In Kelvin's method of finding the resistance of a galvanometer, we

Explanation

Circuit : The metre bridge circuit for Kelvin's method of determination of the resistance of a galvanometer is shown in the figure. The galvanometer whose resistance G is be determined, is connected in one gap of the metre bridge. A variable known resistance R is connected in the other gap. The junction B of the galvanometer and the resistance box is connected directly to a pencil jockey. A cell of emf E, a key (K) and a rheostat (Rh) are connected across AC.

Working : Keeping a suitable resistance R in the resistance box and maximum resistance in the rheostat, key K is closed to pass the current. The rheostat resistance is slowly reduced such that the galvanometer shows about `2//3` rd of the full-scale deflection.
On tapping the jockey at end-points A and C, the galvanometer deflection should change to opposite sides of the initial deflection. Only then will there be a point D on the wire which is equipotential with point B. The jockey is tapped along the wire to locate the equipotential point D when the galvanometer shows no change in deflection. Here, point D is called the balance point and Kelvin's method is thus an equal deflection method. At this balanced condition,
`G/R=("resistance of the wire of length "L_(G))/("resistance of the wire of length "L_(R))`
where `L_(G) equiv` the length of the wire opposite to the galvanometer and `L_(R) equiv` the length of the wire opposite to the resistance box.
If `lambda equiv` the resistance per unit length of the wire,
`G/R=(lambdaL_(G))/(lambda L_(R))=L_(G)/L_(R)" ":. G=R L_(G)/L_(R)`
The quantities on the right hand side are known, so that G can be calculated.