<Tr> <Td> </Td> <Td> This article does not cite any sources . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (February 2017) (Learn how and when to remove this template message) </Td> </Tr> <P> A Kelvin bridge, also called a Kelvin double bridge and in some countries a Thomson bridge, is a measuring instrument used to measure unknown electrical resistors below 1 ohm . It is specifically designed to measure resistors that are constructed as four terminal resistors . </P> <P> 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 . In such resistors, the resistance of the connecting wires or terminals is negligible compared to the resistance value . 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 . </P> <P> To overcome the problems of these undesirable resistances (known as' parasitic resistance'), very low value resistors and particularly precision resistors and high current ammeter shunts are constructed as four terminal resistors . These resistances have a pair of current terminals and a pair of potential or voltage terminals . In use, a current is passed between the current terminals, but the volt drop across the resistor is measured at the potential terminals . The volt drop measured will be entirely due to the resistor itself as the parasitic resistance of the leads carrying the current to and from the resistor are not included in the potential circuit . To measure such resistances, requires a bridge circuit designed to work with four terminal resistances . That bridge is the Kelvin bridge . </P>

Measurement of low resistance using kelvin's double bridge theory