Figure of Merit of Galvanometer Experiment with Readings (Class 12)
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.
Theory:
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,
i_{g} = [EP/(P + Q)] x 1/G x 1/d = [E/(P + Q)] x 1/G
x P/d A/div:
Procedure
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 galvanometer 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
i_{g} = 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
P Ω

Q Ω

Deflection in
galvanometer

P/d

Resistance in R for
half deflection

Left

Right

Mean

Left

Right

Mean

1

999








2

998








3

997








4

996








Mean value
of P/d = ….. Mean G = ….. Ω
i_{g} = E/(P+Q)
x 1/G x P/d = ….. A/div
Result
(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 (S_{i}) 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 northsouth
direction?
Ans: Earth is a huge magnet with its Npole situated near
geographic south and Spole near the geographic north. So a freely suspended magnet
always points along northsouth direction due to the force of attraction of the
opposite poles of earth's magnet.
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