CHAPTER VI 

 MAGNET WINDINGS AND MAGNETS 



1. Given the following data: resistivity 12; ampere-turns 4000; 

 mean length of turn 18 inches; e.m.f. 50 volts. What area of win- 

 should be used and what B. & S. number will answer? (2 win.) 



Note. The area is obtained from the formula for drop in 

 terms of number and mean length of turns, resistivity, current 

 and area. 



2. A magnet requiring 20,000 ampere-turns is used on a 500- 

 volt circuit; the mean length of a turn is 2 feet; the resistivity is 

 12; the circular mils per ampere 1000. Required the area and 

 the B. & S. number of the wire and the number of turns, using the 

 B. & S. size, and remembering that the larger wire will mean more 

 ampere-turns. (5 raw.) 



3. Required the diameter in centimeters of a horseshoe magnet 

 to raise 4000 pounds with B equal to 16,000. (4 min.) 



4. Given a coil of wire 5J inches inside and 6J outside diameter. 

 13,400 ampere-turns are necessary; resistivity is 12.5; e.m.f. is 

 125; find the area of wire necessary and the B. & S. number. 

 (3 min.) 



5. A magnet requires 10,000 ampere-turns for its excitation. 

 If the mean length of a turn be 15 inches, and it be run on a 110- 

 volt circuit, required the area and the B. & S. number of the wire, 

 resistivity to be taken as 12. (3 min.} 



6. What must be the cross-sectional area in sq. cm. of a bar 

 used in a horseshoe magnet to raise 500 pounds, taking B as 

 10,000? (2 min.) 



7. A magnet with a round core 5 inches in diameter has 560 

 turns of No. 15 wire wound 1 inch deep. How many feet more of 

 wire would be necessary if the core, having the same area, were 

 rectangular with one side double the other. What per cent must 

 the area of the wire be increased if the work of excitation is to be 

 the same? (6 min.) 



8. A magnetic circuit consists of 100 cm. of forged steel and 

 | cm. of air, with B equal to 16,000. Required the ampere-turns 



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