254 Mr. J. Parker on the Theory of Magnetism and 



the other repel, they will attract each other ; and the very 

 same properties are true of all the poles. 



Thus it appears that there are two kinds of poles, or of 

 magnetism. Like kinds repel ; unlike kinds attract. For 

 instance, if P and Q repel each other, they are of the same 

 kind. If both P and Q repel a third pole R, R will be of 

 the same kind as P and Q ; if both P and Q attract R, it will 

 be of unlike kind to P and Q . 



The two kinds of magnetism may be distinguished by the 

 signs + and — . It is immaterial which kind of magnetism 

 be considered positive ; but it is generally agreed to take the 

 kind found at that end of a soft bar of iron which, when 

 freely suspended and in stable equilibrium, points to the 

 north. 



If the poles P, Q exert equal forces, both attractive or both 

 repulsive, on any third pole R from which they are equally 

 distant, the poles P, Q, or the quantities of magnetism at P 

 and Q are said to be equal. If the forces be equal, but one 

 attractive and the other repulsive, the poles P, Q are said to 

 be equal and opposite, or the quantities of magnetism at P 

 and Q are said to be numerically equal but of opposite sign. 

 Again, if the pole P exert m times as great a force as Q, and 

 both be attractive or both repulsive, the magnetism at P is 

 said to be +m times that at Q. If one force be attractive 

 and the other repulsive, the magnetism at P is said to be 

 —m times that at Q. Lastly, it is inferred from experiment, 

 supported by theory, that if two poles X, Y be at the same 

 distance as two equal poles P, Q, and the magnetism at X be 

 x times that at P, and that at Y y times that at Q, the force 

 between X and Y will be xy times that between P and Q. 

 The force between X and Y is repulsive if X and Y, or x and 

 y, be of the same sign, that is, if the product xy be positive : 

 the force is attractive if x and y be of opposite signs, or xy 

 negative. 



These results lead to the C.G.S. system of units. If two 

 equal positive poles P, Q, situated at a distance of one centi- 

 metre, repel each other with a force of one dyne, the quan- 

 tity of magnetism at P or Q is defined to be the unit of 

 magnetism. It therefore follows that if two poles X, Y, at 

 which the quantities of magnetism are m and m', be at a dis- 

 tance of r centimetres, the magnetic force between them will 



Ytifn 

 be — -j- dynes, repulsive forces being considered positive and 



attractive negative. 



To complete the fundamental principles of magnetism, we 

 must add the great principle of the Conservation of Magne- 



