4-B] CIRCLE DIAGRAM. 123 



EXPERIMENT 4-B. Circle Diagram for a Circuit with Resist- 

 ance and Reactance. 



I. Introductory. If a circuit with resistance and reactance 

 is supplied with a constant impressed electromotive force, the 

 current will have a certain value and a certain phase position 

 with reference to the electromotive force, as discussed in 

 Exp. 4-A. 



These values of current and phase angle will be changed, if 

 either the resistance or the reactance of the circuit is changed. 



2. In a circuit in which the reactance X is constant, and the 

 resistance R is varied, the value of / and 6 will increase when R 

 is decreased ; as resistance is cut out of circuit, the current will, 

 accordingly, not only be larger but will be more out of phase with 

 reference to the electromotive force. In the limiting cases: when 

 R = o, the current is E^-X and (in the case of inductive react- 

 ance) lags 90 behind the electromotive force; when R=oc, 

 /==o, and = o. 



3. The object of this experiment is to show the change of cur- 

 rent in magnitude and phase, in a circuit with constant inductive* 

 reactance, when the resistance is varied and the impressed elec- 

 tromotive force is maintained constant. It will be found that the 

 locust of the current vector is the arc of a semicircle, as in 

 Fig. 2 ; this is true of any constant potential circuit, in which the 

 reactance is constant and the power consumption is variable as 

 in a transformer (Exp. 5~C) or in an induction motor. 



* (3a). A similar experiment may be performed with capacity react- 

 ance ; see Appendix I., Exp. 4-A. 



A converse experiment may also be made with constant resistance and 

 variable reactance, in which case the diameter of the semi-circle locus is 

 in the direction of E, instead of at right angles to it ; see reference, 3b. 



t(3b). Established by Bedell and Crehore, Alternating Currents, pp. 

 223 and 275. 



