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{{short description|Scale of temperature}}
{{distinguish|Réaumur scale}}
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{{temperature}}
The '''Rømer scale''' ({{IPA|da|ˈʁœˀmɐ}}; notated as '''°Rø'''), also known as '''Romer''' or '''Roemer''', is a [[temperature]] scale named after the [[Denmark|Danish]] [[astronomer]] [[Ole Rømer|Ole Christensen Rømer]], who developed it for his own use in around 1702. It is based on the [[Melting point|freezing point]] of pure [[water]] being 7.5 degrees and the [[boiling point]] of water as 60 degrees.<ref name="dorsey"/>{{rp|365}}

== Degree measurements ==
There is no solid evidence as to why Rømer assigned the value of 7.5 degrees to water's freezing point. One proposed explanation<ref name="dorsey"/>{{rp|367}} is that Rømer initially intended the 0-degree point of his scale to correspond to the [[eutectic temperature]] of ammonium chloride brine, which was the coldest, easily reproducible temperature at the time and had already been used as the lower fiducial point for multiple temperature scales.<ref name="dorsey"/>{{rp|362}} The boiling point of [[water]] was defined as 60 degrees. Rømer then saw that the freezing point of pure water was roughly one eighth of the way (about 7.5 degrees) between these two points, so he redefined the lower fixed point to be the freezing point of water at precisely 7.5 degrees. This did not greatly change the scale but made it easier to calibrate by defining it by reference to pure water. Thus the [[Units of measurement|unit]] of this scale, a Rømer degree, is <math display="inline">100 \div 52.5 = \frac{40}{21}</math> of a [[kelvin]] or [[Celsius]] degree. The symbol is sometimes given as °R, but since that is also sometimes used for the [[Réaumur scale|Réaumur]] and [[Rankine scale]]s, the other symbol °Rø is to be preferred.

== Historical significance ==
Rømer's scale would have been lost to history if Rømer's notebook, ''Adverseria'', were not found and published in 1910 and letters of correspondence between [[Daniel Gabriel Fahrenheit]] and [[Herman Boerhaave]] were not uncovered in 1936. These documents demonstrate the important influence Rømer's work had on Fahrenheit, a young maker and seller of barometers and thermometers.<ref name="dorsey">{{cite journal |last1=Dorsey |first1=N. Ernest |title=Fahrenheit and Roemer |journal=Journal of the Washington Academy of Sciences |date=November 15, 1946 |volume=36 |issue=11 |pages=361–372| publisher=Washington Academy of Sciences}}</ref>

Fahrenheit visited Rømer in Copenhagen in 1708 and while there, became familiar with Rømer's work with thermometers. Rømer also told Fahrenheit that demand for accurate thermometers was high.<ref name="star" />{{rp|4}} The visit ignited a keen interest in Fahrenheit to try to improve thermometers.<ref name="middleton" />{{rp|71}}

By 1713, Fahrenheit was creating his own thermometers with a scale heavily borrowed from Rømer that ranged from 0 to 24 degrees but with each degree divided into quarters.<ref name="star">{{cite book |last1=van der Star |first1=Pieter |title=Fahrenheit's Letters to Leibniz and Boerhaave |date=1983 |publisher=Rodopi |location=Amsterdam |isbn=978-90-04-62867-0 |edition=E-book (from hardcover) |url=https://brill.com/display/title/30269 |access-date=2 December 2023}}</ref>{{rp|31}}<ref>Roger W. Coltey, ''Survey of medical technology'', University of Michigan, 1978, p. 29.</ref> At some point, the quarter degrees became whole degrees and Fahrenheit made other adjustments to Rømer's scale, modifying the freezing point from 7.5 degrees to 8, which, when multiplied by four, correlates to 32 degrees on Fahrenheit's scale<ref name="middleton">{{Cite book |last=Middleton |first=W. E. K. |url=http://archive.org/details/thermometer0000unse |title=A history of the thermometer and its use in meteorology |publisher=Johns Hopkins Press |others=Internet Archive |year=1966|isbn=9780801871535 }}</ref>{{rp|73}} The 22.5 degree point would have become 90 degrees, however, Fahrenheit rounded this up to 24 degrees–96 when multiplied by 4–in order to make calculations easier.<ref name="star" />{{rp|20}}

After Fahrenheit perfected the crafting of his accurate thermometers, their use became widespread and the [[Fahrenheit|Fahrenheit scale]] is still used today in the United States and a handful of other countries.

== See also ==
* [[Outline of metrology and measurement]]
* [[Comparison of temperature scales]]

== Notes and references==
{{Reflist}}

{{Scales of temperature}}
{{Portal bar|Energy}}

{{DEFAULTSORT:Romer Scale}}
[[Category:Obsolete units of measurement]]
[[Category:Scales of temperature]]
[[Category:Danish inventions]]
[[Category:1700s in science]]
[[Category:1700s establishments in Denmark]]