Kelvin–Varley divider

The Kelvin-Varley voltage divider, named after its inventors William Thomson, 1st Baron Kelvin and Cromwell Fleetwood Varley, is a precision voltage divider circuit used to generate an output voltage as a highly accurate ratio of an input voltage, with selectable resolution over several decades. The concept of multi-decade resistor networks for precise voltage and resistance ratio adjustment was developed in the late 19th century as part of the evolution of electrical resistance measurement techniques. Unlike a simple resistive divider, it employs cascaded stages of matched resistor networks to achieve very high linearity and ratio accuracy, and is typically operated in conjunction with a sensitive null detector. In this role, the Kelvin–Varley divider functions as an electromechanical precision digital-to-analog converter for voltage ratio values.

The circuit is used in calibration and metrology laboratories as primary or reference voltage ratio standards, forming part of measurement chains that compare unknown voltages to known standards with minimal dependence on detector calibration. Commercial Kelvin-Varley dividers can achieve resolution, accuracy and linearity of 0.1 ppm (1 in 10 million).