Four-terminal sensing (4T sensing), 4-wire sensing, or 4-point probes method is an electrical impedance measuring technique that uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements than traditional two-terminal (2T) sensing. 4T sensing is used in some ohmmeters and impedance analyzers, and in precision wiring configurations for strain gauges and resistance thermometers. 4-point probes are also used to measure sheet resistance of thin films.
The key advantage of four-terminal sensing is that the separation of current and voltage electrodes eliminates the impedance contribution of the wiring and contact resistances.
Four-terminal sensing is also known as Kelvin sensing, after William Thomson, Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistances. Each two-wire connection can be called a Kelvin connection. A pair of contacts that is designed to connect a force-and-sense pair to a single terminal or lead simultaneously is called a Kelvin contact. A clip, often a crocodile clip, that connects a force-and-sense pair when it closes or slides onto a conductor is called a Kelvin clip.
source: http://en.wikipedia.org/wiki/Four-terminal_sensing
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