Kelvin bridge
A Kelvin bridge (also called a Kelvin double bridge and in some countries a Thomson bridge) is a measuring instrument invented by William Thomson, 1st Baron Kelvin. It is used to measure an unknown electrical resistance below 1 Ω. Its operation is similar to the Wheatstone bridge except for the presence of additional resistors. These additional low value resistors and the internal configuration of the bridge are arranged to substantially reduce measurement errors introduced by voltage drops in the high current (low resistance) arm of the bridge [1].It consist of two sets of ratio arms.First outer set of ratio arms are the known resisters & inner ratio arms are helps to connect the one terminal of galvanometer at appropriate point(which was the disadvantage of Kelvin first bridge). This bridge is used to measurement of low resistance.
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Accuracy
There are some commercial devices reaching accuracies of 2% for resistance ranges from 0.000001 to 25 Ω. Often, ohmmeters include Kelvin bridges, amongst other measuring instruments, in order to obtain large measure ranges, for example, the Valhalla 4100 ATC Low-Range Ohmmeter.
The instruments for measuring sub-ohm values are often referred to as low-resistance ohmmeters, milli-ohmmeters, micro-ohmmeters, etc.
Principle of operation
The measurement is made by adjusting some resistors in the bridge, and the balance is achieved when:
Resistance R should be as low as possible (much lower than the measured value) and for that reason is usually made as a short thick rod of solid copper. If the condition R3·R`4 = R`3·R4 is met (and value of R is low), then the last component in the equation can be neglected and it can be assumed that:
Which is equivalent to the Wheatstone bridge.