1.3 Idiostatic connection
This is connection generally used in commercial
instruments. In this type of connection, needle is connected to any one of the
pairs of quadrant as shown in the Fig 1, directly without external voltage.
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| Fig. 1 Idiostatic connection |
The moving needle is negatively charged, the
left and quadrant is negatively charged and the right and quadrant is positively
charged. The force of attraction on needle due to top and bottom parts of right
hand quadrant cancel each other. So there is no motion of needle due to top and
bottom parts of left hand quadrant also cancel each other.
Thus the right hand positively charged quadrant
attracts the part of the needle near to left hand quadrant while the left hand
negatively charged quadrant repels the part of the needle to right and
quadrant. This rotates the needle and hence the pointer.
1.3.1Teory of Idiostatic Connection
For idiostatic connection, te external voltage
applied to the needle V1 = 0 V. And the potential of quadrant is nothing but
the voltage V1 which is applied to the needle directly.
VL ... V1 =
While the voltage to be measured is V = VM – VL .
Thus using these values in the expression of Tθ ,
The negative sign is neglected as it indicates
the direction of rotation opposite to that which has been assumed.
Note : As torque is proportional to square of
the applied voltage, the scale is nonuniform for idiostatic connection. Note
that as deflecting torque is proportional to the square of the voltage to be
measured, the idiostatic connection is used for a.c. measurement.
1.4 Kelvin Multicellular Voltmeter
It is one of the most important commercial from
an electric voltmeter. It is basically a quadrant electrometer with large
number of needles and only one quadrant. Basically it is used for a voltage
range of 100 to 1000 volts. By modifying basic voltmeter it is possible to
measure voltages of the range of 40 V only.
Thus to obtain a very high force for very small
voltages, a large number of cells are used in te instruments hence it is called
multicellular. The Kelvin multicellular voltmeter is as shown in the Fig 2.
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| Fig. 2 Kelvin Multicellular Voltmeter |
The moving mechanism is suspended with the help
of Phosphor-Bronze suspension. To protect the suspension against the vibration,
the coach spring is used. For the zero adjustment, warm wheel actuated torsion
head is used. The pointer and scale are of edgewise type alongwith oil immersed
damping vane.
It is very essential to use a safely collar above
the pointer to avoid movements of the system caused due to short circuit of
quadrants. Two guard plates are fitted inside inside the case of instrument.
These are electrically connected to the moving mechanism and metal case of the
instrument.
In such instruments, number of cells used
increases as the voltage to be measured decreases. For example to measure
voltage of the order of 150 V, 10 to 15 cells are required while for the
measurement of 3000 V one cell may be sufficient.
The deflection torque for n cells is given by
