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1889
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Many freezing and boiling points were measured during the last two decades of the nineteenth century. Callendar gave a detailed review of gas thermometry at the 1899 meeting of the British Association for the Advancement of Science (BAAS), when he made a proposal for a practical temperature scale. He proposed that a platinum resistance thermometer be adopted as the defining instrument of the scale, and that it be calibrated at the freezing point of water and the boiling points of water and sulphur. Further, he proposed that a particular batch of platinum wire be selected from which the thermometers defining the scale be manufactured.
It was his intention that such a scale be called the British Association Scale of Temperature and that it be related to the ideal temperature scale through chosen gas thermometer measurements of the sulphur point. It is not clear why the British Association did not take up his proposals.
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1911
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The Physikalish-Technische Reichanstalt (PTR, later to become the PTB), Berlin, addressed a circular letter to the BIPM, the National Physical Laboratory (NPL), Teddington, and the Bureau of Standards (BS, which in 1934 become the National Bureau of Standards, NBS, and in 1986 the National Institute of Standards and Technology, NIST), Washington, suggesting that the thermodynamic scale be adopted as the International Temperature Scale, and that a practical realization of it be the 1899 proposal of Callendar. Both the NPL and the BS agreed, the constants of the platinum were specified, and it was proposed that above the upper limit (1100 °C) the scale be defined in terms of the optical pyrometer.
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1913
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At the 5th CGPM, every encouragement was given to this initiative and a Resolution was adopted, asking the Directors of the three laboratories to meet with the aim of coming to a firm agreement on such a scale. The planned meeting did not take place, however, owing to the outbreak of the First World War.
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1923
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By the time discussions resumed, the three national laboratories had put into operation a platinum resistance thermometer scale covering the range from –38 °C, the freezing point of mercury, to 444.5 °C, the boiling point of sulphur, using a quadratic interpolation formula. During the course of a visit to the NPL and the PTR by a representative of the BS, the basis of an international scale was agreed upon. It was to consist of a platinum resistance thermometer to cover the range up to 650 °C, calibrated at 0 °C, 100 °C and the boiling point of sulphur at 444.5 °C. Between 650 °C and 1100 °C the scale was to be defined by a Pt-10 % Rh/Pt thermocouple calibrated at the freezing points of zinc, antimony, silver and gold and using a cubic interpolation formula. Above the gold point, 1063 °C, an optical pyrometer was proposed.
This informal agreement was followed by wider discussions in which the BIPM and the University of Leiden also participated.
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1925
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In 1925 a draft proposal was drawn up, to be put to the CIPM in 1927. In this the range of the platinum resistance thermometer was extended down to -193 °C, and the cubic equation of the thermocouple was replaced by a quadratic equation with calibration points at the freezing points of antimony (630 °C), silver (960 °C), and gold.
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1927
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The 7th CGPM adopted the International Temperature Scale of 1927, which differed very little from the draft of 1925. It was planned to hold an International Thermometry Conference in 1928, at which the question of the status of the International Temperature Scale would be examined in more detail. This Conference, however, did not take place.
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