POWER MEASUREMENT 



321 



i r 



The term I d 2 dt is evaluated by the second reading, for 

 / Jo 



with the connection at b the deflection is 



a = K- 



T o 



2A7J 



(19) 



Db is dependent on the power wasted in the non-reactive resist- 

 ance. 



The necessity for taking the second reading may be obviated 

 by a method given by Miles Walker 4 . 



The connections are shown in Fig. 187. 



r,L=0 



wwvwwvw 



Load 



Vu',. 187. Connections for electrostatic wattmeter. 



Two non-inductive resistances, R and r, are placed in series 

 with the load and in the positions indicated ; the needle is con- 

 nected to some form of potential divider so that its potential 

 with reference to the quadrants is only a fraction of the line vol- 

 tage. The potential divider may be a high resistance, but it is 

 better to provide taps on the secondary of the high voltage 

 transformer, for in this case the potential of the point 6 is not 

 disturbed by the charging current flowing to the needle. 

 Let 



then 



Vac 



- = n 



Vbc 



"*> = " 



~ n) 



The voltage between quadrant (2) and the needle is (see Fig. 187), 

 21 



