SYNCHRONOUS MOTORS. 267 



to allow it to move appreciably from the neutral position 

 between each alternation of the current. Suppose, however, 

 the 2-pole armature shown in Fig. 127 were made to rotate 

 at exactly such a speed that it made half a revolution in the 

 time taken by the current to change from its maximum 

 value in one direction to its maximum value in the opposite 

 direction. In that case the direction of the current would 

 be reversed each time the position of the conductors a, b con- 

 nected to the rings was reversed. Consequently, the polarity 

 of the armature would be the same as before, after half a 

 revolution. 



When followed out in detail, the action of the currents 

 would be as follows : Beginning with the armature in the 

 position shown, suppose that the current is at its maximum 

 value, and flows, as shown by the small arrows. The poles 

 induced in the armature core will be in the positions in- 

 dicated by n, s. As the armature rotates in clockwise 

 direction, in virtue of the attraction between the poles of the 

 armature and those of the field, the current dies down, because 

 it is an alternating current. After a quarter of a revolution the 

 current will have become zero, and the armature poles will at 

 the same time have gradually diminished in strength to zero 

 while approaching the centre of the field poles. After the con- 

 ductors have passed the centre of the field poles, the arma- 

 ture current gradually increases, but flows in the opposite 

 direction, so that the points in the armature marked n and s 

 are now respectively of south and north polarity. With 

 increasing armature current and the simultaneous move- 

 ment of the conductors connected to the slip rings towards 

 the mid-position between the poles, the turning moment 

 increases, until the newly-formed south pole reaches the 

 position s on the diagram, formerly occupied by the opposite 

 point of the armature. The cycle now repeats itself. 



The revolution of the machine would under these con- 

 ditions be maintained by the current. The continuance of 

 the rotation depends upon coincidence of the reversal of the 

 direction of the current and the reversal of the position 

 of the armature. If the current reversed too soon or too 

 late, the polarity induced by the current would oppose the 

 rotation of the armature during part of the revolution. If 

 the rate of alternation of the current were changed relatively 

 to the speed of the armature, the current would sometimes. 



