Instrumentation and Measuremnt(EET 204)
Laboratory Module
EXPERIMENT 2
D’ARSONVAL GALVANOMETER
Objective
To find the internal resistance and the current sensitivity of the galvanometer
Equipment
1 d’Arsonval Galvanometer kit
1 digital multimeter
Potentiometer 20kΩ, 10kΩ
Dc supply
Connecting wires
Introduction
The galvanometer contains a coil of wire in a magnetic field, which will experience a
torque when a current passes through the wire of the coil. The coil is attached to a pointer and
a spring so that the amount of deflection of the pointer is proportional to the current in the wire
of the coil.
The value of the load resistor (R1) will be set to a specified value and the potential
difference provided by the power supply will be varied to obtain a full-scale deflection of the
pointer of the galvanometer. The voltage (VFS) required to obtain full-scale deflection will be
recorded, without changing the applied voltage (VFS). Add a shunt resistor (RS) in parallel with
the galvanometer. Vary the load resistance to get the full-scale deflection in galvanometer.
The new load resistance, R2 will be recorded. In both circuits the potential difference supplied
by the power supply is the same, as is the current passing through the galvanometer (fullscale deflection in both circuits).
Application of Kirchhoff’s rules to the two circuits results in the following expression for
the value of the internal resistance of the galvanometer (Rg).
R S (R1 − R 2 )
R2
The current sensitivity (K) can be obtained from the measurement by using this formula
Rg =
V FS
[N (R1 + R 2 )]
Where, N = Number of major divisions of the galvanometer scale for a full-scale deflection of
the pointer
R1 = Load resistor in circuit 1
R2 = Load resistor in circuit 2
RS = Value of shunt resistor parallel to galvanometer
VFS= Voltage at full-scale deflection of the pointer in galvanometer
K =
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Instrumentation and Measuremnt(EET 204)
Laboratory Module
Procedure
1. Adjust the pointer scale of galvanometer of the d’Arsonval galvanometer trainner kit
exactly to 0.
2. Set the resistance box, R1 to 2.2 kΩ. Keep the voltage knob in minimum position such
that voltage output from the 0-5V terminals are 0V. Connect the circuit as in the Figure
2.1 below.
-
G
+
V
R1
Resistance box
Figure 2.1: Set up of circuit connection
3. Turn ON the supply, then the galvanometer pointer will deflect towards right hand side
or left hand side. Next, vary the 0-5 V potential knob until the galvanometer shows its
maximum deflection, which the pointer should comes to 30 positions (divisions).
4. Now, connect the digital multimeter across the 0-5 v terminal to measure the voltage
and record this value. This voltage is called as VFS.
5. Turn OFF the supply and do not disturb the voltage knob. Now connect the resistance,
RS and construct the circuit as in Figure 2.2 below. Select the 10 Ω for RS. Note that,
now R1 is change to R2. Switch ON the instrument, then the pointer of the
galvanometer will return back by a few divisions.
RS
-
G
+
V
R2
Resistance box
Figure 2.2: The circuit diagram with shunt resistor, RS
6. Next, do not disturb the voltage knot, then adjust the resistance box, R2 until the
pointer scale comes to full scale deflection. Observe the pointer of the galvanometer.
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Instrumentation and Measuremnt(EET 204)
Laboratory Module
7. Turn OFF the supply and disconnect R2. Measure the absolute values of R2 using
multimeter and record the value in the table.
8. Finally disconnect the galvanometer from other components and connect multimeter
across the terminal of galvanometer to measure its internal resistance. Record the
value as Rg in the table.
9. The values of VFS, R1, R2, RS, and Rg. are known, so determine the galvanometer
resistance, Rg by calculation. Then calculate its current sensitivity, K. Repeat the step
from 5 with RS = 5 Ω and 2 Ω and R1 = 1.2 k Ω.
10. Calculate the Mean of calculated Rg and Mean of K in the table below.
11. Plot the Graph of shunt resistor (Rs) for 2.2k and 1.2k versus current sensitivity (K).
Analyse the graph.
Note: Record all value for R2, which is measured from multimeter only
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Instrumentation and Measuremnt(EET 204)
Laboratory Module
EET 204
LABORATORY REPORT
EXPERIMENT 2: D’ARSONVAL GALVANOMETER
NAME
MATRIX NO.
COURSE
TOTAL MARKS
* Laboratory report must be submitted at the end of lab session. All delayed/pending report will be penalised.
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Instrumentation and Measuremnt(EET 204)
Laboratory Module
Name:______________________________ Matrix no.:_____________ Date:_________
RESULT
Table 2.3
N
R1
(Ω)
VFS
(V)
RS
(Ω)
2.2k
10
2.2k
5
2.2k
2
1.2k
10
1.2k
5
1.2k
2
R2
(Ω)
Rg (Ω)
Meas.
Calc.
%e
Mean
of Rg
K
(A/div)
Mean
of K
30
*Record all value for R2 which is measured from multimeter only
Instructor Approval: …………………………….
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Date: ………………
Mark:
/8
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Instrumentation and Measuremnt(EET 204)
Laboratory Module
Name:______________________________ Matrix no.:_____________ Date:_________
CALCULATION
Instructor Approval: …………………………….
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Date: ……………………..
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Instrumentation and Measuremnt(EET 204)
Laboratory Module
Name:______________________________ Matrix no.:_____________ Date:_________
Graph Rs (Ω) v K (A/div)
Instructor Approval: …………………………….
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Date: ……………………
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Instrumentation and Measuremnt(EET 204)
Laboratory Module
Name:______________________________ Matrix no.:_____________ Date:_________
DISCUSSION:
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CONCLUSION:
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Instructor Approval: …………………………….
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Date: ……………………
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Instrumentation and Measuremnt(EET 204)
Laboratory Module
Name:______________________________ Matrix no.:_____________ Date:_________
PROBLEMS
1. A dc meter ampere with internal resistance 99Ω has a 0.1mA of full-scale deflection
current flow through it. Calculate the total current flow through the meter in full-scale
deflection when the shunt resistance is 1Ω.
2. How can the basic d’Arsonval meter movement be used to measure currents?
Draw the circuit.
3. A moving coil meter has a 1 kΩ of internal resistance and gives a full scale deflection
with 1 mA. Determine how it can be used to measure current up to 50 mA ammeter.
Instructor Approval: …………………………….
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Date: …………………….
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