474 ALTERNATING CURRENTS 



of 0.707, lagging. How much energy and how much quadrature current 

 does this load take? If this load is supplied over a line each wire of which 

 has one ohm resistance, what is (a) The total line loss? (b) The line loss 

 due to the energy current? (c) The line loss due to the quadrature current? 



47. In problem 46 how large a condenser would be necessary in order to 

 make the power-factor of the load unity? What is the line loss under these 

 conditions? 



48. A certain transmission line delivers 30 kw. at 2,300 volts and the 

 power-factor of the load is 0.80. The line loss is 5 kw. (a) What current 

 flows over the line? (6) How much is quadrature current? (c) How much 

 is energy current? (d) What is the line loss due to energy current and 

 what is that due to quadrature current? 



QUESTIONS ON CHAPTER HI 



1. Describe the principle of the Siemens Electro-dynamometer. How 

 are its coils connected and what is the relation existing between the turning 

 moment and the current? What are the disadvantages of this type of 

 instrument? 



2. In what way is the indicating electro-dynamometer similar to the 

 Siemens dynamometer? In what ways do the two instruments differ? 



3. Explain how the electro-dynamometer principle may be applied to a 

 voltmeter. What is the general character of the scale divisions? Compare 

 its current with that taken by a direct-current instrument of the same range. 

 Discuss the accuracy of such an instrument when used with direct current. 



4. Describe the. inclined-coil voltmeter, giving the principle upon which 

 it operates. 



5. What difficulty arises when attempt is made to apply the dyna- 

 mometer principle to the alternating-current ammeter? 



6. Describe the construction of a wattmeter and give the principle of 

 its operation. Show how it is connected to a circuit. Give the best method 

 of connecting the potential-circuit, especially when the instrument is used 

 in connection with considerable voltage. 



7. Show two possible methods of connecting a wattmeter in circuit. 

 Discuss the corrections that should be made in each case, if the exact value 

 of the power is desired. What compensation for these errors can be made 

 in the construction of the instrument? 



8. What precautions should be taken against over-loading a wattmeter? 

 How are wattmeters rated and why? 



9. Give the advantages of a polyphase wattmeter over single-pi iasc 

 instruments. How is the polyphase wattmeter constructed? 



10. How are wattmeters calibrated? Give a diagram of connections. 



11. Describe how the Weston iron-vane type of voltmeter utilizes the 

 principle of magnetized iron. Upon what fundamental electrical principle 

 does this instrument operate? How is the instrument damped? 



12. Show how the iron-vane principle has been adapted to an inclined- 

 coil instrument. What two methods are used to damp this instrument? 



