## ~State and Prove Expression of **Meter Bridge**?

**Statement**:

It very sensitive device which is used to measure resistance and resistivity. it is based on the principle of **wheatstone bridge.**

The experimental arrangement for the measurement of resistance and resistivity, by using **Metre Bridge** as shown in figure. it consists of 1 metre long wire made by a constantine which stretched on the wooden board. the ends of the wire is connected two Copper stripes.

There are two gaps one for variable resistance R and another for unknown distance X. A cell is connected across A and C. A galvanometer is connected between Jackey and point as shown in figure.the Jackey can slide over the wire to find the north point .

When the circuit is on and Jackey slide over the wire. let B be the point where **Galvanomete**r shows deflection.

In this condition,

Resistance of AB*X = Resistance of BC*R

or, l*X = (100-l)R. [ Resistance ∝ l ]

so, *X = (100-l/l)R*

By knowing the value of l and R, we can determine the unknown resistance X.

To find the Resistivity, we use the formula,

rho = XA/L

or, rho = Xπd2/4L

Where X = Length of wire used.

d = diameter

By knowing the length by metre scale and diameter micrometre screw gauge ,we can determine the **resistivity** of a wire .

**Questions**

1) **what happens when the copper wire is used in the metre Bridge?**

*Ans*. We know that the copper wire has low resistance .so ,large current flowing through it, and it it is difficult to find null point.

2)** It is better to get null point in the middle of wire of metre Bridge,why?**

*Ans*. Metre Bridge is based on the principle of wheatstone bridge .The sensitivity of wheatstone bridge is high, when all four. resistance are in same order.

if we get null point middle of wire, then resistivity of the ratio of arms are nearly equal. As a result the bridge is more sensitive.

**3)1)Define Drift velocity of electrons? Establish a relation between drift velicity of electrons and current density in a conductor.**

**4)What do you meant by Shunt? Describe its use in converting a Galvanometer into Ammeter.**

**5)State and prove Joule’s laws of heating. **