What is the difference between the historical definition of the centigrade temperature scale and the modern Celsius scale?
In 1742, Anders Celsius published a paper titled (after translation) "Observations on Two Constant Degrees on One Thermometer" in the Transactions of the Swedish Academy of Sciences. The *two constants* were the ice point and the boiling point of water, separated by exactly 100 degrees on that "centigrade" scale. Celsius chose the boiling point as the zero on his original scale. On the other hand, the modern Celsius scale has *only one* fixed point, a zero at 273.15 K, and an increment (unit) of temperature difference identical to 1 K, as Jim Frysinger has explained. Thus, the original "centigrade" scale was, in fact, *different* from the modern Celsius scale. However, the definition of the original centigrade scale, and its immediate successor with the zero reset at the ice point, were effectively discarded by a series of resolutions by the CGPM as Joe has explained. Gene.
