Rømer scale
Second temperature scale in history, influenced Fahrenheit.
The Rømer scale (°Rø), also called Romer or Roemer, is a temperature scale created by Danish astronomer Ole Christensen Rømer around 1702 for his personal use. It sets the freezing point of pure water at 7.5 degrees and the boiling point of water at 60 degrees. Historically, it is the second temperature scale on record, following the Newton scale, which Isaac Newton had introduced just one year earlier.
The reason Rømer chose 7.5 degrees for water’s freezing point is not firmly documented. One theory suggests he originally set zero degrees to the eutectic temperature of ammonium chloride brine—the coldest easily reproducible temperature at the time, already used as a lower reference for other scales—with water’s boiling point at 60 degrees. He then noticed that pure water’s freezing point fell roughly one-eighth of the way between these two points, or about 7.5 degrees. So he redefined the lower fixed point as exactly 7.5 degrees, making calibration easier by relying on pure water without significantly altering the scale. As a result, one Rømer degree equals 100 ÷ 52.5, or 40/21, of a kelvin or Celsius degree. While the symbol °R is sometimes used, it can be confused with the Réaumur and Rankine scales, so °Rø is preferred.
Rømer’s scale might have been forgotten if his notebook, *Adverseria*, had not been published in 1910, and if letters between Daniel Gabriel Fahrenheit and Herman Boerhaave had not been discovered in 1936. These documents reveal how Rømer’s work influenced Fahrenheit, a young maker of barometers and thermometers. Fahrenheit visited Rømer in Copenhagen in 1708 and learned about his thermometers; Rømer also mentioned the high demand for accurate instruments. This visit sparked Fahrenheit’s interest in improving thermometers. By 1713, Fahrenheit was building his own thermometers with a scale heavily based on Rømer’s, ranging from 0 to 24 degrees, with each degree divided into quarters. Later, the quarter degrees became whole degrees, and Fahrenheit made further adjustments: he changed the freezing point from 7.5 to 8 degrees, which, multiplied by four, gives 32 degrees on the Fahrenheit scale. The 22.5-degree point would have become 90 degrees, but Fahrenheit rounded it up to 24 degrees—96 when multiplied by four—to simplify calculations. After Fahrenheit perfected his accurate thermometers, their use spread widely, and the Fahrenheit scal
- field
- Astronomy, temperature measurement
- nationality
- Danish
- known_for
- Developing the Rømer temperature scale
- scale_based_on
- Freezing point of water at 7.5 °Rø, boiling point at 60 °Rø
Lore & Background
There is no solid evidence as to why Rømer assigned the value of 7.5 degrees to water's freezing point. One proposed explanation 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. 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 unit of this scale, a Rømer degree, is 100 ÷ 52.5 = 40/21 of a kelvin or Celsius degree. The symbol is sometimes given as °R, but since that is also sometimes used for the Réaumur and Rankine scales, the other symbol °Rø is to be preferred.
Reader's Guide
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. 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. The visit ignited a keen interest in Fahrenheit to try to improve thermometers. 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. 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. The 22.5 degree point would have become 90 degrees, but Fahrenheit rounded this up to 24 degrees – 96 when multiplied by 4 – in order to make calculations easier. After Fahrenheit perfected the crafting of his accurate thermometers, their use became widespread and the Fahrenheit scale is still used today in the United States and a handful of other countries.
Did You Know?
- The Rømer scale is the second temperature scale in history, after the Newton scale devised by Isaac Newton one year earlier.
- There is no solid evidence as to why Rømer assigned the value of 7.5 degrees to water's freezing point.
- Rømer's scale would have been lost to history if his notebook Adverseria were not found and published in 1910.
- Fahrenheit visited Rømer in Copenhagen in 1708 and became familiar with his work with thermometers.
More in Danish inventions 1-21
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record