83 



coils, and let O F a and O F b represent 

 the fields produced at a given moment. 

 The resultant field is O F, and this 

 revolves round O with a speed corre- 

 sponding to the frequency. In the 

 diagram a cross placed in the circle, 

 representing a wire or coil, signifies an 

 ascending, and a dot a descending cur- 

 rent. When the rotation of the resultant 

 field takes place in the direction of the 

 arrow, the current in B must be approach- 

 ing zero, and that in A its maximum 

 value. 



Suppose, now, the armature is held at 

 rest. The field in sweeping through the 

 conductors C induces in them currents in 

 the direction indicated, and the latter 

 therefore exert a torque. Imagine the 

 field stationary, and the armature re- 

 volved by a belt. In this case, the cur- 

 rent created in the short-circuited coils 

 will produce a torque tending to resist 

 rotation. The lower the resistance of 

 these coils and the greater the speed of 

 rotation, the more power will be required 

 to rotate the armature. It is thus evident 



