Page 140 - Physics - XII

2
where, K RB is called reduction factor.
H

0 N
∴ I ∝ tanθ
The current to be measured by a tangent galvanometer is directly proportional to tangent of defl ection of
the magnetic needle of tangent galvanometer. This is known as tangent law.
From the slope of the graph between I and tanθ, the value of horizontal component of Earth’s magnetic
fi eld can be obtained.

Procedure
1. Assemble the circuit as shown in Fig. 8.2.

K + E – Rh

I

I
T.G.
Reversing
key
+ –
A
Fig. 8.2: Circuit diagram to study the Earth’s magnetic fi eld using a tangent galvanometer (T.G.)

2. Using the spirit level, set the level of the base and the compass needle in compass box of a tangent
galvanometer by adjusting the levelling screw.
3. Rotate the coil of the tangent galvanometer about its vertical axis, till the magnetic needle, its image
in the plane mirror fi xed at the base of the compass box, and the coil, all lie in the same vertical plane.
At this position, the ends of the aluminium pointer should read 0 – 0. If this is not so, then 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.

4. Close the key K, now, the current fl ows through the tangent galvanometer. Read both ends of the
pointer. Now, take the mean of values shown by both ends of the pointer.
5. By using the reversing key, reverse the direction of current. Read both ends of the pointer again and
take the mean of values shown by both ends of the pointer.

6. Take the diff erence between two mean values measured in steps (4) and (5) and observe the diff erence.
If it diff ers by more than 1°, then turn vertical coil slightly until the two values agree. This will set the
plane of the coil exactly in the magnetic meridian.
7. Bring the defl ection in galvanometer around 45° by adjusting the rheostat. Ensure that the defl ection
should be in the range (30° – 60°).
8. Note the readings of the ammeter and the defl ection of the compass needle in the box shown by two
ends of the pointer on the circular scale.
9. Now, reverse the direction of current in the coil of galvanometer. Note the readings of current and
defl ection of needle.
10. Take the mean of four values of defl ection (two values by each end of the pointer) and then fi nd the
value of tanθ, where θ is the mean value of defl ection.

138