Siemens mercury unit
The Siemens mercury unit is an obsolete unit of electrical resistance. It was defined by Werner von Siemens in 1860 as the resistance of a mercury column with a length of one metre and uniform cross-section of 1mm2 held at a temperature of zero degrees Celsius. It is equivalent to approximately 0.953 ohm.
Because glass tube cross sections are often slightly conical, the unit was sometimes constructed by the weight of mercury in 1 m.
In 1881, a similar unit, the siemens was formally defined by the metric system as the unit of electric conductivity, as the inverse of the ohm for resistance. The Siemens mercury unit was superseded in 1884, but continued in use in telegraph and telephone services until World War II.