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Figure of Merit of Galvanometer Experiment (Class 12)

Figure of Merit of Galvanometer Experiment with Readings (Class 12)

To determine the resistance of a pointer type (Weston) galvanometer by half deflection method and find its figure of merit.

Apparatus: The pointer type galvanometer, an accumulator, two resistance boxes Q and R each of range 1 to 1000Ω, a resistance box (P) of low resistance (1 to 10Ω range), rheostat, commutator etc.


The current through the resistors P and Q connected in series with the accumulator of emf E is
I = E/(P + Q)

Therefore, P.d across the resistance P = [E/(P + Q)]P = EP/(P + Q)
Therefore, Current through the galvanometer = [EP/(P + Q)] x 1/G,

If d(divisions) is the deflection in the galvanometer, the figure of merit of the galvanometer,

ig = [EP/(P + Q)] x 1/G x 1/d = [E/(P + Q)] x 1/G x P/d A/div:

Connection are made as shown in the figure. A resistance 1 Q is introduced in P and a resistance 999 Q is introduced in Q. (The values of P and Q are so adjusted that there is appreciate deflection in the galvanometer.)

The circuit is closed and the deflection d (number of divisions on the scale) of the gal-vanometer is noted without introduced any resistance in R (i.e., R = 0). Now, the resistance is introduced in R to reduce the deflection half of the initial value. The resistance introduced in R gives the resistance G of the galvanometer. The commutator is reversed and the experiment is repeated. The mean deflection d and the mean resistance G are calculated. The value (P/d) is calculated.

The whole experiment is repeated for different values of P keeping (P + Q) constant (i.e., 1000 Ω). The mean values of (P/d) and G are calculated. The figure of merit of the galvanometer is calculated using the equation

ig = i/d = [EP/(P + Q)] x 1/G x 1/d = [E/(P + Q)] x 1/G x P/d

Observations and Readings

EMF of the accumulator = E = 2V

P+Q = 1000 Ω



Deflection in galvanometer


Resistance in R for half deflection











































Mean value of P/d = ….. Mean G = ….. Ω

ig = E/(P+Q) x 1/G x P/d = ….. A/div


(i) The resistance of the galvanometer, G.
(ii) Figure of merit of the galvanometer .

Viva Questions of the figure of merit of a galvanometer Experiment

(i) Give the equation for measuring current using a moving coil galvanometer.

(ii) What is meant by figure of merit?

(iii) What is voltage sensitiveness of a moving coil galvanometer?

(iv) What are the advantages of moving coil galvanometer over the tangent galvanometer.

(v) How can you make a galvanometer sensitive?

(vi) What type of galvanometer is used in the laboratory?

Ans: Weston type (or pointed type)

(vii) How are current sensitivity and figure of merit related to each other.

Ans: Current sensitivity (Si) and figure of merit are inversely proportional to each other.

(viii). What is a galvanometer?

Ans: Galvanometer is an electrical instrument used to study very small current.

(ix). What are different types of galvanometer

Ans: i. Moving coil galvanometer

Eg: Western galvanometer

ii. Moving magnet type galvanometer

Eg: Tangent galvanometer

(x). Which of the above galvanometer is better? Why?

Ans: Moving coil galvanometer is better. Because,

i. It is a dead beat galvanometer.

ii. It can be used in any position.

iii. It has a linear scale.

iv. The external magnetic field does not disturb the working of the galvanometer.

(xi). Why the tangent galvanometer named so?

Ans: Tangent law is the basic working principle of tangent galvanometer.

(xii). Why does the presence of an ammeter near a T.G. affect the deflection of the T.G?

Ans: The magnet in the ammeter produces deflection of the compass needle of the T.G.

(xiii). Why a freely suspended magnet always points along north-south direction?

Ans: Earth is a huge magnet with its N-pole situated near geographic south and S-pole near the geographic north. So a freely suspended magnet always points along north-south direction due to the force of attraction of the opposite poles of earth's magnet.

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