The absolute temperature scale in which the temperature measure is based on the average kinetic energy per molecule of a perfect gas. The zero of the Kelvin scale is -273.16 degrees C. The temperature scale adopted by the International Bureau of Weights and Measures is that of the constant volume hydrogen gas thermometer. The magnitude of the degree in both these scales is defined as one one-hundredths the difference between the temperature of melting ice and that of boiling water at 760 mm pressure.