252 



ELECTRICAL MEASUREMENTS 



urements, the calibration should be made by using alternating 

 potential differences. The effects of contact differences of 

 potential are then eliminated. Conditions which distort the 



Variation of Sensitivity with Potential of Needle 

 and Distance between Quadrants 



800 1290 1600 2000 2400 2800 32TX) 3COO 

 Potential of Needle in Volts 



FIG. 137. Illustrating effect of distortion of the electrostatic field at the 

 needle of a quadrant electrometer. 



electrostatic field in which the needle swings cause departures 

 from the theoretical law. For instance, in one of the older de- 

 signs of instrument, a guard tube was used which surrounded 



