34 6 PROBLEMS. [CHAP. 



6. Determine the relation between the total flux set up by the field 

 windings of a dynamo-electric machine and the useful flux that passes 

 through the armature. The ratio of the former to the latter is the 

 dispersion, or leakage, coefficient. 



7. Given a separately excited D.C. motor the armature of which is 

 supplied with current from a series generator. Investigate and explain 

 the conditions affecting the direction of rotation of the generator and 

 the conditions under which the direction will periodically reverse. The 

 motor should have brushes which will not damage the commutator 

 when the direction of rotation is reversed. 



8. In a separately excited motor in which the armature resistance 

 drop is so large that the counter-electromotive force is practically 

 negligible (as in certain watthour meters), determine the relation 

 between speed and field excitation when line voltage is constant. 



9. Design, have made, and test commutating interpoles for some 

 machine which commutates badly. 



10. Find the relation of potential drop to current density between 

 brushes of various materials and slip rings at usual speeds; also at 

 very high speeds. 



ii. Analyze all the losses in a given machine or apparatus. 



12. Make a study of the temperature rise in a machine or appa- 

 ratus by thermometer and by resistance measurements. 



13. Given a differential D.C. motor which runs too fast at full 

 load. Determine a shunt to go in parallel with the series coil that will 

 give a certain speed at no load and full load. Determine whether the 

 same shunt will do for a different speed, and report as to why it will 

 or will not. 



14. Take some point concerning which you find your knowledge 

 inadequate, on some subject you have already studied, and if possible 

 plan an experiment to settle the matter to your own satisfaction. 



15. Take a technical article which proves of interest (as the paper 

 or papers of some A. I. E. E. meeting) and investigate such points as 

 you can in the laboratory. The Digest of the Electrical World and the 

 Question Box of the Electric Journal, can be used to advantage as a 

 source of timely practical problems. 



16. Given a patent specification and claims. Investigate the inven- 

 tion by experiment and study, and report on one or more of the fol- 

 lowing: (i) Its usefulness (from the standpoint of a possible user or 



