Adjustable & Balanced Dividers

A uniform wire has resistance proportional to length. With the output measured from one end to the slider, moving the slider farther from that end increases the output p.d.

At one endpoint the output is 0 V; at the other it is the full input p.d. These endpoint checks catch a reversed fraction.

When the detector reads zero, no current passes through it and the two midpoint potentials are equal. For the labelled balanced bridge:

The two midpoints are at the same potential, so the p.d. across the galvanometer is zero and no current passes through it.