3-B] 



PREDETERMINATION. 81 



in solving alternating current problems ; it is a practical applica- 

 tion* of the elementary principles discussed in detail in Exps. 

 4-A and 4-B. The electromotive force method is general, apply- 

 ing to all classes of alternating current problems, transmission 

 lines ( 56), transformers 

 (Exp. 5-C), etc. For this 

 reason the method will be 

 treated in considerable de- 

 tail. 



17. Unity Power Fac- 

 tor. With a non-inductive 



KH 



load, the power factor of 



, . ., FIG. 3. Electromotive force diagram, at 



the load is unity; the cur- ' 



unity power factor; current m phase with 

 rent which flows is, accord- terminal voltage. 



ingly, in phase with the ter- 

 minal voltage. This is shown in Fig. 3, in which the terminal 

 voltage ET, is in phase with the current /. The armature resis- 

 tance drop, E* = RI f is in the direction of in phase with the 

 current / ; the reactance drop, Ex = XI, is in quadrature with 7. 

 The total generated electromotive force EQ, is accordingly the 

 vector sum of the following three electromotive forces: ET de- 

 livered to the load ; RI to overcomef armature resistance and XI 

 to overcome armature reactance. 



* ( i6a). This application illustrates the way that general principles 

 can be put to practical purposes; the application was first made indepen- 

 dently, and more or less simultaneously, by various engineers. The 

 writer used the method in numerical problems to illustrate the elementary 

 principles of Bedell and Crehore's Alternating Currents in the early nine- 

 ties soon after the issue of that book, and applied it a little later to 

 laboratory data. The data and some of the curves here given are taken 

 from a laboratory outline prepared by the writer for student use and 

 printed in the Sibley Journal, 1897-8, p. 215. 



f (173). The arrows show the direction of the vectors in the sense 

 that BC and CA are electromotive forces to overcome resistance and 

 reactance, respectively; in the reverse sense, CB and AC are the electro- 

 motive forces produced by resistance and reactance. 



7 



