DIRECT-CURRENT DYNAMOS AND MOTORS. - 85 
ampere-turns, or those within the demagnetizing belt; 
see Par. 61, Book 15. This demagnetizing number of 
ampere-turns is equal to 54; of the total armature turns 
multiplied by twice the angle of the brush lead, or: 
NxC * eae, 
_—— eee - Lae 
at = on, Tao vtec (44) 
in which N' = total number of turns on armature; 
C =current flowing in armature; 
2n, = number of parallel armature circuits; 
a = angle of field distortion, or angle of brush 
lead. 
_ The number of armature turns, N’, is equal to the number 
of armature conductors, NV, in case of ring-wound arma- 
tures, and equal to one half the number of conductors in 
case of drum-wound armatures. 
The angle of field distortion in a smooth-core armature is 
approximately equal to half the angle between two adja- 
cent pole corners. In toothed and perforated machines, 
the weakening effect of the armature magnetomotive force 
is checked by the presence of iron surrounding the con- 
ductors, this checking influence being the stronger the 
greater the ratio of tooth section to field density, that is, 
the smaller the tooth density. The angle of lead in 
toothed armatures is from .3 to .9 times the half-pole- 
angle, and for perforated machines from .2 to .8 times 
that angle, according to the flux density in the teeth. 
For the tooth densities usually employed in practical 
machines, the above formula (44) should be multiplied 
by 4 or % for toothed armatures, and by 4 or } for per- 
forated armatures. : 
The indirect influence of the armature field, due to the 
greater magnetizing force required on account of the 
increased flux density in the armature teeth and pole- 
pieces, depends upon the material of, and the average 
density in, the polepieces. The number of ampere-turns 
required to compensate for the indirect effect is usually 
