Tangent Galvanometer A tangent galvanometer galvanometer is an early measuring measuring instrument used for the measurement of electric current. It works by using a compass needle ne edle to compare a magnetic field generated by the unknown current to the magnetic field of the Earth. It gets its name from its operating principle, the tangent law of magnetism, which states that the tangent of the angle a compass needle makes is proportional to the ratio of the strengths of the two perpendicular magnetic fields. It was first described by Claude Pouillet in 1837. A tangent galvanometer galvanometer consists consists of a coil of insulated copper wire wound on a circular nonmagnetic frame. The frame is mounted vertically on a horizontal base provided with leveling screws. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0° to 90°. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax, a plane mirror is mounted below the compass needle.
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 magnet ic 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 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), ( 5), the horizontal intensity of earth’s magnetic field Bh is,
The initial adjustments are done as
follows
The plane of the coil is made vertical by adjusting the leveling screws.
The compass box alone is rotated so that the 90-90 line in the compass box is in the plane of the coil.
The T.G as a whole is rotated till the Aluminium pointer reads 0-0.
Note down the number of turns in the coil. A suitable current current is allowed allowed to pass through the coil. Note down the current as well as the deflection in T.G. Reverse the current and note the deflection again. Repeat the procedure for different values of current. Measure the radius of the coil from its perimeter.
Determine the reduction factor and horizontal intensity of earth's magnetic field.
Procedure
1. Place the tangent galvanometer on an inverted wooden box to raise it above the ferrous metal of the laboratory table. Remove all ferrous metal (such as certain mechanical pencils) from the immediate vicinity of the tangent galvanometer. Turn the thumbscrew to free the pointer, but do not raise the pointer so high that it pushes against the glass plate. Orient the instrument so that the pointer reads zero. If the pointer is bent, each end should be equally close to zero.
2. Place the circular level on top of the glass plate. Level the tangent galvanometer by rotating one or more feet. Remove the level some distance from the instrument. If necessary, rotate the box to re-zero the pointer.
3. Refer to the the circuit diagram in section IV. Set the VOLTAGE and CURRENT control knobs on the triple-output power supply to zero (full counter-clockwise position). Use a banana plug lead to connect the positive terminal of the ammeter to the positive terminal of the power supply (red banana jack). Connect the negative terminal of the ammeter (0.1-A range) to a clip on the wiring board. Connect the 50 turn binding post of the tangent galvanometer to the same wiring board clip. Connect the common binding post of the tangent galvanometer (the post with no number next to it) to another clip of the wiring board. Connect one fixed terminal of the 50 Ω resistor to the same clip. Use a banana plug lead to connect the other fixed terminal of the 50 Ω resistor to the negative terminal of the power supply (black banana jack). Set the ammeter (with its internal magnet) some distance from the tangent galvanometer. 4. Turn the power supply on and rotate the CURRENT control knob to its far counter-clockwise position. While observing the ammeter, adjust the VOLTAGE control knob until that the
o
tangent galvanometer reads 20 . If the ammeter reading fluctuates noticeably, tighten all connections. Record the current. o
o
o
Repeat for angles of 30 , 40 and 50 . Change the ammeter range, if necessary, to keep it on scale.
5. Reverse the direction of the current through through the the coil (NOT THROUGH THE AMMETER) and repeat.
6. Change to the 5 turn binding post of the tangent galvanometer galvanometer and the 1-A range of the ammeter. Replace the 50 Ω resistor by the 5 Ω resistor. Repeat steps 4 and 5.
7. Lock the pointer, but do not move the tangent galvanometer.
8. Use outside calipers and a meter stick to measure the outside diameter of the coil. Measure the actual windings, not the metal frame. 9. Measuring the inside diameter of the windings windings is a bit bit more difficult, because the compass is in the way. Also, we must compensate for the thickness of the metal frame. Measure and record the thickness of the metal which makes up the frame. Using the calipers as outside calipers, open them until they span the inside diameter of the metal frame. Place them on the meter stick; read and record the distance.
Observations and Number of turns in the coil
calculations
=.........
Circumference of the coil, 2πa =…........cm =…........cm Radius of the coil, a
=…........cm =…........cm
Range of Ammeter
=………..A
Least count of Ammeter
=……….. A
1. To find the radius of the coil of Tangent Galvanometer
S.No.
Inner Diameter (d1) (cm)
Outer diameter (d2) (cm)
Mean Diameter (d1 + d2) (cm)
Mean radius (cm)
2. To determine the reduction factor of T.G: Trial No.
Ammete r Readin g (I)A
Pointer deflection in degrees Mean θ (degree )
Direct Reverse θ1
θ2
tanθ tanθ (degree )
θ3
θ4
Note: Take deflection between 30 and 60 degrees.
Graph
K=I/tan (A)
BH (T)
Calculations Take two points A and B wide apart on the straight line graph. Draw perpendiculars from B on the x-axis and that from A on the y-axis intersecting each other at point C. Now, Slope of graph =
So, Slope,
=
=m
Now compute the value of B H by using the relation BH =
= ……………gauss
Result
The value of earth’s magnetic field by using a tangent galvanometer and measuring current by an ammeter is
_________________
Acknowledgement
I would like to express my special thanks of gratitude to my Physics teacher, Mrs. Shikha Jain as well as our principal who gave me the golden opportunity to do this wonderful project which also helped me in doing a lot of Research and i came to know about so many new things I am really thankful to them. Secondly i would also like to thank my parents and friends who helped me a lot in finalizing this project within the limited time frame.
ANKUR PALMIA
Certificate This is to certify that Ankur Palmia (22) student of th class 12 – B, – B, has completed the project titled To
determine the strength of the earth’s magnetic field during the academic year 2014 – – 2015 towards partial fulfillment of credit for the
Physics practical evaluation of CBSE, and submitted satisfactory report, as compiled in the following pages, under my supervision.
Department of Physics
Oxford Sr. Sec. School
Aim
To determine the strength of Earth’s magnetic field using a Tangent Galvanometer.
