Glossary term
Kelvin Bridge
A four-terminal resistance bridge used to measure very low resistance while reducing lead and contact resistance error.
Definition
deviceA Kelvin bridge is a precision bridge circuit for measuring low resistance using separate current and voltage connections to minimize lead-resistance error.
The Kelvin bridge is an extension of the Wheatstone bridge for low-resistance measurement. In milliohm and microohm ranges, lead resistance and contact resistance can be comparable to the unknown resistance. Kelvin four-terminal connection separates the current path from the voltage-sensing path, so the measured voltage drop corresponds more closely to the device under test rather than to test leads and contacts.
A Kelvin bridge measures very low resistance more accurately than a simple two-wire measurement. At low resistance, the resistance of leads, clips, probes, solder joints, and contact surfaces can be large enough to dominate the reading. The Kelvin method reduces that error by separating the current-carrying conductors from the voltage-sensing conductors.
Four-terminal principle
In a four-terminal measurement, current is forced through the outer pair of leads and voltage is sensed across the inner pair. Because the voltage-sense circuit draws very little current, the voltage drop in the sense leads is negligible. The measured voltage is therefore much closer to the voltage across the unknown resistance itself. Resistance is then obtained from:
The Kelvin bridge implements this idea in a balanced bridge arrangement so that very small resistances can be compared with known standards.
Applications
Kelvin bridges and related four-wire instruments are used to measure shunts, busbars, relay contacts, switch contacts, transformer windings, motor windings, cable joints, printed-circuit copper, welds, connector resistance, and material resistivity samples. In power systems and power electronics, milliohms matter because high current turns small resistance into significant Joule heating.
Error sources
Kelvin measurement reduces lead resistance error, but it does not remove all errors. Thermal electromotive forces at dissimilar metal junctions can create microvolt-level offsets. Contact pressure, oxide films, self-heating, ambient temperature, current distribution, and instrument resolution all matter. For very low resistance, reversing current and averaging readings can help cancel thermal offsets.
Common mistakes
A common mistake is using four terminals physically but placing the sense contacts at the wrong points. The measurement should sense exactly the portion of conductor or joint being evaluated. Another mistake is applying too much test current and heating the part, changing the resistance during measurement. Good low-resistance reports state test current, contact geometry, temperature, instrument accuracy, and whether the measurement is two-wire or four-wire.