THE DYNAMO. 



of a two-part commutator. The current which is produced 

 in the armature winding of Fig. 21 is an alternating current, 

 and the two-part commutator serves to reverse the connections 

 of the armature to the external receiving circuit twice during each 

 revolution of the armature, so that the current, which is an alter- 

 nating current in the armature winding, becomes a uni-directional 

 current in the external circuit. The Brush arc lighting dynamo 

 and the Thomson-Houston arc dynamo are examples of ma- 

 chines having open coil armatures. 



The current in any given turn of wire on a closed coil arma- 

 ture is also an alternating current, inasmuch as it flows in one 

 direction through the turn when the turn is on the side d of the 

 armature (see Fig. 1 7), and in the opposite direction through the 

 turn when the turn is on the side c of the armature. 



The above discussion applies to the simplest form of direct-cur- 

 rent dynamo ; namely, a two-pole dynamo with a simple ring 

 winding. Commercial forms of direct-current dynamos, however, 

 may be bipolar or multipolar, they may have ring or drum 

 armatures, and they may have 

 any one of a variety of arma- 

 ture windings, including open 

 coil windings, according to the 

 size of the machine and the 

 conditions of service. 



29. The multipolar direct- 

 current dynamo. Fig. 22 

 shows a ring armature rotat- 

 ing inside of a crown of six 

 inwardly projecting field mag- 

 net poles, NSNSNS. The 



electromotive forces induced in the windings as they sweep across 

 the pole faces cannot produce current in the endless wire that is 

 wound on the ring, inasmuch as the electromotive forces induced 



Fig. 22. 



