THE DESIGN OF ELECTROMAGNETS 



63 



for many hours. The magnet would be designed either for inter- 

 mittent operation, in which case the temperature rise might be 

 calculated as in Case (a) of Art. 12, page 47, or, if left con- 

 tinuously in circuit, a resistance would automatically be thrown 

 in series with the coil windings in order to reduce the PR loss 

 and effect a saving of copper while still maintaining the required 

 pull of 200 Ib. through the reduced air gap. 



Factor of Safety. Seeing that the coils are actually wound with 

 a wire of greater cross-section than the calculated value, the 

 initial pull will be somewhat greater than the specified 200 Ib. 

 The actual ampere-turns are 5,180 X 2 X 1.14 = 11,800, and 

 since the density in the iron is not carried above the "knee" 

 of the B-H curve, the actual flux density in the air gap, instead 



11 800 

 of being 6,570 gausses, will be approximately 6,570 X Q'QQQ = 



8,070 or (say) 8,000 gausses. The initial pull will actually be 



200 X 



(8,000) : 



300 Ib. nearly. This factor of safety of 1.5 



(6,570) 2 



may seem excessive, and if the strictest economy of material is 

 necessary, the coils should be wound with a wire of the calcu- 

 lated size, or, if standard gage numbers must be used, as would 

 generally be the case, the mean length of turn may be modified by 

 providing a greater or smaller depth of winding space. As an 

 alternative, two sizes of wire may be used as explained in Art. 10. 



Most Economical Design. The cost of materials is easily 

 estimated by calculating the weight of iron and copper sepa- 

 rately. For the purpose of comparing alternative designs, it 

 is usual to take the cost of copper as five times that of the iron 

 parts of the magnet. If actual costs are required, the figures 

 would be about 20c. per pound for copper wire, and 4c. per 

 pound for the magnet iron. 



The reader will recollect that this design has been worked 

 through on the assumption that about 6,500 gausses would be a 

 suitable density in the air gap. If many magnets are to be made 

 to the one design, or in any case if the magnet is large and costly, 

 the designer should now try alternative designs, using air-gap 

 densities of (say) 4,000 and 8,000 gausses respectively. By com- 

 paring the three designs, all of which will comply with the terms 

 of the specification, he will be able to select the one which can 

 be constructed at the least cost. This method of working may 

 seem slow and tedious, but it is sure, and if actually tried- 



