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 equal to the reciprocal of one
ohm () and is also referred to as the ''
mho''. The siemens was adopted by the
IEC in 1935, and the 14th
General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.
The unit is named after
Ernst Werner von Siemens. In English, the same word ''siemens'' is used both for the singular and plural. Like other SI units named after people, the name of the unit (siemens) is not capitalized. Its symbol (S), however, ''is'' capitalized to distinguish it from the ''
second'', whose symbol (s) is lower case.
The related property,
electrical conductivity
Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
, is measured in units of siemens per metre (S/m).
Definition
For an element conducting
direct current,
electrical resistance and electrical conductance are defined as
:
where is the
electric current through the object and is the
voltage (electrical potential difference) across the object.
The unit siemens for the conductance ''G'' is defined by
: