APPENDIX C. 



PROBLEMS. 



CHAPTER I. THE ALTERNATOR. 



1. A rheostat of 6 ohms resistance, in a circuit in which a steady 

 current of 10 amperes is flowing, is short-circuited 25 times per 

 second, each time for i/ioo second. What is the equivalent 

 constant resistance of the rheostat ? Ans. 4. 5 ohms. 



Note. It is important that the physical argument of this problem be given by the 

 student. 



2. (a) At what speed must a 4O-pole alternator like Figs. 10 and 

 II be run to give a frequency of 25 cycles per second? (fr) At 

 what speed would the field magnet shown in Fig. 1 2 have to be 

 driven in a suitable stationary armature to give a frequency of 25 

 cycles per second? Ans. (a) 75 revolutions per minute, (fr) 500 

 revolutions per minute. 



3. An alternator is run at a speed of 3,000 revolutions per 

 minute, (a) How many poles must its field magnet have to give 

 a frequency of 10,000 cycles per second? (b) If the machine is 

 constructed according to the design shown in Fig. 13, how many 

 polar projections will be required on each end of the inductor ? 

 Ans. (a) 400 poles, ($) 200 poles. 



Note. See a description of a 10,000 cycles per second alternator by B. G. Lamme, 

 Trans. Am. Inst. Elec., Vol. 23, pages 417 to 428, 1904. 



4. An electromotive force of 100 volts acts on a circuit con- 

 taining resistance only, and the electromotive force is reversed 

 every o.oi second. The resistance of the circuit is 10 ohms. 

 Plot the electromotive force curve and plot the current curve ; 

 and find the average current during half a cycle and the effective 

 current. Ans. Average current 10 amperes, effective current 10 

 amperes. 



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