Physics
Study Material

# Meter Bridge

Meter Bridge works on null point method. Any device or instrument which works on null point method gives very accurate value.

2 minutes long
Posted by Mahak Jain, 27/10/2021

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Meter bridge and potentiometer both are electrical instruments. You can see them in your physics lab or electronics lab. This both instruments works on null point method. Any device or instrument which works on null point method gives very accurate value, that’s why meter bridge and potentiometer gives very accurate value, but still they have errors. Here we will discuss about this both instruments in detail. After reading this article, you will able to solve almost all the numerical related to this two instruments. This instrument is used to measure unknown resistance. It works   on, balanced Wheatstone bridge.

## Structure of Meter Bridge

• R – Unknown Resistance.
• S – Desirable resistance OR known resistance.
• G – Galvanometer (it shows deflection for very small amount of current, when passes through it, this device is a sensitive device.)
• J – Jokey (metal rod with sharp or pointed edge) and (we have to lift and put this on the wire AB. we cannot slide jokey on AB wire, because it will affect the cross section area of the wire.)
• K – key OR switch. (to open and close the circuit)

Wire AB has a uniform cross section and is made of alloy (magainin or constantan), the resistance of this alloy does not change with change in temperature in large magnitude, so the resistance of the wire AB will become temperature independent.

## Meter Bridge Formula

Here, first we will find a null point on wire AB where galvanometer(G) shows no deflections.

• Let D be the point on wire AB where G shows no deflection,  which means there is no current in branch CD.
• If there is no current in branch CD then the potential difference between point C and D is zero.
• According to wheatstone bridge we can say that,

R/x = S/(100-x)

• So, the unknown resistance,

R = xS/100-x