DIRECT-CURRENT DYNAMOS AND MOTORS: '25 
248. The proper core-section for a bipolar machine having been 
found to be 50 square inches, what is the average 
length of each magnet core, if the machine has a smooth 
armature? 
249. What would be the proper length of the magnet cores in 
the preceding example, if the machine has a toothed 
armature? 
250. Give the average length of the magnet cores for a large 
 8-pole smooth-armature machine, if it has been found 
that a total frame area of 1,400 square inches is re- 
quired. 
251. Name the conditions which determine the length of the 
; . air gap. 
252. Between what limits does the radial depth of the air gap 
usually range in machines with toothed armatures? 
253. State the average radial clearances allowed in very small 
and in very large smooth-armature machines, 
254. What should be the bore of the polepieces for a toothed 
armature, 414 inches in diameter? 
255. The height occupied by the winding of a smooth-core ring 
armature, 6 feet in diameter, is 4 inch; find the diame- 
ter of the bore of the polepieces, when allowing 4 inch 
for core insulation, binding wires, etc. 
256. A smooth drum armature, having a core diameter of 13} 
inches, is wound with 4 layers of No. 10 B. & 8S. wire; 
the core insulation consists of two layers of mica-cloth, 
.025 inch thick each, and the binding bands are made 
up of No. 20 B. & 8. phosphor-bronze wire and insu- 
lated from the winding by sheet mica, .012 inch thick; 
give the diameter to which the poles should be bored. 
‘25%. Determine the dimensions of the magnet cores for an in- 
- verted horseshoe-type dynamo similar to Fig. 25, Instr. 
Paper, having a smooth drum armature, 7 in. in diame- 
