Abstract
1. The tests show that the sublimation temperature of solid carbon dioxide may be used as a convenient secondary reference point. 2. Fairly high accuracy and good stability are found when this point is reproduced in an appropriate way. The error involved is about 0.01°C. The method employs a large dewar and operations conducted in a strict sequence. The best results are obtained if supercooling occurs at the start. 3. The presence of air in the atmosphere around the solid affects the sublimation temperature. Methods that do not ensure that this effect is eliminated should not be used. In particular, methods involving solid carbon dioxide in alcohol or other liquids should not be used, nor should powdered carbon dioxide be used without allowing its temperature to settle down. 4. Different batches of dry ice from the same source showed no effect from impurities on the temperature, within 0.01°C limits. 5. The results show that this point may be used to check thermometers, which is a feature of considerable value in work in the range from 0°C to −80°C.
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