70 



ELECTRICAL ENGINEERING 



If a pull P is exerted on the armature and it is moved through 

 a distance dx, the work done is P dx and this is equal to the in- 

 crease in the energy stored in the field; 

 therefore, 



and 



p = 



STT ' ' 



(105) 



this is the pull of the magnet on the 

 armature. 



The pull per square centimeter is 



FIG. 57. Pull of a magnet. 



and this is the tension along the lines 

 of induction in the field and is also 

 equal in magnitude to the pressure at 

 right angles to the lines. 



49. Permeability Curves. If an 

 iron ring, Fig. 56, which has been 

 completely demagnetized, is gradually 

 magnetized, by increasing the current in the exciting coil from zero, 

 the magnetic induction in the ring increases with the magnetizing 

 force as shown in curve 1, Fig. 58. With feeble magnetizing 



Curve 2 



Magnetizing Force X Induction Density 



FIG. 58. Permeability curves. 



forces the gradient of the curve is small and the permeability is low; 

 as the force increases the curve becomes very steep and nearly 

 straight and the permeability increases rapidly and becomes very 

 large; as the force is further increased the curve rounds off and 



