IMPEDANCE. 53 



poles, to the position bringing the coils symmetrically under 

 the poles. 



Repeat these readings with several values of the current, 

 which should finally be increased to the full-load current of 

 the machine. 



Again take the same readings with the alternator fields 

 excited to their normal extent. A field regulator and 

 ammeter may be required in the exciting circuit for the 

 purpose of adjusting the excitation. They are not shown on 

 the diagram. 



Finally, measure the ohmic resistance of the armature with 

 direct current. 



The readings should be entered in the manner indicated 

 below, a separate table being taken for the experiments with 

 the field unexcited and excited. 



Determination of Armature Impedance. 



Alternator No Type 



Output .... amps volts .... frequency 



Normal excitation .... amps. 



For each set of readings corresponding to one particular 

 current, the mean impedance for the various positions of the 

 armature should be calculated and inserted in the table. 

 Strictly, the " square root of mean squares " value should be 

 taken as the effective impedance corresponding to any value of 

 the current, but the error introduced by taking the mean value 

 is very small, and the calculation is much simplified. 



Some difficulty may possibly be experienced in keeping the 

 armature steady in the desired position when the magnets 

 are excited, as it will tend to rotate into the position giving 

 the maximum value of the impedance. With a little care it 

 can usually be held steady with a wooden wedge under the 

 pulley or a similar device. 



