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Home Physics Class 12 Physics To convert the given galvanometer (of known resistance & figure of merit) into an ammeter of desired range & to verify the same
Class 12 Physics Lab Experiments

To convert the given galvanometer (of known resistance & figure of merit) into an ammeter of desired range & to verify the same



Aim

To convert the given galvanometer (of known resistance & figure of merit) into an ammeter of desired range & to verify the same.

Apparatus

A Weston type galvanometer whose resistance & figure of merit are given, a constantan or manganin wire, a battery, one-way key, a rheostat, a milli-ammeter, connecting wires, sand paper etc.

Circuit Diagram

Formulae Used:

To convert a galvanometer which gives full scale deflection for current IGinto an ammeter of range O to IOamperes, the value of required shunt is given by: S= [I G/IO-I G]G

Required shunt resistant S is made using a uniform wire whose, specific resistance is σ (known) & its length:

ι=πr 2S/σ

Observation:

Given resistance of galvanometer, G = 70.8 Ω

Given value of figure of merit, k = 2.19 x 10 -5A div -1

Current for full scale deflection, IG= NOx k = 6.57 x 10 -4A

  • Calculation of value of shunt resistance:
  • *Required range of converted ammeter, IO= 3A

    *Value of shunt resistance, S=[IG/IO-IG]x G=0.0155Ω

    *Computing the length of the wire to make resistance of 0.155Ω

  • Observations for diameter of the wire:
  • Pitch of screw gauge, p = 1 mm

    No. of division of circular scale = 100

    Least count, a = 0.01 mm

    Zero error, e = 0.0 mm

    Diameter of the wire = 0.98 mm, Radius = 0.049 cm

  • Specific resistance of material of wire, ρ=1.92 x 10 -6Ω cm
  • Required length of the wire,cm
  • Required length of the wire,ι= S x π r/2/ σ=0.0155x3.14x(0.049)2/1.72 x 10 -6cm = 60.8cm

  • Table for Verification

    S.No. Galvanometer Reading/Deflectionθ/Current I1=θ x LC Ammeter Reading I2 n x LC Error (I2-I1)A
    1 3 3x 0.1=0.3 6x 0.5=0.3 0.0
    2 5 0.5 11 x 0.05=.55 0.05
    3 7 0.7 15 x 0.05=.75 0.05
    5 9 0.9 19 x 0.05=.95 0.05

    Verification:

    Checking the performance of the converted ammeter: Current indicated by full scale deflection (N O) of converted ammeter. I O= 3A

    Least count of converted ammeter,k' = Io/No= 0.1A/div.

    Result:

    • Current IG for full scale deflection = 6.57 x 10 -4A
    • Resistance of shunt required to convert the galvanometer into ammeter, S=0.0155Ω
    • Required length of wire, ι=60.8 cm
    • As error ι’–ι is very small, conversion is verified.

    Precautions & Sources of Error:

    • All connections should be neat & tight.
    • The diameter of the wire for making shunt resistance should be measured accurately for diameter is taken in two mutually perpendicular directions.
    • The terminal of the ammeter marked positive should be connected to positive pole of the battery. Also ammeter should be in series with circuit.

    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 & Sources of Error:

    1. All connections should be neat & tight.
    2. The diameter of the wire for making shunt resistance should be measured accurately for diameter is taken in two mutually perpendicular directions.
    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.
  • The terminal of the ammeter marked positive should be connected to positive pole of the battery. Also ammeter should be in series with circuit.