Class 12 Physics practical reading To determine the resistance of a given wire using a metre bridge and hence determine the resistivity of the material of the wire.

Aim

To determine the resistance of a given wire using a metre bridge and

hence determine the resistivity of the material of the wire.

Material Required

Metre bridge, a wire (of material whose specific

resistance is to be determined), a resistance box (low value),

galvanometer, a jockey, one-way key, a cell or battery eliminator,

thick connecting wires, sand paper, screw gauge

Theory

To find resistance of wire: A metre bridge is used for this purpose which works on the principle of wheatstone bridge. In the meter bridge circuit, unknown resistance X is in the right gap whereas a resistance box R is in the left gap. The one meter long wire AC acts as other two arms of the bridge. On locating the null point at D on AC, we can write

           P/Q = L/100-L
When the bridge is balanced ( i. e.  no deflection in galvanometer),  then
             P/Q = R/X
             X = RQ/P
             X=R(100-L) /L
 

Circuit diagram

Specific Resistance

The specific resistance, the resistance of the wire having unit length and unit cross-sectional area ( called resistivity), of a material is given by 

         ρ = RA/L = πR².X/L

where r is radius of wire, L is length of wire & X is resistance of wire. 

Procedure

1. Find the average diameter of the wire with a screw gauge. From

this, obtain the value of its 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 as shown in Fig. E 2.1 with unknown resistance

wire of known length in other 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. If there is one-sided deflection, check the circuit again, especially

junctions, for their continuity.

6. Repeat step 4 for four different values of resistance R.

7. Interchange the position of the resistances S and R and repeat

steps 4 to 6 for the same five values of R. While interchanging S

and R, ensure that the same length of wire of resistance S is now

in the gap F. The interchange takes care of unaccounted resistance

offered by terminals.

Precautions

1. The connections should be tight and the connecting wire should be cleaned before making connections. 

2. The jockey should not be pressed too hard and be slided gently to touch the wire throughout while taking reading. 3. The balance point should always be obtained near the mid-point of the wire 

4. The calculated percentage error should not be too large.

Sources of error

1. The wires are not of uniform thickness. 

2. All plugs in the resistance box may not be tight. 

3. Heating of wires due to the continuous flow of current. 

4. Development of contact resistance due to improper connections.