That design choice makes the effective resistance of the bridged portion to be Ri.In the simple Kelvin-Varley decade design, the resistance of each stage decreases by a factor of 5: Ri+1 = Ri / 5.The resistors have to be selected for tight tolerances, and may need to have their resistance values individually trimmed to be equal.This selection or trimming only requires comparing the resistances of two resistors in each trimming step, which is easily accomplished by using a Wheatstone bridge circuit and a sensitive null detector — a galvanometer in the 19th century, or an electronically amplified instrument today.Carbon film resistors have temperature coefficients of several hundred parts per million per kelvin.While these unwanted voltages are small, on the order of a few microvolts per °C, they can cause appreciable errors at the high accuracy of which the Kelvin-Varley circuit is capable.The errors can be minimized through proper design — by keeping all junctions at the same temperature, and by employing only metal pairings with low thermoelectric coefficients (down to the external connectors and cables used; for example, a standard 4 mm plug/socket combination may have a coefficient of 1 μV/°C compared to only 0.07 μV/°C for a "low thermal EMF" grade plug/socket [4]).
