354 Permanent Magnetism [en. xi 



The earth may roughly be regarded as a single magnet of which the two 

 magnetic poles are at points near to the geographical north and south poles. 

 Since the northern magnetic pole of the earth attracts the north-seeking pole 

 of a suspended bar-magnet, it is clear that this northern magnetic pole must 

 be a south-seeking pole ; and similarly the southern pole of the earth must 

 be a north-seeking pole. Lord Kelvin speaks of a south-seeking pole as a 

 " true north " pole i.e. a pole of which the magnetism is of the kind found 

 in the northerly regions of the earth. But for purposes of mathematical 

 theory it will be most convenient to distinguish the two kinds of pole by the 

 entirely neutral terms, positive and negative. And, as a matter of convention, 

 we agree to call the north-seeking pole positive. Thus we have the following 

 pairs of terms : 



North-seeking = True South = Positive, 



South-seeking True North = Negative. 



Law of Force between Magnetic Poles. 



400. By experiments with his torsion-balance, Coulomb established that 

 the force between two magnetic poles varies inversely as the square of the 

 distance between them. It was found also to be proportional to the product 

 of two quantities spoken of as the " strengths " of the poles. Thus if F is the 

 repulsion between two poles of strengths m, m at a distance r apart, we have 



(328). 



It is found that c depends on the medium in which the poles are placed, 

 but is otherwise constant. Clearly if we agree that the strength of positive 

 poles is to be reckoned as positive, while that of negative poles is reckoned 

 negative, then c will be a positive quantity. 



The Unit Magnetic Pole. 



401. Just as Coulomb's electrostatic law of force supplied a convenient 

 way of measuring the strength of an electric charge, so the law expressed by 

 equation (328) provides a convenient way of measuring the strength of a 

 magnetic pole, and so gives a system of magnetic units. A system of units, 

 analogous to the electrostatic system ( 17, 18) is obtained by defining the 

 unit pole to be such as to make c = 1 in equation ('328). This system is 

 called the Magnetic (or, more generally, Electromagnetic) system of units. 

 We define a unit pole, in this system, to be a pole of strength such that when 

 placed at unit distance from a pole of equal strength the repulsion between 

 the two poles is one of unit force. 



