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The siemens (symbol: S) is the 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 Ernst Werner Siemens (von Siemens from 1888; ; ; 13 December 1816 – 6 December 1892) was a German electrical engineer, inventor and industrialist. Siemens's name has been adopted as the SI unit of electrical conductance, the siemens. He foun ...
. In English, the same word ''siemens'' is used both for the singular and plural. Like other SI units named after people, the symbol is capitalized but the name of the unit is not. For the siemens this is particularly important to distinguish it from the second, symbol (lower case) s. The related property, electrical conductivity, is measured in units of siemens per metre (S/m).


Definition

For an element conducting
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
, electrical resistance and electrical conductance are defined as :G = \fracR = \frac where is the electric current through the object and is the
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
(electrical potential difference) across the object. The unit siemens for the conductance ''G'' is defined by :\mathrm = Omega^= mathrm/\mathrm/math> where is the ohm, is the ampere, and is the
volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defin ...
. 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

A historical equivalent for the ''siemens'' is the ''mho'' (). The name is derived from the word '' ohm'' spelled backwards as the reciprocal of one ohm, at the suggestion of Sir William Thomson (Lord Kelvin) in 1883. Its symbol is an inverted capital
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
letter
omega Omega (; capital: Ω, lowercase: ω; Ancient Greek ὦ, later ὦ μέγα, Modern Greek ωμέγα) is the twenty-fourth and final letter in the Greek alphabet. In the Greek numeric system/ isopsephy ( gematria), it has a value of 800. The ...
: . 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. 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 writing the symbol by hand. The usual typographical distinctions (such as italic for variables and roman for units) are difficult to maintain. Likewise, it is difficult to distinguish the symbol 'S' (''siemens'') from the lower-case 's' (''seconds''), potentially causing confusion. So, for example, a pentode’s
transconductance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproc ...
of might alternatively be written as or (most common in the 1930s) or . A handwritten 'S' can also be misread as the frequency-space variable 's', commonly used in transfer functions. The ''ohm'' had officially replaced the old "''siemens'' unit", a unit of ''resistance'', at an international conference in 1881.


Notes and references


External links

* * {{DEFAULTSORT:Siemens (Unit) SI derived units Units of electrical conductance Werner von Siemens