l6o ELEMENTS OF ELECTRICAL ENGINEERING. 



Fig. 108. 



The value of cV' may be reduced below the value given by 

 equation (36) by constructing the pole pieces as shown in Fig. 

 1 08, so that the magnetic circuit CC includes, not only the air 



gaps at a and d, but also the very 

 considerable air space at r, which does 

 not interfere with the flow of the use- 

 ful flux 3>. 



The effect of a long gap space may 

 be produced by using narrow teeth 

 -i and wide slots on the armature. These 

 5 narrow teeth are then highly saturated 

 by the flux 3>, so that their magnetic 

 reluctance is great, and this high re- 

 luctance is the exact equivalent of a 

 long air gap. 



A high reluctance in the path, CC, Fig. 108, of the cross-flux, 

 but not in the path of the useful flux 4>, may be produced by 

 lengthening the gap space under the pole tips, leaving a short 

 gap space under the middle of the pole face. This effect may be 

 accomplished most advantageously in a laminated pole piece by 

 cutting away the tips of half of the laminations, thus greatly re- 

 ducing the sectional area of the iron in the pole tips. Under these 

 conditions the least overcrowding of the flux under a pole tip, as 

 shown in Fig. 105, tends to push the iron of the pole tip beyond 

 magnetic saturation, thus vastly increasing its reluctance. The 

 nearly sparkless operation under all loads, of dynamos of recent 

 design, is largely due to the use of pole pieces of the kind here 

 described. 



The best method of all for reducing armature reaction is by the 

 use of the so-called compensating' winding, which consists of a 

 layer of wires, parallel to the armature shaft, embedded in the pole 

 faces, as shown in Fig. 109, and so connected that the total cur- 

 rent, or a definite fraction of the current, from the brushes of the 

 machine flows through each wire, as indicated by the crosses and 

 dots in Fig. 109. The use of this compensating winding is not 



