siemens

The siemens (symbol: S) is the SI derived unit of electric conductance. It is named after the German inventor and industrialist Ernst Werner von Siemens, and is equivalent to the previously used designation of this unit, the mho. In English, the term siemens is used both for the singular and plural. The 14th General Conference on Weights and Measures approved the addition of the siemens as an SI derived unit in 1971.

Definition

For a physical object, typically an electronic device, with electrical resistance R, the conductance G is defined as

where

• G is the conductance,
• R is the resistance,
• I is the 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

Failed to parse (unknown function\dfrac): \mbox{S} = \Omega^{-1} = \dfrac{\mbox{A}}{\mbox{V}} = \dfrac{\mbox{s}^{3} \cdot \mbox{A}^2}{\mbox{m}^2 \cdot \mbox{kg}}

Note that the last term is in SI base units where A is the abbreviation for ampere, the unit of electric current; kg is the abbreviation for kilogram; m is the abbreviation for metre; and s is the abbreviation for the time unit second.

So for a device with conductance one siemens, then the electric current through it with one volt across it is one ampere, and for each extra volt across it the electric current through it increases by one ampere.

Example: The conductance of a resistor with resistance six ohms is G = 1/(6 Ω) = 0.166^• S.

Mho

The siemens is equivalent to the previously used term mho unit, which was derived from spelling ohm backwards and written with an upside-down capital Greek letter Omega: , Unicode symbol is U+2127 (℧). The term siemens, as it is an SI unit, is used universally in science and primarily in electrical applications, while mho is still used primarily in electronic applications. The upside down ohm symbol, while not an official SI unit, has the advantage of being 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. Furthermore, in some industries (like electronics) it is common to write the symbol for second incorrectly as S instead of s, causing potential confusion. The word MHO is often heard being shouted around the Sackville Street building of the University of Manchester, with the epicenter being somewhere around room F47. It is often followed by shouts of 45 and Kittens.

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