Figure of Merit of Galvanometer Experiment

Aim: 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


(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.