THE DESIGN OF ELECTROMAGNET 



10,000 gausses. We shall select the value of 7,000 for a trial 

 design, and carry this through to completion, although it would 

 usually be preferable to carry at least three designs through to 

 the point where it becomes clear that one of these is distinctly 

 preferable to the others from the point of view of economy; this 

 being the main consideration which the designer must always bear 

 in mind. 



Putting for B, in formula (34), the value 7,000 gausses, the side 

 w of the (square) pole shoe is found to be 1.88 in. Let us make 



13 150 

 this 2 in. ; whence B = '-= = 6,570. The thickness Z 2 of the 



t 



pole shoe must be small in order to keep down the magnetic 

 leakage; a value of K m - f r ^2 should be satisfactory. 



The diameter, d, of the magnet cores under the winding is 

 obtained by assuming a leakage factor and a suitable flux density 

 in the iron. The leakage factor (refer Art. 7, page 28, for defi- 

 nition) might be about 1.5 in a magnet of this type and size; and 

 the density in the core of magnet steel or wrought iron, may be 

 as high as 90,000 lines per square inch. Thus; 



TT 1.5 X 4 X 6,570 X 6.45 



4 d 90,000 



whence d = 1.9 or (say) d = 1.875 in. 



Summing up the quantities so far determined, we have: 



w = 2 in. 

 Z. = K in. 

 d = 1% in. 



B (air-gap density) = 6,570 gausses. 



$ (useful or effective flux per pole) = 6,570 X 6.45 X 2 X 2 



= 170,000 maxwells. 



Depth of Winding. The length I of the winding space, and the 

 thickness of winding, t, will depend upon the ampere-turns neces- 

 sary to produce the desired flux density in the air-gap, and also 

 upon the allowable current density, A, in the copper of the 

 exciting coils. The relation of I to t is not determined by the 

 amount of the ampere-turns, since this only calls for a sufficient 

 cross-section, or product I X t. The thickness t should not exce 

 3 in., because a greater thickness may lead to excessive tem- 

 peratures inside the coil; but the most suitable dimensions of t 

 coil are really determined by the current density, the winding 

 space factor, and the cooling surface necessary to prevent 



