INDUCTANCE AND INDUCED EM I 49 



17. An air magnetic circuit has 753 turns and is threaded by 

 646,000 maxwells when the exciting current is 8.4 amperes. If in 

 0.23 second the current is increased from this value to 97.3 am- 

 peres, what inductive e.m.f. will result? (4 min.) 



18. A dynamo field has 7320 turns, is 9i inches in dian 

 and has a density of 16 kilogausses. If the exciting current is 

 broken in 0.13 second and the density fall to a residual of 900 

 gausses, to what value will the e.m.f. at the terminals of th fi Id 

 circuit rise? Also if 2.6 seconds be allowed for the breaking of 

 the field circuit? (6 min.) 



19. A wrought-iron ring, with 2340 turns, an area of 0.98 sq. 

 cm. and a cut (air gap) across it of } inch, has 14.0 kilogausses for 

 a current of 4.72 amperes, what is the average inductance? (S >< 



20. In problem 19 what would be the inductance if the cut 

 were increased to \ inch by forcing the ends apart? Also if the 

 ends were forced together so as to make the air gap negligible? 

 (6 min.) 



21. A wrought-iron ring, 1.13 feet in inside diameter and 0.872 

 inch diameter of iron, has one layer of No. 30 double-cotton-covered 

 magnet wire. If current enough is used to give a density of 8000 

 gausses, what will be the number of turns, the current and the 

 average inductance? See table of magnet wire on page 104. 

 (5 min.) 



22. If in problem 21 the density is doubled, what will the 

 current be and what will the inductance become? (2 min.) 



23. A ring of cast iron 20 inches in mean diameter and made of 

 2-inch round iron is wound with 3250 turns. Find the mean value 

 of L for 0.2, 0.5, 1 and 2 amperes and plot the curve of inductance 

 with current. (15 min.) 



24. Required the inductance of a winding of one layer of 

 No. 18 double-cotton-covered magnet wire upon a wooden ring, 

 7.5 inches mean diameter and 0.763 inch diameter of cross-section. 

 If the resistance be 0.375 ohm, what is the time constant? (6 



min.) 



25. A certain 100-kv-a. transformer for 5000 to 500-volt trans- 

 formation has a maximum flux of 2.0 X 10 6 with a cross-section of 

 300 sq. cm. The secondary turns are 110 and the primary 1100, 

 the effective exciting current is 0.333 ampere. Required the 

 average inductance of each winding between maximum flux and 

 zero flux. (1 min.) 



