Circuit VR: Resistance Measurement With Four Wires

Measuring resistance sounds like it should be the easiest job in electronics: push a known current through a thing,
read the voltage, and let R = V / I do the rest. That logic is perfectly fineuntil your “thing”
is tiny, your accuracy needs are spicy, or your test leads decide to audition as resistors in the same circuit.

That’s the whole point behind the “Circuit VR” vibe: in a clean mental simulation, wires are perfect and connectors
behave. In the real world, your leads, clips, and contact points are the chaotic side characters who steal the scene.
Four-wire resistance measurement (also called a Kelvin connection) is the plot twist that makes the math honest again.

Why This Even Matters

If you’re measuring a 10 kΩ resistor, a little extra resistance in the leads is usually background noise.
But if you’re measuring milliohmslike a current shunt, a relay contact, a motor winding, a busbar joint,
or a weldthe lead and contact resistance can be the same size (or bigger) than the value you actually care about.
At that point, a standard two-wire measurement isn’t “wrong” so much as it’s measuring a bigger system than you intended.

Four-wire measurement is a practical technique used in precision bench meters, source-measure instruments, data-acquisition
hardware, micro-ohmmeters, and even power supplies that support remote sensing. It’s not fancy for the sake of fancy
it’s what you use when “close enough” stops being close enough.

Two-Wire Resistance: Fine Until It Isn’t

The hidden “bonus resistor” in your leads

In a two-wire measurement, the meter forces a test current through the device under test (DUT) and measures the voltage
using the same two leads. Those same leads also have resistance. So do your banana plugs, alligator clips, probe tips,
and that one connection that “looks fine” but is secretly wearing a thin layer of oxidation like a winter coat.

The result: the meter measures the voltage drop across everything in the current pathDUT + leads + contacts
then reports the resistance of that whole party.

A quick reality check with numbers

Imagine you’re trying to measure a 0.500 Ω resistor. If your leads add 0.100 Ω total (not unusual for ordinary leads),
the meter will report roughly 0.600 Ω. That’s a 20% error, and your resistor didn’t do anything wrong.
Your leads simply insisted on being included.

Now scale the DUT down to 0.010 Ω (10 mΩ). The same lead resistance is suddenly an absolute wrecking ball.
Your measurement becomes mostly “lead trivia” with a cameo from the DUT.

When two-wire is still totally acceptable

Two-wire measurements are still great when:

• The DUT resistance is much larger than lead + contact resistance (think many ohms, kilohms, megohms).
• You’re doing quick go/no-go checks where a small offset is tolerable.
• You can reliably “zero” the leads (and your application allows that workflow).

But if you care about low-resistance accuracy, repeatability, or traceable results, it’s time for the four-wire method.

Four-Wire (Kelvin) Measurement: The “Force and Sense” Trick

Two wires do the work, two wires do the gossip

A four-wire resistance measurement splits the job into two separate paths:

• Force leads (also called current leads): carry the test current through the DUT.
• Sense leads (also called voltage leads): measure the voltage directly across the DUT.

The important part is that the sense leads go to a very high-impedance voltage measurement input. That means the sense
leads carry nearly zero current, so they develop essentially no voltage drop due to their own resistance.

In other words, your meter is still using R = V / Iit’s just measuring V at the DUT
instead of somewhere upstream where the leads and contacts can sneak into the equation.

What changes physically at the DUT

With four-wire, you don’t just “clip onto the part.” You place the sense connection points as close as possible to the
actual resistance you’re trying to measure. On components, that can mean sensing directly at the terminals.
On PCB shunt resistors, that can mean sensing at dedicated Kelvin pads (or carefully chosen points on the pads) so you
measure the shuntnot a chunk of copper trace that happens to be nearby.

What the Instrument Is Doing Under the Hood

The constant-current approach

Most precision resistance measurements use a controlled current source internally. The instrument forces a known current
through the DUT and measures the resulting voltage drop. It then calculates resistance.

The four-wire connection doesn’t change the math; it changes the measurement boundariesso the calculated resistance
corresponds to the DUT rather than the DUT-plus-wiring system.

Choosing the test current without cooking your DUT

Higher current can make the voltage drop larger and easier to measure (good), but it can also heat the DUT (bad),
especially for tiny resistances. Heating changes resistance and can add drift. For sensor elements and precision
references, self-heating is often the quiet source of “why does this number wander?” frustration.

A sensible approach is:

• Use enough current to get a measurable voltage above noise and offsets.
• Not so much current that the DUT temperature changes significantly during the reading.
• Allow time for readings to settle when measuring thermally sensitive parts.

Where Four-Wire Measurement Wins in Real Life

Micro-ohms and milliohms: the natural habitat

Four-wire measurement shows up everywhere low resistance matters:

• Current shunts (battery systems, motor drives, power supplies): milliohms are common.
• Contact resistance (relays, breakers, switchgear): tiny resistances reveal big reliability issues.
• Interconnects and welds (busbars, lugs, crimps, welding joints): measurements can predict heating and losses.
• Cable and connector qualification: the “it should be basically zero” cases where “basically” needs a number.

For extremely low values, specialized micro-ohmmeters may push higher test currents to make the DUT voltage drop easier
to resolve. That’s especially useful in power equipment and heavy connections where you care about micro-ohms because
micro-ohms turn into real heat at high current.

Inductive loads: motors and transformers

Measuring the DC resistance of a motor winding or transformer can be a diagnostic tool, but the resistance is often low,
and the measurement needs to be repeatable. Four-wire measurement helps separate real winding resistance from the leads,
clamps, and connection resistance that otherwise dominate the reading.

Because inductive loads can be noisy electrically and sensitive to connection quality, stable test setup and clean
contact points matter just as much as the wiring method.

RTDs and precision temperature measurement

Resistance Temperature Detectors (RTDs) are basically “precision resistors that get moody with temperature.”
If your temperature measurement depends on resistance, then lead resistance becomes temperature error.

In practice:

• 2-wire RTDs include lead resistance directly (fast and cheap, less accurate, best for short runs).
• 3-wire RTDs compensate by assuming two lead resistances match (common in industrial systems).
• 4-wire RTDs allow the instrument to eliminate lead resistance effects most completely (best accuracy).

If you’re doing high-precision temperature measurement, four-wire RTD configurations reduce the “mystery offset”
that grows with cable length. It’s one of the cleanest examples of Kelvin measurement paying off in daily life:
you’re not trying to measure micro-ohmsyou’re trying to keep temperature honest.

Kelvin Isn’t Just for Ohms: Remote Sense in Power Delivery

Four-wire thinking pops up in a place people don’t always label as “resistance measurement”: remote sensing in power supplies.
When a load draws current through long wires, the wires drop voltage. If the regulator only senses voltage at its own output
terminals, the load can see a lower voltage than intended.

Remote sense uses an extra pair of high-impedance sense wires that connect near the load so the supply regulates the voltage
where it matters. It’s the same philosophy: force wires carry current, sense wires measure the real voltage at the target.

The punchline is simple: if you’ve ever wondered why a bench supply has +S and −S terminals, it’s because someone, somewhere,
got tired of blaming a perfectly innocent power supply for what the cable was doing.

How to Do Four-Wire Measurements Without Summoning Gremlins

Use the right accessories

Four-wire setups are easier with purpose-built accessories: Kelvin clips, tweezers for small components, or probe systems
that bring separate force/sense connections right up to the contact point. If your instrument supports a “four-wire ohms”
mode, it’s worth using the matching lead setbecause the entire point is controlling where the voltage is sensed.

Clean contacts are not optional at low resistance

When you care about milliohms, contamination matters. Finger oils, dust, corrosion, loose banana plugs, and wiggly clips can
add resistance andworsevariability. Clean, secure, repeatable contact is half the measurement.

Be aware of offsets and thermal effects

At very low voltages, small DC offsets can become significant. Dissimilar metals and temperature gradients can generate tiny
thermoelectric voltages that look like “real” signal. Many precision instruments address this with techniques like offset
compensation (often involving measurements with current on/off or current reversal). If your reading seems haunted, it might
just be physics doing jazz improvisation.

EMI and wiring discipline

Sense leads should be treated like measurement signals:

• Keep sense wiring short when possible.
• Route sense leads away from switching nodes and high-current loops.
• Twist sense pairs to reduce pickup.
• Use shielding when measuring microvolts in noisy environments.

PCB shunts: Kelvin layout is a whole thing

If you’re measuring a shunt resistor on a PCB, Kelvin connection isn’t just “nice wiring”it’s layout.
The goal is to sense the voltage at the resistor element, not at a point where copper trace resistance joins the party.
Many current-sense resistors have dedicated Kelvin terminals, and when they don’t, engineers often implement a Kelvin-style
footprint to get the sense traces close to the element.

The practical rule: the high-current path should go through the main pads, while the sense traces should connect in a way
that minimizes extra copper in the measured path. This improves accuracy and reduces temperature sensitivity that can appear
when you accidentally “measure the PCB” instead of the resistor.

A Simple Step-by-Step Workflow

1. Decide if you need four-wire. If the DUT is below a few ohmsor you need high precisionassume yes.
2. Pick an appropriate test current. Enough signal, minimal self-heating.
3. Connect force leads to carry current through the DUT.
4. Connect sense leads as close to the DUT element/terminals as physically possible.
5. Stabilize the setup. Don’t jiggle clips mid-reading. Let the measurement settle.
6. Validate with a known reference if accuracy really matters (or if you enjoy sleeping at night).

Conclusion

Four-wire resistance measurement is one of those techniques that feels like a cheat code the first time it “magically”
fixes your low-ohms readings. But it isn’t magicjust good measurement boundaries. By separating current delivery (force)
from voltage measurement (sense), you remove lead and contact resistance from the result and get the number you actually meant
to measure.

If your work touches shunt resistors, RTDs, inductive loads, power distribution, contact resistance, or anything where
milliohms matter, the Kelvin method is less of an option and more of a life choice. A practical, calm, measurement-stable
life choice.

Hands-On Notes: “Experience” You Can Borrow Without the Bruises (Extra)

Engineers tend to learn four-wire measurement the same way people learn not to touch a soldering iron: once is usually enough.
The classic moment happens when a “0.005 Ω” shunt reads “0.180 Ω” and you start questioning the resistor, the meter, and your
career choicesin that order. The shunt is almost never the villain. The leads are.

One common field scenario is verifying a high-current path: a crimp, lug, or bolted joint that “looks perfect” but runs hot
under load. Low-resistance measurements help identify weak connections early, but only if your setup is repeatable. The trick
is to treat your measurement like a test fixture: same contact points, same clamp pressure, same cleanliness, and enough time
for the reading to stabilize. If you move the clips even a millimeter, you can change what you’re measuringespecially when
the DUT resistance is the same scale as the resistance of the metal you’re touching.

Another real-world lesson: contact resistance is a shape-shifter. A tiny layer of oxidation can add milliohms and also add
inconsistency, which is worse than a fixed offset because it ruins trend data. If you’re measuring connectors or busbars,
clean contact points (and consistent torque on bolts) can matter as much as the instrument. When people say “measurement is a
system,” this is what they meanbecause your results depend on metal surfaces, pressure, and even whether someone handled a
terminal with greasy fingers five minutes ago.

When measuring inductive loads like motor windings, patience is part of the technique. The measurement current can interact
with inductance and noise in ways that make readings jump around, especially if the environment is electrically loud.
Good practice is to secure connections, route sense leads away from high-current loops, and let the instrument average or
settle. If you’re troubleshooting a motor, look for consistency between phases and compare against baseline data. The most
valuable number is often not the absolute ohms value, but the repeatable difference from “normal.”

RTDs add their own flavor of humility. Long lead runs can turn into temperature error if you use the wrong wiring method.
Four-wire RTD wiring is often chosen not because it’s fun to pull extra conductors through conduit, but because it removes a
variable that otherwise changes with cable length, cable temperature, and connector aging. If you’ve ever seen a temperature
reading drift when someone opens a cabinet door (changing airflow around wiring), you understand why lead-resistance
compensation matters.

PCB shunts are where “four-wire” becomes “four-wire plus layout plus tribal knowledge.” A frequent gotcha is accidentally
sensing the voltage drop across a bit of copper trace in addition to the shunt element. That copper drop can be small, but
at high current it becomes meaningfuland it also changes with temperature. Many teams solve this by adopting Kelvin footprints
or four-terminal shunts and by placing the sense connections at carefully chosen pad locations. The payoff is huge: current
measurements become more accurate, drift less with temperature, and match what you expect from the schematicbecause now the
PCB matches the intent.

Finally, a small but powerful habit: always sanity-check your setup with something known. If you’re measuring 2 mΩ, throw in a
known low-value reference (or a shorting bar of known behavior) and confirm your method is stable. Four-wire measurement can
be incredibly accuratebut it’s still honest about whatever you actually connected. Getting “the right answer” starts with
making sure you’re asking the right question of the hardware.