The α–β Transition of Nitrogen

It is well-known that, aside from its triple point, nitrogen exhibits a solidto- solid transition at about 35K that is of some iInterest as a secondary reference temperature. During the recently published highly accuratemeasurements of the triple point of nitrogen (Metrologia 43, 435 (2006)), an extensive study was made also of the solid α–β transition of nitrogen, using both the continuous heating method and the pulse-heating method. This transition is of significantly lower quality than the triple point of nitrogen. A very high thermal resistance and a large time constant characterize the transition. Therefore, even the determination of the self-heating of the thermometer requires a very long time. A value of T90 =35.620K with an expanded uncertaintyU =8mKfor the coverage factor k =2 was found, differing by +6mK from the published CCT-recommended value. The reproducibility of the value was better than±5mK. In addition to the temperature value found for the transition, a comparison is made with previous measurements on this point, and an overview is given of the available information about it