6-A] GENERAL STUDY. 201 



attention being given to polarity; the two secondaries of the 

 other transformer form the remaining two sides. Before clos- 

 ing the square, connect a voltmeter between the two points about 

 to be connected and proceed to connect them only in case the 

 voltmeter reads zero. This precaution should be taken in mak- 

 ing any mesh connection. 



12. A convenient laboratory supply board is obtained from 

 2-phase secondaries, the secondary circuit on each phase consist- 

 ing of four equal coils in series so as to form a 5-wire system 

 on each phase. With the neutrals of the two phases intercon- 

 nected, this gives supply voltages, as Fig. 6. If the total volt- 

 age of each phase is 220 volts, this gives 

 2-phase voltages as follows: 4-wire no 

 and 220 volts; 3-wire 55, no, 77.8 and 

 155.6 volts; also additional single-phase ^ i 

 voltages, 123 and 165 volts. The voltage 

 between any two points can be scaled off 

 from the drawing in Fig. 6, as shown in *s 



the discussion of Figs. 3, 4 and 5. When FlG - 6. Two-phase 

 the transformer secondaries cannot be so l! > b tor * "wiy volt- 



ages. 



subdivided, the result can be obtained by 



connecting across each phase of a 4-wire system an autotrans- 

 former made of four equal coils. Verify these voltages by calcu- 

 lation or by measurement. 



The preceding study has brought out the fact that in poly- 

 phase circuits, the single-phase voltages of interconnected gen- 

 erator or receiver coils are combined geometrically to give re- 

 sultant voltages. Although this was shown particularly for 

 2-phase circuits, it will be understood to be general and to apply 

 as well to a 3-phase circuit or to any circuit whatsoever. 



13. Addition of Currents. Currents, also, when of differ- 

 ent phases, are added* vectorially to obtain the resultant current. 

 To show this proceed as follows: 



* Branch currents, flowing to or from a common point, always combine 

 by addition not by subtraction to give the total current. See Appendix I. 



