POWER MEASUREMENT 



323 



which gives the correction for the power in the resistance R, 

 may become very important. If n is 2 the instrument gives the 

 power without correction. 



When the electrostatic wattmeter is used in connection with a 

 fictitious load (see page 500) in the calibration of power and 

 energy meters, the correction term due to the power loss in the 

 series resistance will be eliminated if the current and voltage 

 circuits are electrically connected at the middle of the resistance, 

 R, as shown in Fig. 188. 



Wattmeter 

 (Under Test 



Source of Current 



Phase Variable 

 FIG. 188. Connections for electrostatic wattmeter with fictitious load. 



In this case, the potential difference between quadrant 2 and 

 the needle is 



d v 



V * b= "2 + n 



(24) 



nD 

 2KR 



This method of connection is used in a highly developed 

 form at the National Physical Laboratory, for calibrating com- 

 mercial instruments. 



The electrostatic wattmeter has been used in measurements of 

 the power lost in dielectrics at high voltages, as in cables run- 

 ning without load, in condensers and in samples of insulating 

 material. 



In testing small samples of insulating materials at high vol- 

 tages, it is necessary to avoid measuring the energy dissipated 

 in the air around the electrode and on the surface of the sample. 



