So basically, your recommendation is to use 'high impedance' to measure electric potential (open circuit
voltage) of the generator is this correct? I don't exactly understand what the purpose of the capacitor is
here. Using a DMM alone showed unreliable reading (0.34 mV). I will definitely try out using 10 GOhm
Even that electrostatic voltmeter has some resistance. I can read 10 to the power of 6 Megohms on the scale.
The higher the resistance between object and instrument, the less it will be influenced.
The trade-off is resolution.
We know almost nothing about your project, and how accurate you want to measure that voltage.
Did you use a circuit board. A fingerprint on it might leak more than a 10Gohm resistor.
I mentioned the capacitor in relation to the Arduino, not the DMM.
The capacitor is slowly charged up to the scaled down voltage from the voltage divider.
The A/D of the Arduino periodically takes a sample.
The capacitor holds the voltage steady while the sample is taken.
If your voltage divider was made with 10k resistors, the cap would not be needed.
A DMM alone will load your circuit with 10Megohm, independent of the voltage range you select on the meter.
10Megohm could collapse the voltage of your circuit to almost zero.
Remember that with a 10Gohm resistor in series with the red lead of a DMM, the voltage you measure is 1/1000 of the original. So 200volt measures as 200mV.