INDUCED ELECTROMOTIVE FORCE. 



129 



directly. This set of copper bars constitutes what is called the 

 commutator. 



Shunt and series field windings. The field magnet of a direct- 

 current generator is usually excited by current taken from the 

 machine itself. The winding of wire on the field magnet may 

 consist of many turns of comparatively fine wire having a con- 



Fig. 89. 



siderable resistance. In this case the terminals of the field wind- 

 ing are connected directly to the brushes of the machine, and 

 from two to ten per cent, of the permissible current output of the 

 generator flows through the field windings and excites the field, 

 the remainder of the permissible current output being available 

 for use in the external circuit. In this case the field winding and 

 the outside receiving circuit are in parallel with each other between 

 the brushes, so that the field winding is in the relation of a shunt 

 to the outside receiving circuit. A direct-current dynamo with 

 its field windings arranged in this way is called a shunt dynamo. 



The winding of wire on the field magnet of a direct-current 

 dynamo may consist of comparatively few turns of heavy wire 

 having a low resistance. In this case the field winding is con- 

 nected in series with the external receiving circuit, the whole cur- 

 rent delivered by the machine flows through the field winding, 

 and from two to ten per cent, of the electromotive force developed 

 by the machine is used to force the current through the field 

 winding, the remainder being available for forcing current through 

 the external receiving circuit. A direct-current dynamo with its 



field windings arranged in this way is called z~sfries dynamo. 

 io 



