Siemens (unit)

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 word "siemens" is used both for the singular and plural.
Definition.
For an element conducting direct current, electrical resistance and electrical conductance are defined as
where is the electric current through the object and is the voltage (electrical potential difference) across the object.
The unit siemens for the conductance "G" is defined by
where is the ohm, is the ampere, and 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.
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.
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 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 μ℧ (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.
The "ohm" had officially replaced the old ""siemens" unit", a unit of "resistance", at an international conference in 1881.