* Principle of Operation: Electrostatic instruments rely on the force of attraction or repulsion between charged plates. This force is directly proportional to the square of the voltage applied, making it a non-linear relationship.

* AC Sensitivity: While AC voltage can create an electrostatic force, the rapidly changing polarity of AC voltage would cause the force to fluctuate rapidly. This fluctuation makes it difficult for the instrument to provide a stable and accurate reading.

* Rectification for AC Measurement: To measure AC voltage with an electrostatic instrument, you would need to rectify the AC signal first. This converts the AC voltage into a DC equivalent that the instrument can then measure.

In summary: Electrostatic instruments are primarily used for DC measurements due to their inherent design and the challenges of accurately measuring rapidly fluctuating AC voltages.