ALTERNATORS. 217 



and the voltage drop with a milli-voltmeter, or by comparing 

 the drop with that in a standard resistance in the same circuit 

 by means of a galvanometer. 



Calculate the ohmic drop ( = armature current x armature 

 resistance) at full load. On the ordinate corresponding to 

 this load set up this voltage drop vertically and draw a 

 straight line through this point and zero. This line then 

 gives the armature ohmic drop at all loads. 



In the case illustrated in Fig. 100 the armature resistance 

 was found to be -14 ohm. At 40 amps, this would corre- 

 spond to a drop of 40 x -14 = 5-6 volts. 



The sloping line drawn at the bottom of the figure shows 

 the ohmic drop for all loads. 



The approximate method of determining the self-induction 

 which is usually more convenient to employ, is described on 

 page 204. The curve, Fig. 100, obtained in the present experi- 

 ment may be employed in the same manner, and will then 

 give the impedance and self-induction of the armature at 

 various loads. 



The curves, Figs. 94 and 100, apply to the same machine, 

 so that they may be compared. From the two curves taken 

 together, the armature impedance at various excitations 

 and loads may be calculated. The exactness of the results, 

 it must be remembered, depends on the possibility of 

 assuming the effects of armature magnetic reactions and 

 eddy current screening to be unimportant. 



(2) Armature Self-induction. The best method of deter- 

 mining the armature drop due to self-induction at various 

 loads has been explained in Experiment VII., page 52. 



From the readings taken in the manner then described, 

 it is easy to separate the loss of voltage due to induction 

 from the drop due to armature resistance, by the formula 

 e> = Cs 2 + g r 2 



Where e= total armature drop obtained in Experiment VII. 

 e s voltage drop due to self-induction. 

 e r = voltage drop due to resistance. 



It must be remembered that the loss of voltage due to 

 armature resistance and armature self-induction cannot be 

 simply added together on the curve to give the total voltage 

 lost due to both causes, since these two voltages differ by 

 90 in phase. 



