PROBLEMS. 499 



the magnetic circuit formed by the core is 100 centimeters. How 

 much current must be passed through a winding of 500 turns of 

 wire to produce a magnetic flux of 1,767,000 lines through the 

 core. Ans. 3.88 amperes. 



Note. The student should plot carefully the <$ and eft" curves for wrought iron, 

 cast iron, and soft steel from the tables given in Art. 12, Appendix A, and use these 

 curves in finding corresponding values of e3 and &C in this and the following problems. 



191. How much flux will be produced through the wrought 

 iron core specified in problem 190 by a current of 8.2 amperes 

 through the 500 turns of wire? Ans. 1,920,000 lines. 



192. The magnetic circuit of a dynamo consists of wrought 

 iron (in armature core and in field cores), cast iron (in field yoke 

 and pole pieces), and air gap. The wrought iron portion is 50 

 centimeters long and 1 20 square centimeters in sectional area, the 

 cast iron portion is 40 centimeters long and 220 square centi- 

 meters in sectional area, and the air portion is 2.5 centimeters 

 long and 300 square centimeters in sectional area. How many 

 ampere-turns are required to force 1,600,000 lines of flux through 

 this circuit, ignoring magnetic leakage? Ans. 12,100 ampere- 

 turns. 



Note. On account of magnetic leakage the magnetic flux through field yoke, field 

 cores and pole pieces is larger, sometimes very much larger, than the flux which 

 flows across the air gaps and through the armature core. Therefore a slightly greater 

 magnetomotive force is required to overcome the magnetic reluctance of field yoke, field 

 cores and pole pieces than would be required if the magnetic leakage were zero. 

 Magnetic leakage is never really negligible, but it can be satisfactorily determined only 

 by tests on a finished machine. 



193. How much magnetic flux would be forced through the 

 magnetic circuit specified in problem 192 by a magnetomotive 

 force of 13,500 ampere-turns. Ans. 1,723,000 lines. 



Note. This problem is to be solved by calculating a series of values of ampere- 

 turns for various assigned values of flux. These results are to be plotted. The flux 

 corresponding to 13,500 ampere-turns may then be taken from this curve. 



194. A straight wrought iron rod 2 inches in diameter and 24 

 inches long is wound with 1,000 turns of wire. Calculate ap- 

 proximately the current required to force 203,100 lines of flux 

 through the rod and back through the air. Ans. 10. i amperes. 



