# how does a Kelvin probe works?

I'm trying to understand how a Kelvin probes works, with 4 wires resistance measurement

Since the wires are twisted, I get that it will cancel any magnetic fields, because the same current goes through them in opposite direction.

However, how can we say there is no resistance made by the wire in those?

• Do you mean probes for 4-wire resistance measurement? In that case the two sense wires only measure voltage (very very little current in those wires) so there is only a negligible voltage drop across the sense wires, so you cancel out the effect of wire resistance. – Arsenal Feb 7 '16 at 22:16
• Please note that Kelvin sensing (the 4-wire connection method) is a very different thing to a Kelvin probe device. The latter is used to perform a measurement of an absolute electric potential, unlike a voltmeter, which measures a potential difference. You should certainly not confuse these two unrelated concepts. – Oleksandr R. Feb 8 '16 at 3:00

## 1 Answer

It has nothing to do with magnetic fields. A Kelvin connection is used when the thing you are trying to measure voltage across has enough current going thru it and the wires to it long enough so that there would be meaningful voltage drop in those wires.

To make a Kelvin connection, hook up the two wires normally. Those carry the current, and each one will have some voltage drop between where you are and the transducer is. The trick is to connect two more wires. These connect immediately to the transducer but are only used for carrying the voltage to measure. That requires very little current, so there is no meaningful voltage drop on those wires.

• If the Kelvin connection has 2 fat wires and 2 thin ones, obviously use the fat ones to carry current, and the thin ones for measurement. – Brian Drummond Feb 7 '16 at 23:31