Siemens (unit)

The siemens (symbol: S) is the SI acquired assemblage of electric conductance and electric admittance. Conductance and admission are the reciprocals of attrition and impedance respectively, appropriately one siemens is according to the alternate of one ohm, and is sometimes referred to as the mho. In English, the appellation siemens is acclimated both for the atypical and plural.1 The 14th General Conference on Weights and Measures accustomed the accession of the siemens as an SI acquired assemblage in 1971.

This SI assemblage is called afterwards Ernst Werner von Siemens. As with every SI assemblage whose name is acquired from the able name of a person, the aboriginal letter of its attribute is high case (S). When an SI assemblage is spelled out in English, it should consistently activate with a lower case letter (siemens), except area any chat would be capitalized, such as at the alpha of a book or in capitalized actual such as a title. Note that "degree Celsius" conforms to this aphorism because the "d" is lowercase. —Based on The International System of Units, area 5.2.

Definition

For a administering or semiconducting aspect with electrical attrition R, the conductance G is authentic as

G = \frac{1}R = \frac{I}V

where I is the electric accepted through the article and V is the voltage (electrical abeyant difference) beyond the object.

The assemblage siemens for the conductance G is authentic by

\mbox{S} = \Omega^{-1} = \dfrac{\mbox{A}}{\mbox{V}}

where Ω is the ohm, A is the ampere, and V is the volt.

For a accessory with a conductance of one siemens, the electric accepted through the accessory will access by one ampere for every access of one volt of electric abeyant aberration beyond the device.

Example: The conductance of a resistor with attrition six ohms is G = 1/(6 Ω) ≈ 0.167 S ≈ 167 mS.

Historical/Deprecated

Since 1860 to the average of 20th century, siemens or siemens mercury unit, was the assemblage of electrical resistance. It was authentic as the attrition of a mercury cavalcade 1 beat continued and compatible 1 mm2 cantankerous exclusive breadth at 0 degrees Celsius. It was agnate to 0.953 ohm approximately. Officially, it accomplished acceptance afterwards 1881, but was broadly acclimated in telegraph and blast casework until World War II.

Mho

Mho is an alternate, non-SI assemblage of application which is agnate to 1 siemens. Mho is acquired from spelling ohm backwards and is accounting with an backward basic Greek letter Omega: \mho, Unicode attribute U+2127 (℧). According to Maver2 the appellation mho was appropriate by Sir William Thomson. The mho was clearly renamed to the siemens, replacing the old acceptation of the assemblage siemens, at a appointment in 1881.3

The appellation siemens, as it is an SI unit, is acclimated universally in science and generally in electrical applications, while mho is still acclimated primarily in cyberbanking applications. Two affidavit are usually givencitation needed for application mho instead of siemens in cyberbanking applications:

The astern Omega and the mho, while not an official SI abbreviation, has the advantage of actuality beneath acceptable to be abashed with a capricious than the letter S back accomplishing algebraic calculations by hand, area the accepted typographical distinctions (such as italic for variables and Roman for assemblage names) are difficult to maintain. Likewise, it is difficult to analyze the attribute S from the lower case s area additional is meant, potentially causing confusion.

The appellation siemens could be abashed with the ample bunch electronics aggregation Siemens.

Relation to resistivity and conductivity

The attrition of a accustomed article depends primarily on two factors: What actual it is fabricated of, and its shape. For a accustomed material, the cross-sectional breadth is inversely proportional to the resistance; for example, a blubbery chestnut wire has lower attrition than an otherwise-identical attenuate chestnut wire. Also, for a accustomed material, the attrition is proportional to the length; for example, a continued chestnut wire has college attrition than an otherwise-identical abbreviate chestnut wire. The attrition R and conductance G of a aqueduct of compatible cantankerous section, therefore, can be computed as

Ohm's law

Ohm's law is an empiric law apropos the voltage V beyond an aspect to the accepted I through it:

V \propto I

(V is anon proportional to I). This law is not consistently true: For example, it is apocryphal for diodes, batteries, etc. However, it is accurate to a actual acceptable approximation for affairs and resistors (assuming that added conditions, including temperature, are captivated fixed). Abstracts or altar area Ohm's law is accurate are alleged "ohmic".

For ohmic materials, the attrition R and conductance G are authentic by:

R = {V\over I}, \qquad G = {I\over V}

Therefore, attrition and conductance are inverses:

G = \frac{1}{R}

(This may not be accurate in AC circuits, see below.)