Historically, it was found that Kelvin is the one needed in the formula relating the volume of a gas to its temperature. Take the ideal gas law, PV = nRT. At constant pressure, V = T (nR/P), where (nR/P) is constant. That is, volume is directly proportional to temperature. Obviously, degrees C cannot be plugged in because at 0°C the formula would say the volume is zero, which it is not. A scientist by the name of Renault found in 1847 that a gas expands by 1/273 of its volume for each degree C as you increase the temperature from 0°C.

By defining an absolute temperature in the following way,

T = measured °C + 273 °C

the correct relation between volume and temperature is established:

V=V0TT0
 

where the subscript zero refers to 0°C. Later it was found that absolute zero had a much deeper theoretical significance than the bottom of the gas scale. It is the point where thermal energy ceases. Absolute zero therefore became the rational bottom of the thermodynamic scale.
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