2. D.TamilArasu
Certificate
This is to certify that the project work titled ………………………………………….
……………………………………………………….............is record of original work
done by ………….…… with registration number …………... under my
supervision and guidance.
The Principal Teacher in charge
Submitted for Practical Examination held on ____________________
3. Internal Examiner External Examiner
Acknowledgement
I would like to express my sincere gratitude to our principal
Mr.Krishnagiri , for helping us with providing all the equipments
for the project work and for moral support. And also to my
Physics Teacher Mr. Ajay Babu for giving us the wounderful
opportunity to do a case study and providing vital support,
guidance and encouragement throughout the project. Without his
motivation and help, the successful completion of this project
would not be possible.
Secondly I would also like to thank my friends who helped me a
lot in finalizing this project within the limited time frame.
4. Contents
*Overview
*Aim of the Project
*Apparatus and Materials required
*Theory
*Procedure
*Procedure for performing the experiment
*Observation
*Graph
*Result
*Precautions
*Source of error’s
*Applications
5. 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.
6. AIM OF THE PROJECT
The aim of the project is to study the Earth’s Magnetic and find its
value (BH) using a tangent galvanometer.
Tangent galvanometer made by Top view of a tangent
galvanometer J.H. BunnellCo.around1890. made about 1950
APPARATUS AND
MATERIALS REQUIRED
* Tangent galvanometer (TG),
* commutator (C),
* rheostat (R),
* battery (E),
* ammeter (A),
* key (k).
8. THEORY
Tangent galvanometer is an early measuring instrument for small
electric currents. It consists of a coil of insulated copper wire
wound on a circular non-magnetic 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=Htanθ (1)
9. When a bar magnet is suspended in two magneticbfields B and Bh,
it comes to rest making an angle θ with the directions of Bh.
Let a current I be passed through the coil of radius R, having
turns N. the magnetic field produced at the centre of the coil is,
F= μ02 πIN/4 πR (2)
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= μ02 πIN/4πR
Htanθ=10-17
*2πIN/R
H=2π*10-17IN/Rtanθ (3)
By substituting the value of current I, from Eq.(3),
Tanθ/I=( μ0/4π)*( 2πN/RH) (4)
Radius of coil of galvanometer R, deflection θ and N, the value of
H can be calculated.
10. Procedure
Connections are made as shown in the figure given below, where
K is the key, E is the battery, A is the ammeter, R is the rheostat, C
the commutator, and T.G is 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 T.G coil relative to the earth’s magnetic
field H.
PROCEDURE FOR PERFORMING THE
EXPERIMENT
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,
11. 3. Now rotate the coil of the galvanometer about its vertical
axis, 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,
5. The ends of the aluminium pointer should read zero-zero. 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 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 10, 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 450. The deflection should not be
outside the range(300-600).
8. Record the readings 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 meter scale at least three times.
12. 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 θ WITH I
s.no
Value of deflection θ
(degree)
mean tan θ Ammeter
reading
(A)
For direct
current
For reverse
current
θ1 θ2 θ3 θ4
Obs corrected
1.
2.
3.
4.
5
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
0.70
1.36
1.04
1.61
1.37
0.15
0.20
0.25
0.30
0.27
0.15
0.20
0.25
0.30
0.27
13. table 2. for radius of tangent
galvanometer
s.no Inner
diameter
d1 (cm)
Outer
diameter
(cm)
Mean
diameter
d
Mean
radius
(cm)
1.
2.
3.
16.0 x 10-2
16.16 x 10-2
16.06 x 10-2
16.40 x 10-2
16.08 x 10-2
16.10 x 10-2
16.20 x 10-2
16.12 x 10-2
16.08 x 10-2
8.10 x 10-2
8.06 x 10-2
8.04 x 10-2
Mean radius of coil R=8.04 x 10-2
GRAPH
14. Slope of straignt line=BC/AC
m= tanθ /I
Now substitute the m in Eq.(4),
m=(μ0*2πN)/(4πRH)
Then,
H=7.6867 x 10-8
T
Result
The value of earth’s magnetic field by using a tangent
galvanometer is
H=7.6867 x 10-8
T
15. 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 deflections.
5. All the readings should be adjusted between 300 and 600.
Sources of error
1. There may a magnetic material around apparatus.
2. The plane of coil will not be exactly in the magnetic
meridian.
Applications
1. T.G can be used to measure the magnitude of the
horizontal component of the geomagnetic field.
2. The principle can be used to compare the galvanometer
constants.
