SOLUTION

Electrostatic instruments are the only instruments that are directly voltage-sensitive and do not depend on a current for their operation. Basically, electrostatic instruments are all voltmeters. Practically such instruments may be used for the measurement of current and power but both the types of measurements require measurement o the voltage across a known impedance.

The main advantage of such instruments is the measurement of high voltages in both a.c. and d.c. circuits without any errors due to eddy current losses and hysteresis.

The operation of all the electrostatic instruments is based on the principle that there exists a force between the two plates with opposite charges. This force can be obtained using the principle that the mechanical work is done is equal to the stored energy if there is a relative motion of plates.

In electrostatic instrument deflecting torque is proportional to

$\theta = \dfrac{1}{2}\dfrac{{{V^2}}}{K}\dfrac{{dC}}{{d\theta }}$

Where K = spring constant
C = capacitance between the plate

Since the deflection is proportional to the square of the voltage to be measured, the instrument can be used on both ac and dc. The instrument exhibits a square-law response and hence the scale is nonuniform.

Advantages of Electrostatic Instrument

1. They can be manufactured with very high accuracy.
2. The instruments may be calibrated with d.c., and yet the calibration would be valid for a.c. also since the deflection is independent of the waveform of the applied voltage.
3. Since no iron is used for their construction, they are free from hysteresis, eddy current losses, and temperature errors.
4. Once the discs are charged, no more current is drawn from the circuit and the instrument represents infinite impedance.