**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 *l*_{1} 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* l*_{2}.

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 (*l*_{1}-*l*_{2}) between the two slide-wire lengths at balance.