If the length of the R1 resistance wire is AB, where A is the (-) end and B is the (+) end, and the movable wiper is at point X at a distance AX on the R3 portion of the resistance wire when the galvanometer gives a zero reading for an unknown voltage, the distance AX is measured or read from a pre-printed scale next to the resistance wire. The unknown voltage can then be calculated: V_U = (Calibration \ Cell \ Voltage) {AX \over AB}
