72 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
Fig. 5, gives the average values for fully loaded machines. 
For an intermediate load the drop can be approximately 
assumed from the ratio of the load currents; that is, if 
on full non-inductive load the regulation is 7% (curve B) 
then on half load it will be 3.5%. 
Par. 130 explains the method of determining the drop 
exactly. 
125. Determination of Voltage Drop.—tThe next step is 
to determine the voltage drop on an inductive load with 
the power factor cos PY = 0, and an explanation of two 
experimental methods will greatly assist in understanding 
which elements affect the inherent regulation. 
126. Prof. Arnold’s Method.—The older method of Prof. 
Arnold deserves attention on account of its great sim- 
plicity, although the results obtained are far from being 
exact; however, it provides a basis for a reliable, corrected 
method. 
The rule of the first method is expressed in the following 
equation, 
where .1,, is the value of the field ampere turns which 
corresponds to the open circuit voltage; 
A,, the value of the field ampere turns which is 
required to generate an open circuit voltage 
equal to that at the terminals, when the 
armature carries a given current with a 
power factor cos Pp = 0; 
and A,,,the field ampere turns which will produce the 
specified current in the short cireuited 
armature. . 
127. Application of Prof. Arnold’s Method.—If the 
normal short circuit current, which lags 90 degrees 
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