Daniel Gabriel Fahrenheit

German scientist

• Born: May 24, 1686
• Birthplace: Gdańsk (now in Poland)
• Died: September 16, 1736
• Place of death: The Hague, Dutch Republic (now in the Netherlands)

In addition to developing the temperature scale that bears his name, Fahrenheit made considerable advances in thermometer technology, greatly increasing the accuracy of measurements of temperature.

Early Life

Daniel Gabriel Fahrenheit (DAHN-yehl GAHB-ree-ehl FAHR-ehn-hit) was born in Gdanńsk (Danzig), a free city under Polish protection located within East Prussia. He was the eldest son of a merchant with connections to the nobility. The early death of his parents forced him to go into business at a young age in order to earn a living. However, his strong interest in the natural sciences led him into scientific studies and experimentation, and he eventually found his place as a maker of fine instruments for scientific measurements. Because his background was that of a tradesman rather than a “natural philosopher,” or scientist, Fahrenheit felt little compulsion to make extensive or detailed records of his researches. As a result, very little is known of his methods or procedures, and they have become the subject of much conjecture and folklore.

Life’s Work

During the first decade of the eighteenth century, Fahrenheit spent considerable time in Denmark, where he visited the famous Danish astronomer Ole R mer, whose most notable achievement was the discovery that light did not propagate instantaneously, as many had supposed, but in fact had a finite speed, albeit an extraordinarily rapid one.

Unlike Fahrenheit, R mer was a keeper of meticulous notes, jotting down his experiments and calculations in a small notebook that ultimately would make its way to the University of Copenhagen after his death. This notebook, and particularly the annotations made in it by his successor, Peter Horrebow, are the best source of information on this critical period in the development of the modern thermometer. R mer was particularly interested in creating a reproducible thermometer, so that experiments and observations from widely differing locales could be reliably compared to one other.

Because the hollow glass tubes that were used in making thermometers were hand-blown, it was impossible to make them physically identical. As a result, it was necessary to find some means other than simple physical comparison to determine when multiple thermometers were in perfect calibration with one another. R mer’s solution was to calibrate each thermometer against known reference points, such as the melting point of ice and the boiling point of water. If the rate of expansion of the measuring fluid (whether mercury, alcohol, or some other substance) were precisely known, and each thermometer was calibrated from the same reference point, it would be possible to ensure that different thermometers would produce identical measurements, even if they were not structurally identical.

The next step was to assign specific numerical values to the various points on the temperature scale being used. R mer experimented with a number of different scales, setting various numbers for the reference points. R mer still had not settled upon a workable scale when Fahrenheit arrived to discuss questions of measurement with him. Historians of science would subsequently argue intensely about the extent of R mer’s role in inspiring Fahrenheit’s work in thermometers and temperature scales, until the discovery of a letter in an archive in Leningrad (St. Petersburg) in which Fahrenheit himself recounted experiments that he and R mer had performed together. These experiments led him to an interest in improving the mechanism of both thermometers and barometers. Although there were some discrepancies between the descriptions of these experiments in Fahrenheit’s letter and those in R mer’s notebook, which have led some scholars to try to diminish the role of R mer in Fahrenheit’s work, the letter was written some time after the fact, and time may have blurred some of the details of Fahrenheit’s memory of the events.

After R mer’s death in 1710, Fahrenheit continued his work in developing a rational scale that would provide convenient numbers for the most commonly observed temperatures. Since he was working primarily with thermometers to be used in meteorological observations, it was desirable to reserve the numbers 0-100 for temperatures that would commonly be observed in the environment. It therefore seemed most prudent for the boiling point of water to be a much higher number than 100, since it represented a temperature significantly beyond those relevant to meteorological observations. By the same token, setting the zero point of the scale at the freezing point of water was undesirable, since it would force meteorologists to use negative numbers to denote the temperatures of northern European winters.

While it is thus apparent why the Fahrenheit scale developed along the general lines that it did, it remains a subject of great conjecture how exactly Fahrenheit determined that 32 and 212 were the best numbers to represent the freezing and boiling points of water. The only definitive account of the process is found in a paper written years after the fact to be delivered at the Royal Society. There, Fahrenheit described his discovery of the supercooling phenomenon, in which water may remain liquid after it has become cooler than its normal freezing temperature, only crystalizing upon the introduction of a trigger, such as air bubbles.

According to his description, Fahrenheit assigned the zero point of his scale to the coldest temperature he could attain with a mixture of ice, water, and either of two kinds of salts. He noted, however, that the experiment worked better in winter than in summer, suggesting that he was providing less than completely accurate information. During his lifetime, much of the work of the instrument maker was still essentially a trade secret, and the habit of secrecy may well have overcome the scientist’s drive to share his discoveries with his colleagues, leading him deliberately to obscure his methods. Many historians have suggested that the melting point of water and the normal body temperature of the human being, also referred in his paper, were Fahrenheit’s real fixed points for determining his scale.

Among Fahrenheit’s customers for thermometers was noted chemist Herman Boerhaave, who bought both a mercury thermometer and an alcohol thermometer from him. After subjecting them to careful experimental observation, Boerhaave noted that they did not always quite agree in their readings. When he informed Fahrenheit of this discrepancy, Fahrenheit became curious as to the reason behind it but could not determine the cause. At length, he concluded that the discrepancy must result from the thermometers being made from two different kinds of glass. He missed entirely the real cause, namely that mercury and alcohol do not expand proportionately to one another.

Two thermometers credited to Fahrenheit have survived to modern times, but their authenticity is doubtful. One, signed by Fahrenheit, has a scale running from 0 to 600, the latter point of which is indicated as being the boiling point of mercury. However, the tube is filled with a darkish liquid that is probably not mercury, and it is often surmised that while the backing board with its engraved temperature marks is probably authentic, the glass parts have been replaced by some unknown hand at a later point.

Significance

Daniel Gabriel Fahrenheit greatly improved temperature-measuring techniques by inventing a scale that could be used to denote temperatures in different times and places and compare them accurately. His immediate goal was to improve the science of meteorology, but the development of a method of objective measurement of temperature and a rational scale by which to measure it was of utmost importance to a great many areas of science and technology, far beyond the meteorological. With a practical thermometer, physicians could determine a patient’s actual temperature, rather than being limited to feeling the forehead and trying to decide if it felt hotter than normal. Chemists could determine the temperature at which reactions took place or control the temperature of a reagent.

Bibliography

Balestrino, Philip. Hot as an Ice Cube. New York: Crowell, 1971. Basic concepts of temperature measurements, including experiments that effectively replicate Fahrenheit’s development of thermometer calibration.

Fahrenheit, Daniel Gabriel. Fahrenheit’s Letters to Leibniz and Boerhaave. Edited, translated, and annotated by Pieter van der Star. Amsterdam: Rodopi, 1983. A collection of Fahrenheit’s correspondence with the Dutch chemist and the German philosopher, reproduced in the original language and translated into English with commentary by the editor.

Frisch, Joy. Temperature. Mankato, Minn.: Smart Apple Media, 2003. Introduction to scientific concepts of temperature, including a laboratory exercise in building a thermometer very similar to Fahrenheit’s original work.

Knowles-Middleton, W. E. A History of the Thermometer and Its Uses in Meteorology. Baltimore: Johns Hopkins University Press, 1966. An older book, but with a wealth of references to primary sources in the original languages, showing the depth of research performed by its author.

Royston, Angela. Hot and Cold. Chicago: Heinemann Library, 2002. Includes a bibliography of other books accessible to young readers.

Walpole, Brenda. Temperature. Milwaukee, Wis.: Gareth Stevens, 1995. Introduction to concepts of temperature and its measurement, including the history of temperature measurement and thermometers.