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DIRECT-CURRENT DYNAMOS AND MOTORS. 
58. Example.—Determine the approximate size and form 
of field magnet required for an armature, 12 inches 
in diameter and 103 inches long, to run at 1,200 
revolutions per minute, 
Solution.— With this size and speed, either the bipolar or 
the four-pole construction may be adopted. Let us con- 
sider both cases, taking the bipolar type first. 
We will assume that it was found in calculating this arma- 
ture that the flux required to cause the desired output is 
® = 5,000,000 lines. This, as can be found by means of 
formula (22), would make the field density about 
B, = 3 ar pe 30,500 lines per square inch, 
which is a good value for an armature of the given size. 
The usual leakage factor for this size of bipolar smooth- 
armature machine is about A = 1.40; consequently, the 
total flux in the field magnet will be 
d' = 1.4 X 5,000,000 = 7,000,000 lines. 
The cores in the present case will preferably be made of 
wrought tron, for which the average practical flux den- 
sity is Bm = 90,000;, hence, the required cross-section of 
the field magnet: 
® 7,000,000 
eo Se SAS ‘ : 
Bu 90,000 square inches 
Sn = 
If circular cores are used, their diameter should con- 
sequently be: 
4 
po Ba V/ Sr x a 777.8 X 1.273 = 10 inches. 
The approximate length of the magnets, in the case of an 
armature with a toothed core, according to Par. 56, is 
between ? and 1} times the diameter, if the latter is be- 
tween 5 and 20 inches. In the present example we will 
