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Home Physics Class 12 Physics To determine the resistance of a galvanometer by half-deflection method & to find its figure of merit
Class 12 Physics Lab Experiments

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



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.

Circuit Diagram

Formulae Used:

  1. 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

    1. 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:

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

    Precautions:

    1. All the plugs in resistance boxes should be tight.
    2. The emf of cell or battery should be constant.
    3. 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.

    Sources of Error:

    1. Plug of the resistant boxes may not be clean.
    2. The screws of the instruments maybe loose.
    3. The emf of the battery may not be constant.