DC Potentiometer - Working Principle - Construction - Applications - Measurements of Various Parameters
DC Potentiometer
A DC Potentiometer is used for the measurement of EMF’s
of different cells and for calibrating the voltmeters, ammeters, wattmeters
etc. These potentiometers consist of a germen silver or manganin wire. This
wire is one meter long and it is stretched between two terminals. This wire is
connected with Rheostat in series. As shown in figure below:
Measurement of EMF by DC Potentiometer
The switch and the slide wire are set to the standard cell
voltage. The standard voltage are 1.01 volt. The switch S is in calibrate
position and the galvanometer switch K is pressed when the rheostat is adjusted
to zero. A 10Kw resistance is included in a circuit to protect the Galvanometer
from overloading. When the null deflection on the galvanometer is come near,
then the protective resistance is shorted to increase the sensitivity of the
galvanometer. The rheostat is adjusted for the deflection of galvanometer. Now,
the switch is closed to connect the unknown emf with the protective resistance
in the circuit. The potentiometer is adjusted by means of a main dial and a
slide wire. The balance is obtained and the value of unknown EMF is measured.
Applications of DC Potentiometer
DC Potentiometer
is used for the measurement of various electric parameters and for the
calibration of different instruments. The uses of DC potentiometer are
described below:
- Measurement of Electric Current
- Measurement of Voltage
- Measurement of Resistance
- Measurement of Electric Power
- Calibration of Ammeter
- Calibration of Voltmeter
- Calibration of Wattmeter
So, let’s discuss all these measurements and calibrations of
instruments with the help of DC Potentiometer one by one:
Measurement of Electric Current
In this method, the current which is to be measured by the DC
potentiometer is passed through the standard resistance / resistor R as shown
in figure. The standard resistor should be of a value by which voltage drop are
caused with the flow of electric current which is to be measured and its value
may not be exceeded to the value of the potentiometer. The following formula is
used to measure the unknown electric current in amperes from this
circuit which is shown in figure is given below:
Measurement of Voltage
In this method of measurement, DC potentiometer measures the
high voltage. These high voltage are measured by the volt-ratio box with the
potentiometer. This box consists of simple resistance with the various tapping
on its input side as shown in the figure. Each input terminal has its own
maximum voltage and these are corresponding to the multiplying factor for the
voltage scale.
High voltage are applied to the input terminal of the volt-ratio
box/voltage divider and these high voltage leads to the potentiometer through
the voltage divider and the two points are taken by the potentiometer from the
volt-ratio box. The high voltage are measured by the potentiometer. The formula
to measure the high voltage is given below:
Where, v are the voltage measured by the potentiometer and k is
the multiplying factor of the volt-ratio box.
Measurement of Resistance
The resistance which is to be measured is connected in series
with the standard resistor. The rheostat is connected in this circuit to
control the electric current and an Ammeter is also connected in series with
the rheostat and it is used to indicate the value of current is within the
limit of the potentiometer or not. The two pole double throw switch is
connected between the unknown resistance which is to be measured and the
standard resistance. When this switch is put into 1 position, the unknown
resistance is connected to the potentiometer. The reading of the potentiometer
is in VR. then,
And when the switch is thrown to the 2 position, the standard
resistance S is connected to the potentiometer. This reading of the
potentiometer is in VS then the value of unknown resistance is
measured correctly. The formula is given below for the measurement of resistance:
Measurement of Electric Power
In this method, the standard resistance is connected across the
load. The voltage drop are divided across the standard resistor then the load
current will be,
Where VS are voltage drop across standard resistor as
measured by the potentiometer. The total power consumed in a circuit is given
below in the form of formula:
Calibration of Voltmeter
In this calibration, a stable DC voltage supply is used and a
potential divider network is used which consists of two rheostats. One rheostat
is used for coarse and the other is used to control the calibrating voltage. These
controls are used to adjust the supply voltage. The volt-ratio box steps down
the voltage across the voltmeter. These voltage are suitable for the voltmeter
reading. The potentiometer measures the accurate value of voltage and if the
reading of the voltmeter and potentiometer does not match with each other then
there will be opposite connections of the voltage supply in the circuit.
Calibration of Ammeter
In this method, standard resistor is connected in series
with the ammeter. This resistor carries high value of current. The
voltage drop and the current is measured across the standard resistor with the
help of potentiometer by dividing the voltage across the standard
resistor with the standard resistor value. The following formula is used to
measure the current from potentiometer:
Where VS is the voltage across the standard
resistance and S is the value of standard resistance.
Calibration of Wattmeter
In this method of calibration, the current coil of the wattmeter
is connected to the low voltage supply and the voltage coil is connected
to the normal supply through the voltage divider. The voltage across the
voltage coil is measured by the potentiometer directly. The current of
the current coil of wattmeter is measured by measuring the voltage drop across the
standard resistance.
The power will be VI where V is the voltage across the voltage
coil and I is the electric current through the current coil of the wattmeter.
