The attracted disc type instruments are
generally used for the measurement of voltage above 20 Kv. The system consists
of two plates such that one plate can move freely while other is fixed. Both
the plates are perfectly insulated from each other. The voltage to be measured
is applies across the plates as a supply voltage as shown in the Fig 1. Due to
the supply voltage, electrostatic field gets produced which develops a force of
attraction between the two plates. Due to the force of attraction, the movable
plate gets deflected. In this mechanism the controlling torque is provided by a
spring.
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| Fig.1 Attracted Disc Type Electrostatic Voltmeter |
1.1Kelving Absolute Electrometer
This instrument is of attracted disc type
electrostatic voltmeter. The basic structure of this instrument is as shown in
the Fig 2.
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| Fig.2 Kelving Absolute Electrometer |
The moving disc is suspended exactly above the
centre of fixed form a micrometer head with the help of a spring. The moving
disc is surrounded by a ring called guard ring, keeping some air gap between
moving disc and ring. This guard ring is very useful in reducing fringing
effects. It is connected electrically with the moving disc.
With a help of a device consisting lenses and
finely pointed rod it is possible to determine zero setting of a disc
consisting a fine cross-air.
In actual practice, the potential to be
measured is supplied to the two disc. Due to the force attraction, the moving
disc is attracted downwards. With the help of micrometer head, the moving plate
is brought back to zero position setting again. This movement required is
observed accurately.
For accurate measurement, it is very important
to calibrate the instrument properly. During the calibration, first instrument
is short circuited. Then the moving plate is set to its zero position. Then
known weights are added to the disc. Then the calibration is done by observing
the movements required to bring the moving disc back to the original zero
position. Thus instrument measures the force of attraction produced by the
potential difference between the two discs. Then the potential difference can
be expressed interms of this force.
1.2 Theory of Attracted Disc Type
Voltmeter
As we have seen already that the force of
attraction F between the two parallel plates with potential difference V
between them is given by,
Let A be the area of each plate and ε =
ε0εr is the permittivity
of the medium. Let d be the distance between the two plates.
Then the capacitance C is given by,
Differentiating equation (2) with respect to x,
The negative sign indicates that with decrease
in distance separation d, capacitance C increase. So neglecting negative sigh,
the force of attraction can be rewritten as,
The potential difference V between the two
plates is given by,
Thus this instrument gives an absolute determination of voltage as it is given interms of force and linear dimension. The deflecting force is adequate only when the voltage to be measured is high. For avoiding the errors due to corona effect, special construction is necessary to ensure good insulation. The superior dielectric strength of a high vacuum is used in modern instruments which enables to get more force a given voltage with very small clearance between the plates.
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