TRANSMISSION OF POWER FOR SUBSTATIONS. E57 
armature were working as a dynamo. As a dynamo this 
current could either be withdrawn through the rings as 
alternating current or through the commutator as continu- 
ous current. So when the same current is.delivered to the 
machine from an external source it may be taken off the 
commutator as continuous current or off the rings as alter- 
nating current if continuous current be supplied from the 
line. ’T'he commutator neither knows nor cares whether 
     
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the current that comes to its leads is generated in the arm- 
ature or poured into it from a distant source. A small 
part of the energy supplied is expended in keeping up the 
rotation of the machine as a motor, the rest is delivered to 
the line as available current. 
This device furnishes a very beautiful and efficient 
method for the conversion of alternating current. Itis 
most available for practical purposes in its polyphase form, 
since although it works admirably with single phase cur- 
rent it cannot start as a motor, nor is it able to give quite 
so good an output. For polyphase work the armature 
winding is tapped not as in Fig. 72 at two points, but at