MAGNET WINDINGS AND MAGNETS 17 



the shunt turn is 79 inches and the total power taken by the 

 shunt at no load is 1.34% of the output of the machine. Required 

 the ampere-turns, the proper area of the shunt wire and the nearest 

 B. & S. size; also the number of turns; take the resistivity as 12.5. 

 (16 min.) 



14. It is desired to build a magnet of rectangular cross-section, 

 width equal to twice the breadth, and to be made of cast iron. 

 Length of gap, f inch; length of path in the iron, 35 inches; total 

 induction, 3 megamaxwells; winding, 2 inches deep; density in 

 gap, 7000. With wire at 20 cents per pound and iron at 2J cents, 

 what is the cost of material, and will it be cheaper to use a density 

 of 6000 or of 7000, and how much? Use No. 12 wire which runs 

 20.5 pounds per 1000 feet, and an exciting current of 10 amperes. 

 Take the volume of the iron as area times length, and its weight as 

 0.26 pounds per cu. in. (20 min.) 



15. An iron-clad magnet, to be used as a clutch, has a central 

 pole 10 cm. in diameter, and a mean diameter of outside ring of 

 20 cm. Required the thickness of the cylindrical pole and the 

 horse-power that the clutch will transmit at 500 r.p.m. if B be 

 taken as 16,000, and if the normal pull is to be taken as ten times 

 the tangential pull. Divide the central pole into a circle 2 cm. in 

 diameter, surrounded by two rings each 2 cm. wide; take the 

 torque as made up of that due to each of these taken at its mean 

 radius, plus that of the outside ring at its mean radius. (15 win.) 



