1. The document describes an experiment to measure the Earth's magnetic field using a tangent galvanometer. A circuit is constructed including a tangent galvanometer, magnet, and current source. Current is passed through the circuit and deflections of the galvanometer needle are measured. 2. Calculations are done to determine the reduction factor of the tangent galvanometer and the horizontal intensity of the Earth's magnetic field at the location. 3. The results found the reduction factor to be 0.19682 A and the horizontal magnetic field intensity to be 7.6867 x 10-5 T.

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LIST OF CONTENTS
S.No Topic Page no.
1. Aim 2
2. Introduction 3-7
3. Apparatus and Materials Required 8
4. Theory 9
5. Procedure 10
6. Observation Table and Calculation 11
7. Result 12
8. Precautions and Sources of error 12
9. Bibliography 13

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AIM:
To study the Earth’s magnetic field using a
compass needle bar magnet by plotting magnetic
field line and tangent galvanometer.
Tangent Galvanometer

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INTRODUCTION:
Earth’s magnetic field also known as the geomagnetic field, is the
magnetic field that extends from the earth’s interior to where it
meets the solar wind, a stream of charged particles emanating
from the sun. Its magnitudeatthe Earth’ssurface ranges from25 to
65 micro teslas 90.23 to 0.65 gauss). Roughly speaking it is the field
of magnetic dipole currently tilted at an angle of about 10 degrees
with respect toEarth’s rotational axis as if these were a bar magnet
placed at that angle at the centre of the Earth.
Unlike a bar magnet, however Earth’s magnetic field changes over
time because it is generated by a geodynamic.
The North andSouth magnetic poles wanderwidely, but sufficiently
slowly for ordinary compasses to remain useful for navigation.
However at irregular intervals averaging several hundred,
thousand years, the Earth’s field reverses and the North and South
magnetic poles relatively abruptly switch places. These reversals
of the geomagnetic poles leave a record in rocks that are of value to
paleomagnetists in calculatinggeomagneticfields in the past.Such
information in turn is helpful in studying the motions of continents
and ocean floors in the process of plate tectonics. The
magnetosphere is the region above the ionosphere and extends
several tens of thousands of kilometres into space, protecting the
earth from the charged particles of the solar wind and cosmic rays
that would otherwise strip away the upper atmosphere including
the ozone layer one stripping mechanism is for gas to be caught in
bubbles of magnetic field which are ripped off by solar winds.

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The intensity of the field is often measured in gauss ( G), but is
generally reported in nanoteslas (nT), with 1G = 100,000nT. A
nanotesla is also referred to as a gamma (γ). The tesla is the SI unit
of the magnetic field, B. The field ranges between approximately
25000 and 65000 nT (0.25 - 0.65G).

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Near the surface of the Earth, its magnetic field can be closely
approximated by the field of a magnetic dipole positioned at the
centre of the earth and tilted at an angle of about 10° with respect to
the rotational axis of the Earth. The dipole is roughly equivalent to a
powerful bar magnet, with its South Pole pointing towards the
geomagnetic North Pole. The North Pole of a magnet is so defined
because if allowed to rotate freely, it points roughly northward (in
thegeographicsense).Sincethenorthpoleofamagnetattractsthe
south poles of other magnet and repels the north pole, it must be
attracted to the south pole.
Bar magnet compass:
A magnetic field is invisible,but it can be detected using a magnetic
compass.Acompasscontainsasmallbarmagnetonapivotsothat
it can rotate. The compass needle points in the direction of the
Earth’s magnetic field or the magnetic field of the magnet.
Magneticfieldscanbe mappedout using smallplottingcompasses:
 Place the plotting compass near the magnet on a piece of
paper.
 Mark the direction of the compass needle points.
 Move the plotting compass to many different positions in
themagneticfield,markingtheneedledirectioneachtime.
 Join the points to show the field lines.
 The needle of a plotting compass points to the south pole
of the magnet.
The Earth’s Magnetic field:

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The behaviour of a compass shows that the earth has a magnetic
field. Scientists believe that this field is produced by convection
currents in the Earth’s core, which is made from iron and nickel.
When a plotting compass is placed in the Earth’s magnetic field, the
northpoleofthecompasswilllineupwiththeEarth’smagneticfield
lines and point to the magnetic south.
Drawing a Magnetic field:
The diagram shows the magnetic field around a bar magnet.
The diagram shows these key features:
The magnetic field lines never cross each other.
The closer the lines, the stronger the magnetic field.
The lines have arrowheads to show the direction of force
exerted by a magnetic north pole.
The arrowheads point from the North Pole of the magnet
to the South Pole.
Tangent Galvanometer:

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Definition
 Tangent galvanometer is the device which was used to
measure small amounts of electric current.
Principle
The tangent galvanometer works on the principle of tangent law.
Tangent law of Magnetism
 The tangent law of magnetism states that the tangent of the
angleofacompassneedlewhichisduetothemovementunder
the influence of magnetic field is directly proportional to the
ratio of strengths of two perpendicular magnetic fields.
 In simpler words, the tangent of the angle made by the moving
needle under the magnetic field directly indicates the strength
of the perpendicular magnetic fields.

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APPARATUS AND MATERIALS REQUIRED:
 Accumulator
 Rheostat
 Ammeter
 Commutator
 Tangent galvanometer
 White paper sheet
 Drawing board
 Adhesive tape
 Bar magnet
 Iron fillings
 Magnetic compass

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THEORY:
 The reduction factor of tangent galvanometer is K= I / tan θ
where I is the current flowing through tangent
galvanometer which produces the deflection θ.
 The horizontal intensity of earth’s magnetic field at a place
Bh = µonK/2r, where n is the number of turns of coil, µo = 4π x
10-7
N A-2
isthepermeability of free space,Kisthe reduction
factor of the tangent galvanometer and r is the radius of the
coil of the tangent galvanometer.
Circuit Diagram:
When a bar magnet is suspended in two magnetic fields B and Bh, it
comes to rest making and angle θ with the direction of Bh.

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PROCEDURE:
The circuit is made as shown in the diagram. The plane of the coil is
madeverticalbyadjustingthelevellingscrews.Theplaneofthecoil
is made parallel to (90-90) in the compass box. The whole tangent
galvanometer is rotated to read (0-0) at the ends of the aluminium
pointer. Now the plane of the coil is in the magnetic meridian.
The commutator keys are put. The rheostat should be adjusted for
deflection in the tangent galvanometer between 10 and 60. For a
current I, the deflections of the pointer θ1 and θ2 are noted. The
commutator is reversed. The deflections of the pointer θ3 and θ4
are noted. The average of the four readings is the deflection θ. From
the theory of tangent galvanometer, I = K tan θ.
By varying the current the experiment is repeated. Using a string
the circumference of the coil is measured. Hence its radius is
found. Let n be the number of turns of the coil. The horizontal
intensity at the place is given by Bh = µonK/2r.
OBSERVATION TABLE:

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CALCULATION:
Mean K = 0.19682
 The reduction factor of TH = 0.19682
 Number of turns of the coil = 50
 Circumference of the coil (S) = 2πr = 50.49 cm
 Radius of the coil r = S/2π = 8.04 x 10-2
cm
 Horizontal intensity at the place Bh = µonK/2r
= 2πnK x 10-7
/r
= 7.6867 x 10-8
T
For different values of current I, deflections are noted and values are
calculated. Knowing K, n and r the value of horizontal intensity Bh can be
calculated.

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RESULT:
1. The reduction factor of Tangent Galvanometer, K = 0.19682 A
2. Horizontal intensity at the place, Bh= 7.6867 x 10-5
T
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 30o
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.
CONCLUSION:
Experiment in tangent galvanometer gives the reduction factor of
galvanometer and horizontal intensity of Earth’s magnetic field.
BIBLIOGRAPHY

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 Tangent Galvanometer (Procedure):Comprehensive Physics
Activities Volume I :Laxmi Publications Pvt Ltd.
 Tangent Galvanometer (Theory): Comprehensive Physics
Activities Volume I : Laxmi Publications Pvt Ltd.
 Tangent Galvanometer (Precautions and Sources of
error):Comprehensive Physics Activities Volume I : Laxmi
Publications Pvt Ltd.
 Galvanometer:http://physics.kenyon.edu/EarlyApparatus/El
ectrical_Measurements/Tangent_Galvanometer/Tangent_Gal
vanometer.html
 Galvanometer:Wikipedia,thefreeencyclopediaen.wikipedia.or
g/wiki/Galvanometer