Page 29 - Physics - XII

where R is the resistance connected in series with the galvanometer and S is the shunt resistance.
Figure of merit K is given as
E 1
K
RG
where θ is the defl ection produced with resistance R and E is the e.m.f. of the cell.

Procedure
A. Resistance of Galvanometer by Half-defl ecti on Method

1. Make the connections accordingly as shown in Fig. 4.1.
2. Clean insulations of wires with the help of sand paper. Check that all the plugs of the resistance boxes
are tight.
3. Now, introduce a high resistance (about 5000 Ω) in the resistance box R and close the key K .
1
4. Adjust the resistance R with the help of resistance box so that the defl ection in the galvanometer is
maximum. Record this value of R and defl ection θ.
5. Now, insert the key K also and without changing the value of R, adjust the value of shunt resistance S
2
so that defl ection in the galvanometer reduces to exactly half the value θ, i.e. θ/2. Note the value of S
and θ/2.
6. Repeat the steps 3 to 5 three times more by taking out diff erent values of R and adjusting S everytime.

B. Figure of Merit

7. Measure the e.m.f. of the battery E with the help of the voltmeter by connecting +ve terminal of the
voltmeter with +ve of the battery and –ve of the voltmeter with -ve terminal of the battery.

8. Make the connections accordingly as shown in Fig. 4.2.
9. Introduce a resistance R from the resistance box and insert the plugs in key K to get the defl ection θ in
the galvanometer. Note these values of R and θ. 1

10. Now, change the value of R from the resistance box and note the galvanometer defl ection again.
11. Repeat the steps 9 and 10 for three more observations.

Observations
Table for resistance of galvanometer by half-defl ection method
S. Resistance Defl ection in the Shunt Resistance Half defl ection Galvanometer Resistance
No. R galvanometer θθ S θ θ/2 RS
(Ω) (divisions) (Ω) (divisions) G= R S

(Ω)
1.
2.

3.
4.

Mean value of the Galvanometer resistance G = _____ Ω

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