GENERAL ACCOUNT OF MAGNETIC ACTIONS 185 



In the last chapter we introduced the idea of magnetic permea- 

 bility as describing the power of conducting the lines of magnetic 

 force possessed by a magnetic body. Though permeability is 

 connected with susceptibility, it implies a somewhat different way 

 of looking at magnetic actions and is not measured in the same 

 way. We may realise the difference, perhaps, by considering the 

 case of a bar inserted in a magnetising solenoid A B, Fig. 133, and 

 placed some distance from a needle N, which it deflects. Before 

 the bar is inserted the space A B contains air, and the lines of 

 magnetic force due to the electric current in the coil are passing 

 along this space and ultimately out into the field, and are deflecting 

 the needle N. Let us suppose that N is brought back to its original 



\ 



M 



FIG. 133. 



position by a magnet M placed to the right. Now let the bar be 

 inserted. Many more lines of force pass through it than through 

 the air which it displaces, and we say that poles are developed on 

 the bar. The needle N is again deflected, and we measure the 

 strength of the poles or the magnetisation of the bar by the in- 

 crease in the lines of force, that is, by the deflection which is due 

 to the substitution of iron for air. We do not consider the 

 magnetisation of the space A B before the bar was inserted, but only 

 the increase when the iron displaces the air. If we inserted an 

 equal bar of wood or of glass instead of the iron, the lines of force 

 would pass through it practically as through the air and the needle 

 would show no appreciable change in deflection or development of 

 polarity in the wood or glass. Hence, though they are the seat of 

 magnetic action, yet as they only have the same action as air, or 

 as a vacuous space, we say that they have no susceptibility their 

 /c = 0. We shall see later that this is not strictly true, for K has 

 some small value for so-called non-magnetic substance, but so small 

 that we could not detect the poles developed in the experiment 

 which we are imagining. 



Susceptibility then depends on the excess of lines of force over 

 and above those passing through air. 



Permeability depends on the total number passing through, not 

 on the excess. Before the iron is inserted the air carries or conducts 

 lines of force, and its permeability is taken as the standard and as 

 having value unity. When the iron is inserted many more lines 

 of force pass through, and when the number is measured in a way 

 to be described later, the permeability, which is denoted by yu, is 

 measured by number of lines through iron/number of lines through 

 air, when the magnetising current is the same. The susceptibility 



