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For the deprecated "mercury unit", see siemens mercury unit. For other uses, see Siemens (disambiguation).

The siemens (SI unit symbol: S) is the unit of electric conductance and electric admittance in the International System of Units (SI). Conductance and admittance are the reciprocals of resistance and impedance respectively, hence one siemens is equal to the reciprocal of one ohm, and is also referred to as the mho. The 14th General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.

The unit is named after Ernst Werner von Siemens. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (S); the lower-case "s" is the symbol for the second. When an SI unit is spelled out in English, it should always begin with a lower case letter (siemens), except where any word would be capitalized.^[1] In English, the same form siemens is used both for the singular and plural.^[2]

Definition [edit]

For a conducting or semiconducting element with electrical resistance R, the conductance G, is defined as

where I is the electric current through the object and V is the voltage (electrical potential difference) across the object.

The unit siemens for the conductance G is defined by

where Ω is the ohm, A is the ampere, and V is the volt.

For a device with a conductance of one siemens, the electric current through the device will increase by one ampere for every increase of one volt of electric potential difference across the device.

The conductance of a resistor with a resistance of five ohms, for example, is (5 Ω)⁻¹, which is equal to 200 mS.

Mho [edit]

"Mho" redirects here. It is not to be confused with Mohs (disambiguation).

Mho is an alternative name of the same unit, the reciprocal of one ohm. Mho is derived from spelling ohm backwards and is written with an upside-down capital Greek letter Omega: , Unicode symbol U+2127 (℧). According to Maver^[3] the term mho was suggested by Sir William Thomson. The mho was officially renamed to the siemens, replacing the old meaning of the "siemens unit", at a conference in 1881.^[4]

The term siemens, as it is a SI term, is used universally in science and often in electrical applications, while mho is still used primarily in electronic applications. Two reasons are usually given^{[citation needed]} for using mho instead of siemens in electronic applications:

The inverted Omega and the mho, while not an official SI abbreviation, has the advantage of being less likely to be confused with a variable than the letter S when doing algebraic calculations by hand, where the usual typographical distinctions (such as italic for variables and Roman for unit names) are difficult to maintain. Likewise, it is difficult to distinguish the symbol S from the lower case s where second is meant, potentially causing confusion.
The term siemens could be confused with the large multinational electronics company Siemens.

References [edit]

^ The International System of Units, section 5.2.
^ NIST Guide to SI Units - 9 Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technology
^ Maver, William: American Telegraphy and Encyclopedia of the Telegraph: Systems, Apparatus, Operation. 1903.
^ http://www.tech-faq.com/siemens.html

Brochure "The International System of Units" issued by the BIPM
Different units named after Siemens

English Journal

Gap distance and interactions in a molecular tunnel junction.

Chang S, He J, Zhang P, Gyarfas B, Lindsay S.SourceDepartment of Physics, ‡Biodesign Institute, and §Department of Chemistry and Biochemistry, Arizona State University , Tempe, Arizona 85287, United States.
Journal of the American Chemical Society.J Am Chem Soc.2011 Sep 14;133(36):14267-9. Epub 2011 Aug 18.
The distance between electrodes in a tunnel junction cannot be determined from the external movement applied to the electrodes because of interfacial forces that distort the electrode geometry at the nanoscale. These distortions become particularly complex when molecules are present in the junction,
PMID 21838292

Single channel characterization of the mitochondrial ryanodine receptor in heart mitoplasts.

Ryu SY, Beutner G, Kinnally KW, Dirksen RT, Sheu SS.SourceDepartment of Pharmacology and Physiology, University of Rochester Medical Center, Rochester, New York 14642, USA.
The Journal of biological chemistry.J Biol Chem.2011 Jun 17;286(24):21324-9. Epub 2011 Apr 27.
Heart mitochondria utilize multiple Ca(2+) transport mechanisms. Among them, the mitochondrial ryanodine receptor provides a fast Ca(2+) uptake pathway across the inner membrane to control "excitation and metabolism coupling." In the present study, we identified a novel ryanodine-sensitive channel i
PMID 21524998

Japanese Journal

Reconstitution in Lipid Bilayer of Smooth Muscle Cation Channels Activated Through a GTP-Binding Protein

DOIRA Naoko,HANANO Toyohisa,ONOUE Hitoshi,NAKANO Hitoo,ITO Yushi,INOUE Ryuji
日本平滑筋学会機関誌 37(2), 39-51, 2001-04
… The reversal potential and unitary conductance(tens of picosiemens)of these channels varied in a manner dependent on Na^+ concentration, but not affected by Cl^-. …
NAID 110002929823