Objective: Precision measurement of meadium reistance.
Fig.1. Carey -Foster Slide-wire bridge.
The circuit connections of this bridge are shown in fig. 1, a slide wire of length L being included between R and S as shown. This bridge is specially suited for the comparison of two nearly equal resistances.
Resistances P and Q are first adjusted so that the ratio P/Q is approxmiately equal to the ratio R/S. Exact balance is obtained by adjustment of the sliding contact on the slide-wire . Let l1 be the distance of the sliding contact from the left hand of the slide wire. The resistance R and S are then interchanged and balance again obtained. Let the distance now be l2.
Then, for the first balance,
Thus the difference between S and R is obtained from the resistance per unit length of slide wire together with the difference (l1-l2) between the two slide-wire lengths at balance.