The unit siemens for the conductance G is defined by
So for a device with conductance one siemens, then the electric current through it with one volt across it is one ampere, and for each extra volt across it the electric current through it increases by one ampere.
Example: The conductance of a resistor with resistance six ohms is G = 1/(6 Ω) 0.167 S.
Since 1860 to the middle of 20th century, siemens or siemens mercury unit, was the unit of electrical resistance. It was defined as the resistance of mercury column of 1 meter long and 1 mm squared at 0 degrees Celsius. It was equivalent to 0.953 Ohm approximately. Officially, it ceased usage after 1881, but was widely used in telegraph and telephone services until WWII.
The siemens was previously referred to by the term mho, which was derived from spelling ohm backwards and written with an upside-down capital Greek letter Omega: , Unicode symbol U+2127 (℧). The term siemens, as it is an SI unit, is used universally in science and primarily in electrical applications, while mho is still used primarily in electronic applications. The inverted Omega, while not an official SI abbreviation, has the advantage of being less likely to be confused with a variable than the letter S when doing algebraic calculations by hand, where the usual typographical distinctions (such as italic for variables and Roman for unit names) are difficult to maintain. Furthermore, in some industries (like electronics) it is common to write, contrary to common established and SI-dictated practice, the symbol S instead of s where second is meant, potentially causing confusion.