Carey Foster bridge

He first described it in his 1872 paper "On a Modified Form of Wheatstone's Bridge, and Methods of Measuring Small Resistances" (Telegraph Engineer's Journal, 1872–1873, 1, 196).

The bridge wire EF has a jockey contact D placed along it and is slid until the galvanometer G measures zero.

The thick-bordered areas are thick copper busbars of very low resistance, to limit the influence on the measurement.

In practical use, when the bridge is unbalanced, the galvanometer is shunted with a low resistance to avoid burning it out.

The two remaining arms are the nearly equal resistances P and Q, connected in the inner gaps of the bridge.

The Carey Foster bridge. The thick-edged areas are busbars of almost zero resistance.
A standard Wheatstone bridge for comparison. Points A, B, C and D in both circuit diagrams correspond. X and Y correspond to R 1 and R 2 , P and Q correspond to R 3 and R X . Note that with the Carey Foster bridge, we are measuring R 1 rather than R X .