The siemens (symbol: S) is the 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
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (bo ...
() and is also referred to as the ''
mho''. The 14th
General Conference on Weights and Measures
The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established ...
approved the addition of the siemens as a derived unit in 1971.
The unit is named after
Ernst Werner von Siemens
Ernst Werner Siemens ( von Siemens from 1888; ; ; 13 December 1816 – 6 December 1892) was a German electrical engineer, inventor and industrialist. Siemens's name has been adopted as the SI unit of electrical conductance, the siemens. He fo ...
. In English, the same word ''siemens'' is used both for the singular and plural. Like other SI units named after people, the symbol is capitalized but the name of the unit is not. For the siemens this is particularly important to distinguish it from the
second, symbol (lower case) s.
The related property,
electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
, is measured in units of siemens per metre (S/m).
Definition
For an element conducting
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
,
electrical resistance and electrical conductance are defined as
:
where is the
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movin ...
through the object and is the
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...
(electrical potential difference) across the object.
The unit siemens for the conductance ''G'' is defined by
: