To find the resistance of a given wire by using meter bridge | Class 12 | Physics Practical Copy | Handwritten with observations

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Aim: To find the resistance of a given wire by using meter bridge.

Meter Bridge:

It consists of one metre long constantan wire AC of uniform cross-sectional area mounted on a wooden board with a scale. The two ends of the wire are attached to terminals A and C. Thick metal strips bent at right angles are used to provide two gaps to connect resistors forming a Wheatstone’s bridge. The terminal B between the gaps is used for connecting galvanometer and other end of the galvanometer is connected to a jockey J.

01

Principal:

A meter bridge works on the principle of Wheatstone’s bridge. it consists of four resistors P, Q, R and X connected in the form of a diamond ABCD. The terminals A and C are connected to two terminals of a cell through a key K. Terminals B and D are connected to a sensitive galvanometer G through a key K1.
If there is no deflection in the galvanometer G, then balance condition for Wheatstone’s bridge is given by
P
Q
 =
R
X
[eq.1]
This relation is used to determine the value of unknown resistance if other three resistance value are known.

02

The unknown resistance X is connected in the one gap and a resistance box Rbox in the other gap of the metre bridge. The terminal B is connected to one terminal of the galvanometer G. The other terminal of the galvanometer is connected to a jockey J which slides along the wire AC. A source of dc current is connected between A and C through a key K so as to provide a constant potential drop along AC.
A resistor (or wire) of known resistance is inserted in the gap by taking out corresponding key from the resistance box RBox. The jockey is moved on the wire AC to obtain a condition of no-deflection in the galvanometer. It happens when the jockey is kept at a point D called the null point.
In this condition,
P
Q
 =
R
X
 =
Resistance of wire of length DC
Resistance of wire of length AD
[eq.2]

03
Unknown resistance X of the wire, having uniform cross-sectional area, is given by

               
X = R
l
100 - l
[eq.3]
where,
R : It is the resistance (from resistance box) in the left gap
X : unknown resistance in the right gap
l : it is the length of the wire of the meter bridge from the zero end A upto the null point D.


04

Procedure:

  1. Find the average diameter of the wire with a screw gauge and from the diameter we can calculate the value of the radius r.
  2. Clean the insulation at the ends of connecting wires with a piece of sand paper. Tighten all plugs of the resistance box (Rbox) by pressing each plug.
  3. Set up the circuit with unknown resistance wire of known length in the right gap.
  4. Next, introduce some resistance R in the circuit from the resistance box. Bring the jockey J in contact with terminal A first and then with terminal C. Note the direction in which pointer of the galvanometer gets deflected in each case. Make sure that jockey remains in contact with the wire

    for a fraction of a second. If the galvanometer shows deflection on both sides of its zero mark for these two points of contact of the jockey, null point will be somewhere on the wire AC. If it is not so, adjust resistance R so that the null point is somewhere in the middle of the wire AC, say, between 30 cm and 70 cm.
  5. 05
  6. If there is one-sided deflection, then check the circuit again, especially junctions, for their continuity.
  7. Repeat step 4 for four different values of resistance R.

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