Siemens mercury unit

{{short description|Obsolete unit of electrical resistance}}

{{distinguish|text=siemens (unit), the SI unit of electrical conductance}}

File:Electric conductance.jpg

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 {{val|1|u=mm2}} held at a temperature of zero degrees Celsius.

{{citation|surname1=Werner Siemens|periodical=Annalen der Physik und Chemie|title=Vorschlag eines reproducirbaren Widerstandsmaaßes |volume=186|issue=5|pages=1–20|date= 1860|language=German|doi=10.1002/andp.18601860502|bibcode=1860AnP...186....1S|url=https://zenodo.org/record/1423670}} 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.Robert Sabine: The Electric Telegraph. Virtue brothers & Company, 1867, 428 pp. [https://books.google.com/books?id=pmBAAAAAIAAJ&dq=Siemens+mercury+unit&pg=PA328 Second Part – V. Units of Resistance – 64. Siemens Mercury Unit] p. 328–333.

The Siemens mercury unit was superseded in 1881 by the ohm; the name "siemens" was later reused for a unit of electric conductance.{{citation|title=Siemens (Unit of Electrical Conductance)|date=6 April 2019 |url=http://www.tech-faq.com/siemens.html}}