Electrical Testing Reference

Low-resistance ohmmeter

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A low-resistance ohmmeter is an instrument that measures small resistances, such as those of contacts, joints and bolted connections, by passing a known direct current through the object and measuring the voltage drop that the current produces across it. [1] Such resistances may be stated in microhms, one microhm being 0.000001 Ω. [2]

What it measures

The quantity measured is electric resistance, whose SI unit is the ohm, equal to the volt per ampere. [3] For the small values found at contacts and connections, the SI prefix micro gives the microhm as a submultiple of the ohm. [2] [1]

Principle

The instrument uses a four-terminal arrangement, also called a Kelvin connection: current is passed through the object by one pair of leads and the voltage drop is measured on a second pair, which removes the error contributed by the series impedance of the leads and suits the method to low resistances. [4] [5] [6] The resistance follows from the ratio of the measured voltage to the known current. [3] [1] The Kelvin bridge is the most widely known four-terminal technique for precision direct-current resistance measurement. [5]

Used in

ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 call for contact resistance measurements, resistance measurements of bolted connections and measurements of battery intercell connection resistance, each a measurement of a small resistance. [7] [8] [9] [10] Calibration of the instruments used in maintenance testing is addressed in section 5.3 of ANSI/NETA MTS-2023. [11]

Notes

  1. Electrical Testing Reference editorial taxonomy, 2026-10 edition. A low-resistance ohmmeter is an instrument that measures small resistances, such as those of contacts, joints and bolted connections, by passing a known direct current through the object and measuring the voltage drop that the current produces across it. Datum instr.low-resistance-ohmmeter.definition
  2. NIST SP 811, web page updated 2025-08-18 edition, clause Table 5. Applying the SI prefix micro, symbol μ, which denotes the factor one millionth, to the ohm gives the microhm, equal to one millionth of an ohm.Value: 0.000001 Ω. Datum qty.ohm.microhm
  3. NIST SP 811, web page updated 2025-08-18 edition, clause Table 3. In the NIST Guide to the SI, the ohm, symbol Ω, is the SI coherent derived unit of electric resistance and is expressed in other SI units as the volt per ampere.Value: V/A. Datum qty.ohm.definition
  4. UC Berkeley Physics Experimentation Lab reference 10, 2001 edition, clause Four terminal measurements. A reference on impedance measurement published by the University of California, Berkeley physics laboratory describes the four-terminal technique as passing current through one pair of leads and measuring voltage on the other pair, which removes the error caused by the series impedance of the leads, and describes it as suited to low impedances. Datum instr.low-resistance-ohmmeter.four-terminal-source
  5. UC Berkeley Physics Experimentation Lab reference 10, 2001 edition, clause Four terminal measurements. The same reference notes that the term Kelvin connection is often used interchangeably with four-terminal, and names the Kelvin bridge as the most widely known four-terminal technique for precision direct-current resistance measurement. Datum instr.low-resistance-ohmmeter.kelvin-term
  6. Electrical Testing Reference editorial taxonomy, 2026-10 edition. In a four-terminal measurement the current is supplied through one pair of leads and the voltage drop is sensed through a separate pair, so that the resistance of the leads and of the current connections is kept out of the result. Datum instr.low-resistance-ohmmeter.four-terminal
  7. ANSI/NETA ATS-2025, clause 7.22.3.B.4. Contact Resistance is addressed in ANSI/NETA ATS-2025. Datum tests.contact-resistance.governed-by.ansi-neta-ats(provenance pending)
  8. ANSI/NETA MTS-2023, clause 7.22.3.B.3. Contact Resistance is addressed in ANSI/NETA MTS-2023. Datum tests.contact-resistance.governed-by.ansi-neta-mts(provenance pending)
  9. ANSI/NETA ATS-2025, clause 7.22.3.B.1. Bolted Connection Resistance is addressed in ANSI/NETA ATS-2025. Datum tests.bolted-connection-resistance.governed-by.ansi-neta-ats(provenance pending)
  10. ANSI/NETA ATS-2025, clause 7.18.1.1.B.4. Battery Intercell Connection Resistance is addressed in ANSI/NETA ATS-2025. Datum tests.battery-intercell-connection-resistance.governed-by.ansi-neta-ats(provenance pending)
  11. ANSI/NETA MTS-2023, clause 5.3. Section 5.3 of ANSI/NETA MTS-2023 deals with the calibration of test instruments used in maintenance testing. Datum instr.low-resistance-ohmmeter.mts-calibration

Data records

  • A low-resistance ohmmeter is an instrument that measures small resistances, such as those of contacts, joints and bolted connections, by passing a known direct current through the object and measuring the voltage drop that the current produces across it. Source: Electrical Testing Reference editorial taxonomy, 2026-10 edition. Verified 2026-10-13. Review status: Agent reviewed. Record instr.low-resistance-ohmmeter.definition
  • In a four-terminal measurement the current is supplied through one pair of leads and the voltage drop is sensed through a separate pair, so that the resistance of the leads and of the current connections is kept out of the result. Source: Electrical Testing Reference editorial taxonomy, 2026-10 edition. Verified 2026-10-13. Review status: Agent reviewed. Record instr.low-resistance-ohmmeter.four-terminal
  • A reference on impedance measurement published by the University of California, Berkeley physics laboratory describes the four-terminal technique as passing current through one pair of leads and measuring voltage on the other pair, which removes the error caused by the series impedance of the leads, and describes it as suited to low impedances. Source: UC Berkeley Physics Experimentation Lab reference 10, 2001 edition, clause Four terminal measurements. Verified 2026-10-13. Review status: Agent reviewed. Record instr.low-resistance-ohmmeter.four-terminal-source
  • The same reference notes that the term Kelvin connection is often used interchangeably with four-terminal, and names the Kelvin bridge as the most widely known four-terminal technique for precision direct-current resistance measurement. Source: UC Berkeley Physics Experimentation Lab reference 10, 2001 edition, clause Four terminal measurements. Verified 2026-10-13. Review status: Agent reviewed. Record instr.low-resistance-ohmmeter.kelvin-term
  • Section 5.3 of ANSI/NETA MTS-2023 deals with the calibration of test instruments used in maintenance testing. Source: ANSI/NETA MTS-2023, clause 5.3. Verified 2026-10-13. Review status: Agent reviewed. Record instr.low-resistance-ohmmeter.mts-calibration

Related entries

Sources

  • ANSI/NETA ATS-2025, Acceptance Testing Specifications for Electrical Power Equipment and Systems, 2025 edition, NETA. Cited: clause 7.22.3.B.4; clause 7.22.3.B.1; clause 7.18.1.1.B.4. Held as: Purchased single-user copy. Publisher page
  • ANSI/NETA MTS-2023, Maintenance Testing Specifications for Electrical Power Equipment and Systems, 2023 edition, NETA. Cited: clause 7.22.3.B.3; clause 5.3. Held as: Purchased single-user copy. Publisher page
  • Electrical Testing Reference editorial taxonomy, Electrical Testing Reference editorial taxonomy of test methods, 2026-10 edition, Electrical Testing Reference. Held as: Own observation.
  • NIST SP 811, NIST Guide to the SI, Chapter 4: The Two Classes of SI Units and the SI Prefixes, web page updated 2025-08-18 edition, NIST. Cited: clause Table 5; clause Table 3. Held as: Government work. Publisher page
  • UC Berkeley Physics Experimentation Lab reference 10, Four Wire Measurement (Appendix A, Why So Many Terminals? Understanding impedance measurement techniques), 2001 edition, The Regents of the University of California. Cited: clause Four terminal measurements. Held as: Public page. Publisher page

Obtaining the documents

Changelog (20 entries)
DateRevisionChangeReasonBy
2026-10-13created (instr.low-resistance-ohmmeter.definition)editorial definition of general physics (Task 10 exemplars)agent
2026-10-13reviewed (instr.low-resistance-ohmmeter.definition)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-13created (instr.low-resistance-ohmmeter.four-terminal)editorial definition of general physics (Task 10 exemplars)agent
2026-10-13reviewed (instr.low-resistance-ohmmeter.four-terminal)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-13created (instr.low-resistance-ohmmeter.four-terminal-source)read from the public university reference (review fix 6)agent
2026-10-13reviewed (instr.low-resistance-ohmmeter.four-terminal-source)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-13created (instr.low-resistance-ohmmeter.kelvin-term)read from the public university reference (review fix 6)agent
2026-10-13reviewed (instr.low-resistance-ohmmeter.kelvin-term)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-13created (instr.low-resistance-ohmmeter.mts-calibration)method fact restated from the licensed-copy digest (Task 10 exemplars)agent
2026-10-13provenance noted (instr.low-resistance-ohmmeter.mts-calibration)locator from the NETA index table of contents (section and table numbers, titles and tables_referenced only); no body text usedagent
2026-10-13reviewed (instr.low-resistance-ohmmeter.mts-calibration)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-131created hand-written exemplar for the article generator (Task 10)agent
2026-10-13created (qty.ohm.definition)read from the government page (Task 10 exemplars)agent
2026-10-13reviewed (qty.ohm.definition)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-13created (qty.ohm.microhm)read from the government page (Task 10 exemplars)agent
2026-10-13reviewed (qty.ohm.microhm)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-13created (tests.battery-intercell-connection-resistance.governed-by.ansi-neta-ats)seeded from the test taxonomyagent
2026-10-13created (tests.bolted-connection-resistance.governed-by.ansi-neta-ats)seeded from the test taxonomyagent
2026-10-13created (tests.contact-resistance.governed-by.ansi-neta-ats)seeded from the test taxonomyagent
2026-10-13created (tests.contact-resistance.governed-by.ansi-neta-mts)seeded from the test taxonomyagent

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Permanent revision URL: https://www.electricaltestingreference.com/instruments/low-resistance-ohmmeter/rev/1/. Current version: https://www.electricaltestingreference.com/instruments/low-resistance-ohmmeter/.

Plain

"Low-resistance ohmmeter." Electrical Testing Reference. Revision 1, 2026-10-13. https://www.electricaltestingreference.com/instruments/low-resistance-ohmmeter/rev/1/.

Legal (Bluebook, internet source)

Electrical Testing Reference, Low-resistance ohmmeter (rev. 1, Oct. 13, 2026), https://www.electricaltestingreference.com/instruments/low-resistance-ohmmeter/rev/1/.

BibTeX

@misc{etr-instruments-low-resistance-ohmmeter-rev1,
  title = {Low-resistance ohmmeter},
  howpublished = {\url{https://www.electricaltestingreference.com/instruments/low-resistance-ohmmeter/rev/1/}},
  organization = {Electrical Testing Reference},
  publisher = {Electrical Testing Reference},
  year = {2026},
  note = {Revision 1; last substantive change 2026-10-13}
}

Wikipedia

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