80 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
be best understood if the machine is looked upon simply 
as a circuit of magnetic lines. 
137. Magnetic Circuits.—In a generator there are as many 
closed magnetic circuits as there are number of poles. For 
instance, Fig. 40 indicates the six closed circuits of a 6- 
pole revolving field type alternator, and as all these circuits 
are alike, it is sufficient to deal with only one of them. 
Such a magnetic circuit consists of the useful and leakage 
paths, the former being represented by the lines through 
Fie. 40.—MAGNETIC CIRCUIT OF A 6-POoLE GENERATOR. 
the wheel rim, pole core, pole shoes, air gap, slotted part of 
the armature and armature core, which return to the 
starting points by passing through the different parts in 
the reverse order. 
The leakage paths are located in the space which surrounds 
the machine body, due to magnetic lines attempting to 
escape through the air, by passing from one part of the 
useful magnetic circuit to another of lower magnetic 
density. 
The magnetic circuits which offer a certain resistance to the 
flow of the lines of magnetomotive force, have in their 
