PhysicsProject
Name:- S.Ramesh Kumar
Roll No:-13
Class:- 12-C
Topic:- Moving coil galvanometer
MOVING COIL GALVANOMETER
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Certificate:This is to certify that S.Ramesh Kumar, a student of
Class 12th (computer Science) has successfully
completed the project on the above mentioned topic
under the guidance of Ms.Angel(Subject Teacher)
during the year 2022-2023 in partial fulfillment of
Physics Practical Examination conducted by
Signature of the Examiner
Signature of subject teacher
MOVING COIL GALVANOMETER
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Acknowledgement:-
The success and final outcome of this
project required a lot of guidance and assistance from
many people. I am extremely privileged to thanks
Mrs.Angel mam (Subject Teacher) for providing me an
opportunity to do the project work and giving me all
support and guidance which made me complete the
project appropriately. She was always supportive and
inspirational for completing this project. I am also
extremely thankful to all my friends for providing me all
the necessary support and guidance.
MOVING COIL GALVANOMETER
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Objective: To study the basic schematic
structure of a moving coil galvanometer and
the basic process underlying the conversion
of a moving coil galvanometer into an
ammeter and a voltmeter.
References: NCERT Class 12 Physics Textbook
 http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
 http://www.brainkart.com/article/Moving-coil-galvanometer
MOVING COIL GALVANOMETER
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Contents:





Introduction on moving coil galvanometer
Construction
Working
General structure of a moving coil galvanometer
Conversion of a Galvanometer into an Ammeter
Conversion of a Galvanometer into a Voltmeter
MOVING COIL GALVANOMETER
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Moving Coil Galvanometer: Introduction:A galvanometer is an electromechanical instrument for detecting
and indicating electric current. A galvanometer works as an
actuator, by producing a rotary deflection (of a "pointer"), in
response to electric current flowing through a coil in a constant
magnetic field. Galvanometers developed from the observation
that the needle of a magnetic compass is deflected near a wire that
has electric current flowing through it, first described by Hans
Oersted in 1820. They were the first instruments used to detect and
measure small amounts of electric currents. Sensitive
galvanometers have been essential for the development of science
and technology in many fields. Galvanometers also had widespread
use as the visualising part in other kinds of analog meters, for
example in light meters, VU meters, etc., where they were used to
measure and display the output of other sensors.
 Principle:When a current carrying coil is suspended in a uniform magnetic
field it is acted upon by a torque. Under the action of this torque,
the coil rotates and the deflection in the coil in a moving coil
galvanometer is directly proportional to the current flowing
through the coil.
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 Construction:-
Schematic Diagram of a Moving Coil Galvanometer
It consists of a rectangular coil of thin
insulated copper wires having a large
number of turns. The horseshoe
magnet has cylindrically concave polepieces. Due to this shape, the magnet
produces radial magnetic field so that
when coil rotates in any position its
plane is always parallel to the direction
of magnetic field. When current flows
through the coil it gets deflected. A
soft iron cylinder is fixed inside the coil
such that the coil can rotate freely
between the poles and around the
cylinder. Due to the high permittivity,
the soft iron core increases the
strength of the radial magnetic field.
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 Working:When a current flows through the coil, a torque acts on it. This torque
is given by the equation
𝑟 = 𝑁𝑖𝐴𝐵 where the symbols have their usual meaning. Since the
field is radial by design, we have taken sin 𝜃 = 1 in the above
expression for the torque. The magnetic torque 𝑟 = 𝑁𝑖𝐴𝐵 tends to
rotate the coil. A spring Sp provides a counter torque 𝑟 = 𝐾𝜑 that
balances the magnetic torque 𝑟 = 𝑁𝑖𝐴𝐵; resulting in a steady
angular deflection 𝜑.
In equilibrium, 𝐾𝜑 = 𝑁𝑖𝐴𝐵 where 𝐾 is the torsional constant of the
spring; i.e. the restoring torque per unit twist. The deflection
𝜑 is indicated on the scale by a pointer attached to the
𝑁𝐴𝐵
spring. We have 𝜑 = ( 𝐾 ) 𝑖.
The quantity given in brackets is a constant for the galvanometer.
Hence, GalvanometerConstant G can be expressed as:𝐺 = 𝑁𝐴𝐵
𝐾
∴ 𝜑 = 𝐺𝑖
∴𝑖∝ 𝜑
So, the current through the coil varies linearly with the deflection
and so, the current flowing through the coil can be known by
measuring the deflection.
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The galvanometer can be used as a detector to check if a current is
flowing in the circuit (this configuration is used in the Wheatstone’s
bridge arrangement). In this usage the neutral position of the
pointer (when no current is flowing through the galvanometer) is
in the middle of the scale and not at the left end. Depending on the
direction of the current, the pointer deflection is either to the right
or the left.
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 Current Sensitivity of Galvanometer:The current sensitivity of a galvanometer is defined as the
deflection produced when unit current passes through the
galvanometer. A galvanometer is said to be sensitive if it produces
large deflection for a small current.
Factors increasing
Current Sensitivity: Increasing the magnetic field B by using strong
permanent horse shoe shaped magnet.
 Increasing the number of turns N. But number of
turns of the coil cannot be increased beyond a
certain limit. This is because the resistance of the
galvanometer will increase subsequently and hence
the galvanometer becomes less sensitivity.
 Increasing the area of the coil A. But it will make the
galvanometer bulky and ultimately less sensitive.
 Decreasing the value of restoring force constant k by
using a flat strip of phosphor
– bronze instead of circular wire of phosphor –
bronze. Quartz fibers can also be used for suspension
of the coil because they have large tensile strength
and very low value
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of K.
 Voltage Sensitivity of Galvanometer:The voltage sensitivity of a galvanometer is defined as the
deflection per unit voltage.
 An interesting point to note is that, increasing the current
sensitivity does not necessarily, increase the voltage sensitivity.
When the number of turns (n) is doubled, current sensitivity is
also doubled (equation 1). But increasing the number of turns
correspondingly increases the resistance (G). Hence voltage
sensitivity remains unchanged.
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Factors increasing Voltage Sensitivity: Increasing number of turns of the coil (N)
 Increasing magnetic field intensity (B)
 Increasing area of the coil (A)
 Decreasing restoring torque per unit twist of the
suspension (k)
 Decreasing resistance (G)
 Advantages of a Moving Coil Galvanometer: The sensitivity of the galvanometer can be increased
by increasing N, B and A whiledecreasing the value of k.
 The instrument has a linear scale.
 Since the instrument uses high value of B, the
deflection is undisturbed by the earth’smagnetic field.
 As the coil is wound on a nonmagnetic metallic
frame, damping is produced by eddycurrents. As a result the
coil quickly assumes the final position.
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 Conversion of
a
Galvanometer
to anAmmeter and a
Voltmeter:-
 Conversion of a Galvanometer into an Ammeter:-
The galvanometer cannot as such be used as an
ammeter to measure the value of the current in a given
circuit. This is for two reasons: (i) Galvanometer is a
very sensitive device, it gives a full- scale deflection for
a current of the order of µA. (ii) For measuring
currents, the galvanometer has to be connected in
series, and as it has a large resistance, this will change
the value of the current in the circuit. To overcome
these difficulties, one attaches a small resistance S,
called shunt resistance, in parallel with the
galvanometer coil; so that most of the current passes
through the shunt.
The value of shunt resistance depends on the fraction
of the total current required to be passed through the
galvanometer. Let Ig
be the maximum current that can be passed
through
the
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galvanometer. The current Ig will give full scale deflection
in the galvanometer.
Galvanometer Resistance = G
Shunt Resistance= S
Current in the circuit = I
∴ 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑡ℎ𝑒 𝑠ℎ𝑢𝑛𝑡 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝐼𝑠 =𝐼 − 𝐼
Since the galvanometer and the shunt resistance are
connected in parallel, the potentialdifference across both of
them is same
∴ 𝐼𝑔. 𝐺 = (𝐼 − 𝐼𝑔). 𝑆
𝐼
∴ 𝑆 = 𝐺. 𝑔
(𝐼 − 𝐼𝑔)
The shunt resistance is very small because Ig is only a fraction
of I.
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The effective resistance of the ammeter Ra is (G in parallel
with S):-
𝐺. 𝑆
=
𝑅𝑎
𝐺+𝑆
Ra is very low and this explains why an ammeter should be
connected in series. When connected in series, the ammeter does
not appreciably change the resistance and current in the circuit.
Hence an ideal ammeter is one which has zero resistance.
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 Conversion of a Galvanometer into a Voltmeter:Voltmeter is an instrument used to measure potential difference
between the two ends of a current carrying conductor. A
galvanometer can be converted into a voltmeter by connecting a
high resistance in series with it. The scale is calibrated in volt.
The value of the resistance connected in
series decides the range of the voltmeter.
Galvanometer Resistance = G
The current required to produce full scale
deflection in the galvanometer = Ig
Range of
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Voltmeter = VResistance to be
connected in series = R
Since R is connected in series with the galvanometer, the
current through the galvanometer,
𝑉
=
∴ 𝐼𝑔
𝑅+𝐺
𝑉
∴𝑅= − 𝐺
𝐼𝑔
From the equation the resistance to be connected in
series with the galvanometer iscalculated. The
effective resistance of the voltmeter is:∴ 𝑅𝑣 = 𝑅 + 𝐺
Rv is very large, and hence a voltmeter is connected in
parallel in a circuit as it draws the least current from
the circuit. In other words, the resistance of the
voltmeter should be very large compared to the
resistance across which the voltmeter is connected to
measure the potential difference. Otherwise, the
voltmeter will draw a large current from the circuit and
hence the current through the remaining part of the
circuit decreases. In such a case the potential
difference measured by the voltmeter is very much
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less than the actual potential difference. The error is
eliminated only when the voltmeter has a high
resistance. An ideal voltmeter is one which
has infinite resistance.
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