$2 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
magnetomotive force passing through the field 
system ; 
r,1, magnetic resistance of the field leakage path; 
F}, flux or number of lines carried by the field leakage 
path; 
r,, magnetic resistance of the air gap; 
F,, flux in the air gap; 
r,, magnetic resistance of the armature system consisting 
of the teeth and' of the core above the toothed part; 
F,, fiux flowing through the armature system; 
r,1, magnetic resistance of the armature leakage path; and 
F, flux in the armature leakage path. 
439. Magnetomotive Force.—The magnetomotive force, 
which may be expressed in ampere turns, is analogous to 
the E.M.F. of an electric circuit; the flux given in 10° © 
lines 3s identical to the electric current, and the reluctance 
is the same for the magnetic circuit as resistance is for the 
electric circuit. 
The symbolical expression of the reluctance is 
on Spas eee (18) 
qx 
where 1 = the length of a magnetic path of a section equal 
to g square inches, and “ — the permeability of the re- 
sistance material. 
The latter symbol / in the above magnetic formula (18), 
is analogous to the specific conductivity of an electrie con- 
ductor. 
It is obvious from Fig. 41, that the leakage lines passing 
through, the dotted paths are entirely. lost, from a useful 
point of view, and in consequence of this, the designer 
must make the resistance of the leakage path high and that 
of the useful circuit comparatively low. 
This qualitative conclusion indicates, that in an electrically 
favorable design, the length of the useful magnetic circuits 
should be as small as possible, the sectional area-ample, and 
