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Kelvin bridge
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A Kelvin bridge (also called a Kelvin double bridge) is a measuring instrument invented by William Thomson, 1st Baron Kelvin. It is used to measure an unknown electrical resistance below 1 O. Its operation is similar to the Wheatstone bridge except that in the circuit are not four but seven resistors.
using a Wheatstone bridge to measure the low resistor, Rx, the non-perfect wires resistances can’t be ignored and substantially affect the measurement.
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A Kelvin bridge (also called a Kelvin double bridge) is a measuring instrument invented by William Thomson, 1st Baron Kelvin. It is used to measure an unknown electrical resistance below 1 O. Its operation is similar to the Wheatstone bridge except that in the circuit are not four but seven resistors.
Functioning
When using a Wheatstone bridge to measure the low resistor, Rx, the non-perfect wires resistances can’t be ignored and substantially affect the measurement. To avoid this, some modifications must be introduced.
If the ratios and are equal and the bridge gets balanced, the Wheatstone condition is again accomplished.
As a result of this modification a new measuring instrument, the Kelvin Bridge, is achieved. There are some commercial devices reaching accuracies of 2% for resistance ranges from 0.001 to 25 ohms. Often, ohmmeters include Kelvin bridges, amongst other measuring instruments, in order to obtain large measure ranges, for example, the
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