THE DESIGN OF ELECTROMAGNETS 



65 



the annular surface forming the outer pole of the magnet should 

 be less than 1 in., it might be necessary, for mechanical reasons, 

 to reduce the diameter in order to obtain a practical design. 

 The total pull required is 4,000 lb., or 2,000 Ib. per pole. The 

 pull per square inch of polar surface is, by formula (16) page 29. 



B 2 



Pounds per square inch 



1,730,000 



whence the area of each pole face is 



A = 



2,000 X 1,730,000 



If B = 6,000 gausses, A = 96 sq. in. ; and if B = 8,000 gausses, 

 A = 54 sq. in. Either of these flux densities would probably 



Steel Casting 



Magnetising Coil 



Retaining Plate 

 I of (Non-magnetic) 

 A Manganese Steel, 

 with Stiffening 

 Ribs 



N/ 



FIG. 22. Circular lifting magnet. 



be suitable for a magnet of the type considered. Assuming a 

 minimum width of 1 in. for the pole face on the outer shell, we 

 have, 



Area of outer ring = 1 X n X 21 

 = 66 sq. in. 



which would provide the required pull if B = 7,240 gausses. 



