In the 19th century, the Siemens? unit or Siemens? mercury unit was a unit of electrical resistance introduced in 1860 by Werner von Siemens himself. One Siemens? unit is approximately 0.9534 ohm. The standard was defined as the resistance of a column of pure mercury 1 meter long with a cross sectional area of 1 square millimeter, at a temperature of 0?C. For everyday purposes, the standard was realized as a German silver wire 3.8 meters long and 0.9 millimeters in diameter.
Siemens manufactured reference wire coils calibrated in this unit.
The advantages of the Siemens unit were that its magnitude was convenient for telegraph engineers and that a high precision standard was fairly easily constructed in the laboratory. The disadvantage was that its definition was entirely unrelated to definitions of units of voltage or current, which complicated calculation.
The siemens unit was one of two units of resistance that competed for acceptance (the other being the B.A. unit of resistance) until the definition of the ohm at the First International Electrical Congress (Paris, 1881), after which the siemens? unit withered.
