FLUX DISTRIBUTION 



125 



the air gap are supposed to lie in planes parallel to this section. 

 Equipotential lines are drawn in directions which seem reason- 

 able to the draughtsman, and tubes of flux, all having the same 

 permeance, are then drawn with their boundary lines perpen- 

 dicular at all points to the equipotential lines. At the first trial 

 it will generally be found that these conditions cannot be fulfilled, 

 but by altering the direction of the tentative equipotential 

 lines the work is repeated until the correct arrangement of 

 lines is obtained. The tube of induction A BCD (Fig. 39) is the 

 first to be drawn. Its permeance in the particular case con- 

 sidered is 0.25 because it consists of four portions in series, each 

 one of which is exactly as wide as it is long (a thickness of 1 cm. 

 measured axially is assumed). Proceeding outward from left 

 to right, and making each section of the individual tube of 

 induction exactly as wide as it is long, the permeance of every 

 one of the component areas in the diagram is always unity, and 

 any complete tube, such as EFGH, has the same permeance (in 

 this example 0.25) as every other tube. The computation of 

 the total permeance between the pole shoe and armature over 

 any given area is thus rendered exceedingly simple. Although 

 the armature surface is represented as a straight line in the 

 accompanying illustrations, the actual curvature of the armature 

 may be taken into account if preferred ; but the error introduced 

 by substituting the developed armature surface for the actual 

 circle is generally negligible. 



In Fig. 40 the flux lines have been drawn to ascertain the 

 effect of the neighboring pole in altering the distribution over 

 the armature surface in the interpolar space. The perpendicular 

 NN' has been erected at the geometric neutral point, and may 

 be considered as the surface of an iron plate forming a con- 

 tinuation of the armature surface AN. Thus ANN' will be an 

 equipotential surface between which and the polar surface 

 the intermediate equipotential surfaces must lie. 



It may be mentioned that in Figs. 39 and 40, and also in 

 the other flux-line diagrams, the pole core under the windings 

 cannot properly be considered as being at the same magnetic 

 potential as the pole shoe, relatively to the armature. The 

 proper correction can be introduced in calculating the flux in 

 each tube of induction; but since the present investigation is 

 confined to the flux entering the armature from the pole shoe, it 

 will not be necessary to make this correction. 



