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1954 |
The 10th CGPM finally adopted a proposal that Kelvin had made back in 1854, namely that the unit of thermodynamic temperature be defined in terms of the interval between the absolute zero and a single fixed point. The fixed point chosen was the triple point of water, which was assigned the thermodynamic temperature of 273.16 °K.
The proposal had already been made in 1948, but at that time there was still a divergence of view as to what value should be assigned to the absolute zero. The question was finally resolved by the CGPM in 1954.
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1961 |
In 1961 it was agreed that the NPL and the Physicotechnical and Radiotechnical Measurements Institute (PRMI), Moscow, would undertake a comparison of platinum resistance thermometers calibrated on four of the most important gas thermometer scales. These were the NPL(1961), NBS(1955), PRMI(1954) and Pennsylvania State University PSU(1954) scales. The results of the comparison provided the basis for the eventual low-temperature part of IPTS-68.
The NBS-55 scale is of particular note since it is an example of the way in which a so-called "wire-scale" can be successfully operated. NBS-55 is a scale based upon gas thermometry carried out in 1939. It was originally maintained on a group of six platinum resistance thermometers, and was known as NBS-39. In 1955 an arbitrary shift of 10 mK was made over the whole of the scale and the name was changed to NBS-55. The successors to these six original NBS-39 thermometers continued to be used to maintain an NBS version of IPTS-68.
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1964 |
The CCT defined a reference function W for interpolation between a number of low-temperature fixed points. The CCT-64 was published as a table under the title "Provisional reference table CCT-64 of W against T for platinum resistance thermometers in the range 12 K to 273.15 K".
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1966 |
A CCT Working Group proposed a 1966 Provisional Scale, taking into account further gas thermometry results for the oxygen boiling point and the hydrogen triple point.
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1968
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The second revision of the Temperature Scale took place, and resolved the curious situation that thermodynamic temperatures were defined in quite a different way from International Practical Temperatures. In the IPTS-68, both thermodynamic and practical units were defined to be identical and equal to 1/273.16 of the thermodynamic temperature of the triple point of water. The unit itself was renamed "the kelvin" in place of "degree Kelvin" and designated "K" in place of "°K".
In outline the IPTS-68 was made up of four parts:
(a) between 13.81 K and 273.15 K;
(b) 0 °C to 630.74 °C;
(c) 630.74 °C to 1064.43 °C; and
(d) above 1064.43 °C.(*)
In part (a) the Scale was defined in terms of a set of six low-temperature fixed points together with a reference function. In the range (b) the Scale was defined in terms of the old Callendar quadratic equation, but modified to take account of new gas thermometry values for the fixed points. In part (c) the defining instrument was the Pt-10 % Rh/Pt thermocouple, calibrated at 630.74 °C and the freezing points of silver and gold, and using a quadratic interpolation formula. Part (d) was defined in terms of the radiation emitted by a black body and described by Planck's equation.
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(*) Thermometrists generally refer to temperatures below 0 °C in kelvin, and those above in degrees Celsius.
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