Siemens mercury unit

Not to be confused with siemens (unit), the SI unit of electrical conductance.
Mercury column resistance unit, about 1860, partially filled. (Munich, 2015)

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.[1] It is equivalent to approximately 0.953 ohm.

Because glass tube cross sections are often slightly conical, for high precision the tube has been tested by inserting a small amount of the liquid mercury and measuring the length auf filled lumen while moving the portion along the tube. Tubes can be selected for at least clean conicality, a correction formula provides the difference in comparison to pure cylindrical tube. The weighing of the content of a 1 m long, filled tube helps to estimate the cross section of the possibly prismatic but non exactly circular cross section.[2]

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 The Siemens mercury unit was superseded in 1884,[3] but continued in use in telegraph and telephone services until World War II.

See also

References

  1. ↑ Werner Siemens (1860), "Vorschlag eines reproducirbaren Widerstandsmaaßes" (in German), Annalen der Physik und Chemie 186 (5): pp. 1–20, doi:10.1002/andp.18601860502
  2. ↑ Robert Sabine: The Electric Telegraph. Virtue brothers & Company, 1867, 428 pp. Second Part – V. Units of Resistance – 64. Siemens Mercury Unit p. 328–333.
  3. ↑ Ohm#Historical_units_of_resistance

External links

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