440 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



bar, except in the case of long, infinitely thin, uniformly mag- 

 netized rods. 



If we mark the poles of any two magnets which possess simi- 

 lar qualities, we find that the two marked poles repel each other, 

 that two unmarked poles also repel each other, but that a marked 

 and an unmarked pole attract each other. The pole which is 

 repelled from the northern regions of the earth is called a posi- 

 tive pole; the other end the negative pole. The negative pole 

 is generally marked N by British instrument-makers, and is 

 sometimes called the north pole of the magnet, whereas it is 

 obviously similar to the earth's south pole. 



The strength of a pole is necessarily defined as proportional 

 to the force it is capable of exerting on any other pole. Hence 

 the force /exerted between two poles of the strengths m and m l 

 must be proportional to the product mm v The force, /, is also 

 found to be inversely proportional to the square of the distance, 

 D, separating the poles, and to depend on no other quantity ; 

 hence we have, unless an absurd and useless coefficient be 

 introduced, 



/-=ijk a) 



from which equation it follows that the unit pole will be that 

 which at unit distance repels another similar pole with unit 

 force; /will be an attraction or a repulsion according as the 

 poles are of opposite or the same kinds. The dimensions of the 



.. . L«M* 

 unit magnetic pole are — =j— • 



6. Magnetic Field. — It is clear that the presence of a magnet 

 in some way modifies the surrounding space, since any other 

 magnet brought into that space experiences a peculiar force. 

 The neighbourhood of a magnet is, for convenience, called a 

 magnetic field ; and for the same reason the effect produced by 

 a magnet is often spoken of as due to the magnetic field instead 

 of to the magnet itself. This mode of expression is the more 

 proper, inasmuch as the same or a similar condition of space 

 may be produced by the passage of electrical currents in the 

 neighbourhood, without the presence of a magnet. Since the 

 peculiarity of the magnetic field consists in the presence of a 

 certain force, we may numerically express the properties of the 

 field by measuring the strength and direction of the force, or, 

 as it may be worded, the intensity of the field and the direction 

 of the lines of force. 



This direction at any point is the direction in which the force 

 tends to move a free pole ; and the intensity, H, of the field is 

 necessarily defined as proportional to the force,/, with which it 



