Siemens (unit)
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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 now-obsolete mho. In English, it is siemens in both singular and plural. The 14th General Conference on Weights and Measures approved the addition of the siemens as an SI derived unit in 1971.
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This SI unit is named after Ernst Werner von Siemens. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (S). But when an SI unit is spelled out, it should always be written in lowercase (siemens), unless it begins a sentence or is the name "degree Celsius".
— Based on The International System of Units, section 5.2.
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[edit] Definition
For a 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 device and
- V is the voltage "drop" (electrical potential difference) across the device.
The unit siemens for the conductance G is defined by 1 S = 1 A/V = 1 A2/W = 1 kg−1·m−2·s3·A2 =1 Ω-1 = 1 kg−1·m−2·s1·C2. So for a device with conductance 1 S, then the current through it with a 1 V voltage across it is 1 A, and for each extra V of voltage across it the current through it increases by 1 A.
Example: The conductance of a resistor with resistance 6 ohms is G = 1/(6 Ω) = 0.166... S.
[edit] SI multiples
| Multiple | Name | Symbol | Multiple | Name | Symbol | |
|---|---|---|---|---|---|---|
| 100 | siemens | S | ||||
| 101 | decasiemens | daS | 10–1 | decisiemens | dS | |
| 102 | hectosiemens | hS | 10–2 | centisiemens | cS | |
| 103 | kilosiemens | kS | 10–3 | millisiemens | mS | |
| 106 | megasiemens | MS | 10–6 | microsiemens | µS | |
| 109 | gigasiemens | GS | 10–9 | nanosiemens | nS | |
| 1012 | terasiemens | TS | 10–12 | picosiemens | pS | |
| 1015 | petasiemens | PS | 10–15 | femtosiemens | fS | |
| 1018 | exasiemens | ES | 10–18 | attosiemens | aS | |
| 1021 | zettasiemens | ZS | 10–21 | zeptosiemens | zS | |
| 1024 | yottasiemens | YS | 10–24 | yoctosiemens | yS |
[edit] Mho
The siemens is equivalent to the now obsolete 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 incorrectly write the symbol for second as S instead of s, causing potential confusion.
