The siemens (symbol: S) is the derived unit of electric conductance, electric susceptance and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is redundantly 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. In English, the same form siemens is used both for the singular and plural.[1]


For a conducting or semiconducting element, electrical resistance R and electrical conductance G are 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.


A name that is used as an alternative to for the siemens is the mho /m/, the reciprocal of one ohm. It is derived from spelling ohm backwards and is written as an upside-down capital Greek letter omega: , Unicode symbol U+2127 (℧). According to Maver[2] the term mho was suggested by Sir William Thomson (Lord Kelvin). The mho was officially renamed to the siemens, replacing the old meaning of the "siemens unit", at a conference in 1881.[3]

NIST's Guide for the Use of the International System of Units (SI) refers to the mho as an "unaccepted special name for an SI unit", and indicates that it should be strictly avoided.[4]

The SI term siemens is used universally in science and often in electrical applications, while mho is still used in some electronic contexts. The inverted capital omega symbol, while not an official SI abbreviation, is 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.[5] So, for example, a pentode’s transconductance of 2.2 mS might alternatively be written as 2.2 or 2200 (most common in the 1930s) or 2.2 mA/V.

Notes and references

  1. ^ NIST Guide to the SI, Chapter 9: Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technology, 2008, retrieved 2017-12-22 
  2. ^ Maver, William: American Telegraphy and Encyclopedia of the Telegraph: Systems, Apparatus, Operation. 1903.
  3. ^ http://www.tech-faq.com/siemens.html
  4. ^ NIST Guide to the SI, Chapter 5: Units Outside the SI, National Institute of Standards and Technology, 2008, retrieved 2017-12-22 
  5. ^ Eugene R. Weiner, Applications of Environmental Aquatic Chemistry: A Practical Guide, p. 109, CRC Press, 2013 ISBN 1439853320

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