Construction and Working of Kelvin Double Bridge

Construction and Working of Kelvin Double Bridge Points : construction and working of kelvin double bridge This method is one of the best available for the precise measurement of low resistances. It is a development of the Wheatstone bridge by which the errors due to contact and leads resistance are eliminated. The connections of the bridge are shown.

In the circuit, “X” is the low resistance to be measured, and “S” is a standard resistance of the same order of magnitude. These are connected in series with a low resistance link “r”, connecting their adjacent current terminals. A current is passed through them from a battery supply. A regulating resistance and ammeter are connected in the circuit for convenience. Q, M, q and in are four known, non-inductive resistances, one pair of which (M and m, or Q and q) are variable.

These are connected to form two sets of ratio arms as shown, a sensitive galvanometer “G” connect in the dividing points of QM and qm. The ratio Q/M is kept the same as q/m, these ratios being varied until zero deflection of the galvanometer is obtained. Then X/S = Q/M = q/m from which “X” is obtained intermesh of S, Q, and M.

By making the resistance of the link “r” very small, and also by making the ratio Q/M as nearly as possible equal to the expression for “X” becomes simply

X = Q/M.s

In order to take into account thermo-electric e.m.f. s, a measurement should also be made with the direction of the current reversed and the mean of the two readings should be taken as the correct value of “X”.

The sensitivity of the bridge is determined by noting smallest variation of the link resistance “r”, which causes an observable deflection of the galvanometer.

In a typical Kelvin bridge, the range of resistance covered is 0.1 micro ohm to 1.0 ohm. The accuracies are as under.

From 1000 µ Ω to 1.0 Ω …… 0.05%
From 100 µ Ω to 1000 µ Ω …… 0.2% to 0.05%
From 10 to Ω to 100 µ Ω ……. 0.5%to0.2%