8 DIRECT CURRENT GENERATORS. [Exp. 



If the speed varied during the run, the external characteristic 

 should be corrected* for speed as before (9). 



The watts output, for any point on the external characteristic 

 is given by the product of current and voltage, and may be 

 plotted as a curve. 



12. If the field coil is connected so that the current from 

 the armature flows through it in the wrong direction, so as to 

 demagnetize instead of building up the residual magnetism, the 

 machine will not " pick up." For each direction of rotation, the 

 proper connection of the field will be found to be independent of 

 the direction of the residual magnetism. Note the effect of pre- 

 vious magnetization (from an outside source) first in one and 

 then -in the other direction, arid the effect in each case of revers- 

 ing the field connections. 



13. Total Series Characteristic. The total characteristic is 

 derived from the series characteristic, so as to show the total 

 generated electromotive force instead of the terminal brush 

 voltage. 



Resistance Data. The only additional data needed are the volt- 

 age drops through the field and armature for different currents; 

 this is plotted as a curve (Fig. 3) which is practically a straight! 

 line. With the armature stationary, current from an outside 

 source is passed through the field or armature (separately) ; the 

 current is measured and the difference of potential at the termi- 

 nals. The ratio E -z- I gives the resistance. This is called meas- 



*(iib). This correction is applied to the external characteristic and 

 not to the total characteristic for convenience. Inasmuch as it is the 

 generated electromotive force which is proportional to speed, to be accurate 

 the correction should be applied to the total and not to the external char- 

 acteristic. 



t ( I3a). This would be a straight line if the resistance were constant. 

 The resistance varies with temperature; see Appendix, 15. The armature 

 resistance also varies with current since it includes the resistance of 

 brushes and brush contact, which depends upon current density. The hot 

 resistance is to be measured after the machine has run awhile, and is to 

 be considered constant. 



