Kelvin Double Bridge

Kelvin Double Bridge

The modified form of Kelvin bridge is shown in Fig. 2.6. The two actual resistances of correct ratio are connected between point m and o. The galvanometer is connected at the junction of these resistances. Due to the introduction of second set of ratio arms, this bridge is termed Kelvin double bridge.

The first set of ratio arm is R₁ and R₂ and the second set of ratio arm is r₁ and r₂. The galvanometer is connected to point n such as to eliminate the effect of connecting lead of resistance R_l between the unknown resistance R_X and standard resistance R₃. The ratio of r₁/r₂ is made equal to R₁/R₂. At bridge balance condition the current through galvanometer is zero, i.e., the voltage drop between point a to b (E_ab) is equal to the voltage drop between the point a-m-n i.e. E_amn