382 ELEMENTS OF ELECTRICAL ENGINEERING. 



corresponding values in amperes of the current are : 65, 45, 

 - 25, o, 25, 45, 65, 75, 78, 75, 65, 45, 25, o, - 25, - 45, - 65, 

 75, 78, 75, and 65. Find the instantaneous values of 

 the power, plot the curves of electromotive force, of current 

 and of power, and find the average power. Ans. Average power 

 is 2138.6 watts. 



12. Plot the curves of instantaneous power in problems 4, 5, 6, 

 and 9, and find the average power in each case. Ans. Average 

 power is 1,000 watts in problem 4, zero in problems 5, 6 and 9. 



13. Plot the electromotive force curve of the alternator specified 

 in problem 8 on the assumption that it has 100 armature conduc- 

 tors spread out in one band 40 wide. 



Note. When a band of conductors is just beginning to enter the space under a 

 pole face the electromotive force is zero (fringes of field ignored), and the electro- 

 motive force rises at a constant rate until the whole band is under the pole face. 



CHAPTER II. ALTERNATING CURRENT MEASUREMENTS. 



14. The spring of a Siemens electrodynamometer is twisted 

 through an angle of 220 to balance the force action of a current 

 of 18.8 amperes. What current will require a twist of 165? 

 Ans. 16.3 amperes. 



15. Given a direct-reading ammeter (or voltmeter) of the direct- 

 current type of which the scale divisions are of equal length so 

 that an observer can read, say, to -^ of an ampere anywhere on 

 the scale, (a) Find the percentage error due to the error of read- 

 ing when the reading is I ampere ; (fr) find ditto when the read- 

 ing is 10 amperes. Ans. (a) 10 per cent. ; (U) I per cent. 



16. An electrodynamometer-ammeter (a Siemens instrument) 

 has a scale of equal parts. The position of the pointer can be 

 read to, say, i of a division anywhere on the scale, (a) Find how 

 many times as great the error in amperes (due to the given error 

 in the reading) is when the reading of the pointer is 25 divisions 

 than when the reading of the pointer is 250 divisions; (b) find 

 the percentage error in each case. Ans. (a) 3.16; (b) i per 

 cent., ^ per cent. 



