UNCLASSIFIED
TM-CAL-010
4-WIRE KELVIN RESISTANCE STANDARD
Elimination of Lead Resistance Error in Low-Resistance Measurements
Prepared by: Mervyn Martin, KO6NNH
Merced, California  •  26 May 2026
Amateur Radio / Electronics — Not for commercial calibration use

Table of Contents

  1. Chapter 1 — General Information
  2. Chapter 2 — Theory of Operation
  3. Chapter 3 — Materials and Construction
  4. Chapter 4 — Assembly Procedures
  5. Chapter 5 — Calibration Procedure
  6. Chapter 6 — Tuning and Adjustment
  7. Chapter 7 — Verification
  8. Appendix A — Calculations and Formulas
  9. Appendix B — Example Results

CHAPTER 1 — GENERAL INFORMATION

1-1. SCOPE

This manual covers the 4-wire Kelvin measurement method for precision resistance calibration. By separating current-forcing leads from voltage-sensing leads, lead and contact resistance is eliminated from the measurement. Applicable for resistances from 0.001 Ω to 10k Ω.

CHAPTER 2 — THEORY OF OPERATION

2-1. KELVIN PRINCIPLE

In a standard 2-wire resistance measurement, the DMM forces current through the same leads it uses to measure voltage. Lead resistance (typically 0.1–0.5 Ω per lead) adds directly to the measured value, introducing significant error for low resistances.

In 4-wire Kelvin measurement, dedicated current-force leads (F+, F−) carry the test current, and separate voltage-sense leads (S+, S−) measure the voltage drop across the unknown resistor only. The voltage-sense leads carry negligible current (DMM input impedance >10 MΩ), so their resistance does not affect the measurement.

Kelvin measurementRx = Vsense / Iforce

CHAPTER 3 — MATERIALS AND CONSTRUCTION

3-1. KELVIN PROBES AND FIXTURES

Equipment
ItemDescription
DMM with 4-wire modeFluke 15B+ does NOT have 4-wire; requires Fluke 87V or bench meter
Kelvin clip probesPaired force/sense clips per terminal (4 total)
Reference resistorsCertified 0.01% standard resistors: 0.1, 1, 10, 100, 1k Ω
4-wire socketDUT mounting with separated F+, F−, S+, S− connections
NOTEThe Fluke 15B+ is a 2-wire meter and cannot perform true Kelvin measurements. For 4-wire work, use a bench DMM with Hi-Force, Lo-Force, Hi-Sense, Lo-Sense terminals, or construct a current-source circuit with separate voltmeter.

CHAPTER 4 — ASSEMBLY PROCEDURES

4-1. DIY KELVIN FIXTURE

  1. Build a simple current source: LM334 or constant-current diode set to 1 mA through unknown resistor.
  2. Measure voltage across resistor with separate high-impedance voltmeter leads.
  3. Rx = V / 0.001 A.
  4. Calibrate current source: force through 100 Ω standard, measure V, actual I = V/100.

CHAPTER 5 — CALIBRATION PROCEDURE

  1. Zero the meter on a shorting bar: connect F+ to S+ and F− to S−. The meter should read <0.01 Ω. If not, apply offset null.
  2. Connect reference resistor in proper 4-wire configuration: F+ to one end, S+ to same end (inner), F− to other end, S− to same end (inner).
  3. Measure and record. Compare to certified value.
  4. Error ppm = (measured − certified) / certified × 106.
  5. Repeat for all decades: 0.1, 1, 10, 100, 1k Ω.

CHAPTER 6 — TUNING AND ADJUSTMENT

NOTEStandard resistors are not adjustable. Select from characterized lot for best accuracy.

CHAPTER 7 — VERIFICATION

  1. Compare 10 Ω standard against Wheatstone bridge (TM-CAL-009).
  2. Agreement within 0.05% confirms both methods are consistent.
  3. Log all results; recertify standards annually.

APPENDIX A — CALCULATIONS AND FORMULAS

4-Wire resistanceRx = Vsense / Iforce
Contact resistance error in 2-wire measurement (example)Error = 2 × Rlead / Rx. For Rlead=0.2Ω, Rx=1Ω: error = 40%
Same measurement 4-wireError ≈ 0 (lead resistance eliminated)

APPENDIX B — EXAMPLE RESULTS

Kelvin vs 2-Wire Comparison
Rx nominal2-Wire (DMM)4-Wire (Kelvin)Lead R (calculated)
0.1 Ω0.42 Ω0.099 Ω0.16 Ω/lead
1 Ω1.34 Ω1.001 Ω0.17 Ω/lead
10 Ω10.34 Ω10.003 Ω0.17 Ω/lead
100 Ω100.35 Ω100.02 Ω0.17 Ω/lead
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