MEASUREMENT OF POTENTIAL DIFFERENCE 257 



of the instrument and C M and CV are the capacities of the con- 

 denser and the voltmeter respectively. As the capacity of the 



(C 1 I ri \ 



instrument, CV, depends on the deflection, the factor-^- 



Cji/ 



will not be a constant and the 

 voltmeter must be calibrated 

 with the condenser in place. 



An alternative method is to 

 join a number of condensers in 



H 



FIG. 142. 

 Fi <;s. 141 AND 142. Condenser multipliers for electrostatic voltmeter. 



series and to place the electrostatic voltmeter around one of them, 

 as shown in Fig. 142. 



Here again the whole arrangement should be calibrated as a 



unit. 



FIG. 143. Electrostatic voltmeter with variable rango. 



A simple form of electrostatic voltmeter with a gravity control, 

 a very obvious development from the quadrant electrometer, 

 is shown in Fig. 143. 



17 



