Kelvin bridge
From Wikipedia, the free encyclopedia
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 Ω. Its operation is similar to the Wheatstone bridge except that in the circuit are not four but seven resistors.
[edit] 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 R3/R4 and R1/R2 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, ohmeters include Kelvin bridges, amongst others measuring instruments, in order to obtain large measure ranges, for example, the Valhalla 4100 ATC Low-Range Ohmmeter
