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.
Definition
For a device with electrical resistance R, the conductance G is defined as
[G = \frac1R = \fracV, ]
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.
1 Ω-1
Example: The conductance of a resistor with resistance 6 ohms is G = 1/(6 Ω) = 0.166... S.
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
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: [\mho], 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.
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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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