|Unit system||SI derived unit|
|Unit of||Electric conductance|
|Symbol||S (= Ω−1)|
|Named after||Ernst Werner von Siemens|
|In SI base units:||kg−1⋅m−2⋅s3⋅A2|
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.
For a conducting element, electrical resistance R and electrical conductance G are defined as
The unit siemens for the conductance G is defined by
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 the siemens is the mho //, 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 the term mho was suggested by Sir William Thomson (Lord Kelvin). The ohm officially replaced the old "siemens unit", which was a unit of resistance, at an international conference in 1881.
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. 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. A handwritten "S" can also be misread as the frequency space variable "s", commonly used in transfer functions.