Fahrenheit temperature scale

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.

Converting between degrees Celsius and Fahrenheit

Degrees Fahrenheit equals nine fifths degrees Celsius, plus 32                Degrees Celsius equals nine fifths of the quantity degrees Fahrnheit minus 32

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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.
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.


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, and a model of its kind.

A. Momber.
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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