Gas-Based Reference Points for Thermometry
For more than half a century after the fabrication of the first real thermometers, the only way to compare the values obtained from measurements made with two different thermometers was to place both instruments in the same “thermal bath,” e.g., air or water (chilled with ice or heated), and compare their readings. The idea that a certain physical state reproduces a unique temperature value, and consequently can be used to calibrate subsequently (or in different locations) different thermometers, was in fact not clearly understood until the second half of the 17th Century, when experimental evidence arose from the readings of the thermometers, and freezing ice was first used for this purposes (Hooke, 1664; Renaldini, 1694). A kind of ice point is reported to be in use for a similar application in ancient China, about 2000 years ago (Chen Xi-guong, 1986). The concept of fixed temperature can also be found in the work of Aristotle (circa A.D. 380), and, following his writings, Galen, the famous Greek physician (A.D. 130–200), introduced a “neutral degree of heat” obtained by mixing equal quantities of ice (his maximum degree of cold) and boiling water (his maximum degree of heat). But the “neutral” degree so obtained was said to be halfway between these extremes, since at that time heat and cold were both considered substances. We know today that such a mixture has actually a temperature of only about 10°C, because of ice enthalpy of fusion and of the variable specific heat of water in that range. These concepts only became clear in the 19th Century.
KeywordsTriple Point Melting Range Melted Fraction Carnot Cycle Melting Plateau
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