THE POLYPHASE CIRCUIT. 



current would require 2w wires. This is true in general ; 

 but for a 3-phase circuit the special relations existing 

 between the currents in the three circuits make it possible 

 to employ in this case three conductors instead of six. 

 The special conditions which make this possible must now 

 be explained. 



Transmission by Three Wires. Let Fig. 143 represent a 

 generator supplying direct current to a distant point P, 



FIG. 143. 



and let us consider the conditions governing the flow of 

 current in the conductor A B. 



Current cannot flow along A B unless this conductor 

 forms part of a closed circuit connected to the generator 

 Hence the circuit must be closed at the end P, and there 

 must be a return conductor from P capaple of transmitting 

 at any moment the same current from C to D as is flowing 

 from A to B. The return current need not flow by a 

 single conductor, but may flow partly through the earth, 

 as in the case of a tramway installation, or may be un- 

 equally divided between a number of conductors. The only 

 absolutely essential condition for the flow of a current 



C. 



FIG. 144. 



from A to B is that the same amount of current shall flow 

 at the same moment from C to Z). This condition applies 

 to any kind of current, whether direct or alternating. 



LetAA\ BB\ CO 1 (Fig/144) represent the six con- 

 ductors carrying a 3-phase current, each conductor shown 



