DIRECT-CURRENT DYNAMOS AND MOTORS. 75 
when a flux density of 80,000 lines per square inch of 
armature cross-section is taken, and when 5 per cent. of 
the length is allowed for insulation between the armature 
laminz. With a smooth core this dimension is laid off 
at PQ, and in a toothed armature at RS. This fixes the 
internal diameter at 
12 — (2 X 34) = 12 — 64 = 5} inches, 
in the former, and 
52 inches minus twice the depth of the teeth 
in the latter case. 
It will thus be noted that the effect of putting slots in the 
armature core, or of varying their depth. is to correspond- 
ingly change the internal diameter of the armature. 
With a drum winding this makes no material difference 
in the design, but with a ring winding it is essential to 
provide sufficient space for the winding to pass through 
| the interior, and also to leave room for the shaft and 
» Spider which carry the armature. If the space left after 
deducting B, and twice the depth of the slots from the 
external diameter is found too small for the above pur- 
poses, an increase in the internal diameter is necessary 
which would raise the density of the lines in the arma- 
ture if the design were left unaltered. ‘To avoid an ex- 
cessive flux density from this cause, the external diame- 
ter of the armature must be correspondingly enlarged, 
with the consequence of increasing the distance BD be- 
tween the field cores, and of lowering the flux density in 
the air gap. 
60. Four-Pole Design.—If the armature given in Par. 58 
is to run in a four-pole field, the armature flux would be 
the same as before, if the flux density in the air gaps and 
the polar embrace were the same. In multipolar ma- 
chines the former is usually somewhat greater, and the 
latter a little less, than in bipolar machines, hence we will 
assume that the armature flux is 5,000,000 lines, as in 
