Before the siemens, electrical conductance was measured in mhos, ohm spelled backwards
In 1883 Lord Kelvin suggested a joke of a name for the reciprocal of resistance: mho, ohm reversed, with an upside-down omega as its symbol. The official SI unit today is the siemens, but engineers still reach for the mho now and then, partly because a handwritten S is so easily confused.
Conductance measures how easily current flows, the mirror image of resistance. For direct current it equals current divided by voltage, so a device with a conductance of one siemens passes an extra ampere for every added volt. A five-ohm resistor, for instance, has a conductance of 200 millisiemens. The unit also covers susceptance and admittance, the reciprocals of reactance and impedance, and conductivity of a material is quoted in siemens per metre.
It honours Ernst Werner von Siemens. Like other SI units named after people, the word stays lowercase, and siemens serves as both singular and plural, but the symbol is a capital S, chosen to tell it apart from the lowercase s of the second. The International Electrotechnical Commission adopted it in 1935, and the General Conference on Weights and Measures added it to the SI as a derived unit at its fourteenth meeting in 1971. Confusingly, an older resistance unit once bore the Siemens name; the ohm officially displaced it at an international conference in 1881.
The US standards body NIST lists the mho as an unaccepted special name and says to avoid it strictly. Yet the siemens rules in science and much electrical work while the mho lingers in some electronics. Its inverted omega is not an official abbreviation, but by hand it is harder to mistake for a variable than an S, and keeping italic variables separate from upright unit symbols is awkward in practice.
So the same valve characteristic can be written several ways. A pentode's transconductance of 2.2 millisiemens might appear as 2.2 millimhos, as 2,200 micromhos, the form most popular in the 1930s, or as 2.2 milliamperes per volt.
Source: Siemens (unit)
