Four-wire (Kelvin) resistance measurement
Below roughly 10 ohms, the resistance of your test leads is no longer a rounding error — it is the measurement. Four-wire (Kelvin) sensing separates the force and sense connections so lead resistance stops contaminating the reading.
Two-wire measures leads, too
A two-wire resistance measurement forces a test current through the leads and measures the voltage across them. That voltage includes the drop across both leads and both contact junctions, so sub-ohm readings are inflated by whatever the leads contribute.
Sense wires remove the drop
Four-wire uses one pair to force the test current and a separate high-impedance pair to sense the voltage right at the device. Because the sense pair draws negligible current, its own resistance contributes almost nothing, and the measured voltage reflects only the device under test.
When two-wire is fine
For anything above a few hundred ohms, lead and contact resistance are typically negligible and two-wire is faster and simpler. Reserve four-wire for low-ohm shunts, connector contact resistance, motor windings, and precisely chosen sense resistors.
Perguntas frequentes
- When does lead resistance start to matter?
- It becomes material below about 10 ohms, where lead and contact resistance are comparable to the resistance being measured. Above a few hundred ohms two-wire is generally sufficient.
- How does four-wire Kelvin sensing work?
- One pair forces the test current while a separate high-impedance pair senses voltage directly at the device, so the sense wires carry almost no current and their resistance does not offset the reading.