352 ELECTROSTATICS AND MAGNETISM. [PT. II. CH. IX. 



that a magnet is polarized ( 120). The intensity of the polariza- 

 tion is called the intensity of magnetization. A magnet may be 

 magnetized solenoidally, and will then appear to be charged only 

 superficially. A long thin magnetic solenoid may be assimilated 

 to two equal and opposite magnetic points. These are called the 

 poles of the solenoid. It is a matter of indifference which one is 

 taken as positive in practice, as the earth is surrounded by a 

 magnetic field whose lines run roughly north and south, the end 

 of any magnetic solenoid which tends to move toward the north is 

 called positive. 



It is to be noticed that a magnetic point never exists alone, 

 but is always accompanied by an equal and opposite point, just as 

 when electricity is generated, equal and opposite amounts always 

 appear simultaneously. In the case of electricity we often lose 

 sight of one of the charges produced, but in magnetism we cannot 

 do so, though we may remove one of the charges as far as we 

 please by making the magnet long enough. It was in this manner 

 that Coulomb, by experiments with a torsion-balance, was able to 

 investigate the forces between magnetic poles, finding that they 

 acted upon each other according to the Newtonian Law of the 

 inverse square. The unit magnetic pole is then defined as the 

 pole which will repel with unit force a similar pole placed at unit 

 distance from it. This definition is the basis of the magnetic 

 system of measurements, which stands in the same relation to 

 magnetic quantities that the electrostatic system does to electric 

 quantities. All the mathematical work that has been done for 

 electricity (with one exception) is then directly applicable to 

 magnetism. Magnetic potential, density, energy, and so on, are 

 defined in a similar way to the corresponding electrical quantities, 

 and their dimensions in the magnetic system are the same as those 

 of the electrical quantities in the electric system. The exception 

 noted is that phenomena of magnetic flow do not exist there are 

 no magnetic conductors, and no dissipation of magnetic energy 

 into heat by flow. It may accordingly seem that the principal 

 part of electrical phenomena, namely the distribution of charges 

 on conductors, forming the subject of electrostatics, has no place in 

 magnetism. While this is true, we shall find that a very important 

 part of electrostatics, namely the consideration of the field in 

 dielectrics, has exact analogues in magnetism, and these are yet to 

 be treated. 



