The total radiant exitance of a black body at the temperature of the triple point of water,
T
tp
(273.16 K), and at a series of other temperatures in the range from about 233 K ( — 40 °C) to 373 K (100 °C), has been measured by using a cryogenic radiometer. From the measurements at
T
tp
a value for the Stefan—Boltzmann constant or has been calculated: ( r = (5.66967 + 0.00076) x 10
-8
W m
-2
K
-4
. This is the first radiometric determination of or having an uncertainty comparable with that calculated directly from fundamental physical constants. This measured value differs from the calculated one by 13 parts in 10
5
, which is less than the combined standard deviations of the measured and calculated values. mbined standard deviations of the measured and calculated values. From the measurements of exitance at the other temperatures, values of the corresponding thermodynamic temperature
T
have been calculated by using Stefan’s fourth-power law. Since the temperature of the radiating black body was also measured by platinum resistance thermometers calibrated on IPTS-68, values of (
T
—
T
68
) were obtained. These range from about — (5 + 1.6) mK at 20 °C to — (28 ±2.5) mK at 100 °C and + (5 + 1.5) mK at —40 °C. The results confirm to within a few millikelvins the departure of
T
68
from
T
above 0 °C already discovered by gas thermometry and show that similar departures, but of opposite sign, exist down to the lowest temperature measured, — 40 °C. The uncertainties associated with these new values of
T
and (
T — T
68
) are similar to those of the best gas thermometry.
Progress towards the determination of thermodynamic temperature with ultra-low uncertainty