POLY PHASE OUURENTS.

85

example, established by a current impulse in the coils thereon,precedes the magnetic effect set up in the armature by the in-duced current in coils f. Consequently the mutual attractionbetween the armature and field-poles is considerably reduced.The same conditions will be found to exist if, instead of assumingthe poles b or c as acting independently, we regard the ideal re-sultant of both acting together, which is the real condition. Toremedy this, the motor field is constructed with secondary polesb' o', which are situated between the others. These pole-piecesare wound with coils o' k ', the former in derivation to the coilsd, the latter to coils k. The main or primary coils i> and e arewound for a different self-induction from that of the coils n' ande', the relations being so fixed that if the currents in n and ediffer, for example, by a quarter-phase, the currents in eachsecondary coil, as 1 / e', will differ from those in its appropriateprimary d or e by, say, forty-five degrees, or one-eighth of aperiod.

Now, assuming that an impulse or alternation in circuit orbranch e is just beginning, while in the branch n it is just fallingfrom maximum, the conditions are those of a quarter-phasedifference. The ideal resultant of the attractive forces of the twosets of poles b c therefore may be considered as progressing frompoles b to poles c, while the impulse in e is rising to maximum,and that in n is falling to zero or minimum. The polarity set upin the armature, however, lags behind the manifestations of fieldmagnetism, and hence the maximum points of attraction in arma-ture and field, instead of coinciding, are angularly displaced.This effect is counteracted by the supplemental poles b' c'. Themagnetic phases of these poles succeed those of poles b c by thesame, or nearly the same, period of time as elapses between theeffect of the poles b c and the corresponding induced effect in thearmature; hence the magnetic conditions of poles b' g ' and ofthe armature more nearly coincide and a better result is obtained.As poles b' o' act in conjunction with the poles in the armatureestablished by poles b o, so in turn poles c b act similarly withthe poles set up by b' c', respectively. Under such conditionsthe retardation of the magnetic effect, of ■ the armature and thatof the secondary poles will bring the maximum of the two morenearly into coincidence and a correspondingly stronger torque ormagnetic attraction secured.

In such a'disposition as is shown in Fig. 68 it will be observed