THE DESIGN OF ELECTROMAGNETS 



49 



cost is the main item, and the total cost of iron and copper is 

 a good guide to the cost of the finished magnet, when it is 

 desired merely to compare alternative designs based on a given 

 specification. 



It is an easy matter to estimate the volume and weight of 

 materials in so simple a design as a lifting magnet, and although 

 formulas can be developed which aim to give the proportions 

 and sizes for the most economical design, these are usually of 

 doubtful value, and it is generally simpler to apply a little 

 common sense and the engineering judgment which will come 

 with practice, and try two or three designs with different pro- 

 portions before selecting the one that seems most suitable in all 

 respects, not omitting the important item of initial cost. 



14. Short-stroke Tractive Magnet. With a design of plunger 

 magnet as shown in Fig. 18, there is not much magnetic leakage, 



_i 



Brass Ri 



FIG. 18. Plunger or iron clad magnet. 



because the travel of the plunger, or length of air gap, is small in 

 comparison with the area of the pole faces. Given a definite total 

 amount of flux to produce the required pull, the cross-section of 

 the various parts of the magnetic circuit is readily calculated. 



