Electrostatic Voltmeter

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Electrostatic Voltmeter

Q. Explain with neat diagrams the principle of operation of an electrostatic voltmeter Discuss its advantages and limitations for high voltage measurements.

Electrostatic Voltmeter

Principle :

In electrostatic fields the attractive force between the electrodes of a parallel plate condenser is given by,

F = I – 𝛿ws /ds I = I 𝛿 / 𝛿S  (1 / 2CV2) I= I1/ 2 V2  𝛿C /  𝛿SI

    = 1 / 2 ∈0
= V2 A /S2

    = 1 / 2 ∈0
= (V/S)2

Where,

V –  Applied voltage between plates

C – Capacitances between the plates,

S –  Separation between plates.

A – Area of cross-section of the plates.

∈0  – Permittivity of the radium (air or free space)

WS– Work done in displacing a plate.

When one of the electrodes is free to move, the force on the plate can be measured by controlling it by a spring or balancing it with a counter weight.

Force < (applied voltage)2

Electrostatic Voltmeter

Construction :

  1. Therefore it can be used for AC and DC measurements. It consist of parallel plate disc type electrodes separated by a small distance.
  2. The hemispherical metal dome D encloses a resistive balance B which measures the force of attraction between the movable disc which charges from one of its arms and larger plate P. 
  3. Movable electrodes M charges with a clearance of above 0.01cm in a central opening in the upper plate union serves as a guard ring.
  4. Diameter of each plates is 1 m.
  5. Light reflected from a mirror carried by balanced beam magnifier its motion and indicated when equilibrium condition is replaced.
  6. As spacing between two electrode is large the uniformity of electric field is maintained by the guard rings H which surrounds the space between discs F and M.
  7. The guard rings Hare maintained at a constant potential in space by C capacitances divider ensuring a uniform special potential distribution resistance.

D – Hemispherical metallic done.

B – Spherical balance.

M – Mounting plate

G – Guard plate

F – Fixed plate diameter in m.

H – Guards rings

C – Capacitances divider

Merits of Electrostatic Voltmeter :

  1. Low loading effect as only electric fields are required to be set up.
  2. Due to high resistances of the medium between plates, the active power loss negligibly small.
  3. Can be used for high frequency 
  4. Used for measurement of AC and DC voltages.

Demerits of Electrostatic Voltmeter :

  1. Not used for small voltage as operating force becomes less.
  2. Expensive, larger size, not robust.
  3. Non uniform scale.
  4. Humidity effects on resistance and consumption increase.
  5. Not suitable for very high frequency above few MHz.

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