DIRECT-CURRENT DYNAMOS AND MOTORS. 73 
take a multiplier somewhat smaller than the average of 
the figures given above, say .9, and we have: 
Lm = 10 X .9 = 9 inches. 
In case of a smooth armature, this length should be in- 
creased by 50 per cent., for the reason stated in Par. 56; 
consequently for the smooth-armature design: 
Im = 9 X 1.5 = 184 inches. 
52. Bipolar Design.—To lay off the complete magnetic cir- 
cuit, draw the pole faces AB and CD in Fig. 25, making 
each an arc of about 135°, corresponding to a polar em- 
Fic. 25.—MAGNETIC CIRCUIT OF BIPOLAR MACHINE, 
brace of 75 per cent. The distance at which these arcs 
are placed from the 12-inch circle representing the arma- 
ture depends upon whether the latter has a toothed or a 
smooth core. In the former case, the air gap HF’ should 
