TREATISE ON ALTERNATING CURRENTS. 



DISTINCTION BETWEEN INDUCTION AND MAGNETIZING FORCE. 



5. When iron is placed in a magnetic field it becomes 

 magnetized. When soft iron is freshly annealed its magnetism 

 disappears if on being removed from the magnetizing field of force 

 it is subjected to mechanical vibration. 



Iron possesses the property of causing the lines of force to pass 

 through it in preference to the surrounding air or whatever the 

 surrounding medium may be. 



That which magnetizes the iron or the Magnetizing* 

 Force is the strength of the magnetic field in which it is placed ; 

 but the number of lines passing normally through each square cen- 

 timetre of the iron, or the Induction in the iron, is many times 

 greater than the strength of the field in which the iron is placed. 



If we represent the field strength by H and induction by B, 

 then H and B are identical when the medium in which B is 

 measured is non-magnetic. When, however, B is measured in a 

 magnetic medium such as iron, we have the relation B = ^H. 



ju is a variable quantity depending upon the value of B, and 

 is called the Permeability of the iron, and for moderate values 

 of B is very large; e.g. in annealed wrought iron when H = 1/6 

 units, B = 5000 (about), so that /LL = 3000. The following table 

 gives the values of B and /m for different values of H for Swedish 

 iron and grey cast iron. 



TABLE I. 



