399-403] Physical Phenomena 355 



Thus the force F between two poles of strengths m, m', measured in the 

 Electromagnetic system of units, is given by 



The physical dimensions of the magnetic unit can be discussed in just the 

 same way in which the physical dimensions of the electrostatic unit have 

 already been discussed in 18. 



Moment of a Line-Magnet. 



402. It is found that every positive pole has associated with it a nega- 

 tive pole of exactly equal strength, and that these two poles are always in 

 the same piece of matter. 



Thus not only are positive and negative magnetism necessarily brought 

 into existence together and in equal quantities, as is the case with positive 

 and negative electricity, but, further, it is impossible to separate the positive 

 and negative magnetism after they have been brought into existence, and in 

 this respect magnetism is unlike electricity. 



It follows that it is impossible to have a body " charged with magnetism " 

 in the way in which we can have a body charged with electricity. A 

 magnetised body may possess any number of poles, and at each pole there is, 

 in a sense, a charge of magnetism; but the total charge of magnetism in the 

 body will always be zero. 



Hence it follows that the simplest and most fundamental piece of matter 

 we can imagine which is of interest for the theory of magnetism, is not a 

 small body carrying a charge of magnetism, but a small body carrying (so 

 to speak) two equal and opposite charges at a certain distance apart. 



This leads us to introduce the conception of a line-magnet. A line- 

 magnet is an ideal bar-magnet of which the width is infinitesimal, the 

 length finite, and the poles at the two extreme ends. Thus geometrically 

 the ideal line-magnet is a line, while its poles are points. 



The strengths of the two poles of a line-magnet are necessarily equal and 

 opposite. The product of the numerical strength of either pole and the dis- 

 tance between the poles is called the " moment " of the line-magnet. 



Magnetic Particle. 



403. If we imagine the distance between the two poles of a line-magnet 

 to shrink until it is infinitesimal, the magnet becomes what is spoken of as a 

 magnetic particle. If + m are the strengths of its ^>oles and ds is the dis- 

 tance between the two poles, the moment of the magnetic particle is mds. 



232 



