|Unit system||SI derived unit|
|Unit of||Electric conductance|
|Symbol||S or ?−1, ?|
|Named after||Ernst Werner von Siemens|
|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 (?−1) 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 an element conducting direct current, 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 ?)−1, which is equal to 200 mS.
Another name for the siemens is the mho . As the reciprocal of one ohm, it is the word ohm spelled backwards, at the suggestion of Sir William Thomson (Lord Kelvin) in 1883. Its symbol is an inverted capital Greek letter omega, Unicode character U+2127.
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 on a blackboard or doing algebraic calculations 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 of 2.2 mS might alternatively be written as 2.2 m? 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.