MAGNETISM AND MAGNETIC CIRCUITS 13 



the watts loss if 1000 cubic centimeters- of this iron were subjected 

 to 25 cycles per second? (12 min.) 

 Note. 1 watt-second = 10 7 ergs. 



17. Plot the curves of permeability with gausses as abscissae 

 for sheet steel and for cast iron. Take the data from the tables 

 on page 105 and plot about 15 points on each. (20 min.) 



18. It is required to design a forged-steel horseshoe to raise 

 1000 pounds. The length of the iron path in the magnet is 50 cm. 

 and there will be two gaps of 0.1 mm. A density of 16,000 is to 

 be used in the steel, which on account of leakage will be reduced 

 to 15,000 in the gaps. On account of greater cross-section, neglect 

 the reluctance of the armature. Required the diameter of the 

 iron and the gilberts needed. If a current of 2 amperes is to be 

 used for excitation, how many turns will be required? (9 min.) 



B 2 A B Z A 



Note. The magnetic pull is - grams or . _ where 



4 7T X c/OJ. I i. I i 



the pull is in pounds, the density in kilogausses and the area 

 (total surface) in square centimeters. 



19. Two parallel surfaces 9X8 and 7 X 5 cm. respectively are 

 10 cm. (normal to surfaces) apart. Find the permeance of the 

 space between them. (1 min.) 



20. Required the permeance between two parallel surfaces 

 13.3 X 21.4 and 9.42 X 21.4 cm. respectively, and 6.31 cm. 

 (normal to surfaces) apart; also the flux if the winding consists 

 of 1270 turns carrying 1.32 amperes. (3 min.) 



21. Two surfaces 9 X 20 cm. lie in the same plane and have 

 their long edges parallel and 2 cm. apart. Required the per- 

 meance of the path between them. If a magnetomotive force, 

 produced by 1000 turns, carrying two amperes, acted between 

 these two surfaces, required the total flux between them. Assume 

 the lines of force to be semicircles. (3 min.) 



Note. P = -log e ^- 2 = 0.733 L logio^' where L = length of 



7T U\ Lf\ 



parallel edges and Z) 2 and A = the distances between the outer and 

 inner edges. (Found by integration of elementary leakage path. 

 See Standard Handbook, Sec. 5, 57.) 



22. Two surfaces 7.12 X ?2.5 cm. in the same plane, and 1.2 

 cm. apart with the long sides parallel. Assuming the lines of force 

 to be semicircles; required the permeance, (o min.) 



