Attracted Disc Type Electrostatic Voltmeter - Kelvin Absolute Electrometer

     An attracted disc type electrostatic instrument is basically known as a portable electrostatic instrument. This device contains two semicircular plates one being stationary and the other rotating. Both the plates are electrically insulated from each other.

     The unknown voltage to be measured is applied across the plates, which results in the development of an electrostatic field between the plates. The electrostatic field developed is attractive in nature. The below shows the portable electrostatic instrument.

Attracted Disc Type Electrostatic Voltmeter

     Due to the electrostatic force of attraction, the moving plate gets attracted towards the stationary plate and gets deflected. The control spring fixed to the rotating plate provides the required controlling torque and the air friction provides the necessary damping.

Kelvin Absolute Electrometer :

     Kelvin's absolute electrometer is an attracted disc type electrostatic voltmeter.

Attracted Disc Type Electrostatic Voltmeter

Construction of Kelvin Absolute Electrometer :

The essential parts of kelvin absolute electrometer are,
  • Fixed plate or disc
  • moving plate or disc
  • Coach spring
  • Guard ring
  • Micrometer head.
     In the above figure, the moving disc is suspended by a micrometer head with the help of a coach spring in a way such that it comes exactly at the center of the fixed plate or disc. This movable disc is surrounded by a guard ring, which is electrically in contact with it. A small gap is maintained between the guard ring and the moving plate. The main function of the guard ring is to reduce the fringing effect. Lenses and finely pointed rod are also used, in order to set the moveable plate to zero setting.

Working of Kelvin Absolute Electrometer :

     A voltage that is to be measured is applied between the two plates. When a potential difference is applied between the plates then the moving plate will be attracted towards the fixed plate. But by rotating the micrometer head the moveable plate can be brought back to its original location. A micrometer which is calibrated in terms of force is used to measure the displacement of the moving plate.

     In order to get the best result, the calibration must be done in a proper way. When calibrating the instrument, initially the instrument is short-circuited, and also the moveable plate is set to its original location i.e., at zero position. Then the known weights are added to the plate, and then the moment of micrometer required to bring back the moveable plate to its original locations are noted for various known weights. Thus the voltage obtained will be in terms of the fundamental units of force, length, and area.

Equation for Force of Attraction :

The expression for the force of attraction between the plates of an electrostatic voltmeter is given by,
Attracted Disc Type Electrostatic Voltmeter
The equation for parallel plate capacitance is given by,
Attracted Disc Type Electrostatic Voltmeter
  • A = Area of plate
  • ε = Permittivity of the medium = ε0 εr
  • d = Distance between two plates
Differentiating the above equation with respect to x,
Attracted Disc Type Electrostatic Voltmeter
The negative sign indicates that, with the decrease in distance between two plates, capacitance increases. Thereby, neglecting the negative sign,
Attracted Disc Type Electrostatic Voltmeter
     Therefore from the above voltage equation which is in terms of force and linear dimensions. We can say that the instrument gives an absolute determination of voltage.

Advantages of Electrostatic Voltmeters :

  • Their power consumption is very small.
  • These can be used where a wide range of voltages is required to measure.
  • They can be used to measure both ac and dc voltages.
  • By using capacitor multipliers, very high voltages (>10kV) can be measured.

Disadvantages of Electrostatic Voltmeters :

  • The scale is not uniform.
  • Electrostatic voltmeters cannot measure low voltages.
  • Cost is high and constructionally not robust.

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