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Home Physics Class 12 Physics To find resistance of a given wire using Whetstone’s bridge (meter bridge)
Class 12 Physics Lab Experiments

To find resistance of a given wire using Whetstone’s bridge (meter bridge)



Aim

To find resistance of a given wire using Whetstone’s bridge (meter bridge) & hence determine the specific resistance of the material.

Apparatus

A meter bridge (slide Wire Bridge), a galvanometer, a resistance box, a laclanche cell, a jockey, a one- way key, a resistance wire, a screw gauge, meter scale, set square, connecting wires and sandpaper.

Circuit Diagram_Meter bridge

Formulae Used:

(i) The unknown resistance X is given by:

X= (100-ι)/ιx R where

R = known resistance placed in left gap.

X = Unknown resistance in right gap of meter bridge.

ι=length of meter bridge wire from zero and upto balance point (in cm)

(ii) Specific resistance (ρ) of the material of given wire is given ρ = XπD 2/4L

Where,

D: Diameter of given wireL:Length of given wire.

Observation Table for length (ι) & unknown resistance, X:

S.No. AResistance from resistance box R (ohm) Length AB = ι cm Length BC = (100-l) cm Unknown Resistance X=R. (100-ι)/ιΩ
1 2 41 59 2.87
2 4 60 40 2.66
3 6 69 31 2.69

Table for diameter (D) of the wire:

Circular Scale Reading

S.No. Linear Scale Reading (N) mm No. of circular scale divisions coinciding (n) Value n x (L.C.) mm Observed diameter D = N + n x L.C.mm
1 0 34 0.34 0.34
2 0 35 0.35 0.35
3 0 36 0.36 0.36
4 0 35 0.35 0.35

Observations:

  • Least count of screw gauge: 0.001 cms
  • Pitch of screw gauge: 0.1 cm
  • Total no. of divisions on circular scale: 100
  • Least Count =Pitch/No. of divisions on circular scale; LC = 0.001 cm
  • Length of given wire, L = 25cm

Calculation:

For unknown resistance, X:

Mean X = X1X2+ X3+ X 4 / 4 = 2.68Ω

Mean diameter, D= D1D2+ D3+ D 4 / 4 =0.035cm

Mean X = X1X2+ X3+ X 4 / 4 = 2.68Ω

Specific Resistance, ρ= X.πD 2/4L=1.03 x 10 -4Ωcm

Result:

Value of unknown resistance = 2.68Ω

Specific resistance of material of given wire = 1.03 x 10 -4Ωcm

Precautions:

All plugs in resistance box should be tight. Plug in key, K should be inserted only while taking observations.

Sources of Error:

  • Plugs may not be clean.
  • Instrument screws maybe loose.