Operation of Kevin Double Bridge in Practice

Operation of Kevin Double Bridge in Practice Points : operation of kevin double bridge in practice The method of operation of the Kelvin double bridge in practice is often somewhat different from that described already, especially when precise measurements of low resistance are to be made.

Instead of varying the ratio arms, keeping the ratio Q/M = q/m, to obtain balance of the bridge, the resistances Q, M, q, and m are often made up of resistance coils whose resistances are fixed and are accurately known, together with their temperature coefficients. The ratios Q/M and q/m thus remain fixed during the test, and are made equal to one another, and roughly equal to the ratio assuming this to be known, approximately. If not known, the ratio can easily be determined approximately by measuring X first of all, using a less accurate method such as the ammeter and voltmeter method, or, better, by the potentiometer method.

Adjustment of the bridge to obtain balance is then carried out by shunting either the unknown resistance box. Assuming balance to be obtained by shunting the unknown by a resistance “x”, let the resistance of “X” and “x” in parallel, at balance, be “X”, then

X’ = Q/M.S
and also
1/X,=1/X+1/X

from which the value of X can be obtained. As before, measurements are made also with the supply current reversed, and the average value of the two results is taken as the final value. Use The Kelvin double bridge is a modified form of the Wheatstone bridge and is used to measure low resistance with increased accuracy. We can measure resistance from 0.00001Ω to 1.0Ω with it.