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Mho

Siemens
Unit system SI derived unit
Unit of Electric conductance
Symbol S (= Ω−1) 
Named after Ernst Werner von Siemens
In SI base units: kg−1m−2s3A2

The siemens (SI unit 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 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 form siemens is used both for the singular and plural.

For a conducting or semiconducting element, electrical resistance R and electrical conductance G are defined as

where I is the electric current through the object and V is the voltage (electrical potential difference) across the object.

The unit siemens for the conductance G is defined by

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

For a device with a conductance of one siemens, the electric current through the device will increase by one ampere for every increase of one volt of electric potential difference across the device.

The conductance of a resistor with a resistance of five ohms, for example, is (5 Ω)−1, which is equal to 200 mS.

Mho /m/ is an alternative name of the same unit, the reciprocal of one ohm. Mho is derived from spelling ohm backwards and is written with an upside-down capital Greek letter Omega: , Unicode symbol U+2127 (℧). According to Maver the term mho was suggested by Sir William Thomson (Lord Kelvin). The mho was officially renamed to the siemens, replacing the old meaning of the "siemens unit", at a conference in 1881.


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