OPERATION OF THE DYNAMO AS A GENERATOR. 93 



The line ab, Figs. 71, 72, 73 and 74, is called the neutral 

 axis of the armature. The line connecting the brushes is called 

 the axis of commutation* When the brushes of a generator are 



Fig. 73. 



shifted by moving the rocker arm, the electromotive force be- 

 tween the brushes, as indicated by a voltmeter, reaches a maxi- 

 mum value when the axis of commutation coincides with the 



Fig. 74. 



neutral axis. In fact the whole of the electromotive force, J5 a , 

 which is induced in each half of the armature, is available between 

 the brushes when the brushes are at the points a and b, Figs. 72 

 and 74. 



* The leads which connect the armature conductors to the commutator bars are 

 supposed to be radial, for the sake of simplicity of statement. In commercial machines 

 these leads are nearly always curved like segments of a spiral. 



