Kelvin bridge

In the scientific community the Kelvin bridge paired with a Null Detector was used to achieve the highest precision.

Resistors above about 1 ohm in value can be measured using a variety of techniques, such as an ohmmeter or by using a Wheatstone bridge.

For resistors of less than an ohm, the resistance of the connecting wires or terminals becomes significant, and conventional measurement techniques will include them in the result.

However, the link between Rs and Rx (Rpar) is included in the potential measurement part of the circuit and therefore can affect the accuracy of the result.

Measurement accuracy is also increased by setting the current flowing through Rs and Rx to be as large as the rating of those resistors allows.

Commercial Kelvin Bridges were initially using galvanometers replaced by micro-ammeters and that was limiting factor of the precision, when voltage difference comes close to zero.

For such use, the error introduced by the mis-match of the ratio in the two potential arms would mean that the presence of the parasitic resistance Rpar could have a significant impact on the very high accuracy required.

Some ohmmeters include Kelvin bridges in order to obtain large measurement ranges.

Instruments for measuring sub-ohm values are often referred to as low-resistance ohmmeters, milli-ohmmeters, micro-ohmmeters, etc.