A temperature scale used in the English-speaking world and parts of Europe, 18ᵗʰ – 21st centuries, having the melting point of ice at 32 degrees and the boiling point of water at 212 degrees.
Download a chart for converting between Fahrenheit and Celsius temperatures visually. (PDF)
The Fahrenheit temperature scale became popular through its use on the first reliable, commercially-available, mercury-in-glass thermometers. Daniel Gabriel Fahrenheit manufactured such thermometers in Amsterdam from about 1717 until his death in 1736. The scale we know as the Fahrenheit scale was the last of three scales he used.
As the zero point on his scale, Fahrenheit chose the temperature of a bath of ice melting in a solution of common salt, a standard 18ᵗʰ century way of getting a low temperature in the laboratory (and in the kitchen, as in an old-fashioned ice cream churn). He set 32 degrees as the temperature of ice melting in water. For a consistent, reproducible high point he chose the temperature of the blood of a healthy person (his wife), which he measured in the armpit and called 96 degrees. (The number arises from beginning with a scale of 12 intervals, like a one-foot ruler, and then doubling the number of steps as instruments become more precise, making 24 intervals, then 48, and finally 96.)¹
Fahrenheit’s successors used the boiling point of water to calibrate their thermometers, which they set at 212 degrees in order to retain the size of Fahrenheit's degree. Fahrenheit himself did make thermometers that could read as high as 600°F, but the largest market was for thermometers for weather observations, and on these the part of the thermometer from 130°F to 212°F would be wasted.
The Fahrenheit scale of temperature remains in use in the United States, but it is now defined in terms of the Kelvin scale.
1. Ernst Cohen discovered in a Leningrad archive a letter in Dutch from Fahrenheit to Herman Boerhaave. In the letter Fahrenheit acknowledges that he got the idea for his thermometers from Ole Rømer, but eventually renumbered the fixed points: “As this graduation is inconvenient and awkward because of the fractions, I decided to alter the scale, and to use 96 instead of 22½ or 90; this I have always used since then.” “Then” was apparently 1717. The translation is Middleton’s (page 71), see the reference in Resources.
Ernst Cohen and W. A. T. Cohen-De Meester.
Chemisch Weekblad, volume 33 (1936), pages 374-393.
D. G. Fahrenheit.
Experimenta et Observationes de Congelatione aquae in vacuo factae a D. G. Fahrenheit, R. S. S..
Philosophical Transactions (London), vol. 33, no. 382, page 78 (March-April 1724).
Translated for Sizes by J. Holland.
Antequam autem experimentorum recensionem aggrediar, necesse ecrit, ut paucis quaedam de thermometris, quae a me construuntur, eorumque scalae divisione, ut & de methodo evacuandi, qua usus sum, mentionem faciam. Duo potissimum genera thermometrorum a me conficiuntur, quorum unum spiritu vini & alterum argento vivo est repletum: Longitudo eorum varia est, pro usu, cui inservire debent: Omnia autem in eo conveniunt, quod in omnibus scalae gradibus concordent, interque limites fixos variationes suas absolvant. Thermometrorum scala, quae meteorologicis observationibus solummodo inserviunt, infra a Zero incipit & 96to gradu finitur. Hujus scalae division tribus nititur terminis fixis, qui arte sequentimodo parari possunt; primus illorum in informa parte vel initio scalae reperitur, & commixtione glaciei, aquae, & salis Armoniaci vel etiam maritimi acquiritur; huic mixturae si thermometron imponitur, fluidum ejus usque ad gradum, qui zero notatur, descendit. Melius autem hyeme, quam aestate hoc experimentum succedit. Secundus terminus obtinetur, si aqua & glacies absque memoratis salibus commiscentur, imposito thermometro huic mixturae, fluidum ejus tricesimum secundum occupat gradum, & terminus initii congelationis a me vocatur; aquae enim stagnantes tenuissima jam glacie obducuntur, quando hyeme liquor thermometri hunce gradum attingit. Terminus tertius in nonagesimo sexto gradu reperitur; & spiritus usque ad hunc gradum dilatatur, dum thermometrum in ore vel sub axillis hominis in statu sano viventis tam diu tenetur donec perfectissime calorem corporis acquisivit. Si vero calor hominis febri vel alio morbo fervente laborantis investigandus est, alio thermometro utendum, cujus scala usque ad 128 vel 132 gradum prolongata est. An autem hi gradus ferventissimo calori alicujus febris sufficiant nondum expertus sum, vix tamen credendum, quod cujusdam febris fervor gradus memoratos excedere debeat. Thermometrorum scala, quorum ope ebullientium liquorum gradus caloris investigatur, etiam a zero incipit & 600 continet gradus, hoc enim circiter gradu Mercurius ipse (quo thermometron repletum est) incipit ebullire.
Yet before I undertake a review of these experiments it will be necessary to say a few words about the thermometers that I have built, and the division of the scale they use, and in addition the method of producing a vacuum I have used. I make two particular types of thermometer, one of which is filled with alcohol and the other with mercury. Their length varies in accordance with the use to which they are put. Yet all use the same scale, and their differences relate only to their fixed limits. The scale of those thermometers that are used only for observations on the weather begins with zero and ends on the 96th degree. The division of the scale depends on three fixed points, which can be determined in the following manner. The first is found in the uncalibrated part or the beginning of the scale, and is determined by a mixture of ice, water and ammonium chloride or even sea salt. If the thermometer is placed in this mixture, its liquid descends as far as the degree that is marked with a zero. This experiment succeeds better in winter than in summer. The second point is obtained if water and ice are mixed without the aforementioned salts. When the thermometer is placed in this mixture, its liquid reaches the 32nd degree. I call this ‘freezing point’. For still waters are already covered with a very thin layer of ice when the liquid of the thermometer touches this point in winter. The third point is situated at the 96th degree. Alcohol expands up to this point when it is held in the mouth or under the armpit of a living man in good health until it has completely acquired his body heat. But if the temperature of a man suffering from fever or some other heating disease is to be investigated, another thermometer must be used, with a scale extended to the 128th or 132nd degree. I have not yet discovered by experiment whether these degrees are sufficient for the most intense heat of some fever, but it is scarcely credible that the heat of any fever should exceed the degrees I have described. When a thermometer is being used to investigate the temperature of boiling liquids, it too starts from zero and contains 600 degrees, for around this point mercury itself (with which the thermometer is filled) begins to boil.
Heathcote² has expressed the opinion that Fahrenheit did not actually use the first fixed point mentioned in his article (and Middleton concurs (page 75), see the reference in Resources).
2. N. H. de V. Heathcote.
Ambix, volume 6, (1958), pages 155-156.
W. E. Knowles Middleton.
A History of the Thermometer and its Use in Meteorology.
Baltimore, MD: Johns Hopkins Press, 1966.
Not only scholarly but delightful. A wonderful read, well-illustrated, a model of its kind.
Daniel Gabriel Fahrenheit. Sein Leben und Wirken.
Schriften der Naturforschenden Gesellschaft in Danzig, Neue Folge, 7th Band, 3rd Heft.
Danzig: Commissions-Verlag von Wilhelm Engelmann in Leipzig, 1890.
Page 108. A very full biographical sketch, to which are appended German translations of some of Fahrenheit's essays, including the one in Philosophical Transactions translated in this entry (page 128). This publication is available through archive.org: www.archive.org/details/schriftendernatu734natu.
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Last revised: 27 July 2011.