Fahrenheit Scale (1724): Daniel Fahrenheit's Mercury Thermometer

The Fahrenheit scale is the temperature scale devised by the German-born instrument maker Daniel Gabriel Fahrenheit, who published it in 1724 in the Royal Society’s Philosophical Transactions. Fahrenheit also made the first reliable mercury-in-glass thermometers, which allowed scientists in different places to compare temperatures accurately for the first time. His scale became the standard in Britain and its empire, and it is still used daily in the United States.

The problem of measuring heat

By the late seventeenth century, many scholars used thermoscopes and early thermometers, often filled with alcohol (“spirit of wine”). But there was no agreement on scales, and instruments made by different craftsmen gave different readings. A temperature recorded in Florence could not be compared with one in London.

The Danish astronomer Ole Rømer had proposed a scale with two fixed points, the freezing and boiling of water. Fahrenheit visited Rømer in Copenhagen in 1708, and his own scale developed from Rømer’s ideas.

Daniel Gabriel Fahrenheit

Fahrenheit was born on 24 May 1686 in Danzig (now Gdańsk, Poland), then a German-speaking city in the Polish–Lithuanian Commonwealth. After his parents died in 1701, he was sent to train as a merchant in Amsterdam, but he turned instead to making scientific instruments. He travelled widely before settling in the Dutch Republic, where he worked for most of his life.

He made alcohol thermometers from about 1709 and his first mercury thermometers around 1714. Mercury had important advantages: it expands evenly over a wide range of temperatures, does not stick to glass and stays liquid well above the boiling point of water. Fahrenheit’s skill was in purifying the mercury and making narrow, uniform glass tubes, so that his thermometers agreed closely with each other.

The scale of 1724

In 1724 Fahrenheit published several short papers in Latin in the Philosophical Transactions, and in the same year he was elected a Fellow of the Royal Society. He described three reference points:

• 0 degrees: the temperature of a mixture of ice, water and sal ammoniac (ammonium chloride), the coldest he could reliably reproduce.
• 32 degrees: the freezing point of water, a mixture of ice and water.
• 96 degrees: the temperature of a healthy man’s body, measured in the mouth or armpit.

On this scale water boiled at about 212 degrees. Fahrenheit also showed that the boiling point of water changes with atmospheric pressure, and in 1724 he reported the supercooling of water, which can stay liquid below its normal freezing point.

After his death, instrument makers recalibrated the scale on two fixed points, 32 degrees for freezing and 212 degrees for boiling water. This is why normal body temperature now reads about 98.6 °F rather than 96 °F. Fahrenheit died in The Hague on 16 September 1736.

Celsius’s rival scale

In 1742 the Swedish astronomer Anders Celsius presented a paper to the Royal Swedish Academy of Sciences proposing a scale of 100 degrees between the freezing and boiling points of water. Oddly to modern eyes, he set 100 for freezing and 0 for boiling.

The scale was soon inverted. In Lyon, Jean-Pierre Christin introduced a centigrade scale with 0 for freezing and 100 for boiling in 1743, and in Sweden the naturalist Carl Linnaeus, author of Systema Naturae, ordered a thermometer with the modern orientation around 1745. In 1948 an international conference officially named the scale after Celsius.

To convert, multiply a Celsius temperature by 9/5 and add 32.

Legacy

Fahrenheit’s thermometers made precise measurement of heat possible for physicians, chemists and meteorologists. His scale was adopted in Britain and the British colonies, including North America, where it became established before the metric system existed.

During the twentieth century most countries, including the United Kingdom, Canada and Australia, changed officially to Celsius as part of metrication. The United States did not, so Fahrenheit remains familiar to hundreds of millions of people. Scientists today use the kelvin, an absolute scale whose degrees are the same size as Celsius degrees.

Timeline

• 1686, 24 May: Daniel Gabriel Fahrenheit is born in Danzig.
• 1708: Fahrenheit visits Ole Rømer in Copenhagen.
• c. 1709: He makes his first alcohol thermometers.
• c. 1714: He makes his first mercury thermometers.
• 1724: He publishes his scale and is elected a Fellow of the Royal Society.
• 1736, 16 September: Fahrenheit dies in The Hague.
• 1742: Anders Celsius proposes his 100-degree scale.
• 1743: Jean-Pierre Christin introduces the modern centigrade orientation in Lyon.
• 1948: The centigrade scale is officially renamed Celsius.

Why it matters

Fahrenheit turned the thermometer from a curiosity into a precise instrument. Reliable, comparable temperature readings were essential for the growth of chemistry, medicine, meteorology and engineering, and the rival Fahrenheit and Celsius scales still shape how people around the world talk about the weather.