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Which of the following is an example of an electrostatic type instrument?
Wattmeter
Hot wire instrument
Kelvin multicellular voltmeter
Energy meter
Kelvin multicellular voltmeter
Electrostatic instruments operate on the principle of electrostatic force of attraction between two charged electrodes. The Kelvin multicellular voltmeter is a classic example of this category, designed to measure high voltages.
IS 1248-1:2003
Electrostatic instruments operate on the principle of electrostatic force of attraction between two charged electrodes. The Kelvin multicellular voltmeter is a classic example of this category, designed to measure high voltages.
TdтАЛ=21тАЛV2d╬╕dCтАЛ тАФ where TdтАЛ is the deflecting torque, V is the applied voltage, C is the capacitance, and ╬╕ is the deflection.
F=21тАЛV2dxdCтАЛ тАФ representing the electrostatic force between parallel plates.
These instruments utilize the force exerted by an electric field on a moving vane. When voltage is applied between a fixed and a moving plate, an electrostatic force is generated that causes a deflection proportional to the square of the voltage. Since the force is independent of the polarity of the voltage, these instruments can be used for both AC and DC measurements.
Electrostatic instruments measure true RMS value regardless of waveform.
They possess extremely high input impedance and draw negligible current.
These are primarily used as voltmeters for high-voltage measurements.
The deflecting torque is proportional to the square of the voltage, resulting in a non-linear scale.
Very low power consumption due to high input impedance.
Can be used for both AC and DC.
No hysteresis or eddy current losses.
Free from errors due to stray magnetic fields.
Non-linear scale (crowded at the beginning).
Low torque-to-weight ratio leading to sluggish response.
Expensive and complex construction for low voltage ranges.
Sensitive to external electric fields.
High voltage measurement in research laboratories.
Standardizing AC voltmeters in precision measurements.
Option A (Wattmeter) is typically electrodynamic type.
Option B (Hot wire instrument) operates on the thermal expansion effect.
Option D (Energy meter) is an induction-type integrating instrument.
C is correct тАФ The Kelvin multicellular voltmeter is the standard example of an electrostatic type instrument.
In competitive exams, remember that electrostatic instruments have a square-law scale, which is why they are often used for high-voltage measurements rather than low-voltage applications where the scale would be too compressed.
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