02 Metre Bridge 1

[ON R.H.S] Experiment No.2 METRE BRIDGE - 1 AIM:- To find the resistance of the given wire using meter bridge and hence determine the specific resistance of its material. APPARATUS:

A meter bridge, galvanometer, one way key, a resistance box, a battery jockey, unknown resistance wire about 1 meter long, screw gauge and connecting wires. THEORY: A meter bridge is the practical application of Wheatstone bridge arrangement as shown in figure below. The four resistances are connected to each other as shown and if the bridge is in balanced state, i.e., there is no deflection in the galvanometer (G),  

=

  

We can use this relation to find the unknown resistance of the given material of wire. The unknown resistance 'X' can be found by Meter bridge which uses the principle of  Wheatstone bridge. The unknown resistance 'X' of the given wire is obtained by relation : X=

 (100 − ) 

And specific resistance of the material of a given a wire is obtained by   .  2 =  where d = diameter of the wire and L = length of wire.

PROCEDURE : Clean the connecting wires using sand paper and make the connection as shown in figure below. Take out some suitable resistance 'R' form the resistance box (R. B.). Touch the jockey at point A; see that there is a deflection in the galvanometer on one side, then touch the jockey on the point C of the wire, the deflection in the galvanometer  should be on the other side. If it is so, your connections are correct.'  Now find the position of null point where deflection in galvanometer becomes zero.  Note length AB (l) BC will be (100 - l). Repeat the above procedure for different values of 'R'. The value of this resistance ' R’ taken out from R. B. should be such that the null point is in between 30 cm and 70 cm in the meter bridge wire. Note the point where the galvanometer shows 0 deflections, this is called the balance point. Measure the length of the given wire using ordinary scale and the radius of the wire using screw gauge.

RESULT: (a) The resistance of the given wire = (b) The specific resistance of the given wire =

Ohm. Ohm-m

PRECAUTIONS:

(i) Clean the connecting wires and the connecting points of metre bridge properly with sand paper. (ii) All connections should be neat and tight. (iii) Balance point should lie between 30 cm and 70 cm. (iv) Do not keep the jockey and the wire in contact for a long time. (v) Hold the jockey perpendicular to the wire of metre bridge. SOURCES OF ERROR :

(i) The screws of the instruments may be loose. (ii) The keys of the resistance box may not be clean and tight. (iii) The wire may not be uniformly thick throughout.

[on L.H.S] Circuit Diagram:

OBSERVATIONS:

Table for unknown resistance (X)

S.  No.

Reading of resistance Box (R) 

Balancing length (l)

cm

X=

 (100 − ) 



01 02 03 04 05 Mean resistance (X) =



Table for diameter of the wire (d) L.C of screw gauge = Zero correction =

Sl.  No 01 02 03

PSR

cm

HSR

Corrected HSR 

Mean diameter  Total mm (PSR+HSR x l.c) (d)

CALCULATIONS: Radius of the given wire (r) = d/2= Length of the given wire (l) =

mm = cm =

Specific resistance of the material of given wire = = =

x 10 x 10

-3 -2

m. m.

   2 



-m