The Temperature Scales

The scientist finds two temperature scales the most convenient. They are the centigrade (or Celsius) scale and the absolute (or Kelvin) scale. Many engineers use the Fahrenheit scale (currently in the United States). The oldest thermometric scale was devised by Gabriel Fahrenheit, a German instrument-maker. In 1714 Fahrenheit invented the mercury-in-glass thermometer. Zero degree on the Fahrenheit scale was selected as the lowest temperature obtainable with a mixture of common salt and water. For a higher reference point he selected the body temperature of a healthy man. The interval between reference points was divided into 96 equal parts. The centigrade scale was devised in 1742 by the Swedish astronomer Anders Celsius, who reasoned that water would make an excellent primary thermal standard. For convenience, he established the zero point on his scale at the freezing point of water and the 100-degree mark at its boiling point. Both temperatures are measured at one atmosphere pressure on air-saturated water. The relationship is (°F) = 1.80 x (°C) + 32.0.

Lord Kelvin in 1848 used his knowledge of the behaviour of gases to predict a temperature that would represent the lowest energetic state in which matter could exist. Theoretical predictions based on thermodynamics indicate that absolute zero can never be attained. Temperatures (T) on the absolute scale are measured in degrees Kelvin (K) in his honour. The size of the degree is identical with that on the centigrade scale. The relationship is (°K) = (°C) + 273.16.