DIRECT-CURRENT DYNAMOS AND MOTORS. 27 
A66. 
26%. 
268, 
269. 
270. 
271. 
272. 
273. 
(274, 
275. 
276. 
the flux required is 20,000,000 lines, and the factor of 
leakage 1.28. 
Find the dimensions of the magnet frame of a 4-pole 
machine for which the following data are given: Ar- 
mature, 40 inches in diameter, 25 inches long, having 
84 slots, 13 inch wide and 1} inches deep; useful flux, 
38,000,000 lines; factor of leakage, 1.3; magnet frame 
to be entirely of cast iron, cores to have rectangular 
cross-section ; width of magnet cores and yoke to be 
equal to length of armature core. 
Note:—To find the length of rectangular cores, determine 
the diameter of the equivalent circular cross-section, 
and apply the rules given in Par. 56, Instruction 
Book, 
Give sketch, drawn to scale, showing the complete mag-- 
netic circuit of the machine considered in the preceding 
example. 
Name the advantages of circular magnet cores over rect- 
angular and elliptical magnets. 
In what type of machines is the employment of rectangu- 
lar cores justified, and why? 
State the only exceptional case in which a practical dy- 
namo designer would employ curved magnet cores. 
Give the formula for the total magnetizing force of > 
dynamo, and name the symbols employed. 
State the fundamental law of the magnetic circuit. 
Define permeance. 
How is the permeance of a magnetic path expressed? 
Give the formula for the magnetomotive force, in gilbents;. 
required to produce a certain density in an inom pati» of 
given length. 
Express the ampere-turns required for an_ iron path, if 
metric units are employed. 
