What is correct: To mention temperature in deg K (°K) or in just K?

Asked by: Prof. C. F. Desai

Answer

The correct one is just K, not degrees K.

The confusion arises due to the other common temperatures scale, the Celsius scale (based on the old Centigrade scale). This scale was derived from getting a mercury-in-glass thermometer and marking the ice-point and steam-point on it. The distance between these points is then divided into 100 divisions or degrees. The units used here are thus called degrees Celsius.

However, Kelvin, the SI unit of temperature, is based on the triple point of water (The temperature where water can exist in all 3 states). 1 K = 1/273.16 of the triple point of water. Kelvin, as an SI unit is similar to the metre and the second, and we don't say degrees metre, or degrees second do we?

Note from the editor: Degree K or °K was actually used in the past but it became obsolete by international agreement in 1967.
Answered by: Simon Hooks, Physics A-Level Student, Gosport, UK