16 ELECTRICAL MACHINERY 



through the armature core, through another air gap and 

 another pole, then through the yoke and so back to the 

 original pole. This complete path is called the magnetic 

 circuit of the machine. 



Magnetic Field due to Current in a Coil. A coil of 

 wire carrying a current generates what is called a magneto- 

 motive force and this m.m.f. will produce a magnetic field 

 through and around the coil. There is a definite relation 

 between the total number of magnetic lines generated by 

 the coil, the current and number of turns in the coil, and 

 the constants of the magnetic circuit. This law of the 

 magnetic circuit may be put in the form of an equation 



where 3> = the total lines of force (or simply total flux) 



through the magnetic circuit; 



N = number of turns of wire in the magnetizing, coil; 

 7 = current flowing in coil in amperes; 

 R = the reluctance of magnetic circuit. 



Reluctance. The dimensions of the magnetic circuit 

 may generally be determined fairly well and it is found 

 that if the whole circuit is made of the same material (as 

 for example of iron) we may write 



(8) 



where I the mean length of magnetic circuit in cms. ; 



A=the average cross-sectional area of circuit in 



sq.cms.; 



ti = a constant, the value of which depends upon 

 the material comprising the magnetic circuit. 



[A is called the permeability of the substance and is equal 

 to one for all ordinary non-magnetic substances as air, 



