Four-terminal sensing

Four-terminal sensing (4T 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. 4T sensing is also known as Kelvin sensing, after William Thomson, Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistances.

2T versus 4T

The key difference between 4T and 2T sensing is that the separation of current and voltage electrodes in 4T allows the ohmmeter/impedance analyzer to eliminate the impedance contribution of the wiring and contact resistances, given that the voltage electrodes have high enough output impedance.

Links

* [http://www.allaboutcircuits.com/vol_1/chpt_8/9.html All About Circuits - Kelvin Resistance Measurement]
* [http://microlab.berkeley.edu/ee143/Four-Point_Probe/ Operations Manual] for a 4-point probe, from UC Berkeley
* [http://www.omicron.de/results/4_probe_measurements_on_liftet_nanowires/ Details of a Nano-Wire application] using 4-point probes