OPERATION OF THE DYNAMO AS A GENERATOR. 83 



electromotive force of a given generator depends upon its speed ;/, 

 and upon the armature flux <i> according to equation (21). The 

 armature flux depends in its turn upon the degree of excitation 

 of the field magnet. Therefore, aside from the comparatively 

 small disturbing action of the current in the armature, the volt- 

 age of a generator depends upon its speed and its field excitation. 



The dependence of the voltage of a generator upon the value 

 of the armature flux, <>, is strikingly shown by the following 

 experiment : Given a generator driven at a constant speed, with 

 a voltmeter connected across its terminals, the receiving circuit 

 being disconnected. When a heavy slab of iron is laid across the 

 pole pieces, the voltage, as indicated by the voltmeter, drops 

 considerably on account of the fact that a portion of the flux from 

 the field magnet poles now passes through the iron slab instead 

 of passing through the armature, and <I> is therefore decreased. 

 The iron slab acts as a magnetic shunt in parallel with the 

 armature core of the generator. When the slab is removed the 

 voltage rises to its original value. 



(b) Secondary condition. The disturbing action of the current 

 in the armature of a generator is comparatively small in its effect 

 upon the voltage of the generator, and therefore it is permissible 

 to consider the voltage of a generator as primarily dependent 

 upon its speed and its field excitation, and as only secondarily 

 dependent upon the armature current. The effect of armature 

 current upon voltage is twofold. In the first place the current 

 in the armature of a generator, under ordinary operating condi- 

 tions, opposes * the magnetizing action of the field windings, and 

 thus tends to lessen the armature flux <E>, and thereby to lessen 

 the actual electromotive force induced in the armature. In the 

 second place a portion of the electromotive force actually induced 

 in the armature is used to overcome the resistance of the arma- 

 ture (and of the series field winding) through which the current 



* The demagnetizing action of the armature current is here referred to. The cross- 

 magnetizing action of the armature current is discussed in Chapter VI. The magnetic 

 action of the armature current is called armature reaction. 



