ELECTRICAL ENERGY. 217 



sary, as the sum of the currents is zero, unless the circuits are unbal- 

 anced. In distributing on the three-phase system a fourth wire can be 

 employed, as it gives an advantage in the amount of copper used. 



In all these alternating systems the great difficulty lies in the fact 

 that the inductance of the circuit causes the current to lag behind the 

 electromotive force. This decreases the amount of energy transmitted 

 by a given current at a given voltage; it causes a drop in the voltage 

 of the line, and it increases the armature reaction of the dynamo for a 

 given current. The total inductance of the circuit is made up of the 

 inductance of the transformers, of the dynamos, of the receiving appa- 

 ratus, and of the line. In the case of transmission to very long dis- 

 tances the line inductance is a large proportion of the total, while the 

 inductance of the receiving apparatus depends upon whether lights or 

 motors are to be supplied and upon the construction of the latter. 

 When the different wires of the multiphase system are fed from wind- 

 ings on the same dynamo armature, then the drop in voltage due to any 

 excess of load on one of these circuits can not be compensated for on 

 the dynamo itself. If the amount of current and the lag of the current 

 is the same for all of the circuits of the system, then it is easy, by a com- 

 pounding winding of the dynamo, or by changing the current in the 

 field winding, if there is no compounding, to keep the voltage constant 

 at either the sending or receiving end. When the load on the different 

 wires of the system is not the same, however, it is, as I have stated, 

 impossible to keep all of the circuits at the proper voltage. Where a 

 two-phase transmission with separate circuits is used, then if the sepa- 

 rate circuits are wound on different armatures each can be regulated 

 to give a constant voltage at the receiving end. This is the case, for 

 instance, in the large dynamos built by the Westinghouse Company for 

 use at the World's Fair in Chicago. The difficulty due to the uneven 

 loading of the circuits is specially marked in the case of the three-phase 

 system, and it is one of the principal objections that have been urged 

 against the employment of this system for distribution. It should be 

 pointed out, too, that it is not enough to balance the quantities of cur- 

 rent for the three branches of the system, but the character of the 

 current must also be considered. A noninductive load on one wire, 

 with an inductive load of equal value on the others, would cause an 

 unbalancing just as if the currents differed in amount. In most of the 

 transmission plants that are being operated and that are proposed it 

 is required to run both lamps and motors from the same circuits, and 

 while a slight variation of potential on the motors would not cause any 

 particular trouble, yet the successful operation of the lamps requires a 

 practically constant voltage. I think, however, and the same grounds 

 have been taken by others, that in any practical transmission of con- 

 siderable size it is possible to so balance the loads that this difficulty 

 will not exist to an extent to cause any serious trouble. When the dis- 

 tributing part of the lines is reached it is usually the custom, when a 

 three-phase transmission is used, to employ four instead of three wires. 



