338 



ELECTRICAL MEASUREMENTS 



The Y-box. The F-box consists of a small case, like that of an 

 ordinary multiplier, containing two resistors in series, both of 

 which have a resistance equal to that of the potential circuit of the 

 wattmeter with which the box is to be used. As a tap is carried to 

 the junction of the two resistors the F-box has three terminals. 

 It is connected in circuit as indicated in Fig. 201. 



If the load is balanced the power is given by P = 3 times 

 reading of wattmeter. 



FIG. 201. Connections of F-box for measuring three-phase power under 



balanced loads. 



It may be convenient to use a F-box with some other instru- 

 ment than that for which it was designed ; in this case the factor 

 is no longer 3. Referring to Fig. 201, by KirchofTs laws the 

 current i v through the potential coil of the wattmeter is 



2R v + r 



where R v is the resistance of the potential circuit of the watt- 

 meter and r is the resistance of each section of the F-box. If the 

 line current be i L the reading of the wattmeter is given by 



1 C T 



TJ ivi 



Therefore, 



Reading = 



For a balanced load (see page 332) . 



1 C T 1 C T 



power == - I i lVl2 dt+ - I i 



.'.P 



2R V + r 



times reading of wattmeter 



(33) 



