nearest the surface. Polarity 

 will also be exhibited by almost any iron body having considerable vertical 

 height in comparison with its other dimensions. A tall steel filing cabinet or 

 an iron stove will attract the N pole of a compass needle at its lower end, 

 and repel the N pole at its upper end. 



haw of Force between Magnetic Poles. — By experimenting with a 

 form of torsion balance, Coulomb proved that the force (F) between two 

 magnetic poles varies inversely as the square of the distance (r) between 

 them. 



It was also found that this force (F) is proportional to the product of 

 the strength of the poles, (lUt and mz). This is expressed by: 



F = 



1 minto 



(1) 



1 



— is a constant depending upon the medium in which the poles are 



placed, ju, = magnetic permeability. For air, /x = 1.0000004 and it is, there- 

 fore, usually taken as = 1, where the poles are in air. Permeability is related 

 to another magnetic quantity termed susceptibility, defined on page 76. The 

 values for susceptibility are given in Table 5. 



An important property of magnetic poles is the fact that those of like 

 sign repel and of unlike sign attract each other. What we speak of as the 

 north magnetic pole of the earth relates to its geographic position. Mag- 

 netically, it is a south pole. This is correct, since the north-seeking end 

 of a compass points toward the (geographic) north, (i.e., unlike poles 

 attract). 



