33 



the terminal electromotive force is Oa, 

 all three being positive, that is, in the 

 direction of the current, whilst the elec- 

 tromotive force of self-induction is 

 negative, and is given by the length Od. 

 As the load decreases, the circle O D 

 becomes smaller, until, when no current 

 is allowed to flow, it has shurnk into a 

 point, whilst there is no loss due to re- 

 sistance, and the points B and C coincide 

 with A. In this case the terminal and 

 induced electromotive forces are identical, 

 and the strength of the field is a mini- 

 mum and proportional to the length O A. 

 At full output the strength of the field 

 is a maximum and proportional to the 

 length O C. Having thus found the 

 limits of the strength of field required in 

 these two cases, it is an easy matter to 

 determine, by the usual formulas con- 

 necting strength of field and exciting 

 power, the strength of the exciting cur- 

 rent for each. It will be immediately 

 apparent from the diagram, that of two 

 machines, the one having a high and the 

 other a low plant-efficiency, the latter 



