ALTER* A TI\t;-('l'KKK\T INSTRUMENTS 





exceed the rating of the instrument, the pointer goes off scale 

 and so warns the user. A wattmeter may be considerably over- 

 loaded and yet the load power-factor be so low that the needle is 

 well on the scale. For this reason a voltmeter and an am 

 should ordinarily be used in conjunction with a wattmeter so 



Load 



Fio. 55. Wattmeter, ammeter and voltmeter connections for measuring power. 



that it is possible to determine whether either the voltage or the 

 current exceeds the wattmeter rating. 



Corrections for the power taken by ammeters and voltmeters 

 are often necessary. For example, in Fig. 55 the I-R loss of the 

 ammeter and the E*/R loss of the voltmeter must be deducted 

 from the wattmeter reading, 

 in addition to the wattmeter .^^^B^ 



potential loss. The ammeter 

 high by the current 

 taken t.y the voltmeter. This 

 voltmeter cm-rent must be 

 subtracted vectorinlli/ from the 

 ammeter reading in order to 

 obtain the truo load current. 



/V///>/m.Nr \\' (ittmeter. 

 Ordinarily, it requires two or 

 more wattmeters to measure 

 the total |H>wer of a two-phase 

 : three-phase circuit. If 

 the load fluctuates, it is diffi- 

 cult to obtain M unit, -imul- 

 UUieOUs readme of two wattn 

 i>.:. in a three-phase circuit, one of the v, is reverses its 



\v:ittuipl4T. 



pair- 



