The magnetic strength of these fields can be considered practically 

 constant inasmuch as it also resumes the same value at every one-sixth 

 of a period (although it may, in the intervals, undergo slight variations 

 which are dampened by the hysteresis and the eddy currents 

 produced in the pole-pieces of the rotor). Therefore, even though the 

 armature (stator) may be stationary, the result is the same as if it had 

 rcrvolving poles which attract or repel the poles of the field (rotor) 

 and we can, henceforth, reason as if we were dealing with the attrac- 

 tions of two systems of magnets presenting the same number of poles 

 which are alternately north and south in polarity. (Fig. 8.) The 

 poles of unlike polarities, of the two sys- 

 tems, attract each other; the others repel 

 each other. Therefore, when at rest, the 

 poles of unlike polarity will always face 

 each other. If the external magnets begin 

 to rotate slowly, starting from rest, they will 

 drag with them the stationary magnets, 

 whose poles tend to remain opposite the 

 poles of unlike polarity. (This result may 

 be obtained by supplying the motor with 

 current obtained from a generator which 



is started from rest and, consequently, gives polyphase currents of 

 increasing frequency.) 



The attractions can only be concordant and continuous when the 

 two systems turn at the same speed; which explains the necessity of 

 synchronism. Otherwise, there would only be successive attractions 

 and repulsions which would neutralize each other. 



The stability of synchronous operation is also easily demonstrated 

 by considering the moment of the motor-couple (i.e., the torque). If 

 the poles of the rotor remain opposite the revolving poles of the stator, 

 the attractions produced are directed radially and consequently they 

 produce a motor-couple or torque which is equal to zero. If, on the 

 contrary, for any reason whatever, the rotor loses or gains speed, some 

 tangential attractions or repulsions will appear, whose resultants tend 

 to bring back the opposite poles of the rotor into coincidence with the 

 poles of unlike polarity of the stator, so long as the poles of the rotor 

 remain near these, because the attractions of unlike poles and the 

 repulsions of like poles act in the same direction; but if the difference 

 in phase amounts to one interpolar space, the poles of like sign of the 

 rotor and stator will be opposite each other, the motor-couple or torque 



