More people are using Celsius or Fahrenheit

Science in dialogue

Why are there different temperature scales in the world?

Temperature scales and units, like other units, can be set arbitrarily. Unlike with sizes such as length or weight, however, you can also choose the zero point for the temperature within certain limits.

A temperature scale can therefore be defined, for example, by specifying at least two so-called fixed points, between which the scale is then divided equally. The two best-known temperature scales are named after the thermometer builders Daniel Gabriel Fahrenheit (1686-1736) and Anders Celsius (1701-1744). Celsius chose the temperatures of an ice-water mixture (0 ° C) and boiling water (100 ° C) as fixed points. Fahrenheit flirted with various fixed points, including the body temperature of healthy people, the temperature of a cold mixture of ice, water and salmiac and the lowest temperature in Gdansk in the extremely cold winter of 1708/1709. On the temperature scale named after him today, an ice-water mixture has a temperature of 32 ° F and boiling water has a temperature of 212 ° F.

Fahrenheit was born in Gdansk and spent many years of his life in German countries, but was also a member of the Royal Society in London and in England highly regarded. It is understandable that the temperature scale named after him was particularly widespread in Anglo-Saxon countries and is still common in the USA today. One advantage of the Fahrenheit scale is that even in cold winters, negative temperatures in ° F hardly ever occur. In most other countries and in the sciences, on the other hand, the Celsius scale has prevailed.

In the second half of the 19th century it was recognized that temperature is a measure of the movement of atoms or molecules. There must therefore be the lowest possible temperature that is present when the atoms or molecules no longer move: it cannot get colder. This temperature is -273.15 ° C. William Thomson (who was later ennobled to Lord Kelvin of Largh) pointed out in 1864 that this temperature can be made the zero point of a temperature scale. However, such a temperature scale was not introduced until 1960, and the unit was called Kelvin, or K for short.

The conversion from ° C to K is very simple: t / ° C = T / K - 273.15. Temperature differences therefore have the same numerical value in both units.

The Kelvin scale is mainly used at low temperatures. In addition, some physical formulas are simpler if the temperature in K is used. The Celsius scale is still used in everyday life because we are used to it and because the numbers are easier to use. But whoever has a fever of 39.5 ° C should of course be in bed just as much as someone who has the same temperature as 312.65 K (or 103.1 ° F in the USA).

The question was answered by Dr. Erich Tegeler, Temperature Department of the Physikalisch-Technische Bundesanstalt