6-A] GENERAL STUDY. 213 



are usually so used that the total power in a 2-phase circuit is 

 twice the product of current per phase, voltage per phase and 

 power factor; the total power in a 3-phase circuit is three times 

 the product of current and voltage per phase, and power factor. 



In a 2-phase system, there is little chance for ambiguity. 



In a 3-phase system, the current and voltage per phase (as 

 defined above) may be either the star (line) current and star 

 voltage, or the delta current and delta (line) voltage. In either 

 case, the total power is three times the power per phase. Using 

 line current, we must use star voltage; using line voltage, we 

 must use delta current. It will be remembered that, if line cur- 

 rent and line voltage are used, the total power is \/3 times their 

 product multiplied by power factor. 



APPENDIX I. 



VECTOR ADDITION OF ALTERNATING CURRENTS AND ELECTRO- 

 MOTIVE FORCES IN A NETWORK OF CONDUCTORS. 



31. Laws of Vector Addition and Subtraction. Any hill may be 

 considered to be up or down according to the direction in which one 

 is walking; the difference in level may be considered positive or 

 negative. In the same way difference of potential may be considered 

 as positive or negative according to the sense in which it is taken 

 that is, according to the direction one takes in proceeding around a 

 circuit or from point to point in a circuit. 



Consider a network of highways in a hilly country. If from any 

 starting point one proceeds by any route or circuit back to the starting 

 point, he will find himself at the original level the plus hills and the 

 minus hills adding up to zero. On different trips he may traverse 

 the same hill in opposite directions, giving it one time a plus and the 

 other time a minus sign. This would be true at any instant, even if 

 the surface were rising and falling, as in an imaginary earthquake or 

 on the surface of the ocean. 



Consider now a network of conductors. If from any starting point 

 one proceeds by any route or circuit back to the starting point, he will 



