siemens (unit)
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| Name | siemens |
| Quantity | electrical conductance, admittance, susceptance |
| Units | 1 S = 1 A/V = 1 Ω^−1 |
| Named after | Werner von Siemens |
siemens (unit) is the SI derived unit of electrical conductance, electrical admittance and electrical susceptance. It quantifies how easily electric current flows through a conductor for a given potential difference and is defined as the reciprocal of the ohm. The unit is widely used in electrical engineering, condensed matter physics and instrumentation.
Definition and symbol
The siemens is defined in the International System of Units as one ampere per volt, expressed as 1 S = 1 A V^−1, which is algebraically equivalent to the reciprocal of the ohm (1 S = 1 Ω^−1). Its single-letter symbol is "S", adopted to honor Werner von Siemens while avoiding confusion with the Siemens company trademark; the choice follows conventions for SI unit symbols like "A" for André-Marie Ampère and "V" for Alessandro Volta. The unit applies to scalar measures of conductance and complex measures of admittance in alternating current contexts used across standards bodies such as the International Bureau of Weights and Measures and the International Electrotechnical Commission.
History and etymology
The name commemorates Werner von Siemens, a 19th‑century German inventor and industrialist associated with early telegraphy and electrical engineering developments that influenced standardization in Prussia and later Germany. Early electrical science used terms like "mho" (ohm spelled backward) popularized in English‑speaking engineering literature and by manufacturers such as the General Electric Company; the siemens was adopted to align with SI nomenclature and international agreements reached through organizations including the International Electrotechnical Commission and the Consultative Committee for Electricity and Magnetism. Decisions in the 20th century by bodies like the International Committee for Weights and Measures led to formal adoption of the unit name and symbol in SI publications.
SI status and multiples
As an SI derived unit, the siemens is coherent with SI base units and integrates with decimal prefixes standardized by the International System of Units; common multiples include the millisiemens (mS), microsiemens (μS), and siemens multiplied by powers of ten such as kilosiemens (kS) and megasiemens (MS). Metrology laboratories such as the National Institute of Standards and Technology and the Physikalisch‑Technische Bundesanstalt maintain traceability chains for conductance standards expressed in siemens and use international comparisons coordinated by the International Bureau of Weights and Measures to ensure global consistency.
Physical interpretation and relation to other units
Conductance measured in siemens is the inverse of resistance measured in ohms, relating to Ohm's law formulated by Georg Ohm as I = V/R, which can be recast as I = G·V where G is conductance in siemens. For alternating current and complex impedance theory developed in works by Oliver Heaviside and others, admittance Y is a complex quantity with real part conductance (S) and imaginary part susceptance (S), linking to reactance in ohms and to impedance matrices used in telecommunications network analysis. In material science, conductivity σ (S·m^−1) relates to conductance by geometric factors familiar from studies at institutions like the Max Planck Society and the Royal Institution.
Measurement and practical realizations
Practical realization of the siemens involves precision measurement of current and voltage using instruments traceable to primary standards maintained by national metrology institutes such as National Physical Laboratory (United Kingdom) and Laboratoire national de métrologie et d'essais. Bridge methods like the Wheatstone bridge lineage tied to Samuel Hunter Christie and Charles Wheatstone are extended by modern four‑terminal sensing, vector network analyzers from companies like Keysight Technologies and cryogenic current comparators pioneered at laboratories including CERN for high‑precision conductance comparisons. Calibration procedures follow protocols from the International Electrotechnical Commission and the International Organization for Standardization.
Applications and examples
Siemens appear across electrical and electronic engineering: characterizing cable and connector losses in Bell Labs‑era transmission line theory; specifying ionic conductivity in aqueous chemistry studies at research centers such as Royal Society of Chemistry‑affiliated labs; quantifying semiconductor channel conductance in devices developed by firms like Intel Corporation and Texas Instruments; and parametrizing physiological bioelectric conductance in biomedical research at institutions like Johns Hopkins University. Typical values range from microsiemens for purified water conductivity measurements to megasiemens for bulk metallic conductors; network analysis of antennas and filters uses admittance parameters (S‑parameters) standardized by the Institute of Electrical and Electronics Engineers.
Standards and conventions
Standards governing the use, symbol and calibration of the siemens derive from SI Brochures produced by the International Bureau of Weights and Measures and technical committees of the International Electrotechnical Commission and Institute of Electrical and Electronics Engineers. Conventions include use of uppercase "S" for the unit symbol, decimal prefixes compatible with ISO 80000 recommendations, and not pluralizing the symbol in specifications. Metrological traceability and interlaboratory comparisons are coordinated through bodies such as the Bureau International des Poids et Mesures and regional metrology organizations like the European Association of National Metrology Institutes.