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 1 mm2 held at a temperature of zero degrees Celsius.[1] It is equivalent to approximately 0.953 ohm.
Glass tube cross sections are typically irregularly conical rather than perfect cylinders, which presented a problem in constructing precise measuring devices. One could make many tubes and test them for conical regularity, discarding the least regular ones; their regularity can be measured by inserting a small drop of mercury into one end of the tube, then measuring its length while sucking it along. The cross-sectional area at each end can then be measured by filling the tube with pure mercury at a fixed temperature, weighing it, and comparing that weight to the relative lengths of the mercury drop at each end. The tube can then be used for measurement by applying a formula obtained from these measurements that corrects for its conical shape.[2]
The Siemens mercury unit was superseded in 1881 by the ohm; the name "siemens" was later reused for a unit of electric conductance.[3]
References
- ↑ Werner Siemens (1860), "Vorschlag eines reproducirbaren Widerstandsmaaßes" (in German), Annalen der Physik und Chemie 186 (5): 1–20, doi:10.1002/andp.18601860502, Bibcode: 1860AnP...186....1S, https://zenodo.org/record/1423670
- ↑ Robert Sabine: The Electric Telegraph. Virtue brothers & Company, 1867, 428 pp. Second Part – V. Units of Resistance – 64. Siemens Mercury Unit p. 328–333.
- ↑ Siemens (Unit of Electrical Conductance), http://www.tech-faq.com/siemens.html
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