Daniel Gabriel Fahrenheit developed sealed mercury thermometers with reliable scales that agreed with each other, revolutionizing the scientific measurement of temperature. By developing a method for calibrating different thermometers to the same scale, he made it possible for different people in different parts of the world to accurately and reliably compare temperature measurements.
Temperature scales of a subjective nature were widely used by physicians in the Renaissance. Such scales could be useful for performing rough diagnoses, but they were inappropriate as instruments of scientific research and measurement. The first thermometer was constructed early in the seventeenth century by Galileo for use in his public lectures in Padua. It was a relatively crude, gas-filled, open glass vessel that enabled Galileo to demonstrate observable differences in temperature between different substances or within the same substance as it was heated or cooled. Knowledge of his device spread rapidly, and in the next century, thermometers of increasing usefulness were constructed by many different people. Ferdinand II de’ Medici is credited with developing the first sealed thermometer, which prevented temperature measurements from being affected by changes in atmospheric pressure.
In 1701, a Danish astronomer named Ole Rømer made a wine-filled (alcohol) thermometer. Rømer used a scale in which the temperature of a mixture of ice and salt water was 0 degrees and that of boiling water was 60 degrees. In the same year, after the death of his parents, Daniel Gabriel Fahrenheit moved to Holland, where he began making scientific instruments. In 1708, Fahrenheit visited Rømer in Denmark and observed Rømer’s methods for calibrating
Fahrenheit made his first alcohol thermometer in 1709. He visited Berlin in 1713 to investigate the expansion of mercury in Potsdam glass thermometers, and in 1714 he made his first reliable mercury thermometer. Fahrenheit sought to ensure that all his thermometers would produce the same measurements, and he picked three specific points on a temperature scale at which to standardize his thermometers. Like Rømer, he established 0 degrees with a mixture of ice and salt water (or ice, water, and sal ammoniac [ammonium chloride]); 32 degrees was set by a mixture of ice and pure water, and 96 degrees was set as the temperature reached when a healthy man placed a thermometer under his armpit or in his mouth.
Fahrenheit produced and calibrated thermometers using the scale he had developed, and his instruments were known to be of high quality, yielding standardized results. His thermometers were widely adopted, and his scale therefore came into wide use. Herman Boerhaave, a noted chemist, bought his thermometers from Fahrenheit and once brought to Fahrenheit’s attention that his alcohol and mercury thermometers read slightly differently. Fahrenheit incorrectly attributed the differences to differences in the glass used for the thermometers’ tubing, rather than to the difference between the rates of expansion of alcohol and mercury.
In 1724, Fahrenheit became a member of the English Royal Society and published the results of his investigations in its journal, Philosophical Transactions. His thermometers were the preferred instruments in Holland and England. After his death in 1736, scientists recalibrated Fahrenheit’s thermometers, setting 212 degrees as the temperature of boiling water. Recalibration then established the normal human body temperature as 98.6 degrees, rather than the 96 degrees used by Fahrenheit.
Fahrenheit’s basic design for the sealed mercury thermometer was not changed significantly after his death. The subsequent history of the instrument revolves around the development and refinement of different scales at which to calibrate it. Thermometers made for use in the late eighteenth century and throughout the nineteenth century often had two or more scales marked upon them, allowing them to be marketed to different areas where different scales were in use. The Fahrenheit thermometer scale never became popular in France, for example.
Anders Celsius, a Swedish astronomer, used René-Antoine Ferchault de Réaumur’s thermometer scale, which assigned 0 degrees to the temperature of ice water and 80 degrees to the temperature of boiling water. Celsius also used a thermometer made by Joseph Nicholas Delisle, which used an inverted scale in which 0 degrees was the boiling point of water. Although not an instrument maker like Fahrenheit, Celsius did perform experiments using thermometers. Celsius suggested a new temperature scale that would place 0 degrees at water’s boiling point and 100 degrees at its freezing point. Celsius worked in very cold climates in Sweden, Russia, and the North Atlantic, and using an inverted scale enabled him to avoid dealing with negative temperatures.
Celsius’s inverted temperature scale was rapidly changed to a direct scale, as the boiling point of water was set at 100 degrees and its freezing point was set at 0 degrees. This change may have been suggested by Jean Pierre Christin in 1743 or 1744, although Pehr Wargentin, secretary of the Royal Swedish Academy of Sciences
Throughout the English-speaking world, the Fahrenheit scale continued to be preferred until the late twentieth century, when most countries switched to the Celsius, or centigrade, scale as part of their move to the metric system. In the early twenty-first century, the Fahrenheit scale continued to be used by most people in the United States. It is still very rare to hear Celsius temperatures used in U.S. news reports or published in U.S. newspapers. In Canada, Celsius temperatures are always used in weather reports and newspapers.
In 1848, William Thomson, Baron Kelvin,
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Halliday, David, Robert Resnick, and Jearl Walker. Fundamentals of Physics. 7th ed. New York: Wiley, 2004. This easy-to-understand, college-level book contains information on how thermometers are calibrated and the relationship between the Kelvin, Fahrenheit, and Celsius scales. -
Hankins, Thomas L. Science and the Enlightenment. New York: Cambridge University Press, 1985. An excellent overview of eighteenth century science in the context of today’s knowledge. -
Middleton, W. E. Knowles. A History of the Thermometer and Its Use in Meteorology. Ann Arbor, Mich.: UMI Books on Demand, 1996. A comprehensive account of the development of thermometers and temperature scales from antiquity to the mid-twentieth century.
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Daniel Gabriel Fahrenheit; Carolus Linnaeus.