CONVERSION OF ALTERNATING CURRENT. 



193 



wire system and maintain 1 1 o volts (direct) between the middle 

 main and each outside main. To do this the current in the 

 middle main must be delivered to the armature at a potential 

 midway between the potentials of the direct-current brushes. 



100 Amperes 



outside main 



Fig. 160. 



The two coils C and C 1 represent the two similar coils of a 

 transformer so wound that the point g is, as it were, the middle 

 of one continuous winding the terminals of which, namely, h 

 and i, are connected to the two alternating-current brushes c 

 and d of the machine. The terminals h and i are at the 

 same potentials as the points e and / of the armature winding 

 respectively. Half of the inflowing current in the middle main 

 flows down hill, as it were, through coil C r to the point / and 

 delivers energy to coil C' . This energy is transformed into 

 coil C and it causes the other half of the current in the middle 

 main to flow up-hill, as it were, through coil C to the point e. 



In case the machine is engine-driven, all of the current enter- 

 ing at e flows up-hill to brush a, and all of the current entering 

 at / flows up-hill to the brush a, because of the electromotive 

 forces induced in the armature winding by its rotation. 



H 



