Four-wire measurement – also called Kelvin measurement after its originator – separates the current path from the sense path. It is the standard method wherever the resistance to be measured is so small that leads and contact points would dominate the result.
Why two-wire measurement reaches its limit
In a two-wire measurement the test current flows through the same leads that are also used to sense the voltage. The instrument therefore sees not only the device under test, but additionally both leads and both contact points.
In practice, leads and contact transitions together easily amount to 50 to 100 mOhm. Anyone trying to test a welded joint of 5 mOhm with that setup is largely measuring their own fixture. And because contact resistances change slightly with every mating cycle, the result also scatters from measurement to measurement.
How four-wire measurement works
The solution is to assign two tasks to two separate pairs of leads. Through the outer pair – the current path – the source feeds a defined test current. Through the inner pair – the sense path – the voltmeter measures the voltage drop directly at the device under test.
The decisive factor is the high input impedance of the voltmeter: because virtually no current flows in the sense path, no appreciable voltage drop occurs across its leads and contact points. Their resistance therefore drops out of the result. The resistance sought is simply the measured voltage divided by the impressed current.
The contact resistance of the test contact therefore does not enter the measurement result – which is what makes four-wire measurement so robust. It is not entirely irrelevant nonetheless: in the current path an excessive transition limits the test current that can be impressed, and in the sense path it degrades the signal-to-noise ratio.
Practical rules for the test setup
- Sense inside, force outside – the voltage tap must lie between the current injection points, otherwise part of the lead is measured as well
- As close as possible to the device – every centimetre between the tap and the measuring object enters the result as an error
- Mind thermoelectric voltages – material transitions generate voltages in the microvolt range; they can be cancelled by measuring with reversed current direction and averaging
- Choose the test current deliberately – a higher current improves the signal but heats the device and distorts the result via the temperature coefficient
- Route four leads all the way to the contact – splitting them only just before the device defeats the whole purpose
When is the effort worthwhile?
As a rule of thumb: below about one ohm there is no way around four-wire measurement. Between one and one hundred ohms it is advisable as soon as reproducibility matters. Above that, two-wire measurement is normally sufficient, because the leads then no longer carry weight in proportion.
Four leads on one pin – the space problem
In theory the matter is simple; in practice it starts at the device under test. A blade terminal on 2.4 mm centres offers no room for two test probes placed side by side, and two contacts applied one after the other do not measure the same spot.
This is exactly what Kelvin probes are for. Two opposing, pre-tensioned contact blades grip the pin from both sides and are separated by an insulating body: one blade for the current, the other for the voltage. One component, one mating movement, two clean paths – with centre distances from 2.40 mm and currents up to 40 A.
Which design suits your device under test is shown in the overview of our Kelvin probes and contacts.
Frequently asked questions
Is Kelvin measurement the same as four-wire measurement?
Yes. The term Kelvin measurement goes back to William Thomson, later Lord Kelvin, who used the principle for his resistance bridge. In test engineering both terms are used synonymously, as are four-point measurement and 4-wire measurement.
Does every instrument need four terminals?
For a true four-wire measurement, yes – the instrument has to bring out the current path and the sense path separately. Many bench multimeters and practically all micro-ohmmeters offer this mode, often labelled 4W or Kelvin.
Does the contact resistance of the probe enter the result?
No. Since virtually no current flows in the sense path, no appreciable voltage drops across the contact either. A low contact resistance nevertheless remains desirable, because in the current path it determines the test current that can be applied.
