Kelvin bridge

Kelvin bridge

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.

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 R_3/R_4 and R_1/R_2 are equal and the bridge gets balanced, the Wheatstone condition is again accomplished.

R_x=R_acdotfrac{R_1}{R_2}

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 [http://www.uiitraining.com/b51a/200/25138measuring_system.htm# Valhalla 4100 ATC Low-Range Ohmmeter]

See also

* Wheatstone bridge
* Ohmmeter
* Maxwell bridge

External links

* [http://www.tpub.com/content/neets/14193/css/14193_94.htm Kelvin Bridge]
* [http://www.cropico.co.uk/images/uploads/KB3_20Handbook.pdf Kelvin Bridge KB3 Data Sheet]
* [http://www.uiitraining.com/b51a/200/25138measuring_system.htm# Valhalla 4100 ATC Low-Range Ohmmeter specifications]
* [http://www.allaboutcircuits.com/vol_1/chpt_8/10.html Excellent explanation with equations]
* [http://books.google.com/books?id=z3lKAAAAMAAJ Methods of Measuring Electrical Resistance - Edwin F. Northrup, 1912, full-text on Google Books] - see chapter 6


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