Index Introduction Materials Required Principle Diagram Theory Procedure Observations Calculations Result Precautions Sources of error Bobliography

Materials Required

Tangent galvanometer (TG), Rheostat (R), Battery (E), Ammeter (A), Key (k) Reversing key Connecting wires Metre scale

Principle

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.

    

Diagram

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 TG is arranged in such a way

that the horizontal component of earth’s

magnetic field (Bh) is in the direction of the

plane of the coil. The magnetic needle is

then under the action of two mutually

perpendicular fields. If θ is the deflection

of the needle, then according to tangent

law,

 

 

Let I is the current passing through the coil

of radius a with n turns, then the field

generated by the current carrying circular

coil is,

 

 

Equating (1) and (2), we get,

                                                               

                    

     

 The left hand side of equation (4) is a

constant and is called the reduction factor

(K) of the given tangent galvanometer.

Now from equation (3) & (5), the horizontal

intensity of earth’s magnetic field Bh is,

 

Procedure:For performing in real life

Connections are made as shown in the figure, 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 Bh .

The magnetic field can be calculated as follows:

1. Make the circuit connections in accordance with the circuit diagram.

2. Using a spirit level, level the base and 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 of the mirror fixed at the base of the compass box and the coil ie all these three lie in the same vertical plane. In this setting, the ends of aluminum pointer should reads 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 mark.

4. By closing the key K, the current flows in the galvanometer. Read the both ends of the pointer. Now reverse the direction of current by using the reserve key. when the mean values of deflections shown by the pointer in 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.

5. By adjusting the rheostat, bring the deflection in galvanometer around 45 degree. The deflection should not be outside the range(30-60).

6. Record the reading of the ammeter and the deflection of the compass needle in a box shown by two ends of pointer of the scale.

7. Reverse the current in the coil of the galvanometer and again record the current and deflection of needle.

8. By changing the value of current, take four more set of reading and plot the graph between I and tan. The graph will be a straight line.

9. Measure the inner and outer diameter of the coil with a half metre scale at least 3 times.

Observations

Range of ammeter r=_____________A

Least count of ammeter = __________A

Zero error in ammeter = __________A

Number of turns used (N) = __________

Table for Variation of with I.

S. No.

Value of deflection, Mean

Tan

Ammeter Reading (A)For direct

currentFor reverse current

1 2 3 4 Observed Corrected1.

2.

3.

4.

5.

Table for radius of tangent galvanometer

S. no. Inner diameter d1

Outer diameterd2

Mean Diameter d

Mean Radius

1.2.3.

Calculations

ResultThe value of earth’s magnetic field by using tangent galvanometer is

H = _________, G = _________T

Precautions1. 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.All the readings must be adjusted between 30o and

600.

Sources of error1.There may be a magnetic material around the

apparatus.2. The plane of the coil will not be exactly in the

magnetic meridian.

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.

Bibliography http://amrita.vlab.co.in/?

sub=1&brch=192&sim=1049&cnt=2 Comprehensive Physics Practical Book www.wikipedia.org/wiki/Galvanometer o Tangent Galvanometer (Procedure) :

Electricity & Magnetism Virtual Lab : Physical

Sciences : Amrita Vishwa Vidyapeetham Virtual Lab

amrita.vlab.co.in/?sub=1&brch=192&sim=1049&cnt=2

o Tangent Galvanometer (Theory) : Electricity & Magnetism Virtual Lab : Physical

Sciences : Amrita Vishwa Vidyapeetham Virtual Lab

amrita.vlab.co.in/?sub=1&brch=192&sim=1049&cnt=1

o Tangent Galvanometer

http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Tangent_Galvan ometer/Tangent_Galvanometer.html

o Galvanometer - Wikipedia, the free encyclopedia

O bse r va t i o ns a n d G r ap h

Radius of the coil, a = 7.72cm Number of turns in the coil = 2 Circumference of the coil,2πa =

48.5cm

Trail

Number

Ammeter

Reading

(A)

Pointer Deflection in DegreesMeanɵ(degree)

tanɵ(degree)

K=1/tanɵ(A)

Bh

(I)Direct Reverse

ɵ1 ɵ2 ɵ3 ɵ4

1.

2.

3.

4.

5.

6.

7.

8.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.11

10

20

30

40

45

50

55

60

10

20

30

40

45

50

55

60

15

30

40

50

55

60

65

70

15

30

40

50

55

60

65

70

12

25

35

45

50

55

60

65

o.22

0.57

0.77

1.00

1.19

1.43

1.73

2.14

2.27

1.75

2.14

2.00

2.10

2.09

2.03

1.87

3.67

2.83

3.46

3.24

3.40

3.38

3.28

3.03

9

3.To de t e r mi n e th e r e d uc t i o n fac to r o f T.G. : Reduction factor K of the tangent galvanometer can be determined. From the graph drawn as,

K=A B BC

101

R e sul t : -

The reduction factor of the given tangent galvanometer,

K = 2 . 03 _ A

Horizontal component of Earth’s magnetic field, Bh = 3 .2 8 x10 4 T

111