How to Calculate Unknown Resistance Using Meter Bridge

A meter bridge is an apparatus used to find the resistance of a coil; you will find it as part of the tools of a physics lab.

Steps

1
Check that the meter bridge wire is connected. If not, just connect both ends of the wire tightly. The apparatus has 5 places for the connections. All gaps for connections are found above the meter bridge wire; two on either sides and one in the middle.
2
Imagine 5 dots plotted on a paper which are coplanar also imagine a line below it which is a little longer than the line which we can get if we join the points. The above connection is similar to this.

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3
Connect a known resistance on one side and unknown resistance on the other this fills 4/5 gaps left for connections. In the remaining gap, connect a galvanometer, high resistance and jockey all in series.
4
Slide the jockey over the meter bridge wire and note down the reading for which you get zero deflection in galvanometer.
5
Let it be R1 now put R2=100-R1.
6
Similarly, calculate another reading by interchanging the known and unknown resistances in the above circuit.
7
Let the new readings be R3 and R4 where R3 is the length obtained for zero deflection and R4=100-R3.
8
Add R1 and R3 and divide it by two. note it down as L. calculate the average for R2 and R4 too let it be M.
9
Repeat the above for various known resistance values. Each time you can calculate the unknown resistance using the formula:unknown resistance(a)=known resistance(b)*L/M. The value is found to be the same for all values of known resistance.