Siemens mercury unit
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The Siemens mercury unit is an obsolete unit of electrical resistance. It was defined by Werner von Siemens as the resistance of a mercury column 1 metre long and uniform 1 mm2 cross sectional area at 0 degrees Celsius. It is equivalent to 0.953 ohm approximately. In 1881 the name "siemens" was reused by the SI unit of electric conductivity, but Siemens mercury unit was widely used in telegraph and telephone services until World War II.