Kelvin bridge

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.

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.

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.

The operation of the Kelvin bridge is very similar to the Wheatstone bridge, but uses two additional resistors. Resistors R₁ and R₂ are connected to the outside potential terminals of the four terminal known or standard resistor R_s and the unknown resistor R_x (identified as P₁ and P′₁ in the diagram). The resistors R_s, R_x, R₁ and R₂ are essentially a Wheatstone bridge. In this arrangement, the parasitic resistance of the upper part of R_s and the lower part of R_x is outside of the potential measuring part of the bridge and therefore are not included in the measurement. However, the link between R_s and R_x (R_par) is included in the potential measurement part of the circuit and therefore can affect the accuracy of the result. To overcome this, a second pair of resistors R′₁ and R′₂ form a second pair of arms of the bridge (hence 'double bridge') and are connected to the inner potential terminals of R_s and R_x (identified as P₂ and P′₂ in the diagram). The detector D is connected between the junction of R₁ and R₂ and the junction of R′₁ and R′₂.

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