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Physics Investigatory Project To study the earth's magnetic
field using a compass needle -bar magnet by plotting
magnetic
field lines and tangent galvanometer.
Physics HL (Achariya Higher Secondary School)
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To study the earth's magnetic field using a compass needle -bar magnet by
plotting magnetic field lines and tangent galvanometer.
An investigatory project submitted towards partial fulfillment of credit for
The All India Senior School Certificate Physics Practical Examination-2025.
Submitted by
DIVYA VYSALI
Grade XII
Under the guidance of
Dr. A.MARTIN JOSEPH
PGT - Physics
ACHARIYA BALA SIKSHA MANDIR
Fishing Harbor Rd, Thengaithittu,
Puducherry – 605110.
2024-2025
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BONAFIDE CERTIFICATE
This is to certify that DIVYA VYSALI bearing Register Number :
____________________ is student of class XII, Achariya Bala Siksha Mandir, Fishing
Harbor Rd, Thengaithittu, Puducherry has completed the investigatory project entitled “ To
study the earth's magnetic field using a compass needle -bar magnet by
plotting magnetic field lines and tangent galvanometer ” during the academic year
2022-2023 towards the partial fulfillment of credit for the Physics Practical Examination of
All India Senior School Certificate Examination and submitted satisfactory report, as
compiled in the following pages, under my guidance.
Internal Examiner
Countersigned by
The Principal
External Examiner
.
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DECLARATION
I hereby declare that the investigatory project work entitled “ To study the earth's
magnetic field using a compass needle -bar magnet by plotting magnetic field
lines and tangent galvanometer ” submitted to the Department of Physics, Achariya
Bala Siksha Mandir, Puducherry is prepared by me. All the works are the result of my
personal efforts. This work has not been previously submitted for any other purpose.
DIVYA VYSALI
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ACKNOWLEDGEMENT
I would like to express a deep sense of gratitude to Mrs. Kavitha ., our Principal
for his coordination, in extending every possible support for the completion of this project.
My sincere thanks to my project guide Dr. A. Martin Joseph for guiding me
immensely through the course of project. He always evinced keen interest in my work. His
conservative advice and constant motivation have been helpful for the successful completion
of the project.
I also thank my parents for their motivation and support. I thank my class mates for
their timely help and support for the completion of the project.
At last, I would like to thank all those who had helped directly and indirectly towards
the completion of this project.
DIVYA VYSALI
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TABLE OF CONTENTS
Overview
Aim of the Project
Apparatus and Materials required
Theory
Procedure
Observations and Graph
To determine the horizontal
component of earth’s magnet Field
(H}
From graph
Result Precautions
Sources of error
Facts
Applications
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OVERVIEW
The tangent galvanometer was first described in
an 1837 by Claude-Servais- Mathias Pouillet, who
later employed this sensitive form of galvanometer
to verify Ohm’s law. To use the galvanometer, it is
first set up on a level surface and the coil aligned
with the magnetic north-south direction. This means
that the compass needle at the middle of the coil is
parallel with the plane of the coil when it carries no
current. The current to be measured is now sent
through the coil, and produces a magnetic field,
perpendicular to the plane of the coil and is directly
proportional to the current.
The magnitude of the magnetic field produced
by the coil is B; the magnitude of the horizontal
component the Earth’s magnetic field is B’.The
compass needle aligns itself along the vector sum
of B and B’ after rotating through an angle Ø from
its original orientation. The vector diagram shows
that tan Ø = B/B’. Since the magnetic field of the
Earth is constant, and B depends directly on the
current, the current is thus proportional to the
tangent of the angle through which the needle has
turned.
AIM OF THE PROJECT
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The aim of the project is to study the Earth’s
magnetic field and find its value (BH) using a
tangent galvanometer.
Tangent galvanometer made byTopview of a
tangentgalvanometer
J.H. Bunnell Co. around 1890.made about 1950.
APPARATUS
REQUIRED
AND
MATERIALS
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 Tangent Galvanometer (TG),
 Commutator (C),
 Rheostat (R),
 Battery (E),
 Ammeter (A),
 Key (K), etc
THEORY
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Tangent galvanometer is an early measuring
instrument for small electric currents. It consists of a coil
of insulated copper wire wound on a circular nonmagnetic frame. Its working is based on the principle of
the tangent law of magnetism. When a current is passed
through the circular coil, a magnetic field (B) is produced
at the center of the coil in a direction perpendicular to the
plane of the coil.
The working of tangent galvanometer is based
on the tangent law. It is stated as when a magnet is
suspended freely in magnetic field F and H, the magnet
comes to rest making an angle θ with the direction H such
that,
F = H tan θ(1)
When a bar magnet is suspended in two Magnetic fields B and Bh, it
comes to rest making an angle θ with the direction of Bh.
Let a current I be passed through the coil of radius R,
having turns N. Then magnetic field produced at the
centre of coil is,
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F=
μ0
2 π ∈ ¿ ¿ (2)
4π
R
Let H is the horizontal component of earth’s magnetic
field and the magnetic needle comes to rest at angle θ
with the direction of H, then according Eq. (1),
Htanθ=
μ0
2π∈ ¿ ¿
4π
R
Htanθ=10−7 2 π ∈ ¿ ¿
R
−7
H=2 π ×10 ∈
¿ ¿
Rtan θ
(3)
by substituting the value of current I , from eq. (3),
( )
μ 2 πN
tanθ
(4)
= 0
I
4 π RH
radius of coil of galvanometer R, deflection θ and N, the
value of H can be calculated.
PROCEDURE
Connections are made as shown in the figure given below,
where K is the key, E the battery, A the ammeter, R the rheostat,
C the commutator, and T.G the tangent galvanometer. The
commutator can reverse the current through the T.G coil without
changing the current in the rest of the circuit. Taking the
average of the resulting two readings for deflection averages
out, any small error in positioning the TG coil relative to the
earth’s magnetic field H .
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PROCEDURE
EXPERIMENT
FOR
PERFORMING
THE
1.Make the circuit connections in accordance with the
circuit diagram.
2.Using spirit level, level the base and the compass
needle in compass box of tangent galvanometer by
adjusting the leveling screw.
3.Now rotate the coil of the galvanometer about its
vertical axis, till the magnetic needle, its image in the
plane mirror fixed at the base of the compass box
and the coil, i.e. all
4.these three lie in the same vertical plane. In this
setting,
…………..9
5.the ends of the aluminium pointer should read zerozero. If this is not so, rotate the box without
disturbing the position of the coil till at least one of
the ends of the pointer stands at the zero marks.
6.By closing the key K, the current flow in the
galvanometer. Read the both ends of the pointer.
Now reverse the direction of current by using the
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reversing key. When the mean values of both
deflections shown by the pointer in the two cases (i.e.
before and after reversing the current) differ by more
than 1o, then turn slightly the vertical coil until the
two values agree. This will set the plane of the coil
exactly in the magnetic meridian.
7.By adjusting the rheostat, bring the deflection in
galvanometer around 45o. The deflection should not
be outside the range (30o-60o).
8.Record the reading of the ammeter and the deflection
of the compass needle in the box shown by two ends
of pointer on the scale.
9.Reverse the current in the coil of galvanometer and
again record the current and deflection of needle.
10. By changing the value of current, take four or
more set of readings and plot the graph between I
and tanθ. The graph will be a straight line.
11. Measure the inner and the outer diameter of the
coil with a half metre scale at least three times.
………….10
OBSERVATIONS
1.Range of the Ammeter –
2. Least count of Ammeter –
3. Zero error in Ammeter –
4. Number of turns used (N) –
Table 1. For variation of θwith I
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S.No.
1.
2.
3.
4.
5.
Value of deflection, θ
(degree)
For direct
For reverse
current
current
Mean
tanθ
θ1
θ2
θ3
θ4
35
49
36
50
45
35
47
36
50
45
35
60
55
65
64
35
64
58
68
65
35
53.6
46.25
58.2
53.8
Ammeter reading
(A)
Obs
0.70
1.36
1.04
1.61
1.37
0.15
0.20
0.25
0.30
0.27
Corrected
0.15
0.20
0.25
0.30
0.27
Table 2. For radius of tangent
Galvanometer
S.No.
1.
Inner
diameter d1
(cm)
Outer
diameter d2
(cm)
Mean
diameter
d
Mean
radius
(cm)
16.0 ×10
16.40 ×10
16.20 ×10
8.10 ×10
−2
2.
16.16 ×10−2
3.
16.06 ×10
−2
−2
16.08 ×10−2
−2
16.10 ×10
−2
−2
16.12× 10−2
16.08 ×10
Mean radius of coil R =8.04 × 10−2
GRAPH
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−2
8.06 ×10−2
−2
8.04 × 10
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BC
Slope of straight line = AC
m=
tanθ
I
Now substitute the m in Eq. (4),
μ 0 2 πN
m = 4 π RH
Then, H =7.6867 ×10 T
−8
RESULT
The value of earth’s magnetic field by using a tangent
galvanometer is
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H = 7.6867 ×10 T
−8
PRECAUTIONS
1.The battery should be freshly charged.
2. The magnetic needle should swing freely in the
horizontal plane.
3.The plane of coil must be set in magnetic meridian.
4.There should be no parallax in noting down the
readings of ammeter and deflection.
5.All the readings should be adjusted between 30 o and
60o.
SOURCES OF ERROR
1.There may a magnetic material around apparatus.
2.The plane of coil will not be exactly in the magnetic
meridian.
FACTS
The tangent galvanometer is an early measuring
instrument for Current
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  The magnetic field produced by a circular coil
carrying current I is Proportional to I .
  The S.I unit of magnetic field is Tesla .
  The magnitude of horizontal intensity of earth’s
magnetic field is3.5x10⁻⁵T .
  For better result while doing tangent galvanometer
experiment, the deflection should be in between 30o60o.
  The value of μ₀ is4πx10⁻⁷NA⁻²
APPLICATIONS
 T.G. can be used to measure the magnitude of the
horizontal component of the geomagnetic field.
 The principle can be used to compare the
galvanometer constants.
 For calibration of secondary instruments.
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