The document discusses the conversion of a galvanometer into an ammeter and voltmeter by using shunt and high resistance, respectively. It explains the principles behind current and voltage measurements, the definitions of ammeter and voltmeter, and their operating features, including their resistance and connection types. It also outlines the differences between ammeters and voltmeters, emphasizing their respective roles in measuring current and potential difference in electrical circuits.

1. Moving Coil Galvanometer
Class 12 th Physics
Priyanka Jakhar
Physics Lecturer
GGIC Vijay Nagar
Ghaziabad U.P.
1 Conversion of Galvanometer into Ammeter and Voltmeter
2 Differences between Ammeter and Voltmeter

2. Shunt :-- Shunt is a low value of resistance which is connected in parallel with the Galvanometer to protect it
from strong current .
A galvanometer can be used for various electrical circuit
For detecting the current flowing in the circuit .
For finding the null point .
If due to any reason or strong current passes through the coil of the Galvanometer then coil of the Galvanometer
may burn due to large heat produced pointer of the Galvanometer may be broken by hitting the stop pin.
In order to save the Galvanometer from the problem ,a low resistance known as Shunt is connected in parallel
with its coil then a major portion of the current pass through the shunt and only a small portion passes through
the coil due to it the galvanometer remain safe.
In order to find the passing through the Galvanometer circuit
i = total current in the circuit
G = resistance of the galvanometer
S = resistance of the shunt
𝒊 𝒈 = current passing through Galvanometer
𝒊 𝒔 = current passing through shunt
Gi
S
𝒊 𝒔 = i - 𝒊 𝒈
𝒊 𝒈
𝒊 𝒔 = i - 𝒊 𝒈
i = 𝒊 𝒔 + 𝒊 𝒈
𝒊 𝒈 = i - 𝒊 𝒔

3. 𝑽 𝑨𝑩 = 𝒊 𝒔S = 𝒊 𝒈G
𝒊 𝒈G = 𝒊 𝒔S = ( i - 𝒊 𝒈 ) S
𝒊 𝒈G = i S - 𝒊 𝒈 S
𝒊 𝒈 = 𝒊 (
𝑺
𝑮+𝑺
)
𝒔𝒊𝒎𝒊𝒍𝒂𝒓𝒍𝒚 𝒊 𝒔 = 𝒊 (
𝑺
𝑮+𝑺
)
Uses
(1) Flow of current through galvanometer can be reduced and galvanometer can be protected
from the heat generated due to flow of large current.
(2) It can be used to measure large current.
( 3) It is used for converting a galvanometer into an ammeter .
( 4) it is also used for increasing the range of ammeter .

4. Conversion of Galvanometer to Voltmeter:
Galvanometer can be converted into voltmeter by connecting
it with a very high resistance.
Voltmeter is a high resistance galvanometer which is used to
measure the P .D. Between the two points of a circuit in volt.
GIg R
V
The value of required resistance depends upon the range of the voltmeter can be
calculated as
V = P.D. to be measured by the galvanometer
G = Resistance of the galvanometer
n = Total number of scale divisions in the Galvanometer
K = figure of merit or current which produces a deflection of one division in the
Galvanometer
𝒊 𝒈 = n k current which produce full scale deflection in the galvanometer.
𝒃𝒚 𝒐𝒉𝒎𝒔 𝒍𝒂𝒘 𝒊 𝒈 =
𝑽
𝑮+𝑹

5. R =
𝑽
𝒊 𝒈
- G
Resistance of voltmeter will be
voltmeter is series combination of the Galvanometer and a high resistance R ,so its resistance
is much greater then that of the galvanometer.
Resistance of voltmeter is very high and the resistance of an ideal voltmeter is infinite .
Voltmeter is always connected in II with those points across which voltage is to be measured
. Voltmeter has high resistance so it draw a very small amount of current and therefor p. d.
between the two points remains unaffected .
Higher is the range of voltmeter to be prepared from a given Galvanometer higher will be the
value of resistance required for the purpose.
An voltmeter of lower range has a lower resistance then voltmeter of higher range.
The range of an voltmeter can be increased as well as decrease .
𝑹 𝑨 = R + G
<
G
<
R + G =
𝑽
𝒊 𝒈

6. Conversion of Galvanometer to Ammeter:
Galvanometer can be converted into ammeter by shunting it with a very small resistance.
An ammeter is a low resistance Galvanometer which is used to measure the current in a circuit
in amperes.
A Galvanometer as such it cannot be used as an ammeter because it has appreciable
resistance and it can measure only a limited current .
A galvanometer can be converted into ammeter by connecting a low value
resistance (known as Shunt ) in parallel with the galvanometer.
The value of Shunt depends upon the range of the required ammeter and
can be calculated as
i = Maximum current to be measured by the galvanometer
G = Resistance of the galvanometer
n = Total number of scale divisions in the Galvanometer
K = figure of merit or current which produces a deflection of one division in the Galvanometer
𝒊 𝒈 = current which produce full scale deflection in the galvanometer.
For measuring the current I ,a shunt of register S is connected in parallel with the Galvanometer so that
out of total current I a part 𝒊 𝒈 passes through the galvanometer while remaining part (i - 𝒊 𝒈) passes through
the Shunt .
Since G and S are connected in parallel so potential difference across them will be same .
Potential difference across the galvanometer and shunt resistance are equal.
GI Ig
I = I - Is g
S

7. 𝒊 𝒈G = ( i - 𝒊 𝒈 ) S
S = (
𝒊 𝒈
i − 𝒊 𝒈
) G
Resistance of ammeter will be
ammeter is parallel combination of the Galvanometer and the shunt resistance so it is less
than that of shunt.
Resistance of ammeter is very small and the resistance of an ideal ammeter is zero .
Ammeter is connected in the circuit in series so that the entire circuit current passes
through it. Because of its small resistance ,ammeter does not change the current in the
circuit to be measured.
Higher is the range of ammeter to be prepared from a given Galvanometer lower is a value
of required shunt .
An ammeter of lower range has a higher resistance as compared to ammeter of higher
range.
The range of an ammeter can be increased however it cannot be decrease .
𝟏
𝑹 𝑨
=
𝟏
𝑮
+
𝟏
𝑺
𝑹 𝑨 < S

8. The ideal ammeter has zero internal resistance. But practically
the ammeter has small internal resistance . The measuring
range of the ammeter depends on the value of resistance.
Symbolic Representation
The capital alphabet A represents the ammeter in the circuit.
Connection of Ammeter in Circuit
The ammeter is connected in series with the circuit so that the
whole electrons of measurand current passes through the
ammeter.
The power loss occurs in ammeter because of the measurand
current and their internal resistance.
The ammeter circuit has low resistance so that the small
voltage drop occurs in the circuit.
Ammeter
The meter uses for measuring the current is known as the ammeter. The current is the flow
of electrons whose unit is ampere. Hence the instrument which measures the flows of
current in ampere is known as ampere meter or ammeter.
A

9. The resistance of the ammeter is kept low because of the two
reasons.
•The whole measurand current passes through the ammeter.
•The low voltage drop occurs across the ammeter.
By the current, the ammeter categorizes into two types.
•AC ammeter- The instrument which measures AC is known as the AC ammeter .
•DC ammeter-The instrument which measures DC is known as the DC ammeter .

10. Voltmeter
The instrument which measures the voltage or potential difference in volts is
known as the voltmeter. It works on the principle that the torque is generated
by the current which induces because of measurand voltage and this torque
deflects the pointer of the instrument. The deflection of the pointer is directly
proportional to the potential difference between the points.
The voltmeter is always connected in parallel with the circuit.
Symbolic Representation of voltmeter
The voltmeter is represented by the alphabet V inside the circle
along with the two terminals.
V

11. The voltmeter always connects in parallel with
the circuit - so that the same voltage drop occurs
across it. The high resistance of the voltmeter combines
with the impedance of the element across which it is
connected.
And the overall impedance of the system is equal to the
impedance that the element had.
Thus, no obstruction occurs in the circuit because of the
voltmeter, and the meter gives the correct reading.
Types of voltmeter
Analogue Voltmeter -AC voltmeter is used in the AC circuit for
alternating voltage measurement.
Digital Voltmeter-The instrument which measures the direct
current is known as the DC voltmeter.

12. Difference between Ammeter and Voltmeter:
S.No. Ammeter Voltmeter
1
It has low resistance. It has high resistance.
2 Resistance is GS / (G + S) Resistance is G + R
3 Shunt Resistance is (G I ) / (I – I )
and is very small.
gg
Series Resistance is (V / I ) - G and is
very high.
g
4 It is always connected in series. It is always connected in parallel.
5 Resistance of an ideal ammeter is zero. Resistance of an ideal voltmeter is infinity.
6 Its resistance is less than that of
the galvanometer.
Its resistance is greater than that of the
voltmeter.
7
It is a current measuring device which
measures current through the circuit .
It is a voltage measuring device which
measure the potential difference between
the two points of the circuit .

13. 8.
9.
10.
Deflecting torque is produced by current to be
measured directly .
deflecting torque is produced by current which is
proportional to the voltage to be measured .
It is not possible to decrease the range of
the given ammeter.
It is possible to decrease the range of the
given voltmeter.
It is more accurate than voltmeter . If accuracy is less than ammeter.
A V