Daniel Gabriel Fahrenheit
Observations are the heart of the scientific method, but human perception is faulty when it comes to observing "absolutes." That is, one may be able to say that this liquid is hotter than that liquid, but not by how much, nor their exact temperatures. For science to be meaningful and its results reproducible, some external mechanism for making (and comparing) measurements that can be used by scientists must exist.
Daniel Gabriel Fahrenheit was a German physicist living in the Netherlands in the early eighteenth century. Like many of his contemporaries, he was interested in a great many phenomena, but the one phenomenon that really seized his attention was the boiling of liquids. Fahrenheit discovered that pure liquids boil at fixed temperatures, which are not influenced by the continued application of heat. He described his reaction to this discovery in his Philosophical Transactions (1724): "I was at once inflamed with a great desire to make for myself a thermometer of the same sort [a water-based thermometer], so that I might with my own eyes perceive this beautiful phenomenon of nature, and be convinced of the truth of the experiment."
Fahrenheit could not make a "thermometer of the same sort," despite repeated attempts. The type of thermometer that he was trying to make used water, open to the atmosphere, as the fluid of expansion. His original thermometers were thus sensitive to air pressure, and acted as both barometers and thermometers at the same time.
Fahrenheit subsequently recognized that both alcohol and mercury expanded with heat. He built closed bulb thermometers that contained alcohol (1709) and mercury (1714), thereby inventing the modern thermometer. With these instruments, he was able to make much more accurate and more consistent measurements of temperature. And he discovered, among other things, that water can be supercooled (cooled below its normal freezing point), and that the boiling point of a liquid is not a constant, but is a function of atmospheric pressure. Fahrenheit used supercooled water (from a mixture of water, ice, and sal ammoniac [ammonium chloride]) to establish his zero point, and his own body temperature to establish what would be 100 degrees. (Of course, we now gauge normal body temperature to be 98.6°F.) Fahrenheit's thermometers were the first to enable accurate and reproducible measurements of temperature.
Until the 1970s, the Fahrenheit scale was in common use in all English-speaking countries. Since then, the Celsius scale has been adopted by most countries (not including the United States).
SEE ALSO Temperature .
Todd W. Whitcombe
Asimov, Isaac (1989). Asimov's Chronology of Science and Discovery. New York: Harper & Row.
Fahrenheit, Daniel Gabriel. Philosophical Transactions, translated. Available from http://webserver.lemoyne.edu/faculty/giunta .