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1878 |
Preparation of metre prototypes was begun. Each national prototype metre was furnished with two mercury-in-glass thermometers, calibrated at the BIPM. The thermometers were made, to the order of the BIPM, by an instrument maker in Paris called Tonnelot. The thermometers were made of verre dur, a particularly good glass from the point of view of stability, and a reproducibility of measurement of a few thousandths of a degree was possible. It became urgent to establish a uniform scale of temperature against which they could be calibrated.
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1884–87 |
Chappuis, at the BIPM, worked to relate the readings of the very best mercury-in-glass thermometers to absolute (i.e. thermodynamic) temperatures. In the first part of his study he considered in detail the constant-volume gas thermometer, using, in turn, hydrogen, nitrogen and carbon dioxide as the working fluids. The estimated uncertainty of his measurements was better than one-hundredth of a degree over most of the range studied, to 100 °C.
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1887 |
The CIPM adopted the constant-volume hydrogen scale (called the normal hydrogen scale), based upon fixed points at the ice point (0 °C) and the steam point (100 °C) as the practical scale for international metrology. This decision was ratified by the 1st CGPM in 1889.
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1888–89 |
Chappuis continued his work at the BIPM with an investigation of the constant-pressure gas thermometer using the same three gases. He concluded that the constant-volume thermometer provided a more convenient practical standard than did the constant pressure thermometer. At the instigation of Griffiths, of Kew Observatory, UK, the work was pursued, using a constant- volume thermometer, to extend the temperature range to higher temperatures. In collaboration with Callendar, Griffiths had been developing a platinum resistance thermometer which was stable to at least 600 °C. Callendar and Griffiths used the boiling point of sulphur, which they deduced to be 444.53 °C, as a third fixed point for calibration, and proposed to the BIPM that a comparison be made between their platinum resistance thermometers and the constant-volume gas thermometer of Chappuis.
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1897 |
This comparison was carried out by Chappuis, in collaboration with Harker of the Kew Observatory. It involved the establishment of a constant-volume nitrogen scale up to the boiling point of sulphur. The Chappuis/Harker measurement of the sulphur point led to a value of 444.70 °C, in very close agreement with the earlier result of Callendar and Griffiths.
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