There are several scales for measuring temperature (Note 1.K). In 1709 the German physicist Gabriel Fahrenheit developed the oldest scale still in common use, taking as zero the lowest temperature he had recorded in the city of Danzig, and fixing the upper limit of the scale at the temperature of the human body, which he (erroneously) took to be 100°F. (It is actually near 96°F.) The boiling point of water at sea level is 212 degrees on such a scale.
However, another scale, based on the properties of water, has now become universal. Zero corresponds to the temperature at which ice forms and 100 degrees is the boiling point of pure water at sea level. It was called the Centigrade scale until 1948, but such a hundred-unit scale may be used for measuring things other than temperature. Also, there is now a convention to relate all units to the names of famous scientists, in this case Anders Celsius (1701-44). The relationship between the Celsius (°C) and the Fahrenheit (°F) scales can be found in Note 1.K.
There is also a third measure of temperature, the Kelvin scale (Note 1.K), commonly used in science. In this book, we use both °C and K. In particular, we use K to designate a temperature difference, so that 8°C and 10°C differ by 2 K, for example.
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