216 ALTERNATORS . 



amps., at 100 volts, when the voltage is regulated by the 

 field rheostat. The field was, of course, kept constant during 

 the experiment. 



The shape of the characteristic depends chiefly on the 

 amount of self-induction in the armature, since this is usually 

 the factor producing the greatest drop in volts. Armatures 

 having a high self-induction will give a characteristic 

 rapidly drooping, and in some cases reaching the horizontal 

 axis, with a current only slightly in excess of the rated 

 full-load current. The point at which the characteristic 

 meets the horizontal axis is the point of zero voltage, and 

 shows the current given by the machine when short-circuited. 

 Early machines made for supplying a number of arc lamps 

 in series at constant current were purposely designed with 

 a steeply falling characteristic, so that the change of current 

 was very slight when the voltage taken by the lamps varied 

 considerably. 



An instructive experiment in which a rapidly falling 

 characteristic may be obtained without the use of a specially- 

 designed alternator is to connect an inductive coil in series 

 with the armature of an alternator of any type available. 

 The alternator is then loaded, and the voltage 

 measured is that of both armature and coil in series. 

 The readings are plotted in the same way as in Fig. 

 100, and show practically the characteristic of an alternator 

 whose armature resistance and self-induction have been 

 increased by those of the coil. For purposes of calculation 

 and illustration of the behaviour of an arc-lamp alternator 

 such a curve is interesting. 



Further Consideration o( Characteristic of an Alternator. 



Drawing a horizontal line through the highest part of the 

 curve (see Fig. 100) the drop of voltage at any load is repre- 

 sented by the vertical distance between the curve and this 

 horizontal line. 



As already stated, the drop in voltage at full load is due 

 to several causes, and it is important to separate them as 

 far as possible. 



(1) Armature Resistance. The loss of voltage due to 

 armature resistance may be separated easily, as follows : 



While the machine is still hot from the run, send a direct 

 current through the armature and determine the resistance, 

 either by accurately measuring the current with an ammeter 



