THE MAGNETIC CIRCUIT ELECTROMAGNETS 23 



be such that the total permeance of these leakage paths has 

 the greatest possible value. It is well to bear this fact in mind, 

 because it enables the experienced designer to make sketches of 

 various probable distributions of the leakage flux, and base his 

 calculations on the arrangement of flux lines which has the 

 greatest permeance. The fact that the leakage flux usually 

 follows air paths means that the permeance of these paths 

 does not depend upon the flux density B; this simplifies the 

 problem because it is not necessary to take into account 

 values of the permeability, ju, other than unity; the difficulty 

 lies in the fact that with the exception of very short air 

 gaps between relatively large polar surfaces it is rarely possi- 

 ble to predetermine the distribution of the stray flux, except 

 by making certain convenient assumptions of questionable 

 value. A designer of experience will frequently be able to 

 estimate flux leakage even in new and complicated designs 

 with but little error, and it is surprising how the intelligent 

 application of empirical or approximate formulas and rules will 

 often conduce to excellent results. The errors introduced are 

 some on the high side and some on the low side, and the averages 

 are fairly accurate; but the estimation of leakage flux like 

 many other problems to be solved by the designer or practical 

 engineer savors somewhat of scientific guesswork; it calls for 

 a combination of common sense and engineering judgment 

 based on previous experience. The following examples and 

 formulas cover some of the simplest cases of flux paths in air; the 

 usual assumptions are made regarding the paths followed by the 

 magnetic lines, but it may safely be stated that, when all possible 

 leakage paths have been considered, and these formulas applied 

 to the calculation of the leakage flux, the calculated value will 

 almost invariably be something less than the actual stray flux 

 as subsequently ascertained by experimental means. 



Case (a) Parallel Flat Surfaces If the length of air gap 

 between the parallel iron surfaces is small relatively to the 

 cross-section, and if the two surfaces are approximately of the 

 same shape and size, the average cross-section (see Fig. 6) is 



A- 



and the permeance is 



- 



(6) 



