Metre - bridge

# Metre - bridge : 

Meter bridge is a device which is based on principle of wheatstone bridge.  With the help of the device the resistance or a specific resistance of material of given wire can be determined. In this device one metre long wire is used and it is based on the principle of wheatstone bridge so it is called metre bridge.

Principle and derivation of formula :

In above electric circuit shows the principle of meter bridge. on completing the circuit, sliding jockey (J) is slides on wire AC till the null deflection obtained in the Galvano -  meter. This is the balance State of the bridge.

let the resistance of unit length (1cm) of resistance wire be x ohm.

And balancing length, AD = L cm 

& length of remaining wire, DC = (100-L) cm

Hence, resistance of wire AD = L.x ohm

& Resistance of wire DC = (100-L).x ohm

In the balancing state of the bridge --

By the wheatstone's principle --

Resistance on armAB   Resistance on armAD

               ÷                      =               ÷

Resistance on armBC   Resistance on armDC

R / S  =  L.x / (100-L).x

S×L  =  R(100-L)

[ S = R.(100-L) / L ] ohm

* Observation table:

Mean resistance, S = (S1+S2+S3)/3

*Precaution:

1) All connecting screws and plug should be tight.

2)  Jockeys should not be rubbed on the wire otherwise cross section of the wire will not remain uniform.

3) Shunt should be removed from the galavano meter near the position of null deflection.