Class 12 Physics Lab Experiments

To determine the resistance of a galvanometer by half-deflection method & to find its figure of merit

May 28 2023

Aim

To determine the resistance of a galvanometer by half-deflection method & to find its figure of merit.

Apparatus

A Weston type galvanometer, a voltmeter, a battery, a rheostat, two resistance boxes (10,000Ωand 500Ω), two one-way keys, a screw gauge, a meter scale, connecting wires and a piece of sandpaper.

Formulae Used:

The resistant of the given galvanometer as found by half-deflection method:

G=R.S/R-S

Where R: resistance connected in series with the galvanometer

S: shunt resistance

Figure of merit: k = E/(R+G)θ

Where E : emf of the cell θ:deflection produced with resistance R.

For Half Deflection:

Sr. No.	Resistance R (Ω)	Deflection in galvanometer (θ)	Shunt Resistance S (Ω)	Half Deflection θ/2	Galvanometer Resistance G=RS/R-S(Ω)
1	4500	30	70	15	71.1
2	9500	14	70	7	70.5
3	5200	26	70	13	70.9
4	5700	24	70	12	70.8

Calculation: Mean G= 70.8Ω

For G : Calculate G using formula.

Take mean of all values of G recorded in table.

For k: Calculate k using formula & record in table.

Take mean of values of k.

For Figure of Merit

Sr. No.	EMF of the Cells E(v)	Resistance from R.B. R Ω	Deflection θ(div.)	Figure fo Merit K=E/(R+G)θ
1	1.5x2 = 3	4500	30	2.18 x 10 ^-5
2	3	9500	14	2.23 x 10 ^-5
3	3	5200	26	2.18 x 10 ^-5
4	3	5700	24	2.16 x 10 ^-5

Mean k= 2.19 x 10 ^-5 A/div.

Result:

Resistance of Galvanometer by half – deflection method: G= 70.8Ω
Figure of merit, k= 2.19 x 10 ^-5 A/div.

Precautions:

All the plugs in resistance boxes should be tight.
The emf of cell or battery should be constant.
Initially a high resistance from the resistance box (R) should be introduced in the circuit. Otherwise for small resistance, an excessive current will flow through the galvanometer or ammeter & damage them.