230 POLYPHASE CURRENTS. [Exp. 



(b) For the given load substitute a load that is non-inductive 

 or is known to have high power factor; if, with certain connec- 

 tions, both wattmeters deflect properly, their readings for these 

 connections are positive. When one connection needs to "be re- 

 versed to obtain proper deflection, one reading is negative. 



(c) Disconnect one* potential circuit from the middle wire Z 

 and connect it to the outside wire, X or Y ; if the wattmeter re- 

 verses, the readings of one of the wattmeters must be considered 

 negative. 



Method (c) can be readily applied during test, when using the 

 two-wattmeter method on a 3-wire system, but does not apply to 

 a system with more than three wires. 



Method (a) is general and can be applied to a system with 

 any number of wires. The polarity of the wattmeter circuits 

 may be marked once for all, instruments of one make being 

 similar. The instruments can be properly connected in the posi- 

 tive sensef in advance and confusion during the test avoided. 



26. Two-wattmeter Method with Balanced Three-Phase 

 Load. As has been already stated, the two-wattmeter method is 

 general for any kind of 3-wire circuit. Detailed proof for each 

 particular case is, accordingly, unnecessary. A discussion of its 

 application to measuring a balanced 3~phase load will, however, 

 prove instructive as an illustration and will serve to make clear 

 the negative reading of one wattmeter at low power factors. 

 Furthermore, it will show a method for obtaining 3-phase power 

 factor. 



27. Fig. 2 is the diagram for a balanced 3-phase load, it being 

 assumed that currents and voltages follow a sine law. For unity 



*(25a). On a 3-phase circuit it is sufficient to do this with one 

 potential circuit only; but in general it should be done, successively, with 

 each potential circuit, a reversal of either instrument indicating that one 

 reading is negative. 



t(25b). This also indicates the direction of the flow of power; see 

 " Polyphase Power Measurements," by C. A. Adams, Elect. World, p. 143, 

 January 19, 1907. 



