Temperature

About 1787 the French physicist, Jacques Charles (1746-1823) noted that a sample of gas at constant pressure regularly contracted by about 1/273 of its volume at 0°C for each Celsius degree drop in temperature. This suggests an interesting question: If a gas were cooled to 273° below zero, would its volume drop to zero? Would it just disappear? The answer is no, because most gases will condense to liquids long before such a low temperature is reached, and liquids behave quite differently from gases.

In 1848 William Thomson (1824-1907), later Lord Kelvin, suggested that it was not the volume, but the molecular translational energy that would become zero at about -273°C, and that this temperature was therefore the lowest possible temperature. Thomson suggested a new and more sensible temperature scale that would have the lowest possible temperature—absolute zero—set as zero on this scale. He set the temperature units as identical in size to the Celsius degrees. Temperature units on Kelvin's scale are now known as Kelvins (abbreviation, K); the term, degree, and its symbol, °, are not used anymore. Lord Kelvin's scale is called either the Kelvin scale or the absolute temperature scale. The normal freezing and boiling points of water on the Kelvin scale, then, are 273K and 373K, respectively, or, more accurately, 273.16K and 273.16K. To convert a Celsius temperature to Kelvin, just add 273.16.

The Kelvin scale is not the only absolute temperature scale. The Rankine scale, named for the Scottish engineer William Rankine (1820-1872), also has the lowest possible temperature set at zero. The size of the Rankine degree, however, is the same as that of the Fahrenheit degree. The Rankin temperature scale is rarely used today.

Absolute temperature scales have the advantage that the temperature on such a scale is directly proportional to the actual average molecular translational energy, the property that is measured by temperature. For example, if one object has twice the Kelvin temperature of another object, the molecules, or atoms, of the first object actually have twice the average molecular translational energy of the second. This is not true for the Celsius or Fahrenheit scales, because their zeroes do not represent zero energy. For this reason, the Kelvin scale is the only one that is used in scientific calculations.