The history of naming temperature intervals can be considered a history of removing the ambiguity introduced by the word degree. In 1742, Anders Celsius, a Swedish astronomer, proposed dividing up the temperature interval between the boiling point of water and the freezing point of water into 100 steps. This is regarded as the origin of the Centigrade temperature scale. The prefix centi-, meaning “100” (as in centipede), is combined with the word grade, which is derived from the Latin word gradus, meaning “step.” Thus, Centigrade means “100 steps.”
1
In 1795, 53 years after Anders Celsius proposed a Centigrade temperature scale, the prefix centi- began to be used in the metric system to mean “1/100,” as in centimeter, centigram, and centiliter. Later, in the 1850s, with the widespread introduction of the metric system, Centigrade started to cause confusion. This is because many European languages have a word similar to grade as their word for degree. For example, German has Grad, Swedish has grad, and Spanish and Italian have grado. Thus, scientific communications developed an ambiguity. When speaking of a Centigrade, did one mean the temperature scale or 1/100 of some degree measure? In order to remove this confusion, scientists agreed in 1948 that the temperature unit degree Centigrade would henceforth be called degree Celsius and the symbol would be °C.
2
In 1954, the definition of the Celsius scale itself was changed. Rather than using the freezing and boiling points of water at 1 atmosphere of pressure, the degree interval Celsius was set equal to the degree interval Kelvin, and 0 °C was set equal to 273.15 degree Kelvin. Thus, the Kelvin scale became the fundamental temperature scale. Its fundamental unit was the degree Kelvin, with the symbol °K. Unfortunately, here also the word degree introduced complications as temperature measurements became finer. For example, the metric system dictates that 0.01 meter is equal to 1 centimeter. However, is 0.01 degree Kelvin equal to 1 centidegree Kelvin, or 1 degree centiKelvin? In order to remove this ambiguity, scientists agreed in 1967 that degree Kelvin would no longer be used to describe the fundamental temperature interval. The fundamental temperature interval would be called simply kelvin (with a lowercase k), and the symbol would be K without any degree symbol (°). The temperature interval in the Celsius scale, however, would retain the word degree, the capitalized C in Celsius, and the symbol °C.
3
Therefore, at the present time, the accepted way to indicate the freezing point of water at 1 atmosphere of pressure in the metric system is 273.15 kelvins, 273.15 K, 0 degrees Celsius, or 0 °C.
