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 () and is also referred to as the ''
mho''. 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
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 foun ...
. 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, 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 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 to ...
(electrical potential difference) across the object.
The unit siemens for the conductance ''G'' is defined by
: