MAGNETIC METHODS 75 



The unit cross-sections in both cases are taken at right angles to the 

 Hnes of force. These relationships are shown in Figure 12. 



(a) (b) 



MAGNETIC BODY 



Fig. 12. — Illustration of magnetic permeability, showing lines of force in a 

 magnetic field (a) in air, and (b) with a magnetic body in the field. 



A magnetic body, such as iron, can be thought of as offering less resis- 

 tance to the passage of lines of magnetic force than air, as depicted in 

 Figure 12. The lines of force will therefore tend to concentrate in the 

 medium of higher magnetic permeability, in order to take the path of 

 least resistance. 



Where a magnetic field is present in air, the flux per unit area 



(^-) 



and the strength of the field H are equal. Since ix = -^ , fx for air = 1. 



If the permeability of a material is less than that of air or its /a<1, the 

 material is diamagnetic. Where a substance shows a permeability greater 

 than air, viz., its ;u.>l, it is paramagnetic, and when /x>>l, it is termed 

 ferromagnetic. 



From Equation 6 it follows that : 



B = [iH (7) 



or flux density of a material in a magnetic field is its permeability times 

 the field strength. 



It also follows that : 



H = ^ (7a) 



The magnetic field equals the flux density it establishes in a body 

 divided by its permeability. 



The quantity l/fi is the constant appearing in Equations 1 and 2. 



Intensity of magnetization. — The intensity of magnetization of a 

 magnet is a measure of its strength. More specifically, the intensity of 

 magnetization / is the amount of magnetization possessed by a magnet. 



