Carey Foster bridge

Use

In the adjacent diagram, X and Y are resistances to be compared, while P and Q are nearly equal resistances forming the other half of the bridge. The bridge wire EF has a jockey contact D placed along it and is slid until the galvanometer G measures zero. Thick-bordered areas are thick copper busbars of very low resistance to limit influence on the measurement. Place a known resistance in position Y and the unknown resistance in position X. Adjust D along EF to null the galvanometer; this position as a percentage of distance from E to F is ℓ1. Swap X and Y and adjust D to the new null point, giving ℓ2. The resistance difference is the resistance of the length of bridge wire between ℓ1 and ℓ2. To measure a low unknown resistance X, replace Y with a copper busbar assumed to have zero resistance. In practical use, when the bridge is unbalanced, the galvanometer is shunted with a low resistance to avoid burning it out; it is used at full sensitivity only when the measurement is close to the null point. To measure the unit resistance of the bridge wire EF, put a known resistance less than that of the wire as X and a copper busbar of assumed zero resistance as Y.