QTESTIONS AND PROBLEMS 475 



13. What change should be made in the construction of the above two 

 types of iron-vane voltmeters in order that they may be used as ammeters? 

 What are the limitations of iron-vane instruments for direct-current 

 measurements? 



14. Discuss the use of direct-current watthour meters upon alternating- 

 current circuits. 



15. Describe the construction of the induction watthour meter. What 

 should be the phase relation existing between the potential-coil flux and the 

 circuit voltage? How is this phase relation obtained? 



16. How is friction compensation effected? Discuss this principle very 

 carefully. 



17. Show by simple sketches how the driving torque is developed. Why 

 does the disc tend to rotate in the direction of the sliding field? 



18. How is the induction watthour meter calibrated? What adjustments, 

 not used for the direct-current type, are necessary? What are the ad- 

 vantages of this type of meter over the direct-current type? 



19. Describe one common type of frequency meter. Upon what principle 

 does it operate? Why are the vibrating reeds kept polarized? 



20. Describe the Tuma phase meter. How is this instrument adapted 

 for power-factor measurements? What control is exerted on the moving 

 system? Why are the coils of the moving system not exactly 90 apart? 

 What modifications of the instrument are necessary in order that it may be 

 used on three-phase circuits? 



21. For what purposes are synchroscopes used? Describe the con 

 tion of some one type. In what way is it related to the phase meter? 



22. What are the commercial uses of the oscillograph. What is its prin- 

 ciple of operation'.' In what way does the moving element differ from that 

 of the ordinary galvanometer? How are the time abscissas obtained? 

 Why is it desirable to immerse the moving element in oil? 



23. Sketch the general arrangements of the laboratory type Riving the 

 relative positions of the lamp, prisms, vibrators, lenses, rotating mirrors, 

 film drum, etc. 



24. Make a diagram of connections showing how the voltage vibrator 

 and the current vil.rator are connected in circuit. 



QUESTIONS ON CHAPTER IV 



1. < live three reasons why three-phase power supply is superior 

 phase supply. 



2. Why is it desirable at times to use Hyinl-oh. notation in the s>hr 

 problems'.' \vh\- i< this system particularlv applicable to polyphase 

 syster 



'e briefly the principles upon which one such system is baaed. 



3. De^rr M en tary three-phase gri. > exist 

 nmoriK the three voltages of such a generator ' II. .w t nay three independent 

 si nd* 1 - phase systems be obtained from such a generator? 



