and 



FLUX DISTRIBUTION 

 M dr 



117 



The average permeance per square centimeter over the slot pitch 

 at center of pole is, therefore, 



Pi + 2P 2 



(57) 



t + s 



where the quantity (t -f- s) is the slot pitch \ expressed in centi- 

 meters. 



The reciprocal of this quantity is the reluctance per square 

 centimeter of cross-section, or the equivalent air-gap length 5. 

 Thus 



(58) 



Consider now Fig 37, which illustrates the case of a highly 

 saturated tooth. The lines of flux are shown parallel over the 



t > 



FIG. 37. Flux lines entering toothed armature. (High flux density.) 



whole of the slot pitch, a condition which is approached but 

 never attained as the density in the tooth is forced up to higher 

 and higher values. It is obviously only when the permeability 



1 If the ventilating ducts are closely spaced, or exceptionally wide, the gap, 

 5 t , for the equivalent smooth-core armature, as given by formula (58), might 

 have to be slightly modified; but the calculation of fringing at the sides of 

 vent ducts is usually an unnecessary refinement. 



